In the 1970s, the Institut Français du Pétrole patented a heterogeneous CoCu catalyst for the production of C2-C6 oxygenates following CO hydrogenation. Currently longer chain oxygenates are produced in the hydroformylation process, which relies on expensive homogeneous noble metal catalysts. Recent developments in the group of Dr. Norbert Kruse suggest that C6+ oxygenates can be produced at both high selectivity and yield using modified heterogeneous CoCu catalysts. However, little is known about the catalytically relevant interaction of Co and Cu beyond the single-atom impurities investigated by Ruban et al. and later Nilekar et al. Thus, our present work uses density functional theory (DFT) calculations to investigate the segregation tendency of Cu/Co(0001) and Cu/Co(10-12) both in the presence and absence of variable coverage carbon monoxide (CO) - the principle FT reactant. Our calculations show that a 1 ML equivalent of Cu in Cu/Co(0001) and Cu/Co(10-12) tends to segregate completely to the surface. We then show that CO adsorption causes a reversal in segregation of both Cu/Co(0001) and Cu/Co(10-12) wherein Co is effectively “pumped” to the surface layer by the presence of CO. For Cu/Co(0001), high coverage CO can induce 100% enrichment of the surface with Co. However, high CO coverage can only induce ~33% Co enrichment of the Cu/Co(10-12) surface. The latter result seems to stem from the formation of highly stable dicarbonyls and tricarbonyls at the step sites, which breakdown if additional surface Co is present; the CO is needed to stabilize this surface Co. Unassisted direct CO dissociation is then investigated on Cu/Co(10-12) and found to be prohibitively endothermic. However, unassisted CO dissociation by Boudouard reaction is potentially feasible and could account for the observed formation of graphitic layers on CoCu under a CO atmosphere.

CO on CoCu: Induced surface (anti)segregation and outcomes on CO dissociation

251st National Meeting (2016)

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