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

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

Barcelona

Encuentro; 3rd General Meeting Redox COST ACTION CM1104; 2014

Formates (HCOO-) are species that appear as spectators or intermediary in chemical reactions of industrial relevance like the water-gas-shift reaction or the methanol steam reforming [1]. Several catalysts have been proposed to increase the rate and efficiency of these reactions. In recent years ceria-based materials have been used as support or promoter to improve the activity, selectivity and/or stability of the catalysts. This is because one of their crucial properties is their oxygen storage capacity by means of redox shift between Ce3+ and Ce4+ under oxidizing and reducing environments. Based on the foregoing, the stability of formates in the CH3OH /CeO2 system was studied. 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 (nu(OCO)) wich are nu_as(OCO) = 1580 cm-1, 1561 cm-1 and 1550 cm-1. Fig. a) shows the temperature evolution of those frequencies where the formate types I and II appear at 500 K together with Ce3+ signal, whereas the type III appears at 450 K with no Ce3+ signal. Within the theoretical framework, we analized the influence of the oxidizing/reducing environment in the stability of the formate species. The phase diagram of formate structures in contact with a gas environment of O2 and H2 was calculated by density-functional theory and atomistic thermodynamics. 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), called Brg-A, no Ce3+ and nu_as = 1535 cm-1. Then at higher methanol chemical potential, as shown in fig. b), a Monodentate with three NN-OHads and another Bridge states with one NN-OHads (Mono-B and Brg-B) develop together with Ce3+ and nu_as = 1564 cm-1 and 1540 cm-1, respectively. Thus we can conclude that the formates experimentally observed correspond to a Monodentate type and two Brigde types.[1]    T. Shido and Y. Iwasawa, Journal of Catalysis, 136, 493 (2003).