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 prec ursors (773 K, one hour) leads to small metallic clusters of nickel wh ich diffuse toward the bulk of the reduced TiO2 support. Back diffusio n of the metallic phase toward the TiO2 surface is achieved by reoxida tion with O2 up to 673 K. This latter process is accompanied by reoxid ation of the nickel to its initial conditions. The results are discuss ed in terms of the well known SMSI state in this type of catalysts.