Oxygen formation in the chlorate process and preparation and deactivation of ozone selective anodes

Abstract: This thesis presents experimental studies concerning two differ-ent electrolytic processes. One part deals with the electrochemicalformation of ozone and focuses on the preparation and deactiva-tion of a highly ozone-selective metal oxide anode (NATO - nickeland antimony doped tin oxide). The preparation of this anode bythermal decomposition of metal chloride salts was investigated anddifficulties and complications of common procedures were identi-fied. The same anodes were also studied regarding the deactivationof their ozone selective properties, identifying possible underlyingmechanisms for this as well as providing indications of the ozoneformation mechanism on the anode. When preparing these anodesby thermal decomposition, the volatility of the precursor salt usedfor the different components needs to be considered. For instanceextensive evaporation of the precursors of tin and antimony canlead to an unreliable preparation process resulting in the difficultyof controlling the properties of the prepared electrodes and a poorreproducibility of the process. The deactivation of the NATOelectrodes was investigated using the ozone current efficiency as amain indicator. The electrodes and the electrolyte were examinedusing electrochemical as well as physical techniques after differentperiods of galvanostatic polarization. The main mechanism behindthe deactivation was identified as the dissolution of antimony fromthe electrode surface. Also contributing, but not as detrimental,seems to be the dissolution of nickel. Both dopants, Ni and Sb, arepresent at the surface of the oxide anode and both seem equallyimportant for enabling the electrochemical ozone formation.The second part of this thesis concerns the decomposition ofhypochlorite, an important intermediate in the industrial chlorateprocess. A connection was found between the formation of chlorateand oxygen, both occurring according to 3rd order kinetics withregard to hypochlorite and both having their highest rates atpH 6 - 7. In the presence of chromium(VI) the hypochloritedecomposition can be modelled as the sum of two parallel reactions:one catalyzed by chromium(VI) and one uncatalyzed reaction.The byproduct oxygen seems to be formed only in the latter. Thusvaddition of chromium(VI) in the electrolyte increases both therate and the selectivity of chlorate formation. These findings areimportant as chromium(VI) needs to be removed from the processdue to its toxicity and in its absence the uncatalyzed decompositionpath would lead to an increase in oxygen formation, resultingin efficiency losses as well as potentially explosive gas mixtures.There is a need for a catalyst that can replace chromium(VI) inthis function.