NO reaction at surface oxygen vacancies generated in cerium oxide

Abstract

NO adsorption on CeO, pre-outgassed at different temperatures (T,) 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 CeO, samples activated by previous heating under N, flow. EPR results indicate that, for T, 2 573 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 CeO, outgassed at different temperatures. The hyponitrites can decompose at ambient temperatures to produce N,O; the process leads to modification (probably reoxidation) of the surface centres initially present. This can be followed by 0, adsorption and EPR spectroscopy, which shows that 0,adsorbed on the modified vacancies is less stable than on CeO, 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 CeO, surface produced N, O for pretreatment temperatures T, 2 573 K, in amounts increasing with J , . The results indicate that surface centres containing associated oxygen vacancies are active sites for this process.The use of CeO, as an additive in many 'three way' catalyst formulations used in car exhausts has caused a great interest in the properties of this oxide.'-3 Initially, CeO, was included to act as oxygen storage, supplying oxygen to the reaction under hydrocarbon-rich conditions and withdrawing oxygen under poor conditions, thus widening the catalyst operation window near the stoichiometric conditions. However, some additional effects of CeO, have been found, such as improving the active metal dispersion4*' or catalysing by itself some of the reactions that take place on the car exhaust c a t a l y ~t . ~? ~ To determine how CeO, participates in these processes, a first aspect to clarify is the effect that the reaction conditions have on the state of the ceria surface. Considering that oxygen vacancies, together with Ce3 .t ions, are expected to be formed easily on CeO, under reducing conditions, these ions can be postulated as possible active sites for redox reactions.In a previous work, we studied the formation of reduced centres involving oxygen vacancies at the CeO, surface as a result of outgassing treatments. 0, adsorption, followed by EPR, was used there as a method to classify the different types of vacancies.8 Thus, on the basis of their EPR signal parameters the 0,species could be grouped into two types, which were assigned to 0, -radicals adsorbed, respectively, at isolated and associated oxygen vacancies. The different EPR parameters of the superoxide radicals formed at the two types of adsorption sites suggested that these had different electron densities, possibly implying different reactivities toward adsorbed species.To clarify this point, in the present work we studied by EPR and FTIR the adsorbed species formed by exposing ...

Palladium doped perovskite-based NO oxidation catalysts: The role of Pd and B-sites for NOx adsorption behavior via in-situ spectroscopy

“…Another type of adsorbed nitrate, revealing vibrational features at 1530 and 1269 cm −1 can be assigned to monodentate nitrates [37]. Two characteristic vibrational modes of bidentate nitrates are also observed on the surface which are located at 1568 and 1246 cm −1 [35,36]. The vibrational bands at 1434, 1322 and 804 cm −1 in Fig.…”

“…Fig. 6c) samples at 323 K. FTIR spectra yield a convoluted set of bands corresponding to various types of nitrates and nitrites with different surface adsorption geometries [34][35][36][37][38][39][40]. Vibrational features appearing at 1649 and 1009 cm −1 are assigned to asymmetric and symmetric stretching modes of bridging nitrate species on the perovskite surface, respectively [34].…”

“…10 illustrates that on the pre-reduced surfaces, abundance of nitrite/nitrito species increases as evident from the relative strengthening of the vibrational frequencies at 1487, ∼1450 and 1330 cm −1 while nitrate-related vibrational features (e.g. 1567 and 1270 cm −1 ) attenuate in a relative fashion [35,36].…”

Cerium oxide nanoparticle: a remarkably versatile rare earth nanomaterial for biological applications

“…Inspired by CeONP's NO exhaust gas treatment ability and Á NO adsorption and decomposing ability in industry, 55 Self and coworkers 56 explored and demonstrated the Á NO scavenging ability of CeONP under physiologically relevant conditions. Surprisingly, compared with the superoxide scavenging properties that prefer a higher level of cerium in the Ce 3 þ state, the Á NO scavenging ability is present in CeONP with a lower Ce 3 þ /Ce 4 þ ratio.…”

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