Deciphering H₂-induced low-temperature NOx adsorption on Ag/Al₂O₃: Expanding the operating temperature range of ethanol-SCR system for effective NOx abatement

Despite the advantages of the catalytic reduction of NOx by hydrocarbons, typical Ag/Al₂O₃ suffers from limited low-temperature activity in response to cold-start periods. In this study, the promotional effect of H₂ on NOx adsorption on Ag/Al₂O₃ was investigated to address the low-temperature NOx emissions. H₂ significantly enhanced NOx accumulation on Ag/Al₂O₃ below 200 °C, with desorption occurring at high temperatures. The H₂-temperature-programmed reduction (H₂-TPR) results showed the formation of silver oxide on metallic Ag species by H₂+O₂, serving as the primary site for H₂-induced NOx adsorption, while the single-atom Ag species (Ag⁺) remained nearly unchanged. Ag/Al₂O₃ with low Ag loading (1–2 wt%) exhibited lower NOx adsorption capacities compared to the case with high Ag loading (4 wt%) owing to the low metallic Ag content. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies identified diverse surface-adsorbed species related to H₂-induced NOx adsorption. Nitrite, bridging nitrate, and bidentate nitrate contributed to low-temperature NOx desorption, whereas monodentate nitrate contributed to high-temperature desorption. Finally, we demonstrated exceptionally-high NOx reduction activity by synergistically combining the NOx trapping ability of Ag/Al₂O₃ with its deNOx capability through ethanol with H₂, covering a wide temperature range of 100–350 °C under transient conditions at a high space velocity of 100,000 h⁻¹.