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The details of Di-Air, a new NOx reduction system using continuous short pulse injections of hydrocarbons (HC) in front of a NOx storage and reduction (NSR) catalyst, have already been reported. This paper describes further studies into the deNOx mechanism, mainly from the standpoint of the contribution of HC and intermediates. In the process of a preliminary survey regarding HC oxidation behavior at the moment of injection, it was found that HC have unique advantages as a reductant. The addition of HC lead to the reduction or metallization of platinum group metals (PGM) while keeping the overall gas atmosphere in a lean state due to adsorbed HC. This causes local O₂ inhibition and generates reductive intermediate species such as R-NCO. Therefore, the specific benefits of HC were analyzed from the viewpoints of 1) the impact on the PGM state, 2) the characterization of intermediate species, and 3) Di-Air performance compared to other reductants.

HEV and PHEV require an improved aftertreatment system to clean the exhaust gas in various driving situations. The efficiency of aftertreatment system is significantly influenced by the residence time of the gas in a catalyst which gas flow has generally strong pulsation. Simulation showed up to 70% reduction of exhaust gas emission if the pulsation could be completely attenuated. A new concept exhaust manifold was designed to minimize pulsation flow by wall impingement, with slight increase of pressure loss. Experimental results with new concept exhaust manifold showed exhaust gas emission were reduced 16% at cold condition and 40% at high-load condition.

Diesel particulate and NOx reduction system (DPNR) is an effective technology for the diesel after-treatment system, which can reduce particulate matter (PM) and nitrogen oxides (NOx) simultaneously. The DPNR has been developed under the Toyota D-CAT (Diesel Clean Advanced Technology) concept. Further improvement of the DPNR is hoped for cleaner air in the future. This paper reviews the results of our study to improve the NOx purification performance on the DPNR. The NOx reduction performance of the catalysts deteriorates due to thermal deterioration and sulfur poisoning. In order to improve the thermal resistance of the catalysts, the suppression of precious metal sintering in the catalyst has been studied. As a result, higher catalytic activity after aging especially under lower temperature conditions was obtained. On the other hands, improvement of desulfurization performance is one of the key technologies in order to keep the high NOx reduction capability of the catalyst.

In this study, instead of investigating NOx storage reaction improvements, the NOx adsorption phenomenon was focused on and analyzed to improve NOx trapping performance at lower temperatures. As a NOx adsorbing material, "Ag" was expected to enhance NOx adsorption and reduce the sulfur regeneration temperature due to the abundance of adsorbed oxygen and moderate basicity. However, when using this material in an actual system, we had to reduce the sulfur regeneration temperature, increase NOx adsorption capacity and improve NOx desorption further. Addition of TiO₂, working as an acidic material, was found to decrease sulfur regeneration temperature. Additionally, it increased the NOx adsorption capacity through improved Ag dispersion which plays an important role in NOx adsorbing. Consequently, a greater NOx trapping performance than NSR catalyst was achieved at lower temperatures.

One primary result of the reduction of platinum group metals (PGM) within a catalytic converter is the decline in NOx conversion efficiency. This paper hypothesizes that the primary factor of this decline to be hydrocarbon (HC) poisoning. To maintain high NOx conversion efficiency as the PGM reduces, Rh activation improvement becomes significant to overcome the HC poisoning. Analysis of the Rh deterioration mechanism found that it is effective to separately arrange Rh and CeO2 on the converter, avoiding the Rh deactivation. By this improvement, we improved the catalyst activity at less than 25% of the original Rh loading.