Highly dispersed Ce(III) species on silica and alumina as new photocatalysts for non-oxidative direct methane coupling

Article abstract of DOI:10.1039/b507698f

Highly dispersed cerium oxide species on silica and alumina, which mainly exist as Ce(III) species, promote non-oxidative direct methane coupling photocatalytically around room temperature, while Ce(IV) species as CeO ₂ particles do not behave as a catalyst for this reaction. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b507698f

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Highly dispersed Ce(III) species on silica and alumina as new
photocatalysts for non-oxidative direct methane coupling
Leny Yuliati,a Tomoyo Hamajima,a Tadashi Hattoria and Hisao Yoshida*b
Received (in Cambridge, UK) 1st June 2005, Accepted 5th August 2005
First published as an Advance Article on the web 2nd September 2005
DOI: 10.1039/b507698f
was much higher than those of other silica-based photocatalysts
previously reported.1–6
Highly dispersed cerium oxide species on silica and alumina,
which mainly exist as Ce(III) species, promote non-oxidative
direct methane coupling photocatalytically around room
temperature, while Ce(IV) species as CeO2 particles do not
behave as a catalyst for this reaction.
As the photocatalyst support we used silica and alumina. Silica
was prepared from Si(OEt)4 by the sol–gel method followed by
calcination in dry air at 773 K for 5 h22 (BET specific surface area
660 m2 g21). Alumina was a reference catalyst, JRC-ALO-8,
donated from the Catalysis Society of Japan, with specific surface
area of 163 m2 g21. Each support was impregnated with an
aqueous solution of Ce(NO3)3 (Kishida), dried at 383 K in an oven
over night and calcined at 773 K in a flow of dry air for 5 h. The
cerium oxide samples supported by silica or alumina are referred
to as Ce/SiO2(x) or Ce/Al2O3(x), where x is the mol% of Ce to
total cation of Ce and Si or total cation of Ce and Al.
Unsupported CeO2 (Kishida) was employed, with specific surface
Direct conversion of methane to hydrogen and higher hydro-
carbons under non-oxidative conditions is thermodynamically
difficult and prohibited at room temperature, e.g., 2CH4 A C2H6
+ H2 DG 5 68.6 kJ mol21. However, it has been reported that
some silica-based photocatalysts such as silica-alumina,1–3 silica-
supported zirconia4,5 and silica supported-magnesia6 showed
activities for this reaction around room temperature when the
catalysts were activated through evacuation at high temperature,
such as 1073 K, before photoreaction.
area of 3 m2 g21
.
On the other hand, rare earth oxides have been widely
investigated for several applications such as lasers, optical
amplifiers, light emitting diodes, etc.7,8 Rare earth ions generally
can be photoexcited by absorbing light via f–d or f–f transitions,
implying their possibilities as photocatalysts. However, only a few
studies on the photocatalysis of rare earth ions have been reported,
e.g. photoactivities of Eu ions for conversion of a-methylstyrene9
and isolated Pr(III) on metal oxide supports and zeolites for
photodecomposition of N2O.10,11 These facts suggest that rare
earth ions have potential abilities to induce various kinds of
reactions photocatalytically.
The reaction tests were carried out in a similar way to previous
studies in a closed quartz reaction vessel (30 cm3).1–6 The standard
procedure was as follows. The catalyst sample (0.2 g) was spread
over the flat bottom of reactor (14 cm2) and treated in a 100 Torr
(1 Torr 5 133 Pa) oxygen atmosphere at 773 K for 1 h,
followed by evacuation at 773 K for 1 h to clean up the catalyst
surface. Methane (200 mmol) was introduced at room temperature.
The catalyst sample was irradiated from beneath by a 300 W
Xe lamp (used at 17.5 A, 15 V) for 3 h. Light intensity measured in
the range of 220–300 nm was about 10 mW cm22 at the
reactor. The products in the gaseous phase were separately
collected and analyzed by gas chromatography. The adsorbed
products at room temperature were thermally desorbed by
heating at 573 K for 15 min, collected and analyzed by gas
chromatography.
Cerium is the most abundant rare earth oxide and one of the
most interesting elements. The roles of cerium in catalysis have
been recognized in three-way catalysis (TWC), fluid catalytic
cracking (FCC), wet catalytic oxidation of organic pollutants12–14
and so on due to their unique properties such as redox and oxygen
release–storage abilities. In addition, since cerium oxide can be
photoexcited by absorbing photoenergy higher than the band gap
(2.95 eV), cerium oxide has been examined as a semiconductor
photocatalyst for some reactions such as water splitting15,16 and
toluene photooxidation.17,18 Cerium has also been employed as a
dopant for the modification of TiO2 photocatalytic activity.18–21
However, photocatalytic activity of the cerium ion itself has not
been revealed yet. In the present study, we first report the
photocatalytic activity of cerium ions as a novel type of
photocatalyst for non-oxidative direct methane coupling, which
The Ce LIII-edge XANES measurements were performed at
room temperature in transmission mode (Ce . 1 mol%) and
fluorescence mode (Ce , 1 mol%) at the BL-9A station of KEK-
PF, Tsukuba, Japan, with a Si(111) double-crystal mono-
chromator in a similar way to the previous study.23 Some samples
were measured at the BL-7C station. Before recording the spectra,
the same pre-treatment was carried out as before the photoreaction
test. Then, the sample was sealed with a polyethylene film in a dry
atmosphere. Diffuse reflectance UV-visible spectra were recorded
at room temperature on a JASCO V-570 equipped with an
integrating sphere covered with BaSO4. Before recording the
spectra, the sample in a specially designed in-situ cell was treated
just as for the pre-treatment mentioned above. Thus, the sample
was transferred to the optical part without exposure to the
atmosphere. X-Ray diffraction (XRD) patterns were recorded on
a Rigaku diffractometer RINT 1200 using Ni-filtered Cu-Ka
radiation (40 kV, 20 mA).
aDepartment of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Furo-cho, Chikusa-ku, 464-8603, Nagoya, Japan
bDivision of Environmental Research, EcoTopia Science Institute,
Nagoya University, Furo-cho, Chikusa-ku, 464-8603, Nagoya, Japan.
E-mail: h-yoshida@esi.nagoya-u.ac.jp; Fax: +81-52-789-5849
4824 | Chem. Commun., 2005, 4824–4826
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Table 1 Results of the non-oxidative direct methane coupling upon photoirradiationa
Gaseous product/1022 mmol
Thermally
desorbed
products
b/1022 mmol
H2 experimental/H2
calculated from
hydrocarbon products
Total yield
c/1022 mmol
Entry
Sampleb
C2H4
C2H6
C3H8
H2/1022 mmol
1
2
3
4
5
6
7
8
SiO2
0.03
n.d.
0.06
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0.09
n.d.
0.10
3.10
7.43
12.0
6.81
7.43
11.0
14.9
0.24
23.0
7.81
7.70
n.d.
0.06
0.21
0.13
0.14
0.22
0.26
0.05
n.d.
1.24
0.29
0.26
n.d.
tr.
0.13
3.15
8.03
12.8
9.93
10.2
14.7
20.6
0.41
25.2
8.61
7.96
n.d.
5.50
16.4
n.d.
12.9
15.4
8.39
n.d.
n.d.
85.1
18.5
27.0
0
Ce/SiO2(0.01)
Ce/SiO2(0.1)
Ce/SiO2(2)
Al2O3
Ce/Al2O3(0.01)
Ce/Al2O3(0.1)
Ce/Al2O3(5)
CeO2
1.7
3.3
0
1.0
1.1
0.5
0
0.33
0.69
2.98
2.56
3.43
5.58
0.17
0.92
0.42
tr.
9
0
10d
11e
12f
a
Ce/SiO2(0.01)
Ce/SiO2(0.1)
Ce/SiO2(0.1)
3.1
1.9
2.0
b
Reaction temperature was ca. 310 K, sample was 0.2 g, initial methane was 200 mmol, irradiation time was 3 h. The italic numbers in
c
parentheses show the content of Ce (mol%) on the supports. These products such as C2H4, C3H6, etc were desorbed by heating at 573 K for
d
15 min after the photoreaction and collected in a liquid nitrogen trap. The sum of the yield of the gaseous phase and the thermal desorption
e
products. Irradiation time was 36 h. Evacuation pre-treatment was performed at room temperature. Oxidation and evacuation pre-
f
g
treatment was performed at 1073 K. tr. 5 trace, n.d. 5 not detectable.
The photoactivities of the samples are shown in Table 1. It was
confirmed that no product was detected in the dark. Upon
photoirradiation, all the samples exhibited formation of C2H6 as
the main gaseous product, without any COx formation. In some
cases, C2H4 and C3H8 were also detected as minor products.
Amorphous silica showed a trace amount of product (entry 1).
Alumina gave a higher yield of hydrocarbons and hydrogen than
silica did (entry 5). The addition of Ce to both the silica and
alumina supports increased the activity (entry 2–4, 6–8), suggesting
that Ce played an important role in the photoreactions. However,
CeO2 itself without supporting material showed only a small
amount of products without hydrogen (entry 9).
loading samples would mainly exist as Ce(IV) cations in CeO2
small particles.
Fig. 2 shows diffuse reflectance UV-visible spectra of the
samples and bulk CeO2. As the bare silica and alumina did not
On bothsilicaand aluminasupports, thesamplewith low loadings
of Ce produced both hydrocarbons and hydrogen, indicating the
possibility that the reactions, methane coupling and consecutive
ones, proceeded catalytically. The amount of hydrogen obtained
over Ce/SiO2 samples was higher than that expected from detected
hydrocarbons. This suggests that the photoreactions over these
samples may also consecutively give higher hydrocarbons that were
not detectable with the thermal desorption procedure employed
here. Meanwhile, it was confirmed that the TON (turnover number)
of the photoreaction over Ce/SiO2(0.01) was 2.5 after 36 h (entry 10),
where the TON was defined as the amount of produced H2 per
amount of Ce ion. A TON more than unity indicates that the
reaction proceeded photocatalytically over the sample. On the
other hand, even though more hydrocarbon products were obtained
over samples with high loadings of Ce, the formation of hydrogen
was not observed (entry 4, 8), suggesting that the whole photoreac-
tion mechanism over the samples with low loadings of Ce and
high loadings of Ce would be different from each other.
Fig. 1 Ce LIII-edge XANES spectra of (a) Ce/SiO2(0.01), (b)
Ce/SiO2(0.1), (c) Ce/SiO2(2), (d) Ce/Al2O3(0.01), (e) Ce/Al2O3(0.1), (f)
Ce/Al2O3(5), (g) CeO2 and (h) Ce(NO3)3.
Fig. 1 shows the XANES spectra of the samples and references.
Ce(NO3)3 was used as a reference for Ce(III) and CeO2 was used
for Ce(IV).23 Samples with low loadings of Ce (0.01 mol%) on both
silica and alumina showed a dominant peak assignable to Ce(III) at
5726.1 eV (Fig. 1a, d), while high loading samples (2 and 5 mol%)
showed peaks similar to those for CeO2 (Fig. 1c, f). This suggests
that Ce species in low loading samples would be highly dispersed
on the supports mainly as Ce(III) cations, while those in high
Fig. 2 Diffuse reflectance UV-visible absorption spectra of (a)
Ce/SiO2(0.01), (b) Ce/Al2O3(0.05), (c) Ce/SiO2(0.1), (d) Ce/SiO2(2) (e)
Ce/Al2O3(5), and (f) CeO2. The intensity of spectrum (b) was multiplied by
three.
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exhibit strong absorption bands, the bands in these spectra should
be due to cerium species. The bulk CeO2, which is a wide-bandgap
semiconductor, showed a large absorption band less than 420 nm
(Fig. 2f), which corresponds to the charge transfer from O (valence
band) to Ce (conduction band). High loading samples (2 and
5 mol%) exhibited a large band (Fig. 2d, e), where the absorption
edge shifted to shorter wavelength than that of bulk CeO2
probably due to the quantum size effect.24,25 This suggests that the
Ce species in the high loading samples would be nano-sized CeO2
particles. Samples with low loadings of Ce exhibited an absorption
band at 265 nm (Fig. 2a–c), that was assignable to Ce(III) species.23
This absorption band corresponds to the charge transfer from
oxygen to Ce(III) (LMCT, ligand-to-metal charge transfer),26 or
the 4f–5d transition of the Ce(III) ion.27,28 The formation of Ce3+
species at low Ce loadings is in agreement with other literature.26,29
XANES and UV-visible spectra suggested that the samples with
low content (0.01–0.1 mol%) would predominantly have highly
dispersed cerium oxide species on the supports, not CeO2 particles.
Samples with high content (2 and 5 mol%) would have mainly
CeO2 nano-sized particles that were also confirmed by XRD
patterns. Since hydrogen was not detected in the photoreactions
over high loading samples, it is suggested that the Ce(IV) species
consumed hydrogen that should be produced in the photoreaction.
The highly dispersed cerium oxide species on both the supports,
which mainly exist as Ce(III) species, would be the photocatalytic
active sites for non-oxidative direct methane coupling.
In conclusion, highly dispersed cerium oxide species on silica
and alumina, which mainly exist as Ce(III) cations, were found to
promote non-oxidative direct methane coupling photocatalytically.
On the other hand Ce(IV) species would inhibit the formation of
H2 and would not catalyse this photoreaction. It would be
expected that the highly dispersed cerium oxide species on any
suitable support would be active for this reaction if stable under a
reductive atmosphere.
The Ce LIII-edge XANES measurement was performed under
the approval of the Photon Factory Program Advisory Committee
(Proposal No. 2003G248). This work was partially supported by a
Grant-in-Aid for Scientific Research on Priority Areas (417) from
the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of the Japanese Government.
Notes and references
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This novel kind of photocatalyst showed much higher
photoactivity (about five times) than silica-based photocatalysts
reported before, such as silica-alumina, silica-supported zirconia,
and silica-supported magnesia.1–6 In the case of these silica-based
photocatalysts, evacuation pretreatment at high temperature
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oxidative direct methane coupling, it is strongly suggested that the
photoactivity of these ceria-based photocatalysts come from the
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4826 | Chem. Commun., 2005, 4824–4826
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