(244b) DFT Analysis of Reactivity of Isolated and Dimeric Cu Sites in SSZ-13 SCR Catalysts
Nitrogen Oxides (NOx) are pollutants generated in all combustion reactions using air, and are a source of acid rain. The reduction of NOx in an oxygen excess environment is a key challenge in meeting more stringent emission regulations. Selective Catalytic Reduction (SCR) of NOx with NH3 is possible through the usage of zeolite catalysts, such as Cu-exchanged SSZ-13. The zeolite lattice presents a variety of potential coordination sites for monomeric, dimeric, and higher aggregates of Cu, and these various species can have very distinct catalytic reactivity. In this work we use a combination of statistical models, thermodynamic models, and density functional theory calculations to quantify both the number and types of Cu sites as a functional of Si/Al and Cu/Al ratios. We contrast the activity of these sites for catalytic NO oxidation and propose an O-bridged Cu-dimer as a likely locus of this activity. In contrast, isolated Cu2+ sites appear to account for SCR actvity, and we propose an SCR mechanism on these sites that involves ammonia activiation on Cu and the cycling between reduced and oxidized Cu states through the catalytic cycle.