| Summary: | 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. === EM2018
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