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The roles of Co-precipitation pH, phase-purity and alloy formation for the ammonia decomposition activity of Ga-promoted Fe/MgO catalysts

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Weidenthaler,  Claudia
Research Group Weidenthaler, Max-Planck-Institut für Kohlenforschung, Max Planck Society;

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Rein, D., Ortega, K. F., Weidenthaler, C., Bill, E., & Behrens, M. (2017). The roles of Co-precipitation pH, phase-purity and alloy formation for the ammonia decomposition activity of Ga-promoted Fe/MgO catalysts. Applied Catalysis A: General, 548, 52-61. doi:10.1016/j.apcata.2017.09.004.


Cite as: http://hdl.handle.net/21.11116/0000-0000-2F5C-6
Abstract
A series of mesoporous MgFe1.75Ga0.25O4 mixed spinel oxides obtained upon calcination of hydrotalcite-like precursors was investigated in the ammonia decomposition reaction at 1 atm after reduction in H2 atmosphere. The corresponding precursors were synthesized from metal salt solutions at five constant pH values in the range between 8.5 and 10.5 by co-precipitation in aqueous media to study the impact of pH variation on the catalyst’s structure and activity. N2 physisorption, thermogravimetric analysis, powder X-ray diffraction, Mössbauer spectroscopy, and temperature programmed techniques (H2-TPR and NH3-TPD) were applied to gather information about the textural, (micro-)structural, and adsorption properties of the samples. While phase purity in the precursor and oxide stages is only observed for pH = 10, undesired by-phases (MgFe2O4 and/or Fe3O4) are additionally formed during co-precipitation at the remaining pH values. This is partly related to an incomplete precipitation of Mg2+ cations in less alkaline environments. In situ XRD measurements during reduction revealed that Fe-Ga alloys are formed between 500 and 600 °C. The absence of by-phases avoids the formation of α-Fe, thus improving the structural and compositional homogeneity of the nitridated samples. This beneficial effect is reflected by the low activation energy (70 kJ/mol) and the enhanced low temperature activity (<450 °C) of the phase pure material in the catalytic decomposition of ammonia.