(370f) CO Hydrogenation Mechanism at Varying Partial Pressures over Co-Based Catalysts

The catalytic hydrogenation of CO according to Fischer-Tropsch (FT) over Co-based catalysts allows producing long-chain hydrocarbons and oxygenates for the use of transport fuel or as feedstock for lubricants, detergents and plasticizers, respectively. Here we will scrutinize which of the mechanistic suggestions for the reaction are compatible with the experimental results obtained by quantitative chemical transient kinetics (CTK) [1,2].

Materials and Methods

Sudden changes in the chemical composition of the reactant feed are produced to follow the construction of the catalytically active phase (build-up) or its scavenging (back-transient) as a function of time. Data have been quantitatively evaluated at any time to count the amounts of C and O atoms accumulating on the catalyst.

Results and Discussion

Figure 1a demonstrates high surface coverages to be reached during atmospheric CO hydrogenation over Co/MnO_x catalysts. In particular, significant carbon accumulation is encountered for a H₂/CO=1 pressure ratio (Fig. 1c). HRTEM shows that Co particles transform into Co₂C for this catalyst ^1. A CO-insertion mechanism occurs at high coverages under these partial pressure conditions since the chain lengthening probability during build-up is directly proportional to the transient CO pressure. Increasing the partial pressure ratio to H₂/CO=2 (Figure 1b, d) shows lower carbon accumulation (Figure 1d). Transient chain lengthening probabilities are now directly proportional to both transient CO pressures and accumulated carbon concentrations. We conclude that chain lengthening via CO insertion and C-C coupling are in competition here. Chain growth via CO insertion likely involves formate/carboxylate-derived surface species as well as alkoxy, in agreement with DRIFTS observations [1]. A completely different behavior is observed for Co/SiO₂ catalysts (Figure 1c, f) demonstrating the importance of support effects.