THE TRANSIENT SPECIES FORMED OVER RU-RUOX/TIO2 CATALYST IN THE CO AND CO+H2 INTERACTION - FTIR SPECTROSCOPIC STUDY

At least six CO stretch vibrational bands at around 2142, 2132, 2085, 2050, 1995, and 1950 cm-1 were detected in the room temperature adsorption of both CO and CO + H2 over titania-supported partially reduced ruthenium. These bands were identifiable with the reported multicarbonyl and the linear-bonded monocarbonyl species and indicate that the different oxidation states of ruthenium serve as independent CO chemisorption sites which may coexist on a real catalyst surface at a particular temperature. The effect of exposure temperature and that of the postexposure thermal annealing on CO vibrational bands suggest that the multicarbonyl species transform progressively to monocarbonyl forms which are in turn identified as precursors to the methylene groups in the presence of hydrogen. Though the presence of hydrogen had no apparent effect on the shape and the frequency of different CO stretch bands, it promoted the above transformations to a considerable extent. In addition to the CO stretch bands, CO + H2 interaction at temperatures above 400 K gave rise to hydrocarbon structures consisting of a chain of methylene groups. When in nascent form, these hydrocarbons were very reactive toward hydrogen but on thermal treatment in the absence of hydrogen they were converted to an inactive form. The presence of these Hydrocarbon chains blocked binding sites responsible for νCO = 2050 cm-1 monocarbonyl species, resulting in the continuous decrease in their concentration and hence in the red shift of corresponding bands with increase in exposure temperature. The promotional effect of oxidized ruthenium and the role of oxygenated surface complexes in CO methanation are discussed in light of the present results. © 1992.