Size-controlled synthesis of a supported Ni nanoparticle catalyst for selective hydrogenation of p-nitrophenol to p-aminophenol

Article abstract of DOI:10.1016/j.catcom.2011.11.015

We reported a general strategy of size-controlled synthesis of supported nickel nanoparticle catalysts using electroless plating technique. The synthesis conditions (e.g. compositions of plating solution and plating temperatures) were optimized to promote plating rate, and nickel nanoparticles with 9 nm diameter and a narrow size distribution were highly dispersed on TiO ₂. The as-prepared Ni/TiO₂ catalyst showed high activity, selectivity and stability in the selective hydrogenation of p-nitrophenol to p-aminophenol.

Full text of DOI:10.1016/j.catcom.2011.11.015

Catalysis Communications 18 (2012) 55–59
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Size-controlled synthesis of a supported Ni nanoparticle catalyst for selective
hydrogenation of p-nitrophenol to p-aminophenol
Zhijie Wu a,b, Juan Chen b, Qun Di b, Minghui Zhang b,
⁎
a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, PR China
Institute of New Catalytic Materials Science, Department of Material Chemistry, College of Chemistry, and Key Laboratory of Advanced Energy Materials Chemistry (MOE),
b
Nankai University, Tianjin, 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
We reported a general strategy of size-controlled synthesis of supported nickel nanoparticle catalysts using
electroless plating technique. The synthesis conditions (e.g. compositions of plating solution and plating tem-
peratures) were optimized to promote plating rate, and nickel nanoparticles with 9 nm diameter and a nar-
row size distribution were highly dispersed on TiO2. The as-prepared Ni/TiO2 catalyst showed high activity,
selectivity and stability in the selective hydrogenation of p-nitrophenol to p-aminophenol.
© 2011 Elsevier B.V. All rights reserved.
Received 17 September 2011
Received in revised form 10 November 2011
Accepted 10 November 2011
Available online 20 November 2011
Keywords:
Electroless plating
Ni/TiO2
Hydrazine
p-Nitrophenol
p-Aminophenol
Hydrogenation
1. Introduction
The reduction of p-nitrophenol (PNP) to p-Aminophenol (PAP) is
a model reaction which has been widely used for the quantification
Nickel nanoparticles, especially supported nickel nanoparticles on
various carriers, have been extensively studied as heterogeneous cat-
alysts [1–3]. These supported nickel nanoparticles are generally pre-
pared by reducing nickel cations impregnated on supports with H2
at a high temperature (e.g. 400 °C) [1–3]. This conventional reduction
method usually leads to large nickel aggregates and broad size distri-
butions when preparing catalysts with high nickel loading (>5 wt.%)
[4–7]. As a result, the aqueous reduction of nickel cations impregnat-
ed/adsorbed on supports with a strong reducing agent (e.g. hydra-
zine) at ambient conditions earns increasing interest because of its
advances on controlling particle morphology, size, and dispersion
[6,7]. In our previous work, we succeeded in controlling the synthesis
of Ni–B nanoparticles (e.g. metal loading, particle morphology, size
and dispersion, etc.) using a modified electroless plating technique
with borohydride as a reductant [8–13]. However, the resulted Ni–B
nanoparticles possess unstable amorphous structure adverse to the
recycle of catalysts in many practical reactions. Here, we attempted
to synthesize supported crystalline nickel nanoparticles by a modified
electroless plating method using hydrazine hydrate as a reductant, in
order to obtain stable and controllable supported nickel particles.
and comparison of the catalytic activity of metal nanoparticles [14].
Nitrophenol compounds are among the most common organic pollut-
ants in industrial and agricultural waste waters, but the PAP is an im-
portant intermediate for the manufacture of analgesic and antipyretic
drugs [15]. The nickel catalysts have been widely used in the hydro-
genation of PNP to PAP [16,17]. Chen et al. found that the unsup-
ported nickel nanoparticle catalyst yields good hydrogenation
activity and selectivity [18,19], but sintering of nickel nanoparticle oc-
curs after hydrogenation [18]. Here, a Ni/TiO2 catalyst with small and
uniform nickel particles for PNP hydrogenation was synthesized by
an electroless plating method, in which TiO2 served as supports to
disperse and stabilize nickel particles.
2. Experimental
2.1. Preparation of Ni/TiO2 catalyst
2.1.1. Seeding 1 wt.% Ni(nuclei)/TiO2
2.1.1.1. Synthesis of 0.1 wt.% Ag/TiO2 per Refs. [8–13]. 12 g TiO2 (anatase,
18 m2/g, 70–120 nm) was added to 425 mL solutions with stirring at
313 K for 4 h. The solution was composed of 4.71×10−2 g L−1
AgNO3, 8.82×10−2 g L−1 ammonia, 1.18×10−2 g L−1 NaOH, and
4.71×10−3 g L−1 HCHO. The resulting Ag/TiO2 was collected and
washed with distilled water, and then dried at 363 K for 4 h.
⁎
Corresponding author. Tel.: +86 22 23507730; fax: +86 22 23507730.
E-mail address: zhangmh@nankai.edu.cn (M. Zhang).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.11.015

56
Z. Wu et al. / Catalysis Communications 18 (2012) 55–59
Seeding 1 wt.% Ni(nuclei)/TiO2 via electroless nickel plating cata-
Blank experiments containing the reactant and solvent (ethanol
and water) without metal catalysts have been carried. No conversion
of PNP (under the detection limit of HPLC) was found, suggesting that
there is no contribution on hydrogenation from autoclave or solvent
(ethanol and water). The initial hydrogenation rate was calculated
as a turnover rate (TOR) of PNP hydrogenation as molar hydrogena-
tion rates per surface Ni atom.
lyzed by Ag/TiO2 [8,11,13]: The plating occurs by the reduction of
nickel ions at the surface of the active substrate immersed into the
plating solution and continues to deposit on the substrate through
catalytic action of the deposit itself [8,13]. For the electroless nickel
plating catalyzed by Ag metal, we have confirmed that the nickel nu-
clei are simultaneously generated by the growth of nickel clusters on
Ag metals [8–13] and the reduction of nickel ion aggregates adsorbed
by supports [13] at the beginning of plating. In a typical synthesis,
1.66 g H2N–CH2CH2–NH2 was dissolved in 80 mL distilled water
under stirring, and 0.71 g NiSO4·6H2O and 4.0 g NaOH was dissolved
in the above solution respectively. Then, 0.55 g KBH4 dissolved in
20 mL distilled water was mixed to compose the plating solution con-
taining 7.1 g L−1 NiSO4·6H2O, 16.6 g L−1 H2N–CH2CH2–NH2,
5.5 g L−1 KBH4 and 40.0 g L−1 NaOH. For plating, 2 g Ag/TiO2 was
added into 20 mL Ni–B plating solution at 80 °C for 5 min, and then
the Ni(nuclei)/TiO2 was separated without washing.
3. Results and discussion
3.1. Characterizations
Fig. 1 shows the XRD pattern of Ni/TiO2 catalyst prepared with ratios
of [N2H4]/[Ni2+]=5 and [OH−]/[Ni2+]=1.25 at 80 °C. Two new re-
flection peaks (Fig. 1) at 2θ=44.59° and 51.90° corresponding to Ni
(111) and (200) crystal plane respectively are observed, suggesting an
fcc structure of metallic nickel particles. Also, the XRD pattern presents
broad peaks corresponding to Ni metal with low intensities. This indi-
cates a small crystalline size of nickel nanoparticles on TiO2.
The morphology of Ni/TiO2 was observed by TEM (Fig. 2). Spherical
nickel nanoparticles are homogeneously dispersed on the external sur-
face of TiO2. The Ni dispersion of Ni/TiO2 is 0.13 corresponding to a
mean particle diameter (dchem) of 9.1 nm. The dchem is consistent with
that measured by TEM characterization (dTEM =9.5 nm in Fig. 2), sug-
gesting a narrow size distribution of Ni nanoparticles. Therefore, we cal-
culated the nickel nanoparticle size based on hydrogen chemisorptions
in the following discussion.
2.1.1.2. Synthesis of 15 wt.% Ni/TiO2. The electroless nickel plating
should first be activated by active substrates preloaded with Ni, Ag,
Au, Pd, or Pt metal [8–13]. For our present plating using N2H4·H2O
as reductant, the Ni metal is found to possess better catalytic activa-
tion than Ag metal in which 3–4-fold plating time was shortened.
Thus, the 1 wt.% Ni(nuclei)/TiO2 was used as the catalyst for the
plating.
For composing plating solution, 15.0 g Ni(CH3COO)2·4H2O dissolved
in 40 mL distilled water was mixed with 40 mL N2H4·H2O (containing
7.5–30.2 g N2H4·H2O) under stirring. After that, 20 mL NaOH (2–6 g
NaOH) solution was added into the above mixture with stirring. To syn-
thesize Ni/TiO2, 2 g Ni(nuclei)/TiO2 was added into 100 mL nickel
plating solution. The suspension was maintained at 60–90 °C for 10–
15 min, and then cooled to room temperature. The black product was
filtered and washed with distilled water until pH=7–8, then washed
with ethanol to remove water and kept in ethanol.
3.2. Formation of Ni nanoparticles
The control of metal particle size is of the utmost importance for
the performance of any industrial supported metal catalyst. Here,
we used TiO2 with low surface area (18 m2/g [9]) to disperse
15 wt.% Ni. Consequently, it is difficult to prepare small Ni particles
by using the conventional impregnation–reduction method [6,7].
The following discussion clearly shows our success in the synthesis
of small Ni particles with high metal loading using the electroless
nickel plating technique.
In our previous work [11,13], the control synthesis of Ni–B parti-
cles by electroless plating has been achieved by designing dispersion
and size of Ni(nuclei). We found that the formation of highly dis-
persed Ni–B particles is dependent on the generation of highly dis-
tributed nuclei on supports, because the growth of nickel particles
occurs via selective deposition of nickel metal on the nuclei [13]. Be-
cause the electroless nickel plating with different reductants pos-
sesses the similar process [20], we regarded that the role of Ni
2.2. Characterizations
X-ray diffraction patterns (XRD) were collected on the Rigaku D/
max-2500 diffractometer employing Cu Kα radiation (λ=1.5418 ).
Transmission electron microscopy (TEM) images were acquired
using a Philips Tecnai G2 high-resolution transmission electron mi-
croscope. The surface-area-weighted cluster diameter, dTEM, was cal-
culated using dTEM =∑nidi3 /∑nidi2 [20].
Chemisorption experiments were carried out in a pulse chromato-
graphic microreactor. ~200 mg catalyst was treated by flowing
100 mL min−1 Ar gas from room temperature to 300 °C (5 °C min−1
ramping rate) and kept for 2 h. Then, the sample was cooled down to
30 °C, and 1% H2/Ar was injected every 5 min until the calculated areas
of consecutive pulses were constant. The dispersion and surface area of
active Ni (SNi) was calculated assuming H/Ni(surf) =1 stoichiometry and
a surface area of 6.5×10−20 m2 per Ni atom [20]. Mean cluster sizes
(dchem) were also estimated as Ref. [20].
2.3. Hydrogenation of PNP
The hydrogenation of PNP was carried out in a high-pressure
stainless steel autoclave. In a typical experiment, 0.4 g catalyst
was dispersed in 40 mL ethanol and 20 mL distilled H2O, and 8 g
p-nitrophenol was transferred to the solution. The reaction was per-
formed at 100 °C and 1.5 MPa H2 pressure with stirring at 800 rpm.
The hydrogenation was monitored by the hydrogen mass flow
meter, and the hydrogenation was stopped until no obvious hydrogen
consumption. The hydrogenation products were analyzed by a High
Performance Liquid Chromatography (HPLC), and their structures
were confirmed by LC–MS. 4-aminocyclohexanol was detected as a
by-product from the further hydrogenation of PAP.
Fig. 1. XRD pattern of Ni/TiO2 catalyst.

Z. Wu et al. / Catalysis Communications 18 (2012) 55–59
57
Fig. 2. (a) TEM image and (b) Ni particle size distribution of Ni/TiO2 sample; and TEM images of Ni/TiO2 catalyst (c) before and (d) after 10-time hydrogenation.
(nuclei)/TiO2 in plating and the plating process are similar with that
in Ni–B plating shown in previous work [8–13]. Thus, the plating
rate was referred to the key factor for size control of Ni particles.
In the present work, when Ni(nuclei)/TiO2 was added to the plat-
ing solution, a mass of nickel particles was deposited through autoca-
talysis of seeded nickel nuclei and the plating rate was drastically
accelerated. More than 180 min for the plating process was required
if there were no nuclei in the initial plating solution, which was
two-fold or ten-fold time extension compared to the plating using
Ag/TiO2 (60 min) [21] or Ni(nuclei)/TiO2 (10–15 min), respectively.
A high plating rate favors generating large amount of nuclei during
inducing period and leads to small and uniform nickel nanoparticles
[8,13], which was also confirmed by the following study of plating
] ]
conditions (e.g., temperature, [N2H4]/[Ni2+ and [OH−]/[Ni2+
Fig. 3. Effect of plating conditions on the particle size and turnover rate (at ~30% conversion of PNP) of resulting Ni/TiO2 samples. (a) change of molar ratios of [OH−]/[Ni2+] with
[N2H4O]/[Ni2+]=5 at 80 °C, (b) change of molar ratios of [N2H4O]/[Ni2+] with [OH−]/[Ni2+]=1.25 at 80 °C, and (c) change of temperatures with [N2H4O]/[Ni2+]=5 and [OH−]/
[Ni2+]=1.25.

58
Z. Wu et al. / Catalysis Communications 18 (2012) 55–59
ratios). Here, we achieved the size-controlled synthesis of Ni nano-
particles (9–20 nm), which are much smaller than our previous
reported Ni particles (~45 nm) using Ag/TiO2 as support [21].
3.2.1. Effect of [OH−]/[Ni2+] ratio
The reduction of nickel ions by hydrazine in basic solution is often
described as [6,7]:
2Ni2þ þ N2H4 þ 4OH−→2Ni0 þ N2 þ 4H2O:
ð1Þ
A high pH value facilitates the reduction of nickel ions by hydra-
zine, and enhances the reduction rate. However, excess OH− ions
lead to a fast precipitation of [Ni(N2H4)3] (OH)2 to large nickel hy-
droxide precipitates on TiO2 and then large nickel particles after re-
duction (Fig. 3a). Therefore, the particle size increases by increasing
pH values ([OH−]/[Ni2+] ratios), and a suitable alkalinity of solution
was required to avoid fast precipitation of nickel hydroxides. Here, a
molar ratio of [OH−]/[Ni2+] equal to 0.8–1.3 is optimized to obtain
small nickel nanoparticles (Fig. 3a).
3.2.2. Effect of [N2H4]/[Ni2+] ratio
Fig. 3b shows that Ni nanoparticle size decreases by increasing ratios
of [N2H4]/[Ni2+], and then remains constant when [N2H4]/[Ni2+] is
higher than 5. For the reduction of nickel ions by hydrazine, the reduc-
tion rate plays a key role in the generation of high-dispersed nanoparti-
cles [6,7]. A high reduction rate promotes the simultaneous deposition
of nickel clusters on all nickel nuclei in a short time, leading to small
and uniform nanoparticles. Here, the deposition rate is determined by
concentrations of nickel ions and hydrazine around nickel nuclei. By in-
creasing concentrations of hydrazine in solution, the concentration of
reductant around nickel nuclei consequently increases and promotes
deposition rate. When excess hydrazine (>[N2H4]/[Ni2+]=5) is ap-
plied, the deposition process is controlled by consuming nickel ions
around nuclei, and a further increase of hydrazine concentration could
not influence the deposition rate. Consequently, the particle size de-
creases with increasing ratio of [N2H4]/[Ni2+], and attains practically
constant size of ca. 9 nm at [N2H4]/[Ni2+]>5 (Fig. 3b).
Fig. 4. The change of initial turnover rates and PAP selectivities for the hydrogenation
on the recycled Ni/TiO2 catalyst.
~100% selectivity of PAP, suggesting the lack of active sites to catalyze
PNP hydrogenation to 4-aminocyclohexanol.
For supported nickel catalysts, Ni–B nanoparticles show two times
turnover rate (TOR) higher than crystalline Ni nanoparticles (Table 1),
which is consistent with the fact that amorphous Ni–B possesses higher
hydrogenation activity than crystalline Ni metal [8–13]. However, the
dilution of active Ni by B elements on Ni–B/TiO2 results in a 60% lower
SNi value than Ni/TiO2. This leads to ~33% longer hydrogenation time
to completely converse PNP, when the same Ni weight amount of cata-
lysts is used (Table 1). As a result, the Ni/TiO2 with a higher SNi value en-
sures a more economical choice to use it as PNP hydrogenation catalyst.
Fig. 3 shows that TOR values are not affected by Ni particle sizes, indicat-
ing the structure-insensitive hydrogenation reaction and the require-
ment of small Ni particle size to promote SNi value for PNP
hydrogenation. Again, the recycling experiments of Ni/TiO2 (Fig. 4)
show bit decrease of TOR values and selectivity, and the HRTEM images
(Fig. 2c and d) indicate no change of particle size before and after 10
times hydrogenation. These results show the good stability of as-
optimized Ni/TiO2 for PNP hydrogenation.
3.2.3. Effect of plating temperatures
For electroless plating, the plating rate accelerates obviously by in-
creasing plating temperatures [8–13], resulting in a small particle size
(Fig. 3c). However, too high temperature (90 °C) leads to the decom-
position of plating solution into nickel aggregates, which results in
growth of particle sizes. Here, the optimal temperature is 80 °C
(Fig. 3c).
3.3. Catalytic performance
Table 1 summarizes the catalytic performance of catalysts in selec-
tive hydrogenation of PNP to PAP. Because Raney Ni possesses the
highest surface active nickel area per gram catalyst (SNi), the shortest
hydrogenation time is required for a complete conversion of PNP.
However, 4-aminocyclohexanol (5.4 mol%) as the by-product is
detected. Unlike Raney Ni, Ni/TiO2 and Ni–B/TiO2 catalysts show
4. Conclusions
The TiO2-supported Ni nanoparticle catalyst was prepared by a
modified electroless plating method using hydrazine hydrate as a re-
ductant. The formation of small and uniform Ni nanoparticles re-
quires a high plating rate by optimizing the ratios of [N2H4]/[Ni2+
]
Table 1
Hydrogenation performance of Ni/TiO2 and Raney Ni catalysts.
Samples
SNi
TOR
Hydrogenation
time (min)
Conversion
(%)
Selectivity (mol%)
PAP
−1
(mol(s molsurf-Ni
)
)
4-aminocyclohexanol
Ni/TiO2
Raney Ni
Ni–B/TiO2
13.5
20.8
5.4
3.23
3.25
6.05
45
37
60
~100
~100
~100
~100
94.5
~100
–
5.4
–
a
a
The catalyst was prepared as Ref. [8].

Z. Wu et al. / Catalysis Communications 18 (2012) 55–59
59
and [OH−]/[Ni2+] in plating solution and the plating temperature.
The as-optimized Ni/TiO2 with 9 nm particle size showed high activ-
ity, selectivity, and stability in hydrogenation of PNP to PAP. Compar-
ing to Ni–B and Raney Ni catalyst, the Ni/TiO2 is a more suitable
choice for the PNP hydrogenation. Our investigation of the control
of nickel nanoparticle size suggests a strategy to optimize metal cata-
lyst for catalytic reactions.
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Acknowledgments
The work was supported by Natural Sciences Foundation of Tianjin
(10JCYBJC04400) and MOE (IRT-0927).
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