Activation of Molecular Oxygen and the Nature of the Active Oxygen Species for CO Oxidation on Oxide Supported Au Catalysts

Abstract

Although highly dispersed Au catalysts with Au nanoparticles (NPs) of a few nanometers in diameter are well-known for their high catalytic activity for several oxidation and reduction reactions already at rather low temperatures for almost 30 years, central aspects of the reaction mechanism are still unresolved. While most studies focused on the active site, the active Au species, and the effect of the support material, the most crucial step during oxidation reactions, the activation of molecular oxygen and the nature of the resulting active oxygen species (Oact), received more attention just recently. This is topic of this Account, which focuses on the formation, location, and nature of the Oact species present on metal oxide supported Au catalysts under typical reaction conditions, at room temperature and above. It is mainly based on quantitative temporal analysis of products (TAP) reactor measurements, which different from most spectroscopic techniques are able to detect and quantify these species even at the extremely low concentrations present under realistic reaction conditions. Different types of pulse experiments were performed, during which the highly dispersed, realistic powder catalysts are exposed to very low amounts of reactants, CO and/or O2, in order to form and reactively remove Oact species and gain information on their formation, nature, and the active site for Oact formation. Our investigations have shown that the active oxygen species for CO oxidation on Au/TiO2 for reaction at 80 °C and higher is a highly stable atomic species, which at 80 °C is formed only at the perimeter of the Au-oxide interface and whose reactive removal by CO is activated, but not its formation. From these findings, it is concluded that surface lattice oxygen represents the Oact species for the CO oxidation. Accordingly, the CO oxidation proceeds via a Au-assisted Mars-van Krevelen mechanism, during which surface lattice oxygen close to the Au NPs is removed by reaction with CO, resulting in a partially reduced TiO2 surface, which is subsequently reoxidized by O2. We demonstrate that this is the dominant reaction pathway for Au catalysts based on reducible metal oxides in general, at typical reaction temperatures, while for less active Au catalysts based on nonreducible metal oxides, this reaction pathway is not possible and the remaining activity must arise from another pathway, most probably a Au-only mechanism. At lower reaction temperature, reactive removal of Oact becomes increasingly inhibited, leading to a change in the dominant reaction pathway.

Probing Oxide Reduction and Phase Transformations at the Au-TiO2 Interface by Vibrational Spectroscopy

“…5a), as evidenced by the vibration band of linearly adsorbed CO 2 located at 2349 cm −1 . In experiments at higher temperatures CO 2 formation in the Au/TiO 2 system has been explained previously with a Mars-van Krevelen mechanism, in which the oxygen atoms of TiO 2 in close proximity to the gold nanoparticles are used to oxidize CO [3]. The extraction of oxygen from the material, accompanied by the formation of oxygen vacancies, is demonstrated by the vibrational band of CO bound to oxygen vacancies forming at 2140 cm −1 [64].…”

“…In this respect, the combination with gold nanoparticles as catalyst or cocatalyst is very common, for example in low temperature CO oxidation [2,3] or for the selective oxidation of alcohols [4][5][6][7]. Since TiO 2 can commonly exist in different crystal structures, with anatase and rutile being the two most common and most studied ones [1], there has been much debate about the (photo)catalytic properties of either of them.…”

“…Furthermore, the catalytically active sites could be located on the gold nanoparticles, on the support, or at the metal-semiconductor interface [3][4][5]. The question arises as to whether nanoparticles supported on the different TiO 2 modifications anatase and rutile differ in their catalytic behavior, and whether the support actually remains unchanged after noble metal deposition.…”

“…Raman spectroscopy is generally very well suited for structural studies of titania, since changes in the phase composition and particle size are well reflected in the spectra [34]. Studies of CO adsorption using infrared spectroscopy are desirable to obtain more information on the (electronic) structure of Au/ TiO 2 : CO has been found to be an ideal probe molecule to characterize the adsorbing facet of TiO 2 [35,36], size, shape [37] and charge [38][39][40][41] of the Au nanoparticles, the crystal structure of TiO 2 [42], and even the Au-TiO 2 interface [3]. Furthermore, CO oxidation on gold catalysts has been studied extensively over the past two decades [38], yet mostly with a focus on the active state of gold but less attention to the underlying titania phase.…”

Controlling activity and selectivity using water in the Au-catalysed preferential oxidation of CO in H2

“…Surface hydroxyl groups on the support are clearly necessary for the most-active catalysts [24][25][26] , and we showed 11 , following work by the Haruta 27,28 , Davis 26,29 , Fujitani 30 and Iglesia 31 groups, that water greatly enhances reaction rates. Several groups also suggested that oxygen vacancies on the support play in important mechanistic role [32][33][34] . More-specific mechanistic PROX investigations by Widmann et al showed that H 2 promotes CO oxidation over Au and the same activated oxygen species is involved in both H 2 and CO oxidation 35 .…”