Aiming at a mechanistic understanding of the CO and CO2 methanation reaction over supported Ru catalysts and the underlying physical reasons, we have investigated the methanation of CO and CO2 over a Ru/zeolite and a Ru/Al2O3 catalyst, in idealized and CO2-rich reformate gases by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements, employing quantitative steady-state isotope transient kinetik analysis (SSITKA) techniques. On the basis of the correlation between COad band intensity/COad coverage, CH4,ad/HCOad/formate band intensity, and the CH4 formation rate under steady-state conditions, HCOad is unambiguously identified as reaction intermediate species in the dominant reaction pathway for CO methanation on the Ru/Al2O3 catalyst. On the Ru/zeolite such species could not be detected. CO2 methanation proceeds via dissociation to COad, which is subsequently methanated. Formation and decomposition of surface formates plays only a minor role in the latter reaction, they rather act as spectator species.

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We have investigated the methanation of CO and CO 2 over Ru/zeolite catalysts with different Ru loading in semi-realistic reformate gases by in situ X-ray absorption spectroscopy (XAS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and kinetic measurements. Increasing the Ru loading causes an increase of the mean particle size from 0.9 nm (2.2 wt.% Ru) to 1.9 nm (5.6 wt.% Ru). At the same time, also the activity for CO methanation increases, while the selectivity for CO methanation, which is constant at 100% for reformate gases with 0.6% CO, decreases at low CO contents. The latter findings are interpreted in terms of a change in the physical effects governing the selectivity for CO methanation with increasing Ru particle size, from an inherently low activity for CO 2 dissociation and subsequent CO ad methanation on very small Ru nanoparticles to a site blocking mechanism on larger Ru nanoparticles. In the latter mechanism, CO 2 methanation is hindered by a reaction inhibiting adlayer of CO at higher CO ad coverages, i.e., at not too low CO concentrations, but facile in the absence of a CO adlayer, at lower CO concentrations in the reaction gas mixture.

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The selectivity for CO methanation is a decisive aspect for the practical application of the methanation reaction for the removal of CO from CO2-rich H2 fuel gases produced via hydrocarbon reforming. We show that increasing the water content in the feed gas, up to technically relevant levels of 30%, significantly increases the selectivity of supported Ru catalysts compared with operation in (almost) dry gas, while in operando EXAFS measurements reveal a gradual decrease in the Ru particle size with increasing amounts of water in the gas feed. Consequences of these findings and related IR spectroscopic data for the mechanistic understanding and practical applications are outlined.