Catalytic Ammonia Synthesis by Supported Molybdenum Nitride: Insight into the Support Effect

Abstract

The influence of the support on the performance of Mo nitrides has been investigated in ammonia synthesis and decomposition. A series of Mo–N catalysts supported on different materials, namely SiO₂ (commercial, SBA-15), Al₂O₃, and CeO₂, were prepared. The results indicated that, despite the high dispersion of Mo species in all catalysts, large disparities in the activity for ammonia synthesis exist. Initial rates of ∼1208, ∼481, and ∼372 µmol g_{active phase}⁻¹ h⁻¹ are obtained over 10-Mo–N/SBA-15, 10-Mo–N/SiO₂, and 10-Mo–N/Al₂O₃ respectively. However, no catalytic activity was registered when Mo species were supported on CeO₂. Furthermore, 10-Mo–N/Al₂O₃ deactivated after few hours of reaction. The surface composition was studied by means of XPS to probe the origin of the catalytic activity differences, and the results indicated that a range of various oxidation states of Mo was detected depending on the support. The difference in catalytic behavior could not be solely explained by the differences in Mo–N species concentrations. In situ EPR analysis exhibited that the mechanism of MoO₃ nitridation could differ depending on the support, leading to the formation of different Mo–N species. The effect of support was, however, not as severe in ammonia decomposition as it was the case of ammonia synthesis.