Reaction of hydrogen with a nonstoichiometric manganese dioxide

Abstract

The reaction of hydrogen with a catalytically active nonstoichiometric manganese dioxide has been examined at temperatures between room temperature and 400 °C employing gas chromatographic procedures. The oxidation rates for heterogeneous oxidation of hydrogen with air at concentrations below 7 vol % were found to be first order with respect to the hydrogen concentration for the temperature range 95 ° to 200 °C. The oxidation rates for depletive oxidation of hydrogen with manganese dioxide at 1950 ppm hydrogen at 100 ° to 300 °C were found to be first order with respect to the manganese dioxide and the data could be fitted to an equation of the form ln [ a (a − x) ] = k″t where a is the initial amount of manganese dioxide present and x is the oxygen removed from the oxide lattice by hydrogen oxidation at t. The activation energy for the heterogeneous hydrogen oxidation was calculated to be 3.9 kgcal/mole compared with an activation energy of 8.7 kgcal/mole for the depletive hydrogen oxidation reaction. The activation energy for depletive hydrogen oxidation is probably enough greater than the activation energy for the heterogeneous oxidation reaction to establish that in air relatively slight demand would be made upon the lattice oxygen. The high catalytic activity and defect nature of this manganese dioxide are indicated by the amount of Mn 3+ initially present. The oxygen of this oxide could be depleted and replenished with a high order of reversibility within the stoichiometric range of MnO 1.96 to MnO 1.52 which corresponds to the range of high activity for hydrogen oxidation.