Influence of the catalyst loading on the activity and the CO selectivity of supported Ru catalysts in the selective methanation of CO in CO 2 containing feed gases

Abstract

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.