In Situ Diffuse Reflectance FTIR Study of CO Adsorbed on a Cobalt Catalyst Supported by Silica with Different Pore Sizes

A series of Co/SiO2 catalysts with different pore sizes were prepared by the incipient wetness impregnation method. Catalytic and characterization results showed that the porosity of the support strongly influenced the adsorption properties and the Fischer−Tropsch (FT) catalytic performance. The large pore size could cause the Co/SiO2 to form a larger cobalt metal crystallite size. The FT activity and the C5+ selectivity increased as the pore size of the Co/SiO2 catalyst decreased. The dependence of adsorption properties and catalytic performance on pore size has been investigated by studying the adsorption of CO and syngas (H2/CO = 2) on Co/SiO2 using in situ diffuse reflectance FTIR (DRIFTS). The linear- and bridge-type CO adsorption was observed at room temperature on Co/SiO2. In the presence of preadsorbed hydrogen, CO adsorption was enhanced remarkably, and more Co adsorption sites were observed. The intensities of both linear- and bridge-type CO adsorption changed significantly with increasing pore size. The peaks at around 1937 and 2035 cm-1 have the same trend toward FT activity, leading to the speculation that CO has adsorbed on the hexagonal cobalt phase, while the peaks at around 1974 and 2054 cm-1 have the opposing trend toward FT activity, which could be due to CO adsorbed on cubic cobalt. The activity of CS-15 in FT synthesis was much lower perhaps due to not only the larger crystallite size but also the higher fractions of cobalt cubic metal phase. When CO and H2 were coadsorbed on the cobalt catalyst, a new band appeared around 2030−2060 cm-1 with increasing temperatures, which was assigned as a new species "hydrocarbonyl" (HCo(CO)), which was also observed during FT reaction. With increasing temperature, the hydrocarbonyl band shifted to lower wavenumber, indicating that the C−O bond became weaker gradually. The activity of CO adsorbed on the catalysts with different supports is in the order of CS-6 > CS-10 > CS-15, which is consistent with the order of CO conversion for FT synthesis.