Dynamic Electronic Structure of a Au/TiO₂ Catalyst under Reaction Conditions

Abstract

The electronic structure of a highly active Au/TiO2 powder catalyst was probed in situ by synchrotron X-ray photoelectron spectroscopy (XPS) in the 10-1 mbar range. The electronic structure of the Au component was found to respond sensitively to changes in temperature and indicated the absence of bulklike metallic Au under the conditions of highest catalytic activity. Concurrent modification of interfacial sites adjacent to Au on the TiO2 support was not evident from the Ti photoemission, but may have been below the detection limit of XPS.

Salt or Co-Crystal? Determination of Protonation State by X-Ray Photoelectron Spectroscopy (XPS)

“…First, caution in the use of XPS may be advised for samples with vapour pressures above $10 À7 mbar, because the measurements require maintenance of ultra-high vacuum conditions <10 À8 mbar (cooling of the sample may achieve sufficient lowering of the vapour pressure). Second, because of the limited escape depth of the detected photoelectron signals ( 10 nm), surface effects may have to be taken into account when contaminating overlayers are present or when the lattice parameters of the crystal structure are comparable in length to the signal escape depth (a tunable, synchrotron X-ray source permits variation of surface sensitivity and thus deconvolution of surface and bulk contributions 27 ). Third, because the sample is irradiated with soft Xrays, a high flux of low-energy photoelectrons is produced, and radiation damage of the sample can occur.…”

Reactivity of Au nanoparticles supported over SiO2 and TiO2 studied by ambient pressure photoelectron spectroscopy

“…Photoemission spectra of the Au4f, Cls, TI2p, Ols, Si2p, core levels were recorded using photon energies of 400 eV (Au and Si), 600 eV (C ), 720 eV (Ti) and 850 (O) eV, respectively, which produce photoelectron emitted approximately the same kinetic energy (300-350 eV), thus, ensuring the same analysis depth. The binding energy scale in all experiments was calibrated using the known value TI3p (37.5 eV) as a reference [28]. A Shirley background was substrated from all spectra before deconvolution.…”

“…The only work published until now using APPES with powder Au/Ti02 catalysts prepared by deposition-precipitation also describes the shifting and broadening of the gold peaks (using Ti3p from the support as a reference). The authors attributed the shift and broadening to the oxidation of gold nanoparticles by the reactant gases [28]. Another possible explanation could be charging effects.…”

Oxidation state of oxide supported nanometric gold

“…For the popular Au/TiO 2 model catalyst an increasing number of reports identifying some cationic Au under operating conditions are emerging. These involve techniques such as time-offlight SIMS [46], DRIFTS [47], and XPS [47,48]. On Al 2 O 3 supports, Au has also been reported to be partly oxidized using a variety of experimental methods including SIMS [46], XRD and FTIR [49] and XANES [50].…”