An investigation has been carried out of CO and CO2 hydrogenation to methane over a Rh foil decorated with submonolayer quantities of AlOx, TiOx, VOx, FeOx, ZrOx, NbOx, TaOx, and WOx. The rate of methane formation was measured at 1 atm and the state of the working catalyst was characterized by XPS immediately after reaction. With the exception of AlOx, each of the oxides was found to enhance the rate of CO methanation relative to that observed over unpromoted Rh. The maximum degree of rate enhancement occurs at an oxide coverage of approximately half a monolayer. AlOx retards the formation of methane in direct proportion to the oxide coverage. FeOx behaves in a manner identical to that of AlOx in the case of CO2 hydrogenation, whereas all of the other oxides studied produce a maximum degree of methanation rate enhancement at an oxide coverage of half a monolayer. The enhancement of CO and CO2 hydrogenation is attributed to the formation of Lewis acid-base complexes between the oxygen end of adsorbed CO (or H2CO formed during the reaction) and anionic vacancies present at the edge of the oxide-metal boundary. The degree of enhancement of CO and CO2 hydrogenation is found to increase with the average oxidation state of the metal in the oxide overlayer. This trend is attributed to the direct relationship between Lewis acidity of an oxide and the oxidation state of the metal in the oxide. The degree of reduction of a given oxide is found to be higher during CO2 hydrogenation than CO hydrogenation.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry