Carbon Number Dependence of Reaction Mechanism and Kinetics in CO Hydrogenation on a Co-Based Catalyst

A detailed mapping of the kinetic parameters involved in the reaction network of CO hydrogenation on a Co-Re/CNT catalyst has been performed. Multicomponent steady-state isotopic transient kinetic analysis (SSITKA) has been used to deconvolute the rates of chain growth and termination to olefins and paraffins with various carbon numbers into the concentration of the surface intermediates and their reactivity, expressed as their rate constants k(g), k(o), and k(p), respectively, at a molecular level. The site coverage of the different products (theta(Cn)) measured by the multicomponent analysis of their isotopic distribution allows the study of their effect on chain growth, as well as on olefin and paraffin formation. The insights into the reaction mechanism were gained from the kinetic dependence of different reaction steps on the hydrogen pressure. The results revealed a significant carbon number dependence of the mechanism for the chain growth and termination to paraffin and olefin reactions: an enol-like intermediate is dominating for the formation of C-3 hydrocarbons, while the alkenyl intermediate is dominating for the formation of C-4 hydrocarbons. Moreover, the SSITKA-assisted kinetic study provided the carbon number dependence of the rate constants of various reaction steps for the paraffin formation and chain growth, both decrease concurrently with increasing carbon number. However, the rate constant for the olefin formation is constant, regardless of the carbon number, suggesting the formation rate of olefins with different carbon number depends mainly on the concentration of the corresponding surface intermediates.