Reactor electrode materials

The electrodes and the separator are the only components in an electrolytic cell which are not to be found in other chemical reactors. Electrode materials have been discussed thoroughly in earlier sections but some comments should be made about separators. In the first place it is clear that a cell should only have a separator if one is entirely necessary. Besides their cost, the inclusion of a separator restricts the electrode geometry and mass transport conditions which are possible, increases the cell resistance substantially and certainly makes the cell design more complex the separator must be gasketed to avoid leaks, there must be separate anolyte and catholyte chambers and therefore twice the number of pipe connections to the cell. 

Polysilicon. Polysihcon is used as the gate electrode material in MOS devices, as a conducting material for multilevel metallization, and as contact material for devices having shallow junctions. It is prepared by pyrolyzing silane, SiH, at 575—650°C in a low pressure reactor. The temperature of the process affects the properties of the final film. Higher process temperatures increase the deposition rate, but degrade the uniformity of the layer. Lower temperatures may improve the uniformity, but reduce the throughput to an impractical level. 

An appreciable increase in working area of the electrodes can be attained with porous electrodes (Section 18.4). Such electrodes are widely used in batteries, and in recent years they are also found in electrolyzers. Attempts are made to use particulate electrodes which consist of a rather thick bed of particulate electrode material into which the auxiliary electrode is immersed together with a separator. Other efforts concern fiuidized-bed reactors, where a finely divided electrode material is distributed over the full electrolyte volume by an ascending liquid or gas flow and collides continuously with special current collector electrodes (Section 18.5). 

The large-scale spread of DAFCs is closely related to the development of efficient anodic and cathodic materials, characterized by very fast electrochemical kinetics, stability at the high current densities in alkaline environments and modest cost. This objective requires cathodes without noble metals and anodes with very low amounts of noble metals. In order to improve the cheapness and sustainability of the processes described above, the most accepted opinion is the possibility of using solar light by means of the introduction of Ti02, pure or doped, into the electrode material formulation. Figure 4.15 shows a typical laboratory-scale photoelectrocatalytic reactor. 

Three-dimensional electrode materials that fit well into parallel-plate [75,91, 92,93] reactors are (i) reticulated metals [75,91-93], (ii) metalized plastics (metalization of polyurethane foams) [94] and (iii) carbon [95]. 

Another application of carbon and carbon hybrids is their use as electrode material in proton exchange membrane (PEM) electrochemical flow reactor for the production of hydrogen peroxide (H202). 

There are pros and cons for each method of electrode preparation. The polycrystalline electrodes are cheap and also are nearest in character to those used in practical reactors inindustiy. However, a polycrystal consists ofinumerable grains (bits) of the electrode material, each having a different crystal orientation and hence a different catalytic property. One way of manufacturing an original metal may differ from another in the distribution of crystal faces of different kinds. Thus, irreproducibility of results in electrode kinetics is not only due to inadequate purification of solution,... 

Fuchigami, Marken and coworkers also reported self-supported anodic methoxy-lation and acetoxylation of several aromatic compounds using a simple thin-layer flow cell reactor (interelectrode gap of 80 pm) (Scheme 4.41) [56]. The current efficiency (CE) of this process was 10% at best because of oxidation of methanol at flow rates lower than 0.03 ml/min. Even though CE increased at a faster flow rate (0.5 ml/min), the yield decreased sharply. The importance of selecting an appropriate choice of electrode material also was noted. 

There are two types of multielectrode reactor monopolar and bipolar cells, as shown in Fig. 15.2. The bipolar configuration has the advantage that the electrical circuit has only to be linked at the ends of the electrode pile the disadvantage is limitation to certain electrode materials when the anode and cathode are of the same material or when they can be easily glued to each other. 

The SPE HDH reactor is flexible in terms of structural materials and functions, e.g. electrode materials could be mesh- or carbon-supported gas diffusion ones the SPE could be a cation or anion exchange membrane (e.g. Nafion 117 or Fu-MATech FT-FKE-S) and the reactor can treat either aqueous or non-aqueous (e.g. a paraffin oil) wastes with or without supporting electrolytes. 

Figure 15 reports experimental results obtained by emission spectroscopy on the relative evolution of the fluorine concentration in CF4—02 and SF6—02 mixtures in a rf diode reactor for various rf electrode materials (Al, Si, Ge) [55,65]. In the case of an electrode material inert with respect to the plasma (Al), addition of 02 to SF6 and CF4 induces, as expected, an increase of the fluorine concentration until the dilution effect lowers the F density. The lower F concentrations observed in the case of Si and Ge electrodes are due to the consumption of fluorine on these surface to form volatile SiF4 and GeF4. In the case of Si with SF6, from 0 to 40% 02 addition reaction is so fast that it consumes all the additional fluorine produced in the plasma. Above 40% 02, the F concentration increases gradually to... 

Principles and applications of electrochemical remediation of industrial discharges are presented by Pallav Tatapudi and James M. Fenton. Essentials of direct and indirect oxidation and reduction, membrane processes, electrodialysis, and treatment of gas streams, and of soils, are complemented by discussions of electrode materials, catalysts, and elements of reactor design. 

Unexpected elements in a plasma polymer often are due to the redeposition of ablated materials. The presence of nitrogen found in a plasma polymer of a monomer that does not contain nitrogen can be traced to contamination of the reactor, which has been used for plasma polymerization of nitrogen-containing monomers [1]. The ablation of electrode material has been utilized to create a graded metal-polymer and polymer-metal interfaces to obtain an excellent adhesion [2,3]. Ablation, therefore, could be utilized in a beneficial way in the engineering of interfaces if we know the nature of ablation and how to control it. 

The anode and cathode should be stable in the electrolysis medium, allow the desired oxida-tion/reduction reactions at the highest possible rates with miiumal by-product formation, and be of reasonable cost. In actuality, the electrodes may corrode or undergo physical wear during reactor operation, which may limit their lifetime. Often, if an expensive electrode material is needed for a given reaction, it can be plated or physically coated on a less costly, inert, and electronically conducting substrate. Common anode and cathode materials are listed in Table 26.8. 

When formulating an electro-organic synthesis on a laboratory scale, the proper choice of reactor, electrolyte composition, and electrode materials must be made. 

The majority of reactors utilise electrolytic cells i.e. the cell reaction is driven by an external power supply general cases have been considered in figure 3 and specific examples will be illustrated in section 5. In certain cases, the electrode materials and conditions may result in a spontaneous reaction i.e. the use of a galvanic cell, particularly in the case of the displacement (cementation) of metals, for example, cupric ions may be removed as a copper deposit on a soluble iron powder in acid solution ... 

In many cases of metal-ion removal, there is a need to maintain a very high fractional conversion in a continuous flow-through reactor. This situation may be accomodated via a number of reactors in series flow both CSTR s [36, 37] and PFR s [45] have been considered. The important requirement of design simplicity often dictates the use of a static cathode geometry incorporating porous, 3-dimensional electrode materials [46], One example is reticulated vitreous carbon which has been extensively investigated in our laboratories [45-49] and elsewhere, e.g. [50, 51]. 

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