Cu-Pd/β-zeolites as highly selective catalysts for the hydrogenation of nitrate with hydrogen to harmless products

Article abstract of DOI:10.1246/cl.2005.678

Cu-Pd/β-zeolites (BEA) are excellent catalysts for the selective hydrogenation of NO₃^- with H₂ to harmless products. 1.2 wt % Cu-1.0 wt % Pd/BEA shows a high selectivity for N₂ + N₂O (14 and 80% at 278 K, respectively), while suppressing NH ₃ production (6%, 3 ppm from 200 ppm of NO₃^-). By coupling the catalyst to Pd/AC, any N₂O produced could be entirely converted to N₂. Copyright

Full text of DOI:10.1246/cl.2005.678

678
Chemistry Letters Vol.34, No.5 (2005)
Cu–Pd/ꢀ-Zeolites as Highly Selective Catalysts for the Hydrogenation of Nitrate
with Hydrogen to Harmless Products
Kyosuke Nakamura, Yasuyuki Yoshida, Ikkou Mikami, and Toshio Okuharaꢀ
Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810
(Received February 21, 2005; CL-050220)
Cu–Pd/ꢀ-zeolites (BEA) are excellent catalysts for the se-
NH4–BEA containing 1.0 wt % Pd and 1.2 wt % Cu by co-ex-
ꢁ
lective hydrogenation of NO3 with H2 to harmless products.
1.2 wt % Cu–1.0 wt % Pd/BEA shows a high selectivity for
N2 + N2O (14 and 80% at 278 K, respectively), while suppress-
ing NH3 production (6%, 3 ppm from 200 ppm of NO3ꢁ). By
coupling the catalyst to Pd/AC, any N2O produced could be en-
tirely converted to N2.
change is described. NH4–BEA (2 g) was added to a mixed aque-
ous solution (150 cm3) of PdCl2 and Cu(NO3)2 and the pH of the
resulting suspension was adjusted to 3.7 by the addition of
0.25 cm3 of aqueous NH3 (4 wt %). Ion exchange was carried
out at 298 K with the suspension being stirred for 12 h. ICP anal-
ysis revealed that the uptake of both Pd2þ and Cu2þ was more
than 99% and the exchange level of NH4þwas 48%. Prior to
use, the resulting solid was dried in air at 333 K for 3 h. A similar
procedure was applied to the other zeolites, giving [Pd–Cu]/zeo-
lites with various contents of Pd and Cu.
The pollution of groundwater by harmful nitrogen-contain-
ing compounds like nitrate, nitrite, and ammonia is an increasing
problem throughout the world. The sources of these pollutants
are mainly fertilizers and animal excreta in agricultural areas.1
To restore the polluted groundwater to drinking water, it is nec-
essary to reduce the levels of these harmful components to the
maximum allowable levels, 50, 0.1, and 0.5 ppm for nitrate, ni-
trite, and ammonia, respectively.2 There have been many reports
of the selective oxidation of NH3 to N2.3
The hydrogenation of nitrate to N2 using H2 (Eq 1) and a
solid catalyst is a novel technology for the purification of pollut-
ed water. Vorlop and co-workers found that bimetallic catalysts,
such as Cu–Pd/Al2O3, were active for the hydrogenation of ni-
trate in water, while monometallic catalysts, like Pd/Al2O3,
were inactive.4 Many studies on the catalytic hydrogenation of
nitrate involve Cu–Pd bimetallic catalysts.5 We have previously
reported the high activity, selectivity and stability of Pd–Cu/
AC.6 However, these catalysts still have unsatisfactory selectiv-
ity for the supply of drinking water.
The reduction of nitrate with a mixture of H2 and CO2 (1:1,
90 cm3 hꢁ1) was performed using a solution of 200 ppm
ꢁ
(3.22 mmol dmꢁ3) of NO3 obtained from NaNO3, in a gas–liq-
uid flow reactor (Pyrex tube, 8 mm i.d.) The reactor was main-
tained at the desired temperature (278, 298, or 333 K) in a water
bath. The pH at the outlet of the reactor was about 6.5 because of
the introduction of CO2, while OHꢁ is formed by the reactions
(Eqs 1 and 2). The gas at the outlet of the reactor was analyzed
using a Micro-GC (Agilent 3000A) equipped with either a
Molecular Sieve 5A column (for N2 and O2) or aꢁHP-PLOT Q
column (for N2O). Concentrations of NO3ꢁ, NO2 and NH3 in
the aqueous phase were determined using a flow injection analy-
sis (FIA) system consisting of a Soma Optics S-3250 detector
and a Sanuki Industry FI-710 analyzer.
When a Na-type zeolite was used as the starting support, the
selectivity to NH3 for the corresponding Pd–Cu zeolite at 333 K
was; BEA (Zeolyst) < BEA (Sud-chemie) < Y < ZSM-5, irre-
¨
spective of the exchange method. Na–mordenite was inert to ion
exchange under these conditions. Thus research was focused on
the BEA zeolite.
Herein, we report the excellent catalytic performance of
Cu–Pd/BEA in the reduction of nitrate to harmless compounds,
specifically to nitrogen (Eq 1) and dinitrogen monoxide (Eq 2).
Furthermore, by coupling Cu–Pd/BEA to Pd/AC, the gas phase
product N2O was reduced to N2. To the best of our knowledge,
Pd–Cu-exchanged zeolites suitable for the purification of water
have not been reported to date.
The time course for the hydrogenation of nitrate (weight
hourly space velocity (WHSV(liq.)) = 50 hꢁ1) at 333 K over
[1.0 wt % Pd–1.2 wt % Cꢁu]/NH4–BEA showed that the near
100% conversion of NO3 continued for at least 60 h, with the
yields of N2 and N2O also remaining constant. On the other
hand, the yield of NH3 was higher at the initial stage (up to
20 h) and then became constant at 25%. The excess of NH3
formed at the initial stage was due to the exchange between
NH4þ in the zeolite and Naþ. The time course demonstrates that
this catalyst produces mainly harmless products, N2 and N2O,
and shows a robust stability.
The catalytic activity and selectivity of various 1.0 wt % Pd–
0.6 wt % Cu/BEA catalysts were measured at 333 K. Pd–Cu/
BEA obtained from Na–BEA (Sud-chemie) produced mostly
¨
NH3 with a selectivity of more than 60%, independent of the
ion-exchange procedure. The use of Na–BEA (Zeolyst) gave a
catalyst with a similar selectivity for NH3. However, Pd–Cu/
BEA from NH4–BEA exhibited lower selectivities for NH3.
[Pd–Cu]/NH4–BEA, in particular, gave the lowest NH3 selectiv-
2NO3ꢁ þ 5H2 ! N2 þ 2OHꢁ þ 4H2O
2NO3ꢁ þ 4H2 ! N2O þ 2OHꢁ þ 3H2O
ð1Þ
ð2Þ
The present study involves the use of various types of zeo-
lite, specifically NH4–BEA (Zeolyst, Si/Al = 12.5, 800 m2
g
ꢁ1), Na-BEA (Zeolyst, Si/Al = 12.5), Na-BEA (Sud-chemie,
¨
Si/Al = 12.5, 586 m2 gꢁ1), Na–mordenite (Tosoh, Si/Al =
18.5, 519 m2 gꢁ1), Na–Y (Tosoh, Si/Al = 5.3, 600 m2 gꢁ1
)
and Na–ZSM-5 (Tosoh, Si/Al = 20, 510 m2 gꢁ1). Pd–Cu-
exchanged zeolites were prepared at room temperature using
aqueous solutions of PdCl2 (1.3 mmol dmꢁ3) and Cu(NO3)2
(3.9 mmol dmꢁ3), or a mixed solution. Both sequential-exchange
and co-exchange procedures were used. Catalyst produced by
co-exchange is designated as [Pd–Cu]/BEA and that from se-
quential-exchange as Pd–Cu/BEA. A typical preparation for
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.5 (2005)
679
ꢁ
Table 1. Hydrogenation of NO3 over [1.0 wt % Pd–Cu]/BEA
(Zeolyst) coupled with 5.0 wt % Pd/AC
Selectivityc/%
Cation
typea
Conv.
/%
Cu/Pdb
ꢁ
N2 N2O NO2
NH3
Na
1
1
2
4
5
40
68
98
89
73
67
85
94
93
89
0
0
0
0
0
2
0
0
0
0
31 (7)d
15 (6)
6 (3)
7 (3)
11 (4)
NH4
NH4
NH4
NH4
aCation of starting BEA. bAtomic ratio. cN-atom base.
dConcentration of ammonia (ppm). Reaction conditions: cat-
alyst 0.4 g, reaction temperature 278 K, nitrate (200 ppm from
NaNO3), WHSV 20 hꢁ1, H2 and CO2 (1:1).
placed at the outlet of the gas phase, the N2O was completely
converted to N2 at 298 K.
Figure 1. Influence of the reaction temperature on the hydroge-
ꢁ
nation of NO3 over [1.0 wt % Pd–1.2 wt % Cu]/NH4–BEA.
Reaction conditions: catalyst 0.4 g, nitrate (200 ppm from
Table 1 summarizes the activity and selectivity of [Pd–Cu]/
BEA(Zeolyst) coupled with 5.0 wt % Pd/AC for the hydrogena-
tion of NO3ꢁ. The WHSV was adjusted to 20 hꢁ1. Table 1 dem-
onstrates that, over [1.0 wt % Pd–1.2 wt % Cu]/NH4-BEA, the
conversion to NH3 was suppressed to 6%, corresponding to
3 ppm, with the selectivity for N2 being 94%. Finally the concen-
trations of Pd and Cu in the solution at the outlet were measured
using ICP. The leaching of both metals was below 0.1% of that
present in the catalyst.
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.
NaNO3), WHSV 15–50 hꢁ1, H2 and CO2 (1:1).
ity at 28%. Therefore, NH4–BEA is the preferred support.
Figure 1 shows the effects of the reaction temperature on the
selectivity over [1.0 wt % Pd–1.2 wt % Cu]/NH4–BEA. For the
examples at 278–298 K, the reaction temperature was lowered
to that of groundwater and the conversion was adjusted to
100% by controlling the WHSV. It is noted that the selectivity
for NH3 decreased to 6 and 12%, at temperatures of 278 and
298 K, respectively.
The influence of the Cu/Pd atomic ratio on the conversion
and selectivity at 278 K for [1.0 wt % Pd–Cu]/NH4–BEA are
presented in Figure 2. The conversion and the selectivity for
the harmless products N2 and N2O were highest at Cu/Pd = 2,
which corresponds to 1.2 wt % of Cu. We have previously re-
ported that Pd–Cu site is indispensable for the activation of
NO3ꢁ.6 Active Pd–Cu species like bimetallic cluster consisted
of Cu and Pd atoms may be formed on Cu–Pd/BEA, showing
high activity and selectivity for N2 and N2O. It is noted that
N2O was always produced with a selectivity of more than 80%
across the range of Cu/Pd ratios investigated. While N2O is
not directly harmful to humans it is a greenhouse gas. In order
to reduce the N2O, another catalyst was coupled to the Pd–Cu
catalysts. When 5.0 wt % Pd/AC (N.E. CHEMCAT. Co.) was
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Published on the web (Advance View) April 9, 2005; DOI 10.1246/cl.2005.678