Surface reactions, functional

Molecular mechanics methods have been used particularly for simulating surface-liquid interactions. Molecular mechanics calculations are called effective potential function calculations in the solid-state literature. Monte Carlo methods are useful for determining what orientation the solvent will take near a surface. Molecular dynamics can be used to model surface reactions and adsorption if the force held is parameterized correctly. 

The principal applications of REELS are thin-film growth studies and gas-surface reactions in the few-monolayer regime when chemical state information is required. In its high spatial resolution mode it has been used to detect submicron metal hydride phases and to characterize surface segregation and difRision as a function of grain boundary orientation. REELS is not nearly as commonly used as AES orXPS. 

As discussed in Chapter 7, this form can provide a good fit of the data if the reaction is not too close to equilibrium. However, most reaction engineers prefer a mechanistically based rate expression. This section describes how to obtain plausible functional forms for based on simple models of the surface reactions and on the observation that aU the rates in Steps 2 through 8 must be equal at steady state. Thus, the rate of transfer across the film resistance equals the rate of diffusion into a pore equals the rate of adsorption equals the rate of reaction equals the rate of desorption, and so on. This rate is the pseudohomo-geneous rate shown in Steps 1 and 9. 

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. 

By applying the machinery of statistical thermodynamics we have derived expressions for the adsorption, reaction, and desorption of molecules on and from a surface. The rate constants can in each case be described as a ratio between partition functions of the transition state and the reactants. Below, we summarize the most important results for elementary surface reactions. In principle, all the important constants involved (prefactors and activation energies) can be calculated from the partitions functions. These are, however, not easily obtainable and, where possible, experimentally determined values are used. 

In the recent past much ejperimental and theoretical effort has been undertaken to understand the microsoopic steps of heterogeneous surface reactions. Ihe main problem oonsists of evaluating the total energy of the reacting coponents (including tiie surface atoms ) as a function of all nuclear coordinates at any reaction time. The solution of this problem is extremely difficult. Detailed studies with model systems, however, can shed same light ipon the various steps of the interaction pattern. 

However, it is known that, even when using construction materials only (no fimc-tional polymer resin or catalyst), bulk reactions can change to surface reactions with the surface acting as a real reactant . Here, the functional groups of the surface act as reactants. Such findings have only recently been identified (see Section 1.6.10). 

Abstract A three-function catalyst model for hydrocarbon SCR of NOx is described, based on experimental evidence for each function, during temperature-programmed surface reactions (TPSR). 

Iron and Stainless Steel. The purpose of XPS investigations on typical corrosion systems like iron or stainless steel, is the determination of the composition of the passive surface layer, if possible, as a function of depth. As a consequence of the technical and economic relevance of corrosion reactions, XPS investigations on corrosion systems are numerous. With respect to the application of XPS, there is no difference between corrosion systems and any other electrochemical surface reaction like oxide formation on noble metals. Therefore, in this paragraph only a few recent typical results of such studies, using XPS, will be mentioned. For a detailed collection of XPS corrosion studies the reader is referred to references [43,104], A review of aqueous corrosion studies, using XPS, was given by McIntyre for the elements O, Cr, Mn, Fe, Co, Ni, Cu and Mo [105], The book edited by M. Froment [111] gives an impression of the research achieved on passivity of metals up to 1983. 

The Japanese company FIS Inc. has developed a 7-probe with a semiconductive cell made of BaSn03. The functional principle is based on the change in conductivity of the probe. The signal is generated by surface reactions with the local atmosphere and also is sensitive for the intermediate products and the free radicals resulting form combustion [6]. 

Fig. 5.2a shows examples of the results obtained on the dissolution of 8-AI203. In batch experiments where pH is kept constant with an automatic titrator, the concentration of AI(III)(aq) (resulting from the dissolution) is plotted as a function of time. The linear dissolution kinetics observed for every pH is compatible with a process whose rate is controlled by a surface reaction. The rate of dissolution is obtained from the slope of the plots. 

The measurement of the surface potential asa function of pH for an oxide provides valuable information for the determination of the parameters which describe the surface reactions. Ionizable surface site theories of the formation of surface charge and potential at an oxide surface in contact with a liquid electrolyte involve many more parameters than can be directly experimentally determined. Additional assumptions are required to evaluate these parameters, which explains why there is often no agreement in the literature about their value. A mathematical treatment of the amphoteric surface site model is given which exhibits the characteristic quantities which can be experimentally measured. It is shown that the measurement of both the surface potential i/>o and the surface charge 00 are required to completely determine these characteristic quantities. This approach is applied to Si02 and AI2O3, two surfaces for which both charge and potential measurements are available. 

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