Photo-and thermogeneration of singlet oxygen by the metal ions deposited on Al2O3 and SiO2

Article abstract of DOI:10.1134/S0036024406050232

The specifics of photo-and thermogeneration of singlet molecular oxygen by metal oxides deposited on silica gel and Al₂O₃ were studied. The deposited oxides were observed to generate equilibrium and superequilibrium concentrations of ¹Δ_gO₂. The V₂O₅/SiO₂ and MoO₃/SiO ₂ systems were found to be most active in both types of generation. A common mechanism of photo-and thermogeneration was proposed. Pleiades Publishing, Inc., 2006.

Full text of DOI:10.1134/S0036024406050232

ISSN 0036-0244, Russian Journal of Physical Chemistry, 2006, Vol. 80, No. 5, pp. 799–802. © Pleiades Publishing, Inc., 2006.
Original Russian Text © N.V. Shcherbakov, A.N. Emel’yanov, E.V. Khaula, A.N. Il’ichev, M.V. Vishnetskaya, Yu.N. Rufov, 2006, published in Zhurnal Fizicheskoi Khimii, 2006,
Vol. 80, No. 5, pp. 918–922.
PHOTOCHEMISTRY
AND MAGNETOCHEMISTRY
Photo- and Thermogeneration of Singlet Oxygen
by the Metal Ions Deposited on Al2O3 and SiO2
N. V. Shcherbakova, A. N. Emel’yanova, E. V. Khaulab, A. N. Il’ichevb,
M. V. Vishnetskayaa, and Yu. N. Rufovb
a Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina 4, Moscow, 117977 Russia
b Gubkin State University of Oil and Gas, Leninskii pr. 65, Moscow, 117917 Russia
E-mail: iov314@list.ru
Received June 16, 2005
Abstract—The specifics of photo- and thermogeneration of singlet molecular oxygen by metal oxides depos-
ited on silica gel and Al2O3 were studied. The deposited oxides were observed to generate equilibrium and
1
superequilibrium concentrations of ∆gO2. The V2O5/SiO2 and MoO3/SiO2 systems were found to be most
active in both types of generation. A common mechanism of photo- and thermogeneration was proposed.
DOI: 10.1134/S0036024406050232
The role of singlet oxygen in chemical reactions has
been most comprehensively studied for homogeneous
systems [1]. That ∆gO2 may play a role in heteroge-
EXPERIMENTAL
1
Preparation of Catalyst
neous systems was pointed out in [2]. It was demon-
strated [3] that, in addition to equilibrium formation,
singlet oxygen was found to be produced in nonequilib-
rium processes. The authors of [3] supposed that such
nonequilibrium processes are associated with the effect
of the electric field created by extrinsic metal ions. It
was discovered [4] that there is a correlation between
the electric field strength of the metal ion and the tem-
perature at which the rate of desorption of singlet oxy-
gen passes through its maximum; more specifically,
this temperature decreases with increasing electric field
strength, a conclusion supported by quantum-chemical
calculations.
The catalysts were oxides of transition and other
metals deposited on silica gel: V2O5/SiO2, MoO3/SiO2,
WO3/SiO2,
ZnO/SiO2,
PbO2/SiO2,
Al2O3/SiO2,
CuO/SiO2, and Bi2O3/SiO2. We also studied Bi2O3/Al2O3
(1, 3, and 6 mol % Bi), V2O5/Al2O3 (1 mol % V), and
MoO3/Al2O3 (1% mol Mo). The supports were Silo-
chrom S-120 and commercial Al2O3 with a specific sur-
face area phase 217 m2/g. The metals were deposited by
precipitation of the corresponding salts from aqueous
solutions. The amount of the salt required for the prep-
aration of the solution was calculated based on the
requirement that the final concentration of the depos-
ited oxide with respect to the silica gel, NM/NSi + NM
should be 1.5 mol %. Silica gel of 0.15- to 0.25-mm
fraction was placed into a solution of the salt to be
deposed, after which the water was evaporated at
110°ë for 12 h under conditions of periodic stirring
(with a magnetic stirrer). Then the sample was calcined
for 5 h at 400 and 500°ë. The deposits of tungsten,
zinc, lead, aluminum, copper, and bismuth oxides were
prepared from the corresponding nitrates. The deposits
of vanadium and molybdenum oxides were prepared
using ammonium vanadate and molybdate, respectively
[9]. A similar technique was used to prepare the alu-
mina-supported deposits.
Some systems have been reported to generate 1∆gO2
when heated. A review of the works on the subject,
including the results obtained by the authors, can be
found in monograph [5]. As noted in [5], the number of
systems capable of generating singlet oxygen during
heating in an oxygen-containing environment is lim-
ited, mainly silica gel with deposited ions of Bi, V, and
Mo. In [6], singlet oxygen was obtained by heating the
Li–Sn–P–O system [6]. In addition, singlet oxygen was
reported to be formed on mechanically crushed quartz
[7] and ZSM-5 zeolite [8].
The EPR spectra were recorded on an EPR-V instru-
ment (Institute of Chemical Physics, Russian Academy
of Sciences) in the X band; CuSO4 · 5ç2O and a solid
solution of Mn2+ chloride in MgO were used as refer-
ences.
The purpose of this paper was to detect the forma-
tion of singlet oxygen during the photo- and thermoex-
citation of silica gel carrying a wider scope of deposited
metals.
799

800
SHCHERBAKOV et al.
c(1∆gO2) × 10–12
,
molecules/(Òm3/g)
120
80
40
0
450
350
I
II
250
t, °C
III
IV
V
VI
25
VII
VIII
IX
1
Fig. 1. Specific concentrations of ∆ O generated at 25–500°C by metal oxides deposited on silica gel: (I) molybdenum oxide,
g
2
(II) vanadium oxide, (III) pure silica gel, (IV) copper oxide, (V) aluminum oxide, (VI) bismuth oxide, (VII) lead oxide, (VIII) tung-
sten oxide, and (IX) zinc oxide.
Technique of Qualitative Determination
of Singlet Oxygen
amounts of singlet oxygen, we performed a series of
test experiments in which these impurities were
removed. The air passed through a NaY-zeolite-con-
taining trap immersed in liquid nitrogen. These experi-
ments revealed that the impurities contained in the air
have no effect on the amount of singlet oxygen pro-
duced.
1
The amount of singlet oxygen ∆gé2 (in this work,
we considered only 1∆gé2, because the 1Σg is known to
be generated in negligibly small amounts under such
conditions [10]) formed on the surface of the samples
in both series was measured using the flow method at a
residual air pressure optimal for minimizing the rate
coefficients for the heterogeneous and homogeneous
quenching of singlet oxygen. This method was
described in detail elsewhere [11]. The chemilumines-
cence detector was calibrated by method given in [3].
RESULTS AND DISCUSSION
Equilibrium Concentration of Singlet Oxygen
The thermogeneration of singlet oxygen (the
increase of the equilibrium concentration with the tem-
perature) was observed for all samples tested. It was
found that, at a given temperature, the different samples
generate different amounts of singlet oxygen (Fig. 1).
This effect depends on the nature of the deposited oxide
and, hence, on the accommodation coefficient of singlet
oxygen [13], which, in turn, affects the rate of the
attainment of the equilibrium 1∆gé2 concentration.
Note that we observed both equilibrium and non-
1
equilibrium concentrations of ∆gé2. The equilibrium
concentrations of 1∆gé2 are generated due to the energy
transfer from the heated surface to 1∆gé2 molecules or
under irradiation (quartz or metal oxides can serve as
such a surface). Thus, the equilibrium concentration of
1∆gé2 depends mainly on the temperature or the inten-
sity of irradiation of the surface. The rate of the attain-
ment of the equilibrium concentration depends on the
nature of the surface [12]. In addition to the equilibrium
concentrations of singlet oxygen, we observed
superequilibrium concentrations of 1∆gé2, which man-
ifest themselves as one or several peaks of desorption of
singlet molecular oxygen from the heated sample.
Within the temperature range covered, the equilib-
1
rium concentrations ∆gé2 increased exponentially
with the temperature for all samples.
At 500°ë, only the vanadium and molybdenum
oxides deposited on silica gel generated molecular oxy-
gen more efficiently than the carrier, i.e., pure silica gel.
To determine how various impurities (CO, ëé2) The deposition of molybdenum oxide leads to an espe-
penetrating into the reactor from the air influence the cially significant increase in the equilibrium 1∆gé2 con-
generation of both equilibrium and nonequilibrium centration. The deposition of the other metal oxides
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 5 2006

PHOTO- AND THERMOGENERATION OF SINGLET OXYGEN
801
inhibits the ability to generate 1∆gé2 as compared to the
c(1∆gO2) × 10–12, molecules/(cm3/g)
pure silica gel. This indicates that these oxides are less
efficient in quenching singlet oxygen, as follows from
the principle of detailed balance. But this conclusion
does not hold for the entire temperature range. For
example, at 350°ë, the equilibrium concentration of
singlet oxygen over the pure silica gel sample is almost
twice as large as that over the sample containing depos-
ited vanadium oxide, although, at 500°ë, the opposite
behavior is observed. The amounts of 1∆gé2 generated
on the Al2O3/SiO2 and CuO/SiO2 samples at 350°ë
exceed those generated on the pure silica gel sample; at
400°ë, these quantities become equal and change
places at higher temperatures.
5
4
3
2
1
0
II VI
V
I
VII III VIII IX IV
Fig. 2. Mean superequilibrium specific concentrations of
1
∆ O emitted from the metal oxides deposited on silica gel
g
2
at 500°ë. For designations, see Fig. 1.
Superequilibrium Amounts of Singlet Oxygen
In contrast to equilibrium generation, the ability to
generate nonequilibrium amounts of singlet oxygen
was observed not for all samples. To investigate this
process, we used deposited oxides not subjected to cal-
cination at 500°ë before the experiment. A diagram
characterizing this process for the samples under study
is displayed in Fig. 2.
c(1∆gO2) × 10–12, molecules/(cm3/g)
20
15
10
5
0
This diagram shows that only four of the tested
oxides deposited on silica gel can generate of
superequilibrium amounts of 1∆gé2: V2O5/SiO2,
Bi2O3/SiO2, Al2O3/SiO2, and MoO3/SiO2. The duration
of nonequilibrium emission ranges from 45 to 60 min
for various samples. The ability to generate superequi-
I
II
III VII IX VI
V
IV
1
Fig. 3. Mean specific concentrations of ∆ O generated by
the deposited metal oxides irradiated with a PRK-4 mercury
lamp. For designations, see Fig. 1.
g
2
1
librium amounts of ∆gé2 increases in the series
In the case of Bi2O3/SiO2 and Al2O3/SiO2, another
MoO3/SiO2 < Al2O3/SiO2 < Bi2O3/SiO2 < V2O5/SiO2.
After this type of generation is finished, only the equi-
librium concentration of singlet molecular oxygen
occurs on the catalysts. Note that this process starts at
high temperatures. For vanadium oxide deposited on
silica gel, we conducted an additional experiment in
which V2O5/SiO2 was heated from 400 to 600°ë in
50°ë steps. As a result, we established that nonequilib-
rium generation occurs at temperatures above 500°ë.
While performing this series of experiments, we
observed characteristic changes in the color of the
V2O5/SiO2 and MoO3/SiO2 samples, a feature indica-
tive of a change in the degree of oxidation of the vana-
dium and molybdenum ions. A mechanism of the gen-
eration of singlet molecular oxygen at the expense of a
change in the degree of oxidation of transition metal
ions was proposed in [13]:
1
mechanism of the generation of ∆gé2 superequilib-
rium amounts is probably operative. At elevated tem-
peratures, these oxides can form silicate compounds
with silica gel, a process accompanied by the release of
active oxygen forms (O and O3 [14]), which, recombin-
ing, yield 1∆gé2 [15].
Photogeneration of Singlet Oxygen
The photogeneration of singlet molecular oxygen
was observed for all samples subjected to UV irradia-
tion (Fig. 3). To measure the ∆gé2 concentration
1
formed during UV irradiation, the sample was placed
into a glass cell covered with a quartz lid through which
UV radiation passed (the design of the experimental
setup was the same except for the reactor being
replaced by a glass cell). The source of UV radiation
was a PRK-4 mercury lamp, with the maximum inten-
sity within 360–365 nm.
–
[Me(n – 1) + –O ]T + 3O2
[Men+=O2–]S + 1O2.
*
1
0
In [13], the initial complex was photoactivated,
whereas in our experiments, it was thermoactivated.
This mechanism of generating singlet molecular oxy-
gen in nonequilibrium amounts is supported by the
observation that the EPR spectrum of O–, a species ini-
tially present on the surface of deposited vanadium and
molybdenum oxides, disappears at elevated tempera-
tures.
The experimental data suggest that only V2O5/SiO2
and MoO3/SiO2 irradiated with a radiation the intensity
maximum of which lies in the UV region yield higher
concentrations of singlet oxygen than pure silica gel
does under the same conditions, in agreement with the
data on the thermogeneration of ∆gé2. These results
suggest that the mechanisms of thermo- and photoge-
1
RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY Vol. 80 No. 5 2006

802
SHCHERBAKOV et al.
c(1∆gO2) × 10–12, molecules/(cm3/g)
50
temperature. The samples were then heated in steps up
to a certain temperature and irradiated. The concentra-
tion of singlet oxygen was measured in the presence
and absence of irradiation, with the difference being
used to evaluate the contribution from photogeneration.
For pure aluminum oxide and for V2O5/Al2O3 and
MoO3/Al2O3, the contribution from photogeneration to
the total amount of 1∆gé2 formed disappeared at a tem-
perature below 300°ë. Somewhat different behavior
was observed for all concentrations of Bi2O3/Al2O3
(1, 3, and 6 mol %). The diagram in Fig. 4 shows how
the contribution from photogeneration to the total equi-
librium concentration of 1∆gé2 decreases with increas-
ing temperature.
45
40
35
30
25
20
15
10
5
0
22
280
400
510
483
T, °C
That the concentration of photogenerated singlet
molecular oxygen decreases with increasing tempera-
ture is associated with an increase in the contribution
from various mechanisms of the dissipation of the
energy of excited states [16], in accordance with the
Jablonski diagram.
1
Fig. 4. Mean specific concentrations of ∆ O produced by
g
2
bismuth oxide deposited on the aluminum oxide (1 at %)
during joint photo- (white bars) and thermo- (gray bars)
excitation.
neration are similar. The only difference is the mode of
–
the activation of the [Me(n – 1)+–O ]T complex: UV irra-
*
ACKNOWLEDGMENTS
1
This work was supported by the Russian Foundation
for Basic Research, project no. 04-03-32513.
diation in the former case and thermal energy in the lat-
ter. For photogeneration, these mechanisms can be pre-
sented as [5]
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