Influence of Cu Active Centers in Cu-SSZ-13 on N2O formation during NH3-SCR
Arthur Shih, Purdue University / Leiden University
Arthur J. Shih1,2,, Juan M. González1,3, Ishant Khurana1, Lucía Pérez Ramírez1, Andres Peña L.1, Ashok Kumar4, Aída Luz Villa3
1 Davidson School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907, USA
2 Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, 2300 RA Leiden, The Netherlands
3 Environmental Catalysis Research Group, Chemical Engineering Department, Engineering Faculty, Universidad de Antioquia, Calle 70, No. 52-21, Medellín, Colombia
4 Cummins Inc., 1900 McKinley Avenue, MC 50183, Columbus, IN 47201, USA
N2O emissions are regulated due to their estimated global warming potential ~300 times that of CO2 and their role in the depletion of stratospheric ozone. The concentration of N2O in the atmosphere has historically hovered around 270 ppb, however in the last 200 years it has increased by ~20% to 331 ppb . In heavy-duty diesel engine systems, N2O is produced primarily from the combustion of fossil fuels and from aftertreatment processes, in particular during the NH3 selective catalytic reduction over Cu-SSZ-13. We synthesized three model Cu-SSZ-13 catalysts with primarily ZCuOH, Z2Cu, and extraframework CuxOy species (where Z represents an anionic site on the zeolite framework) then measured their N2O formation rates during standard SCR [2,3]. We first present evidence that the formation of extraframework CuxOy species after sequential aqueous ion exchange and calcination correlates with the formation of Cu(OH)2 precipitates during ion exchange. These CuxOy species are not active for standard SCR, and unchanged apparent activation energies and reaction orders demonstrate that these CuxOy species do not induce transport limitations to accessible Cu2+ active centers.
During low-temperature standard SCR, N2O formation rates on a per Cu basis were fastest (and exhibited higher selectivities) on ZCuOH, followed by Z2Cu, then extraframework CuxOy. Because N2O formation apparent activation energies were indistinguishable from the standard SCR apparent activation energies associated with the reduction-limited step, we posit that the origin of N2O formation stems from the standard SCR reduction step. Additionally, using sulfur poisons to force the rate-limiting step to the oxidation half-cycle  resulted in an unchanged N2O formation apparent activation energy, further supporting our hypothesis. These N2O formation kinetics coupled with atomic balances and recent literature suggest that during low-temperature SCR, N2O is formed not from the decomposition of ammonium nitrate, but possibly through an ammonium nitrite intermediate during the reduction half-cycle [5-7]. These results suggest that utilizing Cu-SSZ-13 catalysts with higher fractions of Z2Cu active centers in commercial aftertreatment systems can lead to reduced N2O emissions.
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