Surface peroxide formation

Two major pathways exist for this reaction, one bypassing hydrogen peroxide (first pathway) and the other involving intermediate peroxide formation via reaction (15.21) (second pathway). The peroxide formed is either electrochemically reduced to water via reaction (15.22) or decomposed catalytically on the electrode surface via reaction (15.23), in which case half of the oxygen consumed to form it reemerges [in both cases the overall reaction corresponds to Eq. (15.20)]. 

Hole trapping by electron donors bound to the surfaces of the semiconductor particles competes with the e - h+b recombination, allowing e b to react with molecular 02 via Eq. (10.23). Fig. 10.10 shows that the quantum yield, peroxide formation increases with increasing concentration of the electron donor. 

The same overall reaction can be obtained using a reaction mechanism with intermediate formation of Ba peroxide—cf., e.g., Lietti et al. (2001) and Olsson et al. (2001). However, it was shown that the formation of the surface peroxide is an endothermic process with respect to the formation of the N02-Ba0-N02 configuration. Based on that, it was suggested that the... 

In order to avoid chemical compounds at all, it is also possible to apply a high voltage to kill microbes on surfaces. It was found that a direct current kills E. coli cells, probably by heat or by hydrogen peroxide formation [84], Microbial cells can be effectively killed by using pulsed electric fields (PEF), probably by frequently disturbing the cell membrane potential [85], PEF that was found to lower microbial cell numbers in food and drinks was also shown to effectively kill E. coli and Listeria innocua cells attached to polystyrene beads [86], This demonstrates the potential of applying this purely physical method to surfaces as well. 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

Chromic acid-washed molybdenum-glass vessels and boric acid coated surfaces also appear to encourage peroxide formation [34—36], and the hydrogen peroxide so produced must be important in these oxidations since addition of H2 O2 increases the oxidation rate. 

D.R.S. Lean, W.J. Cooper, F.R. Pick (1994). Hydrogen peroxide formation and decay in lake waters. In G.R. Helz, R.G. Zepp, D.G. Crosby (Eds), Aquatic and Surface Photochemistry pp.201-214). CRC Press, Inc, Boca Raton. 

Silva C, Walhout PK, Yokoyama K, Barbara PF (1998) Phys Rev Letts 80 1086 Lean DRS, Cooper WJ, Pick FR (1994) Hydrogen peroxide formation and decay in lake waters. In Helz GR, Zepp RG, Crosby DG (eds) Aquatic and surface photochemistry. Lewis, Boca Raton, chap 16... 

Some variations of this scheme are known. For instance, access of air during the exposure process is possible to a certain extent to increase the yield of peroxide species. However, the risk is that the radiation may also induce the formation of ozone, which will then further oxidize the surface rather than leading to peroxide formation. In contrast, contact with air may be avoided completely after exposure, and... 

Hydrogen peroxide interacts with ferrous ion on MNPs surface with formation of hydroxyl-radicals by Fenton reaction ... 

In this mechanism. Reaction (7-XIII) is the chemical reaction discussed above. Reaction (7-XIV) is the ORR RDS on the Co-N/C catalyst surface. Reaction (7-XV) is peroxide formation. This H2O2 will go two ways (1) further 2-electron reduction to H2O through Reaction (7-XVI), or (2) chemical desorption through Reaction (7-XVII) to form free H2O2, and then entrance into the bulk solution, which can be detected by the ring electrode. The relative proportions of Reactions (7-XVI) and (7-XVII) determine the overall electron number and the amount of H2O2 produced. In Reaction (7-XVI), an x is used to express the proportion that is a 4-electron-transfer process, and in Reaction (7-XVII), (1 — x) to express the proportion of 2-electron process. It can be seen that when x=l, the mechanism will be a completely 4-electron-transfer pathway, and when x = 0, the mechanism will be a completely 2-electron pathway. If x is in between, the ORR will be a mixture of 2- and 4-electron-transfer pathways. 

In Chapter 6, the importance of RRDE fundamentals and practical usage in ORR study is emphasized in terms of both the electron transfer process on electrode surface, diffusion-convection kinetics near the electrode, and the ORR mechanism, particularly the detection of intermediate such as peroxide. One of most important parameters of RRDE, the collection efficiency, is deeply described including its concept, theoretical expression, as well as experiment calibration. Its usage in evaluating the ORR kinetic parameters, the apparent electron transfer, and percentage of peroxide formation is also presented. In addition, the measurement procedure including RRDE preparation, current—potential curve recording, and the data analysis are also discussed in this chapter. 

V vs. SHE in 0.1 mol dm H2SO4 at room temperature [74], The effect of the addition of zirconium was also investigated to enhance the ORR activity [76], The Ba-Nb-Zr-O-N/CB showed higher ORR activity with the ORR onset potential of ca. 0.93 V. The ORR proceeded primarily via a four-electron transfer reaction to water, and the maximum proportion of the hydrogen peroxide formation was less than 12 %. The incorporatiOTi of Ba and Nb into Zr" " matrix may have affected the surface structure and/or state of the catalyst, possibly causing the high ORR activity. 

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