Improving the Hydrogenation Function of Pd/γ-Al2O3 Catalyst by Rh/γ-Al2O3 Addition in CO2 Methanation at Low Temperature

Abstract

Catalytic CO 2 methanation at mild conditions (T ≤ 200 C) was studied over catalysts prepared by mechanically mixing in different proportions Rh(2 wt %)/γ-Al 2 O 3 and Pd(5 wt %)/γ-Al 2 O 3 catalysts. It was found that although Pd/γ-Al 2 O 3 is inert at these conditions, the activity of the mechanical mixtures was up to 50% higher than that of the pure Rh/γ-Al 2 O 3 catalyst. At 200 C, with a H 2 /CO 2 ratio of 4, the rate of reaction was 0.218 × 10 -2 molCH 4 /mol Rh /s for pure Rh/γ-Al 2 O 3 catalyst and increased to 0.318 × 10 -2 molCH 4 /mol Rh /s for a catalyst containing 50 wt % of each catalyst, showing that a synergistic effect operates. In all cases, the selectivity to methane was 100%. No changes in the Rh oxidation state due to the presence of Pd/γ-Al 2 O 3 after pretreatment and reaction was observed. A slight sintering of both metallic particles was observed. The presence of one catalyst did not affect the sintering of the other. No indication of migration of one metal to the surface of the other catalyst that can lead to the formation of bimetallic structures in the mixtures was observed. The nature and reactivity of reaction intermediates was studied by in situ DRIFTS performed at steady state and transient mode. Rh carbonyl hydride was the most abundant carbon-containing species adsorbed on Rh, whereas bridge-bonded CO dominated Pd sites. The reactivity of carbonyl species toward H 2 was greatly influenced by the relative proportions of Pd/γ-Al 2 O 3 and Rh/γ-Al 2 O 3 in the mixtures. Pd/γ-Al 2 O 3 alone cannot hydrogenate the adsorbed CO species, whereas the presence of Rh/γ-Al 2 O 3 significantly increased the reactivity of Pd-CO species, which is proposed to account for the observed synergistic effect. The analysis of apparent activation energies and H 2 and CO 2 reaction orders supports the suggestion that CO 2 is dissociated over both Pd and Rh and that the resulting CO(ads) species react to form methane aided by H species originating from Rh/γ-Al 2 O 3 catalyst. Results show that a nominally inactive catalyst in CO 2 methanation, such as Pd/γ-Al 2 O 3 , can give rise to active surface species and produce methane with high selectivity when contacted with an appropriate promoter, such as Rh/γ-Al 2 O 3 . © 2013 American Chemical Society.