Photocatalytic reactions on chromium containing mesoporous silica molecular sieves (Cr-HMS) under visible light irradiation: Decomposition of NO and partial oxidation of propane

Article abstract of DOI:10.1039/b008527h

Cr-containing mesoporous silica molecular sieves (Cr-HMS) containing tetrahedrally coordinated isolated chromium oxide (chromate) moieties can operate as an efficient photocatalyst for the decomposition of NO and the partial oxidation of propane with mole

Full text of DOI:10.1039/b008527h

Photocatalytic reactions on chromium containing mesoporous silica molecular
sieves (Cr-HMS) under visible light irradiation: decomposition of NO and
partial oxidation of propane
Hiromi Yamashita,*a Katsuhiro Yoshizawa,a Masao Ariyuki,a Shinya Higashimoto,a Michel Cheb and
Masakazu Anpo*a
a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Gakuencho 1-1,
Sakai, Osaka 599-8531, Japan. E-mail: yamashita@chem.osakafu-u.ac.jp
b Laboratoire de Réactivité de Surface, Université P. et M. Curie, UA 1106-CNRS, 4 Place Jussieu Tour 54, 75252
Paris Cedex 05, France
Received (in Cambridge, UK) 23rd October 2000, Accepted 29th January 2001
First published as an Advance Article on the web 13th February 2001
Cr-containing mesoporous silica molecular sieves (Cr-HMS)
containing tetrahedrally coordinated isolated chromium
oxide (chromate) moieties can operate as an efficient
photocatalyst for the decomposition of NO and the partial
oxidation of propane with molecular oxygen under visible
light irradiation.
The results of the XRD analysis indicated that the Cr-HMS
have a mesopore structure7 and that the Cr-oxide moieties are
highly dispersed in the framework of HMS, while no other
phases are formed. Cr-HMS exhibited a sharp and intense pre-
edge peak in the XANES region which is characteristic of Cr-
oxide moieties in a tetrahedral coordination.8 In the Fourier
transforms of EXAFS spectra, only a single peak due to the
neighboring oxygen atoms (Cr–O) were observed, showing that
Cr ions are highly dispersed in the Cr-HMS. Analysis of
EXAFS spectra of Cr-HMS indicated that tetrahedrally co-
ordinated Cr-oxide (chromate) moieties having two terminal
CNO bonds existed as in an isolated state [two oxygen atoms
(CrNO) at 1.57 Å and two oxygen atoms (Cr–O) at 1.82 Å]. The
EPR technique was also applied to investigate the coordination
state of the Cr-oxide species by monitoring the Cr5+ ions formed
under UV irradiation of the catalyst in the presence of H2 at 77
K. After photoreduction with H2 at 77 K, Cr-HMS exhibited a
sharp, axially symmetric signal at around g = 1.9 g∑ = 1.880,
g4 1.945), attributed to the isolated mononuclear Cr5+ ions in
tetrahedral coordination.9
Highly dispersed transition metal oxides incorporated within
the framework of zeolites and mesoporous molecular sieves
show unique reactivities not only for catalytic reactions1 but
also for photocatalytic reactions.2 In particular, highly dispersed
transition metal oxides such as titanium,3 vanadium4 and
molybdenum5 exhibit high photocatalytic reactivities for vari-
ous reactions. However, these metal oxides can operate as
efficient photocatalysts only under UV light irradiation and
exhibit no photocatalytic reactivity under visible light irradia-
tion. To establish the clean photocatalysis system using the most
environmentally ideal energy source—solar light—it is vital to
develop a photocatalyst that can operate efficiently under
visible light irradiation.
In the present study, chromium-containing mesoporous silica
molecular sieves (Cr-HMS) have been prepared and charac-
terized by various spectroscopic methods (XRD, EXAFS, EPR,
UV–VIS, photoluminescence) and their photocatalytic re-
activities under UV and visible light irradiations have been
investigated. It has been found that chromium oxide (Cr-oxide)
highly dispersed on the mesoporous silica can adsorb and utilize
visible light in photocatalytic reactions such as the decomposi-
tion of NO into N2 and O2 and the partial oxidation of propane
with O2.
Cr-HMS mesoporous molecular sieves (0.02, 0.2, 1.0, 2.0
wt% as Cr) were synthesized using tetraethyl orthosilicate and
Cr(NO3)3·9H2O as the starting materials and dodecylamine as a
template.6,7 Calcination of the sample was carried out in a flow
of dry air at 773 K for 5 h. Prior to spectroscopic measurements
and photocatalytic reactions, the catalysts were degassed at 723
K for 2 h, heated in O2 at the same temperature for 2 h and then
finally evacuated at 473 K for 2 h to 1026 Torr. The
photocatalytic reactions were carried out with the catalysts (100
mg) in a flat-bottomed quartz cell (80 ml) connected to a
conventional vacuum system (1026 Torr range). Photocatalytic
reactions were carried out under UV (l > 270 nm) or visible
light (l > 450 nm) irradiation at 273 K using a high pressure
mercury lamp (310–400 nm, 29 W m22; 360–480 nm, 44 W
m22) through water and colored filters. The photocatalytic
decomposition of NO was carried out on a starting amount of
135 mmol and the products in the gas phase were analyzed by
GC. For the photocatalytic oxidation of propane with O2,
reactions were carried out on mixtures containing 240 mmol
propane and 180 mmol O2, and products in the gas phase and the
products desorbed from the catalysts by heating to 573 K were
also analyzed by GC.
As shown in Fig. 1, the UV–VIS spectra of the Cr-HMS
catalysts exhibit three distinct absorption bands at around 250,
360 and 480 nm which can be assigned to charge transfer from
O22 to Cr6+ of the tetrahedrally coordinated Cr-oxide moie-
ties.10 Without Cr ion, HMS exhibit no absorption band above
220 nm. The absorption bands assigned to the absorption of
dichromate or Cr2O3 cluster cannot be observed above 550 nm,
indicating that tetrahedrally coordinated Cr-oxide species exist
in an isolated state. Cr-HMS exhibited photoluminescence
spectra at ca. 550–750 nm upon excitation of the absorption
(excitation) bands at ca. 250, 360 and 480 nm. These absorption
and photoluminescence spectra are similar to those obtained
with well defined, highly dispersed Cr-oxides anchored on to
Vycor glass or silica11 and can be attributed to the charge
transfer processes on tetrahedrally coordinated Cr-oxide moie-
ties involving an electron transfer from O22 to Cr6+ and a
reverse radiative decay, respectively.
Fig. 1 The UV–VIS spectra of Cr-HMS catalysts (a) 2.0 wt%, (b) 1.0wt%,
(c) 0.2 wt%, as Cr.
DOI: 10.1039/b008527h
Chem. Commun., 2001, 435–436
This journal is © The Royal Society of Chemistry 2001
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Fig. 2 The reaction time profile of N2 formation in the photocatalytic
decomposition of NO on the Cr-HMS catalyst at 273 K (2.0 wt% as Cr)
under UV light irradiation (a) l > 270 nm and visible light irradiation (b)
at l > 450 nm.
Fig. 3 The distribution of the photo-formed products in the photocatalytic
oxidation of propane with O2 on the Cr-HMS catalyst at 273 K (2.0 wt% as
Cr) under UV light irradiation (l > 270 nm) for (a) 0.5 h and (b) 2 h, and
visible light irradiation (l > 450 nm) for (c) 2 h.
UV light irradiation (l > 270 nm) of the Cr-HMS in the
presence of NO in the gas phase at 275 K led to the
photocatalytic decomposition of NO and the evolution of N2,
N2O and O2. The Cr-HMS also showed photocatalytic reactivity
even under visible light irradiation (l > 450 nm). As shown in
Fig. 2, the N2 yields increase linearly with the irradiation time.
The reaction stopped immediately when irradiation was ceased.
Formation of these reaction products was not detected under
dark conditions, or from irradiation of the HMS itself without
Cr-oxide. After prolonged irradiation, the amount of decom-
posed NO to form N2 per total number of Cr ions included
within the catalyst exceeded unity [after 96 h on the Cr-HMS
(0.02 wt% as Cr)]. These results clearly indicate that the
presence of both Cr-oxide species (included within the HMS) as
well as light irradiation are indispensable for the photocatalytic
reaction to take place and that the direct decomposition of NO
to produce N2, O2 and N2O occurs photocatalytically on the Cr-
HMS. Although the reaction rate under visible light irradiation
is less than under UV light irradiation, the selectivity for N2
formation (97%) under visible light irradiation is higher than
that of UV light irradiation (45%). These results indicate that
Cr-HMS can absorb visible light and act as an efficient
photocatalyst under not only UV light but also visible light
irradiation, and especially, Cr-HMS can be useful to form N2
under visible light irradiation.
The addition of NO to the Cr-HMS led to an efficient
quenching of the photoluminescence spectrum of the catalyst,
its extent depending upon the amount of NO added. These
results indicate that the charge transfer excited state of the
tetrahedrally coordinated isolated Cr-oxide moieties, (Cr5+–
O2)*, easily interact with NO, and this photo-excited species
plays an important role in the photocatalytic reaction under UV
and visible light irradiation.
On the other hand, light irradiation of the Cr-HMS in the
presence of propane and O2 led to the photocatalytic oxidation
of propane. As shown in Fig. 3, partial oxidation of propane
with a high selectivity for the production of oxygen-containing
hydrocarbons such as acetone and acrolein proceeds under
visible light irradiation, while further oxidation proceeds
mainly under UV light irradiation to produce CO2 and CO. The
selectivity of partial oxidation production under visible light
irradiation observed at 12% propane conversion is higher than
that observed under UV light irradiation at 26% conversion and
even under UV light irradiation for the shorter reaction time
with 11% conversion. These results indicate that the tetra-
hedrally coordinated, isolated Cr-oxide moieties in HMS can
exhibit an efficient photocatalytic reactivity for the oxidation of
propane under visible light irradiation with a high selectivity for
the partial oxidation of propane.
The present results have clearly demonstrated that the Cr-
HMS can absorb visible light and act as an efficient and
selective photocatalyst under visible light irradiation. This
photocatalytic system with tetrahedrally coordinated Cr-oxide
moieties dispersed on mesoporous silica seems to be a good
candidate for the conversion of abundant visible or solar light
energy into useful chemical energy.
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