Confined Metal and Metal Oxide Nanoparticles and Clusters

 

A nanoparticle is a tiny particle typically in the size range of one to one hundred nanometres. Nano-scale systems can exhibit unique quantum mechanical properties due to their size.

 

The European Association for Cooperation in Science and Technology, which recently celebrated its second anniversary, focuses on the science of confined molecular systems. In this episode, we hear about their works to uncover the properties and behaviours of metal nanoparticles and clusters.

 

Visit their site: https://cost-cosy.eu/

 

Read the original research:

 

Image credit: Adobe Stock /Turbomotion046

 

 

Transcript:

 

Hello and welcome to ResearchPod. Thank you for listening and joining us today. In this episode we will be extending our understanding of some of the research activities being undertaken by COST, that is the European Association for Cooperation in Science and Technology, which recently celebrated its second anniversary. You can find the list of the scientific papers relevant to this podcast in the show notes.

 

In previous episodes we discussed the COST research consortium called COSY, which focuses on the science of confined molecular systems. Today we’re going to discuss the work of one of the five working groups of COSY, where the specific area of interest is confined metal and metal oxide nanoparticles and clusters.

 

A nanoparticle is a tiny particle typically in the size range of one to one hundred nanometres. To give you a feel – a nanometre is one billionth of a meter, or one thousandth of the thickness of a piece of paper.

 

 

But the thing about small is that it’s special. Very tiny and confined systems (confined meaning, as opposed to their equivalent bulk material) can prove highly reactive in terms of chemistry, due to their large surface area to volume ratio. And it is also well known that nano-scale systems can exhibit unique quantum mechanical properties due to their size. In our discussion today we’ll talk about systems of just a handful of atoms, up to potentially hundreds or even thousands of atoms.

 

Under the leadership of Dr Alessandro Fortunelli, from the National Research Council in Italy, and co-leadership of Dr Lyudmila Moskaleva from the University of the Free State in South Africa, this collaborative COSY sub-group works to uncover the properties and behaviours of metal nanoparticles and clusters.

 

Dr Fortunelli and Dr Moskaleva explain, “Our group is a diverse pool of researchers working on a large spectrum of topics. We are studying metal nanoparticles and clusters, ranging from single atoms to tens of nanometres, stabilized by molecular ligands or supported on solid substrates. Our interdisciplinary network, encompassing both experimentalists and theorists, is dedicated to unravelling the unique properties of these systems, which hold promise for a variety of applications, including catalysis, photocatalysis, sensing, energy conversion, and medical technologies, including bioimaging and theranostics.”

 

These applications could impact all of us, especially due to applications in environmental protection, fuel and energy technologies, drug synthesis techniques, and in medical research.

 

Including both experimental and theoretical teams, this COSY working group encompasses researchers from many nations and institutions. Their work is providing fascinating insights into the special behaviour of small cluster systems, and today we’re going to uncover some of their progress thus far.

 

Small copper atom clusters are particularly interesting, and exhibit some remarkable characteristics that can have applications in the areas we’ve mentioned. It turns out that clusters of five copper atoms are among the most interesting and can be highly effective catalysts. Actually known as sub-nanometre metal clusters, these extremely tiny systems are prone to oxidation, which is not desired for their practical use here. Oxidation is where the metal atoms can lose one or more electrons, for example to oxygen. This can inhibit the cluster’s effectiveness as a catalyst, and so trying to learn more about the oxidation process is crucial.

 

Some members of the working group, from research centres in Spain, Argentina, France, Italy, and Austria, have studied the thermal stability of “copper-five” clusters. The challenge with such systems is to understand this thermal stability in a real-world oxygen gas environment, where oxidation can cause the systems to give up their desired properties. It has been generally believed that metal clusters such as copper will be prone to further oxidation as their size, that is the number of atoms, decreases.

 

The researchers applied the copper-five clusters to a base, or support material, (known as a substrate) of HOPG – that is highly oriented pyrolytic graphite. They performed spectroscopic experimental studies and were surprised to discover the copper-five clusters resisted what’s known as irreversible oxidation, up to temperatures of about 500 degrees Celsius. The key point here is they discovered that the copper-five clusters can undergo some level of oxidation when exposed to oxygen, but they resist forming oxides that are stable or persistent.

 

To complement the experimental findings, the group also developed advanced theoretical models. Their calculations showed that adsorbed oxygen molecules on copper-five clusters transform into superoxide and peroxide (oxygen variants with electrons missing) due to charge transfer. And associated with this, they were able to show the clusters exhibit ‘large-amplitude breathing modes’, whereby the copper atoms can change place and rearrange the geometry of the cluster with respect to time.

 

They also showed both experimentally and via their theoretical models, that when the adsorbed oxygen molecules vacated the clusters, then the copper-five clusters were able to reclaim the donated charge and resist oxidation. This could offer a vital way forward in terms of optimising their catalytic performance.

 

In another collaboration of María Pilar de Lara-Castells’ AbinitSim Unit at the Spanish National Research Council (CSIC) – the Action Chair’s institution in Madrid and the University of the Free State in South Africa, researchers examined how best to stabilise small metal clusters for catalysis so that their unique properties are maintained.

The researchers wanted to look at the stability of copper-five clusters in catalysis when a graphene base is used. You’ve probably heard of graphene – an extraordinary material with extraordinary properties. It’s a hexagonal sheet lattice of carbon atoms. We say it’s “two-dimensional”, as remarkably it’s just a single atom thick.

 

Graphene sheets are commonly used as a substrate for catalysis. And impurities – so-called carbon vacancies – can be found, or indeed generated if desired, in graphene. These ‘gaps’ or ‘holes’ in the hexagonal lattice structure, are where a carbon atom is missing – and this ‘glitch’ can prove highly advantageous in certain conditions.

 

Using advanced modelling and dynamics simulations at a temperature of about 130 degrees Celsius, the researchers found that copper-five clusters can become ‘trapped’ and stable in the presence of a graphene sheet defect such as a carbon vacancy.

 

The simulations used by the team also confirmed copper-five clusters can dimerise – or ‘pair up’ – to become copper-ten clusters on graphene that is defect-free. And this was shown to agree with other calculations for unsupported copper-five clusters.

 

So if we need copper-five to be a stable catalyst and to stay as copper-five, then the graphene defect method could prove extremely useful. And so clearly this discovery could provide invaluable understanding about the suitability of graphene-based substrates in catalysis.

 

In other work – again using a base of carbonaceous materials akin to oxidized graphene – scientists from the Czech Republic, France, Italy and Switzerland have studied the effectiveness of cobalt ferrite as a catalyst in the conversion of cyclohexane to benzene or cyclohexene. Benzene and cyclohexene play a fundamental role in the chemical industry, and so extending our understanding of their catalytic production from cyclohexane is a major goal.

 

Their production involves the removal of hydrogen atoms from cyclohexane, a process known as dehydrogenation.

 

Using advanced spectroscopic techniques, scientists from the working party wanted to observe exactly how the cobalt ferrite performed as a catalyst in producing either benzene or cyclohexene, with cyclohexene being the preferred product, much more valuable.

 

 

They found that the amount and the type (or shape) of cobalt ferrite applied – whether its atoms were in octahedral or tetrahedral coordination, that is – turns out to influence the route to cyclohexene or benzene. They also discovered that the graphene-oxide base can play a part in terms of the dehydrogenation – in particular, the base can mask or ‘hide’ certain parts of the catalyst so that they don’t participate in the desired reaction at all, orienting the process toward the desired cyclohexene product.

 

What’s very important in catalysis is indeed trying to alter and manipulate various factors to make sure you get exactly what you want – the notion of selectivity. Much of the science of catalysis is learning how different features and selections need to be optimised to get the desired result. This particular research focuses largely on this principle, and is sure to help further selectivity techniques in catalysis more broadly.

 

Meanwhile, the sub-group’s scientists from Russia’s Boreskov Institute of Catalysis in Novosibirsk, and from the University of Barcelona in Spain, are continuing this quest to better understand and control the characteristics of catalytic materials to improve their efficiency in reactions.

 

They have studied the catalytic properties of nanoparticle combinations of platinum and cerium oxide. These are known to be powerful catalysts in the oxidation of carbon monoxide, and in the so-called water gas shift – which is where carbon monoxide and water are converted to carbon dioxide and hydrogen gas.

 

Understanding and optimising the catalysis process here is clearly important with respect to the effective production of hydrogen gas as a clean-burning fuel. We also know of course that carbon monoxide is an unwanted by-product in many industrial processes. More information about CO oxidation at room temperature and below will help with purifying indoor air, and with things like vehicle exhaust gases in cold environments. So we can see that mastering the efficient oxidation of carbon monoxide is going to be of tremendous importance.

 

The team used experimental approaches and computational methods to determine which states of platinum were most effective for carbon monoxide oxidation.

 

Their research demonstrated that small oxidised platinum particles are the active species required for catalysis at temperatures well below zero degrees Celsius. They found that pre-treatment – in this case, adding oxygen – can assist in providing oxidised platinum particles. But they also learned that even without pre-treatment, the reaction conditions can produce the required species for CO oxidation.

 

The clear goal here is to maximise the abundance of active species, in this case platinum oxide, and to provide the most effective conditions for CO oxidation. This research is opening the door to further work in improving the design of catalysts for these reactions. And we must say of course – any developments that reduce carbon monoxide emissions will provide for cleaner air and better public health outcomes for us all.

 

To wrap up our overview of the working group’s progress, we look at gold nanoclusters and a structural property known as chirality.  Imagine a pair of gloves – they are mirror images of each other, but they cannot be overlayed such that they align. That’s chirality, and it exists in molecular and atomic structures. Each variant known as an enantiomer. This feature is also widespread and very important in biological systems, such as sugars, proteins, and DNA.

 

It turns out that chiral materials exhibit unique behaviours when exposed to circularly polarised light, and it is the study of this light-matter interaction – known as chiroptics – which can provide valuable information about the system of interest.

 

Chiral gold nanoclusters have become particularly useful as a versatile tool for applications in biomedicine, catalysis and sensing. A key challenge is to understand how the different cluster structures and behaviours affect their optical properties in order to find optimal designs in different applications. There’s nothing flash about using gold, by the way – it just so happens that it has some stability advantages, as well as some other very useful optical and structural properties. Other systems are available!

 

Researchers from the Universities of Trieste, L’Aquila, Padova, and the Italian National Research Council developed a new approach to predict and understand the chirality of gold nanoclusters. They wanted to study the dynamics of molecules attached to the gold nanoclusters, called ligands, as this can affect the cluster’s properties. Ligands are often attached to metal clusters and provide stability and other functions.

 

Using computational techniques to include the dynamic behaviour (that is, the motion over time) of the ligands, the team were able to compute spectra that matched well with experimental results. And yet when their calculations left out the dynamic behaviour of the ligands they saw no agreement with experiment.

 

This work confirms the clear need to include the ligand dynamics in the calculations for the successful modelling of these systems. It also demonstrates that time and resources may be reduced in future research, as theoretical studies can often be quicker and less costly than full synthesis and experiment.

 

That wraps up our whistle-stop tour of the COSY working group activities, where we’ve seen fascinating and insightful research into catalysis and the behaviours of metal clusters and nanoparticles. This is crucial in key areas of science and industry, and we can see that this broad range of activities is paving the way for major developments across many applications. We’ll hear more from the COSY working group soon, so stay tuned for more on how small metal clusters are going to provide far-reaching impact throughout our daily lives.

 

Don’t forget to check out the links to the original research papers, provided in the notes for this episode, and stay subscribed to ResearchPod for all the latest in research news. See you again soon.

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