Heterogeneous Catalytic Gas-Phase Reactions

For gas phase heterogeneous catalytic reactions, the continuous-flow integral catalytic reactors with packed catalyst bed have been exclusively used [61-91]. Continuous or short pulsed-radiation (milliseconds) was applied in catalytic studies (see Sect. 10.3.2). To avoid the creation of temperature gradients in the catalyst bed, a single-mode radiation system can be recommended. A typical example of the most advanced laboratory-scale microwave, continuous single-mode catalytic reactor has been described by Roussy et al. [79] and is shown in Figs. 10.4 and... 

For a number of liquid-phase reactions, the proper choice of a solvent can enhance selectivity, See, for example, IncL Eng. Chem., 62(9), 16 (1970). In gas-phase heterogeneous catalytic reactions, selectivity is an important parameter of any particular cataly.st. 

Fixed bed reactors are the most common reactors for the study of gas-phase heterogeneous catalytic reactions. They are easy to use and catalysts in powder form can be readily used (usually after sieving the proper particle size fraction). Semi-empirical correlations are used to describe the physical phenomena occurring in these types of reactors [21,22]. However, in the case of very last and for exo- or endothermic reactions, it becomes difficult to measure the intrinsic kinetics with these reactors. To avoid external heat and mass transfer limitations as well as internal diffusion limitations, high flow rates and small particles are necessary. This quickly will lead to an excessive pressure drop over the reactor. 

Example 7.4 Design a packed-bed reactor for the gas-phase, heterogeneous catalytic cracking reaction... 

In the case of three phase heterogeneous catalytic reactions, the rate of the process and its selectivity can be determined either by intrinsic reaction kinetics or by external diffusion (on the gas-liquid and gas-solid interface) as well as by internal diffusion through the catalyst pores. Careful analysis of mass transfer is important for the elucidation of intrinsic catalytic properties, for the design of catalysts, and for the scale up of processes. 

Heterogeneous catalysis takes place in multiphase systems. If a new phase, the supercritical phase, is selectively introduced, remarkable change can be expected. The effects caused by the introduction of a supercritical phase depend on many parameters, such as fluid properties, reaction conditions, and affinity. Successful supercritical phase heterogeneous catalytic reactions can be realized, as long as these parameters are carefully controlled. With greater activities, catalyst lifetimes, or selectivities than for gas phase or liquid phase reactions, some supercritical phase reactions have industrial potentid. 

Strictly speaking, mechanisms for heterogeneous catalytic reactions can never be monomolecular. Thus they always include adsorption steps in which the initial substances are a minimum of two in number, i.e. gas and catalyst. But if one considers conversions of only surface compounds (at a constant gas-phase composition), a catalytic reaction mechanism can also be treated as monomolecular. It is these mechanisms that Temkin designates as linear (see Chap. 2). 

For catalytic reactions the fast and slow variables usually considered are the concentrations of surface intermediates on catalysts and gas-phase reactants, respectively. (In the case of high-vacuum conditions, "a vice versa quasi-stationarity is possible, see below.) But in the equations for heterogeneous catalytic reactions (119)... 

In a recent survey [19] it was noted that a realistic model for catalytic oxidation reactions must include equations describing the evolution of at least two concentrations of surface substances and account for the slow variation in the properties of the catalyst surface (e.g. oxidation-reduction). For the synchronization of the dynamic behaviour for various surface domains, it is necessary to take into consideration changes in the concentrations of gas-phase substances and the temperature of the catalyst surface. It is evident that, in the hierarchy of modelling levels, such models must be constructed and tested immediately after kinetic models. On the one hand, the appearance of such models is associated with the experimental data on self-oscillations in reactors with noticeable concentration variations of the initial substances and products (e.g. ref. 74) on the other hand, there was a gap between the comprehensively examined non-isothermal models with simple kinetics and those for the complex heterogeneous catalytic reactions... 

As a first approximation a convective term in the film region has been negleted, u is the superficial gas velocity and u f denotes the gas velocity at minimum fluidization conditions. Tne specific mass transfer area a(h) is based on unit volume of the expanded fluidized bed and e OO is the bubble gas hold-up at a height h above the bottom plate. Mathematical expressions for these two latter quantities may be found in detail in (20). The concentrations of the reactants in the bubble phase and in film and bulk of the suspension phase are denoted by c, c and c, respectively. The rate constant for the first order heterogeneous catalytic reaction of the component i to component j is denoted... 

In this chapter we summarize results on the application of graph theory to the kinetics of heterogeneous catalytic reactions published up to the present. Special attention is paid to gas-phase reactions over a solid catalyst. A number of more recent results, which have hitherto not been published, are also included. 

On heating, many hydrides dissociate reversibly into the metal and Hj gas. The rate of gas evolution is a function of both temperature and /KH2) but will proceed to completion if the volatile product is removed continuously [1], which is experimentally difficult in many systems. The combination of hydrogen atoms at the metal surface to yield Hj may be slow [2] and is comparable with many heterogeneous catalytic reactions. While much is known about the mobility of H within many metallic hydride phases, the gas evolution step is influenced by additional rate controlling factors. Depending on surface conditions, the surface-to-volume ratio and the impurities present, the rate of Hj release may be determined by either the rate at which hydrogen arrives at the solid-gas inteifece (diffusion control), or by the rate of desorption. 

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

If the catalytic oxidation of alkane molecule starts with the formation of a free radical on the surface of an active catalyst particle and its escape to the gas phase, the complete reaction network includes both homogeneous and heterogeneous steps of the transformation of primary (CnH2n+l) and secondary radicals. Since all these processes are sufficient for the formation of the final products, the analysis of the influence of different factors on the... 

Heterogeneous catalytic reactions constitute around 90% of all processes in the chemical industry. Different types of solid materials are used to catalyze a variety of reactions in the gas phase or in solutions. Some examples are given below. 

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