PNNL

Abstract

The catalytic activities of small-pore Cu-CHA and large-pore Cu-BEA catalysts for the selective catalytic reduction of NO with NH3 were measured at a very high flow rate. Cu-CHA clearly exhibited much higher intrinsic SCR activity and lower N2O formation selectivity. In situ DRIFT spectra were recorded during the adsorption and desorption NO and (NO + O2) on fully oxidized samples in a flow cell. The results were in agreement with what we have reported previously based on in situ transmission IR studies on partially reduced samples. Both suggest that different SCR reaction pathways might exist on these two catalysts and that NO+ could be an important reaction intermediate on Cu-CHA. A detailed IR study with various isotopically labeled gas mixtures of (NO + O2), (15NO + O2), (NO + 18O2) and (15N18O + O2) was conducted to understand the origin of the surface adsorption complexes on Cu-CHA. Formation of NO+ was not the consequence of a simple charge transfer NO + Cu2+ = NO+ + Cu+. Instead, O2 was found to be essential in changing the oxidation state of N from +2 to +3 although it did not participate in new N-O bond formation. The majority of the adsorbed NO+ maintained its origin of the feed gas. HYC is grateful to Johnson Matthey for the support of this collaborative work, and to PNNL for an Alternate Sponsored Fellowship. The authors at PNNL gratefully acknowledge the US Department of Energy (DOE), Energy Efficiency and Renewable Energy, Vehicle Technologies Office for the support of this work. The research described in this paper was performed at the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the US DOE by Battelle.