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• NO disproportionates on stoichiometric ceria to nitrites and trans -hyponitrites. • NO + O 2 coadsorption on ceria leads to formation of different surface nitrates. • NO interacts with reduced ceria forming NO2− and N 3 - species. • The NO2− species interact with NO to produce hyponitrites. • Azide species demonstrate properties very similar to those of surface isocyanates. 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 CeO 2-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 N 2 O are also formed. In presence of oxygen all these species are converted into surface nitrates. Adsorption of N 2 O 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • When adsorbed on ceria NO initially disproportionates to nitrites and hyponitrites. • Further NO adsorption includes formation of non-symmetric nitrites and N 2 O. • Hyponitrites are very easily oxidized when dosing O 2 to the NO/CeO 2 system. • Nitrites first develop and are then oxidized when dosing O 2 to the NO/CeO 2 system. Adsorption of NO and co-adsorption of NO and O 2 on ceria have been re-investigated by FTIR spectroscopy. To provide unambiguous assignments of the IR bands, adsorption of 15NO and co-adsorption of 14NO + 15NO isotopic mixtures have also been studied and DFT calculations performed. At the initial adsorption stages mainly symmetric nitrite species (2472, 1304, ca. 1160 and 822 cm‐1) and trans- hyponitrites ([N 2 O 2 ]2−, ca. 1100 cm‐1) are simultaneously produced as a result of NO disproportionation. At further stages non-symmetric NO 2 − species (1393 and 1014 cm‐1), N 2 O (2240, 1252 cm‐1) and small amount of nitrates are formed. Nitrates are the only surface products of the thermal decomposition of the nitrites and hyponitrites. In the presence of small amounts of oxygen, the hyponitrites are very easily oxidized. In contrast, all nitrites increase in concentration after addition of small amount of O 2 to the NO/CeO 2 system but are gradually oxidized in the presence of higher amount of oxygen giving rise to different surface nitrates. [ABSTRACT FROM AUTHOR]

To design new deNO x materials and processes we need to know in details the mechanism of NO interaction with solid surfaces. Many NO x conversion catalysts contain ceria as a component and cerium changes its oxidation state in the course of redox catalytic processes. Here we show that the interaction between NO and reduced ceria leads to formation of azides (N 3 − , 2044–2042 cm −1 ) and nitric oxide dianion (NO 2− , 1010–980 cm −1 ) with the simultaneous oxidation of Ce 3+ . These species are established for the first time after NO adsorption on solid surfaces and their relative concentrations strongly depend on the sample morphology. At ambient temperature N 3 − does not interact with NO or O 2 alone but easily disappears in the co-presence of the two gases, thus demonstrating reactivity similar to that of isocyanates. In contrast, NO 2− accepts one NO molecule and is converted into hyponitrite (bands in the 1030–970 cm −1 region). At further stages of interaction, N 2 O, N 2 , nitrites and nitrates are formed. The present findings enrich the current opinions of catalytic NO conversion and impose some revisions. [ABSTRACT FROM AUTHOR]