Nitridic and Carbidic Interstitial Pd Nanoparticles for Directed Catalysis

In the field of catalysis, supported metal nanoparticles are an important class of materials; they are a convenient way to thrift expensive metals alongside altering the catalytic properties. However, these materials are extraordinarily dynamic and are particularly sensitive to the fluctuation of environmental conditions experienced during reactions. These structural transformations are not benign and they underpin the working function of this class of catalysts. In this study, we will use our expertise in this area to design new active catalysts for environmental protection and the chemicals industry.

Without ever realising it, on a daily basis most people will be using or buying products made using supported Pd nanoparticle catalysts. These types of materials are an active component in catalytic converters in car exhausts, and used in the manufacture of many fine chemicals. However, their performance can be affected by the reactant gases they interact with, changing the structure of the Pd NPs, where the Pd atoms become interspersed with H, N or C atoms (depending on the gas present). Understanding these new structures, in terms of their formation and stability during reaction can help understand the influence they have on reactions. For example, we recently reported the formation of nitride interstitial Pd NPs (Nature Catalysis, 2, 157, 2019), and its role in directing the oxidation of ammonia to N2. This reaction is of particular importance for diesel engines where unused ammonia can 'slip' through the exhaust, adding to unwanted emissions. We found that in the presence of the Pd nitride, ammonia is converted to N2, whereas without the nitride phase, there is over oxidation to the pollutant NOx.

Our vision is to pre-form interstitial Pd nanoparticle structures and exploit their unique catalytic properties, i.e. they are able to moderate oxidation and hydrogenation chemistry - fundamental to catalysis - to limit over-oxidation and over-reduction products. To achieve this the project must first learn more about how these structures are formed and under what conditions they remain stable. This will be realized by developing element specific spectroscopy that provides direct information on the Pd (XAFS) and the heteroatom environments (MAS NMR) under process conditions. These results will be further supported by DFT modelling studies of realistically sized nanoparticle structures. Ultimately, this will generate a 'rulebook' for how/when these structures are formed and their stability under different conditions (e.g. temperature gas environment).

The major aim of this project is to develop further understanding on the formation of nitridic and carbidic Pd nanoparticles and their stability under process conditions, before demonstrating their performance towards a range of catalytic processes. Catalysis is a significant part of the UK economy and generates > £50 billion annually. However, due to the fundamental nature of the work in this study, impact linked to the development of new catalyst technologies will take time to be realised. In the shorter term there are many more immediate areas where impact will be seen.
The project will firstly deliver new methods for operando spectroscopy that will be of benefit to those that study functional materials - both in industry and academia. These approaches provide greater understanding of structure-function relationships and will hasten the development of new materials for future sustainable technologies. Secondly, the project is expected to lead to new methods for the development of new classes of catalyst based on interstitial insertion of heteroatom in supported palladium nanoparticles, with enhanced activities and selectivities for reactions of sustainability and industrial importance.

From an industrial perspective, Johnson Matthey are supporting the project and are well-placed to adopt methods that are developed throughout the lifetime of the grant. Further direct industrial engagement will be advanced by the membership of the University of Southampton within the Knowledge Centre for Materials Chemistry (KCMC); the KCMC is a standalone entity within the Knowledge Transfer landscape which endeavours to further the involvement of the UKs leading academic establishments in materials chemistry within the wider UK R&D landscape (> £20M of investment in facilities within past 5 years). The University of Southampton have their own KCMC knowledge transfer manager who is responsible for securing further links for academics with additional industrial partners. In essence, the KCMC is able to provide the link to UK R&D to aid industry-academic partnerships and aid effective exploitation of any intellectual property generated. The University knowledge transfer manager will work with the project to set out a framework to bring in additional industrial funding and interaction.

For more general engagement to the wider academic audience, these methods will be presented at international conferences and published in peer-reviewed journals alongside a more bespoke dissemination strategy. This bespoke dissemination includes creating a video of the design process when making a new reactor. This insight through the thought process involved in designing the operando reactors will aid in the adoption and adaption of these approaches.

The development of people is a hugely important area of impact in this project. It is clear that much high level research requires a broad cross-section of skills that one person alone will not be able to master. To aid the PDRAs to build their own collaborative network they will be partially based on the Harwell campus within the UK Catalysis Hub. Here they will be able to make links to the broader UK catalysis community and the world-leading facilities on the Harwell campus. This will create a group of individuals with established networks for their future careers in industry and academia. Impact here will be further achieved through the events (e.g. NMR workshop) the PDRAs will initiate as part of this project.