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

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

Madrid

Seminario; Charla invitada por el Instituto de Catálisis y Petroleoquímica; 2015

Instituto de Catálisis y Petroleoquímica - CSIC

p { margin-bottom: 0.25cm; direction: ltr; color: rgb(0, 0, 0); line-height: 120%; }p.western { font-family: "Liberation Serif","Times New Roman",serif; font-size: 12pt; }p.cjk { font-family: "Droid Sans Fallback"; font-size: 12pt; }p.ctl { font-family: "FreeSans"; font-size: 12pt; }Ceria-basedmaterials are widely used in the field heterogeneous catalysis assupport or promoter to improve the activity, selectivity and/orstability of the catalysts.Thisis because the ability of ceria to rapidly change between Ce4+and Ce3+oxidation states as the environment changes from oxidizing toreducing and vice versa. The active role of ceria in thewater-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 ofthe various mechanisms proposed so far for these reactions involveformate (HCOO) transient species, and it has been argued that theymight be playing an active or spectator roles as intermediates.Thepossibility of elucidating the mechanism of catalytic reactions,which is essential for rational catalyst design, depends very much onthe chances of isolating intermediates in the study of each step inthe catalytic cycle. In particular, IR spectroscopy has been used todiscriminate various kinds of molecularly bonded surface formatespecies [1]. Theassignment of particular features in IR spectra to a chemisorbedspecies is in general not trivial. Strong bonding to the surfacesignificantly affects the vibrational frequencies, and the differentways in which species may anchor to the surface result in complex IRspectra. Creatingand evaluating experimental and theoretical model catalysts thatmimic the real ones in their complexity is expected to aid towardsthe fundamental understanding. Inthe present work, the structure, stability and vibrational propertiesof various types of formate species on a model CeO2(111)surface in equilibrium with a realistic environment containing CH3OH, H2Oand H2,using DFT+U and statistical thermodynamics is investigated. Wecombine this analysis with transmission infrared spectroscopy duringthe temperature-programmed decomposition and reaction of CH3OHon a real CeO2support. In doing this we are able to explain the nature of theobserved formate species and, as will be shown, hydroxyl groups arefound to be crucial for their stabilization.[1]Z.Wu, M. Li, D. R. Mullins, and S. H. Overbury, ACS Catal. 2012,2,2224-2234.