Exhaust electrode

The electrodes and electrode protective coating of the oxygen sensor play a crucial role in determining the performance characteristics and durability (2). The electrodes used are the inner or air-reference electrode and the outer or exhaust gas electrode. The protective coating goes over the outer or exhaust electrode. While a complete discussion of the requirements and properties of the electrodes and electrode protective coating is beyond the scope of this paper, a brief description will be given. 

Figure 8. SEM micrograph of platinum thin film exhaust electrode (bar = 7.5...

Sensor. The control of the exhaust composition was essential to maintain the air-to-fuel ratio close to stoichiometric for simultaneous conversion of all three pollutants. This control came about with the invention of the 02 sensor.21,22 The sensor head of this device was installed in the exhaust immediately at the inlet to the catalyst and was able to measure the 02 content instantly and precisely. It generates a voltage consistent with the Nemst equation in which the partial pressure of 02 (P02)exhaust in the exhaust develops a voltage (E) relative to a reference. The exhaust electrode was Pt deposited on a solid oxygen ion conductor of yttrium-stabilized zirconia (Zr02). The reference electrode, also Pt, was deposited on the opposite side of the electrolyte but was physically mounted outside the exhaust and sensed the partial pressure (P02)ref in the atmosphere. E0 is the standard state or thermodynamic voltage. R is the universal gas constant, T the absolute temperature, n the number of electrons transferred in the process, and F the Faraday constant. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

Aluminum reduction plants Materials handling Buckets and belt Conveyor or pneumatic conveyor Anode and cathode electrode preparation Cathode (haldng) Anode (grinding and blending) Particulates (dust) Hydrocarbon emissions from binder Particulates (dust) Exhaust systems and baghouse Exhaust systems and mechanical collectors... 

Galvanic cells in which stored chemicals can be reacted on demand to produce an electric current are termed primaiy cells. The discharging reac tion is irreversible and the contents, once exhausted, must be replaced or the cell discarded. Examples are the dry cells that activate small appliances. In some galvanic cells (called secondaiy cells), however, the reaction is reversible that is, application of an elec trical potential across the electrodes in the opposite direc tion will restore the reactants to their high-enthalpy state. Examples are rechargeable batteries for household appliances, automobiles, and many industrial applications. Electrolytic cells are the reactors upon which the electrochemical process, elec troplating, and electrowinning industries are based. 

Various furnace movements must be accommodated by the enclosure. Operations such as furnace tilting for tapping and slagging, electrode vertical lift, and direction of furnace roof swing must be coordinated with respect to enclosure shape and the location of the exhaust off-take. 

Application The zirconia oxygen sensor is widely used for combustion control processes and for air/fuel ratio regulation in internal combustion engines. The closed-end portion of the electrode tube is inserted into the exhaust gas stream. In the control of industrial combustion processes, no out stack sampling system is required. 

The deposits formed in internal combustion engines by high-octane petrols may be classed as ashes they consist of mixtures of lead oxides, bromides and sulphates derived from the anti-knock additives and, of course, exert their main corrosive effect on the parts operating at the highest temperature the exhaust valves and the sparking-plug electrodes. 

A number of ion-selective electrodes are available from laboratory supply houses whilst not intended to be an exhaustive list, Table 15.3 serves to indicate the variety of determinations for which electrodes are available. An indication is also given of the lower limit of detection of the electrodes this figure may vary somewhat according to the source of the electrode but full details are furnished by the manufacturer of the effective range of use of each electrode and of likely interferences. 

The consequences of shape change are densification and loss of electrode porosity, increased current density caused by loss of zinc surface area, and finally earlier passivation. Two different forms of pasi-vation can stop the discharge of a zinc electrode before the active material is exhausted. "Spontaneous" passivation occurs... 

As the DI plant becomes exhausted, silica leakage occurs in the treated MU water and the pH falls abruptly. Furthermore, where sodium ion leakage occurs from the cation bed, it produces a regenerative effect on the anion bed, which may also lead to silica leakage. To differentiate between the two phenomena, sodium ion leakage from the cation bed should be assessed directly using selective sodium-ion electrodes, flame photometric analysis, or other appropriate test methods. 

The mercury film electrode has a higher surface-to-volume ratio than the hanging mercury drop electrode and consequently offers a more efficient preconcentration and higher sensitivity (equations 3-22 through 3-25). hi addition, the total exhaustion of thin mercury films results in sharper peaks and hence unproved peak resolution in multicomponent analysis (Figure 3-14). 

The surface electrochemistry of Pt single-crystal electrodes has been exhaustively studied using cyclic voltammetry.100 186 188 197 209 412 753-756,771,-773,779-788,794-796 qq g technique has been proved to be highly... 

As world deposits of petroleum and coal are exhausted, new sources of hydrogen will have to be developed for use as a fuel and in the production of ammonia for fertilizer. At present, most hydrogen gas is produced from hydrocarbons, but hydrogen gas can also be generated by the electrolysis of water. Figure 19-23 shows an electrolytic cell set up to decompose water. Two platinum electrodes are dipped in a dilute solution of sulfuric acid. The cell requires just one compartment because hydrogen and oxygen escape from the cell much more rapidly than they react with each other. 

During the deposition step, some fraction of the total analyte is deposited into the mercury electrode by electrolysis for a given length of time. An exhaustive electrolysis, in which all of the analyte is deposited into the electrode, is time consuming and generally unnecessary, since adequate concentrations can usually be deposited into... 

Other important alternate electrochemical methods under study for pCO rely on measuring current associated with the direct reduction of CO. The electrochemistry of COj in both aqueous and non-aqueous media has been documented for some time 27-29) interferences from more easily reduced species such as O2 as well as many commonly used inhalation anesthetics have made the direct amperometric approach difficult to implement. One recently described attempt to circumvent some of these interference problems employs a two cathode configuration in which one electrode is used to scrub the sample of O by exhaustive reduction prior to COj amperometry at the second electrode. The response time and sensitivity of the approach may prove to be adequate for blood ps applications, but the issue of interfering anesthetics must be addressed more thorou ly in order to make the technique a truly viable alternative to the presently used indirect potentiometric electrode. 

FIG. 6. Vertical cross section of the reaction chamber. Indicated are (I) the grounded electrode, (2) the RF electrode. (3) the dark space shield, (4) the gas supply. (5) the gas exhaust. (6) the position of the sample holder during deposition. (7) the position of the sample holder when loaded, and (8) the lift mechanism. 

The appropriate gas mixture can be supplied to the center of the reactor (4 in Fig. 6) via holes in the lower electrode (2 in Fig. 6). and is pumped out through the space between substrate electrode and the reactor wall to the exhaust (5 in Fig. 6). Alternatively, the gas mixture can be supplied horizontally, parallel to the electrodes, through a flange in the reactor wall, positioned between the electrodes (perpendicular to the plane of the cross section in Fig. 6, not shown). In this case, the gas is pumped out at the opposite side of the supply. 

In the practice of electrolysis one mostly deals with altering and even exhausting redox concentrations at the electrode interface, so-called concentration polarization this has been considered already on pp. 100-102 for exhaustion counteracted by mere diffusion. The equations given for partial and full exhaustion (eqns. 3.3 and 3.4) can be extended to the current densities ... 

Shape of the polarographic curve. The kinetic theory of electrolysis (Section 3.2) for a redox system at a static inert electrode for partial and full exhaustion at the electrode under merely diffusion-controlled conditions leads, for ox + ne - red, to the relationship... 

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