The Effect of H₂ Pressure on the Carbon Path of Methanation Reaction on Co/γ-Al₂O₃: Transient Isotopic and <i>Operando</i> Methodology Studies

Abstract

The effect of H2 partial pressure in a wide range (PH2 = 0.12–0.6 bar, or H2/CO gas ratio of 2–10; PCO = 0.06 bar) on important kinetic parameters of the methanation reaction at 230 °C on a commercially relevant Co/γ-Al2O3 catalyst was investigated using steady-state isotopic transient kinetic analysis (SSITKA) and operando transient DRIFTS-mass spectrometry methodology. The quantification of the dynamic evolution of the net rate of 13CO adsorption and that of active 13CHx-s formation under Fischer–Tropsch synthesis (FTS) reaction conditions upon the 12CO/H2 → 13CO/H2 SSITKA step-gas switch suggested the participation of more than one kind of active −CHx intermediates in FTS and the independence of CO adsorption dynamics on the H2 pressure. Transient operando DRIFTS-MS isothermal hydrogenation studies coupled with kinetic modeling (H-assisted CO dissociation) allowed to estimate the different reactivity (keff) of two linear types of adsorbed CO-s and their relative intrinsic activity (k), as well as the change in the surface coverage of H-s (θH) as a function of PH2 under FTS at 230 °C. A monotonic but small decrease of k with increasing PH2 was observed for both types of linear CO-s. The dependence of TOFCH4 (s–1) on PH2 was better understood based on the determination of the dependence of surface coverages of CO-s and active −CHx species and that of keff (s–1) on PH2, but also on the change of θH with PH2. It was proved that variation of TOFCH4 with PH2 was largely governed by the variation of θΗ with H2 pressure considering an H-assisted CO hydrogenation to CH4 mechanism. Transient isothermal hydrogenation at 230 °C revealed the presence of inactive −CxHy (Cβ) species, while temperature-programmed hydrogenation the presence of other three types of less active (Cγ1–Cγ3) carbonaceous species, the amounts and reactivity of which were determined as a function of hydrogen pressure.