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The mechanism of CO dissociation is a fundamental issue in the technologically important Fischer-Tropsch (F-T) process that converts synthesis gas into liquid hydrocarbons. In the present study, we propose that on a corrugated Ru surface consisting of active sixfold (i.e., fourfold + twofold) sites, direct CO dissociation has a substantially lower barrier than the hydrogen-assisted paths (i.e., via HCO or COH intermediates). This proves that the F-T process on corrugated Ru surfaces and nanoparticles with active sixfold sites initiates through direct CO dissociation instead of hydrogenated intermediates.

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The present density functional theory study provides insight into the reactivity of the surface metal atoms of extended/periodic Rh surfaces, clusters, and nanoparticles toward CO adsorption and dissociation. Our results demonstrate that the defect site in a B 5 configuration is the most active one for CO dissociation on all three considered systems. However, the reactivity of the B 5 site for CO dissociation depends critically on the size of the system. The barrier for CO dissociation barrier on the B 5 site increases for smaller particles. The lowest barrier is found for the B 5 site of a stepped Rh (211) surface. CO dissociation on this site occurred with a barrier below the desorption energy of CO.

This computational study gives an insight into the reactivity of the active sites on the most open Ru(112̅1) surface toward CO adsorption and dissociation. The adsorption sites for the CO molecule on different active sites have been characterized by frequency analysis and are in good agreement with the experimental results obtained from high-resolution electron energy loss spectroscopy (HREELS). Two dissociation paths have been explored where the CO molecule is adsorbed in a preactivated state. The results show that the CO has a barrier of 65 kJ/mol with respect to the adsorbed state. It dissociates at the 4-fold hollow site as predicted by an experimental study. The barrier found for the CO dissociation on Ru(112̅1) surface is significantly lower than that on Ru(101̅5) steps studied earlier.

A critical issue in the Fischer-Tropsch synthesis reaction is the blocking of the active sites for low barrier CO dissociation by the C(1) adsorbed species generated from CO dissociation, which can hinder the further steps in the FT process. Here, we propose a synchronized pathway for low barrier CO dissociation and C-C coupling on a corrugated Ru surface.

An optimum reaction path for CO activation is an important issue in the Fischer-Tropsch synthesis for the production of liquid hydrocarbons from syngas. In the present theoretical study, we show that the CO activation on open Ru and Co surfaces consisting of active six-fold sites is initiated through the carbide mechanism instead of the hydrogen assisted pathway.