Adsorbate surface reactions

The above situation led to the proposal by Rideal [202] of what has become an important alternative mechanism for surface reactions, illustrated by Eq. XVIII-33. Here, reaction takes place between chemisorbed atoms and a colliding or physical adsorbed molecule (see Ref. 203). 

The Langmuir-Hinshelwood picture is essentially that of Fig. XVIII-14. If the process is unimolecular, the species meanders around on the surface until it receives the activation energy to go over to product(s), which then desorb. If the process is bimolecular, two species diffuse around until a reactive encounter occurs. The reaction will be diffusion controlled if it occurs on every encounter (see Ref. 211) the theory of surface diffusional encounters has been treated (see Ref. 212) the subject may also be approached by means of Monte Carlo/molecular dynamics techniques [213]. In the case of activated bimolecular reactions, however, there will in general be many encounters before the reactive one, and the rate law for the surface reaction is generally written by analogy to the mass action law for solutions. That is, for a bimolecular process, the rate is taken to be proportional to the product of the two surface concentrations. It is interesting, however, that essentially the same rate law is obtained if the adsorption is strictly localized and species react only if they happen to adsorb on adjacent sites (note Ref. 214). (The apparent rate law, that is, the rate law in terms of gas pressures, depends on the form of the adsorption isotherm, as discussed in the next section.)... 

How are fiindamental aspects of surface reactions studied The surface science approach uses a simplified system to model the more complicated real-world systems. At the heart of this simplified system is the use of well defined surfaces, typically in the fonn of oriented single crystals. A thorough description of these surfaces should include composition, electronic structure and geometric structure measurements, as well as an evaluation of reactivity towards different adsorbates. Furthemiore, the system should be constructed such that it can be made increasingly more complex to more closely mimic macroscopic systems. However, relating surface science results to the corresponding real-world problems often proves to be a stumbling block because of the sheer complexity of these real-world systems. 

With the aid of (B1.25.4), it is possible to detennine the activation energy of desorption (usually equal to the adsorption energy) and the preexponential factor of desorption [21, 24]. Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters and v dependent on coverage [22]- hr the case of TPRS one obtains infonnation on surface reactions if the latter is rate detennming for the desorption. 

Adsorption of Metal Ions and Ligands. The sohd—solution interface is of greatest importance in regulating the concentration of aquatic solutes and pollutants. Suspended inorganic and organic particles and biomass, sediments, soils, and minerals, eg, in aquifers and infiltration systems, act as adsorbents. The reactions occurring at interfaces can be described with the help of surface-chemical theories (surface complex formation) (25). The adsorption of polar substances, eg, metal cations, M, anions. A, and weak acids, HA, on hydrous oxide, clay, or organically coated surfaces may be described in terms of surface-coordination reactions ... 

An alternative surface reaction which has been suggested is a reaction between an adsorbed oxygen atom with an adsorbed carbon monoxide molecule to form carbon dioxide which is immediately desorbed. The reaction rate is again given by the equation above. 

In the case of coupled heterogeneous catalytic reactions the form of the concentration curves of analytically determined gaseous or liquid components in the course of the reaction strongly depends on the relation between the rates of adsorption-desorption steps and the rates of surface chemical reactions. This is associated with the fact that even in the case of the simplest consecutive or parallel catalytic reaction the elementary steps (adsorption, surface reaction, and desorption) always constitute a system of both consecutive and parallel processes. If the slowest, i.e. ratedetermining steps, are surface reactions of adsorbed compounds, the concentration curves of the compounds in bulk phase will be qualitatively of the same form as the curves typical for noncatalytic consecutive (cf. Fig. 3b) or parallel reactions. However, anomalies in the course of bulk concentration curves may occur if the rate of one or more steps of adsorption-desorption character becomes comparable or even significantly lower then the rates of surface reactions, i.e. when surface and bulk concentration are not in equilibrium. 

From this study and the studies mentioned earlier, it can be concluded that the metathesis of propene is well interpreted kinetically by assuming that the rate is controlled by the surface reaction between the adjacent adsorbed molecules, the two active sites being localized at the same active center or at two neighboring active centers. 

Though as yet in its infancy, the application of laser Raman spectroscopy to the study of the nature of adsorbed species appears certain to provide unusually detailed information on the structure of oxide surfaces, the adsorptive properties of natural and synthetic zeolites, the nature of adsorbate-adsorbent interaction, and the mechanism of surface reactions. 

Even with the simpler Frumkin or Fowler-Guggenheim approach (Eqs. 6.50 and 6.52), treating the coadsorption and surface reaction of different adsorbates leads immediately to mathematically intractable expressions and to the introduction of new parameters, whereas equation... 

Fig. 15-4 Analogy between dissolved ligands and adsorbents (surface-bound ligands) (a) surface acid-base reactions (b) surface complexation of free metals (c) formation of "mixed-ligand" surface complexes.

Step 5. A surface reaction occurs between adsorbed species. The prototypical reaction is... 

When the adsorption step determines the rate, component A no longer retards the reaction. Any A that is adsorbed will quickly react, and the concentration of [AS] sites will be low. Note that the desorption step is now treated as being reversible. Thus, any P in the gas phase will retard the reaction even if the surface reaction is irreversible, kr = 0. 

Example 10.12 Estimate the surface inventory of component A for the catalytic CSTR in Example 10.9. Assume that the surface reaction is ratecontrolling and that A is the only adsorbed species. Suppose A is a moderately large molecule that occupies a site that is 1 nm by 1 nm. 

This equation gives (0) = 0, a maximum at =. /Km/K2, and (oo) = 0. The assumed mechanism involves a first-order surface reaction with inhibition of the reaction if a second substrate molecule is adsorbed. A similar functional form for (s) can be obtained by assuming a second-order, dual-site model. As in the case of gas-solid heterogeneous catalysis, it is not possible to verify reaction mechanisms simply by steady-state rate measurements. 

Power law expressions are useful as long as the approximate orders of reactant concentration are constant over a particular concentration course. A change in the order of the reaction corresponds to a change in the surface concentration of a particular reactant. A low reaction order usually implies a high surface concentration, a low reaction order, and a low surface reaction of the corresponding adsorbed intermediates. In order to deduce (Eq. (1.17b)) the rate of surface carbon hydrogenation, the power law of Eq. (1.18) has been used. 

We have developed a compact photocatalytic reactor [1], which enables efficient decomposition of organic carbons in a gas or a liquid phase, incorporating a flexible and light-dispersive wire-net coated with titanium dioxide. Ethylene was selected as a model compound which would rot plants in sealed space when emitted. Effects of the titanium dioxide loading, the ethylene concentration, and the humidity were examined in batches. Kinetic analysis elucidated that the surface reaction of adsorbed ethylene could be regarded as a controlling step under the experimental conditions studied, assuming the competitive adsorption of ethylene and water molecules on the same active site. 

Kinetic analysis based on the Langmuir-Hinshelwood model was performed on the assumption that ethylene and water vapor molecules were adsorbed on the same active site competitively [2]. We assumed then that overall photocatalytic decomposition rate was controlled by the surface reaction of adsorbed ethylene. Under the water vapor concentration from 10,200 to 28,300ppm, and the ethylene concentration from 30 to 100 ppm, the reaction rate equation can be represented by Eq.(l), based on the fitting procedure of 1/r vs. 1/ Ccm ... 

Calculated results are shovra as solid lines in Fig.4 under the respective experimental conditions, and the assumption that the rate was controlled by the surface reaction of adsorbed ethylene was reasonable under the experimental conditions studied. 

Note that the rates of product formation and reactant conversion indeed have the dimensions of mol per unit of time, and that these rates are proportional to the number of sites, or, in fact, the amount of catalyst present in the reactor. Also, in the case of a second order reaction, e.g. betv een adsorbed species A and B, we write the rate in the form r = Nk0j 0 by applying the mean-field approximation. Here the rate is proportional to both the total number of sites on the surface and the probability of finding a species A adjacent to a species B on the surface, the latter being proportional to the coverages of A and B. In the mean-field approximation A and B are distributed randomly over the N available sites this only tends to be valid when the adsorbents repel each other. Thus the rate is not r= k(N0/ )(N02,) since the reactants need to be on adjacent sites. Another important consideration is that we want the rate to be linearly proportional to the amount of catalyst in the reactor, in accordance with r = Nk0A0B for a second order surface reaction. 

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