Dr. Harun Tüysüz

associated member

Heterogenous Catalysis
Max-Planck-Institut für Kohlenforschung

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  • A Highly-Efficient Oxygen Evolution Electrocatalyst Derived from a Metal-Organic Framework and Ketjenblack Carbon Material
    Öztürk, S. and Moon, G.-H. and Spieß, A. and Budiyanto, E. and Roitsch, S. and Tüysüz, H. and Janiak, C.
    ChemPlusChem (2021)
    The composite of the metal-organic framework (MOF) Ni(Fe)-MOF-74 and the highly conductive carbon material ketjenblack (KB) could be easily obtained from the in-situ MOF synthesis in a one-step solvothermal reaction. The composite material features a remarkable electrochemical oxygen evolution reaction (OER) performance where the overpotential at 10 mA/cm2 and the current density at 1.7 VRHE are recorded as 0.274 VRHE and 650 mA/cm2, respectively, in 1 mol/L KOH. In particular, the activation of nickel-iron clusters from the MOF under an applied anodic bias steadily boosts the OER performance. Although Ni(Fe)-MOF-74 goes through some structural modification during the electrochemical measurements, the stabilized and optimized composite material shows excellent OER performance. This simple strategy to design highly-efficient electrocatalysts, utilizing readily available precursors and carbon materials, will leverage the use of diverse metal-organic complexes into electrode fabrication with a high energy conversion efficiency. © 2021 The Authors. ChemPlusChem published by Wiley-VCH GmbH
    view abstract10.1002/cplu.202100278
  • Dynamics of reactive oxygen species on cobalt-containing spinel oxides in cyclic co oxidation
    Dreyer, M. and Rabe, A. and Budiyanto, E. and Ortega, K.F. and Najafishirtari, S. and Tüysüz, H. and Behrens, M.
    Catalysts 11 (2021)
    Reactive oxygen species (ROS) are considered to be responsible for the high catalytic activity of transition metal oxides like Co3-xFexO4 in oxidation reactions, but the detailed influences of catalyst composition and morphology on the formation of these reactive oxygen species are not fully understood. In the presented study, Co3O4 spinels of different mesostructures, i.e., particle size, crystallinity, and specific surface area, are characterized by powder X-ray diffraction, scanning electron microscopy, and physisorption. The materials were tested in CO oxidation performed in consecutive runs and compared to a Co3-xFexO4 composition series with a similar mesostructure to study the effects of catalyst morphology and composition on ROS formation. In the first run, the CO conversion was observed to be dominated by the exposed surface area for the pure Co-spinels, while a negative effect of Fe content in the spinels was seen. In the following oxidation run, a U-shaped conversion curve was observed for materials with high surface area, which indicated the in situ formation of ROS on those materials that were responsible for the new activity at low temperature. This activation was not stable at the higher reaction temperature but was confirmed after temperature-programmed oxidation (TPO). However, no activation after the first run was observed for low-surface-area and highly crystalline materials, and the lowest surface-area material was not even activated after TPO. Among the catalyst series studied here, a correlation of small particle size and large surface area with the ability for ROS formation is presented, and the benefit of a nanoscaled catalyst is discussed. Despite the generally negative effect of Fe, the highest relative activation was observed at intermediate Fe contents suggesting that Fe may be involved in ROS formation. © 2021 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/catal11111312
  • Holistic View on Materials Development: Water Electrolysis as a Case Study
    Klemenz, S. and Stegmüller, A. and Yoon, S. and Felser, C. and Tüysüz, H. and Weidenkaff, A.
    Angewandte Chemie - International Edition (2021)
    In view of rising ecological awareness, materials development is primarily aimed at improving the performance and efficiency of innovative and more elaborate materials. However, a materials performance figure of merit should include essential aspects of materials: environmental impact, economic constraints, technical feasibility, etc. Thus, we promote the inclusion of sustainability criteria already during the materials design process. With such a holistic design approach, new products may be more likely to meet the circular economy requirements than when traditional development strategies are pursued. Using catalysts for water electrolysis as an example, we present a modelling method based on experimental data to holistically evaluate processes. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202105324
  • Identification of Active Sites in the Catalytic Oxidation of 2-Propanol over Co1+xFe2–xO4 Spinel Oxides at Solid/Liquid and Solid/Gas Interfaces
    Falk, T. and Budiyanto, E. and Dreyer, M. and Pflieger, C. and Waffel, D. and Büker, J. and Weidenthaler, C. and Ortega, K.F. and Behrens, M. and Tüysüz, H. and Muhler, M. and Peng, B.
    ChemCatChem 13 (2021)
    A series of Co1+xFe2–xO4 (0≤x≤2) spinel nanowires was synthesized by nanocasting using SBA-15 silica as hard template, which was characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The Co1+xFe2–xO4 spinels were applied in the aerobic oxidation of aqueous 2-propanol solutions to systematically study the influence of exposed Co and Fe cations on the catalytic properties. The activity of the catalysts was found to depend strongly on the Co content, showing an exponential increase of the reaction rate with increasing Co content. Ensembles of Co3+cus (coordinatively unsaturated) sites were identified as the active sites for selective 2-propanol oxidation, which are assumed to consist of more than six Co ions. In addition, gas-phase oxidation with and without water vapor co-feeding was performed to achieve a comparison with liquid-phase oxidation kinetics. An apparent activation energy of 94 kJ mol−1 was determined for 2-propanol oxidation over Co3O4 in the liquid phase, which is in good agreement with the gas-phase oxidation in the presence of water vapor. In contrast to gas-phase conditions, the catalysts showed high stability and reusability in the aqueous phase with constant conversion in three consecutive runs. © 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH
    view abstract10.1002/cctc.202100352
  • Impact of single-pulse, low-intensity laser post-processing on structure and activity of mesostructured cobalt oxide for the oxygen evolution reaction
    Budiyanto, E. and Zerebecki, S. and Weidenthaler, C. and Kox, T. and Kenmoe, S. and Spohr, E. and Debeer, S. and Rüdiger, O. and Reichenberger, S. and Barcikowski, S. and Tüysüz, H.
    ACS Applied Materials and Interfaces (2021)
    Herein, we report nanosecond, single-pulse laser post-processing (PLPP) in a liquid flat jet with precise control of the applied laser intensity to tune structure, defect sites, and the oxygen evolution reaction (OER) activity of mesostructured Co3O4. High-resolution X-ray diffraction (XRD), Raman, and X-ray photoelectron spectroscopy (XPS) are consistent with the formation of cobalt vacancies at tetrahedral sites and an increase in the lattice parameter of Co3O4 after the laser treatment. X-ray absorption spectroscopy (XAS) and X-ray emission spectroscopy (XES) further reveal increased disorder in the structure and a slight decrease in the average oxidation state of the cobalt oxide. Molecular dynamics simulation confirms the surface restructuring upon laser post-treatment on Co3O4. Importantly, the defect-induced PLPP was shown to lower the charge transfer resistance and boost the oxygen evolution activity of Co3O4. For the optimized sample, a 2-fold increment of current density at 1.7 V vs RHE is obtained and the overpotential at 10 mA/cm2 decreases remarkably from 405 to 357 mV compared to pristine Co3O4. Post-mortem characterization reveals that the material retains its activity, morphology, and phase structure after a prolonged stability test. © XXX The Authors.
    view abstract10.1021/acsami.1c08034
  • In situ total scattering experiments of nucleation and crystallisation of tantalum-based oxides: From highly dilute solutions via cluster formation to nanoparticles
    Onur Şahin, E. and Tüysüz, H. and Chan, C.K. and Moon, G.-H. and Dai, Y. and Schmidt, W. and Lim, J. and Scheu, C. and Weidenthaler, C.
    Nanoscale 13 (2021)
    The exact formation mechanism of tantalum oxides (and in general, metal/mixed metal oxides) from alkoxide precursors is still not fully understood, particularly when forming cluster-like or amorphous materials. The structural evolution of Ta-based oxides was studied in detail using X-ray total scattering experiments along with subsequent pair distribution function (PDF) analyses. Starting from a tantalum alkoxide precursor (Ta2(OEt)10), the formation of hydrolysed TaxOyHz clusters in highly diluted aqueous solution was analysed. From the PDF data, the connectivity and arrangement of TaxOy octahedra in the cluster could be deduced as well as the approximate size of the clusters (<1 nm). Construction of cluster models allowed for identification of common structural motifs in the TaxOyHz clusters, ruling out the formation of chain- or ring-like clusters. More likely, bulky clusters with a high number of corner-sharing octahedra are formed. After separation of the amorphous solid from the liquid, temperature-induced crystallisation processes were monitored via in situ total scattering experiments. Between room temperature and 600 °C, only small rearrangements of the amorphous structure are observed. At about 610 °C, amorphous TaxOyHz transforms directly into crystalline orthorhombic L-Ta2O5 without formation of any crystalline intermediate structures. © 2021 The Royal Society of Chemistry.
    view abstract10.1039/d0nr07871a
  • In-Depth Comparative Study of the Cathode Interfacial Layer for a Stable Inverted Perovskite Solar Cell
    Lee, J. and Tüysüz, H.
    ChemSusChem 14 (2021)
    Achieving long-term device stability is one of the most challenging issues that impede the commercialization of perovskite solar cells (PSCs). Recent studies have emphasized the significant role of the cathode interfacial layer (CIL) in determining the stability of inverted p-i-n PSCs. However, experimental investigations focusing on the influence of the CIL on PSC degradation have not been systematically carried out to date. In this study, a comparative analysis was performed on the PSC device stability by using four different CILs including practical oxides like ZnO and TiOx. A new implemented co-doping approach was found to results in high device performance and enhanced device stability. The PSC with a thick film configuration of chemically modified TiOx CIL preserves over 77 % of its initial efficiencies of 17.24 % for 300 h under operational conditions without any encapsulation. The PSCs developed are among the most stable reported for methylammonium lead iodide (MAPbI3) perovskite compositions. © 2021 The Authors. ChemSusChem published by Wiley-VCH GmbH
    view abstract10.1002/cssc.202100585
  • Large-Scale Production of Carbon-Supported Cobalt-Based Functional Nanoparticles for Oxygen Evolution Reaction
    Bähr, A. and Petersen, H. and Tüysüz, H.
    ChemCatChem (2021)
    A series of Co-based nanoparticles supported on activated carbon was synthesized by using waste tea leaves as a template as well as a sustainable carbon source. The crystal structure of the Co particles was adjusted by post-treatments with H2O2, ethanol vapor, and H2, which result in Co3O4, CoO, and metallic Co phases, respectively. After these different treatments, the composite materials consist of small Co-based nanoparticles with an average crystallite size of 6–14 nm supported on activated carbon with apparent specific surface areas up to 1065 m2 g−1. Correlations between the structure of the materials and their activity for the oxygen evolution reaction (OER) were established, whereby the post-treatment with ethanol vapor was found to yield the most effective electrocatalyst. The material shows good stability at 10 mA cm−2 over 10 h and reaches a mass activity of 2.9 A mgCo−1, which is even higher than pristine ordered mesoporous Co3O4. The superior electrocatalytic performance is ascribed to a high dispersion of Co-based nanoparticles and the conductivity of the activated carbon that facilitate the charge transport. © 2021 The Authors. ChemCatChem published by Wiley-VCH GmbH.
    view abstract10.1002/cctc.202100594
  • Monitoring the Structure Evolution of Titanium Oxide Photocatalysts: From the Molecular Form via the Amorphous State to the Crystalline Phase
    Onur Şahin, E. and Dai, Y. and Chan, C.K. and Tüysüz, H. and Schmidt, W. and Lim, J. and Zhang, S. and Scheu, C. and Weidenthaler, C.
    Chemistry - A European Journal (2021)
    Amorphous TixOy with high surface area has attracted significant interest as photocatalyst with higher activity in ultraviolet (UV) light-induced water splitting applications compared to commercial nanocrystalline TiO2. Under photocatalytic operation conditions, the structure of the molecular titanium alkoxide precursor rearranges upon hydrolysis and leads to higher connectivity of the structure-building units. Structurally ordered domains with sizes smaller than 7 Å form larger aggregates. The experimental scattering data can be explained best with a structure model consisting of an anatase-like core and a distorted shell. Upon exposure to UV light, the white TixOy suspension turns dark corresponding to the reduction of Ti4+ to Ti3+ as confirmed by electron energy loss spectroscopy (EELS). Heat-induced crystallisation was followed by in situ temperature-dependent total scattering experiments. First, ordering in the Ti−O environment takes place upon to 350 °C. Above this temperature, the distorted anatase core starts to grow but the structure obtained at 400 °C is still not fully ordered. © 2021 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/chem.202101117
  • Preparation and Properties of Polystyrene Nanospheres Incorporated Cs3Bi2Br9 Halide Perovskite Disks
    Wang, Y. and Dai, Y. and Tüysüz, H.
    European Journal of Inorganic Chemistry 2021 (2021)
    Halide perovskites are being in the focus of diverse research fields due to their attractive physicochemical properties. Herein, we present a strategy to prepare highly crystalline bismuth-based halide perovskite disk, polystyrene nanospheres incorporated hybrid material and porous halide perovskite. After a proper surface modification of polystyrene, the halide perovskite was grown and crystallized around the sulfonated polystyrene via a facile solution processed synthesis and rapid solidification. By tuning the particle size of sulfonated polystyrene, a range of lead-free hybrid materials was prepared. All the hybrid materials have good visible light absorption with an optical band gap of around 2.6 eV and variable photoluminescence properties. The incorporated sulfonated polystyrene could enhance the stability of halide perovskite in high humidity environments. Additional porous functionality could be brought to the halide perovskite materials through selective leaching of sulfonated polystyrene. © 2021 The Authors. European Journal of Inorganic Chemistry published by Wiley-VCH GmbH
    view abstract10.1002/ejic.202100338
  • Principles of Water Electrolysis and Recent Progress in Cobalt-, Nickel-, and Iron-Based Oxides for the Oxygen Evolution Reaction
    Yu, M. and Budiyanto, E. and Tüysüz, H.
    Angewandte Chemie - International Edition (2021)
    Water electrolysis that results in green hydrogen is the key process towards a circular economy. The supply of sustainable electricity and availability of oxygen evolution reaction (OER) electrocatalysts are the main bottlenecks of the process for large-scale production of green hydrogen. A broad range of OER electrocatalysts have been explored to decrease the overpotential and boost the kinetics of this sluggish half-reaction. Co-, Ni-, and Fe-based catalysts have been considered to be potential candidates to replace noble metals due to their tunable 3d electron configuration and spin state, versatility in terms of crystal and electronic structures, as well as abundance in nature. This Review provides some basic principles of water electrolysis, key aspects of OER, and significant criteria for the development of the catalysts. It provides also some insights on recent advances of Co-, Ni-, and Fe-based oxides and a brief perspective on green hydrogen production and the challenges of water electrolysis. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202103824
  • Rapid Acidic Media Growth of Cs3Bi2Br9 Halide Perovskite Platelets for Photocatalytic Toluene Oxidation
    Dai, Y. and Tüysüz, H.
    Solar RRL 5 (2021)
    Organic ligands with long carbon chains have been widely utilized to mediate the growth of halide perovskite crystals with tunable morphologies. However, the presence of these surfactants on the surface of halide perovskites limits their performance in photocatalytic conversion applications. Herein, a rapid synthetic protocol to prepare Cs3Bi2Br9 platelets with clean surfaces and controllable thickness in a dilute H2SO4 solution after a quick cooling process in liquid nitrogen or mixtures of dry ice is reported. Electron microscopy and X-ray diffraction reveal the preferential exposure of (00l) facets in Cs3Bi2Br9 platelets with variable thickness from 100 to 500 nm. Infrared spectroscopy hints that the selective chemisorption of ethyl acetoacetate on (00l) facets of bismuth perovskites regulates the growth of the crystals. These novel lead-free halide perovskite platelets can drive the photo-oxidation of toluene to benzaldehyde with high selectivity (≥88%) and stability over 36 h. © 2021 The Authors. Solar RRL published by Wiley-VCH GmbH
    view abstract10.1002/solr.202100265
  • Tunable eg Orbital Occupancy in Heusler Compounds for Oxygen Evolution Reaction**
    Yu, M. and Li, G. and Fu, C. and Liu, E. and Manna, K. and Budiyanto, E. and Yang, Q. and Felser, C. and Tüysüz, H.
    Angewandte Chemie - International Edition 60 (2021)
    Heusler compounds have potential in electrocatalysis because of their mechanical robustness, metallic conductivity, and wide tunability in the electronic structure and element compositions. This study reports the first application of Co2YZ-type Heusler compounds as electrocatalysts for the oxygen evolution reaction (OER). A range of Co2YZ crystals was synthesized through the arc-melting method and the eg orbital filling of Co was precisely regulated by varying Y and Z sites of the compound. A correlation between the eg orbital filling of reactive Co sites and OER activity was found for Co2MnZ compounds (Z=Ti, Al, V, and Ga), whereby higher catalytic current was achieved for eg orbital filling approaching unity. A similar trend of eg orbital filling on the reactivity of cobalt sites was also observed for other Heusler compounds (Co2VZ, Z=Sn and Ga). This work demonstrates proof of concept in the application of Heusler compounds as a new class of OER electrocatalysts, and the influence of the manipulation of the spin orbitals on their catalytic performance. © 2020 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202013610
  • A hydrogen-dependent geochemical analogue of primordial carbon and energy metabolism
    Preiner, M. and Igarashi, K. and Muchowska, K.B. and Yu, M. and Varma, S.J. and Kleinermanns, K. and Nobu, M.K. and Kamagata, Y. and Tüysüz, H. and Moran, J. and Martin, W.F.
    Nature Ecology and Evolution 4 (2020)
    Hydrogen gas, H2, is generated by alkaline hydrothermal vents through an ancient geochemical process called serpentinization, in which water reacts with iron-containing minerals deep within the Earth’s crust. H2 is the electron donor for the most ancient and the only energy-releasing route of biological CO2 fixation, the acetyl-CoA pathway. At the origin of metabolism, CO2 fixation by hydrothermal H2 within serpentinizing systems could have preceded and patterned biotic pathways. Here we show that three hydrothermal minerals—greigite (Fe3S4), magnetite (Fe3O4) and awaruite (Ni3Fe)—catalyse the fixation of CO2 with H2 at 100 °C under alkaline aqueous conditions. The product spectrum includes formate (up to 200 mM), acetate (up to 100 µM), pyruvate (up to 10 µM), methanol (up to 100 µM) and methane. The results shed light on both the geochemical origin of microbial metabolism and the nature of abiotic formate and methane synthesis in modern hydrothermal vents. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.
    view abstract10.1038/s41559-020-1125-6
  • A Supported Bismuth Halide Perovskite Photocatalyst for Selective Aliphatic and Aromatic C–H Bond Activation
    Dai, Y. and Poidevin, C. and Ochoa-Hernández, C. and Auer, A.A. and Tüysüz, H.
    Angewandte Chemie - International Edition 59 (2020)
    Direct selective oxidation of hydrocarbons to oxygenates by O2 is challenging. Catalysts are limited by the low activity and narrow application scope, and the main focus is on active C−H bonds at benzylic positions. In this work, stable, lead-free, Cs3Bi2Br9 halide perovskites are integrated within the pore channels of mesoporous SBA-15 silica and demonstrate their photocatalytic potentials for C−H bond activation. The composite photocatalysts can effectively oxidize hydrocarbons (C5 to C16 including aromatic and aliphatic alkanes) with a conversion rate up to 32900 μmol gcat−1 h−1 and excellent selectivity (&gt;99 %) towards aldehydes and ketones under visible-light irradiation. Isotopic labeling, in situ spectroscopic studies, and DFT calculations reveal that well-dispersed small perovskite nanoparticles (2–5 nm) possess enhanced electron–hole separation and a close contact with hydrocarbons that facilitates C(sp3)−H bond activation by photoinduced charges. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201915034
  • Cl2 Adsorption and Desorption over Ordered Mesoporous Carbon Materials as an Indicator for Catalytic Phosgene Formation
    Bähr, A. and Diedenhoven, J. and Tüysüz, H.
    Chemie-Ingenieur-Technik 92 (2020)
    The presented work evaluates the implementation of adsorption and desorption experiments with Cl2 over carbon materials towards the development of more active and stable catalysts for industrial phosgene synthesis. By using a soft templating method as a tool, ordered mesoporous carbon materials with tunable porosity, surface area, and degree of graphitization were synthesized and utilized as model system. The Cl2 adsorption/desorption properties of these materials were studied and compared to commercial activated carbon. To draw correlations between Cl2 adsorption/desorption behavior and catalytic performance, the materials were further tested in the phosgene formation in a plug flow reactor. However, the chemical reaction of Cl2 with carbon during the adsorption/desorption experiment hinders a direct correlation. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cite.202000040
  • Coulometric Titration of Active Sites at Mesostructured Cobalt Oxide Spinel by Surface Interrogation Mode of Scanning Electrochemical Microscopy
    Lorenz, J. and Yu, M. and Tuÿsüz, H. and Harms, C. and Dyck, A. and Wittstock, G.
    Journal of Physical Chemistry C 124 (2020)
    Cobalt-based transition-metal oxides are promising candidates for the oxygen evolution reaction (OER). However, a complex interplay between the catalyst crystal structures and material morphologies as well as the surface reactions hampers a comprehensive understanding of the electrocatalytic OER at those materials. Here, we investigate the amount and reactivity of specific surface sites of a mesostructured cobalt oxide spinel powder by surface interrogation mode of scanning electrochemical microscopy (SI-SECM). The powder material was supplied in cavity microelectrodes and efficiently titrated with an Fe(II)-triethanolamine redox mediator generated at a gold microelectrode in an alkaline electrolyte. Thus, quantification of different surface sites was achieved, and their reactivity showed dependence on the cobalt oxidation state. Titration experiments after adjustable time delays with respect to the generation of the different surface sites indicated that these surface sites are active for the OER. Kinetic analysis revealed two pseudo-first-order decay constants that were related to fast and slow surface sites for the OER. Rate constants were determined for potentials where predominantly a mixed-valence CoIII/IV state might be present as the most active species. These results expand the great potential of the surface interrogation mode on studying the reaction kinetics of distinct surface sites for practically relevant powdered, nonprecious metal catalysts to address a highly relevant challenge in electrocatalysis. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acs.jpcc.9b11114
  • Dual Role of Silver Moieties Coupled with Ordered Mesoporous Cobalt Oxide towards Electrocatalytic Oxygen Evolution Reaction
    Yu, M. and Moon, G.-H. and Castillo, R.G. and DeBeer, S. and Weidenthaler, C. and Tüysüz, H.
    Angewandte Chemie - International Edition 59 (2020)
    Herein, we show that the performance of mesostructured cobalt oxide electrocatalyst for oxygen evolution reaction (OER) can be significantly enhanced by coupling of silver species. Various analysis techniques including pair distribution function and Rietveld refinement, X-ray absorption spectroscopy at synchrotron as well as advanced electron microscopy revealed that silver exists as metallic Ag particles and well-dispersed Ag2O nanoclusters within the mesostructure. The benefits of this synergy are twofold for OER: highly conductive metallic Ag improves the charge transfer ability of the electrocatalysts while ultra-small Ag2O clusters provide the centers that can uptake Fe impurities from KOH electrolyte and boost the catalytic efficiency of Co–Ag oxides. The current density of mesostructured Co3O4 at 1.7 VRHE is increased from 102 to 211 mA cm−2 with incorporation of silver spices. This work presents the dual role of silver moieties and demonstrates a simple method to increase the OER activity of Co3O4. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA
    view abstract10.1002/anie.202003801
  • Investigation of Synergistic Effects between Co and Fe in Co3-xFexO4 Spinel Catalysts for the Liquid-Phase Oxidation of Aromatic Alcohols and Styrene
    Waffel, D. and Budiyanto, E. and Porske, T. and Büker, J. and Falk, T. and Fu, Q. and Schmidt, S. and Tüysüz, H. and Muhler, M. and Peng, B.
    Molecular Catalysis 498 (2020)
    Transition metal oxides are attractive catalyst alternatives in liquid-phase oxidation reactions due to their lower cost and higher abundance compared with conventional noble metal catalysts. We investigated the catalytic properties of a systematic series of Co3-xFexO4 spinel catalysts synthesized by a hard-templating method, which were applied in the liquid-phase oxidation of styrene, benzyl alcohol and cinnamyl alcohol. O2 and tert-butyl hydroperoxide (TBHP) were used as the oxidants in a comparative manner. For alcohol oxidation, TBHP leads to similar or slightly higher selectivity to the corresponding aldehydes compared with O2. For the activation of C=C bonds, TBHP favors the oxidative cleavage pathway, while O2 favors the epoxidation pathway. The comparison of the catalytic performance revealed that the activity of Co3O4 does not benefit from Fe doping using O2 as the oxidant, while the substitution of Fe ≤ 10 % in the spinel structure is beneficial when TBHP is used. This is attributed to the different activation mechanisms of the oxidizing agents, being spin transfer in case of O2 and partial decomposition in case of TBHP. Heterogeneity tests and reusability studies demonstrated the stability of the spinel catalysts. © 2020 Elsevier B.V.
    view abstract10.1016/j.mcat.2020.111251
  • Laser Fragmentation-Induced Defect-Rich Cobalt Oxide Nanoparticles for Electrochemical Oxygen Evolution Reaction
    Yu, M. and Waag, F. and Chan, C.K. and Weidenthaler, C. and Barcikowski, S. and Tüysüz, H.
    ChemSusChem 13 (2020)
    Sub-5 nm cobalt oxide nanoparticles are produced in a flowing water system by pulsed laser fragmentation in liquid (PLFL). Particle fragmentation from 8 nm to 4 nm occurs and is attributed to the oxidation process in water where oxidative species are present and the local temperature is rapidly elevated under laser irradiation. Significantly higher surface area, crystal phase transformation, and formation of structural defects (Co2+ defects and oxygen vacancies) through the PLFL process are evidenced by detailed structural characterizations by nitrogen physisorption, electron microscopy, synchrotron X-ray diffraction, and X-ray photoelectron spectroscopy. When employed as electrocatalysts for the oxygen evolution reaction under alkaline conditions, the fragmented cobalt oxides exhibit superior catalytic activity over pristine and nanocast cobalt oxides, delivering a current density of 10 mA cm−2 at 369 mV and a Tafel slope of 46 mV dec−1, which is attributed to a larger exposed active surface area, the formation of defects, and an increased charge transfer rate. The study provides an effective approach to engineering cobalt oxide nanostructures in a flowing water system, which shows great potential for sustainable production of active cobalt catalysts. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cssc.201903186
  • Nanocast Mixed Ni-Co-Mn Oxides with Controlled Surface and Pore Structure for Electrochemical Oxygen Evolution Reaction
    Priamushko, T. and Guillet-Nicolas, R. and Yu, M. and Doyle, M. and Weidenthaler, C. and Tuÿsüz, H. and Kleitz, F.
    ACS Applied Energy Materials 3 (2020)
    Nanocasting or hard-templating is a versatile method to produce ordered mesoporous mixed transition metal oxides (MTMOs) with promising potential for both oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a comprehensive investigation was conducted on various NixCoyMnzO4 replicated from large pore KIT-6 silica. The materials were calcined at different temperatures to study the influence of the oxide formation and the resulting pore structure ordering, as well as surface properties, on the electrochemical activity and stability of the catalysts. After a comprehensive characterization, electrocatalytic performances of the materials were investigated in detail for OER to find a structure-activity relationship. In OER, a correlation was established between calcination temperature, pore and surface properties, and the overall efficiency and stability. The best sample, NixCoyMnzO4 calcined at 300 °C, provided a reasonable current density (25 mA/cm2 at 1.7 V vs RHE) and an overpotential of 400 mV at 10 mA/cm2, and demonstrated increased current density (above 200 mA/cm2 at 1.7 V vs RHE) once loaded into a Ni foam compared to the bare foam. This sample also remained stable over 15 h. Our results indicate that the calcination temperature greatly affects the porosity, crystalline structure, phase composition, and the activity of the catalysts toward OER. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acsaem.0c00544
  • Tailoring Morphology and Electronic Structure of Cobalt Iron Oxide Nanowires for Electrochemical Oxygen Evolution Reaction
    Budiyanto, E. and Yu, M. and Chen, M. and Debeer, S. and Rüdiger, O. and Tüysüz, H.
    ACS Applied Energy Materials 3 (2020)
    The influence of iron on nanocasting of cobalt oxide nanowires and the performance of these materials for the oxygen evolution reaction (OER) are investigated. Pristine Co3O4 and mixed cobalt iron oxide nanowires with a diameter of 7 nm have been synthesized via a nanocasting route by using SBA-15 silica as a template. A small amount of iron added during the synthesis results in a decrease in the nanowires' array length and induces the formation of a bimodal pore size distribution. Raman spectroscopy, X-ray emission, and high-energy resolution X-ray absorption spectroscopies further show that Fe incorporation alters the electronic structure by increasing the average distortion around the cobalt centers and the amount of Co2+ in tetrahedral sites. These affect the OER activity significantly; the overpotential of pristine Co3O4 at 10 mA/cm2 decreases from 398 to 378 mV, and the current density at 1.7 V increases from 107 to 150 mA/cm2 with the addition of iron at the Co/Fe atomic ratio of 32. Furthermore, post-reaction characterization confirmed that both the morphology and electronic structure of nanowires remain intact after a long-term stability test. Copyright © 2020 American Chemical Society.
    view abstract10.1021/acsaem.0c01201
  • Facile Protocol for Alkaline Electrolyte Purification and Its Influence on a Ni-Co Oxide Catalyst for the Oxygen Evolution Reaction
    Spanos, I. and Tesch, M.F. and Yu, M. and Tüysüz, H. and Zhang, J. and Feng, X. and Müllen, K. and Schlögl, R. and Mechler, A.K.
    ACS Catalysis 9 (2019)
    We report a simple and effective electrochemical method to remove Fe impurities from commercial KOH electrolyte. We therefore utilize a MoS2 catalyst deposited on porous Ni foam as both the anode and cathode in a two-electrode electrolysis setup. After 12 h of constant galvanostatic electrolysis at 100 mA, the Fe impurities from the KOH electrolyte were successfully removed, as confirmed by means of inductively coupled plasma optical emission spectroscopy analysis. In the purified KOH, a Ni-Co3O4 composite oxide catalyst showed no Fe-induced activation. In contrast, we directly observed the uptake of Fe on the Ni-Co3O4 catalyst from the nontreated electrolyte during catalyst operation using a coupled spectroelectrochemical setup. Interestingly, we further identified an influence on the dissolution behavior of Ni and Co in the presence of Fe impurities. Whereas hitherto mainly the activation effect of Fe impurities has been discussed, we hereby show that they additionally suppress corrosion under reaction conditions. Using our fast and low-cost method for the purification of large amounts of electrolyte, catalyst materials can be widely studied without these additional effects induced by Fe impurities in commercial KOH. © 2019 American Chemical Society.
    view abstract10.1021/acscatal.9b01940
  • Highly Active Cobalt-Based Electrocatalysts with Facile Incorporation of Dopants for the Oxygen Evolution Reaction
    Moon, G.-H. and Yu, M. and Chan, C.K. and Tüysüz, H.
    Angewandte Chemie - International Edition 58 (2019)
    In situ formation of electroactive cobalt species for the oxygen evolution reaction is simply achieved by applying an anodic bias to a commercially available cobalt precursor and Nafion binder mixture coated on a glassy carbon electrode. This preparation does not require energy-intensive materials preparation steps or noble metals, yet a low overpotential of 322 mV at 10.2 mA cm −2 and a high current density of more than 300 mA cm −2 at 1.7 V NHE were obtained in 1 m KOH. An operando electrochemical Raman spectroscopy study confirmed the formation of cobalt oxyhydroxide species and the iron stimulated the equilibrium state between Co 3+ and Co 4+ . The iron present in the alkali electrolyte or ink solution effectively activated the cobalt species, and most of the first row transition metals could also enhance the catalytic performance. The concept presented here is one of the simplest strategies for preparing highly active electrocatalysts and is very flexible for the replacement of cobalt by other transition metals. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201813052
  • Lead-Free Cs3Bi2Br9 Perovskite as Photocatalyst for Ring-Opening Reactions of Epoxides
    Dai, Y. and Tüysüz, H.
    ChemSusChem 12 (2019)
    Herein, an innovative approach was developed by using stable, lead-free halide perovskite for solar-driven organic synthesis. The ring-opening reaction of epoxides was chosen as a model system for the synthesis of value-added β-alkoxy alcohols, which require energy-intensive process conditions and corrosive, strong acids for conventional synthesis. The developed concept included the in situ preparation of Cs3Bi2Br9 and its simultaneous application as photocatalyst for epoxide alcoholysis under visible-light irradiation in air at 293 K, with exceptional high activity and selectivity ≥86 % for β-alkoxy alcohols and thia-compounds. The Cs3Bi2Br9 photocatalyst exhibited good stability and recyclability. In contrast, the lead-based perovskite showed a conversion rate of only 1 %. The origin of the unexpected catalytic behavior was attributed to the combination of the photocatalytic process and the presence of suitable Lewis-acidic centers on the surface of the bismuth halide perovskite photocatalyst. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201900716
  • Nitrogen-Doped Mesostructured Carbon-Supported Metallic Cobalt Nanoparticles for Oxygen Evolution Reaction
    Bähr, A. and Moon, G.-H. and Tüysüz, H.
    ACS Applied Energy Materials 2 (2019)
    A series of metallic cobalt nanoparticles supported on mesostructured nitrogen-doped carbons was successfully synthesized through soft-templating by using poly(ethylene oxide)-b-polystyrene (PEO-b-PS) as a structure directing agent. The formation of metallic cobalt nanoparticles and nitrogen-doping into carbon structures were simultaneously achieved by ammonia treatment. The physicochemical properties of the resulting materials and consequently their performance for the oxygen evolution were systematically altered by varying the cobalt loading (5-89 wt %), pyrolysis atmosphere (argon or ammonia), and temperature (600-800 °C). Thereby, up to 37 wt % of the cobalt nanoparticles were confined in the pores of the mesostructured nitrogen-doped carbon materials with a high BET surface area. At temperatures above 700 °C, the cobalt additionally catalyzes the graphitization of the carbon support. The catalyst with a cobalt loading of 37 wt % pyrolyzed at 700 °C under an ammonia atmosphere shows the highest turnover frequency (TOF) of 311 h-1 in the oxygen evolution reaction due to the improved electronic properties of the carbon support from the incorporation of nitrogen atoms combined with a large amount of accessible cobalt sites. This class of materials shows even higher activity in comparison with ordered mesoporous Co3O4. © 2019 American Chemical Society.
    view abstract10.1021/acsaem.9b01183
  • Optimizing Ni-Fe Oxide Electrocatalysts for Oxygen Evolution Reaction by Using Hard Templating as a Toolbox
    Yu, M. and Moon, G. and Bill, E. and Tüysüz, H.
    ACS Applied Energy Materials 2 (2019)
    A specific investigation was carried out to study the influence of the Ni/Fe ratio for oxygen evolution reaction (OER) by using the hard templating method as a toolbox. Various compositions of homogeneously blended Ni-Fe oxide nanoparticles with a primary particle size of around 8 nm were simply prepared by using pore confinement of the tea leaves template. Based on the similar physical properties, including particle size and surface area, for all samples, it was verified that the OER activity in alkali electrolyte was mainly governed by the metal stoichiometry, where a maximum current density was obtained with a Ni/Fe ratio of 32/1. The higher catalytic performance of Ni 32 Fe oxide was attributed to lower reaction resistance and higher intrinsic activity, which are confirmed by electrochemical impedance spectroscopy and surface area analysis, respectively. The lowest overpotential (0.291 V RHE at 10 mA/cm 2 ) as well as the highest current density (over 600 mA/cm 2 at 1.7 V RHE ) was achieved with Ni/Fe = 32/1 loaded on nickel foam due to (i) an uniform distribution of Fe into NiO, (ii) a high conductivity, and (iii) an activation of Ni by neighboring Fe under applying bias. The environmentally benign surfactant-free synthetic procedure and the electrocatalytic system consisting of earth-abundant elements only (Fe, Ni, and O) should be attractive for the development of practical and economical energy conversion devices to split water. © Copyright 2019 American Chemical Society.
    view abstract10.1021/acsaem.8b01769
  • Oxygen Reduction Reaction Activity of Mesostructured Cobalt-Based Metal Oxides Studied with the Cavity-Microelectrode Technique
    Behnken, J. and Yu, M. and Deng, X. and Tüysüz, H. and Harms, C. and Dyck, A. and Wittstock, G.
    ChemElectroChem 6 (2019)
    Cobalt oxides are known as abundant and stable catalysts for the oxygen reduction reaction (ORR) in an alkaline environment. Here, the ORR activity of Co3O4 and mixed metal oxides NiCo2O4 and CuCo2O4 was studied. Synthesis by using the nanocasting procedure resulted in a mesostructured spinel phase with uniform morphology and high surface area. However, the evaluation of the specific activity of this material class is often hampered by limitations in determining the real surface area. The cavity-microelectrode technique did not require the addition of any additives to the catalytic material. Thus, measuring the double layer capacitance was used to assess the surface area. This approach showed comparable and reliable values for all samples and different cavity depths. Furthermore, the in situ derived surface area enabled the determination of the specific ORR activity, which is more accurate than utilizing the geometric and nitrogen absorption derived surface area. Although the activity of Co3O4 was rather low, the presence of Ni2+ and Cu2+ in the mixed metal oxides led to a substantial activity enhancement, possibly by providing additional active sites. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201900722
  • Selective acid leaching: A simple way to engineer cobalt oxide nanostructures for the electrochemical oxygen evolution reaction
    Yu, M. and Belthle, K.S. and Tüysüz, C. and Tüysüz, H.
    Journal of Materials Chemistry A 7 (2019)
    Developing a simple and cost-effective strategy to construct earth-abundant catalysts is in high demand for diverse applications. Herein, a general and facile strategy is developed to engineer cobalt oxide nanostructures via selective acid leaching for the electrochemical oxygen evolution reaction (OER). A leaching process is implemented to selectively remove CoMoO4 by treating mixed Co-Mo oxides in diluted hydrochloric acid solution, resulting in the formation of sub-5 nm particles and a threefold increase in the specific surface area (up to 150 m2 g-1). The leached oxides exhibit superior OER activity to pristine oxides as a result of (i) a larger surface area, (ii) phase purification to expose more active Co3O4 species to the reactant, and (iii) faster charge transfer kinetics for the OER. This strategy can be also applied to a broader range of earth-abundant metals, where a second metal (Li, Ca, and Mg) is selectively leached out, which results in a material with a larger surface area and enhanced catalytic performance for the OER. Moreover, various metal oxides with a high surface area, such as NiO and Fe2O3, can be prepared via this simple synthetic method. This work will pave a new practical way for the production of high surface area catalysts for diverse applications. © The Royal Society of Chemistry 2019.
    view abstract10.1039/c9ta07835e
  • Surface states in bulk single crystal of topological semimetal Co3Sn2S2 toward water oxidation
    Li, G. and Xu, Q. and Shi, W. and Fu, C. and Jiao, L. and Kamminga, M.E. and Yu, M. and Tüysüz, H. and Kumar, N. and Süß, V. and Saha, R. and Srivastava, A.K. and Wirth, S. and Auffermann, G. and Gooth, J. and Parkin, S. and Sun, Y. and Liu, E. and Felser, C.
    Science Advances 5 (2019)
    The band inversion in topological phase matters bring exotic physical properties such as the topologically protected surface states (TSS). They strongly influence the surface electronic structures of the materials and could serve as a good platform to gain insight into the surface reactions. Here we synthesized high-quality bulk single crystals of Co3Sn2S2 that naturally hosts the band structure of a topological semimetal. This guarantees the existence of robust TSS from the Co atoms. Co3Sn2S2 crystals expose their Kagome lattice that constructed by Co atoms and have high electrical conductivity. They serves as catalytic centers for oxygen evolution process (OER), making bonding and electron transfer more efficient due to the partially filled orbital. The bulk single crystal exhibits outstanding OER catalytic performance, although the surface area is much smaller than that of Co-based nanostructured catalysts. Our findings emphasize the importance of tailoring TSS for the rational design of high-activity electrocatalysts. Copyright © 2019 The Authors.
    view abstract10.1126/sciadv.aaw9867
  • An Inverse Opal Structured Halide Perovskite Photocatalyst
    Schünemann, S. and Tüysüz, H.
    European Journal of Inorganic Chemistry 2018 (2018)
    We employ a colloidal crystal templating approach to fabricate ordered macro-mesoporous CsPbBr3 and demonstrate its superior photocatalytic activity compared to its non-templated counterpart in the degradation of an organic pollutant. The presented templating approach reduces charge carrier diffusion pathways and increases the surface area of the halide perovskite. Furthermore, the crystal structure, and the morphology of the templated halide perovskite are stable under photocatalytic conditions, which results in a constant photocatalytic performance in recycling experiments. The presented concept is applicable for other photocatalytic reactions and can thus advance the novel field of halide perovskites in photocatalysis. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/ejic.201800078
  • Au@TiO2 Core–Shell Composites for the Photocatalytic Reduction of CO2
    Pougin, A. and Dodekatos, G. and Dilla, M. and Tüysüz, H. and Strunk, J.
    Chemistry - A European Journal 24 (2018)
    Au/TiO2 catalysts in different geometrical arrangements were designed to explore the role of morphology and structural properties for the photocatalytic reduction of CO2 with H2O in the gas-phase. The most active sample was a Au@TiO2 core–shell catalyst with additional Au nanoparticles (NPs) deposited on the outer surface of the TiO2 shell. CH4 and CO are the primary carbon-containing products. Large amounts of H2 are additionally formed by photocatalytic H2O splitting. Shell thickness plays a critical role. The highest yields were observed with the thickest layer of TiO2, stressing the importance of the semiconductor for the reaction. Commercial TiO2 with and without Au NPs was less active in the production of CH4 and CO. The enhanced activation of CO2 on the core–shell system is concluded to result from electronic interaction between the gold core, the titania shell, and the Au NPs on the outer surface. The improved exposure of Au−TiO2 interface contributes to the beneficial effect. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201801796
  • Coffee-Waste Templating of Metal Ion-Substituted Cobalt Oxides for the Oxygen Evolution Reaction
    Yu, M. and Chan, C.K. and Tüysüz, H.
    ChemSusChem 11 (2018)
    A facile and scalable method using coffee waste grounds as a hard template has been developed to fabricate nanostructured Co3O4 for the oxygen evolution reaction (OER). Co3O4 incorporating metals with different valences (M/Co=1:4; M=Cu, Ni, Fe, Cr, and W) were also prepared with similar sheet-like structures comprising nanosized crystallites. After detailed characterization by X-ray diffraction, electron microscopy, and nitrogen sorption, the oxides were employed as OER electrocatalysts. Substitution of octahedral and tetrahedral sites of the spinel structure with divalent and trivalent transition metals (Cu, Ni, Fe, and Cr) increased the activity of Co3O4 for the OER, whereas incorporation of hexavalent W led to formation of a second crystal phase and significantly higher electrocatalytic performance. Furthermore, this method is easily scaled up for mass production of Co3O4 with the same nanostructure, which is highly desirable for large-scale application. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201701877
  • Glycerol Oxidation Using MgO- and Al2O3-supported Gold and Gold–Palladium Nanoparticles Prepared in the Absence of Polymer Stabilizers
    Dodekatos, G. and Abis, L. and Freakley, S.J. and Tüysüz, H. and Hutchings, G.J.
    ChemCatChem 10 (2018)
    Au and AuPd nanoparticles supported on MgO and Al2O3 were employed for the selective aqueous phase oxidation of glycerol under basic conditions. Catalysts were prepared by sol-immobilization without the addition of a stabilizing agent such as polyvinyl alcohol (PVA), which is generally added to stabilize the noble metal sol prior to immobilization. The obtained materials prepared with and without stabilizing agent were active for glycerol oxidation and showed similar catalytic performances—implying that the stabilizing polymer is not required to obtain active materials. Depending on the support used, it was possible to tailor the selectivity towards the desired oxidation products by using catalysts prepared with or without stabilizing agent. PVA-free Au/γ-Al2O3 exhibited a remarkably high selectivity towards tartronic acid (40 % at 97 % conversion), which was not observed for Au/γ-Al2O3 prepared with PVA (27 % at isoconversion). Selective glycerol oxidation performed under base-free conditions over AuPd/MgO catalysts also corroborated the previous results that the presence of a stabilizing polymer is not required to prepare active catalysts by sol-immobilization. Thus, a facile way to circumvent the inherent drawbacks encountered by the use of polymer stabilizers during catalyst preparation is presented herein. Experimental results suggest that the presence of the polymer stabilizers can affect the reaction pathways and control selectivity. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201800074
  • High surface area black TiO2 templated from ordered mesoporous carbon for solar driven hydrogen evolution
    Xiong, Y. and Gu, D. and Deng, X. and Tüysüz, H. and van Gastel, M. and Schüth, F. and Marlow, F.
    Microporous and Mesoporous Materials 268 (2018)
    Hydrogen reduction of TiO2 to generate surface Ti3+ can significantly increase the photochemical activity under solar-light illumination. However, the low surface areas of commercial TiO2 limit their photocatalytic activities. Herein, we report a high surface area ordered mesoporous black TiO2, which exhibits an improved photocatalytic performance. The TiO2 material was prepared by using a highly ordered mesoporous carbon CMK-3 as a hard template, which possesses very high surface area, large pore volume and uniform mesopores. By using the advantage of pore confinement in the mesoporous carbon template, TiO2-carbon composites were annealed at different temperatures to investigate the influence of the crystallinity of TiO2 on the photocatalytic hydrogen production. TiO2 calcined at 500 °C, having a high surface area (up to 158 m2 g−1), large pore volume (up to 0.62 cm3 g−1), uniform pore size (5–6 nm), and anatase crystal structure, indicated the highest hydrogen generation rate. Since the TiO2 has been treated at a higher temperature in the confinement of the mesoporous carbon, the TiO2 can easily be reduced at 500 °C under hydrogen atmosphere to generate surface Ti3+ species without destruction of the mesostructure and exhibits a high solar-driven hydrogen evolution rate (188 μmol h−1), which is more than two times higher than that of commercial TiO2 (82 μmol h−1). © 2018 Elsevier Inc.
    view abstract10.1016/j.micromeso.2018.04.018
  • Promoting effect of solvent on Cu/CoO catalyst for selective glycerol oxidation under alkaline conditions
    Dodekatos, G. and Ternieden, J. and Schünemann, S. and Weidenthaler, C. and Tüysüz, H.
    Catalysis Science and Technology 8 (2018)
    Cu/CoO catalysts were employed for the selective oxidation of glycerol in the aqueous phase under basic conditions. The effect of the solvent on the catalytic performance was investigated and the impact on the catalyst was thoroughly elucidated. Detailed characterization of the catalysts by HR-TEM, XRD, and XPS analysis before and after the reaction revealed that the addition of co-solvents (ethanol, n-propanol, or tert-butanol) drastically altered the catalyst properties. In particular, the amount of the catalytically active CoO(OH) phase generated during the reaction depends on the co-solvent used. Generally, the co-solvent has a beneficial effect on the catalytic activity and improves the glycerol conversion by a factor of up to 1.8, which could be linearly correlated to the ET(30) solvent polarity. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8cy01284a
  • Reactor Design and Kinetic Study on Adsorption/Desorption of CO and Cl2 for Industrial Phosgene Synthesis
    Bähr, A. and Moon, G.-H. and Diedenhoven, J. and Kiecherer, J. and Barth, E. and Tüysüz, H.
    Chemie-Ingenieur-Technik 90 (2018)
    The aim of this work is the utilization of carbon monoxide (CO) from steel mill gases for the synthesis of phosgene, which can further react with phenol to building blocks for polymers. The effect of impurities like NOx, CO2, and O2 on the activity and stability of the activated-carbon (AC) catalyst and the follow-up products should be evaluated. Therefore, new porous carbon catalysts and a new reactor are designed for a systematic kinetic study on the adsorption and desorption behaviors of CO and chlorine (Cl2) on carbon-based materials to identify AC catalysts with optimum physical properties. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cite.201800016
  • Recent Advances in Thermo-, Photo-, and Electrocatalytic Glycerol Oxidation
    Dodekatos, G. and Schünemann, S. and Tüysüz, H.
    ACS Catalysis 8 (2018)
    Glycerol is a highly versatile molecule because of its three hydroxyl groups and can be transformed to a plethora of different value-added fine chemicals and products. It is an important byproduct in biodiesel production and, hence, produced in high amounts, which resulted in a high surplus flooding the market over the last decades. Thus, glycerol is regarded as a potential platform chemical, and many research efforts were devoted to find active catalysts to transform glycerol to various products via different catalytic processes. The selective oxidation reaction is one of the most promising reaction pathways to produce valuable fine chemicals used in the chemical and pharmaceutical industry. This Review describes the recent developments in selective glycerol oxidation to value-added products over heterogeneous catalysts. Particular emphasis is placed not only on newly developed catalysts based on supported noble-metal nanoparticles but also on catalysts containing nonprecious metals. The idea of using cost-efficient non-noble metals for glycerol oxidation is appealing from an economic point of view. Numerous parameters can influence the catalytic performance of the materials, which can be tuned by various synthetic approaches. The reasons for enhancements in activity are critically examined and put into perspective among the various studies. Moreover, during the past decade, many research groups also reported photocatalytic and, more scarcely, electrocatalytic pathways for glycerol oxidation, which are also described in detail herein and have otherwise found little attention in other reviews. © 2018 American Chemical Society.
    view abstract10.1021/acscatal.8b01317
  • Scalable One-Pot Synthesis of Yolk-Shell Carbon Nanospheres with Yolk-Supported Pd Nanoparticles for Size-Selective Catalysis
    Wang, G.-H. and Chen, K. and Engelhardt, J. and Tüysüz, H. and Bongard, H.-J. and Schmidt, W. and Schüth, F.
    Chemistry of Materials 30 (2018)
    view abstract10.1021/acs.chemmater.8b00456
  • Serpentinization: Connecting geochemistry, ancient metabolism and industrial hydrogenation
    Preiner, M. and Xavier, J.C. and Sousa, F.L. and Zimorski, V. and Neubeck, A. and Lang, S.Q. and Chris Greenwell, H. and Kleinermanns, K. and Tüysüz, H. and McCollom, T.M. and Holm, N.G. and Martin, W.F.
    Life 8 (2018)
    Rock–water–carbon interactions germane to serpentinization in hydrothermal vents have occurred for over 4 billion years, ever since there was liquid water on Earth. Serpentinization converts iron(II) containing minerals and water to magnetite (Fe3O4) plus H2. The hydrogen can generate native metals such as awaruite (Ni3Fe), a common serpentinization product. Awaruite catalyzes the synthesis of methane from H2 and CO2 under hydrothermal conditions. Native iron and nickel catalyze the synthesis of formate, methanol, acetate, and pyruvate—intermediates of the acetyl-CoA pathway, the most ancient pathway of CO2 fixation. Carbon monoxide dehydrogenase (CODH) is central to the pathway and employs Ni0 in its catalytic mechanism. CODH has been conserved during 4 billion years of evolution as a relic of the natural CO2-reducing catalyst at the onset of biochemistry. The carbide-containing active site of nitrogenase—the only enzyme on Earth that reduces N2—is probably also a relic, a biological reconstruction of the naturally occurring inorganic catalyst that generated primordial organic nitrogen. Serpentinization generates Fe3O4 and H2, the catalyst and reductant for industrial CO2 hydrogenation and for N2 reduction via the Haber–Bosch process. In both industrial processes, an Fe3O4 catalyst is matured via H2-dependent reduction to generate Fe5C2 and Fe2N respectively. Whether serpentinization entails similar catalyst maturation is not known. We suggest that at the onset of life, essential reactions leading to reduced carbon and reduced nitrogen occurred with catalysts that were synthesized during the serpentinization process, connecting the chemistry of life and Earth to industrial chemistry in unexpected ways. © 2018 by the authors. Licensee MDPI, Basel, Switzerland.
    view abstract10.3390/life8040041
  • Structural effects on optoelectronic properties of halide perovskites
    Chen, K. and Schünemann, S. and Song, S. and Tüysüz, H.
    Chemical Society Reviews 47 (2018)
    Halide perovskites have prompted the evolution of the photovoltaic field and simultaneously demonstrated their great potential for application in other optoelectronic devices. A fundamental understanding of their structure-property relationship is essential to fabricate novel materials and high-performance devices. This review gives a perspective on different synthetic methodologies for the preparation of halide perovskites and highlights the effects of structural factors such as crystal structure, grain size, nanoscale dimensionality, patterned arrangement, and hierarchical structure on their optoelectronic properties. The main emphasis is given to 0D, 1D and 2D nanostructured materials including their common synthesis methods and key structural properties. Structural factors should be precisely controlled during the material preparation and device fabrication to improve the performance of targeted applications. © 2018 The Royal Society of Chemistry.
    view abstract10.1039/c8cs00212f
  • Structural Engineering of 3D Carbon Materials from Transition Metal Ion-Exchanged y Zeolite Templates
    Moon, G.-H. and Bähr, A. and Tüysüz, H.
    Chemistry of Materials 30 (2018)
    A series of three-dimensional ordered microporous carbon materials (3D CMs) were prepared through a nanocasting route by using transition metal ion-exchanged Y zeolite (M-Y) as template and ethylene gas a carbon source. The different d-π coordination and the formation of metal nanoparticles during thermal treatment altered textural parameters of the final carbon products. After a detailed structural analysis and characterization, the most promising cobalt-carbon sample was further treated with NH3 for nitrogen doping and evaluated for oxygen reduction reaction (ORR). This new class of material indicated good electrochemical stability and similar activity in comparison with those of commercial Pt/C (20 wt %) electrocatalyst. The protocol developed here allows in situ incorporation of diverse transition metals as well as the doping of various heteroelements into a three-dimensional carbon framework and has great potential for different catalytic applications. © 2018 American Chemical Society.
    view abstract10.1021/acs.chemmater.8b00861
  • Effect of Post-Treatment on Structure and Catalytic Activity of CuCo-based Materials for Glycerol Oxidation
    Dodekatos, G. and Tüysüz, H.
    ChemCatChem 9 (2017)
    A series of CuCo-based materials prepared by co-precipitation with varied Co/Cu ratios and different post-treatments were applied in the selective oxidation of glycerol in the aqueous phase under basic conditions. The influence of the post-treatment on the structure of the materials and the catalytic performance was investigated in detail. As-prepared materials without calcination and materials calcined under air with subsequent reduction under ethanol/N2 gas stream showed higher conversion of glycerol compared to samples solely calcined under air or to samples calcined under air with subsequent reduction under H2/Ar gas stream. The main products identified in the liquid phase were glyceric, glycolic, and formic acids. Systematic catalytic studies for differently prepared samples with varied Cu content and subsequent characterization of the materials by N2 physisorption, XRD, TEM, and EDX allowed for the identification of CoO(OH) in contact with CuO as the potentially active phases. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201601219
  • Halide Perovskite 3D Photonic Crystals for Distributed Feedback Lasers
    Schünemann, S. and Brittman, S. and Chen, K. and Garnett, E.C. and Tüysüz, H.
    ACS Photonics 4 (2017)
    Halide perovskites are under intense investigation for light harvesting applications in solar cells. Their outstanding optoelectronic properties such as long charge carrier diffusion lengths, high absorption coefficients, and defect tolerance also has triggered interest in laser and LED applications. Here, we report on the lasing properties of 3D distributed feedback halide perovskite nanostructures prepared via an all-solution process. A colloidal crystal templating approach was developed to precisely control the hybrid halide perovskite structure on the nanoscale. The prepared CH3NH3PbBr3 thin films with inverse opal morphology show narrow lasing emissions with a full width half-maximum as low as 0.15 nm and good long-term stability under pulsed laser excitation above the lasing threshold of 1.6 mJ cm-2 in ambient atmosphere. Furthermore, lasing emission was also observed for CH3NH3PbI3 inverse opals under excitation with a focused laser beam. Unlike other protocols for the fabrication of distributed feedback perovskite lasers, control of the nanostructure of hybrid halide perovskites is achieved without the use of expensive and elaborate lithography techniques or high temperatures. Therefore, the presented protocol opens a route to the low cost fabrication of hybrid halide perovskite lasers. © 2017 American Chemical Society.
    view abstract10.1021/acsphotonics.7b00780
  • High surface area, amorphous titania with reactive Ti3+ through a photo-assisted synthesis method for photocatalytic H2 generation
    Zywitzki, D. and Jing, H. and Tüysüz, H. and Chan, C.K.
    Journal of Materials Chemistry A 5 (2017)
    Amorphous titania-based photocatalysts are synthesized using a facile, UV-light mediated method and evaluated as photocatalysts for hydrogen evolution from water/methanol mixtures. The photocatalysts are prepared through the direct injection of a titanium alkoxide precursor into a water/methanol mixture, with subsequent hydrolysis, condensation, and polycondensation to form TiOx(OH)y species under UV-irradiation. The resulting amorphous titania materials exhibit an overall higher hydrogen evolution rate compared to a crystalline TiO2 reference (P25) on a molar basis of the photocatalyst due to their highly porous structure and high surface area (∼500 m2 g-1). The employed titanium alkoxide precursor did not play a major role in affecting the hydrogen evolution rate or the catalyst surface and morphology. A blue coloration, which is associated with the formation of Ti3+ species, was observed in the amorphous titania but not in P25 upon light irradiation and is enabled by the porous and disordered structure of the amorphous photocatalyst. The Ti3+ species are also used to reduce protons to H2 in the absence of light irradiation or reduce Pt2+ to form Pt nanoparticles. These Pt nanoparticles are smaller and better dispersed on the photocatalyst compared to particles prepared using conventional photodeposition, leading to higher H2 evolution rates. The results show that the direct injection method is a facile approach for the preparation of high surface area titania photocatalysts containing Ti3+ species with good photocatalytic activity for the production of H2. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7ta01614j
  • Iron-Induced Activation of Ordered Mesoporous Nickel Cobalt Oxide Electrocatalyst for the Oxygen Evolution Reaction
    Deng, X. and Öztürk, S. and Weidenthaler, C. and Tüysüz, H.
    ACS Applied Materials and Interfaces 9 (2017)
    Herein, ordered mesoporous nickel cobalt oxides prepared by the nanocasting route are reported as highly active oxygen evolution reaction (OER) catalysts. By using the ordered mesoporous structure as a model system and afterward elevating the optimal catalysts composition, it is shown that, with a simple electrochemical activation step, the performance of nickel cobalt oxide can be significantly enhanced. The electrochemical impedance spectroscopy results indicated that charge transfer resistance increases for Co3O4 spinel after an activation process, while this value drops for NiO and especially for CoNi mixed oxide significantly, which confirms the improvement of oxygen evolution kinetics. The catalyst with the optimal composition (Co/Ni 4/1) reaches a current density of 10 mA/cm2 with an overpotential of a mere 336 mV and a Tafel slope of 36 mV/dec, outperforming benchmarked and other reported Ni/Co-based OER electrocatalysts. The catalyst also demonstrates outstanding durability for 14 h and maintained the ordered mesoporous structure. The cyclic voltammograms along with the electrochemical measurements in Fe-free KOH electrolyte suggest that the activity boost is attributed to the generation of surface Ni(OH)2 species that incorporate Fe impurities from the electrolyte. The incorporation of Fe into the structure is also confirmed by inductively coupled plasma optical emission spectrometry. © 2017 American Chemical Society.
    view abstract10.1021/acsami.7b02571
  • Monodispersed Mesoporous Silica Spheres Supported Co3O4 as Robust Catalyst for Oxygen Evolution Reaction
    Deng, X. and Rin, R. and Tseng, J.-C. and Weidenthaler, C. and Apfel, U.-P. and Tüysüz, H.
    ChemCatChem 9 (2017)
    Monodispersed mesoporous silica spheres (MSS) with fibrous nanostructure and highly open porosity were fabricated by a facile one-pot synthetic route and loaded with Co3O4 nanoclusters for catalyzing the oxygen evolution reaction with Ru(bpy)3 2+–S2O8 2− photosensitizer and sacrificial reagent system. The effect of the loading amount on the morphology and microstructure of Co3O4 was investigated and it was found that lower Co3O4 content in the composite materials results in smaller crystallite size, which in turn leads to significantly enhanced oxygen evolution activity. Furthermore, owing to the monodispersity of the spheres and good accessibility of active species offered by the fibrous pore structure, the material shows a clear advantage over nonsupported Co3O4 nanoparticles and the commonly used ordered mesoporous silica supports such as KIT-6 and SBA-15. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cctc.201701001
  • Photocatalytic Polymerization of 3,4-Ethylenedioxythiophene over Cesium Lead Iodide Perovskite Quantum Dots
    Chen, K. and Deng, X. and Dodekatos, G. and Tüysüz, H.
    Journal of the American Chemical Society 139 (2017)
    The outstanding performance of halide perovskites in optoelectronic applications can be partly attributed to their high absorption coefficient and long carrier lifetime, which are also desirable for photocatalysts. Herein, we report that cesium lead iodide perovskite quantum dots (CsPbI3 QDs) can be used as catalysts to promote the polymerization of 2,2′,5′,2″-ter-3,4-ethylenedioxythiophene under visible light illumination while preserving the quantum dot in the desirable cubic crystal phase. Simultaneously, the generated conducting poly(3,4-ethylenedioxythiophene), PEDOT, encapsulates and stabilizes the morphology of the CsPbI3 QDs. The photocatalytic polymerization clearly depends on the concentration of the CsPbI3 QDs, and the CsPbI3 QDs maintain the desirable perovskite phase when the concentration of the QD increases. Molecular oxygen and 1,4-benzoquinone can serve as electron acceptors during the photocatalytic polymerization reaction. When molecular oxygen is used, the structure of the CsPbI3 QD transforms from cubic to orthorhombic, while usage of 1,4-benzoquinone preserves the cubic phase of CsPbI3 QD. This novel approach enables the one-step formation of CsPbI3/PEDOT composite, which could be promising for the preparation of novel optoelectronic materials and high performance devices. © 2017 American Chemical Society.
    view abstract10.1021/jacs.7b06413
  • Preparation of Waterproof Organometal Halide Perovskite Photonic Crystal Beads
    Chen, K. and Schünemann, S. and Tüysüz, H.
    Angewandte Chemie - International Edition 56 (2017)
    Herein, we report on an innovative method for the preparation of a series of organometal halide perovskite (OHP) photonic crystal beads with pronounced and tunable photonic stop bands by using self-assembled polystyrene spheres as a mold. After infiltration of the mold with OHP precursor solution and slow drying, the OHPs crystallized in the voids of the polystyrene arrays. By controlling the diameter of the polystyrene spheres, the photonic stop band of the OHPs could be precisely tuned. The overlap between the photonic stop band of the beads and the band gap of the OHPs enhances the light harvesting of the perovskite because of the slow photon effect, which arises from the photonic crystal beads. Moreover, the stability of the composite was greatly enhanced by coating with the transparent polymer PDMS without blocking the light propagation. The coated OHP photonic beads kept their composition even after having been in contact with water for 24 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201702556
  • Protocol for the Nanocasting Method: Preparation of Ordered Mesoporous Metal Oxides
    Deng, X. and Chen, K. and Tüysüz, H.
    Chemistry of Materials 29 (2017)
    Ordered mesoporous transition metal oxides have attracted considerable research attention due to their unique properties and wide applications. The preparation of these materials has been reported in the literature using soft and hard templating pathways. Compared with soft templating, hard templating, namely, nanocasting, is advantageous for synthesizing rigid mesostructures with high crystallinity and has already been applied to numerous transition metal oxides such as Co3O4, NiO, Fe2O3, and Mn3O4. However, nanocasting is often complicated by the multiple steps involved: first, the preparation of ordered mesoporous silica as the hard template, then infiltration of the metal precursor into the pores, and finally, formation of the metal oxide and removal of the hard template. In this paper, we provide a complete protocol that covers the preparation of most widely used ordered mesoporous silica templates (MCM-41, KIT-6, SBA-15) and the nanocasting process for obtaining ordered mesoporous metal oxides, with emphasizing cobalt oxide as an example. Characterization of the products is presented, and the factors that can potentially affect the process are discussed. © 2016 American Chemical Society.
    view abstract10.1021/acs.chemmater.6b02645
  • Selective glycerol oxidation over ordered mesoporous copper aluminum oxide catalysts
    Schünemann, S. and Schüth, F. and Tüysüz, H.
    Catalysis Science and Technology 7 (2017)
    Glycerol is a major by-product of the biodiesel production and is therefore produced in high quantities. While currently there are limited possible applications for this highly functionalized molecule, glycerol can be a cheap and abundant feedstock for value-added products that are accessible by selective oxidation. Usually, the selective oxidation of glycerol utilizes expensive noble metal catalysts, such as Au, Pt, and Pd. Here we report the selective oxidation of glycerol in basic media, using ordered mesoporous Cu-Al2O3 catalysts with various Cu loadings prepared by a facile soft-templating method. The materials were characterized in detail by nitrogen physisorption, vis-NIR spectroscopy, EDX, low- and wide-angle XRD, XPS, and TEM. Subsequently the reaction conditions for glycerol oxidation were optimized. The catalytic oxidation of glycerol yields C3 products, such as glyceric acid and tartronic acid, and also C2 and C1 products, such as glycolic acid, oxalic acid, and formic acid. Moreover, the role of the solvent on the catalytic reaction was investigated, and the addition of various co-solvents to the aqueous reaction mixture was found to increase the initial reaction rate up to a factor of three. The trends of the initial reaction rates correlate well with the polarity of the water/co-solvent mixtures. The prepared Cu-Al2O3 catalysts are a more cost-efficient and environmentally viable alternative to the reported noble metal catalysts. © 2017 The Royal Society of Chemistry.
    view abstract10.1039/c7cy01451a
  • Standardized Benchmarking of Water Splitting Catalysts in a Combined Electrochemical Flow Cell/Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) Setup
    Spanos, I. and Auer, A.A. and Neugebauer, S. and Deng, X. and Tüysüz, H. and Schlögl, R.
    ACS Catalysis 7 (2017)
    The oxygen evolution reaction (OER) is the limiting step in splitting water into its constituents, hydrogen and oxygen. Hence, research on potential OER catalysts has become the focus of many studies. In this work, we investigate capable OER catalysts but focus on catalyst stability, which is, especially in this case, at least equally as important as catalyst activity. We propose a specialized setup for monitoring the corrosion profiles of metal oxide catalysts during a stability testing protocol, which is specifically designed to standardize the investigation of OER catalysts by means of differentiating between catalyst corrosion and deactivation, oxygen evolution efficiency, and catalyst activity. For this purpose, we combined an electrochemical flow cell (EFC) with an oxygen sensor and an inductively coupled plasma-optical emission spectrometry (ICP-OES) system for the simultaneous investigation of catalyst deactivation, activity, and faradaic efficiency of catalysts. We tested various catalysts, with IrO2 and NiCoO2 used as benchmark materials in acidic and alkaline environment, respectively. The scalability of our setup will allow the user to investigate catalytic materials with supports of higher surface area than those which are typical for microelectrochemical flow cells (thus, under conditions more similar to those of commercial electrolyzers). © 2017 American Chemical Society.
    view abstract10.1021/acscatal.7b00632
  • Activated-Carbon-Templated Crystalline Tantalates for Photocatalytic Water Splitting
    Grewe, T. and Tuysuz, H.
    Chemnanomat 2 (2016)
    Herein, we report a facile nanocasting route for the preparation of crystalline mesoporous Ta2O5 and NaTaO3 with a high surface area using activated carbon as the structure-directing hard template. The crystalline particles of Ta2O5 and NaTaO3 have mean sizes of 6 and 9 nm, constructing mesoporous networks with surface areas of 91 and 75 m(2)g(-1), respectively. The photocatalytic performances of the tantalum-based replicas were investigated for the water splitting reaction by using methanol as sacrificial agent and better photocatalytic performances were observed in comparison with bulk and amorphous counterparts. Loading the templated Ta2O5 and NaTaO3 with 1 wt% NiOx boosted their photocatalytic activity. The most-active sample, NaTaO3/ NiOx, was also tested for overall water splitting and it produced hydrogen and oxygen in stoichiometric ratios at rates of 129 and 66 mu molh(-1), respectively.
    view abstract10.1002/cnma.201600033
  • Advanced and in situ analytical methods for solar fuel materials
    Chan, C.K. and Tüysüz, H. and Braun, A. and Ranjan, C. and La Mantia, F. and Miller, B.K. and Zhang, L. and Crozier, P.A. and Haber, J.A. and Gregoire, J.M. and Park, H.S. and Batchellor, A.S. and Trotochaud, L. and Boettcher, S.W.
    Topics in Current Chemistry 371 (2016)
    In situ and operando techniques can play important roles in the development of better performing photoelectrodes, photocatalysts, and electrocatalysts by helping to elucidate crucial intermediates and mechanistic steps. The development of high throughput screening methods has also accelerated the evaluation of relevant photoelectrochemical and electrochemical properties for new solar fuel materials. In this chapter, several in situ and high throughput characterization tools are discussed in detail along with their impact on our understanding of solar fuel materials. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/128_2015_650
  • Alkali metals incorporated ordered mesoporous tantalum oxide with enhanced photocatalytic activity for water splitting
    Grewe, T. and Tüysüz, H.
    Journal of Materials Chemistry A 4 (2016)
    Herein, we report a novel synthetic approach for the preparation of alkali (Na, K) metal incorporated ordered mesoporous tantalate composites and their photocatalytic performance for water splitting. With the main focus on sodium based composite materials, a series of samples with ordered mesoporosity and high surface area (108-120 m2 g-1) was prepared by a variation of the Ta/Na ratios through a soft templating route. The structural parameters and properties of the samples were analyzed by low angle XRD, N2-physisorption, TEM and STEM analysis, EDX, XPS, Raman and diffuse reflectance UV-Vis spectroscopy. The incorporation of alkali metals resulted in ordered mesoporous tantalate composites. Furthermore, the addition of alkaline earth (Ca, Ba, Sr) metals to the precursor solution of ordered mesoporous tantalum oxide was attempted. However, alkaline earth metals gave unordered tantalate composites. Photocatalytic investigations for water splitting, by using methanol as a sacrificial agent, indicated that the incorporation of alkali metals enhanced the hydrogen production rate of the photocatalyst whereas addition of alkaline earth metals decreased the hydrogen production. Among the sodium based samples, a Ta/Na ratio of 9 showed the best performance. The efficiency of this sample was further improved through decorated NiOx as co-catalyst. A 2.5 wt% NiOx loading was found to be the optimal loading amount, generating 64 μmol h-1 H2 and 31 μmol h-1 O2 when tested for overall water splitting. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c5ta07086d
  • Co3O4 Nanoparticles Supported on Mesoporous Carbon for Selective Transfer Hydrogenation of α,β-Unsaturated Aldehydes
    Wang, G.-H. and Deng, X. and Gu, D. and Chen, K. and Tüysüz, H. and Spliethoff, B. and Bongard, H.-J. and Weidenthaler, C. and Schmidt, W. and Schüth, F.
    Angewandte Chemie - International Edition 55 (2016)
    A simple and scalable method for synthesizing Co3O4nanoparticles supported on the framework of mesoporous carbon (MC) was developed. Benefiting from an ion-exchange process during the preparation, the cobalt precursor is introduced into a mesostructured polymer framework that results in Co3O4nanoparticles (ca. 3 nm) supported on MC (Co3O4/MC) with narrow particle size distribution and homogeneous dispersion after simple reduction/pyrolysis and mild oxidation steps. The as-obtained Co3O4/MC is a highly efficient catalyst for transfer hydrogenation of α,β-unsaturated aldehydes. Selectivities towards unsaturated alcohols are always higher than 95 % at full conversion. In addition, the Co3O4/MC shows high stability under the reaction conditions, it can be recycled at least six times without loss of activity. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201604673
  • Dual-Templated Cobalt Oxide for Photochemical Water Oxidation
    Deng, X. and Bongard, H.-J. and Chan, C.K. and Tüysüz, H.
    ChemSusChem 9 (2016)
    Mesoporous Co3O4 was prepared using a dual templating approach whereby mesopores inside SiO2 nanospheres, as well as the void spaces between the nanospheres, were used as templates. The effect of calcination temperature on the crystallinity, morphology, and textural parameters of the Co3O4 replica was investigated. The catalytic activity of Co3O4 for photochemical water oxidation in a [Ru(bpy)3]2+[S2O8]2- system was evaluated. The Co3O4 replica calcined at the lowest temperature (150°C) exhibited the best performance as a result of the unique nanostructure and high surface area arising from the dual templating. The performance of Co3O4 with highest surface area was further examined in electrochemical water oxidation. Superior activity over high temperature counterpart and decent stability was observed. Furthermore, CoO with identical morphology was prepared from Co3O4 using an ethanol reduction method and a higher turnover-frequency number for photochemical water oxidation was obtained. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201500872
  • Hollow Nano- and Microstructures as Catalysts
    Prieto, G. and Tüysüz, H. and Duyckaerts, N. and Knossalla, J. and Wang, G.-H. and Schüth, F.
    Chemical Reviews 116 (2016)
    Catalysis is at the core of almost every established and emerging chemical process and also plays a central role in the quest for novel technologies for the sustainable production and conversion of energy. Particularly since the early 2000s, a great surge of interest exists in the design and application of micro- and nanometer-sized materials with hollow interiors as solid catalysts. This review provides an updated and critical survey of the ever-expanding material architectures and applications of hollow structures in all branches of catalysis, including bio-, electro-, and photocatalysis. First, the main synthesis strategies toward hollow materials are succinctly summarized, with emphasis on the (regioselective) incorporation of various types of catalytic functionalities within their different subunits. The principles underlying the scientific and technological interest in hollow materials as solid catalysts, or catalyst carriers, are then comprehensively reviewed. Aspects covered include the stabilization of catalysts by encapsulation, the introduction of molecular sieving or stimuli-responsive "auxiliary" functionalities, as well as the single-particle, spatial compartmentalization of various catalytic functions to create multifunctional (bio)catalysts. Examples are also given on the applications which hollow structures find in the emerging fields of electro- and photocatalysis, particularly in the context of the sustainable production of chemical energy carriers. Finally, a critical perspective is provided on the plausible evolution lines for this thriving scientific field, as well as the main practical challenges relevant to the reproducible and scalable synthesis and utilization of hollow micro- and nanostructures as solid catalysts. © 2016 American Chemical Society.
    view abstract10.1021/acs.chemrev.6b00374
  • Hyperbranched potassium lanthanum titanate perovskite photocatalysts for hydrogen generation
    Grewe, T. and Yang, T. and Tüysüz, H. and Chan, C.K.
    Journal of Materials Chemistry A 4 (2016)
    Semiconductors with hierarchical nanostructured morphologies may be promising as high surface area photocatalysts for producing hydrogen from water. However, there are few scalable synthesis methods that can achieve such morphologies in metal oxide semiconductors such as titanates. Here, hydrothermal methods were used to synthesize nanostructured potassium lanthanum titanate (KLTO) perovskite without using templates or structure-directing agents. The obtained materials were octahedral particles composed of orthogonal hyperbranched nanowires, a morphology that is usually obtained using catalyst-mediated vapor phase methods. Several fundamental materials properties of KLTO were determined for the first time, including the bandgap (3.3 eV), semiconductor type (n-type), flat band potential, and conduction band maximum (-0.265 V and-0.835 V vs. NHE, respectively). The KLTO hyperbranched structures were also investigated as UV-photocatalysts for H2 production and displayed higher activities than P25 TiO2 and KLTO nanoparticles. The H2 production rate for KLTO decorated with 1 wt% Pt using thermal decomposition of K2PtCl4 reached ca. 2.5 mmol h-1 and was stable for 20 h of irradiation. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c5ta07424j
  • Nanocatalysts for Solar Water Splitting and a Perspective on Hydrogen Economy
    Grewe, T. and Meggouh, M. and Tüysüz, H.
    Chemistry - An Asian Journal 11 (2016)
    In this review article, nanocatalysts for solar hydrogen production are the focus of discussion as they can contribute to the development of sustainable hydrogen production in order to meet future energy demands. Achieving this task is subject of scientific aspirations in the field of photo- and photoelectrocatalysis for solar water splitting where systems of single catalysts or tandem configurations are being investigated. In search of a suitable catalyst, a number of crucial parameters are laid out which need to be considered for material design, in particular for nanostructured materials that provide exceptional physical and chemical properties in comparison to their bulk counterparts. Apart from synthetic approaches for nanocatalysts, key parameters and properties of nanostructured photocatalysts such as light absorption, charge carrier generation, charge transport, separation and recombination, and other events that affect nanoscale catalysts are discussed. To provide a deeper understanding of these key parameters and properties, their contribution towards existing catalyst systems is evaluated for photo- and photoelectrocatalytic solar hydrogen evolution. Finally, an insight into hydrogen production processes is given, stressing the current development of sustainable hydrogen sources and presenting a perspective towards a hydrogen-based economy. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/asia.201500723
  • Pentlandite rocks as sustainable and stable efficient electrocatalysts for hydrogen generation
    Konkena, B. and Puring, K.J. and Sinev, I. and Piontek, S. and Khavryuchenko, O. and Dürholt, J.P. and Schmid, R. and Tüysüz, H. and Muhler, M. and Schuhmann, W. and Apfel, U.-P.
    Nature Communications 7 (2016)
    The need for sustainable catalysts for an efficient hydrogen evolution reaction is of significant interest for modern society. Inspired by comparable structural properties of [FeNi]-hydrogenase, here we present the natural ore pentlandite (Fe 4.5 Ni 4.5 S 8) as a direct rock' electrode material for hydrogen evolution under acidic conditions with an overpotential of 280 mV at 10 mA cm -2. Furthermore, it reaches a value as low as 190 mV after 96 h of electrolysis due to surface sulfur depletion, which may change the electronic structure of the catalytically active nickel-iron centres. The rock' material shows an unexpected catalytic activity with comparable overpotential and Tafel slope to some well-developed metallic or nanostructured catalysts. Notably, the rock' material offers high current densities (≤650 mA cm -2) without any loss in activity for approximately 170 h. The superior hydrogen evolution performance of pentlandites as rock' electrode labels this ore as a promising electrocatalyst for future hydrogen-based economy.
    view abstract10.1038/ncomms12269
  • Plasmonic Au/TiO2 nanostructures for glycerol oxidation
    Dodekatos, G. and Tüysüz, H.
    Catalysis Science and Technology 6 (2016)
    Au nanoparticles supported on P25 TiO2 (Au/TiO2) were prepared by a facile deposition-precipitation method with urea and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions. Au/TiO2 samples were characterized in detail by X-ray diffraction, UV-vis spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. The adopted synthetic methodology permits deposition of Au nanoparticles with similar mean particle sizes up to 12.5 wt% loading that allows for the evaluation of the influence of the Au amount (without changing the particle size) on its photocatalytic performance for glycerol oxidation. The reaction conditions were optimized by carrying out a systematic study with different Au loadings on TiO2, reaction times, temperatures, catalyst amounts, O2 pressures and Au particle sizes for photocatalytic reactions as well as traditional heterogeneous catalysis. It has been shown that visible light irradiation during the reaction has a beneficial effect on the conversion of glycerol where the best catalytic results were observed for 7.5 wt% Au loading with an average particle size of around 3 nm. The main product observed, with selectivities up to 63%, was high-value dihydroxyacetone that has important industrial applications, particularly in the cosmetic industry. © 2016 The Royal Society of Chemistry.
    view abstract10.1039/c6cy01192f
  • Preparation of Organometal Halide Perovskite Photonic Crystal Films for Potential Optoelectronic Applications
    Schünemann, S. and Chen, K. and Brittman, S. and Garnett, E. and Tüysüz, H.
    ACS Applied Materials and Interfaces 8 (2016)
    Herein, a facile method for the preparation of organometal halide perovskite (OHP) thin films in photonic crystal morphology is presented. The OHP photonic crystal thin films with controllable porosity and thicknesses between 2 μm and 6 μm were prepared on glass, fluorine-doped tin oxide (FTO), and TiO2 substrates by using a colloidal crystal of polystyrene microspheres as a template to form an inverse opal structure. The composition of OHP could be straightforwardly tuned by varying the halide anions. The obtained OHP inverse opal films possess large ordered domains with a periodic change of the refractive index, which results in pronounced photonic stop bands in the visible light range. By changing the diameter of the polystyrene microspheres, the position of the photonic stop band can be tuned through the visible spectrum. This developed methodology can be used as blueprint for the synthesis of various OHP films that could eventually be used as more effective light harvesting materials for diverse applications. © 2016 American Chemical Society.
    view abstract10.1021/acsami.6b09227
  • Pseudomorphic Transformation of Organometal Halide Perovskite Using the Gaseous Hydrogen Halide Reaction
    Chen, K. and Deng, X. and Goddard, R. and Tüysüz, H.
    Chemistry of Materials 28 (2016)
    Halide exchange is a facile method of adjusting the band gap and optimizing the performance of organometal halide perovskite. During the halide exchange processes, preserving the crystallinity and morphology of highly crystalline materials will be desirable for preparing novel materials with outstanding performance. In this study, the gasous hydrogen halides were used as reactants for halide exchange processes. The mutual conversions among three halides for condense films were realized. Moreover, perovskite inverse opals and perovskite single crystals were also adopted as substrates to illustrate the morphology preservation and crystallinity preservation, respectively. Powder X-ray diffraction and UV-vis diffuse reflectance spectra demonstrated the segregation when smaller ions were substituted by larger ions. Scanning electron microscopy displayed the direct evidence for morphology preservation during the transformation. For the first time, single crystal X-ray diffraction confirmed the single-crystal-to-single-crystal transformation from bromide to chloride analogy, which demonstrated that the presented method can preserve the crystalline framework of large-sized perovskite during the halide exchange. © 2016 American Chemical Society.
    view abstract10.1021/acs.chemmater.6b02233
  • Spent Tea Leaf Templating of Cobalt-Based Mixed Oxide Nanocrystals for Water Oxidation
    Deng, X. and Chan, C.K. and Tüysüz, H.
    ACS Applied Materials and Interfaces 8 (2016)
    The facile synthesis of nanostructured cobalt oxides using spent tea leaves as a hard template is reported. Following an impregnation-calcination and template removal pathway, sheetlike structures containing nanosized crystallites of Co3O4 are obtained. Co3O4 incorporated with Cu, Ni, Fe, and Mn (M/Co = 1/8 atomic ratio) are also prepared, and the materials are thoroughly characterized using X-ray diffraction, electron microscopy, and N2 sorption. The method is applicable to several commercial tea leaves and is successfully scaled up to prepare over 7 g of Co3O4 with the same nanostructure. The oxides are then tested for electrochemical water oxidation, and Cu, Ni, and Fe incorporations show beneficial effect on the catalytic activity of Co3O4, achieving performance comparable to levels from benchmark electrocatalysts. These data suggest that tea leaf templating can be utilized as a facile and promising approach to prepare nanostructured functional catalyst. © 2016 American Chemical Society.
    view abstract10.1021/acsami.6b12005
  • Ammonia decomposition over iron phthalocyanine-based materials
    Tüysüz, H. and Schüth, F. and Zhi, L. and Müllen, K. and Comotti, M.
    ChemCatChem 7 (2015)
    Iron phthalocyanine-based materials have been used herein as efficient catalysts for the ammonia decomposition reaction. These materials showed high activity, even superior to that showed by the commercial nickel-based catalyst and iron-doped carbon nanotubes, which were used as benchmarks in this study. Catalyst stability under reaction conditions appeared satisfactory, because no deactivation phenomena were observed. The type of the phthalocyanine precursor did not affect the catalytic performance; however, the preparation method had a strong effect. If the resulting material was exposed to the reaction conditions, some structural modification occurred. No clear correlation between phase composition and activity could be established because similar nitrogen content and similar crystalline domains in the sample led to different behaviors. However, the results of extensive characterization suggested that catalytic activities and conversion profiles were most likely dependent on material textural properties and thus on the preparation method used. The accessibility of iron species seems to be limited for catalysts prepared under vacuum. These phenomena are most likely responsible for the activation profile and for the low catalytic activity typical of these materials. In contrast, higher accessibility of iron species, typical of materials prepared under argon, would lead to improved and stable catalytic performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201500024
  • Amorphous and Crystalline Sodium Tantalate Composites for Photocatalytic Water Splitting
    Grewe, T. and Tüysüz, H.
    ACS Applied Materials and Interfaces 7 (2015)
    A facile hydrothermal synthesis protocol for the fabrication of sodium tantalates for photocatalytic water splitting is presented. Mixtures of tantalum and sodium ethoxide precursors were dispersed in ethanol, and ammonium hydroxide solution was used as mineralizer. By adjusting the amount of mineralizer, a variety of sodium tantalates with various morphologies, textural parameters, band gaps, crystal phases, and degrees of crystallinity were fabricated. The reaction was carefully monitored with a pressure sensor inside the autoclave reactor, and the obtained samples were characterized using X-ray diffraction, transmission electron microscopy, N2-physisorption, and ultraviolet-visible light spectroscopy. Among the series, the amorphous sample and the composite sample that consists of amorphous and crystalline phases showed superior activity toward photocatalytic hydrogen production than highly crystalline samples. Particularly, an amorphous sodium tantalate with a small fraction of crystalline nanoparticles with perovskite structure was found to be the most active sample, reaching a hydrogen rate of 3.6 mmol h-1 from water/methanol without the use of any cocatalyst. Despite its amorphous nature, this photocatalyst gave an apparent photocatalyst activity of 1200 μmol g-1 L-1 h-1 W1-, which is 4.5-fold higher than highly crystalline NaTaO3. In addition, the most active sample gave promising activity for overall water splitting with a hydrogen production rate of 94 μmol h-1, which is superior to highly crystalline NaTaO3 prepared by conventional solid-solid state route. © 2015 American Chemical Society.
    view abstract10.1021/acsami.5b06965
  • Designing Photocatalysts for Hydrogen Evolution: Are Complex Preparation Strategies Necessary to Produce Active Catalysts?
    Grewe, T. and Tüysüz, H.
    ChemSusChem 8 (2015)
    A facile synthetic route for the preparation of highly active photocatalysts was developed. The protocol involves the preparation of a photocatalyst through the direct injection of metal alkoxide precursors into solutions in a photoreactor. As a proof of concept, a tantalum oxide based photocatalyst was chosen as a model system. Tantalum ethoxide [Ta(OEt)<inf>5</inf>] was injected rapidly into a photoreactor filled with a water/methanol mixture, and a TaO<inf>x</inf>(OH)<inf>y</inf> composite formed and was able to produce hydrogen under light illumination. Compared to commercial and mesostructured Ta<inf>2</inf>O<inf>5</inf> and NaTaO<inf>3</inf> materials, TaO<inf>x</inf>(OH)<inf>y</inf> produced by direct injection shows superior hydrogen production activity. Notably, the samples prepared by direct injection are amorphous; however, their photocatalytic performance is much higher than those of their crystalline equivalents. If Ta(OEt)<inf>5</inf> was dispersed in methanol before injection, an amorphous framework with higher surface area and larger pore volume was formed, and the hydrogen production rate increased further. The addition of a sodium precursor during the injection further boosted the photocatalytic activity. Furthermore, this concept has also been applied to a titanium-based photocatalyst, and a much better hydrogen production rate has been obtained in comparison with that of commercial TiO<inf>2</inf> (P25-Degussa); therefore, the direct-injection synthesis is a flexible method that opens the door to the facile preparation of highly active nanostructured photocatalysts for hydrogen production. Fast and facile photocatalyst preparation: The injection of a metal alkoxide precursor into a methanol/water mixture results in the formation of a highly active photocatalyst. The prepared amorphous photocatalysts show superior activity for hydrogen production over commercial and synthesized nanostructured reference catalysts. This simple and quick method could be an alternative to time- and energy-demanding synthesis routes. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201500774
  • MAXNET Energy - Focusing Research in Chemical Energy Conversion on the Electrocatlytic Oxygen Evolution
    Auer, A.A. and Cap, S. and Antonietti, M. and Cherevko, S. and Deng, X. and Papakonstantinou, G. and Sundmacher, K. and Brüller, S. and Antonyshyn, I. and Dimitratos, N. and Davis, R.J. and Böhm, K.-H. and Fechler, N. and Freakley, S. and Grin, Y. and Gunnoe, B.T. and Haj-Hariri, H. and Hutchings, G. and Liang, H. and Mayrhofer, K.J.J. and Müllen, K. and Neese, F. and Ranjan, C. and Sankar, M. and Schlögl, R. and Schüth, F. and Spanos, I. and Stratmann, M. and Tüysüz, H. and Vidakovic-Koch, T. and Yi, Y. and Zangari, G.
    Green 5 (2015)
    MAXNET Energy is an initiative of the Max Planck society in which eight Max Planck institutes and two external partner institutions form a research consortium aiming at a deeper understanding of the electrocatalytic conversion of small molecules. We give an overview of the activities within the MAXNET Energy research consortium. The main focus of research is the electrocatalytic water splitting reaction with an emphasis on the anodic oxygen evolution reaction (OER). Activities span a broad range from creation of novel catalysts by means of chemical or material synthesis, characterization and analysis applying innovative electrochemical techniques, atomistic simulations of state-of-the-art x-ray spectroscopy up to model-based systems analysis of coupled reaction and transport mechanisms. Synergy between the partners in the consortium is generated by two modes of cooperation - one in which instrumentation, techniques and expertise are shared, and one in which common standard materials and test protocols are used jointly for optimal comparability of results and to direct further development. We outline the special structure of the research consortium, give an overview of its members and their expertise and review recent scientific achievements in materials science as well as chemical and physical analysis and techniques. Due to the extreme conditions a catalyst has to endure in the OER, a central requirement for a good oxygen evolution catalyst is not only its activity, but even more so its high stability. Hence, besides detailed degradation studies, a central feature of MAXNET Energy is a standardized test setup/protocol for catalyst stability, which we propose in this contribution. ©2015 by De Gruyter Mouton.
    view abstract10.1515/green-2015-0021
  • Mesoporous Silica Supported Au and AuCu Nanoparticles for Surface Plasmon Driven Glycerol Oxidation
    Schünemann, S. and Dodekatos, G. and Tüysüz, H.
    Chemistry of Materials 27 (2015)
    Herein, we report for the first time the visible-light-assisted rate enhancement for glycerol oxidation using direct plasmonic photocatalysis. Au nanoparticles were loaded on various mesoporous SiO2 supports, and the catalytic performance was investigated with and without visible-light illumination. Monodispersed mesoporous silica spheres loaded with Au nanoparticles demonstrated a superior photoassisted catalytic rate enhancement compared to Au loaded ordered mesoporous silica (SBA-15, KIT-6, and MCM-41). The enhancement is attributed to the particle size of the Au nanoparticles and better light interaction resulting from the small SiO2 domains. Au loaded monodispersed mesoporous silica spheres exhibit a constant and remarkably small particle diameter of 2 nm at Au loadings of up to 15 wt % as a result of the support's small domain size and efficient pore confinement. The performance of the Au catalyst could be further improved by preparing bimetallic AuCu nanoparticles. Synergistic effects between Au and Cu improved the glycerol conversion by a factor of 2.5 and the dihydroxyacetone selectivity from 80% to 90% compared to monometallic Au catalysts. © 2015 American Chemical Society.
    view abstract10.1021/acs.chemmater.5b03520
  • Morphology-Controlled Synthesis of Organometal Halide Perovskite Inverse Opals
    Chen, K. and Tüysüz, H.
    Angewandte Chemie - International Edition 54 (2015)
    The booming development of organometal halide perovskites in recent years has prompted the exploration of morphology-control strategies to improve their performance in photovoltaic, photonic, and optoelectronic applications. However, the preparation of organometal halide perovskites with high hierarchical architecture is still highly challenging and a general morphology-control method for various organometal halide perovskites has not been achieved. A mild and scalable method to prepare organometal halide perovskites in inverse opal morphology is presented that uses a polystyrene-based artificial opal as hard template. Our method is flexible and compatible with different halides and organic ammonium compositions. Thus, the perovskite inverse opal maintains the advantage of straightforward structure and band gap engineering. Furthermore, optoelectronic investigations reveal that morphology exerted influence on the conducting nature of organometal halide perovskites. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/anie.201506367
  • Pseudomorphic Generation of Supported Catalysts for Glycerol Oxidation
    Deng, X. and Dodekatos, G. and Pupovac, K. and Weidenthaler, C. and Schmidt, W. and Schüth, F. and Tüysüz, H.
    ChemCatChem 7 (2015)
    A catalyst consisting of copper nanoparticles (15-20 nm in size) supported on ordered mesoporous cobalt monoxide was synthesized by the one-step reduction of ethanol from nanocast copper cobalt spinel oxides. The small-angle X-ray scattering patterns showed that the ordered mesostructure was maintained after post-treatment, and the cross-section scanning electron microscopy images showed that the Cu nanoparticles were distributed homogeneously throughout the mesoporous CoO framework. The materials were tested as noble-metal-free catalysts for the oxidation of glycerol under alkaline conditions. The catalytic data showed that the presence of Cu nanoparticles greatly enhanced the catalytic performance. Nothing noble: A catalyst consisting of copper nanoparticles (NPs, 15-20 nm in size) supported on ordered mesoporous cobalt monoxide is synthesized by the one-step reduction with ethanol from nanocast copper cobalt spinel oxides. As a noble-metal-free catalyst for the oxidation of glycerol, the presence of Cu NPs greatly enhances the catalytic performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cctc.201500703
  • Surface plasmon-assisted solar energy conversion
    Dodekatos, G. and Schünemann, S. and Tüysüz, H.
    Topics in Current Chemistry 371 (2015)
    The utilization of localized surface plasmon resonance (LSPR) from plasmonic noble metals in combination with semiconductors promises great improvements for visible light-driven photocatalysis, in particular for energy conversion. This review summarizes the basic principles of plasmonic photocatalysis, giving a comprehensive overview about the proposed mechanisms for enhancing the performance of photocatalytically active semiconductors with plasmonic devices and their applications for surface plasmon-assisted solar energy conversion. The main focus is on gold and, to a lesser extent, silver nanoparticles in combination with titania as semiconductor and their usage as active plasmonic photocatalysts. Recent advances in water splitting, hydrogen generation with sacrificial organic compounds, and CO2 reduction to hydrocarbons for solar fuel production are highlighted. Finally, further improvements for plasmonic photocatalysts, regarding performance, stability, and economic feasibility, are discussed for surface plasmon-assisted solar energy conversion. © Springer International Publishing Switzerland 2015.
    view abstract10.1007/128_2015_642
  • A study on the growth of Cr2O3 in ordered mesoporous silica and its replication
    Grewe, T. and Deng, X. and Tüysüz, H.
    Chemistry - A European Journal 20 (2014)
    A systematic study on the growth of Cr2O3 in three-dimensional cubic ordered mesoporous silica (KIT-6) and its replication through nanocasting is reported. By changing the loading time and amount of precursor, the size and shape of the obtained replica could be controlled to some extent. More interestingly, in contrast to previously published studies, when KIT-6 with an aging temperature of 100 °C, which has a high degree of interconnectivity, was used as a hard template, a cubic ordered mesoporous Cr2O3 replica with an open uncoupled subframework structure and reduced symmetry was obtained. Formation of a replica with different symmetry and uncoupled subframework structure is not only related to the degree of interconnectivity of the parent, but also strongly depends on the type of metal oxide and its growth mechanism in the silica template. Nanocasting of Cr2O3 with a low loading results in a replica with monomodal pore size distribution that has same symmetry as the hard template, whereas increasing the loading amount alters the symmetry of the replica and yields a replica with bimodal distribution. Perfect replicas of cubic ordered mesoporous silica template KIT-6 with the same symmetry and coupled subframework structure were obtained by nanocasting of Cr2O3 at low precursor loading, whereas increasing the precursor loading resulted in a replica with reduced symmetry, uncoupled subframework structure, and bimodal pore size distribution (see figure). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201402301
  • Cobalt-oxide-based materials as water oxidation catalyst: Recent progress and challenges
    Deng, X. and Tüysüz, H.
    ACS Catalysis 4 (2014)
    Inspired by natural processes, there is an enormous interest in light-driven water splitting to convert solar energy into electrical and chemical energy. This approach is thought to be able to eventually solve the main energy problem that society will face more dramatically in the near future. The water oxidation reaction is widely considered a major barrier for utilizing solar energy in artificial photosynthesis. Due to the relatively high overpotential and slow kinetics of the reaction, numerous efforts are made on the development of non-noble metal oxygen evolution catalysts such as transition metal oxides. Among them, cobalt-oxide-based materials have shown decent activity and thus present themselves as a promising candidate. In this perspective, we summarize the state of the art in synthesis of cobalt-oxide-based materials and application as water oxidation catalysts through electrochemical, photochemical, and photoelectrochemical approaches. Additionally, we state the future challenges that are critical to overcome to push the catalyst performance one step further. © 2014 American Chemical Society.
    view abstract10.1021/cs500713d
  • Impacts of geometry, symmetry, and morphology of nanocast Co3O4 on its catalytic activity for water oxidation
    Deng, X. and Schmidt, W. and Tüysüz, H.
    Chemistry of Materials 26 (2014)
    Herein, we report a systematic study on the synthesis of ordered mesoporous Co3O4 nanocast from cubically (KIT-6) and hexagonally (SBA-15) ordered mesoporous silica hard templates. By increasing the number of impregnation cycles, the effect of loading amount on the replica symmetry as well as on its microstructure and textural parameters was investigated in detail by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and N2 sorption. By changing the loading amount of the metal precursor, we could modify the symmetry, pore systems, and morphologies of the replicas. Low loading favors formation of different symmetry in case of replication of cubically ordered mesoporous Co3O4. Increasing the loading amount results in a perfect negative replica of the KIT-6 silica template. Using the 2D ordered SBA-15, the symmetry of the Co3O4 replicas followed that of the template, regardless of its loading amount. However, the degree of the interconnectivity and the length of the nanowires increased. From the cubically ordered Co3O4 replicas the one with lowest symmetry and open pore system performed best as catalyst for water oxidation whereas for hexagonally ordered Co3O4 replicas highest activity was observed with nanowires that have higher degree of the ordering and interconnectivity. The electrocatalytic results for water oxidation showed that hexagonally ordered Co3O4 shows superior activity to the cubically ordered one. © 2014 American Chemical Society.
    view abstract10.1021/cm5023163
  • Influence of Fe doping on structure and water oxidation activity of nanocast Co3O4
    Grewe, T. and Deng, X. and Tüysüz, H.
    Chemistry of Materials 26 (2014)
    Herein, we demonstrate that a perfect replication of a desired composition is not only related to the degree of interconnectivity of the double gyroid ordered mesoporous silica template, there is also an enormous effect from the nature of precursor and its composition. For the first time, the symmetry of ordered mesoporous Co3O4 was tuned with iron doping by using the same batch of cubic ordered mesoporous silica (KIT-6) as a hard template. Nanocasting of the pure Co3O4 results in a negative replica of the silica template that has a monomodal pore size distribution and a dense coupled structure, while incorporation of a small amount of iron lowers the mesostructural symmetry and alters the pore system of the replica. The effect of this remarkable observation was further investigated for electrochemical water oxidation where superior catalytic activities were observed when Co3O4 was doped with small amounts of iron. Furthermore, iron incorporated Co3O4 indicated comparable water oxidation activity with noble metal and cobalt based electrocatalysts. This kind of abundant transition metal based mesostructured material has the potential to be used as promising electrocatalysts for water oxidation. © 2014 American Chemical Society.
    view abstract10.1021/cm5005888
  • Photocatalytic hydrogen production over various sodium tantalates
    Grewe, T. and Meier, K. and Tüysüz, H.
    Catalysis Today 225 (2014)
    The present study reports the preparation of a series of NaTaO3, Na2Ta2O6, composite sodium tantalates and their photocatalytic activities for hydrogen production. We demonstrate that the crystal structure, band gap, morphology and textural parameters of sodium tantalates can be controlled via a feasible hydrothermal route by changing the pH and reaction time. The prepared materials are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), N2-sorption and UV-Vis spectroscopy. Na 2Ta2O6 with pyrochlore-type structure, that has an average particle size of around 27 nm, is synthesized at low alkali concentration. By slightly increasing the alkali concentration, another Na 2Ta2O6 sample with an average particle size of 15 nm and higher surface area is obtained. Further increasing the alkali concentration results in a series of composite materials based on a mixture of Na2Ta2O6 and NaTaO3. In addition, large NaTaO3 cubes are prepared at very high alkali conditions. The catalytic activities of the prepared samples are investigated for photocatalytic hydrogen production and their efficiencies are correlated to the composition, surface area and junction between the two crystal structures of the materials. The highest photocatalytic activity is achieved with Na2Ta 2O6 nanoparticles with the highest surface area. It is noticed that the hydrogen production rate is not only correlated to the high surface areas of the materials, an enhanced H2 production is obtained for composite materials that is attributed to junctions between the pyrochlore and perovskite phases of sodium tantalate. © 2013 Elsevier B.V.
    view abstract10.1016/j.cattod.2013.10.092
  • Plutonium sorption to nanocast mesoporous carbon
    Parsons-Moss, T. and Tüysüz, H. and Wang, D. and Jones, S. and Olive, D. and Nitsche, H.
    Radiochimica Acta 102 (2014)
    Nanocast ordered mesoporous carbons are attractive as sorbents because of their extremely high surface areas and large pore volumes. This paper compares Pu uptake, added as Pu(VI), to both untreated and chemically oxidized CMK-(carbon molecular sieves from KAIST) type mesoporous carbon with that to a commercial amorphous activated carbon. The CMK was synthesized via nanocasting by using cubic ordered mesoporous silica KIT-6 as a hard template, and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR), and nitrogen adsorption. A portion of the CMK was oxidized by treatment with nitric acid, and will be called OX CMK. The three carbon powders have similar particle morphology, and high BET surface areas. The activated carbon is disordered, while the CMK materials show large domains of ordered cubic mesostructure. The CMK material seems to have more oxygen-containing functional groups than the activated carbon, and the oxidation of the CMK increased the density of these groups, especially - COOH, thus lowering the point of zero charge (PZC) of the material. Batch studies of all 3 materials with plutonium solutions, in a 0.1M NaClO4 matrix were performed to investigate pH dependence, sorption kinetics, Pu uptake capacities, competition with ethylenediaminetetraacetic acid (EDTA) in solution, and Pu desorption. Both CMKmaterials demonstrated high Pu sorption from solutions of pH 3 or greater, and the oxidized CMK also showed high sorption from pH 2 solutions. The activated carbon bound less Pu, and at a much slower rate than CMK. All other batch experiments were carried out in pH 4 solutions. The Pu uptake from low-concentration solutions was faster for the oxidized CMKthan for untreated CMK, but inmore concentrated samples (~250 μMPu), the Pu uptake kinetics and apparent capacity were the same for oxidized and untreated CMK. The 23-h Pu uptake capacity of the CMK materials was measured to be at least 58 ± 5mg 239Pu per g CMK carbon, compared to 12 ± 5mg 239Pu per g activated carbon. The presence of EDTA in solution decreased the Pu sorption to CMK. Desorption from all samples occurred in 1M HClO4, usually within 24 h. The Pu interaction with the carbon surface was also probed via X-ray absorption spectroscopy (XAS) on the Pu LIII absorption edge. Spectral fits of the X-ray absorption near-edge structure (XANES) data collected on both types of CMK samples showed that Pu(VI) was reduced to Pu(IV) at the carbon surface. The high affinity of mesoporous carbon for Pu, and the spontaneous reduction of Pu(VI) or Pu(V) to Pu(IV) at these carbon surfaces could be valuable for a variety of applications.
    view abstract10.1515/ract-2014-2138
  • Design of ordered mesoporous composite materials and their electrocatalytic activities for water oxidation
    Grewe, T. and Deng, X. and Weidenthaler, C. and Schüth, F. and Tüysüz, H.
    Chemistry of Materials 25 (2013)
    The controlled synthesis of a series of ordered mesoporous composite materials via solid-solid reaction of ordered mesoporous Co3O 4 with various transition metal precursors is reported. This versatile methodology allows preparation of a range of composites with precisely controllable material compositions. The textural parameters of the heterostructured compounds are highly dependent on the oxidation state of the dopant. Electrocatalytic activities of the prepared materials were investigated as oxygen evolution catalysts for the electrolysis of water. Among the ordered mesoporous composite materials, Co3O4-CuCo 2O4 shows a significant enhancement for electro-catalytic water splitting with a lower onset potential and higher current density. Following these results, a series of ordered mesoporous composite materials based on cobalt and copper oxides with different atomic ratios were prepared through a nanocasting route. Enhanced electrocatalytic performance was obtained for all composite samples in comparison with Co3O4. © 2013 American Chemical Society.
    view abstract10.1021/cm403153u
  • Mesoporous Co3O4 as an electrocatalyst for water oxidation
    Tüysüz, H. and Hwang, Y.J. and Khan, S.B. and Asiri, A.M. and Yang, P.
    Nano Research 6 (2013)
    Mesoporous Co3O4 has been prepared using porous silica as a hard template via a nanocasting route and its electrocatalytic properties were investigated as an oxygen evolution catalyst for the electrolysis of water. The ordered mesostructured Co3O4 shows dramatically increased catalytic activity compared to that of bulk Co3O4. Enhanced catalytic activity was achieved with high porosity and surface area, and the water oxidation overpotential (η) of the ordered mesoporous Co3O4 decreases significantly as the surface area increases. The mesoporous Co3O4 also shows excellent structural stability in alkaline media. After 100 min under 0. 8 V (versus Ag/AgCl) applied bias, the sample maintains the ordered mesoporous structure with little deactivation of the catalytic properties.[Figure not available: see fulltext.] © 2013 Tsinghua University Press and Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s12274-012-0280-8
  • Preparation of amorphous and nanocrystalline sodium tantalum oxide photocatalysts with porous matrix structure for overall water splitting
    Tüysüz, H. and Chan, C.K.
    Nano Energy 2 (2013)
    Herein, we report the preparation of a series of surfactant-free nanostructured sodium tantalum oxide using NaTa(OC3H7)6 as a single precursor. The reaction conditions for the novel synthetic method were optimized and the morphology and crystal structure of the prepared materials were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Condensation and polymerization of NaTa(OC3H7)6 under atmospheric pressure gave a porous amorphous structure that could be converted to crystalline NaTaO3 while crystalline Na2Ta2O6 nanocrystals with a 25nm average particle size could be obtained from a hydrothermal method using NH3 as a base catalyst. In addition, the photocatalytic behaviors of the prepared materials were investigated for overall water splitting into hydrogen and oxygen. Unexpectedly, porous amorphous sodium tantalum oxide showed much better catalytic activity over the crystalline one. The synthesized Na2Ta2O6 nanocrystals also indicated promising activity for overall water splitting without any co-catalyst in comparison to bulk NaTaO3. © 2012 Elsevier Ltd.
    view abstract10.1016/j.nanoen.2012.08.003
  • A crystal structure analysis and magnetic investigation on highly ordered mesoporous Cr 2O 3
    Tüysüz, H. and Weidenthaler, C. and Grewe, T. and Salabaş, E.L. and Benitez Romero, M.J. and Schüth, F.
    Inorganic Chemistry 51 (2012)
    A series of highly ordered mesoporous Cr 2O 3 were prepared through the nanocasting pathway from decomposition of chromium(VI) oxide using KIT-6 as a hard template. The effects of the calcination temperature on the crystal structure, textural parameters and magnetic properties of the material were investigated. It was found that with increasing calcination temperature, surface area and pore volume of the mesoporous Cr 2O 3 decreased slightly. Unpredictably, increasing calcination temperature also influences the lattice parameters of the Cr 2O 3 crystal, and this rearrangement in the lattice parameter leads to changes in the value of the Néel temperature. A spin-flop transition has been observed at a magnetic field smaller than that of bulk material. © 2012 American Chemical Society.
    view abstract10.1021/ic301671a
  • A strategy for the synthesis of mesostructured metal oxides with lower oxidation states
    Tüysüz, H. and Weidenthaler, C. and Schüth, F.
    Chemistry - A European Journal 18 (2012)
    A detailed study on the pseudomorphic conversion of ordered mesoporous Co 3O 4 and ferrihydrite into CoO and Fe 3O 4, respectively, by using alcohol/water vapor as a gentle reducing agent is described. The reduction conditions for the transformation were optimized. In addition, the first one-pot synthesis of mesostructured CoO by using nanocasting with cubic ordered silica as a hard template is demonstrated. As strong as an Ox: A detailed study on the pseudomorphic conversion of ordered mesoporous Co 3O 4 and ferrihydrite into CoO and Fe 3O 4, respectively, by using alcohol/water vapor as a gentle reducing agent is described. The reduction conditions for the transformation were optimized. In addition, the first one-pot synthesis of mesostructured CoO by using cubic ordered silica as a hard template is demonstrated. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201103650
  • Ordered Mesoporous Materials as Catalysts
    Tüysüz, H. and Schüth, F.
    Advances in Catalysis 55 (2012)
    After their discovery in the early 1990s, ordered mesoporous materials have become one of the most widely investigated classes of materials, and applications have been considered in many areas, in particular in catalysis. They have attracted attention because of their unique properties such as high surface areas, controllable compositions, crystallinity, thermal and chemical stability, tailored porosities, narrow pore size distributions, concave surface curvatures, surface functionalities, as well as the opportunities they offer for incorporation of catalytically active and selective species. This chapter is focused on the properties of ordered mesoporous solids that distinguish them from more conventional porous catalytic materials. Emphasis is placed on history, development, and methods of synthesis of ordered mesoporous materials. © 2012 Elsevier Inc..
    view abstract10.1016/B978-0-12-385516-9.00002-8
  • Synthesis of hard magnetic ordered mesoporous Co3O 4/CoFe2O4 nanocomposites
    Tüysüz, H. and Salabaş, E.L. and Bill, E. and Bongard, H. and Spliethoff, B. and Lehmann, C.W. and Schüth, F.
    Chemistry of Materials 24 (2012)
    The nanocomposite Co3O4/CoFe2O4 heterostructured mesoporous material was produced via a simple solid-solid reaction of an iron precursor with ordered mesoporous Co3O 4 that had been prepared via nanocasting from mesoporous silica as hard template. The magnetic behavior of the exchange-coupled antiferromagnetic/ferrimagnetic (AFM/FM) system was investigated via superconducting quantum interference device (SQUID) magnetometry and 57Fe Mössbauer spectroscopy. The low-temperature magnetization loops of the Co3O4/CoFe2O4 heterostructure present exchange bias under cooling in an applied magnetic field. The antiferromagnetic ordering temperature of Co3O4 is increased due to the proximity of the hard magnetic CoFe2O 4 phase. The nanocomposite Co3O4/CoFe 2O4 behaves as an exchange coupled system with a cooperative magnetic switching. © 2012 American Chemical Society.
    view abstract10.1021/cm3005166
  • Fingerprinting the magnetic behavior of antiferromagnetic nanostructures using remanent magnetization curves
    Benitez, M.J. and Petracic, O. and Tüysüz, H. and Schüth, F. and Zabel, H.
    Physical Review B - Condensed Matter and Materials Physics 83 (2011)
    Antiferromagnetic (AF) nanostructures from Co3O4, CoO, and Cr2O3 were prepared by the nanocasting method and were characterized magnetometrically. The field- and temperature-dependent magnetization data suggests that the nanostructures consist of a core-shell structure. The core behaves as a regular antiferromagnet and the shell as a two-dimensional diluted antiferromagnet in a field (2D DAFF) as previously shown on Co3O4 nanowires. Here we present a more general picture on three different material systems, i.e., Co3O4, CoO, and Cr2O3. In particular, we consider the thermoremanent (TRM) and the isothermoremanent (IRM) magnetization curves as "fingerprints" in order to identify the irreversible magnetization contribution originating from the shells. The TRM/IRM fingerprints are compared to those of superparamagnetic systems, superspin glasses, and 3D DAFFs. We demonstrate that TRM/IRM vs H plots are generally useful fingerprints to identify irreversible magnetization contributions encountered in particular in nanomagnets. © 2011 American Physical society.
    view abstract10.1103/PhysRevB.83.134424
  • Templated synthesis of shape-controlled, ordered TiO 2 cage structures
    Deng, Y. and Tüysüz, H. and Henzie, J. and Yang, P.
    Small 7 (2011)
    Based on a combination of colloidal self-assembly and atomic layer deposition, a facile approach is developed to create novel, high-quality, ordered cage structures of anatase TiO 2 with shape and morphology control using Ag nanocrystals of different shapes as templates. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/smll.201100579
  • From glycerol to allyl alcohol: Iron oxide catalyzed dehydration and consecutive hydrogen transfer
    Liu, Y. and Tüysüz, H. and Jia, C.-J. and Schwickardi, M. and Rinaldi, R. and Lu, A.-H. and Schmidt, W. and Schüth, F.
    Chemical Communications 46 (2010)
    Using iron oxide as catalyst, glycerol can be converted to allyl alcohol through a dehydration and consecutive hydrogen transfer. © 2010 The Royal Society of Chemistry.
    view abstract10.1039/b921648k

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