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

The reaction behaviors of CH4 and CH4/O-2 (2/1) with reduced (Rh/SiO2) and oxidized (Rh(O)/SiO2) SiO2-supported rhodium catalysts were investigated at temperatures ranging from 600 to 800 degrees C over a pulse microreactor. The interaction of methane with Rh/SiO2 led to CO, H-2 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 greater than or equal to 0.5%, besides CO, H-2, and surface carbon, CO2 and H2O were formed. Total oxidation of methane occurred over rhodium oxide together with the simultaneous reduction of Rh3+ to Rh-0. 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/O-2 (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 degrees C by performing CH4 + O-2 and CD4 + O-2 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 CHx (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. (C) 1997 Academic Press.