The hydrogen-deuterium exchange reaction on cobalt ferrite

Abstract

The catalytic activity of cobalt ferrite, Co3-xFexO4, for the hydrogen-deuterium exchange reaction and the change in thermoelectric power of the ferrite during the adsorption of hydrogen and oxygen were investigated as functions of the catalyst composition.The exchange reaction was carried out on catalysts with values of x: 1.93, 1.97, 2.02, and 2.07 in a flow reactor in the temperature range 60 [deg] to 130 [deg]C, at 1 atm pressure. It was found that the rate of the reaction was dependent upon the pretreatment of the catalyst while the activation energy increased from about 19 to 24 kcal mole-1 and the pre-exponential factor from 1030 to 1037 min-1 as the compositional parameter, x, progressed from 1.93 to 2.07.The change in thermoelectric power of compressed powder ferrite pellets during the adsorption of hydrogen and oxygen was investigated in the temperature range 88-250 [deg]C on two samples of cobalt ferrite, x = 1.91 and 2.04. At temperatures > 120 [deg]C hydrogen was adsorbed on the cobalt ferrite as an electron donor, and oxygen as an acceptor. However, no change in thermoelectric power was detected at lower temperatures, indicating that in the range in which the catalytic reaction was studied no net electron transfer occurred between the adsorbed molecules and the substrate.It is concluded that the hydrogen-deuterium exchange reaction on cobalt ferrite occurred in two stages. The first is an initial activation step in which the catalytic activity increased with time. This step can be associated with the reduction of the surface of the ferrite and with the exposing of surface cations. In the second stage the exchange reaction proceeded at constant activity. A reaction mechanism involving hydrogen being adsorbed dissociatively on cation-anion couples in such a way as to restore the octahedral coordination of the cations of interest (Co3+, Fe3+) is in accord with the experimental results. The differences in activation energies are qualitatively related to the energies of the Co2+---Co3+ and Fe2+---Fe3+ electron transfers. A theoretically calculated reaction rate, which assumes slow hydrogen desorption, is in accord with the experimental rate.