Research On Four-way Diesel Exhaust Catalyst

In recent years, the diesel vehicles have achieved a growing share of the light-duty vehicle market due to their high efficiency and low operating costs. However, the emission of their pollutants have caused severe environmental and health problems and cannot meet the demands of the more and more stringent legislation. The four main pollutants from diesel vehicles, including soot particulates, nitrogen oxides (NOx), carbon monoxide (CO) and unburned hydrocarbonates (HC), can be simultaneously controlled using aftertreatment catalytic technologies, i.e. four-way catalysis (FWC).On the base of the concept of DPNR (Diesel Particulate NOx Reduction system) proposed by Toyota Corporation, a new four-way catalyst for diesel exhaust was investigated and developed, in which both technologies of catalytic soot combustion and NOx storage and reduction (NSR) were coupled. For the catalyst, alkaline metal K and noble metal Pd were employed as the catalytic components, which were supported by the Mg-Al hydrotalcite-based mixed oxide (MgAlO). The K plays the roles of catalyzing soot combustion and storing NOx, while the noble metal acts as the roles of catalyzing reduction of NOx and oxidation of CO and HC. In the experiments of simulated diesel emission and the bench test of diesel engine, the four pollutants can be reduced under the roles of the catalyst. Furthermore, the catalyst and the catalytic reaction were characterized in details, and thus the reaction mechanism of soot combustion and NOx storage and reduction were expolored.The main works and findings are as following.(1) The K supported MgAlO (K/MgAlO) were obtained and characterized by several techniques. The results show that K was highly dispersed on the surface of MgAlO when the loading amount is below8wt.%. New Lewis basic sites were formed through the interaction between K and MgAlO. Amongst, Mg(Al)-O-K species with the weak basicity were converted from Mg(Al)-OH by the substitution of the proton. While the Mg-O-K species were obtained by the combination with the strongly basic O2-sites. When the loading of K was between5wt.%and8wt.%, the quasi free KOx species, which interact weakly with the support and show stronger basicity than Mg-O-K species, were formed. The increase in the basicity for K/MgAlO can be attributed to the charge transfer from K to the surface oxygen anions, which increased the negative charge of the strongly basic sites.(2) The catalytic activity of K/MgAlO for soot combustion with O2was tested. It was found that presence of K significantly improved soot combustion and depressed the sensitivity to the contact between soot and catalyst. The optimum amount of potassium was below8 wt.%of the supporting amount. Furthermore, a carbon-oxygen complex, ketene group, was observed as a reaction intermediate of soot combustion using ex situ IR. Combined with other characterization, an oxygen spillover mechanism for soot combustion with O2on K supported samples was determined. First, the surface-activated oxygen on K sites spill over to free carbon sites on soot to form the ketene group, which combined with another active oxygen species to give out CO2. Thus, more amount of free carbon sites were exposed, resulting in the depletion of soot. The byproduction CO came from the direct reaction of free carbon sites and gas phase O2. The spillover oxygen may have acted as the spreading of catalysts, which ameliorated loose contact activity. Additionally, the high repeated activity of K/MgAlO was found. This is because the stability of K is greatly improved through the interaction with Al.(3) The catalytic activity of K/MgAlO for soot combustion with NO+O2were also tested. The presence of K improved both soot combustion and NOX reduction. The presence of NOx in O2favors the soot combustion at lower temperature (<300℃). However, a little suppression was observed at higher temperature (>300℃), which was accompanied by a substantial NOx reduction. The reaction intermediates, the ketene group and the isocyanate ions, were observed using the in situ IR technique and thus the reaction mechanism was determined. In the combustion with NO+O2, in addition to the oxygen spillover mechanism mentioned above, two other pathways exist. i) The nitrite route:the NO first combines with surface oxygen on K sites forming nitrites. Then, the nitrites interact with the free carbon sites on soot to produce the ketene group. Finally, the ketene group is further oxidized to CO2by adjacent nitrites, regenerating NO at lower temperatures and/or producing N2at higher temperatures. ii) The NO2route:the NO2forming from NO oxidation directly reacts with the free carbon sites producing the ketene group and isocyanate ion. The latter is further oxidized into N2and CO2by O2or NO2. However, the reactions of NOx with soot are limited by the amount of free carbon sites, which can be provide by the oxidation of soot by O2at higher temperature. Additionally, the formation of nitrates from NOx adsorption might poison the active K sites to a certain extent.(4) The Pd and K co-supported Mg-A1hydrotalcite oxides (Pd-K/MgAlO) were prepared by impregnation method. The results of structure characterization shows that there is a intimated contact between Pd and K+, resulting in that the Pd particles were partly covered by K+and thus the Pd dispersion decreased. A strong chemical interaction exists between K+and Pd, which leads to the formation of Pd-O-K species and suppresses the redox properties of Pd. Furthermore, the interaction disperses the quasi free KOx species into the smaller particle, improving the dispersion of K. The results of activity tests for soot combustion with NO+O2show that Pd-K/MgAlO kept the activity of K/MgAlO for simultaneous catalytic removal of soot and NOx. Due to the existence of the synergism between Pd and K, furthermore, the maximum conversion of NOx reached to45%, which is superior to Pd/MgAlO or K/MgAlO. The higher conversion of NOx during soot combustion is ascribed not only to the reduction of NOx with soot but also to the decomposition of NOx.(5) The activities of Pd-K/MgAlO were tested for NOx storage and reduction. It was found that Pd-K/MgAlO behaves superior capacity for NOx storage, which was evaluated as890.4μmol/g, which is higher than those of both Pd/MgAlO and K/MgAlO. This is because the improvement on K dispersion due to the interaction between Pd and K provides more available K sites to NO storage. Accordingly to results of in situ IR, three pathways were distinguished for NOx storage. ⅰ) The nitrite route:NO is stored on surface K species in the form of nitrites, ⅱ) The nitrite-nitrate route:NO is adsorbed on surface Pd-O-K sites in the form of nitrites, which are progressively transformed into nitrates. ⅲ) The nitrate route:the thermodynamically produced NO2is directly adsorbed to form nitrates. Under the catalytic riles of Pd, the stored NOx on Pd-K/MgAlO can be reduced by H2, in which the initial reduction temperature is250℃.(6) The Pd-K/MgAlO catalyst was coated onto the ceramic honeycomb wall-flow filter and then was packed into a converter. Then, the converter was connected to a diesel engine, and the bench tests were performed to test the activity of the catalyst. Under the static condition, the four pollutants can be eliminated in different degree:ⅰ) the soot was converted by more than90%; ⅱ) the NO was partly converted by about6%when the exhaust temperature was above400℃; ⅲ) the elimination of CO occurred when the exhaust temperature was beyond300℃and the maximum conversion can be reached to95%, but the elimination tent to be suppressed by the presence of soot; iv) the elimination of HC was not significant and the conversion was within10%. In the term of NSR principle, furthermore, the lean/rich transient condition was operated. During the rich stage, the conversion of NO was about20%. In the whole process, the conversion of NO was7.0%while the conversion of CO and HC were81.9%and36.9%, respectively.