Heterogeneous catalysis adsorption step

Adsorption on solids is an important step in the industrially important process of heterogeneous catalysis. Adsorption, which takes place on the surface (including that of the pores) of the solid, should be distinguished from absorption, which occurs throughout its bulk. The latter is illustrated by the taking up of water by anhydrous calcium chloride. 

From the point of view of heterogeneous catalysis the key characteristics of MOFs are porosity and stability of the crystal lattice due to the relatively strong coordinative and Coulombic attraction between the nodes and the linkers [8]. One of the main characteristics of MOFs is the large surface area and open porosity of these materials [9,10]. Since in heterogeneous catalysis adsorption of substrates and reagents on the solid surface is the first step of the reaction, it comes out that large surface area is generally a prerequisite for... 

In particular, reactions in heterogeneous catalysis are always a series of steps, including adsorption on the surface, reaction, and desorption back into the gas phase. In the course of this chapter we will see how the rate equations of overall reactions can be constructed from those of the elementary steps. 

Adsorption of reactants on the surface of the catalyst is the first step in every reaction of heterogeneous catalysis. Flere we focus on gases reacting on solid catalysts. Although we will deal with the adsorption of gases in a separate chapter, we need to discuss the relationship between the coverage of a particular gas and its partial pressure above the surface. Such relations are called isotherms, and they form the basis of the kinetics of catalytic reactions. 

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. 

D.A. Rudd, L.A. Apuvicio, J.E. Bekoske and A.A. Trevino, The Microkinetics of Heterogeneous Catalysis (1993), American Chemical Society, Washington DC]. Ideally, as many parameters as can be determined by surface science studies of adsorption and of elementary steps, as well as results from computational studies, are used as the input in a kinetic model, so that fitting of parameters, as employed in Section 7.2, can be avoided. We shall use the synthesis of ammonia as a worked example [P. Stoltze and J.K. Norskov, Phys. Rev. Lett. 55 (1985) 2502 J. Catal. 110 (1988) Ij. 

Surface faceting may be particularly significant in chiral heterogeneous catalysis, particularly in the N i/P-ketoester system. The adsorption of tartaric add and glutamic acid onto Ni is known to be corrosive and it is also established that modifiers are leached into solution during both the modification and the catalytic reaction [28]. The preferential formation of chiral step-kink arrangements by corrosive adsorption could lead to catalytically active and enantioselective sites at step-kinks with no requirement for the chiral modifier to be present on the surface. 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

As discussed earlier, the first step in heterogeneous catalysis is the adsorption of the molecules of the reactants on the surface of the adsorbent or of the catalyst (inner and outer surfaces). Then, molecular dissociation of at least one or two reacting components takes place, usually preceded by surface diffusion. The next step is a surface reaction, which is... 

Why adsorption, ion exchange and heterogeneous catalysis in one book The basic similarity between these phenomena is that they all are heterogeneous fluid-solid operations. Second, they are all driven by diffusion in the solid phase. Thus, mass transfer and solid-phase diffusion, rate-limiting steps, and other related phenomena are common. Third, the many aspects of the operations design of some reactors are essentially the same or at least similar, for example, the hydraulic analysis and scale-up. Furthermore, they all have important environmental applications, and more specifically they are all applied in gas and/or water treatment. 

There are many more types of elementary processes in heterogeneous catalysis than in gas phase reactions. In heterogeneous catalysis the elementary processes are broadly classified as either adsorption-desorption or surface reaction, i.e., elementary processes which involve reaction of adsorbed species. Free surface sites and molecules from the fluid phase may or may not participate in surface reaction steps. 

The mechanism of heterogeneous catalysis is often complex and not well understood. Important steps, however, involve (1) attachment of reactants to the surface of the catalyst, a process called adsorption, (2) conversion of reactants to products on the surface, and (3) desorption of products from the surface. The adsorption step is thought to involve chemical bonding of reactants to the highly reactive metal atoms on the surface with accompanying breaking, or at least weakening, of bonds in the reactants. 

The adsorption of CO is probably the most extensively investigated surface process. CO is a reactant in many catalytic processes (methanol synthesis and methanation, Fischer-Tropsch synthesis, water gas shift, CO oxidation for pollution control, etc. (1,3-5,249,250)), and CO has long been used as a probe molecule to titrate the number of exposed metal atoms and determine the types of adsorption sites in catalysts (27,251). However, even for the simplest elementary step of these reactions, CO adsorption, the relevance of surface science results for heterogeneous catalysis has been questioned (43,44). Are CO adsorbate structures produced under typical UHV conditions (i.e., by exposure of a few Langmuirs (1 L = 10 Torrs) at 100—200 K) at all representative of CO structures present under reaction conditions How good are extrapolations over 10 or more orders of magnitude in pressure Such questions are justified, because there are several scenarios that may account for differences between UHV and high-pressure conditions. Apart from pressure, attention must also be paid to the temperature. 

Palladium is one of the most frequently used metals in heterogeneous catalysis, used for hydrogenation as well as oxidation reactions. As discussed below, a variety of palladium model catalyst surfaces were used to characterize CO adsorption and the coadsorption and reaction of CO with hydrogen, both under UHV and at atmospheric pressure. Figure 14 shows schematic models of smooth and stepped... 

Tamaru (110) also discusses examples from heterogeneous catalysis in which reaction rates of one step seem to be influenced by the adsorption of other components. For example, Nishimura et al 115) studied the dehydrogenation of ethanol to acetaldehyde and hydrogen over a specially prepared Nb/Si02 catalyst at 523 K. The studies were done in a recirculating closed (batch) reactor. The rate is about constant as time increases, and the IR spectrum of an adsorbed intermediate remains constant. A sudden evacuation of the gas-phase ethanol stops the reaction but does not affect... 

It is generally accepted that heterogeneous catalysis represents a sequence of elementary reactions such as the adsorption of the reactant on the catalyst surface, atomic rearrangements of the adsorbed particles, and desorption of the products, the overall reaction rate being governed by the slowest step of these elementary reactions. The rate of the slowest... 

Of all of these steps, only the adsorption of the reactants, the reaction between the adsorbed species, and the desorption of the product are chemical reactions. The rest are physical processes that can strongly influence not only the apparent rate of the reaction but also the nature of the products obtained. To fully utilize heterogeneous catalysis as a viable synthetic procedure, then, it is important to recognize not only how these different transport steps can influence... 

Since the adsorption of reactants and desorption of products are unavoidable and fundamental steps of heterogeneous catalysis, there is a need of understanding the kinetics of adsorption-desorption phenomena on heterogeneous surfaces. In practically all situations the reaction involves two or more reactants, so that chemically interesting kinetics are multicomponent. This is immediately understood either considering the Rideal mechanism ... 

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