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

The effect of water on the adsorption of NO 2 onto a γ-Al 2 O 3 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 NO 2, nitrites and nitrates are formed, and at higher NO 2 doses only nitrates are observed. The surface nitrates formed were of bridging monodentate, bridging bidentate, and monodentate configuration. At elevated NO 2 pressures, the surface hydroxyls were consumed in their reaction with NO 2 giving primarily bridge-bound nitrates. A significant amount of weakly adsorbed N 2 O 3 was seen as well. Exposure of the NO 2 -saturated γ-Al 2 O 3 surface to H 2 O resulted in the desorption of some NO 2 + NO as H 2 O interacted with the weakly held N 2 O 3, 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 H 2 O to the NO 2 -saturated γ-Al 2 O 3 did not affect the amount of NO x strongly adsorbed on the support surface. In particular, no NO x desorption was observed when the NO 2 -saturated sample was heated to 573 K prior to room-temperature H 2 O exposure. The effect of water is completely reversible; i.e., during temperature-programmed desorption (TPD) experiments following NO 2 and H 2 O coadsorption, the same IR spectra were observed at temperatures above that required for H 2 O desorption as seen for NO 2 adsorption only experiments.