THE REACTION-MECHANISM OF THE PARTIAL OXIDATION OF METHANE TO SYNTHESIS GAS - A TRANSIENT KINETIC-STUDY OVER RHODIUM AND A COMPARISON WITH PLATINUM

The partial oxidation of methane to synthesis gas over rhodium sponge has been investigated by admitting pulses of pure methane and pure oxy gen as well as mixtures of methane and oxygen to rhodium sponge at tem peratures from 873 to 1023 K. Moreover, pulses of oxygen followed by m ethane and vice versa as well as pulses of mixtures of methane and lab elled oxygen were applied to study the role of chemisorbed oxygen and incorporated oxygen in the reaction mechanism. The decomposition of me thane on reduced rhodium results in the formation of carbon and hydrog en adatoms. During the interaction of pure dioxygen with rhodium the c atalyst is almost completely oxidized to Rh2O3. In addition to rhodium oxide, oxygen is also present in the form of chemisorbed oxygen speci es. During the simultaneous interaction of methane and dioxygen at a s toichiometric feed ratio and a temperature of 973 K only 0.4 wt% Rh2O3 is present. The chemisorbed oxygen species are completely desorbed af ter 2 s. A Mars-Van Krevelen mechanism is postulated: methane reduces the rhodium oxide, which is reoxidized by dioxygen. Synthesis gas is p roduced as primary product. Hydrogen is formed via the associative des orption of two hydrogen adatoms from reduced rhodium and the reaction between carbon adatoms and oxygen present as rhodium oxide results in the formation of carbon monoxide. The consecutive oxidation of CO and H-2 proceeds via both chemisorbed oxygen and oxygen present as rhodium oxide. Continuous flow experiments were performed to compare rhodium and platinum. When compared to platinum, rhodium shows a higher conver sion to methane at a comparable temperature and also a higher selectiv ity to both CO and H-2, the difference for CO being most pronounced. T he observed differences in methane conversion and selectivities for th e two catalysts are ascribed to the higher activation energy for metha ne decomposition on platinum compared to rhodium. An additional explan ation for the difference in H-2 selectivity could be the higher activa tion energy for OH formation on rhodium compared to platinum. (C) 1997 Academic Press.