Partial Oxidation of Methane to Synthesis Gas over Ru/TiO2 Catalysts: Effects of Modification of the Support on Oxidation State and Catalytic Performance

The effects of modification of the support on the oxidation state of Ru and the catalytic performance of Ru/TiO2 catalysts under conditions of partial oxidation of methane to synthesis gas have been investigated employing XPS and FTIR techniques. It has been found that the oxidation state of Ru depends on the reaction temperature and on the supporting material and that, under conditions where the direct reaction route is operable, the catalytic performance (conversion of methane and selectivity toward synthesis gas formation) is improved over samples that mainly contain Ru in its metallic form. Doping of TiO2 with small amounts of W6+ cations favors oxygen adsorption on Ru under reaction conditions, thus resulting in stabilization of a fraction of the catalyst in its oxide forms. This is reflected in lower activity and selectivity of this catalyst compared to those of the undoped one. In contrast, Ru supported on undoped and Ca2+-doped TiO2 exists mainly in the metallic form and, as a result, it promotes the direct conversion of methane to synthesis gas. In situ FTIR spectroscopy showed that an adsorbed CO species exists on the surface of the latter two catalysts under reaction conditions, even at temperatures as high as 1073 K. This species, which is not observable over the W6+-doped catalyst, is related to a Ru surface site, which is responsible for the dissociation of methane and formation of CO.