Catalysts for decomposing ozone tail gas

Jounal a Enuironmental Sciennes Val. 15. No 6, pp. 779-782, 2003CN11267tArtide ID: 1001-0742( 2003)06-0779-04 CLC number: X13; X701 Document code: ACatalysts for decomposing ozone tail gasLIU Chang-an, SUN De-zhi, WANG Hui, LI WeiDepartment of Environnental Science and Engineering, Hartin Institute of Technology, Harbin 150001, China, E-mail: sundezhihit yuloronbstract: The preparation of immobilizing catalysts for decomposing ozone by using dipping method wasRD, XPS and tEM wused to characterize the catalysts. The three kinds of catalysts were selected preferentially, and their catalyes were investigated. Theresults showed that tbe catalyst with activated carbon dipping acetate( active components are Mn: Cu=3component propartion incatalys is I5%, calcination temperature is 200T )has the best eatalytie activity for oame decomposing. One gram of catalyst can decompose17.6 g ozone at initial omne concentration of 2.5g/m and the residence time in reactor of 0. I s. The experimental resull- also indicated thathumidity of reaction system had negatise effect on catalytic activityKeywords: ozone tail gas: catalylic decomposition: immabilizng-catalyst; MnO,; CadIntroductionIn recent years ozonation technology has been applied widely. However, the ozone used cannot beutilized completely. The ozone can injure human beings and plants when its concentration exceeded criticalstandard in the air. Presently the restriction of ozone tail gas was evoked extremely. So ozone existing inthe effluent must be removed before being discharged into the atmosphere. The available methods forremoving ozone include thermal destruction, adsorption, catalytic decomposition, chemical oxidation andincineration, There are advantages and disadvantages to each of these techniques(Alebic-Juretic,1992997: 2000: Ouedeni. 1996). The catalytic decomposition is considered to be the preferential methodNot only cun the catalerated, but also the operating is simple and there is no protential secondpollution problem. So thatatalysts have been studied and used. For the purpose of improvingatalytic activity, some noble metals are added, which results in high cost of catalysts, and the lifetime ofatalysts is limited. In addition, manufacturing prucedure of the catalysts is complicated( Christopher1997: Rip, 1982; Taty ana, 1999; Raakitskaya, 2001)The purpose of this paper is to prepare a kind of new catalyst that can decompose ozone effectively andto cut down the cost1 Materials and methods1. 1 Preparation of catalystsThe catalysts of decomposing ozone were prepared by dipping method. The y-AL, 0, and activatedcarbon (AC)which particle sizes are among 0.4-0.8 mm were chosen respectively as carriers of thecatalysts. The chemicals of Mn( NO, )2, Cu(NO, )2, Mn( Ac)2 and Cu( Ac)2 were determined as precursorcalcinated between 200 to 550C, so that the samples of the catalysts are obtained Cu. Then they wereof catalysts, and they were dipped into carriers by different proportion of Mn andThe catalysts are characterized by X-ray diffraction( XRD, D/max-B model, Cuk, as radiant source, a0. 1546 nm, 20=10-90, step length=0. 02, voltage 40 kV), x-photoelectric spectrum( XPSESCA750 model, Mgk, as radiant source, 8 kV and 30 mA)and scanning electron microscope(SEM)1. 2 Catalytic decomposing ozone tail gasThe reaction of decomposing ozone tail gas was performed in a fixed bed reactor of 5-mm in innerdiameter, which is made of quartz. One-gram catalyst was packed in the reactor. The residence time ofozone in the reactor was controlled definitely and residual ozone in effuent was absorbed by sodium sulfitesolution. The ozone concentration was determined bydemetry2 Results and discussion2. 1 Characterization of catalyst中国煤化工cipline Scientific Research Foundation of Harbin InstitutYHCNMHG780LIU Chang-an et alVol, 15CurOof active components in catalyst and calcinationwAtm., improving catalytic activity. Three kinds ofcatalysts that possess better catalytic functionwere delermined by orthogonal test methodhat was: A, ( activated carbon dipping acetateMn: Cu=4 1, active components proportion in~、wa200℃);A2(A2O, dipping acetate:MnCu =4:1, active com40000catalyst was 15 %, calcinations temperature350C):A, (Y-Al20, dipping nitrate: Mn:CuFig. I XRD spectra of catalysts=3: 2, active component proportion in catalystwas 15%, calcinations temperature was 550C). Fig. I and Fig. 2 show three kinds of catalysts spectraof XRD and XPS respectivelyIt can be seen by analyzing Fig. I that A, surface has the formation of Cu,O(20= 36.44 and42.28%)and Cu0(20=3246 and 38.78). CuO crystal was monoclinic structure, and crystal parameterwas:a=4.648, b=3.425 and e=5.129, Cu,0 crystal was cubic structure, and crystal parameter was a=4.2696.However, manganese oxides do not be found on A, surface. This was suggested that manganeseoxides were presented in the high dispersing form and its crystal grain was extraordinary fine, A, and A, inFig. I showed respectively thal surfaces of A, and A, had the formation of Mn0, (20=36.00 and 3954o)Likewise, copper oxides on their surface do not be foundAs can be seen by analyzing Fig. 2 that A, surface has formed Mn,O4, MnO2, Cu2O and Cuo, andA2 and A, surfaces existCu, 0 and Cu0. In addition, it can be seen by analyzing Fig. 2species and valence of manganese and copper oxides in catalyst systern did not changed before andcatalytic reaction. This indicated that manganese and copper oxides played catalytic role~^^-AA、~√Av∽,3∵A""八一书3b.2 ev/diva. the spectra of copper element; b. the spectra of manganese element: I. nonuse dipping catalyst ofdipping catalyst of yAL O中国煤化工CNMHGCatalysts for decomposing ozone tail ge78lThe surface morphological characteristics and void Tablel Surface void content of catalystsstruclure of the three kinds of catalysts are shown in SEM ofThe relevant results are listed Table 1. It can be seenfrom Table I that porosity of A, was the better than these of A, Void content,%38.and A,. So the A, had the stronger adsorptive abilityFig 3 Photograph of catalysts by SEM2.2 Study on catalytic activitylecting one-gram sample in above: Ihree kinds of catalysts respectively was packed in the reactorThe gas contains of 2.5 g/m' ozone went through the reactor in certain speed. The concentration of ozonered to determine removal efficiency of ozone. The experimental results are shown inFig. 4. In the same way, activated carbon and y-AL O, were packed respectively in the reactor as contrastIt can be seen from Fig. 4 that removalficiencies of ozone by using y-Al, O, reachedc only about 30% in reacting 0.036 s. Reducing-()AlzOsreacting time, removal efficiency of ozonedecreased obviously, and it can fall to zero whenreacting time was reduced below 0.024 s. The◆(5)A1eason may be only adsorption of ozone on y.A,O, surface. The experimental results showedthat removal efficiencies of ozone can respectivel8.04 0.026 0. 028 0.030 0.032 0.034 0.036 when reacting time were controlled in 0.036 sReaction time, minThe results indicated that manganese and copperoxides of dipping into y-AL,O, had the betterFig. 4 Comparison of catalysts activitycatalytic activity for decomposing ozoneActivated carbon can also decomposed: from fim. 4 that the senpova ficiency of activate d cartion d减ozone can reach about 97. 2% when reacting time was controlled in 0.036 s. However, activated carbon inpacked bed can be burned, explored and losses weight of 10% while an activated carbon decomposingozone period, because some instable compounds are produced and a lot of heat are released. In the samemanner,A, can completely catalyze to decompose ozone, when reacting time was controlled in 0.036 s,and removal efficiencies of ozone reach also more than 80% for reducing reacting time to 0.024 sMoreover, the original weight of A, was kept during catalytic decomposing ozone. The reason was thatmanganese and copper oxides of dipping on activated carbon on ozone had catalytic function, and activatedcarbon as carrier provides only abundant surfaces, which was favorable for catalytic reaction. So thatbuming, exploding and loss of weight about activated carbon can be avoided during catalytic reaction, Fig4 shows that removal efficiency of ozone could be increase by prolonging reacting time. So that it wasconsidered that three kind of catalysIs had potential availability in industry, particularly, A, was consideredto be the best ideal catalyst for decomposing ozone中国煤化工CNMHG782dvo.15100}·00240.02600280.03000320.0340.03Ozone decomposition amount,&Fig.5 The effeet of relative humidity on catalytic activityFig. 6 Relationship of removal efficiency to ozone2.3 Effect of humidity on catalyticThe effect of relative humidity onefficiencies of ozone was studied by using three kinds ofatalysts in the condition of relative humidity(RH)of 97% and 33% respectively. The experimentalresults are shown in Fig. 5It can be seen by using A2 and A, that removal efficiencies of ozone in relative humidity of 97 %wererespectively deceased 29 and 18.6% than these in relative humidity 33 % provided reacting time of0.036 s. The results indicated that relative humidity can influence obviously catalytic activity of A2 and A,which y-AL, 0, was used as carriers. The reason was explained that essential prerequisite of catalyticdecomposing ozone was that ozone molecule must be adsorbed on the active site of catalyst surace.Theactive components of A, and A,, which are manganese and copper oxides, can strongly adsorb watermolecule. Moreover, y-Al2O, as carriers had hydrophilic property, so that some water molecules on surfaceA2 and A, result in decrease of removal efficiencies of ozone. Correspondingly, removal efficiencies ofozone in relative humidity of 97% decreased only 5. 4% than that of relative humidity of 33 by using Arwhich activated carbon was used as carriers, when reacting time was 0.036 s. The reasons are aslows:(1)Activated carbon had weaker adsorption ability with water molecule than y-Al203.(2Surface of activated carbon had abundant pore, and the active components dispersed highly on the surfaceand pmduce abundant active site, which can catalyze to decompose ozone, although the A, can adsorbsome water moleculesA, and A, of one gram were respectively packed in the reactor. The ozone(2.5 g/m) went throughhe reactor in residence time of 0.1 s. The concentration of ozone in outlet was measured to determlifetime of the catalysts. The experimental results are shown in Fig. 6. When removal efficiency of ozonewas decreased to 80%, the A, and A, had performed 98 and 19.2 h, correspondingly, ozone had beerdecomposed 17.6 g and 3.5 g. In that case, the lifetime of A, was 5 times that of A,3 ConclusionsBased on above experimental results, following conclusions can be obtainedcomponent proportion in catalyst was 15 %, calcinations temperature was wa, Mn:Cu=1:I,activeThe a, catalyst by activated carbon dipping acetate( active components werecot reacted with ozone, so that buming, exploding and weight loss phe, andam i had higher catalyticactivity and using lifetime. One-gram A, can decompose ozone of 17.6avoided. The relative humidity in reacting system has negative effect on catalytic activityReferencesKlasine 1, 1992. Ozone desTruction[J]. Phy Chem,96:493-49rticles[ J]. Environ Monitor and Assessment, 44: 241-247Zhang W, Oyama ST, 1997, Decomposition of三=二=carbon-supported metal oxide catalysts[J]. Applied Catalysis Bnuron,14:117-129Ouedemi A, Iimvorapituk Q. Bes R et al., 1996. Oone decomposition on glass and silica[]. Ozone Sci and Engineenng, 18: 385-415Raakitskaya T L. 2001. 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