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Abstract: The effect of rhenium on the Fischer–Tropsch synthesis activity and selectivity of γ-Al2O3 supported cobalt catalysts was investigated in fixed-bed reactors at T =483K, P =20bar, and H2/CO=2.0. Catalysts containing 20wt.% cobalt and 0 or 0.5wt.% rhenium were prepared by incipient wetness impregnation of different γ-Al2O3 supports with aqueous solutions of cobalt nitrate hexahydrate and for the Re-promoted catalysts, also perrhenic acid. The γ-Al2O3 supports had very different pore characteristics. The post-calcination Co3O4 crystallite size was predominantly controlled by the γ-Al2O3 support pore diameter. Presence of Re had only a minor effect on the crystallite size. For all catalysts, supported Co3O4 was reduced in two steps to Co0 with CoO as intermediate species. However, while reduction of Co3O4 to CoO took place in the same temperature range for all catalysts, the reduction temperature of CoO to Co0 was dependent on the catalyst properties. Large particles present in wide pores were easier to reduce than small particles located in narrow pores. In addition, Re promoted the reduction of CoO. The effect of rhenium as a reduction promoter was less pronounced at increasing pore size and particle size. Re also had a similar positive impact on the cobalt dispersion of the catalysts. Although Re significantly increased the Fischer–Tropsch synthesis cobalt-time yield, it did not modify the site-time yield. The deactivation rates of all the catalysts were also similar up to 100h on stream. Positive correlations were found between the catalyst pore diameter and the C5+ selectivity and between the cobalt particle size and the C5+ selectivity. Re had a consistent positive, albeit small, effect on the C5+ selectivity. [Copyright &y& Elsevier]

Abstract: Au catalysts have been prepared (i) on TiO2, (ii) on carbon nanofibres (CNF) and (iii) on TiO2 deposited onto CNF. Catalysts prepared from deposition-precipitation (DP) and from colloid solutions have been characterised using XRD, TEM, TGA and XAS and tested in the water–gas shift (WGS) reaction. DP yields large Au particles (>50nm) on CNF-containing supports. High Au dispersion on carbon nanofibres requires preparation via other methods such as colloid formation. Au particle growth is more pronounced during the synthesis steps than during thermal treatments. This increase is not observed for the Au particles on TiO2 but only when CNF is present, indicating that the surface properties of TiO2 are altered by the CNF. TiO2 XANES analyses show that distortions in the lattice symmetry of TiO2 are introduced when the oxide is deposited on CNF. The distortion of the TiO2 structure by the CNF may also introduce changes that promote the turnover frequencies. The WGS activity significantly improves when titania is present. This shows that coexistence of Au and TiO2 is needed to obtain high catalytic activity in the WGS reaction, indicating that the active sites are either on the Au–TiO2 interface or that the reaction follows a bifunctional mechanism. [Copyright &y& Elsevier]

Abstract: The catalytic activity and stability in the water–gas shift reaction have been tested for Au-based catalysts prepared by deposition of Au from colloid solutions. The supports that have been used are TiO2, TiO2 supported on carbon nanofibres (CNF) and CNF. Thermal treatments of the samples show that the Au particle size depends on the support material and hence the interaction between the Au particles and the support. In situ X-ray absorption spectroscopic (XAS) measurements during the water–gas shift reaction show no changes in the first Au–Au coordination number for the catalysts containing CNF. Furthermore, improved short-time stability is obtained compared to the AuTiO2 catalysts. The improved stability is achieved by the CNF stabilising small TiO2 particles and hence prevent subsequent sintering of the Au particles. [Copyright &y& Elsevier]

Highlights: [•] High energy XAFS is demonstrated as an attractive option for in situ studies of catalysis at demanding conditions. [•] Both sets of experiments (Re K and LIII edges) indicate that Re is atomically distributed inside the bulk Co particles. [•] The local distortion of the Co environment in intimate contact with the Re atoms may give rise to the enhanced activity of Re-promoted catalysts. [Copyright &y& Elsevier]