Photocatalytic reduction of nitrate over chalcopyrite CuFe0.7Cr0.3S2 with high N2 selectivity

Article abstract of DOI:10.1016/j.jallcom.2015.08.182

Photocatalytic reduction of nitrate (NO₃^-) is a green and potentially inexpensive technique for reducing NO₃^- pollution in ground water. TiO₂-based photocatalysts have been studied extensively for this purpose. In the present study, the semiconducting catalyst CuFe_0.7Cr_0.3S₂ was applied to NO₃^- reduction. Loading this catalyst with metal co-catalysts (Ru, Au, Cu, Ag, Pt, and Pd) greatly increased the rate of NO₃^- reduction and the N₂ selectivity. In addition, there was a synergistic enhancement of the photocatalytic performance when the catalyst was loaded two co-catalysts. For example, the catalyst loaded with Pd and Au at mass fractions of 0.75% and 3%, respectively, could photocatalyze the complete reduction of NO₃^- in a 100 ppm N aqueous solution with 100% N₂ selectivity in less than 5 h with UV irradiation. However, with an inner irradiation from a full-arc Xe lamp, the NO₃^- conversion rate reduced to 0.065 mg N/h, probably because of the low density of the photoexcited electrons. The results show the potential of metal sulfides for photocatalytic reduction of NO₃^-, and the possibility of use of visible light.

Full text of DOI:10.1016/j.jallcom.2015.08.182

Journal of Alloys and Compounds 651 (2015) 731e736
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Photocatalytic reduction of nitrate over chalcopyrite CuFe0.7Cr0.3S2
with high N2 selectivity
Rong Wang, Mufei Yue, Rihong Cong, Wenliang Gao**, Tao Yang*
College of Chemistry and Chemical Engineering, Chongqing University, Chongqing, 400044, PR China
a r t i c l e i n f o
a b s t r a c t
Photꢀocatalytic reduction of nitrate (NO3ꢀ) is a green and potentially inexpensive technique for reducing
Article history:
Received 11 July 2015
Received in revised form
16 August 2015
Accepted 21 August 2015
Available online 24 August 2015
NO3 pollution in ground water. TiO2-based photocatalysts have been studied extensively for this pur-
ꢀ
pose. In the present study, the semiconducting catalyst CuFe0.7Cr0.3S2 was applied to NO3 reduction.
Loading this catalyst with metal co-catalysts (Ru, Au, Cu, Ag, Pt, and Pd) greatly increased the rate of
ꢀ
NO3 reduction and the N2 selectivity. In addition, there was a synergistic enhancement of the photo-
catalytic performance when the catalyst was loaded two co-catalysts. For example, the catalyst loaded
with Pd and Au ꢀat mass fractions of 0.75% and 3%, respectively, could photocatalyze the complete
reduction of NO3 in a 100 ppm N aqueous solution with 100% N2 selectivityꢀin less than 5 h with UV
irradiation. However, with an inner irradiation from a full-arc Xe lamp, the NO3 conversion rate reduced
to 0.065 mg N/h, probably because of the low density of the photoexcited electrons. The results show the
potential of metal sulfides for photocatalytic reduction of NO3ꢀ, and the possibility of use of visible light.
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Metal sulfide
Photocatalysis
Nitrate reduction
N2 selectivity
Co-catalyst
1. Introduction
NO3ꢀ, Kudo et al. used Pt-, Pd-, and Rh-loaded TiO2 catalysts with
no sacrificial reagent [6]. Ranjit et al. studied the photocatalytic
For health reasons, the World Health Organization recommends
that the concentrations of nitrate (NO3ꢀ), nitrite (NO2ꢀ) and
ammonium (NH4þ) in drinking water are below 10, 0.03, and
0.4 ppm, respectively. Ground water is a major source of drinking
water, and methods for removal of excess NO3ꢀ from ground water
activity of metal-loaded TiO2 under UV irradiation in an aqueous
solution containing CH3OH, CH3CH2OH and EDTA [7]. In both cases,
the conversion rates were low, which is not surprising because
ꢀ
many electrons are needed to convert NO3 into N2. Therefore, ꢀa
high density of electrons is needed at the catalytic site when NO3
ions are adsorbed on the surface.
ꢀ
are required. Various methods have been applied to remove NO3
from aqueous solutions, including ion exchange, electrodialysis,
and catalytic processes [1]. Among these methods, catalytic pro-
In 2005, Zhang et al. reported that Ag dꢀoped nanoparticles of
TiO2 could photocatalyze reduction of NO3 at a very high rate
(approximately 50 mg N/h), which is almost one order of magnitude
higher than the rate achieved in traditional physicochemical cata-
lytic processes [8,9]. To the best of our knowledge, most studies have
focused on the photocatalytic activities of modified TiO2 catalysts
[10e30], and only several other oxides have been studied for pho-
tocatalysis of the reduction of NO3ꢀ, such as KxGaxSn8-xO16 (x ¼ 1.8)
[31,32], KTaO3 [33], BaLa4Ti4O15 [34,35], NaTaO3 [33,36], and SrTiO3
[11,37]. Very recently, two chalcopyrite type sulfides (CuInS2
and CuFe1ꢀxCrxS2, 0 ꢁ x ꢁ 0.4) were found to be effective catalysts
under UV irradiation without co-catalysts [38,39]. Preliminary re-
ꢀ
cesses are most promising because the NO3 can be converted into
ꢀ
harmless nitrogen gas. Physicochemical reduction of NO3 by
monometallic or bimetallic co-catalysteloaded catalysts, such as
PdeCu/TiO2, has been studied extensively and the reduction rate
has been optimized to ~10 mg of nitrogen per hour (mg N/h) or
ꢀ
approximately 62 mg of NO3 per hour [2,3]. The selectivity for N2
production can also be optimized by choosing an appropriate
combination of metal co-catalysts such as Cu, Ni, Fe, Sn, In and Ag
[4,5].
ꢀ
Photocatalytic reduction of NO3 is of interest because it could
ꢀ
utilize solar energy. In the first study of photocatalytic reduction of
sults showed that the unmodified metal sulfides could adsorb NO3
and catalyze its reduction. However, the N2 selectivity was not re-
ported. In the present study, we performed a systematic investiga-
tion of the catalytic activities and N2 selectivity of CuFe0.7Cr0.3S2
(CFCS) catalysts loaded with various metal co-catalysts.
* Corresponding author.
** Corresponding author.
E-mail addresses: gaowl@cqu.edu.cn (W. Gao), taoyang@cqu.edu.cn (T. Yang).
http://dx.doi.org/10.1016/j.jallcom.2015.08.182
0925-8388/© 2015 Elsevier B.V. All rights reserved.

732
R. Wang et al. / Journal of Alloys and Compounds 651 (2015) 731e736
CFCS prepared by the solvothermal method has been used to
provide UV- or visible light irradiation (CEL-M500 or CEL-HXF300,
Beijing AuLight Ltd. Co.), which was applied from the top of the
reaction vessel (See Fig. S1 in the Electronic Supplementary Infor-
mation, ESI). Sodium oxalate (0.01 mol/L) was used as the sacrificial
agent. During the reaction, a small amount of the solution was
withdrawn periodically, the catalyst was immediately separated by
centrifugation, and the supernatant was analyzed to determine the
residual concentration of NO3 and NO2 with an UVeVis spec-
trophotometer. Ammonia was not detected in our case. To further
evaluate the visible light activity by enhancing the incident light
intensity, we also applied a setup with an inner irradiation lamp.
The photocatalytic reaction was carried out in a double-walled
quartz cell cooled by water with a 500 W full-arc Xe lamp (CEL-
LAX500) as the light source. A schematic view of the setup can be
found in ESI.
effectively catalyze the reduction of NO3ꢀ under UV irradiation, and
it has a much higher activity than that of unmodified TiO2 [6].
Therefore, there must be adsorption sites for NO3ꢀ on the surface of
CFCS, and the density of photoexcited electrons on these catalytic
sites must be high enough for the reduction of NO3ꢀ. Generꢀally,
ꢀ
NO3ꢀ reduction occurs in ꢀtwo successive reactions, from NO3 to
ꢀ
ꢀ
NO2 and then from NO2 to N2. The first reaction requires two
electrons, and the second reaction requires three electrons. Every
two nitrogen radicals that are produced will combine to form N2,
which then leaves the catalyst surface in the gas phase. In this
study, we aimed to further improve the photocatalytic efficiency
and the N2 selectivity of CFCS by loading it with co-catalysts (e.g.
Ru, Au, Cu, Ag, Pt, and Pd). Eventually, CFCS loaded with 0.75 wt% Pꢀd
and 3 wt% Au offered a high photocatalytic efficiency, and the NO3
in a 100 mL aqueous solution (100 ppm N) was completely con-
verted into N2 in 4.5 h.
3. Results and discussion
2. Materials and methods
The phase purities of the prepared catalysts were all verified by
powder XRD (Fig. 1). There is no standard XRD pattern available for
CuFe0.7Cr0.3S2, instead the XRD pattern for CuFeS2 is provided in
Fig. 1 for comparison purposes because it has a similar pattern to
2.1. Preparations of the catalysts
Black CFCS powder samples were prepared by the thiourea-
oxalic acid solvothermal method as described in the literature
[38]. In this method, CuCl2$2H2O (0.01 mol), FeCl3$6H2O
(0.007 mol) and CrCl3$6H2O (0.003mol) were mixed with oxalic
acid (1.50 g) and an excess of thiourea (0.20 mol). Then, the mixture
was transferred into a 50 mL Teflon autoclave, and the autoclave
was sealed. After heating at 220 ꢂC for 5 d, a black solid was ob-
tained. This solid was collected and washed with water to remove
any contaminant.
Various co-catalysts were investigated to see if they improved
the photocatalytic activity of CFCS. For example, to load CFCS with a
co-catalyst, 0.2000 g of CFCS, 1.4 mL of H2PtCl6$6H2O (1.48 mg/mL),
and 20 mL of distilled water were placed in a 100-mL beaker. This
solution was mixed by ultrasonication for 20 min. Then, dilute
aqueous KBH4 was added to the solution very slowly to obtain Pt
particles. The obtained powder sample was washed with water and
dried at 60 ꢂC in an oven before further characterizations. Addi-
tional co-catalysteloaded CFCS catalysts were prepared using the
same method with RuCl3, AgNO3, HAuCl4$4H2O, PdCl2, and
CuCl2$6H2O aqueous solutions.
2.2. Characterizations
Powder X-ray diffraction (XRD) data were collected on a PAN-
alytical X'pert diffractometer equipped with a PIXcel 1D detector
(PANalytical, Almelo, the Netherlands) with Cu K
a radiation
(1.5406 Å). The operation voltage and current were 40 kV and
40 mA, respectively. Scanning electron microscopy images were
recorded using a JEOL JSM-7800F electron microscope (JEOL, Tokyo,
Japan) at a working distance of 4 mm.
2.3. Photocatalytic activity evaluation
The photocatalytic activities of the prepared catalysts were
mostly tested in a sealed circulation system equipped with a vac-
uum line (LabSolar-IIIAG system, Perfect Light Ltd. Co., Beijing,
China), a 150-mL Pyrex glass reactor (enwrapped with 5 ꢂC recy-
cling water to maintain cool), and a gas sampling port that was
directly connected to a gas chromatograph (GC7900, Shanghai
Techcomp, Shanghai, China). The gas chromatograph was equipped
with a thermal conductivity detector and a column packed with 5A
molecular sieves. Helium was used as the carrier gas to detect the
so-produced N2 online. 500 W Hg or 300 W Xe-lamp were used to
Fig. 1. Powder XRD patterns for the as-synthesized CFCS catalyst and the recycled CFCS
catalysts with metal loaded. In addition, the XRD pattern for the isostructural CuFeS2 is
also given for comparison purposes.

R. Wang et al. / Journal of Alloys and Compounds 651 (2015) 731e736
733
Fig. 2. Scanning electron microscopy images of CuFe0.7Cr0.3S2.
CuFe0.7Cr0.3S2 [38]. The CFCS catalysts both without and with co-
catalysts showed almost identical XRD patterns. In addition, the
catalysts recovered after the photocatalytic experiments showed
similar patterns to the fresh CFCS catalyst, which indicates the
catalysts are very stable in the photocatalytic reactions and are not
degraded. Only a small quantity of Fe2O3 was observed as an im-
purity, and this did not affect the photocatalytic reaction in our
study. SEM images (Fig. 2 and Fig. S3 in ESI) showed that the CFCS
catalyst had nanoscale plates (average diameter 50 nm, thickness
15 nm) on its surface.
The adsorption and desorption of substrates at catalytic sites is a
dynamic equilibrium process. We speculated that loading the
catalyst with Ag, Pt or Pd at a mass fraction of 0.75% would incrꢀease
the number of binding sites and delay the desorption of NO3 by
lowering the potential barrier. This could increase the conversion
ꢀ
rate of NO3 and allow the deep reduction to N2. Loading with co-
catalysts could also facilitate photo-excited charge separation
based on the photocatalytic mechanism, and therefore allow a
higher density of electrons at the catalytic sites. These synergetiꢀc
effects could greatly increase the photocatalytic reduction of NO3
in aqueous solution.
To investigate the activities of the various co-catalysts, aqueous
solutions of NO3ꢀ (25 ppm N) were irradiated for 1 h under UV light
To better understand the effects of the different metallic co-
ꢀ
ꢀ
and the percentage of NO3 converted to NO2ꢀ/N2 was calculated
catalysts on the reduction, NO2 reduction experiments were
(Fig. 3). The results showed that both Ru and Au co-catalysts
decreased the conversion of NO3ꢀ, while Cu did not affect the
reductiꢀon. The co-catalysts Ag, Pt and Pd increased the conversion
of NO3 to between 26.5 and 34.5%. For comparison purposes, an
aqueous solution containing no catalyst was irradiated under UV-
performed (Fig. 4). In most cases, loading with a co-catalyst
increased the conversion of NO2 to N2. The barrier to NO2
ꢀ
ꢀ
adsorption was lowest with Au as the co-catalyst, which gave a
conversion rate of 16.8%. The mechanism of this enhancement
would be similar to that discussed above, with the co-catalysts
aiding both charge separation and adsorption of substrates. By
comparing to Fig. 3, we would expect that the Au cocatalyst has a
strong binding ability to nitrite ions and may be beneficial for the
N2 production.
To investigate co-catalyst loading with two metals, we tried
combining PteAu, AgeAu, and PdeAu. These combinations were
selected because the individual metals all gave good results as co-
ꢀ
light. In this solution, after 2 h of irradiation, the NO3 concen-
tration remained constant within the experimental error. The low
conversion rate in this blank experiment was believed to arise from
the low photon density of the irradiation source used in this study.
Comparison of the catalytic experiments to the blank experiment
ꢀ
indicated that all the catalysts could reduce NO3 with high
efficiency.
ꢀ
Fig. 3. Efficiency of the photocatalytic reduction of NO3 under UV irradiation for 1 h.
ꢀ
The catalyst was unmodified CFCS or CFCS loaded with a metal co-catalyst at a mass
Fig. 4. Photocatalytic conversion of NO2 in aqueouꢀs solution over various photo-
ꢀ
fraction of 0.75%. Photocatalytic conditions: 100 mL of NO3 aqueous solution con-
catalysts. Photocatalytic conditions: 100 mL of NO2 aqueous solution containing
25 ppm N, 0.1 g of catalyst, 500 W Hg lamp, in evacuated system.
taining 25 ppm N, 0.1 g of catalyst, 500 W Hg lamp, in evacuated system.

734
R. Wang et al. / Journal of Alloys and Compounds 651 (2015) 731e736
ꢀ
Fig. 5. Efficiency of the photocatalytic reduction of NO3 under UV irradiation for 1 h.
Each catalyst was loaded with two metal co-catalysts. Photocatalytic conditions:
100 mL of NO3 aqueous solution containing 25 ppm N, 0.1 g of catalyst, 500 W Hg
lamp, in evacuated system.
ꢀ
Fig. 6. Reduction of 100 mL of an aqueous solution of NO3 (100 ppm N) using CFCS
ꢀ
loaded with 0.75% (mass fraction) Pd and 3% (mass fraction) Au. For better under-
standing and easy comparison, the respective contents of NO3ꢀ, NO2 and N2 werꢀe
ꢀ
calculated into the mass of N (mg N). Photocatalytic conditions: 100 mL of NO3
aqueous solution containing 100 ppm N, 0.3 g of catalyst, 500 W Hg lamp, in evacuated
system.
catalysts. Among these co-catalyst combinations, the highest con-
version rate and N2 selectivity were achieved with a mass fraction
loading of 0.75% Pd and 2% Au (Fig. 5). Increasing the mass fraction
of Au, while holding that of Pd constant, resulted in higher N2
selectivity. The N2 selectivity of 52% (in 1 h irradiation) was ach-
ieved with a mass fraction loading of 0.75% Pd and 3% Au. It is
understandable that the increasing the content of Au could increase
the production of N2 because the increasing metal content provides
the majority of the energy in sunlight comes from visible light. For
visible light irradiation, it is advantageous that metal sulfides have
a narrow bandgap because of the high potential of the valence band
arising from their S 2p orbitals. CFCS could show visible light ac-
tivity, and we performed a preliminary experiment to investigate
ꢀ
ꢀ
more catalytic sites and particularly, Au is helpful to adsorb NO2
this. Photocatalytic reduction of NO3 (10 ppm N) was attempted
for deep conversion to N2. With further increasing the mass fraction
loading of 1.5% Pd and 4% Au, a complete conversion of NO3ꢀ and N2
selectivity of 65% were achieved with 1 h of irradiation. From the
industrial view of point, the usage of the noble metal should be as
low as possible for the economic purpose. In our case, we did not
further increase the content of the cocatalysts, which probably
would give an even better performance.
with irradiation from a Xe lamp. The conversion rate was
0.018 mg N/h with a N2 selectivity of 43%. The photocatalytic ac-
tivity was two orders of magnitude lower than that achieved with
UV irradiation even though CFCS has a narrow bandgap energy
(0.7 eV) [38]. This reduction in the conversion rate and selectivity
could have caused by the low density of electrons when irradiated
by the Xe lamp. Therefore, an inner irradiation setup (see Fig. S2 in
ESI) was applied, where the incident light intensity is relatively
larger than the outer irradiation setup (see Fig. S1). Indeed, we
observed an increase of the conversion rate of NO3ꢀ, from
0.018 mg N/h to 0.065 mg N/h (See Fig. 7).
We performed an extended reductionꢀexperiment with a highly
concentrated aqueous solution of NO3 (100 ppm N). In this
experiment, 0.3 g of the CFCS photocatalyst loaded with 0.75 wt%
ꢀ
Pd and 3 wt% Au was used. The NO3 reduction occurred in twꢀo
stages (Fig. 6). In the first stage (roughly the first two hours), NO3
As a heterogenous photocatalyst, the stability of metal sulfides
was always the major weakness and in most cases, people prefer to
use sacrficial agents to avoid the photo-corrosion problem. Here,
we also need to further evaluate this issue. As a representative,
CFCS loaded with 0.75 wt% Pd and 3 wt% Au was carefully studied.
Till now, this recovered particular catalyst has been irradiated by
UV-light for 5.5 h and Xe-lamp for 8 h. We performed the recycling
experiments using the following condition: 0.1 g catalyst, 100 mL of
ꢀ
was not completely converted, and the remaining NO3 coexisted
ꢀ
with thꢀe reduction product NO2 and N2. In this stage, production
of NO2 and N2 occurred simultaneously. These reactions were
apparently zero-order reactions. The observed conversion rate of
ꢀ
ꢀ
NO3 was 4.15 mg N/h. The production rates of NO2 and N2 were
1.67 mg N/h and 2.45 mg N/h, respectively. The calculated N2
selectivity was 59%, which was a little higher than that obtained
using only 0.1 g of the catalyst (52.4%).
ꢀ
NO3 aqueous solution containing 100 ppm N, 500 W Hg lamp, in
When all the NO3ꢀ was reduced, the second stage began, which
only comprises the reduction of NO2ꢀ to N2. The rates for reduction
of NO2 and production of N2 were similar at 1.81 mg N/h and
evacuated system. After each cycle, the resultant supernatant so-
lution was poured out and the cꢀatalyst was washed by water in the
reaction vessel twice. New NO3 aqueous solution was added into
the reaction vessel and after a half-hour dark reaction for adsorp-
tionedesorption equilibrium, the UV-light was switched on. As
shown in Fig. 8, after 5 cycles (15 h irradiation in total) the con-
version rate slightly decrease from 2.01 mg N/h to 1.81 mg N/h
(aboꢀut 10% decreasing). During the recycling exꢀperiments, ~2 mmol
NO3 in total was reduced to either NO2 or N2, and only
~0.55 mmol photocatalyst (CuFe0.7Cr0.3S2) was used. The final
recovered powder sample is ~0.085 g. Moreover, the powder XRD
pattern after long-term experiments is consistent with the initial
one, indicating that it remained as a sulfide, neither oxysulfide nor
ꢀ
1.68 mg N/h, respectively. This reaction was apparently still a zero-
order reaction. It should be noted that the N2 production rate in the
second stage was smaller than that in the first stage. This difference
arises becauꢀse N2 in the first stage is produced from the reduction
of both NO3 and NO2ꢀ, while in the second stage N2 is only pro-
duced from NO2ꢀ. In other words, in the first stage, some of the
ꢀ
NO3 species were strongly bound to the catalytic sites and
therefore can be deeply reduced to N$.
While the CFCS catalyst loaded with a metal co-catalyst
exhibited a very high photocatalytic activity under UV irradiation,

R. Wang et al. / Journal of Alloys and Compounds 651 (2015) 731e736
735
(mostly less than 5 mg N/h), with only one exception, Ag/TiO2 [8,9].
This milestone work of Ag/TiO2 was conducted by F. X. Zhang and
his co-workers, where the conversion rate of nitrate achieved
approximately 50 mg N/h with an inner irradiation of UV-light and
most importantly, the N2 selectivity is close to 100%. There remains
two challenges in this research field, one is to develop visible light
catalysts and the other is to avoid the usage of sacrficial agents
(water is therefore oxidized to O2). Y. Kamiya has developed a dual-
catalyst system (consisted of both photocatalyst Pt/SrTiO3: Rh and
non-photocatalyst SnePd/Al2O3) to achieve a small efficiency
(~0.03 mg N/h) under visible light [37]. A. Kudo recently reported
that BaLa4Ti4O15 with 0.5 wt% Ni as cocatalyst can photocatalyze
the nitrate reduction, and together with the O2 evolution from
water, only when H3BO3 was used as buffer and under the UV-light
irradiation [35].
We believe that our work on CFCS may offer a new opportunity.
When Pd and Au were co-loaded to CFCS, it possesses a quite high
photocatalytic efficiency (4.15 mg N/h and 59% N2 selectivity)
although with the usage of sodium oxalate and under UV light. The
already observed visible light activity (0.065 mg N/h) is a good start
to prompt further research in metal sulfide semiconductors which
usually possess narrow bandgap energies, especially for those
which cannot work as a water reduction catalyst. The next issue is
to find a suitable or unharmful sacrificial agent to stabilize the
photocatalyst and probably some new techniques can be also
applied. For example, people could apply so-called Z-scheme
strategy to avoid the usage of sacrificial agents using a dual-catalyst
system, consisted of a metal sulfide for nitrate reduction and an
oxide for water oxidation (for example, BiVO4, WO3, or TaON)
[40e42].
ꢀ
Fig. 7. Efficiency of the photocatalytic reduction of NO3 under either outer or inner
irradiation from a Xe lamp using the photocatalyst (CFCS loaded 0.75 wt% Pd- 3 wt%
Au). Photocatalytic conditions: 100 mL of NO3 aqueous solution containing 10 ppm N,
0.3 g of catalyst, 300 W Xe-lamp for outer irradiation and 500 W full-arc Xe lamp for
inner irradiation.
ꢀ
sulfate (see Fig. S4 in ESI). Accordingly, we believe the strategy of
using sodium oxalate (0.01 mol/L) as the sacrificial agent is effective
to prevent the photo-corrosion of CFCS during photocatalytic ni-
trate reduction. The slightly decreasing of the activity was attrib-
uted to the small loss of the catalyst powder during the recycling
experiments or the mechanical loss of the loading cocatalysts by
stirring.
4. Conclusions
As stated in the introduction part, most studies of photocatalytic
nitrate reduction were focused on TiO2 or other metal oxides. For
example, the pioneering work was reported by A. Kudo and K.
Domen in 1987 [6]. One problem is that most oxide materials have
wide bandgap energies, which could be active only under UV-light
excitation; another one is that the relative negative conduction
band potential allow the reduction of water, and the so-produced
hydrogen would over-reduce the nitrate to ammonia, which is of
course undesirable. The competition between water reduction and
nitrate reduction sometimes is the major reason to the very low
ꢀ
In conclusion, photocatalytic reduction of NO3 was investi-
gated with CFCS loaded with co-catalysts (Ru, Au, Cu, Ag, Pt, and
Pd). Both the NO3 conversion rate and the N2 selectivity were
enhanced with some of the co-catalysts. The catalyst loaded with
0.75 wt% Pd and 3 wt% Au could completely reduce the NO3 in a
100 mL aqueous solution (100 ppm N) to N2 in less than 5 h under
UV irradiation. However, with an inner irradiation from a full-arc
Xe lamp, the NO3 conversion rate reduced to 0.065 mg N/h,
probably because of the low density of the photoexcited electrons.
The results of this study show metal sulfides have potential for
photocatalytic reduction of NO3ꢀ, and could be extended to the
visible light range.
ꢀ
ꢀ
ꢀ
ꢀ
efficiency of photocatalytic NO3 removal in aqueous solution
Acknowledgment
This work was supported by grants from the Natural Science
Foundation of China (Grant Nos. 91222106 and 21171178), Natural
Science Foundation Project of Chongqing (Grant No.
2014jcyjA50036) and the Fundamental Research Funds for the
Central Universities (Grant No. CQDXWL-2014-005).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jallcom.2015.08.182.
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