Physical Science and Engineering | Physical Science & Engineering

Thursday, March 04, 2021

Porous crystal guides reaction to transform CO2

Metal organic framework helps silver catalyst to turn greenhouse gas into useful chemical feedstock.

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By embedding a silver catalyst inside a porous crystal, KAUST researchers have improved a chemical reaction that converts carbon dioxide (CO2) into carbon monoxide (CO), which is a useful feedstock for the chemical industry.

Carbon monoxide is a building block for producing hydrocarbon fuels, and many researchers are searching for ways to produce it from CO2, a greenhouse gas emitted by burning fossil fuels. One strategy involves using electricity and a catalyst to drive a so-called CO2 reduction reaction. But this reaction typically produces a variety of other products, including methane, methanol and ethylene. Separating these products significantly raises the cost of the process, so researchers hope to guide the reaction to generate a single product.

Osama Shekhah and Mohamed Eddaoudi, chemists at KAUST, in collaboration with Ted Sargent’s group at the University of Toronto, have now fine-tuned the CO2 reduction reaction using metal organic frameworks (MOFs). These porous crystals contain a lattice of metal-based nodes connected by carbon-based linker molecules. By altering these components, researchers can tailor the size of an MOF’s pores and its chemical properties.

The researchers created four different MOFs with the same overall lattice arrangement and grew 5-nanometer-wide nanoparticles of silver inside the pores of each MOF. Then they tested each MOF to find how its structure affected the CO2 reduction reaction. They monitored which products emerged from the process and studied how an activated form of CO — a crucial intermediate in the reaction — bound to the silver catalyst.

The most effective MOF contained zirconium-based nodes connected by molecules of 1,4-naphthalenedicarboxylic acid. Because it has smaller pores, its ability to trap CO2 outperformed its rivals.

The silver nanoparticle in this MOF also bound activated CO in a different way than the others, connecting in a “bridging mode” involving two bonds rather than one. This ensured that CO was less likely to transform into unwanted byproducts. “Controlling the type of the CO intermediate during the reaction has a big influence on the CO selectivity,” says Shekhah. Together, these effects boosted the efficiency of CO production to 94 percent, a dramatic improvement in selectivity.

The researchers hope to build on their strategy, making further tweaks to the MOF’s structure to enhance the CO2 reduction reaction. “We believe that this work paves the way for using MOFs as new supports for improving the activity and product selectivity of the CO2 reduction reaction by directly interacting with the gaseous intermediates and controlling their binding mode,” says Eddaoudi.

- KAUST Discovery

Monday, March 01, 2021

Hot electrons send CO2 back to the future

Catalyst nanoparticles trap an unprecedented range of wavelengths of light to convert carbon dioxide into methane.

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Atmospheric carbon dioxide (CO2) is a major driver of global warming, but this gas could also serve as a valuable resource. Researchers at KAUST have developed an efficient catalyst that uses light energy to convert CO2 and hydrogen into methane (CH4). This counteracts the release of CO2 when methane is burned as a fuel.

Many researchers worldwide are exploring ways to convert CO2 into useful carbon-based chemicals, but their efforts have been limited by low efficiencies that restrict the potential for large-scale application.

“Our approach is based on the synergistic combination of light and heat, known as the photothermal effect,” says postdoc Diego Mateo. He explains that the heat is generated by the interaction of light with the catalyst, so the two forms of energy come from absorbed light.

Some other industrial approaches require heating from external sources to attain temperatures as high as 500 degrees Celsius. The KAUST research demonstrates that the reaction can be achieved using just the photothermal effect of daylight.

The catalyst is built from nickel nanoparticles on a layer of barium titanate. It captures the light in a way that kicks electrons into high energy states, known as “hot electrons”. These electrons then initiate the chemical reaction that sends CO2 back into methane. Under optimum conditions, the catalyst generates methane with nearly 100 percent selectivity and with impressive efficiency.

A major advantage is the wide range of the spectrum of light harnessed, including all visible wavelengths, in addition to the ultraviolet rays that many catalysts are restricted to. This is hugely significant since ultraviolet light comprises only 4 to 5 percent of the energy available in sunlight.

“We strongly believe that our strategy, in combination with other existing CO2 capture techniques, could be a sustainable way to convert this harmful greenhouse gas into valuable fuel,” says Mateo.

Any fuels made from CO2 would still release that gas when they are burned, but the CO2 could be repeatedly recycled from the atmosphere to fuel and back again, rather than being continually released by burning fossil fuels.

The researchers are also looking to widen the applications of their approach. “One strategy for our future research is to move towards producing other valuable chemicals, such as methanol,” says Jorge Gascon, who led the research team. The researchers also see potential for using light energy to power the production of chemicals that don’t contain carbon, such as ammonia (NH3).

- KAUST Discovery

Sunday, February 28, 2021

Pumping perovskites into a semiconductor platform

Fluid injection of perovskite semiconductors creates microwires to build different optoelectronic devices on a single silicon chip.

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Materials called perovskites can be more readily incorporated into silicon-based semiconducting platforms by using a microfluidic pumping technology developed at KAUST.

The perovskites currently being explored for many applications in new technologies are diverse materials sharing the same crystalline structure as the natural mineral perovskite. These semiconducting materials show great promise in a variety of optoelectronic applications, such as light emitters, sensors and solar cells.

Compared to traditional semiconductors, perovskites are soft and unstable. “This makes it difficult to pattern them using standard lithography methods,” says materials scientist Iman Roqan at KAUST.

The challenge tackled by Roqan and her colleagues was to adapt microfluidic technologies to manipulate solutions carrying perovskites to create semiconducting microscale wires.

The first step in the procedure is to create the desired pattern of microchannels in a silicon wafer using a laser interference technique. The key innovation is to pump a solution containing perovskite ions into these microchannels, where the perovskite solidifies into the network of semiconductor wires.

“We are the first researchers to achieve this,” says Roqan, adding that the innovation, which has now been patented, will allow several different perovskite-based optoelectronic devices to be located on one silicon chip. For example, a single chip could embody a photodetector, transistor, light-emitting diode and a solar cell to act as the power supply.

To demonstrate the potential of the technique, the team have built a high-performance photodetector.

“Unlike other options, our fabrication method is extremely simple and cost-effective, while avoiding creating waste materials,” Roqan says. Some metals used in perovskites are toxic, so avoiding their release into the environment is a significant advantage.

There remain challenges to overcome to advance from this initial proof-of-concept stage. For example, the team is working to bring the stability of the devices up to a standard suitable for large-scale industrial applications. Roqan expects that work currently in progress to achieve this is likely to lead to further patents, while also building more complex demonstration devices.

One of the most likely areas for commercial development is in the growing field of "lab on a chip" technology. This can combine processes such as sensing, signaling, microfluid manipulation and chemical separations and combinations to miniaturize many laboratory procedures used in medicine and biology. With miniaturization comes portability, potentially taking a full testing laboratory process out of the lab and into the outside world.

- KAUST Discovery

Thursday, February 18, 2021

Locked MOFs are the key to high porosity

Sophisticated geometry design gives rise to a new form of crystalline material.

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A highly porous metal organic framework, assembled from molecular building blocks designed to lock together in a specific orientation, has been developed by researchers at KAUST.

Metal organic frameworks (MOFs) are crystalline materials made from metal ions connected by organic linkers. Their internal structure is like a repeating array of tiny identical cages, which are ideal for hosting various molecules. MOFs have found potential uses from gas sensing to molecular separations to storage, depending on the dimensions and structure of their pores.

One family of MOFs has been inspired by inorganic porous materials called zeolites. Zeolites are a special class of porous material with myriad applications, explains Norah Alsadun, a Ph.D. student in Mohamed Eddaoudi's lab, who led the research. “However, the ability to fine-tune the pore-aperture size and pore system of a given zeolite, based on a given topology, is extremely challenging,” she says.

By mimicking the zeolite structure with a MOF, however, the pore structure can be readily adjusted by changing the metal and the organic linker. “Our group introduced the use of single-metal-based tetrahedral building units for zeolite-like MOF (ZMOF) construction,” Alsadun says. The basis of the tetrahedron-based ZMOF structure is a pair of triangular pyramids, attached tip-to-tip via a single bond.

The free rotation of the two triangles around the single bond typically leads to the formation of a diamond-like structure. However, the assembly of tetrahedral building units in MOF chemistry can lead to the simultaneous formation of multiple diamond networks, all penetrating each other, thus blocking the pores. “So, we developed a new concept for ZMOF assembly using polynuclear clusters as rigid, directional and locked-in building units,” Eddaoudi says.

Rather than simple single-metal tetrahedra as the main building block, the team used expanded polynuclear clusters in which each edge and corner of the original tetrahedron is replaced by a new face — a geometric concept known as cantellation. As a result, the single bond linking the two tetrahedrons together in the original structure is replaced by three bonds, locking the structure into a specific orientation.

As the team had planned, the locked building blocks generated a ZMOF with a "sodalite", rather than a diamond-like, structure. “The sodalite topology has no room for interpenetration,” says Vincent Guillerm, a senior research scientist in the team. The resultant sod-ZMOF pores measured up to 43 angstroms in diameter, the largest yet reported for a ZMOF.

“Our work proves that these challenging structures can be accessed through our novel design,” Eddaoudi says. “Applying this strategy to construct other ZMOFs is now in progress.”

- KAUST Discovery

Thursday, February 18, 2021

Giving oxygen to the question of air quality

Volatile alkanes can rapidly acquire oxygen atoms in a free radical chain reaction, a process significant for fuel combustion and air pollution.

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The simplest of organic molecules have a much more complex relationship with oxygen than previously thought. Researchers from KAUST and their international collaborators have shown that alkanes participate extensively in autoxidation reactions with oxygen molecules1. The discovery, which overturns current chemical wisdom, has implications for air quality prediction and efficient fuel combustion in engines.

Autoxidation is a chemical process in which oxygen molecules rapidly and sequentially add to organic molecules in a radical chain reaction. The process is critical for the timing of fuel combustion in engines and is a key step in the atmospheric conversion of volatile organic molecules into particulate matter.

“Conventional knowledge suggests atmospheric autoxidation requires precursor molecules with features such as double bonds or oxygen-containing moieties,” says Zhandong Wang, now a professor at the University of Science and Technology of China, formerly a research scientist with Mani Sarathy at KAUST. Alkanes — the primary component of combustion engine fuels and an important class of urban trace gases — do not have these structural features. “Alkanes were thought to have only minor susceptibility to extensive autoxidation,” Wang says.

To overturn this assumption, Sarathy, Wang and colleagues showed that alkanes do undergo extensive autoxidation under the hot high-pressure conditions of combustion2. The team then set out to explore the possibility that alkane autoxidation also occurs under atmospheric conditions.

“In 2016, we collaborated with University of Helsinki researchers to win a KAUST Competitive Research Grant,” says Wang. “That was the beginning of this work.”

The team used a state-of-the-art analytical technique, called chemical ionization atmospheric pressure interface time-of-flight mass spectrometry, to detect products of atmospheric alkane autoxidation. “Strikingly, the yield of highly oxygenated organic molecules containing six or more oxygen atoms was much higher than expected,” Wang says.

Under combustion conditions, the team also observed alkanes that had undergone up to five sequential O2 additions, significantly higher than the three additions they observed previously.

“These findings enrich our understanding of autoxidation processes and will allow us to better perform predictive simulations of combustion engines and atmospheric processes that impact air quality and climate,” Sarathy says.

“We are now working with the department of Health, Safety and Environment in KAUST to better understand atmospheric chemical processes using real-world measurements.” he adds. “Using data acquired at the KAUST-based monitoring station, we are attempting to unravel complex atmospheric chemical processes in the western region of Saudi Arabia.”

- KAUST Discovery

Thursday, February 11, 2021

Get a load of ZIF! Better delivery of cancer immunotherapy

An antibody loaded onto a porous metal organic framework is released by the acidic environment that surrounds tumors, avoiding the adverse effects of administering the antibody alone.

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Loading a cancer immunotherapy drug onto a metal organic framework improves both its delivery and its sustained release for treating leukemia. Furthermore, coating the drug-loaded framework with a cancer cell membrane improves targeted delivery to solid tumors. These findings could lead to safer and more reliable cancer immunotherapies.

“We believe our findings are quite significant because they show that the undesirable side effects of immunotherapy can be modulated by choosing the right delivery vehicle,” says KAUST chemist Niveen Khashab. “They also show that targeted delivery can be realistically established through proper surface functionalization.”

Cancer immunotherapy involves using drugs that modulate the immune system to make it more effective in attacking tumors. Nivolumab is one such drug, which counteracts a cancer immune evasion strategy that involves expressing PD-L1 molecules that deactivate T cells by blocking the PD-1 molecules on their surfaces.

Yet nivolumab, like other “immune checkpoint inhibitors”, can cause immune-related adverse effects, where excessive T cell activation leads to an immune response against the recipient's organs. Scientists are looking for ways to improve the delivery of antibodies like nivolumab so that their release is slow, sustained and targeted, with minimal adverse effects.

Khashab’s group worked with a metal organic framework called zeolitic imidazolate (ZIF-8), a crystalline solid formed of zinc ion subunits attached to organic methylimidazole. “ZIF-8 nanoparticles have shown great potential in cancer drug delivery in the last few years,” says Somayah Qutub, a Ph.D. student in Khashab’s group. “They are composed of zinc ions and imidazole, which are naturally found in the body.” This makes the biocompatible, biodegradable and highly porous nature of ZIF-8 ideal for the safe loading and delivery of drugs like nivolumab.

Khashab’s team loaded nivolumab onto ZIF-8 and tested its effects on leukemia cells. They found that the antibody was slowly released from the ZIF-8 framework when in proximity to a slightly acidic tumor microenvironment. This process prevents the T cells from being deactivated by cancerous cells so that they can attack the leukemia cells.

The team also tested the system for delivering the antibody to solid tumors both in Petri dish experiments and in mice. They encapsulated nivolumab-loaded ZIF-8 with the membrane of the cancer cells that were being targeted. This led to the accurate delivery of the system to the tumor, followed by slow and sustained drug release. Importantly, the mice showed negligible toxicity from the drug, which significantly inhibited tumor growth and prolonged survival.

“Our next step is to improve this system,” says Qutub. “We are combining immunotherapy drugs with other anti-cancer modalities to have a synergic effect. We are also working on optimizing our cancer cell membrane coating with other materials so that the system can be easily reproducible, scalable and compatible with biopharmaceutical industry standards.”

- KAUST Discovery

Wednesday, February 10, 2021

Congratulations Professor Omar!

Professor Omar F. Mohammed has been named a Fellow of the Royal Society of Chemistry.

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Associate Professor of Materials and Chemical Science Programs, Professor Omar F. Mohammed, has been named a Fellow of the Royal Society of Chemistry (FRSC).

The Royal Society of Chemistry (RSC) is the United Kingdom’s professional body for chemists and the largest organization in Europe for advancing the chemical sciences. The designation Fellow of the Royal Society of Chemistry (FRSC) is given to elected Fellows who have made an outstanding contribution to the advancement of chemical sciences, or to the advancement of the chemical sciences as a profession.

Research

“Professor Mohammed's research is directed towards fundamental understanding of charge carrier dynamics in a variety of optoelectronic materials, including semiconductor quantum dots, nanowires and perovskite solar cells with the aid of cutting-edge ultrafast laser spectroscopy and four-dimensional electron imaging.” Read Professor Omar’s full profile.

Wednesday, February 10, 2021

What's the catch? Algal blooms influence fishing booms

The timing of phytoplankton blooms in the Red Sea could help determine next year’s fish catch.

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Satellite images reveal that the timing of algal blooms in the Red Sea may affect the next haul of sardines and squid by commercial fisheries.

Rising temperatures in the Red Sea have changed the timing of phytoplankton blooms. These microscopic algae form the base of many marine food webs and so are critical to ocean biodiversity and the industries that they support on land, such as fisheries and tourism.

A team led by KAUST climate modeler Ibrahim Hoteit has used satellite images to study the phenology of algal blooms in the northern Red Sea and their impact on fish catches. Phenology is the study of key stages in a species' lifecycle, such as when a seed sprouts or a bird migrates. Many phenological events are seasonal and can reveal a lot about the sensitivity of an ecosystem to environmental change. “We suspected that the timing of large phytoplankton blooms could impact fish further up the food chain, including those vital to fisheries,” says Hoteit.

Using ocean color images taken weekly between 1997 and 2018, the team estimated chlorophyll-a concentrations — a proxy for phytoplankton biomass. A model developed by lead author John Gittings was used to calculate the biomass of phytoplankton larger than two micrometers, such as diatoms, as these support food chains important to commercial fisheries. Gittings’ model assumes that smaller phytoplankton do not grow beyond a specific chlorophyll-a concentration, so any extra chlorophyll-a in the system is due to large phytoplankton.

The data revealed that annual winter blooms lasted longer between 2002 and 2008 due to El Niño monsoon winds driving nutrient-rich waters into the Red Sea from the Indian Ocean. However, the 2002-2003 bloom lasted 11 weeks longer than average, suggesting some additional factor. Observations of sea surface height revealed two large eddies in the northern and central Red Sea at the start of this big bloom, which could have caused an upwelling of cold nutrient-rich waters.

The team then gathered fisheries catch data for several species of sardines and squid and found that prolonged algal blooms led to larger catches the following year. This was particularly evident in the bumper catch reported the year after the 2002-2003 bloom event. “We were surprised by the strength of the link,” says Gittings. “It provides compelling evidence that anomalous changes in the timing of food availability may directly impact the success of species higher up the marine food chain.”

Gittings plans to develop a model that can distinguish between different sized phytoplankton and use high-resolution images from next-generation satellite sensors to create a richer picture of phytoplankton communities in the Red Sea.

“Satellite-derived phytoplankton data could serve as an early warning system for fisheries by using changes in bloom timings to predict how next year’s catch may be affected,” suggests Gittings. Such data could help policymakers in Red Sea nations manage this essential ecosystem.

- KAUST Discovery

Tuesday, February 02, 2021

A fine-grained view of dust storms

A new analysis of satellite data gives a detailed view of how extreme dust events have changed over time.

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A satellite-based dataset generated by KAUST researchers has revealed the dynamics of dust storm formation and movements over the last decade in the Arabian Peninsula. Analysis of this long-term dataset reveals the connection between the occurrence of extreme dust events and regional atmospheric conditions, a finding that could help improve weather forecasting and air-quality models.

Dust storms occur when strong winds lift tiny particles of sand into the atmosphere. These events often span several miles and can have an enormous impact on daily life, from damaging buildings and disrupting air traffic to triggering respiratory illnesses and other health problems.

The Arabian Peninsula is a global hotspot of extreme dust events, with storms occurring all year round, typically peaking in March and August. While previous studies that rely on ground-based measurements have explored how these dust storms form, few have captured in detail how they vary across the region.

“Comprehensive and continuous observations are needed to identify extreme dust events over the Arabian Peninsula,” says lead author of the study, Harikishan Gandham.

“Ground-based networks do not provide enough information,” explains Gandham. “We tried to fill this gap by analyzing long-term high-resolution dust data generated by satellite observations.” Gandham and his team used satellite data to analyze extreme dust events that occurred in the Arabian Peninsula between 2003 and 2017. To identify and track dust storms, the researchers used state-of-the-art algorithms to determine where they formed, their frequency and duration, and their spatial extent.

The researchers found a link between intensifying Shamal winds in the north and the formation of extreme dust storms in the Arabian Peninsula. These strong winds transport fine particles from the surrounding desert to the region, leading to thick clouds of dust that can reach altitudes of up to four kilometers above sea level and last for three days or longer.

A total of 49 dust storms occurred in the Arabian Peninsula over 207 days during the 15-year study period — these became more frequent from 2007 to their peak in 2012.

Following 2012, extreme dust events suddenly began to decline due to changing atmospheric conditions, such as heavier winter rainfall brought on by the development of a low-pressure trough in the Red Sea.

“This study generated and validated a much-needed long-term high-resolution dust aerosol dataset for the Arabian Peninsula and adjoining region,” says co-author Ibrahim Hoteit.

“Compared to other global data products available, our high-resolution dataset enables more accurate study of the variability of dust events over our region.”

- KAUST Discovery

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