Influence of the Mn Promoter on the Composition and Activity of the Adsorbed Phase in the Carbon Paths of the CO Hydrogenation Reaction on 20 wt % Co/MnOx-Al2O3: An Operando-SSITKA and Transient Kinetic Study

Herein, we explored the influence of the Mn/Co molar ratio (0.011–0.268) on the composition and activity of the adsorbed phase established during CO hydrogenation at 230 °C (P_T = 1.2 bar) over a 20 wt % Co/MnO_x-Al₂O₃ industrially relevant catalyst as a function of time-on-stream, TOS (2–50 h). Correlations between surface coverages of active and inactive carbonaceous species, site activity (k, s^–1) for methanation and chain growth, and the Mn/Co molar ratio for optimum performance were derived. ¹³CO-SSITKA revealed that the Mn/Co molar ratio and TOS significantly influenced the dynamic net rate of CO chemisorption and that of −CH_x formation, θ_CO, θ_CHx, TOF_CH4, and k (s^–1) of methanation and chain growth kinetic parameters in a diverse way. An optimum Mn/Co ratio of 0.111 was found for chain growth (C₂–C₅ hydrocarbons) and which was related to the dependence of θ_CO and θ_CHx on the Mn/Co ratio. Dynamic hydrogen chemisorption at 100 °C and H₂-TPD studies indicated the increase of θ_H and alteration of H-chemisorption site distribution with increasing Mn/Co ratio. Operando DRIFTS-mass spectrometry transient hydrogenation of two linear type adsorbed CO-s revealed the influence of Mn/Co ratio on their relative hydrogenation activity (k), highlighting the importance of the population of CO chemisorption sites of lower hydrogenation activity toward methane. Structural and topological information for the presence of the MnO_x promoter in Co/γ-Al₂O₃ was obtained via HRTEM/EDX (RGB mapping). Highly dispersed MnO_x clusters were formed on the cobalt surface for low Mn/Co ratios (ca. 0.011), while for higher Mn/Co ratios both MnO_x particles surrounded the Co particles (10–12 nm) and Mn chemically interacted with the Co surface (e.g., Co-MnO clusters and agglomerates). It is proposed that optimum chain growth is the result of a balance between unpromoted and Mn-promoted cobalt surface regions for the present 20 wt % Co/MnO_x-Al₂O₃ catalytic system. This work paved the way for deriving reliable correlations between important kinetic parameters of CO hydrogenation that control the chain growth in FTS for Mn-based and other promoted Co-based FTS catalysts.