Effect of the local structures of v-oxides in MCM-41 on the photocatalytic properties for the partial oxidation of methane to methanol

Article abstract of DOI:10.1016/j.jphotochem.2013.05.005

Vanadium-containing MCM-41 mesoporous molecular sieves with tetrahedrally coordinated VO₄ species have been prepared by two direct synthesis methods under acidic and basic conditions as well as impregnation method. V-MCM-41 prepared in acidic solution and imp-V/MCM-41 exhibit high activity and methanol selectivity for the photocatalytic partial oxidation of methane with NO under UV irradiation at 295 K. However, V-MCM-41 prepared in basic solution shows only a low activity for the coupling reaction of methane. Photoluminescence, UV-vis, photoadsorption and ESR measurements reveal that V-MCM-41 prepared in acidic solution contains VO₄ species exposed on a silica surface, while the catalyst prepared in basic solution contains VO ₄ species confined within a silica layer. The exposed tetrahedral VO₄ species act as active sites for the formation of methanol. Furthermore, it is found that the V₄₊ species and methyl radicals formed by UV irradiation of the exposed VO₄ species with methane interact easily with NO, resulting in the selective formation of methanol.

Full text of DOI:10.1016/j.jphotochem.2013.05.005

Journal of Photochemistry and Photobiology A: Chemistry 264 (2013) 48–55
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Effect of the local structures of V-oxides in MCM-41 on the photocatalytic
properties for the partial oxidation of methane to methanol
Yun Hua,∗, Masakazu Anpob, Chaohai Weia
a The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangdong Provincial Key Laboratory of Atmospheric Environment and
Pollution Control, College of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China
b Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Vanadium-containing MCM-41 mesoporous molecular sieves with tetrahedrally coordinated VO4 species
have been prepared by two direct synthesis methods under acidic and basic conditions as well as impreg-
nation method. V-MCM-41 prepared in acidic solution and imp-V/MCM-41 exhibit high activity and
methanol selectivity for the photocatalytic partial oxidation of methane with NO under UV irradiation at
295 K. However, V-MCM-41 prepared in basic solution shows only a low activity for the coupling reaction
of methane. Photoluminescence, UV–vis, photoadsorption and ESR measurements reveal that V-MCM-41
prepared in acidic solution contains VO4 species exposed on a silica surface, while the catalyst prepared
in basic solution contains VO4 species confined within a silica layer. The exposed tetrahedral VO4 species
act as active sites for the formation of methanol. Furthermore, it is found that the V4+ species and methyl
radicals formed by UV irradiation of the exposed VO4 species with methane interact easily with NO,
resulting in the selective formation of methanol.
Received 20 December 2012
Received in revised form 9 May 2013
Accepted 13 May 2013
Available online xxx
Keywords:
Local structure of V-oxide
Partial oxidation of methane
Photocatalysis
Nitric oxide
Methanol
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
structure–property relationships for this reaction based on dif-
ferent synthesis methods. Zhang et al. [19] have reported that
In recent years, research on the direct use of natural gas as
feedstock to satisfy the increasing demand in intermediate chemi-
cals and energy has attracted much attention due to widespread
concern over fossil fuel consumption. Methane, as the principle
component of natural gas and a by-product of oil refining, has been
considered as an important sustainable feedstock for the chemi-
cal industry. The direct partial oxidation of methane (POM) into
useful oxygenates such as methanol and formaldehyde through a
single catalytic step remains one of the most challenging issues
in catalytic research [1–9]. Although various catalysts have been
studied for the POM into methanol and formaldehyde, supported
vanadium oxide catalysts have exhibited particularly promising
results [10–15]. Spectroscopic studies have suggested that the iso-
lated tetrahedral vanadium oxide species with terminal oxygen
sites (O VO3) often lead to better catalytic performance [16,17]. In
most studies, vanadium oxides supported on silica and mesoporous
molecular sieves are prepared by conventional impregnation meth-
ods [14–18]. However, the relationship between the location as
well as coordination structure of the metal ions and the cat-
alytic property has not been addressed consistently in some cases,
and even researchers have proposed contrary conclusions on the
V-MCM-41 with tetrahedral vanadium oxides dispersed on the
wall surface of MCM-41 showed better catalytic performance than
that with vanadium mainly incorporated inside the framework
of MCM-41, whereas Du et al. [20] have reported that incorpo-
rated V-MCM-41 displayed much higher catalytic activity than the
impregnated V/MCM-41 sample.
On the other hand, in most of these studies, high tempera-
tures (typically above 873 K) are required even for low conversion
levels. The conversion of methane increases with the reaction tem-
perature, however, the selectivity of the C1-oxygenate products
drastically decreases due to over-oxidation of the obtained C1-
oxygenates into carbon oxides at higher reaction temperatures.
There are, thus, strong incentives to realize the selective activation
of methane at low temperatures via new routes such as photocata-
lysis. Photocatalytic systems offer the possibility to promote many
difficult reactions even at room temperature. This would bring
advantages such as low energy consumption, stability of catalyst
and the safety of the reactor. Photocatalytic conversion of methane
in the presence of oxygen has also been reported at room temper-
ature under UV irradiation [21,22]. CO2 as the major product was
observed on a V/SiO2 catalyst besides a trace amount of ethane
and formaldehyde [21], while CO and CO2 were the major products
on a MoO3/TiO2 catalyst [22]. Further research has improved the
activity and the selectivity of vanadia and molybdena-based photo-
catalysts for the POM by employing mild thermal energy together
∗
Corresponding author. Tel.: +86 20 39380573; fax: +86 20 39380588.
E-mail addresses: huyun@scut.edu.cn, huyun7132004@yahoo.com.cn (Y. Hu).
1010-6030/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2013.05.005

Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 264 (2013) 48–55
49
with photoenergy. For example, when the photocatalytic reaction
(100)
was carried out at 493 K, formaldehyde was obtained selectively
on the V2O5/SiO2 [23], VOx/SBA-15 [24], and MoO3/SiO2 [25] cata-
lysts. Both UV light irradiation and a reaction temperature as high
as 500 K were essential for the reactions to proceed.
Meanwhile, the photooxidation of methane into methanol on
V/Vycor glass at 275 K [26] and V-containing MCM-41 at 295 K [27]
was reported in the presence of NO. The former work showed that
the UV irradiation of V/Vycor glass at 275 K in the presence of a mix-
ture of NO and CH4 led to the formation of methanol with a very low
yield. The latter work briefly reported that methanol formation was
observed on V-MCM-41 prepared in acidic solution but not on that
prepared in basic solution. However, no direct evidence showed
the reason that caused the different reaction results and local struc-
tures of the catalysts. In this work, the local coordination structures
of vanadium in MCM-41 introduced by different synthesis methods
are characterized in detail, and the relationship between the local
structure of the V-oxide species and the photocatalytic reactivity
in the POM with NO is investigated. The photocatalytic reaction
scheme is also discussed.
a
b
c
(110)
4
2. Experimental
d
2.1. Catalyst preparation
2
6
8
10
The as-synthesized V-MCM-41 gels were prepared under both
acidic and basic conditions. Under both pathways, cetyltrimethy-
lammonium bromide (CTMABr) and NH4VO3 served as the
template and vanadium ion precursor, respectively. For the acidic
condition pathway, tetraethyl orthosilicate (TEOS) was used as
the Si source and the molar composition of the reaction mix-
ture was 1.0 Si:0.0125–0.5 V:0.2 [C16H33N(CH3)3]Br:160 H2O. The
pH value of the solution was adjusted to 1.0 by HCl solution
and the mixture was vigorously stirred at 295 K for 5 days. For
the basic pathway, silica (Aerosil) as well as sodium silicate was
used as the Si source. The molar composition of the gel was 1.0
Si:0.0125 V:0.27 CTMABr:0.13 Na2O:0.26 tetramethylammonium
hydroxide:60 H2O. The pH value of the reaction mixture was
adjusted to 11.0 with dilute sulfuric acid and the gel was stirred
at 295 K for 24 h. The as-synthesized products formed by the two
synthesismethodswererecoveredby filtration, washedthoroughly
with deionized water, dried at 373 K for 12 h, and finally calcined at
773 K for 8 h under a dry air flow. The samples prepared by acidic
and basic methods were then denoted as V-MCM-41(acid) and
V-MCM-41(base), respectively. MCM-41(acid) and MCM-41(base)
were also synthesized by the above two methods without the vana-
dium source.
2
/ Degree
Fig. 1. XRD patterns of (a) V-MCM-41(acid), (b) MCM-41(acid), (c) V-MCM-41(base),
and (d) MCM-41(base) (V: 0.6 wt%).
Shimadzu UV–vis recording spectrophotometer, Model UV-2200A.
The photoluminescence spectra were measured at 77 K and 295 K
with a Spex Fluorolog-3 spectrophotometer. The electron spin
resonance (ESR) spectra were recorded at 77 K with a JES-RE2X
spectrometer operating in the X-band mode. After the V-MCM-41
catalysts were pretreated under the desired conditions, methane
or NO was admitted onto the sample at room temperature in the
quartz tube and the samples were photo-irradiated at 77 K.
2.3. Photocatalytic reactions
The photocatalytic POM with O2 or NO was carried out under
UV light irradiation (ꢀ > 270 nm) by a 100 W high pressure mer-
cury lamp. Typically, 100 mg of the sample was loaded in a quartz
cell with a flat bottom (30 mL), which was connected to a vacuum
system (10−5 Torr range). After pretreatment, methane (16 ␮mol)
and the oxidizing gases (16 ␮mol) were introduced into the quartz
cell and irradiated for 3 h. A water bath was used to keep the cell at
295 K, thus, the influence of heat on the reaction could be excluded.
After each run, the catalyst bed was heated to 573 K to collect the
products that adsorbed tightly onto the catalyst at room temper-
ature. The desorbed products were condensed in a trap cooled by
liquid nitrogen. The products were analyzed with an on-line gas
chromatographer equipped with a flame ionization detector (FID)
for the hydrocarbons and a thermal conductivity detector (TCD) for
analysis of CO, CO2, N2, and N2O.
The imp-V/MCM-41 and imp-V/SiO2 catalysts were prepared
by the impregnation of an aqueous solution of NH4VO3 into MCM-
41(acid) and MCM-41(base) as well as SiO2 (Aerosil). The samples
were dried at 373 K for 12 h, followed by calcination under a dry air
flow at 773 K for 5 h. Prior to photocatalytic reactions and spectro-
scopic measurements, the degassed catalysts were heated in O2 at
773 K for 2 h, evacuated at 473 K for 2 h, and then cooled down to
room temperature.
2.2. Catalyst characterizations
3. Results and discussions
The vanadium content in these materials was determined by
a Shimadzu atomic absorption flame emission spectrophotome-
ter, Model AA-6400F. The powder X-ray diffraction (XRD) patterns
of the samples were recorded on a Shimadzu XRD-6100 using Cu
K␣ radiation (ꢀ = 1.5417 A). The specific surface areas were deter-
mined according to the multipoint BET method. Diffuse reflectance
3.1. Local structures of catalysts
The XRD patterns of the V-MCM-41 and imp-V/MCM-41 cat-
alysts prepared by different synthesis methods are shown in
Fig. 1. All of the XRD patterns exhibit well-defined 100 diffractions
accompanied with broader, unresolved higher order reflections,
˚
UV–vis spectroscopic measurements were carried out on
a

50
Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 264 (2013) 48–55
Table 1
Physicochemical properties of V-containing MCM-41 (V: 0.6 wt%).
A
b
Catalyst
d100 (Å)
a0 (Å)
BET surface area (m2 g−1
)
V-MCM-41(acid)
V-MCM-41(base)
imp-V/MCM-41(acid)
imp-V/MCM-41(base)
imp-V/SiO2
38.0
37.7
36.6
36.0
–
43.9
43.5
42.3
41.6
–
1124
950
837
931
284
a
indicating that all of the prepared catalysts possess a hexagonal
MCM-41 mesoporous structure [28,29]. Table 1 summarizes the
d100, hexagonal unit cell parameters (a0) and the specific BET sur-
B
b
a
˚
face area for the samples. Mesopores with a0 of about 42 A and a
specific BET surface area of ca. 900 m2 g−1 were retained on the V-
containing MCM-41 catalysts with similar V content. It should be
noted that to obtain the same yield of the solid product (based on
Si), 5 days were needed to stir the starting solution by an acidic
pathway, whereas only 1 day was sufficient by a basic pathway,
i.e., the solid product could be more easily and quickly formed with
a basic pathway.
(0→0) (0→1)
The UV–vis absorption spectra of the catalysts prepared by
different synthesis methods are shown in Fig. 2. The samples exhib-
ited absorption bands at around 245 and 280 nm which can be
assigned to the charge transfer from O2− to V5+ in the tetrahe-
drally coordinated V-oxide species [30,31]. When the V content
increased to 3.6 wt%, a shoulder absorption at around 350 nm was
also observed, suggesting the existence of some polymeric vana-
400
500
600
700
Wavelength / nm
Fig. 3. (A) Photoluminescence spectra and (B) second derivative of the spectra of
(a) V-MCM-41(acid) and (b) V-MCM-41(base). Measured at 77 K; excitation beam:
300 nm (V: 0.6 wt%).
dium species (V
O V) at higher V loadings. No absorption could
be observed at longer wavelengths than 400 nm, indicating that
the V-oxide species is present in a highly dispersed state and an
aggregated V2O5 species is not involved [23,32].
The prepared V-containing MCM-41 catalysts exhibited photo-
luminescence at around 500 nm with a well-resolved vibrational
fine structure measured in vacuum at 77 K (ꢀex = 300 nm), as shown
in Fig. 3a. The emission spectra are attributed to a radiative decay
of the photoexcited tetrahedral VVO4 species (VIVO4∗) formed by
the photoinduced electron transfer from the terminal O2− to V5+
center (Eq. (1)) [33–35].
UV excitation
[V5+ O2−
Ground state of
]
ꢀ
[V4+ O−]∗
(1)
A
photoluminescence Charge transfer
excited triplet state
c
tetrahedral V-oxides
b
a
The second derivative of the photoluminescence spectra of the
respective catalysts is shown in Fig. 3b. Each spectrum obtained
consists of only one emitting species, indicating that each catalyst
contains a single VVO4 species. Table 2 summarizes the vibrational
energy of the V O bond of VVO4 species in the catalysts, calculated
from the energy interval between (0 → 0) and (0 → 1) transition
bands, and V O bond length of the species [36]. It was found that
the VVO4 species on V-MCM-41(base) had the strongest V O vibra-
tional energy and, accordingly, they had the shortest V O length
among the catalysts. This suggests that the V-oxide species on
V-MCM-41(base) had a more distorted tetrahedrally coordinated
structure than on the other catalysts [36–39]. This is probably due
to the confinement of the V-oxide species by the MCM-41 silica
layer.
B
c
b
a
C
Treatment of the V-containing silica with an ammonium acetate
(NH4OAc) solution could remove the extralattice or surface V-
oxide species [40,41]. After washing with an aqueous NH4OAc (1 M)
solution at room temperature for 12 h, V-MCM-41(acid) and imp-
V/MCM-41 exhibited a dramatic decrease in UV–vis absorption and
vanadium content, as shown in Fig. 4 and Table 2. However, lit-
tle effect of the treatment was observed on V-MCM-41(base) and
only small amounts of vanadium were extracted from the catalyst.
These results indicate that the VO4 species on V-MCM-41(acid) and
imp-V/MCM-41 were predominantly present in extralattice pos-
itions, while the VO4 species on V-MCM-41(base) were present in
the framework of MCM-41.
V2O5
e
d
200
300
400
500
600
Wavelength / nm
Fig. 2. Diffuse reflectance UV–vis spectra of (A) V-MCM-41(acid), (B) V-MCM-
41(base), and (C) imp-V/MCM-41 with different V contents. (a) 0.15, (b) 0.6, (c)
3.6, (d) imp-(base) 0.6, and (e) imp-(acid) 0.6 wt%.

Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 264 (2013) 48–55
51
Table 2
Properties of the VO4 species on the catalysts.
Catalyst
V
O vibrational energy (cm−1
)
V
O bond length (Å)a
V content (wt%)
Percentage of surface VO4 species (%)b
Original
After wash
V-MCM-41(acid)
V-MCM-41(base)
imp-V/MCM-41(acid)
imp-V/MCM-41(base)
1031
1060
1033
1044
1.565
1.532
1.563
1.550
0.65
0.64
0.60
0.58
0.11
0.57
0.09
0.12
83.1
10.9
85.0
79.3
O bond length (Å) = 2.751 − 0.00115 × V O vibrational energy (cm−1).
a
V
b
Percentage of VO4 species removed from the catalysts after washing with an aqueous NH4OAc (1 M) solution at room temperature for 12 h.
It was thus seen that the tetrahedral VO4 species on the cata-
3.2. Photocatalytic reactivity
lysts possessed different local structures derived from the different
synthesis methods. In the case of imp-V/MCM-41 synthesis, the
V species were impregnated on a siliceous MCM-41 surface, thus
resulting in the formation of exposed VO4 species anchored on the
MCM-41 surface (Scheme 1a) [42]. In the case of V-MCM-41(acid)
synthesis, solid product formation took 5 times as long as that did in
the case of V–MCM-41(base) synthesis in order to obtain the same
yield of solid products. This implied that the hydrolysis rate of TEOS
in acidic solution is lower than in basic one. Thus the V species sur-
rounded the surfactant micelle prior to Si species in acidic solution.
Calcination of the as-synthesized solid, therefore, led to the forma-
tion of exposed VO4 species on the pore wall surface of MCM-41
(Scheme 1a). In contrast, in V-MCM-41(base) synthesis, solid prod-
uct formation was fast and the hydrolysis rate for both NH4VO3 and
Na2SiO3 were high in basic solution, thus a mixture of hydrolyzed
V and Si species surrounded the surfactant micelle. It is probably
that this promoted the formation of VO4 species confined within
the silica matrix, as shown in Scheme 1b.
The photocatalytic oxidation of methane with O2 or NO at 295 K
was carried out over the V-containing MCM-41 catalysts prepared
by different methods. No products could be detected without the
catalysts or UV light irradiation either in the presence of a CH4–O2
or CH4–NO mixture. Siliceous MCM-41 did not exhibit any reactiv-
ity for the photocatalytic oxidation of methane, indicating that the
presence of vanadium in the catalyst was essential in inducing the
photocatalytic activity.
When O2 was used as an oxidant, only the complete oxida-
tion of methane into CO2 and H2O proceeded under UV light
irradiation on the catalysts prepared by either an acidic or basic
pathway, as shown in Table 3. However, the photocatalytic oxi-
dation of methane with NO on V-MCM-41(acid) resulted in the
selective formation of methanol, accompanied by the formation of
minor amounts of CO2 and acetaldehyde as well as trace amounts
of formaldehyde. N2 and minor amounts of N2O were produced
simultaneously with the selectivity of 74.9% and 25.1%, respec-
tively. Interestingly, the selective formation of methanol from
methane in the existence of NO could not be obtained on the V-
MCM-41(base) catalyst. Methane conversion was quite low and
methanol formation was replaced by C2 production such as C2Hx
and acetaldehyde. On the other hand, the selective formation of
methanol could be observed on both the imp-V/MCM-41 catalysts
synthesized by acidic and basic methods. Furthermore, it was found
that the reactivity of V-MCM-41(acid) was quite similar to that of
A
a
b
B
a
b
C
a
b
200 300 400 500 600
Wavelength / nm
Fig. 4. Diffuse reflectance UV–vis spectra of (A) imp-V/MCM-41, (B) V-MCM-
41(acid), and (C) V-MCM-41(base). (a) Untreated samples and (b) after washing
with NH4OAc (1 M) solution for 12 h at room temperature.
Scheme 1. Structures of VO4 species on (a) V-MCM-41(acid), imp-V/MCM-41, and
(b) V-MCM-41(base).

52
Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 264 (2013) 48–55
Table 3
Results of photocatalytic oxidation of methane over several V-containing catalysts.
Catalyst
Oxidant
CH4 conversion (%)
Selectivity (%)
CH3OH
CO2
C2Hx
CH3CHO
Other
V-MCM-41(acid)
V-MCM-41(base)
V-MCM-41(acid)
V-MCM-41(base)
Imp-V/MCM-41(acid)
Imp-V/MCM-41(base)
O2
O2
NO
NO
NO
NO
7.1
5.0
6.0
0.7
7.1
2.2
–
–
87.6
–
88.4
74.7
98.7
92.5
4.2
10.2
9.1
0.6
1.2
0.6
22.3
0.4
5.9
0.4
1.1
2.6
66.5
2.1
17.0
0.3
5.2
5.0
–
–
–
2.4
V content: 0.6 wt%, reactants: CH4 4 Torr (16 ␮J mol), NO (or O2) 4 Torr (16 ␮J mol), reaction time: 3 h.
imp-V/MCM-41(acid), confirming that the VO4 species in V-MCM-
41(acid) is loaded onto the surface of the catalyst. These results
suggest that the surface isolated tetrahedral VO4 species act as the
active sites in the selective photocatalytic oxidation of methane
with NO into methanol. The low reactivity of V-MCM-41(base) can
be ascribed to the efficient incorporation of the VO4 species within
the framework of MCM-41, thus, inhibiting the efficient interaction
of the VO4 species with gaseous reactants.
The yield of methanol in the photocatalytic POM with NO on
V-MCM-41(acid) and imp-V/SiO2 with various V contents is shown
in Fig. 5. The yield of methanol increased with an increase in the
V content up to 1.2 wt% on V-MCM-41(acid), and then decreased
upon a further increase in the V content. However, in the case of the
imp-V/SiO2 catalysts, the highest yield of methanol was obtained
on imp-V/SiO2 with 0.6 wt%.V content, which was only half of the
highest yield on the V-MCM-41 catalyst. Since the MCM-41 matrix
has a much higher surface area than SiO2 due to its mesoporous
structure, more isolated tetrahedral VO4 species could be obtained
on the MCM-41 support, leading to higher activity than that on SiO2.
In the case of V-MCM-41(base), despite of the V content, the selec-
tive formation of methanol from methane could not be obtained,
although the photoluminescence intensity was similar to that of
V-MCM-41(acid). The result suggested that the local structure of
V-oxide depending on the preparation method strongly affect the
photocatalytic reactivity in this reaction system.
3.3. Reaction scheme
In order to investigate the interaction between NO/CH4 and the
surface of the catalyst, adsorption experiments were carried out at
a low pressure of ca. 0.2 Torr to suppress the influence of further
reactions as much as possible. As shown in Fig. 6A, the adsorption
of NO was not observed on either V-MCM-41(acid) or V-MCM-
41(base) in the dark. Only small amounts of NO were adsorbed on
the catalysts under UV light irradiation. The adsorption of methane
did not occur on either catalyst in the dark, as shown in Fig. 6B.
However, dramatic decreases in the pressure were observed on the
two catalysts under UV irradiation and no desorption was found
even after UV irradiation was discontinued. These results indicate
that methane was chemically photo-adsorbed on the excited sur-
face V-oxide species, leading to the formation of stable complexes
on the surface. These findings were similar to earlier observations
for the photoadsorption of propane on V2O5/SiO2 [43]. Moreover,
the photoadsorption of methane on V-MCM-41(base) was found
to be stronger than that on V-MCM-41(acid), suggesting that V-
MCM-41(base) can easily interact with methane. Following the
methane photoadsorption experiments and evacuation of gaseous
methane, NO was introduced into the vessel in the dark. Over V-
MCM-41(base), UV irradiation caused only a small pressure change.
However, UV irradiation of V-MCM-41(acid) caused an appreciable
decrease in the pressure. These results indicate that the complex
of photo-adsorbed methane with V-oxide species can interact with
NO over V-MCM-41(acid) under UV irradiation. Since only V-MCM-
41(acid) preadsorbing methane caused the photoadsorption of NO,
it can be assumed that the photooxidation of methane with NO is
initiated by the activation of methane on the photo-excited catalyst.
The ESR technique was also applied to investigate the inter-
action between the active species and methane/NO molecules.
As shown in Fig. 7a, the ESR spectrum of the V-MCM-41 cata-
lyst obtained by photoirradiation with H2 at 77 K exhibited axially
symmetrical hyperfine signals of V4+ ions located in tetrahedral
coordination, along with two sharp signals assigned to the hydro-
gen (H·) radicals formed by a H H bond cleavage. This indicates
that the charge transfer excited (V4+ O−)* species can abstract an
H atom from H2 to form (V4+ OH) and H· radicals [44,45]. UV irra-
diation of the two catalysts in the presence of methane at 77 K also
led to the formation of tetrahedral V4+ ions, which was similar to
that in the presence of H2 (Fig. 7b and d). Thus, it can be considered
that methyl (•CH3) radicals was formed by UV irradiation of the
catalysts with methane. After the evacuation of gaseous methane
from the system, NO molecules were introduced into the vessel.
The intensity of the ESR signal due to the reduced V4+ ions scarcely
changed after the addition of NO under dark condition. However,
UV irradiation of the V-MCM-41(acid) system in the presence of
NO resulted in a decrease in the ESR signal intensity of V4+ ions,
as shown in Fig. 7c. On the other hand, the signal of V4+ ions on
the V-MCM-41(base) system showed no changes under UV irra-
diation (Fig. 7e), suggesting that the oxidation of the reduced V4+
The photoluminescence intensity of V-MCM-41, which was
attributed to the isolated tetrahedral VO4 species, is also shown
in Fig. 5. A good correspondence was observed between the yield
of methanol and the intensity of the photoluminescence, indicat-
ing that the isolated tetrahedral V-oxide species plays an important
role in the photocatalytic POM with NO into methanol.
Fig. 5. CH3OH yield (column) on V-MCM-41(acid) and imp-V/SiO2 in the photocat-
alytic partial oxidation of CH4 with NO and the photoluminescence intensity (line)
of V-MCM-41(acid) at 295 K.

Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 264 (2013) 48–55
53
degassing
ON OFF
B
A
NO
CH4
NO
ON
Light
ON
OFF
Light
OFF
OFF
OFF
1.0
1.0
0.9
0
0.9
0
V
Acid
V Base
0
0
10
20
30
10
20
30
40
50
Time / min
Time / min
Fig. 6. Changes in the pressure of (A) NO and (B) CH4 and NO after degassing of CH4 following UV irradiation on V-MCM-41 (0.6 wt%) prepared by different methods.
sites to the original V5+ proceeds on V-MCM-41(acid) rather than
on V-MCM-41(base).
suggesting that the photoformed ·CH3 radicals can easily interact
with NO to form methanol on the V-MCM-41(acid) catalyst.
From these results, the reaction scheme shown in Scheme 2
has been proposed for the photocatalytic POM with NO on the
V-containing MCM-41 catalysts. For V-MCM-41(acid) and imp-
V/MCM-41, the isolated tetrahedral VO4 species are supported on
the surface of MCM-41. The charge transfer excited triplet state
(V4+ O−)* species, especially the O− hole trap center reacts with
CH4, leading to a break in the C H bond of CH4 to form (V4+ OH)
and the ·CH3 radical. Simultaneously, an electron transfer from the
electron trapped center (V4+) to the ␲-anti-bonding orbital of NO
and a reverse electron transfer from the ␲-bonding orbital of NO
to the hole-trapped center (O−) occurs, leading to an easy rupture
of the N O bond [45]. The oxygen species resulting from NO is
expected to act as the oxidative species to oxidize the (V4+ OH)
species to the original (V5+ O) state with a release of the OH
species. The OH species reacts with the ·CH3 radical to form CH3OH
and simultaneously inhibits the coupling of the ·CH3 radicals. On
the other hand, the VO4 species in V-MCM-41(base) are confined
ESR measurements of the •CH3 radicals were carried out with a
slow sweep time by UV irradiation of the catalysts with methane.
Over V-MCM-41(acid), the ESR spectrum (g = 2.002, A = 23.0 G) was
observed with a hyperfine structure and an intensity ratio of
1:3:3:1, assigned to the ·CH3 radicals [26,46], as shown in Fig. 8a.
The signal for the ·CH3 radicals was also obtained over V-MCM-
41(base). After evacuation of gaseous methane, NO was introduced
onto the catalyst. The intensity of the ESR signal of the ·CH3 radi-
cals scarcely changed under dark conditions (Fig. 8b), however,
the ESR signal of the ·CH3 radicals on V-MCM-41(acid) decreased
under UV irradiation (Fig. 8c) while it on V-MCM-41(base)
showed no change under both dark and UV irradiation conditions,
a
g = 2.002, A = 23.0 G
g// = 1.911, A// = 155.7 G
b
g
= 1.994, A = 72.6 G
c
a
d
e
b
c
250
300
350
400
Magnetic field / mT
324 326 328 330 332 334
Magnetic field / mT
Fig. 7. ESR spectra of the photoirradiated V-MCM-41(acid) (a) in the presence of
H2 (10 Torr) and (b) CH4 (10 Torr), (c) after degassing and NO (0.1 Torr) addition
followed by UV irradiation; and of the photoirradiated V-MCM-41(base) (d) in the
presence of CH4 and (e) after degassing and NO addition followed by UV irradiation
at 77 K (V: 0.6 wt%).
Fig. 8. ESR spectra of methyl radicals observed after UV irradiation of the V-MCM-
41(acid) at 77 K (a) in the presence of CH4, (b) degassed at 77 K and followed by the
addition of 0.1 Torr NO, (c) after UV irradiation at 77 K (V: 0.6 wt%).

54
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Acknowledgements
This work was supported by the National Natural Science Foun-
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and Technology Project of Guangzhou City (No. 12C62081602), and
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