Mesoporous Co3O4 catalysts for VOC elimination: Oxidation of 2-propanol

Article abstract of DOI:10.1016/j.apcata.2019.117366

Mesoporous cobalt oxides were prepared by an inverse micelle method and calcined at different temperatures. These materials exhibited superior activity for oxidation of 2-propanol as the model substrate for VOC elimination. Highest active Co₃O₄-350 material showed a maximum turnover frequency of 25.8 h^?1 at 160 °C at a weight hourly space velocity of 60 L g^-1 h^?1. The apparent activation energies of mesoporous cobalt oxides ranged from 69.7 kJ/mol to 115.6 kJ/mol. In situ Diffuse Reflectance Infrared spectroscopy (DRIFTS) revealed that the reaction involves formation of carbonyl and carbonate species on the surface of the catalyst before complete oxidation to CO₂ and H₂O. The activities of the materials were correlated to better low temperature reducibility, large pore volumes, higher Co³⁺/ Co²⁺ ratios, and higher number of surface-active oxygen species.

Full text of DOI:10.1016/j.apcata.2019.117366

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Mesoporous Co3O4 Catalysts for VOC Elimination: Oxidation of
2-propanol
Shanka Dissanayake, Niluka Wasalathanthri, Alireza Shirazi Amin,
Junkai He, Shannon Poges, Dinithi Rathnayake, Steven L. Suib
PII:
S0926-860X(19)30521-6
DOI:
https://doi.org/10.1016/j.apcata.2019.117366
APCATA 117366
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To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
1 August 2019
19 November 2019
26 November 2019
Please cite this article as: Dissanayake S, Wasalathanthri N, Shirazi Amin A, He J, Poges S,
Rathnayake D, Suib SL, Mesoporous Co3O4 Catalysts for VOC Elimination: Oxidation of
2-propanol, Applied Catalysis A, General (2019),
doi: https://doi.org/10.1016/j.apcata.2019.117366
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Mesoporous Co3O4 Catalysts for VOC Elimination: Oxidation of 2-propanol
Shanka Dissanayake a, Niluka Wasalathanthri a, Alireza Shirazi Amin b, Junkai He b,
Shannon Poges a, Dinithi Rathnayake a, and Steven L. Suib a, b,*
aDepartment of Chemistry, University of Connecticut. Storrs, CT 06269-3060
bInstitute of Materials Science, University of Connecticut, Storrs, CT 06269
Graphical abstract
Highlights
1

Mesoporous Co3O4 showed high activity for elimination of 2-propanol at low
temperature.
Better low temperature reducibility, large pore volumes, higher Co3+/ Co2+ amounts, and
higher number of surface-active oxygen species are crucial for higher activity.
Reaction involves formation of carbonyl and carbonate species on the surface of the
catalyst before complete oxidation to CO2 and H2O.
Long term durability and maintaining activity in the presence of water and carbon
dioxide make these catalysts attractive.
The developed catalytic protocol sheds light on non-precious metal catalysts for VOC
elimination.
Abstract
Mesoporous cobalt oxides were prepared by an inverse micelle method and calcined at different
temperatures. These materials exhibited superior activity for oxidation of 2-propanol as the
model substrate for VOC elimination. Highest active Co3O4-350 material showed a maximum
turnover frequency of 25.8 h-1 at 160 °C at a weight hourly space velocity of 60 L g-1 h-1. The
apparent activation energies of mesoporous cobalt oxides ranged from 69.7 kJ/mol to 115.6
kJ/mol. In situ Diffuse Reflectance Infrared spectroscopy (DRIFTS) revealed that the reaction
involves formation of carbonyl and carbonate species on the surface of the catalyst before
complete oxidation to CO2 and H2O. The activities of the materials were correlated to better low
temperature reducibility, large pore volumes, higher Co3+/ Co2+ ratios, and higher number of
surface-active oxygen species.
Keywords: Mesoporous Co3O4; Catalysis; 2-propanol; VOCs oxidation
1. Introduction
Volatile organic compounds (VOCs) are identified as major contributors of global air
pollution[1],[2],[3],[4]. These compounds are precursors for the formation of ozone,
photochemical smog, and secondary aerosols [5],[6],[7].They are released to the environment by
a variety of methods such as industrial processes, transportation, and domestic activities.
2

Industrial processes are comprised of chemical, power and pharmaceutical plants, gas stations,
petroleum refining, printing, shoemaking, food processing, automobile, furniture, and textile
manufacturing [8],[9],[10]. VOCs are the most plentiful and malignant chemical toxins in indoor
air. In addition, most VOCs are further recognized as carcinogens, mutagens, and teratogens to
humans [11],[12],[13].
Stern control of the VOCs evolved is required due to the extremely adverse effect of
VOCs on the environment and human health, in addition to the humongous boost in the extent of
these emitted compounds. Therefore, techniques and materials to moderate the levels of VOCs
are in high demand. Condensation, incineration, biological degradation, adsorption, and
absorption are typical processes for VOC elimination [14]. However, catalytic oxidation which
transforms these toxic compounds to CO2 and H2O is well accepted as one of the most up and
coming technologies for VOC abatement [15],[16],[17]. The utilized catalysts drop the activation
energy of the reactions and total oxidation of pollutants occurs at lower temperatures in
comparison to thermal combustion. Another benefit of catalytic oxidation is the use for odor
regulation and treatment of emissions with evaporated solvents [18],[19]. Control of temperature
for catalytic oxidation using exceptionally efficacious catalysts is a field studied extensively in
recent years. Two leading types of catalysts used are noble metal supported catalysts and
transition metal oxides [20],[21]. In comparison with scarce noble metals, transition metal oxides
are much more inexpensive, practical, and active in some reactions [3],[14]. In terms of
transition metal oxides, cobalt oxides have been demonstrated to be efficient catalysts for VOC
destruction [17],[22]. They have equivalent activities to noble metals and supported noble metals
making them a cheaper substitute [23],[24],[25].
Cobalt (II,III) oxide (Co3O4) exhibits a spinel structure and has a unit cell length of
0.8084 nm. As far as the unit cell is considered, lattice oxygens are cubic close packed where
Co2+ possesses one-eighth of the tetrahedral sites and Co3+ possesses half of the octahedral sites
[26]. Co3O4 materials have been synthesized by a variety of methods and the surface area,
surface active oxygen species, exposure to active crystal facets, and Co3+ ionic species are
reasons for high activity [27],[28]. Co3O4 is utilized as an effective catalyst for oxidation of
CO[29],[30], CH4[31],[32] , C2H2[28],[33], C3H8[34],[35], C7H8[24],[36], and chlorinated
VOCs [22],[37]. The activity of Co3O4 in elimination of the oxygenated VOC 2-propanol has not
been reported. Nonetheless, oxidation of 2- propanol has been studied with modified perovskite
3

catalysts.[16],[17],[38] Furthermore, most of the above reported materials were synthesized by
hard template or precipitation methods. Therefore, it is a necessity to investigate the activity of
Co3O4 synthesized by other methods such as the soft template inverse micelle method which
gives diverse properties to materials for oxygenated VOC removal.
In this study, well-ordered mesoporous cobalt oxides have been synthesized using an
inverse micelle strategy developed by our group which minimizes surfactant inorganic
interactions and solvent involved interactions. A unique NOx chemistry is employed to control
the hydrolysis and condensation of the oxo clusters which ultimately produces mesoporous
materials with monomodal pore size distributions and crystalline pore walls. Unique redox
properties, high surface area, and tunable porosity of these materials were the reasons for
concentrating on cobalt oxide synthesized by this evaporation induced self-assembly method. We
report the outstanding activity of mesoporous cobalt oxide materials for oxidation of 2-propanol
under oxygen deficient conditions. These materials maintain the catalytic activity in the presence
of water vapor (3%) and carbon dioxide (0.9%) in the feed gas. Intensified redox properties and
adjustable porosity of these materials improve the catalytic activity by reducing the solid state
diffusion distances [39]. The synthesized catalysts were characterized with powder X-ray
diffraction (PXRD), N2 sorption, X ray photoelectron spectroscopy (XPS), hydrogen temperature
programmed reductions (H2 TPR), scanning electron microscopy (SEM), and high-resolution
transmission electron microscopy (HR-TEM). Furthermore, cross sections of the nano clusters
were examined by TEM using a focused ion beam to prepare the samples. These characterization
techniques were utilized in order to probe factors that influence the catalytic activity by
establishing possible links between physicochemical properties of these materials and catalytic
activities.
2. Experimental Section
2.1 Chemicals
Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O, ≥98.0%), 1-butanol (anhydrous, 99.8%), poly-
(ethylene glycol)-block-poly(propylene glycol)-block-poly-(ethylene glycol) (PEO20-PPO70-
4

PEO20, Pluronic, P123), and concentrated sulfuric acid(H2SO4, 95−98%) were purchased from
Sigma-Aldrich. Commercial cobalt oxide (bulk Co3O4) purchased from Stremchem Chemical
Inc. company was 99.5% in purity. Concentrated nitric acid (HNO3, 68−70%) and 2-propanol
were purchased from J.T. Baker. All chemicals were directly used as received without further
purification.
2.2 Catalyst Synthesis
The synthesis was done by dissolving 0.0017 mol of Co(NO3)2. 6H2O in a solution containing
0.33 mol of 1- butanol, 0.038 mol of HNO3 and 4.31×10-4 mol of P123 in a 150 mL beaker. A
clear gel was formed after magnetic stirring at room temperature which was placed in an oven at
120 °C for 3 h. Then the product was washed with ethanol four times, centrifuged, and dried in a
vacuum oven. Powders obtained were calcined at 150 °C for 12 h with a heating ramp of 10 °C
per minute.
These samples were further calcined at 250 °C, 350 °C, and 450 °C for 3 h with a heating ramp
of
2 °C per minute and samples were named as Co3O4-X (X=150, 250, 350, and 450) respectively.
2.3 Material Characterization
Powder X-ray Diffraction XRD studies were done on a Rigaku Ultima IV diffractometer using
Cu Kα (λ=0.15406 nm). A beam voltage of 40 kV, and 40 mA was used. Wide angle PXRD
diffraction patterns were collected over a 2θ range of 10º –75º.The nitrogen sorption experiments
were done using a Quantachrome Autosorb iQ2 automated sorption system. All samples were
degassed at 150 °C for 5 h under vacuum prior to the nitrogen sorption experiment. The
Brunauer-Emmett-Teller (BET) method was used for the surface area analysis and the Barrett-
Joyner-Halenda (BJH) method was used on the desorption branch of the isotherm to determine
the pore size distribution.
The morphologies of the metal oxides were studied using a FEI Nova Nano SEM 450 FESEM
with a Schottky emitter at an accelerating voltage of 2.0 kV and a beam current of 1 mA. TEM,
HR TEM and selected area electron diffraction (SAED) images were collected using a Talos
F200X microscope operating at 200 kV. FIB sample preparation was done using an FEI Helios
460F1 FIB. H2 TPR experiments were conducted in a Thermolyne 79300 model temperature
5

programmable tube furnace equipped with a Cirrus MKS Residual Gas Analyzer with a
quadrupole mass selective detector. About 100 mg of the catalyst was packed into a quartz tube
reactor which was mounted into a furnace. After that, physisorbed gases were removed from the
catalyst surface by using a 50 sccm Ar flow for 30 minutes at room temperature. Finally, the
catalysts were heated under a steady flow of 5% H2 in He gas mixture from 100 °C to 700 °C at
a ramp rate of 10 °C min-1 for desorption. TPD experiments were conducted with the use of Ar
(20 sccm) using the same set up. XPS was carried out on a PHI model 590 spectrometer with
multiprobes (ΦPhysical Electronics Industries Inc.). Al-Kα radiation (λ = 1486.6 eV) was used
as the radiation source. Binding energies were measured for C 1s, O 1s, and Co 2p transitions.
The binding energy value of 284.6 eV for C 1s was used to calibrate binding energies. In situ
diffuse reflectance infrared spectroscopy (In situ DRIFTS) was performed on a Nicolet 8700
FTIR spectrometer with an
In situ cell to clarify the intermediate species during 2-
propanol oxidation and its mechanism. The catalyst was packed into the sample holder. After
that, physisorbed gases were removed from the catalyst surface by using a 100 sccm He flow for
30 minutes at 150 °C. After that, samples were pretreated with He for one hour at 150 °C. Then
2-propanol was introduced to the catalysts by passing ultra-zero grade air through a 2-propanol
bubbler for 15 minutes at a flow rate of 20 sccm. Next, the catalyst was heated from 100 °C to
350 °C with an air atmosphere. All spectra were recorded with a 4 cm-1 resolution and 64 scans.
2.4 Catalytic activity tests
The oxidation of 2-propanol over cobalt oxide materials was carried out in a fixed bed flow type
quartz tube reactor (~ 0.4 cm internal diameter) under atmospheric pressure. About 100 mg of
the catalyst was loaded in the middle of the quartz reactor supported by quartz wool and
pretreated with flowing UHP He (50 sccm) for 1 h at 200 °C to clean the catalyst surface. Ultra-
zero grade air (21 % O2) was bubbled in the 300 mL solution of 2-propanol at a flow rate of 20
sccm using a MKS digital mass controller to carry out the catalytic reaction giving a weight
hourly space velocity (WHSV) of 12 Lh-1g-1. The temperature range for the reaction was 100-
340 °C. Reaction products were analyzed after a stabilization time of ~20 minutes on stream.
The product gas stream was analyzed online after passing through a cold trap (dry ice) with SRI
8610C gas chromatographs (GC) equipped with a thermal conductivity detector (TCD), flame
ionization detector (FID), a 6-foot-long molecular sieve 13X packed column, and a 6-foot-long
6

silica gel packed column, and a 6-foot-long Haysep D packed column. Helium was used as the
carrier gas of the GC. The organic products collected in the cold trap were analyzed on an 7820A
GC system equipped with a mass selective detector 5975 series from Agilent Technologies. 2-
propanol concentrations of the feed gas and the reactor outflow were calculated using the moles
of oxygen reacted. The oxygen consumption was calculated based on oxygen concentration
which was calculated using nitrogen as an internal standard [18]. The effect of water vapor was
studied by using a bubbler with 1:1 volume ratio of 2-propanol to water (3% water vapor) in the
stream at 25 °C. Carbon dioxide (0.9% CO2) was introduced by bubbling 1% CO2 balanced with
air through 2-propanol. Saturated vapor pressures of 2-propanol at 25 °C and corresponding flow
rates of air in an open system (1 atm and 25 °C) were used to calculate the initial concentration
of 2-propanol.
3. Results
3.1 Characterization of mesoporous cobalt oxide catalysts
3.1.1 Powder X-ray diffraction (PXRD)
The PXRD patterns for the synthesized mesoporous cobalt oxides calcined at 150 °C,
250 °C, 350 °C, and 450 °C together with the commercial material are given in Figure 1. All
materials except Co3O4-150 conformed to the spinel Co3O4 crystalline pattern (JCPDS 01 043
1003). Co3O4-150 showed diffraction lines that can be indexed to a mixed phase of Co3O4 and
cobalt acetate hydrate (C4H6CoO4·4H2O). However, after calcination at 250 °C the crystal
structure was completely transformed to Co3O4. When the catalysts calcined at 250 °C, 350 °C,
and 450 °C are considered, the wide angle PXRD peak intensities and sharpness gradually
increased with increasing calcination temperatures. The highest intensity and the sharpest peaks
were observed for the commercial sample. The peak intensity is a measure of the crystallinity
whereas the sharpness is a measure of crystallite size. Therefore, the crystallinity and crystallite
size increase as the calcination temperature increases from 250 °C to 450 °C and the commercial
sample has the highest crystallinity and crystallite size.
7

Figure 1. Wide angle powder X-ray diffraction patterns of mesoporous cobalt oxide materials
and commercial Co3O4 material.
3.1.2 N2 Sorption
Figure 2 shows the nitrogen sorption isotherms of the mesoporous cobalt oxide materials
together with the bulk cobalt oxide material. BET surface areas, BJH pore volumes, and pore
diameters are summarized in Table 1. The Co3O4-150 shows a type III adsorption isotherm
indicating the nonporous nature of the samples. The curve is convex to the P/Po axis at higher
relative pressure values. On the contrary, Co3O4-250, Co3O4-350, and Co3O4-450 demonstrate
type IV adsorption isotherms with an H1 hysteresis loop [40]. The occurrence of pore
condensation is implied by the hysteresis loops. This is an indication of the existence of an
ordered mesoporous structure. The highest surface area is for Co3O4-250 (121 m2/g). As the
calcination temperature is raised the surface areas decrease for the materials calcined at 350 °C
8

(83 m2/g) and 450 °C (36 m2/g). The bulk Co3O4 shows a type III adsorption isotherm indicating
the nonporous nature of the samples and surface areas of the mesoporous materials are
significantly higher compared to the bulk material (3 m2/g).
(a)
(b)
Figure 2. N2 sorption measurements: (a) N2 sorption isotherm, (b) BJH pore size distributions.
The BJH pore sizes assuming cylindrical pores are given in Figure 2 (b). When the materials
calcined at 250 °C, 350 °C and 450 °C are considered, the pore sizes increased as the calcination
temperature increases implying that the pores expand as the calcination temperatures increase.
BJH pore volume is highest for Co3O4-350 (0.331 cc/g).
9

Table 1.
Physical properties and surface elemental analysis of the catalysts discussed in the manuscript.
BJH
aCo3+/Co2+
aOads/Olatt
Surface
area
BJH pore
diameter
(nm)
pore
Material
volume
(cc/g)
(m2/g)
-
-
Co3O4 – 150
7
0.034
0.284
0.331
0.237
0.057
1.4
6.6
2.50
2.63
2.00
-
0.67
0.90
0.58
-
Co3O4 – 250 121
Co3O4 – 350 83
Co3O4 – 450 36
Co3O4 – Bulk 3
9.6
17.9
1.7
a Determined from XPS spectra. - Not determined.
3.1.3 Scanning electron microscopy
Scanning electron microscopy and transmission electron microscopy were used to study
the morphology of the materials. Figure 3 gives the SEM images of the mesoporous cobalt oxide
samples. The morphology of the samples consists of spherical clusters with nano size plates. The
opening of the pores are increased with heat treatment. The cluster diameter is about 3 µm. The
rod length is around 1 µm and width is about 200 nm. The enhancement of the pores with heat
treatment could be observed by the SEM images. The uniform porosity of the material calcined
at 450 °C is visible.
10

Figure 3. Scanning electron microscope images of mesoporous cobalt oxide. (a) Co3O4-150 (b)
Co3O4-250 (c) Co3O4-350 (d) Co3O4-450.
3.1.4 Transmission electron microscopy
Figure 4 shows the TEM and HRTEM images of the mesoporous cobalt oxide materials.
TEM analysis reveals obvious size changes of the materials upon increasing calcination
temperature. The particle sizes were 4.6 ± 0.9 nm, 9.0 ± 1.8 nm, and 14.3 ± 1.9 nm for the
calcination temperatures 250 °C, 350 °C, and 450 °C respectively. The HR TEM images of
Co3O4-350 give insight to the mesopores in the material. These images show crystalline pore
walls with well-defined lattice fringes. The nanoparticles acquire a polyhedral shape.
Furthermore, they are randomly packed to form the mesoporous network. This confirms the
crystalline nature of the nanoparticles. The d spacing values of cobalt oxide calcined at 350 °C
turned out to be 0.46, 0.28, 0.24, and 0.20 corresponding to (111), (220), (311), and (400) planes
respectively. Phase identification was done from SAED images from regions of the Co3O4-350
particles by calculating the lattice spacing and then were comparing with the JCPDS values
(JCPDS 01 043 1003) as shown in Figure 4. SAED patterns reveal the pure crystalline nature of
the annealed particles with cubic crystal structure, where (111), (220), (311), (400), (511), and
(440) lattice planes were clearly indexed in agreement with the PXRD data. Figure S1 shows the
11

FIB prepared TEM images of the mesoporous cobalt oxide materials. Feather like morphology
is seen inside the nanoclusters. These images show the origin of the mesoporosity of the
material. FIB prepared HR-TEM images confirm the mesopores inside the spherical
nanoclusters.
(a)
(c)
(b)
(e)
(d)
(f)
(e)
Figure 4. Transmission electron microscope images of (a) Co3O4-250, (b) Co3O4-350, (c)
Co3O4- 450, (d) FIB-TEM Co3O4-350, (e) HR TEM Co3O4-350, (f) SAED pattern of Co3O4-350.
3.1.5. H2 Temperature programmed reduction
H2 temperature programmed reduction was done in order to evaluate the redox properties
of the materials (Figure 5). The reducibility of as prepared samples was evaluated by H2 TPR to
differentiate the activity of surface oxygen. Generally, the reduction of cobalt oxide is known to
occur through two steps. At lower temperature Co3O4 is first reduced to CoO followed by a
second reduction step of CoO to Co at higher temperature [23], [24],[41],[42]. The first reduction
(a)
peaks of Co3O4-350, Co3O4-250, Co3O4-450, and Co3O4-bulk are 273, 305, 326, and 380 °C
12

respectively. Therefore, Co3O4-350 showed the lowest onset reduction temperature among all
samples.
Co O -bulk
3 4
Co O -450
3 4
399
Co O -350
3 4
305
344
Co O -250
3 4
273
421
326
463
380
100
200
300
400
500
600
700
o
Temperature ( C)
Figure 5. H2 TPR profiles of Co3O4-250, Co3O4-350, Co3O4-450, and Co3O4-Bulk.
3.1.6. Catalytic activity evaluation
Turnover frequency (TOF) vs temperature profiles of the mesoporous materials were
used to evaluate the catalytic performance for 2-propanol oxidation. Commercial cobalt oxide is
also included for comparison. The catalytic activities are compared by means of reaction
temperature at which the TOF of 2-propanol is 10%, 50%, and 90% of the maximum TOF
achievable. Table 2 summarizes the catalytic performance of the materials. No CO was detected
in all catalytic experiments and minute amounts of acetone was detected in the cold trap under
experimental conditions. The carbon balance between 2-propanol and total oxidation product
CO2 was found to be over 90%.
13

The catalytic activity was evaluated at 12 L g-1 h-1 and 60 L g-1 h-1 weight hourly space
velocity (WHSV) values. The light off curves of the cobalt oxides at these WHSV are given in
Figures 6 (a) and Figure 6 (b). As far as the lower WHSV is considered, mesoporous cobalt
oxides are more active (Lower T10 temperature) compared to the non-porous cobalt oxide
material. The order of T10 temperatures increases in the order: Co3O4-350 < Co3O4-250 <
Co3O4-450 < Co3O4-Bulk. T90 values increase in the order Co3O4-350 < Co3O4-250 < Co3O4-
450 < Co3O4-Bulk. The order of T10 and T90 follows the same trend at a WHSV of 60 L g-1 h-1
as shown in Figure 6 (b). Obviously, mesoporous materials show lower light off temperatures
and the material calcined at 350 °C exhibits the highest activity at both lower and higher WHSV
values.
Moles of 2-propanol converted were determined by the concentration change as well as the
simultaneous CO2 generation and these results are shown in Figure S5a and S5b. These plots
emphasize the simultaneous generation of carbon dioxide with the conversion of 2-propanol.
Furthermore, the catalytic activity follows the same trend as above. Moreover, we considered
varying percentages of oxygen in the gas stream (21,30, and 40%) at 200 °C where at 40%
oxygen 2-propanol becomes the limiting reagent. Here, the total amount of 2-propanol (6.4%) in
the gas stream gets converted to CO2 (Figure S5c).
CoMn2O4 catalysts have been reported for 2-propanol oxidation and complete conversion
with only 0.2% propanol in the stream was achieved at 350 °C [17]. CoCr2O4 catalysts have been
reported for the complete conversion of 2-propanol at 300 °C under similar conditions [38].
Furthermore, Au/CeO2 catalysts have been reported for 2-propanol elimination with only 1
mol% 2-propanol at 150 °C [43]. Compared to these literature data which even use noble metals
as catalysts, the findings presented here of 4.7% of 2-propanol conversion is promising.
3.1.7. Kinetic Studies
The onset of 2-propanol oxidation is governed by the kinetics of the reaction. To
investigate the kinetics, apparent activation energies were calculated when oxygen conversion
was lower than 20%. Due to the importance of mass and heat transfer, the operating temperatures
are lower than light off temperatures [44],[45]. Moreover, at low oxygen conversions mass
diffusion limitations can be considered as negligible [45],[46]. Apparent activation energy can be
utilized to evaluate the catalytic performance of materials in a reaction and calculated values
14

based on Arrhenius plots given in Figure 6 (c) are shown in Table 2. The calculated apparent
activation energies for the catalysts follow the order Co3O4-350 (69.7 kJ/mol) < Co3O4-250
(108.9 kJ/mol) < Co3O4-450 (115.6 kJ/mol) This trend is perfectly in line with the increasing
order of T90 temperatures of the catalysts. The lower the Ea value, the more feasible the
oxidation of 2-propanol. This result obtained from the kinetic experiment is consistent with the
performance of the catalysts. The activation energy for the mesoporous materials are much lower
than the bulk materials. This is proof of the lack of gas diffusion limitations in materials with
mesopores [18].
15

(a)
(b)
6
5
4
3
2
1
0
30
25
20
15
10
5
Co O -250
3 4
Co O -350
3 4
Co O -450
3 4
Co O -Bulk
3 4
Co O -250
3
4
Co O -350
3
4
Co O -450
3
4
Co O -Bulk
3
4
0
100
150
200
250
300
350
100
150
200
250
300
350
Temperature (oC)
Temperature (oC)
(c)
(d)
6
5
4
3
2
1
0
-13.6
-14.0
-14.4
-14.8
-15.2
-15.6
Co O -250
3 4
Co O -350
3 4
Co O -450
3 4
Co O -Bulk
3 4
Without Contaminant
With H O
2
With CO
2
0.0021
0.0022
0.0023
1/T (K-1)
0.0024
0.0025
100
150
200
250
300
350
o
Temperature ( C)
Figure 6 Activity of cobalt oxides for 2-propanol oxidation. (a) 6.4% 2-propanol in air, WHSV
12 L g-1 h-1 (b) 6.4% 2-propanol in air WHSV 60 L g-1 h-1. TOF = moles of 2 propanol converted
per hour/ moles of catalyst. (c) Arrhenius plots and apparent activation energy calculations with
6.4% 2 propanol in air WHSV 60 L g-1 h-1 (d) Activity for 2-propanol oxidation under the
presence of 3%H2O and 0.9 %CO2 with 6.4% 2-propanol in air, WHSV 12 L g-1 h-1.
16

Table 2. Summary of catalytic activity and calculated activation energies (Ea).
Material
T10 (°C)
T50(°C)
T90(°C)
c Ea (kJ/mol)
a
b
a
b
a
b
Co3O4-250
Co3O4-350
Co3O4-450
Co3O4-Bulk
140
128
199
241
160
137
174
197
171
148
209
250
168
149
188
209
192 177
158 157
218 196
259 217
108.9
69.7
115.6
180.2
a WHSV 12 L g-1 h-1. b WHSV 60 L g-1 h-1. c WHSV 60 L g-1 h-1 and calculated from the
Arrhenius plots for 2-propanol conversion with oxygen conversion below 20%.
3.1.8. Effect of water vapor, carbon dioxide, and results of the stability test
The 2-propanol oxidation under moisture rich conditions (~3%) was performed for the cobalt
oxide calcined at 350 °C at WHSV 12 L g-1 h-1. After sending about 3% moisture into the feed
gas stream, The T90 of the catalyst shifts slightly to a higher temperature of 165 °C from 157 °C
(Figure 6 (d)). Furthermore, the catalytic activity after introducing 0.9% CO2 in a WHSV of 12
L g-1 h-1 was evaluated. Here the T90 temperature shifted to 178 °C. The catalytic stability tests
were conducted on Co3O4-350 to investigate whether the catalysts were deactivated under the
feed gas stream. As shown in Figure S2 Co3O4-350 was extremely stable for 2-propanol
oxidation at 240 °C. No decrease in activity was observed for ~48 h (~2 d) as far as the Co3O4-
350 material is considered.
The sample did not show any transformations according to the PXRD analyses after the
stability tests. Similar PXRD patterns were observed for the catalyst before and after the stability
tests as shown in Figure 7 (a). This indicates that the material is thermally stable under
experimental conditions. The morphology of the material after the stability test was observed
using TEM and SEM (Figure S2). No significant change was observed by TEM and SEM as
17

compared to the fresh catalyst. The absence of phase transformations and segregation were
confirmed by HR-TEM images by the clear lattice fringes. The exposure to high temperature for
a longer period of time does not change the structure or the morphology of the material.
Temperature programmed desorption was carried out for the Co3O4-350 after being tested for
stability. This was done in order to identify the species that were adsorbed on the catalysts after
48 h of reaction. Figure 7 (b) shows the results obtained in this experiment. The only gas
desorbed in the experiment is carbon dioxide (327 °C). This can be attributed to the dissociation
of carbonate species that may be formed during the reaction.
(a)
(b)
Figure 7. (a) PXRD patterns of Co3O4-350 before and after the stability test. (b) CO2 TPD after
stability test.
PXRD patterns of the catalysts were collected after the activity measurements (Figure
S4) and the patterns showed a mixed phase consisting of CoO and Co3O4. The intensity of the
Co3O4 peaks became less and peaks of CoO appeared. This indicates that the materials get
reduced when 2-propanol is oxidized as the reaction temperature is increased to 340 °C. The
stability test confirms that there is no reduction of the material at 240 °C. TPO experiments were
done for the catalysts after the reaction to notice any coke formation during the reaction. No peak
was observed for carbon dioxide indicating that there is no coke formation during the reaction.
3.1.10. X-ray photo electronic spectroscopy
XPS is a technique which is based on the photoelectric effect whereby energy of the incident
photons is expended in the ejection of a photoelectron. This is a useful technique to study the
18

surface composition, metal valence states, and local atomic environments of a material. Figure 8
shows the XPS spectra of the mesoporous cobalt oxide materials. The presence of Co3O4 is
confirmed by Co 2p3/2 and Co 2p1/2 peaks appearing at about 780.0 eV and 795.0 eV
respectively. Since valence states of cobalt may have a high impact on catalytic activity, the Co
2p3/2 peak was deconvoluted to Co2+ and Co3+ peaks as shown in Figure 8 (a). The Co 2p peaks
at binding energies of ~779.6 eV and ~794.7 eV are characteristic of Co3+ whereas Co 2p peaks
at binding energies ~781.4 eV and ~796.5 eV are characteristic of Co2+ [24,41,42]. Table 1
summarizes the Co3+/Co2+ ratios of the catalysts. Figure 8 (b) shows the O 1s spectra of the
samples. These are deconvoluted to 529.5-529.6 eV, 531.1-531.3 eV, and 532.6-532.9 eV
which are ascribed to surface lattice oxygen, adsorbed oxygen species, and chemisorbed water
respectively [24,41,42]. Table 1 shows the Oads/Olatt of the catalysts.
(b)
(a)
Figure 8. (a) Co 2p XPS spectra and (b) O 1s spectra of the Co3O4 catalysts.
19

CH3
CO32-
o
CH3 CO32-
350 C
1126
1156
1207
1230
1260
1278
1321 1424
1372 1463
1565
o
300 C
o
250 C
o
200 C
OH
o
150 C
3377
o
100 C
C=O
1650
1700
1737
CH3
2972
1050 1260 1470 1680 1890 2760
2990
3220
3450
Wavenumber (cm-1)
Figure 9 In situ DRIFTS spectra of adsorbed 2-propanol on Co3O4-350 catalyst at different
temperatures.
DRIFT spectra, carried out on Co3O4-350 is displayed in Figure 9. At 100 °C it is possible to
notice a broad band at about 3263-3500 cm-1, associated with the vibration of surface hydroxyl
group [47],[48]. The peak at 2972 cm-1 represents C-H stretching vibrations from the isopropyl
H3C group. The band at 1700 cm-1 is due to the stretching of aliphatic C=O group [49],[50]. The
bands at 1424,1463, can be attributed to asymmetric vibrations of non-coordinated carbonate
species whereas the bands at 1565 cm-1 can be attributed to vibration of carboxylate species
[51],[52]. The bands at 1321 and 1372 cm-1 were due to the splitting of the symmetric CH3
scissors mode. Furthermore, the isopropyl group shows two characteristic bands at 1126 cm-1
and 1156 cm-1 due to its skeletal vibration [49]. The bands at 1230, 1260, and 1278 cm-1
symmetric vibration of bidentate carbonate species [51],[53]. As far as temperatures from 150 °C
to 350 °C are considered, bands for stretching vibrations of water and splitting of the symmetric
CH3 scissors (1372 cm-1 and 1321 cm-1) remain. Furthermore, bands characteristic of isopropyl
20

group at 1126 and 1156 cm-1 persist with lower intensity. Moreover, bands for carbonate groups
remain in similar intensity until 200 °C and start to increase after 200 °C. Another important
feature is that the intensity of the band at 1700 cm-1 significantly decreases at 150 °C whereas
bands at 1650 and 1730 cm-1 appears. In addition, the band at 1700 cm-1 completely disappears at
200 °C while 1650 and 1730 cm-1 remain. However, these three bands disappear at temperatures
higher than 200 °C.
Discussion
Mesoporous cobalt oxide was used as a catalyst for the total oxidation of 2-propanol in
this study. The synthesis of the cobalt oxide was done using the sol gel based inverse micelle
method. Cobalt nitrate was used as the metal precursor and 1-butanol was used as the interface
modifier and the solvent. Nitric acid hinders the condensation at lower pH. The surfactant used
for the synthesis was P123 which forms inverse micelles which act as nanoreactors. Furthermore,
uncontrolled aggregation is prevented by the individual surfactant molecules by acting as a
barrier between metal oxo clusters. The nitrate ions form nitric oxides (NOx) by thermal
decomposition. The formed NOx species adsorb on the metal oxo clusters to prevent
uncontrolled condensation and they assist the oxidation of oxo clusters.[54] The heat treatment at
150 °C for 12 hours is necessary for the removal of the adsorbed NOx and COO- groups. The
heat treatment at 250 °C produces the desired Co3O4 phase for catalysis. Calcination
temperatures of 250 °C, 350 °C, and 450 °C were used to tune the structural and surface
properties of the materials. The ramp rates and durations of the calcination seem to play an
important role in tuning the properties of the materials synthesized by the inverse micelle method
as emphasized by previous studies [55]. Interparticle voids of randomly packed monodispersed
nanoparticles are responsible for the mesoporosity as evidenced by type IV adsorption isotherms
[56]. The particle sizes were 4.6± 0.9 nm, 9.0 ± 1.8 nm, and 14.3 ± 1.9 nm for Co3O4- 250,
Co3O4-350, and Co3O4-450 respectively indicating the increase of size with increasing
temperature. The mesoporous materials synthesized show high surface areas (up to 121 m2/g)
compared to the commercial cobalt oxide (3 m2/g). Exposure of lattice oxygen to the catalytic
surface is enhanced by the large surface areas and the nanoparticle nature of the materials.
The activity of the catalysts depends on several parameters such as surface area, porosity,
active phase, surface oxygen species, oxidation states of metals, and reducibility [30]. The
21

unique pore structure of the mesoporous cobalt oxides is crucial for the high catalytic activity of
these materials over bulk materials. The adsorption of reactants and diffusion of products is
facilitated by mesopores. The largest pore volume is shown for the highest active Co3O4-350
catalyst which gives the lowest T90 values both in higher and lower WHSV values. A catalyst
with high surface area can expose more active sites for oxidation reactions, accounting for the
higher activity of mesoporous materials over bulk cobalt oxide. Furthermore, this accounts for
low mass diffusional distances in the catalyst giving high activity at low temperature.
The Mars-van-Krevelen mechanism is important for oxidation of VOCs [17],[57],[58].
Here, the organic molecule is oxidized by the lattice oxygens of the oxide, and these sites get re-
oxidized by gas phase oxygen. The presence of metals in different oxidation states which could
be oxidized or reduced is important for this type of mechanism [16]. XPS can be used to identify
the concentration of the difference valence states of elements. The peak area percentages of
different cobalt oxidation states are given in Table 1. The Co3+/Co2+ ratios for catalysts calcined
at 250 °C, 350 °C, and 450 °C were calculated to be 2.50, 2.63, and 2.00. This shows that
Co3O4-350 has the highest amount of Co3+. Temperature programmed reduction experiments are
useful for the evaluation of metal ion concentrations as reported in the literature[23],[24],[42]
As far as
Figure 5 is considered, the intensity of the lowest reduction peak of Co3O4 350 is
higher than other samples. This indicates the presence of higher amounts of Co3+ in Co3O4-350
in conformity with XPS analysis. The higher amount of Co3+ may increase the anionic defect
composition which results from larger surface areas and specific porous channels [27]. The
catalytic activity is related to the higher concentration of Co3+ as detailed in the literature
[23],[24],[30],[41],[59]. Co3+ cationic species assist the total oxidation of formaldehyde [27].
Furthermore, previous studies have found that in CO oxidation reactions, CO molecules interact
preferably with surface Co3+ cations, which is supported experimentally and theoretically
[30],[60]. Therefore, Co3+ cations may be acting as favorable sites for 2-propanol adsorption.
Moreover, low temperature reducibility is a crucial factor for catalytic activity of materials [61].
The highest reducibility is shown by Co3O4-350. Therefore, catalytic activity is closely related to
the low temperature reducibility. According to Figure 8 (b) the ratio of Oads to Olatt was
calculated for the synthesized catalysts as shown in Table 1. The highest ratio is shown by the
Co3O4-350 material. Higher values for Oads/Olatt indicate a higher concentration surface active
oxygen in the material which are responsible for higher activity in oxidation reactions [41].
22

In situ DRIFTS was used to confirm the intermediate species during 2-propanol oxidation
on the surface of the catalyst from 100 °C to 350 °C. At 100 °C the catalyst surface accumulates
peaks for the adsorbed species of 2-propanol such as surface hydroxyl and isopropyl CH3 groups.
Furthermore, the bands for carbonyl and adsorbed carbonate species are also seen at this
temperature. As the reaction temperature is raised to 200 °C, the peaks for the isopropyl alcohol
decreases in intensity. Moreover, a shift in the carbonyl peak is observed at 150 °C and 200 °C.
The C=O vibration is rather sensitive to the physical state of the compound which may be the
reason for the shift of this peak at higher temperature [49]. Furthermore, these peaks for
carbonyls disappear after 200 °C and the peaks for carbonate species increases gradually. At
temperatures lower than 200 °C, the catalysts surface facilitates the formation of carbonyl
species, carbonate species and water. The carbonyl species may be desorbing the surface as
acetone at low temperatures [43]. In addition, complete oxidation of the 2-propanol to carbon
dioxide and water is confirmed by the gradual increase in the intensity of the adsorbed carbonate
species and hydroxyl groups with the increasing reaction temperature.
The synthesized mesoporous cobalt oxides offer high tolerance to water vapor and carbon
dioxide present in VOC streams. Only a slight decrease in activity is observed in catalytic
activity probably due to the competitive adsorption of these molecules and blocking of active
sites. The mesoporous cobalt oxide materials show a CoO phase together with the Co3O4 phase
after the reaction (up to 340 °C). However, no phase change was observed for the catalysts after
the reaction at 240 °C (stability tests). This implies that the catalysts start to undergo reduction at
higher temperatures. This further confirms that at lower temperatures, lattice oxygens are used
for the oxidation and are replenished by oxygen in the feed gas.
5. Conclusions
A soft template method was used to successfully synthesize mesoporous cobalt oxide materials
and their activities were evaluated for low temperature 2-propanol oxidation. The synthesized
materials exhibited impressive catalytic performance and excellent stability for 2-propanol
oxidation. 2-propanol is oxidized to carbonyl species and carbonate species on the surface of the
catalysts before being oxidized to carbon dioxide and water. Furthermore, the activity of
materials was not significantly affected by water vapor or carbon dioxide in the stream. Catalysts
which possess better low temperature reducibility, large pore volumes, higher Co3+/ Co2+
23

amounts and higher number of surface-active oxygen species show better performance. The
inverse micelle method which was utilized in this study is promising for the synthesis of
mesoporous cobalt oxide with high catalytic activity. This work provides a novel method for
developing noble metal free catalysts to eliminate VOCs at lower temperature.
Conflict of interest
The authors declare no competing financial interest.
Acknowledgements
The Authors are grateful for support of the U.S. Department of Energy, Office of Basic Energy
Sciences, Division of Chemical Biological and Geochemical Sciences under Grant DE-FG02-
86ER13622.A000.
24

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