AVS1996 Session SS1-ThA: Hydrocarbon Chemistry on Metals

Thursday, October 17, 1996 1:30 PM in Room 204C

Thursday Afternoon

Start	Invited?	Item
1:30 PM	Invited	SS1-ThA-1 Mimicking Aspects of Heterogeneous Catalysis B. Bent (Columbia University) The impact of vacuum-based surface science studies of low-surface-area materials on heterogeneous catalysis is determined to a large extent by the ability of these model systems to mimic the chemistry that occurs on real catalysts. This talk will describe an approach for mimicking the chemistry of heterogeneous catalysts on metal single crystal surfaces via the formation of reactive intermediates. By freezing out these reactive species at low temperatures, high surface coverages can be generated for spectroscopic study, and bimolecular coupling reactions (which are often difficult to mimic on clean surfaces in vacuum) can be enhanced. Elementary surface reaction steps can also be isolated and probed using physical organic methodologies. Recent results for C-H bond dissociation on copper surfaces and for C-C bond formation on copper and Cu\sub 3\Pt(111) surfaces will be discussed.
2:10 PM		SS1-ThA-3 Direct Dissociative Chemisorption of Methane, Ethane, Propane, and Cyclopropane on Ir(110) D. Kelly, W. Weinberg (University of California, Santa Barbara) We have employed molecular beam methods to investigate the initial probability of direct dissociative chemisorption of methane, ethane, propane, and cyclopropane on Ir(110). In each case the probability of direct dissociative chemisorption increases with increasing translational beam energy, E, from an immeasurable value (< 0.02, as determined by the reflectivity method) at low beam energy, to a value of over 0.20 at E=20 kcal/mol for methane and ethane, and to a value of nearly 0.5 at E = 50 kcal/mol for propane and cyclopropane. The energy above which measurable (> 0.02) direct dissociative chemisorption is observed is approximately 10 kcal/mol for methane, ethane, and propane, and slightly lower than this for cyclopropane, and in each case the increase in the initial probability of direct dissociative chemisorption is approximately a linear function of E. The direct dissociative chemisorption of methane, ethane, and propane exhibits a pronounced kinetic isotope effect, indicating that adsorption involves a mechanism of hydrogen abstraction through tunneling. For cyclopropane, however, C-C bond cleavage is the initial step in the chemisorption mechanism, and consequently no kinetic isotope effect is observed. These observations will be discussed within the context of the bond strengths of the reacting species.
2:30 PM		SS1-ThA-4 Selective C-C Bond Activation in Small Cycloalkanes by Gas Phase Atomic Hydrogen on the Ni(100) Surface K. Son, J. Gland (University of Michigan) Reactions of a series of small cycloalkanes (C\sub n\H\sub2n\, n=3, 4, 5, & 6) with gas phase atomic hydrogen have been investigated on the Ni(100) surface. Gas phase atomic hydrogen induces single C-C bond activation in adsorbed cyclopropane, cyclobutane (1-chlorocyclobutane), and cyclopentane at 100 K and the corresponding n-alkanes are produced in subsequent TPR experiments. No C-C bond activation was observed when cyclohexane was exposed to gas phase atomic hydrogen. These results indicate that C-C bond activation is dominant for the strained cycloalkanes. Coadsorbed surface hydrogen does not induce C-C bond activation in any of these cycloalkanes. Molecular desorption dominates relative to hydrogen addition for this series of cycloalkanes in the presence of surface hydrogen. Based on our HREELS and isotope studies, a two step mechanism is proposed for the formation of n-alkane from adsorbed cycloalkane (C\sub n\H\sub 2n\, n=3-5); Gas phase atomic hydrogen induces a C-C bond activation in adsorbed cycloalkane to form the corresponding n-alkyl adsorbate at 100 K. This n-alkyl intermediate is hydrogenated primarily by surface hydrogen to form n-alkane with heating. For adsorbed cyclohexane, hydrogen abstraction leading to cyclohexene and cyclohexadiene occurs during atomic hydrogen exposure. A smaller amount of hydrogen abstraction was observed for cyclopentane. No hydrogen abstraction was observed for cyclobutane and cyclopropane. Base on these results, we conclude that C-C bond activation and hydrogen abstraction are the two primary competing reactions for adsorbed cycloalkanes during atomic hydrogen exposure. The dominant reaction pathway is controlled by ring strain in the cycloalkanes and the stability of C-C and C-H bonds in the cycloalkanes.
2:50 PM		SS1-ThA-5 \pi\-Allyl Coordination and Allyl Radical Desorption from a Cu(100) Surface A. Gurevich, M. Yang, A. Teplyakov, B. Bent (Columbia University); M. Holbrook, S. Bare (The Dow Chemical Company) Allyl groups (CH\sub 2\CHCH\sub 2\) are proposed as intermediates in a wide range of surface-catalyzed hydrocarbon reactions. However, only in limited cases has this reactive intermediate been detected. We present here temperature programmed desorption spectroscopy (TPD) and near edge x-ray absorption fine structure spectroscopy (NEXAFS) studies concerning the reaction and bonding of allyl groups generated on Cu(100) by the dissociative adsorption of allyl chloride. TPD results show that there is no molecular desorption for exposures up to saturation of the monolayer , indicating that all the molecules on the first layer react. The mass spectrum of the main product obtained by heating a monolayer of allyl chloride agrees with the literature spectrum of allyl radical. In contrast to literature results for Ag(111) and Al(100), where allyl groups couple to form 1,5-hexadiene and disproportionate to produce propene, the dominant reaction for allyl on Cu(100) is allyl radical ejection from the surface at 400 K. On a H atoms-precovered surface, propene is the only product, desorbing at 250K. NEXAFS spectra of allyl chloride adsorbed at 90K indicate the presence of some undissociated allyl chloride or an intermediate species that transforms into allyl at ~150K. The polarization dependence of the 1s->\pi\* transition shows that allyl groups bond with its \pi\ system nearly parallel to the surface. From the position of the \sigma\* shape resonance we calculate a bond length very close to the experimental value for a delocalized allyl species. The spectra do not change until a temperature of 350K is reached, where all the allyl desorbs.
3:10 PM		SS1-ThA-6 A Dramatic Increase in the Selectivity for Methane Formation from the Decomposition of Methanethiol on Ni-covered W(001) D. Mullins (Oak Ridge National Laboratory) More than 85% of the methanethiol adsorbed on W(001) covered with a 2 ML Ni film forms methane as it decomposes. This is the largest selectivity for methane formation yet reported for the decomposition of methanethiol on a metal surface. The methane selectivity starts at 30% on the clean W(001) surface, rises continuously to 70% when the Ni coverage is 1 ML and continues to rise to >85% on the 2 ML film. Only 70% of the adsorbed methanethiol forms methane on Ni(111) and Ni(001). Accompanying the increase in selectivity is a sequential decrease in the methane desorption temperature. The methane desorbs at 350 K from methanethiol on clean W(001), at 290 K from a 1 ML Ni film, and at 210 K from a 2 ML Ni film. On thick (>10 ML), unannealed films, the desorption temperature increases to 270 K and the selectivity is reduced to 70%, which are similar to what is observed on pure Ni. The increase in the amount of methane formed and the decrease in the methane desorption temperature are directly related to a decrease in the C-S bond scission temperature. S 2p soft x-ray photoemission indicates that the C-S bond is cleaved between 300 - 350 K on clean W(001) and between 250 - 300 K on a 1 ML Ni film. On the 2 ML Ni film, there is considerable C-S bond cleavage (>50%) at 100 K. Research sponsored by the Division of Chemical Sciences, Office of Basic Energy Sciences, U.S. Department of Energy at Oak Ridge National Laboratory, managed by Lockheed Martin Energy Research Corp. under contract number DE-AC05-96OR22464.
3:30 PM		SS1-ThA-7 Surface Chemistry of Phenol on Al(111) J. Russell, Jr., A. Leming, R. Morris (Naval Research Laboratory) Phenol is a model compound for studying polyatomic, multifunctional organic molecules relevant to polymer, and petroleum-based fuel constituent interactions with a surface. The reaction of phenol on Al(111) was investigated between 100 and 800 K under ultrahigh vacuum with temperature programmed desorption (TPD), low energy electron diffraction (LEED), and Auger electron spectroscopy (AES). Molecular phenol desorbed at 197 K from multilayer coverages of phenol. The sequence of bond scission in the molecule was determined using deuterium site labelling in the molecule. From the monolayer covered surface, hydrogen desorption was observed around 380 K from the hydroxyl hydrogens. The C-H bonds in the 2,6 positions on the ring were the first to break, yielding hydrogen desorption which began around 450 K and reached a desorption maximum around 510 K. A third hydrogen desorption peak was observed at 680 K and resulted from scission of the C-H bonds on the remaining hydrocarbon fragments on the surface. Surprisingly, benzene production was observed between 450-700 K with desorption peak maxima at 490 and 630 K. While there was evidence for both hydrogen addition and exchange with the ring, there was no evidence for hydrogenation of any of the double bonds. Post-mortem Auger revealed O and C deposition on the surface after heating to 800 K. We describe the reaction paths available to phenol on Al(111).
3:50 PM		SS1-ThA-8 Identification of the Fragmentation Pathways of Ferrocene on Cu(100) C. Waldfried, C. Hutchings, D. Welpitiya, P. Dowben (University of Nebraska, Lincoln) The adsorption of organometallic materials on metal surfaces is of great interest, because of the numerous expected applications in selective area processing. The adsorbed metallocenes are, however, sensitive to their environment and influenced by temperature and ultraviolet radiation. This can result in decomposition of the adsorbed molecules. Therefore it is of immense importance to study the process of fragmentation for the metallocenes adsorbed on metal surfaces. We used angle resolved photoelectron spectroscopy to study the fragmentation of ferrocene [Fe(C5H5)2] adsorbed on a Cu(100) surface. The decomposition of the molecularly adsorbed ferrocene was investigated for exposure to 50 eV synchrotron radiation. Based on symmetry arguments and dipole selection rules we could identify the molecular fragments as the cyclopentadienyl [C\sub 5\H\sub 5\] (C\sub p\) and determine their orientation to Cu(100). The C\sub p\ fragments induce photoemission features at approximately 2.2 eV and 7.0 eV binding energy, and form a relatively strong bond to the Cu(100) surface, as determined from temperature dependent measurements. Our photoemission results are convincingly supported by additional investigations. A similar fragmentation pathway has also been observed for ferrocene on Ag(100).
4:10 PM		SS1-ThA-9 Cyclization and Related Reactions of Iodoethanol on Ag(110) G. Jones, M. Barteau (University of Delaware) The adsorption and reaction of 2-iodoethanol (IEtOH) on a clean Ag(110) surface were studied under ultrahigh vacuum conditions using TPD. Under these conditions alcohols are typically unreactive on the clean Ag(110) surface. However, replacement of one of the \Beta\-hydrogens with a weakly bound (53kcal/mol) iodine atom allowed for IEtOH decomposition by initial C-I scission. TPD results indicate that there are two major reaction channels at 263K and 340K for IEtOH decomposition on the clean Ag(110) surface. The reaction intermediates which give rise to these two pathways were deduced based upon the products observed, the overall product stoichiometry, and the desorption temperatures of the products. The 263K reaction channel has an overall stoichiometry of C\sub 2\H\sub 5\O, corresponding to a hydroxyethyl intermediate. The hydroxyethyl intermediate undergoes a \Beta\-hydride elimination and C-O scission ultimately yielding acetaldehyde, ethylene, water, and ethanol. Unlike unsubstituted alkyl groups on clean Ag(111) and Ag(110), there is no evidence for hydroxyethyl coupling to give butanediol. Thus, the hydroxyethyl cannot be treated simply as a substituted alkyl group, probably due to interactions between oxygen and the surface. The stoichiometry of the 340K pathway is suggestive of an oxametallacycle intermediate. The products of this reaction channel are the same as the 263K channel with the addition of a cyclic ester product, \gamma\-butyrolactone, which to our knowledge has not previously been sunthesized in ultrahigh vacuum. We propose a mechanism initiated by \Beta\-hydride elimination which accounts for the products evolved in our experiments and may explain differences between our results and those previously reported from oxametallacycle intermediates.
4:30 PM		SS1-ThA-10 Structure of Methyl Nitrite on Ag(111) J. Fieberg, J. White (University of Texas, Austin) As part of an ongoing program aimed at elucidating the electron and photon activated chemistry and dynamics of adsorbates, we have undertaken an investigation of methyl nitrite, CH/sub 3/ONO, on Ag(111). Previous data from temperature programmed desorption and x-ray photoelectron spectroscopy have shown there is no thermal dissociation, and the monolayer desorbs at 131 K. Dissociation occurs readily (ejecting NO) when adsorbed CH/sub 3/ONO is irradiated with either 50 eV electrons or photons with energies greater than 3.4 eV. In order to further our understanding of the initial adsorption state and its role in the ejection of NO, we present the structure of CH/sub 3/ONO on Ag(111) as determined by reflection-absorption infrared spectroscopy (RAIRS). At monolayer coverage, the CH/sub 3/ONO RAIRS frequencies are only slightly shifted from the frequencies of the condensed phase, which confirms the weak adsorbate-substrate interaction. Both cis and trans isomers are present as evidenced by band pairs in each region of the spectrum (i.e. N=O stretch at 1610 and 1658 cm/super - 1/). The C-O stretches at 975 and 1026cm/super -1/ are intense and redshifted with respect to the condensed phase. Compared to other bands, the shift of the C-O stretch is larger, which indicates interaction with the substrate takes place mainly through electron donation from the central oxygen atom. Surface selection rules dictate that both isomers assume binding geometries such that the C-O and N=O bonds are tilted away from the surface. This orientation helps explain the facile ejection of NO by photon irradiation. The orientation of molecules in the multilayer and their interaction with the monolayer will also be discussed.