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

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Abstract

Catalytic CO2 methanation at mild conditions (T ≤ 200 °C) was studied over catalysts prepared by mechanically mixing in different proportions Rh(2 wt %)/γ-Al2O3 and Pd(5 wt %)/γ-Al2O3 catalysts. It was found that although Pd/γ-Al2O3 is inert at these conditions, the activity of the mechanical mixtures was up to 50% higher than that of the pure Rh/γ-Al2O3 catalyst. At 200 °C, with a H2/CO2 ratio of 4, the rate of reaction was 0.218 × 10–2 molCH4/molRh/s for pure Rh/γ-Al2O3 catalyst and increased to 0.318 × 10–2 molCH4/molRh/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-Al2O3 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 H2 was greatly influenced by the relative proportions of Pd/γ-Al2O3 and Rh/γ-Al2O3 in the mixtures. Pd/γ-Al2O3 alone cannot hydrogenate the adsorbed CO species, whereas the presence of Rh/γ-Al2O3 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 H2 and CO2 reaction orders supports the suggestion that CO2 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-Al2O3 catalyst. Results show that a nominally inactive catalyst in CO2 methanation, such as Pd/γ-Al2O3, can give rise to active surface species and produce methane with high selectivity when contacted with an appropriate promoter, such as Rh/γ-Al2O3.

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