Metal Nanoclusters as Heterogeneous Catalysts in Flow Reactors

Metal nanoclusters (MNCs) are metal atom clusters with diameters smaller than 2.0 nm, and can be composed of a single or multiple metals. MNCs show promise as heterogeneous catalysts, where the clusters are deposited on a support or encapsulated. These catalysts achieve the high surface area of traditional homogeneous catalysts exhibiting high activity and selectivity, whilst retaining the stability and separation ease of heterogeneous catalysts.

Such characteristics can enable performance that is not achievable using nanoparticle or single atom catalysts. MNC catalysts also drastically reduce the amount of metal required for catalysis which is beneficial for rare or expensive metals and highly desirable from a sustainability perspective.

To date, MNCs have generally been synthesised using wet chemistry methods which require multiple steps, produce waste and are not scalable, preventing commercial application. One preparation technique that can overcome these issues is magnetron sputtering. This physical process can disperse planar metal clusters directly onto supports in a single step, with a high level of control over metal loading and cluster size.

Unlike wet chemistry methods, the process does not produce any chemical waste which is crucial as it becomes more important to reduce the impact of chemical research on the environment. The use of magnetron sputtering will also enable MNC heterogeneous catalysts to be generated at a scale and throughput that does not limit their research.

In parallel to this, heterogeneous catalysis in flow is a growing field and presents huge potential to improve existing chemical processes as well as develop new and more sustainable chemistry. Commonly, packed bed reactors are used, however these can have limitations with respect to mixing, heat transfer and control over the flow of the reaction mixture. As a result, many novel reactor systems have been developed such as 3D-printed inserts and vortex devices.

Proposed solution and methodology

In this project, the two emerging sustainable technologies will be bridged together; MNCs will be fabricated by magnetron sputtering, and then used as heterogeneous catalysts in liquid flow. To the best of our knowledge, MNC catalysts have not yet been demonstrated in liquid flow, and it is not known which flow reactor approach would be best to harness their unique properties.

This is a challenging question because specifics of MNC fabrication will be different for different flow reactor technologies. This project is set to establish MNC catalyst behaviour and performance in flow which is critically important for industrial challenges related to the sustainable use of transition metals for heterogeneous catalysis of flow processes.

To achieve this step change a multidisciplinary approach is required. Heterogeneous MNC catalyst systems will be characterised through imaging and spectroscopy to understand their behaviour. Reaction outputs including product conversion, product purity and reaction rate will be analysed to understand catalyst performance.

Carrying out the reactions in a variety of flow reactors will help to understand how different support types and reaction conditions impact this behaviour and performance. Initially, Palladium MNCs will be investigated with hydrogenation as a model reaction.

By the end of the project we aim to have developed a new generation of high performing and sustainable heterogeneous catalysts for liquid flow with potential for application to industrially important reactions.