Novel single-site gold WGS catalysts may offer pathway to lower-cost production of hydrogen, fuels and chemicals
A team of researchers from universities and national laboratories led by Tufts University has developed catalysts composed of a unique structure of single gold atoms bound by oxygen to several sodium or potassium atoms and supported on non-reactive silica materials. This single-site gold species is active for the low-temperature (< 200 °C) water-gas shift (WGS) reaction that produces hydrogen.
They thus have found that gold is similar to platinum in creating –O and –OH linkages with more than eight alkali ions and establishing an active site on various supports. This finding paves the way for using earth-abundant supports to disperse and to stabilize precious metal atoms with alkali additives for the WGS and potentially other fuel processing reactions. The result could be lower costs. A paper describing their work is now published in Science Express.
The water-gas shift (WGS) reaction (CO + H2O = CO2+ H2) is an important reaction for hydrogen upgrading during fuel gas processing. Emerging applications in fuel cells require active, non-pyrophoric, and cost-effective catalysts. Along with a new group of platinum catalysts with atomically dispersed Pt sites to maximize activity and catalytic efficiency, the lower apparent activation energy Ea for the WGS reaction (~45 kJ/mol) for gold (Au) vs. ~75 kJ/mol for platinum, can be exploited for low-temperature WGS and other reactions. Low-temperature activity is important to avoid multiple-treatment units in practical low-temperature PEM fuel cell systems, whereby the deleterious CO should be totally removed for stable, long-term operation.
… We now show how to use alkali addition to activate and stabilize atomic Au for the WGS reaction even on inert zeolite (KLTL) and mesoporous [Si]MCM-41 silica materials and measure activity comparable to that of Au on reducible oxide supports, and with good stability up to 200 °C.—Yang et al.
In 2010, the Tufts group reported in another paper in Science that alkali ions (sodium or potassium) added in small amounts activate platinum adsorbed on alumina or silica for the low-temperature water-gas shift (WGS) reaction. The alkali ion–associated surface OH groups were activated by CO at low temperatures (~100 °C) in the presence of atomically dispersed platinum. These findings were useful for the design of highly active and stable WGS catalysts containing only trace amounts of a precious metal without the need for a reducible oxide support such as ceria.
Senior author Maria Flytzani-Stephanopoulos, the Robert and Marcy Haber Endowed Professor in Energy Sustainability and professor in the Department of Chemical and Biological Engineering at Tufts, said that the new research suggests single precious metal atoms stabilized with alkali ions may be the only important catalyst sites for other catalytic reactions.
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. Our work showed that 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. 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.— Maria Flytzani-Stephanopoulos
Paper co-authors Professor Manos Mavrikakis at the University of Wisconsin-Madison and Assistant Professor Ye Xu at Louisiana State University used theoretical calculations to predict the stability and thermochemical properties of the single-site configuration.
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 silica materials used as supports. Several graduate students were involved in all aspects of the research both at Tufts and the University of Wisconsin-Madison.
This research is primarily supported by the US Department of Energy under grant #DE-FG02-05ER15730.
Ming Yang, Sha Li, Yuan Wang, Jeffrey A. Herron, Ye Xu, Lawrence F. Allard, Sungsik Lee, Jun Huang, Manos Mavrikakis, and Maria Flytzani-Stephanopoulos (2014) “Catalytically active Au-O(OH)x- species stabilized by alkali ions on zeolites and mesoporous oxides” Science doi: 10.1126/science.1260526
Yanping Zhai, Danny Pierre, Rui Si, Weiling Deng, Peter Ferrin, Anand U. Nilekar, Guowen Peng, Jeffrey A. Herron, David C. Bell, Howard Saltsburg, Manos Mavrikakis, and Maria Flytzani-Stephanopoulos (2010) “Alkali-Stabilized Pt-OHx Species Catalyze Low-Temperature Water-Gas Shift Reactions,” Science 329 (5999), 1633-1636 doi: 10.1126/science.1192449