ESCA and thermodyanmic studies of alkali metal ion exchange reactions on an #alpha#-MnO_2 phase with the tunnel structure

摘要

A manganic acid cation-exchange material with the 2 x 2 type tunnel structure was synthesized by decomposition of a mixture of 80 mol% MnCO_3 and 20 mol% (CH_3)_3COK at 530 ~(o)C, followed by leaching out any other residual ionic impurities using 1M HNO_3. Its chemicla comosition was represented by H_1.7Mn_8O_15.01. The theoretical ion exchange capacity was 2.49 mmol g~(-1), assuming that all the H~(+)s are exchangeable. The bindingenergies of Mn 2p_(3/2) levels were not shifted upon alkali cation exchange of this material. This is clear evidence that no redox process is associated with the cation exchange. The plot of the corrected selectivity coefficient in logarithmic scale vs. the charge fraction X-bar_M was linear in a small range of the charge fraction up to 0.1. This suggests that each cation resides in a similar chemical environment at this concentration level within the tunnel. The slope was -40for Li~(+)/H~(+) exchange, -46 for Na~(+)/H~(+) exchange, -108 for K~(+)/H~(+) exchange, -72 for Rb~(+)/H~(+) exchange, and -60 for Cs~(+)/H~(+) exchange. The corrected selectivity coefficient at infinitesimal exchange increased in the order Li < Na < Cs K approx Rb at 30-60 ~(o)C, indicating that thelargest hydrated cation is the least selective and cations having the same dimension as the tunnel size are the most selective. Thermodynamic functions were evaluated forthe hypothetical ion-exchange reaction with the least steric hindrance.