Temperature dependence of Cu(I) oxidation and Cu(II) reduction kinetics in the selective catalytic reduction of NOx with NH3 on Cu-chabazite zeolites

The selective catalytic reduction (SCR) of NO(x)( )with NH3 over Cu-exchanged chabazite (CHA) zeolites at low temperatures (<523 K) occurs via a redox mechanism in which O-2 oxidizes two NH3-solvated Cu-I sites, and NO and NH3 react together to reduce NH3 -solvated Cu-II sites. Increasing the Cu ion density in Cu-CHA zeolites increases the kinetic relevance of Cu-II reduction relative to Cu-I oxidation during SCR at fixed gas pressures, given the dual-site requirement of Cu-I oxidation but the single-site requirement of Cu-II reduction. Apparent activation energies (E-app) measured at fixed gas pressures increase with Cu ion density, at first glance suggesting that Cu-I oxidation has a lower E-app value than Cu-II reduction, assuming mean-field kinetic behavior wherein barriers are independent of Cu site density. Here, steady-state SCR rates were measured at varying O-2 pressures (1-60 kPa) and temperatures (446-501 K) to isolate E-app values for Cu-I oxidation and Cu-II reduction on Cu-CHA samples of varying Cu ion density (0.065-0.35 Cu per 10(3) A(3)), revealing instead that E-app values depend on Cu density for both Cu-I oxidation and Cu-II reduction steps. E-app values for Cu-I oxidation increase monotonically with Cu density in the full range studied, while E-app values for Cu-II reduction are invariant with Cu density except at the lowest Cu densities (0.065-0.10 Cu per 10(3) A(3)). Moreover, the small differences between E-app values for Cu-I oxidation and Cu-II reduction indicate that their kinetic relevance depends only weakly on temperature for a given Cu-CHA sample. These findings sharply contrast the conclusions from previously reported E-app data measured at fixed gas pressures and interpreted using mean-field kinetic descriptions, and illustrate the importance of measuring rate data over wide ranges of reaction conditions and sample compositions to more precisely describe the non-mean-field kinetic behavior of NH3-solvated Cu ions that become mobilized in zeolites during low-temperature SCR catalysis. (C) 2021 Elsevier Inc. All rights reserved.