The rate of formation of hydrogen peroxide by cathodic reduction of oxygen in porous electrodes made of natural graphite, electrographite, carbon black and active carbon has been studied, as well as the catalytic effect of these materials on spontaneous decomposition of alkaline solutions of hydrogen peroxide. The measured data revealed that the greatest current yields of hydrogen peroxide were attained with electrodes made of low-surface kinds of carbon black which simultaneously had the lowest catalytic effect on the spontaneous decomposition of hydrogen peroxide.Cathodic reduction of oxygen can proceed, according to the character of electrode material and the reaction conditions, either in two consecutive two-electron steps which can be, e.g. for alkaline medium, expressed by equations 02 + H 2 0 + 2 e --;.. HO l + OH-; Ef98 = -0,076 V,HO l + H 2 0 + 2 e --;.. 3 OH -; Ef98 = + 0'878 V, (B) or as a direct four-electron reaction, described by the overall equation 02 + 2 H 2 0 + 4 e --;.. 40H-; Ef98 = + 0'401 V.
(C)Two-electron reduction of oxygen according to reaction (A) proceeding under the formation of hydrogen peroxide as a relatively stable intermediate, takes place e.g. on mercury and on some carbonaceous materials 1 , 2. Direct four-electron reduction according to overall Eq. (C) is known to proceed, at least in part, on electrodes made of noble metals (Pt, Au, Rh)3 -6. The primarily . formed hydrogen peroxide, or the perhydroxyl anion, however, is not a thermodynamically stable product and therefore it can spontaneously decompose to form again gaseous oxygen and water or hydroxyl ions according to the overall equation (again written for alkaline medium) 2 HOi' --;.. 20H-+ °2 ; Ll.Cf98 = -184'09 kJ . (D) This, from the point of view of hydrogen peroxide preparation, unwanted decomposition reaction is considerably catalyzed by various both homogeneous and heterogeneous catalysts, present in the reaction system.
The current yield of hydrogen peroxide during cathodic readuction of oxygen in porous electrodes from various carbonaceous materials is the lower the higher is the number of paramagnetic centers in these materials. A substantial decrease in the concentration of paramagnetic centers by heating to 900 °C in vacuum is only temporary, whereas by heating with alkali metal sulphides it is practically permanent.
The reduction of oxygen to hydrogen peroxide was studied in semihydrophobic trickle electrodes prepared from crushed graphite and teflonized carbon black. The potential of these electrodes was found markedly more positive and the peroxide current yields appreciably higher than for electrodes made of used crushed graphite. The reason for this is the higher surface area of the electrodes containing carbon black and occurrence of a three-phase interface in them. The relevant properties, i.e. electric resistance, electrolyte holdup, polarization curves for the oxygen reduction and pexide current yields, were also studied for trickle electrodes containing graphite felt. The electrochemical activity of electrodes made from untreated felt was found rather low but it increased several times when felt pretreated by oxidation was used.
The effects of the electrode material particle size, electrode thickness, diaphragm properties and gas and electrolyte flow rates on the pressure loss and liquid holdup in the electrode and on the current efficiency of peroxide formation by oxygen reduction were studied in a trickle electrode. The results are discussed using trickle-bed electrode calculations.
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