AVS2001 Session SS1-ThA: Catalysis on Model Systems

Thursday, November 1, 2001 2:00 PM in Room 121

Thursday Afternoon

Start	Invited?	Item
2:00 PM		SS1-ThA-1 Thin Ce_xZr_2-xO₂(111) Films as Model Catalytic Converter Oxygen Storage Materials C.H.F. Peden, T. He, G.S. Herman (Pacific Northwest National Laboratory); Y.-J. Kim (Taejon National University of Technology); S. Thevuthasan, V. Shutthanandan, D. McCready, J. Szanyi (Pacific Northwest National Laboratory) Oxygen storage (OS) materials, usually consisting of ceria (CeO₂) or modified ceria, are used in automobile catalytic converters to effectively damp deviations in the exhaust air/fuel (A/F) ratio in order to optimize the activity of the precious metal catalyst. We are using thin films of CeO₂ and ceria-zirconia (Ce_xZr_2-xO₂) as models for fundamental studies of the oxygen uptake, storage, and release properties of these materials. This presentation will emphasize the characterization (by x-ray diffraction, atomic force microscopy, reflection high-energy electron diffraction, low-energy electron diffraction, x-ray photoelectron spectroscopy (XPS) and diffraction, and Rutherford backscattering spectrometry and high-energy ion channeling) of model CeO₂ and Ce_xZr_2-xO₂ (x = 1.0, 0.9, 0.8, 0.7, and 0.6) thin films will be presented. A wide range of growth parameters using oxygen plasma-assisted molecular beam epitaxy have been used, and successful production of pure-phase, single-crystalline epitaxial oxide films has been achieved for x > 0.6. At higher Zr levels, evidence for phase-separation is observed. We will also report results of XPS and carbon monoxide temperature-programmed desorption experiments that provide evidence for markedly enhanced kinetics of oxygen storage and release upon doping of CeO₂ by Zr.
2:20 PM		SS1-ThA-2 Metal and Oxide Particles on Oxide Supports: Vanadium and Vanadia Deposits on Alumina M. Baeumer, N. Magg, J.B. Giorgi, M. Frank, H.-J. Freund (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) Vanadium in its various oxidation states is a catalytically very interesting system. Oxide-supported vanadia deposits are, for example, used as catalysts for the selective oxidation and dehydrogenation of hydrocarbons. In order to study this multivalent system in more detail, we have carried out model catalytic studies by depositing vanadium under various conditions onto a thin alumina film grown on NiAl(110). Under UHV conditions metallic aggregates are formed. This is connected with extensive changes in the phonon spectrum of the support (intensity damping, frequency shifts and peak broadening). A comparison to other metals, such as Pd, Rh, Ir and Al, reveals that, especially in the low coverage regime, chemical effects at the particle-support interface play an important role. This is also corroborated by XPS. At high coverages, on the other hand, metallic screening seems to gain in importance. If the V deposition is carried out in an oxygen ambient, oxide particles are formed. According to photoelectron spectroscopic data, they have an average oxidation state of +3. Nevertheless, infrared spectroscopy points to the presence of vanadyl groups on the surface of the aggregates actually being a structural element of V₂O₅. For both situations, metal and oxide deposits, the CO adsorption and reaction behaviour will be discussed. IR spectra show that CO adsorption on the vanadia aggregates leads to a blue shift of the CO stretching frequency as compared to the gas phase. Interestingly, an interaction between CO and the vanadyl groups is observed. By contrast, CO adsorption on the vanadium aggregates is connected with a red shift. A comparison to the corresponding IR data of other metals (Pd, Rh, Ir)¹ underscores the strong metal-support interaction. ¹M. Frank and M. Baeumer, Phys. Chem. Chem. Phys. 2 (2000) 3723.
2:40 PM		SS1-ThA-3 Atomic-scale STM Study of Model Catalysts for Hydrodesulfurization J.V. Lauritsen (University of Aarhus, Denmark); S. Helveg, B.S Clausen, H. Topsoe (Haldor Topsoe Research Laboratories, Denmark); F. Besenbacher (University of Aarhus, Denmark) Using scanning tunneling microscopy (STM),¹ we have recently attained novel atomic-scale information on model catalysts for hydrodesulfurization (HDS), an area that currently receives world-wide attention due to new environmental legislations regarding the sulfur content in fuel. The HDS activity is related to MoS₂-like nanoclusters promoted with Co atoms located near the edges. Controversy has, however, prevailed since traditional spectroscopy techniques provide no conclusive information regarding the cluster morphology, catalytically relevant edge structures, active sites or promotional effect of Co. We have successfully synthesized ~30Å wide single-layer MoS₂ clusters on an inert Au(111) substrate as a model system for HDS catalysts. High resolution STM images display an unprecedented view of the atomic details of the MoS₂ nanoclusters, which contrary to expectations exhibit a triangular morphology. We have also been able to resolve the structures of the catalytically active edges, and from interplay with DFT theoretical calculations we have determined the detailed atomic-scale structure.^1,3 The STM images reveal spectacular electronic features near the edge of the triangles, which with input from theory can be associated with electronic edge states. When the MoS₂-based catalysts are promoted with Co, the STM images directly show a morphological transition from triangular to hexagonally truncated structures. This is driven by a preference for Co to be located at only one type of MoS₂ edges.² We are currently investigating the interaction with thiophene, a typical sulfur containing molecule, and preliminary STM results indicate a strong bonding of the molecules to the cluster edges. ¹ S. Helveg, J.V. Lauritsen et al., Phys. Rev. Lett. 84, 951 (2000) ² J.V. Lauritsen et al. J. Catal. 197, 1 (2001) ³ M. Bollinger, J.K. Norskov, private communication.
3:00 PM		SS1-ThA-4 A Temperature Programmed Desorption Study of Propene Adsorption on Gold Islands Dispersed on TiO₂(110) H.M. Ajo (University of Washington); V.A. Bondzie (University of California at Riverside); C.T. Campbell (University of Washington) The adsorption of propene on TiO₂(110) and on gold islands dispersed on TiO₂(110) [Au/TiO₂(110)], both at 120 K, has been studied using temperature programmed desorption (TPD), x-ray photoelectron spectroscopy(XPS) and low energy ion scattering spectroscopy (LEIS). Propene adsorbs on both TiO₂(110) and Au/TiO₂(110), with desorption peak temperatures of ~190 and ~240 K, respectively, for tiny doses of propene. When only 17% of the TiO₂(110) surface is covered by gold islands [17% Au/TiO₂(110)], moderate propene doses populate both the 240 and 190 K TPD peaks, in that order. Since both the dose of propene needed to saturate the 240 K peak and its peak area increase with the gold island coverage, the desorption peak at 240 K is attributed to propene adsorbed at the edges of gold islands. This feature is also seen at about this same temperature even when the gold islands are only one atom thick. Temperature-dependent LEIS results suggest that this propene binds to both a gold island edge and a titanium site. Tiny doses of propene to the 17% Au/TiO₂(110) surface give the 240 K TPD peak but no 190 K feature. This shows that all of the propene desorbs from these island edge sites. Since some propene molecules must initially physisorb on TiO₂(110) sites, but no propene molecules desorb from these sites during TPD, the propene must be mobile enough on the TiO₂(110) surface, either at the dosing temperature or during TPD, to migrate to the gold island edge before desorption (i. e., below 190 K).
3:20 PM	Invited	SS1-ThA-5 Atom-resolved and Nano-scale Structures and Catalyses at TiO₂ and CeO₂ Single Crystal Surfaces Y. Iwasawa (The University of Tokyo, Japan) This paper presents several important topics in surface catalytic chemistry. Atom-resolved and nano-scale structures of TiO_x on TiO₂(110) and (001) surfaces have been visualized by STM and NC-AFM. The surfaces and the TiO_x structures were transformed to new surface structures in a complicated manner induced by heating and adsorption. Structure models are presented. Carboxylic acids were catalytically decomposed on TiO₂(110), and the reaction sites and reaction kinetics were characterized by in-situ STM observation. Nano-structures of Pt and Au on TiO₂(110) were produced by using Pt or Au precursor complexes in different ways. A unique clear-cut size regulation of the Pt particles was found. A new mechanism is presented. Very small Au particles with 0.7 nm height were successfully formed by UV irradiation of the Au-complex adsorbed surface or by UV irradiation of TiO₂(110) before deposition of the Au complex. Very small Au particles are remarkably active for low temperature CO oxidation. Atom-resolved and cluster structures of and around oxygen defects at CeO₂(111) surface have also been imaged by NC-AFM. The oxygen defects were mobile even at room temperature. The phenomenon was entirely different from that observed with the TiO₂ surface, which may be relevant to oxygen reservoir and oxidation activities of CeO₂ in automobile catalysts and oxidation catalysts. These surfaces are dynamic and reactive, depending on temperature and atmosphere, which may be relevant to the origin and mechanism of catalysis.
4:00 PM		SS1-ThA-7 Investigations of Size-Dependent Surface Chemistry on Metal Nanoparticles: Dimethyl Methylphosphonate Reaction on Cu/TiO₂(110) D.A. Chen, J.E. Reddic, J. Zhou (University of South Carolina) We are interested in understanding how metal nanoparticle size affects surface chemistry so that specific particle sizes with the desired reactivity can be identified for catalysis applications. Cu nanoparticles were grown on a TiO₂(110)-(1x2) surface and characterized by scanning tunneling microscopy under ultrahigh vacuum conditions. The Cu nanoparticles deposited on TiO₂(110)-(1x2) exhibit the same "self-limiting" growth behavior previously observed on the unreconstructed titania surface: the particle density increases with increasing coverage while particle size is relatively constant. At all coverages, the Cu particles have a uniform size distribution, and the particle size can be controlled by annealing the surface to higher temperatures. Deposition at room temperature produces particles that are ~25 Å in diameter and ~5 Å high, while annealing to 700 K increases the particle size to an average diameter of 60-70 Å and height of 15-20 Å. We found that a smaller size regime of Cu nanoparticles can be prepared by depositing on this highly reduced titania surface compared to the more stoichiometric titania (1x1) surface. X-ray photoelectron studies of the thermal chemistry of dimethyl methylphosphonate (DMMP) on the smallest Cu nanoparticles (25 Å diameter) show that DMMP decomposition occurs below room temperature. Specifically, P-OCH₃ bond scission is nearly complete at room temperature, but all P-CH₃ bonds are not broken until much higher temperatures (550 K). Both phosphorous and carbon can be removed from the surface by heating to 800 K. Although studies of DMMP reaction on the TiO₂(110)-(1x2) surface show that decomposition of DMMP on titania itself commences around room temperature, our data also suggest that P-OCH₃ bond scission occurs more readily on the Cu nanoparticles.
4:20 PM		SS1-ThA-8 Catalytic Oxidation of Propylene on Stepped Pt(411): In-situ Mechanistic Studies Over an Extended Pressure Range H.D. Lewis, D.J. Burnett, A.M. Gabelnick (University of Michigan); D.A. Fischer (National Institute of Standards and Technology); J.L. Gland (University of Michigan) Surface defects play an important role in reactivity by, for example, lowering activation barriers for dissociation and increasing the bonding energy of adsorbed species. In this work the catalytic oxidation of preadsorbed propylene has been studied in oxygen pressures up to 0.01 Torr on the stepped Pt(411) surface. Using a combination of kinetic and spectroscopic in-situ fluorescence yield soft x-ray techniques we have characterized the oxidation of propylene. In pressures of oxygen, propylene is completely oxidized by 475 K with oxydehydrogenation preceding skeletal oxidation. The 280 K initiation temperature for oxydehydrogenation is independent of oxygen pressure. The temperature where skeletal oxidation begins decreases from 315 K in 0.0005 Torr oxygen to 300 K in 0.02 Torr oxygen. In the temperature range between oxydehydrogenation and skeletal oxidation a reaction intermediate has been spectroscopically characterized. In-situ catalytic oxidation studies with both propylene and oxygen in the gas phase were also studied. With increasing oxygen pressure the concentration of carbon containing surface species decreases showing competitive adsorption. In this catalytic environment, the onset temperature for deep oxidation decreases with increasing oxygen pressures. Taken together, these results suggest that the inhibition of oxygen adsorption is important in limiting this complex reaction system. This new molecular understanding provides a basis for elucidating the mechanism of this complex surface reaction network.
4:40 PM		SS1-ThA-9 A Model Catalyst in Action: A Flow-reactor-STM Study of CO-oxidation on Pt(110) B.L.M. Hendriksen, J.W.M. Frenken (Leiden University, The Netherlands) The activity of a (model) catalyst can depend on its surface structure. In turn, the surface structure can depend on the reaction conditions. We have used a novel high-pressure, high-temperature scanning tunneling microscope, which is set up as a flow reactor, to determine simultaneously the surface structure and the activity of a Pt(110) model catalyst at semi-realistic conditions for CO oxidation. By controlled switching from a CO flow to an O2 flow and vice versa, we can reversibly oxidize and reduce the platinum surface while imaging the surface with our STM. By simultaneously monitoring the gas composition, we have observed that the formation of the oxide has a dramatic effect on the CO2 production rate. Our results show that there is a strict one-to-one correspondence between the surface structure and the catalytic activity.
5:00 PM		SS1-ThA-10 The Structures and Phase Transformations of CO and NO on Rh(111) in the Torr Pressure Range Studied by Scanning Tunneling Microscopy K.S. Hwang, K.B. Rider (University of California, Berkeley); M. Salmeron, G.A. Somorjai (Lawrence Berkeley National Laboratory) Using scanning tunneling microscopy (STM) in a high-pressure reactor cell, we have studied, for the first time, the molecular structure and reaction of CO and NO on Rh(111) in the Torr pressure range. This is a model system for the automobile catalytic converter, where CO is oxidized to CO₂ and NO is reduced to N₂. Numerous reaction studies have been done in various temperature and pressure regimes,¹ but they generally do not yield direct information about molecular surface structure. Traditionally, molecular surface structure studies have been done at low temperature and pressure. These structures are kinetically frozen however, and may be different from high-temperature, high-pressure structures that are in equilibrium with the gas phase. At high coverage, CO forms a (2x2)-3CO structure on Rh(111) with one top-site and two hollow-site molecules in the unit cell.² This structure forms at 300 K from low pressure to at least atmospheric pressure. NO at 300 K forms an analogous structure below 0.03 Torr. Above 0.03 Torr we have discovered a new structure with a (3x3) unit cell. By directly observing the phase transformation between the two structures, we have found the heat of adsorption of the new structure to be 0.9 eV and an energy barrier between the two structures of 0.7 eV. When CO and NO are coadsorbed on Rh(111) at low partial pressures of NO, NO appears to mix randomly among the CO molecules. As the partial pressure of NO increases, the NO segregates into islands. The (3x3)-NO structure nucleates on these islands, though the presence of CO on the surface inhibits the phase transition until the NO partial pressure is three to five times that of CO. At temperatures above 300 K, we have seen evidence of the reaction between CO and NO occurring. ¹ V. P. Zhdanov and B. Kasemo, Surf. Sci. Rep. 29 (1997) 31. ² M. A. Van Hove, R. J. Koestner, and G. A. Somorjai, Phys. Rev. Lett. 50 (1983) 903.