The Effect of Catalysts on Reaction Rate

Catalysts are substances that change the rate of a reaction without being consumed. They help the reaction to occur at a faster rate. Introductory courses often work with the decomposition of hydrogen peroxide as an example of a reaction affected by a catalyst. The decomposition, shown below, occurs very slowly under normal conditions  

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The Effect of Reactant Concentration on Reaction Rates The Effect of Temperature on Reaction Rates The Effect of Catalysts on Reaction Rates... 

The presence of gas-phase water is generally beneficial to the photocatalytic oxidation of aromatic contaminants. In continuous photoreactors, humidity appears to prolong catalyst activity and delay or prevent catalyst deactivation. The effects of humidity on reaction rates, however, appear to vary, depending on the aromatic contaminant concentration and the humidity level. For example. Petal and Ollis [18] examined the continuous photocatalytic oxidation of m-xylene in a powder-layer photoreactor at several different relative humidity levels. The m-xylene photo-oxidation reaction rate was observed to increase for gas-phase water concentrations up to 1000 mg/m (—7% relative humidity). Increasing the humidity level further (up to 5500 mg/m ) produced a gradual decrease in the observed reaction rate, possibly due to increased adsorption-site competition between xylene and water. The reported xylene removal rate for a water concentration of 5500 mg/m was approximately half that seen at 1000 mg/m. ... 

The combination of a good catalyst with a preferred solvent for the desired organic reaction can benefit the reaction system. Several solvents were used to study the effects of solvent on reaction rates and product selectivity on the newly-developed low palladium content catalyst. For N-phenylbenzylamine debenzylation, higher hydrogenation activity was obtained by employing methanol as a solvent. THF (tetrahydrofuran), and cyclohexane were found to be less effective. Studer and Blaser [7] studied the solvent effects on catalytic debenzylation of 4-chloro-N,N-dibenzyl anihne and foimd that the overall reaction rate was slower with the use of non-polar solvents. 

The rate constant A is a composite parameter, k = ELk, where E is the effectiveness factor, L the concentration of active sites on the surface of the catalyst, and k the actual rate constant of the transformation of the adsorbed species. The effectiveness factor which can attain values from zero to one is a measure of retardation of the reaction by diffusion of reactants or products into or out ofthe pores of the catalyst. For our purpose it should have a value of one or near to one and with careful experimentation this can be achieved. According to Thiele (14) the effectiveness factor is a function of reaction rate and effective diffusion coefficient. Both these parameters depend on the structure of the reacting compound and therefore the effectiveness factor will tend to change with the nature of the substituents. The effect of structure on reaction rate is more critical than on diffusion coefficient and if the reactivity within the series of investigated compounds will vary over some orders there is always danger of diffusional retardation in the case of the most reactive members of the series. This may cause curvature of the log kva a plot. 

The salt (buffer) type and concentration may also influence reaction rate. While buffers vary in their effect on the Maillard reaction, it is generally accepted that phosphate is the best catalyst [27], The effect of phosphate on reaction rate is pH dependent with it having the greatest catalytic effect at pHs between 5-7. Potman and van Wijk [27] found the Maillard reaction rate in a phosphate buffered model system increased from 10- to 15-fold compared to a phosphate free reaction system. 

One of the most Important uses of learning models Is In suggesting new experiments. We do not perform experiments at random, and any suggestion as to what might lead to an Improvement is very valuable. Mechanistic models for catalysts or chemical reactions are not very reliable to actually predict reaction rates or even the effect of variables on reaction rate. 

The study of polymerization kinetics allows us to understand how quickly a reaction progresses and the role of temperature on the rate of a reaction. It also provides tools for elucidating the mechanisms by which polymerization occurs. In addition, we are able to study the effect of catalysts on the rates of polymerization reactions, allowing us to develop new and better catalysts based on the measured performance. 

The equations describing the concentration and temperature within the catalyst particles and the reactor are usually non-linear coupled ordinary differential equations and have to be solved numerically. However, it is unusual for experimental data to be of sufficient precision and extent to justify the application of such sophisticated reactor models. Uncertainties in the knowledge of effective thermal conductivities and heat transfer between gas and solid make the calculation of temperature distribution in the catalyst bed susceptible to inaccuracies, particularly in view of the pronounced effect of temperature on reaction rate. A useful approach to the preliminary design of a non-isothermal fixed bed catalytic reactor is to assume that all the resistance to heat transfer is in a thin layer of gas near the tube wall. This is a fair approximation because radial temperature profiles in packed beds are parabolic with most of the resistance to heat transfer near the tube wall. With this assumption, a one-dimensional model, which becomes quite accurate for small diameter tubes, is satisfactory for the preliminary design of reactors. Provided the ratio of the catlayst particle radius to tube length is small, dispersion of mass in the longitudinal direction may also be neglected. Finally, if heat transfer between solid cmd gas phases is accounted for implicitly by the catalyst effectiveness factor, the mass and heat conservation equations for the reactor reduce to [eqn. (62)]... 

For the catalyst system NdV/EASC/DIBAH the impact of water on monomer conversion, Mw, polydispersity and cis- 1,4-content was systematically studied (Table 16) [191], With increasing amounts of water catalyst activity passes through a maximum whereas Mw and Mw/Mn pass through a minimum. It has to be mentioned, however, that the overall effect of water on reaction rate and polymer properties are relatively small. In this study it is also shown that water has no influence on cis-1,4-contents [ 191],... 

The Effect of Catalysts on Chemical Equilibrium. It is a consequence of the laws of thermodynamics—the impossibility of perpetual motion—that a system in equilibrium is not changed by the addition of a catalyst. The catalyst may increase the rate at which the system approaches its final equilibrium state, but it cannot change the value of the equilibrium constant. Under equilibrium conditions a catalyst has the same effect on the rate of the backward reaction as on that of the corresponding forward reaction. 

Predicting the effect of catalysts and reaction conditions on reaction rates... 

Finally, we study the effect of catalyst on the rate of a reaction. We learn the characteristics of heterogeneous catalysis, homogeneous catalysis, and enzyme catalysis. (13.6)... 

The mechanisms in operation for some important classes of alkene metathesis catalyst have been studied density functional theory (DFT) studies have also provided useful insights into reaction mechanisms. In particular, the initiation mechanisms of Grubbs-type and Hoveyda-type complexes have been explored. In each case, insights into the effects of structure on initiation rate have been achieved. 

Nevertheless, these microkinetic models still contain no information about the effect of catalysts on the parameters in rate equations for elementary steps. In addition, there is no verification for important kinetic steps 1 and 2, but they were checked in other cases of elementary reaction steps. 

The effect of catalysts on the ozonation rate is due to the acceleration of ozone decomposition with the production of active free radicals or the acceleration of molecular ozone reactions. The first effect promotes the reaction with respect to ozonation alone, but there is generally a strong dependence on the pH value of the solution. The presence of radical scavengers in the treated water can result in a significant reduction of the efficiency of contaminant removal due to the rapid reaction of these compounds with hydroxyl radicals. This situation is common in the case of wastewaters containing suspended material. A common catalytic element is iron that operates according to Eq. 10.5. ... 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Saturation of the oil with hydrogen is maintained by agitation. The rate of reaction depends on agitation and catalyst concentration. Beyond a certain agitation rate, resistance to mass transfer is eliminated and the rate oecomes independent of pressure. The effect of catalyst concentration also reaches hmiting values. The effects of pressure and temperature on the rate are indicated by Fig. 23-34 and of catalyst concentration by Fig. 23-35. Reaction time is related to temperature, catalyst concentration, and IV in Table 23-13. 

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