Cobalt-Based Fischer-Tropsch Activity and Selectivity as a Function of Crystallite Size and Water Partial Pressure

Using well-defined supported cobalt nanocrystallites in a novel in situ sample presentation device for laboratory X-ray diffractometers, we investigated the extensively studied structure sensitivity of Fischer-Tropsch (FT) catalysts under simulated high conversion conditions, that is, at high water to synthesis gas ratios. This study has to be 4 regarded as a further small step toward a full understanding of the various processes governing FT activity and selectivity. We were able to show, for two different crystallite sizes, that water has an overall enhancing effect on carbon monoxide conversion and surface specific turnover frequency on metallic surfaces and improves the overall product selectivity with a decrease of methane selectivity. For small crystallites oxidation was observed at elevated water partial pressures, which caused a decrease of activity. The selectivity to the undesired product methane is suppressed in favor of chain growth. This influence on the selectivity might originate from water-induced changes on the active sites responsible for chain growth or by an inhibiting effect of water on methanation sites. Due to a stronger effect of water on smaller crystallites, the impact on the methane selectivity reverses previously described trends of increasing methane selectivity with decreasing crystallite size. Secondary olefin reactions, clearly more pronounced on smaller crystallites and under "dry" conditions, are severely suppressed via the addition of water, resulting in a pseudo structure insensitivity of this class of reactions.