Synthesis of shape-controllable cobalt nanoparticles and their shape-dependent performance in glycerol hydrogenolysis

Article abstract of DOI:10.1039/c4ra13395a

Cobalt nanorods were synthesised in polyol using Ir as the nucleation agent and sodium stearate as the surfactant. The aspect ratios of the rods can be facilely mediated by the Ir/Co molar ratios. During rod growth, the solid cobalt alkoxide and stearate intermediates formed at the initial stages and acted as the reservoir to control Co²⁺ reduction and then mediated the subsequent rod growth. Stearate played the critical role in controlling rod formation by its selective and covalent coating on the {10-10} planes, which induces anisotropic growth in the [0002] direction. The cobalt rods presented the hcp phase in the centre part and the fcc phase in the conical tips because of the difference in growth rate between stages, showing the hybrid crystallographic property. The slow growth can also induce the formation of fcc spheres by altering the amount of alkali in polyol. In glycerol hydrogenolysis, the hcp rods, which mainly expose the {10-10} planes, revealed much higher activity and 1,3-propanediol selectivity than the fcc spheres, demonstrating facet-dependent performance as a solid catalyst. To our knowledge, this is the first example of producing 1,3-propanediol using the facet effect of cobalt nanomaterials.

Full text of DOI:10.1039/c4ra13395a

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Synthesis of shape controllable cobalt nanoparticles and the shape
dependent performance in glycerol hydrogenolysis
Qiying Liu1,*, Xiaofeng Cao2,3, Tiejun Wang1, Chenguang Wang1, Qi Zhang1 and Longlong Ma1
1CAS Key Laboratory of Renewable Energy, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences,
Guangzhou, 510640, P. R. China; 2 State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan, 030001, P.R. China and 3 University of Chinese Academy of Sciences, Beijing 100049,
P.R. China
Cobalt nanorods were synthesized in polyol with using Ir as the nucleation agent and sodium stearate as the
surfactant. The aspect ratio of rods can be facilely mediated by Ir/Co molar ratios. During rod growth, the solid cobalt
alkoxide and stearate intermediates formed at initial stages and acted as the reservoir to control Co2+ reduction,
followed by further mediating the rod growth. Stearate played the critical role in controlling the formation of rods by its
selective and covalent coating on {10ꢀ10} planes, which induces the anisotropic growth through [0002] direction. The
cobalt rods presented hcp phase in the center part and fcc phase in the conical tips because of growth rate difference in
stages, showing the hybrid crystallographic property. The slow growth can also induce the formation of fcc spheres by
altering the alkali amount in polyol. In glycerol hydrogenolysis, the hcp rods with the mainly exposed {10ꢀ10} planes
revealed much higher activity and 1,3ꢀpropanediol selectivity than the fcc spheres, demonstrating the facetꢀdependent
performance as solid catalyst. To our knowledge, this is the first example for producing 1,3ꢀpropanediol by using facet
effect of cobalt nanomaterials.
Key words: Cobalt nanorods, heterogeneous nucleation, growth mechanism, glycerol hydrogenolysis, 1,3ꢀpropanediol
Corresponding author:
Qiying Liu, Tel: +86ꢀ20ꢀ87048614, Fax: +86ꢀ20ꢀ87057789, Eꢀmail: liuqy@ms.giec.ac.cn.
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1 Introduction
Nanoꢀmetal materials with different sizes and shapes showed recently the significant applications in catalysis,
selective sensing and optical performance.[1ꢀ4] Among those, cobalt nanomaterials with well defined geometries gained
considerable attention owing to its applications in high density data storage[5], magnetic separation[6] and heterogeneous
catalysis[7,8]. Generally, these cobalt nanomaterials possess different crystallographic planes and relatively discrete
energy band, both of which show the distinct ability to absorb/activate reactant molecule and mediate their catalytic
performance. On the other hand, the shape of cobalt nanomaterials presents orientation effect along exotic magnetic
field, thus altering the magnetic properties. Comparing to the spherical nanoparticles, the oneꢀdimensional cobalt
nanowires showed the significantly enhanced magnetic performance with larger coercivity and higher blocking
temperature.[9] Interestingly, the cobalt nanoparticles with larger sizes possessed higher activity and C5+ hydrocarbons
selectivity in FischerꢀTropsch Synthesis.[10] Moreover, due to the mainly exposed {100} planes of cobalt nanocubes, the
activity was comparable to the spherical cobalt which is mainly exposed {111} planes, but the selectivity for C10+
hydrocarbons was much higher than the latter one, demonstrating the facetꢀdependent catalytic performance.[11,12]
Large numbers of strategies were employed to synthesize cobalt nanomaterials with well defined geometries. By
using ordered mesoporous SiO2, ZrO2, carbon nanotube, polystyrene as the hard templates,[13ꢀ16] and
polyvinylpyrrolidone (PVP), cetyl trimethylammonium bromide (CTAB), sodium dodecyl sulfonate (SDS) as the soft
templates,[17ꢀ19] cobalt nanowires, nanocubes and hollow spheres can be synthesized. In the presence of surfactants,
cobalt nanorods/wires, discs and spherical nanocrystallines were also fabricated via thermal pyrolysis of cobalt
contained precursors in organic solvents[20,21] and reverse microꢀemulsion systems[22]. However, these cobalt
nanosmaterials obtained need multiꢀsteps for hard/soft template, cost/toxic organometallic precursors for thermal
pyrolysis, and complex reaction medium for reverse microꢀemulsion. Comparatively, polyols are effective for
synthesizing metal nanomaterials as solvent, reductant and complexant.[23,24] However, due to the mild reducing
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capability of polyol and negative redox potential of cobalt cation, the synthetic temperature is rather high (the reduction
usually takes place at the boiling point of polyol), which results in exquisite shape and particle size control of cobalt
nanomaterials rather difficult.[25] A solution is to introduce exotic nucleus to facilitate cobalt nucleation at lower
temperature for preferring the relatively separated nucleation and growth process and mediating the shape and size of
metal nanomaterials.[26,27]
In this work, we solvothermally fabricate cobalt nanorods with controllable aspect ratios and nanospheres with
difference sizes by selecting Ir as the heterogeneous nucleation agent in 1,2ꢀpropanediol and sodium stearate. The
structure of nanorods/nanospheres was elucidated and the formation mechanism was proposed. These nanoparticles
were used as the catalyst for selective hydrogenolysis of glycerol to 1,2ꢀ/1,3ꢀpropanediols and the interesting
facetꢀdependent catalysis was obtained.
2 Experimental section
2.1 Materials synthesis
The cobalt nanomaterials were synthesized by a typical solvothermal process. 0.75 g cobalt acetate tetrahydrate and a
certain amount of NaOH were mixed and dissolved in 70 ml 1,2ꢀpropanediol to obtain a purple solution under stirring
and mild heating. A given amount of sodium stearate and H2IrCl6.6H2O dissolved in 5 ml 1,2ꢀpropanediol were then
added to obtain a blue flocculent solution and continue stirring for several minutes. The flocculent solution was
transferred to a 100 ml autoclave line with Teflon and sealed, followed by gradually heating to 150 ℃. After
maintaining for a certain time, the autoclave was cooled to ambient temperature. The black solid was centrifuged,
thoroughly washed with ethanol and deionized water, and finally dried under vacuum at 60 for 5 h.
2.2 Materials characterization
The Powder Xꢀray diffraction (XRD) patterns were recorded on a diffractometer (XPert Pro MPD, Philip) operated
at 40 kV and 100 mA by using nickelꢀfiltered Cu Kα radiation (λ=1.5418 Å), and the low angle XRD pattern was
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obtained on the same instrument with the exception of the operating current 30 mA.
Transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) images
were taken using a JEOL JEMꢀ2100F instrument operated at 200 kV. The samples were ultrasonically dispersed into
ethanol, and drops of the suspension were placed on a carbonꢀcoating copper grid and then dried in air.
SEM images of cobalt materials were recorded using an Sꢀ4800 instrument operated at 30 kV. The sample was
placed on a conductive carbon tape adhered to an aluminum sample holder.
The elemental analyses were performed by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICPꢀAES)
on a PlasmaꢀSpecꢀ spectrometer (LEEMAN, USA). The samples were dissolved into aqua regia, and the solution was
diluted with 2% HNO3 to meet the detect range of the instrument.
The N2 adsorptionꢀdesorption isothermal profile was performed on a Micromeritics ASAP 2010 system at ꢀ196
.
Before measurement, the samples were degassed under vacuum for 6 h at 80 . The surface area was calculated by the
multipoint BraunauerꢀEmmettꢀTeller method.
The FTꢀIR spectra were obtained in the scanning range of 4000ꢀ400 cmꢀ1 by using a Nicolet 6700 FTꢀIR spectrometer
with a resolution of 4 cmꢀ1. The sample was fully grinded with KBr and pressed into wafer before measurement.
The TG curves were conducted on a NETZSCHꢀSTA 409PC DSCꢀSP thermal analyzer under the N2 flow of 30
ml/min with a ramp of 10 /min from room temperature to 630
.
The XPS measurements were performed with a Surface Science Instruments SSXꢀ100 ESCA spectrometer using
monochromatic Al Kα Xꢀrays and an electrostatic hemispherical analyzer. The spectra were recorded with a pass energy
of 110 eV and an Xꢀray spot size of 600 ꢁm. The base pressure in the analysis chamber was around 10ꢀ9 Torr. A flood
gun type charge neutralizer was used to correct for differential charging. For XPS measurements, the samples were
pretreated at 100 ℃ in a reactor cell under N2 flow and then transferred under vacuum to the analysis chamber.
2.3 Glycerol hydrogenolysis
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Hydrogenolysis of glycerol was carried out in a 100 ml autoclave with a mechanical stirring. 0.06 g cobalt
nanomaterials were added to 40 g aqueous solution of 10 wt % glycerol. After removing the residue air in autoclave by
H2 flush, the reactor was pressurized to 3.0 MPa. The reaction system was heated to 200 ℃ and kept at the temperature
for a certain period under vigorous stirring. After reaction, the system was cooled in ice water. For evaluating the
catalytic stability, the cobalt nanomaterials were collected after each run, washed with deionized water and reused
directly for the next cycle. As a reference, Ir was used as the catalyst for this reaction. 0.0015 g H2IrCl6.6H2O (the Ir
amount was calculated as the stoichiometric ones comprised in 0.06 g cobalt) dissolved in 40 ml water was reduced in
the same reactor under H2 at 250
for 0.5 h. After cooling to room temperature in ice water, 4 g glycerol was
introduced into the black colloidal solution and the hydrogenolysis procedure was the same to those mentioned above.
The liquid products were identified by GCꢀMS and quantified by a gas chromatography (Agilent 7890) equipped
with a flameꢀionization detector and a Carbowax 20 M capillary column with 25 m in length and 0.2 mm in diameter.
1,4ꢀButanediol was used as the internal standard. The gas products were collected by a gas bag and were analyzed by a
another gas chromatography equipped with a thermal conductivity detector and a PorapakꢀT (3 m×3 mm) pack column.
Glycerol conversion and product selectivity are defined as the following formulas:
CGolycerol − CGlycerol
(aGlycerol + bGlycerol AGlycerol AS−1 )Ws
Glycerol conversion =
× 100%
(
CGlycerol
=
).
CGolycerol
WSolution
92miCi Mi−1
3(CGolycerol − CGlycerol
(ai + bi Ai AS−1 )WS
Product selectivity (Cꢀmol%) =
× 100% (Ci =
).
WSolution
)
Here, C, W, A and M refer to mass concentration, weight, peak area (in gas chromatography analysis) and molecular
weight of weight of glycerol and product, respectively. The subscript i and s mean product i and the internal standard s.
mi indicates carbon numbers of product i, and a and b are the parameters of product i in correction curves during
quantification analysis.
3 Results and discussion
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3.1 Shape control of cobalt materials
Figure 1 shows the influence of Ir/Co molar ratios on cobalt shape. Without Ir, the cobalt was spheres with the
diameter of about 500 nm. At the Ir/Co molar ratio of 0.01, the spheres disappeared and the cobalt nanorods with the
length of 100ꢀ300 nm and the diameter of about 10 nm were observed instead. On each rod terminal, the conical head
of about 30 nm was observed. As the Ir/Co was increased to 0.025, the rod like shape was preserved, but the length was
decreased to 50ꢀ200 nm and the conical heads were absent in this case. As the Ir/Co ratio was further increased to 0.05,
the rod diameter changed a litter but the length continuously decreased to about 40 nm.
The structural details of rods were characterized by HRTEM. As shown in Figure 2a, the consistent lattice fringes of
0.211 nm, 0.208 nm and 0.188 nm are corresponded to the (10ꢀ10), (0002) and (10ꢀ11) planes, respectively, indicating
the hexagonal closeꢀpacked phase of metal cobalt (hcp, JCPDS No. 5ꢀ727). Based on Figure 2a and the FFT pattern, the
rods grow along [001] plane and the mainly exposed planes are the enclosed {1010} at the lateral directions. The
structure of conical head was revealed in Figure 2b and d. Along the [0002] direction of nanorods, the conical heads
presented as the regular hexagon and the average size was about 28 nm with the side length of about 15 nm, which is
well consistent with the size measurement by TEM (Figure 1b). Interestingly, the HRTEM measurement and relative
FFT pattern showed that the distinct lattice fringes of 0.203 nm are relative to the (111) plane of faceꢀcentered cubic
phase of metal cobalt (fcc, JCPDS No. 15ꢀ806). These fringes are perpendicular to the long axis of nanorods, revealing
the mainly exposed (111) plane. Evidently, the crystallography of the synthetic cobalt nanorods is composed of hcp
phase in centered part and fcc phase in conical tips.
Besides the rods, multiꢀpods nanostructures with three, four and five branches were observed in the cases of the Ir/Co
molar ratios being 0.01 and 0.025 (Figure 3). The length of each branch varied in 30ꢀ200 nm but their diameter was the
similar 10 nm to the isolate rods. In some cases, the branches extruded out from a bending or a straight rod substrate,
forming the quasiꢀtetrapod structure.
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Figure 4 shows the XRD patterns of cobalt nanomaterials synthesized with different Ir/Co molar ratios. When no Ir
was introduced and the synthesis was lasted for 20 h, the diffractions at 2θ= 44.4 °, 51.6 ° and 76 ° are relative to the
(111), (200) and (220) planes, respectively, indicating the fcc phase of the cobalt spheres. As Ir was added and the
synthesis was implemented for 10 h, the fcc phase was disappeared. The distinct diffractions at 2θ= 41.5 °, 44.4 °,
47.4 °, 62.4 ° and 75.9 ° are corresponding to the (10ꢀ10), (0002), (10ꢀ11), (10ꢀ12) and (1100) planes, respectively,
demonstrating that the crystallography of cobalt is hcp phase. The relative (0002) diffraction significantly intensified
with increasing the Ir/Co molar ratio, indicating the preferential orientation of the rods. According to Scherrer equation,
we calculated the consistent diffraction length based on the (10ꢀ10) and (0002) planes in Figure 4d. The lengths along
the [10ꢀ10] and [0002] directions were 12 nm and 43 nm, respectively, which is well consistent with the TEM
measurements (Figure 1d and Figure 2a).
The influence of NaOH on the cobalt morphology was shown in Figure 5. Without alkali, the cobalt was spherical
and the size was about 30 nm. These nanospheres aggregated into the large particles of 100ꢀ300 nm, followed by
further selfꢀassembly into the oneꢀdimensional chain structure. As the appropriate NaOH of 0.2 g was used, the chain
like structure was transferred into the nanorods with the length of 50ꢀ200 nm and the diameter of about 10 nm.
However, as the NaOH amount was further increased to 0.7 g, the cobalt shape changed back to the spheres of 100ꢀ200
nm with a rough surface. Based on the previous reports, the spheres synthesized at high basicity were possibly ascribed
to the slow growth of cobalt materials.[28,29]
Figure 6 shows the XRD patterns of the cobalt nanomaterials synthesized with different NaOH amounts. When no
NaOH was used, the crystallographic phase of the small spheres was the pure fcc structure. As expected, the cobalt
nanorods presented as the pure hcp phase at the NaOH amount of 0.2 g. When the NaOH was further increased to 0.7 g,
the mixed hcp and fcc phases were obtained. The significantly intensified diffraction at 2 θ=44.4 ° is attribute to the
superimposition of the hcp (0002) and fcc (111) planes of cobalt.[28] Based on the previous report, we confirmed that the
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relative content of fcc phase was about 65%.[29]
3.2 Formation mechanism of cobalt materials
To ascertain the growth mechanism of cobalt nanorods, the samples at different time were collected during synthesis
and their respective shape and the crystallographic phase evolution were monitored by TEM and XRD measurements
(Figure 7 and 8). At 0.5 h, the solid recovered was purple and presented as several hundred nanometers with significant
aggregation. Its diffractions below 30 ° indicated the mixed phase of cobalt stearate and cobalt alkoxide.[30,31] No
diffraction relative to metallic cobalt was observed, implying only solid cobalt precursors form at this stage. The two
solid intermediates were produced via the reaction of dissolved Co2+ with stearate and 1,2ꢀpropanediol in the presence
of alkali, as indicated by our and other previous reports. From the diffraction intensities, cobalt stearate seems as the
major one perhaps due to that the more electronegative of carboxyl groups in stearate could easily capture the dissolved
Co2+ to form the cobalt stearate precipitate. The low angle XRD pattern of the solid intermediates displayed four
distinct diffractions at 2θ=1.82 °, 3.64 °, 5.44 ° and 7.25 °, which is responsible for the respective (001), (002), (003)
and (004) facets of laminated cobalt stearate. By using the primary (001) facet, we confirmed that the interlayer space
was 4.85 nm. Considering the aliphatic chain length of 2.5 nm in stearate, this distance is about two times of a single
chain length, indicating the biꢀlayered structure of cobalt stearate.[32]
As the time prolonging to 1 h, the TEM image proposed the formation of mixed particles and rods with the particles
as the majority, apart from the solid intermediates. The particle size was in the range of 5ꢀ20 nm while the rod diameter
and length were about 7 nm and 50ꢀ100 nm, respectively, which demonstrates Co2+ starts to be reduced at this stage.
The formation of large particles was possibly ascribed to the continuous growth during cooling because such size can’t
grow into the rods with the diameter of 10 nm. With the time increasing to 1.5 h, the particles grew into the nanorods at
the expense of the cobalt contained intermediates. The XRD pattern showed that no other diffractions were observed
but a very weak one at 2θ=20 °, indicating the crystallized intermediates transferred to amorphous matter owing to
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consumption. Instead, the hcp cobalt phase was seen by weak diffractions. When the time was increased to 10 h, the
rods continued growing to obtain the length of 200 nm while with the diameter slightly rising to about 10 nm. During
this process, the diffraction of hcp cobalt gradually intensified accompanied with the solid intermediates reduced until
complete consumption. Similarly, the conical tips on one terminal rod at early stages are possibly due to the same
reason for the formation of large particles at 1 h.
During synthesis, the solid intermediates cobalt stearate and alkoxide decrease the dissolved Co2+ concentration in
1,2ꢀpropanediol as Co2+ reservoir and decrease the supersaturation of Co2+ by controlling Co2+ release into the solution.
Here, H2IrCl6 is simultaneously introduced with cobalt acetate. Due to its higher redox potential than Co2+, the Ir4+ ions
can be first and fast reduce to nanoparitcles by 1,2ꢀpropanediol. As heterogeneous nucleation agent, the tiny Ir particles
facilitate the Co2+ reduction to nanocrystals by the same solvent, followed by growing into rods. With regarding to the
absence of Ir particles in XRD patterns (Figure 4, 6 and 8)), we measured the content of Ir residue on cobalt
nanomaterials by means of ICPꢀAES. As shown in Table 1, the Ir/Co molar ratios in the final rods and spheres were
much lower than the initially introduced counterparts, indicating that most Ir particles were removed during washing
procedure. Occasionally, the isolated 2ꢀ3 nm Ir particles were observed by HRTEM measurement (Figure 2c).
In the presence of exotic Ir as the nucleation agent, the reduction of dissolved Co2+ is accelerated and thus induces a
relatively separated nucleation and growth process,[33] which is favorable for the fast growth of nanocrystals into rods.
For the formation of cobalt nanorods, the length is determined by the Co amount for growth. At high Ir/Co ratio, more
Ir particles induce more Co nucleus explosion at nucleation stage, which leads to less cobalt atoms for subsequential
growth and finally obtained the rods with low aspect ratios (Figure 1). In some cases, the conical tips with large size
were observed. Based on the previous reports, its formation is perhaps attribute to the slow growth at final stage where
the Co2+ concentration in solution is very low.[34,35] This slow growth results in the stacking model of reduced cobalt
atoms via the ABCABC… sequence along [111] direction of fcc phase, which is very different from the ABAB…
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sequence along the [0002] orientation of hcp cobalt phase under fast growth.[36] The both factors dominate the
formation of rods having the hybrid hcp and fcc phases.
Based on Figure 5 and 6, the hcp cobalt nanorods can be synthesized at the medium basicity because the low and
high basicities lead to the formation of fcc spheres and mixed fcc/hcp spheres, respectively. As proposed by the
previous reports, H+ is produced by dehydrogenation and accumulated during Co2+ reduction by 1,2ꢀpropanediol.[37,38]
The addition of appropriate NaOH can counteract H+ and push the reaction proceeds, thus accelerating cobalt growth
into the rods. Vice versa, the formation of spheres is owing to the low growth of cobalt.
In addition to the isolated rods, a significant amount of multiꢀpods with isotropic and unisotropic geometries were
obtained at the appropriate Ir/Co ratios (Figure 3). At early stage, the tiny cobalt nanocrystals with specific facets of
high energy could form. These facets possibly serve as the seeds for epitaxial stacking of reduced cobalt atoms from
three dimensions to form the final isotropic multiꢀpods.[39] During fast growth of rod, the stacking faults easily form.[40]
These faults are high energy and favorable for capturing cobalt atoms to gain the second seeds, followed by growth into
the branched rods from the substrate to get the unisotropic multiꢀpods. This hypothesis was evidenced by the fact that
the extrusions were unambiguously observed on some rods (Figure 3d, as labeled by the circles) and survived from rod
synthesis.
Surfactant is widely used for synthesizing wellꢀdefined metal nanostructures via its selective capping effect.[41] Here,
the employed sodium stearate is critical for cobalt nanorods and multiꢀpods. In hcp cobalt, the surface energy of lateral
{10ꢀ10} facets is higher than those of the axial {0002} ones.[42] This induces the selective capping of stearate on them
and mediates the rods growing along the [0002] orientation. Figure 9 shows the FTꢀIR spectra of cobalt nanorods
synthesized at different time. For the pure sodium stearate, the peaks observed were relative to the stretching vibrations
of aliphatic CꢀH (2950ꢀ2850 cmꢀ1) and carboxyl group (ꢀCOOꢀ, 1566 cmꢀ1 and 1428 cmꢀ1), and the scissoring vibration
of CꢀH (1468 cmꢀ1).[32] As the time prolonged, the absorbencies decreased but were still observed at the end 10 h,
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indicating that the cobalt stearate decompose due to Co2+ reduction and release the excessive stearate into solution,
remaining the minority capping on cobalt rod surface. We confirmed the amount of capping stearate on the rods
synthesized at 10 h by TG analysis. As shown in Figure 10, the about 2% of weight loss before 150 ℃ is ascribed to the
removal of physically absorbed water. The second loss at 295 ℃ is about 14%, indicating the thermal decomposition of
stearate. We used XPS to analyze the chemical status of stearate with different aspect ratios (Figure 11). The Co 2p
doublet at the binding energies of 779.1 eV and 794.5 eV indicated the mixed Co and Coδ+ species combined with
carboxylate.[42] This combination is demonstrated by the binding energy of C 1s at 289.4 eV (CꢀO bond). The other C
1s peak at 284.8 eV is responsible for the carbons in aliphatic chain (CꢀC bond) in stearate. As carboxylic acid absorbs
on metal surface, two bonding types, including the monodentate bond with inequivalent oxygen atom or the bidentate
bond with equivalent oxygen atom, are proposed.[43] If the monodentate bond takes place, the O 1s peak corresponding
to C=O would be observed at the binding energy of about 533 eV. However, our result shows that only one peak is
presented at 530.5 eV (CꢀO bond), indicating the bonding model for stearate is bidentate.
3.3 Catalytic Performance of cobalt materials
Glycerol is the byproduct of biodiesel synthesis by transesterification of vegetable oil and methanol.[44] Converting
glycerol into valueꢀadded chemicals can improve the comprehensive benefit. A promising route is to selective glycerol
hydrogenolysis to 1,2ꢀ/1,3ꢀpropanediols which widely used in fine, organic and medicine chemical industries as the
feedstock and/or intermediate. The catalysts for this process mainly depend on the supported noble metals Ru, Rh, Pt,
Pd and transition metals Cu and Ni.[45,46] Over this kind of catalyst, the main product is 1,2ꢀpropanediol. To produce
more valuable 1,3ꢀpropanediol, the medium hydroxyl group needs to be removed. However, due to the steric blockage
the removal of medium hydroxyl group is rather difficult. When using Pt, Ir aided by WOx, ReOx, AlOx as the catalyst,
1,3ꢀpropanediol can be produced with the yields.[47,48] Cobalt platelets and polyhedrons were recently used for glycerol
hydrogenolysis, but only 16% of 1,2ꢀpropanediol yield was obtained.[49]
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Here, we used cobalt nanorods and spheres for this process. The detectable and identified products contain
1,2ꢀ/1,3ꢀpropanediols, acetol, 1ꢀpropanol, ethylene glycol and ethanol. Considering Ir was reported as the active metal,
we prepared the equivalent Ir particles according to the Ir residue on cobalt rod surface and implemented the
comparison experiment under the same condition. The result showed that the glycerol conversion was lower than 3%,
indicating its catalytic performance can be ruled out. As shown in Table 2, the nanorods showed more than 60% of
glycerol conversion with the total 1,2ꢀ/1,3ꢀpropanediols selectivity of more than 65%, and the highest propanediols
yield of 59% was obtained over the rods (Figure 1b) having the largest surface (Table 1). Comparatively, the spheres
showed much lower activities than the rods. Besides activity, the rods presented the remarkably higher 1,3ꢀpropanediol
selectivity and the highest yield of 19% was obtained. The superior performance of rods is dependent on their exposed
{10ꢀ10} facets, which is favorable for glycerol hydrogenolysis to propanediols due to the higher surface energy than
that of the possibly exposed {111} planes on the spheres.[38] Figure 12 compared the activity and 1,3ꢀpropanediol
productivity on unit surface area of rods and spheres. The respective activity and 1,3ꢀpropanediol productivity over the
rods were two and ten times higher than those obtained over the spheres. To our knowledge, this is the first example to
mediate 1,2ꢀ and 1,3ꢀpropanediol selectivities in glycerol hydrogenolysis by controlling the morphology of cobalt
materials.
Figure 13 shows the influence of reaction time on glycerol hydrogenolysis over cobalt rods and spheres. For the rods,
the glycerol conversion increased with increasing reaction time and the maximal 87% was obtained at 16 h. Meanwhile,
the 1,2ꢀ/1,3ꢀpropanediol selectivities presented the volcano feature with the respective maximal 36% and 58% at the
time of 5 h, and then reduced to 41% and 24%, respectively, when the duration was lengthened to 16 h. Besides the
goal products, acetol which is produced by dehydrated removal of tip hydroxyl in glycerol monotonously decreased
with the increased reaction time due to it is further hydrogenated to 1,2ꢀpropanediol.[47] The further hydrogenolysis of
propanediols resulted in the gradually rise of 1ꢀpropanol while with the selectivities below than 10%, indicating this
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kind of side product was suppressed. Additionally, ethylene glycol produced by glycerol dehydrogenation followed by
retroꢀaldolation was observed[48] and the selectivities were monotonously increased to about 10%. Over the cobalt rods,
we obtained significant amount of 1,3ꢀpropanediol in final products. However, we do not detect 3ꢀhydroxyꢀpropanal
even at low glycerol conversion, which is possibly attribute to its fast hydrogenation to 1,3ꢀpropanediol.[48] As the
cobalt spheres were used, the similar phenomena were observed, but the higher 1,2ꢀpropanediol and much lower
1,3ꢀpropanediol selectivities were gained as comparing to the rods. Over both rods and spheres, the gas phase was
analyzed and H2, CO2 and CH4 were detected as the main products, indicating aqueous phase reforming of glycerol and
diols/mononic alcohols takes place in our cases.[50]
The catalytic stability of cobalt rods and spheres was revealed in Figure 14. Over the cobalt rods, the glycerol
conversion slightly decreased from 80% for the first run to 70% for the third run together with the slightly reduced
1,3ꢀpropanediol (from the first 26% to the third 21%) and increased 1,2ꢀpropanediol (from the first 49% to the third
58%) selectivities. As the cycle proceed, comparatively, the glycerol conversion significantly decreased from the first
30% to the third 5% over the cobalt spheres despite the 1,3ꢀpropanediol and 1,2ꢀpropanediol selectivities keeping the
constant 5% and 60%, respectively. Obviously, the rods show superior stability to the spheres in glycerol
hydrogenolysis.
The TEM images of used cobalt materials were shown in Figure 15. After three cycles, the surface of the rods
became rough but the oneꢀdimensional shape was largely kept, indicating the rods in our case are “robust” in this
process. By comparison, the spheres were agglomerated severely into several hundred nanometers, showing that the
spheres are rather morphologically unstable in the present hydrogenolysis conditions. This aggregation is responsible to
the remarkable loss of surface area (Table 1) and is accordingly unreusable in glycerol hydrogenolysis. It’s noted that
the retention of the rod like shape and surface area (Table 1) after reuse may propose the main reason for keeping the
superior performance to the spheres.
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4 Conclusions
Cobalt nanorods were solvothermally fabricated in polyol by using Ir as the exotic nucleation agent and sodium
stearate as the surfactant. The shortened rods were obtained at higher Ir/Co molar ratios, which is possibly due to that
more Ir nanoparticles induce more cobalt nucleus burst out at initial stage remaining less cobalt atoms for the sequential
growth. During rods growth from nanocrystals, cobalt alkoxide and stearate form at early stages and serve as the solid
intermediates to control the dynamic Co2+ reduction. Stearate plays the essential role in controlling the formation of
rods by its selective, covalent capping on the lateral {10ꢀ10} planes and mediating the anisotropic growth along [0002]
orientation. The rods show the interesting hcp phase in the center part and fcc phase in the conical tips. The fcc tips are
produced by the slow growth due to the reduced Co2+ concentration at end stage. The slow growth rate also induces the
formation of fcc nanospheres by controlling the basicity in solution. In glycerol hydrogenolysis, the hcp rods with
mainly exposed {10ꢀ10} planes exhibited two times higher activity and ten times higher 1,3ꢀpropanediol productivity
than the fcc spheres based on unit surface area, showing the interesting facetꢀdependent catalysis.
Acknowledgements
This work is financially supported by the National Natural Science Foundation of China (51376185), the National
Basic Research Program of China (2012CB215304), the Natural Science Foundation of Guangdong Province
(S2013010011612 and S20122040006992) and the Creative Foundation of President of Guangzhou Institute of Energy
Conversion, Chinese Academy of Sciences (y307p21001).
References
[1] M. T. Zhao, K. Deng, L. C. He, Y. Liu, G. D. Li, H. J. Zhao and Z. Y. Tang, J. Am. Chem. Soc., 2014, 136, 1738.
[2] L. C. He, Y. Liu, J. Z. Liu, Y. S. Xiong, J. Z. Zheng, Y. L. Liu and Z. Y. Tang, Angew. Chem. Int. Ed., 2013, 52,
3741.
[3] H. J. Yin, H. J. Tang, D. Wang, Y. Gao and Z. Y. Tang, ACS Nano, 2012, 6, 8288.
14

Page 15 of 35
RSC Advances
View Article Online
DOI: 10.1039/C4RA13395A
[4] J. X. Gong, F. Zhou, Z. Y. Li and Z. Y. Tang, Chem. Commun., 2013, 49, 4379.
[5] S. Singamaneni, V. N. Bliznyuk, C. Binek and E. Y. Tsymbal, J. Mater. Chem., 2011, 21, 16819.
[6] H. Zeng and S. H. Sun, Adv. Funct. Mater., 2008, 18, 391.
[7] Y. Li, Q. Y. Liu and W. J. Shen, Dalton Transctions, 2011, 40, 5811.
[8] V. Polshettiwar, R. Luque, A. Fihri, H. B. Zhu, M. Bouhrara and J. M. Basset, Chem. Rev., 2011, 111, 3036.
[9] K. A. Atmane, F. Zighem, Y. Soumare, M. Ibrahim, R. Boubekri, T. Maurer, J. Margueritat, J. Y. Piquemal, F. Ott, G.
Chaboussant, F. Schoenstein, N. Jouini and G. Viau, J. Solid State Chem., 2013, 197, 297.
[10] G. L. Bezemer, J. H. Bitter, H. P. C. E. Kuipers, H. Oosterbeek, J. E. Holewijn, X. D. Xu, F. Kapteijn, A. J. van
Dillen and K. P. de Jong, J. Am. Chem. Soc., 2006, 128, 3956.
[11] M. Scariot, D. O. Silva, J. D. Scholten, G. Machado, S. R. Teixeira, M. A. Novak, G. Ebeling and J. Dupont, Angew.
Chem. Int. Ed., 2008, 47, 9075.
[12] D. O. Silva, J. D. Scholten, M. A. Gelesky, S. R. Teixeira, A. C. B. D. Santos, E. F. SouzaꢀAguiar and J. Dupont,
ChemSusChem, 2008, 1, 291.
[13] T. A. Crowley, K. J. Ziegler, D. M. Lyons, D. Erts, H. Olin, M. A. Morris and J. D. Holmes, Chem. Mater., 2003,
15, 3518.
[14] J. C. Bao, C. Y. Tie, Z. Xu, Q. Ma, J. M. Hong, H. Sang and D. Sheng, Adv. Mater., 2002, 14, 44.
[15] S. W. Liu, J. J. Zhu, Y. Mastai, I. Felner and A. Gedanke, Chem. Mater., 2000, 12, 2205ꢀ2211.
[16] A. K. Srivastava, S. Madhavi, T. J. White and R. V. Ramanujan, J. Mater. Chem., 2005, 15, 4424.
[17] L. Guo, F. Liang, N. Wang, D. S. Kong, S. M. Wang, L. He, C. P. Chen, X. M. Meng and Z. Y. Yu, Chem. Mater.,
2008, 20, 5163.
[18] J. Zhang, Z. H. Dai, J. C. Bao, N. Zhang and M. A. LópezꢀQuintela, J. Colloid Interface. Sci., 2007, 305, 339.
[19] J. Huang, L. He, Y. H. Leng, W. Zhang, X. G. Li, C. P. Chen and Y. Liu, Nanotechnology, 2007, 18, 415603.
15

RSC Advances
Page 16 of 35
View Article Online
DOI: 10.1039/C4RA13395A
[20] V. F. Puntes, D. Zanchet, C. K. Erdonmez and A. P. Alivisatos, J. Am. Chem. Soc., 2002, 124, 12874.
[21] F. Dumestre, B. Chaudret, C. Amiens, M. Respaud, P. Fejes, P. Renaud and P. Zurcher, Angew. Chem. Int. Ed.,
2003, 42, 5213.
[22] I. Lisiecki, M. Walls, D. Parker and M. P. Pileni, Langmuir, 2008, 24, 4295.
[23] M. A. Mahmoud, M. A. ElꢀSayed, J. P. Gao and U. Landman, Nano Lett., 2013, 13, 4739.
[24] Y. Q. Zheng, J. Zeng, A. Ruditskiy, M. C. Liu and Y. N. Xia, Chem. Mater., 2014, 26, 22.
[25] G. V. Kurlyandskaya, S. M. Bhagat, C. Luna and M. Vazquez, J. Appl. Phys., 2006, 99, 104308.
[26] D. Ung, G. Viau, C. Ricolleau, F. Warmont, P. Gredin and F. Fiévet, Adv. Mater., 2005, 17, 338.
[27] Y. Soumare, C. Garcia, T. Maurer, G. Chaboussant, F. Ott, F. Fiévet, J. Y. Piquemal and G. Viau, Adv. Funct. Mater.,
2009, 19, 1971.
[28] D. Ung, G. Viau, F. FiévetꢀVincent, F. Herbst, V. Richard and F. Fiévet, Prog. Solid State Chem., 2005, 33, 137.
[29] P. Toneguzzo, V. Giau, O. Acher, F. FiévetꢀVincent and F. Fiévet, Adv. Mater., 1998, 10, 1032.
[30] J. M. Rueff, N. Masciocchi, P. Rabu, A. Sironi and A. Skoulios, Chem. Eur. J., 2002, 8, 1813.
[31] N. Chakroune, G. Viau, C. Ricolleau, F. FiévetꢀVincent and F. Fiévet, J. Mater. Chem., 2003, 13, 312.
[32] X. Z. Luo, Z. Q. Zhang and Y. Q. Liang, Langmuir, 1994, 10, 3213.
[33] E. A. Aunmol, P. Kundu, P. A. Deshpande, G. Madras and N. Ravishankar, ACS Nano, 2011, 5, 8049.
[34] Q. Y. Liu, X. H. Guo, T. J. Wang, Y. Li and W. J. Shen, Mater. Lett., 2010, 64, 1271.
[35] M. Ibrahim, C. MarcelotꢀGarcia, K. A. Atmane, E. Berrichi, L. M. Lacroix, A. Zwick, E. WarotꢀFonrose, S.
Lachaize, P. Decorse, J. Y. Piquemal and G. Viau, J. Phys. Chem. C, 2013, 117, 15808.
[36] C. OsorioꢀCantillo, O. PeralesꢀPerez and M. J. F. Guinel, J. Appl. Phys., 2011, 109, 07B531.
[37] S. E. Skrabalak, B. J. Wiley, M. Kim, E. V. Formo and Y. N. Xia, Nano Lett., 2008, 8, 2077.
[38] K. A. Atmane, C. Michel, J. Y. Piquemal, P. Sautet, P. Beaunier, M. Giraud, M. Sicard, S. Nowak, R. Losnoe and G.
16

Page 17 of 35
RSC Advances
View Article Online
DOI: 10.1039/C4RA13395A
Viau, Nanoscale, 2014, 6, 2682.
[39] Y. Dai, X. L, Mu, Y. M. Tan, K. Q. Lin, Z. L. Yang, N. F. Zheng and G. Fu, J. Am. Chem. Soc., 2012, 134, 7073.
[40] Y. Y. Gao, G. M. Xing, W. G. Chu, X. J. Liang, Y. L. Zhao, L. Jing, H. Yuan, Y. Y. Cui and J. Q. Dong, Adv. Mater.,
2008, 20, 1794.
[41] H. G. Liao, K. Y. Niu and H. M. Zheng, Chem. Commun., 2013, 49, 11720.
[42] N. Q. Wu, L. Fu, M. Su, M. Aslam, K. C. Wong and V. P. Dravid, Nano Lett., 2004, 4, 383.
[43] A. Wühn, J. Weckesser and C. Wöll, Langmuir, 2001, 17, 7605.
[44] D. Y. C. Leung, X. Wu and M. K. H. Leung, Appl. Energy, 2010, 87, 1083.
[45] A. M. Ruppert, K. Weinberg and R. Palkovits, Angew. Chem. Int. Ed., 2012, 51, 2564.
[46] M. A. Dasari, P. P. Kiatsimkul, W. R. Sutterlin and G. J. Suppes, Applied Catalysis A, 2005, 281, 225.
[47] Y. Nakagawa, Y. Shinmi, S. Koso and K. Tomishige, J. Catal., 2010, 272, 191.
[48] R. Arundhathi, T. Mizugaki, T. Mitsudome, K. Jitsukawa and K. Kaneda, ChemSusChem, 2013, 6, 1345.
[49] Y. B. Cao, X. Zhang, J. M. Fan, P. Hu, L. Y. Bai, H. B. Zhang, F. L. Yuan and Y. F. Chen, Cryst. Growth Design,
2011, 11, 472.
[50] R. D. Cortright, R. R. Davda and J. A. Dumesic, Nature, 2002, 418, 964.
17

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a
b
d
c
Figure 1. TEM images of cobalt nanomaterials synthesized with different Ir/Co molar ratios at 150 ℃: (a) 0 for 20 h
and (b) 0.01, (c) 0.025 and (d) 0.05 for 10 h. The syntheses were kept under the conditions of 150 ℃, 0.2 g NaOH and
2.30 g sodium stearate.
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(0002)
(111)
a
b
(10-11)
(10-10)
d
c
2 nm
Figure 2. HRTEM images of cobalt nanorod (a), conical head (b), Ir nanoparticels (c) and conical heads by SEM image
(d). Insets in (a) and (b) are the fast Fourier transform (FFT) patterns.
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a
b
c
d
Figure 3. TEM images of cobalt multiꢀpods.
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(10ꢀ11)
•
(0002)
•
(10ꢀ10)
•
(10ꢀ12)
(1100)
•
(d)
•
(c)
(b)
(a)
♦
(111)
(200)
(220)
♦
♦
35 40 45 50 55 60 65 70 75 80
2-Theta (degree)
Figure 4. XRD patterns of cobalt nanomaterials synthesized with different Ir/Co molar ratios: (a) 0, (b) 0.01, (c) 0.025,
and (d) 0.05. Entry (a) was conducted at 150 ℃ for 20 h. (●) and ( ) represent hcp and fcc phase, respectively.
◆
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c
a
b
Figure 5. TEM images of cobalt nanomaterials synthesized with different NaOH amounts: (a) 0 g, (b) 0.2 g, and (c) 0.7
g. Synthetic condition: 2.30 g sodium stearate, Ir/Co= 0.025 (mol), 150 ℃, 10 h.
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(a)
(b)
(c)
35 40 45 50 55 60 65 70 75 80
2-Theta (degree)
Figure 6. XRD patterns of cobalt nanomaterials synthesized with different NaOH amounts: (a) 0 g, (b) 0.2 g, and (c)
0.7 g.
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a
b
c
d
e
Figure 7. TEM images of cobalt nanomaterials synthesized at stages: (a) 0.5 h, (b) 1.0 h, (c) 1.5 h, (d) 3.5 h and (e) 10
h. Synthetic condition: 0.2 g NaOH, 2.30 g sodium stearate, Ir/Co=0.01 (mol), 150 ℃.
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(A)
(B)
*
*
(c)
*
(f)
*
*
(b)
(e)
5
10 15 20 25 30
2-Theta (degree)
(d)
(c)
(b)
•
(a)
(b)
•
♦
•
•
•
•
♦
•
•
(a)
70
10
20
30
40
50
60
80
1
2
3
4
5
6
7
8
2-Theta (degree)
Figure 8. Wide (A) and low (B) angle XRD patterns of cobalt intermediates at different stages: (a) 0.5 h, (b) 1.5 h, (c) 2
h, (d) 3.5 h, (e) 6 h, and (f) 10 h. (*) represents metallic hcp cobalt, (●) and ( ) are the precursors cobalt stearate and
cobalt alkoxide, respectively. Inset in (A) shows the magnified XRD pattern of (b) and (c) in the range of 4ꢀ30 °.
2-Theta (degree)
◆
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sodium stearate
1.5h
10h
4000
3500
3000
2500
2000
1500
1000
500
Wave number (cm-1)
Figure 9. FTꢀIR spectra of cobalt nanorods synthesized at different stages.
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1.00
0.95
0.90
0.85
0.80
0.75
10h
295
100
200
300
400
500
600
Temperature (oC)
Figure 10. TG profile of cobalt nanorods. The rods were synthesized under the following conditions: Ir/Co=0.025
(mol), 2.30 g sodium stearate, 0.2 g NaOH, 150 ℃, 10 h.
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(A)
(C)
(B)
(c)
(b)
(c)
(c)
(b)
(b)
(a)
(a)
(a)
801 798
795 792 789
786 783 780 777 774
294
292
290
288
286
284
282
280
278
538
536
534
532
530
528
526
524
522
Binding energy (eV)
Binding energy (eV)
Binding energy (eV)
Figure 11. XPS spectra of cobalt nanorods with Co 2p core level (A), C 1s core level (B) and O 1s core level (C). (a),
(b) and (c) represent the rods in Figure 1 (b), (c) and (d), respectively.
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3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
10
(B)
(A)
8
6
4
2
0
Figure 12. Glycerol conversion (A) and 1,3ꢀpropanediol productivity (B) on unit surface of cobalt nanomaterials.
29