Chemistry – Page 46 – Chemistry and Physics

October 20, 2022October 21, 2022

Developing stable water-splitting catalysts that don’t require expensive iridium

Synthesis and characterization of Ni-RuO². Credit: *Nature Materials* (2022). DOI: 10.1038/s41563-022-01380-5

Creating a hydrogen economy is no small task, but Rice University engineers have discovered a method that could make oxygen evolution catalysis in acids, one of the most challenging topics in water electrolysis for producing clean hydrogen fuels, more economical and practical.

The lab of chemical and biomolecular engineer Haotian Wang at Rice’s George R. Brown School of Engineering has replaced rare and expensive iridium with ruthenium, a far more abundant precious metal, as the positive-electrode catalyst in a reactor that splits water into hydrogen and oxygen.

The lab’s successful addition of nickel to ruthenium dioxide (RuO₂) resulted in a robust anode catalyst that produced hydrogen from water electrolysis for thousands of hours under ambient conditions.

“There’s huge industry interest in clean hydrogen,” Wang said. “It’s an important energy carrier and also important for chemical fabrication, but its current production contributes a significant portion of carbon emissions in the chemical manufacturing sector globally. We want to produce it in a more sustainable way, and water-splitting using clean electricity is widely recognized as the most promising option.”

Iridium costs roughly eight times more than ruthenium, he said, and it could account for 20% to 40% of the expense in commercial device manufacturing, especially in future large-scale deployments.

The process developed by Wang, Rice postdoctoral associate Zhen-Yu Wu and graduate student Feng-Yang Chen, and colleagues at the University of Pittsburgh and the University of Virginia is detailed in Nature Materials.

Water splitting involves the oxygen and hydrogen evolution reactions by which polarized catalysts rearrange water molecules to release oxygen and hydrogen. “Hydrogen is produced by the cathode, which is a negative electrode,” Wu said. “At the same time, it has to balance the charge by oxidizing water to generate oxygen on the anode side.”

Rice lab advances water-splitting catalystsEngineers develop stable devices that don’t require expensive iridium — A schematic shows the experimental water electrolyzer developed at Rice to use a nickel-doped ruthenium catalyst. Illustration by Zhen-Yu Wu

“The cathode is very stable and not a big problem, but the anode is more prone to corrosion when using an acidic electrolyte,” Chen said. “Commonly used transition metals like manganese, iron, nickel and cobalt get oxidized and dissolve into the electrolyte.

“That’s why the only practical material used in commercial proton exchange membrane water electrolyzers is iridium,” he said. “It’s stable for tens of thousands of hours, but it’s very expensive.”

Setting out to find a replacement, Wang’s lab settled on ruthenium dioxide for its known activity, doping it with nickel, one of several metals tried.

The researchers demonstrated that ultrasmall and highly crystalline RuO2 nanoparticles with nickel dopants, used at the anode, facilitated water-splitting for more than 1,000 hours at a current density of 200 milliamps per square centimeter with negligible degradation.

They tested their anodes against others of pure ruthenium dioxide that catalyzed water electrolysis for a few hundred hours before beginning to decay.

The lab is working to improve its ruthenium catalyst to slot into current industrial processes. “Now that we’ve reached this stability milestone, our challenge is to increase the current density by at least five to 10 times while still maintaining this kind of stability,” Wang said. “This is very challenging, but still possible.”

He sees the need as urgent. “The annual production of iridium won’t help us to produce the amount of hydrogen we need today,” Wang said. “Even using all the iridium globally produced will simply not generate the amount of hydrogen we will need if we want it to be produced via water electrolysis.

“That means we can’t fully rely on iridium,” he said. “We have to develop new catalysts to either reduce its use or eliminate it from the process entirely.”

October 19, 2022October 19, 2022

Converting carbon dioxide to solid minerals underground for more stable storage

A new scientific review article in Nature Reviews Chemistry discusses how carbon dioxide (CO₂) converts from a gas to a solid in ultrathin films of water on underground rock surfaces. These solid minerals, known as carbonates, are both stable and common.

“As global temperatures increase, so does the urgency to find ways to store carbon,” said Pacific Northwest National Laboratory (PNNL) Lab Fellow and co-author Kevin Rosso. “By taking a critical look at our current understanding of carbon mineralization processes, we can find the essential-to-solve gaps for the next decade of work.”

Mineralization underground represents one way to keep CO₂ locked away, unable to escape back into the air. But researchers first need to know how it happens before they can predict and control carbonate formation in realistic systems.

“Mitigating human emissions requires fundamentally understanding how to store carbon,” said PNNL chemist Quin Miller, co-lead author of the scientific review featured on the journal cover. “There is a pressing need to integrate simulations, theory, and experiments to explore mineral carbonation problems.”

Below the ground and in the water

Instead of emitting CO₂ into the air, one option is to pump it into the ground. Putting CO₂ deep underground theoretically sequesters the carbon away. However, gas leaks remain a concern. But if that CO₂ gas can be pumped into rocks rich in metals like magnesium and iron, the CO₂ can be transformed into stable and common carbonate minerals. PNNL’s Basalt Pilot Project at Wallula is a field site dedicated to studying CO₂ storage in carbonates.

Although these subsurface environments are generally dominated by water, the conversion of gaseous carbon dioxide to solid carbonate can also occur when injected CO₂ displaces that water, creating extremely thin films of residual water in contact with rocks. But these highly confined systems behave differently than CO₂ in contact with a pool of water.

In thin films, the ratio of water and CO₂ controls the reaction. Small amounts of metal leach out from the rocks, reacting both in the film and on the rock surface. This leads to the creation of new carbonate materials.

Previous work led by Miller, summarized in the review, showed that magnesium behaves similarly to calcium in thin water films. The nature of the water film plays a central role in how the system reacts.

Understanding how and when these carbonates form requires a combination of laboratory experiments and theoretical modeling studies. Laboratory work allows researchers to tune the ratio of water to CO₂ and watch carbonates form in real time. Teams can see which specific chemicals are present at different points in time, providing essential information about reaction pathways.

However, laboratory-based work has its limits. Researchers cannot observe individual molecules or see how they interact. Chemistry models can fill in that gap by predicting how molecules move in exquisite detail, giving conceptual backbone to experiments. They also allow researchers to study mineralization in hard to experimentally access conditions.

“There are important synergies between models and laboratory or field studies,” said MJ Qomi, a professor at the University of California, Irvine and co-lead author of the article. “Experimental data grounds models in reality, while models provide a deeper level of insight into experiments.” Qomi has collaborated with the PNNL team for three years, and plans to study carbonate mineralization in adsorbed water films.

From fundamental science to solutions

The team outlined key questions that need answering to make this form of carbon storage practical. Researchers must develop knowledge of how minerals react under different conditions, particularly in conditions that mimic real storage sites, including in ultrathin water films. This should all be done through an integrated combination of modeling and laboratory experiments.

Mineralization has the potential to keep carbon safely stored underground. Knowing how CO₂ will react with different minerals can help make sure that what gets pumped underneath the surface stays there. The fundamental science insights from mineralization work can lead to practical CO₂ storage systems. The Basalt Pilot Project represents an important study site that bridges small-scale basic science and large-scale research applications.

“This work combines a focus on fundamental geochemical insights with a goal of solving crucial problems,” said Miller. “Without prioritizing decarbonization technologies, the world will continue warming to a degree humanity cannot afford.”

Miller, Rosso, and Todd Schaef were the PNNL authors of this study. This work was performed in collaboration with MJ Qomi and Siavash Zare of the University of California, Irvine as well as John Kaszuba of the University of Wyoming.

October 18, 2022October 18, 2022

Steel mill gases transformed into bioplastic

Plastic waste from food deliveries is rapidly polluting the environment. An alternative that has emerged is bioplastic, which is also called biodegradable plastic. Bioplastic that uses eco-friendly raw materials emits less pollutants during the production process and has natural decomposition properties. Recently, a Korea-Spain joint research team recreated bioplastic from waste byproducts from gas fermentation from steel mills.

Through joint research with Spain’s Center for Research in Agricultural Genomics (CRAG), a research team led by Professor Gyoo Yeol Jung, Ph.D. candidates Dae-yeol Ye and Jo Hyun Moon, and Dr. Myung Hyun Noh in the Department of Chemical Engineering at POSTECH has developed a technology to generate artificial enzymes from E. coli. The joint research then succeeded in mass-producing itaconic acid, a source material for bioplastic, from acetic acid in E. coli. This study is published in Nature Communications.

Itaconic acid produced by fungi with membrane-enclosed organelles is used as a raw material for various plastics, as well as cosmetics and antibacterial agents. Although its global market value is estimated high at around 130 billion KRW (USD$91 million) this year, its production and utilization have been limited due to the complex production process and high cost of production.

For this reason, studies are being actively conducted to produce itaconic acid with industrial microorganisms such as E. coli. Although E. coli can be produced using inexpensive raw materials and is easy to culture, additional raw materials or processes were required to produce itaconic acid since it lacks membrane-enclosed organelles.

Using biosynthesis, the joint research team developed an artificial enzyme to pave the way for E. coli to directly produce itaconic acid without membrane-enclosed organelles. The research results showed that the newly developed enzyme can be used in E. coli to produce itaconic acid. With this technology, it is now possible to build a microbial cell factory that can easily produce itaconic acid from cheap and various raw materials.

This research result is evaluated as a key original technology for producing itaconic acid from byproduct of gas fermentation products from steel mills, seaweed, as well as agricultural and fishery byproducts such as lignocellulosic biomass. By replacing the raw material from petrochemicals with biosynthesized itaconic acid, the new technology is anticipated to contribute to a carbon-neutral society and significantly expand the itaconic acid market.

October 17, 2022October 17, 2022

A new method to dehydrogenate alkanes at ambient conditions

The chemical term alkanes, or paraffins, refers to organic compounds that consist of single-bonded carbon and hydrogen atoms, such as methane, ethane, and propane, and several other hydrocarbons. Over the years, alkanes have become widely used in organic chemistry, due to their unique chemical properties and their role in producing chemical reactions.

In recent decades, chemists have been exploring the possibility of breaking alkanes down through a process known as “non-oxidative dehydrogenation,” to attain valuable carbon feedstocks and hydrogen fuel. This could ultimately have valuable implications for the energy sector, as it would provide a low-cost method of creating clean fuel for new hydrogen-based energy solutions.

Unfortunately, the C-H bonds in alkanes can generally only be broken down at high temperatures, under ultraviolet light, or using stoichiometric oxidants. This hinders the large-scale adoption of this approach to create hydrogen fuel.

Researchers at Jilin University in China have recently introduced a possible strategy to achieve the non-oxidative dehydrogenation of alkanes in visible to near-infrared light and at room temperature. Their proposed method, introduced in a paper published in Nature Energy, entails the use of platinum/titanium dioxide (Pt/black TiO₂) photocatalysts, in which platinum atoms are close but not fully bonded.

“The initial goal of our study was to replace the commercial TiO₂ used in my previous work with black TiO₂ , to enhance the light-absorption capacity of the catalyst,” Lu Li said. “In our experiments, we utilized solar light energy to drive the non-oxidative dehydrogenation of alkanes at room temperature. The introduction of clean photon energy can overcome the thermodynamic limitation, making alkanes conversion occur at ambient conditions with higher selectivity and durability.”

In initial experiments, the approach by Li and his colleagues achieved highly promising results, as it could efficiently dehydrogenate different alkanes at room temperature and in visible to near-infrared light. For cyclohexane, their photocatalysts enabled the production of H₂with a turnover number of 100,000, with the reaction effectively ongoing for 80 reaction cycles. This is a significantly better result than those achieved by thermal reactions.

For methane, on the other hand, the researchers achieved a conversion rate of 8.2%, with a 65% selectivity to propane. Finally, for C2+ alkanes, Li and his colleagues achieved a fast dehydrogenation (up to 1,440 µmol g⁻¹ h⁻¹) to the corresponding olefins.

“The brand-new ‘single-atom collection’ catalysts can combine the advantages of single-atom catalysts and nano-catalysts,” Li said. “This new class of catalysts may have potential applications in many important heterogeneous catalytic processes.”

In the future, the photocatalysts used by this team of researchers could prove to be highly valuable for enabling the production of hydrogen fuel from alkanes, without the need for high temperatures, UV-light, and stoichiometric oxidants. This could help to lower the cost of hydrogen fuel production, thus potentially facilitating its use for energy applications.

“To extend this work, we are now looking for efficient photocatalysts based on cheap metals,” Li added. “Furthermore, we will extend the substrates to various non-toxic saturated hydrocarbons.”

October 13, 2022October 13, 2022

Developing self-complementary macrocycles with ingenious molecules

Some biological molecules with efficient noncovalent bonding sites can use their bonding properties to create well-defined assemblies from a single class of molecules—i.e., they assemble with each other. These molecules, which are frequently seen in nature, are referred to as “self-complementary assemblies.” For instance, the p24 protein hexamer, which is part of the capsid of the HIV (human immunodeficiency virus), is composed of six protein subunits that complementarily self-assemble using many hydrogen bonds.

This phenomenon allows well-designed molecules to form higher-ordered assemblies without the metal ions that are commonly used as “joints” between monomer molecules. Indeed, many self-complementary assemblies have been reported on the basis of intrinsic hydrogen bonds, π-interactions, and coordination bonds.

However, self-complementary assembly based on host-guest systems is rare and notoriously difficult to control. In order to further our understanding of self-complementary assembly with higher-ordered structures, many new strategies have come to light in recent years.

Now, a team of researchers from the School of Science at Tokyo Institute of Technology (Tokyo Tech) might have just cracked the code to developing these innovative systems. The team, led by Assistant Professor Masahiro Yamashina and Professor Shinji Toyota, has constructed a novel self-complementary macrocycle assembly using an anthracene-based tweezer-like molecule with a pyridinedicarboxamide (PDA) linker as the monomeric species. Their work is described in Nature Communications.

“The molecule we use has an interesting property: It can bond with itself in two ways and form self-complementary structures. Not only does it show head-to-tail π-π interactions between the electron-rich tweezer tail (the anthracene groups) and the electron-deficient head, but also presents hydrogen bonding through the amide (-NH) functional group. By incorporating these two interactions, a preferential direction of self-assembly is achieved, and this guides the formation of the macrocycle,” explains Prof. Yamashina.

This type of dual interaction leads to much more control over the formation of synthetic macrocycles, and in this case, gives rise to a stable self-complementary hexameric structure upon crystallization. These hexamers can further assemble into even bigger self-complementary structures in the right conditions.

“When we added trifluoroacetic acid (TFA), we found that the cyclic hexamers further assemble into two predominant, stable supramolecular structures: rhombohedral grid assemblies and giant spherical cuboctahedra, a so-called hierarchical assembly,” Prof. Toyota says. “The latter structure is particularly impressive as it is formed from 108 monomeric tweezer units.”

Current methods to form supramolecular assemblies require metals that could harm the environment and ecosystems. The metal-free alternative method described here could produce novel supramolecular structures using a simple anthracene-based tweezer molecule. It opens the door to a new range of supramolecular assemblies with optical and electronic functions. This work adds a new important tool in the chemistry toolbox, one that is sure to play a big role in the metal-free supramolecular structures of the future.

October 13, 2022October 13, 2022

‘Smart plastic’ material is step forward toward soft, flexible robotics and electronics

Inspired by living things from trees to shellfish, researchers at The University of Texas at Austin set out to create a plastic much like many life forms that are hard and rigid in some places and soft and stretchy in others. Their success—a first, using only light and a catalyst to change properties such as hardness and elasticity in molecules of the same type—has brought about a new material that is 10 times as tough as natural rubber and could lead to more flexible electronics and robotics.

The findings are published today in the journal Science.

“This is the first material of its type,” said Zachariah Page, assistant professor of chemistry and corresponding author on the paper. “The ability to control crystallization, and therefore the physical properties of the material, with the application of light is potentially transformative for wearable electronics or actuators in soft robotics.”

Patterned sample is being stretched under uniaxial tension. Video was recorded with the sample between cross-polarizers, allowing for visualization of polymer chain alignment. The dark, opaque sections have been hardened. The transparent sections have been left soft and stretchy. Credit: The University of Texas at Austin

Scientists have long sought to mimic the properties of living structures, like skin and muscle, with synthetic materials. In living organisms, structures often combine attributes such as strength and flexibility with ease. When using a mix of different synthetic materials to mimic these attributes, materials often fail, coming apart and ripping at the junctures between different materials.

Oftentimes, when bringing materials together, particularly if they have very different mechanical properties, they want to come apart,” Page said. Page and his team were able to control and change the structure of a plastic-like material, using light to alter how firm or stretchy the material would be.

Patterned island sample is being stretched and relaxed under uniaxial tension. Video was recorded with the sample as seen (left) and between cross-polarizers (right), allowing for visualization of polymer chain alignment. The dark, opaque spots are areas that have been hardened. Credit: The University of Texas at Austin

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Chemists started with a monomer, a small molecule that binds with others like it to form the building blocks for larger structures called polymers that were similar to the polymer found in the most commonly used plastic. After testing a dozen catalysts, they found one that, when added to their monomer and shown visible light, resulted in a semicrystalline polymer similar to those found in existing synthetic rubber. A harder and more rigid material was formed in the areas the light touched, while the unlit areas retained their soft, stretchy properties.

Because the substance is made of one material with different properties, it was stronger and could be stretched farther than most mixed materials.

The reaction takes place at room temperature, the monomer and catalyst are commercially available, and researchers used inexpensive blue LEDs as the light source in the experiment. The reaction also takes less than an hour and minimizes use of any hazardous waste, which makes the process rapid, inexpensive, energy efficient and environmentally benign.

Patterned suture sample is being stretched under uniaxial tension. Video was recorded with the sample between cross-polarizers, allowing for visualization of polymer chain alignment. Credit: The University of Texas at Austin

The researchers will next seek to develop more objects with the material to continue to test its usability.

“We are looking forward to exploring methods of applying this chemistry towards making 3D objects containing both hard and soft components,” said first author Adrian Rylski, a doctoral student at UT Austin.

The team envisions the material could be used as a flexible foundation to anchor electronic components in medical devices or wearable tech. In robotics, strong and flexible materials are desirable to improve movement and durability.

Patterned sample is being melted to show complete transparency and later the opacity returning as the sample cools and returns to a semicrystalline state. Credit: The University of Texas at Austin

Henry L. Cater, Keldy S. Mason, Marshall J. Allen, Anthony J. Arrowood, Benny D. Freeman and Gabriel E. Sanoja of The University of Texas at Austin also contributed to the research.