Oxidation-catalyst electrode

FIGURE 9.4 Cross section of the laminated-type NOx sensor using a YSZ-based oxidation-catalyst electrode. (From Ono et al., 2004. Solid State Ionics. 175, 503-506, with permission.)... 

Figure 9.4 shows a cross-sectional view of a laminated-type sensor for atmospheric nitrogen oxides based on YSZ, acting as an oxidation-catalyst electrode (Ono et al., 2004). The cell was fabricated by firing YSZ sheet on which the oxidation-catalyst platinum anode and the platinum counterelectrode were screen-printed. The porosity of the electrode is adjusted by adding resin powder into each electrode paste. 

Ono, T., Hasei, M., Kunimoto, A., and Miura, N. 2004. Improvement of sensing performances of zirconia-based total NO sensor by attachment of oxidation-catalyst electrode. Solid State Ionics 175, 503-506. 

Ono, T. et al.. Sensing performances of mixed potential type NO sensor attached with oxidation-catalyst electrode. Electrochemistry 71 (2003) 405 07. 

Figure 4.22. Effect of the rate of O2 supply to the catalyst electrode on the increase in the rate of C2H4 oxidation on Pt deposited on YSZ.1,4 Dashed lines are constant faradaic efficiency, A, lines. Reprinted from ref. 4 with permission from Academic Press.

P. Tsiakaras, and C.G. Vayenas, Oxidative Coupling of CH4 on Ag catalyst-electrodes deposited on Zr02(8mol% Y203), /. Catal. 144, 333-347 (1993). 

P.D. Petrolekas, S. Brosda, and C.G. Vayenas, Electrochemical promotion ofPt catalyst-electrodes deposited on Na3Zr2Si2PO 2 during Ethylene Oxidation,/. Electrochem. Soc. 145(5), 1469-1477 (1998). 

Figure 5.2. NEMCA and its origin on Pt/YSZ catalyst electrodes. Transient effect of the application of a constant current (a, b) or constant potential UWR (c) on (a) the rate, r, of C2H4 oxidation on Pt/YSZ (also showing the corresponding UWR transient)3 (b) the 02 TPD spectrum on Pt/YSZ4,7 after current (1=15 pA) application for various times t. (c) the cyclic voltammogram of Pt/YSZ4,7 after holding the potential at UWR = 0.8 V for various times t.

The physical meaning of the parameter 2FNG/I is obvious It expresses the time required to form a monolayer of oxide ions on a surface with NG adsorption sites when the oxide ions are supplied at a rate I/2F. This proves that NEMCA is a surface phenomenon (not a bulk phenomenon and not a phenomenon at the tpb) taking place over the entire gas-exposed catalyst electrode surface. 

Real reasons due to (a) the occurance of very fast (and therefore in most cases diffusion controlled) catalytic reactions on the electrode surface, (b) Formation of non-conducting carbonaceous or oxidic layers on the catalyst electrode surface. 

There is a third real reason for deviations from Eq. (5.18) in the case that a non-conductive insulating product layer is built via a catalytic reaction on the catalyst electrode surface (e.g. an insulating carbonaceous or oxidic layer). This is manifest by the fact that C2H4 oxidation under fuel-rich conditions has been found to cause deviations from Eq. (5.18) while H2 oxidation does not. A non-conducting layer can store electric charge and thus the basic Eq. 5.29 (which is equivalent to Eq. (5.18)) breaks down. 

The variation in quasireference electrode in presence of reactive gas mixtures. This is due to its high catalytic activity for H2 oxidation. Nevertheless the agreement with Eq. (7.11) is noteworthy, as is also the fact that, due to the faster catalytic reaction of H2 on Pt than on Ag and thus due to the lower oxygen chemical potential on Pt than on Ag,35 the work function of the Pt catalyst electrode is lower than that of the Ag catalyst-electrode over the entire UWr range (Fig. 7.8b), although on bare surfaces O0 is much higher for Pt than for Ag (Fig. 7.8b). 

Figure 8.19. Scanning electron micrographs of the Pt-catalyst-electrode deposited on YSZ used for C2H6 oxidation.27 Reprinted with permission from Academic Press.

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... 

Consequently, the observed non-faradaic rate enhancement is due to the acceleration of the catalytic rate of H2 oxidation on the Pt catalyst-electrode. 

Figure 13.9 Reaction scheme for Ci molecule oxidation on a Pt/C catalyst electrode, including reversible diffusion from the bulk electrolyte into the catalyst layer, (reversible) adsorption/ desorption of the reactants/products, and the actual surface reactions. The different original reactants (educts) and products are circled. For removal/addition of H, we do not distinguish between species adsorbed on the Pt surface and species transferred directly to neighboring water molecule (H d, H ) therefore, no charges are included (H, e ). For a description of the individual reaction steps, see the text.

Figure 9. Effect of Ag catalyst-electrode surface area Q on the relative steady-state increase in the rates of epoxidation rt and deep oxidation r2 at constant imposed current i = 100 /jA, constant gas composition, 400°C, Po /Pet 7. Key O, r10/Ars and , ru/Ar%.

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. 

Metalorganic superconductors, 23 851 Metal oxide catalyst formaldehyde manufacture, 12 115-117 Metal oxide catalysts, 10 81 Metal oxide electrodes, silylating agents and, 22 700... 

Figure 19. Fourier transform of the Pt L3 EXAFS acquired during (A) the oxidation and (B) the reduction of a carbon supported Pt catalyst electrode as a function of time. Note that the Fourier transforms have not been phase corrected. The peak at 2.24 A corresponds to the first shell of Pt near neighbors at 2.76 A. The peak at 1.50 A is a combination of the side-lobe from the Pt shell and a shell of O near neighbors at 2.01 A. (Reproduced with permission from ref 43. Copyright 1995 ElsevierSequoia S.A., Lausanne.)...

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