Hydrogenation of CO2 over supported noble metal catalysts

The catalytic activity of supported noble metal catalysts for the CO2 methanation reaction has been investigated with respect to the nature (Rh, Ru, Pt, Pd), loading (0.1-5.0 wt.%) and mean crystallite size (1.3-13.6 nm) of the dispersed metallic phase. It has been found that the turnover frequency (TOF) of CO2 conversion for TiO(2)supported catalysts increases following the order of Pd < Pt < Ru < Rh, with Rh being about 3 times more active than Pd. Selectivity toward methane depends strongly on the noble metal catalyst employed and is significantly higher for Rh and Ru catalysts compared to Pt and Pd, which mainly promote production of CO via the RWGS reaction. Conversion of CO2 at a given temperature increases significantly with increasing Ru loading in the range of 0.1-5.0 wt.%. Results of kinetic measurements show that the CO2 hydrogenation reaction is structure sensitive, i.e., catalytic activity is strongly influenced by metal crystallite size. In particular, for Ru/TiO2 and Ru/Al2O3 catalysts, the normalized turnover frequency (TOF divided by the length of the perimeter of the metal-support interface) increases by two orders of magnitude with increasing ruthenium crystallite size in the range of 0.9-4.2 nm and 1.3-13.6 nm, respectively. FTIR experiments provide evidences that CO2 hydrogenation reaction occurs via intermediate formation of adsorbed CO species (Ru-x-CO, Run+ (CO)(x), (TiO2)Ru-CO) produced via the RWGS reaction. Part of this species interacts with adsorbed hydrogen atoms producing methane, whereas the remaining species desorbs to yield CO in the gas phase. The CO2 hydrogenation pathway does not change with variation of Ru crystallite size. However, the relative intensity of the band due to CO species linearly bonded on reduced Ru crystallites (lux-00) increases significantly with increasing Ru particle size, indicating that Ru-x-CO species could potentially being the reactive surface intermediates.