Papers

AVS2001 Session AS-FrM: Catalysis and Surface Reactivity

Friday, November 2, 2001 8:20 AM in Room 134

Friday Morning

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8:20 AM		AS-FrM-1 Molecular Description of Surfaces of Transition Metal Oxide Catalysts J. Haber (Polish Academy of Sciences) One of the fundamental fields of research is oxidation catalysis by transition metal atoms and ions (TMA, TMI), located at gas/solid oxide or liquid/solid oxide interfaces, because of its importance in chemistry, biochemistry and modern technology. When TM oxide is dispersed on a second oxide phase playing the role of a support, a number of phenomena may take place, as revealed by UPS, XPS, AES, STM, IR and Raman spectroscopies. The TM oxide may be dispersed on the surface in form of isolated TM-oxygen polyhedra, their clusters, a monolayer or small crystallites, the TMI may diffuse into subsurface layer to form solid solution, or may form surface bidimensional compounds. The type of the process will depend on chemical properties of both oxides, their surface free energies, type and structure of the support, method and temperature of preparation. The resulting surface electronic structure determines the conditions of the exchange of electrons between the reacting molecules of the catalytic reaction and the catalyst and hence the catalytic activity. The surface of a catalyst for selective oxidation must be tailored to perform complex multistep operations on reacting molecules, hindering interactions leading to unwanted byproducts. Active sites must be generated which could accelerate following elementary steps: - activate the hydrocarbon molecule, which should be adsorbed in a proper way, by abstraction of hydrogen from the selected site of the molecule; - inject or remove electrons from the surface intermediates by providing high density of states at the energy level corresponding to the C-H redox potential; - perform nucleophilic addition of a surface oxide ion of high enough basicity, which could be easily extracted from the surface in the desorption step of the oxygenated intermediate, e.g. by crystallographic shear mechanism; - enable rapid diffusion of hydrogen and its desorption as water; - easily reoxidize by interaction with gas phase oxygen.
9:00 AM		AS-FrM-3 High-pressure XPS: A New Tool for the In-situ Investigation of Catalysts H. Bluhm, M. Haevecker, A. Knop-Gericke (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany); F. Requejo, D.F. Ogletree, C.S. Fadley, Z. Hussain, M. Salmeron (Lawrence Berkeley National Laboratory); R. Schloegl (Fritz-Haber-Institut der Max-Planck-Gesellschaft, Germany) Photoelectron spectroscopy has been an extremely powerful tool in surface science for decades. It has been extensively used for the ex-situ characterization of catalysts' surfaces. Due to the short mean free path of electrons in a gas phase electron spectroscopies generally must operate in high vacuum. Traditional electron spectrometers are therefore not suited for the much needed characterization of catalytic surfaces under reaction conditions. To overcome these limitations, we have developed a new high-pressure electron spectrometer. Our setup utilizes a differentially pumped electrostatic lens system that refocuses the photoelectrons that are emitted from the sample (which is in a gaseous atmosphere of up to several torr) into the focal plane of a standard electron energy analyzer situated downstream, in the high vacuum region. Using this instrument we have investigated the methanol oxidation over a copper catalyst. The correlation of in-situ XPS spectra of the copper surface and the simultaneously obtained mass spectrometer data (that show the efficiency of the catalytic reaction, i.e. the turnover of the gas phase) allow us to draw conclusions about the electronic state of the catalyst under reaction conditions for the first time.
9:20 AM		AS-FrM-4 XPS Analysis of Silica Supported Copper Catalysts for Fine Chemicals Synthesis K. Wilson, A. Stewie (University of York, UK); S.J. Hutton, A.J. Roberts (Kratos Analytical, UK) Tightening legislation has led to a drive to develop new cleaner heterogeneously catalysed processes for the speciality chemicals industries.^1,2,3 One major area of concern is the wide range of organic synthetic routes that currently rely on the use of inorganic or mineral acids. In recent years, metal trifluoromethanesulfonates or triflates have been reported as a new and interesting type of Lewis acid.⁴ These possess stronger Lewis acidity and higher water tolerances than their halide counterparts, which readily form the hydroxide or oxide.^5,6 Recently we have discovered that silica supported Cu(SO₂CF₃)_n is an effective heterogeneous catalyst for the use in organic synthesis. We find that while silica supported catalysts exhibit high activity in phenol alkylation, versions supported on alumina or amine functionalised silica are relatively inactive. The origin of the high activity of Cu(SO₂CF₃)₂/SiO₂ has been investigated by high resolution XPS analysis of a range of supported and unsupported Cu(II) and Cu(I) salts to determine the oxidation state of the active Cu species. The copper 2p spectrum of free Cu(SO₂CF₃)₂ exhibits a main peak at 936.6 eV which has associated strong satellite transitions. In contrast, silica supported Cu(SO₂CF₃)₂ exhibits a single transition at 933.8 eV without the presence of intense satellite structures. This suggests that electron transfer from SiO₂ could be important in determining catalytic activity. ¹ J.H. Clark, D.J. Macquarrie, Chem. Comm., 1998, 853. ² R. Sheldon, Chem. Ind., 1, 1997, 12. ³ W.F. Hölderich, Stud. Surf. Sci. & Catal. 1993, 75, 127. ⁴ S. Kobayashi, Chem. Lett., 1991, 2187. ⁵ S. Kobayashi, Synlett, 1994, 689. ⁶ K. Wilson, A. Renson, J.H. Clark, Cat. Lett., 1999, 61, 51.
9:40 AM		AS-FrM-5 Effect of Surface Reduction on Decomposition of Dimethyl Methylphosphonate on Tungsten Trioxide Surfaces S. Ma, R.H. Jackson, B.G. Frederick (University of Maine) The adsorption and decomposition of Dimethyl methylphosphonate (DMMP) on oxidized and reduced WO₃ surfaces have been studied with Calibrated Thermal Desorption Spectroscopy (CTDS) and X-ray Photoelectron Spectroscopy (XPS). XPS was used to characterize the surface by monitoring the valence band and W4f regions. The reduced surface, produced by Ar+ sputtering, contained a mixture of W⁶⁺ and W⁵⁺. Treatment of the surface in O2 at high temperature resulted in an oxidized surface. On the oxidized surface, desorption of methanol and dimethylether was detected, but only dimethylether was observed on the reduced surface. These products imply that the P-OCH₃ bond is broken to produce the methoxy intermediate during decomposition of DMMP on reduced and oxidized WO₃ surfaces. This mechanism is similar to the decomposition of DMMP on MgO and Al₂O₃ oxide surfaces.^1,2 No phosphorus-containing compound was observed in CTDS. After CTDS experiments, XPS shows that phosphorus was left on the surface. The adsorption and desorption of methanol and dimethylether on the reduced and oxidized surfaces were compared with the production of these species from the DMMP decomposition. ¹ M.K.Templeton and W.H.Weinberg, J. Am. Chem. Soc., 107 (1985) 97. ² Mark B.Mitchell, V.N.Sheinker and Eric A.Mintz, J. Phys. Chem. B, 101 (1997) 11192 .
10:20 AM		AS-FrM-7 Auger Parameter Studies in Alloys of Transition Metals with Al or Mg M.-L. Abel, P. Tsakiropoulos, J.F. Watts (University of Surrey, UK); J.A.D. Matthew (University of York, UK) Currently, alloys are designed either empirically or in a systematic and cost-effective way, which makes use of experimental and calculated phase equilibria (CALPHAD) data to select alloy composition with the desirable microstructures. The data obtained from this type of calculation are based on first principle calculations (FPC), but although phase diagrams predicted by FPC are topographically correct, they lack sufficient accuracy for practical applications. It is, therefore highly desirable to develop an experimental approach to the study of alloying behaviour. To produce alloys, a metallurgist usually considers the importance of such factors as the electronegativity difference or the atomic size mismatch in controlling the stability of metallic phases. The nature of alloying behaviour can be studied experimentally using the transfer of charge upon alloying. Quantification of charge transfer can be achieved using the Auger parameter and its variation over a wide composition range. This provides means of testing some of the predictions of FPC. In this work, transition metals have been selected because they form the basis of the constitution of conventional and development alloys. The aim of this research is to study hybridisation phenomena and the charge transfer between solvent and solute atoms in alloys. Changes of the Auger parameter determined from X-ray photoelectron experiments, is used as a tool to probe the electronic changes that take place as a result of alloying. The detection of the relevant Auger entries and core level energies is performed using the ESCA 300 in Daresbury (UK) which is fitted with a CrKb source. Core levels such as Si1s or Ti1s are then accessible and allow calculation of the Auger parameter of choice.
10:40 AM		AS-FrM-8 Experimental and Theoretical Study of the Chemical Effects in the Auger Spectra of Aluminium, Copper and Carbon Compounds B. Timmermans, N. Vaeck, F. Reniers (Universite Libre de Bruxelles, Belgium) The bonding and electronic structure information of surface atoms is most often studied by XPS, where the energy shift of a photoelectron peak can be ascribed to a change in the chemical environment. Because three atomic levels are involved in the Auger emission process, the use of Auger spectroscopy to obtain this information from peaks positions and line shape is more difficult. However, new progresses in signal analysis techniques make the extraction of most chemical information intrinsically present in the Auger spectra possible. The advantage of AES over XPS is its higher lateral resolution allowing the study of the surface distribution of the elements (Auger mapping), which, combined with modern computational methods, opens new opportunities in surface analysis. In order to try to correlate the changes in the Auger peaks with the changes in the chemical environment, we performed a full experimental and theoretical study of the Auger lines of aluminium, carbon and copper in several compounds. The changes in the peak energies and shapes were recorded in model spectra. Deconvolution of the peaks was achieved by factor analysis. The experimental results are compared to ab initio calculations based on a new cluster model including chemical, structural and relaxation effects. Al and C compounds, specifically, present important line shape modifications of valence Auger peaks that can be compared with calculations based on the knowledge of valence densities of states. These were obtained experimentally from XPS and theoretically from a full potential linearized augmented plane wave method within DFT. Further improvements came from accounting the final state hole-hole interaction by the well-known Cini-Sawatzky model.
11:00 AM		AS-FrM-9 Investigation of Surface Oxides on Aluminum Alloys by Valence Band Photoemission G. Claycomb, P.M.A. Sherwood (Kansas State University) This paper will address how core level and valence band X-ray photoelectron spectroscopy can be used to study the chemical composition of the surface films on aluminum alloys. Certain alloying elements may preferentially migrate to the surface, thereby altering the composition and consequently the chemistry of the surface. Magnesium and silicon are known to preferentially migrate to the surface of the 6061 aluminum alloy but the chemical nature of the films formed on the surface is not well understood. The surface films can be identified by core level and valence band X-ray photoelectron spectroscopy. It will be shown that the type of magnesium film formed at the surface can be determined by comparing the valence band spectra of the aluminum alloy surface with that of the model compounds, magnesium oxide, magnesium hydroxide and the magnesium aluminum oxide. The effect of boiling water on the surface film will be discussed. It will also be seen that valence band photoemisson spectra, interpreted by band structure calculations, is a very effective tool for the conclusive identification of the surface species present. This material is based upon work supported by Luxfer Gas Cylinder.
11:20 AM		AS-FrM-10 Use of Core- and Valence-Band XPS to Measure the Differences Among Similar Oxide and Mineral Phases F.S. Ohuchi, G. Ghose (University of Washington); M.H. Engelhard, D.R. Baer (Pacific Northwest National Laboratory) The three polymorphs of Al₂SiO₅, sillimanite, andalusite, and kyanite, are geologically important minerals. The differences in their stability relations depend critically on the differences in the chemical bonding of each polymorph of Al₂SiO₅, in which half of the Al-coordinations are 4-fold (tetrahedral) in sillimanite (stable at low P and high T), 5-fold (trigonal bipyramidal) in andalusite (stable at low P and high T), and 6-fold (octahedral) in kyanite (stable at low P and high T), whereas the other half of the Al atoms are in 6-fold (octahedral) and Si in 4-fold (tetrahedral) coordination in all three polymorphs. A challenge is whether we are able to measure such small differences using XPS technique, since bonding differences and electronic structures are fundamental to our understanding of their thermodynamical properties and stability relations. We have therefore undertaken an investigation using a state-of-the-art XPS system by measuring Al(2p), Si(2p), O(1s) core level spectra as well as valence band spectra. Introducing a novel charge neutralization technique, clear differences in each spectrum from three different polymorphs were identified, and the change in the valence band spectra were compared with ab-initio density of state calculations. This technique was further applied to other minerals, such as magnesium-silicates.