AVS1997 Session SS1-FrM: Reactions in Hydrocarbon Monolayers

Friday, October 24, 1997 8:20 AM in Room A1/2-A

Friday Morning

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8:20 AM		SS1-FrM-1 In-Situ Probing of Metal-Liquid-Gas Catalytic Interfaces Using Surface-Enhanced Raman Spectroscopy C.T. Williams, S. Zou (Purdue University); C.G. Takoudis (University of Illinois, Chicago); M.J. Weaver (Purdue University) Transition-metal catalyzed liquid-gas reactions rank among some of the most important industrial processes, including methanol synthesis and hydrogenation reactions. However, molecular-level understanding has been hampered by absence of information regarding adsorbed species under reaction conditions. This has persisted due to a lack of surface-analytical techniques capable of circumventing the liquid phase (reactants, products, solvents) and high gas pressures (typically several atmospheres). Surface-enhanced Raman spectroscopy (SERS), with its surface sensitivity and selectivity, appears ideally suited to probe such catalytic interfaces. Once limited primarily to gold, silver and copper, the effect is readily extended to transition metals by electrodepositing ultra-thin films of the desired catalytic material onto electrochemically roughened gold. These in terfaces display remarkably robust SERS activity, enabling temporal sequences of surface Raman spectra to be obtained over a wide range of reactant pressures (up to 1 atm) and temperatures (up to 500 C), as well as in the presence of liquid. Prospects of utilizing SERS for investigation of interfacial phenomena during metal-liquid-gas catalytic reactions will be discussed. Background on the technique, as well as proposed experimental design will be provided. Several examples of utilizing SERS for examining organic adsorbates (e.g. benzene, benzonitrile, formic acid, methanol) on metal surfaces in situ in the presence of a liquid phase will be provided. Finally, preliminary results for benzene oxidation on palladium will be presented, with emphasis placed on the mechanistic importance of observed adsorbates.
8:40 AM		SS1-FrM-2 Creating Ordered Chiral Templates to Stereodirect Reactions at Metal Surfaces R. Raval, C.J. Baddeley, S. Haq, M. Ortega-Lorenzo, J. Williams (University of Liverpool, United Kingdom) Enantioselective surface reactions represent the ultimate expression of selectivity in catalysis,involving stereodirecting processes where only one optical component of a product is formed. One major route to achieving this stereocontrol is by adsorbing chiral molecules onto a substrate in order to direct the reaction pathway. However, at present, almost nothing is known about the mechanism for this stereocontrol. In a bid to answer this question, we have created model stereodirecting surfaces by the adsorption of chiral modifiers on metal single crystal surfaces and then subjecting them to the reactant. In this paper we report the creation of ordered chiral templates from the adsorption of R,R-tartaric acid and the simplest chiral amino-acid, S-alanine,on Cu(110) and Ni(111). Both are well-known for stereodirecting the enantioselective hydrogenation of ß-keto-esters, with alanine creating one optical isomer and tartaric acid the opposite isomer. A combination of RAIRS, TPD and LEED data show that both molecules create extremely robust overlayers with very long range order, arising from the self-organisation of nanoscopic chiral centres on the surface which provide identical environments for reaction. Amazingly, these chiral templates are stable up to 450K, after which the adlayer is 'exploded' from the surface in a sharp desorption peak. The interaction of the simplest ß-keto-ester, methylacetoacetate (MMA), onto these chiral templates was followed by RAIRS and molecular beam studies. These studies clearly show that a one-to-one surface complex is formed between the adsorbed chiral modifier and the co-adsorbed reactant,induced by hydrogen bonding interactions. This strongly stereocontrolled adsorption geometry for the MMA forces the hydrogenation reaction to occur along one face of of the molecule only.
9:00 AM		SS1-FrM-3 Vibrational Study of CH₂ and CH₃ Radicals on the Cu(111) Surface by HREELS Y.L. Chan (Academia Sinica, Taiwan); P. Chuang (Taiwan University); T.J. Chuang (Academia Sinica, Taiwan) Hydrocarbon CH_x (x=1,2,3) species are important radicals in many heterogeneous chemical reactions. Spectroscopic characterization of such species is essential for studying the reaction mechanisms. In this talk, we will report the study of CH₂ and CH₃ interactions on Cu(111) by XPS,AES, TPS, LEED and HREELS. The methylene and methyl radicals are produced through a high-temperature nozzle source with ketene and azomethane gases, respectively. The presence of the radicals in chemisorbed states is confirmed by the characteristic vibrational frequencies in the HREELS spectra. Most notably, the symmetric and antisymmetric deformation modes of CH₃ appear at 1161 and 1379 cm^-1, and the rock, twist and scissors modes of CH₂ at 815, 978 and 1428cm^-1. The C-H stretch vibrations at high frequencies and the C-metal vibrations at low frequencies are also detected. A striking observation is the formation of aromatic species from CH₂ exposure even at submonolayer coverages at 300K. Apparently adsorbed methylene can aggregate to form the complex species on Cu(111), In this context, CD₂ and benzene are also investigated. To our knowledge, it seems to be the first observation of such reaction on a metal surface. Other studies include the coverage dependence of the reaction and temperature effect on the vibrational spectra. Specular and off-specular measurements have yielded important insight into the electron scattering mechanism and the surface bonding geometry. These aspects will be discussed as well. (NSC 85-2114-M-001-032)
9:20 AM		SS1-FrM-4 Hydrogen Induced C-N Bond Activation in C6 Amines on Ni and Pt Surfaces J.L. Gland (University of Michigan); S.X. Huang (The Sherwin-Williams Co.); A.M. Gabelnick (University of Michigan) Hydrogen plays a number of dominant interrelated roles during C-N bond activation in aniline and cyclohexylamine on nickel and platinum surfaces. Competition between hydrogenation and dehydrogenation of the C₆ ring determines the hydrogen content and orientation of the surface species at reaction temperature. Tilted orientations of hydrogenated C₆ intermediates favor hydrogenolysis. In contrast, adsorption parallel to the surface strongly favors competing dehydrogenation and polymerization reactions. In addition, hydrogen appears to be directly involved in C-N bond activation on both nickel and platinum surfaces. Hydrogen addition to the C-N bond in hydrogenated intermediates results in ammonia formation at 370 K and the formation of adsorbed cyclic C₆ species which remains adsorbed on the surface on both metals. These cyclic species dehydrogenate with increasing temperature to form benzene which generally desorbs at higher temperatures.
9:40 AM		SS1-FrM-5 A Comparison of H Atom Induced C-C Bond Activation on the Nickel and Platinum(111) Surfaces A.T. Capitano, J.L. Gland (University of Michigan) Metal identity plays a dominant role in the control in carbon carbon bond activation reactions induced by gas phase atomic hydrogen. Gas phase atomic hydrogen induces C-C bond activation in adsorbed cyclopropane on both the Ni and Pt (111) surfaces. The reactions are very different on the two metal surfaces. On Pt(111), propane formation occurs via three desorption processes at 170, 190 and 220 K. No multiple C-C bond processes were observed as evidenced by the lack of methane or ethane formation. Based on the products observed, isotopic experiments, and comparison with the literature, we propose mechanisms for each reaction peak. We believe that the 170 K propane peak is caused by surface hydrogenation of an adsorbed propyl formed by C-C bond activation in CP. The 190 K propane peak is caused by reaction with a new reactive form of adsorbed hydrogen that can only be generated by gas phase atomic hydrogen. At 220 K, propane and a new cyclopropane peak are observed after exposure to gas phase atomic hydrogen. In the presence of coadsorbed surface hydrogen, no reaction was observed.On Ni(111), gas phase atomic hydrogen induces propane formation from cyclopropane by two pathways at 130 and 200 K. The low temperature process is caused by propyl hydrogenation. The reaction of subsurface hydrogen with cyclopropane induces propane formation near 200 K. Both of these results are similar to previous work on the Ni(100) surface. Coadsorbed surface hydrogen does not induce C-C bond activation on this surface.
10:00 AM		SS1-FrM-6 Acetylene Cyclization to Benzene on Pd/W(211)* I.M. Abdelrehim, K. Pelhos, T.E. Madey (Rutgers University); J. Eng, Jr., J.G. Chen (Exxon Research and Engineering Company) Pyramidal structures with facets having {211} orientation and nanoscale dimensions are formed when a Pd-covered W(111) surface is heated above ~750K; the structures are identified by LEED and STM. As part of a program to study structure-reactivity relationships in the morphologically-unstable Pd/W system, we are examining a known structure sensitive reaction, acetylene tricyclization to benzene. We find using TPD that clean W(211) yields little or no benzene from acetylene. In contrast, benzene forms reactively from acetylene on Pd/W(211), for Pd coverages ranging from ~0.5 monolayer (ML) to multilayers. A single benzene TPD peak at 460K is observed for the ~0.5 ML and ~1 ML Pd coverages. Upon annealing a 5 ML Pd film to 700K, acetylene adsorption leads to benzene desorption peaks at 190K and 330K, as well as 460K. The appearance of three states is consistent with the coexistence of monolayer Pd/W and 3 dimensional crystalline Pd nanoclusters. On the ~0.5 ML and ~1 ML Pd/W(211) surfaces, HREELS investigations reveal no surface benzene prior to desorption, indicating a reaction rate limited process; in contrast adsorbed benzene forms from 1L acetylene on a multilayer of Pd after flashing to 450K. Furthermore, an additional surface species characteristic of ν(C=C) is observed between 150K and 300K as indicated by loss features at 1211 and 1366 cm^-1; it disappears at temperatures >300K indicating desorption or decomposition. On the other hand, the clean W(211) surface exhibits features for vinylidene and acetylidide following acetylene adsorption at 90K, denoting the extreme reactivity of this surface. *This work is supported, in part, by the DOE/BES.
10:20 AM		SS1-FrM-7 Reaction Mechanisms and Kinetics in Hydrotreating Models on Palladium: A Study of Furan, Thiophene, and Pyrrole on Pd(111) using LITD/FTMS and STM T.E. Caldwell, I.A. Abdelrehim, D.P. Land, C.A. Pearson, G.W. Anderson, S. Chiang (University of California, Davis) The chemistry of sulfur-, oxygen-, and nitrogen-containing compounds on transition metal surfaces is of great interest due to the necessity of their removal from petroleum and biomass hydrocarbon resources. Our recent results, using TDS and laser-induced thermal desorption with FT mass spectrometry, indicate that the low-temperature decomposition mechanisms of these three compounds on clean Pd(111) differ significantly, despite the similarity in their structures. Thiophene decomposition involves cleavage between the C-S bonds, resulting in the deposition of surface sulfur and the formation of a tightly bound C₄ species on the surface which undergoes hydrogenation to form a significant amount of 1,3-butadiene. Furan, on the other hand, decomposes via elimination of α-H (desorbing as H₂) and CO, leaving a C₃H₃ species on the surface. Heating to 320K causes dimerization of some of the C₃ species, forming a small amount of benzene. Deuterium labeling and kinetic studies will be presented to further elucidate the mechanism. Preliminary results of pyrrole decomposition show that HCN is the major reaction product. These results suggest a decomposition pathway involving cleavage of the ring, thus forming an intermediate species which contains HCN. This intermediate appears to remain mostly intact under slow-heating conditions until 500 K where it further decomposes, liberating HCN from the surface.
10:40 AM		SS1-FrM-8 An STM Investigation of the Adsorption and Chemistry of Cyclic Aromatic Hydrocarbons on Pd(111) G.W. Anderson, C.A. Pearson, S. Chiang, T.E. Caldwell, D.P. Land (University of California, Davis) Recently, the scanning tunneling microscope (STM) has been used to identify individual molecular species, and even distinguish among molecular isomers in mixed overlayers on a surface ¹. This ability to image and identify molecules on a surface could yield unprecedented information on the fundamental processes in catalytic reactions on surfaces. In this study we have examined the adsorption and chemistry of furan on Pd(111), using a low temperature STM. Adsorption at 245 K results in a saturated monolayer of furan, which shows the presence of both of 2-fold and 3-fold reconstructions in low energy electron diffraction (LEED) measurements. STM images, measured at ~90 K, show two different adsorption geometries, planar and vertical, for the adsorbed furan molecules. Furan lying flat on the surface appears larger and forms the 3-fold reconstruction, while vertically oriented furan is smaller, and is responsible for the 2-fold reconstruction. The effect of the molecular orientation and binding sites on the surface reactivity and details of the reaction mechanism of the thermal decomposition of furan on Pd(111) will also be discussed. The detailed structure of the furan molecules as observed by STM will be compared to images of other related cyclic aromatic compounds (benzene, pyrrole and thiophene) to examine the possibility of using STM to differentiate these closely related species. Such information is relevant to determining whether STM can be utilized to examine reactions where more than one of these species is present. ¹V.M. Hallmark, S. Chiang, K.-P. Meinhardt and K.Hafner, Phys. Rev. Lett. 70 (1993) p. 3740
11:00 AM		SS1-FrM-9 A Scanning Tunneling Microscopy Study of Benzene Adsorption on Si(100)-(2x1) K.W. Self, R.I. Pelzel, J.H.G. Owen (University of California, Santa Barbara); C. Yan (Applied Materials Corporation); W. Widdra (Technische Universität München, Germany); W.H. Weinberg (University of California, Santa Barbara) Scanning tunneling microscopy has been used to investigate the adsorption of benzene on nominally flat Si(100)-(2x1) substrates. STM images show that benzene adsorbs on top of the dimer rows bonding to the two Si-Si dangling bonds. Bias dependent imaging indicates that the highest occupied molecular orbital of adsorbed benzene lies approximately 1.2 eV below the top of the valence band and that the lowest unoccupied molecular orbital is at least 3.5 eV above the highest occupied molecular orbital. At higher coverages, the benzene molecules are adsorbed on every other dimer along the dimer row and every other dimer across the dimer rows resulting in a local c(4x2) periodicity in agreement with the saturation coverage of 0.25 ML. Supported by QUEST, an NSF Science and Technology Center for Quantized Electronic Structures (Grant No. DMR 91-20007), the NSF (Grant No. DMR-9504400), and the W. M. Keck Foundation
11:20 AM		SS1-FrM-10 Benzene/Si(100): Metastable Chemisorption and Binding State Conversion G.P. Lopinski, R.A. Wolkow (National Research Council of Canada) Previous thermal desorption studies of benzene on Si(100)(2x1) have suggested two types of chemisorbed benzene with similar desorption energies, attributed to adsorption at defects and on terrace sites ¹. In the present work, scanning tunneling microscopy (STM) has been used to observe this system. While room temperature adsorption does involve two types of adsorption, both states are intrinsic to the Si(100) surface (i.e. adsorption at defect sites is not observed). The two states are characterized by distinctly different appearances in the STM images; bright protrusions centered along a dimer row, about one dimer wide (type A), and somewhat dimmer circular features surrounded by a darkened area, occupying the space of two dimer units (type B). Sequential images reveal a slow interconversion between the A and B states at the same position on the surface. The activation barrier for the conversion from A to B is estimated to be 22 kcal/mol while the barrier for the reverse process is approxiamitely 1kcal larger. The more stable, type B state can be assigned to molecules σ-bonded to four silicon atoms, also the lowest energy configuration obtained in semi-empirical calculations ². The metastable type A state may be attributed to π-bonded benzene. For all the molecules studied here no evidence of diffusion is observed at room temperature, indicating a barrier to diffusion comparable to that for desorption, as for benzene on Si(111)³. ¹Y. Taguchi, M. Fujisawa, T. Takaoka, T. Okada and M. Nishijima, J. Chem. Phys. 95, 6870 (1991). ²H.D. Jeong, S. Ryu, Y.S. Lee, S. Kim, Surf. Sci. 344, L1226 (1995). ³R.A. Wolkow and D.J. Moffatt, J. Chem. Phys. 103, 10696 (1995).
11:40 AM		SS1-FrM-11 Vibrational Spectroscopic Studies of Diels-Alder Reactions with a Si(100)-2x1 Surface as a Dienophile. S.F. Bent, A.V. Teplyakov, M.J. Kong (New York University) For the last decade, reactions of hydrocarbons on silicon surfaces have received serious attention. One reason for such an interest is the spatial localization of electron density on surfaces of semiconductors as opposed to the delocalized bands on surfaces of transition metals, which are conventional hydrocarbon conversion catalysts. However, whereas interaction of simple unsaturated compounds (such as ethylene and acetylene) has been studied extensively, more complicated systems have not received as much attention. In this paper we investigate the possibility of a Diels-Alder type cycloaddition reaction between dienes and the silicon dimers of a Si(100)-2x1 surface, which was theoretically predicted by D. Doren and R. Konecny. The reactions of several dienes were studied with a combination of multiple internal reflection infrared spectroscopy and thermal desorption spectrometry. The vibrational spectrum of chemisorbed 2,3-dimethylbutadiene suggests that the product of this chemisorption is the Diels-Alder adduct. The major evidence for the existence of the cycloaddition product is the absence of a band near 3100 cm^-1 in the IR spectrum corresponding to the =C-H stretch, in contrast to the multilayer spectrum, where this band is clearly observed. The results of 2,3-dimethylbutadiene chemisorption will be compared to reactions of other dienes. Our results indicate that the stability of the chemisorption product is affected by post-hydrogenation by atomic hydrogen. The parallels between our experimental results and the theoretical studies of these reactions will be analyzed.