Microkinetic simulation and fitting of the temperature programmed reaction of methanol on CeO2(111): H2 and H2O+V production

The kinetics and mechanism for temperature programmed reaction following adsorption of an adsorbate can be better understood by simulation and fitting with comparison to experiment. A case study is presented here for the chemistry of adsorption of methanol on a CeO2(111) surface followed by heating. The gas products observed are CH3OH, CH2O, H-2, H2O, CO, CO2. At low temperatures (<500 K), there is formation of H-2 and H2O, where the H2O formation is accompanied by lattice oxygen vacancy (V) formation and is thus important in determining the selectivity towards different products. Microkinetic modeling was performed using a recently published method for fitting to gain mechanistic knowledge of the H-2 and H2O+V formation at <500 K. In the kinetic models used here, most of the H-2 and H2O+V formation can be explained by a mechanism in which a metastable state of hydrogen on the surface (H*) acts as an intermediate. Two possibilities were investigated for the source of the metastable H* intermediate: H* from CH bond breaking of methoxies, or promotion of H+ to H* via electron transfer from ionic methoxies absorbed in oxygen vacancies (CH3O-/V). From this study, we consider the latter to be more likely at <500 K. For the H2O formation, it was found to be critical that H2O cannot dissociate directly on oxygen vacancies. Catalytic chemistry was observed in simulations, including catalytic formation of oxygen vacancies. Various features of the experimental results were reproduced, including methoxies being the major carbon containing species on the surface at <500 K.