Structural, Morphological, and Catalytic Properties …
73
showed that the OMS-2 samples synthesized by different methods were characterized
by a high affinity to water molecules and, at RH of 65% (conditions of catalytic
activity testing used by us), they adsorbed different amounts of water (Table 4).
More active OMS-2(MS) and OMS-2(SG) samples adsorbed much less water than
OMS-2(Ref) and OMS-2(SSt) samples. For the latter, competing adsorption of water
vapor reduces their activity in the ozone decomposition.
It is seen that the establishment of correct correlations between any OMS-2 property and catalytic activity in the ozone decomposition is difficult because of the fact
that other parameters do not remain constant. The same conclusion can be drawn
from the analysis of some earlier reported data [8–11, 15, 19, 20, 22, 26, 30–32]. For
instance, the increase in the activity of polymorphic forms of manganese dioxide in
the order β-MnO 2 < γ-MnO 2 < α-MnO 2 was associated with the structure, and, in
this case, S sp increased in the same order from 13.8 to 80.7 m
2 /g [8]. With the same
structure, the decrease in activity in the order α-MnO 2 (nanofibers NFs) > α-MnO 2
(nanorods NRs) > α-MnO 2 (nanotubes NTs) was associated with different morphologies, but, in addition, a number of other parameters changed significantly: S sp and
the number of oxygen vacancies decreased from 80.7 to 33.8 m
2 /g and from 11 to
3.1%, respectively, whereas the average oxidation state (AOS) increased from 3.63
to 3.83 [15]. With an increase in K/Mn ratio for K-α-MnO 2 samples (AOS decreased
from 3.76 to 3.37), the catalytic activity increased significantly, but, at the same time,
such an important parameter as S sp decreased from 61.0 to 51.94 m
2 /g.
Based on both our results, and some reported data [7, 16], we can conclude
that the catalytic activity of cryptomelane in the ozone decomposition and other
redox processes [14, 17, 52] is determined by a combination of properties, including
structure, morphology, crystallite size, and product purity; the content of oxygen
vacancies and AOS; specific surface area and pore volume; the presence of acidic
and basic sites, their ratio, and strength; affinity to water molecules, hydrophilicity,
or hydrophobicity; and pH of the suspension.
4 Conclusions
The structural, morphological, textural, and protolytic properties of OMS-2 samples
obtained by the solid-phase reaction method (OMS-2(SSt)), reflux method (OMS2(Ref)), sol–gel method (OMS-2(SG)), and the method based on KNO 3 and MnSO 4
melting together (OMS-2(MS)) have been studied. The method of OMS-2 obtaining
affects the product phase composition and morphology. OMS-2(SG) and OMS2(MS) samples contain only the cryptomelane phase; their crystallite sizes are 36
and 14 nm, respectively. The cryptomelane crystallite sizes in OMS-2(Ref) and
OMS-2(SSt) samples, containing impurity Mn 2 O 3 and β-MnO 2 , are 15 and 10 nm,
respectively. For OMS-2(SSt), the largest crystal cell expansion has been found. The
morphology of the obtained samples is typical for cryptomelane, i.e., agglomerates
of various shapes and packing densities formed of interconnected fibrous crystals.
73
showed that the OMS-2 samples synthesized by different methods were characterized
by a high affinity to water molecules and, at RH of 65% (conditions of catalytic
activity testing used by us), they adsorbed different amounts of water (Table 4).
More active OMS-2(MS) and OMS-2(SG) samples adsorbed much less water than
OMS-2(Ref) and OMS-2(SSt) samples. For the latter, competing adsorption of water
vapor reduces their activity in the ozone decomposition.
It is seen that the establishment of correct correlations between any OMS-2 property and catalytic activity in the ozone decomposition is difficult because of the fact
that other parameters do not remain constant. The same conclusion can be drawn
from the analysis of some earlier reported data [8–11, 15, 19, 20, 22, 26, 30–32]. For
instance, the increase in the activity of polymorphic forms of manganese dioxide in
the order β-MnO 2 < γ-MnO 2 < α-MnO 2 was associated with the structure, and, in
this case, S sp increased in the same order from 13.8 to 80.7 m
2 /g [8]. With the same
structure, the decrease in activity in the order α-MnO 2 (nanofibers NFs) > α-MnO 2
(nanorods NRs) > α-MnO 2 (nanotubes NTs) was associated with different morphologies, but, in addition, a number of other parameters changed significantly: S sp and
the number of oxygen vacancies decreased from 80.7 to 33.8 m
2 /g and from 11 to
3.1%, respectively, whereas the average oxidation state (AOS) increased from 3.63
to 3.83 [15]. With an increase in K/Mn ratio for K-α-MnO 2 samples (AOS decreased
from 3.76 to 3.37), the catalytic activity increased significantly, but, at the same time,
such an important parameter as S sp decreased from 61.0 to 51.94 m
2 /g.
Based on both our results, and some reported data [7, 16], we can conclude
that the catalytic activity of cryptomelane in the ozone decomposition and other
redox processes [14, 17, 52] is determined by a combination of properties, including
structure, morphology, crystallite size, and product purity; the content of oxygen
vacancies and AOS; specific surface area and pore volume; the presence of acidic
and basic sites, their ratio, and strength; affinity to water molecules, hydrophilicity,
or hydrophobicity; and pH of the suspension.
4 Conclusions
The structural, morphological, textural, and protolytic properties of OMS-2 samples
obtained by the solid-phase reaction method (OMS-2(SSt)), reflux method (OMS2(Ref)), sol–gel method (OMS-2(SG)), and the method based on KNO 3 and MnSO 4
melting together (OMS-2(MS)) have been studied. The method of OMS-2 obtaining
affects the product phase composition and morphology. OMS-2(SG) and OMS2(MS) samples contain only the cryptomelane phase; their crystallite sizes are 36
and 14 nm, respectively. The cryptomelane crystallite sizes in OMS-2(Ref) and
OMS-2(SSt) samples, containing impurity Mn 2 O 3 and β-MnO 2 , are 15 and 10 nm,
respectively. For OMS-2(SSt), the largest crystal cell expansion has been found. The
morphology of the obtained samples is typical for cryptomelane, i.e., agglomerates
of various shapes and packing densities formed of interconnected fibrous crystals.