IR Investigation of the Oxidation of Propane and Likely C 3 and C 2 Products over Group IVB Metal Oxide Catalysts M. A. Hasan, ² M. I. Zaki,* ,‡ and L. Pasupulety ² Chemistry Department, Faculty of Science, Kuwait UniVersity, P.O. Box 5969, Safat, 13060 Kuwait, and Chemistry Department, Faculty of Science, Minia UniVersity, El-Minia 61519, Egypt ReceiVed: June 18, 2002; In Final Form: October 4, 2002 In-situ infrared spectroscopy was used to examine the gas phase and adsorbed species formed during the oxidation of propane gas on ZrO 2 , TiO 2 , and CeO 2 catalysts, versus a reference Pt/Al 2 O 3 catalyst, in the absence and presence of an oxygen atmosphere. The same examination was applied to a number of likely oxidation products, namely, 2-propanol, acetone, and acetic acid (vapor phase). Both examinations were carried out as a function of temperature (up to 400 °C). The results obtained could help in assigning the oxidation pathway and surface sites, which are considered extendable to metal oxide surfaces containing no excess oxygen species. A number of important conclusions were, accordingly, arrived at, the most prominent of them being: (i) the initial catalytic interaction is the oxidative dehydrogenation of propane into propene, rather than the oxidative addition into 2-propanol; (ii) the catalytic activity of the test oxides (ZrO 2 < TiO 2 < CeO 2 ) appears to be directly related to the oxide reducibility and surface acidity; (iii) the catalytic active sites are Lewis acid-base pair sites over which the secondary carbon C-H bond is activated by simultaneous bonding, and the hydrogen abstraction is facilitated by the reducibility of the Lewis acid site; (iv) CeO 2 is a promising catalytic material for the total oxidation of hydrocarbons. Introduction Natural gas-fueled turbines are promising thrusters for electric power generators. 1 The propellant gas is produced via total oxidation (combustion) of the natural gas (essentially methane) in sufficient oxygen atmosphere. Conventionally, the combustion process is flame-activated. 1 The exothermic nature of the process, together with the thermal energy supplied by the flame, results in the buildup of high-temperature regimes (1300-1500 °C) and, hence, high emission levels of NO x in the exhaust. 1,2 Therefore, a catalytic combustion process is sought worldwide to reduce the combustion temperature (<1300 °C) and the environmental risk of the NO x emissions. Metal-oxide-based catalysts are increasingly tested as replace- ments for the active, but unstable, noble metal (e.g., Pt and Pd) combustion catalysts, although with limited success. Noble metals are susceptible to sintering and form volatile oxides under the combustion conditions. 1 Hitherto, however, a few metal oxide composites, mostly assuming perovskite 1-4 and spinel 5 structures, or dispersed on thermally stable ceramic supports (viz., cordierite, mullite, and aluminum titanate), 1,6,7 have been considered promising. The oxidation activity of these metal oxides has been found 2-4 to optimize in an electron-mobile environment established by d-d electron exchange interactions involving metal atoms of different oxidation states. Despite their limited success in combustion (total oxidation) reactions of hydrocarbons, metal oxide catalysts are quite successful in partial oxidation reactions of olefins, 8 light alkanes, 9 and aromatics. 10 This fact may be considered incom- patible with the results of a relatively more recent IR study 11 of catalytic oxidation of C 1 -C 4 hydrocarbons on MgCr 2 O 4 , which shows total oxidation of the test hydrocarbons to be consecutive to their partial oxidation. The study 11 suggests, moreover, that: (i) C-H bonds are the active moieties in oxidizable hydrocarbons; (ii) the C-H activity is inversely related to the bond dissociation energy, but directly related to the bonding symmetry of the carbon atom (tertiary > secondary > primary carbons); (iii) the total oxidation to CO 2 and H 2 O is intermediated by formation (and subsequent oxidation) of several oxygenate compounds (alkoxidic, aldehydic, ketonic, and/or carboxylic species); (iv) on metal oxides exposing strong base sites, the initial catalytic step involves electronic interactions between nonbonding orbitals of surface oxide (O 2- ) sites and antibonding (σ*) orbitals of the C-H bond, whereas it involves the interaction between d-type orbitals of the surface metal (M n+ ) sites and both σ and σ* orbitals of the C-H bond on oxides exposing strong acid sites; and (v) active surface oxygen species could equally be nucleophilic (O 2- ) or electrophilic (O <2- ). It is worth noting that suggesting the involvement of nucleophilic O 2- surface species in total oxidation reactions contradicts long standing findings by Haber, 9,12 whereas sug- gesting Lewis acid sites (M n+ ) to initiate the activation of C-H bonds is in line with earlier results of Al-Mashta et al. 13 as well as recent results of Zaki et al. 14 The formation of π-complexes of olefins with cations, like Pt 2+ , Fe 3+ , Cu 2+ , or Cr 3+ , 15 or even d 0 cations, like Ti 4+ or Zr 4+ , 16 both on the surface and in homogeneous complexes, is also supportive to the latter suggestion. In line with the current research endeavors toward the design of active and durable metal oxide combustion catalysts, the present in-situ IR study of catalytic combustion of propane, 2-propanol (2-propanol), acetone, and acetic acid in rich O 2 atmosphere (at up to 400 °C) on Group IVB metal oxides (TiO 2 , ZrO 2 , and CeO 2 ) was undertaken. Equilibrium reaction products were identified both on the surface and in the gas phase. The * Corresponding author. Tel & Fax: 02-086-360833. E-mail: mizaki@ link.net. ² Kuwait University. ‡ Minia University. 12747 J. Phys. Chem. B 2002, 106, 12747-12756 10.1021/jp0214413 CCC: $22.00 © 2002 American Chemical Society Published on Web 11/14/2002