Spectroscopic and catalytic studies on metal carbonyl clusters supported on Cab-O-Sil: II. Impregnation and decomposition of Ru 3(CO) 12 and the mixture of Ru 3(CO) 12 and Fe 3(CO) 12

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The behavior of Ru 3(CO) 12 (I), H 2Ru 3Fe(CO) 12 (II), a 1:1 Ru 3(CO) 12 and Fe 3(CO) 12 mixture (III), RuFe 2(CO) 12 (IV), and Fe 3(CO) 12 (V) deposited on Cab-O-Sil HS-5 has been compared. (III) and (V) have been studied by Mössbauer spectroscopy and by ir-spectroscopy, and (I)-(V) by temperatureprogrammed decomposition (TPDC) and temperature-programmed reduction (TPR). Decomposition, which is faster in hydrogen than in helium or in vacuum, and is reversible below 400 K, is normally faster for (V) than for (I). At low temperature, CO ligands leave the metal carbonyl cluster (MCC) in one step for (V), whereas they are decomposed stepwise via the formation of subcarbonyl species for (I). In this range the formation of Ru 3(CO) 3 species has been verified. On decomposition of (V), there is some CO adsorption, as indicated by ir spectroscopy and low catalytic activity. This increases when decomposition occurs in helium, and is attributed to the smaller particles stabilized by the metal-carbon species, formed from CO during the decomposition. For (I), decomposition results in a slight oxidation, indicated by weak ir bands in the range of 2100–2140 cm −1. Interaction between Fe and Ru in (III) does not occur in the impregnated phase, but develops during the decomposition, which starts with Fe 3(CO) 12 decomposition and thereby influences the decomposition of Ru 3(CO) 12. However, reduction of iron is also facilitated by the presence of ruthenium, as indicated by Mössbauer spectroscopy. The general feature revealed during decomposition in helium, i.e., the increase of surface carbon, is also operative here, and thus the dispersion of the metal is higher than for decomposition in hydrogen. The mechanism of the decomposition is discussed in terms of the formation of subcarbonyl species for Ru-containing samples and the formation of surface carbon is also considered. The mechanism and possible reaction pathways are given.