photocatalysed reactions of benzhydrol on zinc oxide

E. P. YESODHARAN
1. INTRODUCTION
V. RAMAKRISHNAN
J. C. KURIACOSE
Department of Chemistry
Indian Institute of Technology
Madras
INDIA
The mechanism of the interaction of oxygen with a zinc oxide
surface has aroused considerable interest in the context of the
electrical and optical properties of this oxide. In order to obtain
information to correlate the surface properties and the electronic
structure, the effect of irradiation on the interactions on a zinc
oxide surface has been studied by many authors (1-4). The nature
of the adsorbed oxygen species as well as its role in the
photooxidation reactions of organic substrates has been investigated
in detail. The participation of 02 , present on the surface of
irradiated zinc oxide has been postulated in the case of the oxidation
PHOTOCATALYSED
REACTIONS OF
BENZHYDROL ON
ZINC OXIDE
REACÇÕES
FOTOCATALISADAS DO
BENZIDROL SOBRE
ÓXIDO DE ZINCO
of isopropanol as well as phenols in aqueous suspensions of the
catalyst (5, 6). In the case of isopropanol, besides acetone in varying
amounts, depending on the experimental conditions, a low and
constant concentration of hydrogen peroxide is always found. Under
similar conditions benzylalcohol yields benzaldehyde, toluene and
hydrogen peroxide (7). Without the presence of oxygen no reaction
is detectable, either in the dark or with irradiation. It is assumed that
the reaction gets initiated by an O2 ion followed by a possible
radical reaction occuring without the influence of zinc oxide. The
present work is an extension of our earlier work on cheat and light
induced activation of oxides», with a view to establish the nature of
photocatalysed reactions of benzhydrol as well as the effect of
solvent and the structure of the alcohol on the reaction. The role of
adsorbed water in the photocatalysed reactions of various alcohols is
also investigated.
2. EXPERIMENTAL
The light source used in all the experiments is a Hanovia high
are not
pressure quartz mercury arc. Radiations above 3800
absorbed by zinc oxide as is revealed by its reflectance spectrum.
The lamp is mounted horizontally above a mechanical shaker and
provided with an aluminium foil shade to reflect the light
downwards. The reaction vessel consists of a pyrex tube with an
opening for introducing the solids and liquids and a small side tube
A
Hydrogen peroxide and benzophenone are formed in the photocatalysed
reactions of benzhydrol on zinc oxide. Oxygen, either in solution or on
the surface or both is essential for tfae reaction when the zinc oxide
suspension is irradiated with 3650 A wavelength radiation. A small
induction period in the reaction can be eliminated by the addition of small
amounts of benzophenone. The reaction is faster in acetonitrile than in
cyclohexane. Hydrocarbons like naphthalene and phenanthrene are found
to have a strong inhibiting effect on the activity of the catalyst while
benzene and fluorene are less effective as inhibitors, Adsorbed water
enhances the activity of zinc oxide. It is suggested that Oz is the species
responsible for the reaction. A mechanism consistent with the observations
is proposed and discussed along with other relevant aspects.
164
for bubbling gases. This is provided with two outer jackets, the inner
one to be used for taking external filters such as copper sulphate
solution and the outer one for circulating water at the desired
temperature. 0.35 gm of the catalyst and 30 ml of the liquid under
investigation are taken in the reaction vessel which is then closed and
clamped to the platform of the mechanical shaker and shaken
throughout the experiments. Samples were removed from the
reaction vessel at various intervals and analysed.
Almost all types of commercially available zinc oxide samples were
found equally active as photocatalysts and hence used as such. The
solvent acetonitrile is purified by the well known standard method
and checked by gas chromatography.The benzhydrol (reagent grade)
was recrystallised from ethyl alcohol (8). The resulting white crystals
had a melting range 65-66 ° C. Benzophenone also was recrystallised
from ethylalcohol (9) (m.p. = 47-48 °C). Naphthalene, phenanthrene and fluorene were found pure by melting point and used as
received.
The estimation of hydrogen peroxide was done by iodometry (10).
Benzophenone is determined by gas chromatography using a
carbowax column with hydrogen as the carrier gas and a column
temperature of 210 °C.
•
5
3. RESULTS
MEDIUM : - ACETONITRILE
TEMP , — 29 °C.
The products of the photocatalysed reaction of benzhydrol are
hydrogen peroxide and benzophenone. The product yield increases
with increase in the weight of the catalyst to reach a steady value.
This may be because with increase in the quantity of the catalyst,
the amount of light absorbed also increases until enough zinc oxide
is present to spread completely and uniformly in the reaction vessel
to absorb the maximum amount of light. Further addition of zinc
oxide may increase only its depth and since light does not reach the
interior of the catalyst bulk, it is not effective. This is further
confirmed by the observation that, the bigger the reaction vessel, the
higher the limiting amount of the catalyst.
The reactions were carried out in an acetonitrile medium. The effect
of concentration of the alcohol on the formation of hydrogen
peroxide is given in fig. 1. At higher concentrations, the peroxide
7 4
io
in 3
m
É
xv 2
0
á
C
rnó
t
V
x
00
30
60
150
120
90
Time (min)
210
180
so
MEDIUM-.- ACETONITR ILE
TEMP:- 29 .C.
Fig. 2
4)2CHOH
40-
2C= 0
A:
0.5M
Nil
B.
0.1M Nil
C: 70,00-2 M
Nil
D: 2.5x 10 2 M
E:
0.1M
30—
Nil
5x10 2 M
F. O 1M (no Zn0) 5 x10 2 M
E
e
e
.0 20—
o
C
n
•F
C
o
o
,
D
10 —
i
6
8
Time (hr. )
10
12
14
Fig. 1
Influence of concentration of benzophenone on the reaction
sensitised by it in 0.1 M benzhydrol
concentrations, the reaction is almost independent of it. This agrees
with the observation that initially added benzophenone enhances the
rate first and exerts no more influence towards the later stages of the
reaction (fig. 1, curves B and E).
Formation of benzpinacol has already been shown (8, 11) to
proceed through the triplet of benzophenone interacting with a
ground state benzhydrol molecule. Hence, if a substance which
quenches this excited state is added to the reaction mixture, the
observed selfacceleration of the reaction should disappear. Naphthalene, a well known quencher of the triplet state is thus initially
introduced to the system. But surprisingly not only the benzophenone sensitized reaction, but the zinc oxide catalysed reaction also is
suppressed considerably (fig. 3), The effect of various additives on
the formation of hydrogen peroxide from benzhydrol is given in fig.
4. Phenanthrene with energy levels similar to that of naphthalene is
Effect of concentration of benzhydrol and benzophenone on the
formation of H2 02
goes on increasing with time and levels off later. But at lower
concentration regions there is an induction period followed by a
steady rise in the amount of hydrogen peroxide. Surprisingly, the
amount of benzophenone was less than that of hydrogen peroxide.
Since all the hydrogen required for hydrogen peroxide comes from
benzhydrol by its dehydrogenation as
(C6H5)2CHOH
(C6H5)2C= O+H2
14 —
MEDIUM.- ACETONITRILE
TEMP: — 29 .C.
A
12 —
At
io
X10
e
É
B
t1)2CHOH 4)2c °O C10H8
5x10 -2 M
A
0•IM
B
0.1M
Nil
C
0.1M
Nil
8.10-4 N
Nil
D
0-1M
Nit
4x103M
E:
0.1M
Nil
2:10 2 M
F:
01M
Nil
4x10 2 M
G:
0.1M
5 x10-2 M 4x10 2 M
W
both the ketone and the peroxide should be formed in more or less
equal amounts. This suggested that some other process involving
benzophenone also is taking place simultaneously. The initial
induction period also points to the fact that, the products of the
reaction can accelerate it, resulting in the generation of more
hydrogen peroxide. This is understandable since benzophenone itself
under photochemical conditions interacts with benzhydrol forming
benzpinacol and hydrogen. Considerable amount of benzpinacol is,
in fact, formed in the system. When benzophenone is added initially
to the system the induction period disappears (fig. 1, curve E). At
lower concentrations of benzophenone, the formation of hydrogen
peroxide depends on its concentration (fig. 2) and at higher
Nil
C
E
E
00
2
4
6
8
10
Time (hr.)
F&G
12
14
Fig. 3
Influence of naphthalene on the photocatalysed reactions and
benzophenone sensitized reactions of benzhydrol on ZnO
165
MEDIUM-ACETONITRILE
14
TEMP:- 29 ° C
A , 0.1M BENZHYDROL
(No 0dditive)
B: A+5x10 -2 M BENZENE
1
12
C: A+ 5x 10-2 M FLUORENE
IQ
• A+5x10 2 M NAPHTHALENE
10
E: A+ 5 x 10-2 M PHENANTHRENE
x
õ
E 8
v
a
ó
ã
6
4
D&E
rn
o
v`
spectral analysis showed a broad peak corresponding to adsorbed
water. The photocatalysed reactions of benzhydrol, benzylalcohol
and isopropanol are found to yield higher amounts of hydrogen
peroxide and the carbonyl compound, on zinc oxide, when small
amounts of water are initially added (fig. 6 and 7). A sample of zinc
oxide dried at 400 ° C for 3 hours had a lower activity than the
commercially available one. The higher yield of hydrogen peroxide
compared to the aldehyde or the ketone is understandable, since
water itself, when irradiated over zinc oxide gives the peroxide
which does not decompose to any great extent if organic stabilizers
are present.
24
•
2
A:O-iM BENZYLALCOHOL
8:0.1M BENZHVDROL
C:0.2M ISOPROPANOL
S
21
00
2
4
6
8
10
Time (hr.)
12
14
18
^o
DRY Zn0
1I
D.0-iM BENZVLALCOHOL
E:0.1M BENZHVDROL lWET Zn0
F: 0.2M 150PROPANOL
E
MEDIUM ACETONITRILE
X
TEMP.- Z9 ° C.
w 15
Fig. 4
Effect of additives on the photocatalysed reactions of benzhydrol on
õ
E
12
ZnO
T)
o
as effective as the latter. Fluorene and benzene are found to be less
effective, the latter being very weak as a quenching agent. It looks as
though there is some correlation between the triplet energy of these
hydrocarbons and their efficiency for inhibition. The higher the
energy, the less is the efficiency. The percentage of inhibition caused
by the various hydrocarbons estimated by observing the extent of
reaction after an interval of 6 hours is correlated to their triplet
energies in fig. 5. When they are taken externally the photoreaction
a
9
c
m6
o
^
=
3
0o
24
6
Time (hr.)
8
Fig. 6
140—
MEDIUM:- ACE TONITRI LE
Influence of traces of water on the photocatalytic activity of ZnO —
Formation of H202
TEMP:— 29 °C.
120—
1, NAPHTHALENE
C
o
Z: 100 -3.
2. PHENANTHRENE
FLUORENE
4 BENZENE
C
O
1
80-
¡_ 21
"o-
T;
0, 60 —
o
18
N
A:BENZALDEHYDE
B: BENZOPHENONE DRY ZnO_
C ACETONE
D , BENZ ALDEHY DE
E. BENZOPHENONE
F: ACETONE
WET ZnO
á
40—
É
15 -
MEDIUM .- ACETONITRILE
TEMP.- 29 ° C
20
o
60
a
IO CH2OH - 0 -1M
a
E
(CH 3 ) 2 CHOH-0.2M
^o 12
—
64
68
72
76
80
84
Triplet energy, k cal mole -1
88
0 9
Fig. 5
F
a
o
Percentage inhibition caused by hydrocarbons as a function of their
@ 2 CHOH-0.1M
6
E
B
ó
U
3
triplet energies
goes unaffected. Hence they are not acting as mere filters when
taken internally. The photocatalysed reaction of benzylalcohol also
is suppressed by naphthalene.
It is observed that the reaction is faster when small quantities of
water is introduced into the system when acetonitrile is the medium.
Zinc oxide, the catalyst in this case, when subjected to infra red
166
00
2
4
6
Time (hr.)
8
10
Fig 7
Influence of traces of water on the photocatalytic activity of ZnO —
Formation of carbonyl compounds
4. DISCUSSION
According to the interpretation of several workers (12, 13) the
formation of hydrogen peroxide at irradiated zinc oxide surfaces
involves the reduction of molecular oxygen. ESR studies made by
Sancier (14) have revealed that oxygen has indeed already trapped
the conducti,n electrons of zinc oxide in the dark. But benzhydrol
fails to undergo any reaction in the dark in the presence of zinc
oxide showing that light must produce an excited state of oxygen
and not merely an electron transfer to oxygen. Since the reactions
do not take place either in the dark, or in the presence of light in the
absence of oxygen, both light and oxygen are essential for the
reaction. Similarly, with oxygen, but without the catalyst no
photooxidation is observed. Therefore, the conditions which have to
be fulfilled for the reaction are simultaneous presence of zinc oxide,
oxygen and ultra-violet irradiation.
Oxygen may give rise to different species such as 0 -2 (ads),
0 (ads), 02 (ads), etc. when adsorbed on a solid catalyst (1, 15,
16). Other species like 02 , 0, 0 -* , 02* , 0 - , etc. may also be
formed on the surface of a photocatalyst. For reasons given earlier
(5), 0 cannot be the active species. Barry and Stone (17) have
reported exchange of isotopic oxygen on zinc oxide at room
temperature in the dark. This exchange must take place through
dissociated oxygen and hence atomic oxygen as the active species
can also be excluded. The species that is stable at room temperature
only (18, 19) and therefore weakly fixed to the solid is 02. Hence
02 either in the ground or in the excited state could be the active
species for the reaction. Calvert et al. (13) have ruled out other
excited species of oxygen as possible intermediates. Taking all these
factors into consideration two possible mechanisms can be proposed
for the present reaction.
i.e. H02 + H0 2
---{
H2 02 + 02
The radical (C6 H5)2 COH can compete with this reaction leading
to the formation of benzophenone.
(C6 H5 )2 COH + H02
--i(C6 H5 ) C =
O + H202
SCHEME 2
If 0 — , formed by the donation of an electron to the adsorbed
oxygen from the conduction band of illuminated zinc oxide is the
active species, a polar solvent should favour the reaction. In fact in a
polar solvent like acetonitrile, the reaction is much faster, suggesting
the intermediacy of a polar intermediate. Hence a second scheme of
reaction may be proposed as :
HH C^ C6Hs
O
I
I \ C6H5
H-0
0
—i
/C6H5
H02
H —O
Ì
Ì
®
—C \
C6 H 5
Zn --- O
i
®
Zn --- O
desorption
(C6H5)2C=0 +H202+Zn0
Another possibility where, a (O6 H5 )2 COH radical interacts with a
molecule of oxygen and gives rise to the observed products cannot
be ruled out.
( C 6 H 5 ) 2 C` OH + 02 --+(C6H5)2C(OH)02
SCHEME 1
When the catalyst is illuminated, the Zn-O bonds are excited and
loosened. As a result there is a relative shift of bond electron
towards the zinc atom (2). Thus an adsorption where the cationic
hydrogen bridge is formed between the alcoholic oxygen of the
benzhydrol and zinc atom, now electron-enriched, can be envisaged
as
C6 H5
C6 H 5
(C6 H512 CHOH
ZnO
hv
The anionic bridge is formed between the carbon atom of the
benzhydrol and the electron exhausted excited oxygen of the
surface. Subsequently,
C6H5
(C6 H5 )2 COH can easily lose a hydrogen atom to give
(C6 H5 )2 C = O. The H62 radicals can combine in the presence of
a third body forming H202.
C6 H5
\C /
OZ
H
(b-) (S+)
Zn ---- -0
(C6H5)2 CON
(C6H5)2C(OH)02 H(C6H5)2C = O + H2 0 2
Of the two mechanisms proposed, scheme 2 is more probable in the
present case because it explains the role of a polar solvent in the
reaction.
Once the benzophenone is formed in the reaction it gets
photoexcited under the influence of the absorbed light and converts
to the long lived triplet by a highly efficient intersystem crossing
(11, 20). These triplet molecules can abstract hydrogen from
benzhydrol in a rate determining step forming two free radical
species which dimerise to give benzpinacol.
(C6H5)2C=0 —h ----(C6H5)2C
v
= 0.
(C6H5)2C= O . + (C6 H5)2 CHOH --i (C6 H5 )2 COH +
(C6 H5 )2 COH
2(C6H5)26OH ---o- (C6 H5 )2 — C — C — (C6 H5 )2
H ) COH + H
(C6 H5
Zn -H
(C6H512C(OH)02 + (C6H51 2 CHOH— ► 1(C6H5) 2 C(OH)02H+
ó
0
^((C6H5)2COH+
I
Há 2
OH OH
This will result in the disappearance of benzophenone. But hydrogen
and hence hydrogen peroxide will go on accumulating.
The behaviour of benzophenone can be understood from the effect
of its concentration on hydrogen peroxide formation (fig. 2). At
167
lower concentrations the reaction depends on benzophenone. But in
the higher concentration regions (of the order of 2 x 10 -2 M and
above) it is independent of the concentration of the ketone. When a
sufficient amount of benzophenone is formed from benzhydrol
dehydrogenation the rate is equal to that in the case of initially
added benzophenone. Moreover, the benzpinacol formed in the
system will occupy the surface of the catalyst depriving the alcohol
from access to the surface, thus suppressing the catalysed reaction.
Naphthalene is a well known quencher for the first excited triplet of
benzophenone (21, 22) and hence to its photoreduction to
benzpinacol. The activity of zinc oxide is found to be suppressed to
a great extent by naphthalene. This is further confirmed by the
inhibition of the reactions of benzylalcohol under similar conditions.
The mechanism by which naphthalene quenches the photoreduction
of benzophenone is well established (22) as
a. H2O + O s= - -► OHS + OH S
b. dissociative chemisorption of water accompanied by annihilation of preexisting surface anion vacancies LI (0 2- 1.
(C6H5)2 C= 0 --i (C6 H 5)2 C = 0 • (singlet)
(C6H5 )2 C =
exciplex formation should be the cause of inhibition. The possibility
of naphthalene getting adsorbed on the catalyst resulting in the
nonavailability of sites for dehydrogenation is unlikely since the
different hydrocarbons show different inhibiting efficiency. Yet
another possibility of the hydrocarbon scavenging the active species
of oxygen and causing the suppression of the photoreaction cannot
be ruled out. All these processes could contribute to the quenching
effect by the hydrocarbon.
The presence of water on zinc oxide was found to enhance the
activity. In the case of TiO 2 many authors have shown (23, 24) that
the surface has weakly and strongly bound molecular water and
hydroxyl groups created by the dissociative chemisorption of water.
This can take place in different ways (25).
O (triplet)
(C6 H51 2 C = O (triplet) + C 10 H8--iC10H8 (triplet) +
^(C6H5)2C = O
i.e. H2 O +O producing surface OH and bulk 0HE. The hydroxyl groups may
be responsible for the enhancement of the rate. The mechanism can
be postulated as
h — e (exciton) --•- h + e.
ZnO
This is possible since the triplet energy of naphthalene is 60 k cats
mole -1 whereas that for benzophenone is 70 k cal mole -1 . The
quenching through singlet-singlet energy transfer can be ruled out
since the excited singlet of naphthalene lies at a higher energy level
than that of benzophenone. Oxygen also is reported (22) to be a
quencher for the reaction. But from the observations we have made
it is seen that naphthalene is more efficient as a quencher than
oxygen. The solutions used in the reactions were not deaerated and
hence oxygen is present both in the dissolved form and the adsorbed
state. Still there was a considerable inhibition by naphthalene. The
superiority of naphthalene may be due to the fact that oxygen
slowly reacts with the benzophenone-benzhydrol system while
naphthalene is chemically inert.
The inhibition to the activity of zinc oxide caused by naphthalene
can also be explained in a similar manner. One of the major
phofoeffects on zinc oxide is the transfer of an electron from the
valence band to the conduction band which requires about 3.5 eV of
.energy. The alcohol gets adsorbed by donation of an electron to the
valence band. It is possible that zinc oxide in its excited triplet state
transfers its energy to naphthalene and becomes deactivated. The
suppressing effect decreases es the singlet-triplet gap of the
hydrocarbon increases. Thus the quenching effect is in the order
naphthalene phenanthrene > fluorene. Anthracene could not be
The singlet levels of
used in these studies since it absorbs 3650
all these hydrocarbons lie at much higher levels compared to that of
zinc oxide. Hence singlet-singlet energy transfer is precluded. The
observation that phenanthrene, with a triplet energy similar to that
of naphthalene is equally effective as a quencher while fluorene is
less efficient and that benzene whose triplet energy is 84 k cal
mole -1 which is more than the band gap of zinc oxide is very weak
as an inhibitor supports the energy transfer mechanisms.
It is also possible to envisage the formation of an exciplex between
excited zinc oxide and ground state naphthalene which again is
inefficient as a photocatalyst. It is not clear whether excited zinc
oxide is in the singlet state or in the triplet state. If this is a triplet,
formation of e3 ZnO • — hydrocarbon» exciplex must be considered. On the other hand if it is a singlet, « 1 Zn0 • — hydrocarbon»
+ ❑ x(0 2 )--► OHS +OHL
The presence of adsorbed oxygen in the dark can cause a space
charge to develop at the surface which under illumination favours
the movement of photoholes to the surface. Here they become
trapped at surface states which in the present case is OH ions.
e.,
h + 0H --- -OH.
Now the photoelectron is free to participate in the adsorption of
oxygen resulting in 02. Thus the presence of water assists in the
generation of the active species.
These studies particularly those on the inhibition caused by
naphthalene may be of relevance in the electronic theory of
photocatalysis. The importance of electronic energy levels cannot be
overemphasized when dealing with semiconductors. The energy
levels of surface states with respect to the energy bands of the solid
will determine whether an adsorbed species will become charged or
not and in large part determine the electron exchange (26). Because
of the dominant role played by surface state energy levels in the
electron exchange process, these levels can be controlling factors in
chemical processes such as adsorption and catalysis.
ACKNOWLEDGEMENT
A.
168
We are grateful to the National Science Foundation (U.S.A.) for
financial support. It is a pleasure to record our thanks to Prof. M. C.
Markham for all the stimulating discussions.
REFERENCES
1. J. H. LUNSFORD, Catal, Revs., 8, 135 119731.
2. F. STEINBACH, «Heterogeneous Photocatalysis», Topics in
Current Chemistry, 25, 1 1 7 119721.
3. J. HABER, K. KOSINSKY and M. RUSIEKA, «Photoeffects in
adsorbed species», Disc. Farad. Chem. Soc., B-11 11974).
4. TH. WOLKENSTEIN, Adv. in Catalysis, 23, 157 (1973).
5. J. C. KURIACOSE and M. C. MARKHAM, J. Catalysis, 1, 498
(1962).
6. M. C. MAR KHAM and K. J. LAIDLER, J. Phys. Chem., 57, 363
(1953).
7. E. P. YESODHARAN, V. RAMAKRISHNAN and J. C.
KURIACOSE (in press).
8. W. M. MOORE and M. D. KETCHUM, J. Phys. Chem., 68, 214
(1964).
9. J. N. PITTS JR., R. L. LETSINGER, R. P. TAYLOR, J. M.
PATTERSON, G. RECKTENWALD and R. B. MARTIN, J.
Amer. Chem. Soc., 81, 1068 (1959).
10. C. N. CHARI and M. QUERESHI, J. Indian Chem. Soc., 21, 97
(19441.
11. W. M. MOORE, G. S. HAMMOND and R. P. FOSS, J. Amer.
Chem. Soc., 83, 2789 (1961).
12. J. C. KURIACOSE and M. C. MAR KHAM, J. Phys. Chem., 65,
2232 (1961).
13. J. G. CALVERT, K. THEURER, G. T. RANKIN and W. M.
McNEVIN, J. Amer. Chem. Soc., 76, 2575 (1954).
14. K. M. SANCIER, J. Catalysis, 5, 314 (1966).
15. E. R. S. WINTER, Adv. in Catalysis, 10, 196 (1958).
16. G. C. A. SCHUNT, Chim. Ind., 12, 1307 (1969).
17. T. I. BARRY and F. S. STONE, Proc. Roy. Soc. (London),
335 A, 124 (1960).
18. V. L. RAPPORPORT, Dokl. Akad. Nauk. SSSR., 153, 371
(1963).
19. Y. FUJITA. and T. KWAN, J. Res. Inst. Catalysis, Hokkaido
Univ., 7, 224 (1959).
• 20. H. J. BACKSTROM and K. SANDROS, J. Chem. Phys., 23,
2197 (1955).
21. G. S. HAMMOND and J. LEERMAKERS, J, Phys. Chem., 66,
1148 (1962).
22. W. M. MOORE and M. KETCHUM, J. Amer. Chem. Soc., 84,
1368 (1962).
23. R. I. BICKLEY, G. MUNUERA and F. S. STONE, J. Catalysis,
31, 389 (1973).
24. G. MUNUERA, J. Catalysis, 18, 19 (1970).
25. G. MUNUERA and F. S. STONE, Disc. Farad. Chem. Soc., 52,
205 (19711.
26. S. R. MORISON, Surface Science, 13, 85 (19691.
RESUMO
As reacções fotocatalisadas do benzidrol sobre óxido de zinco dão origem
a peróxido de hidrogénio e benzofenona. O oxigénio, quer em solução ou à
superfície, ou sob ambas as formas, é essencial para a reacção, quando a
suspensão de óxido de zinco é submetida a uma radiação com o
comprimento de onda de 3650 A. Há um pequeno período de indução na
reacção que pode ser eliminado pela adição de pequenas quantidades de
benzofenona. A reacção procma•se a maior velocidade em acetonitrilo do
que em ciclo-hexano. Verificou-se que hidrocarbonetos tais como o
naftaleno e o fenantreno têm um efeito fortemente inibidor sobre a
actividade do catalisador, enquanto o benzeno e o fluoreno são menos
efectivos como inibidores.
A água adsorvida aumenta a actividade do óxido de zinco. Sugere-se que o
02 é a espécie responsável pela reacção. Propõe-se e discute-se um
mecanismo consistente com os resultados experimentais e com outros
aspectos relevantes.
169