Reaction on the catalyst

The metallic catalysts for exliaust pollution control are designed to perform three functions. The air/fuel ratio employed in combustion engines creates exhaust products which are a mixture of hydrocarbons, carbon oxides, and niU ogen oxides. These must be rendered environmentally innocuous by reactions on the catalyst such as... 

There is a third real reason for deviations from Eq. (5.18) in the case that a non-conductive insulating product layer is built via a catalytic reaction on the catalyst electrode surface (e.g. an insulating carbonaceous or oxidic layer). This is manifest by the fact that C2H4 oxidation under fuel-rich conditions has been found to cause deviations from Eq. (5.18) while H2 oxidation does not. A non-conducting layer can store electric charge and thus the basic Eq. 5.29 (which is equivalent to Eq. (5.18)) breaks down. 

Thus it is important to obtain reliable models for catalyst deactivation and to investigate, whether it is possible to decouple the deactivation model from the kinetic model or if it is necessary to treat the catalyst deactivation as one of the surface reactions on the catalyst [45]. 

The catalyst performance depends on the H2 to CCI2F2 feed ratio. The selectivities to CH2F2 and CHCIF2 are influenced by the H2 to CCI2F2 feed ratio, while the selectivity to methane is independent of this ratio. We have previously proposed a reaction mechanism with serial reactions on the catalyst surface and minor readsorption of the intermediate products, which is depicted in figure 8 [4,5]. Thus the kinetics of the reaction follows mainly parallel reaction pathways, in which the selectivities are not influenced by the conversion, and a... 

The reaction on the catalyst surface was followed by in situ i.r. spectroscopy using a Bruker IFS88 FTIR spectrometer for the characterisation of sorbed species and mass spectroscopy for the analysis of gas phase. The state of Pt was further investigated by in situ X-ray absorption spectroscopy (Daresbury, UK, beamline 9.1, transmission mode, Si(220) monochromator, Pt-Lj, edge). Details of catalyst characterisation techniques are reported elsewhere [13,14]. 

Understanding the mechanism of reactions on the catalyst surface requires an adequate description of the surface it must modelled either by infinite slab or by clusters having similar properties. The interesting feature of the M0O3 surface is the existence of three structurally different oxygen atoms, a terminal one O] coordinated to one molybdenum atom, and two bridge-like oxygen atoms On and Om, coordinated to two and three Mo atoms, respectively. 

Fiolitakis, E. Hofmann, H. Dependence of the Kinetics of the Low Temperature Watergas Shift Reaction on the Catalyst Oxygen Activity, to be published in Journal of Catalysis... 

Assuming that the rate determining step is the surface reaction on the catalyst [Eq. (4)], the following rate equation is derived ... 

The main research objectives to industrial applications are the relation of the catalyst acidity to the yield, the location of the rearrangement reaction on the catalyst, the catalyst s deactivation, the best reaction conditions, the best solvent and advanced reactor concepts. 

As the Beckmann rearrangement is believed to be a typical acid-catalysed reaction, many researchers have reported the relationship between the vapour phase reaction catalysis and the acidity of the catalysts tested on non-zeolitic catalysts - i2s- i3i. 318-334 and on zeolitic catalysts Another interesting point for the heterogeneous gas-phase Beckmann rearrangement is the location of the reaction on the catalyst and different studies have been published ° . The outer surface of the catalyst particle seems to be the most probable place for the Beckmann rearrangement supported by the traces of reagents, and notable amounts of by-products found only in the outer layers of the zeolite crystal. Development of new and more efficient catalysts have also been reported " . ... 

In the reactor we are interested in the position in the bed z or height of the bed L, in the pellet we are interested in the position x in the pellet with radius R, in the pore we are interested in distance x down the pore diameter (ipore, nd on the walls of the pore we are interested in reactions on the catalyst particle diameter r/pamcie-... 

Reaction on the catalyst, and diffusion of products to the liquid phase. 

Rates of fluid phase reactions catalyzed by solids also can be represented at least approximately by powers of the concentrations. A more fundamental approach, however, takes into account mechanisms of adsorption and of reaction on the catalyst surface. A few examples of resulting equations are in item 9 of Table 17.2. 

Another example gives us the reactions on the catalyst surface, considered in the last Chapters of this book, where products depart to the gas phase. In... 

The regularities of reactions on the catalyst surfaces are of a very complicated nature and their description is only possible on the basis of schematic and simplified physical models. A model of this kind should, on the one hand, reflect the main features of the phenomenon and, on the other hand, result in comprehensible mathematical expressions. The model of an ideal adsorbed layer or, in terms of the author of the model, Langmuir, simple adsorption (20) is the simplest and historically the first of the models retaining their importance until now. 

Like O radicals on V and Mo oxides discussed above, Oa exhibits a very high reactivity. At room temperature, it readily oxidizes various organic molecules, including methane. This allows one to conduct single turnover reactions on the catalyst surface, providing in particular the synthesis of phenol according to the... 

By suitable adjustments of the quantities of gas and of catalyst, a detailed study can be made of initial reactions on the catalyst surface, and these can often be largely separated from subsequent reactions owing to adsorption of fresh reactants from the gas phase. It is essential to ensure that diffusion phenomena are not rate-determining, since the gas phase is largely static. Suitable tests can be devised, for instance, the catalyst can be spread loosely over the surface of the reaction vessel or it can be tamped into a suitable containing vessel, platinum or otherwise, thus exposing a much smaller superficial area also the size of the capillary leaks can be varied. In all the work subsequently described, these tests have been applied, and it has been demonstrated that diffusion is in no case a rate-determining step. 

If an inert material is initially adsorbed on the fixed bed comprising an appropriate adsorbent and a catalyst, the heat of adsorption—having the same order of magnitude as the latent heat of evaporation—will be released (Figure 15). Since no reaction takes place in this phase, moderate temperature excursions are acceptable, and recycle flows over external heat exchangers or injection of liquid adsorptives may serve as heat sinks. In the subsequent reaction phase, the heat liberated by an exothermic reaction on the catalyst is taken up by the desorption of the inert from the previously loaded neighboring adsorbent particles. As long as this desorption occurs, the heat of reaction will not lead to major temperature increases. Sooner or later, of course, the adsorbent will be depleted and the temperatures will drift upward, at which point the adsorption phase must be repeated. 

Experimental Measurements of Reaction Kinetics. The reaction expressions discussed in the following model the intrinsic reaction on the catalyst surface, free of mass-transfer restrictions. Experimental measurements, usually made with very fine particles, are described by theoretically deduced formulas, the validity of which is tested experimentally by their possibility for extrapolation to other reaction conditions. Commonly the isothermal integral reactor is used with catalyst crushed to a size of 0.5-1.5 mm to avoid pore diffusion restriction and heat-transfer resistance in the catalyst particles. To exclude maldistribution effects and back mixing, a high ratio of... 

Azaruddin et al. [3] carried out dechlorination for removal of organic chlorine composite catalyst system without using hydrogen in the dechlorination reaction. The catalyst was stable in the presence of HCl. The catalyst deactivated with time due to the adsorption of HCl produced during the reaction on the catalyst surface. 

The coupling of heterogeneous reactions on the catalyst surface and homogeneous gas-phase reactions, as discussed in the previous section, is important for the design and operation of a catalytic combustor with maximum temperatures over 900 C, which is the case for gas turbine combustors. It is worth pointing out that the ignition of the fuel-air mixture over the catalyst at much lower temperatures than possible for homogeneous gas-phase combustion is the reason why catalytic combustors can operate at flame temperatures as low as 1100 C [53]. Hence, the formation of thermal NO, which is the most important type of NOx for gas turbine combustors, is practically avoided. 

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