Structural characterisation of silica supported CoMo catalysts by UV Raman spectroscopy, XPS and X-ray diffraction techniques

The structure of Co-Mo catalysts supported on commercial silica, doped with various amounts of sodium ions, was investigated by means of X-ray diffraction (XRD), UV-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Two series of samples were considered. One serieswas prepared by classic incipient wet impregnation (WI) and the other series by co-impregnation in the presence of nitrilotriacetic acid (NTA). The effect of sodium on the catalyst structure depended on the preparation procedure: in the case of the WI catalysts, sodium promoted the transformation of the polymolybdate species into monomolybdate Na2MoO4; in the case of the NTA prepared samples, sodium induced transformation from heptamolybdates, Mo2O72- and mixed CoMo oxides into MoO4 2- units with distorted tetrahedral symmetry without forming Na2MoO4 compound. In addition to purely structural modifications, sodium induced an electronic effect, evidenced by Mo 3d and Co 2p XPS binding energy shifts. The catalytic behaviour of the samples, previously tested in the HDS of thiophene, was explained in terms of structural and electronic changes.