NO Chemisorption on Cu/SSZ-13: A Comparative Study from Infrared Spectroscopy and DFT Calculations

• Published in: ACS catalysis Vol. 4; no. 11; pp. 4093 - 4105
• Main Authors: Zhang, Renqin, McEwen, Jean-Sabin, Kollár, Márton, Gao, Feng, Wang, Yilin, Szanyi, János, Peden, Charles H.F.
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 07-11-2014
• Subjects: Cu-Chabazite; DFT calculations; Environmental Molecular Sciences Laboratory; infrared spectroscopy; selective catalytic reduction; oxidation state; Brønsted acid site; DFT; Cu/SSZ-13; NO chemisorption; dehydration; infrared spectroscopy
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/cs500563s

The locations and energies of Cu ions in a Cu/SSZ-13 zeolite catalyst were investigated by density functional theory (DFT) calculations. For “naked” Cu2+ ions (i.e., Cu2+ ions with no ligands in their coordination spheres other than zeolite lattice oxygen atoms), the more energetically favorable sites are within a 6-membered ring. However, with the presence of various adsorbates, the energy difference between 6- and 8-membered ring locations greatly diminishes. Specifically, Cu2+ ions are substantially stabilized by −OH ligands (as [CuII(OH)]+), making the extra-framework sites in an 8-membered ring energetically more favorable than 6-membered ring sites. Under fully dehydrated high vacuum conditions with different Si/Al and Cu/Al ratios, three chemisorbed NO species coexist upon exposure of NO to Cu/SSZ-13: NO+, Cu2+–NO, and Cu+–NO. The relative signal intensities for these bands vary greatly with Si/Al ratios. The vibrational frequency of chemisorbed NO was found to be very sensitive to the location of Cu2+ ions. On the one hand, with the aid from DFT calculations, the nature for these vibrations can be assigned in detail. On the other hand, the relative intensities for various Cu2+–NO species provide a good measure of the nature of Cu2+ ions as functions of Si/Al and Cu/Al ratios and the presence of humidity. These new findings cast doubt on the generally accepted proposal that only Cu2+ ions located in 6-membered rings are catalytically active for NH3–SCR.