Fundamental aspects of NOx adsorption on BaO

The nature of NO2 and nitrite/nitrate pairs adsorbed on BaO has been studied within the density functional theory. Size effects are investigated by comparing results for the BaO molecule, BaO clusters [(BaO)x, x = 4, 6, 9, 12] and BaO(1 0 0). The adsorption energies show weak dependence on size, which is a manifestation of the fast size convergence of the BaO electronic structure and the local character of the NO2 adsorption bond. Nitrite/nitrate pair formation is associated with a significant energy gain. For BaO(1 0 0), the stability of the pair is insensitive to the nitrite-nitrate separation, a finding that demonstrates a surface mediated non-local mechanism of molecular pair formation on oxide surfaces. The results have implications for the understanding of NOx storage and reduction catalysts.