Controlled formation of nickel oxide nanoparticles on mesoporous silica using molecular Ni4O4 clusters as precursors: Enhanced catalytic performance for dry reforming of methane

Article abstract of DOI:10.1002/cctc.201402983

Supported NiO nanoparticles (NPs) were prepared by impregnation of mesoporous silica SBA-15 with a molecular, metalorganic [Ni₄O₄] cubane cluster as precursor. By using this ligand-stabilized Ni cluster, deposition of four Ni ions in

Full text of DOI:10.1002/cctc.201402983

DOI: 10.1002/cctc.201402983
Communications
Controlled Formation of Nickel Oxide Nanoparticles on
Mesoporous Silica using Molecular Ni4O4 Clusters as
Precursors: Enhanced Catalytic Performance for Dry
Reforming of Methane
Elham Baktash,[a] Patrick Littlewood,[b] Johannes Pfrommer,[c] Reinhard Schomꢀcker,[b]
Matthias Driess,[c] and Arne Thomas*[a]
Supported NiO nanoparticles (NPs) were prepared by impreg-
nation of mesoporous silica SBA-15 with a molecular, metalor-
ganic [Ni4O4] cubane cluster as precursor. By using this ligand-
stabilized Ni cluster, deposition of four Ni ions in close proximi-
ty on the silica support was achieved; this resulted in the for-
mation of small and highly dispersed NiO NPs after heat treat-
ment. These clusters were shown to have a significant influ-
ence on the formation of NiO NPs compared to a conventional
Ni(OAc)2 precursor. After a further reduction step, the materials
were used as catalysts for the dry reforming of methane,
which showed that preorganization of the Ni atoms on the
support surface had a beneficial effect on the methane conver-
sion rate.
high surface area support and, thus, are stable against sinter-
ing.[2,3] Stabilization of small Ni nanoparticles (NPs) is also im-
portant to prevent coking of the catalysts, which is a severe
side reaction observed on larger Ni particles. Different synthet-
ic approaches such as impregnation of high surface area sup-
ports with solutions of Ni salts[4–9] or the preparation of Ni-
based solid solutions[10–12] or Ni-containing perovskites[13–15] and
subsequent reduction to form the Ni0 NPs have been reported
to prepare such materials. If Ni salts are used as precursors in
the first step, NiO nanoparticles are formed on the support
during heat treatment under an atmosphere of air or oxygen,
and it can be assumed that the size and dispersion of these
NiO NPs will have a direct impact on the finally observed parti-
cle size and dispersion of the catalytically active Ni0 species.
The size and dispersion of metal-oxide particles formed from
impregnated metal-salt precursors is, however, difficult to pre-
dict and control and crucially depends on several factors such
as the precursor type and concentration, the heat treatment,
and the surface chemistry of the support. An interesting
option to gain better control of metal-oxide NP formation
could be the use of small molecular metal-oxo clusters instead
of mononuclear metal salts, that is, to start from molecular pre-
cursors, in which a defined amount of metal atoms is already
located in close proximity.[16–18] These single source precursors
(SSP), for which all desired elements of the target material are
preorganized in a metal organic compound, are therefore intri-
guing starting materials for the preparation of supported
metal-oxide catalysts. The herein applied “Ni4 cubane” SSP is
a tetranuclear complex prepared from Ni(OAc)2 and di-2-pyridi-
nylmethanone [(C5H4N)2CO, dpk]; dpk is hydrolyzed into the di-
2-pyridinylmethanediolato [(C5H4N)2C(OH)2, dpd] ligand in
aqueous solution.[16—19]. The preparation of small quantities of
the cubanoid metal cluster [m-(acetato-kO:kO’)]bis(acetato-kO)-
tetrakis{m3-[di(2-pyridinyl-kN)methanediolato-kO:kO:kO]}tetra-
Nickelperchloratehydrate (Ni4) bearing the triscoordinate di-2-
pyridinylmethanediolato ligand was described by several
groups.[18,19] For this work, we adapted the synthetic scheme
to obtain larger quantities of the material on the basis of our
preliminary studies of the respective Co and Zn compounds.[17]
The core of this complex consists of a [Ni4O4] cubane, as
shown in Figure 1, which can also be described as one of the
smallest possible ligand-stabilized NiO nanoparticles. Attaching
these cubane structures to the surface of a high-surface area
support should, after calcination, yield four Ni atoms in close
Methane is the principal component of most natural gas re-
serves and thus one of the major sources of energy for homes
and industry. However, many of these reserves are found in re-
gions far from industrial complexes; therefore, much of the
methane is just flared or vented, a process that wastes hydro-
carbon resources and causes severe environmental problems.
These concerns and the increasing price of oil have led to
global investigation for the conversion of methane into easy
transportable products through processes such as dry reform-
ing of methane (DRM, CH4 +CO2!2H2 +2CO). Syngas, the
product of the DRM reaction can be converted into higher hy-
drocarbons or fuels through Fischer–Tropsch synthesis.[1] Nickel
is known to be an efficient non-noble metal catalyst for the
DRM reaction. For example, high and stable conversions can
be reached if very small Ni particles are dispersed on a robust,
[a] Dr. E. Baktash, Prof. A. Thomas
Functional Materials, Department of Chemistry
Technische Universitꢀt Berlin
Hardenbergstrasse 40, 10623 Berlin (Germany)
E-mail: arne.thomas@tu-berlin.de
[b] P. Littlewood, Prof. R. Schomꢀcker
Chemical Reaction Engineering, Department of Chemistry
Technische Universitꢀt Berlin
Strasse des 17. Juni 124, 10623 Berlin (Germany)
[c] J. Pfrommer, Prof. M. Driess
Metalorganic and Inorganic materials, Department of Chemistry
Technische Universitꢀt Berlin
Strasse des 17. Juni 124, 10623 Berlin (Germany)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402983.
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impregnation and calcination (Figures S1 and S2, Supporting
Information). The patterns show the characteristic (100), (110),
and (200) peaks, which can be assigned to the hexagonal
pore structure of SBA-15;[20] this shows that the ordered meso-
porous structure has been maintained during formation of the
metal-oxide phase. The wide-angle XRD patterns of the Ni4/
SBA-15 and Ni/SBA-15 samples after calcination but before re-
duction are presented in Figures S3 and S4. The characteristic
reflexes for NiO at 2q=37.2, 43.1, and 62.88, corresponding to
the (111), (200), and (22 0) planes of cubic NiO, respective-
ly,[4,21] are observed only for the 5-Ni4/SBA sample even
though the actual Ni content is smaller (1.67 wt%) than that,
for example, in the 5-Ni/SBA sample (2.53 wt%). Still, the ob-
served reflexes are very weak and broad, which points to the
formation of very small NiO NPs. For samples with lower load-
ings, hardly any crystal phase line can be recognized, which
might be a sign of the formation of small, highly dispersed Ni
species. Nitrogen physisorption measurements were performed
for the calcined materials (Figure S5). Corresponding isotherms
show the characteristic type IV isotherm with H1-type hystere-
sis loop for mesoporous materials, which again shows that the
mesoporous structure of SBA-15 was maintained after impreg-
nation and calcination. Figures 2 and S6 show the TEM images
of the catalysts prepared from Ni4 cubane and nickel acetate
precursors, respectively. Formation of few isolated metal-oxide
particles can be observed for the lowest concentration in the
Ni4/SBA-15 series, whereas for Ni/SBA-15, metal-oxide NPs can
be spotted only for the highest concentration (5-Ni/SBA-15).
However, only for 5-Ni4/SBA-15 is a larger amount of NiO
nanoparticles seen, and these NPs are highly dispersed on the
silica support, which also supports the results obtained from
the XRD measurements.
Figure 1. Schematic illustration of the Ni4/SBA-15 catalyst prepared by using
a ligand-stabilized Ni4 cubane precursor.
proximity on the support surface (see Figure 1), which are then
able to form very small Ni clusters upon reduction, possibly to
yield an efficient catalyst for the dry reforming of methane. To
investigate a possible beneficial influence of the preorganiza-
tion of Ni atoms, two series of materials were prepared in this
work. For the first series, a mesoporous silica support (SBA-15)
was impregnated with an aqueous solution of the Ni4 cubane
precursor (“A-Ni4/SBA-15”). For comparison, in a second series
a conventional nickel acetate precursor was used in the same
way (“A-Ni/SBA-15”, A=initial wt% of Ni). The concentration of
the precursors in the aqueous solution was varied to investi-
gate the effect of nickel loading on the structure and activity
of the catalysts. All obtained materials were treated likewise;
thus, first dried at room temperature, then calcined at 5508C
for 4 h, and finally subjected to an in situ reducing atmosphere
(pure H2) at 5008C for 1 h before catalytic tests. The actual
metal contents of the samples were determined after the calci-
nation step by inductively coupled plasma (ICP) and are pre-
sented in Table 1. In almost all cases, the actual Ni content is
lower than the nominal one, probably as a result of a washing
All of the prepared materials were applied as cata-
lysts for the DRM reaction. Figures 3 and 4 show
Table 1. Characterization and test results of the Ni4/SBA-15 and Ni/SBA-15 catalysts.
methane conversion over Ni4/SBA-15 and Ni/SBA-15
after an in situ reduction step at temperatures of 500,
525, 550, and 6008C. The theoretical thermodynamic
equilibrium was also calculated by minimizing the
Gibbs free energy of all species in the system (Fig-
ure S7). For the Ni4/SBA-15 materials at 500 and
5258C, the activity is proportional to the amount of
nickel in the catalyst. It can be seen that 5-Ni4/SBA is
an exceptionally good catalyst for the DRM reaction,
as thermodynamic equilibrium is reached at a low
temperature (5008C), and it also shows stable per-
formance at a higher temperature (6008C) although
equilibrium effects should be noted. Moreover, the
[a]
Sample
Ni
Activity[b] [molCH molNiꢀ1 minꢀ1
]
4
[wt%]
5008C
5508C
6008C
1-Ni4/SBA-15
2-Ni4/SBA-15
5-Ni4/SBA-15
1-Ni/SBA-15
2-Ni/SBA-15
5-Ni/SBA-15
0.52
0.81
1.67
0.51
0.97
2.53
3.2
3.5
3.1
3.9
3.0
1.4
6.0
5.5
3.8
3.9
5.0
2.6
13.9
9.7
5.3
–
8.3
3.4
[a] Metal content from ICP measurements. [b] Reaction rate based on mole CH4 con-
verted per mole Ni per time on stream.
step after impregnation. Notably, variations in the actual Ni
content do not allow for straightforward comparison of the
Ni4 and Ni catalysts prepared from the same initial amount of
Ni. Therefore, in a first step the Ni4/SBA-15 and Ni/SBA-15
series are discussed separately by regarding their catalytic per-
formance in the DRM, whereas a comparison between the two
series is made only on the basis of the actual Ni amount evalu-
ated by ICP. Materials in both series were studied by X-ray dif-
fraction (XRD) measurements. The powder XRD pattern at low
angle provides information about the support structure after
other Ni4/SBA-15 catalysts show initially lower performance
and stability; however, they seem to be further activated at
higher temperatures, especially the 1-Ni4/SBA-15 catalyst. It
can, therefore, be concluded that a certain Ni cluster size has
to be reached to show sufficient catalytic performance in the
DRM and that for the catalysts with highly dispersed Ni parti-
cles a certain temperature is needed to form this critical size
by particle sintering. As seen from the TEM pictures, a consider-
able amount of particles is formed in 5-Ni4/SBA-15, which ex-
plains its high catalytic activity at low temperatures. For the
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Figure 4. Catalysis results of 1-Ni/SBA-15, 2-Ni/SBA-15, and 5-Ni/SBA-15
based on methane conversion; GHSV=36 Lhꢀ1 gꢀ1, CH4:CO2:He = 1:1:8.
From these data, a first assumption can already be made,
and that is that the preorganization of Ni in the Ni4/SBA cata-
lysts has a beneficial effect on the catalytic performance, as ex-
emplified by the methane conversions of the 1-Ni4/SBA-15 and
1-Ni/SBA-15 catalysts, which contain similar Ni amounts, as
seen from the ICP measurements. Also, it can be noted that 5-
Ni4/SBA shows a slightly higher methane conversion than 5-
Ni/SBA, even though the Ni content is much lower in this cata-
lyst (1.67 vs. 2.53 wt%, Table 1). With the exclusion of 1% cata-
lysts, Ni4/SBA-15 also produces higher H2/CO ratios, and 5%
Ni4/SBA-15 is the most selective; furthermore, there is a general
trend between activity and H2/CO ratio (Table S1). To compare
the two catalyst series beyond the qualitative assumptions
made above, the reaction rates were calculated as mole meth-
ane conversion per mole Ni atom per minute (Figure 5,
Figure 2. TEM images of a,b) 1-Ni4/SBA-15, c,d) 2-Ni4/SBA-15, and e,f) 5-Ni4/
SBA-15.
Figure 3. Catalysis results of 1-Ni4/SBA-15, 2-Ni4/SBA-15, and 5-Ni4/SBA-15
based on methane conversion; gas hourly space velocity
(GHSV)=36 Lhꢀ1 gꢀ1, CH4/CO2/He=1:1:8. Eq points show the theoretical
thermodynamic equilibrium for the DRM reaction at each temperature.
Figure 5. Catalysis results based on mole methane converted per mole
nickel per minute.
Ni/SBA-15 series, a similar trend is observed. However, only the
catalysts with higher amounts of Ni show sufficient activity for
the DRM reaction. 2-Ni/SBA-15 and 5-Ni/SBA-15 show stable
performance even though the thermodynamic equilibrium is
not fully reached (Figure 4).
Table 1). The rate for each catalyst was calculated by averaging
over all the activity values at each ramp, excluding the first
point. The catalysts with the highest loadings showed the
lowest rate per Ni content in both the Ni4/SBA-15 and the Ni/
SBA-15 series. This can be expected, as larger particles are ob-
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served in these catalysts, and it can be assumed that only sur-
face Ni atoms take part in the catalytic reaction. Moreover,
a considerable deposition of carbon is found for catalysts with
higher Ni contents, which can also cause lower accessibility of
the Ni atoms and, thus, again lower the reaction rate
(Table S2). For the same reasons, the catalysts with lower Ni
contents show higher rates per Ni atom, especially the 1-Ni4/
SBA-15 catalyst. The rate of the reaction with the use of the
Ni4/SBA-15 series, especially at elevated temperatures, is much
higher than the rate for the Ni/SBA-15 series, which proves the
significant effect of using a preorganized Ni precursor, as it
more quickly forms small but highly dispersed NiO and later Ni
clusters. The heteroleptic nature of the Ni4 complex cation
allows the labile acetato ligands to be easily replaced by other
ligands. This is observed by electrospray ionization mass spec-
trometry (ESI-MS) experiments (see Figure S8), for which the
acetato ligands were replaced by formiato ligands from the
buffer solution and a twofold positively charged ion is ob-
served at m/z= 564 [C46H38N8O12Ni4]2+. The cubanoid cluster
core, however, stays intact, as the four di-2-pyridinylmethane-
diolato ligands are not as easily removed (see thermogravime-
try/differential thermal analysis experiment Figure S9). This
would allow for the deposition of individual Ni4 cluster cations
on SBA-15 by simply replacing the acetato ligands, which
would leave the predefined geometry of four Ni atoms intact.
It can, thus, be assumed that a certain metal cluster size has to
be reached to yield high activity for methane conversion. The
Ni4 cubane precursor seems to combine two advantages by
first allowing the fast formation of small Ni clusters, which are,
however, highly dispersed on the support and, thus, quite
stable during the catalytic reaction.
Experimental Section
Synthesis of the Ni4 complex
Ni(OAc)2·4H2O (2.488 g, 10 mmol) was dissolved in H2O (50 mL) in
a 100 mL Schlenk flask, and the mixture was stirred at 508C under
a N2 atmosphere. Upon complete dissolution of the starting mate-
rial, di-2-pyridinylmethanone (dpk; 0.911 g, 5 mmol) in acetonitrile
(10 mL) was added dropwise to the solution over 15 min. The color
of the solution changed from light green to dark green over the
course of the reaction. The flask was then removed from the oil
bath and cooled to room temperature under a N2 atmosphere. A
solution of NaClO4 (1.40 g, 10 mmol) in water (2 mL) was added to
the mixture and crystallization commenced over 2 days. Green
crystals were obtained from the mother liquor by filtration. The
product was washed with water and acetonitrile and dried in air.
Preparation of SBA-15
SBA-15 was prepared according to
a previously reported
method.[20] Pluronic P123 triblock copolymer (Mav =5800) was dis-
solved in a mixture of distilled water and 2m HCl before the addi-
tion of tetraethyl orthosilicate (TEOS). The mixture was stirred for
24 h at 358C and subsequently treated hydrothermally at 1008C
for 24 h. Finally, the precipitate product was filtered, dried at
1008C, and calcined at 5008C for 4 h.
Synthesis of Ni4/SBA-15 and Ni/SBA-15 catalysts
The incipient wetness impregnation method was used to introduce
the Ni4 SSP molecule and Ni nitrate precursors into the SBA-15
pores. A-Ni4/SBA-15 and A-Ni/SBA-15 (A represents the nominal
content of Ni) was prepared by impregnation with a Ni4 molecule
and Ni nitrate aqueous solutions with different concentrations (Ta-
bles S3 and S4). All samples were vacuum dried at room tempera-
ture and calcined at 5508C for 4 h.
In conclusion, supported nickel oxides were prepared by im-
pregnation of a SBA-15 support by an aqueous solution of two
different Ni precursors, Ni(OAc)2 and a tetranuclear Ni4 cubane
precursor. The application of the cubane precursor yielded
faster formation of small NiO nanoparticles, which are highly
dispersed on the silica support, even at very low precursor
concentrations. The preorganization of the metal atoms in this
cubane precursor is an obvious explanation for the faster for-
mation of NiO particles upon calcination. The resulting materi-
als are very active and stable catalysts for the dry reforming of
methane (DRM) reaction at moderate temperatures (500–
6008C), which is an interesting temperature window to con-
duct the DRM reaction if it is coupled to another exothermic
reaction, for example, the oxidative coupling of methane
(OCM). The heat produced by such exothermic oxidation reac-
tions could be conveniently used to provide the energy for the
endothermic DRM. In this scenario, it is highly desirable to op-
erate the DRM reaction at temperatures 50–100 K below the
OCM reaction (typically conducted at ꢁ7008C), considering
the energy loss caused by heat transfer between the respec-
tive reactors. The catalytic results further show that preorgani-
zation has a beneficial effect on the methane conversion rate
per Ni content.
Acknowledgements
This work is part of the Cluster of Excellence “Unifying Concepts
in Catalysis” (UniCat) coordinated by the Technische Universitꢀt
Berlin, supported by the Deutsche Forschungsgemeinschaft (DFG)
and was conducted in the framework of the BasCat collaboration
between BASF SE, FHI and TU Berlin.
Keywords: dry reforming of methane
catalysis · molecular precursor · nanoparticles · supported
catalysts
· heterogeneous
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E. Baktash, P. Littlewood, J. Pfrommer,
R. Schomꢀcker, M. Driess, A. Thomas*
The knights of Ni: Impregnation of
mesoporous silica with an organometal-
lic Ni4 cubane precursor and subsequent
thermal treatment yield highly active
and stable catalysts for the dry reform-
ing of methane.
&& – &&
Controlled Formation of Nickel Oxide
Nanoparticles on Mesoporous Silica
using Molecular Ni4O4 Clusters as
Precursors: Enhanced Catalytic
Performance for Dry Reforming of
Methane
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