Developing Wide Line Solid State NMR as a Novel Analytical Approach to understand Metals in Catalytic Technology for Fuel Cells

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The Stern report stresses that that the drive to more sustainable energy and the introduction of new technologies in this area must be urgently pursued. Fuel cells have a key role to play in the future energy economy. However, one of central components in many fuel cells is the precious metal electrodes which are very expensive, such that their longevity and high expense often limit fuel cell economics. Understanding the structure of the precious metal nanoparticles that make up these catalytic layers is often difficult due to the highly heterogeneous, disordered nature of these materials. This project will develop wide line solid state NMR of precious metal nuclei as a new characterisation tool for such nanoparticles. It will bring together a group with a long track record of developing new materials applications of solid state NMR and an internationally-leading industrial group that are developing fuel cell catalyst technologies. The project attempts to fulfil the philosophy of the Sainsbury Review to get the core academic science base interacting more directly with industry. Traditionally, platinum catalysts have been used exclusively for low temperature fuel cells due to their overall adequate activity and stability for three key reactions of interest. More recently, alloying of platinum with various other metals has shown improvements in both activity and selectivity leading to a diverse range of catalysts for specific reactions. Currently, more advanced research concepts are focussed on nano core-shell materials, where a platinum (or platinum alloy) shell is deposited onto a different core (e.g. palladium) to give both activity (through electronic modification) and cost benefits. Of the metals of interest, platinum is key as this is the basis for most current active formulations. Palladium is of increasing interest as recent reports have indicated that alloying palladium with base metals such as iron can increase activity for some reactions to that of platinum. Also, it has been extensively used as a core for platinum. Rhodium has not been extensively investigated for fuel cell catalysis, but it does show promise as a promoter for platinum for the electro-oxidation of ethanol. In addition, rhodium is the key catalytic metal for a large range of gas-phase reactions, including the CO-NOx reaction in automotive catalysis and the reforming of hydrocarbon fuels to give hydrogen. A characterisation approach that combines traditional analysis techniques (e.g. XRD, TEM, XPS) along with determination of the catalytic activity will be employed. This data will be merged with the new and potentially unique information that will be provided by solid state NMR. A fully multinuclear approach will be employed to examine 1H (to elucidate surface speciation and proton mobility, the latter via relaxation and pulsed field gradient measurements), as well as 13C and 27Al of the support materials. However the clear focus of the work here will be developing NMR of the metals directly. There has already been progress made on 195Pt which has shown that in such heterogeneous systems very broad spectral lines indeed can be encountered such that traditional pulsed approaches are not possible. With the use of field-sweep approaches accurate lineshapes of even very broad lines can be recorded. This project will take this philosophy, develop it further for 195Pt and take on the very much more challenging task of examining 103Rh and 105Pd. The reports of solid state NMR from the latter two nuclei are extremely scarce providing an indication of their difficulty. However by the use of the state of the art NMR equipment available (e.g. field sweep, very high magnetic field) and the construction of a probe optimised for the static observation of these nuclei it is anticipated that significant progress can be made in observing such nuclei. This would provide a new analytical probe of these technologically important materials.