Heterogeneous catalysis predicted reactions

Heterogeneous catalysis entails reactions between organic molecules and the surface of inorganic materials. Although a great deal of work has been carried out, and continues, to characterize the surfaces of materials, we remain far from being able to predict surface properties for all but the simplest. Even... 

All these steps can influence the overall reaction rate. The reactor models of Chapter 9 are used to predict the bulk, gas-phase concentrations of reactants and products at point (r, z) in the reactor. They directly model only Steps 1 and 9, and the effects of Steps 2 through 8 are lumped into the pseudohomoge-neous rate expression, a, b,. ..), where a,b,. .. are the bulk, gas-phase concentrations. The overall reaction mechanism is complex, and the rate expression is necessarily empirical. Heterogeneous catalysis remains an experimental science. The techniques of this chapter are useful to interpret experimental results. Their predictive value is limited. 

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

The situation described here is based on a simple one-electron model which can hardly be expected to predict the behaviour of complex many-electron systems in quantitative detail. There can be no doubt however, that the qualitative picture is convincing and probably that the broad principles of electronic behaviour in solids have been identified. The most significant feature of the model is the band structure that makes no sense except in terms of the electron as a wave. Important, but largely unexplored aspects of solid-state reactions and heterogeneous catalysis must also relate to the nearly-free models of electrons in solids. 

Research tools and fundamental understanding New catalyst design for effective integration of bio-, homo- and heterogeneous catalysis New approaches to realize one-pot complex multistep reactions Understanding catalytic processes at the interface in nanocomposites New routes for nano-design of complex catalysis, hybrid catalytic materials and reactive thin films New preparation methods to synthesize tailored catalytic surfaces New theoretical and computational predictive tools for catalysis and catalytic reaction engineering... 

Some of the reasons for these drawbacks are inherent to the heterogeneous character. The multiplicity of active sites in terms of surface and bulk structure and their low concentration prevent the easiness of achievement of a reUable structure-activity relationship, a necessary step for improvement of existing catalysts or even for a predictive approach for new catalytic reactions. Despite all these drawbacks, heterogeneous catalysis remains the most applied solution for one simple reason catalyst separation from reagents or reaction products is usually easy which renders industrial processes more easily achievable. 

This chapter is not intended to furnish a comprehensive review of the latest theoretical developments across all types of reaction and catalyst in heterogeneous catalysis. It will instead briefly survey two cross-cutting themes that have in recent years benefited substantially from theoretical insights, especially what first-principles DFT calculations have been able to explain and predict, and that are commanding substantial current interest the design of new catalysts aided by theoretical insights and the elucidation of the molecular level effects of reactive physiochemical environments on catalytic reactivity. The relatively simple and predictable structure of metals... 

Dynamic effects are a potentially important but easily overlooked aspect of heterogeneous catalysis that can nonetheless impact the accuracy of our knowledge and predictions. For example, multiple co-existing meta-stable surface oxide phases have been identified for Pd and Ag interacting with oxygen, which suggests that the catalyst surfaces may be in a state of flux under reaction conditions, adding new uncertainty to the nature of the... 

While many techniques have evolved to evaluate surface intermediates, as will be discussed below, it is equally important to also obtain information on gas phase intermediates, as well. While the surface reactions are interesting because they demonstrate heterogeneous kinetic mechanisms, it is the overall product yield that is finally obtained. As presented in a text by Dumesic et al. one must approach heterogeneous catalysis in the way it has been done for gas phase systems, which means using elementary reaction expressions to develop a detailed chemical kinetic mechanism (DCKM). DCKMs develop mechanisms in which only one bond is broken or formed at each step in the reaction scheme. The DCKM concept was promoted and used by numerous researchers to make great advances in the field of gas phase model predictions. 

In the case of heterogeneous catalysis, a DCKM or microkinetic model must incorporate the added dimension of adsorbed chemical species as well as active versus non-active sites. To obtain the full predictive capability from reactant influent to product effluent, all possible reactions in the system, both heterogeneous and homogeneous, must be accounted for. To properly understand the catalytic reaction sequence, it is possible that seemingly unimportant intermediates on the surface may be what initiate gas phase reactions. To begin this elucidation, the surface chemical species and their properties must be known. 

Different mechanisms and rate-controlling steps may produce rate equations of the same algebraic form, making it impossible to identify mechanism and rate control conclusively on the basis of an empirical rate equation alone. This happens more often in heterogeneous catalysis than in homogeneous reactions. A clearer indication may be gained from studies of the temperature dependence of the coefficients, concentration dependence of initial rates, and tests of model predictions. 

At this time it had become possible to determine experimentally total surface area and the distribution of sizes and total volume of pores. Wheeler set forth to provide the theoretical development of calculating the role of this pore structure in determining catalyst performance. In a very slow reaction, reactants can diffuse to the center of the catalyst pellet before they react. On the other hand, in the case of a very active catalyst containing small pores, a reactant molecule will react (due to collision with pore walls) before it can diffuse very deeply into the pore structure. Such a fast reaction for which diffusion is slower than reaction will use only the outer pore mouths of a catalyst pellet. An important result of the theory is that when diffusion is slower than reaction, all the important kinetic quantities such as activity, selectivity, temperature coefficient and kinetic reaction order become dependent on the pore size and pellet size with which a pellet is prepared. This is because pore size and pellet size determine the degree to which diffusion affects reaction rates. Wheeler saw that unlike many aspects of heterogeneous catalysis, the effects of pore structure on catalyst behavior can be put on quite a rigorous basis, making predictions from theory relatively accurate and reliable. 

Although a decline in research activity in the field of heterogeneous catalysis was predicted in recent years, this in fact did not happen. Instead, stricter environmental requirements and the general trend towards milder reaction conditions have meant that heterogeneous catalysis has increased in importance [5]. This is demonstrated by many examples, including ... 

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