Nanocluster discovery will protect precious metals

Scientists have created a new type of catalyst that will lead to new, sustainable ways of making and using molecules and protect the supply of precious metals. A research team from the University of Nottingham has designed a new type of catalyst that combines features that are previously thought to be mutually exclusive and developed a process to fabricate nanoclusters of metals on a mass scale.

In their new research, published today in Nature Communications, they demonstrate that the behavior of nanoclusters of palladium does not conform to the orthodox characteristics that define catalysts as either homogeneous or heterogeneous.

Traditionally, catalysts are divided into homogeneous, when catalytic centers are intimately mixed with reactant molecules, and heterogenous, where reactions take place on the surface of a catalyst. Usually, chemists must make compromises when choosing one type or another, as homogeneous catalysts are more selective and active, and heterogenous catalysts are more durable and reusable. However, the nanoclusters of palladium atoms appear to defy the traditional categories, as demonstrated by studying their catalytic behavior in the reaction of cyclopropanation of styrene.

Catalysts enable nearly 80 percent of industrial chemical processes that deliver the most vital ingredients of our economy, from materials (such as polymers) and pharmaceuticals right through to agrochemicals including fertilizers and crop protection. The high demand for catalysts means that global supplies of many useful metals, including gold, platinum, and palladium, are become rapidly depleted. The challenge is to utilize each-and-every atom to its maximum potential. The exploitation of metals in the form of nanoclusters is one of the most powerful strategies for increasing the active surface area available for catalysis. Moreover, when the dimensions of nanoclusters break through the nanometre scale, the properties of the metal can change drastically, leading to new phenomena otherwise inaccessible at the macroscale.

The research team used analytical and imaging techniques to probe the structure, dynamics, and chemical properties of the nanoclusters, to reveal the inner workings of this unusual catalyst at the atomic level. The team's discovery holds the key to unlock the full potential of catalysis in chemistry, leading to new ways of making and using molecules in the most atom-efficient and energy-resilient ways.

The research was led by Dr. Jesum Alves Fernandes, Propulsion Futures Beacon Nottingham Research Fellow from the School of Chemistry said, we use the most direct way to make nanoclusters, by simply kicking out the atoms from bulk metal by a beam of fast ions of argon, a method called magnetron sputtering. Usually, this method is used for making coatings or films, but we tuned it to produce metal nanoclusters that can be deposited on almost any surface. Importantly, the nanocluster size can be controlled precisely by experimental parameters, from a single atom to a few nanometres, so that an array of uniform nanoclusters can be generated on demand within seconds.

Dr. Andreas Weilhard, a Green Chemicals Beacon postdoc researcher in the team said, Metal clusters surfaces produced by this method are completely naked, and thus highly active and accessible for chemical reactions leading to high catalytic activity.

Professor Peter Licence, director of the GSK Carbon Neutral Laboratory at the University of Nottingham said, this method of catalyst fabrication is important not only because it allows the most economical use of rare metals, but it does it the cleanest way, without any need for solvents or chemical reagents, thus generating very low levels of waste, which is an increasingly important factor for green chemical technologies.

The University is set to embark on a large-scale project to expand on this work with research that will lead to the protection of endangered elements.

Professor Andrei Khlobystov, the principal investigator of MASI said, our project is set to revolutionize the ways metals are used in a broad range of technologies, and to break our dependence on critically endangered elements. Specifically, MASI will make advances in the reduction of carbon dioxide (CO2) emissions and its valorization into useful chemicals; the production of green ammonia (NH3) as an alternative zero-emission fuel and a new vector for hydrogen storage; and the provision of more sustainable fuel cells and electrolyzer technologies.

Metal nanoclusters are activated for reactions with molecules, that can be driven by heat, light, or electric potential, while tuneable interactions with support materials provide durability and reusability of catalysts. In particular, MASI catalysts will be applied for the activation of hard-to-crack molecules (e.g. N2, H2, and CO2) in reactions that constitute the backbone of the chemical industry, such as the Haber-Bosch process.

Journal Information: Blurring the boundary between homogenous and heterogeneous catalysis using palladium nanoclusters with dynamic surfaces, Nature Communications (2021). DOI: 10.1038/s41467-021-25263-6