Novel NO Trapping Catalysts Derived from Co−Mg/X−Al (X = Fe, Mn, Zr, La) Hydrotalcite-like Compounds

Co2.5Mg0.5/Al1 and Co2.5Mg0.5/X0.5Al0.5 hydrotalcite-like compounds (where X = Fe, Mn, Zr, La) were synthesized by a constant-pH coprecipitation. The derived oxides from hydrotalcites upon calcination at 800 °C for 4 h in static air are mainly of spinel phase, with a surface area of 14.2−23.8 m2/g, where new phase ZrO2 and La2O3 are segregated in Zr- and La-containing oxides, respectively. Incorporation of the fourth element has assisted the reduction of transition-metal cations in the oxide catalysts, which may lead to the enhancement of the NO storage capacity in O2 at 100 °C for all catalysts. However, at 300 °C, only Zr- and La-containing catalysts improve the NO storage performance. Substantially, La-containing catalyst excels over all other catalysts in NO storage capability both at 100 and 300 °C. More remarkably, the NO storage at 300 °C (7.56 mg/g) is much higher than that at 100 °C (4.69 mg/g). NO adsorption/desorption routes have been proposed to explain the NO storage, the NO-to-NO2 conversion, and the reduction (decomposition) of NO to N2O/N2 in O2 on the catalysts. In addition, the negative influences of CO2 or H2O on the NO storage/reduction have been further revealed in this research.