DIFFUSION OF NICKEL AND SURFACE RECONSTRUCTION IN NI/TIO2 CATALYSTS PROMOTED BY H-2 AND O-2 TREATMENTS

XPS combined with Ar+-etching shows that H-2 reduction of Ni/TiO2 precursors (773 K, one hour) leads to small metallic clusters of nickel which diffuse toward the bulk of the reduced TiO2 support. Back diffusion of the metallic phase toward the TiO2 surface is achieved by reoxidation with O2 up to 673 K. This latter process is accompanied by reoxidation of the nickel to its initial conditions. The results are discussed in terms of the well known SMSI state in this type of catalysts.