ETD Collection

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    Reduction of SO2 by CO over gold supported catalysts
    (2018) Ngwenya, Mphumzile Thelma
    Sulphur dioxide is a toxic air pollutant that affects the environment and human life. The catalytic reduction of SO2 to produce the less harmful elemental sulphur has been investigated since the 1940s. This offers a single step flue gas desulphurization process compared to using wet or dry flue gas desulphurization methods. In this research, the catalytic reduction of SO2 by CO was investigated over gold nanoparticles supported on an easily reducible amphoteric metal oxide (TiO2), an alkaline metal oxide (ZnO) and an acidic metal oxide (γ-Al2O3). In comparison, Au/TiO2 catalyst had the highest SO2 conversion of 86.4 % at 300 oC and a gas hourly space velocity of 3 600 mL.gcat -1.h-1 and 2 000 ppm of SO2, with Au/ZnO and Au/Al2O3 having very low average SO2 conversions of 1.8 and 1.4 %, respectively. The optimum reaction temperature was found to be 300 oC as irreversible deactivation of the catalyst by sintering of the Au nanoparticles occurs above 300 oC as observed from the high resolution transmission electron microscopy images of catalysts after the reaction. The products of the reaction, CO2 and sulphur, identified as crystalline sulphur in the form of S8, were observed in the reactor product stream, however, the reaction intermediate carbonyl sulphide was not observed. The optimum feed ratio, CO:SO2, for high SO2 conversions was the stoichiometric ratio of 2:1 over Au/TiO2. The flue gas concentrations of SO2 and CO of 420 ppm and 50 ppm Abstract M. T. Ngwenya University of the Witwatersrand Page iii were also used to investigate the activity of the catalysts. Very high SO2 conversions were observed at these low concentrations despite the limiting CO. The surface analysis of Au/TiO2 after the reaction showed that there was 1 % atomic concentration of S species which was later identified as SOx, where x is 2, 3 or 4; however, there was no accumulation on the catalyst observed after 144 hours. This observation confirmed that the redox reaction mechanism was probably followed with SO2 dissociating into S via adsorbed SOx on the catalysts surface and CO oxidation occurring on the Au-support interface with the excess O adatoms on the support surface. Pre-treatment of Au/TiO2 with 0.5 vol% SO2 enhanced the activity of the catalyst at reaction temperatures less than 150 oC, yet the untreated catalyst showed higher activities at 300 oC. The addition of O2, which is present in flue gas, to the reaction feed stream reduced the conversion of SO2 by 51.9 % as this offered a competing reaction, the direct oxidation of CO. When the O2 feed was removed, the conversion of SO2 increased showing that its effect was reversible.
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    An investigation into the spatio-temporal patterns of modelling SO2, NOx and surface O3 across the Highveld priority area, South Africa
    (2017) Roffe, Sarah Jane
    The Highveld is identified as an air pollution ‘hotspot’ area where pollutant concentrations are elevated due to the high density of industrial and non-industrial air pollution sources. To enhance air quality across the Highveld, it was declared a priority area to manage and monitor pollutants to reduce their negative impact on the environment and society. Hence, the aim of this study was to investigate ambient air pollution across the Highveld Priority Area (HPA), using ground-level SO2, NOx and surface O3 concentrations, meteorological parameters and Moderate resolution imaging spectroradiometer (MODIS) atmosphere products, for January to December 2011, to develop new modelling techniques to aid in the management of air pollution. Results show the annual mean trace gas concentrations of SO2, NOx and surface O3 were 12.14, 14.75 and 28.77 ppb, respectively. SO2 and NOx concentrations were highest during winter at an average of 17.56 and 20.96 ppb, where surface O3 concentrations were highest during spring at an average of 32.82 ppb. Diurnal patterns of SO2 and surface O3 were similar, where a midday peak occurred. NOx concentrations instead showed peaks during traffic hours. Ambient air temperature, solar radiation, relative humidity, wind speed and rainfall levels peaked during summer. Atmospheric pressure was relatively stable throughout the year. Winds typically ranged from N to E up to April and from S to NW from May. Very little variation in SO2 and NOx concentrations was explainable by meteorology, 4 to 29 % and 5 to 23 %, while the influence of meteorology on surface O3 concentrations was more significant, 23 to 53 %. Spatial multiple regression statistical models using a cross validation approach for model validation were made over a number of temporal scales. The model fitting and validation processes indicated that the models were not a good fit as only up to 69, 74 and 58 % of SO2, NOx and surface O3 concentrations with high root means square error (RMSE) values of up to 22.10, 15.56 and 18.59 ppb, respectively, could be explained by the models. This process revealed the potential to model pollutants across the HPA, and as a pilot study future work can be based on this study. It is clear that spatial modelling for pollution estimation and management is necessary as seen by the frequent exceedances of the national and international ambient air quality standards.