The impact of support surface area on the SMSI decoration effect and catalytic performance for Fischer-Tropsch synthesis of Co-Ru/TiO2-anatase catalysts

Abstract

[EN] A series of Co-Ru/TiO2 catalysts (10 wt% Co, 0.5 wt% Ru, nominal loadings) were prepared by impregnation of TiO2-anatase supports synthesized with different specific surface areas (Ti-L: 53 m2 /g, Ti-M: 117 m2 /g, and Ti-H: 148 m2 /g) by tuning the conditions of the hydrothermal synthesis and/or the calcination treatments. The most relevant physicochemical properties of supports and catalysts were determined by a set of techniques including ICP-OES, XRD, N2 physisorption, electron microscopy (FESEM, HAADF-STEM, HR-TEM), H2-TPR, H2 chemisorption, and IR-CO. Oxidized precursors were reduced in-reactor under flowing pure H2 at 400 °C for 10 h and evaluated for Fischer-Tropsch synthesis (FTS) in a fixed bed reactor at 220 °C, 2.0 MPa, and H2/CO molar ratio of 2. These catalysts exhibited the well-known strong metal-support interaction (SMSI) effect reported for TiO2 materials by which partially reduced TiOx species formed during the catalyst reduction step migrate and decorate the surface of the supported metal phases. The extent to which the SMSI effect occurred was found to increase with the surface area of the TiO2-anatase carrier, as supported by H2 chemisorption, TEM, and IR-CO surface titration experiments. As a consequence, the activity per total mass of cobalt or cobalt-time-yield (CTY) of the Co-Ru/TiO2 catalysts gradually declined with the increase in support surface area: Co-Ru/Ti-L > Co-Ru/ Ti-M > Co-Ru/Ti-H. The catalysts, however, displayed similar initial TOFs, implying a negligible influence of the SMSI effect on the initial intrinsic activity of the surface Co0 sites. The high surface area Co-Ru/Ti-H catalyst exhibiting the most pronounced SMSI also presented the lowest C5+ selectivity. This behavior was explained by considering the contribution of two effects: the lower resistance to the intraparticle diffusion of ¿-olefins when increasing the support surface area, as inferred from the olefin-to-paraffin ratios and the values of the diffusionrelated parameter ¿, and the reduction in size of the cobalt ensembles on the terraces of Co0 nanoparticles, connected to the extent of SMSI, on which chain growth events are favored.