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

P.G. LUSTEMBERG; A. BONIVARDI; M.V. BOSCO; H.F. BUSNENGO; M.V. GANDUGLIA-PIROVANO

Bercelona

Workshop; 3rd General Meeting COST Actin CM1104, Reducible oxide chemistry, structure and functions; 2014

Formate species (HCOO-) have been suggested as intermediates or spectators in chemicalreactions of industrial relevance, such as the water-gas-shift and the methanol steamreforming reaction [1]. Several catalysts, some of them based on ceria, have been proposed toimprove the performance of these reactions. One of the main reasons of choosing ceria hasbeen its intrinsic redox properties, but only few works describe the formate formation underthe oxidizing/reducing environment, usually modulated by the reaction itself. In this work, wehave studied the stability of formate groups in the CH3OH/CeO2 system. In situ transmissioninfrared spectroscopy was used to study the temperature-programmed surface reaction(TPSR-IR) of adsorbed methanol. Three types of formates were observed based on the OCOstretching frequencies [(OCO)] (Figure 1). Type III formate (as = 1550 cm-1) was the firstone obtained over 450 K, where no Ce3+ was detected. However, after increasing thetemperature, types I and II (as = 1580 and 1561 cm-1) species emerged accompanied by theappearance of surface Ce3+ species. The phase diagram of formate structures in contact with agas environment of O2 and H2, to simulate oxidizing/reducing conditions, was calculatedusing density-functional theory and statistical calculations. The formate binding structure wasfound to crucially depend on temperature and partial pressures of the reactants in the gasphase. In the absence of methanol the CeO2, CeO2-x, O2/CeO2 and several OH-coveragesphases coexist. As methanol pressure increases, we found a bridge formate with two nextneighbors 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.