Insights into the Nature of Formate Species in the Decomposition and Reaction of Methanol over Cerium Oxide Surfaces: A Combined Infrared Spectroscopy and Density Functional Theory Study

Formation of formate species on oxide surfaces plays a role in reactions for hydrogen production such as the water–gas shift and the steam-reforming of alcohols. It has been suggested that bridge formates are the most common and stable configuration on metal oxides. Ceria-based catalysts are important for these reactions where ceria is a "non-innocent" support. In this work, the nature of the formate species that are formed during decomposition and reaction of methanol on ceria surfaces have been studied using a combination of infrared temperature-programmed surface reaction (TPSR-IR) on a real powder catalyst support and density functional theory (DFT) together with statistical thermodynamics for model CeO2(111) surfaces. The influence of surface oxygen vacancies, hydroxyl groups, and water has been considered. Three different formate species have been identified (450–550 K). Initially, formates are adsorbed on the oxidized surface that is gradually hydroxylated by the release of hydrogen from methoxy groups (>500 K), which leads to a partially reduced surface. On the former, one kind of species is observed, whereas on the latter, the other two kinds appeared. We provide computational evidence that the bonding is only initially of the bridge type but becomes of the monodentate type as the surface concentration of hydroxyl groups rises. The calculated frequencies of the O–C–O symmetric and asymmetric stretching modes for the three structures are in good agreement with those experimentally observed. The existence of monodentate species is discussed in terms of a stabilizing effect of hydrogen bonds. The combined experimental and theoretical results on real and model systems, respectively, thus provide important insights on the reaction of methanol on ceria surfaces.