Room temperature oxidation of methyl orange and methanol over Pt-HCa 2Nb3O10 and Pt-WO3 catalysts without light

Article abstract of DOI:10.1039/c0cc03553j

The layered semiconductor Pt-HCa₂Nb₃O₁₀ can catalyze room temperature air oxidation of methyl orange. It is also more effective than Pt-WO₃ in catalyzing reaction of methanol with air. Pt-HCa₂Nb₃

Full text of DOI:10.1039/c0cc03553j

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Room temperature oxidation of methyl orange and methanol over
Pt–HCa2Nb3O10 and Pt–WO3 catalysts without lightw
Emiliana Dvininov, Upendra A. Joshi, James R. Darwent, John B. Claridge,
Zhongling Xu and Matthew J. Rosseinsky*
Received 30th August 2010, Accepted 18th October 2010
DOI: 10.1039/c0cc03553j
The layered semiconductor Pt–HCa2Nb3O10 can catalyze room
temperature air oxidation of methyl orange. It is also more
effective than Pt–WO3 in catalyzing reaction of methanol with
air. Pt–HCa2Nb3O10 could find wide application as a mild
oxidizing catalyst.
There is a significant interest in photosensitised reactions on
semiconductor particles for solar energy conversion1 and the
removal of dyes and organic pollutants from wastewater.2
Methyl orange is a simple azo dye that has been used as a
model system to probe the chemistry that takes place on
semiconductor particles.3 It is generally considered to be very
stable to light and difficult to oxidize. It was suggested as a
probe system in 1984 and since 1991 there have been more
than 1000 papers using this dye to test photocatalysts.4
Noble metals deposited on semiconductor particles can act
as co-catalysts for reactions such as the reduction and oxidation
of water.1 While studying tungstates and niobates photo-
sensitisers we discovered that platinised semiconductor particles
(Pt–WO3 and Pt–HCa2Nb3O10) can activate oxygen to oxidize
methyl orange in water, in the dark, at room temperature.
These materials also oxidize alcohols with air in water, which
suggests that they may be suitable for the chemical treatment
of wastewater or as catalysts in fuel cells. Previous research has
shown that Pt–WO3 can be used as a catalyst for methanol fuel
cells, where the WO3 works synergistically by helping the
removal of carbon monoxide from platinum.5 Our work shows
that Pt–HCa2Nb3O10 particles, which have been used for
the photooxidation and photoreduction of water,6 are more
effective than Pt–WO3 for the reaction of methanol with air
in water.
Fig. 1 (a) Methyl orange oxidation in water photosensitised by
When Pt–HCa2Nb3O10 is added to an aqueous solution of
methyl orange there is an initial small loss of the dye due to
Pt–HCa2Nb3O10 with light irradiation (l > 300 nm); (b) methyl
orange oxidation in water, in the dark, catalyzed by Pt–HCa2Nb3O10
(20 mg of 1% Pt–HCa2Nb3O10 in 30 ml water, 25 1C).
primarily adsorption onto the low surface area (7.5 m2 gÀ1
)
material. In the absence of platinum or when oxygen is
removed from the solution, by bubbling with nitrogen, there
is no further change in the spectrum. When platinum was
deposited onto the particles, by the photoreduction of Pt4+
ions, and a mixture of the resulting material and methyl
orange solution is subsequently illuminated with ultraviolet
(l > 300 nm) light, the dye was destroyed over a period of
semiconductor photooxidation catalysts. There is no clear
isosbestic point (Fig. 1a), since the reaction proceeds through
a number of steps including demethylation, hydroxyl attack
on the phenyl ring and eventually cleavage of the azo bond to
give a colorless solution.3
Surprisingly, we find that, when a dispersion of Pt–HCa2Nb3O10
is not illuminated, the dye is oxidized by air in the dark.
During the first phase of this process, the reaction proceeds
smoothly with an isosbestic point at 398 nm. LC/MS shows
that the products formed in this phase are due to complete
demethylation of methyl orange (ESIw). The reaction con-
tinues over a period of hours resulting in further oxidation and
loss of colour and proceeds via an intermediate species with an
minutes through
a process that is typical of most
Department of Chemistry, University of Liverpool, Crown Street,
Liverpool, L69 7ZD, UK. E-mail: m.j.rosseinsky@liverpool.ac.uk
w Electronic supplementary information (ESI) available: Detailed
experimental procedures, characterization, SEM and TEM images
and LC/MS data. See DOI: 10.1039/c0cc03553j
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 881–883 881

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Table 1 Rates of oxygen reduction by Pt–HCa2Nb3O10 and Pt–WO3
(10 mg of catalyst and 0.5 mol dmÀ3 methanol in 30 ml water)
Rate of oxygen reduction Band gap BET surface
energy/eV area/m2 gÀ1
Sample
(Â 10À9 mol sÀ1
10
)
Pt–WO3
Pt–HCa2Nb3O10 330
2.7
3.5
2.2
7.5
Scheme 1 The key steps in air oxidation of methyl orange catalyzed
by Pt–HCa2Nb3O10 observed by LC/MS (data in ESIw).
absorption maximum at 374 nm. These spectral changes are
shown in Fig. 1b. This reaction appears milder and more
selective than the photosensitised oxidation of methyl orange
with TiO2 where LC/MS shows a range of products resulting
from hydroxyl attack on the aromatic rings.7
A similar though slower process is seen with Pt–WO3
and again there is no reaction in the absence of platinum or
when air is removed from the dispersion. The main reaction
intermediates for the oxidation of methyl orange are shown in
Scheme 1.
Fig. 3 Structure of HCa2Nb3O10 (left) and WO3 (right).
To further understand the mechanism of these reactions we
have studied the effect of the catalysts on the reaction between
air and methanol. When Pt–HCa2Nb3O10 is added to methanol
in water there is a rapid reaction which reduces the oxygen in a
matter of minutes. This is illustrated in Fig. 2, which shows the
results obtained with an oxygen electrode for Pt–HCa2Nb3O10,
and Pt–WO3. The reaction continues at a similar rate when the
dispersion is purged with air and the experiment repeated.
After 5 repeats this corresponded to turnover numbers of 12
for Pt and 1.5 for Pt–HCa2Nb3O10. No reaction was seen with
platinised titanium dioxide (Degussa P25) in the dark.
The rates for reduction of oxygen are shown in Table 1.
Pt–HCa2Nb3O10 is substantially more effective. The high
catalytic activity demonstrates the ability of these materials
to activate molecular oxygen under ambient conditions.
Fig. 3 shows the different structures of WO3 and HCa2Nb3O10.
Under the conditions of these experiments HCa2Nb3O10 is largely
not delaminated and retains its layered structure as shown in
Fig. 4a. The HRTEM image of Pt–HCa2Nb3O10 (Fig. 4b)
Fig. 4 (a) SEM image of HCa2Nb3O10, (b) HRTEM image of
Pt–HCa2Nb3O10 showing the Pt crystallites. Inset is the FFT of the
image where diffuse Pt reflections (marked by arrows) are observed on
the [001] zone axis diffraction from the niobate sheets. See ESIw for
TEM images in different magnifications.
demonstrates the fine dispersion of crystallized Pt nanoparticles
on the surface of the semiconductor (Pt mean size 3.5 nm on
mean size 1.4 mm HCa2Nb3O10 sheets—data in ESIw).
Several factors may be important in the high catalytic
activity of Pt–HCa2Nb3O10. The material has a higher surface
area than Pt–WO3 and TEM images show that Pt is well
dispersed on this catalyst. The Pt is present as nanoclusters
which may be responsible for the high catalytic efficiency.
HCa2Nb3O10 is similar to niobic acid (H8Nb6O10ÁxH2O)
possessing strong acidity due to its labile protons8 which
makes it a suitable catalyst for partial and total oxidative
reactions. This Brønsted acidity is combined with the presence
of Ca2+ in addition to the d0 centre, which will enhance the
donor power of the lattice oxide compared to niobic acid
and WO3.
To the best of our knowledge this is the first time that
platinum loaded semiconductor particles have been shown to
catalyze the reaction of air with methyl orange. The work also
shows that Pt–HCa2Nb3O10 is substantially more effective
than Pt–WO3 in catalyzing the oxidation of methanol by air
at room temperature. This suggests that these materials could
find wide application as mild oxidizing catalysts with air, and
Pt–HCa2Nb3O10 may be superior to Pt–WO3 as a catalyst for
methanol fuel cells.
Fig. 2 Reduction of oxygen with Pt–HCa2Nb3O10 and Pt–WO3 in
water (10 mg of catalyst, 30 ml water, 25 1C, 0.5 mol dmÀ3 methanol
added after 500 s, 120 mV equivalent to oxygen air saturated water).
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882 Chem. Commun., 2011, 47, 881–883
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Catalytic wet air oxidation is seen as an important route to
meet environmental challenges.9 High temperatures and
pressures are often required. The findings in this paper suggest
that Pt–HCa2Nb3O10 may help in identifying candidates for
such processes under ambient conditions.
W. Chanmanee, C. R. Chenthamarakshan, M. V. B. Zanoni,
P. Kajitvichyanukul and R. Krishnan-Ayer, J. Photochem.
Photobiol., C, 2008, 9, 171.
4 From Web of Science search.
5 S. Jayaraman, T. F. Jaramillo, S.-H. Baeck and E. W. McFarland,
J. Phys. Chem. B, 2005, 109, 22958.
6 (a) O. C. Compton, E. C. Carroll, J. Y. Kim, D. S. Larsen and
F. E. Osterloh, J. Phys. Chem. C, 2007, 111, 14589; (b) Y. Ebina,
A. Tanaka, J. N. Kondo and K. Domen, Chem. Mater., 1996, 8,
2534; K. Domen, J. Yoshimura, T. Sekine, A. Tanaka and
T. Onishi, Catal. Lett., 1990, 4, 339; (c) K. Maeda, D. Lu,
K. Teramura and K. Domen, J. Mater. Chem., 2008, 18, 3539;
(d) H. Hata, Y. Kobayashi, V. Bojan, W. J. Youngblood and
T. E. Mallouk, Nano Lett., 2008, 8, 794; (e) O. C. Compton and
F. E. Osterloh, J. Phys. Chem. C, 2009, 113, 479.
We acknowledge the EU NOVELOX project and EPSRC
(EP/H000925) for financial support, the UK Semiconductor
Photochemistry Network for travel support and enabling
discussions, Chris Ireland and Joanna Clark for helpful
discussions and the Centre for Materials Discovery for SEM
and LC/MS facilities.
7 C. Baiocchi, M. C. Brussino, E. Pramauro, A. B. Prevot,
L. Palmisano and G. Marcı‘, Int. J. Mass Spectrom., 2002,
214, 247.
8 (a) T. Iizuka, K. Ogasawara and K. Tanabe, Bull. Chem. Soc. Jpn.,
1983, 56, 2927; (b) Z. Chen, T. Iizuka and K. Tanabe, Chem. Lett.,
1984, 1085; J. C. Vedrine, G. Coudurier, A. Ouqour, P. G. Pries de
Oliveira and J. C. Volta, Catal. Today, 1996, 28, 3; (c) A. Takagaki,
C. Tagusagawa, S. Hayashi, M. Harad and K. Domen, Energy
Environ. Sci., 2010, 3, 82.
Notes and references
1 (a) A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253;
(b) F. E. Osterloh, Chem. Mater., 2008, 20, 35.
2 (a) R. W. Matthews, Water Res., 1991, 25, 1169–1176; (b) F. E. Albanis,
Appl. Catal., B, 2004, 49, 1; (c) M. R. Hoffmann, S. T. Martin, W. Choi
and D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96; (d) A. Mills,
R. H. Davis and D. Worsley, Chem. Soc. Rev., 1993, 22, 417.
3 (a) G. T. Brown and J. R. Darwent, J. Phys. Chem., 1984, 88, 4955;
(b) C. Galindo, P. Jacques and A. Kalt, J. Photochem. Photobiol., A,
2000, 130, 35–47; (c) K. Dai, H. Chen, T. Peng, D. Ke and H. Yi,
Chemosphere, 2007, 69, 1361–1367; (d) K. Rajeshwar, M. E. Osugi,
9 (a) J. Huang, W. Xiaohong, L. Sen and Y. Wang, Appl. Surf. Sci.,
2010, 257, 116; (b) Y. Zhang, D. L. Li, Y. Chen, X. H. Wang and
S. T. Wang, Appl. Catal., B, 2009, 86, 182.
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This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 881–883 883