Carbon Deactivation of Fischer-Tropsch Ruthenium Catalyst

Carbon deactivation of a 0.5 wt % Ru/ϒ-AI₂O₃ surface-impregnated catalyst was studied by using a Berty continuous-stirred, gas-solid reactor (CSGSR)-gas chromatograph setup. The experimental variables were as follows: temperature, 473-573 K; pressure, 2-6 atm; weight hourly space velocity, 0.85, 16.5 h^-1; H₂/CO feed ratio, 3 and 2; and synthesis time, 0.5-5 h. Carbon deposited in a synthesis run was measured by integrating the methane evolution profile during catalyst reduction at 723 K in H₂. Significant amounts of carbon were deposited, increasing to several monolayers during 5-h synthesis periods. The methanation rate decreased as the synthesis continued, while the selectivity for C₂-C₄ hydrocarbons showed a maximum during the initial stages of deactivation. The kinetic data could be correlated by assuming both hydrogen-assisted CO dissociation and hydrogenation of surface carbon to be rate determining. The turnover numbers for methanation (N_CH4) and carbon deposition (N_C/Ru) are given by eq 3 and 4.