Catalysts general assessement

As indicated earlier, evaluation is probably the most diffi-cut problem. How does one conjecture the value of an arbitrary reaction Thermodynamics can tell one how far a reaction may proceed, but no one can yet predict how fast a reaction will proceed since the rate depends on such things as the effect of catalysts. Also assessing the processing costs is a complex exercise. Generally, evaluation is done very indirectly in these programs by allowing the use only of named chemistry and then, for those reactions which might work, other effects such as stereo hindrance are sometimes examined. 

Catalytic performance for any system, heterogeneous or homogeneous, is generally assessed by measuring the activity (rate of reaction) and selectivity of the catalyst for reactions of Interest. A comparison of reaction rates for catalysts prepared using different synthetic methods or different supports is used to Identify the parameters which Influence catalyst activity and selectivity. For studies involving heterogeneous catalysts and Immobilized catalysts, reactant transport to the active sites must occur at a sufficient rate that the observed activity reveals information about the intrinsic properties of the catalytic site. 

Pilot plants utilizing a single-full-sized reactor tube from a commercial plant are generally used to assess the quaUty and performance of individual catalyst lots and to perform plant or customer ordered process tests. A weU-designed pilot unit is capable of simulating the performance of a commercial plant with great accuracy. 

The general picture of the relative merits of homogeneous and heterogeneous processes has not yet emerged clearly. The homogeneous catalyst system may offer advantages in chemical efficiency but lead to difficulties of catalyst separation and recovery, or catalysts may tend to plate out in the reactor due to slight instability. Materials of construction may have to be different for the two rival plants. All these factors will have to be considered in an economic assessment and detailed studies made of the complete process networks in both cases. 

Some pessimism in assessing the situation in the field of electrocatalysis may also derive from the fact that one of the final aims of work in this held, setting up a full theory of electrocatalysis at a quantum-mechanical level while accounhng for all interactions of the reacting species with each other and with the catalyst surface, is still very far from being reahzed. So far we do not even have a semiempirical— if sufficiently general—theory with which we could predict the catalytic activity of various catalysts. 

In general, it is not strictly correct to conclude that a particular reaction order fits the data based simply on the conformity of data to an integrated equation. As illustrated above, multiple initial concentrations which vary considerably should be employed to assess whether the rate is independent of concentration. Multiple integrated equations should also be tested. It may be useful to show that the reaction rate is not affected by species whose concentrations do not change considerably during an experiment these may be substances not consumed in the reaction (i.e., catalysts) or present in large excess [23,108]. 

A 90% reduction in activation energy, not an unreasonable expectation for catalysts in general, reduces the peak temperature below 0 C. Clearly, only a small amount of catalytic action is required to make dramatic reductions in the release temperature. This implies that, with careful control of the invented process, it should be possible to dial-in the desorption temperature for hydrogen desorption. This allows us to assess how this hydrogen storage media can be applied. 

General Motors has assessed the required activity of a catalyst that costs less compared to the current state-of-the-art Pt activity based on these con-straints. i Assuming that the catalyst layer thickness could be increased to MOO pm from the currently used 10 pm, GM has calculated that the minimum volume activity (i.e., Acm ) for a cathode catalyst that costs less should be at least 10% of the current Pt activity. In reality, this seems rather generous, given the recent trend to reduce catalyst layer thicknesses to optimize high-current performances. The DoE has developed a series of volume activity targets for nonprecious metal catalysts, with the 10% of Pt activity target (300 Acm 3 at 0.8 V, H2/O2) necessary by 2015. 

No single theoretical explanation of compensation behavior has been recognized as having general application. It is appropriate, therefore, to consider in this context the conditions obtaining on a catalyst surface during reaction, with particular reference to the factors that control the rate of product evolution and to the interpretation of kinetic measurements. This discussion of surface behavior precedes a critical assessment of the significance of measured values of A and E. 

In assessing whether a reactor is influenced by intraparticle mass transfer effects WeiSZ and Prater 24 developed a criterion for isothermal reactions based upon the observation that the effectiveness factor approaches unity when the generalised Thiele modulus is of the order of unity. It has been showneffectiveness factor for all catalyst geometries and reaction orders (except zero order) tends to unity when the generalised Thiele modulus falls below a value of one. Since tj is about unity when 0 < ll for zero-order reactions, a quite general criterion for diffusion control of simple isothermal reactions not affected by product inhibition is < 1. Since the Thiele modulus (see equation 3.19) contains the specific rate constant for chemical reaction, which is often unknown, a more useful criterion is obtained by substituting l v/CAm (for a first-order reaction) for k to give ... 

To assess the generality of the decomposition solution method we have applied it to several arbitrary reaction orders, for example, n = 1.73, 0.67, -0.5, -1.0 etc.10,11 These reaction orders have been chosen to represent typical cases of kinetics in heterogeneous catalysis and electrocatalysis where adsorption phenomena play a major role. Values of effectiveness of a plane catalyst pellet for the different reaction orders are shown in Figure 5. Clearly all of the data for positive reaction orders show the expected trend of a decrease in effectiveness with increase in Thiele modulus. Effectiveness values determined for reaction... 

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