Photoinitiated reactions of hydrogen peroxide in the liquid phase

I. Photochem.

PhotobioL

A:

Chem.,

68 (1992)

1

l-33

InvitedReview

Photoinitiated liquid phase

reactions of hydrogen

peroxide

in the

Stanislav Lui%ik and Petr Sedlak Institute of Inorganic 160 00 Praha (Received

Chemistry

Czechoslovak

Academy

of Sciences,

Majakovsk&o

24,

6 (Czechoslovakia)

April

2, 1992;

accepted

April

2, 1992)

Abstract This review introduces the most commonly known types of photoinitiated reactions of hydrogen peroxide. In the first part the photolysis of hydrogen peroxide is discussed. Particular attention is paid to the photocatalytic effects of transition metals and a mechanism of their photocatalytic action is proposed_ In the second part the photoinitiated reactions of hydrogen peroxide with inorganic and organic compounds are discussed. Attention is also devoted to the exploitation of these reactions in environmental protection.

1. Introduction

The importance of the investigation of photochemical reactions of hydrogen peroxide (HaO,) lies on the one hand in an obviously not yet fully appreciated contribution to the deveiopment of theoretical knowledge of the mechanism of photoinitiated and photocatalysed reactions, and on the other hand in a contribution to the answer to many urgent ecological questions. Problems of photochemical reactions of H202 have been summarily dealt with in several studies [l-6]. However, these are either reviews published some years ago [l-3] or are focused only on some narrow aspects of the photochemica1 reactions of Hz02

P-61. The aim of this report is to offer a comprehensive

review of recent studies concerning photoinitiated reactions of H202 in the liquid phase, with particular attention to photocatalysed processes.

2. Photolysis of hydrogen

peroxide

The photolysis of H202 was studied with great attention in the past [2, 31. It was shown [3] that all W radiation absorbed by H202, i.e. radiation of wavelength shorter than 380 nm, is photolytically active. The main products of the photolysis of aqueous solutions of H202 are the same as those arising in its thermal decomposition, i.e. water and oxygen.

lOlO-6030/92/$5.00

0

1992-Elsevier

Sequoia.

All

rights

resewed

2

2.1.

Reaction

mechanhm

-

main

ideas

The effect of radiation on the decomposition of HzOz was observed as early as the end of last century 171. In 1929 Urey et al. [8] suggested for the primary process of the photolysis of H202 its dissociation to hydroxyl radicals (eqn. (1)). According - hY 20H

I%02

(I)

to a reaction scheme suggested by Haber formed should bring about a radical chain OH-+H,O HO;

-

H,O + HO,

+ HzOz -

and Weiss mechanism

19, lo], the OH’ radicals in which the propagation

thus cycle (2)

O,+H,O+OH

(3)

gives high quantum yields (CDS- l)_ The experimentally found situation is more complicated, however, and high quantum yields (@N 1) are not always obtained. The quantum yields of the photolysis of HzOz are markedly affected by the intensity of photolytic radiation 131. It has been shown [3] that high quantum yields are observed only at low radiation intensity. For high radiation intensities the quantum yields were @= 1-2 (H202 molecules per hi) [g-13]. This phenomenon, namely suppression of high quantum yields (i.e. of the chain process), is explained by Weiss [14] by an effective termination of the OH’ and HO,’ radicals caused by their high concentration at a higher intensity of photolytic radiation. However, no such phenomenon has been observed in other reactions with high quantum yields (e.g. oxidation of sulphite or benzaldehyde by dioxygen): in this case, under conditions where a large number of incident quanta were absorbed within a small volume (see e.g. discussion in ref. 15), the quantum yields were high. Weiss [143 suggests and discusses the following termination processes which may reduce the quantum yield: HO;+ON’-

02 + H,O

2H02

-

HzOz + 02

2oI-I

-

HzO-i-0

2OII-

+

H202

oH-+x-

interruption

(4) (5)

(20

-

02)

(6)

(7) of the chain

(8)

According to Weiss, the most probable process is a combination of terminations as shown by eqns. (4) and (7). Assuming a stationary concentration of reactive intermediates and the validity of eqns. (l)-(3) and (S), and using ref. 14, the quantum yield can be adequately described by (9) A linear dependence of quantum yield on the concentration of hydrogen peroxide and on the reciprocal value of the square root of the intensity of absorbed radiation has indeed been found by several authors (for a review see e.g. ref. 3). A note-worthy feature is the relatively narrow range of reaction conditions within which the linearity was verified; this gives rise to the question of whether such results can be regarded as confirming the proposed mechanism. Along with photodissociation (eqn. (l)), two other possibilities of primary photochemical dissociation of l&O2 are considered in the literature:

3 H,O,

=

Hz02 2

H,O+O

(10)

Er+I-Io,

(II)

From the energy point of view, eqn. (10) would be the most advantageous reaction, since an energy of 122.2 kJ mol-1 is sufficient to bring it about - i.e. the energy of radiation of wavelength 979.2 nm. With respect to the Franck-Condon principle, however, reaction (10) is extremely unlikely to occur [6]. In a comparison between reactions (1) and (II), reaction (1) is more advantageous from the energetic viewpoint: it can be accomplished on supplying 213.4 kJ mol-’ (corresponding to the energy of radiation of wavelength 560.6 nm), while reaction (11) requires an energy of 376.8 k.I mol- I, i.e. the energy of radiation of wavelength 317.7 nm. Great attention [l-3, 5, 14, 16-241 has been focused on the so-called Haber-Weiss reaction (eqn. (3)) and on its modification for higher pH (eqn. (12)). 02-+H202-

HO’-tOH-+O,

(12)

This process is the rate-determining step of the whole radical chain mechanism of photolysis of HzOz [25]. The Haber-Weiss reaction is also of interest with respect to the photochemical interaction of HZ02 with biological systems [26-341, where HOi plays an important role in reactions with enzymes and membranes. A comparison between the rate cotistants of the Haber-Weiss reaction, kIIw, as obtained by various authors reveals the fact that they differ in order of magnitude (Table 1). There are two possibfe explanations: either eqns. (3) and (12) do not reflect the objective reality of a given chemical system or the value of the rate constant is affected by a factor not considered in the respective studies. Many authors [35-41] carried out electron paramagnetic resonance (EPR) investigations of frozen solutions of HzOz during UV irradiation_ Although in earlier papers the detection of an unidentified radical [35-381 was usually only stated, in more recent studies the electron spin resonance (ESR) spectra were attributed to the HO’ radical [40], to the superoxide radical HO; [41J or to a mixture of the two [39], in which the content of the individual radicals varies with the concentration of Hjt202. The question as to what extent the analogy of participation of the radicals in the frozen and in the liquid state can be made still calls for further elucidation. Some attention has also been devoted to the photolysis of H,O, in a strongly alkaline medium [42-44]. According to Behar and Czapski [43], HO*undergoes photochemical dissociation under these conditions and reactions (14) and (15) compete

TABLE

1

Literary data on values of rate constants of the Haber-Weiss

tWL3o

PH

kw” (L mol-’ 530 5.0 16 3.7

(M)

8 x lo-’

Natural

3 x 1o-4 0.1-4 l-20

2.0 Natural Natural

l-40

1.0

aAccording to pH, k,

reaction Reference s-‘)

0.01

is either k3 or k L3or a combination of the two.

1201 c211 1221 r231 c243

4

with each other: HO,0-

-oH.+oh”

+HOz-

o-+02 O,-

(13) +

=

-t-HO*-

02-

+ OH-

(14)

09-

(1% 02- +O,+OH-

(16)

Landi and Heidt [42] stabilized the ozonide ion thus formed in the form of sodium ozonide. As has been discussed above, there is a trend in the literature to solve controversies between the radical mechanism of Hz02 photolysis and experimental results by assuming the existence of termination steps. This hypothesis is put forward in connection with the existence of a limit of the intensity of absorbed radiation which separates two types of mechanisms, namely the chain and the non-chain. If for 1~ 1017 quanta L-l -I dependences (17) and (18) are given, then for radiation intensities above the given Firnit eqns. (19) and (20) are valid [3]: -

dW2021

d7

= const xP[HzO~]

(17)

@ = const x 1o.5[H202]

- W,Ozl dr

= const

(18)

XI

@ = const

(20)

In spite of considerable efforts made by a number of authors, the problem of the mechanism of photolysis of “pure” HZ02 has not been satisfactorily solved yet. All the results published so far are affected by the presence of undefined concentrations of photocatalytically active particles, either homogeneously dissolved metal compounds or heterogeneous particles, which makes an exact treatment of the results virtually impossible. (For a discussion of photocatalytic effects see Section 2.2). Owing to the high efficiency of photocatalysts, their removal is demanding and so far nobody has tried it -e.g. in the case of copper{II) ions, concentrations below 10e9 M would have to be reached. 2-2. Photocatalytic effects of transition metals in homogeneous systems 2.2. I. Iron compounds As early as the turn of the century, the effect of complex iron cyanides on HzOz photolysis was discovered ]73_ La1 investigated the kinetics and mechanism of the effect of ferrocyanide [45-49]_ He described a pronounced “after-effect” which he explained by regeneration (reaction (23)) of photochemically generated catalyst (reaction (21)): light &[Fe(C%I

+

2K,[Fe(CN)sH20] WWCNW-I~OI

H20

e

[K,Fe(CN)5H,0]

-t HzOz f &Fe(CN)d

+ KCN

2K2[Fe(CN),H20] *

(21) -t 2KOH

JGIWCNMbOl+

According to this view, the photochemically the thermal decomposition of H202_

formed

(22) &IWCNM

aquapentacyanoferrate

(23) ion catalyses

5

Recently, Ohno [50] also studied the photolysis of hydrogen H202 in the presence of potassium ferrocyanide with respect to the possible use of &/Fe(CN),] as a photochemical electron donor. The catalytic effect of ferricyanide on Hz02 photolysis was investigated by Jain ef al. [Sl]. The nitroprusside ion is [52-551 yet another cyan0 compound of iron which exerts photocatalytic effects on the photolysis of l&C&. The photocatalytic effects were ascribed to the aquapentacyanoferrate ion. Behar and Stein [54] studied H,O, photolysis in the presence of ferric ions at various pH. They showed that the photolysis of H,O, in the presence of ferric ions is initiated by 365 nm radiation, i.e. by radiation which at the concentrations used is not absorbed to any important degree by the H2U, alone or by ferric ions alone. This radiation (365 Am) is, however, absorbed by a complex arising from an interaction between H,O, and ferric ions and having an assumed composition Fe3+HOP-: H 2 0,+Fe3+

e

[FeHO#+

+ HC

(24)

Under the experimental conditions used ([Hz02]=0.16 M, [Fe”‘] =0.005 M), the quantum yields varied in the range 3.3-7.4. According to the authors, the photocatalytic effects can be attributed to the photochemical generation of Fe’+ described by [FeHO,]‘+L

Fe’ f f HO;

(25)

The arising Fe’+

initiates the decomposition

Fe’+ + HzOz +

Fe3+ +OlX+OH-

of Hz02 via the classical Fenton reaction (26)

Kozlov et al. [55] used C(NO&, known as an effective inhibitor of the photolysis of H20z, in the measurement of the rate of initiation of photolytic reactions in the presence of Fe3+ ions. In addition to the generation of Fe’+ suggested by Behar and Stein [54] (eqn. (25)), the authors [55J assume that Fe’+ is also formed according to [FeH0,]2’

+ [FeOH]‘+

-

2Fe2* +0 2 +H,O

(27)

It seems somewhat surprising that initiation with Fe’+ and OX particles arising in a product of the hydrolysis of Fe3+ ions, is not taken the photolysis of [FeOH]‘+, into account in the literature: [FeOH] 2fhv\Fe2+

+ OH

(28)

A pronounced catalytic activity of [Fe”‘EDTA]with respect to H202 photolysis was detected by Kachanova and Kozlov [56]. They explained the photocatalytic effects observed by them using a mechanism which is an analogy of views put forward by Baxendale and Wilson [13] in describing photocatalytic effects of copper(H) ions (see Section 2.2.2): [Fe”‘(EDTA)] [Fe”(EDTA)

- + HO;

-

J”- I- Hz02 -

[Fe”(EDTA)]‘[Fe”‘(EDTA)-

+ H+ i- 0,

(2%

f OH-

(30)

+ OH

The authors [56] also observed a pronounced inhibiting effect of EDTA on the photolysis of H202 but were not able to account for this experimental finding. In ref. 57 the photocatalytic effects of [Fe’Ir(EDTA)]are explained by the photochemical generation of a thermally active iron complex. Lunak and Veprek-Siska [58] report the photocatalytic effect of potassium ferrioxalate cm Hz02 photolysis initiated by 365 nm radiation absorbed virtually only by this complex. Ferrioxalate was photolysed to iron( which proved to persist for a

6 relatively long time in the reaction solution. The kinetics of the reaction was schematically explained by the following mechanism: (a) photochemical generation of the catalyst Fe(III} hv.-ce_I,Fe(H)

(31)

(b) catalysed thermal decomposition 2H,02

=

of H202

2Hz0 -t O2

(32)

(c) decay of the catalyst by thermal reaction Fe(B) -‘,

Fe(III)

(33)

This mechanism enables basic features of the photolysis of Hz02 catalysed by iron(II1) to be explained: (1) high quantum yields (@+ 1); (2) photocatalytic effects of ferric ions; (3) the fact that photolysis can be initiated by radiation that is not absorbed by hydrogen H202 itself; (4) the inhibiting effects of organic compounds, especially those which form strong complexes with catalytically active Fe2+ ions. 2.2.2. Copper compoundr As will be documented in Section 2.2.4, copper ions are the most effective catalyst of HzOz photolysis known so far. The dependence of the quantum yield of photolysis on the addition of copper(B) ions can be described [59] by @= 6.76 x 103[Cu(11)]o.46

(34)

The effect of copper sulphate was detected already at a concentration of approximately 1O-7 M [60]. A classical study dealing with the catalytic effect on HzOz photolysis is the paper by Baxendale and Wilson [13]. They described the mechanism of the photocatalytic effect in terms of eqn. (1) and Cu=+ +HO 2- -

Cu++Oz+HzO

Cu+ +H,O,-

Cu2+ + OH-

Cu+ +HO;-

Cu2+ +HO,-

(37)

OH’+HO;-

H,O + O2

(38)

(35) + OH

(36)

Kinetic consequences of the mechanism suggested by Baxendale and Wilson were investigated by Muhammad and Rao [60]. According to their results, the quantum yields of H202 photolysis depend on the square root of the radiation intensity, which they regard as proof of the correctness of the suggested mechanism. However, a later study [61] shows systematic deviations from this dependence. Berdnikov and coworkers [62-64] also started from the mechanism suggested in ref. 13. They calculated the rate constants for reactions (35) (k35 =3.75 X lo6 L mol-’ s-‘> and (38) (k38=7.3 x lo3 L mol-’ s-l), but convincing evidence in favour of the proposed mechanism was not provided. The reaction between copper(I1) ions and the HO; radical formed in Hz& photolysis was investigated by Meisel et al. [65]. The radical trap used was thorium(IV), Stopka [66] used the EPR method to which forms a stable complex ETh-H0z*14’.

7

investigate the equilibrium between the monomeric and dimeric forms of photocatalytically active copper ions. An investigation was also made 160, 611 of the effect of the concentration of copper(H) ions on the apparent activation energy of the photochemical decomposition of H202. According to ref. 61, the apparent activation energy is 23 kJ mol-’ for the reaction without catalyst added; with 1 X 10e6 M Cu(II). it amounts to 20 kJ mol-I, while for 1~10~~ M Cu(I1) the value is 15 kJ mol-‘. This is in a good agreement with the results introduced in ref. 60, where the apparent activation energy of HzOz photolysis with the addition of 9X 10m7 M Cu(I1) is 19 kJ mol-‘. Lunak et al. [61] proposed the mechanism of eqns. (39)-(42) as an alternative to the views put forward by Baxendale and Wilson. They start from the observed instability of copper in oxidation state 2 + in an aqueous solution of HzO, and assume that the added copper ions are oxidized to catalytlcaIly inactive copper(II1) ions, which in turn undergo photochemical reduction to unstable, but catalytically highly effective copper(H) ions. The whole process can be schematically represented by three steps: (a) photochemical generation of copper in the catalytically active oxidation state Cu(II1) + e-2

Cu(I1)

(39)

(b) catalysed thermal decomposition H Cu(II)

+ 2H,02

6

\

O-0..

of Hz02

1671

AH

Cu’

‘H +

Cu(I1)

+ C& + H,02

+

OH-

(40)

‘o-o’ (c) oxidation of copper(I1) to the catalytically inactive form Cu(I1) - e- -

Cu(II1)

(41)

A question has been put forward [5, 591 following the detection of photocatalytic effects of very low concentrations of copper(I1) ions on HzOz photolysis, namely whether the photoinitiated decomposition would proceed with high quantum yields (@p> 1) without the participation of photocatalytically active transition metal ions. It was found that the photolysis of H202 is affected already by such concentrations which may be expected in the background of every real reaction system [59]. Both experimental results and reported data lead us to the conclusion that in the absence of photocatalytically active transition metals the photolysis of H202 would not proceed by the chain mechanism (@< 1) and that the kinetic chain length is proportional to the concentration of catalytically active copper(I1) ions. 2.2.3. Cobalt compkes Varfofomejev ef al. observed photocatalytic effects of bromopentaarnminecobalt(II1) bromide in the photoinitiated oxidation of dyes (alizarin S) with H202 [68, 691. These catalytic effects are reported in the literature [70,71] as a typical example of photocatalysis where the catalyst is generated by a photochemical change in the oxidation state of the transition metal, It is assumed that Co** ions formed by irradiation in the domain of the charge transfer band are the catalytically effective species: KNNH,),Brl Co’+

‘+hYrCoZC

-t-5NH,+Bi

(42)

It has been shown that in the case of Hz02 photolysis such a view is not correct: ions hardly catalyse the thermal decomposition [72, 731. For this reason the

8 effect of halogenpentaamminecobalt(II1) complexes on the thermal and photochemical decomposition of H202 was investigated [73]. It was found that the photocatalytic effect is exerted only by the bromopentaamminecobaIt(III) and chloropentaamminecobalt(II1) ions. These effects can obviously be explained by the following sequence: [Co(NH&X,‘+~ [CO(NH&]~+

WWHAO,I’+

[Co(NH,),j2+ + O2 -

+X

(43)

]Co(NH&O8+

+ [CWNW,12+ -

(44) P(~3)502co(~3)5P+

(45)

t-x

The photocatalytic activity of halogenpentaammine complexes is associated with the photochemical reduction of cobalt(III) to cobalt(II) proceeding via photoinduced electron transfer from the halide to the central cobalt atom, accompanied by formation of the spin-paired configuration (t2J6(e,) (*E), which according to Valentine [74] is stabilized by reaction with H202_ The metastable state of the pentaamminecobalt(I1) complex is probably an active intermediate 17.51. 2.2.4. Compouncis of the other transition metats Besides the effects of iron, copper and cobalt compounds discussed above, only very few data exist which would inform us which other metal compounds affect the photochemical decomposition of H202. Alexander and Roundhill [76] observed the photocatalytic effect of Re2(CO)r0 on H,02 photolysis and suggested Scheme 1. The photocatalytic effects were investigated both in water and in organic solvents. The authors point out that should the Haber-Weiss mechanism be valid, free hydroxyl radicals would arise in solution, which in the case of work carried out in a two-phase benzene-water system would be followed by the formation of phenol and diphenyl. They report that only 0.3% benzene at most was converted into phenol, while no biphenyl was detected. Hence it cannot be decided [76] if the Haber-Weiss mechanism does indeed play the decisive role in the system. The effect of uranyl salts on the photolysis of H202 was also studied [77-SO]; this was due to an attempt to use this reaction in actinometry [77, 791. For a description

hv

Scheme 1.

9

of the reaction mechanism

Havemann

H202 + UO,= + S

[Hz02-U02]’

[H,O,UO,]=

[UOz”+--H20]

+ -

er al. [77] proposed

+

(46)

+ 302

(47)

Folcher et al, [SO] assumed that in sulphuric acid the photochemically formed radical was stabilized by complexation with UO,“+ to an intermediate HO; which may be employed in the oxidation of various organic compounds_ [UOz-HO;]‘+, The effects of 3d transition metal ions on the quantum yields of Hz02 photolysis were also studied 1813 and compared with the effect on the photoinitiated hydroxylation of salicylic acid. It was stated that pronounced photocatalytic effects were observed with ferric and cupric ions only. The iron(III) ion, which effectively catalyses the photochemical hydroxylation, has only a weak effect on the quantum yields of photolysis, while copper( the most effective homogeneous catalyst of H202 photolysis detected so far, accelerates the photoinitiated hydroxylation only slightly_ This means [75, 811 that the catalytic effect of transition metals is not restricted to the formation of free hydroxyl radicals (in such a case both photolysis and hydroxylation would be accelerated simultaneously) and that an important role in the decomposition or hydroxylation is played by reactions taking place in the coordination sphere of the catalysing ion of the transition metal. The above-reported data contain controversial views on the mechanism of photoinitiated decomposition of Hz02. The controversy can be overcome by assuming complexes of H202 with catalysing transition metals to be reactive intermediates. 2.2.5. Quantum yields and photocatalysis In most cases the existence of high quantum yields (@s- 1) is regarded as proof of the radical chain mechanism, whiie the value of the quantum yield is seen as a measure of the kinetic length of the radical chain. In those reactions where photocatalytic effects are produced by transition metals present in trace concentrations, the existence of high quantum yields is not proof of the chain mechanism. Let us consider for example the photolysis of H202 sensitized (and photocatalysed) by ferrioxalate. We define: (a) the quantum yield of formation of catalyst

@Fe2+ =

d[Fe*+]/dT

~

(48)

abs

where Iabs= d(hv)/dT is the light absorbed i.e. ferrioxalate; (b} the total quantum yield of Hz02 @Hzo*=

by the photocatalytically

active component,

photolysis

d[HzO&d~ 1

where J=d(hv)ld~ is the light absorbed The total quantum yield is directly if I=labs, because d[HzOz]/dT @mo2 Vc= Kz = d[Fe*+]/dT

by the reaction system. related to the mechanism

of photolysis

only

(50)

Here V, indicates how many H,O, molecules are made to react by one Fe2+ ion formed by photochemical reduction of Fe’+, i.e. V, gives the number of catalytic cycles.

In other words, v, is an analogy of the kinetic chain length considered in the radical chain mechanism. The observed dependence of total quantum yields of H202 photolysis on trace amounts of copper ions shows that the measured quantum yield values are determined by the amount of photocatalytically active transition metals in the background of the reaction system. High values of quantum yield (@+ 1) are neither evidence of the chain mechanism of the reaction nor a measure of the kinetic chain length. The quantum yields of H202 photolysis are therefore determined not only by the freeradical propagation cycle, but mainly by the concentration of photocatalytically active transition metals, by the quantum yield of photoinitiated formation of the catalyst of the thermal reaction and by the number of catalytic cycles proceeding on the photochemically generated catalyst of the thermal reaction. 2.3. Heterogeneous photocatulys~ A considerable influence of solid particles on the photolysis of H202 has been known for a long time [82-851. Recently, the interest focused on this region has been predominantly motivated by an effort to utilize semiconductors (particularly TiOz) as photocatalysts in technologies concentrated on the photodestruction of pollutants in waste-water. H202 is either directly added to the system or is formed in the photoinitiated oxidation of water or in the reduction of dioxygen on the semiconductor surface [W-91]. Unlike Salvader and coworkers [92-941, Norton et al. [95] assume Chat H202 does not participate in the rate-determining step in the photo-oxidation of water on the surface of TiOa, being only a side product. A comparative study has been made between the efiect of UV radiation on H202 photolysis photocatalysed by graphite and that photocatalysed by lead dioxide 1961. Mazurkewich et al. [97] investigated the influence of the different crystallographic modifications of lead oxide on its photocatalytic activity in H,Oz photolysis. Some reference to the effect of other metal oxides and hydroxides on the photoinitiated decomposition of H,O, can also be found in the literature. H202 adsorbed on the surface of magnesium oxide and exposed to UV radiation gives rise to HO; radicals ]98]. Shub et aE. [993 described the photocatalytic effect of a suspension of iron(III) oxide on H202 photolysis. Sviridov and Schevchcnko [loo] have compared the effects of oxides and hydroxides of zinc, titanium and zirconium: (1) inhibitors of HzOz photolysisZn(OH),, ZnO (do not catalyse thermal decomposition either), Zr(OH)Z and Ti(OH)4 (catalysts of thermal decomposition); (2) catalysts of photolysis-ZrOz and Ti02 (anatase); (3) an ineffective compound - TiO;? (rutile). The kinetics of oxygen formation on the surface of TiO, was investigated by Augugliaro et al. [go]. The best description of experimental data obtained for a solution bubbled through with helium they see in kinetics of the Langmuir type: dc -

d7 =k4=

WH202]

1+

IC[H&bl

(51)

Here k is the first-order rate constant for the heterogeneous photochemical decomposition coverage of HzOz on the TiO;! surface and K is the of HzO~, + is the fractional absorption constant for H202 on TiO *. The authors [90] report very good agreement between experimental results and calculated values and conclude that the homogeneous decomposition of HzOz plays no important role in this case. They explain the decomposition by employing the following mechanism:

11 TiOl(h+

TiOz 2

f e-)

(52)

0,+2H+

&O,(ads)

+ 2h’

-

H202(ads)

+ 2W

+ 2e- -

(53) 2H,O

(54)

In a solution bubbled through with oxygen, the rate of H202 photolysis was of zero order with respect to H202, The release of oxygen during irradiation of an aqueous suspension of TiO, has been studied in detail [lOl--1061. Jenny and Pichat [106] showed that the rate of the photocatalysed reaction could be fitted numerically using the Langmuir-Hinschelwood mechanism. It can be stated in general that the photolysis of Hz02 in the presence of semiconductors (also the other TiOZ forms) obviously deserves better attention than that focused on it so far. A deeper insight to these processes could assist in finding optimal conditions for the photocatalysed oxidative destruction of pollutants.

3. Pbotoinitiated

reactions with organic and inorganic

substrates

Photochemical reactions of H202 with various compounds have been reported in many papers. Most of these studies, however, report only particular and often merely qualitative data. Deep mechanistic studies are completely lacking. This section provides a survey of the current state of knowledge in the field of photochemical reactions of particular types of compounds with H202. 3.2. General features of the mechantm of hydrogen peroxide reactions As has been discussed in Section 2.1, most photochemical reactions of H202 are assumed to proceed via formation of the hydroxyl radical. Three further possible reactions of this radical have been reported in the literature: (a) abstraction of hydrogen atom RH+OH’-

R’+ H,O

(b) addition to double

(55) (triplet) bond

H+

)c=c<+oH--

(56)

(c) electron transfer Me”‘+OH

-

Me(““)+OH-

(57)

Ingold [107] reports values of the rate constants for the reaction of OH’ radicals in aqueous solution with several typical inorganic and organic substrates. Their values range over a wide interval (8 X 106--8 X lo9 L mol-’ s-l). The following reactions are regarded as the most probable consecutive reactions of radicals K formed in reaction (55) or (56): (a) dimerization R’+R-R’

R-R

(58)

(b) reaction with dioxygen R-+0,

-

RO;

(5%

12 with the subsequent ROz’+RH

-

reaction

ROzH + R

which together with reaction (59) forms a propagation mechanism of the oxidation of hydrocarbons; (c) reactions with transition metal ions (i) oxidation of the radical R R+Mn”+

-

R+ + Mc(“-I)+

with the subsequent R+-t-H,O-

(60) cycle in the radical chain

(61)

reaction

ROH+H+

(62)

leading to formation of the hydroxyl derivative; (ii) reduction of the radical R R+

Me”’

-

R-

+Me(n+‘)+

with the subsequent R-+H20--+

(63)

reaction

RH+OH-

(64)

leading to regeneration of the substrate; (d) isomerization of the molecule. This group represents a broad range of reactions such as displacement of groups along the carbon chain, elimination (e.g. decarboxylation), opening or closure of the ring, etc.

3.2. Aromatic compounds 3.2. I. Aroma tic hydrocarbons Photochemical reactions of the simplest aromatic compound, i.e. benzene, with HZO, were investigated [105-l 161 on the one hand as a model for reactions of more complicated aromatic compounds and on the other hand from an ecological point of view. Addition of the hydroxyl group to the benzene ring leads to the formation of phenol, which, however, is reactive and subject to subsequent oxidation, and the formation of biphenyl and 2,4-hexadiendial was also observed. Loef and Stein [109] showed that the composition of the products is considerably dependent on the presence of dioxygen and on the pH. In the presence of dioxygen the main product formed in a neutral medium is 2,4-hexadiendial, while in an acidic medium it does not arise at all. According to the same authors, in the absence of dioxygen the predominant products should be phenolic compounds. Jacob et al. [llO] investigated the mechanism of the reaction between benzene and HzOz in the presence of dioxygen with Fe3+ or Cu’+ added. They suggested Scheme 2. As can be seen, three reaction products were found: phenol, 2-hydroxy-2,4hexadiendial and 3-hydroxy-2,4_hexadiendial. Their ratio depends, among other things, on the intensity of photolytic radiation [IlO]. The authors explain this finding by a transition from the chain mechanism to the non-chain mechanism (see Section 2.1); a change in the mechanism should take place at a radiation intensity of 3X 1Or7 quanta L-l s-l. An interesting experimental finding can be seen in the different products arising as a result of the addition of copper(U) and iron(IX1) ions; in the presence of coppcr(II), only phenol is formed. In connection with this, the authors try to find

Scheme

2.

out whether the view of the decisive role played by the free hydroxyl radical in the reaction mechanism is justified or not. In addition to the photoinitiated reactions of H202 with benzene, reactions with some other aromatic hydrocarbons were also studied: toluene [llI, 114-1181, xylenes [114, 1161, styrene [1143, naphthalene [119, 1201 and 2,3-dimethylnaphthalene [120]. Jefcoate er al. [ill] investigated the oxidation of benzene and toluene initiated by UV radiation. They focused their attention on the one-electron oxidation of the hydroxycyclohexadienyl radical (substituted in some cases) and compared the rate of oxidation with that of other ways of decay of this radical. The main products were phenol and biphenyl. Their ratio is markedly dependent on the pH of the reaction solution. The dependence observed is explained [ill] by the fact that phenol may be formed by the disproportionation of two hydroxycyclohexadienyl radicals, which unlike dimerization should be an acid-cafalysed reaction. The other fact found by the authors [ill] was that the ratio of the concentrations of u-, m- and p-cresols arising in the UV-initiated reaction between toluene and HzOz is 55113~32,which hardly differs from their ratio in the classical Fenton reaction (55.5:15:29.5). The yield of the cresols decreases distinctly with decreasing pH. Other products of the photoinitiated reaction are phenol, benzaldehyde and benzyl alcohol. The effect of FeQ, K,[Fe(CN)6] and N+[Fe(CN)sNO] on photochemical reactions of HzOz with benzene was investigated [753. In a reaction initiated by UV radiation, FeCI, markedly raises the phenol content in the products, i.e. it cataiyses hydroxylation, while the cyano complexes catalyse photodestruction of the benzene ring. In a reaction initiated by visible light of wavelength 589 nm and photosensitized by methylene blue, there was also a dramatic difference in the photocatalytic activity of individual iron compounds [75]. Reference 116 presents an investigation of the photo-oxidation of aromatic hydrocarbons by H202 and on the surface of TiOa. In connection with the widely accepted view that H,O, is a reactive intermediate of photoinitiated reactions photocatalysed

14

by Ti02 (see Section 2.3), the observed sequence of lifetimes of the particular substrates irradiated by light of wavelength above 290 nm seems quite surprising: with the addition of 3 X 10m3 M TiOz the sequence is cumene Cm-xylene
e-h+ -

Cu

(6%

cu2+

(66)

By using a photochemical reaction between naphthalene and H,O, it is possible 11191, at minor concentrations of Hz02 (2X lop3 M), to obtain a mixture of l- and 2-naphthol. At higher concentrations, hydroxy- and dihydroxybenzoic acids are formed (Scheme 3).

HO OH

OH

Scheme

3.

15 The photolysis of naphthalene, l-methylnaphthalene and 2, 3-dimethylnaphthalene in a micelIar system of sodium dodecylsulphate-cetyltrimethylammonium chloride in the presence of H202 was also studied [lZO]. 3.2.2. Phenol and its derivatives During the hydroxylation of phenols the ortho and para positions are attacked first [121]. The ortho position is preferred owing to the electrophilic character of OH”. Similar results are also obtained in acetonitrile [122]. A comparison was carried out [123] of the reactivity of substituted phenols in their photochemical interaction with H202. The reactivity decreases in the following order: p-Ph >p-AC, p-Me, p-Cl >pCOOH, o-NOz, p-CN, p-r-Bu > m-COOH, p-NC& > 2,4-(COOMe),> 2,4-(NO& Podzorova and Bychkov [124, 1253 studied the interaction of phenol with Hz02 initiated by simultaneous irradiation of the reaction mixture with UV and y rays. They observed a pronounced synergistic influence: the eventual rate was as much as three times higher than the sum of the rates in the case of separate UV and y irradiations. Reference [75] deals with an investigation of the effect of iron compounds on the interaction between phenol and Hz02 initiated with UV and visible irradiation. The composition of the coordination sphere of the catalysts was found to considerabiy affect the selectivity of reaction. This effect is similar for both UV and visible irradiation: in the presence of FeC13 and Fe’“(EDTA), catechol is the main product, while with ferricyanide or ferrocyanide, benzoquinone predominates_ Phenol is a toxic compound which appears in waste-water from a number of sources, e.g. oil refining, electroplating, iron and steel factories. Owing to the difficult degradation of phenol by the usual methods (e.g. biological processes), several authors regarded (and investigated) as promising photochemical reactions between phenols and Hz& [112, 114, 115, 126-1331. Castrantas and Gibilisco [127] examined the effect of pH on the reaction rate. Ho 11261 suggests the following scheme for the degradation of phenol: phenoI2

-

catechol Hz02 hydroquinone

G-G

-

carboxylic acids and aldehydes

pyrogallol 1,2,4-benzenetriol -

carboqlic G-G

acids

and aldehydes

(67)

Recently, attention has been focused on the role played by H20Z in the photooxidation of phenol on the surface of semiconductors such as zinc oxide [128, 1293 and titranium dioxide [130-1341. Peral et al. [128J demonstrated that irradiation of ZnO suspension in an aqueous solution of phenol leads to the formation of H202, the concentration of which decreases with increasing concentration of phenol. The authors arrived at a conclusion that the experimentally observed decrease in the apparent rate constant of decomposition of phenol with its increasing initial concentration can be explained in the following way. At low phenol concentrations (eg. 10e4 M) oxidation proceeds with the participation of hydroxyl radicals, which is a very fast reaction (kapp= 1.4 ~10~’ L moI_’ s-‘)_ In more concentrated solutions the rate of generation of hydroxyl radicals becomes the limiting factor and phenol is then oxidized also in photogenerated holes on the ZnO surface, which is a slower process. In connection with this explanation, however, it should be stated that also in the reaction between Hz02 and phenol in the homogeneous phase the curve representing the dependence of the degraded phenol on the concentration of H202 added to it passes through a maximum [114].

16

Kawaguchi and Uejima [129] observed that in the photocatalysed decomposition of phenol in an aqueous suspension of ZnO saturated with oxygen (air), hydroxyl radicals are formed not only through holes on the surface but also via H202 formed from O;- _ They report that reduction of oxygen to H202 is thermodynamically feasible in photocatalytic processes but its efficiency is rather weak. Augugliaro et al. [130] investigated the photodegradation of phenol on the surface of TiOz (anatase) in the presence of H202. They assume that in the reaction suspension bubbled through with helium, two radicals are formed from H202 which are able to react with the adsorbed phenol: HaOz(ads)

+ h’

-

H202(ads)

+ e- OH

HO;+H+

(68)

OH- + OH-

(69)

.-

CJ%OH(ads) +HO

intermediates

-Co,+H,O

(70)

2

Wei et al. [134] showed that in the photodestruction of phenol on the surface of irradiated TiOz in the absence of H202, ferric ions slightly raised the rate of decomposition while cupric ions had an inhibiting effect. However, both metals had a pronounced catalytic effect if HZO, was added. Sclafani et al. [133] studied the effect of ferric, ferrous and silver ions on the photocatalytic degradation of phenol on the surface of TiOz (anatase and r-utile). The effect of Ag+ is explained by Ag + HaOz(ads)

OHtH,O+Ag”

f H,, -c -

(71)

3.2.3. Carboxylic acids Aromatic carboxylic acids are attacked by the hydroxyl radical mainly on the aromatic ring; this is accompanied by the formation of hydroxyl derivatives. The formation of phenols and benzene is probably a consequence of the splitting off of hydrogen from the carboxylic group followed by decarboxylation. The subsequent processes involve further oxidation accompanied by opening of the aromatic ring and formation of aliphatic carboxylic acids [6]. 3.2.3.1. Benzoic acid Boyland and Sims [119] showed that in the photochemical hydroxylation of benzoic acid with H,O, a mixture of Z-, 3- and 4-hydroxybenzoic and 2,5- and 3,4_dihydroxybenzoicacids is formed in the molar ratio l&5:5:2:8. The mechanism of the reaction was investigated in greater detail by Ogata et al. [135]. They showed that at the initial concentration ratio [H,O,f,/[C,H,COOH], < 10, hydroxybenzoic acids, benzene and phenol predominate among the products. At higher ratios (greater than 25) the benzene ring is destroyed and acetic and formic acids are formed. Maleic, malonic and oxalic acids were also detected as intermediates. The formation of hydroxybenzoic acid is explained 11351 by two consecutive reactions: CJISCOOH

+ OH

[HOGH&OOH]-

+ OH

[HOGH,COOHr -

HOC,&COOH

(72) + H,O

(73)

Reference 136 reports pronounced photocatalytic effects of FeCl, and NaZIFe(CN),NO] on the hydroxylation of HzOz initiated by UV radiation. The authors came to a conclusion that the reaction proceeds via several parallel mechanisms: (a) Radiation is absorbed by H,O,; this is followed by its dissociation accompanied by the formation of free hydroxyl radicals (eqn. (1)) [43 which attack the benzene ring.

17

(b) Radiation is absorbed by benzoic acid. Similarly to ref. 135, energy transfer is assumed to proceed from the excited state of benzoic acid to H202 followed by subsequent dissociation of the latter accompanied by the formation of hydroxyl radicals: C.$&COOH=

(GH,COOH)*

(C&COOH)*

(74) &H,COOH

+ H202 -

+ 2 OH

(75)

(c) Radiation is absorbed by benzoic acid which acts as sensitizer of the reduction of photocatalytically active iron(III) compounds to catalytically active iron(I1) compounds and (77)). Direct photochemical reduction is rather unlikely owing (eqns. (74) (76) to the low content of radiation absorbed by iron complexes [136]. (CeH5COOH)* (Fe(III)L)*

-t Fe(III)L

-

Fe@)

-

GH,COOH

+ LX

f (Fe(III)L)*

(76) (77)

The formation of iron@) in the reaction system containing H202 is in fact generation of Fenton’s reagent. In agreement with the suggested mechanisms (a)-(c) above, the detected [136] quantum yields of hydroxylation increase with increasing concentration of iron(II1). If the prevailing part of the photolytic radiation is absorbed by H202, photocatalytic effects become less pronounced. Reference 137 reports the possibility of hydroxylation of benzoic acid by H202 using the photochemical formation of Fenton’s reagent initiated by 589 nm radiation and sensitized by metliylene blue. The ferric compounds used (FeCL, K3[Fe(CN),] and Na,[Fe(CN),NO]) p ossessed a comparable photocatalytic activity. The content of hydroxybenroic acids thus formed in the reaction products was quite high; this was particularly evident in a comparison with the UV-initiated reaction [136]. This is due to the fact that under such conditions direct photochemical excitation of hydroxybenzoic acids was virtually ruled out. Matthews [138] investigated the hydroxylation of benzoic acid and sodium benzoate on the surface of TiOa (anatase) during W irradiation. He came to a conclusion that the results may be explained by an attack of hydroxyl radicals on the aromatic ring. He assumed that these radicals might arise via cathodically generated [139] HZO, followed by three processes: reaction (1) (photodissociation), reaction (12) (Haber-Weiss) and the Fenton-type reaction H202 + Ti -3+

_

OH’+

OH-

+Ti4’

(78)

To obtain further information regarding such possibilities, an aqueous suspension of sodium benzoate was irradiated in the presence of various concentrations of H202. The fact that the HzOz added did not raise the rate of formation of salicylate ruled out the possibility that reaction (1) is the decisive reaction route. The reaction rate of reactions (12) and (78) is also insufficient [138]. Hence the main source of OH in the system under investigation are light-generated positive holes in the valency band of TiOa particles and not a mechanism involving H202. 3.2.3.2. Hydroxybenzoic acids It has been shown [140] that the rate and quantum yields of the photoinitiated hydroxylation of salicylic acid by HZO, may be considerably increased by adding iron(II1) compounds. The main product is 2,5dihydroxybenzoic acid. The addition of EDTA has an inhibiting effect; it was proved that this effect cannot be explained merely by an internal filtration of radiation but that it, is related to the formation of complexes between iron(III), which is present in the reaction

18

system as an impurity, and EDTA. It should be pointed out that the complex Fe’r’(EDTA) thus formed is photocatalytically less active than ferric ions. References 75 and 81 deal with a comparison of the effect of the 3d transition metal ions on the photoinitiated hydroxylation of salicylic acid and on the photolysis of H202,. The results have been discussed in Section 2.2.4. The possibility of photosensitizing the hydroxylation of salicylic acid by HZ02 during irradiation with visible light is described in ref. 141. The reaction is extremely sensitive to additions of both a photosensitizer (methylene blue) and a photocatalyst (ferric ions). The influence of these reaction components becomes apparent already if they are added in an amount of about lo-’ M. The influence of temperature on quantum yields, so far very little investigated for photocatalysed reactions, is also discussed in ref. 141. It was found that quantum yields of photosensitized reactions increase markedly with increasing temperature. This seems to indicate that the ratedetermining hydroxylation step is a reaction possessing a considerable activation energy. An attack of the hydroxyl radical on salicylic acid can be hardly regarded as such a reaction. The composition of reaction products in the salicylic acid-H,O, reaction mixture in a photochemical reaction depends on the way of initiation. If the mixture is initiated by W radiation, 2,%dihydroxybenzoic acid is mainly formed [140]; if visible light is used for the initiation and methylene blue is the photosensitizer, 2,3-dihydroxybenzoic acid is the main product [141]. The energy transfer in the salicylic acid-methylene blue-ferric ions system was studied by means of laser flash photolysis 1142-1443. It was found that small additions of salicylic acid to an aqueous solution of methylene blue increase the lifetime of MBH*+ (T,). A d ramatic discontinuity in the dependence of lifetime on the amount of salicylic acid added was observed near the concentration 6 x lo-’ M, which corresponds to pH 4.3 of the reaction mixture. This pH is very close to pK, of the leucoform of methylene blue. A similar lifetime of MBH’+(T,) vs. pH dependence was obtained for pH changes brought about by an addition of perchloric acid. In contrast, no pronounced effect of HzOz added to or of oxygen dissolved in the reaction mixture on the quenching of any of the excited states of methylene blue under investigation could be found. The added ferric ions do not affect the concentration of MBH’+ (T,) but do cause a very substantial change in the concentration ratio of these particles and MB’(T,) particles. If without F&l3 added the ratio is 2.1, than e.g. in the presence of 1.6x 1o-4 M FeC13 the ratio increases to over 1000. The formation of photocatalytically active particles (ferrous compounds) may be explained by reaction (791, a competitive reaction with respect to the formation of MB’(T,) (reaction (SO)): MB’(T,)+ MB + (TZ) -

Fe3+. . . Sal MB+ CT,)

MB’*

+Fe*+

. . - Sal

(79) (SO)

By a photochemicaI interaction with Hz02, 3-hydrovbenzoic acid yields [119] a mixture of 25 and 3,4_dihydroxybenzoic acids. In acetonitrile [123], 2,3_dihydroxybenzoic acid is the main product. Formed dihydroxybenzoic acid is further oxidized. Ogata et al. [135] offer Scheme 4 as an adequate description of the photoinitiated interaction between 2,3_dihydroxybenzoic acid and H,O,. 4-Hydroxybenzoic acid is also photochemically hydroxylated by Hz02. In aqueous solutions 11221, 3,4_dihydroxybenzoic acid is predominantly formed (yield 38%): the same acid predominates in acetonitrile [123] but its yield is lower (29%). Kenji and Noritsugu [1453 investigated the photo-oxidative degradation of 3,4dimethoxybenzoic acid in the presence of I&O,. Demethylation is followed by the

19

C02H

COZH

CO2H I

Scheme 4. TABLE

2

Photochemical reactions of hydrogen peroxide with hydrocarbons high pressure mercury arc Substrate

Products

n-Octane

1-Octanol, 2-octanol, 3-octanol, 4-octanol, unidentified products

Cyclohexane

‘Cyclohexanol, cyclohexanone, cyclohexyl acetate

Cyclododecane

Cyclododecanol, cyclododecanone, ck-bicyclo(8_2.0)dodecan-l-01

in ethyl acetate initiated by

hydroxylation of the aromatic ring, with a subsequent opening of the latter. The acid is used as a model compound in the investigation of lignin. Dicarboxylic acids also react with E&O2 when irradiated. Thus, for example, a mixture of monohydroxybenzoic, 2,5- and 3,4-dihydroxybenzoic acids is formed from phthalic acid [119]. 3.3. Aliphatic compounds 3.3.1. Hydrocarbons Data on the photoinitiated oxidation of alkanes and cycloalkanes are very scarce. Sharma et al. [146] studied the photoinitiated oxidation of n-octane, cyclohexane and cyclodecane with HzOz in ethyl acetate solution, but their investigation concerned only on analysis of the products. The results are illustrated in Table 2. There are also a few studies dealing with photochemical interactions between unsaturated hydrocarbons and H+Os 1147, 1481. Kelkar and Sonawane [147] showed that in acetonitrile, addition of the hydroxyl group predominates over alIylic oxidation. Sauer et al. [148] regard atomic oxygen O(‘P) as the reactive intermediate of the photochemical interaction between H202 and cyclopentene. 3.3.2. Alcohols Primary and secondary alcohols undergo a photochemical reaction with H202, giving rise to aldehydes (reaction (81)) or ketones (reaction (82)) and dimerization products of radicals formed (reactions (83) and (84)):

20

R-CH,OH RIR,CHOH 2R,R,C’-OH

&+

R-CH-OH hl’

x

5

R1R2C-OH

-

R-CH= OH’

-_

0

(81)

R1R2C=0

032)

R,R,C(OH)C(OH)R,R,

(83)

Thus, for example, methanol gives formaldehyde and ethyleneglycol, while 2propanol gives acetone with traces of pinacol [149, 1501. For the oxidation of 2propanol a mechanism El511 has been suggested which is similar to the Haber-Weiss cycle in the photolysis of H20,: OH- + (CH&CHOH (CH&C’OH

-

+ Hz02 -

Hz0 + &X-I&C-OH

(84)

(CH&CO

(85)

+ Hz0 I- OH

In the presence of dioxygen, acetone is formed also by the oxidation of the radical (CH,),C’OH: (CH&C’OH

-+ O2 -

(CH&CO

-t HO;

(86)

The formation of hydrogen was detected in the photochemical excitation of HzOz in the matrix of 2-propanol by means of mass spectrometry [152, 1533. The mechanism of its formation was explained by using eqns. (I), (84) and (CH&C’OH

=

H- + (CH&ZHOH

(CH,),CO -

H2

f

+ IF

(87)

(CK)2COH

(88)

Lagenkratz [154, 1551 proved the formation of hydrogen atoms from methanol under UV irradiation in the presence of Hz02 by using an EPR investigation of trapping on a-nitroso-P-hydroxy compounds. The EPR technique was also used to prove the formation of the ‘CH,OH radical from methanol [156-1591 and of the ‘CH-JJHOH radical from ethanol 1158, 1591 during their photochemical interaction with H202_ It was proved that the splitting off of hydrogen from the a-carbon of ethanol is much more probable than from the ficarbon. Attention was also focused on a study of the photochemical interaction between 2-propene-l-01 and hydrogen peroxide [160-1641. The reason may be sought in the application of this compound as a radical scavenger in the photolysis of H202. In this case too the first step consists of the splitting off of hydrogen from the a-carbon accompanied by the formation of two stereoisomeric radicals differing considerably in intensity of EPR signal. In the photoinitiated interaction between H202 and 2-propine-l-01 the first step also is the splitting off of hydrogen from the a-carbon [149]. The system was investigated kinetically and, assuming a stationary state, an expression for the concentration of the radical HC= C-Cl+-OH was derived. Data can be found in the literature on the possibility of cataiytic influence exerted upon photochemical reactions of H202 with Z-propanol. Paszyc investigated the effect of metallic copper [165], while Cundal et ai. [166] describe the photocatalytic effect of rutile. It is interesting that in a nitrogen atmosphere and in the presence of rutile, 2-propanol is oxidized almost quantitatively to acetone. The photochemical oxidation of 2-propanol to acetone can also be catalysed homogeneously [75, 1671. The effects of FeCl,, K,[Fe(CN),], Na,[Fe(CN)SNO], &[Fe(CN),] and &[Fe(GO&] were com-

21 pared. While FeCl, is virtually photocatalytically inactive, the cyano complexes raise the quantum yields by as much as three orders of magnitude. An attempt to use photoinitiated reactions of H,Oz for practical purposes is seen in the oxidation of D-sorb&o1 to L-sorbose [168], Shimizu et al. [169, 1701 reported the synthesis of ethyleneglycol from methanol bubbled through with nitrogen in the presence of H,O, using irradiation by a low pressure mercury arc or by a KrF laser. With irradiation by arc they reached the highest quantum yield e-0.73. In the case of laser irradiation the quantum yields decreased with increasing concentration of H202. High quantum yields (0.78-0.94) were found in the concentration range 6-13 vol.% H202. Selectivity of the formation of ethyleneglycol is virtually independent of the concentration of Hz02 and amounts to 96%-98% for 6-20 vol.% H202. 3.3.3. Cubmylic acids The first step in the interaction between carboxylic acids and HzOz is an attack on the alkyl chain of the acid, e.g. acetic acid yields the radical ‘CH,COOH, identified by EPR [171, 1723. In carboxylic acids with a long chain the y-carbon is the one predominantly attacked 11731. Photolysis of H202 in the presence of carboxylic acids when the concentration of H202 is many times higher than that of the carboxylic acid leads to complete oxidation of the acid to water and carbon dioxide [174]. 3.3.3.1. Formic acici The first attempt to elucidate the mechanism of photoinitiated interaction between HzOz and carboxylic acids was made by Baxendale and Wilson [13]. They found that formic acid in the absence of dioxygen causes a marked increase in the quantum yields of the photolysis of H202, while in the presence of dioxygen the same acid has inhibiting effects. This finding is explained by the authors [13] by assuming reactions (89)-(93). In the first:step the OH’ radical arising in the photochemical dissociation of H,O, reacts with formic acid, giving rise to ‘C02H: OH-+

HCOOH

-

Hz0 -t-‘COOH

(89)

(a) In the absence of dioxygen, %O,H reacts according to eqn. (90) with regeneration of the O)F radical, so that eqns. (89) and (90) form a propagation cycle, thus explaining the high quantum yields observed (@+ 1). Termination is represented by eqn. (91). ‘CC&H + Hz02 OH + ‘C02H

-

CO, + Hz0 -I-OH CO* + Hz0

(90) (91)

(b) In the presence of dioxygen the ‘COZH radical reacts according to eqns. (92) and (93). It can be seen that the quantum yield reaches the limiting value @-OS. *C&H HO;

+ 02 + HO;

HO; -

+ CO*

(92)

Hz02 + Oz.

(93)

3.3.3.2, Acetic acid and acetates Unlike formic acid, acetic acid reduces the quantum yields of H,O, photolysis both in the absence of dioxygen (eqns. (94)-(96)) and in its presence (eqn. (97) [13]: C&I- -t CH&OOH

-

2’CH,COOH

(CHzCOOH)2

-

H,O + ‘CHzCOOH

(94) (95)

22 OH. + ‘CH&OOH Hz0

+ 0,

-

+‘CH,COOH

HOCH,COOH

(96)

-

(97)

HO;

+HOCH,COOH

Ogata et aI. [174] suggested a detailed mechanism for the photochemical interaction between acetic acid and H202_ The reaction is switched off by the elimination of the hydrogen atom by the hydroxyI radical (formation of ‘CH&OOH or CH,COo’) or by a direct photolysis of the acetic acid molecule (formation of methane and carbon dioxide). The presence of glycolic, formic and pyruvic acids was also proved in the products [174]. The photochemical degradation of acetates in waste-water due to the reaction with H202 was investigated by Koubek [175]. He showed that if a low pressure mercury arc (radiation 254 nm) is used together with a slight excess of Hz02 with respect to acetates to be removed, the method is very efficient. 3.3.3.3. Other acids The prevailing products of the photochemical interaction between propionic acid and HzOz in the initial stage of the reaction are cr- and ppropionic acids, while in the later stages of the reaction acetic, formic and glycolic acids predominate 1174, 176, 1771. In the photoinitiated reaction of butyric, valeric and caproic acids with I-LO2 the hydrogen atom in the y position is predominantly split off, succinic acid being the main product in this case: R-CH2-CH2-CH2-COOH R-CH-CH,-CH,-COOH

+ OH + OH

R-CH(OH)-GE-I,-CHz-COOH

-

R-CH-CHz-CH,-COOH

-

+ H,O

(98)

R-CH(OH)-CH,-CH,-COOH

+ HzO,

-

R-CO-CH,-CH,-COOH

(99) + 2 Hz0

(100)

Shtamm et al. [178, 1791 investigated the role played by copper(I1) ions, a very efficient catalyst of the photolysis of H 20 2, on the photochemical interaction between Hz02 and ascorbic acid. They found that although the addition of copper(U) ions raises the total reaction rate during the irradiation, the difference between the reaction rate during irradiation and the rate of thermal reaction does not vary with increasing concentration of copper(R) ions added. This means that copper ions do not catalyse the photoinitiated oxidation of ascorbic acid by Hz02, This finding is in agreement with the view [180] that in this case the reactive intermediate is the superoxide radical. 3.3.4. Other aliphatic compounds Ghosh and Nandy [181] investigated the by H202_ They showed that this reaction is EPR was used in the investigation of the at 77 K [182] and the following mechanism CHSCOCHS CH,COCH; CH,‘+

-t OH z

CH,COCH,

-

CH,COCH;

-I- H,O

CH3’ + CH,CO -

CH,COCH,’

photochemical oxidation of formaldehyde catalysed by tungstic acid. photolysis of H,O, in the acetone matrix was suggested: (101) 002)

+ CH,

(103)

Ogata et al. [183] studied the photoinduced oxidation of diethyleneglycoldimethyl ether (DEDE), ethyleneglycoldimethyl ether (EDE) and ethyleneglycolmonomethyl ether (EME) by H,OZ. The photochemical oxidation and degradation of DEDE and EDE yield methanol, methoxyacetaldehyde, EME, methoxyacetic acid and methyl formate. The authors [183] suggest a mechanism represented by several tens of elementary

23

processes. They see the first step as the elimination of hydrogen by the hydroxyl radical from terminal CH3- groups and from -CH T groups inside the molecule. The ethers under investigation, which play the role of very effective non-ionogenic surfaceactive compounds, represent a grave ecological problem, because their biological degradation is very difficult. The suggested photochemical oxidation by H,O, is a fast and effective method which can be employed for their destruction. 3.4. Nitrogen cornpourtds and biological systems Although photoinitiated reactions of Hz@ with nitrogen compounds have been dealt with in many papers [l&l-253], a deep interest in the kinetics and mechanism of such reactions is rather rare in these studies. The adsorption of nitrogen on the surface of irradiated TiOz and the role played by H202 in this system are discussed in ref. 184. A thermal reaction of ammonia in an alkaline solution with Hz02 yields nitrate, nitrite, hydroxylamine and nitrogen. Radiation of wavelength 254 nm raises the rate of this reaction, but the composition of the products remains unchanged [185, 1861. In an acidic medium the rate of reaction between ammonia and HzOz is low 1185, 1861. The first step is seen as the splitting off of atomic hydrogen from the ammonia molecule [186]; the decrease in the rate in an acidic medium is attributed to the low value of the rate constant of interaction between the Of-F radical and NH,+ ion. Primary and secondary amines irradiated at 77 K give radicals of the R-NH’ or R2N type. The preferentia1 splitting off of the hydrogen atom from the nitrogen atom is related to the ratio between the energies of the N-H (392.3 kJ mol-‘) and C-H (414.5 kJ mol-‘) bonds. It has been shown that by employing a photochemical reaction with H202, a number of nitrogen-containing compounds can be successfully destroyed, while their removal from waste-water by other methods proceeds with great difficulty. These compounds are e.g. cyanides [188, lS9], hydrazine [190-1921, nitrobenzenes [193, 1941, 4-nitrophenol [195], trinitrotoluene [196], trinitrophenols [1971, azo dyes [198, 1991, pyridine [200] and sevine [ZOl]. It is surprising that according to ref. 188, HZ& does not raise the rate of photochemical decomposition of cyan0 complexes of manganese(II), titanium(IV), cerium(IV), (VO)(II) and copper(R). Also, it is note worthy that, under the conditions chosen by the authors of ref. 202, no positive influence of the addition of H,Oz on the photolysis of pyridine could be detected. Degradation of cyclic amines (uracil) proceeds by oxidative decarboxylation [203]. The photochemical interaction between several nitriles and H202 was investigated by employing the EPR method [204]. In all cases a radical arising from the splitting off of hydrogen from cr-carbon was detected. The splitting off of hydrogen from carbon in dipeptides proceeded between the -NH and -COOH groups [2OS, 2061. As has already been reported in Section 3.3.2, nitrogen compounds are used in EPR studies of photochemical reactions of H,O, [154, 155, 207-2111 as radical scavengers. Photochemical reactions between HzOz and biological systems were also investigated. Reactions with the following proteins to form a-amino acids of the NH,-CH(R)-COOH type were described: cystein [212], citruline [213], phenylalanine [214], arginine [21S] and phenylalanine in the presence of riboflavin [216]. Usually the reaction products are simple amino acids, ammonia and urea. Other studies deal with reactions of deoxyribonucleic acid (DNA) [217-2241 with H202 initiated by UV radiation. These studies are mainly concerned with the elucidation of the role assumed by hydroxyl radicals. It is known 12171 that H202 at concentrations above 25 JLM has a harmful effect on DNA. It was found that these concentrations are physiologically relevant;

24 it is assumed, moreover [217, 2181, that Hz02 acts indirectly by its decomposition on hydroxyl radicals. On the other hand, some experimental results [219, 2201 refute the view that photochemically arising H,Oz or radicals obtained by its photolysis play a decisive role in damaging cells of mammals. A similar question regarding the participation of I&O2 arises in the study of the mechanism of the influence of UV radiation on bacteria. With respect to the investigation of the origin of life on Earth, an important reaction is seen in a process opposite to the reactions discussed above, i.e. the photochemical synthesis of amino acids. The UV irradiation of a mixture of ammonia, glucose, HzOz and a catalyst (vanadium(V) oxide, molybdenic acid) gives rise to a mixture of glycine, alanine, lysine and aspartic acid [241]. Among other biologically oriented studies, one may mention photoinitiated reactions of Hz02 with compounds of the R,N+CH,CH(OH)R’Cltype (transfer of nervous impulses) [242] and with enzymes [243, 2443. 3.5. Sulphur and its compounds Elemental sulphur can be oxidized photochemically with l&O2 to sulphuric acid [254-2561. If the reaction is to proceed in an aqueous medium, compounds capable of a sufficiently fast reaction with intermediate radicals or of a photochemical production of such radicals must be present. The effect of alcohols (methanol, ethanol, n-propanol, n-butanol), tetrachloromethane and acetone was demonstrated. Iodine, which acts as a radical scavenger, has an inhibiting effect. Photochemical reactions of hydrogen sulphide, mercaptans and cysteine with H,O, were studied by employing the EPR method [212]. It was shown that in these cases the splitting of the S-H bond predominates (unlike alcohols, where the splitting off of hydrogen atoms in the (Y position is the main process). The photochemical reaction of C1C6H&HI,SCON(~H~)2 (tbiobencarb) was studied with respect to environmental protection 12571. This pesticide is very resistant to sunlight, but its photochemical degradation is considerably accelerated by the presence of H,O, (Scheme 5 [257]). Another ecologically important compound whose photochemical reaction with H202 has been investigated is parathion ((EtO),P(S)-0-C,H,-No,); p-nitrophenol is the main reaction product here [258]. Zhao and Back [259] investigated the photochemical reaction between rhodizonate dianion and H,O, in aqueous solutions. 3.6. Ha logen derivatives So far, not many studies have been devoted to photochemical reactions of halogen derivatives in the presence of H202. Some attention has been focused on these reactions only in connection with removal of pollutants [260-2701. Most authors have reported good possibilities of using the photoinitiated degradation of halogen derivatives with H202 during their removal from surface water. Problems involved in photochemical reactions of H20, in surface water are briefly discussed in Section 4. Kochany and Bolton investigated the mechanism of photochemical interaction of chlorophenols with HzOz by means of the EPR spin-trapping technique [271]. Using the known rate constants of OH- radicals with a spin trap (e.g. 5,5’-dimethylpyrrolidineN-oxide), they obtained the rate constants of interaction between OH radicals and chlorophenols. It was shown that the rate constants for 3-chloro-substituted phenols are distinctly lower than those for 4- or 2-chloro-substituted derivatives. Values of some rate constants of the radicals thus obtained were much higher than would

25

:: CH2fCN(Et)2 0

fi’

CL

cl

u0

.CH

2

CH,SCN(Et)2

S!

OH

N(Et

0 Cl

CH$NHEt

Cl

i CH2SCN(EtJ2 HO

Scheme 5. correspond to diffusion-controlled processes; the authors of ref. 271 explain this finding through the assumed Grotthus-type mechanism for the movement of OH radicals by water. 3.7. Phosphonis compounds There are only a few studies dealing with photochemical interactions between phosphorus derivatives and H202_ Phosphorus-containing compounds used as fertilizers and pesticides are considerably accumulated in the environment. This is undoubtedly the main reason for increasing interest in photoinitiated reactions between H,O, and such compounds [272-2753. Grgtzel et al. [276] showed that photoinitiated reactions between organophosphorus compounds and H202 can be effectively catalysed with TiOz.

4. Ecological

aspects of photochemical

reactions of hydrogen peroxide

4.1. Numral waters The problem of the purity of natural waters has recentIy become one of the paramount problems faced by mankind. The self-purifying capacity of many water streams has already been exhausted, so that many pollutants are accumulated in the circulation of water. One of the mechanisms by which pollutants are degraded in nature consists of reactions of H,02 initiated by solar radiation. The first problem that arises in this case is of course the occurrence of Hz02 in natural waters [277-2821; its concentrations vary considerably, depending not only on the locality but also on the time of day. It was demonstrated that the photochemical degradation of humic acids proceeds with sufficient efficiency under conditions existing in natural waters [280-2821, along with the degradation of fulvic acid [283], pesticides [284, 2851 and many other organic compounds [286J. An important role in photochemical reactions taking place in natural

26 waters is played by compounds of transition metals, particularly of iron [287] and copper [288]. Organic substrates are also effectively degraded on the surface of colloidal particles of iron oxides in natural waters [289]. For a number of selected important pollutants, the role played by oxygen particles in their degradation in natural waters has been evaluated [290]. 4.2.

Waste-water The advantages presented by waste-water treatment using photochemically initiated reactions with H202 are quite obvious. This method makes possible an oxidative degradation of virtually all organic pollutants while keeping the additional burden imposed on the ecosystem at its lowest. This finding is reflected in an increasing number of studies [291-3081 which report the application of the method to particular systems. It is obvious that at a sufficient concentration of HzOz and for a sufficiently long irradiation time the resulting products will be carbon dioxide, water and other inorganic substrates. However, in exceptional cases partial oxidation may also lead to the formation of more toxic products; an example can be seen in the formation of dialdrin from less toxic aldrin [309]. Chang and Zaleiko [310] described the utilization of photocatalytic effects of transition metals, which, owing to the pronounced postirradiation effect, make possible a considerable reduction in the time of irradiation and thus an important saving of energy. Summarily, it may be said that photochemical waste-water treatment in the presence of H202 (in some cases catalysed with ions of transition metals) seems to be a promising procedure.

5. Conclusions At the beginning of the 1930s the idea of the photochemical formation of hydroxyl radicals was put forward and a few years later the radical chain mechanism was suggested. This model of the photochemical reactions of H202 was indiscriminately acknowledged and universally applied to all systems under investigation_ In the course of time, however, particularly after the photocatalytic effects of trace concentrations of transition metals had been detected, doubts arose regarding the justification of such a uniform approach. For example, in the presence of iron ions the oxidation agent attacking the substrate is seen to be the ferry1 ion (FeO*+), i.e. a particle containing iron in the oxidation state 4+, and not the free hydroxyl radical [311]. These unsolved problems along with the ecological importance of photoinitiated and photosensitized reactions of H202 are the reasons underlying the increasing interest in the investigation of such reactions.

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