Dissociative adsorption of CH4 on NiAu/YSZ: The nature of adsorbed carbonaceous species and the inhibition of graphitic C formation

Methane dissociative adsorption and partial oxidation reaction were studied by means of adsorption equilibrium and transient experiments on Au-modified Ni/YSZ powders. It was found that the addition of 1% atomic ratio of Au with respect to Ni on Ni/YSZ cermet catalyst (total metal loading 50 wt%) significantly affects both the kinetics of CH4 dissociative adsorption and the binding strength of the adsorbed CHx species on the Ni surface. The formation of graphitic carbon is significantly inhibited, whereas the hydrogenation of CHx species into CH4, studied by means of temperature-programmed reaction spectroscopy (TPRS), occurs at temperatures 100 K higher than on the unmodified Ni/YSZ surface. This implies higher stability and elongation of the CHx species lifetime on the NiAu/YSZ surface before their decomposition into surface carbidic species. The oxidation reactions involve both the oxidation of CHx species into CHxO and its subsequent decomposition at elevated temperatures (700 K) into CO and H2, whereas carbidic species are selectively oxidized into CO2 at temperatures as low as 500 K. In this respect, the higher CHx surface concentration on the Au-modified Ni/YSZ species will result in higher selectivity toward the production of synthesis gas, with greatly suppressed graphitic carbon formation.