A Simple and Green Procedure to Prepare Efficient Manganese Oxide Nanopowder for the Low Temperature Removal of Formaldehyde

Article abstract of DOI:10.1002/cctc.201700199

Activated reactive synthesis, as a top-down synthesis approach, is proposed for the production of a nanocrystalline, high surface area, manganese (IV) oxide starting from commercial micrometric α-MnO₂. The developed approach consists of two-steps: 1) high energy ball milling (HEBM) produces a nano-sized material, 2) low energy ball milling (LEBM) improves textural properties. During the HEBM step, elementary crystals are observed to evolve from several hundred elongated cylindrical particles to short length cylinders and pseudo spherical particles. Despite fractioning of the elementary crystals, surface area remains low over the HEBM derived solid. The LEBM step, achieves crystal shape modification, giving rise to only pseudo-spherical nanometric particles and significantly increasing the surface area to approximately 60–80 m² g^?1. Catalytic activity of α-MnO₂ powder is affected by the grinding process. Activity is similar to the fresh commercial MnO₂ after HEBM, and increases upon LEBM reaching a maximum for LEBM time of 1 h. Thereafter, activity remains constant despite the further increase in surface area upon prolonged LEBM. The presence of an optimal activity after limited LEBM time is paralleled with the increase of the iron contamination occurring after prolonged LEBM time which impacts manganese reducibility and results in less reactive surface.

Full text of DOI:10.1002/cctc.201700199

Accepted Article
Title: A simple and green procedure to prepare efficient manganese
oxide nanopowder for the low temperature removal of
formaldehyde
Authors: Carmen Ciotonea, Rémy Averlant, Guillaume Rochard, Anne-
Sophie Mamede, Jean-Marc Giraudon, Houshang Alamdari,
Jean-François Lamonier, and Sébastien Royer
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201700199
Link to VoR: http://dx.doi.org/10.1002/cctc.201700199
A Journal of
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ChemCatChem
10.1002/cctc.201700199
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A simple and green procedure to prepare efficient manganese
oxide nanopowder for the low temperature removal of
formaldehyde
Carmen Ciotonea,[a],[b] Rémy Averlant, Guillaume Rochard, Anne-Sophie Mamede, Jean-Marc
[a]
[a]
[a]
Giraudon, Houshang Alamdari, Jean-François Lamonier and Sébastien Royer[a],[b],[c],*
[
a]
[c]
[a],*
Abstract: Activated Reactive Synthesis, as a top-down synthesis
approach, is proposed for the production of nanocrystalline, high
surface area, manganese (IV) oxide starting from commercial
formaldehyde on the human health and on the environment. The
substitution of formaldehyde in industrial processes, including
furniture and wood panel branches, is strongly encouraged.[
Unfortunately, its substitution by other molecules while
maintaining technical properties acceptable in final products is,
even today, not always possible. Formaldehyde stays then an
often encountered indoor air pollutant, since widely used for the
production of building and decoration materials. To date there is
no regulatory limit for occupational exposure to formaldehyde.
The Scientific Committee on Occupational Exposure Limits
(SCOEL) however recommends a 8 h permissible exposure limit
of 0.2 ppm and a 15 min short term exposure limit of 0.4 ppm to
formaldehyde.[2] Such low exposure limit obviously stimulates
research of substitute. However, this approach is sometimes not
feasible with an acceptable delay, and the consumption of
formaldehyde in industrial production process makes necessary
the research of innovative solutions to limit user and consumer
exposure to formaldehyde, and to limit its impact on the
environment.[3,4] Different approaches are then proposed, among
elimination physical process, adsorption, heterogeneous catalysis,
etc. whole taking into account the necessity to limit energy
consumption (the concentration of formaldehyde in air is generally
of few ppmv, and often, like in indoor air, below the ppmv).[
Among the efficient approaches is the adsorption. Active carbons
are shown to be efficient for the formaldehyde removal,[6-10] with
adsorption capacity that can reach 400 mg formaldehyde per g of
adsorbent (with initial formaldehyde concentration ranging from
1]
2
micrometric -MnO . The developed approach consists in a two-step
process: (1) a High Energy Ball Mill (HEBM) step, to produce nano-
sized material, (2) a Low Energy Ball Mill (LEBM) step, to improve
textural properties. During the first HEBM step, elementary crystals
are observed to evolve from several hundred of nanometers
elongated cylindrical particles to short length cylinders and pseudo
spherical particles. Despite fractioning of elementary crystals, surface
area remains low over HEBM derived solid. The second LEBM step,
of attrition type, achieves crystal shape modification, given rise to only
pseudo-spherical nanometric particles. LEBM step allows to
2
-
significantly increase material surface area that reaches 60-80 m g
1
.
2
Catalytic activity of -MnO powder is significantly affected by the
grinding process. Material activity is observed to remain close to the
initial one after HEBM, and increases upon LEBM to rapidly reach a
maximum for LEBM time of 1 h. Thereafter, activity remains constant
despite the further increase in surface area upon prolonged LEBM
time. The presence of an optimal activity after limited LEBM time, with
the absence of activity increase at prolonged time despite the
increase in surface area, is paralleled with the increase of the iron
contamination occurring at prolonged LEBM time that impacts
manganese reducibility and results in less reactive surface.
5]
Introduction
0 to 35 ppm in the feed).[10] Some zeolitic supports are also
2
presenting interesting properties,[
11-13]
with capacities such as 585
Formaldehyde is always
a widely used chemical for the
mg/g over NaY type zeolite (adsorption capacity at 25°C, under 2
hPa of formaldehyde).[14] However, pore network characteristics
production of many goods, despite the consequence of
and surface properties of zeolites are significantly impacting
adsorption capacities.[14] NH
functionalized silica are also
2
promising adsorbents.[
15-17]
with adsorption capacities reported to
[
a]
C. Ciotonea, R. Averlant, G. Rochard, A.-S. Mamede, J.-M.
Giraudon, J.-F. Lamonier,* S. Royer*
be three times higher than those for carbons. Formaldehyde is
removed from air that is efficient from health and security point of
views since it efficiently decreases exposure of users and
consumers to formaldehyde. However, some drawbacks arise
from this technology, such as the necessity to periodically replace
adsorbent, and to treat the adsorbent cartridge for final elimination.
A second way proposed for the formaldehyde elimination is the
low temperature catalytic oxidation process. Advantages of this
approach are (i) process durability, (ii) elimination with limited
energy consumption, and (iii) direct conversion, even at low
Univ. Lille, CNRS UMR 8181, UCCS - Unité de Catalyse et de
Chimie du Solide, 59650 Villeneuve d’Ascq, France
E-mail: jean-francois.lamonier@univ-lille1.fr; sebastien.royer@univ-
lille1.fr
[
[
b]
c]
C. Ciotonea, S. Royer
Université de Poitiers, CNRS UMR 7285, IC2MP, 4 Rue Michel
Brunet, TSA 51106, 86073 Poitiers Cedex, France
H. Alamdari, S. Royer
Department of Mining, Metallurgical and Materials Engineering,
Universitꢀ Laval, Quꢀbec, Quꢀbec G1V 0A6, Canada
Supporting information for this article is given via a link at the end of
the document.
temperature, of formaldehyde into harmless carbon dioxide and
[18]
water.
Supported noble metal based catalysts were initially
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ChemCatChem
10.1002/cctc.201700199
FULL PAPER
proposed for this reaction, [19-26] with good efficiencies since
formaldehyde can be converted at ambient temperature. Titania
supported platinum is shown as the most promising system,[21]
with possibility to improve its activity by sodium doping:[22] 600
(Mn-C) presents a α-MnO
structure (ICDD file n°44-0141), along
2
with weak reflections attributed to KOH as low content impurity
(identified by an * in Figure 1). Upon grinding process, significant
modifications of the diffractogram are observed. After HEBM (Mn-
LEBM-0), the maintaining of the initial crystal structure is
ppm of formaldehyde can be selectively converted into CO
2
and
-1
H
2
O at 15°C, at a GHSV of 120000 h , over 2%Na-1%Pt/TiO
2
.
2
observed, with only α-MnO phase detected. However, a
Platinum is more active than palladium, rhodium and gold.[
21]
significant broadening and decrease in intensity of the reflections
are observed, with mean crystal domain size, as calculated by the
Scherrer equation, decreasing from >50 nm (commercial Mn-C)
to 6 nm (Mn-LEBM-0) (Table 1). Whatever the LEBM duration,
comparable diffractograms are observed. Indeed, only the α-
However, the support properties play a key role on the activity of
the supported phase. As an example, when gold is supported over
3
D ordered macroporous ceria,[27] removal of formaldehyde at
room temperature can be achieved, with selective conversion of
formaldehyde into CO . Due to extensive use of noble metal for
2
2
MnO phase is detected (Figure 1), while diffraction peaks widths
applications other than chemistry and their limited resources,
some of the platinum mine metals are suggested to be exhausted
in the few next decades.[28] Noble metal free catalysts are
proposed to potentially replace noble metal compositions,[29]
unfortunately with significantly lower intrinsic activities. [30-32]
Manganese-containing materials have been rapidly identified as
most promising systems among transition metal containing
and intensities being only slightly altered by the LEBM step time.
While the Mn-LEBM-0 solid presents a crystal domain size of 6.0
nm, the mean crystal size remains in the 4.9-5.8 nm range
whatever the HEBM duration (Table 1).
Table 1. Selected properties of commercial and synthesized materials.
materials for formaldehyde oxidation.[18,33] Xu et al. demonstrated
[b]
[c]
BET
[c]
[c]
Sample
Mn-C
XRD
D
c
S
V
P
D
P
XPS
AON
TPR
AON
phase[
a]
[d]
[e]
that tunnel type cryptomelane is highly efficient for the reaction,[34]
with complete and selective conversion of formaldehyde since
1
40°C (400 ppmv formaldehyde, GHSV of 18,000 mL g-1 h-1).
-
MnO
KOH
Birnessite,
a manganese layered-type material, has been
2
,
>50
6.0
5.0
4.9
5.1
5.8
10.2
14.6
52.5
57.0
65.2
78.0
0.03
0.03
0.08
0.08
0.11
0.13
11.2
3.5
3.5
-
3.6
3.4
-
reported to present high activity for the reaction, due to adequate
textural properties in addition to highly active Mn(IV) surface
oxidation sites:[35] conversion to CO
starts from 100°C (460 ppmv
2
Mn-
LEBM-0
3.9
-1
-1
formaldehyde, GHSV of 30000 mL g h ). Manganese-cerium
mixed oxide is also an efficient material to convert formaldehyde,
with comparable activities than those obtained for channel-type
structures,[36] but with an important effect of the preparation route
on the activity.[37] Indeed, preparation impacts (i) textural
-MnO
2
–
1
2.3
Mn-
LEBM-1
3.6
-MnO2
–
2
3.0
characteristics (surface area, pore size and pore volume), (ii)
homogeneity, (iii) mean manganese oxidation state, (iv) surface
composition, all parameters having an effect, positive or negative,
on the material surface reactivity.
In this work, Activated Reactive Synthesis (ARS) is proposed as
an alternative Top-Down approach to produce efficient oxidation
materials. The approach is, in its principle, considered as
sustainable since no solvent is used during the whole production
process,[38] and abundant and cheap inorganic precursors can be
used for final material production.[39-41] A two steps grinding
Mn-
LEBM-2
3.9
-MnO
2
2
2
–
2
3.3
3.3
3.3
3.5
3.4
3.4
2.8
Mn-
LEBM-5
3.9
–
30.9
-MnO
Mn-
LEBM-10
4.0
–
24.0
process is then developed, based on our experience for the
preparation of perovskite and hexaaluminate,[40-43] to produce
-MnO
nanocrystalline -MnO
2
material, starting from commercial
sample. The study evidenced the potential of the two-step ARS to
modify material morphology, in order to produce material with
adequate properties for low temperature oxidation reaction.
SBA-15
-
-
829
610
1.21
0.83
6.1
-
-
-
Mn/SBA-
4.5-
5.8
β-MnO
2
12.0
4.0
15
[
a] crystalline phase detected; [b] calculated crystal domain size (D
X-ray line broadening, after correction of the instrumental broadening; [c]
specific surface area (SBET), pore volume (V ) and B.J.H. pore size (D
evaluated from N physisorption isotherms; [d] average oxidation number
evaluated from the Mn 3s core level; [e] average oxidation number of
manganese, issued from H consumption of TPR-H experiments.
c
) using
Results and Discussion
Structural properties
p
p
)
2
2
2
X-ray diffraction patterns obtained for the different materials are
presented in Figure 1 for the ball milled materials, and in Figure
S1 for the reference Mn/SBA-15 sample. The commercial sample
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The impact of the two steps of grinding over α-MnO
2
structure is
Textural properties
comparable to precedent results obtained over perovskites and
hexaaluminates.[41-44] For these last two kinds of material, the
reduction of crystal size occurred during the HEBM step, and only
surface area was enhanced during LEBM step. Similarly,
significant decrease in crystal size proceeds during the HEBM
step (decrease in crystal size from >50 nm to 5-6 nm), and mean
crystal domain size remains constant during LEBM.
2
Results of N physisorption are presented in Figure 2 for the ball
milled materials, and Figure S2 for the Mn/SBA-15 reference
sample. Textural properties, obtained by treatment of the
isotherms, are gathered in Table 1.
First of all, the commercial cryptomelane (Mn-C) presents very
low surface area and pore volume (Table 1), values classically
reported in the literature, and related to inter-aggregate porosity
(
isotherm of type II with small H3 hysteresis - Figure 2(a)). Note
that the microporosity, located inside the tunnel structure of the
cryptomelane is not probed by N
dinitrogen molecule.
2
due to the too large size of the
2
Figure 1. High angles diffraction patterns for α-MnO prepared by Ball Milling
process; Bottom: ICDD file n°44-0141 for α-MnO
2
.
Figure 2. (a) N
2
adsorption/desorption isotherms and (b) pore size distributions
for ball milled materials. Isotherms are shifted by 10 cm3
g
-1
for clarity.
Diffractogram obtained over Mn/SBA-15 is presented in Figure S1.
Material presents reflections attributed to β-MnO (ICCD file n°81-
261) with no other crystalline phases detected. Only a broad and
2
After HEBM treatment, only limited modifications in either
isotherm shape or textural properties are observed. The isotherm
recorded for the Mn-LEBM-0 sample remains of Type II, with
small H3 hysteresis, suggesting the maintaining of an external
porosity. The surface area and pore volume remain limited and
comparable to those obtained for the commercial material (14.6
m g and 0.03 cm g , respectively - Table 1). The lack in surface
area enhancement despite the crystal size reduction upon HEBM
is suggested to originate from the formation of dense, poorly
porous, aggregates of nanoparticles. The main difference
between non-milled material and HEBM material (Mn-LEBM-0)
consists in a modification of the pore size distribution. The Mn-C
material presents a broad pore size centered at ~ 11 nm (Table
2
poorly intense additional signal, at 2 ranging from 17° to 32°, is
observed. However, this signal is originating from the amorphous
character of the silica support. Mean crystal domain size of 12 nm
is calculated for the Mn/SBA-15. This size can be easily explained
by the confinement of nanoparticles (NPs) occurring in ordered
solids during thermal treatment. The precursor is located in the
ordered silica pores after the drying step of the impregnation. The
following thermal decomposition, performed in our case at 300°C,
2
-1
3
-1
2
results in the formation of NPs of β-MnO confined in mesopores
for which crystal growth is limited by the space available between
the walls.[ The mean crystal size calculated for the SBA-15
45]
2
supported β-MnO phase, in this study, is higher than the mean
1). The Mn-LEBM-0 sample presents an additional pore
pore size of the silica host (Table 1). This result suggests that a
part of the manganese precursor remains out of the ordered
porosity, or migrates out upon drying. Thermal activation leads to
a part of the manganese oxide not suggested to physical
hindering during calcination. Large external particle formation
contributes to the increase in mean crystal domain size as
calculated by the Scherrer equation.
contribution at lower size, e.g. ~4.0 nm (Table 1, Figure 2(b)). The
formation of such low pore size could be related to the decrease
in crystal domain size (Figure 1) leading to changes in the internal
porosity of the aggregates.[43,44]
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ChemCatChem
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N
2
physisorption isotherms and pore size distributions of SBA-15
and derived Mn-containing catalysts are presented in Figure S2.
As awaited, SBA-15 shows type IV isotherm with H1 hysteresis
loop, characteristic of mesostructured solid with cylindrical
pores.[45] SBA-15 displays sharp pore size distribution, centered
at 6.0 nm (Table 1). Isotherm obtained for Mn/SBA-15 only
differs on the desorption branch, with a visible forced closure step
0
at P/P = 0.53, which highlights the formation of constriction in the
tubular pores of the SBA-15 after manganese oxide particle
formation. Pore size distribution consequently shows the
presence of a pore contribution around 4.5 nm. Finally, upon
2
formation of the MnO particles in the SBA-15 support, surface
area and pore volume are observed to decrease (Table 1), an
evolution coherent with the pore filling of the support by the
manganese phase.[45,46]
Figure 3. Evolution of the specific surface area of ball milling materials with
LEBM time.
Material microstructure
Morphology of the materials are observed by SEM (Figure 4) and
TEM (Figure 5). The commercial low surface area α-MnO
present fibrillar / rod-like shaped cristals (Figure 4(a) and Figure
(a)). The lengths of the fibers are not homogeneous, and are
observed to vary from 100 nm to 500 nm. These elongated
crystals are assembled into large aggregates, of wide size
population from slightly less than 1 μm to 10 μm.
2
sample
With LEBM duration increase, an increase in pore volume is
observed, in parallel with a significant surface area enhancement
6
(
Figure 3), up to a factor 4 when compared with commercial
sample (Table 1). However, the isotherm and hysteresis shapes
remain unchanged, of type 2 with H3 hysteresis type (Figure 2).
An important increase of the surface area is observed during the
first 60 min of LEBM. At prolonged time of LEBM, a roughly linear
Visible modification of the morphology is observed after HEBM
step. First, the micrometric assembly of crystal, after HEBM
2
-1
slow increase in surface area is observed (from 2 h, 57.0 m g -
(
presented for Mn-LEBM-2 sample, exactly similar representative
2
-1
to 10 h, 78.0 m g ). Such evolution could be related to a first
period in which the material microarchitecture is significantly
modified (milling time < 60 min). During this short period, XRD did
not show significant differences in mean crystal domain sizes, and
then, only important modifications of the aggregate size and
internal porosity can explain this evolution in surface area. The
discrepancy of evolution between crystal size and specific surface
area during HEBM and LEBM processes could be related to the
mode of milling applied. Under HEBM process, the generated
surface of crystals, chemically unstable, will cold-weld (no gain in
surface area). However, attrition conditions (LEBM process)
ensure the separation of the elementary crystals and surface
stabilization leading to the formation of aggregates with smaller
size and more porous (increase of SSA – Figure 3; additional low
size pore population – Figure 2(b)). After 60 min of LEBM, the
increase in surface area becomes less rapid, showing that the
process of disaggregation / modification of intra-aggregate
porosity becomes slower. In addition to the increases in surface
area and in pore volume, pore size distribution continues to evolve,
with (i) the increase in 4 nm pore contribution (intra-aggregate
porosity), and (ii) the increase in the broad 15-40 nm pore
contribution (inter-aggregate porosity) (Figure 2(b)).
images being recorded for Mn-LEBM-0 sample) is always from
one to several micrometers (Figure 4(b)). Elongated particles are
always visible, with a core of the aggregate observed to be more
dense. This observation is confirmed by higher resolution TEM
analysis (Figure 5(c)), for which we observed two different shapes
for the elementary particles. The first population of particles
presents elongated shape, with comparable size as in Mn-C, and
they are located on the periphery of the aggregates. A second
population of particle, majoritary, present pseudo-spherical shape,
with size down to 25 nm. Then, HEBM process is observed to
induce a breaking of the elongated particles to form more isotropic
shaped particles (with length of formed particles close to their
width). However, the conversion of the fibrillar particle into
pseudo-spherical is not complete at the end of the HEBM step,
and some of the initial elongated particles remain. The elementary
particle evolution induced during HEBM is coherent with the
observed evolution on pore size distribution (Figure 2(b)). Indeed,
lower size elementary particle will obviously lead to lower pore
diameter inside the aggregates (intra-aggregate porosity, the
porosity observed below 10 nm in Figure 2(b)).
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10.1002/cctc.201700199
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2
Figure 4. Evolution of the α-MnO microstructure upon ball milling process, as observed by SEM. (a) Mn-C; (b) Mn-LEBM-2; (c) Mn-LEBM-5; (d) Mn-LEBM-10.
2
Figure 5. Evolution of the α-MnO microstructure upon ball milling process, as observed by TEM. (a) Mn-C; (b) Mn-LEBM-0; (c), Mn-LEBM-2; (d) Mn-LEBM-5; (e-
f) Mn-LEBM-10.
Morphology evolves significantly upon LEBM process. A first
visible mobification is observed by SEM analysis (Figure 4(c-d),
at different milling times), with the complete disparition of the
decrease in aggregate size is observed, with most of the
aggregates being below 300 nm. By TEM, some residual
elongated crystals are always visible for Mn-LEBM-2 (Figure 5(c)).
However, such particles are no more observed for Mn-LEBM-5
and Mn-LEBM-10 (Figure 5(d-f)). LEBM step achieves the crystal
elongated
particles
(characteristic
of
cryptomelane
microstructure) when LEBM time is above 2 h. Also, a significant
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ChemCatChem
10.1002/cctc.201700199
FULL PAPER
size and shape modification, to obtain after 5 h, only pseudo-
spherical particles. By TEM analysis, the reduction of aggregate
size is also observed, as well as the formation of the <10 nm
porosity (visible between elementary particles in Figure 5(d-e)).
Mn 3s photopeak presents two multiplet split components, due to
the coupling of non-ionized 3s electron with 3d valence-band
electrons. Manganese average oxidation number (AON) can be
extracted from the magnitude of the peak splitting (ΔEs) applying
the correlation established by Galakhov et al.:[50] AON = 8.956 -
2
This observation is in agreement with N physisorption results.
1
.13(ΔEs). Calculated values of AON values are listed in Table 1.
4
+
They range between 3.3 and 3.5, showing the presence of Mn
Surface properties
and Mn3 ions on the material surface, as awaited from XRD
results, with a very limited impact of the milling process (type of
milling and time of milling) on the AON value.
+
Impact of milling process on the material surface characteristics
is evaluated by XPS. When exposed to the atmosphere, surface
of materials contains a noticeable amount of adventitious carbon
contamination. C 1s contamination spectrum classically presents
C-C (285 eV, used as a charge reference), C-O-C (~286 eV) and
O-C=O (~289 eV) components.[47] The three components are
observed at the surface of all materials, commercial and milled
Figure 6 shows the Fe 2p signal, registered in the BE region 705-
740 eV, for selected materials issued from ball milling. First of all,
HEBM step does not induce iron contamination. After LEBM
treatment, two main photopeaks at binding energies 710 eV and
723 eV are clearly observed. The two photopeaks can be
assigned to Fe 2p3/2 (710 eV) and Fe 2p1/2 (723 eV) core levels.
The 2p doublet separation is 13.6 eV. The two peaks with low
intensity at ~718 eV and ~732 eV can be assigned to Fe3+ Fe 2p3/2
and Fe3+ Fe 2p1/2 satellites, respectively.[51] Spectra
decomposition (Figure S5), using the Fe 2p3/2 spectral fitting
(
Figure S3). However, the quantity of carbon increases
significantly after HEBM treatment while it remains constant
during LEBM treatment (Figure S3). O 1s signals are presented
in Figure S4. The O 1s signal can be decomposed in four
components O
I
(530.0 eV), OII (531.2 eV), OIII (532.2 eV) and OIV
component is attributed to O2-
parameters proposed by Biesinger et al.,[52] confirms the
(
534.2 eV) (Figure S3). The O
I
formation of only Fe3 cation in oxide environment, Fe
+
O
3
. The
2
lattice oxygen, while the presence of OII component can be
explained by the presence of oxygen vacancies or defects.[48] The
LEBM treatment then induces a surface contamination with iron,
even at low milling time (Figure 6(b), Mn-LEBM-2). This surface
contamination increases with time of milling since the ratio Fe/Mn
is observed to increase from Mn-LEBM-2 to Mn-LEBM-10 (Figure
O
0
II peak has an area contribution of 18% for Mn-C and Mn-LEBM-
and 26% for Mn-LEBM-2, Mn-LEBM-5, Mn-LEBM-10 samples
suggesting that LEBM induces the formation of little more oxygen
defective surface. The OIII component is ascribed to oxygen
atoms in OH and/or O=C groups, while the presence of OIV
component arises from physisorbed water and/or O-C group.[48,49]
It is noticeable that the OIII peak area contribution increases from
7). However the transition metals (Mn+Fe) surface content, as
evaluated by XPS, remains unchanged, with almost constant
value obtained for (Fe+Mn)/O whatever the milling type and time
(
Figure 7). It can be concluded that Fe is mainly segregating on
the material surface, progressively replacing Mn surface species.
8% for Mn-C to 18-22% for all ball-milled samples. This increase
can be explained by the CO adsorption from ambient air on
2
unsaturated metal sites produced during the milling processes.
0
.8
Fe 2p3/2
Fe 2p1/2
0.7
0.6
0.5
0.4
0.3
0.2
Sat.
(
(
d)
c)
(
(
b)
a)
0.1
0
(Fe+Mn)/O
Fe/Mn
(
a)
(b)
(c)
(d)
(e)
Figure 7. Evolution of Fe/Mn and (Fe+Mn)/O XPS ratio upon ball milling process
a) Mn-C (b) Mn-LEBM-0; (c), Mn-LEBM-2; (d) Mn-LEBM-5; (e) Mn-LEBM-10.
(
740
735
730
725
720
715
710
705
Binding Energy / eV
Redox properties
Figure 6. XPS signal registered in the 705-740 eV B.E. region for materials
prepared by ball milling process. (a) Mn-LEBM-0; (b), Mn-LEBM-2; (c) Mn-
LEBM-5; (d) Mn-LEBM-10.
2
Reducibility of manganese species are evaluated by H -TPR.
Thermograms are presented in Figure 8(a) for the ball milled
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materials, and comparison with reference sample is given in
Figure 8(b).
cryptomelane does not significantly affect the global manganese
oxidation number, in agreement with the preservation of the
cryptomelane structure observed by XRD.
The commercial sample Mn-C exhibits two main reduction peaks
located at 304°C and 370°C, with a shoulder peak at lower
temperature (~280°C) (Figure 8(a)). The low temperature
reduction process (280°C) is assigned to the reduction of Mn4
species in the cryptomelane structure into Mn3+ species leading
The H
2
-TPR profile for the impregnated sample is quite similar,
consumption
showing two reduction steps, with maximum of H
2
+
located at ~345°C and ~445°C (Figure 8(b)). The hydrogen
consumption allowed to conclude in a Mn AON of 4.0 in the
reference sample (Table 1). This value is consistent with the
to the Mn
process is attributed to the reduction of Mn
followed by the reduction of Mn into MnO. Indeed in the
2
O
3
phase formation. The further two-step reduction
2
O
3
into Mn
3
O
4
,
stoichiometric β-MnO
the first reduction peak centered at ~340°C is ascribed to the
reduction of MnO into Mn (3MnO + 2H = Mn + 2H O)
and account for the two-thirds of the total H consumed quantity
while the second peak at higher temperature is assigned to the
reduction of Mn into MnO (Mn + H = 3MnO + 2H O).
2
formation, phase identified by XRD. Then
3
O
4
temperature range studied, MnO is the final state obtained after
reduction of higher valence manganese oxide phase.[53,54] Similar
H reduction profiles are obtained for the ball milled materials. The
2
2
3
O
4
2
2
3
O
4
2
2
temperatures of the two main peaks are shifted to higher
temperatures. After HEBM treatment (Mn-LEBM-0), the
manganese reduction processes take place at temperatures
increased by 30°C compared to temperatures recorded for Mn-
C. The reduction temperatures are also observed to shift to higher
temperature after 2 hours of LEBM treatment, and remains
thereafter unchanged for longer milling times. Over Mn-LEBM-10
material, an additional reduction shoulder at 456°C, and a broad
peak between 500 and 800°C, are clearly observed. These new
contributions, especially at the 500-800°C temperature range, are
also visible for Mn-LEBM-2 and Mn-LEBM-5. They are ascribed
to the reduction of Fe3+ species, identified by XPS analysis and
originating from steel vial and balls contamination. Besides,
temperatures observed in thermograms for this additional
hydrogen consumption matches well with those recorded for the
3
O
4
3
O
4
2
2
Catalytic properties
Figure 9 and Figure S6 show the evolution of the formaldehyde
conversion into CO as a function of the reaction temperature for
the different catalysts. Whatever the catalyst used, HCHO
oxidation is fully selective into CO and H O, and no other
products have been detected during reaction.
2
2
2
2 3
of -Fe O
material reduction.[55]
2
Figure 9. HCHO conversion into CO plotted as a function of the reaction
temperature for commercial and ball milled catalysts.
In comparison with catalytic properties displayed by Mn-C, the
catalytic activity is not enhanced after HEBM treatment (Figure 9).
When LEBM process is applied with moderate time of milling from
2
Figure 8. H -TPR profiles obtained for (a) ball milled materials and (b)
comparison with reference sample.
1
0 minutes to 1 hour, the materials becomes much more active
for the HCHO oxidation. A decrease in T50 (temperature at which
0% of formaldehyde is converted into CO ) of 60°C is
From hydrogen consumption, and assuming that Mn2+ is the final
5
2
oxidation state in the material after reduction at 800°C, Mn
average oxidation numbers (AON) are calculated (Table 1).
Calculated values range between 3.4 and 3.6, in agreement with
AON values issued from XPS analysis (Table 1). These results
confirm that milling conditions used for the modification of
measured between Mn-LEBM-1 and Mn-C (Figure 9). This result
could be directly related to the important increase in specific
surface area observed during the first 60 min of LEBM (Figure 3).
After 1 h of LEBM (Mn-LEBM-1), the material presents similar
activity than the silica supported manganese oxide reference
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ChemCatChem
10.1002/cctc.201700199
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catalyst (Figure S6) that confirms the beneficial effect of the
milling approach on the catalytic performance of commercial -
The good correlation between the intrinsic activity (number of
h-1 m ) and the Fe/Mn atomic ratio
-2
HCHO converted into CO
2
MnO
2
. However when the milling time is further increased, from 1
(Figure 10) supports this interpretation. When the milling time
increases, more iron species are deposited at the material surface.
The intrinsic activity decreases because of the Fe3+ species
progressive surface covering, considering a redox mechanism for
h to 10 h, no significant enhancement in the catalytic activity is
observed (Figure 9), although the SSA always increases (Figure
3). A reason for the difference in evolution between the catalytic
3
+
activity and the accessible surface can be the observed alteration
of the redox properties when LEBM time increases (Figure 8(a))
issued from Fe surface contamination (Figure 6). Indeed, an
increase in reduction temperature is observed when LEBM time
increases, that means a decrease in Mn4+ site reducibility. Indeed,
XPS shows the presence of iron surface contamination after only
HCHO oxidation over transition metal oxides, Fe being less
reducible than Mn4 species will lead to lower activity (Figure 8
(a)).
+
Formaldehyde oxidation, a surface mechanism?
2
h of LEBM, and the iron surface content in observed to increase
From precedent results, it seems obvious that formaldehyde
with LEBM (Figure 6 and Figure 7).
reacts with surface Mn4+ sites to produce CO
2
, and that the
3
+
progressive covering of the surface by less reactive Fe sites
leads to a decrease in material activity when normalized per
surface unit. The carbon balance evolution with reaction
temperature is shown in Figure S7 for ball milled catalysts and
Mn/SBA-15 reference catalyst. U-shapes curves are observed for
all catalysts. When the reaction temperature decreases, the
carbon balance decreases below 100%, showing that, in parallel
with the oxidation reaction, formaldehyde is retained on the
material surface. When material surface is saturated, the carbon
balance increases to 100%, at low temperature. This observation
is valid for either milled materials and reference supported
material.
The IR analysis of Mn-LEBM-2 after catalytic test is performed in
order to identify residual adsorbed species (Figure S8). After
subtraction of the DRIFTS spectrum (post reaction Mn-LEBM-2
and fresh Mn-LEBM-2), five characteristic absorption bands, at
-
1
1
354, 1373, 1578, 2347 and 2857 cm , are observed. These
bands are attributed to formate species.[
56]
Figure 10. Intrinsic activity of ball milled catalyst for formaldehyde oxidation (at
The quantity of adsorbed formate species is evaluated by
integration of the area of the U-shape curve of Figure S7, and
100°C) plotted as a function of the Fe/Mn surface atomic ratio (XPS).
values plotted as a function of the specific surface area of α-MnO
2
ball milling materials (Figure 11). A linear correlation between
specific surface area and quantity of adsorbed formate species is
obtained. The dependence of the formate adsorption capacity
with the material surface area, in addition to the evolution of the
activity with redox and surface properties of material is in line with
a pure suprafacial mechanism for which only surface species are
involved during reaction.
Conclusions
Activated Reactive Synthesis is proposed as an efficient
approach to produce -MnO
surface area, commercial, -MnO
2
nanopowder, starting from low
material. The approach
2
consists in two successive grinding steps. The first step,
performed at high energy, allows to decrease mean crystal
domain size (with significant modifications of the particle shape,
evolving from elongated cylindrical shape to pseudo spherical
shape). Despite the measured elementary particle size reduction,
surface area remains low, and a second grinding step, at low
Figure 11. Quantity of adsorbed formate species plotted as a function of specific
surface area for ball milling derived materials.
energy, is needed to increase accessible surface. Surface areas
up to 60 m2 g-1 are easily obtained. Surface area generation
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ChemCatChem
10.1002/cctc.201700199
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during low energy ball milling occurs through the rearrangement
of the aggregates, with observed size decrease and increase in
intra-aggregated porosity.
Improvement in catalytic activity is measured for milled materials,
which reach comparable activity than obtained for reference silica
the calcined SBA-15 support is immersed in 20 mL of water containing the
dissolved nitrate precursor (amount of precursor is calculated to achieve a
20 wt.% loading in MnO
2
in the final material). After 30 min of mixing, the
solvent is evaporated at 40 °C. Then, the dry solid is calcined at 300°C for
-
1
4
h (temperature increase rate of 1 °C min ). The following material is
obtained: Mn-SBA-15.
2
supported MnO material. However, increasing the milling time at
low energy above 1 h does not allow to increase activity despite
the significant increase in surface area. This discrepancy is
paralleled with the contamination of the material surface by iron
Characterization of solids
Powder X-ray diffraction: X-ray diffraction patterns are recorded over a D8
Advance instrument from Brucker, equipped with a Cu K X-ray source (
= 1.54184 Å). Diffractograms are recorded for 2. values comprised from
10° and 80°, with a step size of 0.04° and an acquisition time for each step
of 2 s. Identification of crystalline phases is performed by comparison with
references issued for PDF database. Cristal domain sizes are calculated
using the Scherrer equation, after correction of the FWHM to account for
the instrumental broadening.
when milling duration increases. Contamination induces
a
decrease in redox capability at low temperature, with partial
replacement of active Mn4+ surface sites by less active Fe3+
surface sites.
Experimental Section
N
2
physisorption: Physisorption experiments are performed at -196°C, on
a Micromeritics Tristar II instrument. Before analysis, a known mass of
50-200 mg of solid is heat-treated under vacuum at 150°C for 6 h.
Specific surface area (SBET) is calculated using the B.E.T. algorithm, on
the linear part of the B.E.T. plot (P/P = 0.1-0.25). Pore volume (V ) is
measured on the adsorption branch of the isotherm, at P/P value of 0.98.
Pore diameter (D ) is calculated applying the B.J.H. model on the
Material syntheses

Raw material: A commercial, microcrystalline, -MnO
Mn-C thereafter) is used as starting material. -MnO
type manganese oxide, is a channel-type microporous material. Channels,
with size of 4.7 Å x 4.7 Å, are formed by 2 x 2 {MnO } octahedra connected
by their edges and corners.[57] Channel presents an XMn
O global
2
sample (denoted
, or cryptomelane-
2
0
p
0
6
p
O16, nH
8 2
desorption branch of the isotherm.
formula in which X is a compensation cation (potassium in our case). In
cryptomelane-type solid, manganese mean oxidation state varies between
3
Scanning Electronic Microscopy: Morphology of material is evaluated by
Scanning Electron Microscopy. Micrographs are recorded on a Hitachi S-
.6 and 3.8,[58] demonstrating the presence of either Mn(IV) and Mn(III) in
the structure. -MnO
2
phase is among the most active phases for
4700 instrument, with a 20k magnification.
formaldehyde low temperature oxidation,[18,34] justifying the use of -MnO
2
as starting raw material for Activated Reactive Synthesis process.
Transmission Electronic Microscopy: Samples morphology evolution is
evaluated by transmission electron microscopy over a JEOL 2100 UHR
instrument, operated at 200 kV with a LaB source and equipped with a
6
Gatan Ultra scan camera. All the samples were embedded in a polymeric
resin (spurr), and cut into sections of 50 nm thick using an ultramicrotome.
Samples are deposited on a carbon grid for observation.
ARS-derived materials: the ARS process consists in a two-step grinding
process. The Mn-C material is first calcined at 400°C to remove adsorbed
water. 7 g of calcined Mn-C are introduced in 50 mL WC crucibles with 3
WC balls of 0.11 cm diameter. The powder is then subjected to a High
Energy Ball Milling (HEBM) in a SPEX 8000 mixer (conditions: 1000 rpm
for 2 h). The objective of this step, at high energy, is to reduce crystal
domain size down to the nanometer length. After HEBM step, the material
is denoted Mn-LEBM-0. Mn-LEBM-0 material is then subjected to a second
step of grinding, at low energy (Low Energy Ball Milling – LEBM) in an
attritor type grinder. Different materials are produced at different LEBM
times: 10 min (Mn-LEBM-0.16), 30 min (Mn-LEBM-0.5), and from 1 h (Mn-
LEBM-1) to 10 h (Mn-LEBM-10) each hour.
X-ray Photoelectron Spectroscopy: Material surface composition and
surface manganese Average Oxidation State (AOS) are both extracted
from X-ray Photoelectron Spectroscopy (XPS). Analysis are performed on
a KRATOS AXIS Ultra instrument equipped with an electron beam of low
energy for charge compensation. X-ray source is of Al K (h = 1486.6 eV)
type. Irradiation is performed at ambient temperature, under ultra-vacuum
conditions (P  10-9 mbar). Determination of the binding energies for the
different elements is performed using binding energy of 1s level of
contamination carbon, arbitrarily positioned at 285.0 eV.
Reference supported material: Ground material properties are compared
with those displayed by a β-MnO
catalyst. Since the preferential growth of -MnO
2
supported mesoporous silica (SBA-15)
crystal leads to channel
2
Redox properties
type morphology, the synthesis in the confined space of the tubular
porosity of the SBA-15 silica is not reliable and would result in significant
pore plugging. Consequently, and due to the excellent properties
displayed by silica supported β-MnO catalyst, it has been selected as
2
noble metal free reference catalyst.[46] The reference is obtained by wet
impregnation of hydrated manganese nitrate precursor over
mesostructured SBA-15 silica support. SBA-15 is prepared following
previously published procedure.[59] 4 g of triblock copolymer is dissolved in
Manganese reducibility is evaluated by Temperature Programmed
Reduction under H (TPR) experiments. Manipulations are performed on
2
a Micromeritics Autochem instrument. A known amount of catalyst (150
mg) is inserted in a U-shaped tubular reactor. The catalyst is activated
under simulated air, up to the material calcination temperature (1 h,
-
1
applying a temperature increase rate of 5 °C min ). After cooling down to
room temperature, a 50 mL min-1 flow composed of 5 vol.% H
in Ar is
.6 mol L-1 HCl solution. After heating of the solution at 40 °C, 8.5 g of
tetraethylorthosilicate is added dropwise to the solution under stirring. After
4 h of ageing, the solution is transferred into a Teflon-lined autoclave for
2
1
stabilized, and a temperature increase ramp of 10 °C min-1 is applied.
Hydrogen consumption is on-line monitored using a Thermal Conductivity
Detector (TCD) previously calibrated. In order to obtain reliable TPR
results, the sensibility factor, K, as well as the shape and resolution
parameter, P, are verified to be in the recommended values with the
applied experimental conditions.[60,61] From the hydrogen consumption are
2
hydrothermal treatment at 100°C for 24 h. After cooling down to room
temperature, the sample is filtered, washed with water, dried at 100°C for
1
2 h, and calcined at 550°C for 4 h (temperature increase rate of 1.5 °C
-
1
min ). Manganese precursor impregnation procedure is as follow: 1 g of
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ChemCatChem
10.1002/cctc.201700199
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extracted manganese Average Oxidation Number (AON), supposing that
the final oxidation state of manganese, at the end of the TPR experiment,
is +II, according to Eq. (1):
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ChemCatChem
10.1002/cctc.201700199
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Activated Reactive Synthesis is
proposed as an efficient, solvent free,
top-down approach for the production
of nanocatalysts. Starting from
microcrystalline – low surface area –
manganese oxide, nanocrystalline -
high surface area - material, highly
active for the low temperature catalytic
oxidation of formaldehyde, is
Carmen Ciotonea, Rémy Averlant,
Guillaume Rochard, Anne-Sophie
Mamede, Jean-Marc Giraudon,
Houshang Alamdari, Jean-François
Lamonier* and Sébastien Royer*
Page No. – Page No.
A simple and green procedure to
prepare efficient manganese oxide
nanopowder for the low temperature
removal of formaldehyde
produced.
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