Zinc oxide-based photocatalyst on woven fiberglass support for destruction of volatile organic compounds in air

Article abstract of DOI:10.1134/S1070363210130098

Method for production of photocatalysts based on zinc oxide deposited to glass fibrous woven material is discussed. The process of destruction of volatile organic compounds acetone in gaseous phase with application of the developed photocatalytic material was investigated.

Full text of DOI:10.1134/S1070363210130098

ISSN 1070-3632, Russian Journal of General Chemistry, 2010, Vol. 80, No. 13, 2010, pp.2762–2767. © Pleiades Publishing Ltd., 2010.
Original Russian Text © S.V. Spitskiy, A.D Parshina, 2010, published in Ekologicheskaya Khimiya, 2010, Vol. 19, No. 4, pp. 205–210.
Zinc Oxide-Based Photocatalyst on Woven Fiberglass Support
for Destruction of Volatile Organic Compounds in Air
S. V. Spitskiy and A. D. Parshina
Saint-Petersburg State University of Technology and Design, Saint-Petersburg, 191186 Russia
Received June 24, 2010
Abstract — Method for production of photocatalysts based on zinc oxide deposited to glass fibrous woven material
is discussed. The process of destruction of volatile organic compounds acetone in gaseous phase with application of
the developed photocatalytic material was investigated.
DOI: 10.1134/S1070363210130098
Volatile organic compounds (VOCs) are widely
used by the modern industry for different applications.
Those compounds are heavy toxicants and have a strong
odour even at low concentrations in the ambient air. The
sources of such emissions being frequently unorganized
therefore influence significantly the quality of life in
adjacent areas. Specifically one of the most widely used
and most harmful components of the gaseous emissions
from diverse anthropogenic sources is acetone. Only in
USA more than 1.5 million tons of acetone are spent an-
nually, a significant part of which evaporates and comes
with gaseous emissions to the atmosphere.
Photocatalytic method is regarded as one of the
most promising technologies for purification of gaseous
emissions. It combines the advantages of adsorptive
and destructive photochemical and catalytic methods
for detoxification of emissions. The method is based on
application of heterogeneous photoactive substance to
absorb the contaminants. The elementary active charged
particles are formed in the structure and on the surface of
the material under irradiation of certain spectral region.
These particles interact with complex organic substance
adsorbed on the material and can promote their destruc-
tion. As a result, a transformation of toxic compounds
of the emissions occurs and less harmful highest oxides
of its corresponding elements appear [2, 3].
Acetone is refered to the fourth class of harmfulness
and can exhibit irritating and toxic effects in human
organism. In the environment it generally exists in form
of vapour in the air. Its maximum allowable concentra-
tion is 0.35 mg·m-3. Aceton is moderately stable in the
natural environment; its photochemical destruction in
atmosphere with hydroxyle radicals or under direct
solar irradiation (UV irradiation) is going relatively
slow with half-destruction period 1/2 = 7180 days [1].
Furthermore, acetone is found to be an intermediate in
destruction of many other complex organic substances.
Taking the above mentioned into account, solution of
the problem of acetone removal from gaseous phase
appears quite challenging task for protection of the
environment.
The photocatalytic systems do not require addi-
tional resources except electricity to power the source
of irradiation. These systems do not normally consist
of moving parts making operational wearing rather
small and decreasing the need for routine service and
repair [4, 5].
Photocatalytic method works at temperature of am-
bient environment which excludes the need for heating
of the treated emissions. Photocatalysts like oxides
of titanium, zinc, tin, iron, and some other transition
metals are widely available and low cost, especially, in
comparison with traditional palinoids-based catalysts.
Sergey Viktorovich Spitskiy, Cand. Sci. (Techn.), Assistant Professor,
Department of Engineering Chemistry and Industrial Ecology, Saint-
Petersburg State University of Technology and Design. Scientific areas of
locus: detoxification of waste water and gaseous emissions, photochem-
istry, photocatalysis, ecotoxicology, environmental management.
Alexandra Dmitrievna Parshina, post-graduate student,
Department of Engineering Chemistry and Industrial Ecology,
Saint-Petersburg State University of Technology and Design.
Scientific areas of locus: detoxification of gaseous emissions,
photocatalysis.
2762

ZINC OXIDE-BASED PHOTOCATALYST ON WOVEN FIBERGLASS
2763
The majority of photocatalytic materials are chemically
stable and irradiated surface of the materials undergoes
self-cleaning, therefore, poisoning of the catalysts ap-
pears to start much more slowly than for traditional
heterogeneous catalysts. All those factors underlie the
rapidly growing interest for photocatalytic processes in
all industrially developed countries [6].
most perspective supporting material for photocatalysts
is fibreglass, which is comparable to polymers, due to
flexibility and to metals, due to mechanical stability. It
also has the inherently high chemical resistance com-
parable to glass [4, 6, 10−12].
In this study we investigated a range of technological
approaches to prepare the photocatalytic materials based
on zinc oxide deposited to fiberglass. In the process of
thermal treatment in air media of the woven fibreglass
impregnated with zinc salt solution,zinc oxide is formed.
It is attached to the carrier by adhesion and partly by
sintering with the glass surface.
The majority of existing technologies and, con-
sequently, the research publications in this area are
based on application of titanium dioxide. It has a very
high chemical stability and low cost. However, appli-
cation of TiO2 is limited by the fact that only one of
four its crystal modifications exhibits photocatalytic
activity [7].
OBJECTS AND METHODS
Ability of zinc oxide to initiate destruction of
organic substances under irradiation is known since
early 20th century with the first research on this
matter published in 1926. Zinc oxide has the same
advantages as TiO2 except the high chemical stabil-
ity of the latter. This has a dominant importance in
selection of catalyst to be used in aqueous phase
where the formation of soluble zinc compounds at
pH different from 7 can be expected. However, this
fact is not important for processes in gaseous phase
where secondary pollution of the treated media by
ions of the catalyst is excepted.
As a precursor for formation of ZnO without any additives
negatively affected the photocatalysis (like halogens or
sulphur compounds which are formed if zinc chloride or
zinc sulphate are used) zinc nitrate was used as the most
widely available zinc salt. Alternatively, zinc acetate was
used as more environmentally friendly reagent, since it
does not give toxic nitrogen oxides in thermal processing.
To prepare the fibreglass photocatalytic material
the samples of fibreglass cloth were impregnated with
saturated solutions of the aforementioned salts for 30
min, after which they were dried and burned in muffle
chamber. The burnt samples were then washed with
distilled water for 2 min to evaluate the fastness of
fixation of zinc oxide to the surface.
Cost of ZnO of the required grade is 20–40% lower
than that of TiO . Moreover, ZnO is more stable in
higher temperatur2es which are required for certain tech-
nologies for immobilization of the catalysts to carrier
materials. There are also some evidences of the higher
photocatalytic activity of ZnO in some applications in
comparison with TiO2 , which is discussed in literature
and was confirmed in our previous studies [8, 9].
It was shown that the optimal temperature for
burning the samples was 350°С for zinc acetate and
300°С for zinc nitrate. At those temperatures the
yield of oxide determined with X-ray fluorescent
spectroscopic measurement of zinc concentration in
samples reached 95−99% of the theoretically possible
stoichiometric values. It was also determined after
quick washing of samples in distilled water that oxide
layer resulting using zinc acetate is 2530% weaker at-
tached to the fibres than that resulting from zinc nitrate.
According to those results, zinc nitrate as a precursor
as chosen, since it required less energy for thermal
treatment and produced the oxide layer more strongly
attached to the support.
Photocatalytic processes take place on the catalyst
surface. Therefore their efficiency primarily depends
on the size and accessibility of the surface for rea-
gents (oxygen, water vapour, organic compounds) and
for irradiation. The latter factor usually limits the ef-
ficiency of heterogeneous photocatalysis in gaseous
phase. To provide enough accessibility of the surface
for irradiation the catalyst should be deposited to a
certain supporting material. The most efficient carriers
hold a thin high-porous layer of catalyst and repeat the
shape of the radiation source for maximal utilization
of the light stream. The supporting material should be
inert and mechanically stable enough to withstand the
destructive activity of the catalyst. Proper fixation of
the catalyst on the carrier is also an essential condition
of the long stable operation of the system. One of the
It was also demonstrated that organic compounds
used in preparation of fibreglass woven materials to
protect fibres decreased the fastness of fixation of the
oxide layer to the supporting material in textile treat-
ment and increased duration of thermal treatment and
consumption of energy, as a consequence. In the samples
where the coupling organic agents were previously re-
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2764
SPITSKIY, PARSHINA
80
Photocatalytic material (3) was placed between the
steel body and the quartz sleeve. At the opposite ends
of the reactor body the inlet and outlet pipes (4) for
gaseous media circulation were located.
70
60
50
40
30
Changes of composition of the gaseous media were
monitored with IR-Fourier spectrometer (7) equipped
with pass-through gas cuvette (6) with the optical path-
way 100 mm and KBr windows 40 mm in diameter.
The cuvette was attached with polyethylene pipes to the
reactor. For circulation of the gaseous phase within the
assembly a microventilator (5) with the output about
5 cm3 sec-1 was used. A chromatographic injection port
(8) was connected to the reactor inlet to allow precise
injection of the studied compounds into the assembly.
Gas cuvette was added to the reactor outlet. Inner
volume of the assembly was completely isolated from
the outer air.
Cinit, g l–1
20
0
500
1000
1500
Fig. 1. Dependence of the specific mass of ZnO (SM) in the
fiberglass samples on the initial concentration of Zn(NO3)2
(Cinit) in the impregnating solution.
moved by burning at 500С oxide layer was distributed
more evenly on the surface (evaluated with microscope
observation) and was 35−40% more strongly attached
to the material compared to the samples without pre-
liminary treatment.
In the experiments with photocatalysts the de-
gree of removal of a substance from gaseous phase
was determined as a % ratio of difference between
its initial and current concentrations to the initial
concentration of the substance. Also the concentra-
tion of carbon dioxide was controlled in the media,
as the basic indicator of destruction of the organic
compounds. The experiments were repeated 3 times
without changing of the photocatalyst, whereas no
significant diminution of the acetone removal rate
was observed. The results (the average means) are
presented in Fig. 4. Time keeping was started from
the moment of switch on the UV lamp. Before this
recirculation of the gaseous media in the assembly
was conducted for 3 minutes to align the concentra-
tion of acetone in the apparatus.
Using the impregnating solutions of a range of con-
centrations (from 300 to 1500 g l-1) the yield of zinc
oxide (measured as zinc) with rather high fastness of
fixation changed according to the dependence shown
at Fig. 1.
Those data suggest the logarithmic character
of the dependence. Maximal amount of zinc oxide
on the supporting material is achieved at the ini-
tial concentration of zinc nitrate in the solution of
1000 g l-1.
RESULTS AND DISCUSSION
It can be seen from the data that the destruction of
acetone and formation of CO occur with gradual de-
celeration of reaction at a dyn2amic dependence typical
for such processes
Samples of the fibreglass photocatalytic material
prepared with the above described method were stud-
ied to determine their ability to destruct VOCs in air
media. For the study an experimental assembly was
used which exterior and construction are presented on
Figs. 2 and 3.
Measured concentrations of carbon dioxide sug-
gest that the amount of CO2 formed at destruction of
acetone is lower than expected from stoichiometry of
the reaction of complete mineralization calculated from
the decay of acetone during the process. This can be
explained with several arguments:
Experimental assembly consisted of the annular
tube reactor with outer body of stainless steel (1) and
inner coaxial quartz sleeve where the source of UV
radiation (2) was placed. The UV radiation source was
the standard mercury low-pressure 8 W lamp Phillips
G8T5 TUV. This type of lamps produces almost mono-
chromatic irradiation in the far UV range (-UV) with
maximum (over 95%) of emission at the wavelength
253.7 nm.
• binding of carbon in form of non-volatile inorganic
compounds on the catalyst surface;
• predominance of adsorption processes over de-
struction in removal of organic matter from gaseous
phase;
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ZINC OXIDE-BASED PHOTOCATALYST ON WOVEN FIBERGLASS
2765
Fig. 2. Experimental assembly exterior.
Fig. 3. Experimental assembly conceptual
scheme.
• multistage reactions of destruction of organic
compounds.
correspond to the amount of carbon atoms in the de-
structed organic molecules However, in practice such
a correspondence did not always take place. Firstly,
destruction of organic compounds in the system was
not complete, as some part of those could be adsorbed
on the inner surfaces of the experimental assembly.
Secondly, some part of the organic matter, particularly,
due to the deficiency of oxygen, could undergo changes
yielding in low-volatile compounds, which did not easily
desorbed from the catalyst in experimental conditions.
Thirdly, part of the resulting inorganic carbon could
be trapped in form of surface carbonate ions accord-
ing to mechanism presented at scheme of destruction
of ethanol:
According to the obtained results, it can be definitely
concluded that photocatalytic process of destruction
is taking place in the studied system. To confirm the
photocatalytic nature of reaction the results were com-
pared to dark experiments (without UV irradiation). The
comparison proves that destruction process (detected by
carbon dioxide build up) does not go without irradiation
of the catalytic material.
Theoretically, the amount of CO2 formed in pho-
tocatalytic destruction of organic compounds should
16000
14000
12000
10000
8000
6000
4000
2000
0
0
1000
2000
UV irradiation time, sec
Acetone Carbon dioxide
3000
4000
5000
6000
Fig. 4. Changes of concentrations of acetone and carbon dioxide in photocata-
lytic process.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol.80 No. 13 2010

2766
SPITSKIY, PARSHINA
carried out with zinc oxide layered on the inner surface
CH3
OH
CH
O
CH3
CH
OH
of the steel reactor body. Zinc oxide has certain adhe-
sion to various surfaces. ZnO coating was formed with
3 g l-1 dispersion of the oxide in water and then dried
at 105°С and cooled to the room temperature. After the
experiments the oxide layer was removed by washing
the surface with 0.1 M HCl solution.
CO2
CH3
C
O
O–
CO–32
When the oxide layer on the inner surface of the
reactor was used as photocatalyst the observed degree
of removal of acetone from the gaseous media in 1 h
was in average 834%. Using the previously developed
fibreglass photocatalytic material degree of removal in 1
h achieved 933%. These results showed a statistically
sound prevalence in efficiency of the developed material.
This prevalence could be attributed to the more highly
developed surface of the woven fibreglass material. This
fact does again confirm the prospectives of using the
woven supports for the catalysts.
CH3
C
O
Possibility of formation of less volatile compounds
in photochemical reactions was discussed by many au-
thors. The following scheme of condensation of organic
compounds under oxygen deficiency can be given as an
example of this reaction [13]:
+ H+
H C
C
O
CH
H C
C
CH
+
3
3
3
2
The photocatalytic materials based on woven glass
are very promising due to their ease of use and low
cost. The important feature is a possibility to use these
materials in filtering devices, for which the overall ef-
ficiency is expected to be higher compared to the reac-
tors similar to the one used in the present study. In the
same time the membrane (filtering) reactors are mostly
required for air disinfection rather than for removal of
VOCs. For the latter task the non-filtering materials
would have longer service life and lower aerodynamic
resistance [14].
OH
OH
H C
C
CH
O
3
3
CH
C
2
3
CH
H C
C
O
CH
+
3
3
CONCLUSIONS
H C
C
3
CH3
Results of the study show the potential efficiency
of photocatalyst based on zinc oxide and of the
photocatalytic materials on the fibreglass woven
support for removal of volatile organic compounds
from the gaseous media (air). Infrared spectrometry
method used for control of gaseous phase speciation
proved to be enough efficient, though it required
deep integration of experimental assembly and the
measuring system. Further studies are also required
for determination pathways of the photocatalytic de-
struction of VOCs in gaseous phase and its products
and intermediates.
CH
C
+
H O
2
O
CH 3
Every reaction similar to those can lead to de-
activation of the catalyst. However, in the process
discussed in this study these reactions are hardly
probable since there always is a multifold stoichio-
metric excess of oxygen in the inner gaseous media,
even at relatively high concentrations of organic com-
pounds. Moreover, even if such low-volatile products
would accumulate on the surface of the catalyst, it
is highly probable that they would be destroyed and
mineralized further when the amount of oxygen in
the system increased.
The study was carried out at Saint-Petersburg state
university of technology and design and at the Labo-
ratory of applied environmental chemistry and in the
University of Kuopio (Mikkeli, Finland) with financial
support of the Federal agency on education of the Rus-
sian Federation (institutional programme “Development
of research potential of the higher school”).
For comparative evaluation of the efficiency of the
developed photocatalytic material an experiment was
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ZINC OXIDE-BASED PHOTOCATALYST ON WOVEN FIBERGLASS
2767
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