Multiply Confined Nickel Nanocatalysts Produced by Atomic Layer Deposition for Hydrogenation Reactions

Article abstract of DOI:10.1002/anie.201503749

To design highly efficient catalysts, new concepts for optimizing the metal-support interactions are desirable. Here we introduce a facile and general template approach assisted by atomic layer deposition (ALD), to fabricate a multiply confined Ni-based nanocatalyst. The Ni nanoparticles are not only confined in Al₂O₃ nanotubes, but also embedded in the cavities of Al₂O₃ interior wall. The cavities create more Ni-Al₂O₃ interfacial sites, which facilitate hydrogenation reactions. The nanotubes inhibit the leaching and detachment of Ni nanoparticles. Compared with the Ni-based catalyst supported on the outer surface of Al₂O₃ nanotubes, the multiply confined catalyst shows a striking improvement of catalytic activity and stability in hydrogenation reactions. Our ALD-assisted template method is general and can be extended for other multiply confined nanoreactors, which may have potential applications in many heterogeneous reactions.

Full text of DOI:10.1002/anie.201503749

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201503749
German Edition: DOI: 10.1002/ange.201503749
Heterogeneous Hydrogenation
Multiply Confined Nickel Nanocatalysts Produced by Atomic Layer
Deposition for Hydrogenation Reactions**
Zhe Gao, Mei Dong, Guizhen Wang, Pei Sheng, Zhiwei Wu, Huimin Yang, Bin Zhang,
Guofu Wang, Jianguo Wang, and Yong Qin*
Abstract: To design highly efficient catalysts, new concepts for
optimizing the metal–support interactions are desirable. Here
we introduce a facile and general template approach assisted by
atomic layer deposition (ALD), to fabricate a multiply con-
fined Ni-based nanocatalyst. The Ni nanoparticles are not only
confined in Al2O3 nanotubes, but also embedded in the cavities
of Al2O3 interior wall. The cavities create more Ni–Al2O3
interfacial sites, which facilitate hydrogenation reactions. The
nanotubes inhibit the leaching and detachment of Ni nano-
particles. Compared with the Ni-based catalyst supported on
the outer surface of Al2O3 nanotubes, the multiply confined
catalyst shows a striking improvement of catalytic activity and
stability in hydrogenation reactions. Our ALD-assisted tem-
plate method is general and can be extended for other multiply
confined nanoreactors, which may have potential applications
in many heterogeneous reactions.
design and engineering of metal–oxide interfaces are critical
in developing highly efficient catalytic systems.[4]
Herein, we employed a facile template-assisted method
based on atomic layer deposition (ALD) to synthesize Ni
nanoparticles not only confined in Al2O3 nanotubes, but also
confined in the cavities of Al2O3 interior wall, to maximize the
metal–support interfaces. The increased interfacial sites and
protecting nanotubes greatly improve the activity and stabil-
ity of confined catalysts for hydrogenation reactions of
cinnamaldehyde and nitrobenzene.
ALD is a powerful technique for depositing nanoparticles
or thin films with outstanding advantages including precise
control of size and thickness and excellent uniformity.[5] It has
emerged as a powerful tool for the atomically precise design
and synthesis of catalytic materials.[6] Ni nanoparticles con-
fined in Al2O3 nanotubes (Ni-in-ANTs) and Ni nanoparticles
on Al2O3 nanotubes (Ni-out-ANTs) were obtained by ALD
using carbon nanocoils (CNCs) as sacrificial templates
(Figure 1). Compared with carbon nanotubes (CNTs), the
T
raditional heterogeneous catalysts, often consisting of
metal nanoparticles supported on oxide solids, are used in
a wide variety of industrial and environmental applications.
To improve our capability of designing highly efficient
catalytic systems, new concepts for optimizing the metal–
support interactions are desirable.[1] Confining metal nano-
particles in porous materials or carbon nanotubes has proven
to be an alternative approach for the design of a novel class of
heterogeneous catalysts with enhanced activity and stability.[2]
Confined catalysts utilizing metal oxide nanotubes have also
been explored.[3] In terms of maximizing the metal–oxide
interaction, the oxide nanotube-confined structure is not the
most ideal structure if the enhanced performance is caused
only by the incorporation of metal nanoparticles inside these
oxide nanotubes, namely spatial confinement effect. Rational
[*] Dr. Z. Gao, Prof. M. Dong, Dr. G. Z. Wang, P. Sheng, Dr. Z. W. Wu,
H. M. Yang, Dr. B. Zhang, G. F. Wang, Prof. J. G. Wang, Prof. Y. Qin
State Key Laboratory of Coal Conversion
Figure 1. Preparation process of catalysts.
Institute of Coal Chemistry, Chinese Academy of Sciences
Taiyuan 030001 (China)
E-mail: qinyong@sxicc.ac.cn
CNCs have lower annealing temperature for convenient
removal and are more suitable for uniform ALD coating,
because of their low thermal stability and abundant active
groups on their surface.[7] The ALD process was carried out in
a hot-wall closed chamber-type ALD reactor. Al2O3 was
deposited at 1508C with trimethylaluminum and deionized
H2O as precursors. NiO was deposited with nickelocene and
O3 as precursors at 2008C. Nickelocene were kept at 758C.
For Ni-in-ANTs, NiO nanoparticles were first deposited onto
CNCs with 150 ALD cycles. Here we used large cycle
[**] We appreciate financial support from the National Natural Science
Foundation of China (21173248, 21403272, 21227002, 21376256,
and 51362010), the Hundred Talents Program of the Chinese
Academy of Sciences, the Hundred Talents Program of Shanxi
Province, and in-house projects of the State Key Laboratory of Coal
Conversion of China (2013BWZ005). XAFS studies were carried out
at BL14W1 beamline at the Shanghai Synchrotron Radiation Facility,
Shanghai Institute of Applied Physics, China.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201503749.
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numbers for NiO ALD to get larger Ni nanoparticles for
convenient observation by transmission electron microscopy
(TEM). Then the as-prepared NiO/CNCs were further coated
by an amorphous Al2O3 film with a thickness of about 20 nm
also by ALD, labeled as Al2O3/NiO/CNCs. In this case, NiO
nanoparticles are confined in the cavities of Al2O3 shell. The
CNCs templates were removed by subsequent calcination in
air (Figure S1 in the Supporting Information). Finally,
a reduction treatment produced Ni-in-ANTs. Here Ni nano-
particles are not only confined in Al2O3 nanotubes, but also
confined in the cavities of Al2O3 interior wall. For Ni-out-
ANTs, we just need exchange the deposition sequence of NiO
nanoparticles and Al2O3 films. The subsequent calcination
and reduction treatments are the same as described above. In
this case, NiO or Ni nanoparticles are supported on the outer
surfaces of Al2O3 wall.
TEM images of Ni-in-ANTs (Figure 2A,B) and Ni-out-
ANTs (Figure 2D,E) clearly show their hollow structures.
The average diameters of Ni nanoparticles are 6.1 and 6.2 nm
for Ni-in-ANTs and Ni-out-ANTs, respectively. None of Ni
nanoparticles are located on the outer surfaces of Al2O3
nanotubes for Ni-in-ANTs. This perfect encapsulation can
hardly be obtained by traditional preparation methods. In
contrast, all of Ni nanoparticles are loaded on the Al2O3 outer
surfaces for Ni-out-ANTs. In principle, the Ni nanoparticles
of Ni-in-ANTs are confined in the cavities of Al2O3 interior
wall (Figure 1). However, this is visible only at the borderline
of the perspective projection of the inner surface of the
Al2O3 shell, because TEM is a 2D imaging technology
(Figure 2C). The Ni contents in the catalysts, measured by
inductively coupled plasma–atomic emission spectrome-
try (ICP-AES), are 11.91% and 10.80% for Ni-in-ANTs
and Ni-out-ANTs, respectively.
Figure 2. TEM images of A,B) Ni-in-ANTs and D,E) Ni-out-ANTs, and
C) HRTEM image of Ni-in-ANTs.
Selective hydrogenation of cinnamaldehyde (CA) was
chosen to evaluate the catalytic performance of Ni-based
catalysts. Figure 3A shows the evolution of CA conver-
sion with the reaction time up to 7 h for the catalysts. The
reaction was carried out at 808C and 2.0 MPa H2 with
10 mg of catalyst in 30 mL of isopropyl alcohol, and
100 mL of cinnamaldehyde. It can be observed that Ni-in-
ANTs exhibits a much higher activity than Ni-out-ANTs.
After reaction for 10 min, the conversion of CA on Ni-in-
ANTs reaches 4.2%, while the reaction product can
hardly be detected for Ni-out-ANTs. After reaction for
7 h, the conversion of CA on Ni-in-ANTs reaches nearly
100%; in comparison, only 17.8% conversion is obtained
on Ni-out-ANTs. Turnover frequency (TOF) is calculated
on the basis of surface Ni atoms, which is determined
according to the CO chemisorption (Table S1). The TOF
values at initial times of 60 min are 0.42 and 0.08 sÀ1 for
Ni-in-ANTs and Ni-out-ANTs, respectively. Hydrogena-
tion of CA to hydrocinnamaldehyde (HCA) is the main
reaction (Figure S2).
Figure 3. A) The evolution of CA conversion with reaction time and B) the
The catalysts were reused to test their stabilities recycling results for the catalysts in the hydrogenation of CA. TEM images
of Ni-in-ANTs C) after reaction for 10 min and D) after the fourth run; and
of Ni-out-ANTs E) after reaction for 10 min and F) after the fourth run.
(Figure 3B). The reaction time is 5 h for each run. Ni-in-
ANTs appears as relatively stable; a slight decrease in the
conversion appears after the catalyst was reused four
times. The conversion of Ni-out-ANTs decreases sharply, only
remaining 1.2% after being reused four times. HCA is the
main product during recycling test (Table S2). In general, the
gradual detachment of nanoparticles from support is a main
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reason responsible for the deactivation. After reaction
for 10 min, Ni nanoparticles are still supported by
Al2O3 nanotubes for both catalysts, indicating that no
obvious detachment has happened in such a short time
(Figure 3C,E). Note that this reveals that the differ-
ence of activity for the two catalysts is not essentially
ascribed to detachment. Most of Ni nanoparticles are
still confined in the Al2O3 nanotubes for Ni-in-ANTs
after the fourth run (Figure 3D). However, after the
fourth run, vast majority of Ni nanoparticles of Ni-out-
ANTs are detached, forming large aggregates (Fig-
ure 3F). Leaching is another reason for deactivation of
supported catalysts. The Ni contents in the reactant
solvent (isopropyl alcohol) after the reaction, mea-
sured by ICP-AES, are 0.39 and 1.35 ppm for Ni-in-
ANTs and Ni-out-ANTs, respectively. Based on the Ni
contents in solution, the Ni content in the re-used
catalysts (including the detached Ni particles) are
11.79% and 10.40% for Ni-in-ANTs and Ni-out-
ANTs, respectively. These results suggest that Ni-
out-ANTs underwent more leaching and much severer
detachment than Ni-in-ANTs.
Figure 4. A) H2-TPR profiles of NiO-in/out-ANTs; B) H2-TPD profiles of Ni-in/
out-ANTs; C) The normalized XANES of NiO-in/out-ANTs and reference
sample; D) The normalized XANES of Ni-in/out-ANTs and reference samples.
The inset in (C) is the amplified image of the pre-edge part of the XANES
Ni-in/out-ANTs using 50 and 300 cycles of NiO
ALD were also prepared (Figure S3). Similarly, Ni-in- spectra.
ANTs shows higher activities than Ni-out-ANTs
(Table S3). Ni-in-ANTs also presents much higher
activities for the hydrogenation of nitrobenzene to aniline
than Ni-out-ANTs (Table S4). Our ALD-assisted template
method is general. It is also suitable for other templates such
as CNTs (Figure S4).
The above results reveal that Ni-in-ANTs does possess
greatly improved catalytic performance than Ni-out-ANTs. In
order to elucidate the reasons, we performed further charac-
terizations for these catalysts. The N2 adsorption–desorption
isotherms of NiO-in-ANTs and NiO-out-ANTs are almost
overlapped (Figure S5), indicating that the two samples
almost possess the same pore structure. Brunauer–Emmett–
Teller (BET) surface areas of NiO-in-ANTs and NiO-out-
ANTs samples are calculated to be 65.0 and 63.6 m2 gÀ1,
respectively. XRD patterns of the two reduced samples reveal
the presence of metallic nickel after reduction (Figure S6).
CO chemisorption experiments were used to measure the
accessibility of Ni nanoparticles. The amount of CO adsorbed
by Ni-in-ANTs (110.7 mmolg-NiÀ1) is about 20% lower than
on Ni-out-ANTs (135.0 mmolg-NiÀ1) (Table S1, Figure S7).
This reveals that Ni-out-ANTs has a higher Ni accessibility
than Ni-in-ANTs.
hydrogen.[8] At the high-temperature domain, the consumed
hydrogen of NiO-in-ANTs is 1.7 times that of NiO-out-ANTs.
These results suggest that, compared with NiO-out-ANTs,
NiO-in-ANTs sample shows stronger NiO–Al2O3 interaction.
Hydrogen temperature programmed desorption (H2-
TPD) tests of Ni-in-ANTs and Ni-out-ANTs were carried
out (Figure 4B). Both the two TPD curves display two kinds
of desorption peaks, indicative of the existence of different
forms of surface hydrogen. The low-temperature desorption
peaks (cenetred at 958C) are ascribed to the hydrogen
adsorbed on the surface of metallic Ni, and the high-temper-
ature desorption peaks above 5008C (maxima at 6208C) are
assigned to spillover hydrogen.[9] The intensity of low-temper-
ature peak of Ni-in-ANTs is lower than that of Ni-out-ANTs,
indicating Ni-in-ANTs shows a lower exposed fraction of Ni
atom than that of Ni-out-ANTs, which is consistent with the
CO chemisorption results. However, the intensity of high-
temperature peak of Ni-in-ANTs is much higher than that of
Ni-out-ANTs. This indicates that the hydrogen spillover
effect for Ni-in-ANTs has been greatly enhanced, compared
with Ni-out-ANTs.
The redox properties of NiO-in-ANTs and NiO-out-
ANTs were compared through hydrogen temperature pro-
grammed reduction (H2-TPR) (Figure 4A). The profile of
NiO-in-ANTs displays a small shoulder peak centered at
3498C and a principal peak centered at 5618C, corresponding
to reduction of bulk NiO and the NiO interacting with Al2O3,
respectively. In contrast, for NiO-out-ANTs, the low-temper-
ature peak at 3948C becomes obvious, while the high-
temperature peak at 5468C shrinks. Quantification of the
TPR curves shows that the total consumed hydrogen of NiO-
in-ANTs (1.93 mmolH2 gÀ1) exceeds that of NiO-out-ANTs
(1.46 mmolH2 gÀ1), which can be attributed to spillover
X-ray absorption fine structure (XAFS) measurements
were also employed to characterize the samples. From the X-
ray absorption near-edge structure (XANES) spectra of NiO-
in-ANTs and NiO-out-ANTs, it can be found that the
intensity of the white line peak for NiO-in-ANTs is lower
than that of NiO-out-ANTs (Figure 4C), and the intensity of
the pre-edge absorption peak of NiO-in-ANTs is higher than
that of NiO-out-ANTs (inset of Figure 4C). The k2-weighted
Fourier transformed-extended X-ray absorption fine struc-
ture (FT-EXAFS) spectra of the two samples are presented in
Figure S8A. The FT-EXAFS spectra on the first shell were
fitted using NiO and NiAl2O4 as model compounds (Fig-
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ure S8C,E). For NiO-in-ANTs, the ratio of the first peak
intensity to the second peak intensity is larger than that of
NiO-out-ANTs. This suggests that the interaction between
NiO and Al2O3 of NiO-in-ANTs is stronger than that of NiO-
out-ANTs, resulting in more surface NiAl2O4 species (Fig-
ure S6). For NiAl2O4, the first peak is stronger than the
second one.[10] The FT-EXAFS curve-fitting results (Table S5)
further confirm this conclusion, which is also consistent with
the H2-TPR results.
From the XANES spectra of the reduced samples (Fig-
ure 4D), it can be seen that the Ni species were not fully
reduced for the catalysts. The XANES spectra were simulated
by a linear function of the references of the reduced (Ni foil)
and oxidic (NiO) state to estimate the proportions of metallic
Ni in the reduced samples (Figure S9). The XANES spectra
can be reproduced by a simple sum of Ni foil and NiO
references. The reduction degree of nickel for Ni-in-ANTs
and Ni-out-ANTs are 80% and 77%, respectively, approx-
imately equal (Table S6). Their k3-weighted FT-EXAFS
spectra are presented in Figure S8B. The FT-EXAFS spectra
on the first shell can be fitted well using NiO and Ni as model
compounds (Figure S8D,F, and Table S7).
From all above characterizations, it can be concluded that
Ni-in-ANTs and Ni-out-ANTs almost possess the same pore
structure, thickness of Al2O3 shell and size of Ni nano-
particles, nickel content, and reduction degree of nickel.
However, Ni-in-ANTs shows greatly enhanced catalytic
performance compared with Ni-out-ANTs. This can be
ascribed to the increased interfacial sites and protecting
nanotubes.
Figure 5. Catalytic performance of the over-coated Ni-out-ANTs cata-
lysts. A) CA conversion obtained after 1 h of CA hydrogenation reaction
for over-coated catalysts; B) the evolution of CA conversion with the
reaction time; C) the recycling results for 5-Al2O3-Ni-out-ANTs ; and
TEM images of 5-Al2O3-Ni-out-ANTs (D) before and (E) after four runs.
First, from the H2-TPR and XAFS results of NiO-in-
ANTs and NiO-out-ANTs, it can be concluded that the
interaction between NiO and Al2O3 of NiO-in-ANTs is
stronger than that of NiO-out-ANTs. This is due to the larger
interface of NiO-in-ANTs than that of NiO-out-ANTs. For
the reduced samples, the CO chemisorption and H2-TPD
results suggest that Ni-out-ANTs show a higher Ni accessi-
bility than Ni-in-ANTs, consistent with the fact that Ni
nanoparticles are confined in the cavities of Al2O3 interior
wall for Ni-in-ANTs (Figure 2C). The confinement of the
cavities of Al2O3 interior wall creates more Ni–Al2O3
interfacial sites. The spillover of the dissociated hydrogen
species is highly dependent on the metal–support interface.[11]
The hydrogen spillover effect for Ni-in-ANTs has been
greatly enhanced, as is confirmed by H2-TPD analysis. It is
well known that hydrogen spillover can exert a great influence
on the catalytic activity in hydrogenation reactions.[12] In
a word, the greatly improved catalytic activity of Ni-in-ANTs
can be ascribed to its increased interfacial sites, which
enhance the hydrogen spillover effect (Figure S10). To further
clarify the role of metal–support interface, we investigated the
catalytic performance of over-coated catalysts prepared by
coating Ni-out-ANTs with different Al2O3 cycles by ALD
(Figure 5A). X-ray photoelectron spectrum results clearly
demonstrate the gradual coverage of Ni nanoparticles by
ALD Al2O3 over-coats (Table S8). For the first few cycles of
ALD, Al2O3 would deposit preferentially onto specific sites,
rather than uniformly blanketing the particles entirely.[13] The
inverse catalysts possess larger metal–support interface. At
the same time, the access of reagents to the embedded Ni
nanoparticles is also maintained. Therefore, the inverse
catalysts with Al2O3 over-coatings in the range of 3–30
cycles show higher activities than the uncoated Ni-out-ANTs.
The sample coated with 5 cycles of ALD Al2O3 (5-Al2O3-Ni-
out-ANTs) exhibits the highest activity (Figure 5B). There is
no visible morphological change for the coated nanoparticles
resulted from the ultrathin coating (Figure 5D). With further
increasing cycle numbers of ALD Al2O3 (over 30), the
particles would be completely encapsulated by Al2O3, result-
ing in decreased catalytic activity, even if such system has
larger metal–support interface. This further confirms that the
increased Ni–Al2O3 interfacial sites are responsible for the
greatly enhanced catalytic activity.
Second, from the TEM analysis of the used catalysts
(Figure 3C–F), and the Ni contents in the reactant solvent
after the reaction, it can be known that the tubular channel
structure of the confined catalysts can inhibit the leaching and
detachment of Ni nanoparticles of Ni-in-ANTs, consistent
with previous results reported in the literature.[14] As leaching
and detachment of Ni nanoparticles will dramatically reduce
active surface areas, Ni-in-ANTs with protecting Al2O3 shells
exhibit dramatically improved catalytic stability than Ni-out-
ANTs. To further demonstrate the protective role of the
Al2O3 shells, 5-Al2O3-Ni-out-ANTs sample was reused to test
its stability (Figure 5C). The conversion of 5-Al2O3-Ni-out-
ANTs decreases sharply. Similar to Ni-out-ANTs, vast
majority of Ni nanoparticles of 5-Al2O3-Ni-out-ANTs are
also detached (Figure 5E). The Ni contents in the reactant
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Van Duyne, P. C. Stair, J. M. Notestein, Nat. Chem. 2012, 4,
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solvent after the reaction, measured by ICP-AES, are
1.29 ppm for 5-Al2O3-Ni-out-ANTs. Without the sufficiently
thick, protective Al2O3 shells, the ultrathin-coated catalysts
still undergo severe leaching and detachment.
In conclusion, we have demonstrated a facile approach to
fabricate a multiply confined Ni-based catalyst with excellent
catalytic performance in cinnamaldehyde and nitrobenzene
hydrogenation reactions. The Ni nanoparticles are not only
confined in Al2O3 nanotubes, but also embedded in the
cavities of Al2O3 interior wall. Our ALD-assisted template
method is general,[15] and can easily be extended for other
multiply confined nanoreactors, which may have potential
applications in many heterogeneous reactions.
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Keywords: atomic layer deposition · hydrogen spillover ·
hydrogenation reaction · interface · multiply confined catalyst
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Received: April 24, 2015
Revised: May 25, 2015
Published online: July 6, 2015
9010 www.angewandte.org
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Angew. Chem. Int. Ed. 2015, 54, 9006 –9010