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 s urface segregation and local environment on the catalytic activity of the PdNi(111) surface for CO hydrogenation. Large Pd segregation to th e surface has been found to affect the activation barrier for dissocia tion of molecules on the surface. It also affects the activation barri ers for different reaction steps in the CO hydrogenation process to fo rm methanol on the Pd-Ni surface. It is argued that for the Pd-Ni syst em, hydrogenation of the adsorbed HCOs species is the rate-limiting st ep. It has then been shown that an addition of even 5 at% of Pd in bul k 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 th at the most active centres for the HCOs hydrogenation step are the cen tre 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.