An investigation of the ozone activity and selectivity of Ni/Sb-SnO b2 s anodes in aqueous acid electrolyte

Abstract

Ozone is finding increasingly wider application across a range of industries, from semiconductor manufacture to water treatment. In principle, electrochemical ozone generation is capable of producing very high concentrations of ozone both in the gas phase and directly into solution, in contrast to Cold Corona Discharge, the present, most generally applied ozone technology.

The aim of the work described in this thesis was to develop highly active and selective anodes for the generation of ozone based on Ni and Sb-doped SnO: Ni/Sb-SnO. 22

Each step of the synthesis of Ni/Sb-SnO-coated Ti mesh anodes was investigated in 2 detail and the electrodes so produced characterized by SEM and EDX, and their activity and selectivity determined using UV-Vis spectroscopy.

Anodes with ozone current efficiencies of up to 50% in aqueous acidic electrolyte were developed, and efficiencies of ca. 30-40% were calculated routinely. However, such impressive efficiencies were derived only when operating the electrochemical cells in single pass mode. When the ozonated electrolyte was injected into the inlet of the electrochemical cell in order to generate gas phase ozone, efficiencies <10% were determined. Such an effect of ozone in solution inhibiting the ozone generating reaction, has not been determined with other electrocatalysts, and suggests that the mechanism of ozone evolution at Ni/Sb-SnO anodes is novel. This inhibiting effect of ozone was 2 investigated in detail and it was concluded that dissolved ozone was displacing a key intermediate or intermediates.

Anode durability is a key issue, particularly when using aqueous acid. It was found that some anodes showed high stability, whilst others activated very quickly. When deactivation occurred was found to be due to physical loss of catalyst and/or etching. A key strategy with respect to the former challenge was to coat the Ti mesh with an Electro Deposited Inter Layer (EDIL), to protect the Ti from oxidation as this would lead to theformation of TiO and spalling of the catalyst layer. The methodology employed was 2 implemented initially by collaborators in Hong Kong University and based on precedent literature. In essence, etched Ti mesh was held at cathodic potentials in ethanolic solutions of SbCl and SnCl; this was reported to produce a protective EDIL containing 34 Sn and Sb. However, it was shown that, no Sn deposited, under the conditions employed. Furthermore, there was no evidence that the EDIL was effective with respect to durability and, indeed, the electrodeposition step introduced significant variability into the catalyst coating process. The reason why some catalysts were durable and others very short lived remains unclear; however, it was postulated that the former involved Ni as NiOOH.

A strategy based on the addition of Au to prevent structural change of the SnO and/or 2 passivation of the SnO surface was investigated. Unfortunately, no beneficial effect (in 2 terms of durability, activity or ozone selectivity) was derived.

2 2 The scale up of the synthesis of the Ni/Sb-SnO anodes from 6.25 cm to 35.0 cm was 2 achieved successfully. Preliminary experiments with an industrial collaborator suggested the technology is transferable.

Preliminary experiments using a prototype water/air cell gave very promising current efficiencies (up to 22%) and showed a major design flaw in the prototype in terms of the compression of the anode/Nafion/cathode membrane electrode assembly.