Surface rate processes

Kang H C and Weinberg W H 1994 Kinetic modeiing of surface rate processes Surf. Sol. 299-300 755... 

The maximum in the steady-state rate at the stoichiometric H2 mole fraction (0.75) indicates that synthesis rate is controlled by a surface rate process, not H2 or N2 chemisorption (13). Greater increase in catalyst activity at higher H2 mole fractions suggests that hydrogen is involved in the rate controlling step. 

Useful for measurement of physicochemical properties of the catalyst, e.g.. diflusivity in the catalyst pellets, adsorplion coefficient, etc., under reaction conditions. Relative importance of adsorption and surface rate processes can be determined from this type of a reactor. 

Boundary and interface conditions must be known if solutions to the conservation equations are to be obtained. Since these conditions depend strongly on the model of the particular system under study, it is difficult to give general rules for stating them for example, they may require consideration of surface equilibria (discussed in Appendix A) or of surface rate processes (discussed in Appendix B). However, simple mass, momentum, and energy balances at an interface often are of importance. For this reason, interface conditions are derived through introduction of integral forms of the conservation equations in Section 1.4. 

Cutlip, M. B., Concentration forcing of catalytic surface rate processes. I Isothermal carbon monoxide oxidation over supported platinum, AIChE J., 25, 502-508 (1979). 

Kang, H.C., Weinberg, W.H. Monte Carlo simulations of surface-rate processes. Acc. Chem. Res. 1992, 25, 253-9. 

Multiple steady state and limit cycle experiments may well contain valuable quantitative information regarding surface rate processes in catalytic reactions and may contribute to improved dynamic catalytic models for process optimization and control. 

The underlying kinetic models and reaction mechanisms are described later. In both models, the variable represents a gas-phase concentration, x, a surface concentration, and is a surface capacitance factor. Therefore, the function Fi(x2 X2) accounts for flow terms (in a CSTR species balance equation) as well as chemisorption and perhaps other reaction terms. The function 2 (x], X2), on the other hand, accounts only for surface rate processes. The variable X2 is a "latent" variable, not measurable in situ in the unsteady state presently. 

Catalytic gas-phase reactions play an important role in many bulk chemical processes, such as in the production of methanol, ammonia, sulfuric acid, and nitric acid. In most processes, the effective area of the catalyst is critically important. Since these reactions take place at surfaces through processes of adsorption and desorption, any alteration of surface area naturally causes a change in the rate of reaction. Industrial catalysts are usually supported on porous materials, since this results in a much larger active area per unit of reactor volume. 

It was determined, for example, that the surface tension of water relaxes to its equilibrium value with a relaxation time of 0.6 msec [104]. The oscillating jet method has been useful in studying the surface tension of surfactant solutions. Figure 11-21 illustrates the usual observation that at small times the jet appears to have the surface tension of pure water. The slowness in attaining the equilibrium value may partly be due to the times required for surfactant to diffuse to the surface and partly due to chemical rate processes at the interface. See Ref. 105 for similar studies with heptanoic acid and Ref. 106 for some anomalous effects. 

For many laboratoiy studies, a suitable reactor is a cell with independent agitation of each phase and an undisturbed interface of known area, like the item shown in Fig. 23-29d, Whether a rate process is controlled by a mass-transfer rate or a chemical reaction rate sometimes can be identified by simple parameters. When agitation is sufficient to produce a homogeneous dispersion and the rate varies with further increases of agitation, mass-transfer rates are likely to be significant. The effect of change in temperature is a major criterion-, a rise of 10°C (18°F) normally raises the rate of a chemical reaction by a factor of 2 to 3, but the mass-transfer rate by much less. There may be instances, however, where the combined effect on chemical equilibrium, diffusivity, viscosity, and surface tension also may give a comparable enhancement. 

The drying a chemical substance is not a simple process. Drying a mass of finely divided solid particles carrying 30 to 40% water, for example the rate of evaporation is constant and high as long as the surfaces exposed are wet. After the surface is dry, the water in the interstices must make its way to the surface, a process of diffusion that is slower than evaporation from a wet surface the rate will then drop. This second part of the process must be modified according to the case with which the material crumbles as it dries, exposing new surfaces. 

Hot Dipped Coatings Major attempts have been made to improve the quality of aluminised steel strip. Requirements on coating thickness and uniformity have been imposed. It is the speed of sheet movement, length of path in the molten bath, temperature and composition of the bath that control the thickness of the intermetallic layer which lies below the aluminium outer surface. The process of intermetallic alloy formation is diffusion controlled, and it is usual that some dissolution of iron into the molten aluminium does occur at a rate, Ac/At, which is given by... 

Wischin s study [201] ofnucleation and growth ofthe product barium during decomposition of BaN6. In other rate processes where the crystallites are small, opaque, of roughened surface, etc., reactant and product cannot be distinguished and microscopic observations yield no useful data. 

Some limitations of optical microscopy were apparent in applying [247—249] the technique to supplement kinetic investigations of the low temperature decomposition of ammonium perchlorate (AP), a particularly extensively studied solid phase rate process [59]. The porous residue is opaque. Scanning electron microscopy showed that decomposition was initiated at active sites scattered across surfaces and reaction resulted in the formation of square holes on m-faces and rhombic holes on c-faces. These sites of nucleation were identified [211] as points of intersection of line dislocations with an external boundary face and the kinetic implications of the observed mode of nucleation and growth have been discussed [211]. 

X= 2) or (P = 0, X = 3) and the distinction between these possibilities is most satisfactorily based upon independent evidence, such as microscopic observations. The growth of compact nuclei inevitably results in the consumption of surfaces and when these outer faces, the sites of nucleation, have been eliminated, j3 necessarily is zero this may result in a diminution of n. The continued inward advance of the reaction interface at high a results in a situation comparable with the contracting volume reaction (discussed below) reference to this similarity was also made in consideration of the Mampel approach discussed above. Shapes of the deceleratory region of a time curves for nucleation and growth reactions and the contracting volume rate process are closely similar [409]. 

Cordes [515] has provided a more general treatment of reaction rates using similar concepts to those discussed by Shannon [514] (the latter model appears as one particular case). Two types of rate process are distinguished bulk decomposition and surface reaction and three classes of bulk decomposition are identified, viz. 

A reaction interface is the zone immediately adjoining the surface of contact between reactant and product and within which bond redistributions occur. Prevailing conditions are different from those characteristic of the reactant bulk as demonstrated by the enhanced reactivity, usually attributed to local strain, catalysis by products, etc. Considerable difficulties attend investigation of the mechanisms of interface reactions because this thin zone is interposed between two relatively much larger particles. Accordingly, many proposed reaction models are necessarily based on indirect evidence. Without wishing to appear unnecessarily pessimistic, we consider it appropriate to mention here some of the problems inherent in the provision of detailed mechanisms for solid phase rate processes. These difficulties are not always apparent in interpretations and proposals appearing in the literature. 

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