Tag: catalysts

Ammonia oxidation – Platinum nanoparticles caught in action

Ammonia oxidation – Platinum nanoparticles caught in action

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Ammonia oxidation is a key reaction in the chemical industry, essential for global agriculture and mining, and it also helps limit emissions of this irritating and polluting gas. A SOLEIL team, in collaboration with researchers from CEA-Grenoble, used advanced techniques on the SixS beamline to observe in real time how platinum nanoparticles—used as catalysts for this reaction—deform and change shape during oxidation.
By combining surface diffraction and Bragg coherent diffraction imaging (BCDI), the scientists revealed that the size, shape, and internal strain of the particles directly influence their catalytic efficiency and selectivity. Their results, published in Applied Catalysis B: Environmental, deepen our understanding of this reaction and pave the way for the design of more efficient and durable catalysts, with major implications for both industry and the environment.

The oxidation of ammonia (NH₃) is a vital industrial process for producing nitric oxide (NO), an essential intermediate in the manufacture of nitric acid (HNO₃)—used in fertilizers, explosives, and dyes. However, ammonia oxidation does not produce NO alone; it also generates nitrous oxide (N₂O) and nitrogen gas (N₂). All three products have industrial relevance, and the challenge lies in maximizing the yield of one or the other—this is known as Selective Catalytic Oxidation.
For over a century, platinum (Pt) has been the reference catalyst for NO production. Interestingly, it is used in the form of micrometric wires woven into metallic gauzes rather than as dispersed nanoparticles—one of the last remaining examples of bulk solid catalysts in industrial use. These wires are metallic and polycrystalline, composed of grains similar in size to the particles studied here. Yet, despite more than a hundred years of research, the precise mechanisms by which platinum particles or crystals influence the selectivity and efficiency of the reaction remain only partially understood, particularly under real operating conditions (high temperature, pressure, and gas mixtures).

Read more on the SOLEIL website

Image: Figure 1: Distribution of platinum nanoparticles. Round (blue) and elongated (red) particles display distinct catalytic behaviors.

Nanoburgers with promising flaws

Nanoburgers with promising flaws

Publication in ACS Nano: DESY team finds surprising defects in tiny metal particles which could stimulate the development of more efficient catalysts

Catalysts are indispensable in many industries: they speed up chemical reactions, making them economically viable. They often consist of tiny particles, just a few nanometres across, to which molecules attach themselves, making it easier for them to form a bond with another reagent. The catalysts themselves are left unchanged. One class of nanocatalysts consists of the precious metals platinum and rhodium and is used, for example, in the purification of waste gases, in hydrogen production and in fuel cells.

The team led by DESY physicist Andreas Stierle has been studying such platinum-rhodium catalysts for quite some time. However, when they analysed the particles again using X-rays, they were surprised to find that some of the nanoparticles are not tiny, homogeneous lumps but consist of an upper and a lower half – like the two halves of a burger bun. Although the two halves are stuck together, the nature of this bond and how it affects the catalytic properties of the nanoparticles was unclear.

To work this out, Stierle’s team designed an experiment at the European Synchrotron Radiation Facility ESRF in Grenoble. ‘It produces an extremely narrow X-ray beam that can be used to study individual nanoparticles,’ explains Stierle. Specifically, the researchers used a method known as Bragg Coherent Diffraction Imaging (BCDI), in which the X-ray beam creates a special diffraction pattern as it passes through the nanoparticle, and this is recorded by a detector. ‘Special algorithms can then be used to reconstruct how the atoms are arranged in the crystal lattice and where they deviate from the regular structure – distortions, defects and dislocations in the crystal lattice,’ explains Ivan Vartanyants, who supervised the reconstructions.

What made their experiment different was that the measurements were performed while the nanocatalysts were active. The group directed a stream of carbon monoxide and oxygen to pass over the nanoparticles, on whose surface the gas was converted into CO2 – at temperatures of more than 400 degrees Celsius. ‘These experiments were extremely difficult; we had to keep the nanoparticles fixed to within ten nanometres so that the X-ray beam always illuminated the entire particle,’ explains first author Lydia Bachmann, who is studying this topic as part of her PhD. ‘To do this, we had to make sure that the conditions were absolutely steady.’

The outcome was unexpected: the experts discovered pronounced crystal defects where the upper and lower halves of the nanoburgers meet. The two boundary surfaces did not fit perfectly on top of each other; atoms were missing around the outer edges. These gaps cause all the atoms in the vicinity to shift, significantly distorting and displacing the crystal lattice.

What was truly remarkable was that these ‘flaws’ had an extremely positive effect on the catalytic properties of the nanoburgers. ‘The defects serve as unique absorption sites for molecules,’ explains co-author Thomas Keller. ‘Molecules such as oxygen adhere very well to them, which increases the effectiveness of the catalyst.’ In the future, these findings could help industry to develop more efficient and effective catalysts – through deliberate ‘defect engineering’ to create as many binding sites as possible on the nanoparticles, where molecules can be converted.

Read more on DESY website

Image: The Nano-Burger in action: The two ‘halves‘ of the platinum-rhodium catalyst interact with reagents in this simulation.

Credit: Science Communication Lab for DESY

Understanding the Role of Manganese in Fuel Production Catalysts

Understanding the Role of Manganese in Fuel Production Catalysts

SCIENTIFIC ACHIEVEMENT

Using specialized equipment at the Advanced Light Source (ALS), including a custom-built reaction cell, researchers uncovered the role of manganese in cobalt manganese oxide catalysts used for fuel production.

SIGNIFICANCE AND IMPACT

This work opens the door to improved catalyst designs that could decrease the production of harmful methane byproducts in a common petrochemical process. 

Sustainable fuel production

First developed in the 1920s, the Fischer-Tropsch synthesis remains a common chemical process used to convert carbon monoxide and hydrogen from coal into liquid hydrocarbons, or fuel. Cobalt is an efficient catalyst for this reaction, and its combination with manganese has been known for decades to further improve the process by promoting the preferential production of long-chain hydrocarbons over methane, a contributor to climate change. However, the molecular scale origin for why manganese improves the efficiency of this reaction remains unclear.

In this work, researchers uncovered the role of manganese in cobalt-manganese-oxide systems by combining well-defined model catalysts with advanced x-ray spectroscopy techniques. These results provide a platform for how customized equipment can answer challenging scientific questions and set the stage for new catalyst designs that may further decrease the production of methane during Fischer-Tropsch synthesis.

Custom-built instrumentation

Numerous studies have investigated the mechanisms for catalytic performance in cobalt-manganese-oxide systems, proposing particular interfaces, mixed oxides, or nanostructures as reasons for the improved efficiency. However, due to the heterogeneity of widely used powder catalysts, resulting in separated domains of cobalt and manganese, the molecular-scale mechanism of these catalysts remains under debate. To circumvent this, the researchers created model catalysts of well-defined cobalt-manganese-oxide nanocrystals and films where the components were intermixed at the sub-nanometer scale.

The model catalysts were investigated using ambient-pressure x-ray photoelectron spectroscopy (APXPS) at Beamline 9.3.2, which is equipped with commercial instrumentation uniquely designed for ambient-condition experiments that mimic real reaction conditions (this instrumentation was previously developed by the researchers, is now available at Beamline 9.0.2 and Beamline 11.0.2, and induced the application of APXPS at other synchrotron facilities). Similarly, to achieve realistic reaction conditions for x-ray absorption spectroscopy (XAS), a custom-built reaction cell was designed for Beamline 8.0.1, allowing experiments that typically occur under high vacuum to be performed under ambient pressure. The challenging and iterative process of perfecting this reaction cell was key to the success of this study, and the reaction cell is now available to other ALS users.

The magic of manganese

Using the custom-built reaction cell for XAS, the researchers were able to observe the real-time breakdown of carbon monoxide during the introduction of hydrogen to the cobalt-manganese-oxide catalyst at ambient conditions. Next, APXPS showed a significant increase in CHx hydrocarbon species after the addition of carbon monoxide and hydrogen on the cobalt-manganese-oxide catalyst surface–which was in stark contrast to the systems without manganese, where the production of cobalt carbide was more dominant instead. In other words, these results demonstrated that the addition of manganese creates more CHx, which ultimately allows for the production of more long-chain hydrocarbons.

The ALS data was complemented by computational density functional theory (DFT) calculations. DFT demonstrated that manganese helps with the production of long-chain hydrocarbons because manganese oxide binds with hydrogen, making it unavailable for reacting with CHx to stop propagation, resulting in less methane and more long-chain hydrocarbons. Moving forward, this work paves the way for improved catalyst designs that can make these reactions even more efficient.

Read more on ALS website

Isolating active sites for more efficient catalysts

Isolating active sites for more efficient catalysts

Scientists are always searching for new catalysts to enable fast, energy-efficient chemical reactions to transform wastes into useful chemical fuels, such as converting carbon dioxide to methane. Single-site catalysts are a promising new class of catalyst, with well-defined, well-designed structures where each reaction site is isolated. The challenge with such materials is that the active catalytic sites, which can be as small as a single atom, tend to aggregate, degrading their efficiency and selectivity. 

Now researchers using the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science user facility at DOE’s Argonne National Laboratory, have demonstrated a design for such single-site catalysts that resist aggregating and retain their high efficiency.

The design starts with polyoxometalate clusters (POMs). These are discrete polyatomic anions in which three or more metals, including molybdenum, share oxygen atoms. Catalytic atoms, often noble metals such as platinum or rhodium, can be embedded with the POM. The POM essentially belts the single-site catalyst atom in place, so it has difficulty interacting with other catalytic sites.

But for the catalyst to be efficient, reacting molecules need to be able reach the active site, so the next step is to disperse them through a support with high surface area. In this case, the researchers used a zirconium-based metal-organic framework (MOF), a porous architecture that contains a high surface area within a small volume. This dual-confinement strategy allows the researchers to achieve a relatively high density of active sites, up to 3.2 weight percent, without the sites sintering together during use.

One previous strategy to prevent aggregation of the catalytic atoms had been to spread them far apart on a support material. That made it difficult to assess the structure of the catalysts. The high catalyst loading that is stable using the POM-MOF approach provides sufficient signal to see the catalyst sites using X-rays. The team used pair distribution function analysis (PDF) a technique that is a specialty of beamline 11-ID-B at the APS. PDF provides the structure and relative geometry of the atoms that make up the active catalyst site. Researchers heated samples of their material to 200 °C and used scattering measurements to determine the structure of the POMs when they entered an active state.

The team also performed X-ray absorption spectroscopy (XAS) at beamline 5-BM-D, the Dupont-Northwestern-Dow Collaborative Access Team (DND-CAT) beamline. This technique provides element-specific information, allowing them to isolate the platinum and the rhodium environments. Combining the PDF and XAS measurements allowed the researchers to figure out the local geometry of the catalyst’s active site during the catalytic reaction. They found, for instance, that the distance between the rhodium or platinum and the molybdenum in the active catalyst was shorter than the metal-to-metal bond lengths in bulk metals. 

Researchers were able to compare their experimental data with computer simulations and say which of almost 100 computer-generated models most closely matched their results. Now that they’ve identified the structure of their single-site catalysts and refined the models, they can undertake further computational studies to explore what other formulations might be most promising, before trying to synthesize them. 

Armed with this new understanding of POMs, scientists can now explore whether there are different versions that might achieve high efficiency while using cheaper or more abundant metals than platinum, making chemical reactions more energy and cost efficient. – Neil Savage

See: Z. Chen1, S.M.G. Rabbani2, Q. Liu3,4, W. Bi3,4, J. Duan3, Z. Lu3, N.M. Schweitzer3, R.B. Getman2, J.T. Hupp2, K.W. Chapman1, “Atomically precise single-site catalysts via exsolution in a polyoxometalate-metal-organic framework architecture,” Journal of the American Chemical Society 2024, 146, 12, 7950-7955 (March 2024)

Author affiliations: 1Stony Brook University; 2Clemson University; 3Northwestern University; 4University of Science and Technology of China

This work was initially supported as part of the Inorganometallic Catalyst Design Center, an Energy Frontier Research Center (EFRC) funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (DE-SC0012702), and subsequently as part of the Catalyst Design for Decarbonization Center EFRC (DE-SC0023383). Q.L. and W.B. acknowledge the financial support as visiting scholars from the National Natural Science Foundation of China (No. 11705205, No. 22175051). This research used the beamline 5-BM-D for X-ray adsorption spectroscopy and 11-ID-B for total scattering measurements at the Advanced Photon Source, a U.S. Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357. This research used ambient pressure XPS of the Center for Functional Nanomaterials (CFN), which is a U.S. Department of Energy Office of Science User Facility, at Brookhaven National Laboratory under Contract No. DE-SC0012704. This work made use of the Reactor Engineering and Catalyst Testing (REACT) core facility of the Center for Catalysis and Surface Science at Northwestern University. The authors acknowledge Leighanne C. Gallington and Qing Ma for the help with remote measurements at the beamline. We thank Ashley R. Head for help with setting up the APXPS measurement. Q.L. thanks Yang Song and Wangsheng Chu for the analysis of XAS.

Read more at Argonne website

Image: A POM consisting of ring of molybdenum (purple)-oxygen (red) octahedra that surrounds a catalytically active rhodium (green) site, all supported on Zr-MOF. When the pristine catalyst (top) is activated under hydrogen at high temperature, its structure changes (bottom) and the Rh protrudes from the Mo-oxo ring.

Green hydrogen: MXenes shows talent as catalyst for oxygen evolution

Green hydrogen: MXenes shows talent as catalyst for oxygen evolution

The MXene class of materials has many talents. An international team led by HZB chemist Michelle Browne has now demonstrated that MXenes, properly functionalised, are excellent catalysts for the oxygen evolution reaction in electrolytic water splitting. They are more stable and efficient than the best metal oxide catalysts currently available. The team is now extensively characterising these MXene catalysts for water splitting at the Berlin X-ray source BESSY II and Soleil Synchrotron in France.

Green hydrogen is seen as one of the energy storage solutions of the future. The gas can be produced in a climate-neutral way using electricity from the sun or wind by electrolytic water splitting. While hydrogen molecules are produced at one electrode, oxygen molecules are formed at the other. This oxygen evolution reaction (OER) is one of the limiting factors in electrolysis. Special catalysts are needed to facilitate this reaction. Among the best candidates for OER catalysts are, for example, nickel oxides, which are inexpensive and widely available. However, they corrode quickly in the alkaline water of an electrolyser and their conductivity also leaves much to be desired. This is currently preventing the development of low-cost, high-performance electrolysers.

MXene as catalysts

A new class of materials could offer an alternative: MXenes, layered materials made of metals, such as titanium or vanadium, combined with carbon and/or nitrogen. These MXenes have a huge internal surface area that can be put to fantastic use, whether for storing charges or as catalysts.

An international team led by Dr Michelle Browne has now investigated the use of MXenes as catalysts for the oxygen evolution reaction. PhD student Bastian Schmiedecke chemically ‘functionalised’ the MXenes by docking copper and cobalt hydroxides onto their surfaces. In preliminary tests, the catalysts produced in this way proved to be significantly more efficient than the pure metal oxide compounds. What’s more, the catalysts showed no degradation and even improved efficiency in continuous operation.

Read more on HZB website

Image: The surface of a Vanadium carbide MXene has been examined by Scanning Electron Microscopy. The beautiful structures are built by cobalt copper hydroxide molecules.

Credit: B. Schmiedecke/HZB

Tender X-rays show how one of nature’s strongest bonds breaks

Tender X-rays show how one of nature’s strongest bonds breaks

Short flashes of an unusual kind of X-ray light at SwissFEL and SLS bring scientists closer to developing better catalysts to transform the greenhouse gas methane into a less harmful chemical. The result, published in the journal Science, reveals for the first time how carbon-hydrogen bonds of alkanes break and how the catalyst works in this reaction.

Methane, one of the most potent greenhouse gases, is being released into the atmosphere at an increasing rate by livestock farming as well as the continuing unfreezing of permafrost. Transforming methane and longer-chain alkanes into less harmful and in fact useful chemicals would remove the associated threats, and in turn make available a huge feedstock for the chemical industry. However, transforming methane necessitates as a first step the breaking of a C-H bond, one of the strongest chemical linkages in nature.

Forty years ago, molecular metal catalysts were discovered that can easily split C-H bonds. The only thing found to be necessary was a short flash of visible light to “switch on” the catalyst and – bafflingly – the strong C-H bonds of alkanes passing nearby were easily broken almost without using any energy. Despite the importance of this so-called C-H activation reaction, it has remained unknown how that catalyst performs this function. Now, experiments at Swiss FEL and SLS have enabled a research team led by scientists at Uppsala University to directly watch the catalyst at work and reveal how it breaks the C-H bonds.

Read more on the PSI website

Image: An X-ray flash illuminates a molecule

Credit: University of Uppsala / Raphael Jay

How a record-breaking copper catalyst converts CO2 into liquid fuels

How a record-breaking copper catalyst converts CO2 into liquid fuels

Researchers at Berkeley Lab, collaborating with CHESS scientists at the PIPOXS beamline, have made the first real-time movies of copper nanoparticles as they evolve to convert carbon dioxide and water into renewable fuels and chemicals. Their new insights could help advance the next generation of solar fuels.

Since the 1970s, scientists have known that copper has a special ability to recycle carbon dioxide into valuable chemicals and fuels. But for many years, scientists have struggled to understand how this common metal works as an electrocatalyst, a mechanism that uses energy from electrons to chemically transform molecules into different products.

Now, a research team led by Lawrence Berkeley National Laboratory (Berkeley Lab) has gained new insight by capturing the world’s first real-time movies of copper nanoparticles (copper particles engineered at the scale of a billionth of a meter) as they convert CO2 and water into renewable fuels and chemicals: ethylene, ethanol, and propanol, among others. The work was reported in the journal Nature.

“This is very exciting. After decades of work, we’re finally able to show – with undeniable proof – how copper electrocatalysts excel in CO2 reduction,” said Peidong Yang, a senior faculty scientist in Berkeley Lab’s Materials Sciences and Chemical Sciences Divisions who led the study. Yang is also a professor of chemistry and materials science and engineering at UC Berkeley. “Knowing why copper is such an excellent electrocatalyst brings us steps closer to turning CO2 into new, renewable solar fuels through artificial photosynthesis.”

Read more on the CHESS website

Image: Artist’s rendering of a copper nanoparticle as it evolves during CO2 electrolysis: Copper nanoparticles (left) combine into larger metallic copper “nanograins” (right) within seconds of the electrochemical reaction, reducing CO2 into new multicarbon products.

Credit: Yao Yang/Berkeley Lab

Building better catalysts to close the carbon dioxide loop

Building better catalysts to close the carbon dioxide loop

The best way to stave off the worst effects of climate change is to reduce CO2 emissions around the world. And one way to do that, says Zhongwei Chen, a professor in the Department of Chemical Engineering at the University of Waterloo, is to capture the CO2 and convert it into other useful chemicals, such as methanol and methane for fuels. Stopping emissions at the source, and further reducing future ones by replacing CO2-producing fuels with cleaner ones “…is a way to close the circle,” Chen says.

In order to turn CO2 into methanol, you need a catalyst to jump-start the electrochemical reaction. Traditionally, these catalysts have either been made out of precious metals like gold or palladium, or base metals like copper or tin. However, they are expensive and break down easily, hindering large-scale implementation. “Right now we can’t meet industrial requirements,” says Chen, who holds a Canada Research Chair. “So we are trying to design catalysts with better activity, selectivity, and durability.”

Read more on the CLS website

Image: Chithra Karunakaran on the SM beamline at the Canadian Light Source

Credit: David Stobbe

Arranging gold nanoparticles precisely in three dimensions

Arranging gold nanoparticles precisely in three dimensions

Metal nanoparticles have a wide variety of applications many of which stem from the fact that extremely small particles a few nanometres to  10’s of nanometres in diameter can have very different properties from those of the same material at a larger scale (a nanometre is just a billionth of a metre). Such particles are used as catalysts, coloring agents and can even  make antibacterial coatings. Some effects are due to the pattern of the particles and the spacing between them, but these are very difficult to control and particles are typically used in solution where they randomly move around like motes of dust in the air.   

In the current work, scientists based at the Bionanoscience and Biochemistry Laboratory at the Malopolska Centre of Biotechnology (MCB), Jagiellonian University showed that an artificial protein structure, a hollow sphere called a TRAP-cage, was able to act as a scaffold and provide regular-spaced points of attachment for small gold nanoparticles. “TRAP-cage is itself tiny, but at around 15 nm in diameter is still big enough to attach multiple  gold nanoparticles” explained Jonathan Heddle the head of the lab, “The protein cage is made of 12 rings, so overall it looks a little like a 12-sided dice – a dodecahedron.”  The researchers showed that there are spaces equivalent to the corners of the dodecahedron that offer just the right environment to snugly fit the gold nanoparticles inside. As a result, instead of randomly floating around, the particles appear to be constrained into a fixed three-dimensional pattern. It is hoped that the ability to arrange metal nanoparticles in this way may be developed further to produce new materials with useful properties.

Read more on the SOLARIS website

Image: The structure of the protein cage (purple) with three of the embedded gold nanoparticles highlighted (yellow) 

Credit: Jonathan Heddle

Nonprecious transition metal nitrides as efficient oxygen reduction electrocatalysts for alkaline fuel cells

Nonprecious transition metal nitrides as efficient oxygen reduction electrocatalysts for alkaline fuel cells

CHEXS users have discovered a class of nonprecious metal derivatives that can catalyze fuel cell reactions about as well as platinum, at a fraction of the cost. A critical part of the fuel cell is the oxygen reduction reaction, an infamously sluggish process that is traditionally sped up by platinum and other precious metals. Now, in a new paper appearing in the journal Science Advances, a team lead by Héctor Abruña (the Émile M. Chamot Professor of Chemistry and Chemical Biology at Cornell University), have reported a new cobalt nitride catalyst material with near identical efficiency to platinum while costing 475 times less (as of February 2022). Carbon-supported cobalt nitride (Co3N/C) achieved a record-high peak power density among reported nitride cathode catalysts of 700 mW cm−2 in alkaline membrane electrode assemblies. The material was demonstrated to remain stable below 1.0V potentials inside working fuel cells, using operando x-ray spectroscopy at the PIPOXS beamline. Operando XANES and EXAFS (A,B) show dramatic changes in valence and bond lengths for potentials above 1V, while below 1V the material remains stable (C,D).

Read more on the CHESS website

New 12 T magnet strengthens energy and magnetism research

New 12 T magnet strengthens energy and magnetism research

Electron paramagnetic resonance (THz-EPR) at BESSY II provides important information on the electronic structure of novel magnetic materials and catalysts. In mid-January 2022, the researchers brought a new, superconducting 12-T magnet into operation at this end station, which promises new scientific insights.

At the THz-EPR end station, unique experimental conditions are provided through a combination of coherent THz-light from BESSY II and high magnetic fields. These capabilities have now been extended by a new superconducting 12 T magnet, acquired through funding from the BMBF network project “ERP-on-a-Chip” and HZB.

Read more on the HZB website

Image: Exhausted but happy: f.l.t.r. – K. Holldack (HZB), A. Schnegg (MPI CEC Mülheim, HZB), T. Lohmiller (HZB, HUB), D. Ponwitz (HZB) after the successful commissioning of the new 12T magnet (green).