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ScienceWeek
MATERIALS SCIENCE: OXYGEN MOLECULES AND METAL OXIDE SURFACES
The following points are made by E. Wahlström et al (Science 2004 303:511):
1) Transition metal oxides and metal-to-oxide interfaces have recently received considerable attention from both fundamental and technological perspectives because of their applications in heterogeneous catalysis (1-5), solar cells, photocatalysis, and biocompatible implants. Reduced metal oxides can be formed through the thermal creation of O vacancies and metal interstitials in originally stoichiometric metal oxides. These defects act as donors of electrons and have a profound influence on the chemical and electrical behavior of transition metal oxides.
2) The authors focus on the rutile TiO2(110) surface, the prototypical transition metal oxide model system for surface science studies. When this oxide is reduced, for example, by annealing in vacuum, O vacancies are formed, which at the surface take the form of missing O atoms in the bridging O rows at the surface. The surface O vacancies constitute the most abundant surface donor that traps electrons ~0.75 eV below the conduction band and act as adsorption sites for simple molecules such as CO and O2. Ti interstitials, which are also formed, are mainly present in the bulk where they control the bulk electronic properties of the reduced material at low temperatures.
3) When O2 molecules are adsorbed on the TiO2(110) surface at low temperatures, scanning tunneling microscopy (STM) images reveal that O2 molecules reside on top of the Ti atoms that constitute the troughs along the [001] direction in between the protruding O rows. The fast-scanning Aarhus STM allows one to follow the dynamics of individual O2 molecules on the TiO2(110) surface. From time-lapsed sequential STM images visualized in the form of STM movies, the authors observed diffusion of O2 molecules only along the [001] direction.
4) In summary: Diffusion of oxygen molecules on transition metal oxide surfaces plays a vital role in the understanding of catalysis and photocatalysis on these materials. By means of time-resolved scanning tunneling microscopy, the authors provide evidence for a charge transfer-induced diffusion mechanism for O2 molecules adsorbed on a rutile TiO2(110) surface. The O2 hopping rate depended on the number of surface donors (oxygen vacancies), which determines the density of conduction band electrons. The authors suggest these results may have implications for the understanding of oxidation processes on metal oxides in general.
References (abridged):
1. M. Haruta, CAT TECH 6, 102 (2002)
2. C. R. Henry, Surf. Sci. Rep. 31, 235 (1998)
3. M. Valden, X. Lai, D. W. Goodman, Science 281, 1647 (1998)
4. A. Sanches et al., J. Phys. Chem. A 103, 9573 (1999)
5. C. T. Campbell, S. C. Parker, D. E. Starr, Science 298, 811 (2002)
Science http://www.sciencemag.org
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ON HETEROGENEOUS CATALYSIS AND SURFACE SCIENCE
The following points are made by G. Ertl and H-J. Freund (Physics Today 1999 January):
1) The economic significance of heterogeneous catalysis is reflected in the fact that the world market for solid catalysts in the automotive, petroleum, and other industries is of the order of US$100 billion per year and growing rapidly.
2) In heterogeneous catalysis, the chemical transformation typically occurs in a flow reactor through which the reacting species pass. Atoms in the surface of the catalyst may form chemical bonds with atoms in impinging molecules, a phenomenon known as "chemisorption". If existing bonds in the impinging molecule break, the process is known as "dissociative chemisorption". The chemisorbed species are mobile on the surface and may bond to other particles, thus leading to new molecules, which eventually leave the surface (desorb) as the desired reaction products.
3) Detailed identification and characterization of these elementary processes of heterogeneous catalysis are hampered by several fundamental problems: a) The reacting systems exist merely as 2-dimensional phases for which most of the usual methods of investigation are not well suited. b) The surfaces of real catalysts are typically inhomogeneous as a result of methods to increase catalytic efficiency. For example, because in heterogeneous catalysis efficiency, in general, increases with total surface area of the solid catalyst, finely divided particles are usually applied to a support material which is only relatively inert. Also, catalytic activity is often further enhanced by the addition of compounds called "promoters". At the present time, analysis of the fundamentals of heterogeneous catalysis is largely dependent on the use of surface science models, real but simple systems such as single crystal surfaces whose structure may be varied by choosing different surface orientations.
Physics Today http://www.physicstoday.org
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CATALYSIS ON OXIDE SURFACES
The following points are made by Helmut Knoezinger (Science 2000 287:1407):
1) Free coordination sites play a major role in catalysis by metal complexes, because reactants may bind to these sites and become activated for catalytic conversion (1). Similar considerations apply to the surfaces of solid catalysts in general and of metal oxides in particular, because the surface atoms are characterized by a "ligand" sphere that differs from that in the bulk. The surface atoms generally have lower coordination numbers than those characteristic for the bulk (2). Their coordination sphere may be completed by adsorbed molecules, and these may be activated for catalytic transformations, in close analogy to processes occurring on metal complexes.
2) The coordination numbers of surface atoms in real catalysts may vary over wide ranges because different crystallographic faces, edges, steps, point defects, and dislocations may be exposed, resulting in an often substantial energetic heterogeneity (4). Oxide surfaces typically expose coordinatively unsaturated site (CUS) cations and CUS oxygen anions, and chemisorption --adsorption involving chemical bond formation --frequently involves both simultaneously. For a specific catalytic transformation, certain geometric and energetic requirements must be fulfilled, so that frequently only a small percentage of all surface atoms may act as active sites.
3) The CUS surface sites on real, high-surface area catalysts can only be characterized indirectly, using probe molecules that can fill the free coordination sites. For example, the carbonyl infrared spectra of CO adsorbed on microcrystalline a-Cr2O3 (5) and on epitaxially grown chromium oxide films show a complex pattern of bands of carbonyl surface complexes characterizing the heterogeneity of chromium sites.
References (abridged):
1. B. C. Gates, Catalytic Chemistry (Wiley, New York, 1992)
2. R. L. Burwell Jr., G. L. Haller, K. C. Taylor, J. F. Read, Adv. Catal. 20, 1 (1969)
3. H. Over et al., Science 287, 1474 (2000)
4. H. S. Taylor, Proc. R. Soc. London Ser. A 108, 105 (1925)
5. D. Scarano, A. Zecchina, A. Reller, Surf. Sci. 198, 11 (1988)
Science http://www.sciencemag.org
ScienceWeek http://scienceweek.com
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