Imaging a catalyst one atom at a time

BY KURT PFITZER
Special to the Rice News

The catalytic processes that facilitate the production of many chemicals and fuels could become much more environmentally friendly, thanks to a new study published this month by researchers at Rice University and Lehigh University.

	LEHIGH UNIVERSITY
	A high-resolution image shows the surface of a sample of tungstated zirconia with low catalytic activity but does not reveal individual tungsten-oxide atoms or clusters. The bar at bottom left is 5 nanometers.

In a Nov. 8 report in Nature Chemistry, the researchers detailed a study of a catalyst called tungstated zirconia that improves the octane content of gasoline. Using a novel electron microscopy imaging technique invented at Lehigh, the team captured detailed images of the catalytic nanoparticles — about 100,000 of which could fit side by side on a human hair. Using information from the images, the researchers were able to design a preparation procedure that increased the activity of the catalyst by more than 100 times.

“By identifying the nano feature that is responsible for the desired catalytic performance, we can then focus research efforts on rationally designing new ways to prepare catalytic materials with only that particular feature,” said study co-author Michael Wong, associate professor in chemical and biomolecular engineering and in chemistry at Rice.

A catalyst is a substance that accelerates the rate of a chemical reaction without being consumed by the reaction. Liquid acid catalysts are widely used in the petrochemical industry, but they sometimes pose environmental concerns due to evaporation, spilling and corrosion, so chemical companies frequently look to replace them with solid catalysts like tungstated zirconia.

LEHIGH UNIVERSITY
High-resolution imaging resolves single tungsten atoms (inside circles) and poly-tungstate species with several tungsten atoms linked by oxygen bridging bonds (inside squares). The bar at bottom left is 2 nanometers.

The tungstated zirconia catalysts measured only about one nanometer, or one-billionth of a meter, across. To shed light on their nanostructure and nanoscale behavior, the Lehigh-Rice team used several microscopic and spectroscopic techniques, including a technique called “aberration-corrected” scanning transmission electron microscopy (STEM) that used a narrow beam of electrons only about one-tenth as wide as the catalytic nanoparticles.

“This new generation of aberration-corrected STEMs enables us finally to see the dimensions of the species we’re studying,” said co-author Israel Wachs, professor of chemical engineering at Lehigh.

“The combination of the imaging and spectroscopy techniques enabled us to make an active catalytic site, deposit it on a catalyst with low activity and show a 100-fold improvement in catalytic activity,” he said. “In short, we’ve been able to design, on demand, the active catalytic sites by molecularly engineering the catalyst.”

Study co-authors include corresponding author Christopher Kiely, professor of materials science and engineering at Lehigh and a pioneer in the use of aberration-corrected STEM; former Lehigh graduate students Wu Zhou and Elizabeth Ross-Medgaarden; and former Rice graduate student William Knowles ’07. The research was funded by the National Science Foundation.

–Kurt Pfitzer is the editor of Resolve Magazine in Lehigh University’s Office of University Communications.