Catalysts Need Proper Support To Clean Up Engine Emissions

If automobile manufacturers had magic wands, they probably would wave them to simultaneously increase a car’s fuel efficiency and reduce its engine emissions. That would be a welcome trick because improving one often comes at the expense of the other.

No one was handing out magic wands at the 7th International Chemical Congress of Pacific Basin Societies, or Pacifichem, in Honolulu. But on Tuesday at a symposium focusing on automobile emissions cleanup, Haris Buwono, a graduate student working with Masato Machida of Kumamoto University, described a catalytic “magic trick” that could help carmakers sidestep the trade-off between fuel efficiency and emissions.

Burning gasoline in excess oxygen can boost fuel efficiency compared with burning stoichiometric mixtures of oxygen and fuel. But in the excess-oxygen condition, which is known as lean-burn because the air-to-fuel ratio is lean in fuel, today’s catalytic converters struggle to scrub nitrogen oxides (NO_x) from the exhaust. So automakers design gasoline engines to operate near stoichiometry to reduce emissions of NO_x, which are involved in reactions that produce ozone and smog in the atmosphere.

Machida reported that rhodium nanoparticles, the main catalytically active material in modern catalytic converters, can do a better job tackling NO_x under lean conditions if the catalyst support is tailored for the job.

Working with Yuki Nagao, a catalyst specialist with Mitsui Mining & Smelting, and others, Machida prepared rhodium catalysts supported on a series of phosphates including ZrP₂O₇, LaPO₄, AlPO₄, and YPO₄. The team tested these supported particles on various mixtures of NO, CO, propene, oxygen, and other gases, which they tailored to simulate exhaust gas mixtures corresponding to a range of air-to-fuel ratios.

Zirconium phosphate showed other promising features as a catalyst support. For example, Machida and coworkers found that excess oxygen readily oxidizes rhodium supported on ZrO₂ to Rh₂O₃, a material with relatively low catalytic activity. When supported on ZrP₂O₇, however, rhodium resists that type of deactivation.

The researchers found that as the air-to-fuel ratio increased above stoichiometry, all of the phosphate-supported rhodium catalysts did a better job of scrubbing NO_x than Rh/ZrO₂, the conventional catalyst. Rh/ZrP₂O₇ worked best, removing about 35% more NO_x than Rh/ZrO₂ at fuel-to-air ratios slightly higher than stoichiometric (ACS Catal. 2015, DOI: 10.1021/cs5020157).

In addition, propene, an exhaust component resulting from incomplete fuel combustion, forms a reactive aldehyde on Rh/ZrP₂O₇. On ZrO₂ it forms a less active carboxylate compound. The aldehyde helps clean up exhaust because it strips oxygen from NO_x. That process reduces NO_x to N₂ and oxidizes the aldehyde to CO₂ and water.

The results are clear, says Seoul National University’s Do Heui Kim, an emissions catalysis specialist: ZrP₂O₇-supported rhodium enhances removal of NO_x from lean-burn exhaust. Kim adds that in addition to studying catalysts in powdered form, the researchers also anchored them on the same type of porous ceramic honeycomb-like brick found in automobiles everywhere. Testing the catalysts under these real-world conditions makes it much easier to translate the results to practical applications, he says.