Evaluation of the Au size effect: CO oxidation catalyzed by Au/TiO₂

The size dependence of activity in gold catalysts was examined. Extended X-ray absorption fine structure was used to determine mean particle size, and a flow reactor was used to assess activity of the catalysts for CO oxidation as a function of temperature. A sequence of calcination steps was used to systematically increase the mean Au particle size while repeated measurements of the activity were conducted. In this way the size dependence could be obtained in a single catalyst to avoid differences due to variations in support, synthesis conditions, Au loading, or incidental impurities. Two Au catalysts with different Au loadings were prepared on TiO₂ by deposition precipitation and used for the measurements. For Au particles with mean particle size, d, in the range of 2-10 nm, the measured TOF at 298 K varies as d^{- 1.7 ± 0.2} and d^{- 0.9 ± 0.2} for the 7.2 and 4.5 wt% Au/TiO₂ (P25) catalysts, respectively. Variation between samples emphasizes the conclusion that the activity is sensitive to many factors that may mask the true structure dependence. It is concluded that the observed decrease in activity with increasing particle size beyond 2 nm is controlled by the population of low-coordinate sites, rather than by size-dependent changes in overall electronic structure of the nanoparticle. No evidence was found for maximum activity for small particle sizes, although arguments are offered for why such a maximum was expected but was not observed.