FTIR Spectroscopy Evidence for the Basicity Induced by Adsorption
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Catalysis Letters Vol. 107, Nos. 3–4, March 2006 (Ó 2006) DOI: 10.1007/s10562-005-0008-4
FTIR spectroscopy evidence for the basicity induced by adsorption E.N. Storozheva, V.N. Sekushin, and A.A. Tsyganenko* V.A. Fock Institute of Physics, St. Petersburg University, St. Petersburg, 198504, Russia
Received 16 November 2005; accepted 24 November 2005
Basicity of silanol oxygen atom increases after adsorption of ammonia or pyridine, as revealed by FTIR spectra of adsorbed CHF3 and CO2. The same effect of ‘‘induced basicity’’ was observed for the dangling hydroxyl groups of ice surface with coadsorbed ethylene and fluoroform. Possible role of the induced acidity and basicity in surface chemistry is discussed. KEY WORDS: FTIR spectroscopy; adsorption; fluoroform; basicity.
1. Introduction Acidic and basic properties of surface sites are known to vary in the presence of adsorbed molecules. Recently, it was shown that the proton-donating ability of silanol groups of silica surface increases in the presence of SO2 or NO2, to such extent that they become capable to protonate coadsorbed ammonia, pyridine or dimethylpyridine [1]. The mechanism of this adsorption induced Brønsted acidity, includes interaction of acid with the oxygen atom of OH group according to the scheme
it and Lewis acid molecule or of H-bond with the adsorbed proton-donating molecules. Unfortunately, characterization of surface basic site strength is not an easy problem up to now. Most of Lewis acids, such as SO3, SO2, CO2, either react with oxide surface with the formation of surface sulfate, sulfite or carbonate species (bisulfates, bisulfites and bicarbonates in the case of reaction with basic OH groups) or reveal weak molecular adsorption with vibrational frequencies close to those in free molecules in the gas phase. OH, NH and even most of CH proton-donating molecules dissoHB
O
B H O
O
B H
S
Si
B
O O
O Si
H
S
O
O S
O
O
Si
Si
Scheme 1.
where B is a base. Interaction of acid with the oxygen atom of OH group leads to strengthening of the H-bond revealed in the lowering of O–H band position. For strong bases, such as ammonia or pyridine, H-bonded species with coordinately bound SO2 are not stable and proton transfer to the base takes place. The same scheme illustrates the increase of basic properties of the hydroxyl oxygen atom induced by the base. In fact, SO2, a weak acid, which at 300 K reversibly interacts with SiO2 surface, becomes chemisorbed in the presence of preadsorbed base. Thus, base interaction with the proton of OH group increases the negative charge or the basicity on oxygen atom, which could be manifested in strengthening of coordinate bond between * To whom correspondence should be addressed. E-mail: [email protected]
ciate on strong basic sites. It was not possible to find one test molecule, which is stable enough to be adsorbed always only in molecular form, and has certain vibrational mode sensitive to the interaction with basic sites to use the frequency shift as a measure of site basicity. Recently fluorof
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