A theoretical study of butadiene adsorption on the Pd-Ni bimetallic system

The calculated electronic perturbation of the butadiene molecule is identical when it is adsorbed either on the Pd-Ni bimetallic system or on the pure Pd surface. However, the calculations provide evidence that the butadiene adsorption strength is much smaller in the former case. The orbital by orbital analysis reveals that the pi-d(z)2 interaction is reduced, whereas the pi*-d(yz) is enhanced with respect to the pure Pd surface. Nevertheless, since the total electron transfers from the pi orbitals to the pi* orbitals within the butadiene molecule are similar, a two-electron analysis alone is unable to explain the large difference between the butadiene adsorption energy on the Pd-Ni surface alloy and that on pure Pd. This difference is partly related to anti-bonding interactions since the Pd-Ni bimetallic system exhibits a larger four-electron destabilization (roughly +25%). Evidence is also presented for a reduced electron exchange between the Pd-Pd adsorption site and its neighboring atoms in the case of the Pd-Ni bimetallic system with respect to the pure Pd and consequently a diminished coupling with the electron reservoir created by the extended surface. Hence, the adsorbing Pd clusters of a bimetallic system appear more isolated from the remaining part of the solid than they do in the pure Pd. (C) 1997 Academic Press.