Catalytic enhancement mechanisms

Four general mechanisms account for the ability of enzymes to achieve dramatic catalytic enhancement of the rates of chemical reactions. 

Porous ceramic membrane layers are formed on top of macroporous supports, for enhanced mechanical resistance. The flow through the support may consist of contributions due to both Knudsen-diffusion and convective nonseparative flow. Supports with large pores are preferred due to their low resistance to the flow. Supports with high resistance to the flow decrease the effective pressure drop over the membrane separation layer, thus diminishing the separation efficiency of the membrane (van Vuren et al. 1987). For this reason in a membrane reactor it is more effective to place the reaction (catalytic) zone at the top layer side of the membrane while purging at the support side of the membrane. 

For the catalytic electrode mechanism, the total surface concentration of R plus O is conserved throughout the voltammetric experiment. As a consequence, the position and width of the net response are constant over entire range of values of the parameter e. Figure 2.35 shows that the net peak current increases without limit with e. This means that the maximal catalytic effect in particular experiment is obtained at lowest frequencies. Figure 2.36 illustrates the effect of the chemical reaction on the shape of the response. For log(e) < -3, the response is identical as for the simple reversible reaction (curves 1 in Fig. 2.36). Due to the effect of the chemical reaction which consumes the O species and produces the R form, the reverse component decreases and the forward component enhances correspondingly (curves 2 in Fig. 2.36). When the response is controlled exclusively by the rate of the chemical reaction, both components of the response are sigmoidal curves separated by 2i sw on the potential axes. As shown by the inset of Fig. 2.36, it is important to note that the net currents are bell-shaped curves for any observed kinetics of the chemical reaction, with readily measurable peak current and potentials, which is of practical importance in electroanalytical methods based on this electrode mecharusm. 

As part of the early work to find alloys ofplatinum with higher reactivity for oxygen reduction than platinum alone, International Fuel Cells (now UTC Fuel Cells, LLC.) developed some platinum-refractory-metal binary-alloy electrocatalysts. The preferred alloy was a platinum-vanadium combination that had higher specific activity than platinum alone.25 The mechanism for this catalytic enhancement was not understood, and posttest analyses26 at Los Alamos National Laboratory showed that for this binary-alloy, the vanadium component was rapidly leached out, leaving behind only the platinum. The fuel- cell also manifested this catalyst degradation as a loss of performance with time. In this instance, as the vanadium was lost from the alloy, so the performance of the catalyst reverted to that of the platinum catalyst in the absence of vanadium. This process occurs fairly rapidly in terms of the fuel-cell lifetime, i.e., within 1-2000 hours. Such a performance loss means that this Pt-V alloy combination may not be important commercially but it does pose the question, why does the electrocatalytic enhancement for oxygen reduction occur ... 

As a second possibility, one may assume that at low potentials OH adsorption occurs exclusively on Ru sites, in agreement with the bifunctional mechanism for CO oxidation on PtRu alloy surfaces. The observed depletion of CO on the Pt sites can then only be explained by rapid surface diffusion of the adsorbed CO molecules to the reactive Ru islands. Because the separation of the active Ru islands is on the nm scale, even a low mobility of the adsorbed CO can account for the observed catalytic enhancement. 

The application of site-directed mutant enzymes to investigate catalytic reaction mechanisms has been considered (subsection 11.4.2). The site-directed mutagenesis has been used to fine-tune enzyme activity, to redesign enzyme specificity/catalysis and to improve enzyme properties such as enhancement of enzyme stability (Table 13.11). 

The above-mentioned chemical catalytic routes lead to racemic AHA mixtures. For the direct use of LA (or its esters) as a solvent or platform molecule for achiral molecules like acrylic acid and pyruvic acid, stereochemistry does not matter. The properties of the polyester PLA, the major application of LA, however, suffer tremendously if d and l isomers are built in irregularly [28]. This is exemplified by atactic PLA, made from racemic LA, which is an amorphous polymer with low performance and limited application. However, when l- and D-lactic acid are processed separately into their respective isotactic L- and d-PLA, as discovered by Tsuji et al., a stereocomplex is formed upon blending these polymers. This polymer exhibits enhanced mechanical and thermal properties [28, 164]. A productive route to D-Iactic acid is, however, missing today. If the chemocatalytic routes to LA are to become viable, enantiomer resolution of the racemate needs to be performed. Given separation success, a cheap source of o-lactic acid will be unlocked immediately, providing an additional advantage over the fermentation route (cfr. Table 2). 

It should be mentioned that in some cases the higher current observed is not dne to the catalytic enhancement of the reaction but is instead a conseqnence of the increased surface area. Nevertheless, this effect is also important, especially when precious metal particles are dispersed in the polymer matrix. Althonghthe condnct-ing polymers are rather stable chemically, there are often problems with the longterm physical stability when gas evolution occurs or intense mechanical stirring is applied. 

It is important to establish if the mechanism of the observed catalytic effect played by an ad-atom is electronic or if the ad-atom is acting as a bifunctional catalyst. The problem arises from the fact that both effects can be considered as short-range effects, that is, the catalytic enhancement is only observed in the surface substrate atoms closest to the adatom. For the electronic effects, it is known that the effect induced by an ad-atom extends to the first and second row or neighboring atoms to the ad-atoms [60]. For the bifunctional catalyst, the pair responsible for the catalytic effect is the combination of an active site in the adatom and an active site in the surface close to it. This way, the determination of the electrocatalytic mechanism requires a detailed knowledge of the distribution of the ad-atom on the surface and the condition under which the catalytic enhancement is found. These questions are currently under investigation. 

In the first case, the activity of the surface is directly proportional to the number of pair ad-atom-surface sites, whereas in the latter case, the activity is proportional to the number of unoccupied surface sites. Since oxidation of formic acid takes place though a parallel-path mechanism, the effects of different levels of poisoning are also considered. For the cases in which the surface is completely covered by poison, both types of ad-atoms (the catalytically effective ad-atoms and the third-body adatoms) produce similar qualitative effects, that is, both types increases the current for the oxidation of formic acid [65]. Of course, the catalytic enhancement is higher in the case of the ad-atoms that modifies the electronic properties of the surface, since the global effect will be the combination of the electronic enhancement and the third-body effect (any ad-atom always acts as a third body). This is the case, for instance, for the Pt(lOO) surfaces modified with ad-atoms [65-67]. For the surfaces with low poisoning, that is, the Pt(lll)... 

XPS measurements of passivated Fe and Ni electrodes that have been exposed to aggressive anions (Ni and Fe to F Fe to Cl , Br, and I ) but have not already formed corrosion pits support this mechanism. The quantitative evaluation of the data clearly shows a decrease of the oxide thickness with time of exposure [22,48], Not only F but also the other halides cause thinning of the passive layer (Fig. 8) [48], The catalytically enhanced transfer of cations from the oxide to the electrolyte leads to a new stationary state of the passive layer. Its smaller thickness yields an increased electrical field strength for the same potentiostatically fixed potential drop, which in turn causes faster migration of the cations through the layer to compensate for the faster passive corrosion reaction (1) at the oxide-electrolyte interface (Fig. 2a). Statistical local changes... 

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