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CO oxidation on a Au/TiO nanoparticle catalyst via the Au-assisted Mars - van-Krevelen mechanism Philomena Schlexer, Daniel Widmann, R. Jürgen Behm, and Gianfranco Pacchioni ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01751 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018
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ACS Catalysis
CO oxidation on a Au/TiO2 nanoparticle catalyst via the Au-assisted Mars - van-Krevelen mechanism Philomena Schlexera,#, Daniel Widmannb, R. Jürgen Behmb,*, Gianfranco Pacchionia a b
Dipartimento di Scienza dei Materiali, Universitá Milano-Bicocca, I-20125, Italy
Institute of Surface Chemistry and Catalysis, Ulm University, D-89069 Ulm, Germany
Abstract Recently, there has been increasing evidence that CO oxidation on TiO2 supported Au catalysts proceeds predominantly via a Au-assisted Mars – van Krevelen mechanism for reaction temperatures of 80 °C and above. We here present results of a combined experimental and theoretical study, aiming at the identification of activated steps in this reaction. O2 multi-pulse experiments, performed in a temporal analysis of products (TAP) reactor at different temperatures between -80°C and 240°C, revealed that the replenishment of surface lattice oxygen vacancies at perimeter sites, at the perimeter of the interface between TiO2 support and Au nanoparticles, proceeds with essentially constant efficiency, independent of the reaction temperature. Hence, this reaction step is barrier-free. Previous studies (D. Widmann and R.J. Behm, Angew. Chem. Int Ed. 50 (2011) 10241) had shown that the preceding step, the formation of a surface lattice oxygen vacancy at these sites, is activated, requiring temperatures above room temperature. Density functional theory based calculations, performed on a Au nano-rod supported on a TiO2 anatase (101) substrate confirmed that the presence of the Au nano-rod leads to a significant reduction of the vacancy formation energy at these sites, resulting in a barrier of only ~0.9 eV for vacancy formation by reaction with adsorbed CO. The reverse process, replenishing the vacancies by reaction with O2, was found to be activated in the case of individual vacancies, but essentially barrier-free for the case of pairs of neighbored vacancies. Consequences of these findings for the mechanism of the CO oxidation reaction on these catalysts, which can be considered as a model system for Au catalysts supported on reducible oxides, are discussed. Keywords: CO oxidation, Mechanism, Mars-van Krevelen mechanism, Activation energy, Oxygen activation, Au/TiO2, TAP reactor measurement, DFT calculation Resubmitted to ACS Catal.: 24.05.2018
* Corresponding author, email:
[email protected] ACS Paragon Plus Environment
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1 Introduction Au catalysts consisting of small Au nanoparticles supported on various metal oxides have attracted considerable attention in the past 30 years due to their high activity for catalyzing various oxidation and reduction reactions already at rather low temperatures. 1-4 Examples include, e.g., the CO oxidation,5-9 the water-gas shift reaction,10-15 the selective and total oxidation of hydrocarbons,16-18 or hydrogenation reactions.19-23 Among these, the oxidation of CO represents the by far most often investigated reaction, and often serves as a prototypical reaction for heterogeneously catalyzed reactions in general.24 For the latter reaction, a number of experimental (for recent examples see, e.g., refs. 9;25-36) and theoretical (for recent examples see, e.g., refs. 37-49) studies could provide a rather detailed, but also contradictory picture of the surface processes contributing to the overall reaction, where the key factors are as follows: i) Under typical reaction conditions, at room temperature and above, CO is mainly adsorbed on the Au nanoparticles (NPs), while at lower temperatures it is also adsorbed on the oxide support. For reaction at very low temperatures
