Development of Clean Diesel NOx After-treatment System with Sulfur Trap Catalyst

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. However, this required the exchange of the sulfur trap catalyst every 40,000 km to maintain a high NOx conversion efficiency, because the sulfur trap catalyst did not have enough sulfur trap capacity. In this paper, we investigated the sulfur trap mechanism, and it was found that the formation of potassium sulfate on the surface of the sulfur trap catalyst during the sulfur trap process inhibited gas diffusion into the surface of the catalyst. We then tried to improve this condition using an oxygen storage material, which could trap sulfur through adsorption without the formation of sulfates. Moreover, the sulfur trap capacity was greatly increased by controlling desorbed SO₂ and transporting it to the sulfur trap material by increasing the temperature. As a result, a system maintaining high NOx conversion efficiency of 80% after 80,000 km mileage accumulation was obtained with our new sulfur trap catalyst concept.