Visible light decomposition of ammonia to dinitrogen by a new visible light photocatalytic system composed of sensitizer (Ru(bpy)32+), electron mediator (methylviologen) and electron acceptor (dioxygen)

Article abstract of DOI:10.1039/b502127h

Visible light decomposition of aqueous ammonia to dinitrogen was successfully achieved by using a new photocatalytic system based on a molecular photoelectron relay composed of a sensitizer (Ru(bpy)₃²⁺), an electron mediator (methylviologen) and an electron acceptor (dioxygen), which can be used as a visible light-driven photocatalyst instead of UV-driven semiconductors. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502127h

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Visible light decomposition of ammonia to dinitrogen by a new visible
light photocatalytic system composed of sensitizer (Ru(bpy)32+),
electron mediator (methylviologen) and electron acceptor (dioxygen)
Masao Kaneko,*a Naoto Katakura,a Chihiro Harada,a Yoshihito Takeia and Mikio Hoshinob
Received (in Cambridge, UK) 10th February 2005, Accepted 19th April 2005
First published as an Advance Article on the web 9th June 2005
DOI: 10.1039/b502127h
TiO2 or other large bandgap semiconductors in order to use visible
light which is abundant in solar irradiation have been conducted,4
but a wavelength capable of being utilized and the catalytic activity
have not been satisfactory enough. To create a new visible light
photocatalyst based on molecules instead of semiconductors, the
present authors attempted to construct a photocatalytic system
using a dye sensitizer, and have had success in photodecomposing
aqueous ammonia with visible light into N2 by use of a
photocatalyst composed of a sensitizer (Ru(bpy)32+), an electron
mediator (methylviologen) and an electron acceptor (O2); the first
results are reported in this paper.
Visible light decomposition of aqueous ammonia to dinitrogen
was successfully achieved by using a new photocatalytic system
based on a molecular photoelectron relay composed of a
sensitizer (Ru(bpy)32+), an electron mediator (methylviologen)
and an electron acceptor (dioxygen), which can be used as
a visible light-driven photocatalyst instead of UV-driven
semiconductors.
Because of the increasing amount of livestock waste, a number of
studies have been carried out to establish an effective waste-
treatment system for the protection of local and global environ-
A 5 ml aqueous solution of 0.1 mM Ru(bpy)32+, 10 mM
K2S2O8, and 3 M NH3 was prepared. The solution was put in a
10 ml cylindrical cell, and deaerated by bubbling argon gas for
30 min. The cell was irradiated in a horizontal direction with
visible light either by a 500 W xenon lamp with UV cutoff (L-42)
and IR cutoff filters (IRA-25S) (light intensity 101 mW cm22) for
spectral measurements (Figs. 1 and 2), or by visible light from a
ments from water pollution.1–3 In particular,
a methane
fermentation system has been extensively developed to diminish
the chemical and biological oxygen demand (COD and BOD)
from cattle waste slurry.4 Digestion gas (biomass gas) from the
fermentation system has been used as fuel for the production of
electricity. Thus, the methane fermentation system is regarded as
one of the promising methods not only for diminution of COD
and BOD but also for creation of biomass energy from waste
slurry. In recent years, emission of NO32 from livestock waste has
been recognized as a serious pollutant of ground and surface
water.5,6 Nitrate ions are the products from enzymatic oxidation of
NH3 that originates from the urea in livestock waste by the
enzymatic reaction of urease emitted from microorganisms.
Removal and collection of NH3 from the cattle waste slurry
before methane fermentation can contribute to the prevention of
high-level nitrate pollution of surface and ground water.
100 W halogen lamp without filters (light intensity 167 mW cm22
)
for gaseous product analysis (Fig. 3). During irradiation on the
above mixture by the halogen lamp, gas bubbles were clearly
observed, and after 1 h irradiation, 220 ml N2 was formed by the
decomposition of NH3. H2 was not formed in this system since
electron accepting K2S2O8 was used in this mixture. It was
confirmed by each blank experiment that both the Ru complex
and K2S2O8 as well as visible light photoirradiation were needed
for the N2 formation from NH3. The gaseous products were
In an earlier study, TiO2 was found to photoelectrochemically
decompose water under UV light irradiation,7 and since then
many organic and inorganic compounds have been decomposed
by this photocatalyst.8–10 The photodecomposition of ammonia in
neutral water has been reported by using TiO2-supported Pt or Pd
catalysts, and nitrogen (and nitrogen oxides) were obtained.11
TiO2/Pt decomposed aqueous ammonia into nitrogen, while pure
TiO2 decomposed NH3 into nitrite and nitrate.12 Some reports are
found on the photodecomposition of ammonia with TiO2 in the
gas phase.13,14 UV light decomposition of ammonia by platinized
TiO2 was achieved to produce N2.15 However, visible light
conversion of ammonia into nitrogen has not been reported both
in liquid and gas phases. The present authors have found that an
aqueous solution of ammonia can be photochemically converted
by UV irradiation into nitrogen and hydrogen with a nearly
stoichiometric 1 : 3 ratio (volume) by using a platinized titanium
dioxide (TiO2) suspension.16 Many attempts at the sensitization of
Fig. 1 In situ visible absorption spectral change of the aqueous solution,
2+
1 mM NH3, 0.1 mM Ru(bpy)3 and 10 mM K2S2O8, at pH 5.3, under
visible light irradiation from a 500 W xenon lamp through a UV cutoff
filter (L-42) and IR cutoff filter (IRA-25S) with a light intensity of
101 mW cm22. The spectrum was measured with a 5 s interval, total
photoreaction time being 600 s after starting irradiation.
*mkaneko@mx.ibaraki.ac.jp
3436 | Chem. Commun., 2005, 3436–3438
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It is therefore concluded that the reaction occurs by the
photochemical oxidation of the Ru(II) complex to Ru(III) by
K2S2O8, leading to the following degradative oxidation of NH3 by
the Ru(III) complex to yield N2 (Scheme 1).
In order to develop a visible light-driven photocatalyst based on
molecules instead of UV light-driven semiconductors, an electron
mediator was used in combination with an O2 electron acceptor
with
a
sensitizer (here Ru(bpy)32+). It is interesting that
methylviologen, a well-known mediator for proton reduction
towards H2 evolution,8–10 can work as an electron acceptor to
produce N2 as follows. At first the photochemical reaction of an
aqueous mixture of NH3 (3 M, ca. 5%), Ru(bpy)33+ (0.1 mM) and
methylviologen (MV2+) (10 mM) (at pH 5 12.3) was investigated
by in situ visible absorption spectroscopy11 during photoirradia-
tion. The in situ visible absorption spectrum under irradiation of
the above mixture is shown in Fig. 2.
Fig. 2 In situ visible absorption spectral change of the aqueous solution,
3 M NH3, 0.1 mM Ru(bpy)3 and 10 mM MV2+, at pH 12.3, under
2+
irradiation from a 500 W xenon lamp through a UV cutoff filter (L-42)
and IR cutoff filter (IRA-25S) with a light intensity of 101 mW cm22
.
The formation of the viologen cation radical (MV+) having
absorption maxima at 395 and 603 nm was clearly observed.
However, N2 formation was not observed under this condition.
Because of the fact that (1) NH3 did not quench the photoexcited
Measured by taking the absorption of the solution before irradiation as
the base line of the spectra.
2+
Ru(bpy)3 complex, and (2) electron transfer from the photo-
excited Ru(bpy)32+ to MV2+ has been well established,8–10 it can be
concluded that in the above system methylviologen works as an
acceptor from the photoexcited Ru complex in place of K2S2O8 to
produce the viologen cation radical: ammonia works as an electron
3+
donor to the formed Ru(bpy)3 resulting in the accumulation of
an oxidized product of ammonia ((NH3)ox, Scheme 2).
+
As for a candidate for (NH3)ox, hydrazine or NH3 might be
2+
possible. However, the mixture of hydrazine, Ru(bpy)3 and
MV2+ produced MV+ very rapidly and N2 also was formed both
under dark and illumination, so that hydrazine would not be
possible as the (NH3)ox species. By stopping photoirradiation on
the mixture shown in Scheme 2, about a 10% decrease of the
absorbance by MV+ was observed showing that all the
components in the mixture are most probably in a dynamic
equilibrium state under irradiation. The structure of (NH3)ox is a
subject to be investigated in the future. In the reaction of Scheme 2
the presence of Pt catalyst (a suspension of platinum black fine
powder) in the solution did not induce H2 formation since the pH
of the solution was basic due to the NH3. However, Scheme 2
shows that NH3 was photochemically decomposed into an
oxidized product ((NH3)ox) by accumulating MV+, the latter of
which is capable of reducing protons to H2 by decreasing the pH
to neutral or to acidic conditions.
Fig. 3 Time dependent evolution of N2 for the mixture NH3(10 M),
Ru(bpy)32+(0.1 mM) and MV2+ (10 mM) under O2 atomosphere.
Irradiated by a 100 W halogen lamp without filter, with a light intensity
of 167 mW cm22
.
analyzed by a gas chromatograph (Shimadzu, GC4C-PT) with a
13X-D molecular sieve column at 80 uC using argon carrier gas.
All the measurements and photochemical reactions were carried
out at 25 uC.
2+
The irradiation of the aqueous solution of NH3 and Ru(bpy)3
by visible light in the presence of K2S2O8 induced the formation of
3+
Ru(bpy)3 by oxidation of the photoexcited Ru complex by
It is important that the presence of O2 in this system induced N2
evolution (Scheme 3) as a result of MV+ oxidation to MV2+. For
instance, irradiation of an aqueous solution of NH3(10 M),
K2S2O8. This photochemical reaction was monitored in situ under
irradiation by the visible absorption spectral change of the aqueous
2+
solution of NH3, Ru(bpy)3 and K2S2O8 using a Shimadzu
Multispeck 1500 diode array UV-Vis spectrophotometer according
to our cell design17 as shown in Fig. 1 where the absorption peak
2+
at 452 nm due to the Ru(bpy)3 disappeared quickly on
irradiation with a simultaneous increase of the peak at 420 nm
due to the formation of Ru(bpy)33+. By a separate experiment it
was confirmed that NH3 does not quench the photoexcited
Scheme 1
2+
Ru(bpy)3 at all. The following oxidation of ammonia by the
photochemically formed 3+ Ru complex would not be rapid since,
under steady state conditions during irradiation, the 3+ Ru
complex was the predominant species. It was also confirmed by a
3+
separate experiment that NH3 decomposes into N2 by Ru(bpy)3
.
Scheme 2
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photoexcited Ru(bpy)3 to MV2+ in a cellulose matrix.18 Such a
molecule-based photocatalyst can exhibit the same function as
semiconductor photocatalysts, working not only by UV light but
also under visible light, which could promise a large variety of
design and applications not only for livestock waste treatment
but also for other photocleaning of the environment in the near
future.
2+
Scheme 3
Ru(bpy)3 (0.1 mM) and MV2+ (10 mM) under O2 atmosphere
(O2 was bubbled for 30 min prior to the reaction) produced 860 ml
N2 after 9 h without accumulating MV+. After the photochemical
reaction in the absence of O2 (Scheme 2), the predominant species
2+
Masao Kaneko,*a Naoto Katakura,a Chihiro Harada,a Yoshihito Takeia
and Mikio Hoshinob
aFaculty of Science, Ibaraki University, Bunkyo, Mito, 310-8512 Japan.
E-mail: mkaneko@mx.ibaraki.ac.jp
2+
are MV+, (NH3)ox, and Ru(bpy)3 as evidenced by the spectra
shown in Fig. 2 for which the spectrum of the mixture before
irradiation was taken as a base line for the spectral change (note
that if the Ru(III) complex is present, the absorbance around
450 nm should be decreased to a negative level). The presence of
O2 (Scheme 3) oxidizes the MV+ very quickly to MV2+ to
accumulate the Ru(III) complex which then oxidizes (NH3)ox
further to N2.
bNanomaterial Processing Laboratory, The Institute of Physical and
Chemical Research, Hirosawa, Wako, 351-0198 Japan
Notes and references
1 T. Kim and W. Park, J. Environ. Sci. Health, Part A, 2004, 39, 1903.
2 D. Corkal, W. C. Schutzman and C. R. Hillard, J. Toxicol. Environ.
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The time dependent N2 evolution by the process depicted in
Scheme 3 is shown in Fig. 3. It shows an induction period for N2
evolution, which could be interpreted by either or both a slow
accumulation of Ru(bpy)33+ and/or slow reaction of (NH3)ox with
the accumulated Ru(bpy)33+. The turnover number of MV2+ was
calculated to be 4.2, and that for the Ru complex 420 per 9 h
supporting the photocatalytic reaction of Scheme 3. The presence
of suspended fine powders of IrO2 dramatically enhanced the N2
evolution rate by more than one order of magnitude.
3 P. Juteau, D. Tremblay, C. B. Ould-Moulaye, J. G. Bisaillon and
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8 A. Fujishima, K. Hashimoto and T. Watanabe, TiO2 Photocatalysis –
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9 Photosensitization and Photocatalysis Using Inorganic and
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Kluwer Academic Publishers, Dordrecht, 1993.
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Kodansha/Springer, 2002.
11 J. Taguchi and T. Okuhara, Appl. Cat., A, 2000, 194–195, 89.
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15 J. Lee, H. Park and W. Choi, Environ. Sci. Technol., 2002, 36, 5462.
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Ammonia was thus photochemically decomposed into N2 in the
presence of a strong acceptor (K2S2O8) by visible light (Scheme 1).
Moreover, NH3 was photochemically converted into an oxidized
product by reducing MV2+ to MV+ by visible light (Scheme 2). The
presence of O2 in Scheme 2 induced visible light decomposition of
ammonia to N2 by a photocatalytic reaction (Scheme 3). This
system shown in Scheme 3 could solve the biomass waste problem
by using visible light in relevance to treatment of nitrogen
compounds as well as the ammonia pollutant problem in our daily
life, and would lead to a solar (visible light) artificial nitrogen cycle.
Use of this kind of visible light-induced electron relay system as
a photocatalyst would in principle be possible also by using solid
state materials as revealed by our earlier work reporting the
photoinduced solid state electron relay from EDTA via the
3438 | Chem. Commun., 2005, 3436–3438
This journal is ß The Royal Society of Chemistry 2005