Perturbation of Adsorbed CO by Amine Derivatives Coadsorbed on the γ-Al2O3 Surface: FTIR and First Principles Studies

Abstract

The coadsorption of CO and triethylenediamine (TEDA) (also called 1,4-diazabicyclo[2.2.2]octane, DABCO) on a high-area gamma-Al2O3 surface has been investigated with use of transmission FTIR spectroscopy. It has been found that TEDA binds more strongly to both Lewis acid sites and to Brønsted Al-OH sites than does CO. Competition experiments indicate that TEDA displaces CO to less strong binding sites. Evidence for weak CO...TEDA interactions is found in which small nu(CO) redshifts are produced. Comparison between different amines such as triethylenemonoamine (TEMA) (also called 1-azabicyclo[2.2.2]octane, ABCO), trimethylamine (TMA), and ammonia indicates that the nu(CO) redshift increases with increasing amine polarizability, indicating that the redshift is mainly due to dipole image damping effects on the CO oscillator frequency. The direct bonding between the exposed N lone pair electrons of the TEDA molecule and CO does not occur. First principles theoretical studies have characterized the bonding of CO with gamma-Al2O3 Lewis acid sites of various types as well as TEDA bonding to both Lewis acid sites and to Al-OH groups. The theoretical studies also indicate that strong bonding of adsorbed CO with TEDA molecules does not occur, and that the observed decrease in the binding energy of CO when coadsorbed with TEDA on gamma-Al2O3 is expected.