New Efficient, Low-Temperature Catalyst for Converting Water and CO to Hydrogen Gas and CO2

Brookhaven Lab chemists Ping Liu and José Rodriguez helped to characterize structural and mechanistic details of a new low-temperature catalyst for producing high-purity hydrogen gas from water and carbon monoxide.

Low-temperature “water gas shift” reaction produces high levels of pure hydrogen for potential applications, including fuel cells

UPTON, NY—Scientists have developed a new low-temperature catalyst for producing high-purity hydrogen gas while simultaneously using up carbon monoxide (CO). The discovery—described in a paper set to publish online in the Journal Science — could improve the performance of fuel cells that run on hydrogen fuel but can be poisoned by CO.

“This catalyst produces a purer form of hydrogen to feed into the fuel cell,” said José Rodriguez, a chemist at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory. Rodriguez and colleagues in Brookhaven’s Chemistry Division—Ping Liu and Wenqian Xu—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.

“This catalyst produces a purer form of hydrogen to feed into fuel cells.”

— José Rodriguez

Because the catalyst operates at low temperature and low pressure to convert water (H₂O) and carbon monoxide (CO) to hydrogen gas (H₂) and carbon dioxide (CO₂), it could also lower the cost of running this so-called “water gas shift” reaction.

“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,” Rodriguez said.

The gold-carbide connection

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,” said Rodriguez, “and the gold-carbide interface has good properties for the water gas shift reaction; it interacts better with water than pure metals.”

Wenqian Xu and José Rodriguez of Brookhaven Lab and Siyu Yao, then a student at Peking University but now a postdoctoral research fellow at Brookhaven, conducted operando x-ray diffraction studies of the gold-molybdenum-carbide catalyst over a range of temperatures (423 Kelvin to 623K) at the National Synchrotron Light Source (NSLS) at Brookhaven Lab. The study revealed that at temperatures above 500K, molybdenum-carbide transforms to molybdenum oxide, with a reduction in catalytic activity.

 

“The group at Peking University discovered a new synthetic method, and that was a real breakthrough,” Rodriguez said. “They found a way to get a specific phase—or configuration of the atoms—that is highly active for this reaction.”

Brookhaven scientists played a key role in deciphering the reasons for the high catalytic activity of this configuration. Rodriguez, Wenqian Xu, and Siyu Yao (then a student at Peking University but now a postdoctoral research fellow at Brookhaven) 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 operandoexperiments 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).

“We modeled different interfaces of gold and molybdenum carbide and studied the reaction mechanism to identify exactly where the reactions take place—the active sites where atoms are binding, and how bonds are breaking and reforming,” she said.

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,” said Liu, but she noted one essential factor: “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.”

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 U.S. 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 U.S. National Science Foundation.

NSLS, NSLS-II, CFN, CNMS, and ALS are all DOE Office of Science User Facilities.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy.  The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time.  For more information, please visit science.energy.gov.

ORNL Researchers make Scalable Arrays of ‘Building Blocks’ for Ultrathin Electronics

OAK RIDGE, Tenn., July 22, 2015—Semiconductors, metals and insulators must be integrated to make the transistors that are the electronic building blocks of your smartphone, computer and other microchip-enabled devices. Today’s transistors are miniscule—a mere 10 nanometers wide—and formed from three-dimensional (3D) crystals.

But a disruptive new technology looms that uses two-dimensional (2D) crystals, just 1 nanometer thick, to enable ultrathin electronics. Scientists worldwide are investigating 2D crystals made from common layered materials to constrain electron transport within just two dimensions. Researchers had previously found ways to lithographically pattern single layers of carbon atoms called graphene into ribbon-like “wires” complete with insulation provided by a similar layer of boron nitride. But until now they have lacked synthesis and processing methods to lithographically pattern junctions between two different semiconductors within a single nanometer-thick layer to form transistors, the building blocks of ultrathin electronic devices.

Now for the first time, researchers at the Department of Energy’s Oak Ridge National Laboratory have combined a novel synthesis process with commercial electron-beam lithography techniques to produce arrays of semiconductor junctions in arbitrary patterns within a single, nanometer-thick semiconductor crystal. The process relies upon transforming patterned regions of one existing, single-layer crystal into another. The researchers first grew single, nanometer-thick layers of molybdenum diselenide crystals on substrates and then deposited protective patterns of silicon oxide using standard lithography techniques. Then they bombarded the exposed regions of the crystals with a laser-generated beam of sulfur atoms. The sulfur atoms replaced the selenium atoms in the crystals to form molybdenum disulfide, which has a nearly identical crystal structure. The two semiconductor crystals formed sharp junctions, the desired building blocks of electronics. Nature Communications reports the accomplishment.

“We can literally make any kind of pattern that we want,” said Masoud Mahjouri-Samani, who co-led the study with David Geohegan. Geohegan, head of ORNL’s Nanomaterials Synthesis and Functional Assembly Group at the Center for Nanophase Materials Sciences, is the principal investigator of a Department of Energy basic science project focusing on the growth mechanisms and controlled synthesis of nanomaterials. Millions of 2D building blocks with numerous patterns may be made concurrently, Mahjouri-Samani added. In the future, it might be possible to produce different patterns on the top and bottom of a sheet. Further complexity could be introduced by layering sheets with different patterns.

Added Geohegan, “The development of a scalable, easily implemented process to lithographically pattern and easily form lateral semiconducting heterojunctions within two-dimensional crystals fulfills a critical need for ‘building blocks’ to enable next-generation ultrathin devices for applications ranging from flexible consumer electronics to solar energy.”

Tuning the bandgap

“We chose pulsed laser deposition of sulfur because of the digital control it gives you over the flux of the material that comes to the surface,” said Mahjouri-Samani. “You can basically make any kind of intermediate alloy. You can just replace, say, 20 percent of the selenium with sulfur, or 30 percent, or 50 percent.” Added Geohegan, “Pulsed laser deposition also lets the kinetic energy of the sulfur atoms be tuned, allowing you to explore a wider range of processing conditions.”

It is important that by controlling the ratio of sulfur to selenium within the crystal, the researchers can tune the bandgap of the semiconductors, an attribute that determines electronic and optical properties. To make optoelectronic devices such as electroluminescent displays, microchip fabricators integrate semiconductors with different bandgaps. For example, molybdenum disulfide’s bandgap is greater than molybdenum diselenide’s. Applying voltage to a crystal containing both semiconductors causes electrons and “holes” (positive charges created when electrons vacate) to move from molybdenum disulfide into molybdenum diselenide and recombine to emit light at the bandgap of molybdenum diselenide. For that reason, engineering the bandgaps of monolayer systems can allow the generation of light with many different colors, as well as enable other applications such as transistors and sensors, Mahjouri-Samani said.

Next the researchers will see if their pulsed laser vaporization and conversion method will work with atoms other than sulfur and selenium. “We’re trying to make more complex systems in a 2D plane—integrate more ingredients, put in different building blocks—because at the end of the day, a complete working device needs different semiconductors and metals and insulators,” Mahjouri-Samani said.

To understand the process of converting one nanometer-thick crystal into another, the researchers used powerful electron microscopy capabilities available at ORNL, notably atomic-resolution Z-contrast scanning transmission electron microscopy, which was developed at the lab and is now available to scientists worldwide using the Center for Nanophase Materials Sciences. Employing this technique, electron microscopists Andrew Lupini and visiting scientist Leonardo Basile imaged hexagonal networks of individual columns of atoms in the nanometer-thick molybdenum diselenide and molybdenum disulfide crystals.

“We could directly distinguish between sulfur and selenium atoms by their intensities in the image,” Lupini said. “These images and electron energy loss spectroscopy allowed the team to characterize the semiconductor heterojunction with atomic precision.”

The title of the paper is “Patterned Arrays of Lateral Heterojunctions within Monolayer Two-Dimensional Semiconductors.”

The research was sponsored by the U.S. Department of Energy, Office of Science. A portion of the work was conducted at the Center for Nanophase Materials Sciences, a DOE Office of Science User Facility at ORNL. Basile received support from the National Secretariat of Higher Education, Science, Technology and Innovation of Ecuador.

UT-Battelle manages ORNL for DOE’s Office of Science. The single largest supporter of basic research in the physical sciences in the United States, the Office of Science is working to address some of the most pressing challenges of our time.

A Global Water Crisis ~ The Seawater Solution: Will Emerging Nanotechnologies Provide the Answers?

“Imagine getting fire-hose volumes and velocities out of your garden hose. Nanotechnology could fundamentally change the economics of desalination.”

Nearly three-quarters of the Earth’s surface is covered by water, but according to the United Nations, more than 97 percent of it is saltwater unsuited for human consumption or agriculture.

The United Nations Population Fund reports that by 2025 two-thirds of the world’s projected population of 7.9 billion may live in areas where access to safe water is limited. “Every time we add a person, it’s not just the water that person consumes but also the additional water for agriculture and industry that you have to use,” says Earl Jones, director of water-scarcity solutions in General Electric Co.’s (GE) Water & Process Technologies unit.

The removal of salt from seawater is an increasingly cost effective answer to the earth’s growing clean-water needs. By 2025, two-thirds of the world’s population may live in areas where access to safe water is limited, reports the U.N.

The removal of salt from water is emerging as one of the best solutions to the world’s water problem, analysts say. According to GOLDMAN SACH S Group Inc. (GS), desalination is a $5 billion global market, with growth of 10 percent to 15 percent a year. Water Desalination Report, a trade journal, reports that more than 12,000 desalination plants are operating World-wide, with 53 percent of the world’s desalination capacity in the Middle East.

“Today, the global capacity is about 40 million cubic meters of desalinated water per day,” says Antoine Frérot, CEO of Veolia Water, the water division of Veolia Environnement (VE). “By 2015, it will be around 70 million cubic meters per day.” Improvements in two technologies are making desalination more cost-efficient, say the experts:

The thermal process, which couples a thermal desalination plant with a power plant to provide the energy, involves evaporating water to remove salt.

Reverse osmosis, the other process, uses semipermeable membranes. About 84 percent of the world’s thermal desalination capacity, which requires more energy than reverse-osmosis facilities, is located in the Middle East, according to Water Desalination Report.  

“We have one huge advantage in the Gulf,” says Phil Cox, CEO of International Power PLC (IPR), which builds, owns and operates thermal desalination plants in that region. “The price of natural gas is extremely low here compared with the rest of the world,” he adds. Outside the Middle East, reverse osmosis is the less expensive alternative, says Jean-Louis Chaussade, CEO of Suez Environment, a unit of Suez SA (SZE). “At our biggest reverse osmosis plants, we operate at roughly 60 cents per cubic meter of use,” says Chaussade.

Aside from GE, International Power, Suez and Veolia, other companies that construct, own and/or operate desalination systems worldwide include The AE S Corp. (AES), Crane Co.’s (CR) Crane Environmental, Siemens AG’s (SI) Power Generation unit and ITT Corp. (ITT). ABB Ltd . (ABB) provides electrical systems for desalination plants, and Met-Pro Corp.’s (MPR) Fybroc division manufactures pumps used in reverse-osmosis plants.

The motivation is there to solve the world’s water needs, the companies say. “According to the U.N., the No. 1 cause of death and illness in developing nations is waterborne diseases,” says GE’s Jones. “We have the technology to fix these problems. It’s very easy to get motivated because of the great opportunity to do good.”  

The Scale Effect  

The world’s largest reverse-osmosis plant in terms of production is Veolia Water’s Ashkelon Seawater Desalination Plant (see illustration) south of Tel Aviv, which has a daily capacity of 320,000 cubic meters per day, according to the company. The plant produces enough water to meet the needs of 15 percent of Israeli households, Veolia reports. “There is a scale effect,” says Veolia Water CEO Antoine Frérot. “At a small desalination plant, the price of water is around $2 per cubic meter. In Ashkelon, the price is 55 cents per cubic meter.”

Other big projects are in the works: General Electric Co.’s (GE) Infrastructure, Water & Process Technologies reports that it plans to open Africa’s largest seawater desalination project in Algiers, Algeria. An international consortium led by Siemens’ Power Generation unit says it plans to build the world’s largest independent water and power project in Riyadh, Saudi Arabia. Uwe Rokossa, Siemens projects sales director for new plants and services in the Middle East predicts: “We will see a continuation of big power and desalination projects.”

Steam Power and Hybrids

Thermal desalination requires steam to boil seawater, GE explains. According to GE’s Earl Jones, the most widely used thermal process is called multistage flash, which heats seawater in a brine tank, immediately converting it to steam. The resulting salt-free steam is captured, cooled and condensed, creating desalinated water, Jones reports. Since only some of the seawater is converted to steam, the process is repeated multiple times in different receptacles, each time using lower atmospheric pressures. The hybrid approach, which combines thermal and reverse-osmosis processes, is an emerging technology, according to Suez Environment, which provides the reverse-osmosis part of the first-ever hybrid facility in the United Arab Emirates. Having both techniques in one plant allows for flexibility, the company says. Suez Environment reports that when demand for electricity from the thermal side’s power plant is low, priority can be given to the less-energy-intensive reverse-osmosis process.

Another form of the hybrid approach involves having a mixture of different membranes inside a reverse-osmosis pressure vessel, says Lance Johnson, senior sales and marketing manager for Dow Water Solutions. “As the water moves down the vessel, the salt concentration increases. At the tail end, where the salinity is highest, you’d have a lower-pressure membrane than at the front end to boost productivity.

Emerging “Nano-Materials” and Membranes

Mixing high and low pressure membranes in a pressure vessel can lower cost.” Applying nanotechnology to membrane science is another promising avenue, according to GE’s Jones, who notes that membranes made out of nanotubes can process water faster than older methods. “Imagine getting fire-hose volumes and velocities out of your garden hose. Nanotechnology could fundamentally change the economics of desalination.”        

Read More:

Nanoscale Desalination of Seawater Through Nanoporous Graphene

Oakridge National Laboratory: Using New Graphene Technology to Desalinate Water

 

Drexel University: Oak Ridge National Labratory: For Energy Storage, MXene Materials Show Increasing Promise

Recently discovered family of 2-D materials could one day yield high-performance batteries, flexible electronics, and more.

A recently discovered family of carbides and nitrides first started to catch the attention of researchers in 2012. Referred to as MXenes (pronounced “maxenes”), the materials turned heads because they are electrically conductive, robust, abundant, and stable as nearly atomically thin sheets—properties that could be useful for making high-performance batteries.

MXenes are now looking even better, as researchers have just shown that these materials are also strong and flexible, exhibit high electrical capacitance, and can easily be prepared as composites and moldable clays. The new discoveries suggest that MXenes may also be useful for applications such as flexible and wearable electronics and are attracting more scientists to this intriguing family of materials. Some of those researchers gathered at last month’s Materials Research Society meeting in Boston to discuss their latest findings and ideas for developing those applications.

The history of MXenes is brief. In 2011, Yury Gogotsi and Michel W. Barsoum, materials science professors at Drexel University, were studying ways to make anodes for lithium-ion batteries that outlast standard graphite anodes. The team’s earlier work suggested that a family of electrically conducting carbides and nitrides were promising candidate materials. Those compounds are known as the MAX phases, where M refers to an early transition metal, A symbolizes main-group elements such as aluminum and silicon, and X represents carbon or nitrogen.

The Drexel team treated Ti₃AlC₂ and other MAX phases with concentrated hydrofluoric acid to selectively remove some of the atoms from the starting materials. The goal was to make enough room in the anode lattice for Li ions to reversibly insert themselves during battery charging and discharging. The process worked. The group ended up with electrochemically active materials that performed admirably in battery tests.

But the Drexel team got more than they bargained for. The group was surprised to learn that the acid treatment had completely removed the Al layers (the A component in MAX) and exfoliated the crystals into microscopic two-dimensionalsheets of Ti₃C₂. Excited by the discovery of new 2-D materials with graphenelike morphology, the team named the materials MXenes. Within a few months, they showed that the acid treatment could be used to make many 2-D materials by exfoliating additional compounds such as Ti₂AlC, Ta₄AlC₃, (Ti_0.5Nb_0.5 )₂AlC, (V_0.5Cr_0.5)₃AlC₂, and Ti₃AlCN (ACS Nano 2012, DOI: 10.1021/nn204153h).

Word began spreading quickly. Thanks in part to graphene’s popularity, Gogotsi says, MXenes “have been riding a wave of excitement about 2-D materials.” He adds that researchers are enthusiastic about MXenes because they see what may potentially be a huge new area of materials science emerging.

That enthusiasm is especially noticeable in Gogotsi’s and Barsoum’s groups. The accelerated pace of research by the combined team has led to publication of several journal papers in just the past few weeks. Those studies have common themes—convenient processing methods and more functional forms of MXenes.

USEFUL

Available only as powders until recently, MXenes (titanium carbide shown above) can now be made as large, flexible, electrically conductive films, an advantage for mobile energy applications.

Credit: Proc. Natl. Acad. Sci. USA

In the early studies in this field, the separated MXene sheets were nanometer-thick flakes with lateral dimensions reaching a few micrometers. Although the particulate (powdered) form of such materials has useful properties, powders have limited function and offer limited processing options.

So the team developed a vacuum filtration method that fuses the flakes into freestanding macroscopic thin films. They also devised procedures for making MXene-polymer composites, which could turn these materials into something with commercial value.

Collectively, the thin films are flexible, foldable, and strong enough to be handled repeatedly without being damaged. They are also electrically conductive, hydrophilic, and highly stable in water. Among other findings, the team observed that pure MXene films conduct electricity better and store more charge than graphene and carbon nanotube “paper” (Proc. Natl. Acad. Sci. USA 2014, DOI: 10.1073/pnas.1414215111). The team also found that polymers such as polyvinyl alcohol mix intimately with titanium carbide (the most studied MXene), forming alternating MXene-PVA-MXene layered structures. The composites are up to 400% stronger than pure MXene films.

Layered

This freestanding titanium carbide-PVA composite film derives its high strength and flexibility from intimate mixing of MXene (dark) and polymer layers (light).

Credit: Proc. Natl. Acad. Sci. USA

The Drexel team has also devised a method for forming MXene-carbon nanotube composite films (Adv. Mater. 2014, DOI: 10.1002/adma.201404140). Similar to the polymer composites, the nanotube composites are strong and flexible films with an alternating layer structure. Gogotsi explains that inserting polymers or nanotubes between the MXene layers enables electrolyte ions to diffuse more easily through the MXenes, which is key for flexible energy storage applications. But unlike polymers, carbon nanotubes also enable electrons to shuttle back and forth. Initial tests show that MXene-carbon nanotube films work well as supercapacitor electrodes, with no degradation in performance in 10,000 charging cycles.

And in another just-published paper, the Drexel group reported a simpler and safer route to MXene films. The team showed that concentrated hydrofluoric acid, a hazardous chemical that has been used until now to prepare all MXenes, can be avoided by treating the starting materials instead with a solution of lithium fluoride and hydrochloric acid. The resulting material can easily be molded like clay to form conductive films or solids of arbitrary shape (Nature 2014, DOI: 10.1038/nature13970).

FLYING HIGH

Drexel’s Chang (Evelyn) Ren displays a “paper” airplane she made from a MXene film, showing that the material is strong enough to be handled and folded repeatedly. Credit: Mitch Jacoby/C&EN

Credit: Mitch Jacoby/C&EN

Several other research groups have also begun studying MXenes. At the University of Bath, in England, for example, Christopher Eames and M. Saiful Islam computationally screened the interactions of Li⁺, Na⁺, K⁺, and Mg²⁺ with a large number of MXenes in search of new high-capacity battery materials. They find that in terms of voltage and charge capacity the most promising M₂C materials contain light transition metals such as scandium, titanium, vanadium, and chromium (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja508154e).

And at Oak Ridge National Laboratory, Paul R. C. Kent and coworkers are also searching for new battery materials—in this case, for anodes for non–Li-ion batteries. They find that for Mg- and Al-ion batteries, bare MXenes have higher charge capacities and enable greater ion mobilities than O-terminated MXenes. They also find that the metal ion storage mechanism is more complicated in MXenes than in other materials. It involves reversible conversion reactions, ion insertion and extraction, and metal plating and stripping (ACS Nano 2014, DOI: 10.1021/nn503921j).

Energy applications aren’t the only ones on the minds of MXene researchers. At Yanshan University, in Qinhuangdao, China, scientists have found that titanium carbide (Ti₃C₂)with hydroxyl group terminations efficiently soaks up lead ions even in the presence of high concentrations of calcium and magnesium ions, suggesting a way to purify drinking water (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja500506k). And according to a just-published study from Peking University, a polymer-brush-grafted form of V₂C responds to changes in temperature and CO₂ concentration, indicating that the hybrid material may function as a sensor (Chem. Commun. 2014, DOI: 10.1039/c4cc07220k).

It’s hard to guess what the next couple of years will bring to this new area of materials science. But it’s clear that this is only the beginning, Gogotsi says. Researchers have examined just a handful of the MAX phase starting materials, yet more than 70 of those compounds are known, he notes. “There is no reason to think that we have seen the best materials with the most impressive properties.”

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