Effect of surface segregation on the catalytic activity of alloys: CO hydrogenation on Pd-Ni(111) surface

A semi-empirical model has been presented for studying the effect of surface segregation and local environment on the catalytic activity of the PdNi(111) surface for CO hydrogenation. Large Pd segregation to the surface has been found to affect the activation barrier for dissociation of molecules on the surface. It also affects the activation barriers for different reaction steps in the CO hydrogenation process to form methanol on the Pd-Ni surface. It is argued that for the Pd-Ni system, hydrogenation of the adsorbed HCOs species is the rate-limiting step. It has then been shown that an addition of even 5 at% of Pd in bulk Ni may increase the activity by as much as four orders of magnitude. So far as the local environmental effect is concerned, it is found that the most active centres for the HCOs hydrogenation step are the centre hollow sites having three-fold symmetry with 3 Pd atoms as nearest neighbours followed by centre hollow sites with two Pd atoms and one Ni atom as nearest neighbours. (C) 1998 Elsevier Science B.V.