DIRECT PARTIAL OXIDATION OF METHANE TO SYNTHESIS GAS BY CERIUM OXIDE

The gas-solid reaction between methane and cerium oxide (CeO2) directl y 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 dec reased the activation energy for the production of the synthesis gas. The hydrogen-exchange reaction between CH4 and CD4 proceeded remarkabl y faster than the oxidation of methane with CeO2 regardless of the pre sence or absence of Pt. Thus, It was suggested that the cleavage of th e C-H bond of methane could not be the rate-determining step. The smal l kinetic isotopic effect (k(H)/k(D)=1.1+/-0.1) in methane conversion suggested that the step involving hydrogen such as the recombination o r desorption of hydrogen could be the rate-determining step. H-2, CO, and a small amount of CH4 were observed in temperature-programmed deso rption experiments for the chemisorbed species generated on CeO2 durin g the reaction with methane. This result along with the in situ FT-IR spectroscopic results suggested that the reaction proceeded not throug h HCHO but probably through carbon intermediate. CO must be produced b y the reaction of the carbon with the lattice oxygen of CeO2. TPD expe riments showed that the presence of Pt remarkably decreased the temper ature for the desorptions of H-2 and CO. The obvious tailing of H-2 fo rmation 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.