Heterogeneously catalyzed surface reactions

Fig. 2.1-10 Overall course ofa heterogeneously catalyzed surface reaction (schematic see text for discussion).

Rate Equations for Heterogeneously Catalyzed Surface Reactions... 

The IR study performed in static controlled atmospheres in the IR cell allowed us to identify a number of adsorbed intermediate and secondary products, together with the main reaction products in oxidation and ammoxidation of toluene and the three xylene isomers. Surface reactions schemes are proposed that account for most of the mechanistic features of the heterogeneously-catalyzed industrial reactions. Our data support the following conclusions ... 

A proper description of heterogeneously catalyzed oxidation reactions must treat several difficult problems simultaneously. First is the characterization of the solid surface in its reactive state. What oxygen species exist on this surface and what reactions does each species undergo What other sites for adsorption are present Second is the problem of reaction path. What steps are involved in the reaction What are the structures and relative energy contents of the intermediates Third is the problem of reaction velocity, a general and difficult problem in all chemistry. What are transition states, activation energies, and reaction probabilities for the various steps ... 

Since the ORR is a surface reaction that involves adsorbed intermediates, it is subdued to the Sabatier-volcano principle of heterogeneous catalysis (Balandin, 1969 Sabatier, 1920). This principle states that in order for a catalyzed surface reaction to proceed, some bonding of adsorbed intermediate(s) is necessary, whereas a bonding that is too strong will block the surface and slow down the reaction. Several examples of reactions in heterogeneous catalysis and electrochemistry that obey these principles are discussed by Parsons (2011). 

The commonly accepted mechanism of heterogeneously catalyzed hydrogenation involves activation of both the hydrogen and the C—C multiple bond adsorbed on the metal surface. First one hydrogen atom is transferred to the least hindered position of the multiple bond to give a half-hydrogenated adsorbed species. This reaction is fully reversible and ac-... 

E. V. Albano. On the influence of reactant s induced surface transformations in the behavior of a heterogeneously catalyzed dimer-monomer reaction model. J Chem Phys 709 7498-7505, 1998. 

A quite analogous treatment may be applied to the kinetics of heterogeneously catalyzed reactions. Consider a surface S that contains so active sites to which reactant A, a gaseous or solute species, may bind reversibly with an equilibrium constant KA ... 

As is well known chemical reactions are accelerated by increasing the temperature. This also applies to heterogeneously catalyzed reactions taking place on the surface of polar sorbents such as aluminium oxide or silica gel (Tables 2.1 and 2.2). Such reactions have also been reported on the moderately polar NH2 layers. Alperin et al. have described the activation of cellulose to yield specific information concerning the substances chromatographed [1]. 

Chapter 10 begins a more detailed treatment of heterogeneous reactors. This chapter continues the use of pseudohomogeneous models for steady-state, packed-bed reactors, but derives expressions for the reaction rate that reflect the underlying kinetics of surface-catalyzed reactions. The kinetic models are site-competition models that apply to a variety of catalytic systems, including the enzymatic reactions treated in Chapter 12. Here in Chapter 10, the example system is a solid-catalyzed gas reaction that is typical of the traditional chemical industry. A few important examples are listed here ... 

Steps 1 through 9 constitute a model for heterogeneous catalysis in a fixed-bed reactor. There are many variations, particularly for Steps 4 through 6. For example, the Eley-Rideal mechanism described in Problem 10.4 envisions an adsorbed molecule reacting directly with a molecule in the gas phase. Other models contemplate a mixture of surface sites that can have different catalytic activity. For example, the platinum and the alumina used for hydrocarbon reforming may catalyze different reactions. Alternative models lead to rate expressions that differ in the details, but the functional forms for the rate expressions are usually similar. 

Example 10.3 Suppose that adsorption is much slower than surface reaction or desorption for the heterogeneously catalyzed reaction A < > P. 

Vibrational spectroscopic studies of heterogeneously catalyzed reactions refer to experiments with low area metals in ultra high vacuum (UHV) as well as experiments with high area, supported metal oxides over wide ranges of pressure, temperature and composition [1]. There is clearly a need for this experimental diversity. UHV studies lead to a better understanding of the fundamental structure and chemistry of the surface-adsorbate system. Supported metals and metal oxides are utilized in a variety of reactions. Their study leads to a better understanding of the chemistry, kinetics and mechanisms in the reaction. Unfortunately, the most widely used technique for determining adsorbate molecular structure in UHV,... 

Our goal is to bridge the gap between studies of well-characterized surfaces in UHV and studies of high pressure, complex, heterogeneously catalyzed reactions. To achieve this goal, we... 

Generally, whenever fluids are processed in a confined space, two different types of phenomena are observed surface and volume effects. An example of a surface effect is a heterogeneously catalyzed reaction occurring at the walls of the vessel, whereas the motion of a fluid due to gravitational forces would be described as a volume effect. In brief, it can be stated that the surface effects gain in importance compared with the volume effects when the size of a reactor decreases. In particular, the reduction of length scale leads to a... 

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

In principle, sites a, IT, and c need not be association sites as depicted by Ogston but could be steric sites that form obstructions such that the adsorbed molecule is chirally directed. Only one active site is actually required providing the remaining two sites (protuberances or cavities) are different from each other and from the active site that catalyzes the reaction. They could be identical providing they are not symmetrically oriented with respect to the active site (not an isosceles triangle). These are the basic concepts for a chiral environment on a surface and they lead to the three basic methods for creating chiral surfaces in heterogeneous catalysis. 

In heterogeneous liquid/liquid reactions, cavitational collapse at or near the interface will cause disruption and mixing, resulting in the formation of very fine emulsions. When very fine emulsions are formed, the surface area available for the reaction between the two phases is significantly increased, thus increasing the rates of reaction. The emulsions formed using cavitation, are usually smaller in size and more stable, than those obtained using conventional techniques and often require little or no surfactant to maintain the stability [8]. This is very beneficial particularly in the case of phase-transfer catalyzed reactions or biphasic systems. 

Steam reforming is a heterogeneously catalyzed process, with nickel catalyst deposited throughout a preformed porous support. It is empirically observed in the industry, that conversion is proportional to the geometric surface area of the catalyst particles, rather than the internal pore area. This suggests that the particle behaves as an egg-shell type, as if all the catalytic activity were confined to a thin layer at the external surface. It has been demonstrated by conventional reaction-diffusion particle modelling that this behaviour is due to... 

Heterogeneously catalyzed hydrogenation is a three-phase gas-liquid-solid reaction. Hydrogen from the gas phase dissolves in the liquid phase and reacts with the substrate on the external and internal surfaces of the solid catalyst Mass transfer can influence the observed reaction rate, depending on the rate of the surface reaction [15]. Three mass transfer resistances may be present in this system (Fig. 42.1) ... 

In heterogeneous catalyzed reaction, the surface of a solid serves as a catalyst while the reactants are in gaseous or liquid phase. One or more reactants get adsorbed on the surface of the solid. This phenomenon is similar to the formation of intermediate complex in homogeneous catalysis. In many cases, heat of adsorption leads to the activation of reactants and the reaction takes place easily. The surface thus provides an alternative path for the reaction to proceed with an accelerated rate. The reaction consists of the following four consecutive steps. 

The solid state and the surface chemistry of some of the solid Fe-phases impart to these oxides and sulfides the ability to catalyze redox reactions. Surface complexes and the solid phases themselves acting as semiconductors can participate in photoredox reactions, where light energy is used to drive a thermodynamically unfavorable reaction (heterogeneous photosynthesis) or to catalyze a thermodynamically favorable reaction (heterogeneous photocatalysis). 

A guide to the manner in which structural theory may be applied to a detailed consideration of the mechanism of a surface-catalyzed reaction is found in papers by Cossee (113), Arlman (114), and Arlman and Cossee (115) concerning the mechanism of the stereoregular heterogeneous catalyzed polymerization of propylene. Particular crystallographic sites are shown to be the active centers at which the reactants combine and ligand field theory is used to demonstrate a plausible relationship between the activation energy for the conversion of adsorbed reactants to the product and the properties of the transition metal complex which constitutes the reaction center. 

---