Ammonia Decomposition and Synthesis over Multinary Magnesioferrites: Promotional Effect of Ga on Fe Catalysts for the Decomposition Reaction
Ortega, K. F. and Rein, D. and Luttmann, C. and Heese, J. and Ozcan, F. and Heidelmann, M. and Folke, J. and Kahler, K. and Schlogl, R. and Behrens, M.
Volume: 9 Pages: 659--671
Published: feb 2017
Magnesioferrite (MgFe2O4)-derived Mesoporous spinels of the type MgFeM3+O4 with M3+=Fe, Al, and Ga obtained upon calcination of hydrotalcite-like compounds were investigated in the ammonia decomposition reaction at 1bar and the synthesis of ammonia at 90bar. The corresponding precursors were synthesized by co-precipitation at 50 degrees C and constant pH of 10.5. N-2 physisorption, PXRD, HR-TEM, H-2-TPR, and NH3-TPD were applied in order to obtain information about the textural, (micro-)structural, solid-state kinetics in reducing atmosphere, and adsorption properties of the samples. While phase-pure layered double hydroxides (LDHs) were obtained for Al and Ga, magnesioferrite as the desired oxide phase and a low fraction of magnetite were formed besides the targeted precursor phase during co-precipitation in the presence of Fe2+ and Fe3+ species. Reduction of the binary and ternary magnesioferrites occurs via two consecutive reactions. Only the second stage is shifted towards higher temperatures after incorporation of Al and Ga. The latter element boosts the catalytic decomposition of ammonia, yielding a 2-fold and 5-fold higher conversion at 500 degrees C compared to the samples containing Fe3+ and Al3+ species, respectively. Insitu XRD measurements showed that this unprecedented promotional effect is related to the generation of (Fe, Ga)Fe3N. This phase, however, is detrimental for the synthesis of ammonia at elevated pressures in which the binary system outperforms the ternary spinels, yielding 30% of the activity obtained with a highly promoted Fe-based industrial catalyst.