A Ni@ZrO2 nanocomposite for ethanol steam reforming: Enhanced stability via strong metal-oxide interaction

Article abstract of DOI:10.1039/c2cc37109j

This communication describes the synthesis of a nanocomposite Ni@ZrO ₂ catalyst with enhanced metal-support interaction by introducing metal nanoparticles into the framework of the oxide support. The catalyst shows high catalytic activity and stability for hydrogen production via steam reforming of ethanol.

Full text of DOI:10.1039/c2cc37109j

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A Ni@ZrO2 nanocomposite for ethanol steam reforming: enhanced
stability via strong metal–oxide interactionwz
Shuirong Li, Chengxi Zhang, Zhiqi Huang, Gaowei Wu and Jinlong Gong*
Received 30th September 2012, Accepted 22nd October 2012
DOI: 10.1039/c2cc37109j
This communication describes the synthesis of a nanocomposite
Ni@ZrO2 catalyst with enhanced metal–support interaction by
introducing metal nanoparticles into the framework of the oxide
support. The catalyst shows high catalytic activity and stability
for hydrogen production via steam reforming of ethanol.
acidity11 and surface oxygen mobility.12 Xu et al. proposed a
‘‘size effect’’ of the zirconia support and found that the reduction
in particle size of zirconia would result in increased surface area,
metal dispersion, as well as strengthened metal–support inter-
action; these features would thus enhance catalytic performance
of nickel particles supported on ZrO2 in dry reforming of
methane.13 However, for catalysts prepared by conventional
impregnation, metal particles were set on a bulk matrix composite
of substrate oxide particles with fixed-frame structure
(Fig. 1a). Under heat treatment, metal particles would suffer
from sintering and grow up readily. Ideally, if frame structure
of the catalyst is composed of comparable nanosized metal
particles and support oxide particles (Fig. 1b), the metal
particles would help in the construction of the pore structure
instead of blocking it resulting in an increased surface area.
Since the amount of substrate oxide particles is dominant in
the catalyst, each metal nanoparticle would be confined by
several surrounding oxide particles. Accordingly, the metal–support
interaction in the catalyst could be strengthened with increased
interfacial area, expectedly leading to improved catalytic
performance of the catalyst in the reforming process.14
Hydrogen production from renewable resources represents a hot
research topic of the last few decades.1 Supported nickel catalysts
are industrially used for hydrogenation and steam reforming, and
have been investigated intensively for many years. Due to their
high activity in C–C bond rupture and low cost, nickel catalysts
have been also proposed recently as promising candidates in
ethanol steam reforming (ESR) for hydrogen production.
However, deactivation with time on stream primarily caused by
sintering and coke deposition remains an unsolved drawback of
this type of catalysts in reforming processes including ESR.2
Stabilization of nickel nanoparticles by solid-phase crystal-
lization and enhancing the removal of surface carbon deposits
by the introduction of oxides with high oxygen mobility are
two typical approaches for the improvement of catalytic
stability of supported catalysts in ESR.3 Nickel catalysts
prepared from crystalline oxide precursors, such as hydrotalcite
compounds,4 perovskite,5 and spinel,6 show enhanced metal
dispersion and metal–support interaction; this, in turn, improves
catalytic activity and stability in ESR. Oxides with high oxygen
mobility—typically rare-earth metal oxides—were reported to
be beneficial for the activation of steam as well as the removal of
surface carbon deposits in reforming.7 Recent attention has been
paid to the removal of surface carbon deposit on catalysts derived
from crystalline oxide precursors.8 However, limited work was
reported regarding the improvement of sintering resistance of
catalysts supported on oxides with high surface oxygen mobility.9
Zirconia is widely used as a catalytic support for its
amphoteric character as well as thermal and chemical stability.10
It is also favored in reforming processes for its moderate
To prepare such a catalyst, nanosized metal particles or their
precursors are needed during the construction of a substrate
matrix. Considering that metallic Ni is prone to oxidization in
air and to avoid the use of hydrazine as a reduction reagent,15
NiO nanoparticles were used in the preparation of Ni@ZrO2
nanocomposites. NiO nanoparticles with a particle size of about
3.3 ꢀ 0.7 nm (Table 1, Fig. S1, ESIz) were prepared by a
surfactant-assisted route16 while the Ni@ZrO2 nanocomposite
(15 wt% Ni loading) was synthesized by a modified hydro-
thermal method in the presence of as-prepared NiO
Key Laboratory for Green Chemical Technology of Ministry of
Education, School of Chemical Engineering and Technology, Tianjin
University, Tianjin 300072, China. E-mail: jlgong@tju.edu.cn;
Fax: +86 22 87401818
w This article is part of the ChemComm ‘Emerging Investigators 2013’
themed issue.
z Electronic supplementary information (ESI) available: Experimental
details and physico-chemical properties of supported nickel catalysts.
See DOI: 10.1039/c2cc37109j
Fig. 1 Schematic illustration of (a) conventional supported metal
catalyst, and (b) nanoconfined metal–oxide composite.
c
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Table 1 Physico-chemical properties of nickel catalysts
NiO nanoparticles.17 The domination of t-ZrO2 could partially
contribute to the higher surface area of the Ni@ZrO2 catalyst.10
However, the surface area of ZrO2 prepared by the precipitation
Particle sizea
SBET
/
VPore
/
DPore
/
Disperꢁsi1onb/
ZrO2 m2 gNi
m2 gꢁ1 cm3 gꢁ1 nm
Ni
method is generally lower than 50 m2 gꢁ1 18
It is noticed that
.
Sample
both catalysts prepared using a similar method showed a similar
pore diameter while the Ni@ZrO2 catalyst possesses a much
larger volume of the mesopore structure compared to the
Ni/ZrO2 catalyst. This could also be the evidence of the formation
of designed structure of the Ni@ZrO2 nanocomposite, in
which NiO nanoparticles substitute part of ZrO2 particles in
the matrix instead of blocking its pore structures.
NiO
ZrO2
Ni/ZrO2
Ni@ZrO2
210
47
40
91
0.47
0.11
0.15
0.33
7.6
6.7
11.3
11.2
N.A. N.A. N.A.
N.A. 13.0
N.A.
1.7
7.0
23.7
10.5
15.7
9.2
a
Calculated from the peak broadenings of Ni(111), m-ZrO2(111), and
b
t-ZrO2(011) using the Scherrer equation, respectively. Calculated
from the H2 pulse chemisorption.
The formation of proposed structure is further evidenced by
electron microscopy analysis. For the supported Ni/ZrO2
catalyst (Fig. 2c), particles with size ranging from 16 to 50 nm
were distributed both at the edge and the inside of another
aggregation of small particles (about 15 nm). EDX images of
selected areas of the sample (Fig. S1, ESIz) attribute the large
particles to nickel, which was further confirmed by the STEM
analysis (see the inset in Fig. 2c). As for the nanocomposite
Ni@ZrO2 catalyst, only a mixture of comparably nanosized
particles was observed; STEM images (the inset in Fig. 2d) of
selected area show that small nickel particles were uniformly
distributed within the ZrO2 matrix.
nanoparticles during the gelation of the ZrO2 precursor. For
comparison, the supported Ni/ZrO2 catalyst (15 wt%) was also
fabricated by a conventional impregnation method (see ESIz
for details). Both of the catalysts were activated in a reducing
stream of H2/N2 (10 ml/40 ml) at 773 K for 1 h prior to catalytic
activity tests.
Physico-chemical properties of the as synthesized nickel
catalysts are listed in Table 1 and Fig. 2. Physisorption of
N2 showed that the specific surface area of the Ni@ZrO2
catalyst is more than twice larger than that of the conventionally
supported one. The formation of different crystalline phases of
zirconia as indicated in their XRD patterns (Fig. 2a) could
account for this disparity.10 Peaks at 28.21 and 31.51 indicate the
domination of monoclinic zirconia (m-ZrO2) in the pure ZrO2
and the supported Ni/ZrO2 catalyst, while the peak at 30.31
indicates the presence of a metastable phase of zirconia in
Ni@ZrO2. Although XRD could not effectively distinguish
tetragonal zirconia (t-ZrO2) from cubic zirconia (c-ZrO2),
t-ZrO2 rather than c-ZrO2 was believed to be stabilized by the
possible presence of Ni2+ during the precipitation of the ZrO2
precursor with the coexistence of the ammonia precipitator and
The surface interaction between NiO and ZrO2 of the
catalysts was evaluated by H2-TPR experiments (Fig. 2b). A
reduction peak at 579 K and a shoulder peak at 693 K were
observed for the supported Ni/ZrO2 catalyst, whereas only an
intense reduction peak at about 723 K was detected for the
nanocomposite Ni@ZrO2 catalyst. Concerning that the formation
of strong chemical bonds between the Ni metal and the ZrO2
has been revealed based on a DFT calculation by Beltran,19
the reduction peak at 579 K was attributed to the reduction of
bulk NiO20 while the reduction peaks at 693–723 K were
related to NiOx species that have strong interaction with
ZrO2. The strengthened interaction between the nickel metal
and the zirconia could be probably due to an increased
interfacial area.19
We should note that a high metal dispersion was also
obtained over the nanocomposite Ni@ZrO2 catalyst. This
could be directly evidenced by the TEM/STEM images of
the reduced nickel catalysts, in which more uniform and
smaller particles of the nickel metal were observed over the
Ni@ZrO2 catalyst compared to Ni/ZrO2 (Fig. 2c and d).
Average Ni particle sizes of 10.5 nm and 23.7 nm for Ni@ZrO2
and Ni/ZrO2 calculated based on the Ni(111) diffraction peaks
(Table 1) also confirm this observation. For quantitative
analysis, H2-TPD experiments (see ESIz for details) were also
performed over the nickel ꢁca1talysts. The results showed a metal
surface area of 7.0 m2 gNi for the nanocomposite Ni@ZrO2
catalyst, which is four times more than that of the supported
one (Table 1).
According to the ESR mechanism, the metal activates the
organic molecules and promotes their reaction with OH
groups provided by water dissociation on the support.21
A
Fig. 2 (a) XRD patterns of as prepared NiO, ZrO2, reduced Ni/
ZrO2, and Ni@ZrO2. (b) H2-TPR profiles of NiO, Ni/ZrO2 and
Ni@ZrO2 catalysts. TEM images of (c) reduced Ni/ZrO2 and (d)
reduced Ni@ZrO2. The insets in (c) and (d) correspond high-angle
annular dark field (HAADF) STEM images of the reduced nickel
catalysts.
catalyst with higher metal dispersion and metal–support interfacial
area is thus expected to offer a better performance in ESR.
A relatively low temperature (i.e. 723 K) was selected for catalytic
activity tests of the nickel catalysts. Varying the gas hour space
velocity (GHSV) would allow us through the changes in conversion
c
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particles. Particularly, the size match of the metal and oxide
could increase the metal–oxide interface length and strengthen
their interaction. Additionally, the confinement effect could
effectively prevent metal particles from sintering. The methodology
reported here may be useful for designing other types of metal
catalysts that are prone to deactivation due to sintering and/or
coke deposition under realistic conditions for reactions such as
reforming, methanation, and dehydrogenation.
We thank the National Natural Science Foundation of
China (21006068, 21206115, 21222604), the Program for
New Century Excellent Talents in University (NCET-10-0611),
the Scientific Research Foundation for the Returned Overseas
Chinese Scholars (MoE), Seed Foundation of Tianjin
University (60303002), the Program of Introducing Talents
of Discipline to Universities (B06006), and China Postdoctoral
Science Foundation for their generous support.
Fig. 3 (a) Catalytic activity at 723 K. Solid symbols: Ni@ZrO2
catalyst; hollow symbols: Ni/ZrO2 catalyst. (b) Catalytic stability at
873 K. The insets are TEM images of the used catalysts.
of ethanol and selectivity of the products to compare the catalytic
activities of the synthesized nickel catalysts. As shown in Fig. 3a,
the decrease in ethanol conversion as well as the emergence of C2+
products (mainly acetaldehyde and a trace amount of acetone) with
an increase in GHSV could be clues of insufficient catalytic activity
of the conventional supported Ni/ZrO2 catalyst for C–H and C–C
bonds cleavage under given conditions.2 Comparatively, a similar
phenomenon was only detected at higher GHSV for the nano-
composite Ni@ZrO2 catalyst, indicating a higher activity towards
ethanol activation and conversion into C1 species on its surface. In
addition, the Ni@ZrO2 catalyst shows a higher selectivity towards
H2 and CO2 production, which could be partially attributed to a
higher WGSR activity promoted by the stabilized tetragonal
zirconia22 as well as the prolonged metal–support interface.14
The catalytic stability of the nickel catalysts was also
investigated as a function of time on stream over a period of
50 h, at 873 K and a GHSV of 50 000 hꢁ1, as depicted in
Fig. 3b. The nanocomposite Ni@ZrO2 catalyst showed a
nearly complete conversion of ethanol during the entire testing
period, whereas a continual decrease in ethanol conversion
after 6 h of reaction was observed over the supported Ni/ZrO2
catalyst. Two major factors could contribute to the excellent
stability of the Ni@ZrO2 catalyst: (i) the geometric confinement
of the surrounding comparable nanosized zirconia particles and
strong interaction between the nickel metal and zirconia that
prevent the nickel metal from sintering,23 and (ii) the richness
of surface active oxygen24 and prolonged metal–support interfacial
perimeter that help in the removal of carbon deposits.25 Ni
particle size of the used Ni@ZrO2 is nearly intact (i.e. 10.8 nm)
based on the calculation using the Scherrer formula from its
XRD pattern (Fig. S2a, ESIz), while an apparent increase in
nickel size (i.e. 25.4 nm) was found for the used Ni/ZrO2
catalyst. TEM images (the inset of Fig. 3b and Fig. S2c and
S2d in ESIz) exhibit that only some amorphous carbon was
formed over the used Ni@ZrO2, while a large amount of
carbon whiskers was observed for the used Ni/ZrO2 catalyst.
TG measurements performed upon the stability tests
(Fig. S2b, ESIz) indicate weight losses of 16.9% and 33.7%
for the used Ni@ZrO2 and Ni/ZrO2 catalysts, respectively.
In summary, we have demonstrated the successful design of
a novel nickel–zirconia nanocomposite for hydrogen production
via steam reforming of ethanol. The introduction of nickel
particles into the framework of an oxide support with high
oxygen mobility could effectively maintain the pore structure of
the oxide support and increase the accessibility of the metal
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Chem. Commun.