Biogas gas derived from biomass is a renewable energy source to produce syngas (CO+H2), which is a useful precursor to produce bioalcohols, and biofuels. Methane is the main component of biogas and activating it is difficult due to the large energy associated with the initial C-H bond breaking step (450 kJ/mol). A catalyst is needed to reduce the activation energy for methane dissociation, which is the rate limiting step for the overall process. Although transition metal catalysts promote this reaction step, they can be highly expensive, and after successive dehydrogenation of the CH3 species, carbon deposits form and block active sites on the metal rendering the catalyst inactive over time. There is thus a need to design transition metal free catalysts that show similar reactivity, are resistant to carbon poisoning, and are economical for implementation in industrial reactors. Cerium dioxide (CeO2) is a well-studied highly reducible oxide with many applications in heterogeneous catalysis. Substitutional cation doping of the surface is generally found to increase the reactivity by promoting formation of oxygen vacancies, increasing oxygen mobility at the surface, which is an important property for partial oxidative coupling of methane gas. Density functional Theory calculations are used to study methane activation by using alkali metal (Mg, Ca, Sr) doping to modify the CeO2 (111) surface to promote C-H bond dissociation. The calculations show that the thermodynamic drive to dissociate CH4 to CH3 + H becomes is most favourable with smaller dopant ionic radius. The computed activation barriers for Mg and Ca doped CeO2 are smaller than on the pure surface so that methane activation is promoted on these doped surfaces. Ca-doped CeO2 has the lowest energy barrier which can be attributed to the stabilisation of the transition state through formation of a Ce-C bond. Plotting the formation energy of the active oxygen vacancy against the adsorption energy of the dissociated species shows a linear correlation, suggesting that the energy of active O vacancy formation can be used as a descriptor in the design of methane activation catalysts. We acknowledge funding from the European Commission 7th Framework program: BIOGO project (grant no: 604296) www.biogo.eu

Effect of dopant ionic radius on methane activation at the alkaline earth metal doped (111) surface of ceria

251st National Meeting (2016)

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