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A.S. Ivanova et al. / Journal of Catalysis 221 (2004) 213–224
of the phase of Bi2Mn4O10 (Table 2). However, the ob-
served dependencies differ in nature (Fig. 15a). Figures 15a
and 15b show that the selectivity of Mn–Bi–O/α-Al2O3 to
N2O varies along with changes in the Mn3+/Mnδ+ ratio
(both in the samples reduced with hydrogen and in the ones
treated with the reaction mixture) as the temperature of cat-
alyst calcination increases. It can be therefore assumed that
oxidation of ammonia proceeds through the stage of activa-
tion of adsorbed ammonia that is accompanied by reduction
of Mn4+ (Mn3+) into Mn2+ followed by its reoxidation with
oxygen. Hence, nitrous oxide can be supposed to form over
oxidized manganese sites Mn4+(Mn3+) and nitrogen over
Mnδ+ sites.
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Fig. 15. Ratio of oxidized to reduced manganese species (a) in catalyst
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Mn–Bi–O/α-Al O and selectivity to N O (b) as a function of calcination
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2
3
2
the time of calcination and temperature to 550 ◦C results in
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surface.
On the other hand, if there is the bismuth oxide in the
Mn–Bi–O/α-Al2O3 catalyst calcined at 400 ◦C, the temper-
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supported by TPR data. For example, the temperatures of re-
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are 348 and 313 ◦C in the Mn–Bi–O/α-Al2O3 and Mn–O/
α-Al2O3 catalysts, respectively, calcined at 400 ◦C (Figs. 9a
and 10a).
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the reaction of ammonia oxidation over Mn–Bi–O/α-Al2O3
varies mainly due to the redox properties of manganese ox-
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can see that Mn3+/Mnδ+ (2 < δ < 3), Mn3+/Mn2+, and
ꢀ
Mn3+
/
(Mnδ+ + Mn2+) ratios increase with calcination
temperature. Therefore, elevation of the calcination temper-
ature makes the degree of the catalyst reduction decreased
after treatment with hydrogen, probably due to formation