Generation of C 2 H 5 Species: Thermal and Photoinduced Dissociation of C 2 H 5 I on Rh(111) F. Solymosi,* L. Bugyi, and A. Oszko ´ Institute of Solid State and Radiochemistry of Attila Jo ´ zsef University and Reaction Kinetics Research Group of the Hungarian Academy of Sciences, † P.O. Box 168, H-6701 Szeged, Hungary Received March 27, 1996. In Final Form: June 5, 1996 X The thermal and photochemistry of ethyl iodide, C2H5I, on Rh(111) surface has been investigated by temperature-programmed desorption, high-resolution electron energy loss, X-ray photoelectron, and Auger electron spectroscopies. C 2H5I adsorbs dissociatively at submonolayer coverage at 90 K and molecularly at higher coverages. The dissociation of a monolayer starts above 170 K and completes below 250 K. Illumination of adsorbed C 2H5I enhanced the extent of the dissociation which is ascribed to the generation of photoelectrons. The primary products of thermal dissociation are adsorbed C2H5 and I. The species C2H5 is stable on the Rh(111) surface up to 200 K: a fraction of C2H5 undergoes dehydrogenation to C2H4 at 200-300 K, and another fraction hydrogenates into C2H6. Dimerization of ethyl species produced by thermal dissociation was not observed. It occurred, however, following the photoinduced dissociation of ethyl iodide at 90 K. Coadsorbed CO induced the desorption of more strongly adsorbed C 2H5I, inhibited the C-I bond breaking, and enhanced the stability of the C2H5 species on Rh(111). 1. Introduction A growing interest is experienced as regards the study of the bonding, structure, and reaction pathways of C x H y fragments (CH 2 , CH 3 , and C 2 H 5 ) on metal surfaces, which provides an important insight into the elementary steps of heterogeneous catalytic reactions involving hydrocar- bons. 1,2 In our laboratory attention was focused on two metals, Pd(100) and Rh(111), which are active catalysts in the synthesis of higher oxygenated compounds, 3 in the production of synthesis gas in the CH 4 + CO 2 reaction, 4 and also in the partial oxidation of CH 4 . 5 In this program we examined the adsorption and dissociation of CH 3 Cl, CH 3 I, CH 2 I 2 ,C 2 H 5 I, and (C 2 H 5 ) 2 Zn on Pd(100). 6-11 Partial dissociation of iodo compounds has been observed even at 90 K. Illumination of adsorbed layers markedly enhanced the extent of their dissociation and made possible genera- tion of CH x fragments in a larger concentration on the surface even at 90 K, far below the temperature of their decomposition. Similar features have been observed in the study of the interaction of CH 3 I and CH 2 I 2 with the Rh(111) surface. 12-14 The reactions of CH x fragments with adsorbed oxygen atoms have been also investigated on both metals, and the formation of CH 2 O (from CH 2 ), CH 3 O (from CH 3 ), and CH 3 CHO (from C 2 H 5 ) were established, in addition to the products of their total oxidation. 13-15 In the present work an account is given of the adsorption and dissociation of C 2 H 5 I on Rh(111). Methods used are temperature-programmed desorption (TPD), high-resolu- tion electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES). 2. Experimental Section The Rh crystal was cut from a single-crystal boule and was a product of the Material Research Corporation (99.99% purity). Initially the sample was cleaned by cycled heating in oxygen. This was followed by cycles of argon-ion bombardment (typically 1-2 kV, 1 × 10 -6 Torr Ar, 1000 K, 3 µA for 10-30 min), and annealing at 1270 K for several minutes. C2H5I was a product of Merck. It was degassed and purified by freeze-pump-thaw cycles. The experiments were performed in two separate ultrahigh vacuum (UHV) chambers with a routine base pressure of 2 × 10 -10 mbar produced by turbomolecular, ion-getter, and titanium sublimation pumps. One chamber was equipped with facilities for AES, HREELS, and TPD. The HREEL spectrometer (VSW, type HA-50) is situated in the lower level of the chamber and has a resolution of 70-100 cm -1 . All spectra reported were recorded with a primary energy of 5.0 eV and at an incident angle of 45°. Work function changes, based on secondary electron energy threshold, were measured with the same electron gun and analyzer used in AES. The second system was a Kratos XSAM 800 instrument, where XPS measurements were performed using Mg KR primary radiation (14 kV, 10 mA). All binding energies are referred to the Fermi level of the Rh(3d5/2), which places the Rh(3d5/2) photoelectron line at 307.2 eV. The pass energy of the electron energy analyzer was set to 40 eV, which gives the full width at half maximum (fwhm) of 1.65 eV for the Rh(3d 5/2) photoelectron line for a clean Rh(111) surface. Collection times † This laboratory is a part of the Center for Catalysis, Surface and Material Science at the University of Szeged. X Abstract published in Advance ACS Abstracts, July 15, 1996. (1) Zaera, F. J. Mol. Catal. 1994, 86, 221. Zhou, X.-Y.; White, J. M. Surf. Sci. Rep. 1991, 13, 73. Lin, J.-L.; Chiang, Ch.-M.; Jenks, C. J.; Yang, M. X.; Wentzlaff, T. H.; Bent, B. E. J. 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Sci., in press. 4145 Langmuir 1996, 12, 4145-4152 S0743-7463(96)00296-X CCC: $12.00 © 1996 American Chemical Society