(739a) Understanding the Active Sites and Mechanism for NOx Standard Selective Catalytic Reduction with Ammonia on Cu-SSZ-13

NO_x selective catalytic reduction (SCR) with ammonia over small-pore copper-exchanged chabazite zeolites (Cu-SSZ-13) is a commercially viable technology to mitigate NO_x emissions from automotive exhaust. The overall goal of this work is to understand the mechanistic details of and the active site requirements for NH₃ SCR by combining operando X-ray absorption spectroscopy (XAS) measurements with other ex situcharacterization techniques and with first principles density functional theory (DFT) calculations.

Cu-SSZ-13 was prepared to contain predominantly isolated Cu²⁺ cations (Cu/Al=0.08, Si/Al=4.5), each of which exchanges for two H⁺ sites as determined by residual H⁺ titration with NH₃ [1]. XAS spectra were collected under operando conditions using a special, x-ray transparent glassy carbon reactor while simultaneously measuring standard SCR reaction rates. X-ray absorption near edge spectroscopy (XANES) under standard SCR conditions of 300 ppm NO, 300 ppm NH₃ and 10% O₂ at 463 K showed the presence of both Cu⁺ and Cu²⁺species at steady state, reflecting the redox behavior of Cu sites during standard SCR.

The effect of each SCR reactant (NH₃, NO, NO₂, O₂)­ on the Cu oxidation state and the steady-state SCR rate was probed by independently varying its concentration in the feed mixture. In the absence of O­₂ and only with both NO and NH₃ present, the complete reduction to Cu⁺ was observed, demonstrating that the combination of NO and NH₃ is responsible for Cu reduction in the standard SCR mechanism. The generation of a proximal Brønsted acid site was observed after reduction of Cu²⁺ to Cu⁺ with NO and NH₃ [2], commensurate with the need for charge compensation of the Al sites. Co-feeding 120 ppm NO₂ while maintaining a constant total NO_x concentration (300 ppm) resulted in a higher steady state fraction of Cu²⁺ than under standard SCR conditions, reflecting the stronger oxidizing nature of NO₂ compared to O₂. This suggests the involvement of NO oxidation in standard SCR to oxidize Cu⁺ to Cu²⁺ through a nitrite, which reacts with NH₄⁺ to form N₂ and H₂O completing the cycle [2]. These data show that standard SCR can proceed via a redox mechanism on isolated Cu²⁺ ions. The combination of NO and NH₃ reduces Cu²⁺ to Cu¹⁺ forming an in situ Brønsted acid site, while NO oxidation is involved in the regeneration of Cu²⁺to complete the catalytic cycle.

We also compare and contrast the standard SCR cycle on Cu²⁺ sites to that on [CuOH]⁺ sites. This study illustrates that ex situ characterization of Cu cations in SSZ-13 show differences in structure and speciation, but operando characterization show similarities among isolated Cu species during standard SCR catalysis.

1. Bates, S.A., Delgass, W.N., Ribeiro, F.H., Miller, J.T., Gounder, R. J. Catal. 312, 26 (2014)
2. Paolucci, C., Verma, A.A., Bates, S.A., Kispersky, V.F., Miller, J.T., Gounder, R., Delgass, W.N., Ribeiro, F.H., Schneider, W.F. Angew. Chem. 126, 12022 (2014)