The Effect of CO and H Chemisorption on the Chemical Ordering of Bimetallic Clusters

The effect of alloying on ligand adsorption energies and how this can modify the segregation patterns of selected binary nanoalloys is studied via first-principles total energy calculations. A model setup is considered, in which high-symmetry 38-atom truncated-octahedral (TO) clusters with compositions A6B32 and B6A32 are used as substrates to bind a single CO molecule or H atom in centroid sites for the following (A,B) pairs: (Au,Pd), (Pd,Pt), and (Cu,Pt), and the relative changes in the energetics of the systems upon ligand coordination are analyzed. We find qualitative similarities but quantitative differences between the CO and H cases (as examples of reducing agents), and a wide variety of behavior for the three (A,B) pairs. In AuPd, Pd-CO bonding is not strongly affected by neighboring Au atoms but the PdcoreAusurface segregation pattern (favored for bare particles) is expected to be inverted in the presence of CO coordinating species. At the other extreme, in CuPt, both Pt-CO and Pt-H bonding is strongly enhanced by neighboring Cu atoms, but the predicted segregation pattern differs from that expected on the basis of results for extended systems due to finite-size effects.