The Process of Chemisorption

The sticking probability S of the hydrogen molecule being the simplest measure of metal surface reactivity, its dependence upon crystal face and coverage may provide ideas for understanding other reactions. In the absence of a precursor state and on an energetically homogeneous surface S will depend upon 9 as 

There is a useful application of the Principle of Microscopic Reversibility (or Detailed Balancing) in the study of surface processes. This is a principle that requires that, when carried out under identical conditions, the reverse of any process should proceed by exactly the same route as the forward process thus whatever energy input is needed for the chemisorption of a hydrogen molecule will be recovered and released when the two atoms recombine and desorb. Measurement of the relaxation of the vibrational and translational energy of the desorbing molecule therefore provides information on the needs in dissociation, and values of S can also be derived.  

As we shall see in what follows, in very many cases there are distinct phases formed as the coverage by hydrogen atoms increases these show characteristic values of heat of adsorption, sticking probability S and other physical properties. The fine structures of S versus 9 plots that are observed when the process of chemisorption is not energetically uniform will be considered in the following section. 

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Adopting the current standpoint that the process of chemisorption can be treated as chemical reaction of adsorption particle with adsorption center accounting for effect of both the reaction on the whole adsorbent and adsorbent on the reaction proper, i.e. accounting for the... 

On experimental level the question regarding the centers of adsorption was addressed in numerous papers. For instance, in [66] the experimental data were used to show that in case of adsorption of hydrogen atoms on the surface of zinc oxide the centers of chemisorption can be provided by regular oxygen ions of the lattice, i.e. the process of chemisorption of H-atoms can be shown as the following sequence of reactions ... 

We should note that we used Ns as Nsaax (which is the concentration of chemisorbed radicals at the moment of activation of the source of radicals) in expressions (2.105) and (2.106). This means that we have assumed that the process of chemisorption is already stationary by this moment of time. 

A metal cluster can be considered as a polynuclear compound which contains at least one metal-metal bond. A better definition of cluster catalysis is a reaction in which at least one site of the cluster molecule is mechanistically necessary. Theoretically, homogeneous clusters should be capable of multiple-site catalysis. Many heterogeneous catalytic reactions require multiple-site catalysis and for these reasons discrete molecular metal clusters are often proposed as models of metal surfaces in the processes of chemisorption and catalysis. The use of carbonyl clusters as catalysts for hydrogenation reactions has been the subject of a number of papers, an important question actually being whether the cluster itself is the species responsible for the hydrogenation. Often the cluster is recovered from the catalytic reaction, or is the only species spectroscopically observed under catalytic conditions. These data have been taken as evidence for cluster catalysis. 

Parallel studies were performed by Liu and Davison (1988), who investigated the process of chemisorption on inverse-supported catalysts, where the surface him is a semiconductor and the underlying support is a metal. Again a key parameter was found to be him thickness, and the substrate was observed to behave as either an acceptor or a donor, depending upon that thickness. The lack of charge self-consistency in this work was addressed by Sun et al (1994a), who also studied the effects of thickness and different metal constituents. 

The process of chemisorption of the substrate molecule at the catalyst surface involves a chemical interaction between the substrate and an active site on the surface. 

In the foregoing sections of this review, attention has been largely confined to a discussion of the interaction of species once they are adsorbed at the catalyst surface. It has been implicitly assumed that, for the processes of chemisorption and reaction, sites possessing the necessary characteristics are available at the catalyst surface. In this last section, consideration is given to the physical and chemical properties of the catalyst which give rise to hydrogenation activity. 

Each of the various processes of adsorption may have desorptions of the reverse forms, for example, dissociative adsorption may have as its reverse, associative desorption. However, the process of chemisorption may not be reversible [ 1.2.2(c)]. Desorption may lead to species other than that adsorbed, for example, ethane dissociatively adsorbed on clean nickel gives little or no ethane upon desorption, 1-butene dissociatively adsorbed to methylallyl and H on zinc oxide gives mainly 2-butenes upon desorption, and some W03 may evaporate from tungsten covered with adsorbed oxygen. 

If A or B dissociates during the process of chemisorption, both the driving force and the adsorption term should be modified (see for example cases l(v) and l(vi) above). For a full discussion of such situations see Hougen and Watson(<0 ... 

Solidification with cement generally is accomplished with a Portland cement and other additives. The quantity of cement can be varied according to the amount of moisture in the waste. Heavy metal cations in the waste form insoluble carbonates and hydroxides at the high pH of the mixture. The surface of the hardened mass can be coated with asphalt or other material to reduce leaching of hazardous components. If the waste is mixed with anhydrous cement and water there is the possibility of ions incorporation in the cement structure during the hydrolysis process. Heavy metal ions could bind with the cement by the process of chemisorption, precipitation, surface adsorption,... 

The process of chemisorption of reactants requires adsorption on the surface of the catalyst. Therefore to maximize the rate the catalytic surface area should also be maximized. This is achieved by dispersing the catalytic species onto a high surface area inorganic carrier. An ideal dispersion of Ni on A1203 is shown in Figure 7.1. 

The sorption of H2 on metals which can take place at both high and low temperatures (for example, —180 to 500°C) is accompanied by dissociation of the H2 to II atoms and is undoubtedly a chemical process of metal hydride formation on the metal surface. The sorption of O2 on charcoal, CO, and N2 on transition metals and of olefins on metals are all accompanied by heat evolutions in the neighborhood of 30 to 100 Kcab and are undoubtedly better viewed as chemical reactions than as loose solvation. For these reasons we shall direct our attention to the process of chemisorption in discussing catalytic reactions. 

Carbon monoxide produced by the reactor will poison the fuel cell. This makes it necessary to include complex CO removal systems [8], Nation membranes are very sensitive to CO. The process of chemisorption (adherence to the surface via a chemical reaction) takes place, with CO competing with H2 on the platinum surface [4],... 

Thus far we have examined only the process of chemisorption, in the sense of it being the first step on the road to chemical reaction on a surface. If we summarize to this point, chemisorption is a chemical interaction between the adsorbate and the surface. The heats and activation energies of chemisorption are typical of those of a chemical reaction, and that is exactly what it is a chemical reaction, albeit three-dimensional on one side of the arrow and two-dimensional on the other side. The activation energies are such that the species involved have sufficient energy to cross the activation energy barrier at temperature levels that are experimentally accessible and of practical importance. 

Thus, when considering the viability of various fuel cell electrocatalysts, it is a requirement that effective chemisorption take place on the electrocatalyst surface. For this reason, there is a large amount of interest in trying to understand the processes involving adsorbed intermediates on electrode surfaces to help in the design of improved electrocatalytic materials. The process of chemisorption on electrode surfaces can be either associative or dissociative. An associative process involves the formation of two single bonds attached to the catalyst surface directly from the c-orbitals from a double... 

It is not easy to decide how to order a summary of the vast amount of information available. The theme which we seek to illustrate is how the processes of chemisorption and desorption and the properties of the adsorbed state depend upon the chemical composition of the surface and its atomic stmcture. The rest of this section (3.2) is sub-divided into the chemisorption process (3.2.2), structural aspects of the chemisorbed state (3.2.3) and energetic aspects including desorption (3.2.4). Such subdivision is highly artificial as there are close interrelations between all subsets however, each technique is chiefly directed towards obtaining information of a speciflc type, and integration of the various results is largely an intellectual or modelling exercise. 

Once again the detail needs to be prefaced by some general considerations. The process of chemisorption is in essence a chemical reaction unfortunately, like many chemical reactions, it is not a simple process, as it does not always lead to a well-defined product. The occurrence of the different structures of the hydrogen atom adlayer exemplifies this. Nevertheless the fact that chemisorption takes place means that the Gibbs free energy of the system must decrease, and that because of the loss of translational entropy there has to be a decrease in the system s heat content chemisorption is thus of necessity exothermic. All manner of thermodynamic parameters can therefore be ascribed to the process and to the resulting state. ... 

The adsorption energy is O-I. The surface acquires a positive charge during the process of chemisorption which leads to a rise in the potential barrier Vs, and therefore a decrease of the work function O. 

Later H. S. Taylor (1890-1974) emphasized the fact that surfaces are never smooth on the atomic scale, and that surface sites are therefore of variable activity. Certain sites, which he called active centers, are particularly active and it is on these that catalysis occurs for the most part. Taylor also showed that the process of chemisorption itself is accompanied by an activation energy, and he referred to this type of adsorption as activated adsorption. 

In heterogeneous systems, particularly those involving a solid phase, reactions happen at the interface between the gaseons medium and the solid phase. The most important are those involving adsorption. When gas is in the presence of a solid, some of its molecules bind to the solid surface. These bonds ean sometimes be of comparable force to covalent bonds. This is described as the process of chemisorption, where the gaseous molecules linked to the solid are called adsorbed molecules. 

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