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Role of oxidative and reductive pathways in the photocatalytic degradation of organic compounds
Colloids and Surfaces A: Physicochemical and Engineering Aspects 151 (1999) 321–327
Role of oxidative and reductive pathways in the photocatalytic degradation of organic compounds E. Pelizzetti *, C. Minero Dipartimento di Chimica Analitica, Universita` di Torino, via Pietro Giuria 5, 10125 Torino, Italy Received 13 December 1997; accepted 19 June 1998
Abstract The recent studies of concurrent oxidative and reductive processes occurring in the photocatalytic treatment are reviewed mainly focusing on the fate of carbon and nitrogen in the presence/absence of oxygen. The unusual possibility of photocatalysis to achieve degradation of organic compounds by concurrent reductive and oxidative reactions is discussed using the average oxidation numbers of C and N in the system. The effects of their initial values and of the nature of the organic substrate are outlined. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Photocatalysis; Degradation of organics; Oxidation processes; Reduction processes; Dehalogenation
1. Introduction In heterogeneous photocatalysis a suspension of semiconductor particles is irradiated with natural or artificial UV light. The excitation promotes an electron from the valence to the conduction band creating an oxidizing site (a ‘‘hole’’, h+ ) and a VB reducing site (an ‘‘electron’’, e− ). These electron–CB hole pairs can then recombine or be captured by reagents present at the surface of the photocatalyst [1,2]. The recognition that semiconductor photocatalysis may lead to oxidative mineralization of organic pollutants came in the early eighties with studies on the stoichiometric photomineralization of halogenated hydrocarbons, halogenated phenols, and salicylic acid carried out by Ollis et al. [3], Pelizzetti et al. [4] and Matthews [5], respec-
* Corresponding author. Fax: 00 39 11 6707 615; e-mail: [email protected]
tively. The mineralization follows Eq. (1):
A
C H O X + x+ x p q y
p−y−2q 4
B
O
2
catalyst, hn
p−y xCO +yH++yX−+ HO 2 2 2
(1)
where X represents a halogen atom. Since then, hundreds of organic compounds have been degraded and exhaustive lists of them can be found elsewhere [6–8]. Since photomineralization to CO is the desired process, O was seen 2 2 as an essential component acting as a scavenger of the photogenerated electrons, forming a superoxide radical ion, and further reacting with transient radicals leading toward CO formation. 2 Although this simplified picture is appropriate for many of the organic compounds investigated, a reductive pathway has been proposed or proven to be crucial for the photodegradation of some organic and nitrogen-containing organic com-
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pounds, depending on the initial average oxidation number of the carbon and nitrogen. As a consequence, the role of oxygen and other electron scavengers has to be considered from this perspective.
2. Average oxidation number of carbon Table 1 reports the oxidation number, n , of the C carbon atom present in some organic compounds. In the case where the organic molecule contains two or more carbons, the value is averaged over all carbon atoms. The oxidation number for –H, –X and –O– is taken as +1, −1 and −2, respectively. For mixtures, n refers to a weighted average C of carbon oxidation states over all compounds present in the system. It is calculated from n =(∑c (n ) )/(∑c ), where c is the concentration C i C i i i of the species in which carbon has the oxidation state (n ) . Ci Although for compounds in which n is low, C direct reaction with holes or hydroxyl radicals leads to oxidation (increase of n ), for compounds C in which the carbon is at the highest oxidation number, the redox nature of the photocatalytic process implies that reductive pathways should be operating. Only for a few photocatalytic dehalogenations, however, has the reductive pathway been proven. Some of them are listed in Table 2. As the degradation reaction progresses, the suspension becomes loaded with several degradation products. The calculation of n requires that all C
Table 2 Photocatalytic processes in which reductive pathways have been recognized Organic substrate
Reference
CF CHBrCl 3 CCl COOH, CHCl COOH, CH ClCOOH 3 2 2 CCl , CHCl , CHBr , CH Cl 4 3 3 2 2 Chlorinated ethanes CH CCl 3 3 CFCl CF Cl 2 2 Benzoquinone C(NO ) 24
[9] [10,11] [12–16 ] [17] [18] [19] [20,21] [22]
the species should be identified and quantified. To diminish the number of possible reaction products, simple molecules such as halomethanes are good candidates. For photocatalytic degradation of CCl , CHCl and CH Cl [15,16 ], if n resulting 4 3 2 2 C from the different C compounds present in the 1 reaction mixture is calculated as a function of time, the plots of Fig. 1 are obtained. The same calculation is reported in Fig. 1 for dodecane, for which the intermediates were not detected at a relevant concentration, and the normalized rate of CO formation is almost equal to that of the 2 hydrocarbon disappearance (12r =r ) [23]. C12H26 CO2 As the final product in the presence of oxygen is CO , n should reach the value +4. This is 2 C evident for dodecane and CH Cl . For CCl n 2 2 4 C should not change from the reactant to the final product (+4 in CCl and +4 in CO ). However, 4 2 in this case an initial decrease of n was observed. C
Table 1 Examples for oxidation number of C in selected organic compounds n a C
Organic compound
+4 +3 +2 +1 0 −1 −2 −3 −4
CO , CCl , CF Cl 2 4 2 2 HOOC–COOH, Cl C–CCl , CF Cl–CCl F 3 3 2 2 HCOOH, CHCl , CO, CF –CHBrCl 3 3 OHC–CHO, CCl NCHCl 2 C, HCHO, C H O , CH CCl , C H O (benzoquinone) 6 12 6 3 3 6 4 2 C H , HOCH –CH OH 6 6 2 2 CH Cl, CH OH 3 3 CH 2 6 CH 4
a Fractional oxidation numbers are common.
Fig. 1. Average oxidation number of carbon, n , for some C organic compounds as a function of the degradation time in aerated solutions at pH 5. From data of Refs. [16,23].
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Evidently, but necessarily, the reduction pathway is largely predominant in the early part of the process. The case of CHCl is intriguing, since a very 3 smooth variation of n was observed during the C initial steps of reaction. This is indicative of concomitant oxidative and reductive pathways even in the presence of oxygen. Despite Eq. (1), some photocatalytic processes may take place even in the absence of oxygen. For example, CCl degradation can be described by 4 CCl +2H OCO +4HCl (2) 4 2 2 which is formally a hydrolysis reaction. Owing to Eq. (2), dehalogenation and CO formation 2 should occur even in the absence of oxygen. The experiments showed that chloromethanes could be fully dehalogenated under anaerobic photocatalytic conditions. However, particularly at pH 10, stoichiometric CO was not formed. A careful 2 analysis of the organic intermediates showed that the concentration of non-halogenated C com1 pounds slowly changed in the last part of the time window investigated [15]. Since complete mineralization of CHCl and 3 CH Cl requires oxygen, their degradation in the 2 2 absence of oxygen could produce chloride but not stoichiometric CO . The evolution of products 2 with oxidation states of carbon <4, i.e. HCHO and HCOOH from CHCl , and CH OH, HCHO 3 3 and HCOOH from CH Cl , has been reported 2 2 [15]. The formation of chloride ions and nonhalogenated compounds can be accounted for only by reactions in which the average oxidation number of carbon remains unchanged. Using stoichiometric concepts, this can be achieved by hydrolysis (see Eqs. (3) and (4), disproportionation reactions involving the carbon atom (see for example Eq. (5) for CH Cl ) or some linear combi2 2 nation of them. CHCl +2H OHCOOH+3HCl (3) 3 2 CH Cl +H OHCHO+2HCl (4) 2 2 2 2CH Cl +3H OHCOOH+CH OH+4HCl 2 2 2 3 (5) Although these reactions do not imply any particu-
323
lar degradation pathway, since photocatalysis operates through electron transfer reactions (concurrently oxidative and reductive), the consecutive redox reactions with e− and ΩOH or with ΩOH and e−, (formally equivalent to reaction with OH−) are equivalent to hydrolysis. This photocatalytically induced hydrolysis was demonstrated to be 106–108 times faster than the corresponding thermal process for chloromethanes [15]. The photocatalytically induced hydrolysis implies that in stoichiometric reactions ( Eqs. (3)– (5) every water molecule corresponds to concurrent reactions with ΩOH/e−/H+ (in any order) and that the ratio of reaction stoichiometric coefficients has to be equal to that between the concentrations of products found in the experimental system. This conceptual framework is useful for understanding complex degradation pathways of organic molecules. For example, in the CHCl degradation, in the 3 initial steps of the process HCOOH, HCHO, CH Cl and CO have been observed as major 2 2 2 products. CO should originate through two oxida2 tive steps (Dn =+2) according to C CHCl +2ΩOHCO +3H++3Cl− (6) 3 2 For HCOOH in which the oxidation state of carbon does not change (Dn =0), the correspondC ing stoichiometric reaction is reaction Eq. (3). CH Cl and HCHO should originate from a 2 2 sequence of two reductive steps represented by the following stoichiometric equations (Dn =−2) C CHCl +2e−+2H+CH Cl +H++Cl− (7) 3 2 2 CHCl +2e−+2H++H OHCHO 3 2 +3H++3Cl− (8) Since the total number of generated and reacted e− must equal that of holes (or ΩOH ), Eqs. (3) and (6)–(8) are already balanced. In other cases the appropriate normalization is required. Under the hypothesis that pathways globally depicted by stoichiometric reactions Eqs. (3) and (6)–(8) have comparable kinetic weight, the concentrations of HCOOH ( Eq. (3), (formed through two possible pathways corresponding to reactions with e− and ΩOH or with ΩOH and e−, then 2e− and 2ΩOH ), CH Cl and HCHO ( Eqs. (7) and (8) (2e− each, 2 2
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respectively) and CO ( Eq. (6), 2ΩOH ) should be 2 in the ratio [CHOOH ]/[(CH Cl + 2 2 HCHO)]/[CO ]=2/1/1. This expectation was rea2 sonably fulfilled at the early stages of degradation at pH 5 [15].
3. Average oxidation number of nitrogen Nitrogen-containing organic compounds belong to different classes of compounds in which nitrogen exhibits a broad spectrum of oxidation numbers (see Table 3). Photocatalytic degradation leads to formation of inorganic nitrogen species, nitrate and ammonium ions being the most common [24]. Nitrite is often observed as a transient species. Table 4 lists some reference works. The ratio of ammonium to nitrate after mineralization of the organics depends mainly on the initial oxidation state of nitrogen and the nature of the nitrogen in the compound (heterocyclic, substituent). Experiments on the possible photocatalytic Table 3 Example for the oxidation number of N in selected compounds n N
Compound
+5 +4 +3 +2 +1 0 −1 −2 −3
NO−, R–ONO a 3 2 NO 2 NO−, R–NO a 2 2 NO N O, R–NO a 2 N 2 NH OH, R–NHOH a, R–NNN–R a 2 NH –NH 2 2 NH , R–NH a, heterocyclic nitrogen (e.g. pyridine) 3 2
a R is an alkyl or aryl group. Table 4 Photocatalytic processes involving nitrogen containing organic compounds Organic substrate
Reference
Miscellaneous Nitrobenzene Nitrosobenzene, phenylhydoxylamine Aniline Trinitrotoluene Tetranitromethane
[24,25] [26,27] [21,28] [21,28,29] [30] [22]
transformations of inorganic nitrogen species in the presence of oxygen [28] showed that ammonium ions are formed from nitrate and nitrite only at trace levels; nitrite is rapidly oxidized to nitrate, and ammonium is slowly transformed to nitrate. The rate of the last process is strongly affected by pH [21,24]. As a consequence, the formation of ammonium ions from nitroderivatives (for example nitrobenzene [21] and tetranitromethane [22]) cannot be explained by photocatalytic transformation of inorganic species and should be assigned, even in the presence of oxygen, to reduction processes occurring at the nitrogen substituent before it is released. This has been demonstrated by investigating the interconversion of nitrogen-containing benzene derivatives in the set nitrobenzene, nitrosobenzene, phenylhydroxylamine and aniline. The analysis of product evolution at short irradiation times revealed that large number of the photocatalytic reactions took place at the nitrogencontaining substituents and involved e− and h+ to comparable extents. For nitrosobenzene degradation, nitrobenzene and phenylhydoxylamine peaked after about 1 min, accounting for about 25% each of the initial concentration of nitrosobenzene (75% degraded). The stoichiometric reaction Eq. (9) exemplifies this reaction course (Dn =0). N +H2O 2C H NO C H NO +C H NHOH (9) 6 5 6 5 2 6 5 The average oxidation number of nitrogen n for N organic compounds present in the reaction mixture varied as a function of irradiation time in the range ±0.1 in the initial 5 min of reaction (see Fig. 2). The value of the average nitrogen oxidation number was calculated using n reported N in Table 3. When most of the nitrogen is released in solution from the organic moiety as inorganic species, its average oxidation number increases significantly only if it was initially at the lowest value (n =−3), as clearly shown in Fig. 3. For initial N intermediate values of n (n =+1 and n =+3), N N N the average oxidation number showed a slight predominance of reductive processes, tending to the value that it had in the organic molecule. Nitrate is formed mainly through nitrite release
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325
contrast, from highly reduced compounds (e.g. aniline) the release of ammonium goes through an initial oxidation step. The balance of these paths depends on the initial nature of the organic compound and is responsible of the [NH+]/[NO−] 4 3 ratio in the early degradation process [28].
4. Effect of the structure of organic compounds
Fig. 2. Average oxidation number of nitrogen, n , for nitrosoN benzene degradation as a function of degradation time in aerated solutions at pH 5. From data of Ref. [21].
Fig. 3. Average oxidation number of nitrogen, n , as a function N of the degradation time for nitrobenzene (NB), nitrosobenzene (NOB), phenylhydroxylamine (PHA), aniline (A) and inorganic compounds in aerated solutions at pH 5. From data of Refs. [21,28]. Values of n are calculated by averaging on the N inorganic nitrogen species released in solution; the number in parentheses is the percentage of the released inorganic nitrogen over the stoichiometric value.
and oxidation. For longer irradiation times the above-mentioned conversion of ammonium ions to nitrate increases the average oxidation number. It appears that, under aerated photocatalytic conditions, the initial redox steps occur largely at the nitrogen substituents. Ammonium ions are released in oxygenated solutions even from organic compounds in which nitrogen is initially at a high oxidation number. For these compounds, reductive pathways operate on the nitrogen substituent and ammonia is released from unstable intermediates, probably quinonoid [21] or aliphatic imino derivatives. In
Despite the above-mentioned importance of the initial average oxidation number, other factors could be relevant for the relative role of reductive/oxidative processes. For example, several structures in which the oxidation number of C or N is the same, show different degradation behavior. For CH Cl for which n =0 it has been reported 2 2 C [16 ] that under air the oxidative pathway is predominant from the beginning of the experiment. In contrast, for benzoquinone, for which n =0, C hydroquinone ( HQ) was formed with an 80% yield (with respect to the initial p-benzoquinone concentration) after 1 min of irradiation [21]. The pbenzoquinone half-time decay was as low as 0.25 min under these conditions. When the experiment was carried out in the presence of 0.01 M tbutanol as a hole scavenger, the same amount of hydroquinone was formed, but consecutive HQ depletion was significantly reduced. In separate experiments on HQ and phenol, HQ was degraded mainly by oxidative processes, yielding trihydroxybenzenes, other products of ring opening, and only traces of p-benzoquinone. When an equimolar mixture of hydroquinone and p-benzoquinone was irradiated in the presence of titanium dioxide, the hydroquinone concentration increased to 160% of its initial value after 1 min and decreased to 60% after 6 min, while the p-benzoquinone disappearance rate was unaffected by the simultaneous presence of HQ. Quinonoid structures appear to be efficient e− scavengers, even in aerated solutions. CB Interestingly, generation of ΩOH and ΩH radicals in sonolysis experiments on aqueous solutions of benzoquinone also produced hydroquinone in considerable yield [31]. The difference in the bond polarization may also play a role for aliphatic structures. CH CCl and 3 3
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HCHO both have n =0. Under aerated conditions C for the first compound some species with lower n have been observed [18], but in the case of C HCHO CH OH was below the detection limit [32]. 3 The photocatalytic transformation of nitrogencontaining related structures such as 4-nitrosophenol (4-NOP) and nitrosobenzene, in which nitrogen has the same oxidation number, showed marked differences in the ratio of ammonium to nitrate ions [21]. After 30 min of irradiation of 4-NOP the concentration ratio [NH+]/[NO−] was as high as 5, and tended to 1 4 3 only after 5 h irradiation. This suggested that the reductive processes were predominant in the photocatalytic degradation of 4-NOP, much more than for nitrosobenzene, for which the same ratio was about 0.6 after 30 min, and about 1 after 5 h. This was attributed to the benzoquinonemonoxime tautomeric form of 4-NOP. These results supported the prominent role of reductive pathways in the degradation of compounds that may exist in a quinonoid tautomeric form.
5. Role of oxygen Oxygen is essential for complete mineralization of organics according to Eq. (1). From the mechanistic point of view, the initial role of oxygen is represented by O +e− OΩ− (10) 2 CB 2 leading to a superoxide radical. A sequence of reactions extensively described in the literature [33,34] involving superoxide, molecular oxygen and adsorbed hydroxyl radical (generated by holes) may lead to H O formation. To assess the 2 2 role of these species, catalase (decomposing hydrogen peroxide) and superoxide dismutase have been added to titania suspensions. It was shown [35,36 ] that: (i) catalase had no effect on the degradation rate of 1,2-dimethoxybenzene, suggesting that H O is not essential to pollutant 2 2 removal; (ii) superoxide dismutase had a detrimental effect on the degradation rate and modified the intermediate distribution. The interaction of superoxide radical with the transient radical cation of
the initial organic substrate may be responsible for this observation. Molecular oxygen is known to add very rapidly to both aliphatic and aromatic radical species. After the direct electron transfer between the organic molecule and the excited semiconductor, or processes mediated by adsorbed ΩOH, the organic radical formed may add O , forming more 2 oxygenated compounds on the route to CO for2 mation. The pathways involving O or OΩ− are 2 2 the sole processes operating in most of photocatalytic degradations where routes for direct reduction/oxidation are not allowed. The dependence of the degradation rate, or the rate of CO formation, on the partial pressure of 2 oxygen [37,38] has been reported to follow a Langmuirian behavior. This is usually valid for compounds having medium or low values of n , C such as CH Cl and C H (see Fig. 1) and phenol 2 2 6 14 or 4-chlorophenol [37,38]. The positive slope in Fig. 1 for CH Cl and C H well represents this 2 2 6 14 case. In the other cases the presence of oxygen can be detrimental. For example, the initial negative slope observed for CCl suggests that oxygen is 4 competitive for reduction with the organic and, as a consequence, has a detrimental effect [15]. The case of CHCl (see Fig. 1) is intermediate: the 3 constant initial slope reflects the negligible effect of the O presence. 2 The ratio between concentrations of substrate, oxidant and surface sites can also play a complex role. We have reported that at low substrate concentration the effect of oxygen on the degradation rate of CHCl was negligible. However, at higher 3 concentration the reverse was observed [39]. In addition to the above phenomenological framework, the effect of oxygen (and other oxidants) on the photocatalytic degradation rates may be better rationalized using kinetic models which take into account all elementary events and experimental conditions [40].
Acknowledgment Financial support of CNR, MURST and Progetto Antartide ( Evoluzione e Cicli
E. Pelizzetti, C. Minero / Colloids Surfaces A: Physicochem. Eng. Aspects 151 (1999) 321–327
Biogeochimici appreciated.
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References [1] E. Pelizzetti, N. Serpone ( Eds.), Homogeneous and Heterogeneous Photocatalysis, Reidel, Dordrecht, 1986. [2] N. Serpone, E. Pelizzetti (Eds.), Photocatalysis. Fundamental and Applications, Wiley, New York, 1989. [3] A.L. Pruden, D.F. Ollis, Environ. Sci. Technol. 17 (1983) 628. [4] M. Barbeni, E. Pramauro, E. Pelizzetti, E. Borgarello, M. Gratzel, N. Serpone, Nouv. J. Chim. 8 (1984) 547. [5] R.W. Matthews, J. Chem. Soc. Faraday I 80 (1984) 457. [6 ] R.W. Matthews, in: E. Pelizzetti, M. Schiavello ( Eds.), Photochemical Conversion and Storage of Solar Energy, Kluwer, Dordrecht, 1991. [7] D.W. Bahnemann, J. Cunningham, M.A. Fox, E. Pelizzetti, P. Pichat, N. Serpone, in: G.R. Helz, R.G. Zepp, D.G. Crosby (Eds.), Aquatic and Surface Photochemistry, Lewis, Boca Raton, FL, 1994, p. 261. [8] D.M. Blake, Bibliography of Work on the Photocatalytic Removal of Hazardous Compounds from Water and Air, Reps. NREL/TP-430-6084 (1994), 473-20300 (1995), and 430-22197 (1997). [9] D.W. Bahnemann, J. Monig, R. Chapman, J. Phys. Chem. 91 (1987) 3782. [10] A. Chemseddine, H.P. Bohem, J. Mol. Catal. 60 (1990) 295. [11] W.H. Glaze, J.F. Kenneke, J.L. Ferry, Environ. Sci. Technol. 27 (1993) 177. [12] W. Choi, M.R. Hoffmann, Environ. Sci. Technol. 29 (1995) 1646. [13] W. Choi, M.R. Hoffmann, J. Phys. Chem. 100 (1996) 2161. [14] W. Choi, M.R. Hoffmann, Environ. Sci. Technol. 31 (1997) 89. [15] P. Calza, C. Minero, E. Pelizzetti, Environ. Sci. Technol. 31 (1997) 2198. [16 ] P. Calza, C. Minero, E. Pelizzetti, J. Chem. Soc. Faraday Trans. 93 (1997) 3765.
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[17] Y. Mao, C. Schoneich, K.D. Asmus, J. Phys. Chem. 95 (1991) 10080. [18] D. Mass, P. Pichat, C. Guillard, Res. Chem. Intermed. 23 (1997) 275. [19] S. Weaver, G. Mills, J. Phys. Chem. B 101 (1997) 3769. [20] C. Richard, P. Boule, New J. Chem. 18 (1994) 547. [21] P. Piccinini, C. Minero, M. Vincenti, E. Pelizzetti, J. Chem. Soc. Faraday Trans. 93 (1997) 1993. [22] C. Minero, P. Piccinini, P. Calza, E. Pelizzetti, New J. Chem. 20 (1996) 1159. [23] E. Pelizzetti, C. Minero, V. Maurino, H. Hidaka, N. Serpone, R. Terzian, Ann. Chim. (Rome) 80 (1990) 81. [24] G.K.C. Low, S.R. McEvoy, R.W. Matthews, Environ. Sci. Technol. 25 (1991) 460. [25] K. Nohara, H. Hidaka, E. Pelizzetti, N. Serpone, Catal. Lett. 36 (1996) 115. [26 ] C. Maillard-Dupuy, C. Guillard, P. Pichat, New J. Chem. 18 (1994) 941. [27] C. Minero, E. Pelizzetti, P. Piccinini, M. Vinceti, Chemosphere 28 (1994) 1229. [28] P. Piccinini, C. Minero, M. Vincenti, E. Pelizzetti, Catal. Today 39 (1997) 187. [29] E. Pramauro, A. Bianco Prevot, V. Augugliaro, L. Palmisano, Analyst 120 (1995) 237. [30] D.C. Schmelling, K.A. Gray, P.V. Kamat, Environ. Sci. Technol. 30 (1996) 2547. [31] N. Serpone, R. Terzian, P. Colarusso, C. Minero, E. Pelizzetti, H. Hidaka, Res. Chem. Intermed. 18 (1992) 183. [32] C. Minero, P. Calza, E. Pelizzetti, in preparation. [33] D.T. Sawyer, Oxygen Chemistry, Oxford University Press, Oxford, 1991. [34] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [35] L. Amalric, C. Guillard, P. Pichat, Res. Chem. Intermed. 20 (1994) 579. [36 ] L. Cermenati, P. Pichat, C. Guillard, A. Albini, J. Phys. Chem. B 101 (1997) 2650. [37] V. Augugliaro, L. Palmisano, A. Sclafani, C. Minero, E. Pelizzetti, Toxicol. Environ. Chem. 16 (1988) 89. [38] A. Mills, S. Morris, J. Photochem. Photobiol. A 71 (1993) 75. [39] C. Kormann, D.W. Bahnemann, M.R. Hoffmann, Environ. Sci. Technol. 25 (1991) 494. [40] C. Minero, E. Pelizzetti, S. Malato, J. Blanco, Solar Energy 56 (1996) 421.
Role of oxidative and reductive pathways in the photocatalytic degradation of organic compounds E. Pelizzetti *, C. Minero Dipartimento di Chimica Analitica, Universita` di Torino, via Pietro Giuria 5, 10125 Torino, Italy Received 13 December 1997; accepted 19 June 1998
Abstract The recent studies of concurrent oxidative and reductive processes occurring in the photocatalytic treatment are reviewed mainly focusing on the fate of carbon and nitrogen in the presence/absence of oxygen. The unusual possibility of photocatalysis to achieve degradation of organic compounds by concurrent reductive and oxidative reactions is discussed using the average oxidation numbers of C and N in the system. The effects of their initial values and of the nature of the organic substrate are outlined. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Photocatalysis; Degradation of organics; Oxidation processes; Reduction processes; Dehalogenation
1. Introduction In heterogeneous photocatalysis a suspension of semiconductor particles is irradiated with natural or artificial UV light. The excitation promotes an electron from the valence to the conduction band creating an oxidizing site (a ‘‘hole’’, h+ ) and a VB reducing site (an ‘‘electron’’, e− ). These electron–CB hole pairs can then recombine or be captured by reagents present at the surface of the photocatalyst [1,2]. The recognition that semiconductor photocatalysis may lead to oxidative mineralization of organic pollutants came in the early eighties with studies on the stoichiometric photomineralization of halogenated hydrocarbons, halogenated phenols, and salicylic acid carried out by Ollis et al. [3], Pelizzetti et al. [4] and Matthews [5], respec-
* Corresponding author. Fax: 00 39 11 6707 615; e-mail: [email protected]
tively. The mineralization follows Eq. (1):
A
C H O X + x+ x p q y
p−y−2q 4
B
O
2
catalyst, hn
p−y xCO +yH++yX−+ HO 2 2 2
(1)
where X represents a halogen atom. Since then, hundreds of organic compounds have been degraded and exhaustive lists of them can be found elsewhere [6–8]. Since photomineralization to CO is the desired process, O was seen 2 2 as an essential component acting as a scavenger of the photogenerated electrons, forming a superoxide radical ion, and further reacting with transient radicals leading toward CO formation. 2 Although this simplified picture is appropriate for many of the organic compounds investigated, a reductive pathway has been proposed or proven to be crucial for the photodegradation of some organic and nitrogen-containing organic com-
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pounds, depending on the initial average oxidation number of the carbon and nitrogen. As a consequence, the role of oxygen and other electron scavengers has to be considered from this perspective.
2. Average oxidation number of carbon Table 1 reports the oxidation number, n , of the C carbon atom present in some organic compounds. In the case where the organic molecule contains two or more carbons, the value is averaged over all carbon atoms. The oxidation number for –H, –X and –O– is taken as +1, −1 and −2, respectively. For mixtures, n refers to a weighted average C of carbon oxidation states over all compounds present in the system. It is calculated from n =(∑c (n ) )/(∑c ), where c is the concentration C i C i i i of the species in which carbon has the oxidation state (n ) . Ci Although for compounds in which n is low, C direct reaction with holes or hydroxyl radicals leads to oxidation (increase of n ), for compounds C in which the carbon is at the highest oxidation number, the redox nature of the photocatalytic process implies that reductive pathways should be operating. Only for a few photocatalytic dehalogenations, however, has the reductive pathway been proven. Some of them are listed in Table 2. As the degradation reaction progresses, the suspension becomes loaded with several degradation products. The calculation of n requires that all C
Table 2 Photocatalytic processes in which reductive pathways have been recognized Organic substrate
Reference
CF CHBrCl 3 CCl COOH, CHCl COOH, CH ClCOOH 3 2 2 CCl , CHCl , CHBr , CH Cl 4 3 3 2 2 Chlorinated ethanes CH CCl 3 3 CFCl CF Cl 2 2 Benzoquinone C(NO ) 24
[9] [10,11] [12–16 ] [17] [18] [19] [20,21] [22]
the species should be identified and quantified. To diminish the number of possible reaction products, simple molecules such as halomethanes are good candidates. For photocatalytic degradation of CCl , CHCl and CH Cl [15,16 ], if n resulting 4 3 2 2 C from the different C compounds present in the 1 reaction mixture is calculated as a function of time, the plots of Fig. 1 are obtained. The same calculation is reported in Fig. 1 for dodecane, for which the intermediates were not detected at a relevant concentration, and the normalized rate of CO formation is almost equal to that of the 2 hydrocarbon disappearance (12r =r ) [23]. C12H26 CO2 As the final product in the presence of oxygen is CO , n should reach the value +4. This is 2 C evident for dodecane and CH Cl . For CCl n 2 2 4 C should not change from the reactant to the final product (+4 in CCl and +4 in CO ). However, 4 2 in this case an initial decrease of n was observed. C
Table 1 Examples for oxidation number of C in selected organic compounds n a C
Organic compound
+4 +3 +2 +1 0 −1 −2 −3 −4
CO , CCl , CF Cl 2 4 2 2 HOOC–COOH, Cl C–CCl , CF Cl–CCl F 3 3 2 2 HCOOH, CHCl , CO, CF –CHBrCl 3 3 OHC–CHO, CCl NCHCl 2 C, HCHO, C H O , CH CCl , C H O (benzoquinone) 6 12 6 3 3 6 4 2 C H , HOCH –CH OH 6 6 2 2 CH Cl, CH OH 3 3 CH 2 6 CH 4
a Fractional oxidation numbers are common.
Fig. 1. Average oxidation number of carbon, n , for some C organic compounds as a function of the degradation time in aerated solutions at pH 5. From data of Refs. [16,23].
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Evidently, but necessarily, the reduction pathway is largely predominant in the early part of the process. The case of CHCl is intriguing, since a very 3 smooth variation of n was observed during the C initial steps of reaction. This is indicative of concomitant oxidative and reductive pathways even in the presence of oxygen. Despite Eq. (1), some photocatalytic processes may take place even in the absence of oxygen. For example, CCl degradation can be described by 4 CCl +2H OCO +4HCl (2) 4 2 2 which is formally a hydrolysis reaction. Owing to Eq. (2), dehalogenation and CO formation 2 should occur even in the absence of oxygen. The experiments showed that chloromethanes could be fully dehalogenated under anaerobic photocatalytic conditions. However, particularly at pH 10, stoichiometric CO was not formed. A careful 2 analysis of the organic intermediates showed that the concentration of non-halogenated C com1 pounds slowly changed in the last part of the time window investigated [15]. Since complete mineralization of CHCl and 3 CH Cl requires oxygen, their degradation in the 2 2 absence of oxygen could produce chloride but not stoichiometric CO . The evolution of products 2 with oxidation states of carbon <4, i.e. HCHO and HCOOH from CHCl , and CH OH, HCHO 3 3 and HCOOH from CH Cl , has been reported 2 2 [15]. The formation of chloride ions and nonhalogenated compounds can be accounted for only by reactions in which the average oxidation number of carbon remains unchanged. Using stoichiometric concepts, this can be achieved by hydrolysis (see Eqs. (3) and (4), disproportionation reactions involving the carbon atom (see for example Eq. (5) for CH Cl ) or some linear combi2 2 nation of them. CHCl +2H OHCOOH+3HCl (3) 3 2 CH Cl +H OHCHO+2HCl (4) 2 2 2 2CH Cl +3H OHCOOH+CH OH+4HCl 2 2 2 3 (5) Although these reactions do not imply any particu-
323
lar degradation pathway, since photocatalysis operates through electron transfer reactions (concurrently oxidative and reductive), the consecutive redox reactions with e− and ΩOH or with ΩOH and e−, (formally equivalent to reaction with OH−) are equivalent to hydrolysis. This photocatalytically induced hydrolysis was demonstrated to be 106–108 times faster than the corresponding thermal process for chloromethanes [15]. The photocatalytically induced hydrolysis implies that in stoichiometric reactions ( Eqs. (3)– (5) every water molecule corresponds to concurrent reactions with ΩOH/e−/H+ (in any order) and that the ratio of reaction stoichiometric coefficients has to be equal to that between the concentrations of products found in the experimental system. This conceptual framework is useful for understanding complex degradation pathways of organic molecules. For example, in the CHCl degradation, in the 3 initial steps of the process HCOOH, HCHO, CH Cl and CO have been observed as major 2 2 2 products. CO should originate through two oxida2 tive steps (Dn =+2) according to C CHCl +2ΩOHCO +3H++3Cl− (6) 3 2 For HCOOH in which the oxidation state of carbon does not change (Dn =0), the correspondC ing stoichiometric reaction is reaction Eq. (3). CH Cl and HCHO should originate from a 2 2 sequence of two reductive steps represented by the following stoichiometric equations (Dn =−2) C CHCl +2e−+2H+CH Cl +H++Cl− (7) 3 2 2 CHCl +2e−+2H++H OHCHO 3 2 +3H++3Cl− (8) Since the total number of generated and reacted e− must equal that of holes (or ΩOH ), Eqs. (3) and (6)–(8) are already balanced. In other cases the appropriate normalization is required. Under the hypothesis that pathways globally depicted by stoichiometric reactions Eqs. (3) and (6)–(8) have comparable kinetic weight, the concentrations of HCOOH ( Eq. (3), (formed through two possible pathways corresponding to reactions with e− and ΩOH or with ΩOH and e−, then 2e− and 2ΩOH ), CH Cl and HCHO ( Eqs. (7) and (8) (2e− each, 2 2
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respectively) and CO ( Eq. (6), 2ΩOH ) should be 2 in the ratio [CHOOH ]/[(CH Cl + 2 2 HCHO)]/[CO ]=2/1/1. This expectation was rea2 sonably fulfilled at the early stages of degradation at pH 5 [15].
3. Average oxidation number of nitrogen Nitrogen-containing organic compounds belong to different classes of compounds in which nitrogen exhibits a broad spectrum of oxidation numbers (see Table 3). Photocatalytic degradation leads to formation of inorganic nitrogen species, nitrate and ammonium ions being the most common [24]. Nitrite is often observed as a transient species. Table 4 lists some reference works. The ratio of ammonium to nitrate after mineralization of the organics depends mainly on the initial oxidation state of nitrogen and the nature of the nitrogen in the compound (heterocyclic, substituent). Experiments on the possible photocatalytic Table 3 Example for the oxidation number of N in selected compounds n N
Compound
+5 +4 +3 +2 +1 0 −1 −2 −3
NO−, R–ONO a 3 2 NO 2 NO−, R–NO a 2 2 NO N O, R–NO a 2 N 2 NH OH, R–NHOH a, R–NNN–R a 2 NH –NH 2 2 NH , R–NH a, heterocyclic nitrogen (e.g. pyridine) 3 2
a R is an alkyl or aryl group. Table 4 Photocatalytic processes involving nitrogen containing organic compounds Organic substrate
Reference
Miscellaneous Nitrobenzene Nitrosobenzene, phenylhydoxylamine Aniline Trinitrotoluene Tetranitromethane
[24,25] [26,27] [21,28] [21,28,29] [30] [22]
transformations of inorganic nitrogen species in the presence of oxygen [28] showed that ammonium ions are formed from nitrate and nitrite only at trace levels; nitrite is rapidly oxidized to nitrate, and ammonium is slowly transformed to nitrate. The rate of the last process is strongly affected by pH [21,24]. As a consequence, the formation of ammonium ions from nitroderivatives (for example nitrobenzene [21] and tetranitromethane [22]) cannot be explained by photocatalytic transformation of inorganic species and should be assigned, even in the presence of oxygen, to reduction processes occurring at the nitrogen substituent before it is released. This has been demonstrated by investigating the interconversion of nitrogen-containing benzene derivatives in the set nitrobenzene, nitrosobenzene, phenylhydroxylamine and aniline. The analysis of product evolution at short irradiation times revealed that large number of the photocatalytic reactions took place at the nitrogencontaining substituents and involved e− and h+ to comparable extents. For nitrosobenzene degradation, nitrobenzene and phenylhydoxylamine peaked after about 1 min, accounting for about 25% each of the initial concentration of nitrosobenzene (75% degraded). The stoichiometric reaction Eq. (9) exemplifies this reaction course (Dn =0). N +H2O 2C H NO C H NO +C H NHOH (9) 6 5 6 5 2 6 5 The average oxidation number of nitrogen n for N organic compounds present in the reaction mixture varied as a function of irradiation time in the range ±0.1 in the initial 5 min of reaction (see Fig. 2). The value of the average nitrogen oxidation number was calculated using n reported N in Table 3. When most of the nitrogen is released in solution from the organic moiety as inorganic species, its average oxidation number increases significantly only if it was initially at the lowest value (n =−3), as clearly shown in Fig. 3. For initial N intermediate values of n (n =+1 and n =+3), N N N the average oxidation number showed a slight predominance of reductive processes, tending to the value that it had in the organic molecule. Nitrate is formed mainly through nitrite release
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contrast, from highly reduced compounds (e.g. aniline) the release of ammonium goes through an initial oxidation step. The balance of these paths depends on the initial nature of the organic compound and is responsible of the [NH+]/[NO−] 4 3 ratio in the early degradation process [28].
4. Effect of the structure of organic compounds
Fig. 2. Average oxidation number of nitrogen, n , for nitrosoN benzene degradation as a function of degradation time in aerated solutions at pH 5. From data of Ref. [21].
Fig. 3. Average oxidation number of nitrogen, n , as a function N of the degradation time for nitrobenzene (NB), nitrosobenzene (NOB), phenylhydroxylamine (PHA), aniline (A) and inorganic compounds in aerated solutions at pH 5. From data of Refs. [21,28]. Values of n are calculated by averaging on the N inorganic nitrogen species released in solution; the number in parentheses is the percentage of the released inorganic nitrogen over the stoichiometric value.
and oxidation. For longer irradiation times the above-mentioned conversion of ammonium ions to nitrate increases the average oxidation number. It appears that, under aerated photocatalytic conditions, the initial redox steps occur largely at the nitrogen substituents. Ammonium ions are released in oxygenated solutions even from organic compounds in which nitrogen is initially at a high oxidation number. For these compounds, reductive pathways operate on the nitrogen substituent and ammonia is released from unstable intermediates, probably quinonoid [21] or aliphatic imino derivatives. In
Despite the above-mentioned importance of the initial average oxidation number, other factors could be relevant for the relative role of reductive/oxidative processes. For example, several structures in which the oxidation number of C or N is the same, show different degradation behavior. For CH Cl for which n =0 it has been reported 2 2 C [16 ] that under air the oxidative pathway is predominant from the beginning of the experiment. In contrast, for benzoquinone, for which n =0, C hydroquinone ( HQ) was formed with an 80% yield (with respect to the initial p-benzoquinone concentration) after 1 min of irradiation [21]. The pbenzoquinone half-time decay was as low as 0.25 min under these conditions. When the experiment was carried out in the presence of 0.01 M tbutanol as a hole scavenger, the same amount of hydroquinone was formed, but consecutive HQ depletion was significantly reduced. In separate experiments on HQ and phenol, HQ was degraded mainly by oxidative processes, yielding trihydroxybenzenes, other products of ring opening, and only traces of p-benzoquinone. When an equimolar mixture of hydroquinone and p-benzoquinone was irradiated in the presence of titanium dioxide, the hydroquinone concentration increased to 160% of its initial value after 1 min and decreased to 60% after 6 min, while the p-benzoquinone disappearance rate was unaffected by the simultaneous presence of HQ. Quinonoid structures appear to be efficient e− scavengers, even in aerated solutions. CB Interestingly, generation of ΩOH and ΩH radicals in sonolysis experiments on aqueous solutions of benzoquinone also produced hydroquinone in considerable yield [31]. The difference in the bond polarization may also play a role for aliphatic structures. CH CCl and 3 3
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HCHO both have n =0. Under aerated conditions C for the first compound some species with lower n have been observed [18], but in the case of C HCHO CH OH was below the detection limit [32]. 3 The photocatalytic transformation of nitrogencontaining related structures such as 4-nitrosophenol (4-NOP) and nitrosobenzene, in which nitrogen has the same oxidation number, showed marked differences in the ratio of ammonium to nitrate ions [21]. After 30 min of irradiation of 4-NOP the concentration ratio [NH+]/[NO−] was as high as 5, and tended to 1 4 3 only after 5 h irradiation. This suggested that the reductive processes were predominant in the photocatalytic degradation of 4-NOP, much more than for nitrosobenzene, for which the same ratio was about 0.6 after 30 min, and about 1 after 5 h. This was attributed to the benzoquinonemonoxime tautomeric form of 4-NOP. These results supported the prominent role of reductive pathways in the degradation of compounds that may exist in a quinonoid tautomeric form.
5. Role of oxygen Oxygen is essential for complete mineralization of organics according to Eq. (1). From the mechanistic point of view, the initial role of oxygen is represented by O +e− OΩ− (10) 2 CB 2 leading to a superoxide radical. A sequence of reactions extensively described in the literature [33,34] involving superoxide, molecular oxygen and adsorbed hydroxyl radical (generated by holes) may lead to H O formation. To assess the 2 2 role of these species, catalase (decomposing hydrogen peroxide) and superoxide dismutase have been added to titania suspensions. It was shown [35,36 ] that: (i) catalase had no effect on the degradation rate of 1,2-dimethoxybenzene, suggesting that H O is not essential to pollutant 2 2 removal; (ii) superoxide dismutase had a detrimental effect on the degradation rate and modified the intermediate distribution. The interaction of superoxide radical with the transient radical cation of
the initial organic substrate may be responsible for this observation. Molecular oxygen is known to add very rapidly to both aliphatic and aromatic radical species. After the direct electron transfer between the organic molecule and the excited semiconductor, or processes mediated by adsorbed ΩOH, the organic radical formed may add O , forming more 2 oxygenated compounds on the route to CO for2 mation. The pathways involving O or OΩ− are 2 2 the sole processes operating in most of photocatalytic degradations where routes for direct reduction/oxidation are not allowed. The dependence of the degradation rate, or the rate of CO formation, on the partial pressure of 2 oxygen [37,38] has been reported to follow a Langmuirian behavior. This is usually valid for compounds having medium or low values of n , C such as CH Cl and C H (see Fig. 1) and phenol 2 2 6 14 or 4-chlorophenol [37,38]. The positive slope in Fig. 1 for CH Cl and C H well represents this 2 2 6 14 case. In the other cases the presence of oxygen can be detrimental. For example, the initial negative slope observed for CCl suggests that oxygen is 4 competitive for reduction with the organic and, as a consequence, has a detrimental effect [15]. The case of CHCl (see Fig. 1) is intermediate: the 3 constant initial slope reflects the negligible effect of the O presence. 2 The ratio between concentrations of substrate, oxidant and surface sites can also play a complex role. We have reported that at low substrate concentration the effect of oxygen on the degradation rate of CHCl was negligible. However, at higher 3 concentration the reverse was observed [39]. In addition to the above phenomenological framework, the effect of oxygen (and other oxidants) on the photocatalytic degradation rates may be better rationalized using kinetic models which take into account all elementary events and experimental conditions [40].
Acknowledgment Financial support of CNR, MURST and Progetto Antartide ( Evoluzione e Cicli
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