Development of Synergic Mixed Metal Oxides for the Combined Catalytic-Absorptive Removal of Nitric Oxide from Diesel Exhaust

The NO removal over oxide sorbents (K2O/Al2O3, CaO/Al2O3 and BaO/Al2O3) at a loading of up to 25 wt.% was investigated using a synthetic diesel exhaust gas containing 1,000 ppm NO, 1000 ppm iso-butane (i-C4H10), 8% O2 and N2 balance. The reactor was operated at 250 oC to 450 oC for the activity study. NO was effectively stored (removal efficiencies between 30-60 %) as nitrate under experimental conditions after surface oxidation to NO2; and the maximum removal efficiency decreased in the order of K2O/Al2O3 (55.4 %) > BaO/Al2O3 (39.0 %) > CaO/Al2O3 (37.0 %). K2O/Al2O3 had higher mobility; but loadings higher than 15 wt.% were detrimental to the morphological properties. Despite higher basicity, CaO/Al2O3 showed lower removal efficiency of NO compared to BaO(10)/Al2O3 due to low catalytic NO2 formation activity. Unlike K2O(10)/Al2O3, BaO(10)/Al2O3 showed increasing NO removal between 350 oC and 450 oC. K2O(5)BaO(5)/Al2O3 resulted in better surface area and volume leading to about 65 % of NO removal. This was attributed to synergic effects between physicochemical, catalytic and NO removal properties of the components. The high performance of K2O(5)BaO(5)/Al2O3 was successfully elucidated based on the characteristics of the sorbent.