Revisiting ceria-NOx interaction: FTIR studies

Cerium is a key component in many DeNO(x) catalysts. An important property of ceria related to these applications is its high oxygen storage capacity. Indeed, in some reaction steps, cerium switches its oxidation state between 3+ and 4+. To design new and effective cerium-based DeNOx catalysts, it is necessary to know in details the interaction between CeO2-x and nitrogen oxides. One of the most informative techniques in this respect is IR spectroscopy. There are many IR studies devoted to this problem. However, as a rule, the efforts are concentrated on investigation of the oxidized forms of ceria and there are few works dealing with reduced oxide. There are also many contradictory assignments and few works use isotopic labelling for better interpretation of the spectra. Here we report a summary of our IR studies on the interaction of nitrogen oxides with ceria paying equal attention to the reduced and non-reduced oxide form. To strengthen the assignments we have used isotopically labelled molecules and DFT calculation. It was found that the initial stages of interaction of NO with oxidized ceria comprise NO disproportionation to symmetric nitrites (formal oxidation state of nitrogen 3+) and hyponitrites (formal oxidation state of nitrogen 1+). At further stages non-symmetric nitrites and N2O are also formed. In presence of oxygen all these species are converted into surface nitrates. Adsorption of N2O on reduced ceria is dissociative and leads to formation of N-2 with simultaneous filling the oxygen vacancies and oxidation of Ce3+. Initial stages of interaction of reduced ceria with NO includes (i) formation of surface azides (N-3(-), showing remarkable similarity with NCO- species) and NO2-. The latter species interact with additional NO molecule being thus converted into hyponitrites. Further stages are associated with formation of nitrites, and, in presence of O-2, nitrates.