Fischer–Tropsch synthesis: The role of pore size for Co/SBA-15 catalysts

Co/SBA-15 (30 wt.%) catalysts with different pore sizes were prepared from the same materials by incipient wetness impregnation and characterized by diffuse reflectance infrared Fourier transform spectroscopy, N2 adsorption–desorption, X-ray diffractometry, temperature-programmed reduction and H2 desorption, oxygen titration as well as transmission electron microscopy. The reduction of the catalysts took place in two stages with Co3O4 reduction to CoO and then to Co0. The first stage reduction was facile regardless of the catalyst pore size while the second stage reduction was much easier on the catalysts with larger pore. After the reduction, cobalt particles were found to be distributed on both the exterior and interior surfaces of the support. Compared to the catalyst with smaller pores, larger pore catalysts have more adsorption sites for CO of both the linear and bridge types. The catalysts with larger pore led to larger cobalt cluster size, lower dispersion and higher reducibility. The larger pores gave rise to more adsorbed CO of the bridge-type on FTS. CO conversion increased and then decreased with the pore size in the range studied. The catalysts with larger cobalt cluster size showed higher C5+ selectivity for the FTS.

When the DRIFTS spectra of 30% Co/SBA-15 with different pore sizes under FTS conditions were collected at 220 °C, a broad peak at 1979 cm−1, attributed to bridge-type CO on cobalt metal sites, was observed. With the increase of catalyst pore size, the intensity of this peak increases significantly. Figure optionsDownload as PowerPoint slide