Elementary reactions catalytic reaction mechanism

The simplest class of catalytic reaction mechanisms are linear ones. This term was introduced by Temkin (see Chap. 2). Linear mechanisms are those that contain only elementary Z, Z -type steps. Hence every reaction involves the participation of only one molecule of the intermediate substance. 

Given a fixed, predetermined set of elementary reactions, compose reaction pathways (mechanisms) that satisfy given specifications in the transformation of available raw materials to desired products. This is a problem encountered quite frequently during research and development of chemical and biochemical processes. As in the assembly of a puzzle, the pieces (available reaction steps) must fit with each other (i.e., satisfy a set of constraints imposed by the precursor and successor reactions) and conform with the size and shape of the board (i.e., the specifications on the overall transformation of raw materials to products). This chapter draws from symbolic and quantitative reasoning ideas of AI which allow the systematic synthesis of artifacts through a recursive satisfaction of constraints imposed on the artifact as a whole and on its components. The artifacts in this chapter are mechanisms of catalytic reactions and... 

The second application is the construction of catalytic reaction mechanisms out of elementary steps, involving only on type of catalytic site. A more useful way to formulate the stoichiometric constraints for these systems is to classify every chemical species as either a terminal species or an intermediate. Terminal species represent stable compounds that can be produced or consumed in significant quantities, while intermediates are short-lived unstable species that participate in the mechanism but are neither raw materials nor final products of the process. 

Microkinetic modeling is a framework for assembling the microscopic information provided by atomistic simulations and electronic structure calculations to obtain macroscopic predictions of physical and chemical phenomena in systems involving chemical transformations. In such an approach the particular catalytic reaction mechanism is expressed in terms of its most elementary steps. In contrast to the Langmuir-Hinshelwood-Hougen-Watson (LHHW) formulations, no rate-determining mechanistic step (RDS) is assumed. 

While flow reactors operate at conditions that closely mirror the operating environment witnessed in practice, these reactors can typically only offer a global kinetic description and lack the details of elementary reaction steps and mechanism that reveal how materials operate on a more fimdamental level. If the goal is catalyst development, then one typically needs more detailed rate expressions that describe elementary reaction steps for the development of a mechanism. The goal is to understand not only more than just the properties of the catalyst but also how it functions. In order to understand such details, transient experiments will provide the most insight into the elementary steps as well as secondary processes (e.g., surface/bulk diffusion) that make up the complex catalytic system. 

Catalytic reaction mechanisms represent the elementary steps at molecular level, but the expression in detail at molecular level is difficult for the catalytic reactions on solid surfaces. So, the usual strategy is to propose applicable reaction mechanisms for key chemical processes occurring on catalyst surface, based on a series of assumptions that must be in agreement with experimental data and could be modified indispensably. 

In general, catalytic reaction mechanisms are considerably more complex than the scheme of (2.113). Reaction intermediates other than adsorbed reactants or products may appear, complicating the expressions for the coverages of all species present on the surface. Moreover, all reaction steps may in principle proceed both in the forward and the reverse direction. The kinetics of such mechanisms is usually treated under the assumption that one of the elementary steps determines the rate while all other steps are essentially at equilibrium. We illustrate the use of this approach by discussing the kinetics of ethylene hydrogenation. 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

Writing out the catalytic reaction between A and B in elementary steps according to the Langmuir-Hinshelwood mechanism, we obtain ... 

Unraveling catalytic mechanisms in terms of elementary reactions and determining the kinetic parameters of such steps is at the heart of understanding catalytic reactions at the molecular level. As explained in Chapters 1 and 2, catalysis is a cyclic event that consists of elementary reaction steps. Hence, to determine the kinetics of a catalytic reaction mechanism, we need the kinetic parameters of these individual reaction steps. Unfortunately, these are rarely available. Here we discuss how sticking coefficients, activation energies and pre-exponential factors can be determined for elementary steps as adsorption, desorption, dissociation and recombination. 

Currently, the density functional theory (DFT) method has become the method of choice for the study of reaction mechanism with transition-metals involved. Gradient corrected DFT methods are of particular value for the computational modeling of catalytic cycles. They have been demonstrated in numerous applications for several elementary processes, to be able to provide quantitative information of high accuracy concerning structural and energetic properties of the involved key species and also to be capable of treating large model systems.30... 

This proposed catalytic mechanism (Chong and Sharpless, 1977) requires four reaction steps (3 bimolecular and 1 unimolecular), which take place on a molybdenum metal center (titanium and vanadium centers are also effective), to which various nonreactive ligands (L) and reactive ligands (e g., O-R) are bonded. Each step around the catalytic cycle is an elementary reaction and one complete cycle is called a turnover. 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

Heterogeneous catalysis is primarily a molecular phenomenon since chemical bonds are created and/or broken (between the molecule and the surface) this implies that surface organometallic fragments are intermediates in any catalytic reaction on a surface. If one can design and synthesize surface organometallic fragments and study their reachvity, especially elementary steps, then one possesses in principle a crihcal tool to better understand the mechanisms of heterogeneous catalysis. 

Rate equations for simple reversible reactions are often developed from mechanistic models on the assumption that the kinetics of elementary steps can be described in terms of rate constants and surface concentrations of intermediates. An application of the Langmuir adsorption theory for such development was described in the classic text by Hougen and Watson (/ ), and was used for constructing rate equations for a number of heterogeneous catalytic reactions. In their treatment it was assumed that one step would be rate-controlling for a unique mechanism with the other steps at equilibrium. 

A knowledge of the kinetic parameters and, in particular, the orders of reaction of a catalysed reaction is important to the accurate definition of the reaction mechanism. However, catalytic hydrogenation reactions proceed through a series of elementary steps, only one of which may be ratedetermining. In consequence, the observed rate expressions give little or no direct information about most of the steps involved and kinetics alone are not sufficient for a precise description of the mechanism. 

Similar reaction sequences have been identified in other chemically reacting systems, specifically catalytic combustion (52, 53), solid-fuel combustion (54), transport and reaction in high-temperature incandescent lamps (55), and heterogeneous catalysis (56 and references within). The elementary reactions in hydrocarbon combustion are better understood than most CVD gas-phase reactions are. Similarly, the surface reaction mechanisms underlying hydrocarbon catalysis are better known than CVD surface reactions. 

In 2001, Mirodatos et al. [89] stressed the importance of transient studies as an alternative to steady continuous reactor operations. A combination of microkinetic analysis together with transient experiments should allow the determination of the global catalytic conversion from elementary reaction steps. Prerequisite for such analysis is the correlation of experimental data with the data of a model. Compliance between the data helps to derive the reaction mechanism. 

For linear mechanisms we have obtained structurized forms of steady-state kinetic equations (Chap. 4). These forms make possible a rapid derivation of steady-state kinetic equations on the basis of a reaction scheme without laborious intermediate calculations. The advantage of these forms is, however, not so much in the simplicity of derivation as in the fact that, on their basis, various physico-chemical conclusions can be drawn, in particular those concerning the relation between the characteristics of detailed mechanisms and the observable kinetic parameters. An interesting and important property of the structurized forms is that they vividly show in what way a complex chemical reaction is assembled from simple ones. Thus, for a single-route linear mechanism, the numerator of a steady-state kinetic equation always corresponds to the kinetic law of the overall reaction as if it were simple and obeyed the law of mass action. This type of numerator is absolutely independent of the number of steps (a thousand, a million) involved in a single-route mechanism. The denominator, however, characterizes the "non-elementary character accounting for the retardation of the complex catalytic reaction by the initial substances and products. 

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