J. Phys. Chem. B 2001, 105, 4973-4978
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The Adsorption and Reaction of HCl on Pd(111) Deborah E. Hunka, Daniel C. Herman,† Liliana I. Lopez,‡ Kara D. Lormand, and Donald P. Land* Department of Chemistry, UniVersity of California, DaVis, California 95616 ReceiVed: February 28, 2001
HCl adsorbed onto clean Pd(111) produces three distinct HCl desorption states in thermal desorption spectroscopy (TDS), all of which are populated simultaneously, even at exposures as low as 0.33 L. One desorption state emanates from molecularly adsorbed HCl (220 K), while the other two (300 and 470 K) emanate from recombination of H plus Cl atoms. The lowest temperature recombinative peak (β at ∼300 K) belongs to recombination of chlorine adatoms with surface hydrogen. This reaction is competitive with the recombinative desorption of H2. Finally, the highest temperature peak with an onset at 470 K (R) is the recombination of the remaining Cl with hydrogen dissolved in the Pd crystal, which emerges from the bulk onto the surface at approximately 470 K. Some of the chlorine adatoms order on the Pd surface to yield a x3 × x3 R 30° overlayer structure at coverages as low as θ ) 0.13 ML. This structure persists through a range of temperatures from 100 to 320 K, when initially adsorbed at 100 K. The order-disorder transition at ∼320 K is reversible.
Introduction The interaction of hydrogen chloride on metal surfaces is of interest in a number of important fields. In electrochemistry, halogens adsorbed to metal electrodes can strongly influence electrochemical reactions and appear to be intimately involved in electron-transfer reactions of transition metal complexes.1,2 In lubricants, chlorinated compounds are used as antiwear additives and are thought to decompose under tribological stress to produce metal chlorides, among other species.3 The detection of chlorinated species using thin-film chemical sensors is important in environmental testing and chemical warfare arenas, and remediation of chlorinated pollutants by metal catalysts is a field with great academic interest and several pilot industrial applications. Our main motivation for this work is that HCl is a common product formed after or during the decomposition of small chlorinated organics on metals,4 a reaction common to many of these fields. Knowledge of the interaction of HCl with the substrate would be useful in interpreting the mechanism of decomposition for halogenated organics and to understand the fate of the products in these systems. Very few UHV studies of HCl on metal surfaces exist. Only three studies, all of which concerned HCl on platinum singlecrystal surfaces, have been reported, although two others on nonmetal surfaces, R-Al2O3 (0001) and β-SiC, also exist.5,6 Wagner et al. found that, regardless of coverage, HCl adsorbs dissociatively on the Pt(111) surface when dosed at temperatures e100 K and that even at exposures as high as 160 L, an HCl ice could not be grown.7 This study also found that a portion of the chlorine ordered on the Pt(111) surface to yield a (3 × 3) diffraction pattern in LEED. In a much earlier study by Garwood et al., no diffraction pattern was seen in LEED when * To whom correspondence should be addressed. E-mail: dpland@ ucdavis.edu; phone: (530) 752-5260; fax: (530) 752-8995. † Present address: School of Medicine, University of North Carolina, Chapel Hill, NC 27599. ‡ Present address: Department of Physics, University of California, Berkeley, CA 94720.
Pt(111) was dosed with HCl at room temperature, but a (2 × 2) structure was observed on the Pt(100) surface.8 The final and most recent study by Fukushima et al. showed STM images of the 3 × 3 Cl overlayer on Pt(111) and a c-4 × 2 overlayer of H3O+ and Cl- when water and HCl were coadsorbed. This study also contained reflection-absorption spectra for the coadsorption experiment (H2O and HCl) in which the O-H stretch and the symmetric bend for H3O+ were identified.9 In this study, the interaction of HCl on Pd(111) has been investigated using low energy electron diffraction (LEED), Auger electron spectroscopy (AES), thermal desorption spectroscopy (TDS), laser-induced thermal desorption coupled with Fourier transform mass spectrometry (LITD-FTMS), and Fourier transform reflection absorption infrared spectroscopy (FTRAIRS). HCl is found to adsorb both molecularly and dissociatively to Pd(111), with the dissociated hydrogen combining with both chlorine and other hydrogen atoms in competing pathways. The H2 that evolves from the surface desorbs at temperatures similar to that for H2 recombinative desorption from H/Pd(111) (∼300 K).10 Some chlorine atoms form a x3 × x3 R 30° overlayer, as observed by LEED from 100 to ∼320 K. These results are similar to those found when Cl2 is adsorbed dissociatively on a clean Pd(111) surface.11,12 Both of these studies report a x3 × x3 R 30° overlayer structure that is assumed to correspond to a coverage of θ ) 0.33. In the study by Tysoe and Lambert, a diffuse x2 ring of 12 diffuse diffraction spots forms upon exposures of chlorine on the order of 15 times those necessary to produce the low exposure LEED pattern. This diffuse x2 ring disappears upon warming the surface to 440 K (and is replaced by the x3 × x3 pattern). This disappearance coincides with the desorption of PdCl2 in TDS and is ascribed by the authors to the formation of (110) oriented microcrystalites of R-PdCl2.12 A recent study by Shard et al. discusses two previously unreported structures of Cl on Pd(111); a 4 × 4 structure at coverages of θ e 0.24 ML, and a highly complicated pattern at a surface coverage of θ ) 0.57
10.1021/jp010790e CCC: $20.00 © 2001 American Chemical Society Published on Web 05/05/2001
4974 J. Phys. Chem. B, Vol. 105, No. 21, 2001
Hunka et al.
ML.13 Neither the x2 ring seen by Tysoe et al. nor the LEED patterns seen by Shard et al. were observed in our study. Experimental Section The UHV chamber in which all experiments are conducted has several techniques available for surface analysis including LEED, AES, FTMS, and FT-RAIRS and is discussed elsewhere.14 The chamber has a base pressure of
