Pyridine Adsorption and Acid/Base Complex Formation on Ultrathin Films of γ-Al2O3 on NiAl(100)

Abstract

Pyridine adsorption on NiAl(100) and ultrathin films of γ-Al 2 O 3 was studied using high-resolution electron energy loss spectroscopy (HREELS). Pyridine adsorbs on NiAl(100) with its molecular axis parallel to the surface plane at low coverages and with its molecular plane inclined more toward the surface normal at higher coverages. On the hydroxylated and nonhydroxylated thin films of γ-Al 2 O 3 , pyridine interacts with the coordinatively unsaturated Al 3 + cations via its nitrogen lone-pair electrons. Pyridine was also found to interact with the surface hydroxyl groups on the hydroxylated γ-Al 2 O 3 thin films, forming C 5 H 5 N-HO complexes. Complex formation causes the OH bond strength to decrease and the OH stretch vibration to shift from 3711 cm - 1 , which is characteristic of uncomplexed, isolated OH, to lower frequency. At low coverages, pyridine only interacts with the more acidic surface OH groups, those located on 3-fold Al 3 + cation sites. This interaction forms a C 5 H 5 N-HO complex, which has an O-H stretch at 2920 cm - 1 . As the coverage is increased, an additional C 5 H 5 N-HO complex is formed from an interaction between pyridine and the less acidic OH groups, which are bonded to 2 Al 3 + cations. The vibrational frequency for O-H stretch of this C 5 H 5 N-HO complex is 3150 cm - 1 . As the intensities for the O-H stretches of the C 5 H 5 N-HO complexes increase, the intensity for the free O-H stretch at 3711 cm - 1 decreases. The interaction with the surface hydroxyl groups is reversible, confirming that the observed shifts in the O-H stretching frequency result from the formation of weakly bonded acid-base complexes. While most of the pyridine desorbs from the γ-Al 2 O 3 thin films by 290 K, annealing the pyridine-dosed γ-Al 2 O 3 thin films to temperatures above 290 K, results in a small amount of pyridine dehydrogenation and the formation of surface OH groups with an O-H stretch at 3742 cm - 1 .