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Fecha20 Jul., 10:30h - 20 Jul., 11:30h de 2015
Aula, ICP
  • Instituto de Física Rosario (IFIR)

On 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

SeminaristasPablo Lustemberg
Ceria-based materials are widely used in the field heterogeneous catalysis as support or promoter to improve the activity, selectivity and/or stability of the catalysts. This is because the ability of ceria to rapidly change between Ce4+ and Ce3+ oxidation states as the environment changes from oxidizing to reducing and vice versa. The active role of ceria in the water-gas-shift (WGS, CO +H2O →CO2 + H2) and the steam reforming of methanol (SRM, CH3OH+H2O → O2+3H2) reactions for hydrogen production has been widely reported. Some of the various mechanisms proposed so far for these reactions involve formate (HCOO) transient species, and it has been argued that they might be playing an active or spectator roles as intermediates.
 The possibility of elucidating the mechanism of catalytic reactions, which is essential for rational catalyst design, depends very much on the chances of isolating intermediates in the study of each step in the catalytic cycle. In particular, IR spectroscopy has been used to discriminate various kinds of molecularly bonded surface formate species [1]. The assignment of particular features in IR spectra to a chemisorbed species is in general not trivial. Strong bonding to the surface significantly affects the vibrational frequencies, and the different ways in which species may anchor to the surface result in complex IR spectra. Creating and evaluating experimental and theoretical model catalysts that mimic the real ones in their complexity is expected to aid towards the fundamental understanding.
 In the present work, the structure, stability and vibrational properties of various types of formate species on a model CeO2(111) surface in equilibrium with a realistic environment containing CH3OH , H2O and H2, using DFT+U and statistical thermodynamics is investigated. We combine this analysis with transmission infrared spectroscopy during the temperature-programmed decomposition and reaction of CH3OH on a real CeO2 support. In doing this we are able to explain the nature of the observed formate species and, as will be shown, hydroxyl groups are found to be crucial for their stabilization.
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