ELUCIDATING THE CO HYDROGENATION MECHANISM OVER COBALT BASED CATALYSTS USING CHEMICAL TRANSIENT KINETICS
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This dissertation presents research performed to advance the fundamental knowledge of the mechanism of the catalytic CO hydrogenation according to the Fischer-Tropsch synthesis. Despite extensive studies on the reaction mechanism, a detailed understanding of the relevant steps is still lacking. Quantitative Chemical Transient Kinetics (CTK) informs us of how the reaction proceeds while the catalytically active phase is being built up. Among the proposed mechanisms, C-C coupling and CO-insertion remain the two main suggested mechanisms. The present work scrutinizes which of these suggestions are compatible with the CTK evidence. We illustrate the importance of surface coverage and support effects. Important mechanistic information is obtained from delay time analysis of the products relative to that of the reactants, provided by CTK. We show that the activated Co/MnOx surface does not provide metallic sites under steady-state conditions. A CO-insertion mechanism occurs at high coverages, while at lower coverages C-C coupling between CHx species can occur. The chemical nature along with the surface coverages yet depend on the H2/CO ratio. For low such ratios, the CO-insertion mechanism dominates, involving formate-derived surface species. Specifically, CO hydrogenation towards methane at different H2/CO ratios shows that two mechanisms co-exist; hydrogenation of formate species when the surface is covered and CHx hydrogenation when the surface still provides metallic sites. Microscopy (TEM) and spectroscopy (XPS) analyses support the CTK conclusions. Some carbon can penetrate the Co surface and induce cobalt carbide formation. We performed CTK analyses with pure Co and Co/TiOx, for comparison purposes. The catalytically active surface is shown to be different for these catalysts. We also compare CTK results of the CO hydrogenation with those of methyl formate (MF) hydrogenation. MF is selected as it contains a formate group and forms methoxy during dissociative adsorption on the surface. Such methoxy species are claimed to be important during Fischer-Tropsch synthesis. CTK studies of the support influence are particularly revealing for Co/SiO2 catalysts. Different from all other catalysts addressed here, the surface coverages over Co/SiO2 do not exceed one monolayer capacity. The impact of this finding regarding the reaction mechanism is still to be explored in future work.