New Insights into the Mechanisms and Active Site Requirements of Low Temperature NOx SCR with Ammonia on Cu-SSZ-13 Zeolites
Rajamani Gounder, Purdue University
We have been collaborating with researcher groups at Purdue (F. H. Ribeiro, W. N. Delgass, J. T. Miller) and Notre Dame (W. F. Schneider) to develop a molecular-level understanding of the active sites and mechanisms for low temperature (473 K) NOx selective catalytic reduction (SCR) with NH3 on Cu-SSZ-13 zeolites. We have developed spectroscopic and titration methods to quantify Cu2+, [CuOH]+, Cu+, and H+ sites on Cu-exchanged zeolites after oxidation and reduction treatments. The density and speciation of extraframework Cu cations depends on the framework Al density (Si/Al ratio) and the distribution of Al atoms (isolated vs. paired Al), which we can control during the synthesis of SSZ-13 zeolites at fixed Si/Al ratio. Although ex situ characterization of Cu cations in SSZ-13 show differences in structure and speciation, operando characterization shows similarities among isolated, cationic Cu species during low temperature SCR catalysis. Standard SCR turnover rates (per isolated Cu, 473 K) are similar on all Cu-SSZ-13 samples and on other Cu-zeolites (e.g., BEA, MFI) when measured in kinetic regimes that are not limited by NO oxidation, indicating that all cationic Cu species catalyze NOx SCR at rates that are insensitive to zeolite topology under these reaction conditions. Insights from experiment and theory are combined to propose plausible SCR mechanisms on different cationic Cu sites, which involve a mechanism for NO oxidation to NO2 on dimeric Cu species formed transiently during the SCR redox cycle.