NO oxidation: A probe reaction on Cu-SSZ-13

• DOI: 10.1016/j.jcat.2014.01.017
• Publication Date: 2014-02-26T17:17:22Z
• Authors: Anuj A. Verma; Shane A. Bates; Trunojoyo Anggara; Christopher Paolucci; Atish A. Parekh; Krishna Kamasamudram; Aleksey Yezerets; Jeffrey T. Miller; W. Nicholas Delgass; William F. Schneider; Fabio H. Ribeiro
• Keywords: 01 natural sciences; 7. Clean energy; 0104 chemical sciences

Abstract The site requirements and mechanism of dry NO oxidation were examined on a series of Cu-SSZ-13 catalysts (silicon/aluminum atomic ratio = 4.5) with Cu:total-aluminum (Cu/Al tot ) atomic ratios ranging from 0.02 to 1.6. Catalysts with Cu/Al tot atomic ratio tot atomic ratio from 0.2 up to 0.5. Hydrated Cu-SSZ-13 catalysts with Cu/Al tot atomic ratio –1 under ambient conditions that we assign to a d – d transition of an isolated, hydrated Cu 2+ ion. X-ray absorption near edge structure (XANES) measurements on the same catalysts under ambient conditions quantitatively match a [Cu(H 2 O) 6 ] 2+ reference. The 12,500 cm –1 feature intensity is constant above Cu/Al tot atomic ratio = 0.2, implying that the additional Cu ions adopt other configurations. Catalysts with Cu/Al tot atomic ratio > 0.2 also showed an increasing percentage of Cu x O y species (clustered Cu 2+ ions x ⩾ 2, y ⩾ 1) as quantified by XANES under ambient conditions. Saturation of these isolated Cu 2+ sites at Cu/Al tot atomic ratio = 0.2 is consistent with the expected number of 6-membered ring Al f pair sites available to accommodate them. The hydrated isolated Cu 2+ ions in catalysts with Cu/Al tot atomic ratio 2+ ions under NO oxidation conditions and do not contribute measurably to the rate of NO oxidation. In contrast, in situ XANES experiments show that the Cu x O y species remain present under NO oxidation conditions (300 ppm NO, 150 ppm NO 2 , and 10% O 2 , at 300 °C) and contribute linearly to the rate of NO oxidation per mole Cu (at 300 °C). We used density functional theory (DFT) calculations to compare the ability of isolated Cu ions and Cu dimers (Cu 2 O y ) species to support NO oxidation. Only the Cu dimers can accommodate adsorption and dissociation of O 2 necessary to catalyze NO oxidation. We hypothesize that activated oxygen enables NO to form NO 2 in a kinetically-relevant step. These findings reveal that dry NO oxidation (300 ppm NO, 150 ppm NO 2 , and 10% O 2 ) can be used as a probe reaction to identify clustering of Cu ions on Cu-SSZ-13.