Simultaneous removal of nitrite and ammonia as dinitrogen in aqueous suspensions of a titanium(iv) oxide photocatalyst under reagent-free and metal-free conditions at room temperature

Article abstract of DOI:10.1039/c4ra09900a

Photocatalytic reduction of nitrite (NO₂^-) in an aqueous suspension of metal-free titanium(iv) oxide in the presence of ammonia (NH₃) as a hole scavenger gave dinitrogen (N₂), indicating that NO₂^- and NH₃ are simultaneously removed from water as N₂ at room temperature without the use of an extra reagent. This journal is

Full text of DOI:10.1039/c4ra09900a

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Simultaneous removal of nitrite and ammonia as
dinitrogen in aqueous suspensions of a titanium(IV)
oxide photocatalyst under reagent-free and metal-
free conditions at room temperature†
a
Cite this: RSC Adv., 2014, 4, 51576
Received 5th September 2014
Accepted 7th October 2014
Hiroshi Kominami, Ken Kitsui,b Yuki Ishiyamaa and Keiji Hashimotoa
*
DOI: 10.1039/c4ra09900a
www.rsc.org/advances
ꢀ
Photocatalytic reduction of nitrite (NO2ꢀ) in an aqueous suspension of aqueous suspension, i.e., NO2 was almost completely con-
metal-free titanium(IV) oxide in the presence of ammonia (NH3) as a verted to N2 and NO3ꢀ, both the values of NB and ROB being
hole scavenger gave dinitrogen (N2), indicating that NO2ꢀ and NH3 are close to unity.34
simultaneously removed from water as N2 at room temperature
5NO2 + 2H+ / N2 + 3NO3 + H2O
(1)
(2)
(3)
ꢀ
ꢀ
without the use of an extra reagent.
ꢀ
2NO2 + 8H+ + 6eꢀ / N2 + 4H2O
In nature, the nitrite ion (NO2ꢀ) is a product formed in the
procꢀess of ammonia (NH3) oxidation by nitrite bacteria. Since
NO2 reacts with secondary amines to yield nitrosoamines,
3NO2 + 3H2O + 6h+ / 3NO3 + 6H+
ꢀ
ꢀ
From the point of view of denitrication of waste water,
conversion of NO2 to N2 without formation of NO3 is most
favorable.
Two feasible methods for photocatalysis of TiO2 for conver-
sion of NO2ꢀ to N2 in aqueous suspensions are shown in Fig. 1.
ꢀ
which are carcinogens, ingestion of a large amount of NO2
ꢀ
ꢀ
should be avoided. Nitrite ion is oen formed as a relativeꢀly
stable intermediate in the processes of nitrate ion (NO3
)
reduction by catalysis,1–9 electrocatalysis10–17 and photo-
catalysis9,18–33 using dihydrogen (H2), electrically supplied elec-
trons and photogenerated electrons, respectively. However,
reduction of NO3ꢀ and NO2ꢀ by the above methods has mainly
ꢀ
As shown in Fig. 1(a), NO3 was selectively reduced back to
NO2ꢀ over Ag–TiO2 in the presence of a hole scavenger (HS) (eqn
(4) and (5)), and NO2ꢀ was totallꢀy reduced to N2 by combination
with disproportionation of NO2 catalyzed by Pd-TiO2.35
+
or partially yielded NH3 or ammonium ion (NH4 ), in which
nitrogen is over-reduced.
ꢀ
In our previous study, photocatalytic reaction of NO2 in
eꢀ ꢀ h+ + HS / eꢀ + HSox+
(4)
(5)
aqueous suspensions of bare and metal-loaded titanium(IV)
oxide (TiO2) particles was examined in the absence of electron
and hole scavengers under irradiation of UV light.34 In the bare
TiO2 system, disproportionation of NO2ꢀ to dinitrogen (N2) and
nitrate (NO3ꢀ) with both nitrogen balance (NB) and redox
balance (ROB) close to 100% within experimental errors was
observed (eqn (1)), although the reaction was slow. Eqn (1)
consists of two half reactions caused by photogenerated elec-
trons and holes as shown in eqn (2) and (3), respectively.
Palladium-loaded TiO2 (Pd-TiO2) particles exhibited an
extraordinarily large rate of disproportionation of NO2ꢀ in their
NO3 + 2eꢀ + 2H+ / NO2 + H2O
ꢀ
ꢀ
If HS is added together with NO2ꢀ to an aqueous suspension
of TiO2 or Pd-TiO2 and holes are consumed by the hole scavenger
ꢀ
(eqn (4)), the oxidation aspect of disproportionation of NO2
(eqn (3)) may be suppressed and only N2 can be obtained without
formation of NO3ꢀ as shown in Fig. 1(b). However, oxidation of
NO2ꢀ by holes partly occurred (eqn (3))even in the presence of HS
(sodium oxalate), and ideal selective conversion of NO2ꢀ to N2 in
an aqueous suspension of Pd-TiO2 as shown in Fig. 1(b) was not
achieved.35 From the point of view of practical application, metal-
free reaction and the use of a cheap HS are highly desired in
addition to high product selectivity to N2 in NO2ꢀ conversion. In
the present study, we investigated various kinds of HSs enabling
us to achieve selective conversion of NO2ꢀ to N2 and found that
NH3 was an excellent HS for conversion of NO2ꢀ to N2.
aDepartment of Applied Chemistry, Faculty of Science and Engineering, Kinki
University, Kowakae, Higashiosaka, Osaka 577-8502, Japan. E-mail: hiro@apch.
kindai.ac.jp
bGraduate School of Science and Engineering Research, Kinki University, Kowakae,
Higashiosaka, Osaka 577-8502, Japan
† Electronic supplementary information (ESI) available: Experimental procedure
and Fig. S1. See DOI: 10.1039/c4ra09900a
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were under their detectiꢀon limits. Aer photoirradiation for 28
+
h, almost 100% of NO2 and 92% of NH4 were removed and
455 mmol of N2 was evolved, the values of NB being close to unity
ꢀ
+
(Fig. 2(b)). These results indicate that NO2 and NH4 reacted
with almost 1 : 1, forming N2. The oxidation states of nitrogen
in NO2ꢀ, NH3 and N2 are +3, ꢀ3 and 0, respectively. Therefore,
this reaction includes change in the oxidation states of nitrogen
ꢀ
+
in NO2 and NH4 (+3 and ꢀ3, respectively) to the same
ꢀ
oxidation state (zero), i.e., redox reaction between NO2 and
NH4 , and formation of an N2 product. We used various
+
commercially available metal (double) oxides for photocatalytic
ꢀ
+
reaction of NO2 and NH4 under the same conditions, and
results obtained aer 5 h photoirradiation are shown in Fig. S1
(ESI†), indicating that TiO2 (P 25) exhibited the highest N2 yield
among the metal oxides used in this study.
It is known that NaNO2 in a concentrated and boiling
aqueous solution reacts with ammonium chloride in the solu-
ꢀ
tion to form N2. Based on the thermal reaction of NO2 and
+
NH4 in an aqueous solution, the simultaneous removal of
ꢀ
NO2ꢀ and NH4+ and formation of N2 observed under the present
conditions can be shown by eqn (6).
Fig.
1 Photocatalytic reduction of NO2 to N2 in an aqueous
suspension of metal-loaded TiO2 in the presence of a hole scavenger.
(a) Combination of two photocatalytic systems: disproportionation of
NO2ꢀꢀ to N2 and NO3ꢀ catalyzed by Pd-TiO2 and selective reduction of
ꢀ
+
ꢀ
NO2 + NH4 / N2 + 2H2O
(6)
NO3 to NO2 catalyzed by another metal-loaded TiO2 in the pres-
ence of a hole scavenger and (b) simple photocatalytic system: hole
scavenging with a sacrificial reagent and reduction of NO2 to N2 by
photogenerated electrons on Pd particles.
ꢀ
A large molar heat capacity at constant pressure of liquid
water (75 J Kꢀ1 molꢀ1 at 298 K) means that removal of NO2ꢀ and
NH4+ in waste water by boiling needs a large amount of energy.
Therefore, the thermal reaction cannot be applied for the
removal of NO2ꢀ and NH4+ in waste water from the point of view
of practical application. Since a moderate supply of heat to
waste water would be possiblꢀe by using exhaust heat in facto-
ꢀ
Results of photoirradiation of UV light to NO2 in an
aqueous suspension of bare TiO2 (AEROXIDE® TiO2 P25, Evo-
+
nik) in the presence of NH3 (as NH4 ) for 6 h are shown in
+
ries, thermal reaction of NO2 and NH4 in the dark at 348 K
Fig. 2(a). Only sodium nitrite (NaNO2), ammonium sulfate
((NH4)2SO4), water, TiO2 and argon were present before the
reaction. The amounts of NO2ꢀ and NH4+ (both 500 mmol) were
was examined to compare with the results of photocatalytic
reaction at 298 K, and the results are shown in Fig. 3(a).
ꢀ
+
ꢀ
Concentrations of NO2 and NH4 slightly decreased and a
small amouꢀnt of N2 was evolved, indicating that thermal reac-
gradually decreased with photoirradiation, and NO2 (285
mmol) and NH4+ (260 mmol) were removed from the liquid phase
aer photoirradiation for 6 h. Corresponding to the decreases
+
tion of NO2 and NH4 occurs at 348 K. However, the reaction
ꢀ
+
rate was much smaller than that of the photocatalytic reaction
(Fig. 2(a)). Since NO2ꢀ absorbs UV light, photochemical reaction
(not photocatalytic reaction) might occur under the present
conditions. To elucidate ꢀthe effect of photochemical reaction, a
solution containing NO2 and NH4+ was irradiated by UV light
in the absence of TiO2 at 298 K, and the results are shown in
in NO2 and NH4 , N2 (278 mmol, corresponding to 556 mmol
nitrogen) was formed. During the reaction, NB was close to
100% within experimental errors as also shown in Fig. 2(a). The
amounts of other detectable species with GC (NO, H2 and O2)
ꢀ
+
Fig. 3(b). Only a small decreases in NO2 and NH4 and
formation of N2 were observed, indicating that the contribution
of photochemical reaction was negligible under the present
conditions. From results of control experiments shown in Fig. 3,
it can be concluded that a photocataꢀlyst (TiO2) and UV light
+
were essential for the reaction of NO2 and NH4 to N2 with a
sufficient reaction rate. In factories producing and re-producing
precious metals, a large amount of waste water containing NH3
is formed in another process and should be denitricated as
well as waste water containing NO2ꢀ. Therefore, simultaneous
removal of NO2ꢀ and NH4+ to N2 over a bare TiO2 photocatalyst
by appropriate mixing of two kinds of waste water is an ideal
method from the point of view of practical application for the
following reasons: (1) since NH3, which should be detoxicated,
Fig. 2 Time courses of photocatalytic conversions of NO2ꢀ (squares)
and NH4+ (circles) in aqueous suspensions of bare TiO2 particles at pH
7 under irradiation of UV light for (a) 6 h and (b) 28 h at 298 K. Triangles:
N2, diamonds: NO3ꢀ, open squares: N2O, reversed triangles: NB.
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irradiation of UV light in the presence of NH3. Since NH3, which
should be detoxicated, works as a reducing reagent (hole
ꢀ
scavenger) and an extra reagent for NO2 reduction is not
required, this reaction system is ideal from the point of view of
practical application.
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