Kinetics of the photocatalytic oxidation of N(III) and S(IV) on different semiconductor oxides

C~,mosp/~r¢, Vol. 38, No. 6, pp. 1265-1271, 1999 @ 1999 Elsevier Science Ltd. All fights reserved 0045-6535/991 $ - see front matter

Pergamon

PH: S004~S$35(98)00527-X KINETICS OF THE PHOTOCATALYTIC OXIDATION OF N(III) AND S(IV) ON DIFFERENT SEMICONDUCTOR OXIDES

Xavier Dom6nech,* Jos6 Peral

Departament de Qufmica, Universitat Autbnoma de Barcelona, 08193 Bellaterra, Spain

ABSTRACT The results of the nitrite and sulphite oxidation in aerated aqueous media in the presence of different semiconductor oxides acting as photocatalyst is investigated. Particular attention has been paid to the analysis of the kinetics of nitrite and sulphite oxidation over TiO2, ZnO and different Fe(ITI) oxides which are present in the natural environment. From the obtained experimental results it has been ascertained that nitrite photooxidation follows a Langmuir-Hinshelwood kinetics, the TiO2 with the higher anatase content being the most efficient photocatalyst. For sulphite photooxidation, the ferric oxides show a high photoactivity, along with an strong adsorption of reaction products. A kinetic law is derived taking into account the experimental results. INTRODUCTION The presence of solid particulate substances in the atmosphere is well known; these substance participate in different chemical heterogeneous processes [1]. Among the chemicals that constitute these solid materials, different metal oxides, particularly TiO2, ZnO and different ferric oxides can be found at relatively high concentrations [2]. These oxides behave as semieonducting materials, being able to perform photocatalytic processess [3,4]. In fact, upon absorption of light of energy greater than the semiconductor bandgap, electron transition from the valence band to the conduction band is produced, thus generating electron-hole pairs. Besides recombination, some charge-carriers migrate to the semiconductor surface where they remain trapped in the hydroxilated surface of the metal oxide [3]: h+ + >MeaOH---~>Men+lOH" e- + >MenOH--~ >Men4oH. In these surface sites, both electrons and holes, are able to react with suitable adsorbed redox species [3,4]. Particularly, TiOz, ZnO and ferric oxides can photocatalyze oxidation processes in aerated conditions in which adsorbed oxygen acts as electron scavenger producing 02"'[3]: 1265

1266 >Me"qOH + 02 ---)Me" OH + 02-'. This anion radical undergoes protonation, leading to the formation of hydrogen peroxide [5]: 02-' + H*--+ HOe 2 HOe -+ H20: + O> These photocatalytic processes can take place in the atmospheric environment. Some authors pointed out the important role of sunlight-absorbing semiconductor solids suspended in cloud water concerning the generation of acid rain [6,7]. In the present work, the photocatalytic oxidation of nitrite and sulphite to nitrate and sulphate, respectively, over different semiconductor oxides with known surface properties is investigated. The aim of this study is to obtain kinetic data for the understanding of the relevance of these semiconductor materials in environmental photochemical processes, particularly those that generate acid rain.

Experimental Different semiconductor oxides were used as photocatalyst. Table 1 shows relevant surface characteristics of the materials used. Different iron-doped samples (between 0.2 and 10 wt%) were prepared by incipient wetness impregnation of TiO2 Degussa P-25 with aqueous solutions of Fe(III) nitrate [8]. ~-Fe203 was synthesized by hydrolysis at 95°C of FeC13 solution in acid medium, ot-FeOOH, ~-FeOOH and "/-FeOOH ~ere prepared lk~llowing the procedures described by Ansari [9] and Domingo [10]. The photocatalytic experiments were performed in an open thermostated cylindrical Pyrex cell of 100 mP capacity. The reaction mixture inside the cell was maintained in suspension by means of magnetic stirring. As light source, a 250 W xenon lamp (Applied Photophysics) was used. The intensity of the incident light inside the cell, measured employing an uranyl actinometer, was 6x10 -6 Einsteins rain -t. The concentration of nitrite, nitrate and sulphate in solution were measured by ion chromatography (Metrohm 690) using a Hamilton anion column PRP-X100. The analysis of iron species in solution was performed using polarography (Metrohm 626) with background solutions of 0.1 tool dm -3 NaCI; Zn(II) and H202 were also determined polarographically using a NH3/NH4 + solution as background electrolyte. The concentration of sulphite was determined by visible spectroscopy using disulphide as colour former. RESULTS AND DISCUSSION

Nitrite photooxidation From the experiments carried out using different semiconducting oxides, it has been deduced that TiO2-P25 and ZnO are the most efficient. Thus, for 2x10 -4 tool dm <~ nitrite aqueous solutions in the presence of photocatalyst (4 g d m 3), 55 and 22% of nitrite is oxidized to nitrate after 15 rain of illumination for TiO2P25 (pH=5.5) and ZnO (pH=7.9), respectively. In fact, in both cases the percentage of nitrite conversion increases with time attaining a limiting value after 25 rain of irradiation; this limiting value is 60% for TiO2

1267 and 35% for ZnO. Also, in both cases the yield of the photooxidation increases with the mass of the semiconductor and with light intensity. For TiO2 suspensions, a linear dependence of nitrite oxidized on light intensity is observed at low intensities (up to 10% of full lamp intensity), while for ZnO suspensions, a linear behaviour up to 50% of full lamp intensity is observed. In these cases a ratio of 0.33 and 0.13 mols of nitrite reacted per incident photon for TiO2 and ZnO suspensions, respectively, is observed.

Table 1: Pro ~erties of the different semiconducting oxides tested. Metal oxide

mean particle size (~tm)

specific surface area (m2g"1)

point of zero charge

TiO2 P-25 ~

0.03

59.2

6

80% anatase

TiO2 AAb

0.25

10.1

7.6

92% anatase

TiO2 ARc

0.50

19.7

4.0

89% rutile

TiO2-Fed

50-180

29

6.5-7

mostly anatase

ZnOe

0.40

34.6

8.0

(~-Fe203

0.05

37

4.9

(x-FeOOH

0.10

83

6.4

I3-FeOOH

0.10

60

5.4

~,-FeOOH

0.05

127

3.9

crystallographic structure

a Degussa P-25

b Aldrich-Anatase c Aldrich-Rutile d Iron-doped TiO2. The mean particle size and the point of zero charge depends on the iron content. e Probus Formation of H202 was detected in ZnO suspensions under illumination, attaining a limiting concentration of 3x10 3 mol dm3 after 30 min irradiation of a 2.1×10 .4 tool dm -3 nitrite solution at pH=7.9. This value is about 30% greater than the value obtained in the absence of nitrite. The produced H202 can react with conduction band electrons, generating more OH radicals that favour the nitrite oxidation: H202 + e ~ OH' + OH" NO2- + 2OH" ~ NO3- + H20.

In the TiO2 system, H202 probably remains adsorbed onto the semiconductor surface [1 l]. The yield of nitrite oxidation is very dependent on pH for both TiO2 and ZnO suspensions. In fact, the percentage of nitrite oxidized decreases with increasing pH, which is mainly due to the progressive increment of the negative surface charge of the semiconductor particles at higher alkaline media, which inhibits the approach of nitrite anions to the reactive centers. The experimental data of nitrite photooxidation at different initial concentrations (between 0.001 and 0.0153 mol dm-3 at pH=5.5) using both TiO2 and ZnO suspensions shows that the process follows a Langmuir-Hinshelwoodkinetics:

1268

R-

kKC I+KC

where R is the initial rate constant, C the nitrite concentration and k and K are the reaction rate constant and the equilibrium adsorption constant, respectively. From the linear plots of I/R vs. l/C, the following values are deduced: k= 2.3×10-4mol dm 3 rain -t and K= 0.29×103 dm~mol -I for TiO2 suspensions and k=l.5×10 -~ tool dm ~ rain -j and K=0.17x103 dm -3 mol ~ for the ZnO system. On the other hand, the yield of nitrite oxidation has been studied using TiO2 with different crystallographic structures: TiO2-P25, TiO2AA and TiOzAR (see experimental section). From experimental data the following initial rates have been determined: 0.34, 0.12 and 0.067 ~mol min -1 for P-25, AA and AR photocatalysts, respectively (experimental conditions: 2. l× 10-4 tool dm -3 initial nitrite solutions with 4 g dm of semiconductor at pH=6.5). In Figure 1, the initial rates normalized to the corresponding specific areas are represented as a function of the TiO2 anatase content of the semiconductor. As can be observed, an increment of the rate of the process takes place with increasing the anatase content, being more significant for TiO2 with high anatase ratio in the crystallographic structure. Finally, different iron-doped TiO2 photocatalysts have been studied. In these cases, for all the samples tested, nitrite photooxidation follows a zero-order kinetics. The values of the respective rate constants are summarized in Table 2. As can be seen, the TiO2 sample doped with 0.5% of iron is the photocatalyst with higher activity. The increment of the efficiency of the process with increasing the iron content of the photocatalyst up to 0.5% can be explained considering that Fe 3+ acts as an electron trapping center, which increases the lifetime of the photogenerated electrons [12]. On the other hand, the decrease of the rate constant from 0.5% to higher iron contents can be attributed to the formation of iron spots that decrease the active sites needed for the reaction [12]. The observed kinetic law suggests that the availability of holes at the surface is the rate determining step. In fact, comparing with bare TiO2, a significant increment of electron-hole recombination in iron-doped TiO2, which reduces the life-time of the charge carriers, is observed [ 12].

Table 2: Rates of nitrite photooxidation over different iron-doped TiO2 samples. pH = 6.5, [TiO2] = 2 g 1-l, 25°C.

TiO2 (%Fe)

k (~M min a)

0.2

4.6

0.5

7.1

1.0

6.5

2.0

5.0

5.0

2.1

10

0.8

1269

0.0140

0.0120

~D

0.0100

tO

0.0080 0

.qo 0

0.

0.0060 /

0.0040

0.0020 0.00

Y

I

I

I

I

0.20

0.40

0.60

0.80

1.00

% anatase

Figure 1: Initial rates of nitrite photooxidation normalized to specific surface areas (pmol m 2 g-i min-t) as a function of TiO2 anatase content (2.1 x 10-4 tool dm "3 nitrite solutions containing 4 g dm 3 of semiconductor at pH=6.5. Temperature 25°C).

Table 3: Sulphate and Fe(II) in solution after illumination of 0.001 mol dm 3 sulphite solutions in presence of different iron oxides at initial pH of 4.5 Iron oxide SO47"×10"3 (tool dm'3~ Fe(II)xl0"3 (tool dm "s)

pHn~l

¢x-Fe203

0.77

0.035

3.3

c~-FeOOH

0.77

0.025

3.5

I~-FeOOH

0.78

0.01

2.9

~/-FeOOH

0.73

O.11

3.7

Sulphitephotooxidation The photocatalytic oxidation of sulphite over ~-Fe203, c~-FeOOH, I3-FeOOH and 7-FeOOH has also been investigated. The experiments were performed in 1.7 g d m 3 suspensions of the corresponding iron oxide in the presence of 0.001 tool dm "3 of sulphite at pH--4.5 and at 25°(]. In addition, the photoreduction of lattice

1270 Fe(IIl) to Fe(II) was taking place, since Fe(II) in solution has been detected after illumination. Table 3 summarizes some of the experimental results obtained. A kinetic analysis has been performed from the experimental results of the time-course of sulphite eliminated on different iron oxides. From this analysis, it is suggested that sulphate is strongly adsorbed onto the semiconductor surface. Considering the Langmuir isotherm, the fractional coverage of the surface by the reactant 0(HSO3-) can be expressed as, KCmo, O(HSO~) =

1+ KCmo + KC>~

where K and K' are the equilibrium adsorption constants of HSO3 and SO42, respectively. Assuming K'>>K tm fact, the formation constant of the Fe*+-SO42 complex in aqueous solution is far greater than the corresponding Fe~+-SO32 complex [13]l, then, K('mo, O(HS03) -

K" Cso~

Considering that HSO3- is oxidized following a first order kinetics then, dx dt

-k

l-a

where x is the fraction of sulphite oxidized at time t. The plots (l-x)-ln(l-x) vs. t, that corresponds to the integrated form of the last equation, gives straight lines for all the iron oxides tested. The following rate constants were obtained: 3.9 s l (correl.coeff. 0.995), 0.2t s -~ (correl.coeff. 0.956), 2.3 s 1 (correl.coeff. 0.977) and 0.13 s J (correl.coeff. 0.968), for ot-Fe20~, ot-FeOOH, y-FeOOH and [3-FeOOH, respectively. If these rate constants are divided by the corresponding specific surface area, the following trend in photocatalytic activity is deduced: ot-Fe203>>~-FeOOH = "{-FeOOH >> o~-FeOOH. It must be pointed out that a certain degree of sulphite oxidation occurs under dark conditions. In that case, it is assumed that the iron oxide is dissolved giving Fe 3+, which in turn oxidizes sulphite to sulphate: 6H + + Fe20~ --4 2Fe > + 3H20 H20 + Fe 3+ + HSO3- ---) Fe 2+ + SO42 + 3H + both Fe ~+ and Fe 2+ have been detected in solution and their concentrations were from 0.1 to 0.25 mM. Under these experimental conditions (no light, initial pH=4.5, mass of semiconductor=1.7 g dm -3 and 25°C), it has been found that with 13- and "f-FeOOH sulphite oxidation follows a zero-order kinetics with rate constants 2.8×10 4 and 1.7×10 .4 mol dm 3 s i, respectively, while with ~-Fe203 and ~-FeOOH, the kinetics are firstorder with rate constants of 0.035 and 0.1 I s q, respectively.

CONCLUSIONS The photocatalytic oxidation of nitrite to nitrate over ZnO and TiO2 occurs at relatively high yields following a Langmuir-Hinshelwood kinetics. For ZnO suspensions, the production of H202 at significantly high concentrations has been observed. In the case of TiO2 samples, the yield of nitrite photooxidation strongly

1271 depends on anatase content. For iron-doped TiO2, the reaction follows a zero-order kinetics with varying rate constants depending on semiconductor iron content. On the other hand, sulphite is readily oxidized in illuminated suspensions of iron ( I ~ oxides. In this case, the kinetics of the photocatalytic oxidation is affected by the adsorption of sulphate onto the photocatalyst surface. The rate of oxidation is dependent on the crystallographic structure of the semiconductor; the rate constant normalized to the specific area of the photocatalyst decreases in the following order: o~-FeEO3>>~-FeOOH = ~t-FeOOH >> ot-FeOOH. ACKNOWLEDGEMENTS This work was financially supported by CICYT (AMB96-0742), to whom we are very grateful. REFERENCES 1. S.E. Manahan, Environmental Chemistry (4 th Edn), p. 356, Lewis Pub., Boston (1991). 2. Y. Hori, S. Suzuki, Rate of heterogeneous photocatalytic oxidation of NO2- in cloud water droplets. Estimated for hypothetical environmental conditions, Chem. Lett., 1987, 1397-1400. 3. M. Hoffmann, S. Martin, W. Choi, D. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev., 95, 69-96 (1995). 4. A. Mills, S. Le Hunte, An overview of semiconductor photocatalysis, J. Photochem. Photobiol A, 108, 1-35 (1997). 5. O. Legrini, E. Oliveros, A. Braun, Photochemical processes for water treatment, Chem. Rev., 93, 671-698 (1993). 6. M. Hoffmann, D. Jacob, SO2, NO and NO2 Oxidation mechanisms:Atmospheric considerations (De J. Calvert), p. 101, Butterworth, Boston (1984). 7. J. Calvert, A. Lazrus, G. Kok, B. Heikes, J. Walega, J. Lind, C. Cantrell, Chemical mechanisms of acid generation in the troposphere, Nature, 317, 27-35 (1985). 8. J.A. Navio, M. Maclas, M. Gonzfilez, A. Justo, Bulk and surface characterization of powder iron-doped titania photocatalysts, J. Mat. Sci., 25, 3036-3042 (1992). 9. A. Ansari, Sintesis y propiedades fotocataliticas de los 6xidos y oxo-hidr6xidos de hierro, Ph.D. Thesis, Universitat Autbnoma de Barcelona, Barcelona, Spain (1995). 10. C. Domingo, Obtenci6n de 6xidos de Fe(I~) (HI), mediante oxidaci6n de Fe(II), Ph.D. Thesis, Universitat de Barcelona, Spain (1992). 11. C. Kormann, D. Bahnemann, M. Hoffmann, Photocatalytic production of H202 and organig peroxides in aqueous suspensions of TiO2, ZnO and desert sand, Environ. Sci. Technol., 22, 798-806 (1988). 12. J.A. Navio, F. Marchena, M. Roncel, M. de la Rosa, J. Photochem. Photobiol. A, 55, 319-322 (1991 ). 13. R.M. Smith, A.E. Martell, Stability Critical Constants, Vol. 4, Plenum Press, New York (1974).