Salt-induced formation of hollow and mesoporous CoOx/SiO2 spheres and their catalytic behavior in toluene oxidation

Article abstract of DOI:10.1039/c5ra27488e

Hollow and mesoporous CoO_x/SiO₂ spheres (denoted as CoO_x/hSiO₂ and CoO_x/mSiO₂) were synthesized via salt-assisted ultrasonic spray pyrolysis. Precursor solutions containing sodium silicate solution, a mineral acid (hydrochloric acid or nitric acid) and a cobalt salt were ultrasonically aerosolized and pyrolyzed. Results showed that CoO_x/SiO₂ spheres with hollow or mesoporous structure can be fabricated by using the NaCl and NaNO₃ salts as in situ formed templates. Significantly, this approach avoids the need for post-calcination for template elimination, instead permitting aqueous removal with water. The influence of the sodium salts on the characteristics of CoO_x/SiO₂ spheres was investigated by means of XRD, nitrogen physisorption, SEM/TEM, ICP-MS, UV-Vis, XPS and H₂-TPR. On the basis of the experimental results, a possible mechanism on the formation of hollow and mesoporous spheres was proposed. The CoO_x/SiO₂ spheres were tested as catalysts for toluene oxidation. The mesoporous CoO_x/mSiO₂ was found to exhibit superior activity to hollow CoO_x/hSiO₂, probably attributed to a combination of several factors, including the predominant existence of a Co₃O₄ active phase, high surface Co³⁺ content, and easy reducibility of Co³⁺ at low temperature.

Full text of DOI:10.1039/c5ra27488e

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Page 1 of 35
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DOI: 10.1039/C5RA27488E
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Salt-induced formation of hollow and mesoporous CoOx/SiO2 spheres
and their catalytic behaviors in toluene oxidation
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Liang-Yi Lin1,2∗ and Hsunling Bai2
1Department of Energy, Environmental and Chemical Engineering, Washington
University in St. Louis, St. Louis, MO 63130, United States
2Institute of Environmental Engineering, National Chiao Tung University,
Hsinchu 300, Taiwan
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∗ Corresponding author. Eꢀmail: liangyi1102@gmail.com Tel.: 1ꢀ314ꢀ637ꢀ6523
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ABSTRACT
Hollow and mesoporous CoOx/SiO2 spheres (denoted as CoOx/hSiO2 and
CoOx/mSiO2) were synthesized via saltꢀassisted ultrasonic spray pyrolysis. Precursor
solutions containing sodium silicate solution, a mineral acid (hydrochloric acid or nitric
acid) and a cobalt salt were ultrasonically aerosolized and pyrolyzed. Results showed that
CoOx/SiO2 spheres with hollow or mesoporous structure can be fabricated by using the
NaCl and NaNO3 salts as inꢀsitu formed templates. Significantly, this approach avoids the
need of post calcination for template elimination, instead permitting aqueous removal
with water. The influence of the sodium salts on the characteristics of CoOx/SiO2 spheres
was investigated by means of XRD, nitrogen physisorption, SEM/TEM, ICPꢀMS, UVꢀVis,
XPS and H2ꢀTPR. On the basis of the experimental results, a possible mechanism on the
formation of hollow and mesoporous spheres was proposed. The CoOx/SiO2 spheres were
tested as catalysts for toluene oxidation. The mesoporous CoOx/mSiO2 was found to
exhibit superior activity to hollow CoOx/hSiO2, probably attributed to a combination of
several factors, including predominant existence of Co3O4 active phase, high surface Co3+
content, and easy reducibility of Co3+ at low temperature.
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Keywords: ultrasonic spray pyrolysis, hollow, mesoporous, molten salt, cobalt oxide
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1. INTRODUCTION
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Catalytic oxidation has been proposed as an effective and energyꢀsaving technology
for eliminating volatile organic compounds (VOCs) emitted from industrial processes and
transport vehicles.1–3 Recently, cobalt oxide (Co3O4) has attracted much attention to
control the emissions of VOCs, and it has been considered as a promising alternative to
costly noble metalꢀbased catalysts because of its high activity, relatively low price and
availability.4,5 However, the insufficient catalytic performance at low temperatures,
narrow operatingꢀtemperature window and poor thermal stability of Co3O4 largely limit
their performance and impede commercialization of VOCs removals.6–9
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Several studies have been conducted to improve the catalytic activity of
11 cobalt oxide by depositing it on various supports, such as Al2O3, ZrO2, TiO2, SiO2
12 and CeO2, etc.10,11 Among available support materials, ordered mesoporous silicas
13 synthesized by surfactantꢀtemplating methods have been investigated as promising
14 candidates for catalysis because of their own unique characteristics of high surface
15 area and regular pore structure, which can be used to uniformly disperse the active
16 metal/metal oxide particles.12–15 Numerous mesoporous silicas, such as MCMꢀ41,
17 SBAꢀ15 and KITꢀ6 loaded with cobalt oxides for VOCs oxidation have been
18 reported.16–20 The performance of the supported cobalt oxide catalyst is highly
19 dependent on different synthetic methods as well as precursors.21,22 Moreover, the
20 crystallinity, oxidation state and surface reducibility of supported cobalt oxide
21 species have been shown to be critical to the activity of the catalyst.23,24 For
22 preparing highly active catalysts in VOCs oxidation, it is important to obtain
23 Co3O4 oxide phase with high surface reducibility.
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Although these novel cobalt oxide/mesoporous silica composite catalysts
have contributed to recent progress in VOCs removals, they may pose problems in
practical field applications because of their high cost. Generally, these composites
are synthesized through solutionꢀbased batch processes that take several days in a
high pressure vessel and require further postꢀtreatments, such as solvent extraction
and calcination for template removals.22,26 Therefore, they are difficult to apply to
commercial scale production due to their long processing times, significant batchꢀ
toꢀbatch variation and high energy consumption.
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Compared to solutionꢀbased methods, the aerosolꢀassisted chemical vapor
10 decomposition, also known as spray pyrolysis, has distinct advantages of rapid
11 synthesis, high product purity, and more importantly, a continuous and scalable
12 process.27–31 This process involves atomization of a liquid precursor into fine
13 aerosol droplets that are delivered to a heated zone where solvent evaporation and
14 precursor decomposition take place, leading to the deposition of final solid
15 product. The detailed descriptions on this technique can be found in several
16 reviews.31–33 Recently, Debecker and coꢀworkers34 synthesized a series of
17 mesoporous WO3ꢀSiO2ꢀAl2O3 microspheres by the approach combining spray
18 pyrolysis and evaporation induced selfꢀassembly (EISA) of poly(alkylene oxide)
19 block copolymer. Nevertheless, most of the aerosolꢀbased methods reported
20 previously were carried out using expensive silicon precursors (e.g. silicon
21 chloride and silicon alkoxide) and sacrificial templating agents (e.g. surfactant
22 polymers and colloidal polystyrene) as starting precursors,35 which would increase
23 the manufacturing cost and add pollution to the environment.
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In this study, we first report the saltꢀassisted ultrasonic spray pyrolysis
fabrication of cobalt oxide/silica hollow and mesoporous spheres (detonated as
CoOx/hSiO2 and CoOx/mSiO2) using sodium silicate and cobalt nitrate hydrate as
precursors. Compared with traditional aerosolꢀassisted EISA method for preparing
mesoporous silica supported catalysts where costly precursors are employed, the
synthesis strategy demonstrated herein is much more inexpensive and energyꢀ
saving because no extra templating agents, or highꢀtemperature calcination are
required for template removals. The influence of salts on the characteristics of
produced spheres is studied in detail by various characterization tools. On the basis
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10 of the experimental results, the formation mechanism of hollow and mesoporous
11 spheres is proposed. Further, our experimental results reveal that, compared with
12 pristine Co3O4, the CoOx/mSiO2 demonstrates greatly improved catalytic
13 performance and longꢀterm thermal stability for toluene removal.
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2. EXPERIMENTAL SECTION
2.1 Catalyst preparation
Hollow and mesoporous CoOx/SiO2 composites were continuously synthesized by a
singleꢀstep aerosol process in a homeꢀmade apparatus depicted in Scheme 1. The sodium
silicate solution, which is recovered from the silicate waste of liquid crystal display (LCD)
manufacturing industry, is used as a lowꢀcost silicon source in this study.36,37 First, the
sodium silicate solution was prepared by mixing LCD industrial waste powder with 6 M
NaOH solution at room temperature for 3 h. The elemental composition of raw waste
powder and silicate supernatant after extraction with NaOH solution are summarized in
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Table S1. Thereafter, a certain amount of HCl or HNO3 was added to the 189 ml of waste
silicate supernatant to bring down the pH value to 2 with constant stirring. Meantime, a
calculated amount of cobalt nitrate (dissolved in 20 ml of DI water) was added into the
above acidified silicate solution and the combined mixture was stirred for 30 min. The
molar composition of the precursor mixture was 1 SiO2: 2 Na: 0.055 Co: 280 H2O: 8 HCl,
and 1 SiO2: 2 Na: 0.055 Co: 280 H2O: 3.7 HNO3, for CoOx/hSiO2 and CoOx/mSiO2,
respectively.
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The precursor mixture was nebulized by an ultrasonic atomizer (1.8MHz) as carried
by highꢀpressure air (35 psi). The temperature of the heating reactor was controlled at 500
oC, and the total synthesis time of this continuous flow process was approximately 5 s to
generate the CoOx/SiO2 particle. The asꢀsynthesized material was collected downstream
of the reactor with a high efficiency filter. Finally, they were recovered by washing and
filtration with DI water followed by drying in an oven at 110 oC.
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To verify the activities of CoOx/SiO2 catalysts, the aerosolꢀmade CoOx/HMSP
(hexagonally mesoporous silica particle), which was prepared using commercial silicon
precursor via traditional surfactantꢀtemplated method, was prepared with similar
procedures described in the synthesis of CoOx/SiO2 materials. Tetraethoxysilane (TEOS)
and cetyltrimethylammonium bromide (CTAB) was selected as the silicon source and
mesostructureꢀdirecting template, respectively. The molar gel composition of the mixture
was 1 SiO2: 0.18 CTAB: 0.055 Co: 10 ethanol: 80 H2O: 0.008 HCl. The asꢀprepared
CoOx/HMSP powder was then placed in a muffle furnace for 6 h at a calcination
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temperature of 550 C to remove the residual organic template. Detailed description on
the synthesized procedure can be referred to Wang and Bai.38,39 The cobalt loading on the
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silica support for all supported catalysts was ca. 5 wt.% according to our experimental
results.
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The pure Co3O4 used in this work was prepared by calcination of Co(NO3)2．4H2O
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at 500 oC for 4 h.
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2.2 Characterization
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The chemical composition of raw waste powder was determined by energyꢀdispersive Xꢀ
ray spectroscopy in a scanning electron microscopy (SEM, HITACHIꢀS4700). The
elemental cobalt content in the samples was analyzed by a SCIEX ELAN 5000
inductively coupled plasmaꢀmass spectrometer (ICPꢀMS). The morphology and structure
of the samples were characterized by using SEM (HITACHIꢀS4700), transmission
electron microscopy (TEM, Hitachi Hꢀ7100, Japan) and Xꢀray diffraction (XRD, Rigaku
D/MAXꢀB, Japan, using CuKα radiation n source at 1.54A˚). Nitrogen physisorption
isotherms of the synthesized materials were measured at ꢀ196 ◦C using a surface area and
pore diameter distribution analyzer (Micromertitics, ASAP 2020, USA). Pore volumes
were obtained from the volumes of nitrogen adsorbed at P/Po=0.95 or in the vicinity.
The UVꢀVis spectra were measured by a spectrophotometer (HITACHI U3010)
equipped with a diffuse reflectance integrating sphere coated with aluminum oxide which
served as a reference material. All the samples were recorded in the spectral range
between 900 and 275 nm. Xꢀray photoelectron spectroscopy (XPS) measurement was
performed on a PHI Quantera with a monochromatic Al Kα source and a charge
neutralizer. All the binding energies were calibrated to C 1s peak at 284.8 eV of the
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surface adventitious carbon. H2ꢀTPR experiments were performed with an AutoChem II
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2920 analyzer. The samples were preꢀtreated in air at 500 C for 2 h. After that, the
experiments were carried out from 50 oC to 900 oC at a heating rate of 10 oC/min in 10%
H2/Ar. The H2 consumption was determined by a thermal conductivity detector (TCD).
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2.3 Catalytic removal of gaseous toluene
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Catalytic tests were performed by a vertical and downward flow reactor system (i.d. = 0.8
cm). Catalysts were tested in 16ꢀ30 mesh (595ꢀ1190 ꢁm) powdered form, and a total of
0.1 g catalyst was packed into the middle of the glass reactor supported with thin layers
of glass wool on both sides. The reactant feed was composed of 1000 ppm toluene, O2
and N2 (balance). The toluene/O2 molar ratio was 1/200, and the total inlet flow rate was
controlled at 100 cm3/min, which corresponded to a space velocity (SV) of 20,000 hꢀ1 at
room temperature (25 °C). Prior to each catalytic reaction test, the samples were
pretreated under the reaction feed by heating at 120 °C for 2 h, and then cooled to room
temperature. Then, reaction temperature was increased stepwise from 100 °C to 500 °C.
The catalytic activity was evaluated in terms of toluene conversion. When the complete
conversion for toluene oxidation was obtained, the temperature was kept for 33 h to test
the reaction stability of the catalyst. To evaluate the reusability of the catalyst, the reuse
runs were implemented after the stability test. Each run was kept at 250 °C for 2 h, then
the reaction was cooled down to the room temperature. The gas composition after the
reaction was analyzed by a gas chromatograph equipped with a flame ionization detector
(FID) and a TCD detector.
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3. RESULTS AND DISCUSSION
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3.1 Catalyst characterization
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The wideꢀangle (10≦2θ≦80°) powder XRD patterns of asꢀprepared CoOx/hSiO2 and asꢀ
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prepared CoOx/mSiO2 are depicted in Fig. 1A. It can be seen that several diffraction
reflections are found in both asꢀprepared CoOx/hSiO2 and CoOx/mSiO2, which can be
indexed on crystalline NaCl and NaNO3, respectively. Such an observation can be
explained by considering the acidification reaction of sodium silicate by HCl or HNO3. In
the acidification reaction, the silica condensation reactions would occur to form a
siloxane linkage between surface silanol groups, which can be represented as:40
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Si(OH)4 + HOꢀSi(OH)3 →(OH)3SiꢀOꢀSi(OH)3 +H2O
(1)
And the acidification reaction of sodium silicate solution with HCl and HNO3,
respectively, can be expressed as:
Na2O．xSiO2 + 2HCl → xSiO2 ↓ + 2NaCl + H2O
Na2O．xSiO2 + 2HNO3 → xSiO2 ↓ + 2NaNO3 + H2O
(2)
(3)
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At this time, soluble sodium salts of NaCl or NaNO3 were inꢀsitu formed. During the
aerosol process (T= 500 oC), the rapid water evaporation would result in the solidification
of these sodium salts, which were then embedded in SiO2 spheres.
For washed CoOx/SiO2 samples (Fig. 1B), there is a broad reflection at 23o observed
for both two samples, indicating that silica materials are presented in amorphous phase.41
The lack of visible diffraction reflections due to crystalline NaCl or NaNO3 components
reveals that these sodium salts can be easily removed by washing with water. Moreover, it
is noticeable that three weak diffraction reflections at 2θ = 36.9, 59.4 and 65.2° are found
in CoOx/mSiO2. These diffraction reflections can be assigned to the spinelꢀstructured
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Co3O4.22 The presence of visible Co3O4 reflections reveals that the Co3O4 particles in
CoOx/mSiO2 might be presented in larger particle size or in agglomerated form. On the
contrary, both CoOx/hSiO2 and CoOx/HMSP show no observable reflections of Co3O4.
This could be attributed to that the smallꢀsized Co3O4 are uniformly distributed and the
size of Co3O4 particles are below the detection limit of the XRD instrument.42 The lowꢀ
angle (1≦2θ≦10°) powder XRD pattern (Fig. 1C) of CoOx/HMSP shows two wellꢀ
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defined diffraction reflections of (100) and (110) at 2θ of 2.4o and 4.3o, revealing the
wellꢀordered hexagonal mesostructure of CoOx/HMSP.39
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The particle morphologies of CoOx/SiO2 materials are revealed by SEM and TEM
images. As seen in Fig. 2A and 2B that CoOx/hSiO2 consists of mostly hollow shells or
fractured hollow shells (while remaining the spherical shape). In addition, many of these
shell structures contain comparatively rough and porous surface. In contrast, as illustrated
in Fig. 2D and 2E, spherical particles with wellꢀdefined morphologies are observed on
CoOx/mSiO2. It is noteworthy that some wellꢀdispersed tiny particles are locating on the
external surface or sitting inside the hollow spheres (Fig. 2C). The tiny particles could be
assigned to the presence of cobalt oxide particles. And it is found from the HRTEM
images (Fig. 2F) that the cobalt oxide particles mainly exist between the interconnected
pores of CoOx/mSiO2.
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Selected area electron diffraction (SAED) image (Fig. 2F, inset) confirms the
crystalline nature of the large Co3O4 particles (CoOx/mSiO2 sample). By contrast, no
clear diffraction pattern was observed for the CoOx/hSiO2 sample, suggesting that the
cobalt oxides are poorly crystallized. These findings are consistent with the XRD
measurements (Fig. 1A). The absence of diffraction reflections indicates that the
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crystalline domains are very small (at most a few nanometers) and points to
polycrystallinity. The CoOx/hSiO2 and CoOx/mSiO2 were further characterized through
mapping analysis to investigate the Co distribution. As shown in Fig. S2, it can be seen
that Co species are wellꢀdispersed in both hSiO2 and mSiO2 matrices. On the basis of the
above observations, it may conclude that the nature of sodium salt has significant impacts
not only on the morphology, but also on the crystallinity of the cobalt oxide particle of
CoOx/SiO2 composites.
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The pore structures of the prepared materials are further analyzed by the nitrogen
physisorption measurement, with results displayed in Fig. 3A. The CoOx/hSiO2 exhibits
type IV isotherms that possess twoꢀstep adsorption isotherm. This type of hysteresis loop
has been reported to be associated with hollow particles with nanoporous shell.43 An
adsorption at P/Po values in the range of 0.3ꢀ0.7 could be related to the capillary
condensation of nitrogen inside nanopores in the shells, while the second adsorption at
P/Po=0.7ꢀ0.9 corresponded to the filling of huge hollow cores, as observed in Fig. 2A and
2B. For CoOx/mSiO2 sample, a typical singleꢀstep type IV isotherms with H2ꢀtyped
hysteresis loop is observed, which is indicative of mesoporous material having networks
of interconnected pores.44,45 Similarly, CoOx/HMSP shows a singleꢀstep type IV
isotherms with clear hysteresis loops at a relative pressure of P/Po = 0.25−0.45, which is
attributed to the presence of intraꢀparticle mesoporosity originated from the surfactant
template.46,47
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The pore size distributions of CoOx/hSiO2 and CoOx/mSiO2 calculated by using
NLDFT method are shown in Fig. S1, and their physicoꢀchemical parameters derived
from nitrogen physisorption measurement are summarized in Table 1. It is observed that
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the formation of multiꢀmodal porous particles are obtained for both CoOx/hSiO2 and
CoOx/mSiO2. Such phenomenon presumably results from nonuniform solute distribution
within the droplet.48 Compared with CoOx/SiO2 samples, the CoOx/HMSP, which was
prepared through surfactantꢀtemplating route, clearly exhibits a narrow pore size
distribution, suggesting the existence of uniform mesoporosity. And it also exhibits the
highest surface area and largest pore volume among all catalysts studied (Table 1).
The effect of sodium salts on the physicoꢀchemical properties is also investigated
and the results derived from nitrogen physisorption measurement and ICPꢀMS analyses
are summarized in Table 1. It is seen that both asꢀprepared CoOx/SiO2 samples show high
Na contents, low specific surface area and no porosities. After washing process with
water, the surface area and porosities of both samples are significantly increased;
meanwhile, the Na content in the asꢀprepared CoOx/hSiO2 and CoOx/mSiO2 samples are
reduced to 0.42 and 0.44 wt. %, respectively, in the washed CoOx/SiO2 samples. The
above results clearly reveal that the NaCl and NaNO3 salts, which are inꢀsitu formed
during the acidification process of sodium silicate, can act as effective porogen, which
can be then readily removed by washing with water. On the other hand, the cobalt content
in the CoOx/hSiO2 and CoOx/mSiO2 is 3.09 and 3.95 wt. %, respectively, which is lower
than the theoretical content (5 wt. %) in the precursor solution. This is probably due to
the incomplete decomposition of cobalt precipitates that would result in some of the
cobalt content being carried away during the washing process. Thus, one can conclude
that the sodium salts have significant influences not only on the textural properties, but
also on the chemical composition of the CoOx/SiO2 samples.
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The UVꢀvis spectra of the supported cobalt catalysts are shown in Fig. 3B. For
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CoOx/hSiO2 and CoOx/mSiO2 samples, two absorption bands at 370ꢀ390 and around 700
nm are found, which can be related to the Co3+ in the octahedral coordination in the
Co3O4 and to the Co2+ in the tetrahedral coordination in the Co3O4, indicating the
presence of spinelꢀstructured Co3O4.24,49 For CoOx/HMSP, the wide bands at 525, 585 and
640 nm, typical for tetrahedrally coordinated Co2+ due to the metalꢀligand charge transfer,
are observed. They could be included in amorphous cobalt oxide clusters and Co2+ꢀ
silicate species.24 Thus, the spectra indicate the formation of various cobalt oxide species,
which are in different interaction with the support.
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The XPS analyses were conducted to determine the oxidation state of surface cobalt
species, with results shown in Fig. 3C and Table 1. In the Co XPS spectra, the peaks
located at 794.8−797.5 and 779.6ꢀ781.2 eV can be ascribed to Co 2p1/2 and Co 2p3/2 spinꢀ
orbital peaks, respectively.22 It can be seen the binding energy of Co 2p3/2 for
CoOx/mSiO2 is 779.6 eV, and the spinꢀorbit separation between Co 2p3/2 and Co 2p1/2,
ꢂ(Co 2p3/2ꢀ2p1/2) of CoOx/mSiO2 is 15.2 eV (Table 1). The above results are indicative of
the existence of Co3+ in Co3O4.22 For CoOx/hSiO2 and CoOx/HMSP samples, the binding
energies of Co 2p3/2 are increased to 780.9 and 781.2 eV, respectively, accompanying
with intense satellite peaks at ca. 785.5 eV, which have been reported as the evidence of
the progressive formation of Co2+ species. Meanwhile, the ꢂ(Co 2p3/2ꢀ2p1/2) of
CoOx/hSiO2 and CoOx/HMSP increase to 15.8 and 16.3, respectively, demonstrating the
decrease in the surface Co3+ content.22,50 Besides, the relative surface contents of Co3+
(Co3+/ (Co3++ Co2+)), which was calculated from peak I/peal II area in Fig. 3C, decreases
in the order of CoOx/mSiO2 > CoOx/hSiO2 > CoOx/HMSP. Accordingly, one can
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conclude that the synthetic procedure greatly influences the oxidation state of surface
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cobalt species. The surface Co3+ content is the highest on CoOx/mSiO2, while
CoOx/HMSP exhibits the lowest content of surface Co3+.
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The surface reducibility of the catalysts studied by the H2ꢀTPR measurement are
displayed in Fig. 3D. For CoOx/mSiO2 sample, four main reduction peaks centering at
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279, 482, 720 and 771 C can be clearly observed. By analogy with previous studies, the
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first peak is indicative for the reduction of Co3+ to Co2+, while the second peak is
associated with the reduction of CoO to metallic Co.20,21,50 The broad peak between 600
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to 900 C is attributed to the reduction of the remaining Co2+ and highly dispersed
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tetrahedral coordinated Co2+ꢀsilicateꢀlike species with strong interaction with the
support.51,52 Compared with CoOx/mSiO2, CoOx/hSiO2 sample displays a progressive
shift of the H2 consumption peak to higher temperatures, indicating decreased reducibility.
The TPR profile of CoOx/HMSP differs significantly from those of other samples. Three
weak reduction peaks can be found at 262, 380, and 535 oC, respectively, which could be
ascribed to the reduction of Co3+ and CoO to metallic Co. Additionally, a sharp reduction
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peak at 730 C with a shoulder at 806 C are also observed. They might be mainly
originated from the reduction of Co2+ and Co2+ꢀsilicateꢀlike species, and the above
observation is in good agreement with UVꢀVis and XPS results. It has been reported that
the nature of the support affects the particle size, and the lowꢀsurfaceꢀarea supports
promote the agglomeration of cobalt particles as larger Co3O4 crystals, which are easily
reduced in comparison with smaller particles.23,49 According to this, the highꢀsurfaceꢀarea
HMSP support may provide a suitable environment for the formation of smallꢀsized and
highly dispersed cobalt oxide species, which interact strongly with the support, and thus
had a detrimental effect on the surface reducibility.
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It has been reported that reduction of Co3+ species is one of the crucial factors that
determines the activity of cobalt oxide catalyst.22,53 Wu et al.21 showed that Co3O4ꢀCeO2
mixed oxides with a lower reduction temperature of Co3+ presented higher catalytic
activity in methane oxidation. It is clear that high surface reducibility favors high
catalytic activity since the reduction of reactive surface species frequently occurs at
relatively lower temperature. In this study, for the reduction of Co3+ in the TPR profiles,
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CoOx/mSiO2 has a relatively lower reduction temperature (279 C) and the highest H2
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consumption of Co3+ amount among all supported catalysts. This indicates that
CoOx/mSiO2 has the highest amount of easily reducible Co3+. On the other hand, in spite
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of the lowest reduction temperature of Co3+ (262 C), CoOx/HMSP shows the lowest H2
consumption amount of Co3+, suggesting that it contains the least amount of easily
reducible Co3+ species.
3.2 Formation mechanism
It is of particular interest to understand the formation mechanism of CoOx/hSiO2 and
CoOx/mSiO2 particles considering that the hollow shell and mesoporous morphologies are
obtained without the use of extra templates. On the basis of the aforementioned results, a
possible formation pathway of CoOx/hSiO2 and CoOx/mSiO2 particles is proposed in
Scheme 2. At first, precursor sols were firstly prepared via the acidification of LCD
wasteꢀderived sodium silicate solution by adding either HCl or HNO3; meanwhile, the
soluble sodium salts of NaCl or NaNO3 were inꢀsitu formed. Subsequently, cobalt nitrate
was added as a metal precursor.
o
As the precursor was aerosolized and entered a heated zone (500 C), the rapid
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water evaporation resulted in a temperature gradient within the aerosol droplet, with the
surface of the aerosol droplets being heated first and then heat being transferred to the
core. Secondly, when heating of the droplet proceeded, water containing dissolved salts
like NaCl diffused from the interface to the core due to the improved ion concentration at
the interface because of water evaporation; simultaneously, the silica and cobalt species
would crossꢀlink with each other and form cobaltꢀsilicate species by isolating the
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structural hydroxyl neighbors during the thermal treatment at 500 C. The cobaltꢀsilicate
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aggregate acted as a shell, and salts began to nucleate and be fixed by primary units.
Thirdly, when water evaporated completely, NaCl crystallized and aggregated as a core,
while the thermal decomposition of the precursor of cobaltꢀsilicate might yield cobalt
hydroxide, which was subsequently transformed to CoO/Co3O4. Finally, CoOx/hSiO2
hollow particles were produced after the removal of NaCl by washing with water.
In contrast to CoOx/hSiO2, CoOx/mSiO2 particle with wellꢀdefined morphology and
mesoporous structure is observed. Similarly, the NaNO3 diffused from the interface to the
core due to rapid water evaporation during aerosol process. As the process continued, the
NaNO3 (melting point = 308 oC) would become molten and served as a solvent, diffusing
back into the interface in response to the concentration gradient. After washing with water,
CoOx/mSiO2 having interconnected mesopores were obtained.
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Moreover, unlike NaCl, which is thermally stable up to 800 C, NaNO3 might be
being thermally decomposed in the present case. This was further confirmed by the
TG/DTG analyses. As seen in Fig. S3A, both asꢀprepared CoOx/hSiO2 and CoOx/mSiO2
o
samples showed initial weight losses from 100 to 300 C, which can be ascribed to the
evaporation of moisture on the catalyst surface, and the decomposition of cobalt
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precursor.54 Interestingly, it is noted that there is a drastic weight loss from 300 to 800 oC
on asꢀprepared CoOx/mSiO2 (Fig. S3A and 3B), which can be attributed to the thermal
decomposition of NaNO3.55 In spite of the rapid heating rates experienced by the droplet
in aerosol process, the residence time of the droplet in the heated zone (5 s) was too short
for complete decomposition of NaNO3. And this was verified by the XRD analysis (Fig.
1A), where diffraction reflections of crystalline NaNO3 were clearly observed.
Consequently, one may deduce that only a certain amount of NaNO3 was being thermally
decomposed in aerosol process.
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The decomposition products from NaNO3, such as O2, may further catalyze the
oxidation of Co2+ yielding Co3+, accompanying the formation of spinelꢀstructured Co3O4.
Although oxidizing gases may be produced from the inherent decomposition of cobalt
nitrate, the predominant existence of Co2+ species on both CoOx/hSiO2 and CoOx/HMSP
samples, as revealed by XPS studies, suggests that the impact of these low concentration
gases on the nature of supported cobalt species is negligible. The above results clearly
demonstrate that the NaCl and NaNO3 salts, which were inꢀsitu formed during the
acidification process of sodium silicate, played crucial roles on affecting the pore
structure, oxidation state and surface reducibility of CoOx/SiO2 materials.
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19 3.3 Toluene oxidation over various catalysts
20 In the blank experiment (without any catalyst), no conversion of toluene was
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21 detected below 500 C under the conditions of toluene concentration= 1000 ppm
22 and the GHSV= 20,000 hꢀ1, revealing that the direct thermal incineration of toluene
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was negligible under the adopted conditions. Fig. 4A displays the toluene
conversion as a function of temperature over CoOx/hSiO2, CoOx/mSiO2,
CoOx/HMSP and Co3O4 catalysts. Obviously, the toluene conversion increased
with the rise in reaction temperature and toluene was completely oxidized over all
catalysts below 500 oC. The final products in the reaction were only CO2 and H2O,
and no other byꢀproducts were detected.
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The activities of the catalysts are evaluated by T50 and T90, corresponding to
the temperature at 50% and 90% of toluene conversion, and decrease in the order
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of CoOx/mSiO2 (185 and 230 C) > Co3O4 (230 and 245 C) > CoOx/hSiO2 (260
10 and 290 oC) > CoOx/HMSP (400 and 430 oC). For VOCs oxidation over supported
11 cobalt catalysts, it is wellꢀknown that the catalytic performance is associated with a
12 number of parameters of catalyst, such as the crystallinity of supported cobalt
13 oxide, nature of the support, and reducibility of the cobalt species.21,22,56 The
14 support material having high surface area and large pore volume is reported to be
15 favorable for dispersing active particles and improving the efficiency of the
16 catalyst.57,58 In this work, however, it is worth pointing out that the catalytic
17 activity of CoOx/HMSP is much inferior to those of CoOx/hSiO2 and CoOx/mSiO2,
18 even though it consists of wellꢀordered mesostructure, and much higher surface
19 area and pore volume (872 m2/g, 0.61 cm3/g) than those of CoOx/hSiO2 and
20 CoOx/mSiO2 samples (411 m2/g, 0.34 cm3/g; 471 m2/g, 0.47 cm3/g). In this sense,
21 SBET, Vpore and pore structure are minor factors for affecting the toluene oxidation
22 in these supported cobalt catalysts.
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The metal content is one of the crucial parameters that affect the activity of
the supported catalyst. As shown in Table 1, the cobalt contents of the samples
decrease in the order of CoOx/HMSP > CoOx/mSiO2 > CoOx/hSiO2. However, the
order of the cobalt content is not fully in line with the results of catalytic tests. It is
therefore deduced that the cobalt content is not the key factor determining the
activity of these catalysts. Alternatively, the UVꢀvis spectra, XPS and H2ꢀTPR
analyses suggest that the amount of spinelꢀstructured Co3O4 with respect to Co3+
species decreases in the order of CoOx/mSiO2 > CoOx/hSiO2 > CoOx/HMSP,
which is perfectly consistent with the order of catalytic tests. According to the
10 literature, it is known that the predominant formation of Co3O4 oxide phase with
11 facile reducibility of Co3+ species on the support is advantageous for efficient
12 VOCs oxidation,16,17,24 which can well elucidate the highest catalytic activity of
13 CoOx/mSiO2 catalyst. For CoOx/hSiO2 sample, its openꢀhollow structure with
14 nanoporous shell might allow the toluene molecules access to the active cobalt
15 sites more easily; however, its relatively weaker surface reducibility resulted in a
16 higher reaction temperature.
17
On the opposite, the poorest performance of CoOx/HMSP should be related to
18 its highest amount of hardly reducible Co2+ and Co2+ꢀsilicateꢀlike species, which
19 are considered inactive in the studied reaction. Considering the results of structureꢀ
20 performance relationship, one can conclude that the presence of Co3O4 oxide phase
21 with high amount of easily reducible Co3+ species is favorable for high catalytic
22 activity on toluene oxidation. Besides, it is also found that the catalytic activity
23 over CoOx/mSiO2 is higher than unsupported Co3O4 catalyst. Thus it is reasonable
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to suggest that mSiO2 support shows a significant promoting effect for the activity
of cobalt oxide catalysts, and this could be related with the higher dispersion and
enhanced reducibility of cobalt oxide species on mSiO2.59,60
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3.4 Stability of CoOx/mSiO2 catalyst
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Fig. 4B depicts the timeꢀdependent catalytic performance of CoOx/mSiO2 under
o
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conditions of 1000 ppm toluene at a temperature of T100 (250 C). Since unsupported
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Co3O4 showed almost complete removal of toluene at 250 oC (Fig. 4A), its durability was
also investigated for comparison. Initially, the conversion of toluene over unsupported
Co3O4 slightly decreased from 98.6% to 95.1% and can be well sustained during the
following 16 h. Then it quickly decreased to 62% within 2 h and then completely
deactivated after running for 29 h on stream. Conversely, the toluene conversion over
CoOx/mSiO2 catalyst almost kept constant at 94% after running for 33 h, revealing its
excellent tolerance and catalytic stability. Fig. 4C shows the reuse performance of
CoOx/mSiO2 during 5 cycles of operation. It can be seen that the toluene conversion was
very stable and exhibited within 1% attrition during the 5 cycles of operation. On the
basis of the obtained results, one can conclude that the synthesized CoOx/mSiO2
possessed good stability and durability.
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The XRD and nitrogen physisorption analyses of the used Co3O4 and CoOx/mSiO2
samples were tested and shown in Fig. S4 and Table S2. The XRD patterns show that the
crystallinity of used Co3O4 sample diminishes considerably as compared with that of
fresh one. It is known that water is produced during VOCs oxidation and always has a
negative effect on the catalyst structure.61,62 In this work, the deactivation of unsupported
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Co3O4 catalyst could be associated with the collapse of active Co3O4 component by the
presence of moisture. Conversely, there is no significant difference observed in the XRD
and surface area results between fresh and used CoOx/mSiO2 samples. Moreover, the Co
2p region XPS spectra (Fig. S5) also reveal that after the catalytic test in toluene
oxidation, the signal intensities of used CoOx/mSiO2 only weakened slightly with respect
to fresh sample, and the binding energies were nearly unchanged, indicating that active
cobalt oxides can be wellꢀstabilized and effectively preserved on mSiO2 support during
consecutive reaction. This high stability of the active cobalt oxide particles together with
the mesostructure preservation are the key factors for the excellent activity of
CoOx/mSiO2 catalyst.
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4. Conclusions
By adopting a rapid and singleꢀstep saltꢀassisted aerosol method, hollow and
mesoporous CoOx/SiO2 spheres were synthesized without using extra pore templates. The
XRD, nitrogen physisorption, ICPꢀMS, UVꢀVis, XPS, and H2ꢀTPR results clearly showed
that the sodium salts, which were inꢀsitu formed during the acidification of sodium
silicate solution, had significant impacts on the crystallinity, cobalt content, textural
structure, oxidation state and surface reducibility of CoOx/SiO2 catalysts. The catalytic
behaviors of CoOx/SiO2 catalysts were further investigated in catalytic oxidation of
o
toluene at temperature range of 100ꢀ500 C. It was demonstrated that mesoporous
CoOx/mSiO2 performed better than the hollow CoOx/hSiO2, probably due to a
combination of several factors of predominant existence of Co3O4 active phase, high
surface Co3+ content, and easy reducibility of Co3+ at low temperature. This facile method
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could be extendable to the preparation of various hollow and mesoporous silica supported
composites for a wide range of environmental applications.
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Acknowledgment
5
The authors gratefully acknowledge the financial support from the Ministry of
Science and Technology of Taiwan, R.O.C. through grant No.: NSC 102ꢀ2221ꢀEꢀ009ꢀ
009ꢀMY3.
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Table 1
Physicochemical parameters of cobaltꢀbased samples.
SBET
VPore
Na content
Cobalt content
(wt. %)
Binding energy (eV)
ꢂ(Co 2p3/2ꢀ2p1/2)
Sample name
(m2/g)
(cm3/g)
(wt. %)
13
0.04
0.08
7.35
7.48
ꢀ
ꢀ
ꢀ
ꢀ
Asꢀprepared CoOx/hSiO2
20
Asꢀprepared CoOx/mSiO2
411
0.34
0.47
0.61
0.03
0.42
3.09
3.95
4.88
ꢀ
15.8 (796.7ꢀ780.9)
15.2 (794.8ꢀ779.6)
16.3 (797.5ꢀ781.2)
ꢀ
Washed CoOx/hSiO2
471
0.44
Washed CoOx/mSiO2
872
ꢀ
ꢀ
Calcined CoOx/HMSP
13
Co3O4
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DOI: 10.1039/C5RA27488E
Figure Caption
Scheme 1. Schematic drawing illustration the aerosol route and the washing process of
CoOx/hSiO2 and CoOx/mSiO2 particles.
Fig. 1. (A) Wideꢀangle XRD patterns of asꢀprepared CoOx/hSiO2 and asꢀprepared
CoOx/mSiO2; (B) Wideꢀangle and (C) lowꢀangle XRD patterns of washed
CoOx/hSiO2, washed CoOx/mSiO2, CoOx/HMSP and Co3O4.
Fig. 2. SEM, TEM and selected area electron diffraction images of CoOx/hSiO2 (A, B and
C) and CoOx/mSiO2 (D, E and F).
Fig. 3. (A) Nitrogen physisorption isotherm, (B) UVꢀVis spectra, (C) XPS spectra and (D)
H2ꢀTPR profiles of CoOx/hSiO2, CoOx/mSiO2, and CoOx/HMSP.
Scheme 2. Scheme representation showing the formation mechanism of CoOx/hSiO2 and
CoOx/mSiO2 through saltꢀassisted aerosol process.
Fig. 4. (A) Toluene conversion over (a) CoOx/hSiO2, (b) CoOx/mSiO2, (c) CoOx/HMSP,
and (d) Co3O4 (B) Timeꢀonꢀstream stability of toluene removal over
CoOx/mSiO2 and Co3O4 catalysts at operation temperature of 250 °C. (C)
Recycling tests for toluene removal over CoOx/mSiO2 at operation
temperature of 250 °C. (Reaction condition: toluene concentration: 1000
ppm, toluene/O2 molar ratio was 1/200 and SV: 20,000 hꢀ1).
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DOI: 10.1039/C5RA27488E
Scheme 1 Schematic drawing illustration the aerosol route and the washing process
of CoOx/hSiO2 and CoOx/mSiO2 particles.
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