Search

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 staistical 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.

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.

The surface structure and reactivity of vanadia supported on ceria catalysts were studied by Raman spectroscopy and methanol adsorption and temperature-programmed surface reaction (TPRS) of the adsorbed species monitored in situ by infrared spectroscopy. Monomeric and polymeric vanadia surface species and CeVO4 were identified on the catalysts. Methanol adsorption reveled that mostly reduced Ce3+ sites are present in the surface of the catalysts, produced as a result of the reductive solid reaction of VOx species with the ceria surface. Adsorbed methanol/methoxy decomposed stepwise to formate and, finally, carbonate/carbon dioxide. The reactivity is linked to interphase sites, Ce–O–V, which could favor the abstraction of hydrogen during the methoxy transformation to formates.

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

Ceria-based materials have a wide variety of applications including ceramics, gas sensors, solid state electrolytes, automotive exhaust catalysts, among others [1]. Particularly, ceria is used in the field heterogeneous catalysis as support or promoter to improve the activity, selectivity and/or stability of the catalysts. One of the crucial properties of a ceria-based material is its oxygen storage capacity by means of redox shift between Ce3+ and Ce4+ under oxidizing and reducing environments, which reduces the carbon formation and/or facilitate the oxidation of pollutants in three way catalysts (exhaust catalysts). Since then, other industrial reactions have been studied on such materials, for instance the water-gas-shift reaction (WGSR, CO + H2O → CO 2 + H2) and the steam reforming of methanol (SRM, CH3OH + H2O → CO2 + 3 H2). Numerous reaction mechanisms have been proposed to determine the limiting step of each reaction [2], where the possible reaction pathways involve formate species (HCOO −) formation as either spectator or intermediate [2]. The formates species can be coordinated differently on oxide surfaces, classifying the species according to the number of bondings with the Ce atom (Monodentate, Bidentate, Bridge). IR measurements performed under various experimental conditions have shown three types of formate species for the WGS and SRM reactions. From the theory, various efforts were made to assign the IR peaks to different formates species but in any case the experimental conditions consistently considered [3]. The aim of this work is to study the influence of oxidizing/reducing environment on the stability of various formate species. For this we include in our calculations H, OH, O2 and H2O co-adsorbed in the vicinity of formate, being calculated the Gibbs free energy from data obtained by the DFT + U and the results are summarized in phase diagrams depending on the hydrogen and oxygen chemical potentials