Gas adsorptive properties of manganese dioxides

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Manganese oxide minerals adsorb some kinds of gas molecules on their surface and released them on heating. The released gas from natural and synthetic manganese oxide minerals was studied by Q-pole mass spectrometer. CO_2 was observed in the temperature range 100-350 ﾟC, 400-450ﾟC and 600-700ﾟC. The released CO_2 gas between 100 and 350 ﾟC derived from the surface adsorption. Natural and synthetic manganese dioxide minerals adsorb CO_2 gas in the air readily.Manganese hydroxide mineral was synthesized by the reduction of ramsdellite (gamma-MnO_2) using hydrothermal techniques in order to find the new manganese oxide which adsorb CO_2 gas. Powdered ramsdellite and manganese metal was sealed in a Pt tube with water and keep it at 150 ﾟC, 1000 kb for 10 days. The reaction is written as follows. 8MnO_2 + 2H_2O + Mn -> 4Mn_2O_3(OH) + MnO_2. Manganese metal is added to reduce the oxygen pressure in the tube and stabilize manganese ion as Mn^<3+> and Mn^<4+>. The X-ray powder diffraction profile of the product is quite similar to that of ramsdellite and groutite( gamma-MnOOH). The crystal structure, refined by the Retveldt method, is isostructure to ramsdellite and groutite. The ideal formula for the product is Mn^<3+>_2Mn^<4+>_2O_6(OH)_2. In the process of reduction, ramsdellite + new phase or new phase + groutite is observed. Therefore it is obvious that the solid solution between ramsdellite and groutite is absent.