Adsorption and vibrational spectroscopy of ammonia at mordenite: ab initio study

• Published in: The Journal of chemical physics 2004-06, Vol.120 (21), p.10263-10277
• Main Authors: Bucko, T, Hafner, J, Benco, L
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.1737302

The adsorption of ammonia at various active centers at the outer and inner surfaces of mordenite, involving Brønsted acid (BA) sites, terminal silanol groups, and Lewis sites has been investigated using periodic ab initio density-functional theory. It is shown that ammonia forms an ammonium ion when adsorbed at strong BA sites. The calculated adsorption energies for different BA sites vary in the interval from 111.5 to 174.7 kJ/mol depending on the local environment of the adduct. The lowest adsorption energy is found for a monodentate complex in the main channel, the highest for a tetradentate configuration in the side pocket. At weak BA sites such as terminal silanol groups or a defect with a BA site in a two-membered ring ammonia is H bonded via the N atom. Additional weak H bonds are formed between H atoms of ammonia and O atoms of neighboring terminal silanol groups. The calculated adsorption energies for such adducts range between 61.7 and 70.9 kJ/mol. The interaction of ammonia with different Lewis sites is shown to range between weak (DeltaE(ads)=17.8 kJ/mol) and very strong (DeltaE(ads)=161.7 kJ/mol), the strongest Lewis site being a tricoordinated Al atom at the outer surface. Our results are in very good agreement with the distribution of desorption energies estimated from temperature-programmed desorption (TPD) and microcalorimetry experiments, the multipeaked structure of the TPD spectra is shown to arise from strong and weak Brønsted and Lewis sites. The vibrational properties of the adsorption complexes are investigated using a force-constant approach. The stretching and bending modes of NH(4) (+) adsorbed to the zeolite are strongly influenced by the local environment. The strongest redshift is calculated for the asymmetric stretching mode involving the NH group hydrogen bonded to the bridging O atom of the BA site, the shift is largest for a monodentate and smallest for a tetradentate adsorption complex. The reduced symmetry of the adsorbate also leads to a substantial splitting of the stretching and bending modes. In agreement with experiment we show that the main vibrational feature which differentiates coordinatively bonded ammonia from a hydrogen-bonded ammonium ion is the absence of bending modes above 1630 cm(-1) and in the region between 1260 and 1600 cm(-1), and a low-frequency bending band in the range from 1130 to 1260 cm(-1). The calculated distribution of vibrational frequencies agrees very well with the measured infrared adsor