Scientists in the US and China have developed a new low-temperature catalyst for producing high-purity hydrogen gas while simultaneously using up carbon monoxide (CO) via the water-gas shift (WGS) reaction. The discovery—described in a paper in the journal Science—could improve the performance of fuel cells that run on hydrogen fuel but can be poisoned by CO.
The WGS reaction (CO+H2O = H2+CO2) is an essential process for hydrogen generation and CO removal in various energy-related chemical operations. The reaction is favored at a low working temperature. Application in fuel cells requires a WGS catalyst to be highly active, stable and energy-efficient and match the working temperature of on-site hydrogen generation and consumption units.
Low-temperature efficient catalysts for the WGS reaction, especially those operating under 423 K [150 ˚C], are of interest for applications in fuel cells, especially those use H2 generated by hydrocarbon reforming processes that are contaminated with CO, which deactivates the catalysts. For the heterogeneous catalysis, besides Cu based catalysts which display low activity at low temperature, Pt group noble metals and Au supported on reducible metal oxides, like ceria or FeOx which contain oxygen vacancies, are commonly used.
; In order to achieve high WGS activity at low temperature, we searched for catalysts that could dissociate water efficiently and reform the generated oxygen-containing species (reaction of surface oxygen or hydroxyl with CO*) at low temperature. We report that Au confined over face centered cubic (fcc) structured α-MoC is at least one order of magnitude more active than previous reports for the WGS reaction below 423 K. The α-MoC substrate facilitates epitaxially-grown atomic Au layers with altered electronic structure for favorable bonding with CO. Its synergy with adjacent Mo sites in α-MoC can effectively activate water at low temperature.—Yao et al.
The international team synthesized gold layered clusters on an α-MoC substrate to create an interfacial catalyst system for the ultra-low-temperature WGS reaction. Water was activated over α- MoC at 303 K (30 ˚C), while CO adsorbed on adjacent Au sites was apt to react with surface hydroxyl groups formed from water splitting, leading to a high WGS activity at low-temperatures.
This catalyst produces a purer form of hydrogen to feed into the fuel cell. With low temperature and pressure, the energy consumption will be lower and the experimental setup will be less expensive and easier to use in small settings, like fuel cells for cars.—José Rodriguez, a chemist at BNL and co-corresponding author
Rodriguez and colleagues were among the team of scientists who helped to characterize the structural and mechanistic details of the catalyst, which was synthesized and tested by collaborators at Peking University in an effort led by Chemistry Professor Ding Ma.
The catalyst consists of clusters of gold nanoparticles layered on a molybdenum-carbide substrate. This chemical combination is quite different from the oxide-based catalysts used to power the water gas shift reaction in large-scale industrial hydrogen production facilities.
Carbides are more chemically reactive than oxides, and the gold-carbide interface has good properties for the water gas shift reaction; it interacts better with water than pure metals. The group at Peking University discovered a new synthetic method, and that was a real breakthrough. They found a way to get a specific phase-or configuration of the atoms-that is highly active for this reaction.—José Rodriguez
Rodriguez and colleagues conducted structural studies using x-ray diffraction at the National Synchrotron Light Source (NSLS) while the catalyst was operating under industrial or technical conditions. These operando experiments revealed crucial details about how the structure changed under different operating conditions, including at different temperatures.
With those structural details in hand, Zhijun Zuo, a visiting professor at Brookhaven from Taiyuan University of Technology, China, and Brookhaven chemist Ping Liu helped to develop models and a theoretical framework to explain why the catalyst works the way it does, using computational resources at Brookhaven’s Center for Functional Nanomaterials (CFN).
Additional studies at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences (CNMS), the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory, and two synchrotron research facilities in China added to the scientists’ understanding.
This is a multipart complex reaction. The interaction between the gold and the carbide substrate is very important. Gold usually bonds things very weakly. With this synthesis method we get stronger adherence of gold to molybdenum carbide in a controlled way.—Ping Liu
That configuration stabilizes the key intermediate that forms as the reaction proceeds, and the stability of that intermediate determines the rate of hydrogen production, she said.
The Brookhaven team will continue to study this and other carbide catalysts with new capabilities at the National Synchrotron Light Source II (NSLS-II), a new facility that opened at Brookhaven Lab in 2014, replacing NSLS and producing x-rays that are 10,000 times brighter. With these brighter x-rays, the scientists hope to capture more details of the chemistry in action, including details of the intermediates that form throughout the reaction process to validate the theoretical predictions made in this study.
The work at Brookhaven Lab was funded by the US DOE Office of Science.
Additional funders for the overall research project include: the National Basic Research Program of China, the Chinese Academy of Sciences, National Natural Science Foundation of China, Fundamental Research Funds for the Central Universities of China, and the US National Science Foundation.
NSLS, NSLS-II, CFN, CNMS, and ALS are all DOE Office of Science User Facilities.
Siyu Yao, Xiao Zhang, Wu Zhou, Rui Gao, Wenqian Xu, Yifan Ye, Lili Lin, Xiaodong Wen, Ping Liu, Bingbing Chen, Ethan Crumlin, Jinghua Guo, Zhijun Zuo, Weizhen Li, Jinglin Xie, Li Lu, Christopher J. Kiely, Lin Gu, Chuan Shi, José A. Rodriguez, Ding Ma (2017) “Atomic-layered Au clusters on α-MoC as catalysts for the low-temperature water-gas shift reaction”, Science doi: 10.1126/science.aah4321