Adsorption of Bromine on Gold Nanoclusters

Small metal nanoclusters are extremely effective as catalysts, with rates that rival those of enzymes in biological systems. The first step in a catalytic reaction is the adsorption of a precursor molecule. The neutralization of alkali projectiles during low energy ion scattering (LEIS), which is acutely sensitive to the local electrostatic potential a few {\AA}'s above the surface, is used here to probe Au nanoclusters grown on SiO$_{\mathrm{2}}$ as they are reacted with Br$_{\mathrm{2}}$. Previous work had demonstrated very efficient neutralization in scattering from small catalytically active Au clusters, which was interpreted as an indication that the bare clusters are negatively charged. X-ray photoelectron spectroscopy and LEIS show little or no Br signal after exposing SiO$_{\mathrm{2}}$ and Au foil to Br$_{\mathrm{2}}$, suggesting that adsorption does not occur because the Br-Br bond does not break. Dissociative adsorption occurs rapidly, however, when small Au nanoclusters are reacted with Br$_{\mathrm{2}}$. 1.5 keV Na$^{\mathrm{+}}$ ions scattered from the Au clusters show a decrease in the neutralization probability as Br is reacted, indicating that adsorption results in charge being transferred from the cluster to the Br adatom.