Analysis and Modeling of NOx Reduction Based on the Reactivity of Cu Active Sites and Brønsted Acid Sites in a Cu-Chabazite SCR Catalyst

The NOx-reducing activity of a Cu-chabazite selective catalytic reduction (SCR) catalyst was analyzed over a wide temperature range. The analysis was based on the ammonia SCR (NH₃-SCR) mechanism and accounted for Cu redox chemistry and reactions at Brønsted acid sites. The reduction of NOx to N₂ (De-NOx) at Cu sites was found to proceed via different paths at low and high temperatures. Consequently, the rate-limiting step of the SCR reaction at Cu sites varied with the temperature. The rate of NOx reduction at Cu sites below 200°C was determined by the rate of Cu oxidation. Conversely, the rate of NOx reduction above 300°C was determined by the rate of NH₃ adsorption on Cu sites. Moreover, the redox state of the active Cu sites differed at low and high temperatures. To clarify the role of the chabazite Brønsted acid sites, experiments were also performed using a H-chabazite catalyst that lacks Cu sites. NOx reduction via the NO₂-NH₃ reaction was found to occur at Brønsted acid sites at high temperatures (up to 600°C). We also analyzed the chabazite catalyst's activity towards NH₃ oxidation, which significantly affects NOx reduction at high temperatures. Cu sites were required for NH₃ oxidation; NH₃ was not oxidized in their absence. However, the formation of the by-product NO increased as the content of Brønsted acid sites in the Cu-chabazite catalyst decreased. It was therefore suggested that Brønsted acid sites contribute to the reduction of NO formed during NH₃ oxidation. Numerical studies were conducted to develop an SCR reaction model that incorporates these processes. The resulting model accurately predicted the outcomes of NOx reduction experiments under diverse conditions including some involving transient temperature changes.