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

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/Altot) atomic ratios ranging from 0.02 to 1.6. Catalysts with Cu/Altot atomic ratio<0.2 exhibit immeasurable NO oxidation rates (per mole Cu), while NO oxidation rates increase monotonically with Cu/Altot atomic ratio from 0.2 up to 0.5. Hydrated Cu-SSZ-13 catalysts with Cu/Altot atomic ratio<0.2 exhibit a near infrared feature at 12,500cm–1 under ambient conditions that we assign to a d–d transition of an isolated, hydrated Cu2+ ion. X-ray absorption near edge structure (XANES) measurements on the same catalysts under ambient conditions quantitatively match a [Cu(H2O)6]2+ reference. The 12,500cm–1 feature intensity is constant above Cu/Altot atomic ratio=0.2, implying that the additional Cu ions adopt other configurations. Catalysts with Cu/Altot atomic ratio>0.2 also showed an increasing percentage of CuxOy species (clustered Cu2+ ions x⩾2, y⩾1) as quantified by XANES under ambient conditions. Saturation of these isolated Cu2+ sites at Cu/Altot atomic ratio=0.2 is consistent with the expected number of 6-membered ring Alf pair sites available to accommodate them. The hydrated isolated Cu2+ ions in catalysts with Cu/Altot atomic ratio<0.2 are quantitatively converted to dehydrated isolated Cu2+ 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 CuxOy species remain present under NO oxidation conditions (300ppm NO, 150ppm NO2, and 10% O2, 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 (Cu2Oy) species to support NO oxidation. Only the Cu dimers can accommodate adsorption and dissociation of O2 necessary to catalyze NO oxidation. We hypothesize that activated oxygen enables NO to form NO2 in a kinetically-relevant step. These findings reveal that dry NO oxidation (300ppm NO, 150ppm NO2, and 10% O2) can be used as a probe reaction to identify clustering of Cu ions on Cu-SSZ-13.