Effect of H2O on the adsorption of NO2 on gamma-Al2O3: an in situ FTIR/MS study

Abstract

The effect of water on the adsorption of NO2 onto a gamma-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 gamma-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 gamma-Al2O3 did not affect the amount of NO, strongly adsorbed on the support surface. In particular, no NO, 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