Abstract
The mobility of NH3-solvated Cu ions within the zeolite framework has been recently identified as a key factor for the kinetics of the selective catalytic reduction of NOx with NH3 (NH3-SCR) over Cu-zeolite catalysts at low temperatures. Here, we utilize in situ impedance spectroscopy to explore the mobility of NH3-solvated CuII ions, i.e., CuII(NH3)n, in Cu-SSZ-13 and Cu-ZSM-5 zeolites with varied Cu ion exchange levels, and observed that both the zeolite framework (CHA or MFI) and the Cu exchange level influence the high-frequency dielectric relaxation processes that are associated with the short-range (local) motion of CuII(NH3)n. Our results suggest that the local motion of CuII(NH3)n species is favored within the CHA framework due to the unique cage structure, and thereby contribute to the overall ion conductivity at high frequencies, which, on the contrary, is not observed for ZSM-5, where NH3-solvated Cu2+ ions do not experience a comparable constrained space for local motion. This study sheds new light on the mobility of Cu active sites under NH3-SCR related reaction conditions and may contribute to an advanced understanding of the underlying mechanism.
Highlights
Introduction“Three-way” catalysts, being successfully employed for NOx abatement in gasoline-powered vehicles, are not sufficiently effective in the O2 -rich exhaust environment typically in “lean-burn” engines (e.g., diesel engines)
Nitric oxide and nitrogen dioxide (NO and NO2, commonly indicated as “NOx ”) are harmful atmospheric pollutants and are byproducts of anthropogenic high-temperature combustions [1,2].“Three-way” catalysts, being successfully employed for NOx abatement in gasoline-powered vehicles, are not sufficiently effective in the O2 -rich exhaust environment typically in “lean-burn” engines
The Cu-exchanged zeolites will be hereon named as “Cu(x)-SSZ-13” and “Cu(x)-ZSM-5”, respectively, where x refers to the Cu/Al ratio determined by inductively coupled plasma optical-emission spectroscopy (ICP-OES) (Table 1)
Summary
“Three-way” catalysts, being successfully employed for NOx abatement in gasoline-powered vehicles, are not sufficiently effective in the O2 -rich exhaust environment typically in “lean-burn” engines (e.g., diesel engines). Cu-zeolites show outstanding low-temperature performance and low sensitivity to NO2 concentration in the exhaust flow [4,5] Due to their better performance (in terms of both activity and hydrothermal stability) when compared to large- and medium-pore zeolite materials, Cu-exchanged small-pore chabazite (CHA) zeolites (e.g., Cu-SSZ-13 and Cu-SAPO-34) are the most common choice for NH3 -SCR [2,3,4,5,6,8,9].
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