NO reaction at surface oxygen vacancies generated in cerium oxide

Abstract

NO adsorption on CeO₂ pre-outgassed at different temperatures (T_v) has been studied by EPR and FTIR. The results are compared with those obtained by measuring the products of the reaction, performed at 323 K, between NO and CeO₂ samples activated by previous heating under N₂ flow. EPR results indicate that, for T_v 573 K, NO molecules are adsorbed in radical form at 77 K and react at 298 K with surface oxygen vacancies generating diamagnetic products. FTIR spectra show correspondingly the formation of hyponitrites, nitrites and nitrates upon NO adsorption at 298 K on CeO₂ outgassed at different temperatures. The hyponitrites can decompose at ambient temperatures to produce N₂O; the process leads to modification (probably reoxidation) of the surface centres initially present. This can be followed by O₂ adsorption and EPR spectroscopy, which shows that O₂^– adsorbed on the modified vacancies is less stable than on CeO₂ without NO adsorbed, its reaction leading in some cases to the formation of peroxynitrate-type radicals. The reaction at 323 K of NO on the pretreated CeO₂ surface produced N₂O for pretreatment temperatures T_F 573 K, in amounts increasing with T_F. The results indicate that surface centres containing associated oxygen vacancies are active sites for this process.