Reduction of nitrate and nitrite ions over Ni-ZnS photocatalyst under visible light irradiation in the presence of a sacrificial reagent

Article abstract of DOI:10.1246/cl.2002.838

Ni-doped ZnS photocatalysts (Zn_0.999Ni_0.001S) with a 2.4 eV energy gap showed activities for the reduction of nitrate and nitrite ions to nitrite, ammonia, and dinitrogen under visible light irradiation (λ > 420 nm) in the presence of methanol as a reducing reagent. The reduction of nitrate ions competed with that of water to form dihydrogen. The concentration of nitrate ions and loading a platinum cocatalyst affected the selectivity for the reduction products of nitrate ions.

Full text of DOI:10.1246/cl.2002.838

838
Chemistry Letters 2002
Reduction of Nitrate and Nitrite Ions over Ni-ZnS Photocatalyst under Visible Light Irradiation
in the Presence of a Sacrificial Reagent
Osamu Hamanoi and Akihiko Kudoꢀ
Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601
(Received May 9, 2002; CL-020400)
Ni-doped ZnS photocatalysts (Zn0:999Ni0:001S) with a 2.4 eV
energy gap showed activities for the reduction of nitrate and
nitrite ions to nitrite, ammonia, and dinitrogen under visible light
irradiation (ꢀ > 420 nm) in the presence of methanol as a
reducing reagent. The reduction of nitrate ions competed with that
of water to form dihydrogen. The concentration of nitrate ions and
loading a platinum cocatalyst affected the selectivity for the
reduction products of nitrate ions.
dinitrogen were determined using an on-line gas chromatography
(Shimadzu, GC-8A, TCD, Ar carrier). The amounts of produced
nitrite ions and ammonia in supernatants were determined by the
Griess-Romijin method (Wako, a Griess-Romijin reagent) and an
indophenol method, respectively. The colorimetric analyses were
carried out using a UV-vis-NIR spectrometer (Jasco, UbestV-
570).
Table 1 shows the amounts of reduction products of nitrate
and nitrite ions on Zn0:999Ni0:001S photocatalysts under visible
light irradiation (ꢀ > 420 nm) of 20 h and the effect of loading
platinum cocatalysts on the photocatalytic activity. The values in
brackets in Table 1 indicate the amounts of electrons consumed
for each production. The Zn0:999Ni0:001S photocatalyst produced
dihydrogen from an aqueous methanol solution without nitrate
ions (Run 1). The reduction of nitrate and nitrite ions (the
equations (1) and (2)) competed with that of water to form
dihydrogen (the equation (3)).
Water pollution by nitrate and nitrite ions is getting a serious
social issue. Various chemical processes for the elimination of
nitrate and nitrite ions in aqueous media have been extensively
studied.1{15 A photocatalytic system is one of candidates for the
elimination processes.7{15 TiO2 photocatalysts which work only
under UV irradiation have been widely employed for that
reaction.12{15 As far as the authors know, however, an efficient
photocatalyst for the elimination of nitrate and nitrite ions under
visible light irradiation has not been reported. One of the authors
has reported that Cu or Ni-doped ZnS photocatalysts possessed
high activities for dihydrogen evolution from aqueous solutions
in the presence of a sacrificial reagent such as sulfite ions under
visible light irradiation (ꢀ > 420 nm) even without a platinum
cocatalyst.16;17 In the present paper, the reduction of nitrate and
nitrite ions was studied under visible light irradiation using the Ni-
doped ZnS photocatalyst with a highly reducing ability.
NO3ꢁ þ 2Hþ þ 2eꢁ ! NO2ꢁ þ H2O
ð1Þ
ð2Þ
ð3Þ
CB
NO2ꢁ þ 7Hþ þ 6eꢁ ! NH3 þ 2H2O
CB
2Hþ þ 2eꢁ ! H2
CB
The dihydrogen evolution was suppressed when the reaction was
carried out in an aqueous KNO3 solution (1 mol dmꢁ3) (Run 2).
Nitrate ions were mainly reduced to nitrite ions. Ammonia and
dinitrogen were also formed. The reduction products were not
detected under darkcondition inthepresence ofthe photocatalyst.
It has been reported that platinum loaded on a TiO2 photocatalyst
is an effective cocatalyst to enhance thereduction of nitrate ions to
ammonia (the equation (2)).12 Therefore, the effect of the
platinum cocatalyst on the photocatalytic reduction of nitrate
ions on Zn0:999Ni0:001S photocatalysts was examined. In the
present system, dihydrogen evolution was enhanced by the
loading of a platinum cocatalyst while the reduction of nitrate ions
was suppressed (Run 3). However, the ratio of ammonia
formation to nitrite formation by the reduction of nitrate ions
was increased in the presence of the platinum cocatalyst
A Zn0:999Ni0:001S photocatalyst was prepared by a coprecip-
itation method. An aqueous sodium sulfide solution (1 mol dmꢁ3
,
)
100 ml) was mixed with an aqueous Zn(NO3)2 (0.3 mol dmꢁ3
and Ni(NO3)2 (3 ꢂ 10ꢁ4 mol dmꢁ3) solution (100 ml). The
obtained precipitation was washed with water and it was heat-
treated at 773 K for 3 h in a N2 gas flow. Photocatalytic reactions
were carried out using aqueous KNO3 and KNO2 solutions
(300 ml) containing methanol (6.25% in vol.) of a reducing
reagent in a gas-closed circulation system. 0.5 g of the
Zn0:999Ni0:001S photocatalyst was dispersed in the solution in a
Pyrex reaction cell, and irradiated with visible light using a 300 W
Xe lamp (ILC technology, CERMAX LX-300) and a cut-off filter
(HOYA, L42). The amounts of evolved dihydrogen and
compared with that in the case of a naked Zn0:999Ni0:001
photocatalyst. Although the total efficiency of the reduction of
S
ꢁ
ꢁ
Table 1. Reduction of NO3 and NO2 on Zn0:999Ni0:001S photocatalysts under visible light irradiation (ꢀ > 420 nm)
Run
Reactant
Concentration
/mol dmꢁ3
Pt cocatalyst
/wt%
Amounts of products after 20 h of irradiation (electron consumed)/ꢁmol
ꢁ
H2
NO2
NH3
(—)
N2
ꢁ
1
2
3
4
5
no NO3
ꢁ
0
1
1
0.01
0.01
0
0
1
0
1
214
60
212
37
(428)
(120)
(424)
(74)
—
250
14
—
—
(—)
(500)
(28)
(—)
(—)
—
21
11
41
65
—
2.7
0.79
2.4
(—)
(27)
(7.9)
(14)
(57)
NO3ꢁ
(168)
(88)
(246)
(390)
NO3ꢁ
NO2ꢁ
NO2
96
(192)
9.5
Catalyst; 0.5 g, solution; aqueous CH3OH solutions (6.25 vol%), light source; 300 W Xe lamp with a cut-off filter.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
839
nitrate ions was low in the presence of the platinum cocatalyst, the
number of electrons consumed for the ammonia formation
overwhelmed that for nitrite formation. It indicated that the
platinum cocatalyst enhanced the further reduction of nitrite to
ammonia (the equation (2)). The Pt cocatalyst loading does not
always improve photocatalytic activities for photocatalysts which
show high activities even without cocatalysts. In the reduction of
nitrate ions, the decrease in the total efficiency by the loading of
the Pt cocatalyst was due to a shielding effect of incident light, a
covering effect of active surface of a Zn0:999Ni0:001S photo-
catalyst, and the formation of recombination sites at the interface
between Pt and Zn0:999Ni0:001S.
Reduction of nitrite ions was alsoexamined. The efficiency of
ammonia formation was increased compared with the reduction
of nitrate ions on the naked Zn0:999Ni0:001S photocatalyst (Run 4).
It indicated that the reduction of nitrite ions was a rate
determining step in the reduction of nitrate ions. The loading of
a platinum cocatalyst enhanced the ammonia and dinitrogen
formations in the reaction of nitrite ions (Run 5). The total
efficiency was also increased by the Pt cocatalyst loading. In this
case, a positive effect of an active site formation for dihydrogen
and ammonia formation by the loading of the Pt cocatalyst was
superior to the negative effects mentioned above.
the reduction of nitrate ions was consumed for the reaction of the
equation (2).
In conclusion, it was found that Zn0:999Ni0:001S was an active
photocatalyst for the reduction of nitrate and nitrite ions under
visible light irradiation accompanied with the oxidation of an
organic compound, methanol. The selectivity of the reduction
products depended on the concentration of nitrate and the
presence of a platinum cocatalyst.
This work was supported by Kurita Water Industries Ltd.,
Core Research for Evolutional Science and Technology
(CREST), a Grant-in-Aid (No. 14050090) from the Ministry of
Education, Culture, Science, and Technology, and Tokyo Ohka
Foundation for the Promotion of Science and Technology.
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Figuꢁre 1. Dependence of photocatalytic reduction of
NO3 over Zn0:999Ni0:001S in the presence of CH3OH
upon KNO3 concentration under visible light irradiation
(ꢀ > 420 nm). (a): H2, (b): NO2ꢁ, (c): NH3, (d): N2 ꢂ 10.
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