NOx selective catalytic reduction (SCR) with ammonia over small-pore copper-exchanged chabazite zeolites (Cu-SSZ-13) is a commercially viable technology to mitigate NOx 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 . 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 , commensurate with the need for charge compensation of the Al sites. Co-feeding 120 ppm NO2 while maintaining a constant total NOx 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 . 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.
- 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)