J. Phys. Chem. B 2005, 109, 6331-6333
6331
Adsorption of Triethylenediamine on Al2O3-III: Bonding to Lewis Acid Al3+ Sites Sunhee Kim, Oleg Byl, and John T. Yates, Jr.* Department of Chemistry, Surface Science Center, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: NoVember 30, 2004; In Final Form: January 27, 2005
The adsorption of triethylenediamine (TEDA) on Lewis acid (Al3+) sites of the highly dehydroxylated Al2O3 surface has been observed by FT-IR spectroscopy. This was done by monitoring the competitive adsorption of TEDA and CO on the Al3+ sites. A stoichiometric replacement of Al3+-CO species was observed as Al3+-TEDA surface species were formed.
I. Introduction The triethylenediamine (TEDA, also named 1,4-diazabicyclo[2.2.2]octane, DABCO) molecule is useful for functionalizing oxide and carbon surfaces by adsorption via one of the tertiary amine N atoms.1-5 We recently demonstrated that TEDA bonds, via hydrogen bonding, to surface Al-OH groups.6a,6b This occurs in two stages where first a single TEDA molecule forms a hydrogen bond, Al-OH‚‚‚(TEDA), and then a second TEDA molecule forms an Al-OH‚‚‚(TEDA)2 species. The bonding enthalpy for the Al‚‚‚(TEDA) species formation is -15.6 ( 0.5 kJ mol-1. This report demonstrates that the TEDA molecule also bonds to Al3+ Lewis acid sites on the Al2O3 surface. The work was done using chemisorbed CO as a marker for the Al3+ sites. Previous work has shown that CO behaves as a donor molecule when it bonds to Al3+ sites7,8 and that the vibrational frequency of ν(C-O) may be used to measure the relative Lewis acidity of the Al3+ sites.9,10 The ν(C-O) frequency of the adsorbed CO molecule is observed to shift up by about 50 cm-1 from its gas-phase value when bound to a Lewis acid Al3+ site. This is thought to be due to the electric-field effect on the CO molecule (Stark effect). II. Experimental Methods The experimental methods used in this work are described in detail in ref 6a. The ultrahigh vacuum system for IR measurement is designed such that the IR beam passes through a tungsten grid into which a powdered Al2O3 sample is pressed. The temperature of the grid is controlled by cryogenic cooling and resistive heating. The cell is maintained at a base pressure
