Mechanistic studies of methane partial oxidation to syngas over SiO₂-supported rhodium catalysts

The reaction behaviors of CH₄ and CH₄/O₂ (2/1) with reduced (Rh/SiO₂) and oxidized (Rh(O)/SiO₂) SiO₂-supported rhodium catalysts were investigated at temperatures ranging from 600 to 800°C over a pulse microreactor. The interaction of methane with Rh/SiO₂ led to CO, H₂ and surface carbon formation. The conversion of methane increased with the increase in rhodium loading. During the interaction of methane with Rh(O)/SiO₂ catalysts having rhodium loading ≥0.5%, besides CO, H₂, and surface carbon, CO₂ and H₂O were formed. Total oxidation of methane occurred over rhodium oxide together with the simultaneous reduction of Rh³⁺ to Rh⁰. The partial oxidation of methane as well as the decomposition of methane took place over the resulting metallic rhodium. Within the first pulse of CH₄/O₂ (2/1), rhodium oxide in the oxidized catalysts was reduced to metallic rhodium. From the second pulse onward, both methane conversion and CO selectivity were similar to those observed over the reduced catalysts. The results indicate that metallic rhodium is the active site for methane partial oxidation and chemisorbed oxygen species have participated in the methane oxidation reaction. We suggest that OH groups and/or certain low coordinated oxygens of SiO₂ were also involved in the oxidation of methane, especially at low rhodium loadings. Deuterium isotope effects in the methane partial oxidation reaction were investigated at 700°C by performing CH₄ + O₂ and CD₄ + O₂ reactions alternately over the Rh/SiO₂ catalysts. Normal deuterium isotope effects of similar magnitude were observed on the overall reaction of methane oxidation as well as on the CO and CO₂ formation reactions with insignificant change in product selectivities. The results indicate that methane dissociation is a key step and that CO and CO₂ are formed via some common intermediates, viz. surface CH_x (x = 0-3) species. The activation energies for methane dissociation with or without the involvement of chemisorbed oxygen on a Rh(111) surface were calculated by means of the bond-order conservation Morse-potential approach.