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

Abstract

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.