Simultaneous Catalytic Removal Of Diesel Soot And Nitrogeon Oxides Over Hydrotalcites-Derived Oxides Catalysts

The high efficiency, economy and durability of diesel engines have resulted in widely use in various power systems, trucks, buses, ship and nonroad vehicles in recent years. Meanwhile, the pollutants emitted by diesel engines have been causing severe environmental and human health problems. More and more stringent regulations have been established by many countries. Then the emission control on diesel engines has been an important research topic in energy and environmental area.Nitrogen oxides (NOx) and particulate matters (PM) are the main harmful substances. Since the reduction of NOx and PM cannot be accomplished by engine modifications alone, aftertreatment technology should be developed. A promising process to meet this demand is the simultaneous catalytic removal of NOx and soot. In this paper, mesoporous mixed oxides derived from hydrotalcites have been firstly used for NOx-soot removal under simulated diesel emission conditions. The detailed works are as follows:1. Mg-Al hydrotalcites (HT) nanometer materials have been prepared by two different methods: conventional co-precipitation (the CP method) and co-precipitation assisted by ultrasound (the US method). Mixed oxides (LDO) was derived from LDH calcined at 450℃for 6 h. The results showed that well crystallized LDH of high purity could be obtained by the US method within acceptable times. Ultrasonic treatment could also accelerate anions exchange in the interlayer space. The mixed oxide, whose precursors were prepared by 20 minutes of ultrasonic treatment, showed mesoporous structures with monomodal pore size distribution and a very small amount of micropores. Its BJH desorption pore size distribution exhibited a narrow peak with maxima at 4 nm with SBET of 168.8 m²Â·g^-1 and pore volume of 0.37 cm³Â·g^-1. The amount of micropores decreased and the pore size distribution become broader when the ultrasonic radiation time exceeds 40 minutes. The pore size distribution of LDO can be controlled by ultrasonic treatment.2. MAlO (where M = Ni²⁺, Co²⁺ and Cu²⁺) mixed oxides derived from hydrotalcites and potassium-promoted MAlO catalysts (designated as K/MAlO) have been studied for soot oxidation. The catalysts were characterized by XRD, N₂ adsorption, TPR, FT-IR and TPO techniques. The hydrotalcites calcined at 800℃have large surface areas in the range 17-88 m²/g and uniform mesoporous features, which resulted in high activity for diesel soot oxidation under the conditions of tight contact between soot and catalyst powders. Potassium increased the activity due to the improvement of surface mobility. The presence of NO_x considerably enhanced the catalytic soot oxidation rate. The enhancement was attributed to the acceleration of soot oxidation due to NO₂ as a strong oxidizing agent and intermediates of nitrate and/or nitrite species formed on the catalyst surface.3. Co-Al mixed oxides (CAO) was prepared by co-precipitation method from hydrotalcites (HT) as precursors, and their catalytic activity was investigated for the simultaneously catalytic removal of NOx and diesel soot particulates by the Temperature-programmed Reaction (TPR) technique. All HT samples present well crystallized, layered structures, no excess phases were detected. A nonstoichiometric spinel phase was formed by calcining the CAO at 500℃and 800℃, irrespective of the Co/Al ratio. Both the activity of soot oxidation and the selectivity to N₂ formation of CAO catalysts calcined at 800℃were higher than that at 500℃. The observed difference in the catalytic performance was related to the redox properties of the catalysts and the crystallite size of HT precursors. The active species might come from Co₃O₄, which acted for redox-type mechanism for soot oxidation in the NO_x-soot reaction.4.Mesoporous mixed oxides catalysts were derived from Cu substituted Mg/Al hydrotalcites at the ratio of M²⁺/M³⁺ = 3. CuO was the predominant phase in the catalyst, while traces of spinels were also detected. Cu-containing mixed oxides had large surface areas in the range 20-100 m²/g and 80% of catalyst pores were mesopores. The catalysts showed high activity for the simultaneously catalytic removal of NO_x and diesel soot particulates. Both the activity of soot oxidation and the selectivity to N₂ formation increased with the increase of Cu content or calcined temperature. Compared with pure CuO, hydrotalcite derived catalysts had high activities, which maybe related to the mesoporous structure and large suface areas. The optimal calcined temperature was 800 ℃and the appropriate Cu/Mg molar ratio was 3.0. Then 3.0Cu-800 may be a good catalyst with high activity (T_i = 260℃, S_N2/C = 4.37%, S_N2O = 16.6%).5. The apparent activation energy (E_a) for CO₂ formation and T_i value under noncatalytic soot combustion was 158.4 kJ·mol^-1 and 530℃respectively. Both E_a and T_i value decreased when simultaneous NOx-soot removal reactions took place over Cu-containing mixed oxides catalysts. Large amounts of N₂ were formed at the same time. The compensation effect between the apparent activation energy and the pre-exponentail factor were observed over Cu-containing mixed oxides.6. Effects of reaction conditions including soot content, concentrations of inlet gas, total flow rate, heating rate, contact conditions and reuse of catalyst were investigated over 3.0Cu-800 samples. Soot content, total flow rate and heating rate hardly affect the ignition temperature of soot, while NO and O₂ concentration positive affect the catalytic acitivity. The contact between catalyst and soot was a very important factor for the catalytic performance. The relatively high activity over 3.0Cu-800 catalyst under loose contact correlated with the low melting point and high partial pressure of CuO phases. The catalytic performace decreased during catalyst reuses due to the coverage of active sites by the adsorption inermediates species in the reactions.7. The kinetics analysis of non-steady TPR results for the simultaneous NOx-soot removal has been revealed to be possible when a substantial amount of the charged soot remained in the soot/catalyst mixture. Based on the above experiments, the reaction orders of CO₂ and N₂ formation in NO-O₂-soot reactions at lower temperatures were calculated by Arrhenius-type plot. The power law expressions of reaction rates of CO₂ and N₂ formation were obtained.8. The reaction mechanisms of simultaneous NOx-soot removal over 3.0Cu-800 were discussed based on the NOx adsorption-desportion behavior, catalytic experiments and the results by other researchers. In the proposed mechanisms, the effect of NOx adsoption-desportion on the reaction was considered. The coexisting of O₂ promoted the NOx adsorption. NO₂ and the adsorption species formed on the catalysts surface increased the reation rate.In conclusion, transition metal containing hydrotalcites derived catalysts showed high activity for the simultaneously catalytic removal of nitrogen oxides and diesel soot particulates. This technology can be regarded as a combined process of PM trapping, soot oxidation and NOx reduction by soot, and, if realized, this should be the most desirable aftertreatment of desel exhausts because it is capable of simultaneously removing both harmful substances.