Characterization and reactivity in CO oxidation of gold nanoparticles supported on TiO2 prepared by deposition-precipitation with NaOH and urea

Au/TiO2 catalysts were prepared by deposition-precipitation with NaOH (DP NaOH) (∼3 wt% Au) and urea (DP Urea) (∼8 wt% Au), and calcined at various temperatures between 100 and 400 °C. They were characterized by UV-visible absorption, XANES, EXAFS, and TEM. Their activity was tested in the reaction of CO oxidation at 5 °C. After preparation, the gold species are in the oxidic state III. They begin to transform into metallic gold under air at ∼100 °C for DP NaOH and ∼150 °C for DP Urea. At 200 °C, all the gold is metallic for the two preparation methods. The particle size increases from 1.5 to 3.5 nm with the calcination temperature. The catalytic activity for CO oxidation increases with the percentage of metallic gold, and it is maximum after calcination at 200 °C for both types of samples. The activities (per mole of Au) and TOF are the same for the two types of catalysts. After higher calcination temperatures, the catalytic activity drops. The decrease of activity is mainly due to a change of particle shape with the calcination temperature rather than to the increase of the particle size. At a pretreatment temperature of 200 °C, the particles present small facets with rounded parts, and their outer surface contains a large proportion of low-coordinated sites (steps and corners) while at higher temperatures of pretreatment, they are truncated octahedra with smooth facets.