FTIR spectroscopic study of the interaction of CO₂ and CO₂ + H₂ over partially oxidized Ru/TiO₂ catalyst

Abstract

At least three distinct linearly bound carbonyl species are identified in the adsorption of CO₂ or CO₂ + H₂ over Ru---RuO_x/TiO₂ catalyst. The relative concentration and the growth of these species depend on metal oxidation state, presence of hydrogen, reaction temperature, and duration of exposure. The presence of preadsorbed or coadsorbed hydrogen promotes formation of x173-1 type species, the RuO_x-(CO)_adspecies develop only on prolonged exposure to a dose of CO₂ or CO₂ + H₂. The oxygen or the hydrogen ligand bonded to ruthenium facilitates C---O bond scission. The widely reported lower temperature requirement for the CO₂ methanation reaction as compared to that of CO is attributed to the high reactivity of nascent carbonyl species which give methane directly via "active" carbon formation. As shown earlier (Gupta et al., J. Catal. 137, 437 (1992)), the CO methanation requires multistep transformations, making the process energy intensive, particularly in the 300-450 K temperature range. The studies using ²H and ¹³C labeled adsorbates helped in the identification of oxygenated surface species having vibrational bands in the 1000-1800 cm^-1 region. These species are regarded as inactive side products formed on the support and/or at the Ru-support interfaces.