S Tolerant Pt-Pd Based Diesel Oxidation Catalyst for LDD Euro 4 Application 2012-28-0009

This paper describes the development results of S tolerant Pt-Pd-based Diesel Oxidation Catalyst (DOC) which can be applied for passive DOC application, targeting Euro 4 and India BS4 emission standards with a view of the fact that in India the sulfur content is different in the 13 main cities compared to rest of the country.

In order to develop a cost-effective DOC to meet Euro 4 and India BS4 legislation, Pt-Pd-based DOC was studied.

Firstly, the effect of Pd used together with Pt in catalytic oxidation performance was studied. DOCs having different Pt to Pd ratios were evaluated in the engine exhaust. The results revealed that CO (Carbon Monoxides) and HC (Hydrocarbons) oxidation activity over Pt-Pd DOC were significantly improved as compared to Pt-only DOCs. It was also revealed that there is an optimum Pt to Pd ratio to give the best light-off performance under conditions tested. Advantage of Pd use with Pt was also confirmed in terms of thermal stability. The Pt-Pd DOC was able to suppress more of PGM (Platinum Group Metals) crystallite size growth as compared with Pt-only DOC at higher temperature aging. However, higher Pd ratio in DOC with conventional washcoat technology resulted in more rapid deactivation in CO and HC oxidation activity than a Pt-only DOC, in a steady state activity test conducted in the presence of SO₂.

To improve S tolerance over Pt-Pd DOC, effect of washcoat technology was studied considering thermal stability and SO₂ adsorption/desorption properties. In laboratory TPD (Temperature Programmed Desorption) testing, it was found that one of the newly developed washcoat technologies significantly improved SO₂ desorption property. On this new technology, adsorbed SO₂ was desorbed at lower temperature than commercially available conventional washcoat technology. Residual SO₂ in catalyst bricks after S poisoning test was carried out with XRF (X-ray Fluorescence Analysis) analysis to confirm residual S in the catalyst after sulfation. The result revealed that S content in newly developed washcoat technology was the lowest among the washcoats tested. These results indicated that newly developed washcoat technology has significantly different SO₂ adsorption and desorption properties. It suppresses SO₂ adsorption and enhances desorption of SO₂ adsorbed.

Catalytic activity of new Pt-Pd DOC was evaluated and it was confirmed that the new DOC significantly improved S tolerance because it maintained stable CO and HC oxidation activity in a steady state test conducted in the presence of SO₂. In addition, it was demonstrated that its catalytic activity can be recovered from sulfur poisoning when it was exposed to high temperature condition in the engine exhaust.