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CHEMISTRY: ON BOND-SELECTIVE CATALYSIS

The following points are made by A.C. Luntz (Science 2003 302:70):

1) When an atom and a molecule encounter each other in the gas phase, vibrational excitation of the molecule along the reaction coordinate generally increases the reactivity substantially. When the reacting molecule has several different chemical bonds, vibrational excitation of a specific bond can be used to control the reaction outcome (1,2).

2) There are two principal requirements for bond-selective chemistry: a means to excite a specific bond (or local mode), and localization of the energy in this bond until reaction occurs. The first condition is usually met by laser excitation of a local mode. The second requires that the internal vibrational redistribution rate is slow relative to the reaction rate.

3) Both conditions can often be met for fast direct gas-phase reactions (1), but it has been unclear whether the second condition can be satisfied for the activated dissociation of molecules at a gas-surface interface. These reactions are the rate-limiting steps in many technologically (and economically) important heterogeneous catalytic processes. Beck et al. (2) have unequivocally answered this question in the affirmative by measuring the activated dissociation of methane (CH4) prepared in different vibrational modes on a Ni(100) surface.

4) Surface scientists have tried for decades to understand the activated dissociation of CH4 on transition metal surfaces. The reaction has attracted so much interest because CH4 dissociative adsorption on a supported Ni catalyst limits the rate of the industrially important steam reforming process [which converts natural gas to a mixture of CO and H2 ("syn gas")]. A detailed microscopic understanding of this step may lead to a better catalyst. However, the activated dissociation of CH4 on a metal surface is such a complex multidimensional dynamical process that the microscopic description of this process remains controversial (3).

5) There are two main competing views of this process. One is that activated dissociation is a direct dynamical process, much like that in a direct gas-phase bimolecular reaction. The challenge then is to reduce the dimensionality of the dynamical treatment to the essential features (3). The other view is that activated dissociation is a purely statistical process (4). This is also a well-known mechanism for gas-phase reactions, but only when long-lived collision complexes are formed so that the energy is randomized in the collision complex before reaction. The statistical picture greatly simplifies the description of the reaction dynamics. Both experiment and theory provide evidence that the CH4-surface collision complex is short-lived (approximately 10^(-13) s), and the justification for such a mechanism is therefore based on the high dimensionality of the CH4-surface collision complex. In this case, it is suggested that the high density of states makes the internal vibrational redistribution fast compared to the short collision lifetime.(5)

References (abridged):

1. F. F. Crim, Acc. Chem. Res. 32, 877 (1999)

2. R. D. Beck et al., Science 302, 98 (2003)

3. A. C. Luntz, J. Harris, Surf. Sci. 258, 397 (1991)

4. V. A. Ukraintsev, I. Harrison, J. Chem. Phys. 101, 1564 (1994)

5. P. M. Holmblad, J. Wambach, I. Chorkendorff, J. Chem. Phys. 102, 8255 (1995)

Science http://www.sciencemag.org

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ON CATALYSIS AND WILHELM OSTWALD

In the final quarter of the nineteenth century, Germany was leading the world in the study of the physical changes associated with chemical reactions. The outstanding worker in this field of physical chemistry was the Russian-German chemist Friedrich Wilhelm Ostwald (1853-1932). It was thanks to him, more than to any other individual, that physical chemistry came to be recognized as a discipline in its own right. By 1887, he had written the first textbook on the subject and founded the first journal to be devoted exclusively to the field.

Fittingly enough, Ostwald was among the first Europeans to discover and appreciate the work of Josiah Willard Gibbs (1839-1903). Ostwald translated Gibbs's papers on chemical thermodynamics into German in 1892. Ostwald proceeded to put Gibbs's theories to use almost at once in connection with the phenomenon of catalysis.

Catalysis (a word suggested in 1835 by J.J. Berzelius [1779-1848]) is a process whereby the rate of a particular chemical reaction is hastened, sometimes enormously so, by the presence of small quantities of a substance which does not itself seem to take part in the reaction. Thus, powdered platinum will catalyze the addition of hydrogen to oxygen and to a variety of organic compounds, as Humphry Davy (1778-1829) (the isolator of sodium and potassium) discovered in 1816. Again, acid will catalyze the breakdown to simpler units of a number of organic compounds, as G.S. Kirchhoff (1824-1887) first showed in 1812. At the conclusion of the reaction, the platinum or the acid is still present in its original quantity.

Ostwald prepared, in 1894, a summary of someone else's paper on the heat of combustion of foods, this summary to appear in his own journal. He disagreed strongly with the conclusions of the writer, and to buttress his disagreement discussed catalysis. He pointed out that the theories of Gibbs made it necessary to assume that catalysts hastened reactions without altering the energy relationships of the substances involved. The catalyst, he maintained, must combine with the reacting substance to form an intermediate that breaks up to give the final products. The breakup of the intermediate released the catalyst, which thus resumed its original form.

Without the presence of this catalyst-combined intermediate, the reaction would proceed much more slowly, sometimes so slowly as to be imperceptible. Hence, the effect of the catalyst was to hasten the reaction without itself being consumed. Furthermore, since a molecule of catalyst was used over and over, a small quantity of catalyst was sufficient to hasten a great deal of reaction.

This view of catalysis is still held today. It has helped to explain the activity of the protein catalysts ("enzymes") which control the chemical reactions in living tissue.

Adapted from: Isaac Azimov: A Short History of Chemistry. Doubleday 1965, p.155.

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