AVS2001 Session SS3-ThP: Surface Reactions Poster Session
Thursday, November 1, 2001 5:30 PM in Room 134/135
Thursday Afternoon
Time Period ThP Sessions | Topic SS Sessions | Time Periods | Topics | AVS2001 Schedule
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SS3-ThP-1 Thermal Behavior of NO on Stepped Pd(112)
K. Irokawa, S. Ito, K. Okada, T. Okuya, H. Miki (Science University of Tokyo, Japan) The thermal behavior of NO on a stepped Pd(112) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy(XPS) and Auger electron spectroscopy(AES). Ramsier et al. reported that at low coverage NO adsorbed molecularly at terrace sites on Pd(112) with the NO axis perpendicular to the terrace and as increasing the coverage NO adsorbed at step sites with the NO tilted to downstairs.1 It was found that four peaks appeared at 2.7, 9.4, 11.2 and 14.8eV below the Fermi level in a UPS spectrum, when the surface was saturated with NO at 300K.2 The peak appeared at 11.2eV originates from 1π+5σ orbital of NO molecules adsorbed at the step sites of Pd(112) and the three remaining peaks originate from 2π, 1π+5σ and 4σ orbitals of NO molecules adsorbed at the terrace sites. The peak at 11.2eV vanished at 383K with increasing temperature, although the remaining peaks disappeared at 423K. This result indicates that an activity of NO dissociation at the step edge of the Pd(112) surface is much higher than the terrace. The N atoms desorbed from the surface at 700K. This behavior observed by UPS is consistent with results of XPS and AES. |
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SS3-ThP-2 Simulation of Lateral Interactions in the Dissociation of NO on Rh(100) by Dynamic Monte Carlo Simulations
A.P. van Bavel, J.J. Lukkien, J.W. Niemantsverdriet (Eindhoven University of Technology, The Netherlands) The kinetics of the dissociation of NO on Rh(100) is largely determined by lateral interactions as a recent study using Temperature Programmed Desorption (TPD) and Temperature Programmed Static Secondary Ion Mass Spectrometry (TPSSIMS) by Hopstaken et.al.1 clearly shows. At zero coverage limit the NO dissociation is completed around 200 K. At increasing initial NO coverages the dissociation is retarded due to strong repulsions between NO and its decomposition products. At saturation coverage the NO dissociation is even fully retarded until a few NO molecules desorb from the surface, thereby creating the necessary vacancies for dissociation. The created vacancies are immediately filled with the atoms formed. This way an auto-catalytic process is developed, since the atoms cause more stronger repulsions and thereby more desorption. Due to the essential role of interactions and to the possibility of island formation, the kinetics cannot satisfactorily be described using a mean-field approach. Therefore, we have developed a model to describe the dissociation of NO on Rh(100) by means of Dynamic Monte Carlo simulations. We have included pairwise additive interactions between neighbouring species and diffusion of all adsorbates. Repulsion between NO and its decomposition products is larger than the mutual repulsion between NO molecules. This - in combination with the higher mobility of NO - leads to segregation in the adlayer to form mixed Nads+Oads islands and compression of the NO in islands. The Monte Carlo simulations provide a means to estimate the magnitude of the interaction between neighbouring adsorbate species. |
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SS3-ThP-3 Multi-directional N2 Desorption in Thermal Dissociation of N2O on Pd(110) and Rh(110) at Low Temperatures
H. Horino (Environ. Earth Sci. Hokkaido University, Japan); T. Matsushima (CRC, Hokkaido University, Japan) Multi-directional N2 desorption was found in N2O dissociation on Pd(110) and Rh(110) below 170 K by angle-resolved TDS. N2 desorption sharply collimates off the surface normal in the (001) plane. Hot-atom-assisted N2 desorption is proposed in aligned N2O(a) dissociation. N2O(a) is mostly dissociated during heating procedures, emitting N2(g) and leaving O(a). N2 showed four desorption peaks. Pd(110);β1-N2 peaks around 150 K, β2-N2 134 K, β3-N2 123 K, and β4-N2 110 K. β4-N2 was clearly seen at low N2O exposures. It sharply collimated at ±50° off the surface normal. β3-N2 was significant and revealed inclined sharp emission centered at ±43° off the surface normal. A similar distribution was also found with β1-N2 found at high N2O exposures, whereas β2-N2 showed a cosine distribution. The preference of each N2 peak was sensitive to pre-adsorbed O(a). Rh(110);β1-N2 peaks at 165K, β2-N2 140 K, β3-N2 120 K and β4-N2 110 K. β3-N2 and β4-N2 collimated at θ=±33° and ±75°, respectively. These were seen at small exposures. β1-N2 showed a cosine distribution. β2-N2 desorption collimated at θ=±30°. Sharp inclined desorption possesses high kinetic energy. Prior to dissociation, N2O(a) must lie on the surface. For inclined desorption, a surface parallel momentum must be transferred from nascent hot oxygen atoms to desorbing N2. Larger inclined angles and higher kinetic energy may be expected on Rh(110) because higher hot-atom energy comes from the stronger metal-O bonding. |
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SS3-ThP-4 Adsorption and Reaction of SO2 with Cu(110) and Cu(110)-p(2x1)-O
A.R. Alemozafar, X.-C. Guo, R.J. Madix (Stanford University) Sulfur dioxide (SO2) is infamous for its role as an environmental pollutant and in most circumstances a catalyst poison. Over the past twenty years SO2 has been investigated on a number of single crystal metal surfaces, yet there is little SO2/Cu(110) work. The results of our study which combines STM and TPRS to advance the understanding of the reactions of SO2 on Cu(110) are reported. STM images reveal the formation of c(2x2), p(2x2) and c(4x2) surface structures when SO2 interacts with the clean Cu(110) surface. The p(2x2) and c(4x2) structures form small domains, approximately 3-4 lattice units across, while the c(2x2) structures are considerably larger. The LEED pattern resulting from this reaction is a diffuse c(2x2), consistent with the domain sizes revealed by STM. STM studies of the dissociative adsorption of D2S on Cu(110) reveal a c(2x2) sulfur structure on the surface with a corrugation identical to that observed upon SO2 interaction with the Cu(110) surface, indicating that the c(2x2) moieties are due to sulfur adsorption. The p(2x2) and c(4x2) moieties are distributed randomly throughout the scan area in equal proportions, and STM shows similar corrugations of these two phases suggesting that they are the same SOx species, stable up to 450 K (determined) by separate TPRS experiments. With the use of isotopic labeling the TPRS work suggests that the SOx species is SO3, with SO3 bound to the surface via one of its oxygen atoms. This stoichiometry is consistent with the 1:2 ratio of the fraction of the surface covered by S and SOx when the clean surface is exposed to SO2. Further, from our STM images the binding site for the SO3 can be determined to be a four-fold hollow. With our STM we have also probed the mobility of surface species. The real-time movie reveals the mobility of both SO3 and oxygen rows along the [001] and [110] azimuths, respectively. |
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SS3-ThP-5 NEXAFS Investigation of SO2 Reactions on Oxygen-modified Ni (100) Surfaces
C.M. Kim (Kyungpook National University, Korea) The surface reaction of SO2 and O on a Ni (100) surface has been investigated using a Near Edge X-ray Absorption Fine Structure (NEXAFS) technique and X-ray Photoelectron Spectroscopy (XPS). Four different surfaces were studied; clean Ni(100), p(2x2)_O/Ni(100), c(2x2)_O/Ni(100), and NiO(111)/Ni(100). Chemisorbed SO2 was formed at 160 K on all four surfaces. Upon heating, SO2 was decomposed to SO and atomic sulfur on clean Ni(100). On p(2x2)_O/Ni(100) and c(2x2)_O/Ni(100), however, SO3 was formed in the temperature range of 200 to 400 K. Sulfur K-edge NEXAFS results showed that SO3 was adsorbed with C3-axis perpendicular to the surface. On the NiO(111)/Ni(100) surface, both SO3 and SO4 were formed. |
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SS3-ThP-6 Mechanism of O2 Ejection from Pt(111) at 100K Induced by Gas-phase D Atom
J.-Y. Kim, J.S. Choi, S.J. Lee, J. Lee (Seoul National University, Korea) Rettner and Lee1 have shown that the gas-phase H(D) atom incident on an O2-adsorbed Pt(111) surface at 85K induces prompt desorption of O2 with a translational energy well in excess of the surface temperature. To elucidate the mechanism of this nonthermal desorption of O2, we have performed detailed kinetic studies using a D atom beam generated in a hot tungsten capillary tube at 1900K. Real-time monitoring of the gas-phase desorption products and post-reaction TPD measurements have been made with a QMS to find out 1) O2 and D2O desorb simultaneously with different kinetics, 2) the initial desorption rate of O2 is proportional to the O2 coverage, 3) at submonolayer O2 coverages, the O2 desorption rate increases with time following a step-like initial jump, which is more pronounced at a lower coverage, and 4) post-reaction TPD spectra show multiple D2O desorption peaks with increasing D atom exposure. Based on these observations, we conclude that O2 desorption occurs by site displacement of primary as well as secondary hot D atoms, which competes with D2O formation reaction.
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SS3-ThP-7 CO Adsorption on the c(2x2)-Mn/Cu(100) Surface Alloy: Magnetically Driven Restructuring
M. Grüne (Universität Bonn, Germany); G. Boishin (Universität Linz, Austria); C. Becker, J. Breitbach, A. Frey, T. Pelster, K. Wandelt (Universität Bonn, Germany) The c(2x2)-Mn/Cu(100) surface alloy is stabilized by the large magnetic moment of the Mn atoms yielding a substantial exchange energy.1 We have investigated the adsorption of CO on this alloy at 100 K by means of HREELS, UPS, LEED, and work function measurements. CO chemisorption passes through two subsequent stages. In no stage a CO-induced superstructure LEED pattern is seen. Initial adsorption of mainly side-on-CO, accompanied by adsorption at defects, leaves the substrate order intact. Subsequent adsorption of CO ontop Mn irreversibly destroys the long-range order of the substrate. This takes place by lateral interdiffusion, as can be shown by the application of UPS symmetry selection rules. We propose that the loss of the translational symmetry is related to a suppression of the local magnetic moment by ontop-CO adsorption. The reduction of the mnagnetic energy contribution causes a lifting of the energetic exclusion of Mn nearest neighbours, leading to a considerable entropy gain by lateral intermixing. |
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SS3-ThP-8 The Adsorption and Dehydrogenation of Cyclohexane and Benzene on Pt Islands on ZnO(0001)-O
A.W. Grant, L.T. Ngo, C.T. Campbell (University of Washington) The dehydrogenation of perdeuterated cyclohexane and benzene on Pt/ZnO(0001)-O model catalysts were studied with temperature programmed desorption (TPD), ion scattering spectroscopy (ISS), and X-ray photoelectron spectroscopy (XPS). Pt grows as 2-dimensional (2D) islands on ZnO(0001)-O until they cover ~50% of the surface, and then 3D islands form. Thus, the reactivity of these Pt islands can be studied as a function of their thickness and lateral dimensions. On Pt(111), most of the adsorbed cyclohexane converts to benzene (> 300 K) decomposing to H2 and adsorbed carbon. Two H2 TPD peaks, at ~360K and ~540 K, are due to desorption of H lost in producing adsorbed benzene and C-H bond scission in benzene, respectively.1 Perdeuterated cyclohexane desorbs molecularly at ~200 K from Pt-free ZnO(0001)-O, and ~240 K from the Pt islands, where decomposition also occurs. The Pt island thickness affects the decomposition reaction of the resulting adsorbed hydrocarbons dramatically. |
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SS3-ThP-9 Acetylene on Cu(110): Trimerization and Chemical Bonding
H. Öström, L. Triguero, K. Weiss, D. Nordlund, H. Ogasawara (Uppsala University, Sweden); L.G.M. Pettersson (Stockholm University, Sweden); A. Nilsson (Uppsala University, Sweden and Stanford University) We have studied the chemical bonding of acetylene on Cu(110) and the well known trimerisation reaction to benzene by high resolution X-ray photoelectron spectroscopy (XPS), near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray emission spectroscopy (XES). At liquid nitrogen temperature the XP spectra show two different C 1s peaks which correspond to two nonequivalent acetylene species adsorbed in different sites. By heating the sample, one species transforms into the other. This species disappears above room temperature due to the trimerisation of acetylene to benzene. We monitored the reaction with time resolved XPS, finding that benzene leaves the surface as soon as it is formed, in agreement with previous results. The adsorption geometry of the different adsorbate species have been determined by polarization dependent NEXAFS spectra, which shows that both species lie down on the surface, with different molecular alignment. XES shows that both species are chemisorbed on the surface with their electronic structure significantly distorted from the gas phase. The experimental results are completed by ab-initio cluster model calculations performed in the framework of density functional theory (DFT). |
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SS3-ThP-10 Thermal Chemistry of cis-1,2-Dichloroethene on Pd(111)
D.M. Jaramillo, D.E. Hunka, D.P. Land (University of California, Davis) The decomposition of halogenated compounds on metal surfaces has generated significant interest due to the facile remediation of halocarbons by metal particles. Of particular importance are the reactions of toxic and/or carcinogenic compounds, such as chloroethenes. We have elucidated some mechanistic information about the decomposition of cis-1,2-dichloroethene on palladium. After adsorption on Pd(111) at 100 K, cis-1,2-dichloroethene thermally decomposed by 400 K to yield chlorine and hydrocarbon fragments. The only decomposition products observed by temperature programmed desorption (TPD) and laser-induced thermal desorption (LITD) were hydrogen chloride and hydrogen. Very little HCl was formed on the surface and only for exposures above 0.3 L. However, the presence of chlorine, observed by Auger electron spectroscopy (AES), on the surface above 650 K for exposures below 0.4 L indicates that decomposition occurred even though no HCl was observed. Possible surface intermediates were identified using Fourier transform reflection-adsorption infrared spectroscopy (FT-RAIRS). |
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SS3-ThP-11 Adsorption of Cyclopentene and Cyclohexene on Ordered Sn/Pt(111) Surface Alloys
J. Breitbach, D. Franke, G. Hamm, F. Jaeger, C. Becker, K. Wandelt (University of Bonn, Germany) The adsorption of cyclopentene (C5H8) and cyclohexene (C6H10) on Pt(111) and two Sn/Pt(111) surface alloys has been investigated using HREELS, UPS, LEED and TPD. The two ordered Sn/Pt(111) alloys were prepared by annealing a Sn film deposited onto Pt(111). Depending on the temperature of annealing the surface exhibited a (2x2) or (√3x√3)R30° LEED pattern corresponding to a surface composition of Pt3Sn and Pt2Sn, respectively.1 At temperatures below 250K C5H8 and C6H10 adsorb intact on the pure Pt(111) surface. The di-σ-bonding of the molecules is signified by the absence of the olefinic CH-stretching mode that is identified for the undisturbed molecules in the multilayer. Upon heating part of the C5H8 and C6H10 desorb and the remaining amounts are converted to C5H5 and C6H6, respectively. On the alloy surfaces the decomposition of C5H8 and C6H10 is completely suppressed. As the Sn concentration is increased, there is a marked decrease in the C5H8 desorption temperature from 278K on Pt(111) to 243K on the (2x2) alloy and to 192K on the (√3x√3)R30° alloy. This behaviour is in close analogy to the behaviour of ethylene on Sn/Pt(111)2 and hints to similar adsorption geometries of cyclopentene and ethylene on the pure Pt(111) surface. The adsorption of C6H10 is more dramatically influenced by alloying: On the (2x2) surface C6H10 is still di-σ bonded, while on the (√3x√3)R30° surface C6H10 is physisorbed. It can be concluded that C6H10 adsorbs on Pt-threefold hollow sites on Pt(111), which are not present on the (√3x√3)R30° surface. |
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SS3-ThP-12 Adsorption and Reaction of s-Triazine on Al(111)
V.J. Bellitto, B. Bartlett, J.M. Valdisera, J.N. Russell, Jr. (Naval Research Laboratory) Polycyanurates, cyanate ester resins with low-k-dielectric properties, are useful for the fabrication of microelectronic devices. Formed by the trimerization of monomers with cyanate functionalities, the polymer linkage in polycyanurates is a triazine ring. To understand how this linkage interacts with aluminum, a material used as interconnects in microelectronics, we examined the chemical interaction of 1,3,5-Triazine (C3H3N3) and its isotopomer (C3D3N3) on Al(111) using infrared reflection absorption spectroscopy (IRRAS), x-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). A multilayer of triazine was produced by dosing the Al(111) surface while held at 140 K. Based on IRRAS measurements, triazine was randomly oriented in the multilayer. The multilayer desorption peak temperature occurred at 173 K, leaving a monolayer of triazine on the surface. At surface temperatures between 205 K and 300 K, similar IRRAS spectra were observed, showing modes at 1564 and 1341 cm-1 and the absence of a mode at 737 cm-1. Symmetry analysis of the IRRAS spectrum indicates the triazine molecular plane is tilted with respect to the Al(111) surface in the adsorbed monolayer, bonding through one of the nitrogen lone pairs. Thermal decomposition product desorption began around 425 K. HCN and H2 desorption were observed, but surprisingly CH4 and C2H4 desorption were also detected. Monitoring the decomposition products of C3D3N3 confirmed the product assignments. Consistent with the desorption results, around 425 K the beginning of carbide and nitride formation was observed with XPS. Above 750 K, decomposition product desorption ceased. A broad phonon mode was observed at ~800 cm-1 due to the formation of AlCxNy. |
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SS3-ThP-13 The Chemistry of 1,1-Dichloroethene on Pd(111) Investigated by TDS, LITD-FTMS, STM and FTRAIRS
D.E. Hunka (University of California, Davis); D.C. Herman (University of North Carolina, Chapel Hill); K.D. Lormand, A. Loui, S. Chiang, D.P. Land (University of California, Davis) Chloroethene contamination in ground water is a concern from both an environmental and health standpoint. All six chloroethenes are contained in over half of the sites listed on the EPA’s National Priorities List as well as possible carcinogens. One promising method of remediating these pollutants is using zero valent metals to degrade these halocarbons. Both iron and iron palladium bimetallic clusters have been shown to effectively decompose several small chlorocarbons, including dichloroethenes (DCEs). However, no systematic studies on palladium alone have been performed to date. In this study, the chemistry of 1,1-dichloroethene on clean Pd(111) has been investigated using thermal desorption spectrometry (TDS), laser induced thermal desorption Fourier transform mass spectrometry (LITD/FTMS), scanning tunneling microscopy (STM) and Fourier transform reflection absorption infrared spectroscopy (FTRAIRS). TDS and LITD-FTMS results indicate a coverage dependent decomposition mechanism. Coverages above 0.32 L show a stepwise decomposition initiated by C-Cl bond scission in which two successive stable surface intermediates are produced. These intermediates are proposed to be monochloroethylidyne and chlorovinylidene, respectively. The decomposition of 1,1-DCE in coverages below 0.32 L are initiated by C-H bond cleavage, and produce one stable surface intermediate, proposed to be dichloroethylidyne. All surface intermeidiates will be investigated and confirmed by FTRAIRS. Finally, STM reveals that adsorption and decomposition of 1,1-DCE happens preferentially at step edges. |
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SS3-ThP-14 Structural Study on (CH3)2S/Cu(100) by Near Edge X-ray Absorption Fine Structure and X-ray Photoelectron Spectroscopy
S. Yagi (Nagoya University, Japan) Adsorption behavior of a molecule on metal surface has been interested in a catalytic and surface reaction fields. In this study, we have studied an adsorption structure of the (CH3)2S on Cu(100) surface by use of polarization dependent S K-edge Near Edge X-ray Absorption Fine Structure (NEXAFS) and S 1s X-ray Photoelectron Spectroscopy (XPS) techniques. The Cu(100) crystal was cleaned by means of the Ar+ bombardment and annealing up to 800 K. The cleanliness and order of the surface were verified by XPS and LEED. Research grade (CH3)2S molecule was introduced with an exposure of 0.4 L to the Cu(100) at 90 K, in order to obtain a submonolayer phase. S K-edge NEXAFS and XPS measurements were carried out at the soft X-ray double crystal monochromator beamline BL-3 on Hiroshima Synchrotron Radiation Center. By comparing the edge-jump ratio between the submonolayer phase and the S atomic adsorption phase, the S amount was estimated to be 0.2 ML. Noticeable polarization dependence can be seen in the NEXAFS spectra, first feature (1s-σ*(S-C)) is enhanced at normal incidence (electric vector is parallel to the surface). This result imply that the (CH3)2S molecule is lying on the Cu(100) surface without the cleavage of the S-C bonds. A significant chemical shifts of the S 1s XPS peak were observed to a lower binding energy side in the submonolayer phase compared to that of the multilayer. It found that the charge transfer occurs from the substrate to the S atom of the molecule. |
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SS3-ThP-16 Reactions of Perchlorate on Titanium/Titanium Oxide Surfaces Studied by LITD/FTMS.
K.D. Lormand, D.E. Hunka, D.P. Land (University of California, Davis) Organic contaminants in water supplies have been a concern for decades, due to possible deadly health effects. Perchlorates, in particular, have posed a major concern as of late due to their irreversible and damaging affects on the human thyroid,1 long residence time in water sheds, and resistance to existing catalysts used in water treatment.2 Catalysts, such as palladium, iron, and platinum, have been found to be quite effective in reducing the concentration levels of most halocarbon residues, but are ineffective on perchlorates. However, preliminary studies of oxidized surfaces of titanium exposed to ultraviolet radiation have been seen to reduce perchlorates in aqueous solutions effectively. Reaction mechanisms of many halocarbon residues on palladium and platinum catalysts have already been elucidated using laser induced thermal desorption and conventional thermal desorption with FT mass spectrometry (LITD/FTMS) on clean surfaces in ultra high vacuum (UHV). Though these studies reveal accurate reaction mechanisms, they are done in an ultra clean environment and do not fully incorporate atmospheric gases into the equation. We have recently designed a new LITD/FTMS chamber to allow the rapid introduction of samples from reaction in aqueous solution into UHV for analysis. This allows for more inclusive reaction mechanisms to be determined due to the incorporation of atmospheric water and oxygen. The reaction of perchlorate is investigated on both titanium metal as well as titanium oxide using LITD/FTMS. While titanium itself is less reactive, the oxides show increased activity and studies of varying oxide layers are presented. |
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SS3-ThP-17 Surface-Termination-Dependence of the Reactivity of Single Crystal Hematite with Carbon Tetrachloride
N. Camillone III, K. Adib, K.T. Rim, J.P. Fitts, G.W. Flynn (Columbia University); S.A. Joyce (Pacific Northwest National Laboratory); R.M. Osgood, Jr. (Columbia University) We describe ultrahigh vacuum Auger electron spectrometric measurements of the uptake of chlorine following the exposure of single crystal hematite to CCl4 at room temperature. We compare the surface chemistry of two distinct terminations of α-Fe2O3: the Fe3O4 "selvedge" and the α-Fe2O3 / FeO "biphase." For Fe3O4 (111)-2x2 we estimate that saturation levels of Cl of at least ~ 27 % of a monolayer are attained at relatively low exposures of on the order of 0.1 L. No significant amount of carbon uptake is detected. Low energy electron diffraction measurements suggest that, dependent upon preparation procedures, at least two types of α-Fe2O3 / FeO biphase structures can be formed. Interestingly, no significant Cl or C adsorption is detected for either of these biphases, revealing a marked difference in the reactivity of the terminations. Comparison of these results with the surface structure of these terminations suggests that the active site for the dissociative adsorption of CCl4 on Fe3O4 (111)-2x2 must comprise both an iron cation and an oxygen anion that is uncapped by iron cations. Modification of the biphase termination by thermal treatment, as well as the electron-stimulated and thermal desorption of Cl from the saturated Fe3O4 (111)-2x2 selvedge will be discussed. Finally, the relationship between these results and our recent STM measurements on this system will be presented. |
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SS3-ThP-18 Optical and STM-based Excitation of Adsorbed Molecules
L. Bartels (University of California at Riverside); D. Moeller, T.F. Heinz (Columbia University); E. Knoesel (Rowan University); G. Meyer, S.W. Hla (Free University Berlin, Germany); A. Liu (University of California at Riverside); K.H. Rieder (Free University Berlin, Germany) Optical and STM-based excitation of adsorbed molecules Photodesorption and other photochemical reactions have been studied for a long time. Using femtosecond lasers, lately minute details of such surface reactions could be revealed that result in desorbing species. Simultaneously, scanning tunneling microscopy has matured from a pure imaging technique to a highly precise and powerful adsorbate and surface manipulation tool. Here experiments are shown, in which the diffusion of individual adsorbed molecules is induced by electron attachment from an STM tip and by optical excitation. In both cases the resultant diffusive motion on the surface is measured by STM. In both cases diffusion pattern are found, which do not occur under equilibrium thermal conditions. |
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SS3-ThP-19 A Comparative Study of the Adsorption of Acetylene, Ethylene and Benzene on the Pure Pd(111) Surface and the Ordered Pd2Sn Surface Alloy on Pd(111)
G. Hamm, T. Schmidt, J. Breitbach, D. Franke, C. Becker, K. Wandelt (University of Bonn, Germany) The adsorption of acetylene, ethylene and benzene on the pure Pd(111) surface and the ordered Pd2Sn surface alloy on Pd(111) has been investigated with TPD, LEED, UPS and HREELS. The surface alloy with (√3 x √3)R30° periodicity corresponding to the Pd2Sn composition was produced by annealing of multilayer amounts of Sn vapor deposited onto Pd(111).1 Below 300K benzene chemisorbs intact on the pure Pd(111) surface, bonding via its π-electron system. In the range 300-500K most of the adsorbed benzene desorbs while a small part is dehydrogenated leaving a CCH species on the surface. For the first time an ordered superstructure of benzene has been found at room temperature. On the alloy, benzene can only be physisorbed. Ethylene is most probably di-σ bound on the pure Pd(111) surface below 250K, whereas ethylidyne is the dominant species after the adsorption of ethylene at 350K. In the temperature range from 150 to 300K most of the ethylene desorbs. At the same time part of the molecules undergo a three step conversion into ethylidyne above 250K. Due to the absence of appropriate conversion sites and the weak adsorption, this reaction is totally suppressed on the alloy. Acetylene chemisorbs on both surfaces. While the electronic structure of the adsorbed molecule is nearly identical on the pure Pd(111) surface and the alloy, vibrational spectroscopy reveals marked differences. Benzene is reactively formed from adsorbed acetylene on pure Pd(111), exhibiting two desorption peaks at 200K and 500K, which are ascribed to tilted and flat lying benzene. The majority of the acetylene is, however, converted to ethylidyne near room temperature via a vinylidene intermediate. In contrast to Pd(111), both reactions are suppressed on the alloy surface resulting in a single acetylene desorption peak at about 160K. |