Vibrational spectroscopy in catalysis: The power of synergy between theory and experiment
1Instituto de Catálisis y Petroleoquímica, CSIC, Madrid, Spain
2Instituto de Física de Rosario (CONICET-UNR) and Univ. Nacional de Rosario, Rosario, Argentina
3Instituto de Desarrollo Tecnológico para la Industria Química (CONICET-UNL) and Univ. Nacional del Litoral, Santa Fe, Argentina
The complexity of real (powder) catalysts hinders the fundamental understanding of their structure, which is essential to establish the relationship between structure and reactivity. Moreover, the possibility of understanding the mechanism of catalytic reactions depends very much on the chances of isolating intermediates in the study of each step in the catalytic cycle. Understanding catalyst structure and reaction mechanism can be obtained by a reductionist approach consisting in creating and evaluating experimental and theoretical model catalysts that mimic the real ones in their complexity. The feasibility of such an approach to date is undoubtedly due to recent advancements in characterization techniques and theoretical methods. In-situ vibrational spectroscopy offers a very powerful experimental tool box, allowing investigation of catalysts surfaces at the molecular level and reaction intermediates on both real and model catalysts than can help bridge the gap between them. Yet, the interpretation of vibrational spectra is far from trivial and the synergy between theory and experiment is essential to it.
In this talk we will demonstrate the capabilities of vibrational spectroscopy analysis for obtaining structure-reactivity relationships and molecular level insight about catalytic reaction intermediates. Specifically, we will discuss the example of an important class of catalysts formed when one metal oxide is deposited on a second metal oxide such as VOx/CeO2 - a very active catalyst for oxidative dehydrogenation (ODH) reactions. We will show how a combination of scanning tunneling microscopy (STM), photoelectron spectroscopy (PES), infrared reflection absorption spectroscopy (IRAS), and density functional theory (DFT) calculations, enables the elucidation of the surface functional groups existing on the catalyst surface . The relation between structure and reactivity for ODH reactions will be briefly discussed .
A second example deals with the elucidation of the nature of the formate (HCOO) species upon methanol (CH3OH) decomposition on a CeO2 support; such species have been proposed as intermediates in the production of hydrogen by the water-gas shift (CO+H2O→CO2+H2) and methanol steam reforming (CH3OH+H2O→CO2+3H2) reactions over CeO2-based catalysts. We will analyze 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 O2, using DFT and statistical thermodynamics. We combine this analysis with transmission infrared spectroscopy during the temperature-programmed CH3OH decomposition reaction on a real CeO2 support. In doing this we are able to explain the nature of the observed formate species while bridging the above-mentioned gap. Hydroxyl groups are found to be crucial for the species stabilization.
 M. Baron et al., Angew. Chem. Int. Ed. 48, 8006 (2009)
 M. V. Ganduglia-Pirovano et al., J. Am. Chem. Soc. 132, 2345 (2010)
 P. G. Lustemberg et al., unpublished