CATALYTIC CARBON-DIOXIDE METHANATION BY ALUMINA-SUPPORTED MONONUCLEAR AND POLYNUCLEAR RUTHENIUM CARBONYLS
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The catalytic activity and selectivity toward methanation of carbon dioxide using several alumina-supported ruthenium-cluster-derived catalysts have been studied over the temperature range 180250 C. The ruthenium clusters used in these investigations include Ru3(CO)12, KHRu3(CO)n, [PPN][HCO2Ru3(CO)10], H4Ru4(CO)12, Kh3Ru4(Co)12, [Ppn][H3Ru4(Co)12], and Ru6-C(CO)17(PPN = bis(triphenylphosphine)nitrogen(l+)). Comparative studies were made with the mononuclear complexes Ruc13 And Ru(Co)5. The latter species provides a low-valent, organometallic, mononuclear ruthenium source. Catalysts were supported by impregnation over alumina (partially dehydroxylated at 150 C in vacuo) and activated in hydrogen at 200 C. Catalyst characterization included diffuse-reflectance infrared spectroscopy, surface area determination, oxygen chemisorption, and electron microscopy. In general, the cluster-derived catalysts were more active than the analogously prepared catalyst obtained from Ruc13; e.g., at 180 C the catalyst derived from Ru6C(Co)17was 22 times more active than that derived from Ruc13. The activity of catalysts derived from supported neutral species was observed to increase as the number of ruthenium atoms present in the precursor complex increased; i.e., Ru(co)3 < ru3(co)12< H4Ru4(CO)12< Ru6C(Co)17, Catalysts derived from supported anionic ruthenium cluster derivatives were less active than their neutral counterparts, displaying a great deal of sensitivity to the nature of the accompanying cation. The diffuse-reflectance FTIR spectra of all the supported, activated catalysts originating from low-valent ruthenium derivatives exhibited the same band pattern in the v(CO) region, whereas the RuCl3-derived catalyst displayed a quite different v(CO) infrared spectrum upon addition of carbon monoxide. 1986, American Chemical Society. All rights reserved.