Adsorption and Reaction Processes

Mathematical modeling of the photocatalytic processes is an important task. This allows to establish the reactor performance from a conversion of model pollutants perspective as well as on the basis of the calculation of energy efficiencies. 

We have already described in Chapter V phenol photoconversion in slurry photocatalytic systems with adsoiption assumed at quasi-equilibrium. In many cases, however, photocatalytic reactors ai e operated under non-equilibrium conditions the pollutant concentrations in the solid phase and in the fluid phase are significantly fai away from the adsorption equilibrium values. As demanded by cai eful modeling, accounting for adsorption at non-equilibrium conditions is needed. 

To achieve this, separate species balances for both the liquid phase (water solution) and the solid phase (Ti02 particles) have to be considered. 

Equation (7-2) states that the observed pollutant solid phase concentration is the result of the competition processes adsorption, desorption and reaction. Furthermore, equation (7-1) establishes that the observed bulk fluid phase species concentration is the result of the difference between the adsoiption and desorption rates. 

one has to consider the combined set of equations (7-1), (7-2) and (7-3), in addition to the various adsorption and reaction parameters involved in these equations, to provide an adequate phenomenological description of pollutant concentration changes in photocatalytic reaction systems with respect to the reaction time. 

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Of these, the most extensive use is to identify adsorbed molecules and molecular intermediates on metal single-crystal surfaces. On these well-defined surfaces, a wealth of information can be gained about adlayers, including the nature of the surface chemical bond, molecular structural determination and geometrical orientation, evidence for surface-site specificity, and lateral (adsorbate-adsorbate) interactions. Adsorption and reaction processes in model studies relevant to heterogeneous catalysis, materials science, electrochemistry, and microelectronics device failure and fabrication have been studied by this technique. 

Nicolau YF, Dupuy M, Brunei M (1990) ZnS, CdS, and Zni j Cdj S thin films deposited by the successive ionic layer adsorption and reaction process. J Electrochem Soc 137 2915-2924... 

The versatile IR method may be extended to extremes of both temperature and pressure as a probe of adsorption and reaction processes on surfaces. The extension of IR spectroscopy to the study of weakly-bound surface species at low temperatures opens up the possibility of stabilization of transient surface species which are Involved in surface chemistry at high temperatures. 

Nicolau, Y. F. 1985. Solution deposition on thin solid compound films by a successive ionic-layer adsorption and reaction process. Appl. Surf. Sci. 22-23 1061-1074. 

Recent advances have led to the development of microcalorimeters sensitive enough for low-surface-area ( 1 cm2) solids [71]. This instrumentation has already been used in model systems to determine the energetics of bonding of catalytic particles to the support, and also in adsorption and reaction processes [72,73],... 

Although many physical processes of interest to chemical reaction engineers involve absorption, heterogeneous reaction, surface mass transport, and interfacial mass transfer at moving and deforming interfaces, their main focus is concerned with the phenomena occurring at two particular types of interface systems. These are (1) the adsorption and reaction processes taking... 

The simplest case to be analyzed is the process in which the rate of one of the adsorption or desorption steps is so slow that it becomes itself rate determining in overall transformation. The composition of the reaction mixture in the course of the reaction is then not determined by kinetic, but by thermodynamic factors, i.e. by equilibria of the fast steps, surface chemical reactions, and the other adsorption and desorption processes. Concentration dependencies of several types of consecutive and parallel (branched) catalytic reactions 52, 53) were calculated, corresponding to schemes (Ila) and (lib), assuming that they are controlled by the rate of adsorption of either of the reactants A and X, desorption of any of the products B, C, and Y, or by simultaneous desorption of compounds B and C. 

Principal differences between catalysis by dissolved electrolytes and by resins are that with resins as catalysts catalysis overlaps with diffusion, adsorption, and desorption processes, while this is not the case with electrolytes (Naumann, 1959). Also, the matrix of the resin with fixed ionic group may have some influence on the course of reaction. 

The interaction between NO and soot takes place through adsorption and reduction processes involving N02. The reaction mechanism starts with N02 adsorption on soot, forming C-N02 and C-ONO complexes. Spontaneous desorption produces CO, accompanied by a reduction of the soot mass, and NO or H2. 

Hougen- Watson Models for Cases where Adsorption and Desorption Processes are the Rate Limiting Steps. When surface reaction processes are very rapid, the overall conversion rate may be limited by the rate at which adsorption of reactants or desorption of products takes place. Usually only one of the many species in a reaction mixture will not be in adsorptive equilibrium. This generalization will be taken as a basis for developing the expressions for overall conversion rates that apply when adsorption or desorption processes are rate limiting. In this treatment we will assume that chemical reaction equilibrium exists between various adsorbed species on the catalyst surface, even though reaction equilibrium will not prevail in the fluid phase. 

A survey of the literature shows that although very different calorimeters or microcalorimeters have been used for measuring heats of adsorption, most of them were of the adiabatic type, only a few were isothermal, and until recently (14, 15), none were typical heat-flow calorimeters. This results probably from the fact that heat-flow calorimetry was developed more recently than isothermal or adiabatic calorimetry (16, 17). We believe, however, from our experience, that heat-flow calorimeters present, for the measurement of heats of adsorption, qualities and advantages which are not met by other calorimeters. Without entering, at this point, upon a discussion of the respective merits of different adsorption calorimeters, let us indicate briefly that heat-flow calorimeters are particularly adapted to the investigation (1) of slow adsorption or reaction processes, (2) at moderate or high temperatures, and (3) on solids which present a poor thermal diffusivity. Heat-flow calorimetry appears thus to allow the study of adsorption or reaction processes which cannot be studied conveniently with the usual adiabatic or pseudoadiabatic, adsorption calorimeters. In this respect, heat-flow calorimetry should be considered, actually, as a new tool in adsorption and heterogeneous catalysis research. 

Note that the redox reaction of electron transfer via adsorption intermediates requires the adsorption and desorption processes to occur as the preceding and... 

Electrodeposition is by its nature a condensed phase process, whereas most studies of ALE have been performed using gas phase or vacuum methodologies, CVD or MBE. A solution phase deposition methodology related to ALE has been developed in France by Nicolau et al. [27-32] (Fig. 2), in which adsorbed layers of elements are formed by rinsing a substrate in aqueous solutions containing ionic precursor for the desired elements, sequentially, in a cycle. After exposure to each precursor, the substrate is copiously rinsed and then transferred to a solution containing the precursor for the next element. The method is referred to as successive ionic layer adsorption and reaction (SILAR). Reactivity in SILAR appears to be controlled by the rinsing procedure, solution composition, pH, and specifically... 

Transport of solutes and gases through the soil is much slower than through soil-free water because of the restricted cross-sectional area for transport through the soil pore network and because of adsorption and reaction on soil surfaces (Chapter 2). Redox conditions are therefore closely linked to transport processes. 

In heterogeneous catalysis, the catalyst often exists in clusters spread over a porous carrier. Experimentally, it is well established that reactivity and selectivity of heterogeneous reactions change enormously with cluster size. Thus, theoretical studies on clusters are particularly important to establish a basis for the determination of their optimal size and geometry. Cluster models are also important for studying the chemistry and reactivity of perfect crystal faces and the associated adsorption and desorption processes in heterogeneous catalysis (Bauschlicher et al, 1987). 

Successive Ion-Layer Adsorption and Reaction (SILAR) Process... 

Fig. 18. Scheme of adsorption, desorption and reaction processes on the surface of the NSRC during lean and rich conditions (Koci, 2005) (see Plate 3 in Color Plate Section at the end of this book). 

Non-linearities arising from non-reactive interactions between adsorbed species will not be our main concern. In this section we return to variations of the Langmuir-Hinshelwood model, so the adsorption and desorption processes are not dependent on the surface coverage. We are now interested in establishing which properties of the chemical reaction step (12.2) may lead to multiplicity of stationary states. In particular we will investigate situations where the reaction step requires the involvement of additional vacant sites. Thus the reaction step can be represented in the general form... 

The simple model just discussed shows multistability even when the system is clean but requires the involvement of a poison for oscillations. One reason for this is that the latter is needed to provide a second independent surface concentration, so we theij. have a two-variable system. It was mentioned in 12.3.1 that implicit in the rate law used above may be the adsorption of a second reactant which participates in the reaction step. The latter did not provide a second concentration variable there since its adsorption and desorption processes were assumed to be on a very much faster (instantaneous) timescale. 

Since mass action law for elementary reactions in ideal adsorbed layers (including also adsorption and desorption processes) coincides in its form with mass action law for elementary reactions in volume ideal systems, general results of the theory of steady-state reactions are equally applicable to volume and to surface reactions. They are very useful when the reaction mechanism is complicated. 

Adsorption and desorption processes are particular cases of stage 1 namely, when substance Bt is absent, and I coincides with Aj, rA is the rate of adsorption and rB is the rate of desorption. Since the equilibrium of stage 2 is maintained as a result of mutually reverse elementary reaction, the particles I very frequently leave some surface sites and appear on others. 

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