It seems water can play an important role when using nanocatalysts made of gold nanoparticles combined with metal oxides. From a July 27, 2020 news item on ScienceDaily,
Nanocatalysts made of gold nanoparticles dispersed on metal oxides are very promising for the industrial, selective oxidation of compounds, including alcohols, into valuable chemicals. They show high catalytic activity, particularly in aqueous solution. A team of researchers from Ruhr-Universität Bochum (RUB) has been able to explain why: Water molecules play an active role in facilitating the oxygen dissociation needed for the oxidation reaction. The team of Professor Dominik Marx, Chair of Theoretical Chemistry, reports in the high-impact journal ACS Catalysis on 14 July 2020.
Most industrial oxidation processes involve the use of agents, such as chlorine or organic peroxides, that produce toxic or useless by-products. Instead, using molecular oxygen, O2, and splitting it to obtain the oxygen atoms needed to produce specific products would be a greener and more attractive solution. A promising medium for this approach is the gold/metal oxide (Au/TiO2) system, where the metal oxide titania (TiO2) supports nanoparticles of gold. These nanocatalysts can catalyse the selective oxidation of molecular hydrogen, carbon monoxide and especially alcohols, among others. A crucial step behind all reactions is the dissociation of O2, which comprises a usually high energy barrier. And a crucial unknown in the process is the role of water, since the reactions take place in aqueous solutions.
In a 2018 study, the RUB group of Dominik Marx, Chair of Theoretical Chemistry and Research Area coordinator in the Cluster of Excellence Ruhr Explores Solvation (Resolv), already hinted that water molecules actively participate in the oxidative reaction: They enable a stepwise charge-transfer process that leads to oxygen dissociation in the aqueous phase. Now, the same team reveals that solvation facilitates the activation of molecular oxygen (O2) at the gold/metal oxide (Au/TiO2) nanocatalyst: In fact, water molecules help to decrease the energy barrier for the O2 dissociation. The researchers quantified that the solvent curbs the energy costs by 25 per cent compared to the gas phase. “For the first time, it has been possible to gain insights into the quantitative impact of water on the critical O2 activation reaction for this nanocatalyst – and we also understood why,” says Dominik Marx.
Mind the water molecules
The RUB researchers applied computer simulations, the so-called ab initio molecular dynamics simulations, which explicitly included not only the catalyst but also as many as 80 surrounding water molecules. This was key to gain deep insights into the liquid-phase scenario, which contains water, in direct comparison to the gas phase conditions, where water is absent. “Previous computational work employed significant simplifications or approximations that didn’t account for the true complexity of such a difficult solvent, water,” adds Dr. Niklas Siemer who recently earned his PhD at RUB based on this research.
Scientists simulated the experimental conditions with high temperature and pressure to obtain the free energy profile of O2 in both liquid and gas phase. Finally, they could trace back the mechanistic reason for the solvation effect: Water molecules induce an increase of local electron charge towards oxygen that is anchored at the nanocatalyst perimeter; this in turn leads to the less energetic costs for the dissociation. In the end, say the researchers, it’s all about the unique properties of water: “We found that the polarizability of water and its ability to donate hydrogen bonds are behind oxygen activation,” says Dr. Munoz-Santiburcio. According to the authors, the new computational strategy will help to understand and improve direct oxidation catalysis in water and alcohols.
In discussions about water desalination and carbon nanomaterials, it’s graphene that’s usually mentioned these days. By contrast, scientists from the US Department of Energy’s Lawrence Livermore National Laboratory (LLNL) have turned to carbon nanotubes,
There are two news items about the work at LLNL on ScienceDaily, this first one originated by the American Association for the Advancement of Science (AAAS) offers a succinct summary of the work (from an August 24, 2017 news item on ScienceDaily,
At just the right size, carbon nanotubes can filter water with better efficiency than biological proteins, a new study reveals. The results could pave the way to new water filtration systems, at a time when demands for fresh water pose a global threat to sustainable development.
A class of biological proteins, called aquaporins, is able to effectively filter water, yet scientists have not been able to manufacture scalable systems that mimic this ability. Aquaporins usually exhibit channels for filtering water molecules at a narrow width of 0.3 nanometers, which forces the water molecules into a single-file chain.
Here, Ramya H. Tunuguntla and colleagues experimented with nanotubes of different widths to see which ones are best for filtering water. Intriguingly, they found that carbon nanotubes with a width of 0.8 nanometers outperformed aquaporins in filtering efficiency by a factor of six.
These narrow carbon nanotube porins (nCNTPs) were still slim enough to force the water molecules into a single-file chain. The researchers attribute the differences between aquaporins and nCNTPS to differences in hydrogen bonding — whereas pore-lining residues in aquaporins can donate or accept H bonds to incoming water molecules, the walls of CNTPs cannot form H bonds, permitting unimpeded water flow.
The nCNTPs in this study maintained permeability exceeding that of typical saltwater, only diminishing at very high salt concentrations. Lastly, the team found that by changing the charges at the mouth of the nanotube, they can alter the ion selectivity. This advancement is highlighted in a Perspective [in Science magazine] by Zuzanna Siwy and Francesco Fornasiero.
Lawrence Livermore scientists, in collaboration with researchers at Northeastern University, have developed carbon nanotube pores that can exclude salt from seawater. The team also found that water permeability in carbon nanotubes (CNTs) with diameters smaller than a nanometer (0.8 nm) exceeds that of wider carbon nanotubes by an order of magnitude.
The nanotubes, hollow structures made of carbon atoms in a unique arrangement, are more than 50,000 times thinner than a human hair. The super smooth inner surface of the nanotube is responsible for their remarkably high water permeability, while the tiny pore size blocks larger salt ions.
There’s a rather lovely illustration for this work,
An artist’s depiction of the promise of carbon nanotube porins for desalination. The image depicts a stylized carbon nanotube pipe that delivers clean desalinated water from the ocean to a kitchen tap. Image by Ryan Chen/LLNL
Increasing demands for fresh water pose a global threat to sustainable development, resulting in water scarcity for 4 billion people. Current water purification technologies can benefit from the development of membranes with specialized pores that mimic highly efficient and water selective biological proteins.
“We found that carbon nanotubes with diameters smaller than a nanometer bear a key structural feature that enables enhanced transport. The narrow hydrophobic channel forces water to translocate in a single-file arrangement, a phenomenon similar to that found in the most efficient biological water transporters,” said Ramya Tunuguntla, an LLNL postdoctoral researcher and co-author of the manuscript appearing in the Aug. 24 edition of Science.
Computer simulations and experimental studies of water transport through CNTs with diameters larger than 1 nm showed enhanced water flow, but did not match the transport efficiency of biological proteins and did not separate salt efficiently, especially at higher salinities. The key breakthrough achieved by the LLNL team was to use smaller-diameter nanotubes that delivered the required boost in performance.
“These studies revealed the details of the water transport mechanism and showed that rational manipulation of these parameters can enhance pore efficiency,” said Meni Wanunu, a physics professor at Northeastern University and co-author on the study.
“Carbon nanotubes are a unique platform for studying molecular transport and nanofluidics,” said Alex Noy, LLNL principal investigator on the CNT project and a senior author on the paper. “Their sub-nanometer size, atomically smooth surfaces and similarity to cellular water transport channels make them exceptionally suited for this purpose, and it is very exciting to make a synthetic water channel that performs better than nature’s own.”
This discovery by the LLNL scientists and their colleagues has clear implications for the next generation of water purification technologies and will spur a renewed interest in development of the next generation of high-flux membranes.
Earth is 70 percent water, but only a tiny portion—0.007 percent—is available to drink.
As potable water sources dwindle, global population increases every year. One potential solution to quenching the planet’s thirst is through desalinization—the process of removing salt from seawater. While tantalizing, this approach has always been too expensive and energy intensive for large-scale feasibility.
Now, researchers from Northeastern have made a discovery that could change that, making desalinization easier, faster and cheaper than ever before. In a paper published Thursday [August 24, 2017] in Science, the group describes how carbon nanotubes of a certain size act as the perfect filter for salt—the smallest and most abundant water contaminant.
Filtering water is tricky because water molecules want to stick together. The “H” in H2O is hydrogen, and hydrogen bonds are strong, requiring a lot of energy to separate. Water tends to bulk up and resist being filtered. But nanotubes do it rapidly, with ease.
A carbon nanotube is like an impossibly small rolled up sheet of paper, about a nanometer in diameter. For comparison, the diameter of a human hair is 50 to 70 micrometers—50,000 times wider. The tube’s miniscule size, exactly 0.8 nm, only allows one water molecule to pass through at a time. This single-file lineup disrupts the hydrogen bonds, so water can be pushed through the tubes at an accelerated pace, with no bulking.
“You can imagine if you’re a group of people trying to run through the hallway holding hands, it’s going to be a lot slower than running through the hallway single-file,” said co-author Meni Wanunu, associate professor of physics at Northeastern. Wanunu and post doctoral student Robert Henley collaborated with scientists at the Lawrence Livermore National Laboratory in California to conduct the research.
Scientists led by Aleksandr Noy at Lawrence Livermore discovered last year  that carbon nanotubes were an ideal channel for proton transport. For this new study, Henley brought expertise and technology from Wanunu’s Nanoscale Biophysics Lab to Noy’s lab, and together they took the research one step further.
In addition to being precisely the right size for passing single water molecules, carbon nanotubes have a negative electric charge. This causes them to reject anything with the same charge, like the negative ions in salt, as well as other unwanted particles.
“While salt has a hard time passing through because of the charge, water is a neutral molecule and passes through easily,” Wanunu said. Scientists in Noy’s lab had theorized that carbon nanotubes could be designed for specific ion selectivity, but they didn’t have a reliable system of measurement. Luckily, “That’s the bread and butter of what we do in Meni’s lab,” Henley said. “It created a nice symbiotic relationship.”
“Robert brought the cutting-edge measurement and design capabilities of Wanunu’s group to my lab, and he was indispensable in developing a new platform that we used to measure the ion selectivity of the nanotubes,” Noy said.
The result is a novel system that could have major implications for the future of water security. The study showed that carbon nanotubes are better at desalinization than any other existing method— natural or man-made.
To keep their momentum going, the two labs have partnered with a leading water purification organization based in Israel. And the group was recently awarded a National Science Foundation/Binational Science Foundation grant to conduct further studies and develop water filtration platforms based on their new method. As they continue the research, the researchers hope to start programs where students can learn the latest on water filtration technology—with the goal of increasing that 0.007 percent.
As is usual in these cases there’s a fair degree of repetition but there’s always at least one nugget of new information, in this case, a link to Israel. As I noted many times, the Middle East is experiencing serious water issues. My most recent ‘water and the Middle East’ piece is an August 21, 2017 post about rainmaking at the Masdar Institute in United Arab Emirates. Approximately 50% of the way down the posting, I mention Israel and Palestine’s conflict over water.
A Feb. 27, 2017 article on Nanowerk describes research which could turn living plants into solar cells and panels (Note: Links have been removed),
Plants power life on Earth. They are the original food source supplying energy to almost all living organisms and the basis of the fossil fuels that feed the power demands of the modern world. But burning the remnants of long-dead forests is changing the world in dangerous ways. Can we better harness the power of living plants today?
One way might be to turn plants into natural solar power stations that could convert sunlight into energy far more efficiently. To do this, we’d need a way of getting the energy out in the form of electricity. One company has found a way to harvest electrons deposited by plants into the soil beneath them. But new research (PNAS, “In vivo polymerization and manufacturing of wires and supercapacitors in plants”) from Finland looks at tapping plants’ energy directly by turning their internal structures into electric circuits.
A Feb. 27, 2017 essay by Stuart Thompson for The Conversation (which originated the article) explains the principles underlying the research (Note: A link has been removed),
Plants contain water-filled tubes called “xylem elements” that carry water from their roots to their leaves. The water flow also carries and distributes dissolved nutrients and other things such as chemical signals. The Finnish researchers, whose work is published in PNAS, developed a chemical that was fed into a rose cutting to form a solid material that could carry and store electricity.
Previous experiments have used a chemical called PEDOT to form conducting wires in the xylem, but it didn’t penetrate further into the plant. For the new research, they designed a molecule called ETE-S that forms similar electrical conductors but can also be carried wherever the stream of water travelling though the xylem goes.
This flow is driven by the attraction between water molecules. When water in a leaf evaporates, it pulls on the chain of molecules left behind, dragging water up through the plant all the way from the roots. You can see this for yourself by placing a plant cutting in food colouring and watching the colour move up through the xylem. The researchers’ method was so similar to the food colouring experiment that they could see where in the plant their electrical conductor had travelled to from its colour.
The result was a complex electronic network permeating the leaves and petals, surrounding their cells and replicating their pattern. The wires that formed conducted electricity up to a hundred times better than those made from PEDOT and could also store electrical energy in the same way as an electronic component called a capacitor.
I recommend reading Thompson’s piece in its entirety.
ORNL researchers discovered that water in beryl displays some unique and unexpected characteristics. (Photo by Jeff Scovil)
That striking image from the Oak Ridge National Laboratory (ORNL; US) depicting a new state for water molecules looks like mixed media: photography and drawing/illustration. Thankfully, an April 22, 2016 news item on ScienceDaily provides a text description,
Neutron scattering and computational modeling have revealed unique and unexpected behavior of water molecules under extreme confinement that is unmatched by any known gas, liquid or solid states.
In a paper published in Physical Review Letters, researchers at the Department of Energy’s Oak Ridge National Laboratory [ORNL] describe a new tunneling state of water molecules confined in hexagonal ultra-small channels — 5 angstrom across — of the mineral beryl. An angstrom is 1/10-billionth of a meter, and individual atoms are typically about 1 angstrom in diameter.
The discovery, made possible with experiments at ORNL’s Spallation Neutron Source and the Rutherford Appleton Laboratory in the United Kingdom, demonstrates features of water under ultra confinement in rocks, soil and cell walls, which scientists predict will be of interest across many disciplines.
“At low temperatures, this tunneling water exhibits quantum motion through the separating potential walls, which is forbidden in the classical world,” said lead author Alexander Kolesnikov of ORNL’s Chemical and Engineering Materials Division. “This means that the oxygen and hydrogen atoms of the water molecule are ‘delocalized’ and therefore simultaneously present in all six symmetrically equivalent positions in the channel at the same time. It’s one of those phenomena that only occur in quantum mechanics and has no parallel in our everyday experience.”
The existence of the tunneling state of water shown in ORNL’s study should help scientists better describe the thermodynamic properties and behavior of water in highly confined environments such as water diffusion and transport in the channels of cell membranes, in carbon nanotubes and along grain boundaries and at mineral interfaces in a host of geological environments.
ORNL co-author Lawrence Anovitz noted that the discovery is apt to spark discussions among materials, biological, geological and computational scientists as they attempt to explain the mechanism behind this phenomenon and understand how it applies to their materials.
“This discovery represents a new fundamental understanding of the behavior of water and the way water utilizes energy,” Anovitz said. “It’s also interesting to think that those water molecules in your aquamarine or emerald ring – blue and green varieties of beryl – are undergoing the same quantum tunneling we’ve seen in our experiments.”
While previous studies have observed tunneling of atomic hydrogen in other systems, the ORNL discovery that water exhibits such tunneling behavior is unprecedented. The neutron scattering and computational chemistry experiments showed that, in the tunneling state, the water molecules are delocalized around a ring so the water molecule assumes an unusual double top-like shape.
“The average kinetic energy of the water protons directly obtained from the neutron experiment is a measure of their motion at almost absolute zero temperature and is about 30 percent less than it is in bulk liquid or solid water,” Kolesnikov said. “This is in complete disagreement with accepted models based on the energies of its vibrational modes.”
Water is a unique liquid and researchers from Germany and the Netherlands can detail at least part of why that’s so according to a Sept. 18, 2015 news item on Nanowerk,
A team of scientists from the Max Planck Institute for Polymer Research (MPI-P) in Mainz, Germany and FOM Institute AMOLF in the Netherlands have characterized the local structural dynamics of liquid water, i.e. how quickly water molecules change their binding state. Using innovative ultrafast vibrational spectroscopies, the researchers show why liquid water is so unique compared to other molecular liquids. …
With the help of a novel combination of ultrafast laser experiments, the scientists found that local structures persist in water for longer than a picosecond, a picosecond (ps) being one thousandth of one billionth of a second ((1012 s). This observation changes the general perception of water as a solvent.
… “71% of the earth’s surface is covered with water. As most chemical and biological reactions on earth occur in water or at the air water interface in oceans or in clouds, the details of how water behaves at the molecular level are crucial. Our results show that water cannot be treated as a continuum, but that specific local structures exist and are likely very important” says Mischa Bonn, director at the MPI-P.
Water is a very special liquid with extremely fast dynamics. Water molecules wiggle and jiggle on sub-picosecond timescales, which make them undistinguishable on this timescale. While the existence of very short-lived local structures – e.g. two water molecules that are very close to one another, or are very far apart from each other – is known to occur, it was commonly believed that they lose the memory of their local structure within less than 0.1 picoseconds.
The proof for relatively long-lived local structures in liquid water was obtained by measuring the vibrations of the Oxygen-Hydrogen (O-H) bonds in water. For this purpose the team of scientists used ultrafast infrared spectroscopy, particularly focusing on water molecules that are weakly (or strongly) hydrogen-bonded to their neighboring water molecules. The scientists found that the vibrations live much longer (up to about 1 ps) for water molecules with a large separation, than for those that are very close (down to 0.2 ps). In other words, the weakly bound water molecules remain weakly bound for a remarkably long time.