Effect of H₂O on the Adsorption of NO₂ on γ-Al₂O₃:  an in Situ FTIR/MS Study

Abstract

The effect of water on the adsorption of NO2 onto a γ-Al2O3 catalyst support surface was investigated using Fourier transform infrared spectroscopy (FTIR) and mass spectrometry (MS). Upon room-temperature exposure of the alumina surface to small amounts of NO2, nitrites and nitrates are formed, and at higher NO2 doses only nitrates are observed. The surface nitrates formed were of bridging monodentate, bridging bidentate, and monodentate configuration. At elevated NO2 pressures, the surface hydroxyls were consumed in their reaction with NO2 giving primarily bridge-bound nitrates. A significant amount of weakly adsorbed N2O3 was seen as well. Exposure of the NO2-saturated γ-Al2O3 surface to H2O resulted in the desorption of some NO2 + NO as H2O interacted with the weakly held N2O3, while the bridging monodentate surface nitrates converted into monodentate nitrates. The conversion of these oxide-bound nitrates to water-solvated nitrates was observed at high water doses when the presence of liquid-like water is expected on the surface. The addition of H2O to the NO2-saturated γ-Al2O3 did not affect the amount of NO x strongly adsorbed on the support surface. In particular, no NO x desorption was observed when the NO2-saturated sample was heated to 573 K prior to room-temperature H2O exposure. The effect of water is completely reversible; i.e., during temperature-programmed desorption (TPD) experiments following NO2 and H2O coadsorption, the same IR spectra were observed at temperatures above that required for H2O desorption as seen for NO2 adsorption only experiments.

Fine-Tuning the Dispersion and the Mobility of BaO Domains on NOx Storage Materials via TiO2 Anchoring Sites

“…28,[34][35][36][37] The most prominent band after the first few initial NO 2 doses in Figure 3a at 1230 cm -1 and the shoulder at about 1320 cm -1 are attributed to the nitrites on the alumina surface. Concomitant with the formation of nitrites, further NO 2 doses lead to the oxidation of the surface nitrite species to nitrates.…”

“…34,35,[44][45][46][47] The high-frequency section of the IR data in Figure 4a (ν OH > 3650 cm -1 ), shows the development of several negative bands located at 3730, 3760, and 3775 cm -1 and shoulders at ∼3690 and 3795 cm -1 during the NO 2 (g) adsorption process on pure alumina. Appearance of these negative features and their relative intensity changes can be explained by a gradual consumption of the isolated hydroxyl groups on the alumina surface with increasing surface NO x coverage.…”

“…TPD profiles of the 8Ba/Al and 20Ba/Al samples with different Ba loadings upon NO 2 (g) adsorption at 323 K (Figures 9 and 10) can be readily analyzed on the basis of one of our former reports 28 as well as other reports in the literature. 34,36,[49][50][51] It is visible in Figures 9 and 10 that the first major desorption feature for the 8(20)Ba/Al samples appears at 389 K due to the decomposition of the N 2 O 3 /NO + species, which is followed by a desorption feature associated with the decomposition of the nitrates that reside on the exposed γ-Al 2 O 3 sites (600-650 K). Next the decomposition of the surface (bidentate) nitrates on the BaO monomers/dimers, which are strongly interacting with the support material (650-700 K), takes place by releasing mostly NO 2 .…”

“…Finally, the decomposition of the bulk nitrates takes place at T > 900 K), mostly in the form of NO + O 2 . 28,34,36,[49][50][51] TPD data for the 8(20)Ba/Al samples can be used as a useful benchmark for elucidating the TPD results obtained for the 8(20)Ba/Ti/Al(P1/P2) materials. It is evident that TPD profiles corresponding to the 8Ba/Ti/Al(P1) system (Figure 9a,b) have much closer resemblance to the TPD data of the 20Ba/Al sample (the bottommost profiles in Figure 10a,b) than that of the 8Ba/ Al system (Figure 9a,b).…”

Thermal and photochemical reactions of NO2 on chromium(iii) oxide surfaces at atmospheric pressure

“…[22][23][24][25][26][27][28] Adsorbed water on oxide surfaces often has a significant effect on these heterogeneous reactions. [21][22][23]26,27,29,30 Enhanced surface uptake of NO 2 and HNO 3 has been reported on hydrated surfaces or in the presence of water vapor compared to dehydrated surfaces.…”