Influence of thorium on the surface structure and CO hydrogenation activity of silica-supported nickel catalysts

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X-ray photoelectron spectroscopy (ESCA or XPS), ion scattering spectroscopy (ISS), X-ray diffraction (XRD), H 2 chemisorption, and gravimetric analysis have been used to characterize a series of Ni/Th/SiO 2 catalysts. The information obtained from surface and bulk techniques has been compared with CO hydrogenation activity and selectivity of the supported nickel catalysts. Comparison of measured ESCA Th/Si intensity ratios with values predicted by a theoretical model shows that the particle size of the Th phase increases from 1.4 to 4.2 nm as the Th/Si atomic ratio increases from 0.0036 to 0.023. ESCA and XRD results also indicate that nickel is present as poorly dispersed NiO on the calcined unpromoted catalyst. Reducibility of the Ni phase in all catalysts exceeds 97%. ESCA data show that Th addition has little effect on Ni metal dispersion up to a Th/Si atomic ratio of 0.015. For higher Th loadings, the Ni dispersion increases from approximately 6 to 9% with increasing Th loading. H 2 chemisorption data suggest that Ni dispersion increases from 4.8 to 9.8% with increasing Th content. The turnover frequency (TOF) for CO hydrogenation calculated using ESCA estimates of Ni dispersion increases by a factor of 9 as the Th/Si atomic ratio increases from 0 to 0.015. For higher Th loadings the TOF decreases, but it remains seven times the value observed for the unpromoted catalyst. These results have been explained in terms of the Th surface coverage indicated by ISS. Th addition also increases olefin selectivity and the yield for higher hydrocarbons.