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.