Fischer-Tropsch Synthesis on alumina-supported ruthenium catalysts. II Influence of morphological factors

A Ru/\u3b3-Al2O3 (0.87% wt./wt.) catalyst has been studied for the Fischer-Tropsch synthesis (FTS) at 5 \ub7 103 Pa, 508 K with a H2/CO ratio of 2/1. The same catalyst was also sintered at 973 K for different times in order to change the metal dispersion DM of Ru (DM from 18 to 93%). The aim of the present work is to verify which atoms of Ru crystallites are most active in FTS. For this study a 'model' of truncated hexagonal bipyramid for the crystalline agglomerates has been assumed. It was verified that atoms with higher nearest neighbors i (i,8 and 9) on flat faces and edges are more active than those on vertexes and edges at lower i values (i,5 to 7). We found that larger crystallites decompose CO more easily than smaller ones, and that the turnover number for CO disappearance and for CH4, C2, C3, C5-C15 formation does not depend strongly on metal dispersion. However, on the contrary, we found that light olefins (C2 and C3) are easily formed on large crystallites. The above findings reveal that FTS could be considered as a structure sensitive reaction for the initial step, but are structure insensitive for the overall reaction