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

The effect of H-2 partial pressure in a wide range (P-H2 = 0.12-0.6 bar, or H-2/CO gas ratio of 2-10; P-CO = 0.06 bar) on important kinetic parameters of the methanation reaction at 230 degrees C on a commercially relevant Co/gamma-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 (CO)-C-13 adsorption and that of active (CHx)-C-13-s formation under Fischer-Tropsch synthesis (FTS) reaction conditions upon the (CO)-C-12/H-2 ->(CO)-C-13/H-2 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 H-2 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 (theta(H)) as a function of P-H2 under FTS at 230 degrees 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 P-H2 was better understood based on the determination of the dependence of surface coverages of CO-s and active -CHx species and that of k(eff) (s(-1)) on P-H2, but also on the change of theta(H) with P-H2. It was proved that variation of TOFCH4 with PH2 was largely governed by the variation of theta(Eta) with H-2 pressure considering an H-assisted CO hydrogenation to CH4 mechanism. Transient isothermal hydrogenation at 230 degrees C revealed the presence of inactive -CxHy (C-beta) species, while temperature-programmed hydrogenation the presence of other three types of less active (C gamma(1)-C gamma(3)) carbonaceous species, the amounts and reactivity of which were determined as a function of hydrogen pressure.