Catalysis is the alteration of the rate of a chemical reaction using substances known as catalysts, which can repeatedly participate in the elementary steps of a chemical process changing the number and character of such steps; the catalyst is regenerated after the transformation is completed. A positive catalysis promotes the chemical reaction and a negative catalysis retards it. The term "catalysis" is more frequently applied to the former.
Catalysis is classified in terms of phase as heterogeneous and homogeneous. In homogeneous catalysis, reactants and catalyst are in the same phase while in heterogeneous catalysis, reactants and catalyst are in different phases and the reaction proceeds at the interface. Diffusion is an indispensable stage of catalysis in the heterogeneous system. Catalysis with colloid particles in the liquid phase, that is, a micellar catalysis, is intermediate between the two kinds mentioned above. Here, the reaction begins on the surface of solid catalyst, subsequently proceeding to the bulk of the reaction medium.
The important properties of catalysts include catalytic activity, selectivity, sensitivity to small amounts of extraneous substances (promoters and modifiers), resistance to external factors (e.g., reaction medium, temperature, pressure, mechanical action) and regeneration after the reaction terminates.
The catalytic activity is determined by the change of the rate of the chemical reaction under the action of a catalyst. A specific catalytic activity is an activity per unit volume or mass of the catalyst. Heterogeneous catalytic reactions are characterized by the number of reaction recycles, that is, a specific activity related to a single active center. The reaction rate in the homogeneous catalysis is proportional to the catalyst concentration.
Under catalysis, the activation energy EA is normally lower than in the noncatalytic reaction. A reduction in EA in the catalytic reaction in relation to the noncatalytic reaction is schematically shown in Figure 1.
The catalyst does not change the chemical equilibrium, but it alters the rates of the direct and reverse reactions, thereby facilitating faster attainment of an equilibrium state.
Each catalyst alters the rates of specific reactions, i.e., catalysts are selective in action.
One of the main points in various catalytic theories is an insight into the formation of intermediate compounds of a catalyst and a reagent, and their decomposition (with catalyst regeneration) in the course of a reaction.
Two mechanisms of catalytic action via formation of an intermediate complex are suggested for the A+B→C+D reaction types. One is accomplished in stages
and the other is associative (synchronous)
Heterogeneous catalytic reactions are characterized by an intricate macrokinetics. Macrokinetic stages commonly considered are transport of reagents from the flow core to the outer surface of the catalyst grain, diffusion in the grain pores, adsorption of initial reagents on the catalyst grain surface, the chemical interaction proper that may proceed in a few stages, desorption of reaction products from the catalyst surface, diffusion of products from the grain inner surface and transport of products from the outer surface of the catalyst particle to the flow core of the reaction mixture. The stage with the lowest rate determines the mechanism of the entire process. The kinetic characteristics of heterogeneous catalysis depend not only on the nature and the state of the catalyst surface, but also on the laws of heat and mass transfer.
Depending on the nature of the rate controlling stage, outer and inner kinetic, outer and inner diffusion, and adsorption limiting macrokinetic regions of heterogeneous catalysis are distinguished.
There exists no unified theory of heterogeneous catalysis that can account for all the properties of all the catalysts and which can help select an appropriate catalyst for a given reaction. The well-known theories fall into three groups.
The first includes the geometrical theories of Taylor, Balandin, and Kobozev that are based on the geometrical correspondence of catalytically active centers; the role of such centers can be played by several atoms or ions on a nonhomogeneous catalyst surface, and depends on the arrangement of atoms in the reactant molecules.
The second group covers the electronic theories of Schwab, Dowden, Turner, and Volkenshtein that proceed from the fact that reactant molecules adsorbed on the catalyst surface, exhibit free valences due to redistribution of electron density and gives rise to a new bond. Reagent molecules dissociate and atoms are added to the surface. An atomic migration on the surface and the interaction of molecules that are weakly bound to the surface may result in formation of the reaction products that are later desorbed.
These two groups of theories rest on physical approaches. Recently, an ever-increasing preference is given to chemical theories that consider a catalyst as a reagent forming, with the reactants, unstable surface intermediate complexes which, in the course of the reaction, are decomposed into the reaction products and the starting catalyst.
All the heterogeneous catalysis theories assign an essential role to the stage of reactant adsorption on the catalyst surface. Adsorption is assumed to result in the weakening of the bonds in the molecules of the reactants and the production of surface intermediate compounds that decompose into the products and are desorbed.
For heterogeneous catalytic reactions of the A+B→C type, two adsorption mechanisms are considered. One is described by the Rideal-Eley model, involving formation on the catalyst surface of an intermediate complex with the atoms of one of the reagents , with subsequent interaction of an adsorbed atom and the molecule of the other reagent incident from the main gas flow ([AZ]*+B→C+Z). The other mechanism is described by the Langmuir-Hinshelwood model, based on the assumption made for deriving the Langmuir adsorption isotherm. In terms of this model, starting reagents are adsorbed on the catalyst surface to produce surface compounds. Migration of compounds on the catalyst surface causes the [AZ]+[BZ]→C+2Z transformations. In this case, it is assumed that the rate of the chemical reaction is lower than the rates of reagent and product adsorption and desorption.
Currently, the study of complex compound catalysis is proceeding vigorously. Interaction of atoms and ions of a transition metal with those of other substances leads to the formation of complex metal compounds with one or several ligands, the orbitals of which are overlapped by the orbitals of the central atom or a metal ion. The catalytic effect of complex catalysts is accounted for by addition in the coordination sphere of reagent molecules with substitution of less strongly bound ligands. The catalytic reaction is promoted by a mutual orientation of reagents, their polarization in the central atomic field, an easier electron transition due to participation of the central atom, reduction of EA and compensation of the energy of bond rupture by a simultaneous formation of other bonds. An advantage of complex catalysts is the alteration of catalytic properties via the addition of other ligands. This makes it possible to achieve high specificity and selectivity of the catalysts. Complex compounds can serve as catalysts in both homogeneous and heterogeneous catalysis.
Catalysis underlies many industrial processes. Catalysts are employed for industrial synthesis of ammonia from N2 and H2, production of HNO3, and for production of H2SO4 by contact technology. The conversion processes CH4 + H2O → CO + 3H2, C + H2O → CO + H2, CH4 + CO2 → 2CO + 2H2, and C + CO2 → 2CO also proceed in the presence of catalysts. Catalytic techniques are used in organic and petrochemical synthesis, production of petroleum derivatives (catalytic cracking and reforming), polymers, methanol, acetic acid, etc. In recent years, catalysts are widely employed in developing environmentally-important processes. Thus, they are used for cleaning waste gases in power generation and metallurgy (elimination of detrimental impurities such as NOx, SOx, H2S) and for eliminating CO and NOx from exhaust gases of internal combustion engines.
Rideal, E. (1968) Concepts in Catalysis, New York.
Satterfield, C. (1981) Heterogeneous Catalysis in Practice, New York.