Density functional theory study of methanol decomposition on the CeO2(110) surface

Methanol decomposition on the stoichiometric CeO2(110) surface has been investigated using density functional theory slab calculations. Three possible initial steps to decompose methanol by breaking one of three bonds (O-H, C-O, and C-H) of methanol were examined. The relative order of thermodynamic stability for the three possible bond scission steps is C-H > O-H > C-O. We further isolated transition states and determined activation energies for each of the bond-breaking modes using the nudged elastic method. The activation barrier for the most favorable dissociation mode, the O-H bond scission, is 0.3 eV on the (110) surface. An even lower activation barrier (< 0.1 eV) has been obtained on the CeO2(111) surface for the same bond-breaking mode. We also calculated pre-exponential factors based on the harmonic approximation and obtained overall rate constants at 300 and 500 K for all three initial decomposition steps. In contrast to the order of thermodynamic stability, the calculated bond breaking barriers indicated a different favorable bond breaking order: O-H > C-O > C-H. Our results are consistent with experimental observation that methoxy is the dominant surface species after the stoichiometric CeO2 surface was exposed to methanol. The experimentally observed methanol chemistry was determined by the kinetics of the initial dissociation steps rather than the thermodynamic stability of product states.. The surface coverage of methanol was found to affect the relative stability between molecular and dissociative adsorption modes: Dissociative adsorption modes are preferred for methanol coverages up to 0.5 ML but only molecular adsorption was found to be stable at a full monolayer coverage.