Excellent porous environmental nanocatalyst: Tactically integrating size-confined highly active MnO: X in nanospaces of mesopores enables the promotive catalytic degradation efficiency of organic contaminants

Article abstract of DOI:10.1039/c9nj05092b

The confinement of refined catalytically active substances in special nanospaces is beneficial for promoting reaction processes by taking advantage of the quantum confinement effect and enlarged atomic efficiency. Herein, we present a redox nanoporous molecular sieve nanocatalyst containing size-confined active MnO_x in the siliceous nanospace of mesopores via a functional micelle-assisted in situ embedding strategy. We also demonstrate its excellent environmental catalytic efficiency toward the oxidative degradation of methyl orange in the liquid phase (8 min) and volatile toluene in the gas phase (T₉₀/T₁₀₀ = 225/240 °C, under the mass space velocity of 60?000 ml g^-1 h^-1 and concentration of 1000 ppm, which surpass noble metal catalysts and most composite catalysts). This was achieved by virtue of eliminating diffusion limitations, and great active atom-efficiency. In this synthetic strategy, the well-made resultant MnO_x had a refined size and greater interaction with diffusing reactant molecules without impeding their diffusion and mass transfer. Excellent and durable catalytic efficiency was demonstrated, due to the superior characteristics concerning the great dispersion of MnO_x and favorable structure, thereby confirming that high catalytic removal efficiency depends on the amount of accessible MnO_x rather than the loading of active species. Our findings provide the first example of a heterogeneous environmental catalyst that affords highly efficient catalytic elimination of organic contaminants in both liquid and gaseous systems.

Full text of DOI:10.1039/c9nj05092b

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Excellent porous environmental nanocatalyst:
tactically integrating size-confined highly active
MnOx in nanospaces of mesopores enables the
promotive catalytic degradation efficiency
of organic contaminants†
Cite this: New J. Chem., 2019,
43, 19020
a
Fu Yang, ‡*a Shuying Gao,‡b Yun Ding,c Sheng Tang,
Haifeng Chen,a
cd
a
Jianjun Chen,
*
Jianfeng Liu,e Zhen Yang,e Xiaocai Hue and Aihua Yuan
*
The confinement of refined catalytically active substances in special nanospaces is beneficial for
promoting reaction processes by taking advantage of the quantum confinement effect and enlarged
atomic efficiency. Herein, we present a redox nanoporous molecular sieve nanocatalyst containing size-
confined active MnOx in the siliceous nanospace of mesopores via a functional micelle-assisted in situ
embedding strategy. We also demonstrate its excellent environmental catalytic efficiency toward the
oxidative degradation of methyl orange in the liquid phase (8 min) and volatile toluene in the gas phase
(T90/T100 = 225/240 1C, under the mass space velocity of 60 000 ml gÀ1 hÀ1 and concentration of
1000 ppm, which surpass noble metal catalysts and most composite catalysts). This was achieved by
virtue of eliminating diffusion limitations, and great active atom-efficiency. In this synthetic strategy, the
well-made resultant MnOx had a refined size and greater interaction with diffusing reactant molecules
without impeding their diffusion and mass transfer. Excellent and durable catalytic efficiency was
demonstrated, due to the superior characteristics concerning the great dispersion of MnOx and
favorable structure, thereby confirming that high catalytic removal efficiency depends on the amount of
accessible MnOx rather than the loading of active species. Our findings provide the first example of a
heterogeneous environmental catalyst that affords highly efficient catalytic elimination of organic
contaminants in both liquid and gaseous systems.
Received 10th October 2019,
Accepted 5th November 2019
DOI: 10.1039/c9nj05092b
rsc.li/njc
Therefore, the removal of VOCs in air and large organic mole-
cules in the liquid phase is highly necessary for the protection
1. Introduction
The organic contaminants present in outlet water and environ- of the environment and for green chemistry.1–4 For instance,
mental air have been bothering us for a long time. In particular, catalytic combustion treatment of VOCs emerged as an effica-
the emissions of volatile organic compounds (VOCs) to the cious end-step technique as compared to other processes such
atmosphere are regarded as a great threat to human health. as adsorption tactics. The design and development of highly-
efficient functional catalysts, especially transition metal-based
a School of Environmental and Chemical Engineering, Jiangsu University of Science
catalysts are crucial by virtue of the enhancement of reaction
activity.5–9
and Technology, Zhenjiang 212003, Jiangsu, P. R. China.
E-mail: fuyang@just.edu.cn, aihua.yuan@just.edu.cn
Nanometric space confinement of the heterogeneous
reaction process exerts a significant positive effect on the
improvement of micromolecule reaction activity, because of
the involved molecular diffusion, mass transferring factors and
contact activation of reactant molecules.10–13 Many endeavors
have been devoted to this research by creating positive active
catalytic species in the confined reaction micro-region, aiming
to achieve the high atom utilization of active species. However,
the regulation of the size-matched catalytic active species and
applicable reaction space for the enhancement of reaction
b State Key Laboratory of Materials-Oriented Chemical Engineering, College of
Chemical Engineering, Nanjing Tech University, Nanjing 210009, Jiangsu,
P. R. China
c National Engineering Laboratory for Multi Flue Gas Pollution Control Technology
and Equipment, School of Environment, Tsinghua University, Yancheng 224001,
China. E-mail: chenjianjun@tsinghua.edu.cn
d State Key Joint Laboratory of Environment Simulation and Pollution Control,
School of Environment, Tsinghua University, Beijing 100084, China
e Shanghai Waigaoqiao Shipbuilding Co., Ltd, Shanghai 200137, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9nj05092b
‡ These authors equally contributed to the work.
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activity remains a big challenge during the heterogeneous groups based on the coordination-chemical driving-force effect.
catalytic reaction, especially for environmental catalysis. The metal species interacted strongly with the siliceous wall and
Mesoporous silica has emerged as a promising catalytic were stabilized by the interactions and were thereby dispersed in
nanomaterial, thanks to the excellent surface area and suitable the cluster-like state. These resulting samples were systematically
pore size and volume.14–16 Various active metal species have characterized to identify their physicochemical properties using
been contrived for encapsulation in the nanopores of meso- their corresponding test techniques based on the nanometric
porous silica17 and the post-impregnation method is still the level. Ensuingly, the catalytic evaluation of these catalysts was
most widely employed method to achieve this purpose.18 However, further explored using the catalytic oxidative degradation of
we have noticed that the distribution and position of active metal MO in the liquid phase by stimulating the generation of sulfate
species are still not well controlled during the post-impregnation radicals from peroxymonosulfate and toluene in air that typi-
process because of the non-uniformity of the active metal species, cally make up VOCs gas. To our satisfaction, the synthesized
which easily aggregate under high-temperature driven force.19,20 optimal 3Mn-MS achieved the best catalytic degradation effi-
Therefore, the atom efficiency of active metals tends to be ciency (T90/T100 = 225/240 1C) of toluene among almost all
insufficient. Although some small enough nanoparticles have monometallic Mn-based catalysts, even outperforming most
been confined in the mesopores of silica, the diffusion chan- known well-designed multi-metal composite catalysts. In this
nels of the original two-dimensional interconnected mesopores regard, this study deals with the role of manganese dispersity
might be blocked completely or partially, owing to the presence and content on the catalytic oxidation degradation properties of
of metal particles in a certain position. Therefore, one of the large organic molecules and VOCs over Mn-bearing siliceous
reactant molecules has to turn back, or be delayed, when several mesoporous molecular sieves.
molecules simultaneously meet at the blocked pore, even missing
the chance for contact with other active species, especially for the
2. Experimental section
2.1 Chemicals and materials
stream-directed heterogeneous gas–solid reaction. The nanosphere-
like mesoporous silica is more susceptible to this issue due to
the sealing pore in the center of the sphere.21,22 Therefore, the
Dodecyl amine (DDA, Z98%), methyl orange (MO), and potassium
modification of mesoporous silica nanospheres with metal oxides
peroxymonosulfate sulfate (PMS) were commercially obtained from
for achieving high catalytic efficiency is a challenge but is significant
Aladdin. Manganese chloride tetrahydrate (MnCl2Á4H2O, Z99%),
for heterogeneous catalysis.
tetraethylorthosilicate (TEOS, AR), toluene (AR) and methanol
Special techniques like sonication,23,24 surface modification,25,26
(MeOH, Z99%) were purchased from Sinopharm Chemical
etc.27–29 have been proposed to improve the dispersing process
Reagent Co., Ltd. All chemicals were commercially available
and control of active species, but the results remain unsatisfac-
and were utilized without further purification.
tory as compared to the expectation. For instance, Sun et al.30
utilized the interface of the P123 micelle and the pore wall of
2.2 Synthesis of the targeted Mn-bearing nanoporous
SBA-15 as the confined space to embed the metal precursors by
molecular sieve catalyst
mechanical grinding. The metal species were capable of being
encapsulated and were dispersed in a larger reaction space
after the thermal process, but some large metal nanoparticles
in a mixed solution, including deionized water (62.5 ml) and
were still located outside the pore, inevitably due to the inexact
mechanical process at the nanometric level. Additionally, this
process involves significant mechanical energy consumption,
thereby wasting energy and adding to the synthetic cost.
TEOS (10.73 ml) was added dropwise under stirring and kept
Another strategy that incorporates isolated metal atoms into
The synthetic process for the Mn2+-involved assembly of
xMn-MS was as follows: DDA (1.85 g, 0.01 mol) was dissolved
ethanol (25 ml), with additional stirring for 30 min at 45 1C.
Controlled amounts of MnCl2Á4H2O (1, 2, 3, 4 and 5 mmol) were
introduced, and the solution became brown and translucent.
for another 4 h with stirring and was static for 18 h at 45 1C.
Finally, the resultant catalyst samples were attained after
the skeleton of silica is capable of avoiding the blocking of
mesopores,31–33 but most of these metal atoms are easily
centrifugation and washing several times with water and ethanol
embedded in the internal skeleton of silica due to the similarity
and were then annealed for 6 h at 550 1C in air. The resulting
catalysts corresponding to the controlled manganese amounts
were labeled as xMn-MS (x = 1, 2, 3, 4, and 5), whose Mn/Si molar
ratios were in theory 2.08, 4.17, 6.24, 8.32 and 10.4 (%).
of the atom size between the transition metal and silicon,
thus preventing real catalytic activity. Hence, the cluster-like
metal species seem to be favorable for the decoration of
mesopores but is limited by the synthetic method owing to
its instability.
2.3 The assessment of catalytic performance and stability
Herein, we propose micelle-induced assembly tactics to
trigger Mn cations to directly enter the interface of the siliceous
Oxidative degradation of MO in the liquid phase. The
wall and micelles through chemical coordination and counterion- catalytic performance of the prepared catalysts was evaluated
media interactions, which exclude the additional post-introduction in the catalytic oxidative degradation of large organic molecules
process of metal active species (see Scheme 1). Adjustable amounts with the assistance of PMS. The reaction was conducted in a
of Mn precursors were allowed to enter the interface of the siliceous 100 ml beaker containing 100 ppm of MO solution at room
wall and micelle via coordination to the micelle surface amino temperature. A certain amount of PMS (12 mmol LÀ1) was
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Scheme 1 (a and b) Synthetic depiction and coordination evidence of the Mn2+–N group for a highly active porous nanosphere integrated with size-
confined MnOx.
introduced before the reaction. Afterwards, 40 mg of catalyst the coordination of Mn2+ with the amino surfactant and
was dispersed in the mixture to stimulate the degradation counterions-media interactions between the negative siliceous
reaction for 60 min in a continuous process. At a certain oligomers and metallomicelles. The DDA molecules firstly
interval, the concentration of MO was analyzed by virtue of assemble into the micelles with the enriched surface amines
a UV detector. Additionally, to evaluate the stability of the that can capture the metal Mn ions through the coordination
optimal catalyst, the catalyst was collected by centrifugation interactions. This functional Mn2+-micelle became the cationic-
after one trial for the subsequent cycle testing.
like micelle liquid-crystal template and easily matched with
Oxidative degradation of toluene in air. The catalytic oxida- the negative siliceous oligomers via the counterions media.
tive elimination of toluene in the gas phase was conducted As a result, the confined MnOx species were generated in situ
using the synthesized Mn-based catalysts in a continuous-flow in the nanoporous silica. UV-visible spectra were utilized to
fixed-bed tubular reactor at atmospheric pressure (Scheme S1, identify the variation in the metal ion coordination environ-
ESI†). Specifically, 100 mg of catalyst (40–60 mesh), derived ment before and after adding DDA molecules, which was
from tableting and sieving, was further mixed with commercial evidenced by the presence of an emergent UV absorbance peak
quartz sand (40–60 mesh) and fixed in the reactor filled with at 270 nm (Scheme 1a), indicating the realized coordination
the quartz wool plug and equipped with a K-type thermocouple. effect for the Mn2+ and DDA molecule.34 Further direct
The reactant gas mixture consisted of 1000 ppm toluene vapor evidence of the coordination was observed via the color varia-
(produced by compressed air bubbling from liquid toluene in a tion of the solution due to the formation of metallomicelles
simple ice-water isothermal bath); one stream of air was used (inset in Scheme 1b).
as the balance with a total flow rate of 100 ml minÀ1, and
Structural and morphology characterizations
its weight hourly space velocity (WHSV) was calculated as
60 000 ml gÀ1 À1. The reactor bed was preheated from room The FESEM technique has been used to determine the micro-
h
temperature to 100 1C prior to each test, with an additional scale morphology and the variation in the regularity of
1 h temperature plateau to prevent overestimation of the Mn-based series samples; the representative SEM images of
toluene conversion; the programmed temperature control was several comparable samples are presented in Fig. 1. Typically,
conducted from 180 to 300 1C. The concentration of toluene the defined nanospherical particles are visibly displayed with
and possible products involving various reaction temperatures regard to each sample, but it is noticeable that the surface state
in the outlet were evaluated by an online CEAULIGHT GC 7290 including particle size and spherical regulation show a certain
gas chromatograph (Flame Ionization Detector (FID)) under variation on account of the adjustable Mn loading in
steady-state conditions after 30 min of reaction stabilization.
the assembling procedure. In the case of the low-loading
Mn-bearing sample (1Mn-MS and 2Mn-MS), the spherical state
of the sample seems far from satisfactory as compared to that
of the high-loading Mn-containing sample (3Mn-MS and
4Mn-MS). Specifically, the 3Mn-MS and 4Mn-MS samples have
3. Results and discussion
The synthetic strategy, as depicted in detail in Scheme 1, more uniform nanosphere morphologies. This phenomenon
follows the functional micelle-induced self-assembly route via absolutely demonstrates the participation of Mn cations in the
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and conceivable cluster-like state in the confined nanopores of
silica. However, the existence and presence of Mn species in the
mesoporous silica were confirmed based on the elemental
mapping results (Fig. 2e), in which the Mn was well dispersed
in the selected region. The non-directive porous structure of
sample 3Mn-MS was precisely identified, and was in good
agreement with the N2 adsorption/desorption results, benefi-
cial for the diffusion and mass transfer of reactant.
To identify the porous character of the series samples, the
low-angle XRD technique was used to examine the resultant
samples. As shown in Fig. 3, a typical mesoporous phase 100
diffraction peak located at 2y = ca. 21 was observed, clearly
indicating the mesostructure of the resultant targeted
materials.35,36 The 100 diffraction peak position showed a
remarkable shift towards high angle, along with the trend of
decreased intensity with gradually-increasing Mn concen-
tration in the catalyst, demonstrating the decreasing d100 space
distance and damage to the mesostructure to some extent. This
is associated with the fact that Mn ions participate in the
assembly process for mesoporous silica20,37 and the gradual
insertion of MnOx in molecular sieves.
The porous properties were determined using N2 adsorption/
desorption isotherm measurements. The typical IV type curve
for these samples is presented in Fig. 4a, which is in accordance
with that of typical mesoporous materials.38 However, the curve
gradually transformed to type I-like behavior with the enhanced
Mn loading in the mesochannel, verifying the appearance and
enrichment of micropores owing to the aggregation and occupa-
tion of MnOx species in mesochannels. The pore size distribution
calculated from the BJH model also reconfirmed the decreasing
trend of pore size because of the occupation of more MnOx species
in the mesochannel. The detailed textural parameters are dis-
played in Table 1, thus, the increase in pore wall thickness with the
increasing Mn loading can be clearly observed, further indicating
the adhesion of MnOx species on the pore wall (Scheme 2).34,39
These results demonstrated the introduction and confinement of
cluster-like MnOx species in mesochannels. However, what we
noticed was the disappearance of mesopores, especially for the
4Mn-MS and the 5Mn-MS to some extent, indicating the possible
blocking and closing of pores after over-introducing MnOx species.
Fig. 1 Representative SEM images of various comparable series samples,
(a) 1Mn-MS, (b) 2Mn-MS, (c) 3Mn-MS, (d) 4Mn-MS, and (e) 5Mn-MS, and a
specific simulated diagram (f).
liquid crystal template assembling procedure for nanoporous
silica. It was observed that the over-dosage of Mn in the self-
assembly process caused the morphology distortion (5Mn-MS),
which favors the above-proposed conclusion.
Transmission electron microscopy is a technique that is
used to determine the morphology and microscale structure
and provides the greatest evidence for identifying the nano-
structural sample. As shown in Fig. 2a–c, the representative
nanospherical structures of 3Mn-MS were obviously observed
in several typical TEM images, and the relatively uniform size
(100–200 nm) of the nanospheres is obvious. Note that the size-
confined MnOx particles failed to be observed based on
the selected microscale regions, indicating their ultra-fine size
Fig. 2 (a–c) Representative TEM images of 3Mn-MS; (d) specific simulated diagram and (e) its SEM-EDX elemental mapping results; scale bar 1 mm.
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samples, we identified the uncalcined samples using the wide-
angle XRD technique (Fig. 5a). The results demonstrate the con-
sistent crystalline state of the siliceous support before and after
calcination. Besides, the crystalline state of MnOx was not detected
in the curves of the uncalcined samples, signifying the oxidative
ions state of Mn in the interface between the siliceous wall and
micelles of initial as-synthesized samples.
To determine the presence and coverage of the introduced
MnOx on the siliceous surface, the FT-IR spectra ranging from
990 cmÀ1 to 880 cmÀ1 of these resultant series catalyst samples
of xMn-MS are presented in Fig. 6a. The variations in the
intensity of the peak at 956 cmÀ1 (due to surface Si–OH groups)
were obviously discerned with the enhanced loading of MnOx
on the surface. The gradual decrease in intensity with the
gradual incorporation of MnOx species on the siliceous surface
is regarded as the specific evidence for the consumption of
surface silanol groups, owing to the strong chemical interaction
between the inherent surface silanol groups and MnOx
species.42 In addition, the coverage of MnOx on the surface of
the siliceous (Fig. 6b) wall was further identified based on the
calculation of the numbers of Si–OH groups, in light of the
decreased intensity of the IR absorption peak and their respec-
tive surface areas (Fig. 6c and Table 1). In contrast, the inter-
acting Si–OH group numbers of 3Mn-MS accounted for
the biggest proportion despite the higher loading of MnOx in
4Mn-MS and 5Mn-MS, thus indicating its highest exposed
degree of introduced unit MnOx species (normally, the number
of Si–OH groups is demonstrated for ca. 4 per nm2).43
Fig. 3 Low-angle XRD patterns of a series of Mn-based mesoporous
molecular sieve samples xMn-MS.
Wide-angle XRD was utilized to ascertain the crystal state
of the whole material and the probable phase state of active
Mn species; their corresponding pattern results involving
uncalcined and calcined catalysts are displayed in Fig. 5a and
b. As observed, the broad diffraction peak ranging from 20–301
is attributable to the amorphous siliceous carrier.40 A weak
swell at 351 was identified in the case of the high concentration
Mn-bearing samples(Fig. 5b) and was assigned to the low-
crystalline degree of size-confined MnOx species (Mn3O4, PDF#
24-0734) inside the nanopores of mesoporous silica.41 To probe
the state of Mn species in the as-synthesized Mn-containing series
Fig. 4 (a) N2 adsorption/desorption isotherms and (b) pore size distribution of series xMn-MS molecular sieve samples.
Table 1 Textural properties and parameters of series Mn-bearing porous silica samples
SBET (m2 gÀ1
)
Va (cm3 gÀ1
)
Dp (nm)
a0 (nm)
d100 (nm)
dW (nm)
a
b
c
d
Samples
Mn (%)
Pure HMS
1Mn-MS
2Mn-MS
3Mn-MS
4Mn-MS
5Mn-MS
0
868
862
814
800
466
380
0.78
1.08
0.72
0.54
0.29
0.61
3.49
2.73
2.44
2.14
1.58
1.57
4.96
5.30
5.10
4.70
4.59
—
4.30
4.59
4.39
4.07
3.98
—
1.47
2.57
2.66
2.56
3.01
—
2.2
4.1
6.2
7.5
8.2
a
b
Dp represents the most probable pore size from the BJH pore size distribution. a0 represents the unit cell parameter, a0 = 2d100/O3.
c
d
d100 represents the d-spacing of the mesopore, d100 = l/2 sin y. dw represents the pore wall thickness of mesopore, dw = a0 À dp.
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demonstrate the coexistence of Mn2+ and Mn3+. The weak
shoulders at around 359 nm could be attributed to d–d transi-
tions of the 5B1g - 5B2g and 5B1g - 5Eg of Mn3+ in Mn2O3,
and the weak peak at around 616 nm corresponds to the
6A1g - 4T2g transition of Mn2+ in MnO. Taken together, the
evidence indicates the presence and state of the size-confined
MnOx in Mn3O4.
Scheme 2 Description of the introduction of MnOx species into the
The direct association of the reducibility of the transition
metals with catalytic activity was further indicated by a H2
temperature-programmed reduction process technique (H2-TPR).
mesopores of the siliceous molecular sieves.
DRUV-vis spectra (Fig. 7a) were utilized to further identify Fig. 7b presents the H2-TPR profiles of the series catalysts.
the metallic state of the introduced Mn species on the surface One visual peak, ascribed to the reduction of the Mn species
of the siliceous pore wall. To our knowledge, Mn3O4 can (Mn3O4 - MnO), was observed at ca. 350 1C,46 corresponding to
be recognized as a type of Mn2O3ÁMnO. Specifically, the two the weak interaction of MnOx on the porous wall, which accounts
bands near 311–323 nm are ascribed to the charge transition for MnOx species introduced with relatively lower reducibility. In
(O2À - Mn3+) in Mn3O4, where octahedral Mn coordinates with contrast, the remarkable shift from the reduction peak of Mn3O4 at
oxygen. The second band centered at ca. 512 nm corresponds to the higher temperature (ca. 420–550 1C) is ascribed to the strong
the crystal field transition of Mn2+ (6A1g - 4T2g) as observed in interaction between MnOx and the siliceous wall, associated with
Mn3O4 and MnO.44,45 Thus, these two broad bands at around the higher redox capacity.26 Notably, Mn-bearing 3Mn-MS with two
311–323 and 512 nm in the DR UV-vis spectra of xMn-MS adjacent reduction peaks correspond to the above-mentioned
Fig. 5 Wide-angle XRD patterns of uncalcined (a) and calcined (b) series of xMn-MS catalyst samples.
Fig. 6 (a) FTIR spectra of xMn-MS samples; (b) schematic description of the surface status of MnOx on the siliceous wall; (c) coverage degree and
calculated area of coverage of MnOx.
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Fig. 7 Diffuse UV-vis spectra (a) and H2-TPR profiles (b) of xMn-MS samples.
two kinds of MnOx. However, interestingly, the MnOx ascribed sample of 3Mn-MS, Mn2p1/2 and Mn2p3/2 correspond to the
to stronger redox ability in 3Mn-MS has the highest proportion binding energy of 641.65 eV and 653.95 eV respectively, agreeing
as compared to its counterparts, despite the greater loading of with the typical Mn3O4 species.47 The binding energies of Mn2p
Mn in 4Mn-MS and 5Mn-MS.
before and after calcination showed an obvious shift (0.35 eV)
To identify the chemical environment of the various elements related to the transformation of metal atoms from ionic status to
in the catalysts, the uncalcined HMS-DDA, u-3Mn-MS, and the metal oxide phase. This phenomenon indicates that the metal
calcined 3Mn-MS were determined by the high-resolution XPS species underwent surface-interactions on the pore wall rather
technique. Fig. 8 presents XPS spectra of N1s (Fig. 8a), Mn2p than forming a stable framework.
(Fig. 8b), and O1s (Fig. 8c) in the u-3Mn-MS and 3Mn-MS. For
Additionally, the coordination decoration and interaction of
Mn2p spectrum of u-3Mn-MS, Mn2p1/2 and Mn2p3/2 with the Mn ions on N groups of DDA micelles in the resultant sample
binding energy value at 642 eV and 654 eV, respectively. For the are further ascertained using the XPS spectrum of N1s core
Fig. 8 XPS spectra of (a) N1s, (b) Mn2p, and (c) O1s core levels, and (d) full survey of several comparable samples including calcined 3Mn-MS and
uncalcined u-3Mn-MS samples.
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level (Fig. 8a). The N1s spectrum of uncalcined DDA-bearing might cause obvious aggregation and block in the mesopore to
HMS showed a dominant peak with binding energy of some extent, with more and more Mn species appearing on the
399.60 eV, and a shoulder at 401.3 eV corresponding to the pore surface. Additionally, the excessive introduction of Mn
free amino moiety that interacted with the siliceous wall by species in the nanopores of silica obviously destroyed the
hydrogen-bonding in the DDA molecule.48 In contrast, u-3Mn- porous structure, which gave rise to the physical structural
MS showed two characteristic N1s peaks at 399.5 and 401.7 eV, properties. In particular, we observed that the mesoporous
where the binding energy of 401.7 eV differs from that diffraction peak of sample 5Mn-MS almost disappeared at
previously mentioned for interactions by hydrogen-bonding. 2y = B21, indicating the absence of regular mesopores in the
The obvious shift (0.4 eV) to high binding energy and the resulting samples. The MnOx species were embedded in closed
obviously-enhanced intensity were observed, which indicated pores without the real exposed active sites that can come in
the variation in N atoms in the chemical state and coordination contact with the reactants in the reaction system. Therefore, the
environment before and after introducing Mn2+ into the assembly 3Mn-MS afforded the appropriate TOF value as compared to
process. In addition, the obvious decrement in the amount of free other counterparts at relatively high metal loading (Fig. 9e),
amino groups because of the introduction of Mn ions was based on the comprehensive consideration of Mn loading and
evidenced by the increased peak intensity of nitrogen atoms reaction conversion rate. On the other hand, according to the
interacting with Mn ions, which was strongly attributed to the H2-TPR results, the surface MnOx species afforded different
Mn2+ coordinated to the N atoms of the DDA molecule where states corresponding to discriminative reduction temperatures.
the electron-enriched nitrogen atoms shared electrons with Mn In particular, the surface interacted MnO species in the 3Mn-MS
ions, thereby resulting in the lowered electron density of the took up an obviously large proportion as compared to that of
nitrogen atoms.49 Additionally, the existence and absence of other Mn loading samples. We also observed that the 3Mn-MS
the N1s peak in the u-3Mn-MS and 3Mn-MS XPS survey spectra afforded the highest catalytic activity, which may be due to the
verified the removal of surfactant template by calcination.
surface interacted MnOx species.
We proceeded to evaluate the catalytic stability of the
selected optimal catalyst via recycling the used catalyst; the
Catalytic analysis
The catalytic activity of the MnOx-cluster-embedded nanoporous catalytic results are presented in Fig. 9d. To our delight,
silica was systematically evaluated using the oxidative degrada- the catalytic activity was retained for at least 5 cycles. In view
tion process of MO aqueous solution. Herein, peroxymonosulfate of the slight drop in the catalytic activity, we checked the Mn
emerges as the oxidant agent, which has been demonstrated to content retained in the catalyst using the ICP measurements.
be oriented over the functional active sites of mesoporous silica. The leaching of the Mn element was observed from 6.2 wt% to
The oxidative sulfate radicals (SO4 À) possess higher oxidation 5.1 wt%, which should account for the loss of catalytic activity
ꢀ
electric potential (2.5–3.1 eV) with respect to hydroxyl radicals after recycling several times.
(2.7 eV), which is beneficial for accelerating the oxidation reac-
tion process.50 On the other hand, the catalytic active sites are
regarded as the promotive factors, with the conductive cluster-
like MnOx affording more exposed active centers in the pores,
which is beneficial for enhancing the catalytic activity.
We performed the degradation test using the series of
Mn-functionalized molecular sieve samples under identical
reaction conditions (Fig. 9). The reaction process of MO degra-
dation involving 1Mn-MS at different times was determined
(Fig. 9a). Before the degradation, we assessed the catalytic
performance of the pure HMS sample in the MO aqueous
solution, which gave a feeble catalytic activity for the oxidation
degradation in the absence of the PMS (Fig. 9b). The results
demonstrated that the catalytic activity should be mainly
derived from the Mn species. On the other hand, the xMn-MS
À
À
Mn2+ + HSO5 - Mn3+ + SO4 + OHÀ
(1)
(2)
(3)
(4)
(5)
ꢀ
À
À
Mn3+ + HSO5 - Mn4+ + SO4 + OHÀ
ꢀ
Mn3+ + HSO5 - Mn4+ + SO4 + OH
À
2À
ꢀ
À
À
Mn4+ + HSO5 - Mn3+ + SO5 + H+
ꢀ
À
À
Mn(3+/4+) À OH + HSO5 - Mn(3+/4+) À HSO5 + OHÀ
Mn(3+/4+) À HSO5 + 2H2O - Mn(3+/4+) À OHÀ + SO4 + O2 +
À
À
ꢀ
4H+
(6)
(7)
(8)
SO4 + OHÀ - SO4 + OH
À
2À
ꢀ
ꢀ
À
À
2ꢀOH + SO5 - SO4 + H2O + O2
ꢀ
Based on the reported literature, we tentatively speculate
catalysts showed evolutive catalytic performance (Fig. 9b and c) that the formation (Scheme 3) of oxidative radicals, including
corresponding to the variation in reaction time under identical sulfate radicals and hydroxyl radicals, is derived from the
reaction conditions, which is associated with the different transformation of Mn2+ and Mn3+ into Mn4+ species of surface
physicochemical states of catalysts. Note that the catalytic MnOx, which is thermodynamically favorable for activating
À
À
activity affords an interesting variational trend with increased HSO5 by Mn2+ and Mn3+ to generate active ꢀOH and SO4
,
ꢀ
Mn-loading in silica, in which the catalytic performance was referring to eqn (1)–(3).51,52 In the meantime, Mn4+ was changed
enhanced from the 1Mn-MS to 3Mn-MS, while a decreased back into Mn3+, acÀcompanied by the generation of SO5 by
trend in catalytic activity was demonstrated as the Mn loading reacting with HSO5 (eqn (4)).53,54 Thus, the hydroxyl species
was further raised to 8.32% (Mn/Si). In conjunction with the produced in eqn (1) and (2) as well as those from the dissociation
aforementioned characterizations, we suggest that the MnOx of physically adsorbed H2O molecules could bind to the surface of
À
ꢀ
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Fig. 9 (a) The catalytic degradation of MO using 1Mn-MS as displayed by dynamic UV spectra. (b and c) Catalytic degradation performance of
comparable catalysts, and (d) the recycling test results of 3Mn-MS. (e) TOF values of the involved comparable samples, and (f) EPR spectra of 2Mn-MS and
3Mn-MS (reaction conditions are as follows: 10 ppm DMPO, 20 ppm MO, 0.1 g LÀ1, 1 mM PMS, pH of 7, 25 1C, and reaction time of 5 min).
Mn(3+/4+) sites of the catalyst to generate Mn(3+/4+)–OH, which catalyst in toluene combustion in air was further assessed to
À
would then interact with HSO5 by ligand displacement to form explore its potential in VOC treatment. Before the catalytic
Mn(3+/4+)–HSO5À (eqn (5)). Afterward, Mn(3+/4+)–HSO5À dissociated experiment, the adsorption of toluene on the resultant
into SO4 À in eqn (6).55 Partial ꢀOH was also produced by reacting Mn-bearing catalysts was firstly measured on the basis of the
ꢀ
À
OHÀÀ and SO4 (eqn (7)). In the meantime, some ꢀOH induced by pressure-swing process, as shown in Fig. 10a, which could
ꢀ
À
SO5 resulted in SO4 in eqn (8).56,57 To confirm the presence indicate the diffusing behavior of toluene in the real catalyst’s
ꢀ
À
ꢀ
of both ꢀOH and SO4 radicals, EPR spectral studies were pore. As a result, the variation in the adsorption amount in view
performeÀd and are displayed in Fig. 9f. As expected, the ꢀOH of the increased Mn loading could be observed clearly, implying
ꢀ
and SO4 signals were recognized as the specific evidence for the rational decrement due to the progressive introduction of
affirming the proposed mechanism process. Additionally, MnOx species into the reaction channel. In particular, the
the intensity of signals for the tested 3Mn-MS and 4Mn-MS observation of the capillary condensation around P/P0 B 0.2
further sustained the active site numbers-determined catalytic with regard to the 1Mn-MS, 2Mn-MS, and 3Mn-MS verified the
performance.
normal mesoporous channel without complete blocking after
Apart from the above study concerning the liquid oxidation the controlled introduction of Mn species.58 However, the
degradation of MO, the catalytic capacity of the Mn-based loss of toluene adsorption in the 4Mn-MS and 5Mn-MS is
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Scheme 3 Schematic catalytic oxidative degradation of MO in aqueous solution.
Fig. 10 Toluene adsorption isotherms of a series of Mn-based catalyst (a), the catalytic degradation efficiency of toluene using a series of Mn-based
catalysts (b). Arrhenius plots of 3Mn-MS, 4Mn-MS, and 5Mn-MS (c), and life test of catalyst 3Mn-MS under the optimal reaction conditions (d).
remarkable as compared to several other counterparts. This from 180–300 1C, which is displayed in Fig. 10b. The scalari-
indicates the possible closing and blocking of reaction channels in form increment in catalytic performance is clearly observed
the presence of larger toluene molecules, which is in good agree- with increased reaction temperature. However, the reaction
ment with previous characterization analyses. The greater adsorp- trend in the presence of the increasing loading of Mn species
tion capacity of 5Mn-MS compared to 4Mn-MS could be due to the gives rise to a similar phenomenon to the above liquid reaction
greater packing porosity among interparticles rather than meso- systems, which is associated with the pore blocking of excessive
pores, which agrees well with the N2 adsorption/desorption results. MnOx species. This can further account for the higher catalytic
The catalytic oxidative degradation of toluene over all of the efficiency of 3Mn-MS as compared to other counterparts
samples in the fixed-bed reactor was evaluated in the range because of the best degree of exposure of the active substance
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on the external surface coupled with the unplugged diffusing
channel. Note that the pristine catalytic conversion of toluene
before 180 1C was still observed below 40%, implying that there
was catalytic progress even at a lower reaction temperature;
however, the catalytic efficiency was undesirable from a prac-
tical point of view. The dramatic enhancement in catalytic
conversion was demonstrated after 200 1C, especially for the
3Mn-MS, which was directly related to the more accessible
active centers that appeared on the catalyst. Apart from the
produced H2O molecules, according to the GC results, CO2
emerged as the only product at 240 1C. However, at a lower
temperature, CO was partially visible, and other intermediates
were weak or absent. As a result, the T90 and T100 can be
achieved at 225 1C and 240 1C, which is far superior to several
other comparable catalysts. Notably, the catalytic efficiency of
these used catalysts follows the sequence: 3Mn-MS 4 4Mn-MS 4
5Mn-MS 4 2Mn-MS 4 1Mn-MS. To our delight, when com-
pared to the reported catalyst systems (Table 2), the superior
catalytic efficiency of 3Mn-MS was the most outstanding
current monometal-bearing Mn-based catalyst, even higher
Scheme 4 Illustration of the encouraging catalytic diffusion reaction
pathway in catalytic toluene degradation in a fixed-bed reactor using
unidirectional airflow: (a) blocked diffusion channel; (b) free diffusion
channel.
than those well-designed composite catalysts obtained by The apparent activation energies (Ea) calculated by the Arrhenius
incorporating multiple active metal centers. plots for the three samples with advantageous activity, including 3,
To ascertain the catalytic stability and life of the resultant 4, 5Mn-MS, were further introduced to evaluate the catalytic activity
optimal catalyst, the catalytic assessment was carried out over for these samples, and are presented in Fig. 10c. Specifically, note
600 min under identical reaction conditions, and the near that the Ea values follow the order: 3Mn-MS (45.3 kJ molÀ1) o
100% conversion activity in the successive catalytic cycles 4Mn-MS (64.1.8 kJ molÀ1) o 5Mn-MS (85.4 kJ molÀ1). Obviously,
indicates the durable catalytic performance in the present the Ea values follow an inverse trend with respect to the catalytic
catalytic system. To explain the existing promotive catalytic activities. The results clearly indicate that the differences in the
efficiency of 3Mn-MS in toluene conversion, in conjunction Ea values are most likely related to the differences in the number of
with the aforementioned toluene adsorption results, we propose active surface sites on the catalyst.70
the possible efficient catalytic model based on the diffusion-
Finally, the real adsorption process of surface-adsorbed
eliminated pathway (Scheme 4b) as compared to its counterpart species after introducing toluene and the ensuing catalytic
with the pore-blocked catalyst (Scheme 4a). This is associated reaction process of toluene combustion were understood
with the fact that the blocked pore fails to accommodate the by virtue of in situ DRIFTS experiments performed over the
unidirectional flow of the toluene reaction molecule under high 3Mn-MS catalyst. As shown in Fig. 11a and b, the in situ
space velocity, and the back-mixing effect cannot exist in the gas– adsorption process of toluene on the active MnOx centers was
solid phase reaction, unlike the interim liquid phase reaction.
clearly observed and identified by the changing spectra of
At a more fundamental level, this catalytic process followed toluene at 1543 cmÀ1 corresponding to the skeletal C–C stretching
a first-order reaction mechanism with respect to toluene vibration of the aromatic ring. Before introducing oxygen, the
concentration, which has been affirmed by wide investigations.70 relatively pure spectral characteristics of toluene adsorption were
Table 2 Comparison of the catalytic degradation of toluene, based on various known catalysts and 3Mn-MS
Catalysts
Conv. tem. (1C)
Initial concentration (ppm)
Dosage (mg)
Ref.
Mn4Al0.7Zr0.5Ce0.1(im)/CTA-L
Cu1.5Mn1.5O4
OMS
Mn3O4
Mn2O3
T90 = 250
T100 = 270
T90 = 311
T90 = 270
T90 = 274
T90 = 320
T90 = 255
T100 = 225
T90 = 258
T100 = 240
T90 = 267
T90 = 235
T100 = 270
T90 = 240
T90/T100 = 225/240
500
1000
1000
1000
1000
1000
1000
1000
1000
500
1000
1000
1000
1000
1000
500
350
100
400
100
100
—
300
100
200
240
-
59
60
61
62
61
62
63
64
65
66
67
68
69
61
LaMnO3
CuMn/La-4 mol%
Ce0.03MnOx
LaMnO3/d-MnO2
Mn/R-SBA-15
Co1.5Mn1.5O4
MnxZr1-xO2
Co-Mn BHNCs
Pd-Al2O3
3Mn-MS
100
100
100
This work
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Fig. 11 Time-dependent in situ DRIFTS 2D (left) and 3D (right) spectra of toluene adsorption over 3Mn-MS (a and b), after inputting 20% O2/N2 at 225 1C
for the identification of toluene degradation in the different time courses (c and d).
observed (Fig. 11a), and the adsorption process almost reached the whole degradation process. Specifically, the toluene mole-
saturation after 10 min of dynamic adsorption, indicating the cule was firstly oxidized to benzaldehyde by the surface oxygen
rapid completion of toluene adsorption by the high exposure of derived from MnOx, and then further oxidized to benzoate and
surface MnOx and its accessibility. Besides, the CQO stretching other deep-oxidized products with the assistance of introduced
vibration (1718 cmÀ1) attributed to benzaldehyde was synchro- external oxygen, which is in accordance with some recent
nously identified, accompanied by the presence of toluene,71 and reports.72
increased by the additional time, which was associated with the
surface oxygen-participated oxidation process derived from the
catalyst. Note that the other detectable product is sightless,
indicating no occurrence of the successive deep oxidation. After
4. Conclusions
the long purge of N2 at the realized reaction temperature, the In summary, we successfully developed a fine-sized MnOx-
physically-absorbed toluene was removed to exclude the interrup- modified nanoporous functional siliceous molecular sieve hetero-
tion of free toluene molecules for the ensuing reaction process. geneous catalyst through the in situ embedding of cluster-like
The in situ spectra of toluene adsorption after inputting 20% MnOx species into mesochannels during the catalyst assembly
O2/N2 are further shown in Fig. 11c and d for ascertaining the process. The confined MnOx species were of ultra-tiny size and
toluene degradation under the optimal reaction condition. had strong interactions with the siliceous surface, and the porous
As expected, relatively complex IR spectra were obtained owing siliceous materials showed nanospherical morphology with the
to the reaction proceeding after the introduction of sufficient assisted assembly process of Mn ions and were influenced by
oxygen. A significant decrement in intensity corresponding to the the introduced Mn concentration in the samples. Furthermore,
skeletal C–C stretching vibration of toluene was demonstrated, the pore size of the siliceous material was tailored gradually on
indicating the degradation of toluene in 12 min, thereby account of the encapsulation and occupation of MnOx clusters in
accounting for the excellent catalytic capacity of surface- the mesochannel. As a result, the oriented functional catalysts
incorporated MnOx through the proposed tactical integration. affording the confined MnOx active species demonstrated
In addition, the symmetric vibration of the carboxylate band excellent catalytic activity for motivating peroxymonosulfate to
(1383 cmÀ1) was clearly discerned, further indicating the pro- oxidatively degrade MO (within 8 min), and the thermal catalytic
ceeding of successive deep oxidation from the benzaldehyde to degradation of toluene (T90/T100 = 225/240 1C) in air, superior to
the subsequent benzoate and so on. This result also explains most known composite catalysts. This provides significant insight
the different oxidation mechanisms of toluene derivatives into the environmental catalytic process by virtue of efficient
for the demand of oxygen at different oxidative steps during catalyst-design engineering.
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in Confined Space for Oxygen Evolution Reaction, ACS
Sustainable Chem. Eng., 2019, 7, 2837–2843.
Conflicts of interest
There are no conflicts to declare.
11 T. Zeng, X. Zhang, S. Wang, H. Niu and Y. Cai, Spatial
Confinement of a Co3O4 Catalyst in Hollow Metal–Organic
Frameworks as a Nanoreactor for Improved Degradation of
Organic Pollutants, Environ. Sci. Technol., 2015, 49, 2350–2357.
12 Y. Zhang and S.-J. Park, Au-pd bimetallic alloy nanoparticle-
decorated BiPO4 nanorods for enhanced photocatalytic
oxidation of trichloroethylene, J. Catal., 2017, 355, 1–10.
13 Y. Zhang and S.-J. Park, Incorporation of RuO2 into
charcoal-derived carbon with controllable microporosity
by CO2 activation for high-performance supercapacitor,
Carbon, 2017, 122, 287–297.
14 K. Chen, H. Fang, S. Wu, X. Liu, J. Zheng, S. Zhou, X. Duan,
Y. Zhuang, S. C. E. Tsang and Y. Yuan, CO2 hydrogenation
to methanol over Cu catalysts supported on La-modified
SBA-15: the crucial role of Cu-LaOx interfaces, Appl. Catal.,
B, 2019, 251, 119–129.
Acknowledgements
The authors acknowledge the project Funded by the National
Natural Science Foundation (51672114 and 21908085), Natural
Science Foundation of Jiangsu Province (BK20161357 and
BK20190961), Foundation from Marine Equipment and
Technology Institute for Jiangsu University of Science and
Technology (HZ20190004), High-tech Ship Research Project of
the Ministry of Industry and Information Technology, China
(No. [2017] 614), and Scientific Research Foundation of Jiangsu
University of Science & Technology (1112921902).
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