Photocatalytic Degradation of Gaous Formaldehyde Using TiO2 Film
T E T S U R O N O G U C H I A N D
A K I R A F U J I S H I M A*
Department of Applied Chemistry,Faculty of Engineering, The University of Tokyo,7-3-1Hongo Bunkyo-ku,Tokyo,
113-8656,Japan
P H I L L I P S A W U N Y A M A
Kanagawa Academy of Science and Technology,1583Iiyama, Atsugi,Kanagawa243-0297,Japan
K A Z U H I T O H A S H I M O T O
Rearch Center for Advanced Science and Technology,
The University of Tokyo,4-6-1Komaba,Meguro-ku,Tokyo, 153-8904,Japan
The photocatalytic degradation of gaous formaldehyde s a major cau of sick building syndrome s was studied using a TiO2thin film.TiO2thin films have many unique photoinduced properties,for example,lf-cleaning,anti-fouling,and anti-bacterial functions.UV illumination of the TiO2thin film placed in a gaous formaldehyde/air environment resulted in the total mineralization of formaldehyde to CO2and H2O.We invoked a Langmuir-Hinshelwood kinetic model to analyze the dependence of reaction rates on the concentration of formaldehyde.In addition,the overall decomposition rate constant for formaldehyde was comparable to that of acetaldehyde(a standard test reactant)for initial concentrations of up
to1000ppmv.However,the apparent adsorption constant K app of formaldehyde onto TiO2was ca.2.5times larger than that of acetaldehyde.Thus in the low concentration regime,the reactivity of formaldehyde appeared to be greater than that of acetaldehyde.In like manner,a dark adsorption experiment also showed the high adsorption capacity of TiO2for formaldehyde.Therefore,we conclude that TiO2rves as both a good adsorbent and a photocatalyst for the elimination of gaous formaldehyde.
Introduction
七下古诗词Much work has been done in recent years on the photo-catalytic treatment of environmental pollutants found in the gas pha for possible application to decontamination, purification,and deodorization of the atmospheres(1-9). In addition,new TiO2-bad lf-cleaning materials such as TiO2-coated glass(10-13),tile,paper(14),etc.have been developed or are under development.One of the possible targets for the materials is the elimination of indoor air pollution,for example,the destruction of formaldehyde.This is becau the intensity of ultraviolet(UV)light in a typical indoor environment is considered to be adequate for the decomposition of very low concentrations of pollutants(15, 16).Formaldehyde is one of the major sources of indoor air pollution.Formaldehyde is an irritant that produces allergic symptoms at very low levels,and it is also a carcinogen.In recent years,formaldehyde emission from building renova-tions(new carpets,painting,etc.)is considered to be one of the major caus of the problem known as“sick building syndrome”.This condition is characterized by drowsiness, headaches,sore throats,and mental fatigue in many oc-cupants of modern office buildings,shopping centers, schools,and newly built hous(17).We report herein a study of the TiO2-mediated photodecomposition of form-aldehyde in the gas pha.The decomposition of acetal-dehyde photocatalyzed by TiO2was ud as a standard test reaction(13)for the evaluation of the efficiency of the TiO2 thin film photocatalyst.
Experimental Section
The TiO2thin film on soda lime glass was prepared from a STS-21sol(40wt%anata TiO2,pH8.5,20nm particle diameter,50m2g-1surface area)by a spin coating method (18).The sol was obtained from Ishihara Sangyo Co.After being spin coated,the film was calcined at450°C for1h. The thickness of the translucent anata TiO2film was ca.体恤的拼音
1.7µm.
All photodecomposition experiments were conducted at room temperature using a aled Pyrex glass photocatalytic reaction vesl(500cm3).The TiO2thin film(3×3cm2)was placed at the bottom of the reaction vesl,and an appropriate concentration of formaldehyde(prepared by heating para-formaldehyde)or acetaldehyde was injected into the vesl. Typical initial reactant concentrations were in the range of 30-2000ppmv(parts per million by volume).After adsorp-tion equilibrium was attained(approximately1h),UV light from a Hg-Xe lamp(Hayashi Tokei,Luminar Ace210)pasd through a365-nm band-pass filter was irradiated onto the TiO2film through a transparent silica plate window.The UV light that uniformly illuminated the whole area of the TiO2 film was transferred from the light source using a1-m fiber light pipe.The UV light intensity at the film surface was approximately1.0mW/cm2(measured by a UV intensity meter:TOPCON UVR-1).The photodecomposition pro-cess were followed by gas chromatography[Shimadzu GC-8A equipped 流行钢琴
with a2m Porapaq-Q column and a flame ionization detector(FID)].N2was ud as the carrier gas. Due to the lack of respon of the FID to formaldehyde and CO2,an additional methanizer(Shimadzu MTN-1loaded with reduced shimalite-Ni catalyst)that reduces formaldehyde to methanol(HCHO+H2f CH3OH)and CO2and/or CO to methane(CO2+4H2f CH4+2H2O)was attached to the GC system between the column and the detector.The experi-mental error in the determination of formaldehyde and CO2 was approximately5%,respectively.Residual byproducts that remained on the TiO2surface were extracted with a0.01 M NaOH aqueous solution and were measured using a liquid chromatograph(Toyosoda HPLC system with a UV-8010 optical detector and a Shodex Ionpak KC-811column). Results and Discussion
Typical results for the photodecomposition of formaldehyde (280ppmv)are shown in Figure1.The decomposition reaction was initiated by UV illumination of the TiO2film. As the initial concentration of formaldehyde decread,a parallel increa in the concentration of CO2was obrved. There were no traces of formaldehyde(in the photoreactor)
*Corresponding author telephone:+81338129276;fax:+813
38126227;e-mail:u-tokyo.ac.jp.
10.1021/es980299+CCC:$15.00©1998American Chemical Society VOL.32,NO.23,1998/ENVIRONMENTAL SCIENCE&TECHNOLOGY93831 Published on Web10/28/1998
after about80min reaction.Accordingly,the expected stoichiometric CO2concentration was generated.
Next,we investigated the influence of initial reactant concentration on the rate of decomposition of formaldehyde. Furthermore,we compared the rate of decomposition of formaldehyde with that of a standard test reactant,acetal-dehyde(13),using the same TiO2film sample.In Figure2 the dependence of initial decomposition rates on the initial gas concentrations for formaldehyde and acetaldehyde are prented.It is evident that formaldehyde decompod faster than acetaldehyde in the low concentration region.To analyze the differences in detail,we ud a Langmuir-Hinshelwood(L-H)kinetic model.The L-H model has been shown to provide a quantitative kinetics treatment of many solid-gas-pha reactions(19).The rate r of a unimolecular surface reaction will obey the following equation:
where k is the surface rate constant,K app is the apparent adsorption coefficient,and C eq is the reac
tant equilibrium concentration.By fitting plots of Figure2to the linear form of eq1as shown in the int in Figure2,we obtained k values of0.19and0.16µmol min-1and K app values of0.51and0.21µmol dm-3for formaldehyde and acetaldehyde,respectively. The calculated results emphasize the differences in K app values which are related to the adsorption strength of each compound onto TiO2.On the contrary,there are no remarkable differences in k values that directly correspond to the reactivity of each compound.The results suggest that the differences in decomposition rates in the low concentration region between formaldehyde and acetalde-hyde can be attributed to the differences in adsorption strength.That is,in the low reactant concentration region, the reaction kinetics is“mass-transfer limited”.Mass transfer is incread for higher K app values becau the driving force for mass transfer from the air to the solid surface is greater. As a result,formaldehyde decomposition in the low con-centration region proceeds faster than the decomposition of acetaldehyde due to its greater adsorption onto TiO2pro-moted by a higher adsorption strength(20).
Although CO2is the final oxidation product,intermediates and/or byproducts are however generated.Byproducts were extracted from the TiO2surface by aqueous NaOH solution, and the extracts were analyzed by HPLC.The product distribution in the photocatalytic reactions of formaldehyde and acetaldehyde at ca.75%reaction are shown in Table1. Formic acid in the ca of f
ormaldehyde and acetic acid, formic acid,etc.in the ca of acetaldehyde remained on the TiO2surface as partial oxidation byproducts.Nevertheless, with prolonged UV illumination,all of the intermediates and byproducts were oxidized to CO2.
The major oxidative and reductive process in the photodegradation of formaldehyde can be written as follows (1,21):
Thus,the complete oxidation of one formaldehyde molecule to CO2requires four holes stoichiometrically.Furthermore, we estimated the apparent quantum yield(φapp)[4mol of HCHO degraded]/[incident photons(365nm)])for the
FIGURE1.Plot of concentration versus irradiation time for the degradation of formaldehyde photocatalyzed by a TiO2film.Open circles(O)reprent the disappearance of formaldehyde whereas filled circles(b)show the evolution of CO2with reaction time, respectively.Reaction conditions were as follows:initial con-centration of formaldehyde,ca.280ppmv,incident light intensity, I)1.0mW cm-2;temperature,≈22°C;and relative humidity,40%. FIGURE2.Dependence of reaction rates on the initial reactant concentrations.Open circles(O)reprent acetaldehyde whereas the filled circles(b)reprent formaldehyde.The reaction rates were calculated within10min of reaction.The initi
al concentration of formaldehyde and acetaldehyde was varied from30to2000ppmv, respectively,under otherwi standard reaction ,I )1.0mW cm-2;temperature,≈22°C;and relative humidity,40%. The int shows the reciprocal plot of eq1.The following data were extracted from the least-squares fit of the curves:(a) formaldehyde,intercept)5.3µmol min-1,slope)10.3dm-3min-1, and r)0.986;(b)acetaldehyde,intercept)6.2µmol min-1,slope )28.9dm-3min-1,and r)0.992.The data were ud to estimate the surface rate constant k and the apparent adsorption coefficient K app for the respective compounds on TiO2(e text for details).TABLE1.Product Distribution at about75%Reaction during the Photodegradation of Formaldehyde and Acetaldehyde Mediated by a TiO2Film,Respectively a
product(%)
CO2HCOOH CH3COOH others HCHO8020
CH3CHO5020255
a Reaction conditions as outlined in Figure1.
r)
kK app C eq
1+kK app C eq
(1)
TiO298hνe-+h+(2) oxidation
HCHO+H2O+2h+f HCOOH+2H+(3) HCOOH+2h+f CO2+2H+(4) reduction
O2+4e-+4H+f2H2O(5)
38329ENVIRONMENTAL SCIENCE&TECHNOLOGY/VOL.32,NO.23,1998
性动decomposition of formaldehyde bad on the initial decrea of formaldehyde concentration shown in Figure1.φapp was ca.27%.
We also performed dark adsorption experiments to determine the adsorption capacity of TiO2for formaldehyde. Compared to a familiar adsorbent(activated carbon),TiO2 powder(Degussa P25)showed a high adsorption affinity for formaldehyde as shown in Table2.Moreover,in terms of unit surface area,P25even surpasd the adsorption capacity of activated carbon.
In summary,we have demonstrated that formaldehyde can be efficiently photooxidized to CO2on a TiO2thin film photocatalyst.In the concentration range we investigated, our findings suggest that the rate of photooxidation of formaldehyde is greater than that of acetaldehyde in the low concentration region.This phenomenon is attributed to the higher adsorption capacity of TiO2for formaldehyde.In-terestingly,TiO2also shows a greater adsorption capacity for formaldehyde as compared to a conventional adsorbent, activated carbon.
Acknowledgments
This work was supported by Grant-in-Aids for Priority Area Rearch on“Electrochemistry of Ordered Interfaces”from the Ministry of Education,Science,Sports and Culture,Japan.Literature Cited
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Received for review March25,1998.Revid manuscript received September9,1998.Accepted September24,1998. ES980299+
TABLE2.Adsorption Constants(K)and Maximum Adsorption
Quantities per Unit Surface Area(Q m)for the Dark Adsorption
of Formaldehyde a
全网营销方案P25activated carbon
K(µM-1)0.570.11
生肖的意思Q m(µmol m-2) 5.0 2.0
a Dark adsorption experiments were conducted at room temperature
using the same vesl ud in the photodegradation experiments.The
powder form of TiO2(Degussa P25,surface area:50m2/g)and activated
carbon(surface area:700m2/g)were ud as adsorbents.
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