MnOx-loaded Mesoporous Silica for the Catalytic Oxidation of Formaldehyde. Effect of the Melt Infiltration Conditions on the Activity – Stability Behavior

Article abstract of DOI:10.1002/cctc.201901902

Conventional melt infiltration (over calcined SBA-15 silica support) and optimized melt infiltration (over surfactant-containing SBA-15 support) were used for the first time to prepare MnO_x nanoparticles encapsulated within the support pores. A comprehensive study on the evolution of textural, structural, and morphological characteristics of the MnO_x-silica composites as well as their redox and surface properties is reported herein by varying synthesis parameters, such as manganese loading (5, 10, 20 and 30 Mn wt.%), and post-treatment temperature (300 and 500 °C). The catalytic performances of the materials prepared in this work have been evaluated in the catalytic oxidation of formaldehyde. The results show a high performance of the SBA-15 supported manganese oxide at low- to moderate- temperatures of reaction (Temperature at 50 % of HCHO conversion into CO₂ ranging from 110° to 150 °C), activity being related to Mn oxidation state (from 2.3 to 4) and MnO_x phase location, confined with or without 2D spatial distribution in the support porosity or formed at the external surface of SBA-15. A high stability (60 hours), under dry air and 50 % relative humidity air, is reported for MnO_x-based nanocatalysts prepared by conventional and optimized melt infiltration procedures.

Full text of DOI:10.1002/cctc.201901902

Accepted Article
Title: MnOx loaded mesoporous silica for the catalytic oxidation of
formaldehyde. Effect of the melt infiltration conditions on the
activity – stability behavior
Authors: Guillaume Rochard, Carmen Ciotonea, Adrian Ungureanu,
Jean-Marc Giraudon, Sébastien Royer, and Jean-François
Lamonier
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To be cited as: ChemCatChem 10.1002/cctc.201901902
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MnOx loaded mesoporous silica for the catalytic oxidation of
formaldehyde. Effect of the melt infiltration conditions on the
activity – stability behavior
Guillaume Rochard,[a] Carmen Ciotonea,[a] Adrian Ungureanu,*[b] Jean-Marc Giraudon,[a] Sébastien
Royer,[a] Jean-François Lamonier*[a]
Abstract: Conventional melt infiltration (over calcined SBA-15 silica
support) and optimized melt infiltration (over surfactant-containing
SBA-15 support) were used for the first time to prepare MnOx
comparable technical properties. Unfortunately, in several
production domains, as in wood and furniture fields, no chemical
substituents affording comparable properties with HCHO (paste,
final resistance and so on) have been found. Therefore, one
actual challenge is to develop efficient processes for HCHO
degradation (in industry and in indoor air), in order to improve the
indoor air quality in agreement with the current environmental
nanoparticles encapsulated within the support pores.
A
comprehensive study on the evolution of textural, structural, and
morphological characteristics of the MnOx-silica composites as well
as their redox and surface properties is reported herein by varying
synthesis parameters, such as manganese loading (5, 10, 20 and 30
Mn wt. %), and post-treatment temperature (300 and 500 °C). The
catalytic performances of the materials prepared in this work have
been evaluated in the catalytic oxidation of formaldehyde. The results
show a high performance of the SBA-15 supported manganese oxide
at low- to moderate- temperatures of reaction (Temperature at 50 %
of HCHO conversion into CO2 ranging from 110° to 150°C), activity
being related to Mn oxidation state (from 2.3 to 4) and MnOx phase
location, confined with or without 3D spatial distribution in the support
porosity or formed at the external surface of SBA-15. A high stability
(60 hours), under dry air and 50% relative humidity air, is reported for
MnOx-based nanocatalysts prepared by conventional and optimized
melt infiltration procedures.
regulations [3]
.
From a technical point of view, HCHO elimination can be
performed by either adsorption or combustion. Regarding the
adsorption process, porous materials such as activated carbons,
[4] aminated silicas [5] or zeolites (e.g., faujasite) [6] are recognized
as efficient adsorbents of HCOH. The highest adsorption
capacities are reported for NaY zeolite, with more than 0.4 mL
HCHO stored per gram of adsorbent. However, adsorption has its
own limitations such as the need of a regeneration step [7] and the
high-energy cost for the incineration [4b]. Owing to some obvious
advantages such as: (i) a selective transformation of HCHO into
harmless compounds (CO2 and H2O) and (ii) a lower energy
consumption compared to thermal decomposition due to a lower
operating temperature,[8] catalytic total oxidation appears as one
of the most practical, efficient, and promising way for HCHO
abatement, representing thus a key strategy to mitigate climate
Introduction
change and protect human health. Among the different studied
[8,9]
catalysts
supported noble metals (Pt, Pd and Au) showed
Among the volatile organic compounds (VOC), formaldehyde
(HCHO) is a common molecule found in indoor environments
such as offices, houses, schools and industrial plants [1]. Long-
term exposures to HCHO is evidenced to be at the origin of severe
respiratory system diseases and even cancer [2]. Considering the
adverse effect of HCHO to human health, initiatives emerged
during the last decade to limit release of HCHO in the environment,
the most efficient from industrial/process point of view being the
replacement of formaldehyde by other chemicals having
good performances at low/room temperature in the catalytic
[10]
oxidation of formaldehyde
which can be further improved by
alkaline doping [11]. It is worthy of mention that the undoped
supported noble metal catalysts are generally activated at
temperatures of around 50 °C to be selective to CO2 [12]. Despite
these promising results, nowadays, there is a continuous growth
of interest for the catalysts containing transition metal oxide
[14]
(TMO) nanoparticles (NPs) (e.g., mixed MnOx-CeO2 [13], MnOx
,
TiO2-CeO2 [15] or Co3O4 [16]) due to their high catalytic capability as
well as their cost-effectiveness, as compared with catalysts based
on noble-metals. Nevertheless, the results showed that total
oxidation reaction generally takes place at higher temperatures
[a]
[b]
Dr. G. Rochard, Dr. C. Ciotonea, Prof. S. Royer, Dr. J.-M. Giraudon,
Prof. J.-F. Lamonier
Univ. Lille, CNRS, ENSCL, Centrale Lille, Univ. Artois, UMR 8181 -
UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille,
France
(>100°C) than those for noble metal supported catalysts (<50°C)
[17]
.
Amongst these TMO nanocatalysts, those based on
manganese oxides (MnOx) are the most promising candidates,
but, at the same time, they are the most challenging to design
owing both to the redox properties of manganese – several
oxidation states, and to the multiple oxide polymorphs found for
every manganese oxidation state, each of them manifesting a
distinct catalytic performance in the VOC oxidation. For instance,
among the different polymorphs of MnO2 [18], birnessite (δ-MnO2)
and cryptomelane (-MnO2) were found as the most active
E-mail: jean-francois.lamonier@univ-lille.fr
Prof. A. Ungureanu
“Gheorghe Asachi” Technical University of Iasi, Faculty of Chemical
Engineering and Environmental Protection, 73 D. Mangeron Bvd.,
700050 Iasi, Romania.
E-mail: aungureanu@tuiasi.ro
Supporting information for this article is available on the WWW
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10.1002/cctc.201901902
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FULL PAPER
phases. Complete HCHO oxidation at temperatures of around
140 °C and even around 100 °C was reported for cryptomelane
[19] and high surface area birnessite, respectively [20]. Furthermore,
in the case of supported MnOx catalysts, difficulty arises because
the performances of such nanomaterials in terms of activity,
selectivity and stability, generally depends on a multitude of
properties that have to be simultaneously controlled:
morphostructure (e.g., morphology, size, dispersion, and spatial
distribution of MnOx NPs), texture (e.g., specific surface area,
pore volume, and pore size distribution), resistance against NPs
control over the activity-stability behavior of MnOx-SBA-15
catalysts during HCHO oxidation. The performances of the
catalysts were discussed in light of the physicochemical
characterizations performed on the calcined and spent catalysts.
Table 1. Textural and structural properties of SBA-15 support and Mn containing
catalysts.
Mn
Load.1
(wt.%)
2
3
4
5
6
7
SBET
Sμ
Vp
Vμ
Dpores
Dcryst
(nm)
Sample
SBA-15
(m²/g)
(m²/g)
(cm3/g)
(cm3/g)
(nm)
aggregation upon thermochemical preparation steps
–
calcination/activation, reducibility of oxide catalysts, extent of
metal oxide-support interactions, oxidation state of manganese,
surface chemical composition and, presence of acid-base
promoters and so forth [18], which, in turn, are influenced by the
applied synthesis method and particularly, by the physico-
chemical properties of the catalytic support, chemical nature and
crystallographic characteristics of the active nanophase and its
-
782
569
201
157
1.2
0.8
0.016
0.016
6.9
-
20Mn-
MIc-500
19.6
4.8-
7.0
10
20Mn-
MIc-300
19.5
426
125
0.6
0.016
5.4-
7.1
9
SBA-15
-
786
592
197
155
1.2
1.0
0.009
0.008
7.3
-
local
environment,
and
thermal
conditions
upon
calcination/activation steps. For the reasons set out, as compared
with HCHO oxidation performed over bulk MnOx, only a few
catalytic studies were reported on supported MnOx particles.
MnOx NPs have been loaded on different supports such as
cellulose [21], activated carbon [22] or graphene nanosheets [23]. As
a very relevant example, -MnO2 NPs confined within the
mesopores of ordered SBA-15 by classical impregnation or two-
solvent methods were reported as highly efficient catalysts for the
elimination of formaldehyde, achieving a complete conversion at
130 °C, an activity which is similar with that shown by tunnel and
lamellar bulk manganese oxides, despite a lower Mn content for
supported catalyst (i.e., 15 wt.% Mn)[24]. Such good performance
was ascribed to the beneficial adsorption of HCHO over the
surface silanol groups of SBA-15 support, synchronized with an
effective HCHO conversion into CO2 over the MnO2 NPs
encapsulated within the pores of SBA-15. Nevertheless, the
stability of these catalysts was not in detail investigated.
5Mn-
MIc-300
4.7
5.3-
7.2
10
10Mn-
MIc-300
11.4
28.5
514
433
129
96
0.8
0.6
0.003
0.04
5.4-
7.3
11
11
30Mn-
MIc-300
5.1-
6.8
SBA-15
-
-
782
525
201
158
1.2
0.7
0.011
0.022
6.3
-
20Mn-
IWI
5.2-
7.0
9
20Mn-
MIa-
300
19.2
527
135
0.8
0.008
4.8-
7.3
9
1, Mn loading as determined by ICP-OES; 2, Specific surface area (BET
5
method); 3, Microporous surface area (t-plot method); 4, Total pore volume;
,
Micropore volume (t-plot method); 6, Mean pore size (BJH method); 7, MnOx
average crystal domain size calculated by the Scherrer equation.
Motivated by these promising results, we report a strategy to
obtain original catalysts for the total oxidation of formaldehyde,
active and resistant to deactivation, by the confinement of small
and homogeneous MnOx NPs in the mesochannels of SBA-15
through the melt infiltration (MI) approach. Such a method was
used successfully to get highly dispersed NiO, CuO, Co3O4, and
Fe2O3 NPs on ordered mesoporous silica [25]. A series of catalysts
obtained with different Mn loadings (5, 10, 20 and 30 wt.% Mn)
and at two calcination temperatures (300 or 500 °C) was prepared
through a MI procedure adapted from our recent studies on NiO-
confined NPs [25c, 25e], using Mn(NO3)2•4H2O as precursor taking
advantage of its low cost, limited toxicity, and low melting
temperature (i.e., 37°C). By the MI approach proposed herein,
which basically involves the time-controlled diffusion of the melted
manganese nitrate precursor within the pores of either the
surfactant-free, calcined SBA-15, or the as-synthesized SBA-15
occluded with Pluronic P123 surfactant, a selective localization of
highly dispersed MnOx NPs inside the mesopores can be
achieved after controlled calcination and optimal Mn loading, with
positive effects on the activity and/or stability of catalysts during
reaction. This offers a unique opportunity to have an enhanced
Results and Discussion
Effect of calcination temperature on the catalyst properties
Properties of 20Mn-MIc-Z (Z=300, 500) samples.
The nitrogen adsorption/desorption isotherms of parent SBA-15
and 20Mn-MIc-Z are displayed in Figure 1. All the materials
present type IV isotherms with H1 hysteresis, as awaited for
ordered mesoporous materials with
a narrow pore size
distribution of the cylindrical pores [26]. For MI-derived materials,
the ordered structure is retained which indicates that the support
is stable during the MI grinding process. A delay on the closure of
the desorption branch of the isotherms is observed for the MnOx-
containing materials. This delay is related to the formation of so
called “ink-bottle” shaped pores, formed when confined NPs are
plugging the main mesopores [27] or when some constrictions form
at the entrance of the pores. Indeed, as showed in Figure 2, SBA-
15 exhibits a narrow pore size distribution centered at 6.9 nm
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whereas, after MI and calcination, a bimodal population is
observed for both MI derived materials.
range (a characteristic that should have been associated to the
formation of external aggregates of MnOx particles).
The textural properties were evaluated based on the
adsorption/desorption isotherms and the results are summarized
in Table 1. The BET surface area as well as the microporous
surface area decreased for the MnOx containing materials by
comparing to the parent SBA-15 support. As previously reported
by Ungureanu et al. [28] for Ni-Cu particles confined into SBA-15
pores, this trend can be related to the confinement of MnOx NPs
within the mesopores which can block the secondary porosity
(micro- and small mesopores). The decrease is much more
pronounced for 20Mn-MIc-300, suggesting that the increase in
calcination temperature induces the reopening of the porosity
fraction during sintering of initial dispersed MnOx NPs. This
reopening of the porosity is mostly visible on the total pore volume
and total surface area, than on the micropore surface area.
Figure 1. Adsorption/desorption isotherms (vertical shift of 150 cm3.g-1 each) for
SBA-15 and 20Mn-MIc-Z prepared at different calcination temperatures.
Figure 3. HR-TEM images for 20Mn-MIc-Z prepared at different calcination
temperatures
The low-angle XRD patterns obtained for the SBA-15 and the
20Mn-MIc-300 samples (Figure S1) both exhibit three well-
resolved peaks indexed to the (100), (110) and (200) reflections
of the ordered 2D hexagonal structure of P6mm symmetry, as
classically reported [26]. This confirms, as already observed, that
the melt infiltration procedure and the further decomposition of the
precursors to form the oxide MnOx phase, does not significantly
alter the mesostructure long range ordering. Indeed, the decrease
of the (100) reflection intensity, while the (110) and (200)
reflections remains visible, is not correlated to the decrease of the
pore network ordering, but is associated to the specific
localization of the MnOx NPs inside the mesopores, thus reducing
the electron density contrast between the pores and silica walls
Figure 2. Pore size distribution (vertical shift of 0.5 cm3.g-1.nm-1) for SBA-15 and
20Mn-MIc-Z prepared at different calcination temperatures.
The formation of the second porosity of ~ 5 nm diameter is then
assigned to the formation of NPs filling and obstructing the main
mesopores of the SBA-15 support. Most of the active phase is
suggested to be located inside the mesopores, since the
isotherms remains of type IV (with a long, almost horizontal,
plateau associated to the N2 capillary condensation), with no
significant increase in N2 volume adsorbed in the 0.9-1.0 P/P0
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[28]. The wide-angle XRD patterns of the 20Mn-MIc-300 sample
(Figure S2) revealed a crystallized material with three main
reflections between 35° and 65° which can be attributed to -
MnO2 (Akhtenskite, JCPDS 30-0820) [10c]. The broad diffraction
peak at ~ 22° is attributed to the amorphous silica. [25b, 27] Heating
the sample at 500 °C under air led to a more intense reflection at
2 = 28° which can be assigned to the presence of the -MnO2
crystal structure (Pyrolusite, JCPDS 001-0799) [17b, 29]. Then a
mixture of -MnO2 and -MnO2 phases forms after the MI and
decomposition of manganese precursor into SBA-15 porosity.
The size of MnO2 particles (9-10 nm) remains in the range of SBA-
15 mesopore size [26], suggesting that crystallization took place in
the confining space of the main mesopores of the silica support.
Representative TEM images are illustrated in Figure 3. As a first
observation, the SBA-15 mesopores do not collapse during the
MI preparation whatever the calcination temperature, confirming
the N2-physisorption results. For both materials, the MnO2 NPs
can be observed under the form of confined NPs, beside fewer
external larger NPs. Particle size distribution of MnO2 NPs
represented in Figure S3 shows that the proportion of particles of
size < 10 nm is about 75 % for the sample calcined at 300 °C, a
comparable value being obtained for the sample calcined at
500 °C, confirming a satisfying confinement, with limited migration
of manganese oxide phases outside of the mesopores when
calcination temperature increases. However, a growth of the
particles of size above 10 nm is observed with the increase of the
calcination temperature: the average size of the external particles
is ~23 nm at 500 °C compared with ~17 nm for a calcination at
300 °C. Hence, calcination at a higher temperature promoted the
growth of MnO2 NPs located at the external surface of SBA-15
grains.
the silica surface even after calcination at 500 °C, the ~106.2 eV
component is not visible on the Si 2p spectra of 20Mn-MIc-500. In
order to estimate the Mn average oxidation state (AOS), the Mn
3s XPS region was analyzed (Figure S6). A correlation between
the Mn AOS and the energy difference between the Mn 3s main
peak and its satellite (ΔEs) has been proposed by Galakhov et al.
[32] Equation (1) was used to calculate the Mn AOS (Table 2):
Mn AOS = 8.95 - 1.126 x ΔEs (eV)
(1)
The decreases in Mn AOS with calcination temperature increase
is confirmed, and values are in agreement with those obtained by
H2-TPR analyses. The O 1s XPS spectra of 20Mn-MIc-300 and
20Mn-MIc-500 are presented in Figure S7. In the O 1s region, two
photoelectron peaks can be clearly identified. The O-I component
located at ~529.8 eV may be considered as resulting mainly from
Mn-O-Mn bonds in MnO2 [33] while the dominating O-II component
at ~532.9 eV is related to O2- species in SiO2 (Si-O-Si
environments).[34] The contribution of the O-I component to the O
1s envelop is ~10 % whatever the calcination temperature. For
20Mn-MIc-300 sample, Mn/Si atomic ratio value is close to the O-
II/O-I atomic ratio value, thus confirming the O 1s attribution.
However, in comparison with the theoretical ones, Mn/Si atomic
ratio value is lower indicating that not all Mn species are detected
by XPS analysis.
Table 2. Summary of the XPS results obtained for Mn containing samples.
B.E. (eV)
Es
(eV)
Mn
AOS
Sample
Mn/Si1
O-I/O-II
O 1s
Si 2p
20Mn-
MIc-300
O-I : 529.4
O-II : 533.0
103.9
106.2
0.14
(0.27)
4.36
4.58
5.12
4.66
4.37
5.93
4.0
3.8
3.2
3.7
4.0
2.3
0.10
0.11
0.03
0.07
0.10
0.04
Redox properties and surface state of 20Mn-MIc-Z samples.
The H2-TPR profiles are shown in Figure S4. The hydrogen
consumption profile obtained for 20Mn-MIc-300 is typical of a
MnO2 phase reduction, being accomplished in two steps. During
the first step, the reduction of MnO2 into Mn3O4 takes place (65 %
of total H2 consumption), while during the second step, the
reduction of Mn3O4 into MnO can be observed (35 % of total H2
consumption), which is consistent with the theoretical reduction
values (67 % and 33 %, respectively). For 20Mn-MIc-500, the
proportion area of the first reduction peak is much lower (51 % /
49 %) suggesting the formation of an oxide phase with a lower
mean valence (MnOx with x < 2). This result can be explained by
the partial reduction of MnO2 when increasing the calcination
temperature [24, 30]. Based on the H2-consumption, the Mn average
oxidation state (AOS) was calculated, assuming a complete
reduction to MnO occurs in the studied temperature range. The
Mn AOS calculated for 20Mn-MIc-300 is close to 4, in agreement
with the presence of MnO2 phases identified by XRD analysis. For
20Mn-MIc-500, the Mn AOS is lower (3.6) demonstrating the
possible presence of a MnOx amorphous phases (unidentified by
XRD) with lower Mn oxidation state.
20Mn-
MIc-500
O-I : 529.5
O-II : 532.9
0.16
(0.27)
103.6
103.7
103.9
104.0
103.6
O-I : 529.6
O-II : 533.2
0.05
(0.06)
5Mn-MIc
10Mn-
MIc
O-I : 529.4
O-II : 533.3
0.10
(0.12)
30Mn-
MIc
O-I : 529.3
O-II : 533.2
0.16
(0.47)
20Mn-
MIa
O-I : 530.4
O-II : 532.7
0.09
(0.27)
1 values in brackets correspond to the expected bulk ratio
The Si 2p XPS spectra (Figure S5) of the 20Mn-MIc-300 and
20Mn-MIc-500 are composed of one main peak at BE ~ 103.8 eV,
characteristic of Si4+ in SiO2. For the sample calcined at 300 °C,
a smaller Si 2p component at BE ~106.2 eV indicates the
presence of Si-OH species [31]. Despite the remaining of Si-OH on
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calcination temperature. Thus, the highest activity can be
correlated to the highest Mn AOS. Therefore, for the following
studies, a calcination temperature at 300 °C has been kept
constant for the preparation of MnOx-SBA-15 catalysts.
Effect of manganese content
Properties of XMn-MIc-300 (X=5,10,20,30) samples.
The nitrogen adsorption/desorption isotherms at -196 °C as well
the pore size distribution of SBA-15 and Mn-MIc samples with
different Mn loadings are displayed in Figures S8-S9. The delay
in the closure of desorption branches becomes more significant
as the Mn content increases, up to 20 wt.%. With Mn loading, the
initial pore size distribution of the parent SBA-15 (7.3 nm) is
progressively replaced by smaller pore size distribution (~ 5.3 nm).
Based on the formation of such second porosity of ~5 nm
diameter (Figures S8-S9), previously attributed to MnO2 NPs
occluding partially the silica main mesoporosity, it can be
suggested that the optimum for Mn infiltration is 20 wt.%. Besides,
specific surface area decreased progressively from 25 % for 5Mn-
MIc to 46 % for 20Mn-MIc compared to SSA of SBA-15, while no
significant modification in SSA value is observed with a further
increase of Mn loading up to 30 wt.% (Figure 5). The pore volume
follows the same evolution as the specific surface area.
Figure 4. HCHO conversion into CO2 as a function of the reaction temperature
for MnOx/SBA-15 catalysts (20 wt.% Mn) prepared at different calcination
temperatures.
The XRD patterns are shown in Figure S10. As previously
described for 20Mn-MIc sample calcined at 300 °C, whatever the
Mn loading, the visible XRD peaks can be whole attributed to a
mixture of ε-MnO2 and β-MnO2. The average crystallite size
values range between ~10 to 12 nm (Table 1), suggesting that
MnO2 is well dispersed even at very high metal loadings.
Table 3. Catalytic properties of Mn oxides supported on SBA-15 in
formaldehyde total oxidation
2
3
Activity1
/mmolHCHO conv. molMn-1 h-1
T90
T50
Sample
(°C)
(°C)
20Mn-MIc-300
20Mn-MIc-500
5Mn-MIc-300
10Mn-MIc-300
30Mn-MIc-300
20Mn-IWI-300
20Mn-MIa-300
17.5
10.3
12.4
15.8
13.8
17.5
6.2
160
160
175
168
167
210
176
120
133
152
135
114
119
144
1 : Measured at 120 °C, 2 : Temperature at 90 % of HCHO conversion into CO2,
3 : Temperature at 50 % of HCHO conversion into CO2
Catalytic properties of 20Mn-MIc-Z samples. The catalytic
performances of 20Mn-MIc-300 and 20Mn-MIc-500 catalysts
were evaluated in the reaction of formaldehyde total oxidation by
decreasing the temperature from 200 °C to 50 °C. HCHO was
converted into CO2 and H2O without any other byproducts. The
curves of HCHO conversion into CO2 as a function of the reaction
temperature (light-off curves) are displayed in Figure 4. In the
temperature range 75 °C -150 °C, 20Mn-MIc-300 presents the
highest activity (Table 3), highlighting the important role of the
calcination temperature. The two samples with same MnOx
loading presented similar textural and structural properties but are
distinguished by the Mn AOS which decreased with the
Figure 5. Evolution of the specific surface area and pore volume of MnOx/SBA-
15 materials calcined at 300°C with the Mn loading.
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more difficult to be reduced [25a]. The more important formation of
such large – external - NPs for 30Mn-MIc-300 is in agreement with
the Mn loading limit, as it has been observed by HR-TEM (Figure
6).
The Si 2p, Mn 3s and O 1s XPS spectra of the XMn-MIc samples
are reported in Figures S5, S6 and S7, respectively. The Si 2p
spectra are similar regardless of the Mn loading (Si4+ from SiO2).
The Mn AOS value, calculated from Es, is lower for 5Mn-MIc-
300 sample (3.2) than for samples of higher Mn loadings (3.7-4.0)
(Table 2). Considering that H2-TPR leads to a 4.0±0.1 Mn AOS
whatever the loading, the lower surface oxidation state obtained
for 5Mn-MIc-300 sample could be related to the smaller MnOx
particle size generated and exclusive localization inside the
mesoporosity. As previously proposed, the two peaks in the O 1s
region are attributed to O2- species in MnOx (O-I component) and
in SiO2 (O-II component). The contribution of the O-I component
to the O 1s envelope increased with the Mn content up to 20 wt.%,
while the Mn/Si atomic ratio values are close to the theoretical
ones for 5Mn-MIc-300 and 10Mn-MIc-300, suggesting that the Mn
species are homogeneously distributed within the SiO2 porosity at
the lower Mn loadings (Table 2). In contrast, Mn/Si atomic ratio
values departs from the theoretical ones for Mn loadings of > 15
wt.% (Table 2), in agreement with the formation of larger MnO2
particles not entirely detected by XPS and a less homogeneous
dispersion in the support porosity.
Figure 6. HR-TEM images for MnOx/SBA-15 materials calcined at 300 °C (5,
30 wt.% Mn).
TEM images for the Mn-loaded materials were collected (Figure
6). The manganese oxide NPs are confined within the mesopores
of the SBA-15 for 5Mn-MIc (Figure 6), 20Mn-MIc (Figure 3) and
30Mn-MIc (Figure 6). Even if Mn-containing NPs confined in the
main mesopores remain the main phase for all the catalysts, the
proportion of external phase is observed to increase with the Mn
loading, as observed comparing particle size distributions
reported in Figure S11. In addition to the higher proportion of
external phases, their size also increase with the MnOx loading
(15 nm, 17 nm and 34 nm for 5, 20 and 30 wt.% Mn, respectively).
Catalytic properties of XMn-MIc-300 catalysts
The evolution of the HCHO conversion into CO2 as a function of
the temperature (light-off curve) obtained with the different XMn-
MIc-300 catalysts is displayed in Figure 7. By increasing Mn
loading, the light-off curves are shifted to lower temperatures.
While the T90 values are quite similar, the T50 is shifted from
152 °C for 5Mn-MIc-300 to 114 °C for 30Mn-MIc-300, respectively
(Table 3). The activities (mmole of HCHO converted into CO2 per
hour and per mole of Mn) have been calculated at T = 120 °C in
order to evaluate the influence of Mn loading on the catalyst
efficiency (Table 3). 120°C was selected considering that, at this
temperature, the 5Mn-MIc-300 is active while the other catalysts
do not present complete HCHO conversion degree. A maximum
of activity is reached for 20Mn-MIc-300 catalyst. This result
suggests that the highest MnOx dispersion is associated to the
highest activity in formaldehyde total oxidation (17.5 mmolHCHO conv.
molMn-1 h-1). The MnO2 particles from the external silica grain are
larger and exposed less active sites for the formaldehyde
oxidation in agreement with the lowest redox properties of such
Mn phase (Figure S12). For volatile organic compounds oxidation
at low temperature, it is generally accepted a redox mechanism
in which the reduction of active species of catalysts (here Mn4+) is
the rate determining step while the reoxidation of the active sites
occurs in a faster way. Such a mechanism can then explain the
parallel between the activity and reducibility obtained in this study.
When the Mn loading increases, the difference between the
average crystal domain size as determined by XRD and the TEM
issued average particle size becomes more important. For 30Mn-
MIc-300 sample, after calcination at 300°C, formation of
aggregates of elementary particles are observed, located at the
external surface of the silica grain and inside the silica main
mesoporosity. It leads to an average size, observed by TEM, of
~30 nm. XRD however demonstrated the maintaining of an almost
constant elementary crystallite size of ~11 nm (Table 1), a value
mostly dependent temperature of activation that conditions the
crystal growth.
Redox properties and surface state of XMn-MIc-300 samples.
The H2-TPR profiles of XMn-MIc-300 are shown in Figure S12. All
the TPR profiles are representative of MnO2 reduction with 67 %
- 33 % distribution between the areas of the two reduction peaks.
The reduction occurs as: MnO2 → Mn3O4 → MnO. The Mn AOS
is close to 4 for all the samples, in agreement with the presence
of MnO2 phases identified by XRD (Table 2). In the case of 30Mn-
MIc-300, a shoulder is observed at higher temperature (T =
435 °C). This could be explained by the presence of larger MnO2
NPs located at the external surface of the silica grain, which are
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MnOx NPs. Besides, SSA decrease is less pronounced for 20Mn-
MIa (527 m2.g-1), in comparison with 20Mn-MIc (426 m2.g-1)
(Table 1). The low-angle XRD pattern of 20Mn-MIa-300 (Figure
S1) confirms, as already concluded for the preparation 20Mn-MIc-
300, that the pore network long range ordering is not affected by
the preparation steps (presence of three reflections in the 0.8-2.0°
2 region, Figure S1). Wide-angle XRD patterns of MnOx-SBA-15
samples (20 wt.% Mn) prepared by different synthesis methods
are displayed in Figure 9. No significant differences were
observed IWI and MIc samples, both materials presenting mixed
-MnO2 and -MnO2 phases. On the contrary, the presence of
[35]
Mn3O4 (Hausmannite, JCPDS 80-0382)
phase is observed
when using melt infiltration on the as-made SBA-15, followed by
a calcination at 300 °C (Figure 9). The presence of such mixed
Mn3+/Mn2+ oxide phase can be explained by the carbon oxidation
of P123 during the calcination who can induce the reduction of
Mn4+ species into Mn3+ and Mn2+ species. TEM images, and
corresponding particles size distributions obtained for the different
MnOx-SBA-15 samples are displayed in Figure 10 and Figure S13.
Starting from calcined SBA-15 and using IWI and MIc methods
lead to MnO2 NPs localized inside and outside the mesoporosity
even if the external phase remains minor. The size of the external
MnO2 NPs is ~17 nm, while the size of the mesopore confined
NPs is fitting with the mesopore size (Figure 10). For the sample
prepared by MIa method, infiltration degree within the porosity of
the SBA-15 is significantly improved, with only Mn3O4 NPs located
inside the main mesopores, and a uniform spatial distribution of
NPs obtained as well. No external aggregates of Mn3O4 can be
observed (Figure 10), and approximatively 80% of the particles
are of 5-10 nm size (Figure S13). Unexpectedly, a new type of
MnOx NPs formed in an egg-shell shape were generated by MIa
when the Pluronic P123 is kept within the porosity for infiltration
step. These results demonstrate that infiltration of the molten
precursor can be accomplished within the unique nanospaces
between the surfactant molecules and the silica walls, as already
suggested in recent investigations.[25a,36] Mn3O4 NPs appear to
cover the walls of SBA-15 mesopores while keeping the
mesopores opened. As a consequence, the presence of such new
particles together with the unblocked mesoporosity results in
improved textural characteristics, as higher surface area and pore
volume.[25c]
Figure 7. HCHO conversion into CO2 as a function of the reaction temperature
for MnOx-SBA-15 catalysts with different Mn loadings, calcined at 300 °C.
Redox properties and surface state of 20Mn-YYY-300 samples
H2-TPR analyses are shown in Figure S14. The Mn AOS values
for 20Mn-IWI-300 sample is of 4, confirming the formation of
MnO2 phases as already observed for 20Mn-MIc-300. Besides,
typical MnO2 reduction profiles with two reduction peaks were
observed for these samples. 20Mn-MIa-300 shows a completely
different feature. The Mn AOS is of 2.7, far below the values
measured for the two other catalysts, but in agreement with the
presence of the Mn3O4 phase as identified by XRD (Figure S15).
XPS analysis confirmed the decrease in Mn AOS on the surface
over 20Mn-MIa-300 sample (Table 2). Interestingly, when
comparing the O 1s signal of 20Mn-MIc-300 and 20Mn-MIa-300
samples, a shift to higher BE (+1 eV) of the O-I component is
observed (Figure S7). This result is in agreement with works of
Tang et al. which observed the BE decrease of lattice oxygen
(Olatt) bonding with Mn when Mn valence decreases [37]. Likewise,
Figure 8. Adsorption/desorption isotherms (vertical shift of 150 cm3.g-1) for
MnOx-SBA-15 samples (20 wt.% Mn) prepared by different synthesis methods.
Effect of synthesis method of the catalyst performances
Properties of 20Mn-YYY-300 (YYY = MIc, MIa, IWI approaches)
The nitrogen adsorption/desorption isotherm of catalysts
prepared by different synthesis methods are displayed in Figure
8. No notable change in isotherm shape is observed, and the only
difference is on the desorption branch closure and the contribution
of the partially occluded porosity. When compared to the MIc and
IWI derived materials, the higher pore size (7.3 nm) proportion in
the 20Mn-MIa-c300 sample is more important, suggesting that the
porosity of SBA-15 is less occluded by the mesopore-confined
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it must be pointed out that Mn/Si atomic ratio value substantially
departs from the theoretical ones for 20Mn-MIa-300 sample
(Table 2) indicating that a significant part of the Mn is not XPS
visible which is consistent with a part of Mn phase being located
in the core of the silica grain.
20Mn-MIc-300 ~ 20Mn-IWI-300 > 20Mn-MIc-500 > 20Mn-MIa.
The activity is then well correlated with the evolution of the Mn
AOS in the different materials, as shown in Figure 12. Therefore,
despite the remarkable Mn3O4 dispersion and spatial distribution
of NPs, 20Mn-MIa-300 catalyst showed a limited catalytic activity
due to the low Mn AOS. This result suggests that the Mn3+/Mn2+
redox couple is far less efficient than the Mn4+/Mn3+ couple in the
total formaldehyde oxidation, in agreement with literature on bulk
20a]
MnO2 and Mn3O4 catalysts for HCHO oxidation.[14,
When
compared with the catalysts proposed in the literature, even if the
comparison is difficult considering the differences in terms of
setup configuration and reaction conditions, these catalysts
presents comparable T50 (<150°C) than reported for pure –
undoped - manganese oxide [8, 14, 24]
.
Figure 9. Wide-angle XRD patterns for MnOx-SBA-15 samples (20 wt.% Mn)
prepared by different synthesis methods.
Figure 11. HCHO conversion into CO2 as a function of the reaction temperature
for MnOx-SBA-15 catalysts (20 wt.% Mn) prepared by different synthesis
methods.
Figure 10. TEM images for MnOx-SBA-15 samples (20 wt.% Mn) prepared by
IWI and MIa methods.
Catalytic properties of 20Mn-YYY-300 catalysts
The HCHO conversion into CO2 is plotted as a function of reaction
temperature (light-off curve) for the MnOx-SBA-15 catalysts
prepared by different procedures (Figure 11). According to the T50
values, the following order of performance can be established:
Figure 12. Evolution of the catalyst activity measured at 120 °C as a function of
Mn AOS as determined by H2-TPR.
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lower temperature (130 °C) (Figure 14). In the presence of water
at 130 °C, a limited deactivation is measured over 20Mn-MIc-300
with a loss of only 15 % of its initial activity after 10 h of reaction,
and a stable activity reached at the end of the test. Then, the
decrease of the reaction temperature added to the presence of
water in the reaction feed has a positive effect on the 20Mn-MIc-
300 catalyst stability. After reaction under moist atmosphere, the
Mn AOS (calculated from TPR experiment, Figure S18) is
decreasing to 3.8 after reaction. The limited decrease in activity is
obviously related to the slight decrease in Mn AOS but the main
difference between dry and humid air reaction is on the obtaining
of a stable activity during stability test when water is added to the
reaction feed. On the contrary, a significant deactivation is
measured with the 20Mn-MIa-300 (45 % in 10h). In agreement
with the deactivation observed, the calculated Mn AOS value of
20Mn-MIa after test under moist conditions at 130 °C is of 2.2
(Figure S18). Considering the limited variation of the Mn AOS
between before and after test, the observed activity decrease is
associated to the water presence. Consequently, the reduced
catalyst prepared by the MIa approach is more sensitive to the
water presence, while a beneficial effect is observed for the
catalyst prepared by MIc procedure.
Figure 13. Evolution of the relative activity for MnOx-SBA-15 catalysts (20 wt.%
Mn) prepared by different synthesis methods at a reaction temperature of
170 °C.
Effect of the preparation route on the catalyst stability
Stability tests have been further performed at 170 °C during 60 h
and the activity evolution (%conv. divided by initial %conv.) is
plotted as a function of time in Figure 13. Moderate deactivation,
i.e. less than 10%, was measured for 20Mn-IWI and 20Mn-MIc
catalysts. However, considering the shape of the curves obtained
for the two catalysts, a decrease of the activity is awaited with the
increase in reaction time. On the contrary, 20Mn-MIa-300 catalyst
shows a remarkable stability. After a short period of activity
stabilization, of 15 h, the activity remains stable until the 60 h of
reaction. XRD patterns of catalysts after the durability tests
(Figure S15) do not allow to detect any evolution when compared
to the fresh catalysts: no change in crystalline phase detected; no
change in average crystal domain size as measured from X-ray
line broadening. The absence of any significant change in crystal
domain size is not surprising considering the low reaction
temperature applied for the reaction, and the high stability
observed for mesopore confined NPs [28]. In addition, H2-TPR
analysis (Figure S16) did not showed any significant modification
of the reduction profile in the case of 20Mn-MIc-300. Hydrogen
consumed quantification evidenced however a decrease of the
Mn AOS from 4.0 (before reaction) to 3.8 after the stability test at
170 °C during 60 h (Figure S16). Similar evolution is measured
when studying the Mn AOS on the 20Mn-MIc-300 surface (Mn 3s
spectra, Figures S6 and S17), with a slight decrease observed. In
the case of 20Mn-MIa-300 catalyst, Mn AOS for the catalyst after
test remains at a comparable value (a very small increase being
even measured from Mn 3s XPS spectra evolution).
Consequently, the progressive decrease in manganese oxidation
state is proposed to be at the origin of the catalyst activity
decrease (case of MIc and IWI preparation approaches).
Figure 14. Evolution of the relative activity for MnOx-SBA-15 catalysts (20 wt.%
Mn) prepared by different synthesis methods at a reaction temperature of
130 °C at a R.H. of 50% at 25 °C.
Conclusions
MnOx NPs confined within the SBA-15 pores were successfully
synthesized by melt infiltration. MnO2 oxide phases have a
heterogeneous distribution under the form of both confined and
extra-porous nanoparticles, when melt infiltration is applied on
calcined support (MIc), the final solids displaying comparable
properties than when the catalyst is prepared by incipient wetness
impregnation (IWI). A 20 wt.% Mn is an optimal loading to avoid
In order to test the catalysts under more realistic conditions,
stability tests were performed under 50 % relative humidity, at
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XRD measurements were carried out with a Bruker D5000 vertical
goniometer equipped with Cu Kα anticathode (=1.5418 Å) and a graphite
monochromator. A proportional counter and a 0.04° step size in 2 were
used. The integration time was 2 seconds per step and the scan range
was from 10 to 80° in 2. The assignment of the various crystalline phases
was done by comparison with files in PDF card (Powder Diffraction Files
structure data base). The average crystallite sizes (Dcryst) of manganese
oxides are determined using the X-ray line broadening using the Scherrer
consequent formation of NPs at the external surface of the SBA-
15 silica grains. When the catalyst is prepared by melt infiltration
over the support containing the template (MIa), a far better
dispersion is obtained (without formation of external phase), in
addition to an improved 3D spatial distribution in the support
porosity. However, applying the MIa approach leads to the
formation of less oxidized Mn3O4 phase. Mn AOS was evidenced
to directly impact the catalytic activity and is demonstrated the key
parameter for obtaining high performances in HCHO catalytic
oxidation. Thus, low calcination temperature of 300 oC is preferred
for the catalyst activation before reaction. Despite a lower activity
due to a decreased Mn AOS (2.3), the MIa catalyst is displaying
an excellent stability when reaction is performed at high
temperature (170°C) under dry air while the catalyst prepared by
MIc suffers from deactivation due to Mn AOS decrease (from 4 to
3.8) upon reaction. However, the MIc catalyst presents better
stability at lower temperature (130°C), and when water is added
to the reaction feed. Consequently, presence of water and a lower
reaction temperature are acting positively for the maintaining of
the Mn AOS upon reaction time. An opposite effect of water is
observed at low temperature on the Mn3O4 phase, and the
catalyst prepared by MIa suffers in this case from significant
deactivation (45 % in 10h).
equation after Warren’s correction for instrumental broadening: Dcryst
=
0.89 .  / ( / cos()) (Dcryst is the mean crystallite size (nm),  is the X-ray
wavelength,  is the line broadening at half the maximum (FWHM) and 
is the Bragg angle). For calculation, the (100) reflection of MnO2 (2 =
37.1°) and (211) of Mn3O4 (2 = 36.1°) were used. N2 adsorption-
desorption isotherms were measured at 77K, using a TriStar II 3020
analyzer. Pore volume was determinate at P/P0 = 0.98. The BET method
was used to calculate the specific surface areas, and pore size distribution
were calculated using the Barrett-Joyner-Halenda (BJH) model. Before
measurements, the samples were heated at 100 °C for 3 hours under
vacuum. Elemental analyses were performed by inductively coupled
plasma-optic emission spectroscopy 720-ES ICP-OES (Agilent) with
axially viewing and simultaneous CCD detection. The sample preparation
was made by dissolving 10 mg of dried and ground samples catalyst in 1,5
mL of concentrated nitric acid and 500 µL of hydrofluoric acid solution.
Solutions were heated at 50 °C and stirred during 24 hours in a sonication
bath to ensure complete dissolution before analysis. Transmission
Electronic Microscopy (TEM) was performed on a JEOL 2100 instrument
(operated at 200 kV with a LaB6 source and equipped with a Gatan Ultra
scan camera) equipped with Hypernine (Premium) detector (active area:
30mm2). Before analysis, the samples were embedded in a resin and then
sliced using an ultramicrotome (cuts of ~100nm thick). The X-ray
photoelectron spectroscopy (XPS) analyses were performed with a Kratos
Axis Ultra, equipped with a dual Mg/Al anode. As excitation source was
used the monochromatized Al Kα radiation (1486.6 eV). The sample were
analysed as pellets, mounted on a double-sided adhesive tape. The
pressure in the analysis chamber was in the range of 10-9 mbar during data
registration. The binding energies of the different core levels were referred
to the C1s component set at 284.8 eV. Atomic concentrations were
calculated from peak intensity using the sensitivity factors provided with
the CasaXPS software (version 2.3.16 PR 1.6). The binding energy values
are quoted with a precision of ± 0.15 eV and the atomic percentage with a
precision of ± 10%.
Experimental Section
Catalyst synthesis:
The SBA-15 support was synthesized according to the procedure
[26]
proposed by Zhao et al.
4 g of Pluronic P123 (poly(ethylene oxide)-
block-polypropylene oxide)-block-polyethylene oxide)- block, PEO20–
PPO70–PEO20, with MW = 5800 g mol−1, BASF Corp) was dissolved in a
1.6 M solution of HCl at 40 °C. 8.5 g of a silica precursor, tetraethyl
orthosilicate (TEOS, Sigma-Aldrich), was added dropwise to the solution,
and the stirring was maintained for 24 h. The resulting gel was subjected
to hydrothermal treatment for 48 h at 100 °C. After recovering by filtration,
washing, and drying, the uncalcined SBA-15 is obtained (used for the MIa
procedure). SBA-15 is calcined at 550 °C during 6 h (1.5 °C.min-1), in order
to obtain the template free support used for MIc and IWI procedures).
Redox properties of the samples were evaluated using a temperature-
programmed reduction apparatus form Micromeritics (model AutoChem II).
50 mg of the sample was placed in a quartz reactor and heated under
flowing air (50 mL.min-1) at 150 °C for 0.5 h, and then cooled down to room
temperature. Then, the reactor was heated from 40 to 1000 °C with a
heating rate of 10 °C min−1 in a gas mixture (5 vol.% H2 + 95 vol.% Ar, 50
mL.min−1). The amount of H2 consumption was assessed using a TCD.
Melt infiltration method is applied, starting from Mn(NO3)2•4H2O (98%,
Sigma-Aldrich) as precursor. The solid precursor was mixed with support
(uncalcined SBA-15 - MIa procedure; calcined SBA-15 – MIc procedure).
Catalysts are prepared at different Mn loadings (5, 10, 20 and 30 wt%).
Procedure adopted involves: (1) the grinding at room temperature for 15
min of precursor and support; (2) a diffusion heat treatment at 38 °C for 12
days; (3) a thermal stabilization by calcination at 300/500°C for 6 h, using
a heating rate of 1.5 °C min−1. The samples were labelled XMn-MIc(or
MIa)-Z, with X is the Mn loading (5, 10, 20 or 30 wt.%), and Z the
calcination temperature applied (300 or 500 °C). Incipient wetness
impregnation (IWI) is also used, starting from Mn(NO3)2•4H2O (98%,
Sigma-Aldrich) as precursor. The appropriate amount of precursor (to
obtain 20 wt.% Mn loading) is dissolved in water (volume adjusted to the
pore volume of the silica). The aqueous solution of the precursor is mixed
with the calcined support, and is dried at 25 °C for 5 days [28]. The solid is
then calcined at 300 °C for 6h using a heating rate of 1.5 °C min−1 in order
to obtain the catalyst denoted 20Mn-IWI.
The catalytic oxidation of formaldehyde was performed in a fixed-bed
reactor (internal diameter = 10 mm) loaded with the catalyst. Before each
test, the catalyst was heated 1 hour at 300 °C under O2 (20 vol.%) / N2 flow
(100 mL.min-1) to remove water and surface impurities. Gaseous
formaldehyde was generated from para-formaldehyde in a permeation
tube placed in a permeation chamber (Dynacalibrator, VICI Metronics Inc.).
By adjusting the gas carrier flow (O2 (20 vol.%)/N2) and the chamber
temperature, a stable formaldehyde concentration can be generated. The
inlet concentration of formaldehyde was fixed at 100 ppm, and the total
flow rate for the reaction was 100 mL.min-1. HCHO tests were performed
with 200 mg of catalyst (GHSV = 30000 h-1). The reactor temperature was
decreased from 300 °C to 40 °C (0.5 °C.min-1), and the effluent gases were
analysed online with a Varian CP-4900 Micro-GC equipped with a TCD
and COX type column (1 m). The formaldehyde conversion (XHCHO, %) was
Catalyst Characterization:
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10.1002/cctc.201901902
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evaluated using the following equation: XHCHO (%) = [CO2] / [HCHO]ini
×
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[HCHO]ini being the initial concentration of formaldehyde determined at the
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Stability tests were performed on the same setup. The temperature were
stabilized at the desired temperature (170 °C or 130 °C), and the reaction
prolonged during 60 hours. Some experiments are performed under moist
air at a relative humidity of 50% at 25 °C.
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Acknowledgements
This research is supported by an European Program INTERREG
V France—Wallonie—Flanders (FEDER) (DepollutAir). Chevreul
institute (FR 2638), Ministère de l’Enseignement Supérieur et de
la Recherche and Région Hauts-de-France are also
acknowledged for supporting this work. The authors thank
Martine Trentesaux, Pardis Simon, Olivier Gardoll and Laurence
Burylo for their contribution in XPS, H2-TPR and XRD
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Keywords: VOC catalytic oxidation ; Formaldehyde ;
Manganese oxides ; SBA-15 ; Melt infiltration
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10.1002/cctc.201901902
ChemCatChem
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Conventional and optimized melt
Guillaume Rochard,[a] Carmen
SBA-15
SBA-15 + P123
infiltration were used to prepare MnOx
nanoparticles encapsulated within the
silica pores for the catalytic oxidation
of formaldehyde
Ciotonea,[a] Adrian Ungureanu,*[b] Jean-
Marc Giraudon,[a] Sébastien Royer,[a]
Jean-François Lamonier*[a]
Optimized
Classical
MeltInfiltration
MeltInfiltration
Page No. – Page No.
Mn3O4
MnO2
MnOx loaded mesoporous silica for
the catalytic oxidation of
+ O2
formaldehyde. Effect of the melt
infiltration conditions on the activity –
stability behavior
+
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Author(s), Corresponding Author(s)*
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Title
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