Active oxygen on a Au/TiO2 catalyst: formation, stability, and CO oxidation activity.

Since their introduction by Haruta, oxide supported Au catalysts with Au nanoparticles (NPs) of few nanometers in diameter have attracted enormous interest because of their high activity for various oxidation and reduction reactions most prominently the CO oxidation reaction. Mechanistic details and hence the physical origin of their high activity, however, are still controversial. Focusing on the CO oxidation reaction, a number of different effects and active sites have been proposed as being responsible for the observed high activity, both from experimental and from theoretical work, but so far agreement has not been reached. Most controversial in oxidation reactions are the activation of molecular oxygen, the active site for this reaction step, and the nature of the catalytically active oxygen species present under working conditions. Stiehl et al. had shown that molecularly adsorbed oxygen can be deposited on both Au(111) and Au NPs supported on TiO2(110) at 77 K [16] and moreover, that this oxygen can directly react with CO. The molecular species desorbs, however, upon heating to 170 K. Carretin et al. could identify a -superoxide and peroxide species on pure and Fe-doped Au/TiO2 catalysts upon interaction with O2 at 253 K at atmospheric pressure, which disappeared when changing to CO/O2 reaction mixtures, implying that these species represent the active oxygen species. Stable, molecularly adsorbed oxygen species, mostly located at the perimeter of the interface between a Au cluster and TiO2 support [10;11;15;18;19] or at low-coordination sites of the Au clusters, were identified also in a number of theoretical studies and proposed as active oxygen species, which can react with coadsorbed CO with rather low activation energies via a coadsorption complex. In most cases, the dissociation of adsorbed O2,ad species without interaction with coadsorbed CO was found to be highly activated. Recently, Kotobuki et al. demonstrated in temporal analysis of products (TAP) reactor measurements that active oxygen species, active for facile reaction with CO, can be deposited on Au/TiO2 catalysts at 80°C by exposure to thermal O2 pulses and that these species are stable against desorption at that temperature. They showed that the oxygen storage capacity (OSC) and also the CO oxidation activity of these catalysts during continuous CO oxidation in a micro reactor scale with the length of the perimeter of the interface between TiO2 support and Au NPs. Accordingly, active oxygen species on sites along the perimeter of the interface between Au NPs and TiO2 support were proposed as active species, both for reaction in the TAP reactor and during continuous reaction at atmospheric pressure in a micro reactor. It is important to realize that this oxygen species can hardly be identical with the molecularly adsorbed oxygen species identified in the above experimental and theoretical studies, since the calculated adsorption energies would be too low to stabilize them at 80°C, and in the work by Stiehl et al., desorption of the molecular O2 species was observed at 170 K. [16;17] Stable active oxygen species and a correlation between OSC and CO oxidation activity were reported also for other oxide supported Au catalysts, indicating that this species, which contrasts most proposals for the CO oxidation mechanism, is a general feature for CO oxidation on oxide supported Au catalysts. However, the nature of the active oxygen species, in particular whether it is a molecular or atomic species, could not be clarified in these studies, leaving this central question unresolved. Because of the very low amount of these oxygen species of about 1% of the total amount of surface oxygen, spectroscopic identification of this species is hardly possible. Furthermore, it is also open whether these oxygen species are adsorbed at the perimeter, or whether they represent surface lattice oxygen adjacent to the Au NPs, which is activated by the presence of the Au NPs. In the present communication, we report new results which allow us to clearly identify the nature of the active oxygen species and which provide strong evidence for their location on the catalyst surface. This is based on multi-pulse measurements performed in a TAP reactor at temperatures between 80°C and 400°C. Prior to each experiment, the Au/TiO2 catalyst was pre-treated by in situ calcination in 10% O2/N2 at 400°C (O400) to prepare a well defined, fully oxidized catalyst. Afterwards, the micro reactor was evacuated, and exposed alternately to sequences of CO/Ar pulses and O2/Ar pulses (1:1, 1 10 molecules per pulse each) to determine the amount of stable adsorbed oxygen that can be reversibly deposited and reactively removed under these conditions (OSC). This procedure allows us to determine even very low amounts of active oxygen stored on a catalysts surface very precisely. Raw data of the signals during the initial reduction and subsequent re-oxidation of the Au/TiO2 catalyst after O400 pretreatment (reaction temperature 80°C) and the following cycle, are shown in Fig. 1. In agreement with previous findings, CO2 is produced solely during the CO/Ar pulses over the oxidized catalyst, and not during the subsequent O2/Ar pulses, indicating that CO is reversibly adsorbed under these conditions and desorbs instantaneously after the CO pulse. The consumption of the educt gases, calculated from the missing mass spectrometric intensity compared to that after saturation, is highest in the beginning of each sequence and decreases with ongoing pulse number, until it reaches the zero level and the oxidation state of the catalyst surface is not changed [ ] D. Widmann, Prof. Dr. R.J. Behm Institute of Surface Chemistry and Catalysis Ulm University Albert-Einstein-Allee 47, 89081 Ulm (Germany) Fax: (+49) 731-502 5452 E-mail: [email protected] Homepage: www.uni-ulm.de/en/nawi/iok