Study of formate species in the CH3OH/CeO2 reaction: combining IR spectroscopy and statistical thermodynamics techniques

Abstract

Trabajo presentado en el Meeting COST Action CM1104-Reducible oxide chemistry, structure and functions, celebrado en Barcelona (España) del 12 al 14 de noviembre de 2014.Formate species (HCOO-) have been suggested as intermediates or spectators in chemical reactions of industrial relevance, such as the water-gas-shift and the methanol steam reforming reaction [1]. Several catalysts, some of them based on ceria, have been proposed to improve the performance of these reactions. One of the main reasons of choosing ceria has been its intrinsic redox properties, but only few works describe the formate formation under the oxidizing/reducing environment, usually modulated by the reaction itself. In this work, we have studied the stability of formate groups in the CH3OH/CeO2 system. In situ transmission infrared spectroscopy was used to study the temperature-programmed surface reaction (TPSR-IR) of adsorbed methanol. Three types of formates were observed based on the OCO stretching frequencies [(OCO)] (Figure 1). Type III formate (as = 1550 cm-1) was the first one obtained over 450 K, where no Ce3+ was detected. However, after increasing the temperature, types I and II (as = 1580 and 1561 cm-1) species emerged accompanied by the appearance of surface Ce3+ species. The phase diagram of formate structures in contact with a gas environment of O2 and H2, to simulate oxidizing/reducing conditions, was calculated using density-functional theory and statistical calculations. The formate binding structure was found to crucially depend on temperature and partial pressures of the reactants in the gas phase. In the absence of methanol the CeO2, CeO2-x, O2/CeO2 and several OH-coverages phases coexist. As methanol pressure increases, we found a bridge formate with two next neighbors hydroxyls adsorbed (NN-OHads) and no Ce3+, named Brg-A (1535 cm-1). At higher methanol chemical potential (Figure 2), a monodentate formate with three NN-OHads and a bridge state with one NN-OHads, named Mono-B and Brg-B, respectively, appeared together with the formation of Ce3+ species (1564 and 1540 cm-1, respectively). We conclude that the experimentally observed formate species correspond to those monodentate and brigde types.Peer Reviewe