Electrode silver catalyst

Methanol oxidation on Ag polycrystalline films interfaced with YSZ at 500°C has been in investigated by Hong et al.52 The kinetic data in open and closed circuit conditions showed significant enhancement in the rate of C02 production under cathodic polarization of the silver catalyst-electrode. Similarly to CH3OH oxidation on Pt,50 the reaction exhibits electrophilic behavior for negative potentials. However, no enhancement of HCHO production rate was observed (Figure 8.48). The rate enhancement ratio of C02 production was up to 2.1, while the faradaic efficiencies for the reaction products defined from... 

In Figure 4 we have presented the experimental Tafel plots of air electrodes with catalysts from pure active carbon and from active carbon promoted with different amounts of silver. The obtained curves are straight lines with identical slopes. It must be underlined that the investigated electrodes possess identical gas layers and catalytic layers, which differ in the type of catalyst used only. Therefore, the differences in the observed Tafel plots can be attributed to differences in the activity of the catalysts used. The current density a at potential zero (versus Hg/HgO), obtained from the Tafel plots of the air electrodes is accepted as a measure of the activity of the air gas-diffusion electrodes the higher value of a corresponds to higher activity of the air electrode. 

Figure 5. Polarization curves of air electrodes with catalysts from active carbon and with active carbon promoted with 5% and 30% of silver.

In Figure 11, we have presented the AE - I curves of air electrodes with catalysts from active carbon and active carbon promoted with different amounts of silver (from Figure 5). It is seen that the transport hindrances in the electrodes with catalysts from pure active carbon and with active carbon promoted with 5% of silver are near to each other. The transport hindrances in the air electrodes with catalyst containing 30% of silver are much higher. That s why catalysts containing large amount of silver are suitable to be used in air electrodes operating at comparatively low current densities. 

When oxygen is pumped to the catalyst the activity of oxygen on the silver catalyst-electrode increases considerably because of the applied voltage. It thus becomes possible to at least partly oxidize the silver catalyst electrode. In a previous communication it has been shown that the phenomenon involves surface rather than bulk oxidation of the silver crystallites (17). The present results establish the direct dependence of the change in the rates of epoxidation and combustion Ari and Ar2 on the cell overvoltage (Equations 2,3, and 5) which is directly related to the surface oxygen activity. 

The relative increase Ar /r Q in the rates of epoxidation (i=l) and combustion (i=2) is proportional to A/S, where A is the electrolyte surface area and S is the surface area of the silver catalyst electrode. Thus with a reactor having a low value of S (reactive oxygen uptake Q =.4 10 7 mol O2) a threefold increase in ethylene oxide yield was observed with a corresponding 20% increase in selectivity. 

Another important point to guarantee the long-term stability of the electrode is the procedure used to manage shutdowns. The experience gained through the laboratory tests shows that during shutdown the cell must be maintained under polarisation conditions to avoid the probable dissolution of the silver catalyst and its re-deposition... 

E. Gulzow, N. Wagner, and M. Schulze [2003] Preparation of Gas Diffusion Electrodes with Silver Catalysts for Alkaline Fuel Cells, Fuel Cells—From Fundamentals to Systems 3, 67-72. 

To understand the mechanism of the degradation of the physical and electrochemical components, two different pore systems must be distinguished first, the pore system in the electrode formed by the interparticle space between the catalysts (that allows for the gas transport), and second, the pore system in the silver catalyst. In the first type of pore system, the hydrophobic character is dominant. 

The German company Siemens later modified these electrodes with skeleton metal catalysts. Small amounts of titanium were added to the anodic nickel catalysts, and nickel, bismuth, and titanium were added to the cathodic silver catalysts. Fuel cells with such electrodes and a matrix electrolyte operated at 95°C and a current density of 400 mA/cm had a working voltage of 0.8 to 0.9 V. 

Silver is a good catalyst for four-electron oxygen reduction in alkaline solutions. In 1962, Justi and Winsel developed an oxygen electrode consisting of skeleton (Raney) silver. Alkaline fuel cells with such electrodes were actually manufactured in subsequent years, but later, in view of the higher demands placed on space-flight fuel cells, the silver catalysts were replaced by platinum catalysts. 

The Siemens cell was similar except that the air electrode was fabricated with two layers a hydrophilic layer of porous nickel on the electrolyte side for oxygen evolution and a hydrophobic layer [carbon black bonded with Teflon (PTFE) and catalyzed with silver] on the air side for oxygen reduction. The dual porosity helped to shield the silver catalyst from oxidation. As many as 200 cycles were achieved. ... 

Porous anodes of nickel, stainless steel, stainless steel with iron-iron oxide mixtures, lithiated manganous oxide, and silverized catalysts (Rh, Co, Zn, ZnO, Mn02, C02O3, lithiated NiO) have been tested. The difference in the reactivity of these electrodes for the H2 oxidation is not large at temperatures between 500 °C and 700 °C. The presence of oxides improves the performance of the anode for hydrogen gas containing some carbon monoxide. Stainless steel, nickel, silver, lithiated nickel... 

Oxidation States. The common oxidation state of silver is +1, ie,, as found in AgCl, which is used with Mg in sea- or freshwater-activated batteries (qv) AgNO, the initial material for photographic materials, medical compounds, catalysts, etc and silver oxide, Ag20, an electrode in batteries (see Silver compounds). Few compounds are known. The aqua ion [Ag(H2 O), which has one unpaired electron, is obtained... 

Cuesta A, Lopez N, Gutierrez C. 2003. Electrolyte electroreflectance study of carbon monoxide adsorption on polycrystalline silver and gold electrodes. Electrochim Acta 48 2949-2956. Date M, Hamta M. 2001. Moisture effect on CO oxidation over Au/Ti02 catalyst. J Catal 201 221-224. 

Since the reaction between hydrogen and oxygen is very slow at room temperature, catalysts are incorporated in the carbon electrodes. At the anode, suitable catalysts are finely divided into platinum or palladium at the cathode, cobaltous oxide, or silver. The two halfreactions shown above yield the overall result as ... 

Various carbon-based catalysts were tested in the investigated air gas-diffusion electrodes pure active carbon [6], active carbon promoted with silver [7] or with both silver and nickel. Catalysts prepared by pyrolysis of active carbon impregnated with a solution of the compound Co-tetramethoxyphenylporphyrine (CoTMPP) are also studied [8],... 

From Figure 4 it is visible that the electrodes with silver in the catalyst are more active than that with pure active carbon catalyst. Moreover, the increase of the amount of promoting silver in the catalyst results, as expected, in a higher activity of the electrode. 

Active carbon promoted with small amount of silver is used as catalyst in the air electrodes of these cells. In Figure 15 we presented the discharge curve of the zinc-air cell ZV3000 at constant current 1 A. 

The exchange current density of Pt-metals is relatively small, but they have high stability. Very high cost does not permit to use them in the batteries of wide application. Noticeably higher activity, very good stability and lower costs are demonstrated by silver. The most inexpensive catalyst is activated carbon that has very high surface area. This type of catalyst is used in some batteries. Activity of carbon electrode can be improved by additive of oxide (e.g. Mn02) or pyropolymers. 

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