The relationship between the composition and structure of Ni/Sb-SnO₂ and electrochemical ozone activity

Abstract

This thesis presents work seeking to elucidate the active site and the mechanism of ozone generation at nickel and antimony-doped tin oxide (NATO) electrodes. To this end, tin oxide (TO, SnO2), antimony-doped tin oxide (ATO, Sb-SnO2) and nickel-antimony doped tin oxide (NATO, Ni/Sb-SnO2) nanopowders were prepared via a hydrothermal (HT) method and either left uncalcined or calcined at 300, 400 and 700 oC. The nanopowders were characterised using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), measurement of surface area by the Brunauer Emmett Teller (BET) technique, thermogravimetric analysis (TGA), diffuse reflectance Fourier Transform Infra-Red spectroscopy (DRIFTS) and X-ray Photoelectron Spectroscopy (XPS). The electrochemical ozone activity and selectivity of the powders were also determined in 0.5M H2SO4 and compared to those of ceramic anodes prepared via conventional methods. All the nanopowders showed a single cassiterite phase with crystallite sizes that varied with composition and calcination temperature. The BET surface areas of the nanopowders decreased with increasing calcination temperature and also on doping with Sb and Ni. The BET surface areas in general were smaller than those calculated from XRD, suggesting the agglomeration of crystallites to form larger grains. Addition of Sb to undoped SnO2 resulted in a significant increase in the number of crystallites per grain. Co-doping with Ni initially caused a large reduction in the number of crystallite per grain, but not back to the undoped value, with additional Ni having little or no effect. The ozone activities and selectivity of the nanopowders were studied by UV-Vis spectroscopy in 0.5M H2SO4 by deposition onto Ti foil substrates and using a UV-Vis cuvette as the electrochemical cell. The data so obtained were compared to results using a ceramic Ni/Sb-SnO2 anode prepared via the conventional method. All the Ni/Sb-SnO2 nanopowders calcined at 400 oC were inactive with respect to ozone, whilst the Ni/Sb-SnO2 nanopowders calcined at 700 oC were all active, showing comparable current densities and ozone current efficiencies to those observed using the ceramic anodes. This was the first work to show ozone generated with high selectivity and activity at Ni/Sb-SnO2 nanopowders. Durability studies on a ceramic anode showed no change in ozone activity or selectivity over a 10 day period, supporting the results of earlier such studies in Newcastle and strongly suggesting that the Ni species responsible for ozone evolution at Ni/Sb-SnO2 is not located at the surface.A key aspect of the research programme was the study of undoped SnO2 calcined at 400 oC and 700 oC using BET, XRD, TGA-MS and in-situ variable temperature DRIFTS. BET showed the relatively high surface area and nanometer scale of the SnO2 particles, whilst XRD confirmed the nano dimension of the crystallites and showed only the cassiterite phase. TGA analysis indicated four temperature regions over which mass loss was observed. These and the in-situ DRIFTS studies revealed the existence of various forms of water associated with specific crystal facets of the SnO2, as well as existence of isolated O-H groups and adsorbed oxygen species. For the (100) facets, hydrogen bonding does not occur, and water absorption is less strong than for the (111) and (110) facets where hydrogen bonding does occur. On the (100) facets, the hydrogen atoms of the OH groups are located in cavities in the plane of the O atoms, and hence are unavailable for hydrogen bonding. In contrast, the H atoms on the (111) and (110) facets are available. The samples calcined at 700 oC showed significantly less adsorbed water than those calcined at 400 oC, and this could be attributed to lower coverage by OH on the former. The reversible uptake of oxygen was observed in the TGA studies, and this seeded the development of the final model. Electronic absorptions were also observed and the data rationalised in terms of the existence of both free electron absorptions, and absorptions from oxygen vacancy states. XPS of the Sb-containing nanopowders (i.e. Sb-SnO2 and Ni/Sb-SnO2) showed Sn in the +4 oxidation state, whilst Sb was present as both Sb(III) and Sb(V), and Ni as Ni(II) and Ni(III). Combining these studies with TGA-MS, it was shown that Sb(V) ions substitute for Sn(IV) in the lattice, with a preference for centrosymmetric coordination sites whilst the Sb(III) ions occur at the grain boundaries or surface. The Sb(V) ions confer electronic conductivity on the SnO2 whilst both Sb(III) and Ni are essential for O3 generation. The Ni occupies Sn(IV) sites in the subsurface region at concentrations below the detection limit of XPS. A model was postulated on the basis of the data, as well as a mechanism for ozone generation. The remediation of the Reactive Blue dye (RB50) in 0.5M H2SO4 was studies using both powder and ceramic anodes. Decolourization of RB50 solution was achieved within minutes of electrolysis, with COD and TOC removal of more than 80%. In addition to identifying a possible mechanism for ozone formation, the work reported in this thesis resulted in the production of active nanopowders which will allow the fabrication of high surface-area anodes with the potential to exceed the space-time yield of β-PbO2 anodes, permitting the application the Ni/Sb-SnO2 anodes in the treatment of real waters.A key aspect of the research programme was the study of undoped SnO2 calcined at 400 oC and 700 oC using BET, XRD, TGA-MS and in-situ variable temperature DRIFTS. BET showed the relatively high surface area and nanometer scale of the SnO2 particles, whilst XRD confirmed the nano dimension of the crystallites and showed only the cassiterite phase. TGA analysis indicated four temperature regions over which mass loss was observed. These and the in-situ DRIFTS studies revealed the existence of various forms of water associated with specific crystal facets of the SnO2, as well as existence of isolated O-H groups and adsorbed oxygen species. For the (100) facets, hydrogen bonding does not occur, and water absorption is less strong than for the (111) and (110) facets where hydrogen bonding does occur. On the (100) facets, the hydrogen atoms of the OH groups are located in cavities in the plane of the O atoms, and hence are unavailable for hydrogen bonding. In contrast, the H atoms on the (111) and (110) facets are available. The samples calcined at 700 oC showed significantly less adsorbed water than those calcined at 400 oC, and this could be attributed to lower coverage by OH on the former. The reversible uptake of oxygen was observed in the TGA studies, and this seeded the development of the final model. Electronic absorptions were also observed and the data rationalised in terms of the existence of both free electron absorptions, and absorptions from oxygen vacancy states. XPS of the Sb-containing nanopowders (i.e. Sb-SnO2 and Ni/Sb-SnO2) showed Sn in the +4 oxidation state, whilst Sb was present as both Sb(III) and Sb(V), and Ni as Ni(II) and Ni(III). Combining these studies with TGA-MS, it was shown that Sb(V) ions substitute for Sn(IV) in the lattice, with a preference for centrosymmetric coordination sites whilst the Sb(III) ions occur at the grain boundaries or surface. The Sb(V) ions confer electronic conductivity on the SnO2 whilst both Sb(III) and Ni are essential for O3 generation. The Ni occupies Sn(IV) sites in the subsurface region at concentrations below the detection limit of XPS. A model was postulated on the basis of the data, as well as a mechanism for ozone generation. The remediation of the Reactive Blue dye (RB50) in 0.5M H2SO4 was studies using both powder and ceramic anodes. Decolourization of RB50 solution was achieved within minutes of electrolysis, with COD and TOC removal of more than 80%. In addition to identifying a possible mechanism for ozone formation, the work reported in this thesis resulted in the production of active nanopowders which will allow the fabrication of high surface-area anodes with the potential to exceed the space-time yield of β-PbO2 anodes, permitting the application the Ni/Sb-SnO2 anodes in the treatment of real waters.