Mechanistic Studies of Methane Partial Oxidation to Syngas over SiO2-Supported Rhodium Catalysts

Abstract

The reaction behaviors of CH4and 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/SiO2led to CO, H2and surface carbon formation. The conversion of methane increased with the increase in rhodium loading. During the interaction of methane with Rh(O)/SiO2catalysts having rhodium loading ≥0.5%, besides CO, H2, and surface carbon, CO2and 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 SiO2were 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+O2and CD4+O2reactions alternately over the Rh/SiO2catalysts. Normal deuterium isotope effects of similar magnitude were observed on the overall reaction of methane oxidation as well as on the CO and CO2formation reactions with insignificant change in product selectivities. The results indicate that methane dissociation is a key step and that CO and CO2are 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.