Ind. Eng. Chem. Fundam. 1986, 2 5 , 2-9
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only because of his fundamental research in reactions related to industrial chemistry and his efforts to develop chemical engineering principles to facilitate reaction rate analysis, but also because of his close liaison with many industrial research laboratories. As a consequence,he has had a great awareness of intriguing industrial research problems for which independent programs were difficult to justify because of the constraints that exist in industrial research, but for which explanations were of such importance that academic research could readily be justified. Thus, his many contributions to kinetics and catalysis have
repeatedly been of importance to engineers and researchers in industry, as well as those in academia. The two of us have had the pleasure and the challenge to work with Michel as graduate students. We were aware of the relevance of the many projects which existed in Michel's group and had the privilege of discussing our research with many of the industrial researchers who visited his laboratory. Consequently, we learned firsthand about the synergism that can occur when fundamental research is directed toward industrial problems.
School of Chemical Engineering Purdue University West Lafayette, Indiana 47907
W. NICHOLAS DELGASS*
Department of Chemical Engineering The Pennsylvania State University University Park, Pennsylvania 16802
M. ALBERT VANNICE*
Influence of Technology on Catalytic Science John H. Sinfelt Corporate Research Science Laboratories, Exxon Research and Engineering Company, Annandale, New Jersey 0880 I
During the twentieth century, technological advances in catalysis have been enormous. These advances have generally stimulated research leading to significant progress in the science of catalysis. Areas of catalysis providing good examples include ammonia synthesis in the chemical industry and catalytic cracking and reforming in the petroleum industry. As the science of catalysis has developed in response to the stimulus of technology, it has had an increasingly greater effect in the reverse direction, Le., in shaping the technology. This feedback phenomenon is highiy desirable for maximizing progress in both science and technology.
Around the year 1900 the German chemist Wilhelm Ostwald defined a catalyst as a substance which alters the rate of a chemical reaction without appearing in the end products. This insight provided a basis for scientific inquiry into the subject and paved the way for the widespread investigation and application of catalytic phenomena. Since the time of Ostwald, the science of catalysis has progressed steadily. The scientific progress has been accompanied by enormous technological advances, ranging from the Haber process for ammonia synthesis to present-day processes for the catalytic cracking and reforming of petroleum fractions. More often than not, the technological advances have led to significant scientific advances. In this brief account, we consider several examples. Evolution of Catalytic Science-Examples Ammonia Synthesis. In the period between 1900 and World War I in Germany, a process was developed for the direct synthesis of ammonia from nitrogen and hydrogen through the use of an iron catalyst. The process was
conducted at high temperatures, in the vicinity of 450 "C, to obtain a sufficiently high reaction rate. However, since the reaction is exothermic, the high temperature is undesirable from the standpoint of thermodynamics. To compensate for the adverse effect of the high temperature on the equilibrium, pressures of 100 atm and higher were employed. According to the principle of Le Chatelier, first enunciated in 1884 and hence a relatively new idea at the time, a reaction involving a decrease in the number of molecules should be aided by an increase in pressure. Since two molecules of ammonia are produced from four molecules of reactants (one molecule of nitrogen and three molecules of hydrogen), it was therefore anticipated that the production of ammonia would increase if the reaction was conducted at high pressures. The research of Haber provided a beautiful demonstration of the Le Chatelier principle. Since the ideas of chemical thermodynamics were relatively new at the time (along with the science of physical chemistry in general), the successful research of Haber may be regarded as a highly important milestone
0196-4313/86/1025-0002$01.50/0 1986 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 25, No. 1, 1986 3
in advancing this fundamentally important scientific discipline. The successful development of the ammonia synthesis process stimulated a great deal of research to understand the surface chemical processes involved in the reaction. The research continues to this day, almost a century after the original work. In 1934 Emmett and Brunauer (8) obtained important data to support the hypothesis that nitrogen chemisorption is the rate-controlling step in the reaction. The chemisorptionof nitrogen on an iron catalyst was determined as a function of time over a wide range of temperatures. Rates of chemisorption were shown to correspond closely to rates of ammonia synthesis from nitrogen and hydrogen at the same temperatures. The activation energy of the ammonia synthesis reaction was therefore identified with the temperature dependence of the rate of chemisorption of nitrogen on the catalyst. Thus,nitrogen chemisorptionon the iron catalyst provided an example of activated adsorption, a concept proposed by Sir Hugh Taylor in the 19209. Since 1934, many studies of the kinetics of ammonia synthesis have been made, but the essential conclusion of the early work has not changed, namely, that nitrogen chemisorption is the slow step in the synthesis. The essential features of the kinetics of ammonia synthesis can be described by a simple analysis in which the reaction is treated as a sequence of two steps (3). First, the nitrogen molecule is chemisorbed dissociatively on the catalyst surface
N2 + 2s
-+
2N-S
(1)
where S is a surface atom and N-S is a chemisorbed nitrogen atom. The latter reacts readily with hydrogen to form ammonia in the second step
2N-S
+ 3H2
2NH3 + 2s
(2)
Equilibrium is established in this step during ammonia synthesis, and hence the concentration of chemisorbed nitrogen atoms is determined by the concentration of ammonia and hydrogen over the catalyst. At low conversions of nitrogen and hydrogen to ammonia, the concentration of N-S is much smaller than that corresponding to equilibrium with Nz in the gas. If we assume that the active catalytic sites are all equivalent, it can readily be shown that the rate of ammonia synthesis is given by the expression
where k is the rate constant for nitrogen chemisorptionand
K is the adsorption equilibrium constant relating the concentration of N-S to the concentration of NH, and H2 in the gas phase. The expression describes the rate of reaction in the forward direction. To account for the reverse reaction, which becomes increasingly important as the extent of conversion increases toward the equilibrium value, it is necessary to modify eq 3 by including a second term. Although eq 3 ignores the nonuniformity of the surface, it describes the essential kinetic features of the ammonia synthesis reaction. If one takes account of surface nonuniformity (32, 46, 48), the form of the rate expression is not very different from eq 3, as Michel Boudart has emphasized repeatedly. The exponent 2 in the denominator becomes 2a, where a is the ratio of the change in activation energy to the change in heat of adsorption on a nonuniform surface (32). In addition, the parameters k and K in eq 3 then refer to their values at a surface coverage equal to zero.
( IRON 1
C02 CHEMISORPTION (ALKALI PROMOTER)
ADSORPTION (TOTAL SURFACE)
I
Figure 1. Selective chemisorption for determining the surface composition of an alkali (K20)-promotediron catalyst (9).
In the course of research on ammonia synthesis there have been a number of advances in surface science which have been very important for catalysis. One of these was a method for determining the total surface area of a porous solid from measurements of the low-temperature physical adsorption of a gas. The method, due to Brunauer, Emmett, and Teller (5), has been widely used by catalytic chemists for almost half a century for the characterization of catalytic materials. Another was the demonstration of selective chemisorption as a probe for obtaining information on the surface composition of a catalyst containing more than one component. In the classical iron catalyst used in ammonia synthesis, there is a small amount of an alkali promoter (K,O)which constitutes about 1.5% of the catalyst mass. Although the amount of promoter is small, it can cover a large fraction of the catalyst surface if it concentrates there, since the surface layer of the catalyst contains only about 1% of the total mass. Emmett and Brunauer (9) in 1937 approached the problem by first measuring the amount of physically adsorbed nitrogen required to form a monolayer on the entire catalyst surface. Next, they used carbon Monoxide chemisorption as an indicator of the fraction of the surface that consisted of iron. Finally, they used the chemisorption of carbon dioxide as an index of the amount of alkali promoter in the surface. The results of this combination of measurements are depicted schematically in Figure 1. As can be seen from the figure, the results indicated that a very substantial amount of the promoter was present at the surface of the catalyst. Thus, selective chemisorption measurements were very useful in determining the surface composition. The method of selective chemisorption has been extended to a whole host of metal catalysts (23,44,45)other than the ammonia synthesis catalysts originally investigated by Emmett and Brunauer. It has been very important for the characterization of a variety of supported metal catalysts which are widely employed in industrial processes. In such catalysts the metal component is dispersed on the surface of a refractory material which is commonly known as a carrier or support. The refractory material is frequently an oxide such as silica or alumina with a high surface area. In some cases of great industrial importance, the surface associated with the metal represents only a small fraction of the total surface area of the catalyst. To determine the metal surface area, one measures the extent of chemisorptionof a gas such as hydrogen or carbon monoxide, often at room temperature. In such measurements, chemisorption occurs very selectively on the metal. Selective chemisorption measurements have also found application in the determination of the surface compositions of metal alloys used as catalysts. In general, the
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