Fischer-Tropsch synthesis: Foregoing calcination and utilizing reduction promoters leads to improved conversion and selectivity with Co/silica

Foregoing calcination and utilizing direct reduction of cobalt nitrate led to the formation of smaller cobalt oxide nanoclusters in stronger interaction with silica support as intermediates of the activation process to Co° nanoparticles; this was demonstrated by TPR, TPR-MS, TPR-XANES, and TPR-EXAFS experiments using hydrogen. These intermediate cobalt oxides included a spinel (e.g., Co₃O₄) formed from oxidation of Co²⁺ species by NO₂, which in turn converted to CoO prior to formation of the metal. To improve the reducibility, metal promoters such as Pt, Re, Ru, and Ag were added. Hydrogen chemisorption and EXAFS experiments revealed smaller nanoparticles; Co-Co metal coordination numbers were significantly lower for the H₂-activated Co metal nanoparticles when direct reduction of the nitrate was used relative to H₂-activated air calcined catalysts. Comparing at the same space velocity, the best catalysts were Re and Pt promoted 12%Co/SiO₂ catalysts utilizing direct reduction of the nitrate, where initial conversions in a CSTR were up to 3.8 times higher and 71% higher than unpromoted and Pt promoted air calcined catalysts, respectively. At these conditions, methane production was lower (6.8 and 8.0% for Re and Pt promoted catalysts, respectively, by direct reduction versus 12.5 and 10.1% for unpromoted and Pt promoted air calcined catalysts) and C₅₊ selectivity was higher (81.2% and 81.5% for Re and Pt promoted catalysts, respectively, by direct reduction versus 73.4 and 78.8% for unpromoted and Pt promoted air calcined catalysts). The uncalcined catalysts were slightly less stable than the calcined samples, with the only exception being the rhenium promoted sample, where no visible deactivation was observed; this catalyst also had the highest catalytic activity on a per gram catalyst basis.