Desorption of reactants

Reactions for the synthesis of fine chemicals differ in many aspects from the hydrocarbon reactions that constitute today the major application of zeolites and other micro- or mesoporous catalysts, as they often involve the transformation of molecules with several functional groups. Chemoselectivity is therefore of prime importance. These reactions are generally operated in rather mild conditions and condensed media (rather than vapour phase) to avoid undesired secondary reactions. The use of solvents can have major impacts on the activity and selectivity of these catalysts as they may affect the adsorption and desorption of reactants and products on these catalysts. 

The reaction mechanism is based on the Langmuir-Hinshelwood model for heterogeneous catalysis. This represents sorption and desorption of reactants and products as equilibria the ligand-exchange reaction is considered rate limiting. By... 

Consider the effect of surface diffusion on the effectiveness factor for a first-order, irreversible, gaseous reaction on a porous catalyst. Assume that the intrinsic rates of adsorption and desorption of reactant on the Surface are rapid with respect to the rate of surface diffusion. Hence equilibrium is established between reactant iii the gas in the pore and reactant adsorbed on the surface. Assume further that the equilibrium expression for the concentration is a linear one. Derive an equation for the effectiveness factor for each of the following two cases ... 

It has been proposed that the acid-base nature of catalyst and support can modify the adsorption/desorption of reactants and products [2,3,5]. In this way, and according to both the catalytic and the physicochemical properties, it can be concluded that the incorporation of potassium initially increases the selectivity to propylene as a consequence of lowering the numberof acid sites. 

In conclusion, this paper shows the effect of the addition of different metal oxides (K, Bi, P and Mo) on the catalytic behavior of an alumina-supported vanadia catalysts in the ODH of propane. In all cases, the addition of small amounts of metal oxide (MeA/ atomic ratio of 0.1) increases the selectivity to propylene, probably as a consequence of the elimination of non selective sites (Lewis acid sites) on the surface of the support. However, only in the case of K-doped catalysts the selectivity and the yield of propylene increases with the metal content. The varition of the acid-base character of catalysts and its influence on the adsorption/desorption of reactants and products could be responsible of the different performances obsen/ed. In this way. 

The kinetic modelling of complex multiphase catalytic reactions needs a careful consideration of various complexities of adsorption and desorption of reactants and products. In such cases the kinetic model developed based on the initial rate data may not be adequate to explain the integral batch reactor performance. Hence it was thought appropriate to use mainly the integral rate data for developing a suitable kinetic model. Different rate equations were derived based on various Langumir-Hinshelwood mechanisms and a few of them are given below. 

Equilibrium adsorption-desorption is established if the rate of surface reaction is very slow compared to rates of adsorption and desorption of reactants. 

Kinetic modeling used for process development and process optimization has a historical tradition. Quite often power law models are still used to describe kinetic data. Such phenomenological expressions, although useful for some applications, in general are not reliable, as they do not predict reaction rate, concentration and temperature dependence outside of the range of the studied experimental conditions. Thus, in catalysis, due to the complex nature of this phenomenon, adsorption and desorption of reactants as well as several steps for surface reactions should be taken into account. Models based on the knowledge of elementary processes provide reliable extrapolation outside of the studied interval and also make the process intellectually better understood. 

As mentioned above, the Hougen-Watson kinetic rate law is based on single-site adsorption/desorption of reactant A (i.e., A - - ct o- Act) ... 

The co-adsorption of reactants at below normal reaction temperature, with temperature-programmed desorption of reactants, intermediates and products, has also received increasing attention in recent years. GSC techniques again find use in this context. 

At sufficiendy high sweep rates, one can measure currents used to charge the double layer, and for the processes of adsorption or desorption of reactants or intermediates involved in the charge-transfer step. The latter current is called the pseudocapacitive current. 

It is important to note that most industrial-scale reactions operate in a mass transfer-limited mode where the rates of adsorption and desorption of reactants and products from the catalyst surface depend on the conditions of temperature, pressure... 

The kinetics of the overall electrochemical reaction is not only determined by the electron transfer process but also by transport processes, mainly diffusion, furthermore adsorption/desorption of reactants and reactions in homogeneous solution preceding or following the charge transfer. If new phases are formed in the course of an electrolysis (e.g. with metal deposition at a solid surface or with gas evolution), the kinetics of phase formation (mainly the nucleation step for depositions) also controls the overall reaction rate. All partial processes have their own special temperature dependence. 

An important step in heterogeneous catalysis is the adsorption and desorption of reactants and products of the reaction. Important information on the mechanism of ammonia synthesis has come from the study of the adsorption of H2, N2 and NH3. The adsorption of H2O and O2 is interesting because of the role of H2O as a poison for NH3 synthesis. 

Hsu et al. [6] state that lumped kinetic models developed by the top-down route have limited extrapolative power . To remedy this situation, many researchers have developed complex reaction schemes based on chemical first principles that involve thousands of chemical species. We can classify them into mechanistic models and pathway models. Mechanistic models track the chemical intermediates such as ions and free radicals that occur in the catalytic FCC process. Transition state theory helps in quantifying the rate constants involved in adsorption, reaction and desorption of reactant and product species from the catalyst surface. Froment and co-workers [19] have pioneered the use of such models in a refinery context and have developed a model for catalytic cracking of vacuum gas oil (VGO). Hsu et al. [6] claim that using this method is challenging because of its large size and reaction complexity. 

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