Preparation and characterization of Pt-promoted NiY and CoY catalysts employed for 4-nitrophenol reduction

Article abstract of DOI:10.1016/j.apcata.2013.08.047

Abstract The reduction of 4-nitrophenol (4-NP) by NaBH₄ was used as a probe reaction to investigate the promotion effect of platinum to CoY and NiY catalysts. Subsequent catalysts preparation by ion exchange method, they were characterized by X

Full text of DOI:10.1016/j.apcata.2013.08.047

Applied Catalysis A: General 468 (2013) 175–183
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Preparation and characterization of Pt-promoted NiY and CoY
catalysts employed for 4-nitrophenol reduction
Zeinhom M. El-Bahya,b,∗
a Chemistry Department, Faculty of Science, Taif University, Taif, Saudi Arabia
b Chemistry Department, Faculty of Science, Al-Azhar University, Nasr City 11884, Cairo, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
The reduction of 4-nitrophenol (4-NP) by NaBH4 was used as a probe reaction to investigate the pro-
motion effect of platinum to CoY and NiY catalysts. Subsequent catalysts preparation by ion exchange
method, they were characterized by XRD, FTIR, TEM, EXD and surface area measurements at −196 ◦C.
The metal particle size calculated from XRD patterns and TEM images was ≈20 nm for PtY and ≈8–14 nm
for Pt-promoted CoY and NiY catalysts. The catalytic efficiency toward the hydrogenation reaction was
in the order of PtY < NiY < CoY < PtNiY < PtCoY. Using Pt-promoted CoY catalyst, the reaction rate of 4-NP
reduction at 28 ◦C was found to be 54 times higher than that of PtY catalyst. Moreover, after reduction
of the employed catalysts, a significant improvement in the catalytic performance was observed toward
the hydrogenation reaction. The specific rate constant of the hydrogenation reaction in the presence of
reduced PtCoY was 3.7, 50 and 201 times more than that in the presence of unreduced PtCoY, CoY and
PtY catalysts, respectively. Both of the catalysts promotion with platinum and their reduction extremely
decreased the apparent activation energy (Ea), whereas it decreased in case of reduced PtCoY to ≈one
third of its value in case of PtY. The catalytic activity was found to have a logarithmic relation with the
catalyst mass.
Received 24 May 2013
Received in revised form 24 August 2013
Accepted 26 August 2013
Available online xxx
Keywords:
4-Nitrophenol
Reduction
4-Aminophenol
NaY zeolite
Platinum
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
and cheaper cost catalysts never ends. The presence of transition
metal as a second element (d electrons are available) can increase
Faujasite zeolites have many potential properties with their
novel cage structure, high surface area and their high affinity for
adsorption of enormous reactants and products while being used
as supports for catalysts in several reactions. Precious metals-
loaded faujasite zeolites have attracted much attention as they
are interesting host–guest materials. Specifically, it is of interest
to investigate the activity of platinum supported on NaY zeolite
for the development of these catalysts for the industrial applica-
tions of organic pollutants removal by hydrogenation reactions.
Bimetallic platinum with a second metal such as PtFe/Al2O3 [1],
PtCo/Al2O3 [2], supported PtNi [3,4], and Pt-Cr/ZSM-5 [5] have
been prepared using different preparation methods such as succes-
sive impregnation, incipient wetness and competitive ion exchange
methods. They have been used in some reactions such as CO oxida-
tion, isomerization reactions and alkane conversion. Although, the
interaction between the noble metals and the zeolitic structure can
lead to the generation of electronic deficient sites which have high
activity toward hydrogenation reactions, seeking of higher activity
the Pt electronic density and, consequently, enhance the catalytic
activity [5]. In the previous oxide promoted catalysts, the activity
is likely to be controlled via the Pt-promoter interface and so it
is essential that the oxide and Pt should be associated together to
increase the catalytic activity [6] with high dispersion. Such disper-
sion is difficult to be achieved by traditional preparation methods
such as impregnation, incipient wetness or co-precipitation. One of
the ways to attain high dispersion is to use ion exchange method
which can homogenously distribute ions on the surface. In a previ-
ous work [7], we used successive ion exchange method to prepare
PtFeY catalyst and study its catalytic activity toward CO oxidation.
As far as we know, Pt was not used as promoter to CoY and NiY cata-
lysts using ion exchange method. Therefore, it will be interesting to
use such method to prepare Pt-promoted CoY and NiY catalysts and
investigate their catalytic activity toward the removal of hazardous
materials.
Upgrading water quality is getting more interest with increas-
ing the industrial applications. Phenolic compounds are considered
as priority pollutants since they are harmful to organisms even at
low concentrations [8,9]. Among various phenolic compounds, 4-
nitrophenol (4-NP) is one of the frequently occurring by-products,
which is toxic to the environment. On the other hand, 4-NP is
∗
Corresponding author. Tel.: +20 224129790.
E-mail addresses: zeinelbahy2020@yahoo.com, zeinelbahy2020@hotmail.com
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.08.047

176
Z.M. El-Bahy / Applied Catalysis A: General 468 (2013) 175–183
reduced to form 4-aminophenol (4-AP) which has great commer-
cial importance as an intermediate for the preparation of analgesic
and antipyretic drugs [10–12]. In view of the hazardous effect of
4-NP and the growing demand for 4-AP, direct catalytic reduction
of the former to the later becomes important.
4-nitrophenolate
4-nitrophenol
NaBH4 has the potential to be a useful hydrogen storage com-
pound (NaBH4 + 2H2O → NaBO2 + 4H2). The speed of releasing of
hydrogen can be raised by adding metal containing catalysts [13].
Reduction of 4-NP with NaBH4 in the aqueous medium is kinetically
inert reaction, and generally it occurs in the presence of metal con-
taining catalysts such as colloids of platinum nanoparticles [14,15].
Reduction of 4-NP to 4-AP with an excess amount of NaBH4 has
often been used as a model reaction to examine the catalytic perfor-
mance of metal nanoparticles [16]. It has been extensively studied
involving various noble metal nanocatalysts such as Au, Ag, Pt and
Pd [17]. Compared with Au, Pt, Pd and Ag cobalt and nickel are not
expensive. Moreover, they can be used as binary metallic system
with precious metals such as Pt to enhance its activity. Based on
our literature survey, the use of Pt-promoted CoY and NiY catalysts
for the 4-NP reduction has not been reported. So, employing these
catalysts for such reaction is interesting in view point of applied
and industrial importance.
210
250
290
330
370
410
450
490
Wavelength, nm
Fig. 1. UV–vis absorption spectra for the change of 4-NP to 4-AP in the presence of
NaBH4.
The apparent lesser reports on the metal oxides-promoted PtY
catalyst for the 4-NP reduction hearten the author to prepare Pt-
promoted CoY and NiY catalysts using successive ion exchange
method and characterize the prepared catalysts with different tech-
niques. The catalysts will be employed in the reduction of the
hazardous organic pollutant 4-NP to form the useful 4-AP.
size (D) of the obtained powders; where ꢁ represents the X-ray
wavelength (1.54 A), ꢀ is the Bragg’s angle and ˇ (in radians) is the
pure full width of the fraction line at half of the maximum capacity.
IR spectra were recorded in the solid state as KBr pellet on JASCO
FTIR-600 Plus with a spectral resolution of 2 cm−1 and accumula-
tion of 100 scans at room temperature.
˚
N2 adsorption–desorption isotherms were used to examine
the porous properties of each sample by using nitrogen as the
adsorbent at −196 ◦C. The measurements were carried out in
a Quantachrome AS1Win. (Quantachrome instruments Version
2.01). Before analysis, all samples were pretreated in vacuum
at 300 ◦C for 2 h. The surface area was calculated using the
Brunauer–Emmett–Teller (BET) method based on adsorption data
in the partial pressure (P/P0) range of 0.02–0.25. The total pore
volume was determined from the amount of nitrogen adsorbed at
P/P0 = ca. 0.95. Pore size and pore volume were obtained via t-plot
analysis of the isotherm data.
Transmission electron microscope (TEM) micrographs of the
prepared catalysts were measured using JEOL JEM-1010 trans-
mission electron microscope at an accelerating voltage of 60 kV.
Samples were previously ground and ultrasonically dispersed in
water. The solids were then deposited over a thin carbon film sup-
ported on a standard copper grid.
2. Experimental
2.1. Catalyst preparation
All the employed reagents in this study were of analytical grade.
To prepare platinum, cobalt and nickel supported NaY solids, 4 g
of NaY zeolite (Toyota Company Ltd., Japan. CBV, SAR = 5.1) were
soaked in 0.01 M solution of Pt(NH3)4Cl2·H2O (Mitsuwa’s Pure
Chemicals), Ni(NO3)2·6H2 and Co(NO3)2·6H2 (Aldrich), respec-
tively. The mixtures (pH ≈ 6.2) were stirred for 24 h at room
temperature, and then the solution was filtered off. This was
repeated three times to increase the degree of ion exchange. After
filtration, the suspensions were washed with de-ionized water,
dried, and finally calcined at 500 ◦C in air for 3 h. The resulted solids
were inferred as PtY, CoY and NiY for platinum, cobalt and nickel
supported NaY zeolite, respectively.
The binary systems PtCo and PtNi catalysts supported on NaY
zeolite were prepared by successive ion exchange method. In two
separated beakers, 2 g of the previously prepared powders CoY and
NiY were added to 0.01 M of Pt(NH3)4Cl2·H2O solution at pH 10.
Then, the mixtures were stirred at 80 ◦C for 1 h, filtered, washed
with distilled water, dried at 100 ◦C for 24 h and finally calcined at
500 ◦C for 3 h. The obtained solids were referred as PtCoY and PtNiY
for Pt-promoted CoY and NiY catalysts, respectively.
EDX analysis of the prepared samples was accomplished to esti-
mate the weight ratio (wt%) of the metal ions by using an X-ray
energy dispersive spectroscopy (JOEL, Model: JSM-5600, Japan).
2.3. Catalytic reduction of 4-nitrophenol
The performance of 4-NP reduction in the presence of NaBH4
was investigated in batch mode. Typically, 100 ml aqueous solution
of 4-NP (0.72 mmol) was mixed with NaBH4 (1.5 mmol) as a reduc-
ing agent. The concentration of the NaBH4 was chosen to exceed the
concentration of the 4-NP by far. It is known that the light yellow
4-NP exhibits a strong absorption peak at 317 nm (Fig. 1), however,
after addition of NaBH4 to its aqueous solution (increase of alka-
linity), 4-nitrophenolate ions (dark yellow) are formed with a new
absorption band at 400 nm [19–21].
2.2. Catalyst characterization
The parent NaY and the prepared catalysts were characterized
by various techniques such as X-ray diffraction (XRD), FTIR, surface
texture parameters measurements, TEM and elemental analysis by
EDX technique.
XRD patterns of parent NaY and the prepared samples were
collected at room temperature with an X-ray diffractometer, D8
Advance (Bruker axs), with a Cu K␣ radiation source (30 kV and
20 mA) in the 2ꢀ range of 5–60◦. Hall-equation–Scherer’s formula
D = 0.9ꢁ/ˇ cos ꢀ [18] was used to determine the average crystallite
4-nitrophenol + NaBH4 → 4-Nitrophenolate
The temperature of the mixture (4-NP + NaBH4) was kept con-
stant at the desired temperature using water bath (t 1 ◦C) and

Z.M. El-Bahy / Applied Catalysis A: General 468 (2013) 175–183
177
As prepared catalysts
Pt(111)
(a)
Pt(111)
PtCoY
PtNiY
CoY
NiY
PtY
NaY
5
10
15
20
25
30
35
40
45
50
55
60
2θ, degree
Reduced catalysts
PC
(b)
PtCoY
PN
C
PtNiY
NiY
N
CoY
C
P
PtY
P
NaY
38
43
48
53
2θ, degree
331
220
(c)
(d)
331
220
311
311
CoY
NiY
RedPtCoY
PtCoY
10
12
14
16
10
12
14
16
2 θ, degree
2θ, degree
Fig. 2. Powder XRD patterns of parent NaY, as prepared and reduced catalysts. N = Ni0, C = Co0, PN = PtNi, PC = PtCo.
the reacting suspension was continuously magnetically stirred. To
3. Results and discussions
3.1. Characterization
the mixture, unless otherwise stated, a sample weight of 20 mg
of the employed catalyst was added and stirred at room temper-
ature to be the zero time of the reaction (t0). The disappearance
of 4-nitrophenolate dark yellow color was monitored using UV–vis
spectrophotometer (Varian carry 50) by withdrawing 3 ml of mix-
ture in different time intervals during the reaction. The solid
suspension was separated from the solution by filtration and the
quantitative determination of 4-nitrophenolate was performed by
recording the absorption spectra at different time intervals. In
agreement with previous literature [22–24], the kinetics of the
reduction can be treated as pseudo-first order reaction with respect
to 4-nitrophenol and applying the equation [ln(C0/C) = kt] where, C0
and C are the extinction of the peak maxima at 400 nm at the begin-
ning of the reaction (t = 0) and after time t, respectively. The slope
of the plot ln(C0/C) vs t(min) gives the reaction rate constant (k,
min−1).
3.1.1. X-ray diffraction
The crystallinity of the prepared samples was confirmed by
powder XRD analysis and all the measurements are shown in
Fig. 2a. The patterns of parent NaY and the other prepared cata-
lysts showed nine main lines at 2ꢀ = 6.18◦ (1 1 1), 10.9◦ (2 2 0), 12.6◦
(3 1 1), 16.4◦ (3 3 1), 19.48◦ (3 3 3), 21.2◦ (4 4 0), 24.5◦ (4 4 4), 27.88◦
(7 3 1) and 32.32◦ (8 4 0). Other small peaks at 23.64◦ (5 3 3), 26.66◦
(5 5 1), 30.52◦ (6 6 0), 31.64◦ (5 5 5), 33.34◦ (7 5 3), 35.02◦ (8 4 4),
38.72◦ (10 2 2), 42.38◦ (8 6 6), 48.82◦ (11 7 2), 52.82◦ (10 8 2), 53.50◦
(9 9 7) were also detected. According to the JCPDS powder diffrac-
tion database file [PDF# 38-0239], these XRD lines are typical to
NaY zeolite which is also in a good agreement with previous litera-
ture [25,26]. These lines were observed in all the measured patterns

178
Z.M. El-Bahy / Applied Catalysis A: General 468 (2013) 175–183
270
of all the samples. This indicates that the zeolitic structure order-
ing remains unchanged after incorporation of metal ions into NaY
zeolite. The Pt containing sample showed small lines at 2ꢀ = 40.4◦
and 47◦ which are characteristic of Pt(1 1 1) and Pt(2 0 0) phases,
respectively [PDF# 87-0647]. However, the patterns did not show
any of Co or Ni oxides phases which may be due to the low concen-
tration of Co and Ni oxides or the encapsulation of these oxides in
the NaY zeolite cages.
HY Ads.
NiY Ads.
CoY Ads.
PtCoY Ads.
Red PtCoY Ads.
HY Des.
NiY Des.
CoY Des.
PtCoY Des.
Red PtCoY Des.
250
230
210
190
The representative XRD patterns of the reduced Pt-free and Pt-
promoted CoY and NiY catalysts are shown in Fig. 2b. The pattern
of reduced NiY catalyst shows small diffraction peaks at 2ꢀ = 44.5◦
(1 1 1) and 52◦ (2 0 0) which correspond to cubic Ni0 [PDF# 88-
2326]. Moreover, small peaks at 2ꢀ = 46◦ (1 1 1) and 54◦ (2 0 0)
which correspond to Co0 [PDF# 88-2325] in the pattern of reduced
CoY catalyst were observed. Small peak due to PtCo at 2ꢀ = 44◦
(1 1 1) [PDF# 43-1358] was detected in the catalyst containing
platinum and cobalt. Furthermore, PtNi alloy may be detected at
2ꢀ = 44.5◦ in case of PtNiY catalyst. According to Ahmadi et al. [27]
and Takenaka et al. [28], it is well acceptable to form PtCo alloys
over Y zeolite after reduction with H2 gas at 400 ◦C.
Quayle and Lunsford [29] reported that an empirical derived
relationship exists between the relative 331, 311, and 220 peak
intensities and cation location in faujasite type zeolites. If the inten-
sities are in the order of I3 3 1 > I2 2 0 > I3 1 1, the cations are randomly
distributed within the lattice. However, if I3 3 1 > I3 1 1 > I2 2 0, the
cations assume positions at sites I− and II. Site I− is the sodalite
cavity, while site II is at the center of a single six-ring (S6R) or dis-
placed from this point into a supercage [30]. Using these empirical
criteria, the diffraction patterns, Fig. 2c, suggest that the Co2+ is
probably located in random positions while Ni2+ may be located
at sites I− and II. However, after reduction of NiY, the Ni did not
take specified sites (I− and II) and it is suggested that it is probably
located in random positions. In case of bimetallic system, Fig. 2d
shows that the ions in the unreduced sample PtCoY are randomly
distributed while reduction directs the ions in specific positions
170
150
130
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P/P0
Fig. 3. Nitrogen adsorption-desorption isotherms for parent NaY and different
metals loaded NaY catalysts.
exhibited broad peak centered at 3440 cm−1 and other peak at
1637 cm−1 which are attributed to stretching vibrations of surface
hydroxyl groups and the absorption frequency of H OH, respec-
tively [34,35]. The spectra showed also peaks typical for Y zeolite
at 1173, 1057, 823, 745, 589 and 459 cm−1 [36]. The bands related
to external linkages between TO4 tetrahedra and sensitive to the
framework structure were observed at 1173, 823, and 589 cm−1
.
Moreover, the structural insensitive vibrations caused by internal
vibrations of TO4 tetrahedra of NaY were observed at 1057, 745 and
459 cm−1 [36]. As described in XRD patterns, FTIR data gave another
verification that the structure of NaY zeolite is kept intact after
incorporating of metal ions and calcination at 500 ◦C. In coherence
with XRD data, FTIR spectra did not show any peaks characteristic
of M O bonds which may be due to the low concentration of these
oxides.
since I2 2 0 > I3 3 1 > I3 1 1
.
The relative crystallinity was estimated by comparing the main
peaks intensities with that of the parent NaY zeolite (100%). The
relative crystallinity of the prepared samples decreased as summa-
rized in Table 1. The Pt particle size was calculated by Scherrer’s
equation and it was found to be ≈20 and ≈16 nm in PtY and PtCoY,
respectively, Table 1. This indicates that nanosizd Pt particles are
easily prepared by the employed method. The decrease in parti-
cle size in bimetallic system is in a good agreement with previous
reports [3,7]. However, it was reported in literature that Pt parti-
cle size increased after addition of secondary metal prepared by
incipient wetness [31,32] or vapor deposition [33] methods.
3.1.3. Surface texture characteristics of the prepared solids
The effect of incorporating metal ions to NaY zeolite on the sur-
face adsorption capacity and pore structure can be clearly seen in
the N2 sorption isotherms, Fig. 3. For the sake of clarity, some of the
isotherms were omitted. According to the IUPAC classification, the
isotherms of all the samples resembled type IV closely or faintly,
suggesting the coexistence of mesopores and micropores [37]. The
isotherms have relatively high adsorption below relative pressure
(P/P0) of 0.1 due to micropore filling. With increasing nitrogen
pressure (indicated by an arrow), a gradual increase in adsorp-
tion was observed up to P/P0 = 0.7, and then followed by a sudden
increase of adsorption until P/P0 = 0.95. The overall adsorption was
the highest in NaY zeolite (265 cm3/g) while it was the lowest in
case of CoY (184 cm3/g). This is reflected in the specific surface area
values (SBET) which are listen in Table 1. Overall, the porosity of
3.1.2. FTIR spectroscopy
IR spectra (not shown) of parent NaY zeolite and the Pt-
promoted CoY and NiY catalysts in the range of 4000–400 cm−1
Table 1
Physical properties of the prepared catalysts.
SBET (m2/g)
Pore size cc/g
Pore radius (A)
Particle sizea (nm)
Metalb (wt%)
Pt
˚
Catalyst
Crystallinity (%)
Co
Ni
NaY
PtY
NiY
CoY
PtNiY
PtCoY
RedPtY
RedPtCoY
100
76
82
84
71
77
83
97
719
556
526
486
525
572
–
0.151
0.118
0.113
0.105
0.099
0.122
–
4.548
4.544
4.540
4.665
4.661
4.553
–
–
20
–
–
2.22
–
–
–
–
2.4
–
1.03
–
1.11
–
–
2.61
–
2.02
–
–
–
14
16
18
14
2.89
2.64
2.26
2.72
–
–
–
–
–
a
Particle size calculated from TEM images.
Weight ratio calculated from EDX.
b

Z.M. El-Bahy / Applied Catalysis A: General 468 (2013) 175–183
179
Fig. 4. TEM images of: (a) Co-free and (b) Co-promoted PtY catalysts.
these samples appears to be dominated by both micropores and
mesopores, however, it decreased after addition of metal ions, as
described by the decrease of pore volume and pore radius (Table 1).
The presence of micropores is consistent with the zeolitic struc-
tural ordering as shown previously in XRD patterns. In all cases,
the porosity was persisted even after preparation of monometallic
or bimetallic systems.
as expected, did not show any catalytic performance toward the
reduction of 4-nitrophenole in the presence of NaBH4. Interestingly,
after the addition of metal containing zeolite (PtCoY) to the solution
(4-NP + NaBH4), there was a continuous fading of color until discol-
oration of solution. This was clear from the decrease of the peak
located at 400 nm characteristic of 4-nitrophenolate, Fig. 5. Also,
two new peaks at 295 and 230 nm due to 4-aminophenole (4-AP)
increased with the time progress. It is noticed that, there is no pro-
portional increase in the 4-aminophenol peak intensity with that of
4-nitrophenol; probably due to the difference in the molar extinc-
tion co-efficient of 4-nitrophenolate and 4-aminophenol [38]. The
UV-vis spectra show an isosbestic point (315 nm), illustrating that
the catalytic reduction of 4-NP yields only 4-AP without byproducts
[17,39].
3.1.4. TEM micrographs
The TEM images of PtY and PtCoY are shown in Fig. 4. It is clearly
presenting that the particle size of PtY was ≈18 nm, Fig. 4a. Signifi-
cant morphological changes were obtained after promotion of CoY
with platinum (PtCoY). The micrograph of PtCoY, Fig. 4b, showed
that the particle size decreased to ≈8 nm. This indicates that the
dispersion in PtCoY catalyst is more than that of Co-free PtY cata-
lyst. This is in coherence with the data obtained from XRD patterns
and in agreement with previous literature [3,7].
0 min
4-nitrophenolate
1 min
3.1.5. EDX analysis
2 min
4 min
The EDX analysis of the prepared catalysts determined the
amount of metal ions loaded on NaY zeolite. The data are sum-
marized in Table 1. It showed that the ratio between Pt/Ni = 1.4
while the ratio of Pt/Co = 2.6. The difference between Pt/Co and
Pt/Ni may be due to the difference in ion exchange ability and
capacity between Na and Co or Ni ions.
8 min
4-nitrophenol
12 min
15 min
3.2. Catalytic reduction of 4-nitrophenol
The reduction of 4-NP over the prepared samples in the presence
of NaBH4 was previously investigated for the efficient production
of 4-AP [21–24]. In addition to the previous important reason, this
reaction was used as a model reaction to examine the effect of pro-
motion with platinum on the catalytic performance of CoY and NiY
catalysts. The results showed that the prepared samples efficiently
assisted the catalytic reduction of 4-NP. Moreover, no hydrogena-
tion reaction was observed without using catalyst. This confirms
the absence of any noncatalytic reduction of 4-NP. Before investi-
gating the catalytic activity of metal supported zeolite, NaY zeolite,
210 250
290
330 370 410
450
490
Wavelength, nm
Fig. 5. Time-dependant UV–vis absorption spectral changes of the 4-NP reduc-
tion at 28 ◦C using 100 ml aqueous solution containing 20 mg PtCoY catalyst, 4-NP
(0.72 mmol) and NaBH4 (1.5 mmol).

180
Z.M. El-Bahy / Applied Catalysis A: General 468 (2013) 175–183
2.5
3
2.5
2
NaY
NiY
PtY
PtY
NiY
2
CoY
CoY
PtCoY
PtNiY
PtNiY
PtCoY
1.5
1.5
1
1
0.5
0.5
0
0
0
5
10
15
20
Time, min
0
5
10
15
20
25
30
35
40
Time, min
Fig. 7. Pseudo-first order plots of 4-NP reduction in the presence of NaBH4 over
5 mg of reduced catalysts at 28 ◦C using 100 ml aqueous solution containing 4-NP
(0.72 mmol) and NaBH4 (1.5 mmol).
Fig. 6. Pseudo-first order plots of 4-NP reduction in the presence of NaBH4 over
parent NaY and different metals loaded NaY catalysts at 28 ◦C using 100 ml aqueous
solution containing 20 mg catalyst, 4-NP (0.72 mmol) and NaBH4 (1.5 mmol).
increased in the presence of the reduced metallic Ni amounts.
Moreover, the presence of Pt enhances the reduction of Ni cations
and forms more metal particles [3,42]. The reduction of metal ions
over Y zeolite may be the reason of increasing the catalytic activ-
ity in bimetallic catalysts, namely, PtCoY or PtNiY. The presence
of the reduced forms of these metals facilitates the adsorption
of reactants, electron transfer from catalyst to phenolate ion and
desorption of the products. It will be interesting to investigate the
reduction of 4-NP in the presence of the reduced catalysts (higher
basic sites) and aqueous solution of NaBH4.
3.2.1. Reduction of 4-nitrophenol in the presence of different
catalysts
Fig. 6 shows the pseudo-first order kinetics of 4-NP reduction
in the presence of NaBH4 at 28 ◦C. Straight lines were observed in
the plots of lnC0/C vs t (min) after short period of time. During the
first stage of the reaction, abrupt decrease in the concentration of 4-
nitrophenolate ion was observed which may be due to the increase
of H2 evolution in the first few seconds of reaction.
Although the dark yellow color of 4-nitrophenolate started to
fade with time, the color still exists even after 60 min of reaction
in case of simple metal oxides loaded on NaY zeolite (PtY, NiY and
CoY). However, in Pt-promoted CoY and NiY systems (PtCoY and
PtNiY), the fading of color was faster than that the case of sup-
ported simple oxide catalysts. Moreover, after using PtCoY, the
color disappeared within 8 min of the reaction progress indicating
its completion. The reaction rate constant values (k, min−1) were
calculated from the slope of the straight line portion and summa-
rized in Table 2. It is clear that the catalytic performance was in
the order of PtY < NiY < CoY < PtNiY < PtCoY. The k value (min−1) of
4-NP reduction at 28 ◦C in the presence of PtCoY is 54 and 14 times
more than that in the presence of PtY and CoY catalysts, respec-
tively. Hence, the combination of Pt and Co greatly enhanced the
catalytic activity toward the reduction reaction. Considering that
all the metal atoms participate in the reaction to liberate H+ from
NaBH4, the ratio of 4-NP concentration (0.72 mmol) and the num-
ber of moles of metal atoms, measured by EDX, ([4-NP]/[M]) was
calculated and summarized in Table 2. It is clear that after comple-
tion of reaction, the turn over number of this reaction is 31.6, 8.3
and 12 in case of PtY, CoY and PtCoY catalysts, respectively.
In agreement with Ismail et al. [40], the careful inspection of
the k (min−1) values, Table 2, showed that the catalytic activity did
not have direct correlation with SBET, pore diameter and pore vol-
ume, Table 1. However, the results indicated that the rate constants
increased from 4 × 10−3 min−1 to 216 × 10−3 min−1 as the metal
nanoparticle size decreased from 20 nm to 8 nm in PtY and PtCoY,
respectively. It was reported that, in low particle size region, a
decrease in the particle size leads to an increase in the fraction
of low coordination metal sites such as vertices, edges, and kinks,
which can promote adsorption of the reactants and facilitate the
reaction [41].
3.2.2. Effect of catalyst reduction
Since the basic metallic sites are believed to play a great role
in the reduction of 4-NP by NaBH4, it is interesting to reduce the
prepared catalysts and compare their catalytic activity toward the
4-NP hydrogenation reaction. Reduction of the catalysts was car-
ried out using 80 Torr of pure H2 in static vacuum conditions at
400 ◦C for 30 min and in the presence of liquid nitrogen trap. The
catalytic activity of the reduced catalysts toward the reduction
(4-NP → 4-AP) was tested and the reaction was too fast to follow
when employing 20 mg catalyst. Therefore, the amount of catalyst
was reduced to 5 mg while keeping other parameters unchanged.
The first order plots of 4-NP reduction are presented in Fig. 7. The
catalytic activity was compared by calculating the specific rate con-
stant as k (min−1)/mass (g) (Table 2). It is clear that the activity of
the reduced catalysts is 8.5 and 3.7 times more than that of the
unreduced ones in case of PtY and PtCoY, respectively, at 28 ◦C. As
mentioned in XRD data, it is expected to form PtCo and PtNi alloys
over Y zeolite after reduction with H2 gas at 400 ◦C. Kuroda et al.
[16] reported that, the mechanism of the hydrogenation of nitro
compound indicated that the process is influenced by factors such
as proton availability and electron transfer to the nitro compound.
Since the reaction is carried out in aqueous medium, protons are
available and in excess. The relay of the electrons from the strong
−
nucleophile BH4 (because of its diffusive nature and high elec-
tron injection capability) to the acceptor 4-NP via metal particles
helps to overcome the kinetic barrier of the reaction and controls
the reaction speed, Scheme 1. It can be noted that the presence
of metallic form of the employed elements facilitates the relay-
ing of electrons to 4-NP from borohydride ion to nitrophenolate
ion. Similar data were obtained by other work groups when using
Ni/Al2O3, Ni/TiO2 [43] and CuO [44]. These data confirms the high
activity of metallic particles in the hydrogenation of 4-NP to 4-AP
which may be due to the ease of hydrogen adsorption over metallic
sites and 4-AP desorption out of these sites. The greatest change
in apparent rate constant was noticed for PtNiY which increased
The addition of platinum to either CoY or NiY leads to the for-
mation of electron rich sites compared to those of PtY, CoY or NiY
catalysts. Additionally, it decreases the acidic characters [3,7,42] of
the surface of the catalyst. Lu et al. [43] reported that the activ-
ity of Ni-containing catalysts toward the hydrogenation of 4-NP

Z.M. El-Bahy / Applied Catalysis A: General 468 (2013) 175–183
181
Table 3
Comparison of apparent rate constants for 4-nitrophenol reduction over reduced
PtCoY with different reported catalyst systems.
Catalyst
Apparent rate constant (min−1
)
Reference
Ti/Al2O3
Pure Pt
Raney Ni
Ni–Pt (64:36)
Ni–Pt (20:80)
Microgel-Pd
Ni–Pt (94:6)
Red PtCoY
3 × 10−3
7.2 × 10−3
9 × 10−3
[45]
[46]
[38]
[38]
[38]
[47]
[38]
This work
[48]
28.8 × 10−3
54 × 10−3
90 × 10−3
115.8 × 10−3
201 × 10−3
210 × 10−3
215 × 10−3
222 × 10−3
264 × 10−3
PNIPAa gels (Ag)
PAMAMb-Pd
PAMAMb-Au
SPBc-Pd
[20]
[49]
[38]
a
PNIPA denotes Poly(N-isopropylacrylamide).
PAMAM denotes poly(amidoamine).
SPB denotes Spherical Polyelectrolyte Brushes.
b
c
catalyst is rather high in comparison to almost all previous find-
ings in the literature [20,45–49] including those obtained with Pt
and Ni (Table 3).
3.2.3. Effect of mass of catalyst
As previously mentioned, the catalyst PtCoY exhibited the high-
est catalytic activity. Therefore, the effect of the amount of the
catalyst PtCoY on the rate of 4-NP reduction was studied using
different catalyst masses in the range of 5–50 mg while keeping
the other parameters unchanged. Fig. 8a shows the pseudo-first
order plots of 4-NP in the presence of NaBH4 and different masses
of PtCoY catalyst. As expected, an enhancement of the catalytic
efficiency was noticed with increasing the amount of catalyst. The
relation between the rate constant and the catalyst mass was found
to be linear when using N-CMC and AuNPs [22,50]. Chen and Ray
[51] showed that the catalytic activity gave limiting value at high
catalyst concentration. On the other hand, the relation between
catalyst mass and the catalytic activity showed a volcano shape in
the photocatalytic degradation of 4-NP over Fe-MFI catalyst [52].
In the present work, the catalytic activity increased progressively
in logarithmic relation with the increase of catalyst weight and the
plot of log(k) vs the catalyst mass shows a linear relation, Fig. 8b.
Scheme 1. Proposed scheme of 4-NP reduction in presence of NaBH4 and the pre-
pared catalysts.
16 times after reduction whereas PtCoY increased 3.7 times after
reduction. However, the rate constant of reduced PtCoY is the high-
est amongst the employed catalysts. Additionally, the turn over
number increased to 114 and 47.6 in case of reduced PtY and PtCoY
catalysts, respectively.
It is noteworthy to mention that, the apparent rate constant
obtained for 4-NP reduction in the presence of reduced PtCoY
Table 2
4-NP reduction experiments over the prepared catalysts.a
Catalyst
PtY
Temperature (◦C)
Mass (mg)
10−3 × k (min−1
)
Ksp (min−1 g−1
)
Ea (kJ/mol)
[4-NP]/[M]
31.6
28
35
40
28
35
40
28
35
40
28
35
40
28
35
40
28
35
40
28
28
28
28
35
40
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
5
5
5
5
5
5
5
5
5
4
13.3
32.2
12
35
61
16
29
40
47
126
297
216
346
503
8.7
19
0.2
0.665
1.61
0.6
1.75
3.05
0.8
1.45
2
2.35
6.3
14.85
10.8
17.3
25.15
1.7
3.8
6.6
2.3
1.6
38.6
40.2
73
136
105
60
28
NiY
CoY
8.3
7.2
12
119
55
PtNiY
PtCoY
117
114
Red
PtY
33
Red CoY
Red NiY
Red PtNiY
Red
11.5
8.4
193
201
365
599
–
–
–
–
–
–
48
47.6
PtCoY
119.8
a
Reaction conditions: 100 ml aqueous solution containing 4-NP (0.72 mmol) and NaBH4 (1.5 mmol); reaction temperature (28–40 ◦C); catalyst mass (5–20 mg).

182
Z.M. El-Bahy / Applied Catalysis A: General 468 (2013) 175–183
Fig. 8. (a) Pseudo-first order plots of the reduction of 4-NP in the presence of NaBH4 and different amounts of PtCoY catalyst at 28 ◦C using 100 ml aqueous solution containing
4-NP (0.72 mmol) and NaBH4 (1.5 mmol); (b) the relationship between the rate constant (k, min−1) and the catalyst mass.
In addition, the turn over number increased to 114 and 47.6 after
catalyst reduction in case of RedPtY and RedPtCoY, respectively.
This refers to the ability of the prepared catalysts to complete the
reaction even with relatively low metal concentration.
and PtCoY decreased to 117 and 48 kJ/mol, respectively, Table 2.
The decrease of the activation energy of the hydrogenation reac-
tion in presence of RedPtCoY indicates that the proceeding of
the reaction over the reduced catalysts is more facile than that
in case of unreduced catalysts. This strengthens the idea that,
the presence of more basic sit−es in the catalyst surface increases
the electron relay from BH4 to 4-NP and reaction becomes
easier.
3.2.4. Effect of temperature
The effect of temperature on the pseudo-first order rate con-
stant of the catalytic hydrogenation of 4-NP to 4-AP in the presence
of NaBH4 and the different prepared catalysts was studied in the
temperature range of 28– 40 ◦C. The activation energy values were
calculated by using Arrhenius equation:
4. Conclusions
ꢀ
ꢁ
In conclusion, reactive and efficient Pt-promoted CoY and
NiY catalysts were successfully prepared by successive ion
exchange method. They were used for the catalytic reduction
of 4-nitrophenol to 4-aminophenol with high catalytic perfor-
mance. Taking PtY catalyst as a reference (the least K value),
the relative catalytic performance calculated from the specific
rate constant values (Ksp(catalyst)/Ksp(PtY)) was in the order of
PtY(1) < NiY(3) < CoY(4) < RedNiY(8) < RedPtY(8.5) < RedCoY(11.5) <
PtNiY(11.7) < PtCoY(54) < RedPtNiY(193) < RedPtCoY(201). Mean-
while, the apparent activation energy was the highest in case of
PtY (136 kJ/mol) while it decreased to ≈1/3 of this value in the
case of RedPtCoY catalyst. The experimental data showed that, the
presence of low oxidation states or metallic forms on the catalyst
surface has an important role in the catalytic reduction of 4-NP.
Ea
ln k = ln A −
(2)
RT
where k is the rate constant, Ea is the apparent activation energy, R
is the ideal gas constant (R = 8.314 J K−1 mol−1) and T is the absolute
temperature (K). The Ea was calculated from the slope of the plot
(ln k vs. 1/T), Fig. 9, and the data are summarized in Table 2. The
highest activation energy (136 kJ/mol) was noticed for the 4-NP
reduction in the presence of PtY catalyst, however, Ea decreased
when using PtNiY and PtCoY to 119 and 56 kJ/mol, respectively.
As expected, Ea of 4-NP hydrogenation in presence of reduced PtY
0
-1
PtY
Appendix A. Supplementary data
CoY
-2
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2013.08.
047.
NiY
-3
PtNiY
-4
-5
-6
PtCoY
R PtY
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