(351c) Spectroscopic and Kinetic Study of Isolated Copper Cations in SSZ-13 Used for NOx Standard Selective Catalytic Reduction (SCR) with Ammonia

Small-pore copper chabazite (CHA) zeolites are promising catalysts for NO_x removal from automotive exhaust because of their hydrothermal stability and selectivity in reducing NO_x with NH₃ under oxygen-rich conditions. This study aims to identify the different copper species in SSZ-13 as a function of Si/Al and Cu/Al ratio using X-ray absorption spectroscopy (XAS), UV-Visible (UV-Vis) spectroscopy, NH₃ titrations of residual H⁺sites [1], and density functional theory (DFT) calculations.

Copper was exchanged onto SSZ-13 samples with Si/Al ratios of 4.5, 15 and 25 by liquid-phase ion exchange of Cu(NO₃)₂ precursors. Theoretical simulations of aluminum siting in SSZ-13, assuming a random Al distribution that obeys Lowenstein’s rule, indicate that the fractions of Al present as paired Al (2 Al) in a double 6-membered ring (D6R) are 0.22, 0.09 and 0.05 for Si/Al ratios of 4.5, 15 and 25 respectively. DFT calculations indicate that Cu preferentially exchanges as isolated Cu²⁺ cations at paired Al atoms in D6R locations of SSZ-13, and then as isolated [CuOH]⁺ species at single Al atoms. NH₃ titrations of residual H⁺ sites [1] were also consistent with the predominant exchange of Cu²⁺ on the Cu/Al=0.08 and Si/Al=4.5 sample, and of [CuOH]⁺on the Cu/Al=0.41 and Si/Al=25 sample.

Under ambient conditions, however, X-ray absorption near edge structure (XANES) spectra indicate the presence of hydrated Cu²⁺ species irrespective of Si/Al or Cu/Al ratio (Cu/Al <0.5), while extended x-ray absorption fine structure (EXAFS) data indicate a coordination number of ~4 and undetectable signatures of copper coordination with the zeolite. Exposure of these samples to different gas treatments, both oxidizing and reducing, enabled differentiating between Cu²⁺ and [CuOH]⁺ species.  XANES and EXAFS showed significantly greater extent of reduction to Cu⁺ on samples with predominantly [CuOH]⁺ species compared to those with predominantly isolated Cu²⁺ ions, demonstrating the easier reducibility of [CuOH]⁺species.

Standard SCR turnover rates (per Cu, 473 K) were similar on all SSZ-13 samples when measured in equivalent kinetic regimes and also similar to those on other copper exchanged zeolites (e.g., BEA, MFI), indicating that both Cu²⁺ and [CuOH]⁺ species are able to catalyze standard SCR at similar rates.

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)