Catalytic dissociation of N2O on pure and Ni-doped MgO surfaces

The catalytic activity of pure and Ni-doped MgO surfaces in N2O decomposition has been investigated theoretically with DFT B3LYP cluster model calculations. The barrier to abstraction of O from N2O to form a surface peroxo group, O22-, is found to be rate limiting on both pure and Ni-doped MgO. In the presence of Ni impurities, however, the barrier is reduced from 1.37 eV to 1.19 eV, thus accounting for the experimentally observed increase in catalytic activity as function of the Ni content. Dissociation reaction was found to take place also by direct interaction with surface Ni ion with a competing activation barrier of 1.22 eV. However, this latter process is less exothermic (1.45 eV vs 2.19 eV). The O abstraction by a surface oxygen is followed by O diffusion and recombination with final desorption of an O2 molecule from the surface. This process occurs preferentially on pure MgO while requires higher barriers in the presence of Ni. This can explain the observed decrease of catalyst activity when the Ni concentration becomes higher than 50%.