Scientists build nanobowls protect catalysts during biofuel refining

Scientists build nanobowls to protect catalysts during biofuel refining

11:39 AM, 29th October 2012
Scientists build nanobowls to protect catalysts during biofuel refining
Computer graphic showing a fructose molecule (white, gray and red chain-like structure) within a zirconium oxide nanobowl (at centre). Other nanobowls in the array are unoccupied. The red atoms are surface oxygen and the blue atoms are zirconium.

NEW YORK, US: Scientists from the Institute for Atom Efficient Chemical Transformations (IACT), an Energy Frontier Research Centre led by Argonne National Laboratory (ANL), and including Northwestern University, the University of Wisconsin and Purdue University, are using a layering technique developed for microchip manufacturing to build nanoscale “bowls” that protect miniature metal catalysts from the harsh conditions of biofuel refining. Furthermore, the size, shape, and composition of the nanobowls can easily be tailored to enhance their functionality and specificity.

In recent years, nanoparticles of metals such as platinum, iridium and palladium supported on metal oxide surfaces have been considered as catalysts to convert biomass into alternative fuels as efficiently as possible. Unfortunately, under typical biorefining conditions where liquid water may reach temperatures of 200 degrees Celsius and pressures of 4,100 kilopascals, the tiny metal nanoparticles can agglomerate into much larger particles which are not catalytically active. Additionally, these extreme conditions can dissolve the support.

“We needed a method to protect the catalysts without reducing their ability to function as desired during biorefining. Our solution was to use atomic layer deposition (ALD), a process commonly employed by the semiconductor industry to lay down single-atom thick layers of material, to build a ‘nanobowl’ around the metal particle,” said Jeffrey Elam, Principal Chemist, ANL’s Energy Systems Division.

To create a matrix of nanobowls containing active catalysts, the researchers first use ALD to deposit millions of metal nanoparticles (the eventual nanocatalysts) onto a support surface. The next step is to add an organic species that will only bind to the metal nanoparticles and not to the support. This organic “protecting group” serves as the mould around which the nanobowls are shaped.

“Again using ALD, we deposit layer upon layer of an inorganic material known as niobia (niobium pentoxide) around the protecting group to define the shape of the nanobowls in our matrix. Once the desired niobia thickness is reached, we remove the protecting groups and leave our metal nanoparticles sheltered in nanobowls that prevent them from agglomerating. In addition, the niobia coating protects the substrate from the extreme conditions encountered during biorefining,” said Elam.

According to Elam, the nanobowls themselves can be made to enhance the overall functionality of the catalyst matrix being produced. “At a specific height, we can put down ALD layers of catalytically active material into the nanobowl walls and create a co-catalyst that will work in tandem with the nanocatalysts. Also, by carefully selecting the organic protecting group, we can tune the size and shape of the nanobowl cavities to target specific molecules in the biomass mixture.”

Elam and his colleagues have shown in the laboratory that the nanobowl/nanoparticle combination can survive the high-pressure, high-temperature aqueous environment of biomass refining. They also have demonstrated size and shape selectivity for the nanobowl catalysts. The next goal, is to precisely measure how well the catalysts perform in an actual biomass refining process.

© AVS News

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