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.