Gold Oxide Films Produced by RF-Plasma Oxidation at Various Temperatures: XPS, TPD and Reactivity Study
The metals of Ib group (copper, silver) are widely used as active components of catalysts in the processes of complete and partial oxidation. Till recently gold was not used since it was believed totally inactive. However recently it was surely showed that nanosized gold particles supported on the reducible metal oxides possess high activity in many catalytic reactions. The pathways of oxidation are determined to a great extent by oxygen species, so their nature is a key factor in the understanding of the reaction mechanisms.
Unfortunately the coordination and state of oxygen are not established because of certain difficulties in the application of the surface analysis methods (XPS, UPS, AES etc.) in the investigation of oxygen species on the surface of the supported gold catalysts. Therefore investigations of the gold oxides species without oxide supports or on the inert oxide supports are of high importance. Oxygen species on the gold surface can be investigated on the surface of massive samples (foils, single crystals, films) only using the activation of oxygen. The use of oxygen RF-plasma is one of the most efficient oxidation techniques.
The experiments were performed using electron spectrometer VG ESCALAB HP equipped with XPS and TPD methods. The polycrystalline gold foil (99.99%) was used as a sample. The home-made RF-generator (13,6 MHz, 600W) equipped with alumina electrode was used for phase oxygen activation. The oxidation process was performed at different sample temperatures: 300, 500, 525 K. The adsorbed surface oxygen was characterized by means of XPS (Au4f and O1s spectra) and TPD, the reactivity was tested in situ using flow of CO by means of dynamic-XPS at different temperatures and “step-by-step” surface reduction by dose of CO.
It was found that RF-plasma in 25Pa O2 atmosphere leads to the intensive gold foil oxidation with formation of Au2O3 oxide layers. The maximum oxide film thickness was estimated as 10 nm. Produced gold oxide was characterized by one oxidized gold state – Au3+ with Eb (Au4f7/2) = 85,5 -85,7 eV and two main oxygen forms with Eb(O1s) = 529,3 eV and 530,5 eV. The ratio of oxygen forms changed during surface oxidation. Oxidation at samples temperature close to Tdec of gold oxide leads to diffusion oxygen form formation.