Papers

AVS1997 Session SS1-WeA: Reactions on Model Catalysts

Wednesday, October 22, 1997 2:00 PM in Room A3/4

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		SS1-WeA-1 Synthesis of Ammonia over a Ruthenium Single Crystal S. Dahl (Technical University of Denmark); E. Törnqvist, P.A. Taylor (Haldor Topsoe Research Laboratories, Denmark); I. Chorkendorff (Technical University of Denmark) Ruthenium based catalysts for ammonia synthesis have received considerable interest for a long time¹ and recently it has been shown, that it is possible to make a catalyst that shows promising activity and stability.² In this work, the synthesis of ammonia over a Ru(0001) single crystal is studied in a combined high pressure and ultrahigh vacuum system. The ammonia production rate from a stoichiometric (N₂:3H₂) gas mixture at a pressure of approximately 2 bar, is measured in the temperature range 623-873 K by leaking gas from the reactor through a quarts orifice to a mass spectrometer. The rate is measured by two methods. The first method is under constant gas flow, where the crystal is heated to the desired temperature and the ammonia concentration is measured after steady state is reached. By the other method the reactor is operated in batch mode and the concentration of ammonia is measured as a function of time at the desired crystal temperature. At temperatures up to 773 K the latter method gives rise to an initial linear increase in the ammonia concentration. At higher temperatures the increase in concentration is not linear because the gas composition is getting close to the equilibrium for the reaction. From the high temperature measurements the reaction order with respect to the ammonia concentration is found to be -1. The activation energy obtained by the two methods are in excellent agreement and the overall it is found to be 101 ± 4 kJ/mol. After synthesis, the primary adsorbed species is nitrogen and there is evidence of the presence of hydrogen in the bulk of the crystal. ¹Tennison S.R., in: Catalytic Ammonia Synthesis, 1st ed., Jennings J.R. (Plenum Press, NewYork, 1991) p. 303. ²Rosowski F., Hornung A., Hinrichsen O., Herein D., Muhler M. and Ertl G. Appl. Catal. A 151, 443 (1997).
2:20 PM		SS1-WeA-2 The Catalytic Chemistry of Small Hydrocarbons on Palladium and Oxygen-Modified Molybdenum. W.T. Tysoe (University of Wisconsin, Milwaukee) The reaction pathways for acetylene trimerization catalyzed by palladium and olefin metathesis catalyzed by oxygen-modified molybdenum are investigated using a range of surface sensitive techniques. It is found that benzene is formed from acetylene via the intervention of a metallacyclic C₄H₄ intermediate. At high temperatures (above 650 K) olefin metathesis is proposed to proceed via the direct dissociation of carbon-carbon bonds forming alkylidenes which then react associatively to form metathesis products. In both cases, the catalytic reaction proceeds in the presence of a carbonaceous layer. In the case of acetylene cyclotrimerization, this consists of vinylidene and for olefin metathesis a relatively thick carbonaceous layer that has both hydrocarbon and graphitic portions. It is shown that the hydrocarbon part of this layer can be removed by reaction in high pressures (several Torr) of hydrogen. The activation energy for this process on molybdenum surfaces is about 6 kcal/mol. It is also demonstrated that the equilibrium thickness of the carbonaceous layer is lower when, for example, metathesis is carried out in the presence of hydrogen. Correspondingly, the rate of metathesis and cyclotrimerization are found to be accelerated by the addition of hydrogen to the reaction mixture. The implication of these phenomena on alkene and alkyne hydrogenation reactions catalyzed by transition metals will also be discussed.
2:40 PM		SS1-WeA-3 Mechanism and Kinetics of Methanol Synthesis over a Zn-promoted Cu(111) Surface J. Nakamura, I. Nakamura, T. Uchijima (University of Tsukuba, Japan); T. Fujitani (National Institute for Resources and Environment, Japan) We have studied the mechanism and kinetics of methanol synthesis by the hydrogenation of CO₂ over a Zn-promoted Cu surface using XPS, STM, and IRAS apparatuses each equipped with a high pressure reactor ( 1 - 18 atm). Previous kinetic studies at 18 atm showed that Zn evaporatively deposited on Cu(111) promoted methanol formation by a factor of 13, and the turnover frequencies (TOF) and the activation energy were in agreement with those for Cu/ZnO powder catalysts. Thus, the Zn-deposited Cu(111) was regarded as a good model of Cu/ZnO catalysts. We have then focused the role of Zn in the reaction mechanism and the active sites created upon the deposition of Zn. Post-reaction surface analyses by XPS showed that the oxidation states of Zn and Cu during the reaction were in metallic, and the Zn was bound to a formate intermediate. The coverage of the formate intermediate was proportional to that of Zn, suggesting that the role of Zn was to stabilize the formate and promoted the hydrogenation of formate. STM images of Zn-deposited Cu(111) showed that Zn atoms were substituted with Cu atoms, leading to a Cu-Zn surface alloy. Thus, the surface alloy is the active surface for methanol synthesis. To quantitatively elucidate the role of Zn, kinetics of formation, decomposition, and hydrogenation of formate were measured on Zn-free and Zn-deposited Cu(111) surfaces using the XPS and IRAS apparatuses. It was clearly shown that formate species formed on bare Cu surfaces and migrated to the Zn atoms, where formate was hydrogenated to methoxy species. That is, the active Cu-Zn surface alloy has bifunctional roles to catalyze methanol synthesis. We then constructed a kinetic picture of methanol synthesis over the Zn/Cu(111) surface to express the promotional effect of Zn generally observed in Cu/ZnO catalysts.
3:00 PM		SS1-WeA-4 CO Oxidation Over Model Au/TiO₂ Catalysts S. Pak, H. Wang, X. Lai, D.W. Goodman (Texas A&M University) Model Au/TiO₂ catalysts were prepared under ultrahigh vacuum (UHV) conditions with Au loadings varying from 0.25 to 5 equivalent monolayers (ML_eq.) supported on thin TiO₂ films. Catalyst preparation and composition were monitored with Auger electron spectroscopy (AES). Reactant and product adsorption as well as catalyst morphology were studied with infrared reflection absorption spectroscopy (IRAS). Reaction kinetics was carried out at 40 Torr total pressure in an elevated reactor contiguous to the surface analysis chamber. Freshly synthesized Au/TiO₂ catalysts were active for CO oxidation at 300K, and exhibited reproducible behaviour at temperatures greater than 350K. The apparent activation energy for the reaction between 350-450K varied from 1.7 kcal/mol for the clean TiO₂ film to ca. 5 kcal/mol for 1.5-5 ML_eq. Au. The specific rates of reaction (product molecules/site/sec) were dependent on the Au loading with a maximum occuring at 0.5-1 ML_eq. Au suggesting that CO oxidaion over Au/TiO₂ is structure sensitive. Scanning tunneling microscopy (STM) and IRAS results show that annealed Au clusters on TiO₂ undergo significant morphological changes upon adsorption of CO.
3:20 PM		SS1-WeA-5 Realistic Surface Science Models of Industrial Catalysts J.W. Niemantsverdriet (Eindhoven University of Technology, The Netherlands) The last decades have seen a tremendous evolution in surface sensitive methods which provide atomic level information on structure, composition, oxidation state, some even combined with high spatial and temporal resolution. However, catalysts usually consist of small particles hidden inside the pores of a support, which, moreover, is usually an electrically insulating oxide. This implies that many of the tools in surface science can not be applied. Thus, one has to resort to model systems in order to do surface science on catalysts. The challenge in devising model systems is to develop techniques for the preparation of supported particles with sufficient definition, such that they are not only suitable for the study of fundamental issues (structure - activity relationships, particle size effects) but also realistic enough that practical questions from the field of industrial catalysis can be addressed ¹. In this respect, the preferred method of preparation is to apply the catalytic precursor materials via wet-chemical impregnation techniques on thin film oxide supports ². Measuring catalytic activities and selectivities provides the ultimate test for the validity of such a model system. Strategies to achieve this will be discussed and illustrated with examples from our work on model catalysts for hydrodesulfurization ³, exhaust treatment and ethylene polymerization. ¹P.L.J. Gunter, J.W. Niemantsverdriet, F.H. Ribeiro and G.A. Somorjai, Catal. Rev. - Sci. Eng.,38 (1997) 77. ²R.M. van Hardeveld, P.L.J. Gunter, L.J. van IJzendoorn, W. Wieldraaijer, E.W. Kuipers and J.W. Niemantsverdriet, Appl. Surface Sci. 84 (1995) 339. ³A.M. de Jong, V.H.J. de Beer, J.A.R. van Veen and J.W. Niemantsverdriet, J. Phys. Chem. 100 (1996) 17722.
4:00 PM		SS1-WeA-7 Nanofabrication of Model Catalysts S. Johansson, K. Wong, V.P. Zhdanov, B. Kasemo (Chalmers University of Technology, Sweden) Electron beam lithography offers new opportunities to manufacture nearly ideal 2D arrays of catalyst particles in the nm regime, with controllable shapes, sizes, interparticle distances, and support material etc., of the catalyst particles. The present work examplifies the manufacturing of such model catalysts, with the specific examples taken from Pt particles down to 10 nm size, deposited on different oxide supports. Pt-particles can be reshaped and crystallized through catalytic treatment, in a way that resembles a selforganizing system. The experimental opportunities and demands to study the catalytic activity of the manufactured structures are considered, e. g. the necessity to design proper microreactors. Finally the modelling of the kinetics of such nanostructures is discussed( with specific examples) in relation to the so called pressure and structure gaps in catalysis. It is shown that in some cases the resulting kinetics is unique for the nano-scale particles, and cannot be predicted from the kinetics of macroscopic single crystals.
4:20 PM		SS1-WeA-8 Preparation of Supported Nanoparticles on Thin Oxide Substrates. A.M. De Jong, A.F.P. Engelen (Eindhoven University of Technology, The Netherlands); G.J. Kramer, W. Wieldraaijer (Shell Research and Technology Centre Amsterdam, The Netherlands); J.W. Niemantsverdriet (Eindhoven University of Technology, The Netherlands) The preparation of supported particles with nanometer dimensions is of interest for a number of disciplines, among which heterogeneous catalysis. Nanoscale particles supported on low surface area oxide materials are used in fundamental studies in catalysis ¹ and in short contact time catalysis. Those particles can be applied to flat supports by spincoating a solution containing a precursor onto the support ². In this constribution we illustrate this approach with the preparation of nanosize rhodium particles on thin film silica supports. An aqueous solution of rhodium chloride is applied on 3 nm thick silica films by spincoating. The particle size and the number of particles can be controlled by varying the concentration of the rhodium chloride and the rotation speed. The loading of the rhodium can accurately be predicted from the rotation speed and the evaporation time of the film of solution, and is independently determined by Rutherford Backscattering Spectrometry. Atomic Force Microscopy provides the number density of particles present on the support and the size distribution as reflected by the height of the rhodium particles. Increasing the rotation speed during impregnation leads to a decrease in particle size. Angle Dependent X-ray photoelectron spectroscopy is used to monitor the effect of reduction and thermal treatment on the size and morphology of the rhodium particles. The results indicate that careful control of solvent evaporation time during spincoating impregnation provides a means to control the size of small metal particles on flat substrates that are of interest as models for supported catalysts. ¹P.L.J. Gunter, J.W. Niemantsverdriet, F. Ribeiro, G.A. Somorjai, Cat. Rev. Sci. Eng. 38, 79 (1997). ²R.M. van Hardeveld, P.L.J. Gunter, L.J. van IJzendoorn, W. Wieldraaijer, E.W. Kuipers and J.W. Niemantsverdriet, Appl. Surf. Sci. 84, 339 (1995).
4:40 PM		SS1-WeA-9 Reaction-induced Surface and Structural Changes in Perovskite Catalysts for Oxidative Dehydrogenation of Alkanes A.G. Sault, N.B. Jackson, J.E. Miller, T. Nenoff (Sandia National Laboratories) The catalyzed reaction of alkanes with oxygen to form water and alkenes is known as oxidative dehydrogenation (OD). This reaction offers a potential advantage over conventional dehydrogenation in that water is a product rather than hydrogen, and the overall reaction is exothermic. As a result, OD can theoretically achieve high conversions at lower temperatures than conventional dehydrogenation. An important challenge in the development of OD catalysts is achieving adequate selectivity since total combustion is far more favorable than OD. Catalysts for this reaction generally consist of mixed metal oxides. Since lattice oxygen is known to participate in the reaction, catalysts are sought with surface oxygen atoms that are labile enough to perform dehydrogenation, but not so plentiful as to promote complete combustion. Also, catalysts must be able to replenish surface oxygen by transport from the bulk. Perovskite based catalysts appear to fulfill these requirements. We are studying BaCeO₃ perovskites doped with elements such as Y, La, Nd, Ca, Sr, and Mg. During OD of ethane, the perovskite structure is not retained in any of the materials yet analyzed, and a mixture of carbonates and oxides is formed, as revealed by XRD. XPS shows strong enhancement of the Ba signal at the expense of Ce after reaction. Dopants effect this enhancement, with Y causing a depletion of Ce before reaction, while for BaCeO₃ and the Ca doped material, Ce levels only decrease after reaction. A strong enhancement of the oxygen signal is also observed after reaction suggesting the formation of BaO₂ at the surface. While both the Ca and Y doped materials showed enhanced total combustion activity below 873 K, only the Ca doped material displays enhanced ethylene production at 973 K. Structural and surface changes are being correlated with activity in order to understand the factors affecting catalyst performance, and to modify catalyst formulations to improve conversion and selectivity.
5:00 PM		SS1-WeA-10 Synthesis and Characterization of Nickel Sulfide Catalysts A. Olivas (CICESE, Mexico); J. Cruz-Reyes (UABC, Mexico); M. Avalos (Instituto de Fisica-UNAM, Mexico); V. Petranovskii (CICESE, Mexico); S. Fuentes (Instituto de Fisica-UNAM, Mexico) Nickel sulfide catalysts have been traditionally used in the oil processing industry. However, there are a few applications in other areas, like fine chemistry. These catalysts are often used in hydrogenation reactions. In this work, various nickel sulfide catalysts obtained by homogeneous sulfide precipitation were prepared by varying the homogenizing time and sulfiding temperature. Structural characterization studies by transmission electron microscopy (TEM) and X-ray diffraction (XRD) show the formation of several metastable crystalline phases [NiS (millerite), NiS_1.03, Ni₃S₂, Ni₇S₆, Ni₉S₈] depending on the preparation conditions. The influence of such phases on the catalytic behavior of these samples as compared to the stable Ni₃S₂ phase will be discussed. The change in morphology and surface area of the catalysts with preparation conditions are also studied by scanning electron microscopy (SEM) and Brunauer-Emmett-Teller method (BET).