The spectroscopic characterization of intermetallic synthesis gas conversion catalysts and the correlation of their activity with surface structure

Two series of Ni x Si y (NiSi 2 , Ni 3 Si 2 , Ni 2 Si and Ni 5 Si 2 ) and Ni x Th y (Ni 3 Th 7 , Ni 2 Th, NiTh and Ni 5 Th) intermetallics have been prepared by melting the component metals in an induction furnace. Surface and bulk characterization were carried out by the combined use of several techniques: Ion Scattering Spectroscopy (ISS), X-ray Photoelectron Spectroscopy (XPS, ESCA) and X-ray diffraction (XRD). The results show that the surfaces of the untreated Ni x Si y and Ni x Th y alloys are all nickel-poor compared to the bulk. Oxidation treatment further decreases the surface Ni content of Ni x Si y alloys. Surface enrichment in nickel is, however observed in the case of Ni x Th y intermetallics upon oxidation. Oxidized Ni x Si y alloys which are reportedly active in methanation (Ni 5 Si 2 , Ni 2 Si) show a relative surface enrichment in nickel and evidences of partial decomposition whereas silicon-rich alloys are essentially stable. Oxidized Ni 5 Th, which exhibits the greater methanation activity within the Ni x Th y system, shows equally the highest surface concentration in nickel. The correlation between Ni surface content and CO conversion data will be discussed in terms of the relative configuration of the Ni phase with respect to ThO 2 .