Evidence for an active oxygen species on Au/TiO 2(1 1 0) model catalysts during investigation with in situ X-ray photoelectron spectroscopy

• Published in: Catalysis today Vol. 181; no. 1; pp. 20 - 25
• Main Authors: Dumbuya, K., Cabailh, G., Lazzari, R., Jupille, J., Ringel, L., Pistor, M., Lytken, O., Steinrück, H.-P., Gottfried, J.M.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 12.02.2012
• Subjects: Carbon monoxide; Catalysis; Gold; In situ photoelectron spectroscopy; Oxidation; Oxygen; Titania; Gold; Oxygen; In situ photoelectron spectroscopy; Oxidation; Catalysis; Carbon monoxide; Titania
• ISSN: 0920-5861; 1873-4308
• DOI: 10.1016/j.cattod.2011.09.035

[Display omitted] ► Au/TiO 2(1 1 0) planar model catalysts for CO oxidation. ► In situ X-ray photoelectron spectroscopy in O 2 and O 2/CO mixtures. ► Au 4f shifts smaller than 1 eV suggest transient molecular adsorption of O 2. ► X-ray induced formation of gold oxide evidenced by Au 4f shift larger than 2 eV. ► Gold oxide reacts with CO. The influence of oxygen (O 2) and carbon monoxide (CO) on Au nanoparticles supported on TiO 2(1 1 0) in the size range of 2–3 nm has been studied using X-ray photoelectron spectroscopy (XPS) and in situ (high pressure) XPS at 300 K for O 2 and/or CO pressures of 0.1–1 mbar. These experiments were aimed at revisiting Au 4f core level shifts as reported in the literature and most importantly, to establish the dependence of the core-level shifts on the knowledge that there exists a maximum in reactivity for CO oxidation. Two samples were prepared with a coverage corresponding to that maximum (Au coverage 0.14–0.2 ML, particle size estimated to ∼2–2.5 nm) while a third sample was expected to be less reactive (Au coverage 0.4 ML, particle size estimated to ∼3.3 nm). At elevated O 2 pressures, a new Au 4f component at higher binding energy (2.4–2.6 eV relative to the Au(0) bulk signal) evolved at all particle sizes. Its appearance was attributed to a radiation-induced activation of oxygen and simultaneous oxidation of gold. The activation was much more efficient on the ∼2–2.5 nm particles. The relative intensity of the oxide component depended strongly on O 2 pressure and, thus, on the equilibrium coverage of O 2. While not present in 0.1 mbar O 2 regardless of exposure time and particle size, it dominated the Au 4f spectrum of particles ∼2–2.5 nm in size at 1 mbar oxygen pressure. This pressure-dependent formation reconciles previously conflicting XPS data. Finally, the activated oxygen species were very reactive toward CO as manifested by the rapid disappearance of the new Au 4f component in a 1:1 mixture of CO and O 2. The rates of evolution and consumption of this component were found to depend on gold coverage (and thus, particle size) and were highest for the smaller particles.