Selective photocatalytic reduction of nitrate to nitrogen molecules in an aqueous suspension of metal-loaded titanium(IV) oxide particles

Article abstract of DOI:10.1039/b502909k

Nitrate was photocatalytically reduced to nitrogen molecules with a high selectivity in a basic aqueous suspension of palladium and copper-loaded titanium(IV) oxide powders in the presence of oxalate anion as a hole scavenger. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b502909k

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Selective photocatalytic reduction of nitrate to nitrogen molecules in an
aqueous suspension of metal-loaded titanium(IV) oxide particles
Hiroshi Kominami,*a Takao Nakaseko,a Yumiko Shimada,a Akitoshi Furusho,a Hiroyuki Inoue,a
Shin-ya Murakami,a Yoshiya Keraa and Bunsho Ohtanib
Received (in Cambridge, UK) 28th February 2005, Accepted 13th April 2005
First published as an Advance Article on the web 28th April 2005
DOI: 10.1039/b502909k
Degussa P-25 was used as a TiO2 photocatalyst because it is
known to be one of the most active photocatalysts. Bare TiO2
powder (50 mg) was suspended in a solution (5 cm3) containing
nitric acid (or sodium nitrate, 50 mmol), OA (or sodium oxalate,
200 mmol) and a metal source (mainly metal chloride) in a test
tube. No effects of metal source (chloride or sulfate) were observed
in this study. Using 5 M sodium hydroxide solution, pH of the
suspension was adjusted to the desired value. The tube was sealed
with a rubber septum and then photoirradiated at l .300 nm by a
400 W high-pressure mercury arc (Eiko-sha, Osaka, Japan) under
argon with magnetic stirring. During photoirradiation, the test
tube was set in a water bath kept at a continuous 298 K to avoid
thermal reaction. In the early stage of irradiation, the metal source
was reduced by photogenerated e2 and the metal was deposited on
TiO2 particles, resulting in the formation of metal-loaded TiO2.
Concentrations of NO32, NO2 and NH3 (in the form of NH4 )
in the liquid phase were determined by absorption spectro-
photometry (TOA Electronics LASA-20, Tokyo). The amounts of
N2, hydrogen and carbon dioxide in the gas phases of reaction
mixtures were measured using a Shimadzu GC-8A gas chromato-
graph equipped with MS-5A (N2 and H2) and Porapak QS (CO2)
columns. Some blank experiments revealed that both TiO2 and
photoirradiation were indispensable to drive all reactions observed
in this study.
Nitrate was photocatalytically reduced to nitrogen molecules
with a high selectivity in a basic aqueous suspension of
palladium and copper-loaded titanium(IV) oxide powders in
the presence of oxalate anion as a hole scavenger.
Nitrate (NO32) is a final product of nitrogen atom oxidation and
is one of the most important components of fertilizer. However,
2
NO3 is toxic in humans, and the recent gradual increase in the
concentration of NO32 in ground water mainly due to agricultural
effluents has became a serious problem. European Drinking
Water Directive has set the maximum admissible concentration
2
of NO3 in drinking water to 50 mg dm23, and the World
Health Organization has set the concentration to 25 mg dm23 as
a guide level.1
There are several methods for reducing NO32 to other nitrogen
2
compounds. Photocatalytic reduction (or decomposition) of NO3
2
+
by photogenerated electrons (e2) is one of the methods, and this
method has been extensively studied by many researchers2–12
because solar power can be used as an energy source for driving
the reaction. However, it has been found that titanium(IV) oxide
(TiO2) itself has no or almost no photocatalytic activity. Li and
Wasgestian6 reported that oxalic acid (OA) added to a suspension
2
of bare TiO2 containing NO3 acts as a hole (h+) scavenger and
accelerates photocatalytic reduction of NO3 by e2 to ammonia
2
Table 1 shows effects of pH on the photocatalytic reduction
2
(12 h irradiation) of NO3 in a suspension of Cu–TiO2 in the
+
2
(NH3) or ammonium ions (NH4 ) (eqn. (1)). Reduction of NO3
by carbon dioxide anion radical (CO2 2), which is an intermediate
?
presence of OA. When Cu–TiO2 was used in this reaction system
under acidic conditions, i.e., without pH control (ca. pH 1), OA
was oxidized by two holes to produce twofold greater molar
species formed by partial oxidation of OA by h+, has also been
proposed.11,12
2
2
22
+
amount of carbon dioxide, whereas NO3 was reduced by eight
+
NO3 + 4C2O4 + 10H+ 5 NH4 + 8CO2 + 3H2O
(1)
photogenerated electrons to form NH3 (or NH4 ) (eqn. 1).
From an environmental point of view, photocatalytic reduction
2
of NO3 to non-toxic nitrogen molecules (N2) is preferable
2H+ + 2e2 5 H2
(3)
because other nitrogen compounds such as nitrite (NO22) and
NH3 are more toxic than NO32. However, selective photocatalytic
reduction of NO32 to N2 under UV irradiation at a wavelength of
.300 nm has not been reported, indicating that controlling the
extent of NO32 reduction by photocatalysis is very difficult. Here
we briefly show that NO32 is effectively and selectively reduced to
N2 in an aqueous suspension of both copper- and palladium-
loaded TiO2 particles under basic conditions in the presence of OA
(eqn. (2)).
Reduction of NO32 by e2 competes with that of H+ (eqn. (3)),
and the selectivity was determined by hydrogen overvoltage
(HOV) of metal loaded on the TiO2 particles because metal works
as a reduction center in this reaction.9 In the case of Cu–TiO2,
reduction of H+ to H2 was completely inhibited due to the large
2
HOV of Cu metal, and an NO3 ion reacted with four oxalate
anions to yield NH3 and eight CO2 molecules (eqn. (1)).
NO3 + C2O4 + 2OH2 5 NO2 + 2CO3 + H2O
(4)
2
22
2
22
2NO3 + 5C2O4 + 8OH2 5 N2 + 10CO3 + 4H2O
2
22
22
At pH 5, NO22, which was a two-electron reduction product,
was formed as well as NH3 (Table 1). Nitrogen balance after
photoirradiation at pH 5 was 75%, suggesting that another
(2)
*hiro@apch.kindai.ac.jp
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Chem. Commun., 2005, 2933–2935 | 2933

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Table 1 Photocatalytic reduction of nitrate in an aqueous suspension of 0.5 wt% metal-loaded P-25 TiO2 in the presence of oxalic acid for 12 h
2a
2
2
NO2
selectivityb/%
Metal
loaded
NO3
mmol
/
NO2
mmol
/
NH3/
mmol
N2/
mmol
H2/
mmol
Nitrogen
balance/%
N2
selectivityc/%
Hydrogen
overvoltaged/V
pH
Cu
Cu
Cu
Pt
Pd
Au
Ag
1
5
28
24
36
48
47
42
38
24
22
0
5
13
0
0
6
13
2.4
1.0
20
8
0
0
0
0
0
0
0
0
0
0
0
0
0
0
10
13
3
1
0
26
14
6
0
0
0
96
75
98
96
94
96
102
93
98
b
0
13
99
0
0
0
0
0
0
0
0
95
98
0.43
0.43
0.43
0.01–0.09
0.04
0.18
11
11
11
11
11
11
11
0
50
100
4.6
1.5
0.30
Pd–Cue
Pd–Cuf
a
Nitrate remaining after the reaction for 12 h (initial amount: 50 mmol). Based on consumption of photogenerated electron. Calculated from
2
c
the equation 1006 2NO22/(10N2 + 2NO2 + 8NH3 + 2H2) Based on consumption of photogenerated electron. Calculated from the equation
1006 10N2/(10N2 + 2NO2 + 8NH3 + 2H2) Ref. 18–20. 0.5 wt% Pd-0.5 wt% Cu. 1.0 wt% Pd-0.5 wt% Cu.
2
d
e
f
intermediate species was produced, although the species was not
2
for 24 h. Fig. 1 also shows that the reaction rate (both N2
2
evolution and NO3 reduction) gradually decreased, especially
identified. At pH 11, NO2 was the sole product with a high
nitrogen balance, indicating that protonation of NO32, i.e.,
after 16 h. When sodium carbonate was added to the suspension
before photoirradiation, the reaction rate was suppressed.
Therefore, the gradual decrease in the reaction rate was attributed
to adsorption of dissolved CO322 arising from OA. When a small
amount of sulfuric acid (5 M) was injected into the suspension
after 24 h irradiation, CO2 evolved from the suspension. The test
tube was opened to release CO2 and sealed again after Ar
bubbling, and then NaOH solution was injected to maintain the
pH of the suspension at 11. This process of removing CO322 from
the suspension is hereafter called ‘‘pH swing’’. After pH swing,
production of NH3, was suppressed in basic conditions. The
oxidation product of oxalate anion, CO2, did not evolve into the
22
gas phase and remained in the suspension as carbonate (CO3
2
due to the basicity of the suspension. Therefore, one NO3 ion
)
2
reacted with one oxalate to yield one NO2 ion and two CO3
22
anions (eqn. (4)). Two hydroxide anions are consumed in the
reaction. In fact, a slight decrease in pH of the suspension was
observed after photoirradiation. These results indicate that the
2
extent of photocatalytic reduction of NO3 by e2 can be
2
controlled by pH of the suspension of Cu–TiO2.
the suspension containing Pd–Cu–TiO2 and unreacted NO3
(18 mmol) and OA was photoirradiated again for 20 h. As shown
in Fig. 1, 6.6 mmol of N2 (corresponding of 13.2 mmol for nitrogen
atoms) evolved after this second photoirradiation, indicating that
22
22
C2O4 + 2OH2 5 2CO3 + H2
(5)
2
The effects of type of metal on the photocatalytic reduction of
NO32 at pH 11 are also summarized in Table 1. The effects of gold
and silver were almost the same as the effect of Cu. When
palladium (Pd) and platinum (Pt) were loaded on TiO2 particles,
only H2 formation was observed. In this case, photogenerated e2
was captured only by H+, i.e., photocatalytic decomposition of
oxalate13,14 (eqn. (5)) proceeded. The promotive effect of loading
of noble metals having a small HOV on H2 liberation from
aqueous alcohol solutions has been observed in previous studies
and has been attributed to the efficient reduction of H+ by e2 on
the metal surface.15–17 The same mechanism accounts for the
results observed in the Pd– and Pt–TiO2 system.
subsequent photocatalytic reduction of NO3 to N2 is possible if
22
CO3 is removed from the suspension.
2
To investigate the formation of N2 from NO3 in the Pd–Cu–
2
TiO2 system, photocatalytic reaction of NO2 in an aqueous
suspension of Cu–TiO2, Pd–TiO2 or Pd–Cu–TiO2 was carried out
It should be noted that when both 0.5 wt% Pd and 0.5 wt% Cu
2
were loaded on TiO2 powder, five-electron reduction of NO3 to
N2 occurred (eqn. (2)) with a high nitrogen balance without
2
formation of NO2 or NH3 as shown in Table 1. An increase in
Pd loading to 1 wt% resulted in an increase in N2 yield. The yield
of H2 formed by reduction of H+ by e2 was negligible as well as in
the Cu–TiO2 system.
Fig. 1 shows the time courses of yields of N2 and NO22 and the
2
amount of NO3 in the aqueous suspension of Pd–Cu–TiO2
2
powders. The amount of NO3 remaining in the suspension
decreased monotonously with photoirradiation. A small amount
of NO22 was observed in the early stage of photoirradiation, but
2
Fig. 1 Time course of photocatalytic reduction of nitrate (initially
50 mmol) in an aqueous suspension of Pd–Cu–TiO2 powders in the
presence of oxalate (200 mmol). At ‘‘pH swing’’, a small amount of sulfuric
acid was added to remove CO2 and then NaOH solution was added to
increase the pH of the suspension to 11. Circles: nitrate, triangles: nitrite,
squares: nitrogen molecules.
the yield of NO2 was decreased with prolongation of photo-
irradiation. The yield of N2 increased with photoirradiation with a
good nitrogen balance, and finally 16.5 mmol of N2 (corresponding
to 33.0 mmol of nitrogen atoms) was formed after photoirradiation
2934 | Chem. Commun., 2005, 2933–2935
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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
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Hiroshi Kominami,*a Takao Nakaseko,a Yumiko Shimada,a
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and Bunsho Ohtanib
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aDepartment of Applied Chemistry, Faculty of Science and Engineering,
Kinki University, Kowakae, Higashiosaka, Osaka, 577-8502, Japan.
E-mail: hiro@apch.kindai.ac.jp; Fax: +81-6-6727-4301;
Tel: +81-6-6721-2332
bCatalysis Research Center, Hokkaido University, Sapporo, Japan.
E-mail: ohtani@cat.hokudai.ac.jp; Fax: +81-11-706-9133;
Tel: +81-11-706-9132
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This journal is ß The Royal Society of Chemistry 2005
Chem. Commun., 2005, 2933–2935 | 2935