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ScienceWeek
CHEMISTRY: ON MOLECULAR DYNAMICS IN HETEROGENEOUS CATALYSIS
The following points are made by H. Ueba and M. Wolf (Science 2005 310:1774):
1) Heterogeneous catalysis is a process by which chemical reactions are fostered on the surfaces of small solid particles. Although many products used in daily life are based on the products of such catalytic processes, our understanding of the underlying elementary steps and reaction dynamics is still far from complete. One of the major goals in surface chemistry, therefore, is to obtain microscopic insight into the dynamics of making and breaking chemical bonds during surface reactions. The challenge is to observe molecular motions and energy redistributions with a time and spatial resolution sufficient to capture the key details. Whereas studies with femtosecond resolution [1] and optimum control of chemical reactions [2] in the gas phase are well established, a comparable level of sophistication is lacking in the analysis of surface reactions. However, new work [3] presents a direct analysis of the time evolution of the most elementary reaction taking place on a surface: the lateral motion of a molecule (carbon monoxide) on a platinum surface.
2) Adsorbate motion on a surface constitutes the most fundamental step for many surface chemical reactions, because it is the primary way for adsorbates to meet a reaction partner or to reach an active site before reaction takes place. As molecules adsorb on specific sites (e.g., on top of surface atoms of the substrate), they move by site-to-site hopping if they can overcome the energetic barrier. Heating the surface is the simplest way to provide the adsorbate with the necessary energy to diffuse. In such a thermally activated process, however, the energy is evenly distributed over all electronic and vibrational degrees of freedom of the adsorbate/surface system. This makes it difficult to identify the elementary reaction steps.
3) This drawback can be circumvented with the use of femtosecond laser pulses to drive surface diffusion, as demonstrated recently by two independent groups [4,5]. The essence of this approach is that with ultrafast lasers heating and cooling occur so rapidly that the different degrees of freedom are not equilibrated, allowing one to distinguish their contribution to the reaction. In previous work, the adsorbate motion was not followed in real time, but now Backus et al [3] present a real-time study of the lateral motion of molecules on a metal surface. The authors use a pump-probe scheme: A femtosecond pump pulse induces the motion of CO over the surface, and the motion is followed in real time with variably delayed probe pulses. The probe consists of femtosecond surface vibrational spectroscopy to look "inside" the CO molecules at the C-O stretch vibration, as these are excited and displaced as a result of femtosecond laser excitation. The selection of a nanostructured stepped platinum surface, and the fact that the internal CO stretch frequency depends on the precise location of the CO molecule on this type of surface, enable simultaneous high temporal and spatial resolution.
References (abridged):
1. A. Zewail, in Nobel Lectures in Chemistry 1996-2000, I. Grenthe, Ed. (World Scientific, Singapore, 2003)
2. H. Rabitz, R. de Vivie-Riedle, M. Motzkus, K. Kompa, Science 288, [824] (2000).
3. E. H. G. Backus, A. Eichler, A. W. Kleyn, M. Bonn, Science 310, 1790 (2005)
4. L. Bartels, F. Wang. D. Moller, E. Knoesel, T. Heinz, Science 305, 648 (2004). [648]
5. K. Stépán, J. Güdde, U. Höfer, Phys. Rev. Lett. 94, 236103 (2005)
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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CHEMISTRY: ON HETEROGENEOUS CATALYSIS
The following points are made by Gabor A. Somorjai (Nature 2004 430:730):
1) Catalysts have the remarkable property of facilitating a chemical reaction repeatedly without being consumed. In principle (if not in practice), a catalyst can function as long as reactants are available. Enzymes -- which catalyse all aspects of cell metabolism -- are the supreme masters of this art. We have used them from earliest times for leavening bread, curdling cheese and brewing beer.
2) In the 20th century, a seemingly very different type of catalysis -- heterogeneous catalysis, which uses small metal particles supported on solid surfaces -- became the foundation for much of the chemical industry. It plays a central role in generating the feedstocks for making the synthetic materials that we use every day, from fuels to fertilizers. New experimental techniques have brought fresh insights into this form of catalysis, and it now seems that there are more similarities between enzymes and heterogeneous catalysts than initially meets the eye.
3) Heterogeneous catalysis usually involves the interaction between a reactant and the catalytic surface of small metal particles. This in turn activates chemical bonds within the reactant, resulting in their dissociation or rearrangement. For decades, studies of heterogeneous catalysts were largely empirical, involving little understanding at the molecular level. Theories of how they function emphasized the need for complementarity between reactant structure and the geometrical arrangement of atoms in the static catalyst surface. Now, more advanced methods of observation have allowed us to see these molecules during reactions in considerably more detail, and have revealed that this static view of catalysis is, in fact, wrong.
4) The advent of ultrahigh-vacuum surface science in the 1960s provided an entry into the world of adsorbed molecules. However, it is only recently that modern techniques such as high-pressure scanning tunnelling microscopy have allowed experiments to be done under realistic (high pressure and temperature) conditions. Exposure to reactants results in local restructuring of the catalytic surface. The formation of bonds between the metal atoms of the catalyst and the adsorbed molecules produces heat, providing energy to loosen the bonds between the metal atom and its neighbors. This allows it to move from its original position and thus strengthens the chemical bond between the adsorbed molecule and the metal. The fewer neighbors a surface metal atom has, and the stronger the bonding to the adsorbed molecules, the greater is the degree of restructuring that occurs. Also, it is not always the original reactant that bonds with the metal catalyst; it can be a derivative if the metal-adsorbate complex can form stronger bonds this way.
5) The formation of strong chemical bonds at these active metal sites intuitively suggests that such sites will be blocked and prevented from participating in new reactions. But in fact the very same sites that are most active in bond activation are also catalytically the most active, and remain so. They can carry out bond activation repeatedly. The reason for the continued catalytic activity is that it is not only the metal atoms that are on the move. Strongly adsorbed reactants on the metal surface are also mobile. In a model reaction (the hydrogenation of ethene to form ethane) used to understand the industrially important hydrogenation process, an important derivative species, ethylidyne, retains the ability to move over the surface despite being fairly strongly bound. The ethylidyne is not part of the catalytic process, but remodels active sites and then moves on, allowing ethene, which is adsorbed only weakly, to bind briefly and become rapidly hydrogenated.(1-3)
References:
1. Somorjai, G.A. Introduction to Surface Chemistry and Catalysis (Wiley, New York,1994)
2. Thomas, J.M. & Thomas, W.J. Principles and Practice of Heterogeneous Catalysis (Wiley-VCH, Berlin, 1997)
3. Tsou, C.-L. Science 262, 380 - 381 (1993)
Nature http://www.nature.com/nature
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