The nature and binding strength of carbon adspecies formed during the equilibrium dissociative adsorption of CH4 on Ni–YSZ cermet catalysts

The equilibrium dissociative adsorption of CH4 was studied over Ni–YSZ cermet catalysts for a deeper insight regarding the nature and binding strength of generated carbon species on the Ni–YSZ surface. Three main carbon species were detected by the reaction of carbon adspecies with H2 to produce CH4 or with O2 to produce CO and CO2. Carbidic species (Cc) are reactive with H2 and O2 at temperatures below 600 K while adsorbed carbon (Ca) species in equilibrium with CHx species react with H2 and O2 above 600 K. Graphitic carbon layers (Cg) are formed upon CH4 adsorption above 700 K and its main characteristic is the absence of any reactivity with H2, to form CH4. The binding energy of Ca species with respect to graphite decreases with increasing coverage ranging between 7.32±0.03 and 6.5±0.04 eV for the low (<0.2 ML) and high (≈1 ML) coverage, respectively. The presence of 1% wt of Mo either suppresses the formation of adsorbed graphitic layers which are not reactive with H2 or enhances the reactivity of adsorbed hydrogen atoms toward CH4 at temperatures higher than 800 K, thus revealing the positive effect of Mo in inhibiting the formation of adsorbed graphitic layers.