Investigating the interactions between Pt nanoparticle catalysts and their substrates

The widespread use of platinum as a catalyst has led to wide-ranging research efforts to understand its activity, improve its effectiveness, and also to obtain similar performance from less expensive materials.This project proposes a set of investigations to better understand the way in which Pt nanoparticles interact with the carbon materials that are commonly used as a substrate to the catalyst, with the expectation that conclusions and strategies can be developed that would be applicable widely across supported metal catalysts. The Applicant has extensive experience (>20 papers) in the area of particle impact electrochemistry ("PIE"), having worked in the group that pioneered the technique, and proposes to utilise this method in this study [e.g. N.V.Rees, Electrochem. Commun. 2014,43,83]. Despite some theoretical work, there are no comprehensive studies into the control of the particle-substrate interaction - e.g. varying contact time, numbers of contacts (single vs multi-bounce), etc. Understanding of the factors that contribute to the particle-substrate interaction will have the following direct applications:
The project will consider two themes.

Theme 1: Establishing flexibility in kinetic determination
The traditional means of determining the catalytic activity of nanoparticles are becoming recognised as unreliable due to the presence of a support: the overpotential method is affected by the porosity and thickness of a support layer [Sims et al, Sens Actuators B 2010, 144, 153] and the Koutecky-Levich approach to determine kinetics via rotating disk voltammetry has also been shown to be sensitive to its surface roughness [Masa et al, NanoRes 2014,7,71].
The use of PIE, therefore, is perhaps one of the simplest means to unambiguously measure the kinetics of a catatyic process. However, in order to reliably investigate multiple electron transfers (which are commonly coupled to proton transfer), it will be necessary to have greater understanding and a degree of control over the particle-substrate collision in order to observe the sluggish reactions relevant to energy generation (e.g. oxygen reduction, alcohol oxidation).
In order to study such effects, a walljet reactor (WJR) will be designed and tested, based on an impinging jet arrangement in order to enable straightforward control over the parameters affecting the particle-electrode collision. It will also facilitate separation of reacted and unreacted particles (for theme 2). The well-defined hydrodynamics also simplify computational modelling of the NP system.

Theme 2: Developing a novel fabrication methodology
It has been shown [Y.-G. Zhou et al, ChemPhysChem 2011,12,2085; Chem.Phys.Lett. 2011,511,183] that the equivalents of monolayer and multilayer shells can be electrodeposited onto a core nanoparticle during its collision with a substrate electrode when potentiostatted in the underpotential deposition (upd) and bulk deposition regions respectively. However, no separation of reacted/unreacted particles was possible and so no physical charaterisation on the resulting core@shell particles was ever conducted.
The WJR will effect such a separation and so the homogeneity of deposited overlayers will be investigated, with frutehr work conducted to improve homogeneity where needed.
Control of the particle-substrate contact should enable a degree of control to be applied to the depositon of mulitple layers: useful for core-shell combinations where no upd region exists and where specific shell thicknesses are desired. Careful characterisation will also shed light on the homogeneity of these shell layers, and . Fabrication via PIE is in principle upscaleable via impinging jet arrangements, and work will be conducted to demonstrate and develop this.

The Impact of this research will be primarily academic in the short term, but there is potential for significant wider impact in the 5-10 year horizon in the Industrial context.

1) Academic impact
This will be in two parts, linked by an increased understanding and control of the dynamic interaction of nanoparticles (NPs) with the carbon substrate.
(i) Flexibility in kinetic measurement
The kinetic processes that can be observed and quantified via particle-impact electrochemistry (PIE) are limited by the contact time of the NP with electrode. Establishing an experimental control of the contact time between NP and electrode will enable multi-electron transfer redox processes to be interrogated. At present the maximum reported is a 2 electron process, whereas for widespread use in catalysis research a higher number of electron-transfer processes are required (e.g. 4 for the full reduction of oxygen, 6 for methanol oxidation, etc).
(ii) Fabrication methodology
Control over contact time and collision parameters will also have impacts for the fabrication of core@shell architectures, for cases where either (a) no underpotential deposition region exists for the desired combination of metals, or (b) a narrow range of shell multilayers are needed. The particle-impact electrodeposition (PIED) method does not generally involve additional dispersion agents (capping agents, surfactants, etc), and can be operated in protic and aprotic solvents, inert atmospheres etc for sensitive or reactive species.

In both cases, the research will focus on Pt as the nanoparticle of interest, due to its widespread use in catalysis. However, the results are expected to be generally applicable of all conducting/semiconducting nanoparticles.
Impact will be achieved through rapid publication of results in high impact journals, and presentation at appropriate national/international conferences such as ISE 2015 (Taipei) or 2016 (The Hague), International Symposium on Electrocatalysis ECAT, ECS 2015 (Chicago).

2) Industrial Impact
Although fundamentally electrochemical in nature, this proposal has cross-disciplinary reach due to the occurrence of nanoscale charge transfer processes across the physical and natural sciences, and engineering. In principle, the methodology to be developed has application wherever a charge-transfer process (and accompanying reactions) occurs at a nanometric object or particle.
The ubiquity of charge-transfer catalysts across Industry, from synthetic applications to energy applications creates the potential for significant industrial, and economic impact to be achieved by this programme of research in the mid to long term. The global catalysis market has been estimated to be worth $19.5bn by 2016, and with a significant proportion of catalysts involved in charge-transfer reactions and processes, it is reasonable to foresee that improvements in catalytic design, fabrication or implementation arising from insights gained in the proposed research will translate into realisable economic impact.