(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 NOx removal from automotive exhaust because of their hydrothermal stability and selectivity in reducing NOx 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 , 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  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.
- Bates, S.A., Delgass, W.N., Ribeiro, F.H., Miller, J.T., Gounder, R. J. Catal. 312, 26 (2014)
- 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)