Adsorption of NO on Pd-Exchanged Mordenite: Ab Initio DFT Modeling

• Published in: Journal of physical chemistry. C 2008-08, Vol.112 (32), p.12349-12362
• Main Authors: Grybos, R, Hafner, J, Benco, L, Raybaud, P
• Format: Article
• Language: English
• Subjects: C: Surfaces, Interfaces, Catalysis
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp8009723

Pd-exchanged mordenite (Pd-MOR) is considered as a promising catalyst for the selective catalytic reduction of nitrogen oxides (NO x ) by hydrocarbons. Previously (J. Phys. Chem. C 2007, 111, 6454), we have investigated the structure of the active sites in Pd-MOR (Si/Al = 11) using periodic ab initio density functional calculations. In this Article, we present the results of investigations of NO adsorption in Pd-MOR for a representative set of 16 different active sites. Our results show that the adsorption strength varies considerably with the structure of the active site, that is, the location of the divalent extraframework cation and the coordination of the cation by the charge-compensating Al/Si substitution sites. Adsorption energies for NO range between 1.21 and 2.70 eV. The adsorption energies are highest for cations located at large distances from the Al sites. Strong binding of the adsorbate depends on the availability of empty palladium electronic levels just above the Fermi energy, favoring donation of electron density from the singly occupied antibonding π* molecular orbital of NO into empty d states of palladium. NO is adsorbed in a bent mode, with a Pd−N−O angle of ca. 130°. The calculated N−O stretching frequencies (corrected for anharmonicity) for the adsorption complexes are in very good agreement with the experimental position and width of the infrared adsorption band.