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

• Published in: ACS catalysis Vol. 12; no. 24; pp. 15110 - 15129
• Main Authors: Vasiliades, Michalis A., Govender, Nilenindran S., Govender, Ashriti, Crous, Renier, Moodley, Denzil, Botha, Thys, Efstathiou, Angelos M.
• Format: Journal Article
• Language: English
• Published: American Chemical Society 16.12.2022
• Subjects: operando methodology; Al2O3-supported Co; transient DRIFTS-CO hydrogenation; methanation reaction; SSITKA
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.2c04269

The effect of H2 partial pressure in a wide range (P H2 = 0.12–0.6 bar, or H2/CO gas ratio of 2–10; P CO = 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 13CH x -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 −CH x 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 (k eff) 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 P H2 under FTS at 230 °C. A monotonic but small decrease of k with increasing P H2 was observed for both types of linear CO-s. The dependence of TOFCH4 (s–1) on P H2 was better understood based on the determination of the dependence of surface coverages of CO-s and active −CH x species and that of k eff (s–1) on P H2, but also on the change of θH with P H2. It was proved that variation of TOFCH4 with P H2 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 −C x H y (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.