The Degradation Of Gaseous VOCs On TiO₂-based Photocatalyst:the Degradation Pathways And Deactivation Mechanism

Volatile organic compounds(VOCs)do great harm to human body.A certain concentration of VOCs can damage human liver,kidney,brain and nervous system in a short time.Some VOCs such as benzene,toluene and formaldehyde have been listed as carcinogens.Additionally,some VOCs are the precursors of PM2.5 formation in oxidizing atmosphere.Therefore,how to achieve the degradation of low concentration VOCs in the atmospheric environment is of great significance to ensure environmental safety and suppress the frequent occurrence of haze.At present,there are many treatment technologies for solving air pollutants.Among these,photocatalytic technology has become one of the most promising technologies because of its mild reaction conditions,direct use of sunlight and deep mineralization of pollutants.Many kinds of photocatalytic materials are used in photocatalysis technology.Among them,Ti O2 has been widely considered as an ideal candidate due to its non-toxicity,excellent photochemical stability and high oxidation potential.Recently,a lot of researches have been done on the modification of Ti O2,but most of these works mainly focused on the photocatalytic degradation of aqueous pollutants including rhodamine B,methyl orange,phenol and NOx.Since the composition and emission of VOCs are complex and regional,the research on the application of Ti O2 towards VOCs degradation is rare.There are several main problems to be solved in the application of Ti O2 during VOCs removal:1)improving the utilization rate of light energy,inhibiting the recombination rate of electron-hole pairs and increasing the adsorption capacity.2)competitive adsorption and degradation under the coexistence of VOCs.3)service lifetime and deactivation-activation mechanism of Ti O2,and reaction paths of VOCs over catalyst.In view of the above problems existing in the degradation process of Ti O2 during VOCs elimination,the main works of this paper is as follows:(1)Investigation on the degradation features of gaseous acetaldehyde,o-xylene and their mixture over Tm³⁺-TiO₂ catalystHerein,Tm³⁺modified Ti O2 photocatalysts with high specific surface area and excellent performance were prepared by a simple sol-gel method.The specific surface area of the modified catalyst increases with increasing the amount of Tm³⁺,thereby promoting the adsorption capacity of VOCs.Additionally,the special 4f electronic structure of Tm³⁺improves the separation efficiency of photo-induced electron-hole pairs.These characteristics make the 0.5 mol%Tm-Ti O2 sample exhibit excellent photocatalytic performance towards single VOC and the mixed VOCs.Interestingly,the adsorption capacity of acetaldehyde under single conditions is greater than that of o-xylene,while the opposite phenomenon is observed under mixed system.The generation of reactive species and the difference in adsorption behavior leads to significant differences in degradation pathways and intermediate products under the two modes.Gas chromatography-mass spectrometry(GC-MS)analysis showed that o-xylene molecules were first oxidized to o-methylbenzaldehyde,then converted into butanol,and finally mineralized into carbon dioxide and water.However,for the mixed VOCs,new intermediates such as phthalein,o-methylacetophenone,naphthalene and5-methyl-4-octanone are detected on the surface of the catalyst.This work provides an effective strategy to modify Ti O2-based photocatalyst for the target VOCs degradation.It also has an important guiding significance for how to design a photocatalyst for the efficient degradation of mixed VOCs in the future.(2)Visible-light degradation of acetaldehyde and full light degradation of other VOCs on Er³⁺-TiO₂ photocatalystA series of Er-doped Ti O2 photocatalysts,using Er(NO3)3�6H2O precursor ranging from 0.5 to 2mol%,were synthesized via sol�gel method with TIP solution as reactants.The structure and properties of as-prepared samples were characterized by SEM,TEM,XRD,UV-vis,XPS,FT-IR,TG,PL,EIS and photocurrent.The as-synthesized Er-doped Ti O2 nanoparticles demonstrated improved photocatalytic activities for the photodegradation of selected volatile organic compounds(VOCs),including acetaldehyde,o-xylene,and ethylene.Additionally,a high photodegradation of acetaldehyde was recorded in the visible region(?>420 nm).XPS analysis revealed that a handful of Er in anatase Ti O2 was presented in the form of Er2O3,which benefited to increase the specific surface area ranging from 59.28 to 110.34 m²/g and consequently enhance the adsorption capacity for acetaldehyde,o-xylene and ethylene by 3.3,3.7,and 3.8 times,respectively,in contrast to the undoped Ti O2.ESR results showed that Er doping caused production of oxygen vacancies(Vo)and Ti³⁺,which promoted the photocatalytic activity of the catalyst.The Ti O2 samples containing0.5?1.5%Er content exhibited higher photoactivity in comparison with other samples.The highest removal efficiency of acetaldehyde and o-xylene within 100 min was 99.2%and 84.6%,respectively,and ethylene degradation efficiency reached 22.4%within 180min.Furthermore,the visible light elimination efficiency of acetaldehyde was 25.5%.Finally,the results suggested that Er doping predominantly inhibited the catalyst poisoning in the degradation of o-xylene.(3)A new strategy for resisting deactivation mechanism of rGO/Er³⁺-Ti O2 catalyst during the photodegradation of flowing o-xyleneThe poor efficiency and stability of photocatalysts are the fundamental issues to be addressed for photodegrading VOCs in practical applications.Herein,0.5wt%rGO/0.5mol%Er³⁺-Ti O2(0.5rGO/0.5ET)exhibited the excellent performance and a remarkable durability towards gaseous VOC(o-xylene),while pure Ti O2(PT)and 0.5mol%Er³⁺-Ti O2(0.5ET)showed poor catalytic activity and deactivation after 60min.The results suggested that intermediates accumulation was primarily responsible for the deactivation of PT and 0.5ET.In contrast,the activity of 0.5rGO/0.5ET was not decreased although more intermediates were generated compared with other samples.The experiments revealed that rGO provide more adsorption sites for the intermediates,which helped 0.5rGO/0.5ET resist the deactivation even after photodegradation of 10 h and 4 cycles.Therefore,rGO played a vital role in keeping the stable activity of the composites.This study is helpful to understand the deactivation/activation mechanism of photocatalysts and provided a promising strategy to make catalyst in activation for industrial application.(4)Role of reactive oxygen species(ROS)in VOCs degradationClarifying the specific role of reactive oxygen species(ROS)in the photocatalytic oxidation paths of VOCs is the extremely important research significance for guiding the design and synthesis of effective catalysts and the generation of useful intermediate products for industrial raw materials.Interestingly,the results of gas quenching and sacrificial agent trapping shows that�OH plays a major role in the degradation of o-xylene and styrene in our study.However,�O2^-is dominant species during the process of acetaldehyde degradation.This might be due to the different properties of aromatics and aldehydes.Additionally,we present a detailed role of holes,�OH and�O2^-during the flowing gas-phase o-xylene,styrene and acetaldehyde.Specifically,�O2^-dominate the formation of some intermediate products containing benzene ring and ketones in photocatalytic reaction routes during aromatics elimination.But VOC molecules tends to form carbon chain compounds under the action of�OH and holes.This is ascribed to the different oxidation abilities of ROS.For acetaldehyde removal,�O2^-facilitate generating acids while�OH favor forming ketones.This work is useful to understand the role of ROS played in gas-solid reactions in depth,which can be exploited to select the reasonable catalyst to degrade the different properties of VOCs and develop the designated photocatalysts for industrial application.It also provides a new idea of�turn waste into treasure�for VOCs removal.