A novel 3D oxide nanosheet array catalyst derived from hierarchical structured array-like CoMgAl-LDH/graphene nanohybrid for highly efficient NOx capture and catalytic soot combustion

Abstract

A newly designed 3D oxide nanosheet array catalyst, CoMgAlO-array, with small-sized active Co₃O₄ species (5.7 nm) highly dispersed on a Mg/Al-oxide matrix was obtained by calcining the hierarchical structured array-like CoMgAl-layered double hydroxide (LDH)/graphene hybrid prepared by a modified coprecipitation method. The superior redox property of CoMgAlO-array contributes to its much higher NO_x storage capacity (NSC) and catalytic soot combustion activity than CoMgAlO. For NO_x storage, the highly dispersed Co₃O₄ phases of CoMgAlO-array efficiently facilitate the adsorption of gaseous NO and then oxidation to chelating bidentate nitrate and bridging bidentate nitrate as the major adsorbed species at 300 °C with NSC of 8.8 mg g⁻¹, while at 100 °C (NSC: 10.4 mg g⁻¹), despite the larger amounts of nitrites and nitrates formed, CoMgAlO-array with higher oxidation ability can rapidly convert initially formed bridging bidentate nitrite to the much more stable monodentate nitrate. More remarkably, the formed nitrates over CoMgAlO-array can be rapidly reduced within 1 min in 0.7% H₂/N₂ at 300 °C and the catalyst exhibits excellently recyclable NO_x storage/reduction abilities. For soot combustion, on one hand, CoMgAlO-array possesses stronger NO oxidation ability due to the small-sized Co₃O₄ phase; on the other hand, the unique hierarchical structure of the catalyst with larger external surface area can provide much more contact sites with gaseous NO and solid soot, thus greatly improving the catalytic activity with lower characteristic temperature (T_m) for maximal soot conversion and activation energy than CoMgAlO in both tight contact and loose contact modes. The NO_x storage/soot combustion mechanism and the function of Co in 3D oxide nanosheet array catalyst are proposed and discussed on the basis of these observations.