The thesis is devoted to the establishment of regularities of electrocatalytic processes in low-concentration NaCl solutions, and to the development on this basis of the effective electrocatalysts for the synthesis of sodium hypochlorite.
It is shown that the electrocatalytic activity of the electrodes in relation to anode processes in low-concentration chloride solutions is attributed to the strength of the bond of chemisorbed oxygen-containing particles of different nature with the anode surface. Participation of labile oxygen-containing particles in the oxidation of Cl– increases the rate of hypochlorite formation and leads to inhibition of undesirable reactions of oxygen evolution and formation of chlorates and chlorites.
Studies of the kinetic regularities of synthesis of sodium hypochlorite in low-concentration NaCl solutions allowed to establish the correlation between selectivity for the formation of NaClO and NaClO3, and the activity of the electrocatalyst to the reaction of oxygen evolution in balk solutions. The obtained correlations are for both oxide and metal electrocatalysts and indicate that the oxidation processes of Cl– and the formation of O2 go through the same stages and are related. The found correlations are volcano-curve, which indicates participation of oxygen-containing particles in the formation of HClO or ClO–, the presence of which on the surface causes the OER to flow in a certain region of potentials. In this case, CE (NaClO)> 90% and CE (NaClO3) <1% is realized on the anodes on which in 1.0 M HClO4 at 20 mA/cm2 for metal composite electrocatalysts it is 1.56-1.66 V, and for oxide electrocatalysts with a developed surface it is 1.49-1.58 V. Significant formation of chlorates, however, occurs only at the anodes where in 1.0 M HClO4 the OER potential is greater than 1.66 V for metal, or 1.58 V for oxide electrocatalysts.
In this work has been developed the method of obtaining the highly efficient metal anode for the synthesis of sodium hypochlorite in low-concentration NaCl solutions, which consists in electrochemical deposition of 1-2 mg/cm2 of platinum, 0.5-1.0 mg/cm2 of palladium and subsequent heat treatment at 500- 5300С. On these anodes during the electrolysis of 0.15 M NaCl, the current efficiency of NaClO is 94-96% with a current efficiency of NaClO3 less than 0.5%.
According to X-ray diffraction analysis, palladium in the heat-treated electrocatalytic coating based on metals Ti/Pd and Ti/Pt-Pd and oxide Ti/SnO2-Pd and Ti/SnO2-Pt-Pd is in the form of PdO oxide. X-ray photoelectron spectroscopy has shown that the PdO surface has a high affinity for hydroxylation - adsorption of H2O and ОН–. Most likely, this property of palladium compounds provides its high electrocatalytic activity in the reaction of formation of hypochlorite, where labile oxygen-containing particles participate. Electrocatalysts based on SnO2 modified simultaneously with Pd (5-15 at.%) and Pt (5-10 at.%) demonstrated maximum efficiency in the synthesis of sodium hypochlorite in low-concentration NaCl solutions when using oxide anodes.
It is shown that on the reduced surface of platinum and platinum-plated titanium the current efficiency of hypochlorite reaches 90%. Thus, platinum-plated titanium electrodes with a surface platinum content of 2 mg/cm2 can be used in non-diaphragm electrolyzers of flow and storage type for electrolysis of low-concentration NaCl solutions to obtain NaClO solutions not contaminated with unwanted impurities. The electrolysis must be carried out in the modes of periodic reverse current at current densities of 20-40 mA/cm2. In this case CE (NaClO) can be 1.5 times increased, and CE (NaClO3) is almost 5 times reduced. To increase the selectivity of NaClO synthesis it is advisable to use platinum-plated porous titanium heat-treated at 4000C as electrodes.
Key words: sodium hypochlorite, hypochloric acid, active chlorine, electrocatalysis, electrochemical kinetics, anodic processes, electrosynthesis.
