Scalable Integration of Highly Uniform MnxCo3?xO4 Nanosheet Array onto Ceramic Monolithic Substrates for Low-Temperature Propane Oxidation

Article abstract of DOI:10.1002/cctc.201700795

The redox reaction between KMnO₄ and Co(NO₃)₂ was designed and readily utilized for the scalable integration of spinel Mn_xCo_3?xO₄ nanosheet arrays in three-dimensional ceramic honeycombs by controlling the reaction temperature. The Co²⁺ can reduce MnO₄ ^? to form Mn–Co spinel oxide nanosheet arrays uniformly on the channel surface of cordierite honeycomb. The novel platinum group metal free oxide nanosheet array integrated ceramic honeycomb monolith shows good low-temperature catalytic activity for propane oxidation, with the 50 % conversion temperature achieved at 310 °C, which is a much lower temperature than that over the wash-coated commercial Pt/Al₂O₃. These integrated Mn–Co composite oxide nanoarrays may hold great promise for the construction of advanced monolithic catalyst for high-performance and low-cost emission control.

Full text of DOI:10.1002/cctc.201700795

Accepted Article
Title: Scalable integration of highly uniform MnxCo3-xO4 nano-sheet
array onto ceramic monolithic substrates for low temperature
propane oxidation
Authors: Wenxiang Tang, Zheng Ren, Xingxu Lu, Sibo Wang, Yanbing
Guo, Son Hoang, Shoucheng Du, and Pu-Xian Gao
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: ChemCatChem 10.1002/cctc.201700795
Link to VoR: http://dx.doi.org/10.1002/cctc.201700795
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10.1002/cctc.201700795
ChemCatChem
Scalable integration of highly uniform MnxCo3-xO4 nano-sheet array onto
ceramic monolithic substrates for low temperature propane oxidation
Wenxiang Tang,a Zheng Ren,a Xingxu Lu,a Sibo Wang,a Yanbing Guo,a Son Hoang,a
Shoucheng Dua and Pu-Xian Gao a, *
a
Department of Materials Science and Engineering & Institute of Materials Science,
University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136, USA
*Corresponding author e-mail: puxian.gao@uconn.edu
Abstract: The redox reaction between KMnO4 and Co(NO3)2 was designed and
readily utilized for scalable integration of spinel MnxCo3-xO4 nano-sheet arrays with
three-dimensional (3D) ceramic honeycombs by controlling the reaction temperature.
-
The Co2+ can reduce MnO4 to form Mn-Co spinel oxide nano-sheet arrays uniformly
on the channel surface of cordierite honeycomb. The novel PGM free oxide
nano-sheet array integrated ceramic honeycomb monolith shows good low
temperature catalytic activity for propane oxidation, with the 50% conversion
temperature achieved at 310 ºC which was much lower than that over the wash-coated
commercial Pt/Al2O3. These integrated Mn-Co composite oxide nano-arrays may hold
great promise for the construction of advanced monolithic catalyst for
high-performance and low-cost emission control.
Keywords: Metal Oxide, Nano-sheet array, Monolithic Catalyst, Propane Oxidation,
Scalable Integration
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Due to the multiple valency states of manganese ions, manganese oxide with
mixed-valent (Mn) framework can be achieved by a series of reactions such as
-
reduction or oxidation of Mn2+ cation[1], MnO4 species[2, 3], and
-
reduction-oxidation between Mn2+ and MnO4 ions[4, 5]. Some oxidants such as
K2Cr2O7[6], (S2O8)2-[7, 8], NaClO[8] and carbon[9], have been applied for oxidizing
Mn2+ to yield MnOx. Although some of their redox potentials are close to Mn4+/Mn2+
(1.23 V), the reaction could be still proceeded by controlling the reaction temperature
and solution acidity. Potassium permanganate, a strong oxidizer, has been usually
selected as a manganese precursor for synthesis of manganese oxide related materials
under acidic condition. Because standard redox potential of MnO2/Mn2+ (1.224 V) is
-
much higher than MnO4 /MnO2 (0.595 V), manganese salts with lower manganese
-
oxidation state (Mn2+) have been usually used for reducing MnO4 to obtain
manganese oxide such as octahedral molecular sieves (OMS)[5, 10, 11]. Besides the
manganese ion-pairs such as Mn4+/Mn2+, some other ion-pairs (Ce4+/Ce3+, Co3+/Co2+
-
and Cu2+/Cu+) may be considered for the reduction of MnO4 into related MnOx or
composite oxides. However, due to their incompatible standard redox potential near
room temperature, using other metal ions with low oxidation state for the reduction of
-
MnO4 has been usually ignored so far.
Mn-Co spinel oxide, an important solid solution composite oxide with a general
formula of (Co, Mn)(Co, Mn)2O4, has attracted great attention as heterogeneous
catalysts and battery anode materials, owing to its favorable features such as low cost,
easy accessibility, high stability, redox-active metal centers, and environmental
friendless[12-14]. Much research has been reported on the synthesis of powder-based
Mn-Co composite oxides with significantly promoted activity through nanostructuring.
However, it is usually accompanied with compromised performance after the
assembly of nanostructured powder-based devices. In ceramic honeycomb monolith
with separate 3D channels, the direct integration with hierarchical Mn-Co oxide array
structure remains a challenge due to the difficulty to grow nanostructures uniformly in
deep channels. Up to now, no study is reported on the integration of Mn-Co oxide
array structures on the ceramic monolith with 3D channels. In the past few years, our
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group have successfully grown various arrays of metal oxide nanostructures such as
nanorods, nanowires and nanotubes onto 3-D channeled honeycomb substrates[15-23]
which has well-defined structures and good uniformity enabling superior properties,
excellent stability, and high materials utilization efficiency.
Herein, we successfully designed and utilized a redox reaction between KMnO4
and Co(NO3)2 at controllable temperature for directly growing Mn-Co composite
oxide nanostructure arrays onto 3-D channeled cordierite honeycomb substrates. With
uniform deposition in the form of nanostructure arrays, the Pt-group metal (PGM)
free Mn-Co spinel oxide integrated honeycomb monolith can be scaled up in full size
with good catalytic propane oxidation activity at low temperature superior to the
Pt/Al2O3 wash-coated monolithic catalysts. The materials loading can be dramatically
reduced on the nanostructure array monolithic catalysts, while the well-defined
structural characteristics and distribution enabled by array nanostructures have
allowed effective mass transport and enabled improved reaction chemistry and
kinetics[15].
2. Experimental
2.1 Materials preparation
Cobalt nitrate hexahydrate (Co(NO3)2•6H2O) was applied as a reducing agent to react
with potassium permanganate (KMnO4), which could make the formation of Mn-Co
composite oxide on the surface of ceramic cordierite. Before growth, ceramic
cordierite honeycomb was ultra-sonicated for 30 minutes in ethanol to remove the
residual contamination and washed with DI water, then dried at 90 ºC for further use.
The cordierite honeycomb substrate (1cm×2cm×3cm, mesh 600) was suspended into
40 mL mixed aqueous solution of Co(NO3)2 and KMnO4. To investigate the reducing
effect of Co(NO3)2 on KMnO4, different molar ratios of Co(NO3)2/KMnO4 (the units
of both Co(NO3)2 and KMnO4 are mmol) were used such as 0.5/4, 1/4, 2/4, 4/4, 6/4
and 8/4, where the as-received samples were denoted as 0.5Co-4Mn, 1Co-4Mn,
2Co-4Mn, 4Co-4Mn, 6Co-4Mn and 8Co-4Mn, respectively. The mixed solution was
then transferred into an electrical oven for hydrothermal synthesis at 95 ºC for 12
hours. Importantly, no reaction could take place before the temperature is developed.
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After the reaction, the substrate was withdrawn from the solution and carefully
washed to remove the residual precipitate, then dried at 90 ºC for 12 hours. Both the
monolith and collected powder from the solution were transferred into a muffle
furnace and treat at 500 ºC for 2 hours in air.
2.2 Materials characterization
X-ray diffraction (XRD) patterns of the as-synthesized Mn-Co nanostructured film
were measured on BRUKER AXS D5005 X-ray diffractometer system using Cu-Ka
radiation in the diffraction angle (2θ) range 10-80. The BET surface areas and pore
size distributions of all samples were obtained using the N2 adsorption-desorption
method on an automatic surface analyzer (ASAP 2020, Micromeritics Cor.). For each
measurement, all samples were degassed at 150 ºC for 6 h. The morphology and
structure were recorded on a scanning electron microscope (SEM, FEI Teneo
LVSEM). The microstructures of selected sample were obtained by using
transmission electron microscopy (TEM, FEI Talos S/TEM) with an accelerating
voltage of 200 kV. Hydrogen temperature programmed reduction (H2-TPR) was
carried out in a U-shaped quartz reactor under a gas flow (5%H2 balanced with Ar,
25mLmin-1) on a Chemisorption system (ChemiSorb 2720, Micromeritics Cor.). In
each run, sample with 49 channels (7 mm×7 mm ×10 mm) was used and the
temperature was raised to 750 ºC from room temperature at a constant rate of 10 ºC
min-1.
2.3 Catalytic test
Catalytic propane combustion was selected as a probe reaction to illustrate activity of
as-prepared nanostructured 3D Mn-Co composite film. Hydrocarbons generated from
mobile and stationary combustion sources, such as automobiles, petrochemical, and
power generation plants, play an important role in the formation of photochemical
smog and ozone pollution and some are difficult to remove like propane. The propane
oxidation tests were carried on a BenchCAT system (Altamira Instruments), and an
Agilent Micro-GC were equipped for separating and detecting gaseous species in the
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exhaust stream. The reactant gas was composed of 0.3% C3H8, 10% O2 and balanced
with N2, and the total flow rate of 50 ml min-1. Typically, honeycomb sample with 25
channels (5 mm×5 mm ×10 mm) was used to test and the space velocity (SV) was
about 12,000 h-1. The total weight of the monolithic nanostructured honeycomb was
around 0.1 g and the actual catalytic active materials were about 5-20 mg.
3. Results and discussion
-
3.1 Reaction between MnO4 and Co2+
-
For the first time, we designed and conducted the reaction between MnO4 and Co2+,
resulting in the incorporation of Co species into manganese oxide framework and then
formation of Mn-Co composite spinel oxide. In earlier study of the Co3O4@MnO2
hybrid nanowire array structure on the stainless steel plate, three processes have been
involved, including growth of Co3O4 nanowires array; deposition of a carbon layer on
the Co3O4 nanowires; and then MnO2 shell formation through reduction of KMnO4
with assistance of carbon layer[24], a rather complex and less controllable process. In
our study, the explorative experiments showed no obvious change after mixing the
KMnO4 and Co(NO3)2 solution for several days at room temperature, indicating that
-
the reaction between MnO4 and Co2+ is very slow at room temperature. According to
the Nernst equation (E=Eϴ-(RT/(nF))lnQ), the cell voltage E will be influenced by
standard voltage Eϴ, temperature T and reaction quotient. From the standard reduction
-
potentials of Co3+/Co2+ (1.92 V) and MnO4 /MnO2 (1.679V), the standard voltage for
-
the reaction between Co2+ and MnO4 will be positive, indicating the standard free
energy change will be negative with the reaction proceed at room temperature (see
detail calculations in Supporting Information). However, the reaction at room
temperature seems to be negligible due to the large activation barriers to the reaction.
With the temperature rise, Gibbs free energy change will be more negative and the
activation barriers can be overcome. As a result, a significant reaction can take place
when increasing reaction temperature to 95 ºC in an hour. As shown in Fig. 1g, the
-
molar ratio of Co2+/MnO4 has a great effect on the reduction-oxidation process. When
-
using 4Co2+/4MnO4 as the initial molar ratio of reactants, the solution after synthetic
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reaction looks more colorless and transparent which means the reaction completed
under this condition. Through the purple and red color of the solution after synthetic
reaction, it can be inferred that one of the reactants is surplus when the molar ratio of
-
Co2+/MnO4 is lower or higher than 1/1.
3.2 Morphologies and microstructures
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)
were used for studying the morphological features of as-prepared Mn-Co composite
oxide nano-arrays. As shown in Figs. 1a-f, the SEM images suggest that all
nano-arrays with porous architectures were assembled by numerous nano-sheet array
which were perfectly grown on the ceramic surface of honeycomb cordierite. Such
porous array structures can provide very large active surface areas to facilitate the
diffusion of reactant molecules when they are applied as catalysts. With increasing
-
molar ratio of reactants (Co2+/MnO4 ), the assembled nano-sheets become denser due
to the higher deposition ratio on the surface. As listed in Table 1, the weight loading
ratios are varied from 3% to 20%. This value kept stable when the molar ratio of
-
-
Co2+/MnO4 is higher than 4mmol/4mmol which means all MnO4 has been reduced
by enough Co2+. With higher molar ratio, the Co2+ will be surplus which can be
estimated by the solution color as displayed in Fig. 1g.
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Fig. 1 (a-f) SEM images of as-prepared nanofilms (top view) on monolithic cordierite
substrate with different ratio of Co(NO3)2 and KMnO4 (scale bars are 1 μm); (g)
Photographs of the solution after synthetic reaction (scale bar is 1 cm).
Table 1. Loading ratio, BET surface area, pore diameter, pore volume and T50 (the
reaction temperatures for propane conversions of 50%) of the as-prepared Mn-Co-O
nanofilms based monoliths.
Co(NO3)2/K
MnO4
Pore
volume
（cm3
g-1）
Loading
ratio
BET
Pore
T50
Sample
surface area diameter
(Reactants
ratio)
（ºC）
(wt.%)
(m2 g-1)
3.2
（nm）
9.6
0.5Co-
4Mn
0.5 mmol/4
mmol
3.9
0.009
0.025
>500
495
1Co-4
Mn
1 mmol/4
mmol
10.4
8.1
9.1
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2Co-4
2 mmol/4
mmol
18.7
20.5
19.7
19.6
24.1
35.1
35.6
35.6
8.8
4.5
4.6
5.0
0.052
0.057
0.058
0.060
413
345
310
357
Mn
4Co-4
Mn
4 mmol/4
mmol
6Co-4
Mn
6 mmol/4
mmol
8Co-4
Mn
8 mmol/4
mmol
One of the as-synthesized samples (6Co-4Mn) was selected to check carefully to get
more information as shown in Fig. 2. Low magnification SEM image (Fig. 2c) clearly
demonstrates the nanofilm covered on the surface of substrate is highly uniform,
-
confirming the reaction between Co2+ and MnO4 is versatile way to construct highly
uniform nanostructured Mn-Co spinel film. Fig. 2b shows the cross section of
cordierite monolith and the nanofilm has a thickness of 1 μm with good adhesive
property. Figs. 2e present the selected area SEM-EDX results on sample 6Co-4Mn.
Besides the general composition of cordierite (Al, Si, Mg, O), the Mn, Co and K
species were clearly observed on the nanofilm. Across different selected areas EDX
spectra, the compositions revealed are similar, indicating the uniformity of nanofilm
covered on the cordierite surface. The molar ratio of Co/Mn is about 1/1 which is also
close to the ICP result (Co/Mn= 0.972). A certain amount of K was found that may be
assigned to the incorporation of K from KMnO4 into Mn-Co composite oxide.
The nano-sheet like morphology was further illustrated by observation of TEM
images which are presented in Fig. 3. Interestingly, it can be seen that all the
nano-sheets are inner-connected which can give a superior mechanical stability to the
nanofilm when it suffers the flushing of gas flow. Also, the space formed among the
nano-sheet array will be good for capture and diffusion of reactants[15, 19, 25-27].
Moreover, every single nano-sheet has a size of several hundred nanometers and a
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thickness of several nanometers, respectively. The high resolution TEM (HR-TEM)
image (Fig. 3e) shows the novel polycrystalline properties and every nano-sheet
contains large number of inner-connected nanocrystals with a size of several
nanometers. Clearly, a typical inter-planar spacing observed at 0.25 nm can be
ascribed to the preferentially exposed {311} facet of (Co, Mn)(Co, Mn)2O4 (JCPDS
018-0410), which is similar to the other reports [28]. Furthermore, the TEM-EDS
mapping analysis were performed to identify the element dispersion of composite
oxide as shown in Fig. 4. The elemental mapping results show a uniform distribution
of Co, Mn, O and K in each piece of Mn-Co composite nano-sheet, and all the related
peaks can be found in the spectrum. The uniform elemental distribution shown here
suggests that the Mn-Co composite oxide is uniformly synthesized by this approach.
Fig. 2 SEM images of (a, c) top view and (b) cross-section view of the Mn-Co-O
nano-arrays on monolithic cordierite substrate (sample 6Co-4Mn); (d) a selected area
SEM-EDX spectrum of the Mn-Co-O nano-arrays (sample 6Co-4Mn).
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Fig. 3 (a, b and c) TEM, (d) SAED and (e) HR-TEM images of sample 6Co-4Mn.
Fig. 4 TEM images (a), elemental mapping images (b-Co; c-Mn; d-O; e-K) and
EDX-data of the Mn-Co-O nanofilm (sample 6Co-4Mn).
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3.3 X-ray diffraction
The XRD patterns of the as-prepared nano-sheet array based monolithic cordierite are
represented in Fig. 5. The main diffractions of blank cordierite can be indexed to
Zn2Al4Si5O18 (JCPDS 032-1456) and Mg2Al4Si5O18 (JCPDS 012-0303). Because the
highly strong peaks of cordierite substrate, no clearly different peaks were observed
on the monolith covered with Mn-Co composite oxide nano-sheet array films. In
order to understand more information of the as-prepared samples, the powders
obtained from every reaction were collected for doing X-ray diffraction analysis and
the results are shown in Fig. 5b. For samples achieved at low molar ratio of
-
Co2+/MnO4 (0.5Co-4Mn and 1Co-4Mn), the weak peaks can be ascribed to KMn8O16
(JCPDS 029-1020). With increase of the Co/Mn precursor dosage, the peaks related to
KMn8O16 disappeared. Even all the samples have been treated at high temperature
(500 ºC), only one main peak at about 37.5º can be observed which can be assigned to
the {311} plane of (Co, Mn)(Co, Mn)2O4 (JCPDS 018-0410) spinel structure, which
is consistent with the HR-TEM results (Fig. 3e).
Fig. 5 (a) XRD patterns of Mn-Co-O nanofilms on monolithic cordierite substrate; (b)
XRD patterns of Mn-Co-O powders collected from synthetic reaction.
3.4 N2 physisorption
Fig. 6 shows the nitrogen adsorption and desorption isotherms of nano-array based
monoliths. A typical type IV characteristics can be observed on all samples with
well-developed H1 type hysteresis loops, which are attributed to the capillary
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condensation in mesopores, indicating the existence of mesoporous structure of
as-prepared nanofilms[14, 29]. Table 1 presents the textural parameters of all
as-prepared samples. The average pore diameters are from 4.5 nm to 9.6 nm. The
results of the BET analysis demonstrate that the surface area of Mn-Co composite
oxide nano-sheet arrays with substrate can be high to 35 m2 g-1. Considering the
ultra-low surface area of cordierite honeycomb and loading ratio, the surface area of
nano-arrays can be calculated by the equation Snano-array=Smonolith/Wnano-array (where the
Smonolith is the total surface area of nano-array on substrate, Wnano-array is loading ration
of nano-array on substrate), With this calculation, the BET surface area of pure
nano-array can be as high as 175 m2g-1 (35 m2 g-1/20%), offering more active sites for
catalytic reaction.
Fig. 6 (a) N2 adsorption–desorption isotherms and (b) BJH pore-size distribution of
the as-prepared Mn-Co-O nano-sheet arrays on monolithic cordierite substrate.
3.5 H2-TPR analysis
A redox mechanism is usually applied to explain the catalytic oxidation reaction on
the metal oxides, which contains oxidizing hydrocarbon by the surface (or lattice)
oxygen of catalyst and regeneration of surface (or lattice) oxygen by gaseous oxygen
[30, 31]. The different oxidation states of Mn and Co species can provide serious
redox pairs that will effectively promote the oxidation and reduction processes during
the oxidation of hydrocarbons. H2-TPR is an ideal method to investigate the
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reducibility of the catalysts and the results are displayed in Fig. 7. For single
manganese oxide and cobalt oxide, the reduction processes include MnOx → Mn3O4
→ MnO and Co3O4 → CoO → Co, where every further reduction step needs higher
temperature[32-34]. As shown in Fig. 7, the reduction part happened at lower
temperature (300-350 ºC) due to the reductions of MnOx to Mn3O4 and CO3O4 to CoO,
and the higher-temperature (>350 ºC) reduction can be assigned to the reductions of
Mn3O4 to MnO and CoO to Co[13, 35, 36]. Moreover, the signals at lower
temperature (<300) can be ascribed to the highly active oxygen species generated by
the strong synergistic effect in Mn-Co spinel oxides, which is similar to other reports
[32, 35-37]. Moreover, it can be seen that the low-temperature reducibility is
promoted by increasing the molar ratio of reactants, indicating more incorporation of
Co is beneficial for generating more active oxygen species.
Fig. 7 H2-TPR profiles of the as-prepared Mn-Co-O nano-sheet array based monoliths
3.6 Catalytic combustion of propane
Fig. 8a presents the catalytic conversion of propane as a function of reaction
temperature. The thermal decomposition of gaseous propane at low concentration is
difficult and little conversion of propane was observed below 500 °C when only blank
cordierite was loaded in the reactor. With the in-situ grown Mn-Co composite oxides
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into the cordierite honeycomb channels, the conversion of propane can take place at
lower temperature. With increase of the molar ratio of reactants, the catalytic
performance is significantly promoted. For an example, the temperature for 50%
conversion of C3H8 into CO2 over catalyst 1Co-4Mn can be achieved at 495 °C while
the same conversion over sample 4Co-4Mn is lowered to 345 °C. From the sample
0.5Co-4Mn to 4Co-4Mn, the promoted catalytic activities can be attributed to the
increased loading ratio of Mn-Co composite oxide nano-sheet arrays. Compared to the
samples with similar loading ratio (4Co-4Mn, 6Co-4Mn and 8Co-4Mn), catalyst
6Co-4Mn exhibits the best catalytic propane oxidation activity with 50% conversion
temperature at 310 °C. Fig. 8b shows the catalytic activity of 6Co-4Mn at different
space velocity (SV). With increase of the SV, the propane conversion decreased
significantly, indicating a longer contact time is beneficial for improving the catalytic
performance. It is noteworthy that under similar reaction conditions over wash-coated
catalyst with commercial 1%Pt/Al2O3, the temperature for 50% conversion of C3H8
into CO2 is about 370 °C as shown in Fig. 8c, which is 60 °C higher than that over the
array based catalyst. The better performance in the Mn-Co composite oxide
nano-sheet array based catalyst can be due to the good redox properties of Mn-Co
spinel oxide and effective diffusion condition of array nanostructures. Moreover, the
performance can be repeated perfectly over used catalyst as shown in Fig. 8d,
indicating the good stability of the synthesized Mn-Co array catalyst.
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Fig. 8 (a) catalytic propane oxidation performance of the as-prepared Mn-Co-O
nanofilms based monoliths; (b) catalytic propane oxidation performance over the
sample 6Co-4Mn at different space velocity; (c) comparison of catalytic activities
between the sample 6Co-4Mn and washcoated Pt/Al2O3; (d) catalytic propane
oxidation performance of the sample 6Co-4Mn over 1st and 2nd runs.
4. Conclusions
-
In summary, the reaction between Co2+ and MnO4 at 90 ºC has been firstly developed
as a facile strategy for the integration of Mn-Co composite oxide nano-sheet array
-
onto the 3D cordierite honeycomb substrate. The Co2+ can reduce MnO4 to grow
Mn-Co spinel oxide nano-sheet arrays uniformly on the channel surface of cordierite
honeycomb. The novel nanostructure shows good low temperature catalytic activity
for propane oxidation, with the 50% conversion temperature achieved at 310 ºC which
was much lower than that over the wash-coated commercial Pt/Al2O3. These
integrated functional Mn-Co composite oxide nano-arrays may hold great promise for
the construction of advanced monolithic catalyst for high-performance and low-cost
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emission controlling devices.
Acknowledgements
The authors are grateful for the financial support from the US Department of Energy
(Award Nos. DE-EE0006854 and DE-FE0011577) and the US National Science
Foundation (Award No. CBET-1344792).
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