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

Abstract

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 (PT = 1.2 bar) over a 20 wt % Co/MnOx-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 −CHx formation, θCO, θCHx, TOFCH4, 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 θCO and θCHx on the Mn/Co ratio. Dynamic hydrogen chemisorption at 100 °C and H2-TPD studies indicated the increase of θH 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 MnOx promoter in Co/γ-Al2O3 was obtained via HRTEM/EDX (RGB mapping). Highly dispersed MnOx clusters were formed on the cobalt surface for low Mn/Co ratios (ca. 0.011), while for higher Mn/Co ratios both MnOx 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/MnOx-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.