USE OF XPS AND AR+ DEPTH PROFILING TO DETERMINE THE DISPERSION DEGREE OF NI IN NI/TIO2 AND NI/SIO2 CATALYSTS

Two series of Ni/TiO2 and Ni/SiO2 catalysts prepared by different methods have been characterized by XPS and Ar+ sputtering. Depth profiles have been obtained from the intensity of the Ni 2p photoelectron peaks as a function of the sputtering time. The analysis has been carried out for samples in their precursor, calcined and reduced states, in order to obtain information on the changes in dispersion of the active phase through the whole activation process. In the case of reduced Ni/SiO2 Catalysts, comparison with results by other techniques (ie. H-2 chemisorption, x-ray diffraction and transmission electron microscopy) has shown that the sputtering method may give a semiquantitative assessment on the dispersion degree of nickel. However, for Ni/TiO2 catalysts in the SMSI state after reduction in H-2 at 773 K only the Ar+ sputtering method was able to provide information on the distribution of nickel. In this case the depth profiles were consistent with partial solution of the nickel in the TiO2 matrix.