PERKIN MEDAL
Exxon's Sinfelt Wins 1984 Perkin Medal for Catalysis Research Joseph Haggin, C&EN Chicago
The 1984 Perkin Medal has been awarded to a specialist in catalysis who adamantly disagrees that catalysis was ever a black art. John H. Sinfelt, senior scientific adviser in the corporate research science laboratories of Exxon Research & Engineering Co., is the recipient, cited for his pioneering work in catalysis over the past 25 years. He is particularly noted for his work on the development of bimetallic catalysts and the elucidation of structures in highly dispersed catalytic systems. The Perkin Medal, which recognizes an individual for outstanding contributions to applied chemistry, is awarded jointly each year by the Society of the Chemical Industry, the American Chemical Society, Société Chimie Industrielle (American Section), the American Institute of Chemical Engineers, and the Electrochemical Society. This year's award was made at a gala dinner in New York City last week. Among the practical applications of Sinfelt's work has been the development of a platinum-iridium catalyst now used worldwide to make high antiknock, low-lead gasoline. He also made important contributions to a process now used to produce p-xylene, an intermediate for polyester fibers, films, and plastics. Sinfelt regards his academic training in chemical engineering at the University of Illinois and Pennsylvania State University as a good background for his work in catalysis. One reason is the unusual organization of the chemical engineering and chemistry departments at Illinois where Sinfelt obtained his M.S. and Ph.D. degrees. There students of chemistry and chemical engineering experience a common curricu-
lum in many respects. The synergism that this produces is of considerable scientific and technical benefit and undoubtedly provides a good research perspective, Sinfelt says. In his case, at least, the transition from chemical engineering to catalysis research was never a problem. After joining Exxon in 1954, Sinfelt's first project was concerned with the catalytic reforming of hydrocarbons. The catalyst was platinum deposited on alumina. This work introduced him to the concept of bifunctionality in catalysis and was responsible for his sustained interest in metal catalysts. In the early 1960s he encountered bimetallic catalysts and his interests focused on the problems of varying selectivity with catalyst composition and preparing bimetallics in highly dispersed form. This led to the development of bimetallic "clusters" whose structural identity is still
being debated. Sinfelt notes that the term cluster has been used by chemists for a long time to mean different things. Sinfelt makes a firm distinction between bimetallic catalysts that are alloys and those that are not. Bimetallic alloys are solid solutions. They usually impart their own peculiar properties to catalysts, particularly in highly dispersed systems. However, some bimetallic catalysts don't form alloys but have their own unique features. Copper-nickel catalysts, for example, form alloys but ruthenium-copper and osmiumcopper catalysts do not. The modification of such catalytic properties as selectivity with changes in composition can be illustrated by some of Sinfelt's work with the copper-nickel alloy system in the early 1970s. These metals catalyze such reactions as cycloparaffin dehydrogenation and ethane hydrogenolysis. At very low concentrations of copper, both reactions have an appreciable rate. As the copper concentration is increased in the catalyst, the dehydrogenation of cyclohexane is virtually unaffected and the rate of hydrogenolysis of ethane sharply diminishes. The two reactions are essentially different in that hydrogenolysis involves the rupture of carbon-carbon single bonds, whereas dehydrogenation ruptures carbon-hydrogen bonds. Eventually, at copper concentrations above 80%, the rates of both reactions drop sharply. However, by using a small a m o u n t of copper, hydrogenolysis can be preferentially suppressed without greatly affecting the rate of dehydrogenation. The interpretation of these results depends on a knowledge of the compositional variation in the bimetallic catalyst and the effects of March 19, 1984 C&EN
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Perkin Medal dispersion. Sinfelt explains that in copper-nickel catalysts the copper preferentially concentrates on the surface. Even if the bulk concentration of copper is small, its surface concentration may be sufficient to change the catalysis drastically. This surface concentration effect is manifest even in highly dispersed systems where the nature of the surface may be in doubt. Sinfelt invented the term "bimetallic cluster" to designate highly dispersed catalytic entities composed of two metals. They are usually smaller than 50 Â and in some cases are so small that they approach single atoms in size. Clusters are usually supported on carriers with high surface areas such as porous silica and alumina. Most of the clusters that have been investigated in Sinfelt's laboratory are combinations of either two Group VIII metals or of a Group VIII metal and a Group IB metal. Even though the dispersion of the clusters is very great there appears to be a kind of surface enrichment phenomenon similar to that in the copper-nickel bulk alloys. The phenomenon is often present even when the bimetallic systems do not form alloys as in the r u t h e n i u m - c o p p e r and osmiumcopper clusters. Why bimetallic catalysts function the way they do is not completely known. Among the theoretical explanations that have been offered are the existence of some set of multiple sites on the catalyst surface that function together or the possibility of electronic interactions at the optimum catalyst composition. The bond strength of chemisorbed intermediates is another factor that may sometimes influence catalyst function. The imperfect knowledge of what is happening on the surface of the catalyst and the details of the surface structure itself are obviously matters of great catalytic importance. One technique that Sinfelt and his associates have been using to determine some of these factors is extended x-ray absorption fine structure (EXAFS). They use the technique to help determine the structure of clusters in high dispersion. The technique provides considerable information about the number 26
March 19, 1984 C&EN
and type of atoms in the vicinity of a particular atom. It also can be used to determine interatomic distances and the deviations of these distances from their equilibrium values. EXAFS data can be determined for each element in a complex material. Though the technique has been known for a long time, using it to investigate noncrystalline structures has only recently been appreciated. It is particularly useful for catalysis researchers because for many of the highly dispersed catalysts of commercial importance, EXAFS may be the only method capable of yielding much structural information. EXAFS is element-specific and is based on the backscattering of ejected photoelectrons from atomic cores. In the case of osmium-copper bimetallic clusters, for example, the metals have limited bulk miscibility but the two species interact strongly. The analytical results suggest that osmium atoms in the clusters coordinate predominantly with other osmium atoms. Copper atoms coordinate equally well with both species. More or less the same thing was observed with copper-ruthenium clusters. The indications are that the environment about a ruthenium atom in a copper-ruthenium bimetallic catalyst is not very different from that in a ruthenium reference catalyst. Theories in catalysis have been abundant over the years. Sinfelt doesn't regard that as necessarily a sign that catalysis is a black art. He rather resents anyone describing catalysis in that way. In fact, he says, catalysis has always been distinctly theoretical and has exhibited a rather orderly development, at least since 1901. That is the year he regards as marking the formal beginning of catalysis science. There were significant developments in catalysis before then, but 1901 marks the period of Wilhelm Ostwald, whom Sinfelt regards as the first formal catalytic chemist. Since then there has been a progression of milestones including the work of Paul Sabatier and Irving Langmuir; industrial successes, such as a catalytic production of ammonia; and a proliferation of catalytic research following World War I. Like most other scientific disciplines, catalysis has pro-
gressed from its origins in a more or less orderly fashion, if not at a uniform rate. One problem that the discipline of catalysis does have is the artificial and sometimes deep chasm that exists between catalytic theorists and pragmatists, Sinfelt says. Both have made great contributions to catalysis and continue to do so. They are certainly not natural antagonists even if they are sometimes regarded as such. Higher mutual regard at the professional level would benefit both groups and assist the science of catalysis immeasurably, he suggests. Other than an occasional swim, Sinfelt doesn't engage in hobbies or athletics. His work is his hobby, he says, and he spends a lot of his "leisure time" organizing his future work. He expects to maintain his long-standing interest in catalysis by metals, particularly in the modification of specificity with composition. Such nonmetallic catalysts as the zeolites may be important, but Sinfelt has arbitrarily limited his work to the metals to avoid spreading himself too thin. He is also keenly interested in kinetics, a subject he believes has been neglected by catalytic chemists. Kinetics is what catalysis is all about, and Sinfelt hopes to increase his involvement in the formalities of chemical kinetics in the future. Other than the "black art" label and the chasm between theorists and pragmatists, what possibly disturbs Sinfelt most about catalysis is that it is sometimes considered distinct from chemistry. Catalysis is intrinsically chemical in nature, and good catalysis is good chemistry. There are those who regard catalysis as a kind of microchemistry, meaning that it represents a detailed knowledge of chemistry constituting a new level of understanding beyond the more familiar macrochemistry. With the historical perspective of catalysis that is now emerging, it seems safe to suggest that catalysis will be an increasingly unifying force in chemical research. Considering the significance of the Perkin Medal, it also seems safe to suggest that Sinfelt's work will be another milestone in that history. D CIRCLE 28 ON READER SERVICE CARD — •
