The catalysts considered in this study were mixed-oxides materials; the main component, mullite, was mixed with oxides of other elements, such as samarium (Sm), gadolinium (Gd), cerium (Ce) and strontium (Sr). Three materials were prepared; their formulas were SmMn2O5, Mn7CeSmSrO14.83 and GdSrCeMn7O14.83.
To test their performance the oxidation of nitrogen oxide was monitored in an atmosphere of 10 % O2, and for a temperature range between 50 and 350 oC. The performance of the potential catalysts was compared with a standard Pt catalyst.
High Catalytic Activity
Professor Kyeongjaie Cho, one of the researchers involved in the project, shared the following comments on the results with Decoded Science:
“All three mixed oxides tested showed an activity higher than that of the platinum; Mn7CeSmSrO14.83 and GdSrCeMn7O14.83, in particular, were the most active catalysts; they reached the peak of their activity just above 300 oC, with a conversion about 45% higher than that of Pt-based catalysts.
These results are very important, as they show us that it is possible to replace precious metals such as platinum with different, less precious oxides.”
Importance of the Catalyst Structure
In their study, professor Cho and his coworkers also performed some theoretical calculations, to try to establish a correlation between the reaction mechanisms and the structure of the employed materials. Professor Cho told Decoded Science: “We found out that the presence of Mn-Mn dimers was crucial for the NO oxidation. Strontium, on the other hand, increased the surface area of the material; this led to an increase in the catalytic activity. Cerium was also important, as it favored the dissociation of molecular oxygen O2 into the reactive O* species. These findings can also help in the development and/or optimization of other materials.”
Reducing Diesel Emissions: Real-Life Applications
The researchers performed some additional tests with the new catalyst, Mn7CeSmSrO14.83, in an environment simulating diesel engine exhaust. They deposited the mullite-based catalyst on the surface of the platinum catalyst normally used in diesel engines. They then tested this composite material for NO oxidation. They performed the reaction test on a gas mixture containing, as well as NO and O2, also other gaseous species, such as CO, CO2, C3H6, C3H8 and n-decane.
The results showed that the presence of the Mn7CeSmSrO14.83 improved the NO oxidation, and that the other functions of the catalysts (i.e. CO and hydrocarbons oxidation) were not affected by the presence of the new catalyst.
Real-life applications in existing equipment could reduce emissions from diesel engines significantly. As Professor Cho said, “this shows that potentially these kind of materials could be employed in real systems.”
Wang, W. et al. Mixed-Phase Oxide Catalyst Based on Mn-Mullite (Sm, Gd)Mn2O5 for NO Oxidation in Diesel Exhaust. (2012). Science. 337, 832-835. Accessed August 26, 2012.
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