60
T. Rakitskaya et al.
by using some compositions complicated by MnO 2 anchoring on porous supports
[27, 28]. However, as was reported [19, 23], the number of oxygen vacancies must
be optimal. Deactivation of these catalysts is a result of a competitive adsorption
of water molecules [19] and a participation of surface OH groups in the reaction
with ozone with the formation of water molecules blocking surface active sites [22].
Poisoning of MnO 2 catalysts by water vapor necessitates a systematic study of water
vapor adsorption by them to find out the conditions required for their application in
devices for air purification from ozone [29].
As was reported [8–11], among polymorphous forms of manganese dioxide under
the same conditions, α-MnO 2 (OMS-2, cryptomelane) shows the highest activity in
the ozone decomposition. It is known that the selected method and conditions of
OMS-2 synthesis considerably affect the product purity, its structural, morphological,
and textural parameters as well as physicochemical properties. Ozone decomposition
over OMS-2 was intensively studied [11, 19, 20, 22, 26, 30, 31]. OMS-2 samples
were mainly synthesized by hydrothermal method; however, the precursors [15, 19,
30], synthesis temperature and duration, methods of precipitate treatment, and drying
temperature [11, 19, 20, 22, 26] were different. The hydrothermal method of OMS-2
synthesis is considered as hardly realized in industrial scales [32], and another method
based on thermal decomposition of manganese carbonate at 300 °C for 6 h has been
proposed. A suspension method of OMS-2 synthesis from KMnO 4 and Mn(Ac) 2
lasted for 48 h was also reported [22]. Unfortunately, it was impossible to compare
the catalytic activity of cryptomelane samples even through they were synthesized
by the same, for instance, hydrothermal, method because, in the cited works, they
were tested under varied conditions: initial ozone concentrations, C
in
O 3
, from 14 ppm
(30 mg/m
3 ) [8, 26] to 120 ppm (257 mg/m
3 ) [23, 32], temperatures from 0 °C [25]
to RT, specific volume flow rates, W sp , from 540 L/g × h [20, 22, 32] to 1320 L/g
× h [26], and RH of the ozone–air mixture from 5 to 90% [19, 23, 26, 30, 32]. In
many cases, the latter parameter was not shown despite the fact that water vapor
adsorption, as stated above, resulted in catalyst deactivation. Time dependences of
the ozone decomposition degree, η, %, and assertions that η →100% [20, 30–32] do
not allow to evaluate ozone final (outlet) concentrations C
f
O 3
, and their conformance
to MPC O 3 in the air of working areas. In all cases considered, a gradual decrease
in OMS-2 activity leads to C
f
O 3
>> MPC O 3 . Using amorphous MnO 2 -synthesized
by the KMnO 4 reduction with formic acid as an example, we first have studied the
protective capability of this catalyst at C
in
O 3
= 1.5 mg/m
3 (0.7 ppm) and have shown
that the protective time (τ MPC ) (a period of time during which the air purification
from ozone below MPC O3 is provided) depends on the precipitate treatment methods
[33]. Thus, in spite of numerous reported data concerning catalytic properties of
polymorphic forms of manganese dioxide, an influence of the synthesis methods for
manganese oxide (especially OMS-2) obtaining on their catalytic behavior in the
reaction of ozone decomposition must be systematically studied.
The aim of the work was to synthesize cryptomelane samples by using four
methods, i.e., solid-state (SSt), reflux (Ref), sol–gel (SG), and melting (MS), to
T. Rakitskaya et al.
by using some compositions complicated by MnO 2 anchoring on porous supports
[27, 28]. However, as was reported [19, 23], the number of oxygen vacancies must
be optimal. Deactivation of these catalysts is a result of a competitive adsorption
of water molecules [19] and a participation of surface OH groups in the reaction
with ozone with the formation of water molecules blocking surface active sites [22].
Poisoning of MnO 2 catalysts by water vapor necessitates a systematic study of water
vapor adsorption by them to find out the conditions required for their application in
devices for air purification from ozone [29].
As was reported [8–11], among polymorphous forms of manganese dioxide under
the same conditions, α-MnO 2 (OMS-2, cryptomelane) shows the highest activity in
the ozone decomposition. It is known that the selected method and conditions of
OMS-2 synthesis considerably affect the product purity, its structural, morphological,
and textural parameters as well as physicochemical properties. Ozone decomposition
over OMS-2 was intensively studied [11, 19, 20, 22, 26, 30, 31]. OMS-2 samples
were mainly synthesized by hydrothermal method; however, the precursors [15, 19,
30], synthesis temperature and duration, methods of precipitate treatment, and drying
temperature [11, 19, 20, 22, 26] were different. The hydrothermal method of OMS-2
synthesis is considered as hardly realized in industrial scales [32], and another method
based on thermal decomposition of manganese carbonate at 300 °C for 6 h has been
proposed. A suspension method of OMS-2 synthesis from KMnO 4 and Mn(Ac) 2
lasted for 48 h was also reported [22]. Unfortunately, it was impossible to compare
the catalytic activity of cryptomelane samples even through they were synthesized
by the same, for instance, hydrothermal, method because, in the cited works, they
were tested under varied conditions: initial ozone concentrations, C
in
O 3
, from 14 ppm
(30 mg/m
3 ) [8, 26] to 120 ppm (257 mg/m
3 ) [23, 32], temperatures from 0 °C [25]
to RT, specific volume flow rates, W sp , from 540 L/g × h [20, 22, 32] to 1320 L/g
× h [26], and RH of the ozone–air mixture from 5 to 90% [19, 23, 26, 30, 32]. In
many cases, the latter parameter was not shown despite the fact that water vapor
adsorption, as stated above, resulted in catalyst deactivation. Time dependences of
the ozone decomposition degree, η, %, and assertions that η →100% [20, 30–32] do
not allow to evaluate ozone final (outlet) concentrations C
f
O 3
, and their conformance
to MPC O 3 in the air of working areas. In all cases considered, a gradual decrease
in OMS-2 activity leads to C
f
O 3
>> MPC O 3 . Using amorphous MnO 2 -synthesized
by the KMnO 4 reduction with formic acid as an example, we first have studied the
protective capability of this catalyst at C
in
O 3
= 1.5 mg/m
3 (0.7 ppm) and have shown
that the protective time (τ MPC ) (a period of time during which the air purification
from ozone below MPC O3 is provided) depends on the precipitate treatment methods
[33]. Thus, in spite of numerous reported data concerning catalytic properties of
polymorphic forms of manganese dioxide, an influence of the synthesis methods for
manganese oxide (especially OMS-2) obtaining on their catalytic behavior in the
reaction of ozone decomposition must be systematically studied.
The aim of the work was to synthesize cryptomelane samples by using four
methods, i.e., solid-state (SSt), reflux (Ref), sol–gel (SG), and melting (MS), to