XPS INVESTIGATION ON Cu-Co MIXED OXIDE CATALYSTS

Copper-cobalt hydroxysalts with cu:co = 0:100, 30:20 and 50:50 atomic ratios have been prepared by coprecipintion, at constant pH, from solutions of copper and cobalt nitrates added to a solurion of NaHCO3. New metastable precurson containing only Cu+2 and Co+2 species have been obtained. They were decomposed at 723 K for 24 hrs both in air and in nitrogen. The surface properties of the mixed oxides so far prepared were investigated by X Ray-Photoelecton spectroscopy (XPS) using Al-Ka radiation (hv=1486.6 eV) as source. Depending both on the composition and on the calcination atmosphere, the following species were revealed : Cu+l, Cu+2, Co+2, Co+3 confirming that the phases detected by XRD were CoO, CuO, Co3O4, CuxCo3-xO4, CuCoO2 and CuxCo1-xO. The 2p3/2 transidons of both Cu+2 and Co+2 ions are characrcrized by a main peak with a satellite peak at higher binding energy whose intensity decreascs when Co+2 and Co+3 are simultaneously present. With regard to the latter two ions, the binding energy of the Co+3 main peak is lower than one of the Co+2 main peak whereas the reverse would be expected. This anomalous result can bc explained considering that, in spite of the higher oxidation state, Co+3 could be experience a smaller positive charge ( less ionic in-character bonding). With respect to copper species, Cu+l is formed only under N2 calcination and is present in the CuCoO2 compound; on the other hand, Cu+2 is present in samples calcined either in air and in N2. Both satellite--main peak energy separation and intensity ratio of the 2p3/2 transition have showed that Cu+2 is more ionic when is contained in phases other than CuO. At last, the surface quantitarive analysis has evidenced that for rhe Cu:Co=30:70 and 50:50 samples calcined in N2 the Cu-Co surface composition is simitar to the Cu/Co bulk ratio meaning that an homogeneous distriibution of copper and cobalt is reached both in the bulk and at the surface, whereas for the corresponding samples calcined in air a cobalt enrichment occurs.