The Effect of H2Pressure on the Carbon Path of Methanation Reaction on Co/γ-Al2O3: Transient Isotopic and Operando Methodology Studies

Abstract

The effect of H2partial 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-Al2O3catalyst 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/H2SSITKA step-gas switch suggested the participation of more than one kind of active -CHxintermediates in FTS and the independence of CO adsorption dynamics on the H2pressure. 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 PH2under FTS at 230 °C. A monotonic but small decrease of k with increasing PH2was observed for both types of linear CO-s. The dependence of TOFCH4(s-1) on PH2was better understood based on the determination of the dependence of surface coverages of CO-s and active -CHxspecies and that of keff(s-1) on PH2, but also on the change of θHwith PH2. It was proved that variation of TOFCH4with PH2was largely governed by the variation of θ-with H2pressure considering an H-assisted CO hydrogenation to CH4mechanism. 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.