Infrared Spectroscopic Study of the Adsorption of HCN by γ-Al2O3: Competition with Triethylenediamine for Adsorption Sites

The adsorption and vibrational properties of chemisorbed HCN on Lewis acid sites, Lewis base sites, and Brønsted Al−OH acid sites on a partially hydroxylated γ-Al 2 O 3 surface have been obtained by a combination of FTIR and density functional theory studies. The vibrational modes from the molecular and dissociative adsorption of HCN were assigned by using deuterium and 13 C-labeled D 13 CN molecules at 170 K. In addition, η 2 (C, N)−HCN bonding is also found from the ν(C N) vibrational spectra. Good correlation of the calculated vibrational frequencies for the adsorbed species with experimental data is found. The effect of triethylenediamine (TEDA) (also called 1, 4-diazabicyclo [2.2.2]octane, DABCO) on the adsorption of hydrogen cyanide (HCN) on the high area γ-Al 2 O 3 surface has been investigated using transmission FTIR spectroscopy. During HCN adsorption on TEDA-functionalized surfaces, there is no spectral change or emerging feature in either the TEDA or HCN spectral regions, indicating that no direct interaction occurs between these two molecules. Instead, we found that TEDA competes with HCN for the active sites on γ-Al 2 O 3 . The observed ν(C⋮N) mode on a TEDA-precovered surface is due to the HCN adsorption on Lewis base sites (Al− O −Al) which are less affected by TEDA preadsorption.