Homogeneous, Heterogeneous, and Enzymatic Catalysis S. Ted Oyamal and Gabor A. Somorjai Catalvsis Prosram of the Center for Advanced Materials, Lawrence Berkeley Laboratory, University of California, Berkeley. CA
Catalysts have been employed by mankind since antiquity in such activities as wine-, bread-, and cheesemaking. In many cases i t was found that the addition of a small portion from a orevious batch. a "starter". was'necessarv to begin s an a c c o k t the next production. 1d 1835~ e r z e l i u published that tied together earlier observations hv chemists such as ThBnard, D&, and Diihereiner by suggesting that minute amounts of a foreign substance were able to affect greatly the course of chemical reactions, both inorganic and biological. Berzelius attributed a mysterious force to the substance that he called catalytic (1-3). In 1894 Ostwald proposed that catalysts are substances that accelerate the rate of chemical reactions without themselves heine consumed durine the reactions ( 4 , 5 ) .This definition is sell applicable today. The scooe of catalvsis is enormous (6). . . Catalvsts are widely used i n the com&ercial production of fueis, chemicals, foods, and medicines. They also play an essential role in processes in nature, like nitrogen fixation, metabolism, and photosynthesis. Classlflcatlon Catalysts can be protons (7,8), ions ( 9 - l l ) , atoms (12,13), molecules (12), or larger assemblages. Traditionally, catalysts have been classified as homogeneous, heterogeneous, and enzymatic, reflecting this increasing hierachy of comolexitv. Protons, ions, atoms, and molecules may be considered examples of homogeneous catalysts (14).In addition, metal complexes and organometallic compounds are important members of this class of catalysts (15-17). As the name implies, these catalysts are part of agas phase or dissolved in a liquid phase together with the reactant of the reaction. . . In contrast to homogeneous catalysts, heterogen~ouscatnlystsare usually solid surfaces,attarhed tosolid surfaceli,or of insoluble matrices such as polymers, and are, thus, phase-separated from the fluid medium surrounding them. Reeardless of their form. the active catalvtic comnonent is locked a t the interface between the solidUandthekuid and mav consist of a wide diversitv of soecies. Examoles are one or two atoms of the total surface (?8),a larger ensemble of such surface atoms (19-23), an organometallic compound attached to the surface by covalent bonds (241, or a molecular cluster lying on the surface (25,26). Enzymatic catalysts are like liquid-phase homogeneous catalysts in being dissolved in a liquid media, but enzymatic catalysts are of biologicalorigin and possess the highest level of complexity among the three types (27-33). Ironically, as mentioned in the opening sentence, they were probably the first catalysts utilized by man. Enzymatic catalysts are proteins composed of repeating units of amino acids, often twisted into helices, and in turn folded into three-dimensional structures. The protein structures often surround a central organometallic structure. Fundamentals The action of catalysts will be illustrated by an example, the water gas shift reaction catalyzed by iron and chromium oxides.
H,O
+ CO-
H,
+ CO,
This reaction is used in the production of hydrogen in several commercial processes. I t is an example of a heterogeneous catalytic reaction, hut the principles derived from it are also applicable to homogeneous and enzymatic catalytic reactions. A simplified scheme for the reaction is presented below.
In the first step, one of the reactants, HzO, reacts with an empty catalytic site, denoted by *, to produce a product, Hz, and a reactive intermediate consisting of an oxygen atom associated with the site, denoted by O*. In the second step, the other reactant, CO, reacts with the intermediate to produce the product, CO,, and regenerates the catalytic site, *. The energetics associated with this process are given in the figure. A key aspect of this scheme is that i t represents a cycle that occurs many times as the reaction proceeds. Each repetition of the cycle is called a turnouer. A good catalyst will have millions of turnovers. In contrast, a stoichiometric reactant will have only one. Several important points are to be made concerning the energetics and scheme presented above. 1. The energy level diagram shows that the catalyzed reaction has a lower octiuation barrier (34)than the uncatalyzed thermal reac-
tion. This is the origin of the enhancement in the rate, and it applies both in the forward and reverse directions of the reaction. 2. Regardless of the details of the mechanism and the energetics of the transformationof reactants into products, their relative enerdo not change (35).This means that gies, as shown by the thermodvnamic eouilibrium between them does not chanee. . Catalysts increase the rate of approach to equilibriwm but not the thermodynamic equilibrium value itself. 3. As shown by the overall reaction stoichiometry, there is no net consumption or production of the catalytic site, *. The reaction proceeds by repetition of the catalytic cycle or chain, with the catalytic species remaining unchanged at the end. This explains the observation noted earlier that miniscule amounts of catalyst can aive rise to large amounts of product. I. The intermediate, O', must be neither loo stab/
