Catalyst/water interface

Unfortunately, there has hardly been any ab initio work on the structure of catalyst/polymer interface. On the other hand, there is a bulk of literature on the structures of catalyst/water interfaces. 

Neutral PET hydrolysis usually takes place under high temperature and pressure in die presence of alkali metal acetate transesterification catalysts.28 It is diought diat the catalytic effect observed on the part of zinc salts is the result of electrolytic changes induced in die polymer-water interface during the hydrolysis process. The catalytic effect of zinc and sodium acetates is thought to be due to die destabilization of die polymer-water interface in the hydrolysis process. 

Many organic electrode processes require the adsorption of the electroactive species at the electrode surface before the electron transfer can occur. This adsorption may take the form of physical or reversible chemical adsorption, as has been commonly observed at a mercury/water interface, or it may take the form of irreversible, dissociative chemical adsorption where bond fracture occurs during the adsorption process and often leads to the complete destruction of the molecule. This latter t q)e of adsorption is particularly prevalent at metals in the platinum group and accounts for their activity as heterogeneous catalysts and as... 

The evolving structural characteristics of CLs are particularly important for further analysis of transport of protons, electrons, reactant molecules (O2), and water as well as for the distribution of electrocatalytic activity at Pt-water interfaces. In principle, the mesoscale simulations allow relating these properties to the choices of solvent, ionomer, carbon particles (sizes and wettability), catalyst loading, and hydration level. Explicit experimental data with which these results could be compared are still lacking. Versatile experimental techniques have to be employed to study particle-particle interactions, structural characteristics of phases and interfaces, and phase correlations of carbon, ionomer, and water in pores. 

Aqueous surfactants are another class of catalysts. Substantial rate enhancement is seen in the reaction occurring at the micellar hydrocarbon-water interface, which is ascribed to a concentration of the reactant in the micellar pseudo-phase. Chiral p-nitrophenyl esters derived from phenylalanine are hydrolyzed by a histidine-containing dipeptide at a micellar interphase, at which a very high enantiomer discrimination, kR/ks up to 30.4 at 0°C, is observed (49). As shown in Scheme 20, the enantioselectivity is expressed at the stage at which a transient, zwitter-ionic tetrahedral intermediate leading to the acylimidazole is formed,... 

Fig. 8. The application of vesicles for photocatalytic water decomposition in sacrificial systems (a) — dihydrogen evolution in the vesicle cavity. Pt metal catalyst is anchored to the inner membrane // water interface (b) — dioxygen evolution in the bulk solution. Manganese oxide catalyst is anchored to the outer membrane // water interface of the vesicle...

The use of a two-phase system with added phase transfer catalyst and the use of a microemulsion are two alternative approaches to overcome reagent incompatibility problems in organic synthesis. Both routes have proved useful but on entirely different accounts. In phase transfer catalysis the nucleophilic reagent is carried into the organic phase where it becomes highly reactive. In the microemulsion approach there is no transfer of reagent from one environment to another the success of the method relies on the very large oil-water interface at which the reaction occurs. 

The rate equation with strongly acidic catalysts is also second order in silanol and first order in catalyst (75). The mechanism is proposed to proceed via protonation of silanol, followed by an electrophilic attack of the conjugate acid on nonprotonated silanol. The condensation processes outlined in reactions 16a and 16b for sulfonic acids is also an applicable mechanism for the acid catalysis. The condensation polymerization in emulsion catalyzed by dodecylbenzenesulfonic acid is second order in silanol, but the rate has a complex dependence on sulfonic acid concentration (69). This process was most likely a surface catalysis of the oil-water interface and was complicated by self-associations of the catalyst-surfactant. 

The backing in this model is treated with the Stefan-Maxwell equation to yield the partial pressure of oxygen at the backing/catalyst layer interface from the total pressure Ptot in the gas flow channel, the backing characteristic current density, 7b, and the mole fractions of water vapor and of oxygen, Xws and Xon, respectively ... 

The advantage of the emulsion method of polymerization lies in the wide choice of catalysts that may be used. In mass reactions such agents as oil-soluble peroxides, diazoamine derivatives, and diazothioethers must be used. Emulsification permits the use of water-soluble catalysts, as well as mixtures of these with oil-soluble materials. The reaction then occurs at the monomer-water interface, with a high probability that the free radicals formed will immediately react with the adjacent monomer. Use of quarternary emulsifiers such as CTAB or Emulsol 607 in such... 

Figure 8.20. Simplified scheme for the oxidation of H2S by O2 mediated by a variety of bacteria. The gradient zone between O2 and H2S is the environment of many colorless sulfur bacteria, among which the type Beggiatoa often reach high population densities and form white mats on the mud or sediment-water interface. If light penetrates at the zonation between O2 and H2S, phototrophic, often colorful, sulfur bacteria grow. Reduced sulfur can also be oxidized abiotically, for example, by Fe(III)(hydr)oxides or even by O2 in the presence of metal-ion catalysts.

Polyphenol thin films were obtained using Langmuir—Blodgett technique.21b-24 A monomeric mono-layer was formed from p-tetradecyloxyphenol and phenol at the air—water interface in a Langmuir trough, which was polymerized by HRP catalyst in the subphase. The polymerized film could be deposited on silicon wafer with a transfer ratio of 100% for the Y-type film with a thickness of 27.8 A. 

Some other C—C bond coupling reactions in micellar systems should be mentioned here. Monflier et al. [72] described, in both papers and patents, the telome-rization of 1,3-butadiene into octadienol in a micellar system by means of a palladium-phosphine catalyst. Water-soluble and amphiphilic phosphines have been used and the surfactants were widely varied. The authors have shown that the promoting effect of surfactants appeared above the CMCs of the surfactants, and they conclude that micellar aggregates were present in the reaction mixture. Cationic, anionic, and nonionic surfactants gave this micellar effect but the combination of the highly water-soluble TPPTS and the surfactant dodecyldimethylamine hydrocarbonate was found to be best. A speculation about the location of reactants shows that the reaction probably occurs in the interface between the micellar pseudophase and water. 

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