γ-Al2O3-supported ruthenium carbonyls: Reactivity towards H2 and catalytic activity in CO2 methanation

Abstract

Ru₃(CO)₁₂ was used as a precursor to synthesize γ-Al₂O₃-supported Ru carbonyls via an impregnation method. The interaction between the precursor and the support was influenced by the degree of hydroxylation of the γ-Al₂O₃ surface, resulting in distinct Ru carbonyl species depending on the thermal treatment of the support. The reactivity of these supported Ru carbonyls toward H₂ at increasing temperatures was investigated using infrared spectroscopy coupled with mass spectra of the gaseous products. The results indicate that Ru₃(CO)₁₂ adducts on Lewis acidic sites of γ-Al₂O₃ react with H₂ to form various Ru carbonyl species. Some of these species generate methane via a stoichiometric reaction with H₂. Samples containing supported Ru(CO)₃ and Ru(CO)₄ species were found to be active for CO₂ methanation at 140 °C. However, under reaction conditions, these initial carbonyls rapidly transformed into supported Ru dicarbonyl species, which remained stable during steady-state catalysis. To assess the possible role of supported Ru dicarbonyl species in the reaction, step changes in the reactor feed composition were performed while monitoring the infrared spectra of the samples and the mass spectra of the effluent gases. The data indicate coexisting pathways for CO₂ methanation. The dominant route involves CO₂ activation on the support, in which formate species act as intermediates in methane formation. Although the involvement of Ru dicarbonyl species in the catalysis cannot be ruled out, their presence may primarily result from a stoichiometric reaction between supported Ru and the CO₂/H₂ mixture.