Electrochemical Synthesis of Ammonium Persulfate (NH₄)₂S₂O₈ Using Oxygen-Depolarized Porous Silver Cathodes Produced by Powder Metallurgy Methods

The stage-by-stage mechanism of the reduction process proceeding in an acidic environment in the cathodic depolarization of ammonium sulfate (NH₄)₂SO₄ (including the primary process such as synthesis of hydrogen peroxide H₂O₂ on a porous silver cathode by equation $2{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{S}\mathrm{O}}_4^{2-}=={\mathrm{S}}_2{\mathrm{O}}_8^{2-}+{\mathrm{O}}_2\uparrow +{\mathrm{H}}_2\mathrm{O}$ and the main final process such as $2{\mathrm{NH}}_4^{2+}+{\mathrm{S}}_2{\mathrm{O}}_8^{2-}=\left({\mathrm{NH}}_4\right){}_2{\mathrm{S}}_2{\mathrm{O}}_8\Big)$ along with the anodic reaction, $4{\mathrm{H}\mathrm{O}}_2^{-}=3{O}_2\uparrow +2{\mathrm{H}}_2\mathrm{O}+4\mathrm{e}$, on a platinum plate has been established. The silver cathodes were produced by powder metallurgy methods (compaction of silver nanopowder with particles 10–30 nm in diameter at 44.13 MPa followed by sintering of the samples in a purified hydrogen atmosphere at 450°C). The silver nanopowder was obtained as individual acicular nanocrystals that branched out and were held in pairs on the Pt plate in electrolysis in a 2% AgNO₃ solution with addition of 1% nitric acid. The nanostate of the silver powder was verified with the BET method (Brunauer, Emmett, Teller) by low-temperature nitrogen adsorption. The ammonium persulfate synthesis kinetics and mechanism indicate that the replacement of commercial lead cathodes with oxygen-polarized silver cathodes leads to a 17.5% decrease in the electricity consumed. The polarization properties of the cellular silver cathode remain unchanged for 100 h of (NH₄)₂SO₄ electrolysis.