To better understand the role of water in the selective methanation of CO in CO2-rich reformate gases on Ru/Al2O3 catalysts, the influence of exposing these catalysts to H2O-rich reformate gases on their reaction characteristics in transient experiments was investigated by employing kinetic and in situ spectroscopic measurements as well as ex situ catalyst characterization. Transient exposure of the ruthenium catalyst to wet reaction gas (5 or 15% H2O) results in significantly enhanced activity and selectivity for CO methanation in subsequent reactions in dry reformate compared with activation and reaction in dry reformate directly. Operando X-ray absorption spectroscopy results reveal that this is in accordance with a significant decrease in ruthenium particle size, which is stable during subsequent reaction in dry reformate. The implications of these data and additional results from in situ IR spectroscopy on the role and influence of H2O on the reaction, also in technical applications, are discussed.

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 H-2 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.

We have investigated the methanation of CO and CO2 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 0.9nm (2.2wt.% Ru) to 1.9nm (5.6wt.% 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 CO2 dissociation and subsequent COad methanation on very small Ru nanoparticles to a site blocking mechanism on larger Ru nanoparticles. In the latter mechanism, CO2 methanation is hindered by a reaction inhibiting adlayer of CO at higher COad 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.

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 kcoverage, 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.

We herein report a water-based sol-gel approach towards porous mixed Si/Ti oxides using co-precipitated glycol-modified precursors. By adjusting synthesis parameters such as the pH value and the Si/Ti ratio of the precursor, the morphology as well as the Si/Ti-composition of the resulting mixed oxide particles can be varied in a wide range. The behaviour of the mixed oxides as substrates for Au catalysts and the performance of the resulting catalysts in the CO oxidation reaction was investigated and compared to catalysts supported on mesoporous anatase and rutile synthesized analogously. For comparable Au particle sizes and Au loadings, the composition of the mixed oxide support was found to significantly affect the reactivity and reaction behaviour, with mixed oxide supported Au catalysts synthesized at pH=5 or 10 and with a Si/Ti-ratio of 1:19 and 1:34 exhibiting the maximum activity. In contrast to the enhanced activity, the mixed oxide supports do not lead to a significant improvement in deactivation behaviour and catalyst stability.