Structure sensitivity of the Fischer–Tropsch activity and selectivity on alumina supported cobalt catalysts

Identifying the active site on the surface of a heterogeneous catalyst is one of the biggest challenges in the field of catalysis research. Especially, in the case of structure sensitive and heterogeneously catalyzed reactions, the knowledge of the active site/ensemble would result in a great advantage in the quest to design tailored catalyst displaying the desired activity and selectivity. In the Fischer–Tropsch synthesis, the multitude of reaction products as well as the large number different reaction pathways does result in additional difficulties in the search for the active site/ensemble. In the here presented work, we were able to conduct a thorough study of the CO hydrogenation reactions on nano-sized alumina supported cobalt crystallite model catalysts. By evaluating the full product spectrum, it was possible to deconvolute the structure sensitivity of the various reactions and to gain further insight into the nature of the present reaction mechanisms. It was therefore possible to measure decreasing carbon monoxide turn over frequency with decreasing cobalt crystallite site, paralleled with an increased selectivity toward methane and branched hydrocarbons at a constant olefin selectivity. Although these trends were observed to be independent of time on stream, the activity did change drastically upon the initial exposure to reaction conditions. CO-TPD studies provided direct evidence for the observed size dependencies.