Catalytic response

This XPS investigation of small iron Fischer-Tropsch catalysts before and after the pretreatment and exposure to synthesis gas has yielded the following information. Relatively mild reduction conditions (350 C, 2 atm, Hg) are sufficient to totally reduce surface oxide on iron to metallic iron. Upon exposure to synthesis gas, the metallic iron surface is converted to iron carbide. During this transformation, the catalytic response of the material increases and finally reaches steady state after the surface is fully carbided. The addition of a potassium promoter appears to accelerate the carbidation of the material and steady state reactivity is achieved somewhat earlier. In addition, the potassium promoter causes a build up on carbonaceous material on the surface of the catalysts which is best characterized as polymethylene. 

If the system were to remain in this situation, no kinetic information concerning the follow-up reaction would be available. In the other limiting case, the catalytic response is governed by the follow-up reaction, while electron transfer acts as a preequilibrium. The response is thus a function of the dimensionless parameter... 

Attaching the catalyst molecules to the electrode surface presents an obvious advantage for synthetic and sensor applications. Catalysis can then be viewed as a supported molecular catalysis. It is the object of the next section. A distinction is made between monolayer and multilayer coatings. In the former, only chemical catalysis may take place, whereas both types of catalysis are possible with multilayer coatings, thanks to their three-dimensional structure. Besides substrate transport in the bathing solution, the catalytic responses are then under the control of three main phenomena electron hopping conduction, substrate diffusion, and catalytic reaction. While several systems have been described in which electron transport and catalysis are carried out by the same redox centers, particularly interesting systems are those in which these two functions are completed by two different molecular systems. 

The next step consists of assembling these various rate-limiting factors so as to provide a description of the resulting catalytic responses of these multilayered coatings. 

Assuming that pure kinetic conditions are fulfilled, the Q profile is confined within a thin layer adjacent to the electrode surface. It therefore follows from the condition (0[S]/0x) c=o = 0 that [S] may be regarded as constant throughout the reaction layer and equal to its value, [S], at the electrode surface. Within this framework, we consider the case where the catalytic response is controlled by the enzymatic reaction. Equations (6.233) may be simplified upon consideration that [S] = C( and also from the fact that pure kinetic conditions implies that 0[Q]/0t = 0. It follows that... 

Immobilizing the catalyst on the electrode surface is useful for both synthetic and sensors applications. Monomolecular coatings do not allow redox catalysis, but multilayered coatings do. The catalytic responses are then functions of three main factors in addition to transport of the reactant from the bulk of the solution to the film surface transport of electrons through the film, transport of the reactant in the reverse direction, and catalytic reaction. The interplay of these factors is described with the help of characteristic currents and kinetic zone diagrams. In several systems the mediator plays the role of an electron shuttle and of a catalyst. More interesting are the systems in which the two roles are assigned to two different molecules chosen to fulfill these two different functions, as illustrated by a typical experimental example. 

The major part of the reports discussed above provides only a qualitative description of the catalytic response, but the LbL method provides a unique opportunity to quantify this response in terms of enzyme kinetics and electron-hopping diffusion models. For example, Hodak et al. [77[ demonstrated that only a fraction of the enzymes are wired by the polymer. A study comprising films with only one GOx and one PAH-Os layer assembled in different order on cysteamine, MPS and MPS/PAH substrates [184[ has shown a maximum fraction of wired enzymes of 30% for the maximum ratio of mediator-to-enzyme, [Os[/[GOx[ fs 100, while the bimolecular FADH2 oxidation rate constant remained almost the same, about 5-8 x 10 s ... 

The boundary between cases I-II has been explored by Flexer et al. [69] with LbL self-assembled GOx multilayers and poly(bipyridine-pyridine) redox polymer (PAH-Os). Figure 2.29 shows the catalytic response in excess glucose as a function of the number ofself-assembled polymer-enzyme bilayers. For the first bilayers (thin films)... 

The organic conductor properties of tetrathiaflulvalenetetracyanoquino-dimethane (TTF-TCNQ) as a material for constructing electrodes, viz. its catalytic response and resistance to passivation, are of special interest for the determination of biological compounds, which usually have slow electrode kinetics and a low sensitivity, and tend to foul electrode surfaces. The response of a TTF-TCNQ microarray sensor inserted in a flow system for... 

Electrochemical Behavior, Catalytic Response to Glucose, and Selectivity to Pharmaceutical Drugs... 

Polypyrrole thin film doped with glucose oxidase (PPy-GOD) has been prepared on a glassy carbon electrode by the electrochemical polymerization of the pyrrole monomer in the solution of glucose oxidase enzyme in the absence of other supporting electrolytes. The cyclic voltammetry of the PPy-GOD film electrode shows electrochemical activity which is mainly due to the redox reaction of the PPy in the film. Both in situ Raman and in situ UV-visible spectroscopic results also show the formation of the PPy film, which can be oxidized and reduced by the application of the redox potential. A good catalytic response to the glucose and an electrochemical selectivity to some hydrophilic pharmaceutical drugs are seen at the PPy-GOD film electrode. 

Effect of pH. The relationship between the catalytic current of a PPy-GOD film (2000 A) and the pH of the solution was recorded in the pH range of 3-11. When the pH was less than 3, no catalytic current was observed, but the current was increased by increasing the pH from 3 to 7. When pH was more than 7.5, the current decreased. The PPy-GOD film electrode showed a good catalytic response to glucose in the solutions of pH ranging between 6 and 8. 

Figure 6. Catalytic response of PPy-GOD film to glucose, (a) Current response at PPy-GOD film electrodes with successive additions of 2.5 mM glucose solution (1) 80 A film and (2) 2000 A film, in PB buffer (pH 7.4) at 1.0 V. (b) Calibration curves. 1 80 A film, 2 2000 A film.

Wright and co-authors (Wright et al. 1999) studied the properties of a polyethyle-neoxide myoglobine-modified electrode in the reductive dehalogenation of hex-achloroethane in ethanolic solutions, and the observed catalytic response resulted linearly dependent on the bulk concentration of the substrate. 

The different catalytic responses of peroxidase in dioxane and methanol versus acetone are intriguing. It is clear that the effects of water-miscible solvents on enzymatic catalysis are not equivalent and for the first time quantitative kinetic data have been obtained which highlight this. However, the cause of this effect remains unresolved. We are continuing and expanding this kinetic study to include other solvents, both water-miscible and immiscible, and other phenols. This future study will enable rational and quantitative approaches for peroxidase-catalyzed phenolic polymerizations to be based on optimal solvent and phenol choices. From a more fundamental standpoint, this work has shown that enzymes may be more active in organic media than in water as long as optimal conditions are employed. There is no reason to believe peroxidase is unique in this respect. 

Figure 12. Voltammetric response of a GOx-SWCNT-modified glassy carbon electrode in the absence (red) and presence (blue) of 0.5 mM ferrocene monocarboxylic acid. The catalytic response (green) after the addition of 50 mM glucose is also shown. From reference 89.

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