d Electron Density and Reactivity of the d Band as a Function of Particle Size in Supported Gold Catalysts

The electronic properties of supported gold nanoparticles were determined using X-ray absorption near-edge spectroscopy (XANES) at the gold L III -edge. The whiteline intensity decreased with decreasing particle size, suggesting that smaller particles contained more d electrons. No change in the whiteline intensity, however, was detected due to the support composition. Full-multiple scattering calculations show that hybridization of the s, p, and d orbitals in bulk gold created holes in the gold d band, transferring a small amount of 5d electron (and 6s electron) density to the 6p orbitals, leading to d electron counts smaller than 10 and a larger whiteline. Calculations for Au atoms with different gold−gold coordination numbers were consistent with the experimental XANES results, indicating that smaller particles have increased d electron density. In addition, the d band in smaller particles is narrower and shifted closer to the Fermi level. These results are consistent with the higher adsorption coverages of H 2, CO, and O 2 and the increased reactivity of Au nanoparticles. The effect of particle size on intrinsic reactivity of small gold particles and that of the support are discussed.