JOURNAl. OF CATAI.YSIS 146, 173-184 (1994) FTIR Spectroscopic Study of the Interaction of CO 2 and CO 2 -q- H 2 over Partially Oxidized Ru/TiO 2 Catalyst N. M. Gupta,* V. S. Kamble,* V. B. Kartha,t R. M. Iyer,* K. Ravindranathan Thampi,:~ and M. Gratzel + *Chemistl 3' Division and tSpectroscopy Division, Bhabha Atomic Research Centre, Bombay 400 085, India; and .~Physical Chemistry Institute 11 Swiss Federal Institute of Technology, Lttttsatttte, CH 1015. Switzerland Received April 14, 1993; revised September 23, 1993 At least three distinct linearly bound carbonyl species are identi- fied in the adsorption of CO, or CO2 + H 2 over Ru-RuOx/TiO 2 catalyst. The relative concentration and the growth of these species depend on metal oxidation state, presence of hydrogen, reaction temperature, and duration of exposure. The presence of pread- sorbed or coadsorbed hydrogen promotes formation of Ru°-(CO)ad H / and Ru \ CO type species, the RuOx-(CO)a d species develop only on prolonged exposure to a dose of CO 2 or CO 2 + H 2 . The oxygen or the hydrogen ligand bonded to ruthenium facilitates C-O bond scission. The widely reported lower temperature requirement for the CO2 metha- nation reaction as compared to that of CO is attributed to the high reactivity of nascent carbonyl species which give methane directly via "active" carbon formation. As shown earlier (Gupta et al., J. Catal. 137, 437 (1992)), the CO methanation requires multistep transformations, making the process energy intensive, particularly in the 300-450 K temperature range. The studies using 2H and '3C labeled adsorbates helped in the identification of oxygenated surface species having vibrational bands in the 1000-1800 cm -I region. These species are regarded as inactive side products formed on the support and/or at the Ru-support interfaces. ~ 1994 Aca- demic Press, Inc. INTRODUCTION Some of the earlier studies have shown that CO, + H 2 interact with noble metals to form adsorbed CO in addition to formate or bicarbonate type oxygenated species (1-8). CO2 is also reported to be dissociatively adsorbed over supported and single crystals of noble metals, such as Ru and Rh, even in the absence of hydrogen (I, 9-13). Some studies, on the other hand, have shown that the CO2 adsorbs very weakly over clean Rh at room temperature 173 (14, 15) and its dissociation is assisted by the seggregation of certain impurities at metal surface (15). Contradictory views have, similarly, been expressed regarding the role played by CO or by oxygenated species in the overall COz methanation reaction. On the one hand, these species are considered as reaction intermediates (7), and on the other, they are regarded as catalyst poisons (8). Some other in- triguing aspects include the lower temperature require- ment for the methanation of CO 2 as compared to that of carbon monoxide (6, 16-22), even though the former reac- tion is believed to occur via CO as an intermediate. In a recent publication (23) we reported on the surface species formed in the exposure of CO or CO + H2 over titania supported partially oxidized ruthenium (Ru-RuOx/ TiO2). The reduced and the oxidized metal sites, which were shown to coexist at a given temperature, act as independent CO chemisorption sites. It was also shown that the multicarbonyl species formed by interaction of CO or CO + H2 undergo transformation to monocarbonyl form, which in turn acts as precursor for the methylene groups, which subsequently form hydrocarbons. The na- ture of the transient species was found to depend on the catalyst temperature and on the availability of hydrogen. In the present paper we report on the surface species formed in the exposure of Ru-RuO.,/TiO2 catalyst to CO2 and CO2 + H2 at various temperatures. This catalyst is found to show high CO2 methanation activity, particularly at low temperatures (7, 24, 25). For example, in a typical experiment employing a flow through microcatalytic reac- tor at 450 K, the methane yields from the CO2 + H2 and the CO + H2 reactions were found to be 70% and 7%, respectively, under identical test conditions (25). The em- phasis in the present study was to investigate the chemis- try of reaction intermediates by evaluating the time and the temperature dependent modifications in the transient species using FTIR spectroscopy. Based on these data, some of the abovementioned fundamental aspects of CO2 hydrogenation reaction are discussed. 0021-9517/94 $6.00 Copyright © 1994 by Academic Press, Inc. All rights of reproduction in any form reserved.