Chemistry Letters Vol.33, No.7 (2004)
909
trite was retained even at high conversions (>90%). In a separate
experiment, the hydrogenation reactions of nitrite in the pres-
ence of the Cu–Pd cluster catalysts were significantly slower
than those of nitrate, which indicated that the Cu–Pd cluster cat-
alysts are essentially inactive against nitrite under these condi-
tions. It can, therefore, be concluded that the low activity of
the Cu–Pd cluster for nitrite is responsible for the high selectivity
to nitrite in the hydrogenation of nitrate.
Table 1. Hydrogenation of nitrate in the presence of Cu–Pd
bimetal supported on active carbon
b
Metal ratio Conv.
a
Selectivity / %
ꢁ
c
Type
RNO3
ꢁ
/RNO2
ꢁ
(
Stabilizer)
/%
N2
NO2
NH3
Cu4Pd(PVP) 36.6
Cu2Pd(PVP) 92.0
0
0
86.4 13.6
85.1 14.9
7.4
10.4
14.0
6.7
Cluster CuPd(PVP) 90.9
CuPd2(PVP) 69.9 10.4 70.0 19.6
Pd(PVP)
CuPd1:6(SC) 93.6
4.4 82.9 12.8
—
—
—
—
0.0
28.0
2.5 87.2 10.3
0
0
0
.2
.1
CuPd5
CuPd5
Pd
99.5 76.7
2.8 20.4
24.7 13.3 65.4 20.4
2.7 15.9 12.7 71.4
1.3
1.3
0.0
0.4
Conven-
tional
Cu
2.2 16.4 74.6
9.0
a
c
PVP: poly(vinylpyrrolidone), SC: sodium citrate, bN-atom base.
ꢁ ꢁ
ꢁ ꢁ
RNO3 : reaction rate of NO3 reduction, RNO2 : reaction rate of NO2
reduction. Catalyst: 0.05–1 g, Reaction temperature: 333 K, aqueous
solution of nitrate (200 ppm from NaNO3) or nitrite (148 ppm
NaNO2): 0.01–0.07 dm h , H2 (1 atm): 0.0978 dm h .
3
ꢁ1
ꢁ3 ꢁ1
0
20
40
60
80
100
Pd atoms are atomically mixed.10 Presumably, the Cu–Pd pair is
active for the selective reduction of nitrate to nitrite. The pres-
ent study clearly demonstrates the unique catalytic function of
the Cu–Pd pair.
The particular catalysis of the Cu–Pd cluster can be em-
ployed in the development of an effective process for nitrate re-
duction in industrial wastewater and groundwater. Because ni-
trite is selectively hydrogenated to nitrogen using precious
Cu content / mol%
7
Figure 2. Changes in the reaction rate for the hydrogenation of
ꢁ
ꢁ
NO3
as function of Cu content. Catalyst: 0.5–1 g, reaction tempera-
( ) and NO2 ( ) in the presence of [Cum–Pdn]PVP/AC
ture: 333 K, solution of nitrate (200 ppm from NaNO3) or nitrite
3
ꢁ1
(
148 ppm NaNO2): 0.01–0.06 dm h , H2 (1 atm): 0.0978
3
ꢁ1
.
dm h
5
The influence of the Cu content in the Cu–Pd catalyst of
Cum–Pdn]PVP/AC on the reaction rates for the hydrogenation
metal catalysts such as Pd, the combination of the two hydroge-
[
nation processes would allow the transformation of nitrate to ni-
trogen. In other cases, because cleanup process using nitrite as
oxidant for industrial wastewater containing ammonia has be-
of nitrate and nitrite is shown in Figure 2, where the reaction
rates were determined from the W/F dependences of the conver-
sions. The activity for the hydrogenation of nitrate was greatly
dependent on the content of Cu, in which the maximum activity
was obtained for the catalyst with a Cu content of 50% (Cu/Pd =
1
1
come practical, this process of nitrate hydrogenation in the
presence of the Cu–Pd cluster catalyst can be developed.
1
). It is important to note that the reaction rate for the hydroge-
nation of nitrite was significantly slower than that of nitrate. At
0% Cu content, the reaction rate of the nitrate hydrogenation
References
1
2
R. Kubo, J. Phys. Soc. Jpn., 17, 975 (1962).
G. Schmid, V. Maihack, F. Lantermann, and S. Peschel,
J. Chem. Soc., Dalton Trans., 1996, 589.
5
was 14 times higher than that of nitrite; in contrast, the reaction
rate for the hydrogenation of nitrite was scarcely affected by the
content of Cu. The reaction rate of nitrate hydrogenation
3
4
5
6
R. Elghanian, J. J. Storhoff, R. C. Mucic, R. L. Letsinger, and
C. A. Mirkin, Science, 277, 1078 (1997).
G. Sch o¨ n and U. Simon, Colloid Polym. Sci., 273, 202
(1995).
S. H o¨ rold, K.-D. Vorlop, T. Tacke, and M. Sell, Catal.
Today, 17, 21 (1993).
O. M. Ilinitch, L. V. Nosova, V. V. Gorodetskii, V. P.
Ivanov, S. N. Trukhan, E. N. Gribov, S. V. Bogdanov, and
F. P. Cuperus, J. Mol. Catal. A: Chem., 158, 237 (2000).
Y. Yoshinaga, T. Akita, I. Mikami, and T. Okuhara, J.
Catal., 207, 37 (2002).
ꢁ1
ꢁ1
(
2.0 mmol g-cat
h ) for [Cu–Pd1:6]SC/AC was about 10 times
that for [Cu–Pd]PVP/AC, while those of the nitrite hydrogenation
were comparable for these catalysts.
The catalytic data for the hydrogenation of nitrate in the
presence of the Cu–Pd clusters, conventionally prepared bimet-
allic Cu–Pd, and monometallic Pd and Cu catalysts are summa-
rized in Table 1. High selectivity to nitrite was obtained for the
Cu–Pd cluster catalysts regardless of the Cu/Pd ratio, in which
7
[Cu–Pd1:6]SC/AC showed the highest selectivity to nitrite
(87% selectivity at 94% conversion). On the other hand, the con-
8
9
N. Toshima and Y. Wang, Chem. Lett., 1993, 1611.
Y. Tawarasako, T. Hirai, M. Kumasawa, and M. Komatsu,
Jpn. Kokai Tokkyo Koho, 10-188681 (1998) to Catalysis
& Chemical Ind. Co., Ltd.
ventionally prepared Cu–Pd/AC catalyst was highly active and
somewhat selective to nitrogen, but not to nitrite. Our results also
showed that the monometallic Cu and Pd catalysts were inactive
for the hydrogenation of nitrate.
In the case of the Cu–Pd clusters, it has been proposed that
Cu–Pd bonds are preferentially formed; specifically, the Cu and
10 C.-R. Bian, S. Suzuki, K. Asakura, L. Ping, and N. Toshima,
J. Phys. Chem. B, 106, 8587 (2002).
11 I. Joko and T. Nakahara, Shokubai, 39, 590 (1997).
Published on the web (Advance View) June 21, 2004; DOI 10.1246/cl.2004.908