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

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) Auδ+–CO in the region of 2140–2137 cm−1 and (iv) Auδ′+–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 Auδ+ 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 Auδ+ sites to metallic gold. The oxidation of gold particles was more efficient when performed with a NO + O2 mixture. It resulted in creation of Auδ′+ sites with a higher positive charge than that of the Auδ+ 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.