The effect of gold particle size on Ausingle bondAu bond length and reactivity toward oxygen in supported catalysts

Au catalysts with different metallic particle sizes and supported on silica, alumina, titania, zirconia, ceria, and niobia were prepared, and the reduced catalysts were characterized by EXAFS spectroscopy. As the Ausingle bondAu coordination number decreased, the interatomic bond length decreased. The Ausingle bondAu bond length contraction appears to be independent of the support type. A correlation between the dispersion of Pt catalysts determined by hydrogen chemisorption and the EXAFS Ptsingle bondPt coordination number was established and used to determine the dispersion of fully reduced Au catalysts. In addition, the Au particle size was estimated using a literature correlation of the EXAFS coordination number. For particles larger than about 40 Å, there was little change in the metallic bond length, whereas in catalysts with gold particles smaller than 30 Å, the Ausingle bondAu distance decreased with decreasing particle size, with a maximum contraction of about 0.15 Å. Decreasing particle size also brought a decrease in the intensity of the white line of the XANES spectrum. Both the decrease in bond distance and white line intensity were consistent with an increase in the d-electron density of Au atoms in very small particles. Au particles smaller than about 30 Å were also reactive to air, leading to oxidation of up to 15% of the atoms of the gold particles, depending on the size; larger particles were not oxidized. These oxidized Au atoms in small particles are suggested to be active for CO oxidation.