FTIR Spectroscopic Characterization of NO_xSpecies Adsorbed on ZrO₂and ZrO₂−SO₄^2-

Abstract

The nature of the NO x species produced during the adsorption of NO at room temperature and during its coadsorption with oxygen on pure and sulfated zirconia has been investigated by means of in situ FTIR spectroscopy. The adsorption of NO on both samples occurs through disproportionation leading to the formation of nitrous acid; water molecules; nitro species; and anionic nitrosyls, NO -. A mechanism for the formation of these adsorption forms is proposed. The NO -species are stable on the surface of zirconia, whereas on the sulfated sample, they are readily oxidized by the SO 4 2-groups. The process of NO disproportionation is favored by wet surfaces and occurs with participation of the tribridged (ZrO 2 ) and terminal (ZrO 2 -SO 4 2-) hydroxyl groups. Coadsorption of NO and O 2 on pure zirconia leads to the formation of various kinds of nitrate species. The presence of sulfate ions reduces the amount of surface nitrates and decreases their thermal stability. An analysis of the combination bands of the nitrate species shows that this spectral region can be used for structural identification of bidentate and bridged nitrates.

Routes of formation and composition of NOx complexes adsorbed on palladium-promoted tungstated zirconia

“…The weak band at 2230 cm −1 which is detected also in the spectrum taken during the room-temperature evacuation (Fig. 7) is attributed to adsorbed NO + species [44][45][46]. The Zr-OH hydroxyls at 3740 and 3693 cm −1 are perturbed giving rise to positive absorption in the region of H-bonded OH groups.…”

“…4B): the absorption at 1196 cm −1 decreases in intensity, and the lowfrequency component of the split ν 3 vibration of the bidentate nitrate is resolved at 1243 cm −1 . The spectrum of NO adsorbed on the ZrO 2 sample contains features that differ from those observed on monoclinic zirconia [44,46]. This could be associated with differences in the morphology, degree of hydroxylation and presence of impurities.…”

“…The band at 1950 cm −1 , which is absent from the spectrum at the beginning of the process, is assigned to NO adsorbed on Zr 4+ sites with increased electrophilicity caused by induction effect from adjacent electronwithdrawing NO x species [47]. The shoulder at about 1915 cm −1 and the weak absorption centered at 1327 cm −1 are attributed to the ν(N O) and ν s (NO 2 ) modes of adsorbed N 2 O 3 , respectively [44,45,51]. The ν as (NO 2 ) stretching vibration appears usually at 1590-1550 cm −1 [45] and overlaps with the band at 1601 cm −1 .…”

“…4A. The intense band at 1196 cm −1 is assigned to the ν(NO) stretching mode of anionic nitrosyl, NO − [44]. Since Zr 4+ ions are considered to be irreducible under these conditions, it is proposed that these species are formed by disproportionation of NO on surface O 2− sites:…”

Spectroscopic characterization of tungstated zirconia prepared by equilibrium adsorption from hydrogen peroxide solutions of tungsten(VI) precursors

“…Amorphous ZrO x (OH) 4-2x was prepared by hydrolysis of ZrCl 4 (Merck, for synthesis) with concentrated (25%) solution of ammonia as already described [23]. After washing (negative test for Cl -ions), the precipitate was dried at 383 K in air for 24 h. According to XRD, the solid (specific surface area of 298 m 2 /g) was amorphous.…”