Materials Sciences and Applicatio ns, 2010, 1, 97-102
doi:10.4236/msa.2010.12017 Published Online June 2010 (http://www.SciRP.org/journal/msa)
Photochemical Properties of Precipitated Solid
Aerosol Produced by Burning of Titanium
Microparticles under Ambient Air
Valery Zakharenko, Sophia Khromova
Boreskov Institute of Catalysis, Pr. Lavrentieva, Novosibirsk, Russia.
Received February 26th, 2010; revised May 4th, 2010; accepted May 7th, 2010.
ABSTRACT
In order to neutralize a drastic pollution of the environment (technogenic catastrophe) it is suggested to use technogenic
technologies of chemical compound decontamination. One in such technologies can be the techno logy using metal oxide
solid aerosols which are active in removal of pollutant compounds and obtainable by combustion under ambient air of
appropriate metal particles, for example, aluminum, magnesium, titanium and etc. It is shown that the titanium dioxide
out of an solid aerosol, obtained by pyrotechnic mixture combustion containing titanium microparticles has optic,
chemical and photocatalytic properties close to properties of titanium dioxide produced by a different way. The
production of s uch aeros ol in direct place of a techn ogenic catastrop he can be m ade for the cleanin g of atmosphere near
a pollution source.
Keywords: Pyrotechnic Mixture, Precipitated Aerosol, Tio2, Photoadsorption, Photocatalysis, Troposphere
1. Introduction
The Earth’s atmosphere is ability for self-cleaning from
dangerous compounds because of the photocatalytic and
photosorption processes on the particle aerosol surface in
the troposphere. These processes are anticipated to pri-
mary affect the intensity of acid rains, concentration of
greenhouse and ozone- depleting g ases, as well as cleaning
up harmful compounds from the atmosphere.
However, in case of man-caused catastrophe, which
accompany a local emission of pollutants, the decon-
tamination of the atmosphere by natural aerosols may be
ineffective. For the neutralization of drastic pollution of
the environment can be used technogenic technologies of
a chemical compound decontamination. One in such
technologies can be the technology using metal oxide
solid aerosols which are active in removal of pollutant
compounds and obtainable by combustion under ambient
air of appropriate metal particles, for example, aluminum
[1], magnesium [2] and etc. It is known that magnesium
oxide photosorbs efficiently halogen-containing organic
compounds [3] and titanium dioxide photocatalyzes an
oxidation breakdown a large number of organic com-
pounds polluting atmosphere [4,5].
The pyrotechnic technology of a metal oxide prepara-
tion is mobile, available and inexpensive way of a solid
aerosol production near a pollution source. The technical
implement may be look like on a firework, lighting flare
and the like. It allows locating a contaminatin g impact of
chemical compounds. For a development of such tech-
nology the study of photochemical properties of the tita-
nium dioxide producing by pyrotechnic mixture combus-
tion under ambient air was carried out.
2. Experimental
Powdery titanium dioxide was prepared by combustion
pyrotechni c mixture consisting of ammo nium perchl orate,
hydroxy-terminated polybutadiene (HTPB) and titanium
particles (particle size from 60 to 90 m) under ambient air.
The highly dispersed titanium dioxide powder had the
specific surface area of 6 m2g-1. After a treatment of the
oxide surface at temperature 625 K for 30 min in air the
specific surface area was decreased to 4 m 2g-1. The crystal
structures of rutile and anatase were confirmed by X-ray
diffraction of the prepared titanium oxide powders.
A suspension of TiO2 in the distilled water was sup-
ported on the internal wall of a quartz glass reactor, whic h
was transparent for the light with wav elength longer than
185 nm. The TiO2 layer was dried at room temperature in
air for a week. Then, without any additional treatment, the
reactor was sealed to the high vacuum system for the

98 Photochemical Properties of Precipitated Solid Aerosol Produced by Burning of Titanium Microparticles under Ambient Air
photoadsorption experiments. The reactor was evacuated
to 10-5 Pa at room temperature for seve ral hours.
Freon 134a (asymmetric tetrafluoroethane, CH2FCF3)
was provided by the State Scientific Center of Applied
Chemistry (St. Petersburg). Freon 22 (CHF2Cl) was pro-
duced by the Ural factory “Halogen”. Before experiments
this compounds were cleaned additionally by freezing.
The chemical composition of the freons during the course
of the experiment was monitored with a mass spec-
trometer via their sampling through a leak valve.
The pressure into the reactor volume was measured
with the Pirani gage, which was calibrated with oxygen. A
necessary correction was introduced for the sensitively
with respect to Freons. The composition of the gas phase
was measured with a mass spectrometer made from an
monopol e analyzer APDM-1. The ki netics of the observed
processes were registered by measuring the alteration in
amplitude of the most intensive peak in the mass spectra
of the corresponding substances.
A DRSh-250 high pressure mercury lamp illuminated
the reactor through heat water and interference filters. An
RTN-20S thermopile was used to measure the radiation
intensity. In some experiments, the unfiltered radiation of
the mercury lamp at > 300 nm was used; for this purpose
the interference filter was replaced by colored glass UV
filter. The total density of the radiation flux was 1 mW
cm-2 through this filter.
The effective quantum yield was calculated as the ratio
of the number of photoadsorption or photodesorption
molecules to the number of quanta penetrated through the
external wall of the reactor.
The diffuse reflectance spectra were taken in air on a
SPECORD M40 spectrophotometer. In the studies a
powder-like magnesium oxide was used as the reference
standard.
3. Results and Discussion
3.1 Optical Properties of Powdered Titania
Samples
We studied the absorption properties of dispersed titania
after storage in air for a long period of time and after
activation by heating at 625 K in air for 30 min with re-
spect t o the light from diff erent parts of the solar spectrum.
The experiments with the samples contacted with the air
for a long time are especially important for evaluating
theprospects of titania prepared by this method for appli-
cation in processes leading to purification of the environ
ment during various man-caused catastrophes.
There are three stable crystalline modifications of tita-
nia encountered in the nature in the form of rutile, anatase
and brookite crystals. Titania samples of all these crys
talline modifications with stoichiometric chemical com-
position are white powers. The absorption of light with
energies below the band gap of titan ia in rutile modifica-
tion (Eg = 3.05 eV) is minimal by all such powders
(Figure 1, inserts a) and b)). This absorp tion cor respo nds
to the impurity absorption bands for single crystals or
surface absor p tion for f in e ly dispersed powders.
According to these data, there is a bend on the absorp-
tion edge curve for powdered titania samples consisting
from a mixture of anatase and rutile modifications (Fig-
ure 1, insert s a) and b ), cu rv es A, B, and C) . Su ch a b end
is not observed for pure anatase (Figure 1, inserts c),
curve D) or rutile (Figure 1, insert c), curve E) samples.
Titania produced by Degussa contains rutile, an atase and
some amorphous phase [6]. The absorption band edge of
anatase powder is shifted by 0.1-0.2 eV to shorter wave-
lengths compared to that of rutile powder (3.0 and 2.8-2.9
eV, respect i v e ly).
If the titania stoichiometry is changed due to partial
reduction of Ti4+ to Ti3+ and formation of lattice oxygen
vacancies du ring the oxide synt hesis, the pow ders becom e
colored and absorption in the surface band of the oxide
appears. Figure 1 (insert c), curve E) shows the diffuse
reflectance spectrum of TiO2 sample prepared at Bore-
skov Institute of Catalysis, which had substantially non-
stoichiome t ri c composition and gree n color. Acco r di n g t o
the results of the XRD studies, such sample contained
titania in rutile modification. The greenish titania sample
studied by us consisted of TiO2 in rutile modification
mixed with ana tase. It was shown by EPR t hat this sample
(which was use d i n the phot oa dso rpt ion e xpe ri m ents) was
not stoichiometric and contained Ti3+ ions. The meas-
urements of diffuse reflectance optical density (Figure 1,
insert c), curve F) also proved that of sample was not
stoichiometric. The titania sample was not subjected to
any treatment during registration of the spectra. A similar
sample was used for deposition on the reactor walls. The
diffuse reflectance optical density spectrum of titania
heated in air at 620 K for 30 min is shown in Figure 1
(insert d), curve H). Its spectrum is compared with that of
the titania sampled from the gas mixture after combustion
of the pyrotechnic mixture using a vacuum impactor
(Figure 1, insert d), curve G).
Thus, the absorption of titania powders prepared by
combustion of pyrotechnic mixture containing titanium
microparticles in the region before the TiO2 intrinsic ab-
sorption edge (corresponding to the region of the solar
troposphere irradiation spectrum) depends on the synthe-
sis method and after-treatment of the samples.
3.2 Chemical Properties of Titania Prepared by
Combustion of Titanium Microparticles in
Air
When the reactor with deposited TiO2 was evacuated at
room temperature, water and carbon dioxide were the
ain products released from the surface of the titania m

Photochemical Properties of Precipitated Solid Aerosol Produced by Burning of Titanium Microparticles under Ambient Air 99
1,5 2,0 2,5 3,0 3,5 4,0
0,0
0,5
1,0
1,5
Fig1inseta
B
A
Ener gy/e V
Optical density
1.5
1.0
0.5
0.0
1.5 2.0 2.5 3.0 3.5 4.0
Opt ic a l d e ns i ty / A.U .
1,5 2,02,5 3,0 3,5 4,0
0,0
0,4
0,8
1,2
1,6
2,0
b)
D
C
Energy / eV
Optical density / A.U.
2.0
1.6
1.2
0.8
0.4
0.0
1.5 2.0 2.5 3.0 3.5 4.0
(a) (b)
1,5 2,0 2,53,0 3,5 4,0
0,0
0,2
0,4
0,6
0,8
1,0
Optical density / A.U.
Energy / eV
0,04
0,08
0,12
c)
Optical density / A.U .
F E
1.0
0.8
0.6
0.4
0.2
0.0
0.12
0.08
0.04
1.5 2.0 2.5 3.0 3.5 4.0
1,5 2,0 2,5 3,0
0,30
0,32
0,34
0,36
0,38
0,40
Optical density / A.U.
Energy / eV
0,14
0,16
0,18
0,20
0,22
0,24
0,26
0,28
d)
Optical density / A.U.
G
H
0.40
0.38
0.36
0.34
0.32
0.30
0.28
0.26
0.24
0.20
0.18
0.16
0.14
1.5 2.0 2.5 3.0
(c) (d)
Figure 1. The diffuse reflection spectra for TiO2 samples of different mode of production; (a) A-plasmotrone production,
specific surface area 3 m2g-1 (Russia); B-TiO2 of anatase crystal structure after an air treatment at 1270 K, 8 m2g-1 (Russia,
“Reachim”); (b) C-plasmotrone, 50 m2g-1 (Germany, “Degussa”); D-plasmotrone, 100 m2g-1 (France); (c): E-greenish TiO2 of
rutile crystal structure, (Russia, Institute of Catalysis); F-TiO2 used in the present work (mixture of rutile and anatase crystal
structures); (d): G-used in the present work TiO2 after an air treatment at 620 K during 30 min; H-TiO2 collected immediately
after pyrotechnic mixture combustion
powder. During the experiments described below water
vapor was always present in the reactor. Wh en the reactor
volum e was conn ecte d t o th e mea surement vol ume, water
vapor was trapped in the reactor volume in a trap with a
cooling liquid (eth yl alcohol cooled to 173 K). Prolong ed
evacuation of the reactor volume for 1 h in high-vacuum
installation through a trap with the co oling liqu id resulted
in partial removal o f carbon diox ide adsorb ed on the tita-
nia surface. As a result, quasi-equilibrium filling of the
surface with CO2 was established. The kinetics of ap-
proaching to the equilibrium CO2 pressure is shown in
Figure 2, curve A. This kinetics is presented as the de-
crease of the amount of CO2 molecules adsorbed on the
titania surface (desorption). The kinetic curve follows the
first-order equation with the time constant 1  6 min.
No other gases except for CO2 and H2O were observed to
be released in measurable amounts in the dark at room
temperature from the surface of titania prepared by com-
bustion of the pyrotechnic mixture containing ammonium
perchlorate and organic stabilizer under ambient air.
Much more processes take place on the titania surface
under visible light illumination than in the dark. In edition
to the dark desorption of CO2, its photodesorption was
observed with the first-order kinetics similar to that of the
dark desorption (2  5 min) by 20 times larger amount of
desorbed CO2 (Figure 2, curve B). The photodesorption
kinetics is pres ented as t he decrease i n the amount of C O 2
molecules adsorbed on the titania surface. Figure 3 pre-
sents the data of the mass-spectrometric analysis of
changes in the intensity of the peak with mass 44 (the
main peak i n the m ass spect rum of car bon di oxi de) duri ng
illumination with light passing through the interference
filters 436 nm and 308 nm. The kinetics of CO2 photo-
desorption during illumination through the interference
filter 436 nm (region of impurity surface absorption on
TiO2, Figure 1) is different from that observed during

100 Photochemical Properties of Precipitated Solid Aerosol Produced by Burning of Titanium Microparticles under Ambient Air
Figure 2. The kinetics of the desorption СО2 from TiO2
surface. A-decrease of number of СО2 molecules on TiO2
surface (dark desorption); B-decrease of number of СО2 (in
addition to dark desorption) under illumination of TiO2
surface through the UV filter (photodesorption)
024681012
0
2
4
6
8
10
12
Evacuation
Evacuation
308 nm
436 nm
Intens ity of 44 mass/ A. U.
Time/min
Figure 3. The kinetics of the desorption СО2 from TiO2
surface under light with wavelengths 436 nm and 308 nm
(mass-spectrometry data)
300 400 500 600 700
0,00
0,04
0,08
0,12 C
B
A
Optical density
Q
uan
t
um y
i
e
ld
(%)
Wa vele ngt h / nm
0,04
0,08
0,12
0.12
0.08
0.04
0.00
0.12
0.08
0.04
Optical density / A.U.
Figure 4. Spectral dependencies for titanium dioxide, pre-
pared by burning of titanium particles under ambient air.
A-photodesorption СО2 after first illumination; B-photo-
desorption СО2 after the long-time illumination through the
UV filter; C-diffuse reflection spectrum for TiO2 which is
the same as located on the wall of a reactor
illumination through the interference filter 308 nm (region
of intrinsic TiO2 absorption, Figure 1) by faster decrease
of the CO2 desorption rate. In the former case, the kinetics
corresponds better to the first-order reaction kinetics.
Meanwhile, in the latter case during illumination with
ultraviolet light (Figure 2, curve B) the kinetics is des-
cribed by a linear equation.
The measurement of the spectral dependencies of the
CO2 photodesorption quantum yield showed that the
highest quantum yield was observed in the region of the
TiO2 intrinsic absorption (Figure 4, curves A and B). A
diffuse reflectance spectrum of the titan ia powder used in
this study is shown for comparison (Figure 4, curve C). If
the time of the TiO2 surface illumination was increased to
several hours, the CO2 photodesorption quantum yield
decreased.
The CO2 photodesorption in the region of the TiO2 in-
trinsic absorption (including the 308 nm filter) is appar-
ently caused by deep oxidation of adsorbed hydrocarbons
with the oxygen from the air [7]. The processes pre-
dominating in the region of the surface absorption are
related to the electron transfer from the oxide valence
band to the surface and discharging of the surface com-
pounds (e.g. CO3-, CO2-) with the electron transfer to the
conduction band. In the former case, the CO2 photode-
sorption results from discharging of the surface carbonate
and carboxylate groups [8] with a mobile hole of the va-
lence band. In the latter case, the CO2 photodesorption to
the gas phase is related to direct discharging of these
compounds. The efficiency of the absorption with the
formation of electron-hole pairs is substantially higher
than that in the surface absorption band (see the titania
diffuse reflectance spectrum, Figure 1, inser t c), c urve F).
The quantum yi eld of p hoto deso rptio n in t he regi on o f the
oxide intrinsic absorp tion is also much higher than in th e
surface absorption band (Figure 4).
No photoadsorption of Freon 22 (CHF2Cl) and Freon
134a (CF3CH2F) was observed under illumination with
unfiltered light of the DRSh-250 mercury lamp or illu-
mination through a UV filter over titania prepared under
ambient conditions and stored in air for a long time. No
photoadsor pti on of t hese halocarb ons was o bserve d in the
presence of dry air as well. According to the literature data
no photocatalytic oxidation of methane is observed on
TiO2 [9,10]. However, photoadsorption of methane and
hydrogen was observed on rutile after high-temperature
oxygen-vacuum treatment [11-13]. Photoadsorption of
halogenated methane and ethane derivatives (Freon 22
and Freon 134a, respectively) was quite efficient on MgO
and CaCO3 exposed in air for long time [3,14]. Photo-
catalytic oxidation of carbon monoxide [7] and some
other gas-phase reaction, e.g. photocatalytic oxidation of
C2+ alkanes [9, 10,15] , arom atic hydr ocarbons [16], photo-
catalytic destruction of chlorinated ethane derivatives
[17,18], etc., occur efficiently on the TiO2 surface.

Photochemical Properties of Precipitated Solid Aerosol Produced by Burning of Titanium Microparticles under Ambient Air 101
In our experiments we observed oxygen photoadsorption
on the TiO2 surface from the air dried by passing through a
trap cooled with liquid nitrogen. Photodesorption of carbon
dioxide takes place simultaneously with the oxygen
photoadsorption. The initial rate of oxygen photoadsorption
exceeds that of the CO 2 photodesorption. So, the total pres-
sure registered by a Pirani manometer decreases (photoad-
sorption). The kinetics of the oxygen concentration decrease
(oxygen photoadsorption) determined from changes in the
intensity of peak with mass 32 (mass-spectrometer data) is
shown in Figure 5, curve A. This kinetics is described by a
first-order equation with time constant 3  3 min. This
process ends only when all oxygen from the reaction vol-
ume is photoadsorbed. Even during the first illumination the
amount of photoadsorbed oxygen exceeds that of photode-
sorbed CO2 by a factor of 5. The kinetic of CO2 photode-
sorption when oxygen is present in the gas phase is charac-
terized by much longer time constant (4 > 10 min, Figure 5,
curve B) than those of the dark desorption (1) and photo-
desorption of CO2 (2).
According to the spectral dependencies of the quantum
yield of oxygen photoadsorption from dried air, the
photoadsorption was more efficient in the region of the
intrinsic TiO2 absorption (Figure 6, curves B and C). The
edge of the quantum yield spectrum of oxygen photoad-
sorption was shifted to longer wavelengths (550 nm)
compared to that of the CO2 photodesorption (500 nm,
Figure 4).
So, it appears that the oxygen photoadsorption on the
titania sample prepared by combustion of titanium parti-
cles in air does not result from the oxygen interaction with
the surface electron sites induced by illumination. The
oxygen photoadsorption (consumption of oxygen from the
gas phase) occurs as a reaction of oxygen molecu les with
compounds adsorbed from the air earlier, i.e. after syn-
0510 15 20 25
0
500
1000
1500
2000
B
A
Intensity / A.U.
Time / min
Figure 5. The kinetics of the photoinduced processes on TiO2.
A-photoadsorption of oxygen from the mixture of dried air
and Freon 134a (the alteration in amplitude of peak for m/z 32
in the mass spectra); B-increase of number of СО2 molecules
under illumination of TiO2 surface through the UV filter
(photodesorption, mass-spectrometric data)
200 300 400 500 600 700
0,02
0,04
0,06
0,08
0,10
0,12
0,14
0,16
Optica l
densi ty
Wavelength / nm
0,00
0,04
0,08
0,12
0,16
0,20
Quantum yield (%)
C
B
A
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.20
0.16
0.12
0.08
0.04
0.00
Optical density / A. U.
Figure 6. Spectral dependencies for titanium dioxide, pre-
pared by burning of titanium particles under ambient air.
A-diffuse reflection spectrum for TiO2 which is the same as
located on the wall of a reactor; B-photoadsorption of oxy-
gen from the dried air; C-photoadsorption of oxygen from
the mixture of dried air and Freon 134a
thesis and storage of the titania sample. The registered
oxygen photoadsorp tion k inetics is close to the first order
kinetics. Meanwhile, the photoadsorption kinetics of
simple gases on metal oxides (photoadsorption on surface
sites generated by illumination) after th e thermal vacuum
treatment of the oxides is more complex [9,11,19]. The
reaction of oxygen with adsorbed compounds is also
proven by the change of the time constant of CO2 photo-
desorption in the presence of oxygen in the reaction
volume and by the increase of the amount of CO2 formed
in this case. The consumption of oxygen from the gas
phase under illumination for oxidation of organic sub-
stances adsorbed on TiO2 was earlier suggested by Solo-
nitsyn [20].
4. Conclusions
Thus, the optical and chemical properties of titanium
dioxide prepared by combustion of a pyrotechnic mixture
are close to those of titania prepared by other methods.
The band gaps of TiO2 samples prepared by different
methods are the same. Absorption in the spectral region
below the intrinsic absorption edge, which is related to
changes in the titania stoichiometry, is also observed.
Photodesorption of CO2 and photoadsorption of oxygen
are observed on TiO2 in wide range of the solar tropo-
sphere irradiation spectrum. No reaction of the titania
surface with methane and its halogenated derivatives as
well as ethane and its halogenated derivatives was ob-
served. Meanwhile, we observed pho tocatalytic oxid ation
of carbon monoxide.
It is natural to expect that titania prepared by combus-
tion of the pyrotechnical mixture in air will be active in the
same reactions where phot ocatalytic oxidat ion is obser ved.
These are oxidation of ethylene and its halogenated de-
rivatives, oxidation of organic acids, alcohols, etc. If such
processes take place under conditions close to those ob-

102 Photochemical Properties of Precipitated Solid Aerosol Produced by Burning of Titanium Microparticles under Ambient Air
served in the combustion zone, they can contribute to
cleaning of the environment from various gaseous pol-
lutants.
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