Oxidation of gold metal particles supported on TiO₂: an FTIR study by means of low-temperature CO adsorption

Abstract

Two Au/TiO₂ samples with different gold loadings (0.7 and 4.0 wt.% Au) were prepared by deposition-precipitation with urea and calcined at 673 K. TEM revealed gold particles of 3.2 and 3.9 nm for the 0.7 and 4.0 wt.% samples, respectively. The samples were subjected to different red-ox treatments and then the state of gold was determined by the FTIR spectra of CO adsorbed at low temperature. Several kinds of gold carbonyl species were detected during the experiments: (i) Au⁰–CO at around 2107 cm⁻¹; (ii) Au⁺–CO at ca. 2175 cm⁻¹; (iii) Au^δ+–CO in the region of 2140–2137 cm⁻¹ and (iv) Au^δ′+–CO (δ^′ > δ) at around 2155 cm⁻¹. The 4.0 wt.% sample contained mainly metallic gold after evacuation at 673 K. Subsequent interaction with oxygen at 373 K leads to oxidation of a fraction of the surface metallic gold sites to Au^δ+ sites. These sites were considered as cations located on the surface of the metal particles with a partially positive charge δ+ (0 < δ < 1) because of electron transfer from the gold bulk. Evacuation at 673 K leads to back reduction of the Au^δ+ sites to metallic gold. The oxidation of gold particles was more efficient when performed with a NO + O₂ mixture. It resulted in creation of Au^δ′+ sites with a higher positive charge than that of the Au^δ+ sites. In this case the oxidation involved a higher number of Au⁰ sites. A similar treatment of the 0.7 wt.% Au sample, however, resulted in formation of “isolated” Au⁺ species. The results indicate that small metal particles are more easily oxidized by a NO + O₂ mixture. A model of the formation of the different sites, explaining well the experimental results, is proposed.