Understanding of the activity difference between nanogold and bulk gold by relativistic effects

Abstract

It is a challenge to thoroughly understand the astonishing difference in catalytic activity between nanogold and bulk gold for some oxidation reactions. In this work, the Au-O interactions in various surroundings were investigated by DFT calculations and compared with the Ag-O interactions. We have found the three points. First, only Au-O bond can be significantly strengthened by the linear O-Au-O structure. Second, the Au-O bond is always stronger than the Ag-O bond when the bonds are embedded in common surroundings. Third, the Au-O bond becomes weaker than the Ag-O bond when the number of neighboring Au atoms becomes large, because the Au-O interactions are suppressed by the presence of neighboring gold atoms. The origin of these three points can be attributed to wider spatial extension of d orbitals of gold, induced by strong relativistic effects. The strong relativistic effects make nanogold with smaller coordinate numbers highly active due to the ease in forming strong Au-O bonds, especially for the O-Au-O bond, whereas gold atoms in bulk with larger coordination numbers chemically inert due to the strong suppression by neighboring gold atoms destabilizing the O-Au-O bond.