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H2PtCl6 aqueous solution containing Pb(CH3COO)2Á3H2O to
give the cathode denoted as Pt/Pt black.
photodecomposition was performed under a high ammonia
concentration (1 M) as proved by another experiment,
showing that oxidations of water and NH3 are competitive.
The reactions at the photoanode and cathode in the BPCC can
be summarized as follows (refer also Fig. 3):
The FTO/TiO2 and Pt/Pt black electrodes were soaked in
5 ml of 1.0 mM NH3 aqueous solution containing 0.1 M
KNO3 in a 10 ml cylindrical cell made of Pyrex glass. In this
cell air leakage was negligible. Ar gas was bubbled into the
solution for 20 min and the FTO/TiO2 was irradiated
with a 500 W xenon lamp (500 mW cmÀ2, UV-A region
ca. 20 mW cmÀ2).
Photoanodic reactions:
TiO2 + UV light - TiO2(h+) + TiO2(eÀ)
2NH3 + 6h+ - N2 + 6H+
(1)
(2)
(3)
The time dependence of the short circuit photocurrent was
measured by a potentiostat/galvanostat and function genera-
tor (Hokuto Denko). The cylindrical cell was sealed with a
rubber septum, through which the gas phase was sampled by a
syringe. The gases were analyzed by a gas chromatograph
(Shimadzu GC2014) with a molecular sieve 5A column at
40 1C using Ar carrier gas.
In the photoelectrochemical decomposition of ammonia by
the BPCC, oxidative potential was generated by the holes at
the nanoporous TiO2 and reductive potential at the cathode as
shown later in Fig. 3. We attempted nitrate reduction by this
2H2O + 4h+ - O2 + 4H+
Cathodic reactions:
À
2NO3 + 12H+ + 10eÀ - N2 + 6H2O
(4)
O2 + 4H+ + 4eÀ - 2H2O (at a later stage after 90 h) (5)
2H+ + 2eÀ - H2 (does not occur, see Fig. 1)
(6)
By excitation of TiO2 by UV irradiation whose energy is larger
than the bandgap of the semiconductor, charge separation
takes place producing electrons in the conduction band (CB)
and holes in the valence band (VB) (eqn (1)). The NH3 and
H2O molecules react with holes on the TiO2 to produce N2 and
O2, respectively (eqn (2) and (3)). The electrons left in the CB
(eqn (1)) are transported to the Pt/Pt black cathode via outer
BPCC using NH3 in the presence of excess nitrate (NH3
=
1 mM, NO3À = 100 mM, 6200 ppm). In the photodecomposi-
tion of ammonia on the photoanode, cathodic reaction was
investigated under Ar atmosphere. As mentioned in the
introductory section, ammonia can be oxidatively decomposed
to dinitrogen gas (N2) quantitatively at a TiO2 thin film
photoanode.6 If a 1 mM NH3 aqueous solution (5 mL)
(NH3 = 5 mmol) was completely photodecomposed, 2.5 mmol
N2 should be produced from NH3. The gases evolved under Ar
À
circuit, and NO3 would be reduced there to produce N2
(eqn (4)). At a later stage after 90 h, the formed O2 is partially
reduced at the cathode to H2O (eqn (5)). If there were no
electron acceptor in the cell, proton reduction to H2 would
take place, but H2 was not formed.
are shown in Fig. 1. As reported earlier, by using platinized
À
À
TiO2 powders or BPCC with ammonia solutes without NO3
,
Thus, when NH3 was present (Fig. 1), NO3 was photo-
chemically reduced to N2 (eqn (2) and (4)). After 142 h, the
N2 and H2 were photoelectrochemically evolved under Ar
atmosphere at the molar ratio of nearly 1 : 3.12,13 However,
it is evident from Fig. 1 that, when nitrate anions were present
(NO3À/NH3 = 100), H2 did not evolve, but N2 was evolved in
excess over the calculated value from the NH3 (broken line in
Fig. 1). The only possible interpretation would be that the
nitrate anions were reduced to N2 at the cathode. Addition-
ally, it was also important that oxygen was evolved due to the
water oxidation at the TiO2 photoanode donating reduction
power to the cathode. It should be noted that the evolved O2
reached a maximum and then decreased, which can be inter-
preted by reduction of the formed O2 producing H2O at the
cathode as reported before in our previous paper.6 Water
oxidation is a four-electron process and N2 production from
two molecules of NH3 is a six-electron process. In addition, it
is of importance that oxygen was not evolved when the
total evolved N2 was 5.6 mmol, and the total O2 5.7 mmol.
Since the formed O2 produced
À
a reduction power of
4 mol eÀ/mol O2 reducing 2NO3 to N2 by a 10 eÀ process,
it is calculated that 2.3 mmol N2 (= 5.7 mmol  (4 eÀ/10 eÀ))
À
came from NO3 reduction by electrons donated from H2O.
The remainder, 3.3 (= 5.6–2.3) mmol N2 is calculated to come
from NH3 oxidation (eqn (2)) + NO3À reduction (eqn (4)), to
which it is calculated that 4.1 mmol NH3 contributed; from
this, NH3 decomposition yield was calculated to be 82%
(= (4.1/5) Â 100) after 142 h, as summarized in Table 1.
The same photoelectrochemical reaction as Fig. 1 was
conducted in the absence of ammonia and O2 (under Ar),
and the results are shown in Fig. 2. In this case H2 was formed
in addition to N2 and O2. When the holes formed by UV light
excitation (eqn (1)) were used for water oxidation evolving O2
(eqn (3)), the electrons left inÀthe CB are transported to the
cathode and then reduce NO3 and H+ forming N2 (eqn (4))
and H2 (eqn (6)), respectively. From the above-mentioned
reactions of O2 formation (4eÀ process), proton reduction to
À
H2 (2eÀ process), and 2NO3 reduction to N2 (10eÀ process),
the amount of the evolved N2 was calculated from the evolved
O2 and H2. After 48 h the formed N2 (1.6 mmol) was much less
than the calculated value (2.6 mmol), which could be inter-
À
preted by slow reduction of NO3 to gaseous N2. After 96 h
Fig. 1 Time dependence of the formed gases in NH3 (1 mM)–KNO3
À
the formed N2 (2.9 mmol), in contrast, exceeded the calculated
value (2.1 mmol). We have reported that TiO2 can produce
reduction power also through oxidation of Ti4+ to Ti5+
(100 mM, NO3 6200 ppm)–water (5 mL) under Ar at pH 9.74.
Broken line, theoretical amount (2.5 mmol) of evolved N2 by complete
decomposition of 5 mmol NH3 (1 mM, 5 mL).
ꢀc
This journal is The Royal Society of Chemistry 2009
3232 | Chem. Commun., 2009, 3231–3233