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New Catalysts May Provide Path to Low-Cost Production of Future Fuels
Gold stabilized by alkali ions on inert supports are promising new catalysts for low-cost hydrogen production

New catalysts designed by Tufts University School of Engineering researchers and collaborators from other university and national laboratories have the potential to greatly reduce processing costs in future fuels, such as hydrogen. The catalysts, composed of single gold atoms bound by oxygen to sodium or potassium atoms and supported by a wholly unique structure comprised of non-reactive silica materials, demonstrate comparable activity and stability with current catalysts used in producing highly purified hydrogen.

The work, which appears in the November 27, 2014 edition of Science Express, points to new avenues for producing single-site supported gold catalysts that could produce high-grade hydrogen for cleaner energy use in fuel-cell powered devices, including vehicles.

"In the face of precious metals scarcity and exorbitant fuel-processing costs, these systems are promising in the search for sustainable global energy solutions," says senior author Maria Flytzani-Stephanopoulos, the Robert and Marcy Haber Endowed Professor in Energy Sustainability.

Professor Flytzani-Stephanopoulos' election to the NAE recognizes her as a leader in the field of clean energy technologies and underscores her importance to our community as one of our most valued faculty members in engineering for sustainability research, one of our school's three strategic cross-disciplinary focus areas.

Flytzani-Stephanopoulos's research group (NanoCEL) has been active in designing catalysts requiring a lower amount of precious metals to generate high-grade hydrogen for use in fuel cells. The water-gas shift reaction, in which carbon monoxide is removed from the fuel gas stream by reacting with water to produce carbon dioxide and hydrogen, is a key step in the process. Catalysts, such as metal oxides prepared with precious metals like platinum and gold, are used to lower the reaction temperature and increase the production of hydrogen.

The Tufts group was the first to demonstrate that atomically dispersed gold or platinum on supports, such as cerium oxide, are the active sites for the water-gas shift reaction. (doi:10.1126/science.1192449). Metal nanoparticles are "spectator species" for this reaction.

New catalyst designFlytzani-Stephanopoulos says the new research suggests single precious metal atoms stabilized with alkali ions may be the only catalyst sites for other catalytic reactions. "If the other particles are truly 'spectator species', they are therefore unnecessary. Future catalyst production should then focus on avoiding particle formation altogether and rather be prepared solely with single-site atomic dispersion on various supports," says Flytzani-Stephanopoulos.

The just published research describes how single gold atoms dispersed on non-reactive supports based on silica materials can be stabilized with alkali ions. As long as the gold atoms, or cations, are stabilized in a single-site form configuration, irrespective of the type of support, the precious metal will be stable and operate for many hours at a range of practical temperatures.

"This novel atomic-scale catalyst configuration achieves the maximum efficiency and utilization of the gold," says Flytzani-Stephanopoulos. "These single-site gold cations were active for the low-temperature water-gas shift reaction and stable in operation at temperatures as high as 200°C."

Paper co-authors Professor Manos Mavrikakis at the University of Wisconsin-Madison and Assistant Professor Ye Xu at Louisiana State University used quantum mechanics theoretical calculations to elucidate the likely atomic-scale structure and bonding with the active site, as well as to evaluate the thermochemical properties of the Au-Ox(OH)y-Naz ensembles, which retain the gold atom in cationic state. Weak adsorption of CO and facile H2O dissociation are key features predicting good reactivity in the water-gas shift reaction. Researchers Larry Allard at Oak Ridge National Laboratory and Sungsik Lee at Argonne National Laboratory used atomic resolution electron microscopy and x-ray absorption spectroscopies, respectively, to demonstrate the existence and stability of the single-site gold species. Co-author Jun Huang, a lecturer at the University of Sydney, synthesized and characterized the mesoporous silica materials used as supports. Several graduate students were involved in all aspects of the research both at Tufts and Wisconsin.

According to first-author Ming Yang, a doctoral student working in Flytzani-Stephanopoulos's group, the preparation of these single-site gold catalysts does not involve ion exchange in the zeolites. "It cannot be achieved without due consideration of the support surfaces," says Yang. For example, solid-state impregnation was used to add the alkaline component NaOH on these silica-rich supports.

"Armed with this new understanding, practitioners will be able to design catalysts using just the necessary amount of the precious metals like gold and platinum, dramatically cutting down the catalyst cost in fuels and chemicals production processes," Flytzani-Stephanopoulos says.

The paper appears in the November 27 edition of Science Express. (doi:10.1126/science.1260526). This research is primarily supported by the U.S. Department of Energy under grant # DE-FG02-05ER15730.