The effect of preparation route of commercial Co/γ-Al2O3 catalyst on important Fischer-Tropsch kinetic parameters studied by SSITKA and CO-DRIFTS transient hydrogenation techniques

Abstract

Two commercial Co-based catalysts supported on carbon-coated and uncoated γ-Al2O3 (during Co deposition) of different Co particle size (∼7 and 10 nm) and surface structure were investigated to address the effect of preparation route on important kinetic parameters of the methanation reaction (5 vol% CO, H2/CO = 2, P = 1.1 bar) by advanced transient isotopic and operando DRIFTS experiments. 13CO-SSITKA-Mass Spectrometry experiments performed at 230 °C and after 32 h on TOS, showed that the surface coverage (θCO) and mean residence time (τCO, s) of reversibly adsorbed CO-s are not influenced by the different preparation route, while θCHx, TOFCH4 (s−1) and the τCHx (s) related to the active carbonaceous species were slightly increased with increasing Co particle size (in the 7–10 nm range). On the other hand, the kinetic rates of CO conversion and CH4 production (μmol g−1 s−1) were found to increase with decreasing Co particle size. The TOFCH4,ITK (s−1) estimated on the basis of the measured active CHx-s and CO-s species was found very similar for the two Co particle sizes. The effective rate constant of hydrogenation of CHx-s (keff, s−1) after short time on stream (1 h) was found to increase with increasing Co particle size as the result of the different preparation route applied. Transient isothermal hydrogenation (TIH) experiments following the 13CO-SSITKA-MS gas switch measured the concentration of inactive CHx (Cβ) species formed after 32 h of reaction and which were found to be readily hydrogenated at 230 °C. Temperature-programmed hydrogenation (TPH) experiments estimated the concentration of other types of inactive carbonaceous species (Cγ), which were hydrogenated at elevated temperatures (250–600 °C). The amounts of Cβ and Cγ species were found to significantly increase with increasing Co particle size, and their kinetics of hydrogenation was dependent of Co particle size. These results provided evidence for the lower deactivation rate observed under industrial FTS conditions over the Co/γ-Al2O3 (carbon-coated) catalyst. Operando SSITKA-MS-DRIFTS studies followed by transient isothermal hydrogenation coupled with kinetic modeling highlighted the influence of Co particle size on the relative reactivity (keff) of two different types of adsorbed CO-s towards hydrogen. Catalyst characterization was performed by HAADF-STEM, H2-TPR and transient hydrogen chemisorption at 100 °C followed by TPD.