Abstract
Research and/or Engineering Questions/Objective
Worldwide increase in number of gasoline vehicles requires extra consumption of precious metals to purify CO, HC and NOx from the automotive exhaust gas. The catalyst is mainly composed of purification sites (e.g., Pd, Rh) supported on carrier materials (e.g., alumina, Ce-Zr mixed oxides). Compared to Pd, Rh has higher NOx purification activity when supported on conventional carriers. However, the less abundance of Rh on the earth crust limits its use as an automotive catalyst to some extent. This research objective is to improve NOx purification activity of Pd using newly developed carrier material toward the development of Rh-free catalysts.
Methodology
To improve the NOx purification activity of Pd, strong NOx adsorption and high reactivity with reductants over Pd and/or carrier material must be desired. We have shown that apatite-type mixed metal oxide, which has high oxide ion conductivity and the presence of both acidic and basic sites over the surface, exhibits partial oxidation activity for HC species and the resultant oxidized species react with NOx easily. In this study, NOx desorption temperatures over Pd/apatite, Pd/alumina and Rh/zirconia were investigated using NO-TPD equipped with mass spectrometer. NO-CO reactivity over these catalysts was also investigated using FT-IR at 573K. The reactivity was defined as the starting time of the reaction between gaseous CO and adsorbed NO species over the catalysts. Vehicle test was also carried out to check the practical performance of Pd/apatite.
Results
Desorption temperatures of NO from the surface of Pd/apatite, Pd/alumina and Rh catalysts were 693K, 599K and 534K, respectively. Similar trend was observed for NOx desorption temperatures. As suggested by the NO-TPD experiments, Pd/apatite has strongest NOx adsorption property among the three catalysts. Starting time of NO-CO reaction over Pd/apatite, Pd/alumina and Rh/zirconia catalyst were 7.4 seconds, 12.7 seconds and 5.3 seconds, respectively. These results indicate that NO reduction by CO over Pd/apatite is quite faster than that over conventional Pd/alumina and is close to that over Rh/zirconia catalyst. Even in the vehicle test, Pd-only catalysts using Pd/apatite showed high purification performance toward the evaluation of the strictest emission standard in the world LEVIII SULEV30.
Limitations of this study
For the developed Pd/apatite catalyst, tolerances toward space velocity and S and/or P poisoning are not fully studied yet. The tolerance of Pd/apatite toward space velocity, S and P poisoning will be investigated in the future work.
What does the paper offer that is new in the field including in comparison to other work by the authors?
Conventionally, automotive catalyst for gasoline engine vehicle is constructed from Pd, Rh and carrier materials. Our previous work focused on development of Pd/Rh catalysts. However, in consideration of quite lower resource volume of Rh compared with Pd, we focused on the function improvement of Pd catalyst and then developed newly Pd-only automotive catalyst in this study.
Conclusions
In order to develop Rh free catalyst, using apatite-type mixed metal oxide as Pd carrier, NO-CO reactivity of Pd catalysts is improved drastically compared with conventional Pd/alumina catalyst and its performance is close to that of Rh catalysts. Pd-only catalyst using Pd/apatite has a high possibility of meeting the emission standard LEVIII SULEV30. The results of this study suggest that the NO reductivity over Pd catalyst is improved when Pd is combined with an apatite-type mixed metal oxide. Pd/ apatite is one of the most potential candidates toward development of Rh-free automotive catalysts.
KEYWORDS : Automotive after treatment; Apatite-type mixed metal oxide; Pd only catalyst