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Neutrons & Catalysis NEUTRONS & CATALYSIS by Juergen Eckert and Phillip J. Vergamini atalysis—the ability of some ture of chemicals, could not function catalysts, is still. as it always has been, substances to alter the rate of without catalysts. It has been estimated, largely empirical. chemical reactions without for example, that catalysts are involved In the last few years, however, so- being consumed-was rec- at some point in the production of 60 to phisticated new analytical and compu- c tational techniques have helped stimu- ognized more than 150 years ago and 70 percent of all industrial chemicals. has been applied on an industrial scale Yet the store of knowledge about how late a renaissance in catalysis research. since the beginning of this century. catalysts work is surprisingly small. The Powerful economic forces have mo- Modem industrial chemistry, especially search for a catalyst for a particular re- tivated the study of catalysis as well: petroleum processing and the manufac- action, or for ways to improve existing the need for new sources of energy and Neutrons and Catalysis chemicals, changes in the availability Unfortunately, these details of struc- of’ raw materials, potential restrictions ture and dynamics cannot easily be de- on the availability of noble-metal cat- termined in a “real-world” situation— alysts, and the desire for new products that is, during an actual catalytic reac- have pointed up the need for a clearer tion. Catalytic processes usually pro- understanding of catalytic processes. ceed under conditions that preclude the Much of the research into cataly- direct application of many powerful an- sis is directed toward metals. because alytical techniques-or at least make they catalyze many important reac- such application very difficult. Consid- tions. Metals may be catalytically ac- erable effort has therefore been devoted tive in the form of finely divided parti- to the study of so-called model systems, cles, organometallic compounds in so- which are designed to reproduce the lution, or ions bound to large biologi- critical relationships as accurately as cally active molecules, such as enzymes. possible. One useful model system is The catalysis may be heterogeneous in a single crystal of a metal for which the sense of involving more than one the surface arrangement of atoms is phase (solid metal and gaseous reac- known. Others that have been widely tants, for example) or it may be homo- used are synthetic molecules consisting geneous in the sense of involving only of a metal atom (or a cluster of metal one phase (such as a solution). What- atoms) surrounded by stabilizing lig- ever the form, when the metal binds to ands, usually carbonyl groups (CO) or a reactant molecule, it almost always other more complex organic groups. alters the chemical bonding in the re- When a reactant molecule such as ethy- actant, If that alteration is favorable to lene or benzene binds to such a syn- some particular reaction, then the metal thetic molecule, we can assume that, to is a catalyst for that reaction. some degree, the configuration of the To understand catalytic activity, or resulting complex resembles that of the to tailor a catalyst to do a specific job, same reactant adsorbed on a metal sur- we need to know the individual steps in face. The complex can be studied with the catalytic process in great detail, For several spectroscopic techniques, and its example, consider the hydrogenation of crystalline form can be characterized by ethylene to ethane, x-ray’ and neutron diffraction, which re- veal details of its architecture with great accuracy. The more closely the properties of the which can serve as a prototype of re- model system approximate the proper- actions used in producing synthetic fu- ties of the real-world system, of course, els. The production of synthetic fuel the better. As a result, model systems from coal, for example, involves various are often structurally modified to refine series of reactions, including the step- their properties and bring them closer in wise hydrogenation of carbon to form line with the more complex system of acetylene (HCCH), ethylene, and ethane, interest. However. such modifications as well as the stepwise hydrogenation can complicate the structural character- of carbon chains with more than two ization of the model system. For exam- carbon atoms. The hydrogenation of ple, as the model system becomes larger ethylene shown above is a particularly and more complex. the chances increase useful react ion to study because it can that some portions of the molecule will be carried out at moderate temperatures be disordered or less easily defined. The in the presence of a metal catalyst. The necessity of modeling the disorder can various steps to the reaction are repre- decrease the precision of the results for 116 Los Alamos Science Summer 1990 Neutrons and Catalysis the metal-hydrogen interaction, which for a particular aspect of the problem, ing is in locating the all-important hy- is the feature of most interest. In effect, and neutron scattering is one of these. drogen atoms and highlighting the vibra- the results become slightly fuzzy and However, even the most intense neu- tional and rotational motions associated less precise. tron sources produce fluxes far below with them. This strength is a result of Besides being useful in the study of those commonly available from sources the fact that neutrons scatter as strongly catalysis, metal complexes are highly of photons (x rays, ultraviolet, visible from hydrogen as from most other el- suitable for theoretical studies of chemi- light, and infrared), and so neutron ements (see “Neutron Scattering—A cal bonding between the bound molecule scattering is not one of the principal Primer” by Roger Pynn). Although it (ligand) and the metal atoms. They tools of surface science. Nevertheless, is nearly impossible to “see” hydrogen are therefore of fundamental interest when the systems include hydrogen atoms in the presence of heavy met- to researchers studying chemical bonds als using x rays, x-ray diffraction can from first principles. Finally, metal- sometimes implicitly locate hydrogen cluster complexes can stabilize cer- atoms bound to or interacting with metal tain molecules that are unstable in pure atoms. If a site in a metal complex is form. For example, cyclobutadiene can usually filled, an apparent vacancy at be stabilized by binding to iron car- that site, together with other physical bonyl, Fe(CO)3; and ethylidyne, CH3-C and chemical evidence, can lead to the (a highly reactive intermediate formed inference that hydrogen occupies the po- in the hydrogenation of ethylene), can sition. Neutron scattering, however, is be isolated by reacting with cobalt car- needed to confirm the actual presence of bonyl to form the metal-cluster complex hydrogen. Thus, the structures of com- CH3C–C03(CO)9. pounds of interest are typically deter- The kind of information available mined by first applying x-ray diffrac- through the study of model compounds tion to locate the heavier atoms and is illustrated by the case of the clus- then neutron diffraction to obtain pre- ter compound HFeCo3(CO)12. Diffrac- cise metal-hydrogen distances and bond tion studies show that the single hydro- angles. gen atom is located at a site of three- Historically, single-crystal neutron fold symmetry, that is, just outside A CLUSTER COMPOUND diffraction has been more difficult than the triangle formed by the three cobalt x-ray diffraction. Neutrons can travel atoms (Fig. 2). The vibrational spec- Fig. 2. The HFeCo3(CO)12 complex, which large distances through material without trum of hydrogen in this compound is contains a single hydrogen atom (blue) being scattered, so neutron diffraction very similar to that of hydrogen atoms located against an equilateral triangle of requires a much larger crystal. This chemisorbed on a nickel or a platinum cobalt atoms (red), can serve as a model problem has been partly alleviated by surface. Since the vibrational spectrum system for hydrogen atoms chemisorbed on the availability of more intense sources of a molecule or atom strongly reflects a metal surface. In particular, comparison of of neutrons. Furthermore, the time-of- the way in which it is bound to other vibrational spectra can help establish whether flight wavelength measurements possi- atoms, the similarity here allows the or not the hydrogen on the metal surface is ble at pulsed-accelerator-based neutron inference that hydrogen chemisorbed also located at sites with threefold symmetry. sources makes all neutrons in each pulse on a catalyst surface is located at a site (Adapted from a figure in an article by R. G. usable. Area detectors make it possi- of threefold symmetry. We can further Teller, R. D. Wilson, R. K. McMullan, T. F. ble to collect large volumes of data at infer that the catalytically active sur- Koetzle, and R. Bau. Journal of the American one time and make feasible full struc- face is the so-called (111) plane of the Chemical Society 100: 3071, 1978.) tural determination from polycrystalline metal, because that is the only crystal material. plane having threefold symmetry. This or molecules containing hydrogen—as For the observation of molecular vi- information could not have been easily do the more important types of com- brations, optical techniques (infrared obtained in any direct way. pounds involved in industrial catalytic absorption and Raman scattering) are How does one then study the model processes—neutron scattering is ex- far more common and much easier to systems? There are many experimen- tremely useful. use than neutron scattering. Once again, tal techniques, each especially suited The singular utility of neutron scatter- however. the difference in the nature of Los Alamos Science Summer 1990 117 Neutrons and Catalysis the interaction between the scatterer and the ring hydrogens is replaced by a The Hydride Ligand the probe makes neutron-scattering vi- methyl group), the remaining ring hy- brational spectroscopy advantageous in drogens can be replaced by deuterium The first example we want to dis- certain cases.
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