The Studies On The Photocatalytic Oxidation Properties Of VOCs On Bi₂O₃ Nanoparticles

Today condensed state physicists,chemists and material scientists are Paying attention to semiconductor nanoparticles because of its especial optical property ,catalytic activity,mechanical property,magnetic property and so on. In recent years, application of the semiconductor nanoparticle to environmental cleanup has been one of the most active areas in heterogeneous photocatalysis. This is inspired by the potential application of semiconductor metal oxides and sulfides photocatalysts to the complite destruction of organic compounds in polluted air and wastewaters. We can choose excellent photocatalyst based on specific requirement.Usual photocatalyst,under TiO₂, ZnO, Fe₂O₃, SnO₂,SrTiO₃,Bi₂O₃,CdS,WO₃, ZnS,PbS, etc, are semiconductors, i.e. they have a moderate energy band-gap between their valence and conduction bands. Under illumination by photons of will enegy higher than band-gap energies, the valence band electrons can be excited to the conduction band, creating highly reactive electron-hole pairs. After migration to the solid surface, these may undergo electron-transfer processes with adsorbates of suitable redox potentials. At moderate conditions (room temperature, one atmosphere pressure and with molecular oxygen as the only oxidant), the above mentioned semiconductors have proved to be effective photocatalysts for the thermodynamically favored transformations of many organics to CO₂ and H₂O. TiO₂ is the most one to attract people's attention because if has many advantages such as nonpoisonous,high catalytic activity,high oxidation ability,good stability,low cost,and so on. However the band-gap of TiO₂ is wide(Ebg=3.2eV),and it only absorbs light wave of wave-length≤387nm.Thus,Studying and preparing new photocatalyst or improving photocatalytic efficiency is the important assignment. One of the most active fields of research in heterogeneous photocatalysis using semiconductor particles is the development of systems capable of using natural sunlight to degrade a large number of organic and inorganic contaminants. It has been reported that the band-gap energy of Bi2O3 semiconductor is 2.8ev and Bi2O3 semiconductor can absorb light longer wave-length.Anthony.H.has applied Bi2O3 semiconductor to photocatalytic degradate organic chlorine pollutant in water.CUI Yu-ming and WANG Jun-Zhen have tried to apply Bi2O3 semiconductor to water treatment.However, reports on studying gas-solid heterogeneous photocatalytic oxidation of air pollution on Bi2O3 semiconductor has never been found. In this paper, Bi2O3 nanoparticles were prepared by means of the ammonia precipitation,polyol mediated methods and microemulsion chemical method. The structure and properties of Bi2O3 nanoparticles were studied by means of many modern techniques such as TEM, XRD, BET, IR, XPS and UV-Vis spectrum. Bi2O3 particles prepared by means of the three methods all present quantum size. The crystallite dimension of Bi2O3 nanoparticles prepared by microemulsion chemical method is smallest than that of Bi2O3 nanoparticles prepared by other two methods. Results show that the order of surface areas of Bi2O3 is microemulsion chemical method> polyol mediated method>the ammonia precipitation method>comercial. Bi2O3 nanoparticles show quantum dimesion effect,which obviously influence the photocatalytic activity of nanoparticles. The XRD data of Bi2O3 nanoparticles has been compared with that of standard card.It is found that the structure of commercial Bi2O3 is mainly monoclinic crystal structure.However, the structure of Bi2O3 nanoparticles prepared by the about three methods we used mainly tetragonal crystal structure(Fig1.1). The difference of crystal form result in different electron combining energy of Bi2O3 nanoparticles.The valene band is same,but the conduction is different,which cause different the energy of band-gap.Defferent separating rate of electrons and holes,which result in defferent photocatalytic activity of Bi2O3 nanoparticles.With the same crystal form, the grain size of Bi2O3 particles is not different,the photocatalytic activity of Bi2O3 particles is different. Becausethe quantum dimesion effection and election combining energy effection of nanoparticles result in band-gap energy increasing.the band gap become wider and strong separating electrons from holes,which make the oxidation-reduction potential of electrons and holes stronger,thus the photocatalytic activity of Bi2O3 nanoparticles are improved. The results show that the higher thecalcining temperature,the higher the photocatalytic activity of Bi2O3 nanoparticles.Thus,the crystal form of Bi2O3 nanoparticles act on the photocatalytic activity of Bi2O3 nanoparticles in evidence. With the live way changing, people spend more time indoor whether living or working.However the decorating material of modern building contain a great deal of chemical, containing VOCs.The toxicity,stimulatory function,carcinogenic action and special smell of VOCs are able to cause uncomfortable reaction of humanbody.Especially,aromatic compound have high toxicity and can be absorbed by body ,which can damage liver,hemopoietic organ and nervous system,at last,cause aplastic anemia or leucocythemia.Animal experiment show that benzene has embryonic toxicity.Thus,air pollution in rome has been one of the most active area in heterogeneous photocatalysis.The gas-solid heterogeneous photocatalytic oxidation can take place under usual 2θ/(°) Fig1.1 XRD patterns of Bi2O3 photocatalysts prepared at different method and calcined at 750℃Bi2O3: a.commercial; b.the ammonia precipitation method; c.the microemulsion chemical method; d.the polyol mediated method.atmospheric pressure and room temperature,this reaction neednot stirring, at the same time,photocatalysis of gas-solid heterogeneous photocatalytic reaction is easely recoveried and have less effect on environment.In this paper, three systems, C6H6-O2-Bi2O3, C7H8-O2-Bi2O3 and C8H10-O2-Bi2O3 were studied, respectively. The reactant, intermediates and products were analyzed quantitatively and qualitatively by means of GC and GC-MS techniques. The reaction mechanism, reaction kinetics and the deactivation and regeneration of the Bi2O3catalyst were investigated.Facts show:the photocatalytic activity of Bi2O3 0n triphenyl-contaminants is highter then that of TiO2,at the same time,Bi2O3 particles is easelier than that of TiO2 particles. For C6H6-O2-Bi2O3 system, the oxidation of benzene did not proceed in the absence of either Bi2O3 or light irradiation. However, when the gas mixture containing benzene and oxygen enter the photoreactor with Bi2O3 as the catalyst, the photocatalytic oxidation of benzene occurred.. Benzene can be completely oxidized with almost stoichiometric CO2 and H2O as the final products. The intermediates detected were quinine,phenol,polycyclic aromatic compound, 1,2-butadiene, 13-butadiene and1-allylene.The reaction rate can be expressed by the Lang muir -Hinshelwood rate equation: Where r is the reaction rate (μM/min), C is the concentration of reactant (μM), k is the rate constant (μM/min), K is the adsorption constant (μM-1), I is the light intensity (mW/cm2), αis a constant, and subscripts B, W and O2 represent Benzene, water and oxygen, respectively. From the rate equation described above, we can see that αOOOOBBwwBBrk0 (1 KKCCKC)(1 KKCC)I2222=+++the reaction rate of benzene increased as the initial concentration of benzene and oxygen, and the light intensity increased, but was inhibited by the increased moisture content. The photocatalytic activity of Bi2O3 can be sustained definitely, which can be attributed to the products in the system.Products may occupy the active centers of photocatalys surface to reduce photocatalyst activity. For C7H8-O2-Bi2O3 system, toluene can be completely oxidized with almost stoichiometric CO2 and H2O as the final products. The intermediates detected were benzene, oxybenzene, phenylcarbinol, phenylaldehyde,phenylformic acid and xenene compound.The reaction rate can also be expressed by the Langmuir -Hinshelwood rate equation: Where r is the reaction rate (μM/min), C is the concentration of reactant (μM), k is the rate constant (μM/min), K is the adsorption constant (μM-1), I is the light intensity (mW/cm2), αis a constant, and subscripts T, W and O2 represent Toluene, water and oxygen, respectively. From the rate equation described above, we can see that the reaction rate of toluene increased as the initial concentration of toluene and oxygen, and the light intensity increased, but was inhibited by the increased moisture content. The photocatalytic activity of Bi2O3 can be sustained definitely, which can be attributed to products in the system.Products may occupy the active centers of photocatalytic surface to reduce photocatalyst activity. Thus we can calcine photocatalyst to recover it's photocatalytic activity. For C8H10-O2-Bi2O3 system, xylene can be completely oxidized with αOOOOTTwwTTrk0 (1 KKCCKC)(1 KKCC)I2222=+++αOOOOXXwwXXrk0 (1 KKCCKC)(1 KKCC)I2222=+++almost stoichiometric CO2 and H2O as the final products. The reaction rate can be expressed by the Lang muir -Hinshelwood rate equation: Where r is the reaction rate (μM/min), C is the concentration of reactant (μM), k is the rate constant (μM/min), K is the adsorption constant (μM-1), I is the light intensity (mW/cm2), αis a constant, and subscripts X, W and O2 represent Xylene, water and oxygen, respectively. From the rate equation described above, we can see that the reaction rate of xylene increased as the initial concentration of xylene,oxygen, and the light intensity increased, but was inhibited by the increased moisture content. The photocatalytic activity of Bi2O3 can be sustained indefinitely, which can attributed to the complete oxidation of xylene in the system.After photocatalytic oxidation reaction finished,the surface of photocatalysts maintain clean. Photocatalyst is key part of photocatalytic reaction.The activity of photocatalyst is a decisive factor which decide on photocatalytic reaction can be industrialized.Peoper have been striving to improving photocatalytic activity and extending excitation wavelength.In summary,extending excitation wave length of photocatalyst and reducing composite probability of election-hole pair are important methods.Peoper have studied composite semiconductor for long time.Research show that composite semiconductor exist different energy level which can seperate changes and improve photocatalytic activity.In this paper,we have prepared Ti2O3/Bi2O3 and Ni2O3/Bi2O3 nanoparticles, and have studied the structure and properties of Ti2O3/Bi2O3 and Ni2O3/Bi2O3 nanoparticles by means of many modern techniques such as TG-DTA, XRD, XPS and UV-Vis spectrum. The photocatalytic activity of composite semiconductor nanoparticles is...