A team of scientists from the Institute for Atom Efficient Chemical Transformations (IACT)—an Energy Frontier Research Center (earlier post) led by Argonne National Laboratory (ANL), and including Northwestern University, the University of Wisconsin and Purdue University—is using atomic layer deposition (ALD) to build nanoscale “bowls” that protect metal catalysts from the harsh conditions of biofuel refining.
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 °C and pressures of 4,100 kilopascals (597 psi), the tiny metal nanoparticles can agglomerate into much larger particles which are not catalytically active. Additionally, these extreme conditions can dissolve the support.
The team, led by Jeffrey Elam, principal chemist in ANL’s Energy Systems Division, will present its research during the AVS 59th International Symposium and Exhibition, held 28 Oct. to 2 Nov. 2012, in Tampa, Fla.
...the nanoparticle size, composition, and local environment can be tailored on the atomic scale to tune the reactivity, selectivity, and thermal stability of the catalysts. ALD is a thin film growth technique that uses alternating cycles of self-limiting chemical reactions between gaseous precursors and a solid surface to deposit material in an atomic layer-by-layer fashion. By combining ALD processes for metal oxides, noble metals, and other materials relevant to catalysis, it is possible to engineer nanostructured catalysts with unique properties by depositing a sequence of discrete layers or particles which each perform a specific function.
Although the idea of using ALD to prepare catalysts is not new, recent advances in ALD technology, coupled with innovative ideas for nanofabrication, have rekindled this field and now offer potential solutions to long-standing problems in catalyst synthesis.—Lu et al.
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 mold 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.—Jeffrey Elam
Elam says that 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.—Jeffrey Elam
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, he says, is to precisely measure how well the catalysts perform in an actual biomass refining process.
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Junling Lu, Yu Lei and Jeffrey W. Elam (2012). Atomic Layer Deposition of Noble Metals – New Developments in Nanostructured Catalysts, Noble Metals, Yen-Hsun Su (Ed.), ISBN: 978-953-307-898-4, InTech, doi: 10.5772/33082