In a case of the Goldilocks story retold at the molecular level, scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory and Northwestern University have discovered a new path to the development of more stable and efficient catalysts. The research team sought to create “nanobowls” – nanosized bowl shapes that allow inorganic catalysts to operate selectively on particular molecules.
Catalysts are vitally important substances that enable the production of everything from petroleum to soap. In 2009, Argonne and Northwestern, along with the University of Wisconsin at Madison and Purdue University, jointly founded the Institute for Atom-Efficient Chemical Transformations(IACT) to research new catalyst designs to improve the efficiency of producing fuels from biomass. IACT is one of the Energy Frontier Research Centers funded by DOE’s Office of Science to accelerate research toward meeting our critical energy challenges.
“Nanobowls are intended to mimic the selective enzymes found in nature,” said Argonne chemist Jeffrey Elam. “We can tailor the nanobowl size and shape to accept certain molecules and reject others.”
Although nanobowls and enzymes both use a lock-and-key mechanism, they serve different purposes and operate in dramatically different environments. “Enzymes are composed of organic materials suitable for the relatively low-temperature, low-pressure environments of living organisms,” Elam said. “But the extremely harsh conditions necessary for biomass conversion would cause the enzyme proteins to unravel. In contrast, the nanobowls are inorganic, and this makes them very durable.”
According to Elam, the design’s effectiveness correlates with the size and depth of the bowl; if the bowl is too large or shallow, practically any molecule can access the catalyst, which can lead to uncontrolled and often undesirable side reactions. Likewise, if the bowl is too small or deep, even the intended molecule will not fit into the bowl. However, if the nanobowl structure is “just right,” only the intended molecule will reach the catalyst and react.
The trick to building a nanobowl with a specific shape and depth is to use a nano-sized template. In the first proof-of-concept nanobowl experiments, bulky organic molecules called calixarenes were used as the template. They were grafted onto a titanium dioxide surface that served as both the catalyst and the “table” for the nanobowl to rest on. Next, the walls of the bowl were built around the template, one atomic layer at a time, using atomic layer deposition (ALD), a technology borrowed from the semiconductor industry. Once the scientists grew the nanobowl to the proper height, they burned away the organic template, leaving behind a cavity with the same shape.
Since the titanium dioxide is in the form of a nanopowder with lots of surface area, the experiment required the scientists to create millions of these nanobowls. Fortunately, the processing techniques that Elam and his colleagues employed can be scaled up so that successful nanobowls identified in these bench-scale studies can eventually have a real-world impact.
The next step, Elam said, involves applying the knowledge gained in these studies to make nanobowl catalysts tailored for biofuel production. “The overarching problem in these reactions is to selectively remove oxygen without breaking carbon-carbon bonds.”
Elam and his colleagues also used the 12ID-C X-ray beamline at the laboratory’s Advanced Photon Source to characterize the structure of the nanobowls.