A combined X-Ray photoelectron and M�ssbauer emission spectroscopy study of the state of cobalt in sulfided, supported, and unsupported Co$z.sbnd;Mo catalysts

Alumina-supported and unsupported Co-Mo catalysts, as well as Co metal, Co9S8, and CoMo2S4 samples, have been studied using X-ray photoelectron spectroscopy (XPS) and Mössbauer emission spectroscopy (MES). The main aim of the study was to examine the feasibility of using XPS to characterize the different Co-containing phases which may be present in sulfided Co-Mo catalysts. The Co phase distributions in the catalyst samples studied by XPS were determined by means of MES. The different cobalt phases observed in the catalysts were Co-Mo-S and Co9S8, and for the supported catalysts cobalt in the alumina lattice was also observed. Although Co metal, Co9S8, Co-Mo-S, and CoMo2S4 are structurally and chemically different and give rise to very different MES spectra, the Co 2p spectra of these compounds are similar. It is shown, however, that by a combination of accurate determinations of binding energy differences and comparisons of peak shapes it is possible to distinguish the different Co phases in the catalysts by XPS. The Co 2p binding energies of Co9S8 are about 0.5 eV smaller than those of Co-Mo-S, and also the Co 2p peak shapes are different. Using XPS, Co9S8 can only be distinguished from Co metal by a detailed comparison of the Co 2p peak shapes. The close similarity between the Co 2p spectra of Co-Mo-S and CoMo2S4 suggests that the electronic state of Co in Co-Mo-S is similar to that in CoMo2S4. However, the MES results show that the two phases are structurally different.