CRYPTOMELANE MODIFIED WITH TRANSITION METAL IONS: STRUCTURE AND CATALYTIC ACTIVITY IN THE REACTION OF OZONE DECOMPOSITION

Cryptomelane, OMS-2, and cryptomelane modified with transition metal ions, M/OMS-2 (M = Cu2+, Co2+, Fe3+), were synthesized by the reaction of KMnO4 reduction with manganous salts (MnCl2 and MnSO4) by the reflux method. The samples thus synthesized were characterized by X-ray diffraction and IR spectroscopic methods and their catalytic activity was tested in the reaction of ozone decomposition. Cryptomelane, OMS-2, is the main phase of the synthesized samples, bixbyite, Mn2O3, and pyrolusite, β-MnO2, are impurity phases presented in OMS-2 and M/OMS-2 at a quantity lower than 10 %. The doped metal ions did not create a new phase however they influenced the size of cryptomelane crystallites and it became almost two times lower. Our IR spectral investigations show that, in the region of stretching vibrations of OH groups in the associated water molecules, the IR absorption is very weak for all M/OMS-2 samples as opposed to OMS-2. IR spectroscopic analysis also showed that incorporation of the transition metal ions into the cryptomelane structure differently affected the vibrations of Mn-O and Mn-OH bonds. The doped Cu2+ ion practically caused no change in the OMS-2 spectrum whereas new absorption bands characteristic of Co-O and Fe-O bonds appeared in the spectra of Co/OMS-2 and Fe/OMS-2. The absorption band in 650-400 cm-1 region of the Fe/OMS-2 spectrum is complex. The catalytic activity of M/OMS-2 samples depended on the nature of the current transition metal ion doped into tunnel channels of cryptomelane. The Cu/OMS-2 sample exhibits a tendency to the steady-state mode of ozone decomposition whereas, in the case of the Cu/OMS-2 and Fe/OMS-2, the ozone concentration at the reactor outlet rapidly increases. Comparison of the activity of these three M/OMS-2 samples and OMS-2 resulted in the following order: Co/OMS-2 > OMS-2 > Cu/OMS-2 > Fe/OMS-2. Thus, the accelerating effect was observed only in the case of cobalt (II) ions