Palladium interface reactions with

G. Interface Reactions with Gold, Silver, and Palladium... 

A schematic representation for the reaction pathways is given in Figure 27 [88]. At the surfaces or perimeter interfaces of gold NPs O2 and H2 react with each other to form H2O2. Gold is catalytically as active as palladium for the direct synthesis of H2O2 [89-91]. Figure 28 draws a... 

The previous extension of solvent mixtures involved solvent interfaces. This organic-water interfacial technique has been successfully extended to the synthesis of phenylacetic and phenylenediacetic acids based on the use of surface-active palla-dium-(4-dimethylaminophenyl)diphenylphosphine complex in conjunction with dode-cyl sodium sulfate to effect the carbonylation of benzyl chloride and dichloro-p-xylene in a toluene-aqueous sodium hydroxide mixture. The product yields at 60°C and 1 atm are essentially quantitative based on the substrate conversions, although carbon monoxide also undergoes a slow hydrolysis reaction along with the carbonylation reactions. The side reaction produces formic acid and is catalyzed by aqueous base but not by palladium. The phosphine ligand is stable to the carbonylation reactions and the palladium can be recovered quantitatively as a compact emulsion between the organic and aqueous phases after the reaction, but the catalytic activity of the recovered palladium is about a third of its initial activity due to product inhibition (Zhong et al., 1996). 

However, the electric potential of the electrocatalyst at its interface with the electrolyte (and thus the facility for charge transfer) can be easily and extensively altered at will to control rate and selectivity. For instance, a decrease of electrode potential by about 0.15 V can change the product selectivity for vinyl fluoride and chloride reduction on palladium by as much as 80% (31). In contrast, gas phase parallel reductions, with 5 kcal/mol difference in activation energies, would require a temperature increase from 500 K to 730 K for a comparable selectivity change. We should note here that the electrocatalytic specificity of the above reductions is quite similar to that of conventional heterogeneous catalytic reactions, but differs from that of conventional electrolytic reduction on noncatalytic electrodes (32). 

Although adsorbed molecules of extractants are well oriented at the interface, the interfacial reactions occur more slowly, as recalculated for the same volume, than reactions in the aqueous phase. The extraction of palladium(II) from HCl solutions with dialkyl sulfides is a classic example of a very slow process. Modifying the extractant structure by adding a hydrophilic hydroxyl group and/or a phase transfer catalyst, e.g., trialkylamine or quaternary ammonium salt, increases the rate of extraction. The addition of sulfonic and phosphoric acids to hydroxyoximes may enhance the rate of copper extraction due to the formation of reverse micelles and the development of the microscopic interface. The a-acyloin oximes that form an intermediate complex that has a five-member ring with copper(II) increase the rate of extraction of aromatic hydroxyoximes. The addition of surfactants may cause both retardation and acceleration of extraction. If the interfacial tension is decreased, surfactants cause an increase in the interfacial surface area in dispersed systems. When they adsorb at the interface, they cause an additional... 

The method in which palladium complexes with hydrophilic phosphines are used in a biphasic system of water-organic solvent can be considered complementary to the standard protocol. In this case, boronate and palladium catalyst reside in the aqueous phase, while halide substrate is in the organic phase. In order for the reaction to run, the latter should be partitioned into the aqueous phase. Alternatively, oxidative addition may occur at the interface. Due to the low efQcienc of both methods, high loads of palladium catalyst and phosphine are required. Recycling is possible but is hampered by the accumulation of inorganic salts (hahde, borate) in the aqueous layer. 

Since the response of a hydrogen sensor is mainly governed by the hydrogen adsorption reaction on the palladium-semiconductor interface, the decrease in film thickness would lead to increased response and recovery rates. It was established that such a device with 10-100-nm thick palladium layer could easily detect 10 ppm of hydrogen in air in the range 27-300 °C (Lundstrom et al. 1975 Choietal. 1986). 

Practical applications of [C mimBFJ/TX-lOO/cyclohexane microemulsions include reaction media for kinetic studies and synthesis of metal NPs. For instance, the rate constant for aminolysis of an ester at the interface of this IL/oil microemulsion is between two and four times higher than in traditional W/oil microemulsions. However, the rate constant is at least five times higher in pure water than in the [C mim][BFJ [32]. An example is a medium to prepare stable silver NPs, with an average diameter of approximately 3 nm, in the absence of any other auxiliary solvent in the whole process [33], [C mim][BF ]/TX-100/cyclohexane microemulsions were recently used as a new method for the preparation of monodispersed palladium NPs [34],... 

Of the noble metals, palladium exhibits the highest activity for CO oxidation.As already mentioned for Pd-promoted CeCoO catalysts, the reaction over a noble metal supported on ceria involves a cooperative effect between the metals and the oxide, the so-called dual site mechanism. A recent study by temporal analysis of products (TAP) experiments for Pt supported over ceria showed two independent sites with different activities. The high activity site was associated with the metal/support interface and the low activity one was located on the support. The different sites were characterized by two different activation energies. Moreover, at variance with the stable number of high activity sites, the number of the low active sites increased with reaction temperature. 

---