Characterization and catalytic behavior of Co/SiO2 catalysts: Influence ofdispersion in the Fischer-Tropsch reaction

The study of hydrogen evolution from temperature-programmed desorption (TPD ) experiments appears to be so complex that it casts some doubt on the reli ability of chemisorption techniques for the estimation of cobalt dispersion in a series of Co/SiO2 catalysts. Co particle sizes were examined by magne tic measurements (MMs), transmission electron microscopy (TEM), and extende d X-ray absorption fine structure (EXAFS). EXAFS shows that Co in the catal ysts crystallizes in fee crystal lattice and that relatively small clusters are formed. The deduced average size is almost an order of magnitude small er than that obtained by MMs and TEM, suggesting a complex morphology of Co particles. Study of size sensitivity in the Fischer-Tropsch reaction shows that intrinsic activity and chain growth probability first increase and th en stabilize on increasing the particle size, thus providing a quantitative determination of the critical diameter separating these two zones, i.e., c a. 6 nm. Study of the surface species after reaction by TPH experiments ind icates that (i) deactivation may be due to a loss of catalytically active c obalt; (ii) in the course of the reaction, neither formates nor acetates ar e detected in the catalyst; and (iii) four carbon-containing species are ob served (alpha, beta, gamma, and delta). Species beta has been speculated to be the active phase of the reaction on the basis of its parallel variation with intrinsic activity. This intermediate, which is strongly interacting with the cobalt phase, does not contain oxygen atoms. The delta species mig ht be associated with graphitic residue formation, whereas no specific role has been attributed to the gamma species. (C) 2001 Academic Press.