Surface Catalysis Intrinsic Kinetics

Surface catalysis is involved in a large majority of industrial catalytic reactions. The rate laws developed in this section are based on the following assumptions  

These assumptions are the basis of the simplest rational explanation of surface catalytic kinetics and models for it. The preeminent of these, formulated by Langmuir and Hinshelwood, makes the further assumption that for an overall (gas-phase) reaction, for example, A(g) +...- product(s), the rate-determining step is a surface reaction involving adsorbed species, such as A s. Despite the fact that reality is known to be more complex, the resulting rate expressions find wide use in the chemical industry, because they exhibit many of the commonly observed features of surface-catalyzed reactions. 

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The insolubility of enzymes in monophasic organic systems has a controlling influence on the kinetics of enzymatic catalysis in organic media. Insolubilized enzymes are subject to intraparticle and external diffusional limitations which can mask the true, intrinsic kinetics of catalysis. These limitations are particularly severe for highly active and purified enzymes such as horseradish peroxidase. One way to overcome this problem is to increase the surface area of the enzyme in contact with the organic solvent. 

TPD experiments can be carried out either in ultra high vacuum or at ambient pressure under flow condition. Thus they can bridge the material and pressure gaps between surface science and heterogeneous catalysis. Despite the popularity of TPD as a catalyst characterisation method its application in kinetic analysis for porous samples is often being discouraged. There are indeed important methodical considerations such as the selection of the reactor model and the intrinsic kinetic model and the evaluation of mass transfer limitations. Furthermore, experimental data with sufficient information content should be collected in a carefully selected and standardised manner, since the pretreatment and the adsorption step prior to TPD significantly influence the TPD patterns. 

Catalysis enhances the sustainability of today s world. Raw material utilization, energy efficiency, elimination of hazardous synthesis routes, pollution abatement and so on are a few examples of issues that are typically addressed by catalysis and, hence, contribute to safer, cleaner, more reliable, and more economical chemical processes (1). Catalyst development and improvement require an elaborate testing of candidate catalytic materials. Not only the physical properties, such as porosity, crystallinity, and surface composition are investigated but also and even more importantly, the functional properties such as the kinetics need to be determined. The observed kinetics is constituted by a sequence of different steps. When investigating a reaction mechanism in detail, operating conditions are selected at which the physical transport phenomena are not rate limiting. At such operating conditions, the observed kinetics are entirely determined by the chemical adsorption and reaction phenomena and, hence, they correspond to so-called intrinsic kinetics (2). 

Catalytic reaction engineering is a scientific discipline which bridges the gap between the fundamentals of catalysis and its industrial application. Starting from insight into reaction mechanisms provided by catalytic chemists and surface scientists, the rate equations are developed which allow a quantitative description of the effects of the reaction conditions on reaction rates and on selectivities for desired products. The study of intrinsic reaction kinetics, i.e. those determined solely by chemical events, belongs to the core of catalytic reaction engineering. Very close to it lies the study of the interaction between physical transport and chemical reaction. Such interactions can have pronounced effects on the rates and selectivities obtained in industrial reactors. They have to be accounted for explicitly when scaling up from laboratory to industrial dimensions. 

Unfortunately, later the development of two areas— homogeneous kinetics and heterogeneous catalysis—occurred almost independently, which caused serious intrinsic discrepancies. For instance, the traditional chain theory implies the participation of surfaces also in chain termination, which determines the existence of the low-pressure ignition limit. In the framework of this approach, two regimes—diffusional and kinetic—are distinguished. In the latter case the parameter that describes the process is the probability of surface decay of chain carriers per one collision. It is worth noticing, however, that this assumes only a disappearance of active species from the gas phase, without any analysis of its mechanism and even stoichiometry. This is why the heterogeneous termination reactions are usually represented in kinetic models as a formal reaction ... 

Nanoparticles are commensurable in size with boron s radius of excitons in semiconductors. This governs their optical, luminescence and redox properties. Since the intrinsic size of nanoparticles is commensurable with that of a molecule, this ensures specifics of the kinetics of chemical processes on their surface." Current investigations are concentrated on the study of boundary regions between nanoparticles and the polymer because these interfaces are responsible for the behavior of adsorption and catalysis. 

In the simplest case, the catalytic activity is proportional to the number of active sites Nt, intrinsic rate constant and the effectiveness factor. Catalyst deactivation can be caused by a decrease in the number of active sites, changes in the intrinsic rate constant, for example, changes in the ability of surface sites to promote catalysis, and by degradation in accessibility of the pore space. When the reaction and deactivation rates are of different magnitudes, the reactions proceed in seconds, while the deactivation can require hours, days, or months moreover, the deactivation does not affect the selectivity. The concept of separable deactivation kinetics is applied. The reaction rates and deactivation are treated by different equations. A quantity called activity, (a), is introduced to account for changes during the reaction. 

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