Mechanistic origins of the promoting effect of tiny amounts of Rh on the performance of NiOx/Al2O3 in partial oxidation of methane

The influence of Rh concentration (0.005–0.15 wt.%) in NiOx(3 wt.%)/Al2O3 on the mechanism and kinetics of partial oxidation of methane (POM) to syngas was elucidated by combining steady-state catalytic tests over 120 h on stream with catalysts characterization and temporal analysis of products. All bi-metallic catalysts showed POM performance close to the thermodynamic one. The reaction mechanism does not depend on the nature of metals but is influenced by their reduction state. From a kinetic point of view, rhodium species (both reduced and oxidized) possess significantly higher activity toward methane conversion than the nickel ones. Rhodium fulfills a double role in the bi-metallic catalysts. Irrespective of its concentration, it promotes the reduction of Ni2+ in inactive NiAlxOy to metallic Ni directly under POM conditions. This is a reason for the higher activity and selectivity of low Rh-loaded (0.005 wt.%) NiOx(3 wt.%)/Al2O3 catalyst compared to its respective mono-metallic counterparts. Owing to the high activity of rhodium species, they alone determine the POM performance of higher Rh-loaded catalysts.