Catalyst electrodes

In the single-chamber type reactor (Fig. 4.1b) all three electrodes (catalyst-working (W), counter (C) and reference (R)), electrode are all in the same chamber and are all exposed to the reactants and products.1 3 In this case the counter and reference electrodes must be made from a catalytically inert (e.g. Au) material for otherwise the catalytic rate on them will obscure the measured (via gas-chromatography or mass-spectrometry, Fig. 4.2) rate on the catalyst-working electrode. 

Figure 10A. Effect of electrode-catalyst potential and oxygen (a) and hydrogen (b) partial pressure on the rate of hydrogen oxidation on Pt/graphite in 0.1 M KOH (a) and 0.1 M LiOH (b) Fv=500 cm3 STP/min. Reprinted with permission from Nature, McMillan Magazines Ltd.3...

Fig. 5. Change of the open-circuit potential with time for steam reforming of methanol over the 30 wt% Ni-SDC and 30 wt% Ni-YSZ electrode-catalyst. Upper Ni-SDC lower Ni-YSZ. Operating conditions 800 °C, 1 atm, H20/CH30H = 2, space time = 0.37 s [9].

Sucessful application of the air electrode requires solving some key problems the air electrode catalyst, the alkaline electrolyte carbonization, the oxygen reaction with anode materials, an influence of an air humidity on an electrode behavior. 

The capaciatance C depends on the gas-electrode-electrolyte interline "area" but not on the total electrode surface area S. If the porosity of all the electrode catalysts used is the same, which is a reasonable assumption since they were all prepared by the same calcination procedure, it follows that the interline "area" is proportional to the flat electrolyte surface area A, i.e. the constant X equals X A, where X is another constant which does not depend on any macroscopic dimension. 

Carbon Monoxide The presence of CO in a H2-rich fuel has a significant effect on anode performance because CO affects Pt electrodes catalysts. The poisoning is reported to arise from the dual site replacement of one H2 molecule by two CO molecules on the R surface (40, 41). According to this model, the anodic oxidation current at a fixed overpotential, with (ico) and without (in2) CO present, is given as a function of CO coverage (0co) by Equation (5-11) ... 

As an application of Pt nanowires in heterogeneous catalysis, we performed preferential oxidation (PROX) of CO as a test reaction [32]. The PROX reaction is useful for PEM fuel cells for the selective removal of contaminating CO from hydrogen gas, because CO works as a strong catalyst poison for Pt electrode catalysts (Figure 15.24). H2 produced in steam-reforming and the water-gas shift reaction needs further to be purified in the PROX reaction to selectively oxidize a few% CO towards inert CO2 in a H 2-rich atmosphere, to reduce the CO content to <10ppm. Under the PROX conditions, the facile oxidation of H2 to H2O may also occur, thus the catalyst selectivity for CO oxidation over H2 oxidation is an... 

Figure1.4 Activation barrierforan electrochemical reaction. K is the decrease in activation energydue to the electrode catalyst and anFE is that due to the electrode potential E.

Some other uses of silver metal include its applications as electrodes, catalysts, mirrors, and dental amalgam. Silver is used as a catalyst in oxidation-reductions involving conversions of alcohol to aldehydes, ethylene to ethylene oxide, and ethylene glycol to glyoxal. 

Most electrochemical kinetic measurements have been made on polyciystals, i.e., norma] metals. However, metal surfaces, in reality, consist of many facets— patches—on each of which the crystals have what is called a specific orientation. In this section, some results of electrodic measurements are described in which the surfaces of the electrode catalysts are no longer the ill-defined mixture of many kinds of ciystal orientations found in polyciystals. The results described here will be those obtained on crystals prepared in such a way that one ciystal face only—having a specific, known orientation—is exposed to the solution. 

So, in describing factors in electrocatalysis that can be understood without resorting to quantal concepts, it can be said in summary that an electrode catalyst can be rationally chosen only if one knows what the rds is in the electrode reaction. Then... 

When ions migrate through a solid electrolyte, they diffuse from this onto the gas-exposed surface of the metal electrode. These ions form a double layer (and hence a potential difference) at the metal/gas interface. I Iowcver, this potential difference (which varies with the electrode potential) in turn changes the work function at the gas/metal interface. The ease of availability of electrons in the bonding of radicals adsorbed from the gas phase onto the electrode increases as the electronic work function of the solid decreases. The chemical reaction rate of the catalyzed reaction depends on the bonding strength of these radicals to the electrode catalyst, which involves electrons from the metal and is therefore dependent on the work function of the metal this itself is a function of the electrode potential. In this way, a dependence of the rate of the chemical reaction upon the potential of the working electrode can be rationalized. 

Fig. 8.1. Minuscule quantities of impurities in solution adsorbed at active sites on solid electrodes and reduced the activity of the electrode catalyst.

At 500°C the reaction rate over the platinum electrode-catalyst appeared to be independent of the e.m.f. of the cell. At 550°C two reaction rate branches were observed, depending on whether the catalyst had been pretreated in oxidising or reducing conditions (see Figure 9). The e.m.f. of the cell also exhibited two branches dependent upon pretreatment (see Figure 9), in a similar manner to other SEP work on oxide catalysts.86,87 It was suggested that the catalyst state (i.e., catalyst oxygen content or 5) was a function of the catalyst history. Different catalyst states corresponded with different catalyst reactivities and the e.m.f. of the cell reflected the catalyst state. 

Synthesis gas production. Alqahtany et al.92 have studied synthesis gas production from methane over an iron/iron oxide electrode-catalyst. Although the study was essentially devoted to fuel cell operation, for purposes of comparison some potentiometric work was performed at 950°C. It was found that under reaction conditions Fe, FeO or Fe304 could be the stable catalyst phase. Hysteresis in the rates of methane conversion were observed with much greater rates over a pre-reduced surface than over a pre-oxidised surface possibly due to the formation of an oxide. 

In this section the use of amperometric techniques for the in-situ study of catalysts using solid state electrochemical cells is discussed. This requires that the potential of the cell is disturbed from its equilibrium value and a current passed. However, there is evidence that for a number of solid electrolyte cell systems the change in electrode potential results in a change in the electrode-catalyst work function.5 This effect is known as the non-faradaic electrochemical modification of catalytic activity (NEMCA). In a similar way it appears that the electrode potential can be used as a monitor of the catalyst work function. Much of the work on the closed-circuit behaviour of solid electrolyte electrochemical cells has been concerned with modifying the behaviour of the catalyst (reference 5 is an excellent review of this area). However, it is not the intention of this review to cover catalyst modification, rather the intention is to address information derived from closed-circuit work relevant to an unmodified catalyst surface. 

It is unnecessary to use expensive materials such as Pt as electrode catalysts. 

Electrolyte Currant transport temperature, c Electrode catalyst Fuel Oxidant State of development... 

An ideal SC-SOFC has the same OCV and I-V output as a double-chamber cell, given a uniform oxygen partial pressure. A difference in catalytic properties of the electrodes must be sufficient to cause a substantial difference in oxygen partial pressure between the electrodes. For the ideal SC-SOFC, one electrode would be reversible toward oxygen adsorption and inert to fuel, while the other electrode would be reversible toward fuel adsorption and completely inert to oxygen [30], Advances in electrode catalyst materials are needed for SC-SOFC to have the same performance as conventional double-chamber SOFC with a significant reduction in complexity and cost. 

This cell works optimally at 80 °C using relatively inexpensive materials. When it is switched on in the cold, it produces about one-quarter of the power finally produced after it warms up. This is an advantage compared with other types of fuel cells operating at intermediate (200 °C) or high (650 to 1000 °C) temperatures, which need an auxiliary power source to start them and warm them up. The alkaline environment means that a wide range of electrode catalysts are available, while cells using acid solutions can only use noble metal electrode materials, which is a distinct economic disadvantage for terrestrial applications. 

The electrode-catalyst (Pt-Ir) is chosen so that i0 for the 02 reaction is particularly small and therefore oxygen evolution will be suppressed and most of the current will go to the oxidation of the organics. 

Under closed-circuit conditions, the electrochemical reactions involve a number of sequential steps, including adsorption/desorption, surface diffusion of reactants or products, and the charge transfer to or from the electrode. Charge transfer is restricted to a narrow (almost one-dimensional) three-phase boundary (tpb) among the gaseous reactants, the electrolyte, and the electrode-catalyst. 

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