Chemisorption reaction rates

Let us now consider a more complicated reaction and devise the chemisorption reaction rate form. Dehydrogenation reactions are of the form... 

The reaction kinetics approximation is mechanistically correct for systems where the reaction step at pore surfaces or other fluid-solid interfaces is controlling. This may occur in the case of chemisorption on porous catalysts and in affinity adsorbents that involve veiy slow binding steps. In these cases, the mass-transfer parameter k is replaced by a second-order reaction rate constant k. The driving force is written for a constant separation fac tor isotherm (column 4 in Table 16-12). When diffusion steps control the process, it is still possible to describe the system hy its apparent second-order kinetic behavior, since it usually provides a good approximation to a more complex exact form for single transition systems (see Fixed Bed Transitions ). 

To give an idea of the wide rai e of catalytic systems that have been investigated where chemisorption data were essential to interpret the results, some of the author s papers will be discussed. Measurements were reported on the surface areas of a very wide range of metals that catalyze the hydrogenation of ethane. In the earliest paper, on nickel, the specific catalytic activity of a supported metal was accurately measured for the first time it was shown also that the reaction rate was direcdy proportional to the nickel surface area. Studies on the same reaction... 

CO2, N2 and N2O production as a function of the catalyst potential, UWR> obtained at 62IK for fixed inlet pressures of NO and CO. A sharp increase in reaction rate and product is observed as the catalyst potential is reduced below 0 V, i.e., upon Na supply to the Pt catalyst. The selectivity to N2, Sn2, is enhanced from 17% to 62%. This dramatic enhancement in catalytic performance is due to (a) enhanced NO vs CO chemisorption on Pt with decreasing potential and (b) Na-induced dissociation of chemisorbed NO. 

The three remaining steps (chemisorption of reactants, reaction on the surface, and desorption of adsorbed products) are all chemical in nature. It is convenient to employ the concept of a rate limiting step in the treatment of these processes so that the reaction rate becomes equal to that of the slowest step. The other steps are presumed to be sufficiently rapid that quasiequilibrium relations may be used. The overall rate of conversion will then be determined by the interaction of the rate of the process that is rate limiting from a chemical point of view with the rates of the physical mass transfer processes discussed above. 

Sabatier and Balandin had predicted a relationship between catal)dic activity and heat of adsorption. If a solid adsorbs the reactants only weakly, it will be a poor catalyst, but if it holds reactants, intermediates or products too strongly, it wiU again perform poorly. The ideal catalyst for a given reaction was predicted to be a compromise between too weak and too strong chemisorption. Balandin transformed this concept to a semiquantitative theory by predicting that a plot of the reaction rate of a catal)Tic reaction as a function of the heat of adsorption of the reactant should have a sharp maximum. He called these plots volcano-shaped curvesl This prediction was confirmed by Fahrenfort et al." An example of their volcano-shaped curve is reproduced in Fig. 9.1. They chose the catalytic decomposition of formic acid... 

Recent reviews by Ertl and Engel have summarized most of the chemisorption and low pressure catalytic findings . In general, the reaction proceeds through a Langmuir-Hinshelwood mechanism involving adsorbed CO and O atoms. Under reaction conditions typical in most high pressure, supported catalyst studies, and most low pressure UHV studies, the surface is almost entirely covered by CO, and the reaction rate is determined by the rate of... 

The input parameters for the model are the thermodynamics of the gas phase, chemisorption energy and spectroscopic properties for the intermediates, the kinetic parameters for the rate limiting step and the number of active sites on the catalyst. No reference to experimental data for catalytic reaction rates are made in the determination of the input parameters. 

Normally, catalytic activity is expressed as the reaction rate per unit area of active surface (expressed as metre per gram) under given conditions. In a chemical reaction, catalytic conversion is defined as the fraction of reactants converted to products and selectivity is a function of the rate of formation of a desired product with respect to the overall conversion of the initial reactants. The reactant molecules transfer to the catalyst surface where adsorption may occur on an active site , with possible rearrangement of their bonds leading to a chemical adsorption (chemisorption), gas-catalyst reaction and the subsequent desorption of new species. The active site or phase is of high activity and selectivity for the desired products. Thus, the nature of the active sites is important. In many cases, it is not enough to have just activity. Selectivity to desired products is important and often modifiers or promoters are needed both to improve the... 

There have been many attempts to relate bulk electronic properties of semiconductor oxides with their catalytic activity. The electronic theory of catalysis of metal oxides developed by Hauffe (1966), Wolkenstein (1960) and others (Krylov, 1970) is base d on the idea that chemisorption of gases like CO and N2O on semiconductor oxides is associated with electron-transfer, which results in a change in the electron transport properties of the solid oxide. For example, during CO oxidation on ZnO a correlation between change in charge-carrier concentration and reaction rate has been found (Cohn Prater, 1966). 

In the field of nonmetallic catalysts, particularly of oxides, Hauffe and co-workers (14a) used only semiconductors for which information concerning electronic and ion defects was available from measurements of electrical conductivity, thermoelectric properties, and Hall effect. These workers obtain a quantitative correlation between the reaction rate, the amount of chemisorption, and the number of electron defects of the catalysts. Since every catalyzed reaction is initiated by a chemisorption process involving one or several of the reacting gases, and because the nature of this chemisorption process determines the subsequent steps of the reaction, it seems appropriate to begin with a discussion of the mechanism of chemisorption. 

First we study the surface structure and chemisorption characteristics of crystals cut along different crystallographic orientations. Then a well-chosen chemical reaction is studied at low pressure to establish correlations between reactivity and surface structure and composition. Below 10 4 Torr the surface can be monitored continuously during the reaction with various electron spectroscopy techniques. Then the same catalytic reaction is studied at high pressures (1-100 atm) and the pressure dependence of the reaction rate is determined using the same sample over the nine orders of magnitude range. Finally, the rates and product distributions that were determined at... 

Recently, in order to understand processes on the catalyst surface, in particular structural formations, it has become a frequent practice to apply theories accounting for the interaction of adsorbed atoms. An important microscopic model of such a type is the lattice gas model. Its specific peculiarity is that this model accounts for the interaction of the nearer surface molecules (lateral interactions). It is this model that was applied in refs. 86 and 87. They should be specially emphasized as having exerted a great influence on the interpretation of thermodesorption experiments. The lattice gas model is used, e.g. in a series of investigations by Tovbin and Fedyanin [88, 89] devoted to the kinetics of chemisorption and reactions on catalyst surfaces. In terms of this model, one can interpret the complicated reaction rate dependences of surface coverage observed experimentally... 

Alternative techniques do exist, however, for obtaining information regarding the distribution and number of catalytic components dispersed within or on the support. Selective gas adsorption, referred to as chemisorption, can be used to measure the accessible catalytic component on the surface indirectly by noting the amount of gas adsorbed per unit weight of catalyst. The stoichiometry of the chemisorption process must be known in order to estimate the available catalytic surface area. One assumes that the catalytic surface area is proportional to the number of active sites and thus reaction rate. This technique has found use predominantly for supported metals. A gas that will selectively adsorb only onto the metal and not the support is used under predetermined conditions. Hydrogen and carbon monoxide are most commonly used as selective adsorbates for many supported metals. There are reports in the literature of instances in which gases such as NO and O2 have been used to measure catalytic areas of metal oxides however, due to difficulty in interpretation they are of limited use. 

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