Doctor of Philosophy (PhD)
Higher alcohols produced by catalytic conversion of synthesis gas are potential octane enhancers for transportation fuels. Copper-cobalt catalysts have emerged as alternatives to the ethanol-selective, but expensive, rhodium-based ones. The reducibility of the catalyst and the CO adsorption mode are important factors in directing the selectivity of CO hydrogenation toward methanol, hydrocarbons, or higher alcohols. In this study, Cu/SiO2, Co/SiO2, and bimetallic CuCo/SiO2 catalysts, as well as their tin-promoted analogues, were synthesized and characterized by TPR, in situ XRD, in situ XANES, and in situ DRIFTS. Cobalt addition to Cu/SiO2 created an amorphous fraction of CuO that was more easily reducible than crystalline CuO. CO adsorption on copper sites, in terms of adsorption frequency and stability, was similar regardless of the presence or absence of cobalt. CO adsorbed on copper was thermally unstable, desorbing below typical CO hydrogenation temperatures. Copper addition to Co/SiO2 increased reducibility of Co3O4, and to greater extent of the intermediate CoO, which in Co/SiO2 was less readily reduced due to interaction with the support. In general, linearly adsorbed CO bound more strongly to the cobalt sites of CuCo/SiO2 than of Co/SiO2. Thus, on CuCo/SiO2, CO could dissociate at the same sites where it also linearly adsorbed, leading to increased probability of CO insertion and ethanol formation. A second type of site on Co/SiO2 was responsible for direct CO dissociation, leading to high activity and hydrocarbon selectivity on this catalyst, as well as faster deactivation. The main effect of Sn addition was to increase dispersion of copper and cobalt on all the catalysts. This increased the reducibility of copper species and decreased that of cobalt species. On CuCoSn/SiO2, copper promoted cobalt reduction, such that this catalyst was both well reduced and highly dispersed. Sn addition increased the amount and stability of CO adsorbed on copper and cobalt sites of CuCoSn/SiO2 and on copper sites of CuSn/SiO2. However, the CO hydrogenation activity of the Sn-promoted catalysts decreased relative to the Sn-free catalysts. Fischer-Tropsch type chain growth became less favorable upon Sn addition, but CO insertion was still favored on CuCoSn/SiO2, making this catalyst more ethanol-selective than CuCo/SiO2.
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Smith, Miranda Louise, "Copper-Cobalt Catalysts for Conversion of Syngas to Ethanol and Higher Alcohols" (2012). LSU Doctoral Dissertations. 2516.
Spivey, James J.