Research On The Photocatalytic Oxidation Of Low-concentration Sulfur Dioxide

Sulfur dioxide?SO₂?,largely emitted from fossil fuel combustion in power plants and other industrial processes,is one of the common toxic air pollutants which can cause many serious environmental problems such as acid rain,photochemical smog,and formation of fine particulates.Exploring more efficient and economic techniques has become one of the most attractive approaches in gas purification for coal-fired industries.For low concentration SO₂,a commercial available photocatalyst P25-TiO₂ was used in this study to investigate its photocatalytic oxidation performance in the ultra-low emission control of SO₂.Firstly,TiO₂ catalyst that has been immobilized on glass beads was used in a fixed bed reactor under UV irradiation in order to study its performance in SO₂ removal.Experimental studies on the influences of different operation conditions on the catalytic efficiency were conducted.The possible reaction and deactivation mechanisms of the catalyst were also discussed.The results indicated that UV irradiation significantly affect the oxidation process.The increase of oxygen content and reaction temperature would benefit the photocatalytic oxidation of SO₂.The existence of NO in flue gas promoted the adsorption of SO₂ on the catalyst surface.On the contrast,SO₂ photocatalytic oxidation process was weakened due to the increasing of NO concentration.Water content had more complicate effect on the catalytic process.The activity of the catalyst was remarkably improved in the presence of water.However,as the accumulation of water on catalyst surface,the adverse effect became obvious which caused the efficiency in the later stage of the photocatalytic process to decrease with the increasing of water content.Quantitative analysis with IC showed that more than 97%of the SO₂ oxidation products were adsorbed on the catalyst surface,which caused the rapid deactivation of the catalyst.Secondly,a kinetic model considering both the reaction and deactivation of the catalytic process was established.The kinetic parameters including reaction rate constant,deactivation rate constant,and residual activity of the catalyst were obtained based on the model.Calculation results showed the variation of some important variables such as catalyst activity,and reaction rate against the catalyst bed position,as well as the reaction time,respectively,which can provide useful supplementary information for the experiments.According to the obtained kinetic parameters,the influences of oxygen,water,and temperature on the photo-reaction was discussed in depth.Results indicated that the water-related radicals were more reactive than the oxygen-related radicals on the irradiated TiO₂ surface.In addition,the presence of water significantly reduced the residual activity of the deactivated catalysts,while the influences of oxygen and temperature can be negligible.The apparent activation energy of the photocatalytic reaction was determined to be 29.0±1.2 kJ/mol at relatively low temperatures,which reveals that SO₂ can be oxidized by photocatalysis much easier when compared with other oxidation approaches.Thermodynamic analysis was also performed to study the nature of SO₂ adsorption on the catalyst surface.At last,aiming at the regeneration of deactivated catalyst,water washing,acid washing,alkali washing,and thermal treatment?calcinating?were studied to investigate the regeneration performances of the deactivated catalyst with different methods and regeneration times.XPS,BET,and UV-Vis DRS were used to analysis the regeneration mechanisms.It was found that Na₂CO₃ washing showed the best regeneration performance due to the interaction between the accumulated Na₂CO₃ and TiO₂ which significantly promoted the catalytic efficiency.The thermal regeneration could basically recover the activity of the deactivated catalyst without causing the spalling of TiO₂ from the substrate.The sulfate products of the photooxidation,which resulted in the deactivation of catalyst,would become beneficial to the photocatalytic reactions to some extent after the calcinating treatment.