SUPPORTIVE CATALYST [+]Enlarge Credit: ACS Catal.

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.

REAL CATALYST [+]Enlarge Credit: ACS Catal.

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 ZrP 2 O 7 , LaPO 4 , AlPO 4 , and YPO 4 . 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 ZrO 2 to Rh 2 O 3 , a material with relatively low catalytic activity. When supported on ZrP 2 O 7 , 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/ZrO 2 , the conventional catalyst. Rh/ZrP 2 O 7 worked best, removing about 35% more NO x than Rh/ZrO 2 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/ZrP 2 O 7 . On ZrO 2 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 N 2 and oxidizes the aldehyde to CO 2 and water.