ROLE OF OXYGEN VACANCIES IN THE DECOMPOSITION OF N2O OVER YBA2CU3O7-DELTA AND GD2CUO4 OXIDE SYSTEMS

The activity of high T(c) oxides, YBa2Cu3O7-delta (0.7>delta>0.1) and that of nonsuperconducting Gd2CuO4 was evaluated for N2O decomposition in the temperature range of 450-800 K. The structural and compositional changes were examined at different stages of reaction using X-ray powder diffraction, thermogravimetry and iodometric titration methods. The N2O decomposition on the oxygen-deficient tetragonal form of high T(c) cuprates commenced at 450 K and at each temperature these oxides showed much higher catalytic activity than the oxygen-rich orthorhombic homologues. The higher activity was related to the number of vacant oxygen lattice sites. The oxygen produced during N2O decomposition led to the tetragonal --> orthorhombic transformation. On the other hand, the tetragonal gadolinium cuprate showed poor activity and N2O decomposition was observed only at temperatures above 650 K. The results support a reaction mechanism involving adsorption of N2O at oxygen vacancy sites adjacent to copper ions, followed by a change in copper valence state and release of nitrogen. An equilibrium between N2O adsorption at vacancy sites and the generation of such sites on thermal release of lattice oxygen decided the catalytic behaviour of the oxide systems studied at higher temperatures.