CO Oxidation on a Au/TiO₂ Nanoparticle Catalyst via the Au-Assisted Mars–van Krevelen Mechanism

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 multipulse experiments, performed in a temporal analysis of products (TAP) reactor at different temperatures between −80 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 (Widmann and Behm Angew. Chem. Int. Ed. 2011, 50, 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 nanorod supported on a TiO2 anatase (101) substrate confirmed that the presence of the Au nanorod 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.

The Key Role of Nanocasting in Gold‐based Fe2O3 Nanocasted Catalysts for Oxygen Activation at the Metal‐support Interface

“…This is in agreement with our earlier work, where we showed that this is partly due to the polarisable nature of the Au 10 cluster which stabilises the change in oxidation state of surface Fe cations on the removal of an oxygen anion and partly due to charge transfer to the Au 10 cluster itself. A similar effect has also been found in calculations for Au supported on other oxide surfaces, for example Au/TiO 2 . For the grooved surface, several different types of surface O are present.…”

“…A similar effect has also been found in calculations for Au supported on other oxide surfaces, for example Au/TiO 2 . [35] For the grooved surface, several different types of surface O are present. We have concentrated on two co-ordinate oxygen anions as these give rise to low defect creation energies.…”

Design of Ceria Catalysts for Low‐Temperature CO Oxidation

“…Incorporation of aliovalent dopants into the lattice of host oxide often facilitated the formation of oxygen vacancies on the catalyst surface and induced oxygen species, which affected the CO oxidation of the oxide catalysts . Among the three typical reaction mechanisms describing catalytic CO oxidation such as Langmuir‐Hinshelwood (LH), Eley‐Rideal (ER), and Mars‐van Krevelen (MvK), the MvK mechanism in which the surface is an active reaction component has been particularly used to describe the consecutive processes of CO oxidation on metal oxide catalysts . As shown in Figure , this mechanism consists of two CO oxidation processes with different sources of the oxidizing oxygens.…”

“…[42][43] Among the three typical reaction mechanisms describing catalytic CO oxidation such as Langmuir-Hinshelwood (LH), Eley-Rideal (ER), and Mars-van Krevelen (MvK), the MvK mechanism in which the surface is an active reaction component has been particularly used to describe the consecutive processes of CO oxidation on metal oxide catalysts. [19,[44][45][46][47] As shown in Figure 1, this mechanism consists of two CO oxidation processes with different sources of the oxidizing oxygens. The first one makes use of a lattice oxygen on the clean CeO 2 surface, while the second one uses the oxygen atom from an oxygen molecule which adsorbs onto the surface by recovering an oxygen vacancy.…”

Oxide‐Supported Gold Clusters and Nanoparticles in Catalysis: A Computational Chemistry Perspective

“…Recently, PBE+U calculations performed on an Au nano‐rod supported on a TiO 2 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 for CO oxidation following a MvK mechanism of only ∼0.9 eV. The reverse process, replenishing the vacancies by reaction with O 2 , was found to be activated in the case of individual vacancies, but essentially barrier‐free for the case of pairs of neighboured vacancies …”