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Diesel engines with relatively good fuel economy are known as an effective means of reducing CO₂ emissions. It is expected that diesel engines will continue to expand as efforts to slow global warming are intensified. Diesel particulate and NOx reduction system (DPNR), which was first developed in 2003 for introduction in the Japanese and European markets, shows high purification performance which can meet more stringent regulations in the future. However, it is poisoned by sulfur components in exhaust gas derived from fuel and lubricant. We then developed the sulfur trap DPNR with a sulfur trap catalyst that traps sulfur components in the exhaust gas. High purification performance could be achieved with a small amount of platinum group metal (PGM) due to prevention of sulfur poisoning and thermal deterioration.

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.

An unprecedented phenomenon that achieves high NOx conversion was found over an NSR catalyst. This phenomenon occurs when continuous short cycle injections of hydrocarbons (HCs) are supplied at a predetermined concentration in lean conditions. Furthermore, this phenomenon has a wider range of applicability for different catalyst temperatures (up to 800 degrees Celsius) and SVs, and for extending thermal and sulfur durability than a conventional NOx storage and reduction system. This paper analyzes the reaction mechanism and concludes it to be highly active HC-deNOx by intermediates generated from adsorbed NOx over the base catalysts and HCs partially oxidized by oscillated HC injection. Subsequently, a high performance deNOx system named Di-Air (diesel NOx aftertreatment by adsorbed intermediate reductants) was demonstrated that applies this concept to high speed driving cycles.

The Diesel Particulate and NOx Reduction System (DPNR) is an effective technology as a diesel after-treatment system, which can reduce particulate matter (PM) and nitrogen oxides (NOx) simultaneously. However, it requires desulfurization control since the DPNR catalyst is poisoned by sulfur components in the exhaust gas from the fuel and lubricant. Desulfurization control causes some degree of fuel penalty and thermal deterioration of the DPNR catalyst because it requires control of rich air fuel ratio and high temperature simultaneously. In this paper, we investigated a new system with a sulfur trap catalyst which can trap sulfur components included in the exhaust gas as sulfates (Sulfur trap DPNR). In this system, desulfurization control is not performed because the sulfur poisoning of the DPNR catalyst is drastically suppressed by the sulfur trap catalyst. In the present DPNR, periodic desulfurization control is required.