Palladium-copper/hydrophobic active carbon as a highly active and selective catalyst for hydrogenation of nitrate in water

Article abstract of DOI:10.1246/cl.2007.994

Cu-Pd supported on active carbon (AC), in which Cu/Pd ratio is strictly controlled, is an excellent catalyst for selective hydrogenation of NO ₃^- to harmless products in the presence of H₂ at low partial pressure. 3.6 wt% Cu; 2.0 wt% Pd/AC, coupled with another Pd/AC catalyst, showed high selectivity to N₂ (98% at 298 K), while suppressing NH₃ production (2% selectivity, 0.5 from 100 ppm of NO₃^-). Modification of the Cu-Pd/AC catalyst with PTFE results in further enhancement; the activity more than tripled, while suppression of NH₃ production was retained. Copyright

Full text of DOI:10.1246/cl.2007.994

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94
Chemistry Letters Vol.36, No.8 (2007)
Palladium–Copper/Hydrophobic Active Carbon as a Highly Active
and Selective Catalyst for Hydrogenation of Nitrate in Water
1
1
2
1
ꢀ1
Yi Wang, Tomohiro Kasuga, Ikkou Mikami, Yuichi Kamiya, and Toshio Okuhara
1
Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810
2
Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi 332-0012
(Received May 1, 2007; CL-070464; E-mail: oku@ees.hokudai.ac.jp)
Cu–Pd supported on active carbon (AC), in which Cu/Pd
ratio is strictly controlled, is an excellent catalyst for selective
hydrogenation of NO3 to harmless products in the presence
pressure H2 (0.05 atm) is used. To the best of our knowledge, this
is the first report of catalytic hydrogenation of nitrate under such
reaction conditions.
ꢁ
of H2 at low partial pressure. 3.6 wt % Cu; 2.0 wt % Pd/AC,
coupled with another Pd/AC catalyst, showed high selectivity
to N2 (98% at 298 K), while suppressing NH3 production (2%
ꢁ
ꢁ
2
NO3 þ 5H2 ! N2 þ 2OH þ 4H2O
ð1Þ
ð2Þ
ꢁ
ꢁ
NO3 þ 4H2 ! NH3 þ OH þ 2H2O
ꢁ
selectivity, 0.5 from 100 ppm of NO3 ). Modification of the
Cu–Pd/active carbon catalysts were prepared by an impreg-
nation method using PdCl2 and Cu(NO3)2 from Wako Pure
Chem. Co. The four types of active carbon (AC) listed in
Table 1 were used as supports. An aqueous solution of
Cu–Pd/AC catalyst with PTFE results in further enhancement;
the activity more than tripled, while suppression of NH3 produc-
tion was retained.
ꢁ
1
ꢁ3
PdCl2 (1:13 ꢂ 10 mol dm ) was added to AC and stirred
for 30 min at room temperature. After filtration and drying at
373 K for 12 h, an aqueous solution of Cu(NO3)2 (1:66 ꢂ
The pollution of groundwater by harmful nitrogen-contain-
ing compounds such as nitrate is an increasing problem through-
out the world. The use of drinking water containing high concen-
trations of nitrate causes various diseases, including blue baby
syndrome and diabetes. Thus, the reduction of nitrate in polluted
groundwater to an allowable level (25 ppm recommended by
WHO) is absolutely necessary for the provision of safe drinking
water.
ꢁ1
ꢁ3
10 mol dm ) was introduced to the solid at room temperature.
The resulting wet solid was dried at 373 K for 12 h, followed by
calcination at 523 K for 3 h. The catalysts were reduced using
NaBH4 before the reaction. As a reference, Cu–Pd/Al2O3
(JRC-ALO-4) was prepared.
The reduction of nitrate with H2 was performed using a gas–
ꢁ
3
liquid flow reactor (Pyrex tube, 10 mm i.d.) at 298 K. NO3 ,
prepared from NaNO3 (Wako Pure Chem Co.) (100 ppm,
The hydrogenation of nitrate with H2 to form N2 (eq 1)
over a solid catalyst has attracted much attention in the area of
purification of polluted water. In the catalytic hydrogenation,
the formation of NH3 (eq 2) is a critical problem; the allowable
level of NH3 in drinking water is 0.5 ppm. Many studies have
been carried out on the catalytic hydrogenation of nitrate using
ꢁ
3
3.22 mmol dm ), and gas (H2:CO2:He = 5:50:45 and 50:50:0
for low- and high-partial-pressure H2, respectively; flow
rate 80 cm h ) were fed into a reactor under atmospheric
pressure. The gas at the outlet of the reactor was analyzed by
TCD-GC (Shimadzu GC-8A), with a molecular sieve 5A column
for N2 and O2 and a Porapak Q column for N2O. Concentrations
3
ꢁ1
1
Cu–Pd bimetallic catalysts since the discovery of Cu–Pd/Al2O3
2
ꢁ
ꢁ
as an active and selective catalyst by Vorlop and colleagues.
of NO3 , NO2 , and NH3 in the aqueous phase were measured
using a flow injection analysis (FIA) system.
However, few catalysts with acceptable activity, stability, and
selectivity have been developed. Herein, we report the develop-
ment of a catalyst consisting of Cu–Pd/active carbon, in which
the Cu/Pd ratio has been controlled and a hydrophobic active
carbon selected, as a highly active, selective, and stable catalyst
for hydrogenation of nitrate in water. In this study, with a view to
profitable and practical use of the reaction system, low-partial-
ꢁ
Table 1 summarizes the activity (removal rate of NO3 ) and
selectivity and the AC surface areas of catalysts in which the
amounts of Cu and Pd were adjusted to 3.6 and 2.0 wt %, respec-
tively. The data for Cu–Pd/Al2O3 are also listed. In this Table,
selectivities were evaluated at near 100% conversion. The
AC-supported Cu–Pd catalysts showed relatively high activities
Table 1. Hydrogenation of nitrate with H2 over various Cu–Pd catalystsa
PH d
SA
2
e
Activity
f
Selectivity /%
ꢁ
Catalystb,c
2
ꢁ1
ꢁ1 ꢁ1
/atm
/m g
/mmol h
g
N2
N O
2
NO2
NH3
(NH ppm)
3
Cu–Pd/AC (coconut shell, Wako)
Cu–Pd/AC (coal, Kuraray)
Cu–Pd/AC (coconut shell, Kuraray)
Cu–Pd/AC (wood chip, Wako)
Cu–Pd/Al2O3
0.05
1022
1041
934
1393
166
—
0.12
0.10
0.09
0.04
0.06
0.27
0.53
93
84
65
70
34
41
1
0
0
23
16
58
0
0
0
0
0
0
0
0
7
16
12
14
7
55
24
b
1.8
4.5
4.0
3.9
1.8
15.1
6.4
Cu–Pd/AC (coconut shell, Wako)
Cu–Pd/Al2O3
0.50
—
75
a
ꢁ1
ꢁ
Reaction conditions: Temperature, 298 K; nitrate, 100 ppm (from NaNO3); WHSV = 54–102 h ; H2/NO3 = 9. Loading amounts
c
of Cu and Pd were 3.6 and 2.0 wt %, respectively. Information in parentheses is the type of active carbon used and the manufacturer.
e
d
f
Partial pressure of hydrogen. Surface area. Selectivity on the basis of N atom at near 100% conversion.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.8 (2007)
995
Figure 2. Time courses for hydrogenation of nitrate (100 ppm)
over 3.6 wt % Cu; 2 wt % Pd/AC coupled with 5 wt % Pd/AC
Figure 1. Influence of Cu content in x wt % Cu; 2 wt % Pd/AC
on conversion and selectivity to NH3.
placed at the gas outlet. Conversion ( ), and selectivity to N2
ꢁ
(
), N2O ( ), NO2 ( ), and NH3 ( ).
at a low partial pressure of H2 (0.05 atm). The catalyst using
coconut shell AC (Wako) showed the highest activity. While
Cu–Pd/Al2O3 was highly active under the reaction conditions
with high-partial-pressure hydrogen (0.50 atm), Cu–Pd/AC
ꢁ
NO3 remaining (17 ppm) was well within the allowable level
(25 ppm). The Cu–Pd/AC catalyst showed stationary conversion
and selectivity from the beginning of the reaction, which were
retained for at least 50 h. The concentrations of Pd and Cu in
the solution at the outlet of the reactor were measured using
ICP. The amounts of both metals leached were less than 0.1%
of the amounts present in the catalyst during the reaction.
In order to further enhance the activity of the Cu–Pd/AC
catalyst, we modified the catalyst using polytetrafluoroethylene
(
coconut shell, Wako) showed double the activity of Cu–Pd/
Al2O3 when low-partial-pressure H2 (0.05 atm) was used; the
reaction orders with respect to H2 were 0.4 and 0.9, respectively.
In addition, it should be emphasized that the use of Cu–Pd/AC
(coconut shell, Wako) resulted in very low production of NH3
compared to other Cu–Pd/AC catalysts.
5
The present reaction system consisted of three phases: gas
hydrogen), liquid (nitrate), and solid (catalyst). Efficient contact
(PTFE). Since PTFE is a water-repelling agent, it was expected
(
that the use of this material would results in increased hydropho-
bicity in the catalyst. This proved to be the case; the reaction
rate increased steeply with increasing PTFE content. It was not-
ed that the reaction rate of the catalyst modified with 5 wt %
PTFE (0.33 mmol h
that of the PTFE-free catalyst (0.12 mmol h
ity to NH3 (8% at 55% conversion) was comparable to that of
the PTFE-free catalyst, although the addition of excess amounts
of PTFE (10 wt %) resulted in the formation of significant
amounts of NH3 (26% selectivity). These results demonstrate
that modification of the catalyst with PTFE is an effective
way to enhance catalytic activity. The enhanced activity due to
this modification (5 wt % PTFE) was retained for at least 50 h
without any deactivation.
between the solid catalyst and the reactant gas is essential
for progression of the reaction in such a system. Investigation
via water adsorption isotherms showed that the surface of AC
ꢁ1
ꢁ1
(
coconut shell, Wako) was much more hydrophobic than that
g ) was about three times greater than
ꢁ1 ꢁ1
g ), and selectiv-
4
of Al2O3. This leads to efficient contact between the catalyst
and the hydrogen gas, which results in the high activity of the
Cu–Pd/AC (coconut shell, Wako) catalyst with low-partial-
pressure H2.
In the series of catalysts represented by the formula (x wt %
Cu; 2 wt % Pd/AC), the influence of Cu content on conversion
and selectivity to NH3 was investigated (Figure 1). In this graph,
the horizontal axis represents the Cu/Pd atomic ratio. The
catalyst without Cu (2 wt % Pd/AC) showed very low activity,
but the addition of even a small amount of Cu (Cu/Pd = 0.1)
resulted in a significant enhancement in activity. When the
Cu/Pd ratio was more than 0.33, the activity became nearly
constant. However, the formation of NH3 was suppressed to an
increasing extent as the Cu/Pd ratio increased, and the minimum
selectivity to NH3 (2%) was obtained when Cu/Pd ratio was 3.0,
i.e., 3.6 wt % Cu; 2.0 wt % Pd/AC.
This work was partly supported by a project of Core
Research for Evolutional Science and Technology (CREST) at
Japan Science and Technology Agency (JST), and a Grant-in-
Aid (No. 15360425) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan.
ꢁ
Figure 2 shows time courses for the hydrogenation of NO3
References and Notes
over a 3.6 wt % Cu; 2.0 wt % Pd/AC (coconut shell, Wako)
catalyst. In this experiment, either no catalyst or a 5 wt % Pd/
AC (N.E. Chemcat Co.) catalyst was placed at the gas outlet.
When this Pd/AC catalyst was absent, N2O was produced with
selectivity of more than 35%. Although N2O is not directly
harmful to humans, it is a greenhouse gas, and, therefore, its
production is undesirable. However, placing of the Pd/AC cata-
lyst at the gas outlet resulted in complete conversion of N2O to
N2, using unreacted H2. It should be emphasized that the amount
of NH3 formed (0.5 ppm, 2% selectivity at 83% conversion)
was at the allowable level (0.5 ppm), while the amount of
1
U. Pr u¨ sse, K.-D. Vorlop, J. Mol. Catal. A 2001, 173, 313, and
references there in; Y. Yoshinaga, T. Akita, I. Mikami, T.
Okuhara, J. Catal. 2002, 207, 37.
S. H o¨ rold, K.-D. Vorlop, T. Tacke, M. Shell, Catal. Today
1993, 17, 21.
K. Nakamura, Y. Yoshida, I. Mikami, T. Okuhara, Chem.
Lett. 2005, 34, 678.
Supporting Information is available electronically on the
CSJ-Journal Web site; http://www.csj.jp/journals/chem-lett/.
T. Shimizu, in Applications of Fluoro Functional Materials,
ed. by M. Yanabe, CMC publishing, Tokyo, 2006, p. 3.
2
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Published on the web (Advance View) June 30, 2007; doi:10.1246/cl.2007.994