Reduction of nitrates dissolved in water over palladium-copper catalysts supported on a strong cationic resin

Article abstract of DOI:10.1016/j.molcata.2005.08.057

Catalysts containing 4 and 1 wt.% of palladium and copper, respectively, supported on cationic resin (DOWEX 1 × 4, gel type poly(styrene-co- divinylbenzene) with -N(CH₃)₃⁺Cl^- groups, particle size 100-200 mesh) were prepared and tested in the liquid-phase hydrogenation of nitrates in water. The catalyst was prepared by reduction of anionic chloro complexes of palladium(II) and copper(II) immobilized in the resin. Different reduction agents were employed, namely sodium borohydride in ethanol, or in water, and dihydrogen in either water (0.1-0.5 MPa) or methanol (0.5 MPa) solution of Na₂CO₃ (2.5%, w/w). The characterisation by means of X-ray microprobe analysis (XRMA), X-ray powder diffraction (XRPD) and transmission electron microscopy (TEM) revealed the presence of palladium metal nanoclusters and copper compounds. A cationic resin as the support was expected to facilitate the transport of the nitrate anions into the catalyst in comparison to previously employed anionic resins. This was fully proved by the generally higher specific activity of Pd-Cu catalyst supported on cationic resin. The most active and relatively stable catalyst was obtained by reduction of the immobilized metal precursors with H₂ in aqueous Na₂CO₃ at 0.5 MPa. The catalysts exhibited different stability with respect to leaching of metals under duty. Owing to the mechanical stirring of the reaction mixture, the catalytic particles were gradually ground and handling the reaction mixture with the suspended catalyst became rather problematic.

Full text of DOI:10.1016/j.molcata.2005.08.057

Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
Reduction of nitrates dissolved in water over palladium-copper
catalysts supported on a strong cationic resin
a
a,b,∗
a
c
Dana Ga sˇ parovi cˇ ov a´ , Milan Kr a´ lik
, Milan Hronec , Andrea Biffis ,
c
c
Marco Zecca , Benedetto Corain
Department of Organic Technology, Slovak University of Technology, Radlinskeho 9,
SK-812 37 Bratislava, Slovak Republic
a
b
VUCHT j.s.c., Nobelova 34, SK-836 03 Bratislava, Slovak Republic
Dipartimento di Scienze Chimiche, Via Marzolo 1, 35131 Padova, Italy
c
Received 25 April 2005; received in revised form 30 May 2005; accepted 30 August 2005
Available online 20 October 2005
Abstract
Catalysts containing 4 and 1 wt.% of palladium and copper, respectively, supported on cationic resin (DOWEX 1 × 4, gel type poly(styrene-co-
+
−
divinylbenzene) with –N(CH Cl groups, particle size 100–200 mesh) were prepared and tested in the liquid-phase hydrogenation of nitrates
3 3
)
in water. The catalyst was prepared by reduction of anionic chloro complexes of palladium(II) and copper(II) immobilized in the resin. Different
reduction agents were employed, namely sodium borohydride in ethanol, or in water, and dihydrogen in either water (0.1–0.5 MPa) or methanol
(0.5 MPa) solution of Na
2 3
CO (2.5%, w/w). The characterisation by means of X-ray microprobe analysis (XRMA), X-ray powder diffraction
(XRPD) and transmission electron microscopy (TEM) revealed the presence of palladium metal nanoclusters and copper compounds. A cationic
resin as the support was expected to facilitate the transport of the nitrate anions into the catalyst in comparison to previously employed anionic
resins. This was fully proved by the generally higher specific activity of Pd-Cu catalyst supported on cationic resin. The most active and relatively
stable catalyst was obtained by reduction of the immobilized metal precursors with H in aqueous Na CO at 0.5 MPa. The catalysts exhibited
2 2 3
different stability with respect to leaching of metals under duty. Owing to the mechanical stirring of the reaction mixture, the catalytic particles
were gradually ground and handling the reaction mixture with the suspended catalyst became rather problematic.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Nitrates; Reduction; Pd-Cu catalyst; Cationic resin
(partial reduction) and NH + (over-reduction). To this purpose,
1
. Introduction
4
bimetallic Pd-Cu and Pd-Sn catalysts on inorganic supports (e.g.
Al O , pumice) apparently offer the most promising prospects
Removal of low-concentrated nitrates from water represents
2
3
a serious ecological problem. The limits currently accepted for
nitrates and nitrites in drinking water are 50 and 0.1 mg/l, respec-
tively. The content of nitrates in water can be lowered by several
methods, such as ion exchange, electrodialysis, reverse osmosis
or microbiological treatment [1]. However, the catalytic reduc-
tion of nitrates with hydrogen, with no formation of solid or
liquid wastes, is a very attractive alternative and a subject of
extensive investigation [2–13]. The target is the selective con-
[3–6].
Recently [14], we prepared and tested palladium-copper
catalysts supported on Dowex 50 W × 4 (particle size
100–200 mesh). This is a gel-type sulfonated poly(styrene-co-
divinylbenzene) resin, containing acidic groups ( SO H). The
3
concept of these catalysts rested on the possibility to trap pro-
+
duced NH in the form of NH , by ion exchange. However,
3
4
the restoration of the acidic sites with hydrochloric acid led to
dissolution of the metals, mainly copper, with gradual loss of
catalytic activity.
−
−
version of NO3 to N2 and water, with no formation of NO2
Therefore, we decided to prepare Pd-Cu catalysts supported
on anion exchangers (Dowex 1 × 4), in view of our large experi-
ence gathered in recent years in the field of resins-supported
∗
Corresponding author. Tel.: +421 2 5249 5242; fax: +421 2 5249 3198.
E-mail addresses: milan.kralik@stuba.sk, mkralik@vucht.sk (M. Kr a´ lik).
1
381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.08.057

D. Ga sˇ parovi cˇ ov a´ et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
259
The catalysts were characterised by means of X-ray micro-
probeanalysis(XRMA)[17], X-raypowderdiffraction(XRPD),
scanning electron microscopy (SEM) and transmission elec-
tron microscopy (TEM) [24]. X-ray powder diffraction patterns
were obtained using the Diffractometer PHILIPS with an X-ray
source Cu K␣; λ = 0.154 nm. To prevent the surface oxidation
of copper in contact with air [10], the samples of reduced cata-
Nomenclature
c
n
n
t
concentration (mol/m3)
−
nitrite content in the catalyst (mol)
nitrate content in the catalyst (mol)
duration of an experiment
NO2 ,cat
−
NO3 ,cat
3
V
volume of reaction mixture (m )
◦
lysts were dried in nitrogen atmosphere at 60 C and embedded
x
−
nitrate conversion (%)
extent of reaction (mol)
selectivity (%)
NO3
in poly(methyl methacrylate) matrix. X-ray microscope anal-
ysis and scanning electron microscopy were performed in a
Cambrige Stereoscan 250 EDXPW 9800. Image analysis was
carried out with an Image Pro Plus Program. Transmission
electron micrographs of catalysts were obtained with a JEOL
ξ
σ
Subscripts
0
t
at the start
at the time
2010 electron microscope with GIF. Water swollen samples
of the analysed catalysts were ground with pestle and mortar.
Then, a drop of the obtained suspension was deposited onto
a carbon grid and the resin let to dry in the air before the
analysis.
metal catalysis [15–22] and on the fact that the mobility of
anions inside of swollen cationic resin is higher in comparison
to anionic ones [23].
2.2. Catalytic tests
2
. Experimental
Catalytictestswerecarriedoutinathermostaticbatchreactor,
3
four-necked flask (volume, 500 cm ) equipped with a magnetic
or KPG stirrer, inlets for hydrogen–nitrogen (45 mole% of H2)
and 0.1 M HCl supply, a capillary for sampling and off-gas. The
flow rate of the H2–N2 mixture was 64 ml/min and the total pres-
sure 1 bar. Hydrochloric acid was fed throughout the catalytic
experimentstomaintainthepHat5–6. Atypicalstartingreaction
2
.1. Catalysts preparation and characterisation
All chemicals were of analytical grade purity, supplied by
Fluka (Dowex 1 × 4, CuCl2·2H2O), Safina Jesenice (PdCl2,
4
0 wt.% solution in HCl/H2O) or Lachema Brno.
The Dowex 1 × 4 microgel resin (particle size 100–200 mesh,
3
mixture consisted of 0.25 dm KNO3 aqueous solution contain-
tetraalkylammonium groups) was washed with 1 M KCl, dis-
tilled water and cold methanol and finally dried at 50 C at
3
−
ing 100 mg/dm NO3 and 0.5 g of catalyst. The catalytic runs
◦
◦
were carried out at 25 C. Samples were periodically collected
reduced pressure (10 kPa). The ion exchange capacity of the
resin was 4.18 mmol/g. The chloride form of Dowex 1 × 4 anion
exchanger (3.8 g) was metallated by ion exchange (3 h, mild stir-
ring) upon using 20 ml of a solution of PdCl2 (1.1109 g, 40 wt.%
solution in HCl/H2O) and CuCl2 (0.1789 g) in 4 M aque-
ous HCl (13.33 g Pd/l and 3.33 g Cu/l) according to reactions
withsamplingintervalsrangingfrom5to30 min. Recoveredcat-
alysts were treated with 0.25 M NaCl to displace any nitrate and
−
nitrite ions entrapped inside the support. After filtration, NO3
−
and NO2 ions were quantitatively determined in the filtrate as
well as in the reaction mixture.
(1) and (2).
2.3. Product analysis
+
−
2−
2
(P)-N(CH3)3 Cl (s) + [PdCl4]
→
The determination of nitrates, nitrites and ammonia was per-
formed by VIS absorption spectrophotometry [25]. NO3 ions
{(P)-N(CH3)3 }2[PdCl4] (s) + 2Cl−
+
2−
−
(1)
(2)
were determined with salicylic acid in the presence of sulfuric
−
+
−
−
acid (maximum absorption at 410 nm), NO2 ions via reaction
(
P)-N(CH3)3 Cl (s) + [CuCl3]
with sulfanilic acid and N-(1-naphtyl)ethylenediamine dichlo-
ride (maximum absorption at 550 nm)andNH4+ ions byreacting
with Nessler’s reagent (maximum absorption at 425 nm). The
samples of water were analysed for palladium and copper by
atomicabsorptionspectroscopy(AAS)onaCarlZeissJenaAAS
+
−
−
→
(P)-N(CH3)3 [CuCl3] (s) + Cl
The obtained material was dried at 50 C under reduced pres-
sure to constant weight. The metal loading was 4.1 and 1.0 wt.%
of palladium and copper, respectively. The cationic metallated
resin was reduced with 0.066 M solution of sodium borohydride
in ethanol (C1) or with 1.163 M solution of sodium borohydride
in water (C2), for 1 h at room temperature with occasional stir-
ring.
Suspensions of the metallated resin in 2.5 wt.% water or
methanol solution of Na2CO3 at 25 C (20 ml of solution per
gram of dry resin) were treated for 1 h with hydrogen at 0.1 MPa
aqueous suspension, C3) or 0.5 MPa (aqueous suspension, C4;
◦
3
atomic absorption spectrometer.
2
.4. Calculation of the selectivities
−
+
TheselectivitytoN2 (σN ), NO2 (σNO2
−
)andNH4 (σ
+
)
NH4
2
formation in the hydrogenation of nitrates was calculated on the
basis of the following expressions:
◦
ξN2
(
σN =
(3)
2
methanol suspension C5).
Σ

2
60
D. Ga sˇ parovi cˇ ov a´ et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
ξ
ξ
+
−
(Fig. 1). Higher concentration of metals (mainly palladium)
NH4
σ
σ
+
−
=
=
(4)
(5)
NH4
NO2
on the surface was observed in the case of catalysts reduced
at lower pressure (Fig. 1b), which probably relates with the
transport of anions during the reduction. At low pressure,
the concentration of hydrogen inside the catalyst is low and
the metal ions have the possibility to move to the periph-
ery of the particles, where they are reduced. On the con-
trary, the metal distribution in the particles of the Pd-Cu cat-
alyst reduced with NaBH4 appears to be uniform, shown by
XRMA map of the whole cross-section of the catalyst particles
Σ
NO2
Σ
Σ = ξN + ξ
+
+ ξNO2
−
2
NH4
=
V(cNO3
−
− cNO3
−
,t) − nNO3
−
(6)
(7)
(8)
,0
,cat
Moles of NO3 converted to NH4+:
−
(Fig. 1a).
ξ
+
= VcNH4
+
,t
NH4
The presence of either monometallic or mixed Pd and Cu
−
−
Moles of NO3 converted to NO2 :
crystalline microphases in catalysts C1, C3 and C4 was inves-
tigated by XRPD (Figs. 2 and 3). No peaks were detected in
the XRP diffractogram of C1 (Fig. 2). It suggests that any
metal phase is either amorphous or so well dispersed (i.e. very
small particle size) that they could not be detected. The X-ray
powder diffraction analysis of C4 showed a peak of metallic
ξ
−
= nNO2
−
+ VcNO2
−
,t
NO2
,cat
−
Moles of NO3 converted to nitrogen (ξN ) were calculated from
2
Eqs. (6)–(8).
◦
palladium(1 1 1) at 40.1 (Fig. 2). The estimated size of crys-
3
. Results and discussion
tallites is 3.8 nm. No characteristic peak for copper species or
palladium/copper alloys is observed.
The reduction procedures and characterisation of the pre-
XRPD analysis of the Pd-Cu catalyst reduced with hydrogen
at lower pressure (C3) exhibited much more complex diffrac-
togram. In addition to the diffractions signals of metallic palla-
pared Pd-Cu catalysts are summarised in Table 1.
The chlorometallates of palladium(II) and copper(II) immo-
bilized in the cationic resin are much more resistant to reduction
than palladium(II) and copper(II) cations fixed in anionic resins.
In fact, the metal precursors in the Dowex 1 × 4 cationic resin
are not reduced with hydrogen in methanol or water even at pres-
sure 5 MPa. However, in the pre-treated catalyst, the reduction
of palladium and copper precursors was achieved after about
◦
◦
dium(1 1 1) at 40.1 and 46.5 , we observed a strong diffraction
◦
at 37.96 (Fig. 3). This peak can be associated to the mixed
palladium(II) oxide Na2Pd3O4. Poorly visible signals, if any,
◦
◦
were found at 42.40 and 48.93 , and can be attributed to Cu3Pd
alloys. This observation is slightly in contradiction with our pre-
vious results where no such kind of diffractions was monitored.
No match in the XRPD database was found for other signals
1
0 min from the start of the catalytic test, as the consequence
of the pH increase during the reaction. In fact, the increase of
pH enhances the reducing power of H2, as shown in the Nernst
equation (9):
(marked as A, B and C in Fig. 3). Broadly speaking, it is possible
to conclude that at relatively low hydrogen pressure, Pd(II) and
Cu(II) were not completely reduced, which was also reflected
by grey colour of the catalyst.
TEM pictures of C4 after catalytic test (Fig. 4) revealed that
this catalyst contains relatively large aggregates (20–100 nm)
of smaller (<10 nm) irregularly shaped nanoclusters. We specu-
late that in this catalyst, the active component in the reduction
of nitrates is metallic palladium in the form of relatively small
nanoclusters embedded in a matrix of an amorphous material
+
−
2
H + 2e → H2,
R · T
◦
E = −
· pH = 0.0256 · pH (25 C)
(9)
F
Accordingly, we found that the addition of an appropriate
base like Na2CO3 in methanol or water was enough to outweigh
the reluctance of the metal precursors towards the reduction with
hydrogen.
XRMA analysis of the Pd-Cu catalysts reduced with hydro-
gen in the presence of 2.5 wt.% water solution of Na2CO3
exhibited a non-uniform (peripheral) distribution of metals
(hydrated oxo-compounds of copper(I), copper(II) and/or metal-
liccopper), whichareresultingintheobservedlargeraggregates.
Metallic copper should be present, if any, only in very small
amount. In fact, it can be easily oxidised upon exposition to air
Table 1
Pd-Cu catalysts for nitrate hydrogenation and some properties thereof
Catalyst
Reduction conditions
Coloura
Distributionb
Pd nanocluster sizec
C1
C2
C3
C4
C5
NaBH4, EtOH
NaBH4, H2O
H2, 0.1 MPa, Na2CO3, H2O
H2, 0.5 MPa, Na2CO3, H2O
H2, 0.5 MPa, Na2CO3, MeOH
B
B
G
B
B
E
P
VP
P
P
n.d.
6.7
3.2
3.8
2.9
a
B: black, G: grey.
E: even, P: peripheral, VP: very peripheral.
Nanometer, from XRPD analysis.
b
c

D. Ga sˇ parovi cˇ ov a´ et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
261
Fig. 1. XRMA analysis of Pd-Cu catalysts supported on Dowex 1 × 4, reduced with: (a) sodium borohydride (0.066 M solution in ethanol) (C1); (b) H2 in aqueous
◦
◦
Na2CO3 (2.5%, w/w) at 25 C and 0.1 MPa (C3); (c) H2 in aqueous Na2CO3 (2.5%, w/w) at 25 C and 0.5 MPa (C4).
before the reaction, or by nitrates, during the catalytic process
10,26].
The activity of catalysts C1–C5 in hydrogenation of nitrate to
nitrite, dinitrogen and ammonium ion is illustrated in Figs. 5–8,
respectively. The marked decrease in the concentration of nitrate
at the beginning of the reaction over Pd-Cu/Dowex 1 × 4 cata-
lysts (Fig. 5) is caused by the ion exchange ability of the support.
Lowering concentration of nitrates, driving force for its trans-
[
port is also lowered and total conversion of nitrates would be
reached at infinity time. Therefore, conversions reached after
certain time are never quantitative, as demonstrated by the
amount of nitrate that still remains in the resin after the catalytic
experiments.
In general, higher activity was achieved with the catalysts
prepared by hydrogen reduction of the metal precursors. The
most active catalyst is C3, which was obtained under hydrogen
at 0.1 MPa. Its relatively high activity can be accounted for by
lower mass transport restrictions due to peripheral location of
catalytic sites (see Fig. 1). However, this catalyst is plagued
by low stability and significant metal pollution of the solution
was observed during the experiment (black colour of water from
−
3
0
.5 g of catalyst used contains 2.1 × 10 mol of cationic cen-
ters and the total amount of nitrate ions present at the beginning
−
3
of the catalytic experiments is 0.4 × 10 mol. While the rapid
nitrate adsorption creates an increase in the local concentra-
tion, on the other hand, it constitutes a major hurdle to the
intraparticle migration towards the Pd active centers. In addi-
tion, there is equilibrium among individual ions in the resin.

2
62
D. Ga sˇ parovi cˇ ov a´ et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
Fig. 2. X-ray powder diffraction patterns for C1 (reduction with sodium boro-
hydride, 0.066 M in ethanol) (a) and C4 (H2 in aqueous Na2CO3, 2.5%, w/w, at
◦
2
5 C and 0.5 MPa) (b). Dashed lines indicate the position of typical diffraction
peaks for the possible metal species.
released metals). Due to its low stability, this catalyst was not
investigated any further.
Mass transport limitations are more effective in C4 than in
C3, the former is apparently a little less active than the latter
(87% nitrate conversion versus 93% at 135 min). When the spa-
tial distribution of the metals is quite homogeneous, as in the
case of catalyst C1, the apparent activity of the catalyst is even
lower, in spite of the smaller size of metal nanoclusters. This
suggests that the reaction takes place under diffusion regime,
whereby the process rate is not sensitive to the metal nanoclus-
ter size. In fact, whereas the metal nanocluster size increases
in the order C5 < C3 < C4 < C2 (palladium nanocluster diam-
eter not available for C1), the apparent activity increases as
Fig. 4. Transmission electron micrographs of Pd-Cu catalyst supported on
◦
Dowex 1 × 4, reduced with H2 in aqueous Na2CO3, 2.5% (w/w), at 25 C and
0.5 MPa (C4), after the catalytic test.
data for C1 are referred to 175 min and not 135 min as for the
other ones).
The reduction of nitrate ions produces nitrite and hydroxide
ions that are fixed on the cationic resin as well, and in close prox-
imity of the active Pd sites. It is known that with increasing pH,
the rate of reaction both of nitrate and nitrite reduction decreases
(C1) < C5 < C2 < C4 < C3.
The data in Fig. 8 show that the selectivity to nitrogen for-
mation decreases as the apparent catalyst activity increases (it
should be borne in mind that the final conversion and selectivity
◦
Fig. 5. Nitrate concentration in water vs. time for nitrate reduction (25 C) over
Pd-Cu catalysts (4 wt.% Pd, 1 wt.% Cu), 0.5 g of a catalyst reduced with: (ꢀ)
sodium borohydride, 0.066 M in ethanol (C1); (᭹) sodium borohydride in water
◦
Fig. 3. X-ray powder diffraction patterns for C3 (H2 in aqueous Na2CO3, 2.5%,
w/w, at 25 C and 0.1 MPa). Dashed lines indicate the position of typical diffrac-
tion peaks for the possible metal species.
(C2); (ꢁ) H2 in aqueous Na2CO3, 2.5% (w/w), at 25 C and 0.1 MPa (C3); (ꢀ)
◦
◦
H2 in aqueous Na2CO3, 2.5% (w/w), at 25 C and 0.5 MPa (C4); (ꢁ) H2 in the
◦
presence of Na2CO3, 2.5% (w/w) in methanol, at 25 C and 0.5 MPa (C5).

D. Ga sˇ parovi cˇ ov a´ et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
263
◦
◦
Fig. 6. Nitrite concentration in water vs. time for nitrate reduction (25 C) over
Fig. 7. Ammonia concentration in water vs. time for nitrate reduction (25 C)
over Pd-Cu catalysts (4 wt.% Pd, 1 wt.% Cu), 0.5 g of a catalyst reduced with:
(ꢀ) sodium borohydride, 0.066 M in ethanol (C1); (᭹) sodium borohydride
Pd-Cu catalysts (4 wt.% Pd, 1 wt.% Cu), 0.5 g of a catalyst reduced with: (ꢀ)
sodium borohydride, 0.066 M in ethanol (C1); (᭹) sodium borohydride in water
◦
◦
(
C2); (ꢁ) H2 in aqueous Na2CO3, 2.5% (w/w), at 25 C and 0.1 MPa (C3); (ꢀ)
in water (C2); (ꢁ) H in aqueous Na CO , 2.5% (w/w), at 25 C and 0.1 MPa
2
2
3
◦
◦
H2 in aqueous Na2CO3, 2.5% (w/w), at 25 C and 0.5 MPa (C4); (ꢁ) H2 in the
(C3); (ꢀ) H in aqueous Na CO , 2.5% (w/w), at 25 C and 0.5 MPa (C4); (ꢁ)
2
2
3
◦
◦
presence of Na2CO3, 2.5% (w/w) in methanol, at 25 C and 0.5 MPa (C5).
H in the presence of Na CO , 2.5% (w/w) in methanol, at 25 C and 0.5 MPa
2
2
3
(C5).
and the formation of nitrite and ammonium ions increases [27].
Fig. 9 represents situation for catalyst particles with the uniform
and very peripheral distribution of reduction centers.
When the reduction of nitrates proceeds in the catalyst with
uniform distribution of catalytic centers (C1), the local con-
centration of the formed hydroxyl groups is low; consequently,
the driving force for transport to bulk liquid is also low, and
they remain mainly inside the catalysts. It is in agreement with
the data shown in Table 2. In contrast, for the very peripheral
distribution of catalytic centers (C3), the reduction takes place
−
very close to surface, the local concentration of OH ions and
the driving force for the transport of hydroxyl groups are high.
Thus, the concentration of hydroxyl groups in bulk is higher than
in the case of catalyst with the uniform distribution. In order to
maintain pH equal to 5–6, more acid is required (Table 2).
Hydroxyl groups are also transported inside the body of the
catalyst and in opposite way nitrates migrate to edge and are
Fig. 8. Final nitrate conversion and selectivity to N2, NO2 and NH4+ ions in
the hydrogenation of nitrates in water (25 C) after experiments with C1–C5
catalysts.
−
−
◦
reduced on the catalytic centers. The OH ions, which are
remaining inside the catalyst particle, negatively affect the rate
Fig. 9. Concentration (c) profiles of hydroxyl groups for catalyst with uniform (a) and peripheral (b) distribution of catalytic centers.

2
64
D. Ga sˇ parovi cˇ ov a´ et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
Table 2
Neutralisation of OH anions produced in the hydrogenation of nitrates
−
a
(×10−3 mol)
b
(×10−3 mol)
c
−3
−
ads/ncatd (%)
OH
Catalyst
n
−
n
+
n
−
ads (×10 mol)
n
OH
H
OH
C1
C2
C3
C4
C5
0.30
0.37
0.56
0.43
0.36
0
0.30
0.32
0.01
0.33
0.26
14
15
0.2
16
12
0.05
0.55
0.1
0.1
a
−
−
−
n
−
−
: the total amount of OH produced in experiment (1 mol OH is formed when 1 mol of NO3 is transformed to nitrogen, or oxides of nitrogen, 2 mol of
OH
−
+
OH is produced when NO3 is transformed to NH4 ).
b
c
d
+
n
+
: total amount of H fed throughout the catalytic experiment as 0.1 M HCl.
H
−
n
−
ads: total amount of OH fixed on the cationic resin.
OH
−
The ratio between the total amount of OH fixed on the cationic resin and the number of cationic centers present in the amount of catalyst used.
Table 3
4
Pd
NaBH in ethanol (C1). The concentration of palladium (c )
Final nitrate, nitrite and ammonia concentration in the hydrogenation of nitrates
after experiments with fresh (1) and recycled (2 and 3) catalyst C4
in the treated water from experiments with C1, C2, C4 and
C5 catalysts was below the detection limit of the used method
(0.1 ppm).
a
−
−
NH4+ (mg/l)
Experiment
NO3 (mg/l)
NO2 (mg/l)
In order to elucidate the effect of mechanical stress in a reac-
tor with a magnetic stirrer, a set of experiments with a KPG
stirrer was performed (Table 5). The catalytic data show the dif-
ferent effects of diffusion when a KPG stirrer was used instead
of a magnetic stirrer. The higher catalytic activity, as well as
a higher selectivity towards dinitrogen in the experiments with
the magnetic stirrer, can be explained by the “sponge effect” and
by grounding of catalyst. The “sponge effect” means a periodi-
cal pressing and releasing of catalyst particles by the blades of
stirrer at bottom of the reactor. This effect increases the rate of
transportation of the liquid inside the catalysts particles, and of
course higher transport between the catalyst body and bulk liq-
uid is achieved. Thus, a sort of convective transport contributes
to diffusion transport, and the overall transport is increased.
Ontheotherhand, therelativelyhighmechanicalstressbrings
out the production of fines. As a consequence, the specific sur-
face of the solid increases and this improves both the activity
and the N2 selectivity, simply because diffusion paths are shorter
and the surface for solid/liquid exchange of materials is larger.
Under these conditions, products effuse from the catalyst to bulk
water more readily and the chance of over-reduction is dimin-
ished. The catalytic data (Table 5) are in agreement with this
hypothesis, as well as metal leaching data (detection limits:
cPd < 0.1 mg/l, cCu < 0.013 mg/l). The latter in fact was com-
pletely suppressed when the KPG stirrer (slower mass transport
rates) was employed.
1
2
3
1.1
1.1
1.1
0.4
0.7
2.0
10.0
20.0
14.9
a
−
2
50 ml of distilled water with 100 mg/l NO3 (KNO3), 1.5 g of catalyst,
◦
H2–N2 64 ml/min, 1 bar, 25 C, 0.1 M HCl, pH 6.
of the reduction of nitrates. If transport of hydrogen is also
taken into account, it is obvious that the catalyst with peripheral
distribution should be more active than the catalyst with uni-
form structure. Higher local concentration of hydroxyl groups
decreases the selectivity to nitrogen, which is also fairly docu-
mented by catalyst C3 (type b) and catalyst C1 (type a) (Fig. 8).
Whereas the catalytic activity of these materials is generally
quite low (TON less than 2 mol/mol and TOF less than 1 h ),
recycling is of the greatest importance in connection with tech-
nical application of the catalytic abatement of nitrates in water.
When recycled catalyst was used for the second and third runs,
a comparable activity with the first run was observed, but the
concentrations of NO2 and NH4 in the final reaction mixture
were somewhat higher (Table 3).
Analysis of the treated water from experiments with C1, C2,
C4 and C5 catalysts by means of AAS revealed the presence of
dissolved copper when magnetic stirring was applied (Table 4).
−
1
−
+
II
The largest Cu leaching (cCu) was observed in the sample
from the catalytic test carried out with the catalyst reduced with
Table 4
Final nitrate (mNO3
−
) and nitrite content (m
−
) in the used Pd-Cu catalysts, concentration of Cu dissolved in the treated water (cCu), nitrate conversion (x
−
)
NO3
NO2
and selectivity (σ) to N2, NO2 and NH4+ over Pd-Cu/Dowex 1 × 4 catalysts (4 wt.% Pd, 1 wt.% Cu)
−
a
Catalyst; duration of
an experiment (min)
m
−
(mg)
m
−
(mg)
cCu (mg/l)
x
−
NO3
(%)
σ (%)
NO3
NO2
−
NH4+
N2
NO2
C1; 175
C2; 135
C3; 135
C4; 135
C5; 135
4.5
4.2
0.9
1.8
4.6
0
0
0.03
0.03
0
0.06
0.03
–
0.04
0.02
71
81
93
87
73
96
86
50
76
77
0
4
0.2
0.9
0.8
0.2
13.8
49.1
23.2
22.8
b
a
−
◦
2
50 ml of distilled water with 100 mg/l NO3 (KNO3), 0.5 g of catalyst, H2–N2 64 ml/min, 1 bar, 25 C, 0.1 M HCl, pH 6.
b
Not analysed.

D. Ga sˇ parovi cˇ ov a´ et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
265
Table 5
Final nitrate (mNO3
−
) and nitrite content (m
−
) in the used Pd-Cu catalysts, concentration of Cu dissolved in the treated water (cCu), nitrate conversion (x
−
)
NO2
NO3
and selectivity (σ) to N2, NO2 and NH4+ over C1 and C4 catalysts (4 wt.% Pd, 1 wt.% Cu) under different stirring conditions
−
a
Catalyst; duration of an
m
−
(mg)
m
−
(mg)
cCu (mg/l)
x
−
NO3
(%)
σ (%)
NO3
NO2
experiment (min); blender
−
NH4+
N2
NO2
C1; 175; magnetic
C1; 175; KPG
C4; 135; magnetic
C4; 135; KPG
4.5
7.2
1.8
2.5
0
0
0.03
0.23
0.06
0
0.04
0
71
61
87
83
96
94
76
71
0
0
0.8
2.2
4
6
23.2
26.8
a
−
◦
2
50 ml of distilled water with 100 mg/l NO3 (KNO3), 0.5 g of catalyst, H2–N2 64 ml/min, 1 bar, 25 C, 0.1 M HCl, pH 6.
for C4. The decrease in activity can be due to some catalyst
poisoning brought out by CO2. A possibility is the formation
of CuCO3·Cu(OH)2 (malachite), a sparingly soluble copper(II)
compound, which lowers the number of active copper sites.
The increase of the partial H2 pressure has no effect for C4:
both the apparent activity and the selectivity of the catalysts did
not change when pure H2 was used instead of the H2/N2 mix-
ture. By contrast, at relatively high H2 pressure, the selectivity
of C1 appreciably dropped down, indicating that under these
conditions over-reduction is favoured. This suggests that CO2
“
protected” C4 from over-reduction. It is however interesting
to observe that in the presence of CO2, the activities of C1 and
C4 were to some extent levelled off, at both low and high H2
pressure. This could be perhaps explained by a much more effi-
cient embedding of palladium in the copper carbonate phase,
which tends to cancel the differences in the metal nanoclusters
sizes. According to our expectations, AAS analysis showed on
the undetectable contents of copper and palladium in the treated
water from these experiments.
Fig. 10. Final nitrate conversion and selectivity to N2, NO2 and NH4+ ions in
the hydrogenation of nitrates in water (25 C) after experiments with C1 catalyst.
−
◦
We have also investigated the effect of the nature of the acid
(CO2 instead of HCl) and of hydrogen partial pressure on the
catalytic performance of C1 and C4. The KPG stirrer was used
for minimizing the mechanical stress (Figs. 10 and 11).
4. Conclusions
The substitution of CO2 for HCl produces a drop of the appar-
ent catalytic activity, which is somewhat larger for C4 (Fig. 11).
In both cases, the lower catalytic activity is accompanied by
a higher N2 selectivity. Again, the effect is more appreciable
The Pd-Cu/Dowex 1 × 4 catalysts proved the hypothesis of
positive effect of higher mobility of anions in body of the swollen
cationic resin. The catalysts described herein were approxi-
mately four times more active in the hydrogenation of nitrates
than those prepared on anionic resins. However, in compari-
son with inorganic catalysts, with metal particles located on
the outer surface, the overall mass transport resistance was still
appreciably high and they exhibited lower catalytic activity. This
problem could be circumvented, at least partially, with the use
of a fixed bed reactor with the external recycling of the treated
water (increase of the overall liquid–solid mass transport coeffi-
cient). This reactor configuration would also help in preventing
excessive mechanical stress on the catalyst particles, which is
one of the causes of enhanced metal leaching.
Acknowledgements
This work was supported by funds of the Slovak VEGA
projects 1/9142/02 and 1/2459/05. We acknowledge Dr.
Vladim ´ı r Jor ´ı k from the Department of Inorganic Chemistry,
Slovak University of Technology, for X-ray powder diffraction
analysis.
Fig. 11. Final nitrate conversion and selectivity to N2, NO2 and NH4+ ions in
the hydrogenation of nitrates in water (25 C) after experiments with C4 catalyst.
−
◦

2
66
D. Ga sˇ parovi cˇ ov a´ et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 258–266
[15] M. Kr a´ lik, M. Hronec, V. Jor ´ı k, S. Lora, G. Palma, M. Zecca, A. Biffis,
References
B. Corain, J. Mol. Catal. A: Chem. 101 (1995) 143.
16] A. Biffis, B. Corain, M. Zecca, C. Corvaja, K. Je ˇr a´ bek, J. Am. Chem.
Soc. 117 (1995) 1603.
17] A. Biffis, B. Corain, Z. Cvengro sˇ ov a´ , M. Hronec, K. Je ˇr a´ bek, M. Kr a´ lik,
Appl. Catal. A: Gen. 124 (1995) 355.
18] M. Hronec, Z. Cvengro sˇ ov a´ , M. Kr a´ lik, G. Palma, B. Corain, J. Mol.
Catal. A: Chem. 105 (1996) 25.
19] R. Fi sˇ era, M. Kr a´ lik, J. Annus, V. Kr a´ tky, M. Zecca, M. Hronec, Collect.
Czech. Chem. Commun. 62 (1997) 1763.
20] M. Kr a´ lik, M. Zecca, P. Bianchin, A.A. D’Archivio, L. Galantini, B.
Corain, J. Mol. Catal. A: Chem. 130 (1998) 85.
21] D. Belli, A.A. D’Archivio, L. Galantini, S. Lora, A. Biffis, B. Corain,
J. Mol. Catal. A: Chem. 157 (2000) 173.
[
[
[
[
[
[
[
[
[
[
[
1] A. Kapoor, T. Viraraghavan, J. Environ. Eng. 123 (1997) 371.
2] K.D. Vorlop, T. Tacke, Chem. Ing. Tech. 61 (1989) 836.
3] U. Pr u¨ sse, K.D. Vorlop, J. Mol. Catal. A: Chem. 173 (2001) 313.
4] A. Pintar, T. Kajiuchi, Acta Chim. Slovenica 42 (1995) 431.
5] O.M. Ilinitch, F.P. Cuperus, L.V. Nosova, E.N. Gribov, Catal. Today 56
(
2000) 137.
[
[
[
[
6] F. Deganello, L.F. Liotta, A. Macaluso, A.M. Venezia, G. Deganello,
Appl. Catal. B: Environ. 24 (2000) 265.
7] G. Strukul, R. Gavagnin, F. Pinna, E. Modaferri, S. Perathoner, G. Centi,
M. Marella, M. Tomaselli, Catal. Today 55 (2000) 139.
8] A.E. Palomares, J.G. Prato, F. Marquez, A. Corma, Appl. Catal. B:
Environ. 41 (2003) 3.
[
[
22] B. Corain, M. Kr a´ lik, J. Mol. Catal. A: Chem. 173 (2001) 99.
23] P. Mears, in: J. Crank, G.S. Park (Eds.), Diffusion in Polymers, Aca-
demic Press, New York, 1968, p. 373.
9] Y. Yoshinaga, T. Akita, I. Mikami, T. Okuhara, J. Catal. 207 (2002) 37.
[
10] F. Epron, F. Gauthard, C. Pin e´ da, J. Barbier, J. Catal. 198 (2001)
09.
3
[
[
[
[
24] F. Artuso, A.A. D’Archivio, S. Lora, K. Je ˇr a´ bek, M. Kr a´ lik, B. Corain,
Chem. Eur. J. 9 (2003) 5292.
[
11] U. Pr u¨ sse, V. Morawsky, A. Dierich, A. Vaccaro, K.D. Vorlop, in: B.
Delmon, et al. (Eds.), Stud. Surf. Sci. Catal., Prep. Catal. VII, vol. 118,
Elsevier, Amsterdam, 1998, p. 137.
25] M. Hor a´ kov a´ , P. Lischke, A. Gr u¨ nwald, Chemick e´ a fyzik a´ lne met o´ dy
anal y´ zy vod (in Czech), SNTL-ALFA, Praha, 1989.
26] A. Edelman, W. Schiesser, H. Vinek, A. Jentys, Catal. Lett. 69 (2000)
[12] A.E. Palomares, J.G. Prato, F. Rey, A. Corma, J. Catal. 221 (2004) 62.
[
13] J. S a´ , H. Vinek, Appl. Catal. B: Environ. 57 (2004) 245.
[
14] D. Ga sˇ parovi cˇ ov a´ , M. Kr a´ lik, M. Hronec, Collect. Czech. Chem. Com-
mun. 64 (1999) 502.
11.
27] G. Centi, S. Perathoner, Appl. Catal. B: Environ. 41 (2003) 15.