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

Abstract

The selective catalytic reduction (SCR) of NOx with NH3 over Cu-exchanged chabazite (CHA) zeolites at low temperatures (<523 K) occurs via a redox mechanism in which O2 oxidizes two NH3-solvated CuI sites, and NO and NH3 react together to reduce NH3-solvated CuII sites. Increasing the Cu ion density in Cu-CHA zeolites increases the kinetic relevance of CuII reduction relative to CuI oxidation during SCR at fixed gas pressures, given the dual-site requirement of CuI oxidation but the single-site requirement of CuII reduction. Apparent activation energies (Eapp) measured at fixed gas pressures increase with Cu ion density, at first glance suggesting that CuI oxidation has a lower Eapp value than CuII reduction, assuming mean-field kinetic behavior wherein barriers are independent of Cu site density. Here, steady-state SCR rates were measured at varying O2 pressures (1–60 kPa) and temperatures (446–501 K) to isolate Eapp values for CuI oxidation and CuII reduction on Cu-CHA samples of varying Cu ion density (0.065–0.35 Cu per 103 Å3), revealing instead that Eapp values depend on Cu density for both CuI oxidation and CuII reduction steps. Eapp values for CuI oxidation increase monotonically with Cu density in the full range studied, while Eapp values for CuII reduction are invariant with Cu density except at the lowest Cu densities (0.065–0.10 Cu per 103 Å3). Moreover, the small differences between Eapp values for CuI oxidation and CuII 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 Eapp 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.