Microwave radiation solid-phase synthesis of phosphotungstate nanoparticle catalysts and photocatalytic degradation of formaldehyde

Article abstract of DOI:10.1016/j.molcata.2006.08.043

Photocatalysts M₃PW₁₂O₄₀ (M = NH₄⁺, Cs⁺, Ag⁺, Cu²⁺) were prepared by microwave radiation solid-phase synthesis method. The results from SEM and XRD characterization re

Full text of DOI:10.1016/j.molcata.2006.08.043

Journal of Molecular Catalysis A: Chemical 262 (2007) 149–155
Microwave radiation solid-phase synthesis of phosphotungstate
nanoparticle catalysts and photocatalytic degradation of formaldehyde
a
a
a
a
Qian Deng , Wenhui Zhou , Xiaomei Li , Zhenshan Peng ,
b
a
a,∗
Shaoliang Jiang , Ming Yue , Tiejun Cai
a
College of Chemistry & Chemical Engineeing, Hunan University of Science & Technology, Xiangtan 411201, PR China
b
Institute of New Catalytic Material Science, College of Chemistry, Nankai University, Tianjin 300071, PR China
Available online 23 August 2006
Abstract
Photocatalysts M
+
+
+
2+
3
PW12
O
40 (M = NH
4
, Cs , Ag , Cu ) were prepared by microwave radiation solid-phase synthesis method. The results
40 is 15–80 nm. Diffuse reflectance UV–vis spectra of
40. The
from SEM and XRD characterization reveal that the average particle size of M
40 shows that the absorption band shifted from 260 to 270–350 nm and become broad in comparison with those of H
results of photocatalytic degradation of formaldehyde prove that the shift of UV–vis absorption band is proportionally related to the activity of the
catalyst. The Cu catalyst which has the maximal red shift to 350 nm of its UV–vis absorption band is found to be the most efficient
catalyst. When the concentration of formaldehyde is 32.79 mg m , the conversion of the formaldehyde is basically completed under the flow rate
3
PW12O
M
3
PW12
O
3
PW12O
3
40 2
(PW12O )
−
3
−1
of 40 ml min over 0.1 g M
is easy. The catalysts have promising applications in the photocatalytic degradation of organic contamination.
2006 Elsevier B.V. All rights reserved.
3 3
PW12O40 catalyst. The M PW12O40 catalysts prepared by this method are stable and the regeneration of the catalysts
©
Keywords: Phosphotungstate; Photocatalyst; Nanoparticle; Microwave; Solid-state synthesis
1
. Introduction
porouspolyoxotungstate–anatasecanphotocatalyticallydecom-
posed the organophosphorus pesticide with visible-light excita-
tion [22]. The addition of phosphotungstate in PVC membrane
induces photocatalytic degeneration of plastic membrane, which
provides an effective way to eliminate white pollution [23].
The outstanding photocatalytic, electrical, magnetic proper-
ties of nano-materials and their increasing applications in many
fields are drawing much attention to their preparation method.
You [24] was the first who prepared polyoxometalates by room-
temperature solid-phase synthesis. Its procedure is simple and
can easily be repeated. However, it takes much time to grind
the reactant and consumes large amount of organic solvents to
eliminate the soluble reactant in the last step.
The microwave technique has its unique merit as the chemical
reaction heat source [25]. First, microwave has strong penetrat-
ing power and the reaction temperature can be increased fast
and evenly. Second, only the polarized material can absorb the
microwave. Third, it is a “non-thermal reaction” between the
microwave and the chemicals. As a result, it reduces the reaction
temperature. The microwave radiation solid phase synthesis has
many advantages such as rapid reaction, minimized subsequent
handling, wide ranges of application, high production rate, and
so on. The studies mentioned above are mostly aimed at reduc-
As our living standard is rapidly improving and the pace of
industrialization is accelerating, the environmental pollution is
becoming a serious problem. The photocatalytic degeneration of
organic contamination is one of the effective ways to eliminate
environment pollutant. At present, the research of this topic is
mainly concentrated on the titanium dioxide (TiO2) and poly-
oxometalates (POM) catalysts [1–11].
POM has excellent photochemical nature, and is being
explored in the environmental protection domain. Among them,
the photocatalytic activity of tungstate system is quite promi-
nent. For example, some aromatic derivatives and chlorinated
aceticcanbephotolyticmineralizedwithnearvisibleorUVlight
in the presence of PW12 or SiW12. These catalysts are, at least,
as effective as the titanium dioxide [12–14]; The PW12/TiO2
catalyst can degrade many kinds of aqueous dye [15–20]. The
activity dramatically increases when the particle size of the cat-
alysts is in the range of nanometer [21]. Nanosize and bimodal
∗
Corresponding author. Tel.: +86 732 8291 350; fax: +86 732 8290 017.
E-mail address: tjcai53@163.com (T. Cai).
1
381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.08.043

1
50
Q. Deng et al. / Journal of Molecular Catalysis A: Chemical 262 (2007) 149–155
ing organic pollutant in liquid phase nowadays. The application
of POM catalysts for the degeneration of gas phase pollutant is
still limiting.
H3PW12O40·19H2O + Cs2CO3
Cs3PW12O40·yH2O + H2O ↑ + CO2↑
→
Indoor air pollution seriously harms the human health and
has caused strong concerns. Among many indoor pollutants,
formaldehyde is one of the major culprits. Formaldehydes
introduced in home remodelling materials and processes have
becomehugehealthhazard, andarebecominganimportanttopic
of environment management.
H3PW12O40·19H2O + AgNO3
Ag3PW12O40·yH2O + H2O ↑ + NO2↑ + O2↑
→
H3PW12O40·19H2O + CuSO4·5H2O
In our study, phosphotungstate nanoparticle photocatalysts
were prepared by the microwave radiation solid phase synthe-
sis and photocatalytic performance of the catalysts was stud-
ied using simulated polluted air containing formaldehyde and
methyl alcohol. The correlations between the surface property
or the reaction conditions and the activity of photocatalytic
catalysts were investigated for the purpose of searching a pho-
tocatalytic method of formaldehyde management.
→ Cu3(PW12O40)2·yH2O + H2O ↑ + SO3↑
.2. Catalytic reaction
2
The photocatalytic activity tests were performed in a inter-
mission (Fig. 1a) and continuous-flow (Fig. 1b) fix-bed photore-
action system, respectively. 0.1 g catalysts were spread on the
bottom of the quartz reactor (ϕ/8 mm; L/300 mm). The quartz
reactor was in a constant temperature box irradiated under the
1
8 W UV lamp (λ = 253.7 nm). Atmospheric pressure air stream
2
. Experimental
was bubbled in a vessel filled with formaldehyde aqua (AR
reagent, containing formaldehyde 37–40% and methyl alcohol
2
.1. Catalysts preparation
−
1
18–20%) at a flow rate of 40 ml min . The UV lamp was not
The (NH4)3PW12O40 catalyst was prepared by first mix-
turned on until the concentration of the feed gas was stable. The
gas mixture passed through the catalyst bed equipped with a
six-port valve. The products were detected by gas chromatogra-
ing H3PW12O40·19H2O and (NH4)2C2O4·2H2O in molar ratio
under grinding in carnelian mortar for 5 min, powder turned into
dope. Then it was ground for 10 more minutes. The mixture was
then placed into porcelain crucible and treated in the microwave
oven for 5 min, with gases spillage immediately. Finally, the
phy (GC) analysis (the CO is detected using the saturated lime
2
aqua).
◦
product not purified was calcined in air for 2 h at 250 C, which
2.3. Life and regeneration tests
is (NH4)3PW12O40 photocatalyst.
The preparation of Cs3PW12O40, Ag3PW12O40 and Cu3
The life test was performed in continuous-flow fix-bed pho-
toreaction system (Fig. 1b) under the condition as shown in
Section 2.2. The concentration of organic compounds was mea-
sured every 30–60 min. After 72 h of life test the activity of
photocatalysts was still stable.
(
2
PW12O40)2 catalysts: H3PW12O40·19H2O and H2C2O4·
H2O were mixed by molar ratio (2:3) under grinding in car-
nelian mortar for 5 min. Then, the same molar Cs2CO3, AgNO3
and CuSO4·5H2O were added respectively into mortar, and con-
tinued grinding for 10 min. Then the next treatment was the same
as that for (NH4)3PW12O40 catalyst.
After photocatalytic reaction, the color of the catalysts
changed into the blue except Ag PW O that change into the
3
12 40
The reactions were shown as follow:
black. Passing the air over the catalyst bed after the life test, the
color of the catalysts was restored basically (the Ag3PW12O40
is a grey color, which may have a few of Ag). The reactivity of
catalysts was recovered by regeneration in air. It is shown that
H3PW12O40·19H2O + (NH4)2C2O4·2H2O + O2
→
(NH4)3PW12O40·yH2O + H2O ↑ + CO2↑
Fig. 1. Gas–solid photoreactor system: (a) intermission reactor and (b) continuous-flow reactor. (1) Air, (2) manometer, (3) valve, (4) desiccator, (5) mass flowmeter,
6) the air occurrence machine, (7) formaldehyde aqua, (8) photo-catalytic reactor, (9) UV lamp, (10) quartz reactor, (11) fan, (12) fixed amount tube, (13) six-port
valve, (14) tail gas absorb, (15) put empty, (16) carring gas, (17) gas chromatography, (18) GC work station, and (19) constant flow pump.
(

Q. Deng et al. / Journal of Molecular Catalysis A: Chemical 262 (2007) 149–155
151
the activity of photocatalysts is stable and the regeneration of
the catalysts is easy.
2
.4. Catalyst characterization
IR spectra of the catalysts were taken on a Perkin-Elmer
000 FT-IR spectrometer by KBr tablet method in the range
000–450 cm . Diffuse reflectance UV–vis spectra (UV/DRS)
2
4
−
1
of the solids were recorded at room temperature on a Perkin-
Elmer Lambda35 UV spectrometer equipped with an integrating
sphere and using BaSO4 as reference. The range of scanning
wavelength is 355–220 nm. The BET measurements were car-
ried out on a Chembet-3000 BET adsorption instrument under
an atmosphere of mixed H2–N2 containing 15% H2 (v/v) with
◦
−1
a temperature increasing rate of 10 C min and the final tem-
◦
◦
perature of 700 C. The samples were pretreated at 250 C under
Ar atmosphere for 30 min. The surface structure of the catalysts
was studied by scanning electron microscopy (SEM) on a FEI
Sirion200 microscope. X-ray diffraction (XRD) patterns were
measured by a MSAL XD-2 powder X-ray diffractometer using
˚
Cu K␣ radiation (λ = 1.54056 A). The 2θ angles were scanned
from 5 to 70 at a rate of 4 min .
◦
◦
◦
−1
The TG-DTA curve of the catalysts was obtained on a
WCT-IA differential thermal balance with a heating rate of
Fig. 2. FT-IR spectra of(a)H3PW12O40, (b)(NH4)3PW12O40, (c)Cs3PW12O40,
◦
−1
◦
1
3
3
0 C min . The final temperature was 570 C.
(
d) Ag3PW12O40, and (e) Cu3(PW12O40)2.
. Results and discussion
1
100 cm 1,which are attributed to [PW12O40] of the Keggin
− 3−
structure (Table 1) [29]. The bend vibration of H–O–H bond
and symmetry and dissymmetry flex vibration of O–H bond in
crystal water are marked by the very strong absorption peaks at
.1. Preparation of catalysts
In the preparation process of catalysts, H2C2O4·2H2O
−
1
3
434.81 and 1615.30 cm
.
was acted as a superficial dispersant and chelator of
H3PW12O40·29H2O. As a result, the superficial energy of
H3PW12O40·29H2O was restrained and it prevented sintering
of the particles. In addition, the crystal water in the reactants
created countless puny “pond”, which controlled the parti-
cle size effectively [26]. (NH4)2C2O4·2H2O, Cs2CO3, AgNO3
and CuSO4·5H2O could dissolve into the crystal water as
+
+
+
+
Fig. 2b–e show that the M3PW12O40 (M = NH4 , Cs , Ag ,
Cu ) retain the Keggin structure [29]. The four characteristic
absorption peaks display blue or red shift after M replaces H . It
suggests that the chemical bond in the [PW12O40] is affected
by the M , which strengthen or weaken in different degree.
The very strong absorption peaks at 3434.81 and 1615.30 cm
2+
+
+
3
−
+
−1
are marked the bend vibration of H–O–H bond and symmetry
+
M and acid radical anion. In the effect of electromagnetic
field of microwave and the “pseudo-liquid” phase [27] of
PW12O40]3 , M or M diffused into [PW12O40]
3−
and
−
+
2+
[
formed the M3PW12O40·yH2O [28].
Because of the restricting “pond” size, the particle size of
M3PW12O40 is controlled. The microwave’s strong penetrat-
ing power allows heating the sample inside-out at the same
time quickly and evenly, thus the reactants H2O, NO2, SO3and
CO2 are vaporized out instantaneously. The slipstream pre-
vents the agglomeration of particles, forming nanoparticles of
M3PW12O40.
3
.2. Characterization of the catalysts
The catalysts were characterized by FT-IR, UV/DRS and TG-
DTA, SEM and XRD, and their results are shown in Figs. 2–6
and Tables 1 and 2.
FT-IR spectrum of H3PW12O40 is shown in Fig. 2a, where
the catalyst has four strong absorption peaks between 700 and
Fig. 3. Diffuse reflectance UV–vis spectra of (a) H3PW12O40, (b) (NH4)3
PW12O40, (c) Cs3PW12O40, (d) Ag3PW12O40, and (e) Cu3(PW12O40)2.

1
52
Q. Deng et al. / Journal of Molecular Catalysis A: Chemical 262 (2007) 149–155
Fig. 4. SEM spectra of (a) (NH4)3PW12O40, (b) Cs3PW12O40, (c)Ag3PW12O40, and(d) Cu3(PW12O40)2.
and dissymmetry flex vibration of O–H bond in crystal water.
This indicates that there is some crystal water in M3PW12O40
nanoparticles.
The diffuse reflectance UV–vis spectra of M3PW12O40
+
+
+
2+
(
M = NH4 , Cs , Ag , Cu ) and H3PW12O40 catalysts are pre-
sentedinFig. 3. ThespectrumofH3PW12O40 presentsanintense
absorption band with a maximum at 260 nm, corresponding to
the charge transfer (CT) from O2p to W5d occurred at W–O–W
bonds, which has been described as the characteristic band of
Keggin-type H3PW12O40 [30]. This band shifts from 260 to
2
70–350 nm and has been widened and strengthened obviously
+
n+
after the H has been replaced by M .
+
+
+
2+
Among M3PW12O40 (M = NH4 , Cs , Ag , Cu ), substi-
+
n+
tution of H with M benefits the electron-transfer from the
PW12O403 to cation, due to their larger size than H . The
+
+
induction effect is advantage for the electron-transfer from
PW12O403 toM , resultinginalowerdensityofelectroncloud
−
n+
for W,O Oc,O . Thus, bond length of W–Oc W–O increases
b,
d
b
and the LUMO energy decreases [27]. The lower charge transfer
Fig. 5. XRD spectrum of Cu3(PW12O40)2.
energy barrie from the HOMO → LUMO could be expected, so
Fig. 6. TG-DTA spectra of (a) H3PW12O40, (b) (NH4)3PW12O40, (c) Cs3PW12O40, (d) Ag3PW12O40, and (e) Cu3(PW12O40)2.

Q. Deng et al. / Journal of Molecular Catalysis A: Chemical 262 (2007) 149–155
153
Table 1
The FT-IR fingerprint data of Keggin structure H3PW12O40 and phosphotungstate
P–Oa–W (cm 1)
−
W
Od (cm
−1
)
W–Ob–W (cm
−1
)
−1
W–Oc–W (cm )
Catalysts
H3PW12O40
1086.57
1080.35
1079.51
1061.51
1080.76
987.47
987.29
985.36
979.10
983.01
887.96
890.92
889.58
887.62
891.08
811.67
821.01
803.59
797.12
800.29
(
NH4)3PW12O40
Cs3PW12O40
Ag3PW12O40
Cu3(PW12O40)2
that light absorption strengthened. The influence decreases as
tension of Cu3(PW12O40)2 particles is great, so agglomeration
phenomenon of the catalyst grain is severe, bringing on its BET
surface area is the least (70.3 m g ). This phenomenon makes
the catalyst to form the network structure with tiny bore features,
which was confirmed by the TG-DTA (Fig. 6e).
2
+
+
+
+
+
Cu → Ag → Cs → NH4 . The significant influence of Ag
2+
2
−1
and Cu may also relates to their stronger oxidation ability
3−
after the combination with PW12O40 . The electron transfer
n+
O → M is still produced, which has longer wave-length than
n+
that of the characteristic band [27]. Because the O → M and
The BET surface area of the H3PW12O40 is very small
2
−1
O → W band overlaps so that the UV–vis absorption band is
broadened. Fig. 3 shows that there is a red shift of the main
peak and strengthened absorption corresponding to spectrum b
(2.4 m g ) originally. But after forming salt, the BET sur-
face areas of the M3PW12O40 increase consumedly, causing the
catalyst activity sites to enlarge. The BET surface areas of the
catalysts are related with the calcined temperature showing in
Table 2.
+
+
+
(
NH4 salt) and spectrum c (Cs salt), whereas, spectrum d (Ag
2+
salt) and spectrum e (Cu salt) become a strong broad peak.
◦
It indicated that M3PW12O40 catalysts increase the optical
absorption and broaden the range of wavelength. The optical
response of nanoparticle of M3PW12O40 catalysts expands into
near visible light region. This is in favor of the formation of
charge stream and increases the utilization of light energy.
When the catalysts were calcined at 250 C, the BET surface
areas of the catalysts reach maximum value. The areas decrease
rapidly when the calcined temperature increases. According to
thermodynamics, the nanoparticle is one kind of quasi-steady
structure, so one unit volume average free energy elevates when
the particle size reduces, which reduces its thermal stability.
The crystal particle growth process may be considered as the
smaller particle migrates to the bigger one, then the bigger parti-
cle enwraps the smaller one. This process needs to overcome the
interfacial energy, which is as the activating energy. We assumed
that the catalyst obtains the energy to overcome the interfacial
+
+
+
The SEM patterns for the M3PW12O40 (M = NH4 , Cs , Ag ,
2
+
Cu ) catalysts are shown in Fig. 4. The SEM pattern shows
that the M3PW12O40 catalyst is in a form of anomaly spher-
ical particles and shows some extent of agglomeration. There
are some defect sites on the edge and corner site, in which
the valence bond is unsaturated and has the surplus valence
bond power. These defect sites usually act as adsorptive and
catalytic sites. The average particle size of (NH4)3PW12O40,
Cs3PW12O40, Ag3PW12O40 and Cu3(PW12O40)2 is 40, 15, 40
and 80 nm, respectively.
◦
energy at about 250 C. When the temperature increases, the
particles agglomerate and its surface area is rapidly reduced, so
the catalytic active sites are reduced. This phenomenon indicates
that the activity of photocatalyst is dependent on its surface area
in certain degree. For same catalyst, the photocatalytic activity
is related with its surface area obviouslly, which is proved by
the catalytic activity of degradation of formaldehyde.
The XRD spectrum of Cu3(PW12O40)2 is shown in Fig. 5.
The crystallinity of the Cu3(PW12O40)2 is very good. Accord-
ing to Scherrer’s equation (D = 0.89λ/β cos θ), the particle size
calculated is about 76 nm, which is in accordance with the SEM
result above.
The Cs3PW12O40 (Fig. 4b) catalyst is in a form of spheroid
grain, and its size is small and uniform. The average particle
size of Cs3PW12O40 is about 15 nm. The BET surface area is
2
−1
2
13.5 m g . The particle shows a tendency to agglomerate to
lower the surface energy.
The particle size of the Ag3PW12O40 (Fig. 4c) catalyst is
smaller, and the average size is about 40 nm. It has larger BET
2
−1
surface area (179.7 m g ).
The Cu3(PW12O40)2 (Fig. 4d) catalyst is in a form of tiny
spheroid grain, and the average size is about 80 nm. The surface
The TG-DTA curves of H3PW12O40 catalyst are shown
in Fig. 6a. An endothermic peak at T < 130 C accompanying
◦
largest weight loss shows the existence of free lattice water
◦
and adsorbed water. Another endothermic peak at 130–300 C
Table 2
BET surface areas (m g ) of M3PW12O40 (M = NH4 , Cs , Ag , Cu2+) after
2
−1
+
+
+
accompanying weight loss is the emergence of a combined state
lattice water which is most likely the hydrated proton. The
exothermic peak at 599 C without weight loss indicates that
calcined at different temperatures
◦
◦
Temperature ( C)
the catalyst is decomposed.
The TG-DTA patterns of M3PW12O40 exhibit two to three
endothermic peaks at 60–235 C corresponding the desorption
2
00
250
300
70.7
131.9
83.2
43.8
350
400
◦
(
NH4)3PW12O40
90.5
134.8
213.5
179.6
70.3
47.6
79.8
49.2
30.9
39.5
42.9
37.2
24.2
Cs3PW12O40
Ag3PW12O40
Cu3(PW12O40)2
127.3
48.3
51.8
of absorbed water and lattice water (Fig. 6b–e). Except for the
Cs3PW12O40 catalyst, there is a keen-edged exothermic peak
◦
at above 527 C without weight loss but for the decomposition

1
54
Q. Deng et al. / Journal of Molecular Catalysis A: Chemical 262 (2007) 149–155
Table 3
The photocatalytic activities of catalysts to the formaldehyde degradation
Catalysts Conversion (%)
−3
−3
−3
−3
6
9.82 mg m
32.79 mg m
14.85 mg m
5.347 mg m
100
(
NH4)3PW12O40
18.32
24.63
25.16
39.36
∼100
∼100
∼100
∼100
Cs3PW12O40
Ag3PW12O40
Cu3(PW12O40)2
100
100
100
−3
The values 69.82, 32.79, 14.85, 5.347 mg m denotes the formaldehyde concentration.
of NH4+ (Fig. 6b) on the patterns of all other catalysts, which
is the decomposition temperature of the M3PW12O40 catalyst.
The decomposition temperature of the Cs3PW12O40 catalyst is
of the methanol reduces under ultraviolet light after the
reaction. Methyl ether, methyl formate and formic acid are
observed in the products. The formaldehyde peak vanishes
in GC. There are CO2 and no formaldehyde in the tail
gas, showing that the conversion of the formaldehyde is
completed. The formaldehyde is almost oxidized to formic
acid, part of formic acid is further oxidized to CO2 and
H2O.
◦
above 650 C.
On the TG-DTA of Cu3(PW12O40)2 (Fig. 6d), a small
◦
exotherm appears after 300 C because the growing size of the
catalyst grains causes the surface energy to release. At about
◦
3
30 C, because of the breakage of the tiny bore structure, the
catalyst releases a great deal of surface energy, showing an
exothermic peak on the DTA curve. It confirms the result of
SEM.
(3) When the formaldehyde initial concentration is
−
3
14.58 mg m
(over the (NH4)3PW12O40 catalyst the
−3
initial concentration of formaldehyde is 5.347 mg m ) at
−
1
a flow rate of 40 ml min , the concentration of methanol
reduces, but a few of methyl ether and methyl formate
still appear in the exhaust gas. In the tail gas, there are no
formaldehyde and formic acid, but more CO2 is determined,
showing that the formaldehyde is mainly oxidized to formic
acid and most of the formic acid farther turns into CO2
3
.3. Catalytic activity for formaldehyde degradation
The photocatalytic formaldehyde degradation reaction was
carried out in a intermission fix-bed photoreaction system
Fig. 1a) first. It is found that the activities of the catalysts that are
(
◦
and H O. The remainder of the formic acid reacts with
calcinated at 250 C for 2 h are the best, agreeing with the BET
results. The catalytic activity for formaldehyde degradation is
evaluated under this condition in a continuous-flow fix-bed pho-
toreaction system (Fig. 1b). The results of the catalytic activity
of the photocatalysts are shown in Table 3.
2
methanol to produce methyl formate. The formaldehyde is
basically mineralized.
In summary, the activity of the catalysts has been found
to increase in the order of (NH4)3PW12O40, Cs3PW12O40,
Ag3PW12O40, Cu3(PW12O40)2. It indicates that M3PW12O40
catalysts increase the optical absorption and broaden the range
of wavelength. The optical response of M3PW12O40 catalysts
expands into near visible light region. This is in favor of
the formation of charge stream and increases the utilization
of light energy. The more shift of UV–vis absorption band,
the higher activity of the above catalysts is. For example, the
Cu3(PW12O40)2 catalyst has the maximal red shift to 350 nm
and very strong absorption of its UV–vis absorption band, so it
has some photocatalytic activity in the indoor nature light. More-
over, the particles of Cu3(PW12O40)2 agglomerated to lower
the surface energy and to form the network structure with tiny
bore, which may have a better shape-selective catalysis. So the
Cu3(PW12O40)2 catalyst is identified to have the best photocat-
alytic activity.
No changes of concentration and the composition of
feed gas are found without the ultraviolet illumination over
(
NH4)3PW12O40, Cs3PW12O40 and Ag3PW12O40 catalyst,
respectively. However, the composition of the feed gas obviously
changes under indoor nature light rays over the Cu3(PW12O40)2.
There are a small peak of formic acid on the curve of GC and
the concentration of formaldehyde is decreased, which indicates
that the Cu3(PW12O40)2 can be excitated by nature light.
(
1) When the initial formaldehyde concentration is
− −1
3
6
9.82 mg m at a flow rate of 40 ml min , the amount of
formaldehyde and the methanol reduce under ultraviolet
light after the reaction. Methyl ether, methyl formate
and formic acid are observed in the products. There are
some formaldehyde but no CO2 in the tail gas. Formic
acid is the product of formaldehyde oxidation. Formic
acid and methyl alcohol produce methyl formate. Part of
the methanol produces methyl ether. The conversion of
formaldehyde is increased according to (NH4)3PW12O40,
Cs3PW12O40, Ag3PW12O40 and Cu3(PW12O40)2, among
which the catalytic effect of Cu3(PW12O40)2 is the best and
the conversion of formaldehyde over it reaches to 39.36%.
2) When the initial formaldehyde concentration is
4. Conclusion
+
+
+
2+
Photocatalysts M3PW12O40 (M = NH4 , Cs , Ag , Cu )
have been prepared by microwave radiation solid-phase syn-
thesis method. SEM and XRD characterization reveals that
the average particle size of M3PW12O40 is 15–80 nm. Diffuse
reflectanceUV–visspectrashowthatrelativetoH3PW12O40, the
(
−3 −1
3
2.79 mg m at a flow rate of 40 ml min , the amount

Q. Deng et al. / Journal of Molecular Catalysis A: Chemical 262 (2007) 149–155
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155
absorption band of M3PW12O40 shift from 260 to 270–350 nm
[
5] C. Tanielian, R. Seghrouchni, C. Schweitzer, J. Phys. Chem. A 107 (8)
and the absorption band is strengthened obviously. The pho-
tocatalytic degradation of formaldehyde has been performed
over M3PW12O40. The results prove that the larger shift of
UV–vis absorption band the higher activity of the catalyst is. The
Cu3(PW12O40)2 catalyst has the highest photocatalytic activity,
because its red shift of UV/DRS absorption band is 90 nm and
it has a network structure with a better shape-selective catalysis.
The Cu3(PW12O40)2 catalyst shows some photocatalytic activ-
ity under visible light. When the concentration of formaldehyde
(
2003) 1102.
[
[
[
6] T. Yamase, E. Ishikawa, Langmuir 16 (23) (2000) 9023.
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[
[
[
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3
is 32.79 mg m , the conversion of the formaldehyde is basi-
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−
1
(
cally completed under the flow rate of 40 ml min over 0.1 g
M3PW12O40 catalyst. The life of M3PW12O40 catalysts pre-
pared by this method is long and the catalysts can be regenerated
easily. The catalysts have preferable prospective application in
the photocatalytic degradation of organic contamination.
[
[
[
[
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01.
2
Acknowledgement
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The authors gratefully acknowledge the funding of this
project by Hunan Provincial Natural Science Foundation of
China (05JJ30143).
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