Direct partial oxidation of methane to synthesis gas by cerium oxide

The gas-solid reaction between methane and cerium oxide (CeO2) directly produced a synthesis gas with H-2/CO ratio of 2. The addition of Pt black remarkably accelerated the formation rates of H-2 and CO and decreased the activation energy for the production of the synthesis gas. The hydrogen-exchange reaction between CH4 and CD4 proceeded remarkably faster than the oxidation of methane with CeO2 regardless of the presence or absence of Pt. Thus, It was suggested that the cleavage of the C-H bond of methane could not be the rate-determining step. The small kinetic isotopic effect (k(H)/k(D)=1.1+/-0.1) in methane conversion suggested that the step involving hydrogen such as the recombination or desorption of hydrogen could be the rate-determining step. H-2, CO, and a small amount of CH4 were observed in temperature-programmed desorption experiments for the chemisorbed species generated on CeO2 during the reaction with methane. This result along with the in situ FT-IR spectroscopic results suggested that the reaction proceeded not through HCHO but probably through carbon intermediate. CO must be produced by the reaction of the carbon with the lattice oxygen of CeO2. TPD experiments showed that the presence of Pt remarkably decreased the temperature for the desorptions of H-2 and CO. The obvious tailing of H-2 formation in the reaction of CeO2 with methane pulse also indicated that the recombination or desorption of hydrogen was the rate-determining step. It was suggested that Pt accelerated this step probably through a reverse spillover mechanism. (C) 1998 Academic Press.