The activation routes of C-O bonds during Fischer-Tropsch synthesis remain controversial, with direct CO* dissociation and hydrogen-assisted activation as plausible routes. Recent kinetic and isotopic data and theoretical treatments have provided evidence for a distinct preference for H-assisted paths.H2O increases rates of CO activation on Co catalysts; such effects do not reflect removal of site-blocking chemisorbed carbon (C*) nor the result of an increase in CO diffusivity through the intrapellet condensed phase (as these experiments were run at conditions free of any CO mass transfer limitations). Here, we show that H2O also increases CO activation turnover rates and product molecular weight on Ru catalysts, and propose a mechanism for H2O-assisted CO activation based on density functional theory (DFT) calculations. H2O mediates the formation of COH* via a H+-shuttling reaction in which H2O reacts with H* to form short-lived H3O+ intermediates which protonate the O-atom of CO*. COH* then reacts with H* to form HCOH** which undergoes C-O activation to form CH* and OH* species. Kinetic studies suggest that the H2O-mediated CO activation path has a ?Gact 8 kJ mol-1 lower than the anhydrous path, in good agreement with the DFT-predicted difference in ?Gact of 2 kJ mol-1.C-C bond formation routes during Fischer-Tropsch synthesis also remain controversial, with both CHx* and CO* as plausible chain-growth monomers. The increase in product molecular weight with increasing H2O is due an increase in the relative rate of propagation over termination for growing chains. H2O increases the coverage of CHx* intermediates and decreases the barrier of C-O activation of RCO* intermediates (formed from CO*-insertion), thus increasing the rate of propagation over termination for both C-C bond formation routes.

Effects of water on the mechanism of Fischer-Tropsch synthesis on Ru catalysts

248th National Meeting (2014)

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