Structural, Morphological, and Catalytic Properties …
71
The contribution of the Mn 2 O 3 and β-MnO 2 impurity phases was estimated using
the reported data [51] on the kinetics of ozone decomposition by a sample containing
100% Mn 2 O 3 (curve 5) and a sample containing a mixture (30% Mn 2 O 3 and 48.7%
β-MnO 2 ) [29] (curve 6). It is arguable that the contribution of these phases to the
reaction of ozone decomposition by the synthesized OMS-2 cryptomelane samples
can be neglected and cryptomelane, as it was earlier reported [8], makes a decisive
contribution to the OMS-2 activity.
It is obvious that the kinetic curve profiles for different cryptomelane samples are
different. Curves 2 and 4 for OMS-2(SG) and OMS-2(SSt) samples are characterized
by a sharp increase in the final concentration of ozone in the initial section, while the
time required to reach 50% decomposition of ozone (τ 1/2 ) is 6 times longer in the
case of OMS-2(SG). Curves 1 and 3 for OMS-2(MS) and OMS-2(Ref) samples are
characterized by a section in which no ozone concentration is detected at the reactor
outlet. The duration (τ 0 ) of this section is the longest in the case of OMS-2(MS).
In addition, from curves 1 and 3, it is possible to determine the catalyst protective
time (τ MPC ). The longest τ MPC (1050 min) is observed for OMS-2(MS). Then, C
f
O 3
increases to 0.28 mg/m
3 , and this value is maintained for a long time.
In Table 4, the most important characteristics of cryptomelane samples are systematized, as well as the parameters characterizing their catalytic activity in the ozone
decomposition.
X-ray diffraction analysis has shown that only two products, namely, OMS-2(MS)
and OMS-2(SG), are pure; the other two samples, except the cryptomelane predominant phase, contain impurities, i.e., Mn 2 O 3 and β-MnO 2 . Although the contribution
of these impurity phases to the kinetics of ozone decomposition is insignificant
(Fig. 6), it can be concluded that with an increase in the content of the cryptomelane
phase, the amount of decomposed ozone, Q exp (O 3 ), increases. At the same cryptomelane content (100%), the OMS-2(MS) sample is more active than the OMS-2(SG)
sample and its crystallite size is smaller. The OMS-2 samples obtained by different
methods have some structural differences, namely, the unit cell volume changes and
the largest expansion has been found for OMS-2(SSt). The samples have typical
cryptomelane morphology. They consist of fibrous crystals forming microspheres of
different packing densities: loosely (OMS-2(MS)) and tightly packed (OMS-2(Ref),
OMS-2(SG), and OMS-2(SSt)) urchin-like spheres. The sample with loose packing
of nanofibers is the most active in the ozone decomposition. Two types of kinetic
curves of ozone decomposition (Fig. 6) are due to structural and adsorption differences. OMS-2(MS) and OMS-2(Ref) samples have approximately the same values
of S sp and average pore diameter (d m ), and these parameters are greater than those
for the second pair of samples, OMS-2(SG) and OMS-2(SSt).
The synthesized OMS-2 samples have demonstrated different mechanisms of
water molecules protolysis and change in pH of the suspension in a wide range.
The low-active OMS-2(SSt) sample is characterized by the highest pH level and
the presence of basic sites inactive in the ozone decomposition [32, 53]. Luo et al.
[24] characterized cryptomelane as having hydrophobic properties with a maximum
adsorption value of 1,1 mmol/g according to water vapor adsorption. Our studies [29]
71
The contribution of the Mn 2 O 3 and β-MnO 2 impurity phases was estimated using
the reported data [51] on the kinetics of ozone decomposition by a sample containing
100% Mn 2 O 3 (curve 5) and a sample containing a mixture (30% Mn 2 O 3 and 48.7%
β-MnO 2 ) [29] (curve 6). It is arguable that the contribution of these phases to the
reaction of ozone decomposition by the synthesized OMS-2 cryptomelane samples
can be neglected and cryptomelane, as it was earlier reported [8], makes a decisive
contribution to the OMS-2 activity.
It is obvious that the kinetic curve profiles for different cryptomelane samples are
different. Curves 2 and 4 for OMS-2(SG) and OMS-2(SSt) samples are characterized
by a sharp increase in the final concentration of ozone in the initial section, while the
time required to reach 50% decomposition of ozone (τ 1/2 ) is 6 times longer in the
case of OMS-2(SG). Curves 1 and 3 for OMS-2(MS) and OMS-2(Ref) samples are
characterized by a section in which no ozone concentration is detected at the reactor
outlet. The duration (τ 0 ) of this section is the longest in the case of OMS-2(MS).
In addition, from curves 1 and 3, it is possible to determine the catalyst protective
time (τ MPC ). The longest τ MPC (1050 min) is observed for OMS-2(MS). Then, C
f
O 3
increases to 0.28 mg/m
3 , and this value is maintained for a long time.
In Table 4, the most important characteristics of cryptomelane samples are systematized, as well as the parameters characterizing their catalytic activity in the ozone
decomposition.
X-ray diffraction analysis has shown that only two products, namely, OMS-2(MS)
and OMS-2(SG), are pure; the other two samples, except the cryptomelane predominant phase, contain impurities, i.e., Mn 2 O 3 and β-MnO 2 . Although the contribution
of these impurity phases to the kinetics of ozone decomposition is insignificant
(Fig. 6), it can be concluded that with an increase in the content of the cryptomelane
phase, the amount of decomposed ozone, Q exp (O 3 ), increases. At the same cryptomelane content (100%), the OMS-2(MS) sample is more active than the OMS-2(SG)
sample and its crystallite size is smaller. The OMS-2 samples obtained by different
methods have some structural differences, namely, the unit cell volume changes and
the largest expansion has been found for OMS-2(SSt). The samples have typical
cryptomelane morphology. They consist of fibrous crystals forming microspheres of
different packing densities: loosely (OMS-2(MS)) and tightly packed (OMS-2(Ref),
OMS-2(SG), and OMS-2(SSt)) urchin-like spheres. The sample with loose packing
of nanofibers is the most active in the ozone decomposition. Two types of kinetic
curves of ozone decomposition (Fig. 6) are due to structural and adsorption differences. OMS-2(MS) and OMS-2(Ref) samples have approximately the same values
of S sp and average pore diameter (d m ), and these parameters are greater than those
for the second pair of samples, OMS-2(SG) and OMS-2(SSt).
The synthesized OMS-2 samples have demonstrated different mechanisms of
water molecules protolysis and change in pH of the suspension in a wide range.
The low-active OMS-2(SSt) sample is characterized by the highest pH level and
the presence of basic sites inactive in the ozone decomposition [32, 53]. Luo et al.
[24] characterized cryptomelane as having hydrophobic properties with a maximum
adsorption value of 1,1 mmol/g according to water vapor adsorption. Our studies [29]