Computational and Fourier Transform Infrared Spectroscopic Studies on Carbon Monoxide Adsorption on the Zeolites Na-ZSM-5 and K-ZSM-5: Evidence of Dual-Cation Sites

详细信息    查看全文

• 作者：C. Otero Are&aacute ; n ; M. Rodrí ; guez Delgado ; K. Frolich ; R. Bul&aacute ; nek ; A. Pulido ; G. Fiol Bibiloni ; P. Nachtigall
• 刊名：Journal of Physical Chemistry C
• 出版年：2008
• 出版时间：March 27, 2008
• 年：2008
• 卷：112
• 期：12
• 页码：4658 - 4666
• 全文大小：368K
• 年卷期：v.112,no.12(March 27, 2008)
• ISSN：1932-7455

文摘

A combination of variable-temperature Fourier transform infrared (FTIR) spectroscopy with calculationsperformed at the periodic density functional theory (DFT) level was used in the investigation of carbonmonoxide adsorption on zeolites Na-ZSM-5 and K-ZSM-5. On the basis of a very good agreement betweenexperimental and calculated frequencies and adsorption enthalpies, it is shown that the IR absorption bandappearing in the intermediate frequency range for adsorbed CO (2155 and 2150 cm^-1 for Na-ZSM-5 andK-ZSM-5, respectively) is due to the formation of linearly bridged CO adsorption complexes on dual-cationsites (M⁺···CO···M⁺, M = Na, K). The population of such adsorption complexes increases with increasingcation radius and with decreasing Si/Al ratio. Bridged adsorption complexes are slightly more stable thancarbonyl complexes formed on isolated extraframework metal cations. Adsorption enthalpies and CO stretchingfrequencies of carbonyl complexes formed on isolated extraframework metal cations were found to dependon the metal cation coordination with the zeolite framework. This dependence is particularly apparent forNa-ZSM-5, where cations located on the intersection sites are coordinated to only two framework oxygenatoms and CO adsorption on these sites is up to 8 kJ/mol more stable than adsorption on the channel wallsites; CO stretching frequencies of carbonyls formed on intersection sites are up to 7 cm^-1 higher thanfrequencies of carbonyls formed on channel wall sites.