Resolving atomic ordering differences in group 11 nanosized metals and binary alloy catalysts by resonant high-energy X-ray diffraction and computer simulations

Resonant high-energy X-ray diffraction coupled to atomic pair distribution function analysis and computer simulations is used to study the atomic-scale structure of group 11 nanosized metals and binary alloy catalysts. We find that nanosized Cu is quite disordered structurally whereas nanosized Ag and especially Au exhibit a very good degree of crystallinity. We resolve Cu-Cu and Ag-Ag atomic correlations from Au-involving ones in Au-Cu and Au-Ag nanoalloys and show that depending on the synthetic route group 11 binary alloys may adopt structural states that obey or markedly violate Vegard's law. In the latter case, Cu and Ag atoms undergo substantial size expansion and contraction by as much as 0.3 and 0.03 Å, respectively, while heavier Au atoms remain practically intact. The size change of Cu and Ag atoms does not follow Pauling's rule of electronegativity predicting charge flow toward the more electronegative Au but occurs in a way such that Cu/Au and Ag/Au atomic size ratios in the nanoalloys become closer to one. Atomic size adjusting and the concurrent charge redistribution result in a synergistic effect of oxygen inactive Au and oxygen very active Cu and Ag leading to nanoalloys with very good activity for low-temperature oxidation of CO.