M.T. Nguyen Dinh, et al.
Applied Catalysis A, General 595 (2020) 117473
considered as an intergrowth of pyrolusite (2.3 Å × 2.3 Å) and rams-
2.4. Catalytic oxidation of toluene
dellite (2.3 Å × 4.6 Å) along c axis and is classified in the same type of
Catalytic oxidation of toluene over the PEMD catalysts was carried
γ-MnO
lacks of Mn atoms in [MnO
pyrolusite could be considered as an intergrowth, known as ‘De Wolff’
structural defects within the matrix of ramsdellite of the ε-MnO
2
but exhibits more structural defects “microtwinning” due to the
out in a continuous-flow fixed-bed microreactor constructed of stainless
steel with 9 mm-inner diameter (BTRS- Parker). 0.1 g of catalyst was
loaded into the reactor with gas hourly space velocity GHSV of
6
] frameworks [13,19]. The presence of
2
6
0,000 mL/g.h. Toluene was fed by passing a nitrogen flow into an
structure [19–21]. Interestingly, in the case of 6-1-6 and 6-2-6 the XRD
peak of the (100) crystallographic plane at 2θ = 37.3° is shifted to
higher angle compared to that of 6-0-6 and 6-1-12 (Fig. 1c) indicating
structural modifications caused by the pyrolusite defects in the rams-
isothermal saturator containing toluene liquid at 5 °C. The saturator
outlet gas was mixed with airflow to maintain the total flow rate of
100 ml/min and the inlet toluene concentration of about 500 ppm.
Prior to each run, the catalyst was pretreated at 250 °C during 2 h in an
air flow and cooled down to the ambient temperature. For each test, the
reactor temperature was raised from 30 °C to 300 °C with a heating rate
of 1 °C/min. For the evaluation of effects of humidity, different amounts
of vapor water were introduced to the reaction medium in maintaining
the same flowrate and the inlet concentration of toluene. The feed and
reaction products were analyzed by an on-line Gas Chromatography
dellite matrix [13]. Additionally, Mn
(JCPDS Card n° 076–7064), which possibly due to the decomposition of
MnO during annealing at 400 °C. A small amount of Mn is also
detected for Mn-6-0-6 at 2θ 28.8°, 31.0°, 32.3°, 36.1°, 44.4°, 58.5°, and
59.8° (JCPDS card n° 75–1560), which is previously generated during
2 3
O phase is detected at 2θ 33.0°
2
3 4
O
synthesis at 180 °C. Moreover, MnCO
Mn , and MnO for Mn-6-6-6 as revealed in Figs. 1A and B (line e).
Trace of remaining MnCO is also found at 31.4° for all samples. Ac-
cording the TG and DTA results (Fig. S1), the highly exothermic com-
bustion of carbon species cause the successive decomposition of MnO
into Mn and Mn (only for Mn-6-6-6) at 400 °C for 4 h.
3 5 8
is decomposed into Mn O ,
2
O
3
2
(
7890B, Agilent - Wasson) equipped with a flame ionization detector
3
(
FID) and a thermal conductivity detector (TCD). The overall conver-
sion of toluene (ηtoluene) was calculated by the formula as follows:
2
2
O
3
5 8
O
[
C7H8]in − [C7H8]out
ηC7H8
=
× 100
[
C7H8]in
(1)
3.2. Morphologies of the dried and PEMD samples
Where [C
7
H
8
]
in and [C
7
H
8
]
out represent for the toluene concentration
in the inlet and outlet gas streams, respectively. A blank experiment is
carried out in the microreactor filled with quartz wool and no reaction
occurred at the studied condition.
Fig. 2 exhibits SEM images of the dried and ε-MnO2 microcubes
samples. The similar morphologies before and after annealing could be
observed (see Fig. S2 for low magnification). The cubic-shaped micro-
crystals range from 5 to 10 μm in size and rise with the increase of
glucose concentration. Thus, glucose significantly impacts on the sur-
face roughness. Without glucose, smooth cubic and monoclinic-shaped
crystals are formed (Fig. 2a). Increasing the glucose amount results in
nub-shaped nanocrystals with the size of several dozen nanometers on
each façade, as shown in Figs. 2b-g. It seems that the carbonization of
glucose under the hydrothermal condition roots for the morphologies of
the annealed Mn-6-1-6 and Mn-6-2-6 samples, which are modified with
numerous sponge holes between nub-shaped nanocrystals (Figs. 2f, g)
and with the enlargement of hole sizes. Herein, the gluconic acid de-
3
. Results and discussion
.1. The formation and structure of the dried and PEMD samples
As illustrated in Scheme 1, the porous ε-MnO with different
3
2
morphologies and pore size structure are produced by the hydrothermal
synthesis at 180 °C via the control of manganese-glucose-urea ratios
from 6-0-6 to 6-6-6. It should be noted that urea functions an essential
role as it assists in forming MnCO
3
microcubes while glucose is quickly
carbonized into carbonaceous materials at this temperature. The for-
3
rived from the carbonization of glucose could corrode MnCO micro-
mation of both MnCO
posite of carbon-MnCO
to produce porous ε-MnO
found to obtain an appropriated porous structure of the ε-MnO
3
and carbonaceous materials creates the com-
microcubes which are then annealed at 400 °C
crystals during the hydrothermal reaction, generating their rough sur-
face [22,23]. In shorter hydrothermal reaction time of 1 or 3 h, the
surface of MnCO particles is partially corroded (see Fig. S3 a, b).
3
Several places on their facades still being smooth. For higher glucose
amount of Mn-6-6-6, the cubic-shaped particles are not retained and
covered by carbon species and transformed by annealing into spherical-
shaped microcrystals as shown in Figs. 2d and 2 h. The excess of glu-
cose leads to create carbon colloidal spheres observed next to these
particles [24].
3
2
. The desired manganese-glucose-urea ratio is
mi-
2
crocubes exhibiting the high catalytic activity. The higher amount of
carbonaceous species leads to produce stronger reducibility and more
4
+
Mn
2
fraction of the final ε-MnO catalysts.
Figs. 1A and B display the XRD patterns of samples before and after
the calcination, respectively. All the diffraction peaks of the dried
samples before calcination (Fig. 1A) can be attributed to rhodochrosite
In order to detect the existence of carbon species inside the PEMD
particles, EDX measurements are performed on the microcubes, as
shown in Fig. S4. The elemental composition of dried samples is also
given in Table S2. The higher glucose concentration results in the in-
creasing of carbon to manganese (C/Mn) and carbon to oxygen (C/O)
ratios, which are from 4.7 to 5.5 and from 0.5 to 0.7, respectively. In
contrast, oxygen to manganese (O/Mn) ratio reduces from 10.2–8.1
suggesting that the presence of carbon species inside the cubic micro-
crystals, knowing that the penetration of X-ray is up to 2 microns by
EDX technique. Hence, the carbonaceous species may be co-pre-
MnCO
3 3 4
(JCPDS Card n° 044–1472). A trace of Mn O is detected at 2θ
of 36.1° for the sample without the presence of glucose because urea is
2
−
3-
pyrolyzed to form carbonate species such as CO
3
or HCO , which
precipitates and MnCO is
by dissolved oxygen [15]. At lower
hydrothermal temperature than 180 °C, no traces of Mn is formed
not shown). After annealing at 400 °C, MnCO is decomposed into
MnO (except for Mn-6-6-6) as shown in Fig. 1B (line a–d). The main
2
+
react with Mn
ions to generate MnCO
3
3
partially transformed to Mn
3 4
O
3 4
O
(
3
2
peaks at 37.3°, 42.5°, 56.2°, and 66.0° could be attributed to the re-
flections of (100), (101), (102), and (110) planes of hexagonal akh-
3 3
cipitated with MnCO resulting in the formation of MnCO -carbonac-
tenskite-type ε-MnO
and space group P63/mmc (JCPDS card n° 030-0820) [16]. A trace of
2
with cell parameters a = 0.285 nm, c = 0.448 nm
eous composite. Both carbon species inside and outside of the micro-
crystals are simultaneously decomposed during the annealing step.
Notably, it turns out that the burning of the internal carbon species
generates a large space inside the microparticles, as shown by TEM
images in Figs. 3a and b, indicating the role of carbonaceous species as
the soft template for the production of high surface area materials. The
HR-TEM images provided in Figs. 3c and d also show that the lattice
pyrolusite-type β-MnO
2
is observed at 2θ 28.6 (d = 0.31 nm) and 72.2°
(
d=0.13 nm) (JCPDS card no. 81-2261). The formation of pyrolusite-
type β-MnO
2
or a mixture of aktenskite and pyrolusite from the an-
was also previously found [17,18]. ε-MnO is also
nealing of MnCO
3
2
highly disordered by thermal treatment and consists of the arrangement
of single and double [MnO ] octahedral building blocks [16]. ε-MnO is
6
2
2
spacing of 0.42 nm and 0.24 nm are characteristic of ε-MnO , which is
3