Experimental and density functional theory studies on Cu/Ba-coimpregnated γ-Al₂O₃ for low-temperature NO_x storage and adsorbent regeneration

Although the lean-NO_x trap (LNT) has been used to reduce NO_x emissions from diesel-powered vehicles, LNT application is limited because its performance degrades at low temperatures (e.g., during cold starting). Therefore, to enhance low-temperature NO_x storage and adsorbent regeneration, γ-Al₂O₃ was coimpregnated with both Cu and Ba (Cu–Ba/γ-Al₂O₃). The experimentally measured NO_x storage capacities (NSCs) and NO_x storage efficiencies (NSEs) were compared. Density-functional-theory (DFT) calculations were performed to reveal the NO_x storage mechanism. The Cu/Ba-coimpregnated γ-Al₂O₃ improved both NSC and NSE of NO storage and enhanced NSE of NO₂ storage at initial stage. In addition, it desorbed NO_x at lower temperatures than the conventional Ba-impregnated γ-Al₂O₃ (Ba/γ-Al₂O₃). The in-situ diffuse reflectance infrared Fourier-transform spectroscopy analysis and DFT calculations for NO storage showed that NO adsorption was superior on the Cu-compound surfaces and that stable hyponitrite was stored on the Ba-compound surfaces. In NO₂ storage, Cu/Ba coimpregnation offered high preferential NO₂ coverage on the CuO surface and produced the most stable ionic nitrate on the Ba-compound surfaces. The experimental and theoretical results confirmed that the Cu/Ba-coimpregnated adsorbent exhibited both superior NO_x storage and adsorbent regeneration compared to the conventional Ba-containing LNT adsorbent.