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

摘要

The NO_x-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 (NH_3-SCR) mechanism and accounted for Cu redox chemistry and reactions at Bronsted acid sites. The reduction of NO_x to N_2 (De-NO_x) 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 NO_x reduction at Cu sites below 200°C was determined by the rate of Cu oxidation. Conversely, the rate of NO_x reduction above 300°C was determined by the rate of NH_3 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 Bronsted acid sites, experiments were also performed using a H-chabazite catalyst that lacks Cu sites. NO_x reduction via the NO_2-NH_3 reaction was found to occur at Bronsted acid sites at high temperatures (up to 600°C). We also analyzed the chabazite catalyst’s activity towards NH_3 oxidation, which significantly affects NO_x reduction at high temperatures. Cu sites were required for NH_3 oxidation; NH_3 was not oxidized in their absence. However, the formation of the by-product NO increased as the content of Bronsted acid sites in the Cu-chabazite catalyst decreased. It was therefore suggested that Bronsted acid sites contribute to the reduction of NO formed during NH_3 oxidation. Numerical studies were conducted to develop an SCR reaction model that incorporates these processes. The resulting model accurately predicted the outcomes of NO_x reduction experiments under diverse conditions including some involving transient temperature changes.