On the catalytic decomposition of nitrous oxide over metal oxides

Abstract

In literature a lot of data has accumulated on the activation energy of of N 2O decomposition over a large number of different catalysts. If one constructs a histogram of the partition of the data one observes that the values of activation energies seem to gather themselves into a series of rather sharply defined regions. These regions are spaced regularly with an approximate interval of 3.5 kcal/mol. This empirical observation fits the suggestion made previously that the change of activation energy for a series of related catalysts for one and the same reaction occurs in a stepwise manner. This increment should correspond to an energy quantum of the vibration mode that distorts the molecule to reaction. A quantitative treatment is attempted to determine the anharmonicity constant of that vibration mode from all data presented. One than gets about 7 cm −1 compared to 3.2 cm −1 for the gaseous molecule. Introducing the expression E a = E + RT one obtains the anharmonicity constant 3.4 cm −1, in close agreement with the above value from molecular spectroscopy. The findings support the theories of resonance in catalysis, indicating a selective transfer of energy from the catalyst to the vibration mode of the reactant that has to be activated to reach the ‘activated state’. Using this model a quantitative description of the ‘compensation effect’ has been made. The isocatalytic temperature, characteristic of a series of catalysts obeying the compensation rule, can be expressed as a function of the vibration frequency of the molecule, ν, and that of the energy pool of the catalyst, ω. The results of this treatment are applied to the material of N 2O. It follows that the experimentally determined isocatalytic temperature, Θ, can be calculated if one uses the value of the N-O stretching frequency that is obtained from the treatment of the activation energy data. The dependence of E a on lattice parameters follows from the presented model. The model presumes a special relation between adsorption enthalpy and activation energy, somewhat different from the one emerging from the equilibrium model that is usually applied.