INSP - UPMC - 4 place Jussieu - 75005 Paris - Barre 22-12, 4e étage, salle 426
Gianfranco Pacchioni - Dipartimento di Scienza dei Materiali - Università Milano Bicocca
CO oxidation by supported gold nanoparticles is one of the most intensively studied catalytic reactions. Most of the attention has been concentrated on the low-temperature CO oxidation, where the reaction occurs well below room temperature (RT) via direct interaction of the activated O2 molecule with adsorbed CO to form CO2. However, at RT or above, the reaction follows another path because the O2 molecule desorbs from Au at 170 K [1 ]. In this case, the CO molecule is directly oxidized by a lattice oxygen at the gold/oxide interface. The process can occur also in absence of ambient gas-phase oxygen, as demonstrated by a series of dedicated experiments [ 2]. The role of O2 is that to re-oxidize the oxide support, in a gold-assisted Mars van Krevelen mechanism. In this process, the role of the oxide reducibility is essential, as the first step of the reaction is the formation of an oxygen vacancy (oxygen extraction by gas-phase CO). In this talk we consider various aspects of this process, as derived from Density Functional Theory calculations on periodic models of the real Au/TiO2 and Au/ZrO2 catalysts. In particular, we will demonstrate the key role of the gold nanoparticle (and in general of supported metals) in reducing the formation energy of oxygen vacancies at the metal/oxide interface. By computing the profile for the entire reaction, we can also address the differences related to the activities of titania and zirconia supports, two oxides characterized by very different reducibility. The results point towards the importance of distinguishing between bulk reducibility and surface reducibility, since this latter can be strongly affected by phenomena such as deposition of metal nanoparticles or oxide nanostructuring.
[1 ] Gong, J. ; Mullins, C. B. Surface Science Investigations of Oxidative Chemistry on Gold. Acc. Chem. Res. 2009, 42, 1063 – 1073.
[2 ] Widmann, D. ; Behm, R. J. Activation of Molecular Oxygen and the Nature of the Active Oxygen Species for CO Oxidation on Oxide Supported Au Catalysts. Acc. Chem. Res. 2014, 47, 740 – 749.