Influence of the catalyst loading on the activity and the CO selectivity of supported Ru catalysts in the selective methanation of CO in CO2 containing feed gases

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 0.9 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 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.

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► Particle size dependent CO methanation rate and CO selectivity.
► Increasing Ru particle size → increasing CO methanation rate, but decreasing CO selectivity.
► Larger Ru nanoparticles readily dissociate CO2 if at low COad coverage.
► Small Ru nanoparticles show an inherently low CO2 methanation activity, preventing CO2 dissociation even at low COad coverage.