Studys On Microwave Discharge Electrodeless Lamp System For Molecular Oxygen Activation Mechanism And Desulfurization Performance

The odorous sulfide gas has gained more attention,issued from manure composts,pulp and paper facilities,distilleries and wastewater treatment plants.The sulfide gas is recognized to pose environment and human health hazards owing to extremely low odor threshold and high toxicity.Photocatalysis has been widely applied for the purification of gaseous pollutants.However,TiO2 is rapidly deactivated and low quantum efficiency is low when the photocatalytic oxidation of sulfur compound begins under traditional UV lamp irradiation.Thus,the photocatalytic technology is greatly limited in the desulfurization process.An effective and long-term photocatalytic system for treatment of these odorous sulfides is highly desired.The reactive oxygen species(ROS)play significant roles in the photocatalytic degradation of organic pollutants.However,the generation of ROS in gaseous photocatalysis is rarely researched.In this work,we propose that optimizing the photocatalytic pathway by enhancing the generation of ROS in gas phase in order to achieve efficient and long-term photocatalytic desulfurization performance.The microwave discharge electrodeless lamp(MDEL)is used as experimental platform to explore the molecular oxygen activation mechanism based on the fact that microwave can not only generate ultraviolet light but also activate the catalyst.The characterization method of ROS during gaseous photooxidation process is developed,and the quantity and species of photocatalytic systems before and after introducing different catalysts are compared in detail.The photocatalytic degradation mechanisms of dimethyl sulfide(DMS)and intermediates dimethyl sulfide(DMDS)are investigated through analyzing the characteristics of photocatalytic system.The work provides basic data reference for the development of photocatalytic oxidation technology for desulfurization.We can obtain the following results from the paper.(1)The characterization method of ROS during gaseous photooxidation process was developed.ROS are determined by the method of diphenylcarbazide capture and oxidation-diphenylcarbazone extraction spectrometry.Four kinds of chemicals,N2,O2,H2O and DMS,are used as molecular probes to explore the generation process of ROS and their photo-oxidation performances.Results show that the types and quantities of the generated ROS depended on the atmosphere and humidity in the photolytic process.The positive effect of oxygen on the photodegradation and mineralization was attributed to the generation of ozone(O3),excited singlet oxygen atom[O(1D)]and excited singlet oxygen molecule(1O2).In contrary,high humidity poses an adverse effect on the luminous intensity,which results in the decreasing quantity of ROS and generation efficiency of OH.Thus,the highest mineralization rate is reached in dry air,and 1O2 and O(1D)are proved to play a major role in the degradation of DMS.(2)TiO2 is modified by HZSM-5 and TiO2 nanoparticles is highly dispersed on HZSM-5.TiO2 nanoparticles are mainly immobilized on the framework of HZSM-5 owing to the polymerization of acid sites.TiO2 particles are anatase-type with mean crystalline sizes of 19 nm.TiO2/HZSM-5 can present strong acidity and photocatalytic property.(3)A new photocatalytic system is constructed.TiO2/HZSM-5 is introduced into the MDEL photo-oxidation system,and the generation mechanism of ROS in the photocatalytic system was systematically studied.Compared with photo-oxidation system and P25,the concentrations of ROS production were obviously increased by introducing TiO2/HZSM-5 into MDEL photo-oxidation system.The main ROS is changed from superoxide radical(O2)to 1O2 and O(1D),and the proportion of 1O2 and O(1D)is increased sharply from 11.21%to 73.64%after TiO2 loading HZSM-5 in dry air.The amounts of ROS are increased with increasing humidity.·OH is major ROS in humid air.(4)The mechanism of adsorption and degradation of DMS by the new photocatalytic system is studied,and the deactivation of the catalyst is inhibited through optimizing the photocatalytic pathways during photocatalytic desulfurization process.The adsorption isotherms are changed from Langmuir to Freundlich isotherm models at different humidity after TiO2 loading HZSM-5.It suggests that the adsorption system is heterogeneous system in which the adsorbed DMS are of multilayer form after TiO2 loading HZSM-5.The adsorption capacity of TiO2/HZSM-5 is decreased with increasing humidity.Compared with traditional UV lamp,microwave has a remarkable effect on the desorption of the absorbed DMS molecules.TiO2(18%)/HZSM-5 exhibits a high and stable photocatalytic efficiency of 92.5%without catalyst deactivation for 600 min under MDEL irradiation.Moreover,humidity has a positive effect on the DMS removal efficiency for the photocatalytic system,and 100%of DMS removal efficiency is maintained for 600 min at 90%humidity.More ROS are produced in the MDEL and TiO2(18%)/HZSM-5 photocatalytic system.When DMS molecules are added into the system,microwave has a strong desorption effect on the absorbed substances.These DMS molecules are converted to methanesulfonic acid on the surface,and further mineralized into SO2 and SO3 by 1O2,O(1D)and OH.This process induced by microwave radiation can achieve efficient and long-term photocatalytic degradation of sulfide.(5)DMDS has been recognized to be a refractory intermediate during DMS degradation process,which exists in the gas and solid phase,greatly limiting the mineralization rate of DMS.The metals Ce/Co are chosen as synergistic components due to their storage and activation of oxygen.The ternary functional photocatalysts of Ce/Co combined TiO2 loading on HZSM-5 are constructed,which is expected to promote ROS production and improve the degradation of DMDS and mineralization of DMS.Results show that the surface areas and pore volumes of Ce-TiO2/HZSM-5 are higher than Co-TiO2/HZSM-5.Ce enters into TiO2 lattice while Co enters into the framework of HZSM-5.Thus,more oxygen vacancies are produced by Ce-TiO2/HZSM-5,resulting in higher concentration of ROS generation under MDEL irradiation.The DMS removal efficiency of 95%is maintained by 0.2%Ce-TiO2/HZSM-5 under MDEL irradiation,and no obvious deactivation is seen.The DMS removal efficiency is increasing with increasing O2 concentration and humidity.The main products were gaseous SO2 and SO3 when 0.2%Ce-TiO2/HZSM-5 and MDEL are together applied to degrade DMDS.The amounts of SO2 and SO3 of 0.2%Ce-TiO2/HZSM-5 is twice as high as TiO2/HZSM-5,enhancing the mineralization rate of DMDS.The intermediates DMDS is not observed in the solid and gas phase of 0.2%Ce-TiO2/HZSM-5 for DMS degradation under MDEL irradiation.The amount of sulfate on the catalyst surface is decreased by 23.18%and the content of main products of SO2 and SO3 is increased by 33.28%than TiO2/HZSM-5.The results are attributed to the fact that the Ce modification can further enhance the production of ROS and more DMS molecules are preferentially oxidized into SO2 and SO3.