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Influence of hydrogenperoxide and methanol on thephotocatalytic degradation of1,3‐dihydroxybenzeneA. Neren Ökte a d , Marianne Sowa Resat b e & Yuksel Inel ca Department of Chemistry , Bogazici University , Bebek/Istanbul, 80815, Turkeyb Department of Chemistry , Bogazici University , Bebek/Istanbul, 80815, Turkeyc Department of Chemistry , Bogazici University , Bebek/Istanbul, 80815, Turkey Phone: +90–212–263 1540/1610/1619Fax: +90–212–263 1540/1610/1619 E-mail:d Department of Physical and Theoretical Chemistry ,Universität Essen , FB 8, Essen, D‐45117, Deutschland E-mail:e Pasific Northwest National Lab , PO Box 999, MS K8–88,Richland, WA, 99352 E-mail:Published online: 19 Sep 2008.
To cite this article: A. Neren Ökte , Marianne Sowa Resat & Yuksel Inel (2001) Influence ofhydrogen peroxide and methanol on the photocatalytic degradation of 1,3‐dihydroxybenzene,Toxicological & Environmental Chemistry, 79:3-4, 171-178, DOI: 10.1080/02772240109358986
To link to this article: http://dx.doi.org/10.1080/02772240109358986
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Toxicological and Environmental Chemistry, Vol. 79, pp. 171-178 © 2001 OPA (Overseas Publishers Association) N.V.Reprints available directly from the publisher Published by license underPhotocopying permitted by license only the Gordon and Breach Science
Publishers imprint.Printed in Malaysia.
INFLUENCE OF HYDROGEN PEROXIDE ANDMETHANOL ON THE PHOTOCATALYTIC
DEGRADATION OF1,3-DIHYDROXYBENZENE
A. NEREN ÖKTE†, MARIANNE SOWA RESAT‡
and YUKSEL INEL*
Department of Chemistry, Bogazici University, Bebek/Istanbul, 80815 Turkey
(Received 6 March 2000; Revised 21 July 2000)
Effects of scavengers; hydrogen peroxide, H2O2 (an electron scavenger), and methanol, CH3OH(a hole scavenger) are examined for the photocatalytic degradation of 1,3-dihydroxybenzene(1,3-DHB), by following the carbon dioxide (CO2) formation in a gas recycling reactor. Addi-tion of H2O2 as an electron trap enhances the CO2 formation rate for 1,3-DHB, but there is alimit for the concentration of H2O2 above which hydroxyl radicals (*OH) are consumed in otherreactions rather than taking part in oxidative routes. CH3OH being an hole scavenger decreasesthe degradation rate by reacting with photogenerated holes as well as *OH radicals.
Keywords: Titanium dioxide; 1,3-dihyroxybenzene; hyrogen peroxide; methanol; kinetics
1. INTRODUCTION
When illuminated with light of energy higher than the band gap, electronsand holes are formed in a semiconductor and are capable of initiatingchemical reactions. Hoffman et al. (1995) proposed a general mechanism
*Corresponding author. Tel.: +90-212-263 1540/1610/1619. Fax: +90-212-287 2467.E-mail: [email protected].
†Present address: Department of Physical and Theoretical Chemistry, Universität Essen,FB 8, D-45117 Essen, Deutschland. E-mail: [email protected].
‡Present address: Pasific Northwest National Lab, PO Box 999, MS K8-88, Richland,WA 99352; E-mail: [email protected].
171
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172 A.N. ÖKTE et al.
for heterogeneous photocatalysis on TiO2. Briefly, oxidation of organicsover titanium dioxide in aerated aqueous solution can involve direct reac-tions with *OH radicals and hydroperoxyl radicals (HO2). The formationof *OH radicals can be achieved by two routes, (a) via reaction of the valenceband holes (h^b) with either adsorbed water (H2O(ads)) or with surfacehydroxyl groups (OH~), forming surface adsorbed #OH radicals (*OH(ads))
4-hi/->h+b + ecb (1)
h+ + H20(ads) -» #OH + H+ (2)
h+ + OH- -» 'OH(ads) (3)
or (b) via H2O2 from Superoxide ion (O2*). It is generally accepted thatsurface-adsorbed oxygen (O2(adS)), delays the electron-hole recombinationprocess by trapping the conduction band electron (ejb) as a Superoxide ion.
Q . _- > Q-» /^\
Like O2, H2O2 can scavenge electrons and form *OH radicals. Whatever theroutes of formation, these oxidizing species, and, in particular *OH radicals,are known to react rapidly with most organics as expressed by Pelizetti et al.(1993).
The purpose of the present study is to examine the effects of H2O2 as anelectron scavenger and CH3OH as a hole scavenger for the photocatalyticdegradation of a substituted phenol; 1,3-dihydroxybenzene (1,3-DHB).1,3-DHB behaves similarly to monophenols and undergoes all of the typicalreactions of phenol, which forms one class of organic contaminant.
2. EXPERIMENT
2.1. Reagents and Sample Preparation
Degussa P25 grade titanium dioxide was used as the photocatalyst. Theaverage particle size was 30 nm. The BET surface area was 50±15m2/g.X-ray diffraction data has confirmed that the TiO2 is predominantly in theanatase form.
1,3-DHB was from Fluka chemical and used without further purification.Reagent grade H2O2 (30%) and CH3OH (99.5%) were obtained from Merk.
Singly distilled water was deionized and used for the preparation of allsolution. Solutions of 1,3-DHB are prepared as 100 uM at the natural pH
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EFFECTS OF H2O2 AND CH3OH ON 1,3-DHB 173
(pH 5.4), a catalyst concentration of 1 g/L of TiO2, and a 144ml/min of flowrate of air were used for all experiments. Prior to the photodegradationexperiments, the suspension (containing 200 ml of the substrate and 1 g/Lof TiO2) was stirred for 30 minutes in the dark to achieve an adsorptionequilibrium for the substrate on the photocatalyst. Solutions were alwayskept in the dark in order to prevent any interference from ambient lightprior to irradiation.
2.2. The Gas Recycling Reactor
All photodegradation experiments were performed in a gas recycling reactoras shown by Inel and Ökte (1996). Briefly, the reactor consists of a 36.2 cmlong Pyrex tube with an inner diameter of 3.5 cm. To allow temperatureregulation via water circulation, there is a sealed inner glass tube in thegas recycling reactor with a thickness of 2 cm. The TiO2 suspension iscontained in the annulus formed between the two tubes. The gas above thesuspension is pumped using a Cole-Palmer peristaltic pump. A sintered glassdisk, placed at the bottom of the reactor, provides circulation of air andprevents settlement of the suspension. All connections were made withTygon tubing.
The reactor is located in an irradiation box (70 cm in length, 22 cm inwidth) containing six 20 W black light fluorescent lamps (General ElectricF 20 T 12/BLB) that provide light of wavelength 320-440 nm. The lampsare positioned to surround the reaction vessel from three sides. Lampscan be lit individually as well as in conjunction with each other. The frontside of the irradiation box was designed to operate as a door with thereactor attached. This maintains a uniform geometry throughout the experi-ments. A fan was placed at the top of the box and air was also continuallycirculated through the box in order to eliminate any heating effects of thelamps.
2.3. Analysis
The amount of CO2 produced during reactions was determined by aShimadzu (Gow Mac) gas Chromatograph equipped with a thermal con-ductivity detector and a Porapak N column. Helium was used as a carriergas at a flow rate of 60ml/min. Calibrations were carried out usingmeasured volumes of CO2 added to the gas phase loop under identicalconditions with the experiments. Sampling was made from the top of thereactor on which a mini inert valve is attached.
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174 A.N. ÖKTE et al.
3. RESULTS AND DISCUSSION
3.1. Effect of H2O2
Different concentrations of hydrogen peroxide were used to investigateits effect on the formation of CO2 for the photocatalytic degradation of1,3-DHB (C6H4(OH)2). Overall, addition of H2O2 is found to enhance thedegradation of 1,3-DHB and lead to faster rates of CO2 evolution. Thiseffect is enhanced as H2O2 concentration is increased but seems to reacha limiting value around 5xlO~3M with little significant change beingobserved upon further increase of H2O2 concentration (Figure 1).
The enhancement in the degradation rates observed upon H2O2 additioncan be explained as follows: When H2O2 is added to the system, 'OHradicals are formed by the reaction of H2O2 with electrons (equation (5)).These radicals are then able to react with adsorbed 1,3-DHB ([CeH4
(OH)2](ads)) (equation (6)) and produce CO2 as a final product of thedegradation process.
H2O2 + e~ -+ *OH + OH~ (5)
[C6H4(OH)2](ads) + 'OH -> CO2 + H2O (6)
If excess H2O2 is used, 'OH radicals may either combine together to ter-minate the free radicals (equation (7)) or react with H2O2 to produce HO2
radicals, which are much less reactive compared to *OH radicals (equation(8)). Thus, the amount of *OH radicals for the oxidation process decreases.This results in a decrease in the amount of CO2 evolved compared to thecase with low concentrations of H2O2.
•OH + 'OH -> H2O2 (7)
H2O2 + #OH — H2O + HO; (8)
When the experimental results are compared with photocatalytic mineral-ization of phenol in the presence of H2O2 (Akmehmet and Inel, 1993), itis observed that increasing the H2O2 concentration from 1 x 10~3M to1 x 10~2M decreases the formation rate of CO2 (Table I).
The kinetics of reactions in the presence of H2O2 are different from thosein a system without H2O2. Auguliaro et al. (1990), investigated phenoldegradation in the presence of H2O2 and O2. On the basis of their results,H2O2, O2 and phenol species adsorb onto the TiO2 surface according totheir chemical nature, to their relative concentrations and to the physico-chemical properties of the sites. They explained the destiny of photogener-
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EFFECTS OF H2O2 AND CH3OH ON 1,3-DHB 175
10 15 20
Time (min)
25 30
FIGURE 1 Effect of H2O2 concentration on the formation of CO2. Conditions: [TiO2] =lg/L, Flow Rate: 144ml/min, [1,3-DHB]O= 100|iM, pH = 5.4, 70= 10.8 x 10~6einstein/min,T= 298 K.
TABLE I Comparison between the rate of phenolrate of 1,3-DHB degradation in the presence of H2O
[H2O2] (M)
01 x 10"3
5xlO~3
1 x 10"2
Venoi (nM/min)*
5.0515.6721.1519.19
degradation and the2
fci,3-DHB (nM/min)
9.0616.5824.1422.92
•(Akmehmet and Inel, 1993).
ated pairs according to the energetic levels of the adsorbed species and onthe rates of the charge transfer processes. They hypothesised that the paral-lel reactions of O2 and H2O2 with photogenerated pairs show a second orderkinetics for phenol degradation.
For 1,3-DHB photodegradation the same parallel reactions are valid sincedegradation does not occur in the absence of O2 from the reacting mixture.When the effect of concentration is discussed, it is assumed that the O2
adsorption step does not play an important role in the degradation rate dueto the presence of sufficient O2. However, in the presence of H2O2, O2
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176 A.N. ÖKTE et al.
adsorbed on the TiO2 surface competes with H2O2 in the process of electrontrapping. Both processes, i.e., the reactions between H2O2 and electrons, andthe reactions between O2 and electrons, generate radical species able todetermine the oxidation of adsorbed 1,3-DHB. As a result, the formationof CO2 from 1,3-DHB degradation can occur through two parallel reactionpathways which is also assumed for phenol photodegradation by Auguliaroet al. (1990)
^ (kl/H2O2 + k2/o2)/i,3-DHB (9)(kl/H2
where kj is the rate constant of radical species produced by H2O2 photo-decomposition, k2 is the rate constant of radical species produced by O2
photoreduction,/H2o2 is the fractional coverage of H2O2 on TiO2,/o2 is thefractional coverage of O2 on TiO2, and/ji3.DHB is the fractional coverage of1,3-DHB on TiO2 (equation (9)). So, d[CO2]/d? becomes;
d[CO2] ^ k1KH2o2[H2O2] + k2Ko2[O2] K[C6H4(OH)2]at 1 + KO2[O2] + KH2o2[H2O2] 1 + K[QH4(OH)2]0 ^ >
where KH2O2 is the adsorption constant of H2O2 on TiO2, Ko2 is theadsorption constant of O2 on TiO2, and K is the adsorption constant of1,3-DHB on TiO2 (equation (10)).
The simultaneous presence of O2 and H2O2 improves the 1,3-DHBdegradation unless an excess concentration of H2O2 is used. Although bothO2 and H2O2 compete for adsorption onto the TiO2 surface and for thecharge transfer processes, they inhibit the electron-hole recombination reac-tion and they produce radical species essential for the degradation of 1,3-DHB. The two independent mechanisms for utilising O2 and H2O2 makesthe 1,3-DHB degradation kinetics second-order.
3.2. Effect of CH3OH
In photocatalytic reactions, the initiation of mineralization reactions startswith the attack of *OH radicals to the adsorbed molecules. In the presenceof CH3OH, *OH radicals are known to react with CH3OH through abstrac-tion of an hydrogen atom (equation (11)). Then, in the presence of oxygen,formaldehyde (HCHO) is formed as the stable product (equation (12)) (Sunand Bolton, 1996).
*OH + CH3OH -> *CH2OH + H2O (11)
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EFFECTS OF H2O2 AND CH3OH ON 1,3-DHB 177
•CH2OH + O2 -> HCHO + (12)
Therefore, the amount of 'OH radicals which are available to react withadsorbed molecules decreases.
By the introduction of CH3OH into the reacting solution of 1,3-DHB, weaim to follow the progress of the degradation process with a decreasednumber of hydroxyl radicals. In one hour irradiation time, it is observedthat rate of formation of CO2 in the presence of CH3OH is 2.86 timessmaller than that of CO2 in the absence of CH3OH (Figure 2). Therefore,the CH3OH decreases the yield of the mineralization reaction.
4. CONCLUSION
It is well known that electron or hole scavengers can affect the rate ofphotocatalytic degradation. Based on this, the rate of formation of CO2
for 1,3-DHB is investigated in the presence of CH3OH as a hole scavengerand in the presence of H2O2 as an electron scavenger. In the presence of
o*1
250
200
150
100
50
• without CH3OH
• with CH3OH
Time (min)
FIGURE 2 Effect of CH3OH addition on the formation of CO2. Conditions: [TiOJ = 1 g/L,FlowRate:144ml/min,[l,3-DHB]o= 100nM, pH=5.4, / 0 = 10.8 x 10"6einstein/min, r=298K.
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178 A.N. OKTE et al.
CH3OH, the formation of CO2 decreases since 'OH radicals are known toreact with CH3OH and thus the amount of *OH radicals which are ableto react with adsorbed 1,3-DHB molecules decreases. The enhancementin the rate of formation of CO2 upon addition of H2O2 is due to formationof *OH radicals between the reaction of H2O2 with photogeneratedelectrons. However, it is also possible that 'OH radicals can combinetogether or react with H2O2 in the presence of excess H2O2. Thus, there isa limit for the H2O2 concentration beyond which the amount of *OHradicals for the oxidation process decreases.
A ckno wledgements
This research was supported by the AVICENNE Research Programme ofthe European Community (Project No: 074).
References
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Auguliaro, V., Davi, E., Palmisano, L., Schiavello, M. and Sclafani, A., Influence of HydrogenPeroxide on the Kinetics of Phenol Photodegradation in Aqueous Titanium DioxideDispersion. Applied Catalysis, 65, 101-116 (1990).
Hoffman, M.R., Martin, S.T., Choi, W. and Bahnemann, D.W., Environmental Applicationsof Semiconductor Photocatalysis. Chem. Rev., 95, 69-96 (1995).
Inel, Y. and Ökte, A.N., Photocatalytic Degradation of Malonic Acid in Aqueous Suspensionsof Titanium Dioxide: An Initial Kinetic Investigation of CO2 Photogeneration. J. Photo-chem. and Photobiol. A: Chem, 96, 175-180 (1996).
Pelizetti, E., Minero, C., Hidaka, H. and Serpone, N., Photocatalytic Processes for SurfactantDegradation., In: D.F. Ollis and H. Al-Ekabi (Eds.), Photocatalytic Purification andTreatment of Water and Air, Amsterdam, The Netherlands, pp. 261-273 (1993).
Sun, L. and Bolton, J.R., Determination of the Quantum Yield for the Photochemical Genera-tion of Hydroxyl Radicals in TiO2 Suspensions. J. Phy. Chem., 100, 4127-4134 (1996).
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