Tandem Reaction on Ru/Cu-CHA Catalysts for Ammonia Elimination with Enhanced Activity and Selectivity

Abstract

Ammonia emissions from vehicles and power plants cause severe environmental issues, including haze pollution and nitrogen deposition. Selective catalytic oxidation (SCO) is a promising technology for ammonia abatement, but current catalysts often struggle with insufficient activity and poor nitrogen selectivity, leading to the formation of secondary pollutants. In this study, we developed a bifunctional Ru/Cu-CHA zeolite catalyst for ammonia oxidation, incorporating both SCO sites (Ru) and selective catalytic reduction sites (SCR, Cu). Various characterizations, including HAADF-STEM, XAFS, and H₂-TPR, revealed that Cu²⁺ cations are dispersed within the CHA zeolite, while RuOx clusters and nanoparticles are present both inside and on the surface of the zeolite. Operando DRIFTS-MS, in situ Raman spectroscopy, and DFT calculations confirmed that NH₃ adsorbed on Cu²⁺ Lewis acid sites efficiently reduced RuO₂ with a lower energy barrier, significantly enhancing the low-temperature activity of the Ru/Cu-CHA catalyst. Additionally, Cu²⁺ cations further facilitated the elimination of byproducts (NOx) via the tandem SCR reaction, thus greatly improving the nitrogen selectivity. This synergistic effect contributed to high catalytic activity (>94% at 200 °C) and excellent nitrogen selectivity (>90% even at high temperatures above 325 °C) for Ru_2.5/Cu-CHA during practical ammonia elimination in the presence of NOx and water vapor.