AVS2001 Session SS1-ThP: Catalysis on Model Systems 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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SS1-ThP-1 A Model Catalyst in Motion: Restructuring of a Pt(110) Surface at Atmospheric Pressure
B.L.M. Hendriksen, J.W.M. Frenken (Leiden University, The Netherlands) The surface structure of a catalyst can depend on the reaction conditions. However, most model studies have been performed at well-defined, but strongly non-realistic, conditions such as ultrahigh vacuum or very low pressures. We have used a novel high pressure, high temperature scanning tunneling microscope, which is set up as a micro-flow reactor, to study a platinum (110) surface at semi-realistic conditions for CO oxidation, i.e. high pressure and temperature. Already a low partial pressures of CO induces the lifting of the Pt(110)-(1x2) missing row reconstruction [T. Gritsch et al., Phys. Rev. Lett. 63, 1086 (1989)] to form a tiger-skin like structure consisting of (1x1) patches. As soon we apply an atmospheric pressure of CO at 425 K, this intermediate structure coarsens to form smooth (1x1) terraces, as we have observed in STM-movies. |
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SS1-ThP-3 Lateral Interactions in Elementary Surface Reactions between CO and NO on Rhodium Surfaces
M.J.P. Hopstaken, A.P. van Bavel, J.W. Niemantsverdriet (Eindhoven University of Technology, The Netherlands) Unraveling catalytic mechanisms in terms of elementary reactions and determining the kinetic parameters of such steps is at the heart of understanding catalytic reactions at the molecular level. Here we report the use of temperature programmed desorption and static secondary ion mass spectrometry to study reactions between NO and CO on Rh(100) and (111). On both surfaces the reaction rates of the different elementary steps depend highly on coverage. At low coverage, dissociation of NO is completed around 250 K and 340 K for the Rh(100) and the Rh(111) surface, respectively. When the surface is saturated with NO, dissociation only starts when some NO desorbs first, i.e. when empty sites become available. However, inhibition of NO dissociation at intermediate coverages cannot be explained by site blocking alone, but is due to lateral interactions with other adsorbates such as N, O, and NO. Studying the effect of coadsorption of these species enables an estimate of the magnitude of these lateral interactions. The combined influence of lateral interactions and site blocking leads to explosive behavior in the CO + NO reaction on saturated surfaces. The explosion is triggered by the desorption of a small amount of CO. The liberated sites enable the dissociation of NO and the subsequent reaction of O-atoms with CO creates even more free sites. The process is autocatalytic in the free sites and becomes explosive. These explosions have been observed in real time with TPD and SIMS. |
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SS1-ThP-4 Molecular Beam Study of the N2O + CO Reaction on Rh(111)
S. Wehner, F. Zaera (University of California, Riverside) Rhodium is well known for its unique ability to reduce NO to N2. It is this property that makes it indispensable in the three way catalysts used in cars to clean their exhausts. Former studies in this laboratory using molecular beams have shown that the catalytic reduction of NO by CO takes place at the periphery of surface islands of adsorbed nitrogen atoms, and most likely involves the formation a N-NO intermediate. Here, results from a kinetic study on the conversion of N2O + CO mixtures on Rh(111) surfaces are presented. It was found that the overall behaviour is similar to that of the NO + CO reaction. The reaction rate with both nitrogen oxides reaches a maximum near 500 K for stoichiometric beams, but in the case of N2O the rate-limiting step is the formation of CO2, not the production of N2 as when NO is used. These results will be contrasted with our previous work on the NO + CO and CO + O2 systems to get an overall picture of the elementary steps involved in the cleaning of car exhausts. |
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SS1-ThP-5 Oxygen Defect Structures and Diffusion of Surface Oxygen Atoms on CeO2(111) Surface Studied by Noncontact Atomic Force Microscopy
Y. Namai, K. Fukui, Y. Iwasawa (The University of Tokyo, Japan) CeO2 is widely used as a component of automobile catalysts, where CeOx (x≤2) is believed to work as a buffer of active oxygen of the catalysts to the most efficient region for oxidation of CO and hydrocarbons in exhaust gas. Migration of surface oxygen atoms to active sites for the oxidation and surface structural changes in the reduction-oxidation cycles are crucial issues to understand the role of CeOx in an atomic scale. Noncontact Atomic Force Microscopy (NC-AFM) is a recently developed technique to visualize surface structures in an atomic scale. We have applied NC-AFM to a CeO2(111) surface and succeeded in obtaining atom-resolved images for the first time. By annealing an Ar ion-sputtered CeO2(111) surface at 1173 K for 1 min, hexagonally arranged oxygen atoms with a constant separation of 0.38 nm were observed by NC-AFM. Oxygen point vacancies were found on the surface as dark depressions. Further annealing of the surface at 1173 K in vacuum increased the density of point vacancies and multiple defects began to appear from total annealing period of 4 min. Triangular defects which consist of neighboring three oxygen vacancies and line defects which consist of 2-4 oxygen vacancies along the [10-1] direction, the [0-11] direction, and the [1-10] direction were visualized by NC-AFM. Successive NC-AFM observation revealed that oxygen atoms on slightly reduced CeO2(111) surfaces are mobile even at room temperature. Mobility of the surface oxygen atoms seems to depend on the density of surface oxygen defects. Such mobile oxygen atoms may be a key species in the oxidation reactions. |
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SS1-ThP-7 Adsorption Geometry of Modifiers in Chiral Catalysis
J. Kubota, F. Zaera (University of California, Riverside) It has been recently determined that heterogeneous hydrogenation catalysts such as platinum can be made enantioselective by the use of molecular modifiers. For instance, alpha-ketoesters such as ethyl pyruvate can be selectively hydrogenated by cinchona-modified platinum catalysts to produce the corresponding optically-pure (R)- or (S)-alpha-hydroxoesters (ethyl lactates from the pyruvate). In these, the adsorption geometry of the modifier appears to be critical to the performance of the catalyst. Here we report on infrared studies on the characterization of the adsorption of those modifiers from the liquid phase onto platinum surfaces. A number of cinchona molecules were probed, and the effects of concentration and solvent on the adsorption were investigated. |
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SS1-ThP-8 Combustion of Hydrogen Over a Palladium Catalyst Studied with Laser Induced Fluorescence Imaging
A. Johansson, M. Forsth, A. Rosen (Goteborg University and Chalmers University of Technology, Sweden) Catalysis is of great fundamental, practical and economical interest in today's society. Noble metals, such as palladium, is widely used as heterogeneous catalysts for reduction of emissions of car exhausts. It is therefore important to study the catalytic property of this metal. Combustion of hydrogen with oxygen was chosen as a model system to make it as simple as possible, avoiding hydrocarbons. The oxygen and hydrogen molecules adsorb and react via reaction intermediates to form water. An important intermediate is the OH molecule. The hydroxyl radical OH also has spectroscopic properties which make it convenient to study. Laser Induced Fluorescence (LIF) was used to investigate the gas-phase concentration of the OH molecule. An ICCD camera was used as detector to study the OH concentration profile in two dimensions outside the palladium surface. The polycrystalline palladium foil was resistively heated up to 1300K and nearby temperatures. The chamber was evacuated with roots and turbo pumps. The hydrogen and oxygen gas flow was regulated with two mass flow controllers, one for each gas. The mass flow controllers were calibrated with a quadropole mass spectrometer connected to the chamber. The working pressure in the chamber was varied from 0.1-100 Torr. The hydrogen/oxygen reaction on a hot palladium surface was also modelled using the Chemkin software package. Estimations of important reaction parameters could be achieved by comparing experimentally obtained OH-profiles with modelled profiles. Also, sensitivity analysis of the obtained model indicated which reactions that are rate-limiting and which reactions that are less important from a catalytic point-of-view. These last results should be taken as advises as to where future experimental efforts should be made. The water production rate is measured with micro calorimetry. |
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SS1-ThP-9 Scanning Tunneling Microscopy Studies of the Growth and Morphology of Cu Clusters Deposited on TiO2(110)
J. Zhou, J.E. Reddic, D.A. Chen (University of South Carolina) The growth of metals on oxide surfaces has become an important area due to the variety of applications involving metal-oxide interfaces. We have conducted scanning tunneling microscopy (STM) studies of Cu clusters grown on single-crystal TiO2(110) (rutile) surfaces under ultrahigh vacuum conditions as a model for understanding the early stages of metal-on-oxide growth. Previous investigations of Cu deposited on TiO2(110)-(1x1) have shown that the Cu clusters exhibit 'self-limiting' growth in the sense that with increasing coverage, the Cu cluster densities increase but the cluster sizes remain approximately constant. Our current studies of Cu grown on a partially reconstructed TiO2(110)-(1x2) demonstrate that surface defects play an important role in determining the size and spatial distribution of the Cu clusters. Growth on the (1x2) surface is also 'self-limiting' and produces very uniform size distributions at all Cu coverages. However, the average cluster size at room temperature on the (1x2) surface (25Å diameter, 5Å height) is much smaller than on the (1x1) surface (~40Å diameter, 6-8Å height), and this difference is attributed to the decreased adatom mobility on the more defective (1x2) surface. Furthermore, the Cu clusters show no preference for residing at the titania step edges, which are the highest coordinate sites, even when the surface has been annealed to high temperatures (700-800 K) to increase the diffusion rate of the Cu adatoms. To further investigate this growth behavior, the initial stages of Cu cluster growth will be investigated by depositing Cu at low temperatures (25K-100 K) to prevent Cu adatom diffusion. The surface will then be heated to initiate adatom diffusion, and changes in the Cu clusters will be imaged by STM. We will also explore how the morphology of the Cu clusters is changed by oxidation at various temperatures. Both Cu deposition and oxidation studies can be performed during STM imaging. |
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SS1-ThP-10 LITD-FTMS Study of Dehydrogenation of Cyclohexane on Al2O3-supported Pt Clusters
M.M. Ivey, M.F. Luo, J.C. Hemminger (University of California, Irvine) We present a study of the adsorption and subsequent thermally activated dehydrogenation of cyclohexane on oxide-supported Pt clusters by use of Fourier transform mass spectrometry (FTMS) in combination with laser-induced thermal desorption (LITD).1 Pt clusters were generated by thermal evaporation of Pt onto an Al2O3 ultra thin film of 10 Å thickness that was grown on a NiAl(001) surface through oxygen adsorption at high temperature.2 Both cyclohexane desorption and dehydrogenation are observed. The branching between the desorption and dehydrogenation paths was monitored using a combination of AES, TDS and LITD. The cyclohexane dehydrogenation on Pt clusters behaves in a manner significantly different from that on Pt(111) single crystal surfaces. We will show the reactivity/desorption behaviour for this system for a range of Pt surface loadings. This will be discussed in the light of the dependence on Pt average cluster size which was determined by an AES quantitative analysis of CO chemisorption, assuming hemispherical cluster shapes.
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