TOUCAN: TOwards an Understanding of CAtalysis on Nanoalloys

Nanoparticles differ in many ways from their "bulk" or "liquid" structures. Nanoparticles of transition metals have been widely used for accelerating important chemical reactions, thanks to their high surface to volume ratios and increasing surface energy when the cluster size decreases to a few tens of nanometers. Of particular interest are bi- and multi-metallic nanoparticles (the so-called "nanoalloys") due to the richness of structures and mixing patterns that they can exhibit and the control of chemical and physical properties that this affords.

Computational tools play a central role in designing and tailoring of nanomaterials, allowing us to find the "magic" nanoparticles for target applications, since the computing power will recreate and investigate in-silico the experimental conditions in order to suggest optimal candidates to industrial partners. A fundamental use of first-principles simulations in nanoalloy science focuses on the chemical reactions that they induce. It has been experimentally shown that heteroepitaxial grown strained over-layers can present chemical properties different than those of the unstrained surface of the same elements. This fact has been confirmed by first-principles calculations. However, at the nanoscale, due to their peculiar surface and bulk geometries, as well as various chemical orderings, even the characterization of chemisorption sites on nanoalloys is not an easy task.

In this proposed research programme, binary nanoalloys will be investigated for their potential catalytic properties. This project will focus mainly on Pt- alloys (i.e. PtAg and PtAu), Ni-alloys (i.e. AgNi, NiPt), Pd-alloys (i.e. PdAg, and PdPt), Co-alloys (i.e. AgCo and CoPt) and Fe-alloys (i.e. FePt and FeCo). Specific chemical reactions with a strong influence in the field of sustainable energy will be considered, such as CO2-capture, biomass processes - e.g. involving dissociation of CO and CH4, and NH3 dissociation for hydrogen production.

Two conditions must be fulfilled for a spontaneous chemical transformation to occur in the laboratory: (1) the final state must have a lower free energy than the initial state, and (2) there must be at least one pathway that allows the transformation to take place within a reasonable time. In simple chemical reactions, the transformation pathway (the reaction coordinate) is often well understood, which makes it possible to compute reaction rates and predict how external influences (such as catalysts) will affect these rates. However, there are many transformations, including
structural relaxation and nucleation in solids, where the trajectory can follow complex paths that correspond to cooperative or sequential motion of many degrees of freedom. From the point of view of computer simulation, such pathways correspond to rare events, because the waiting time required for the process of interest to occur is very large compared to the time taken for the event itself. To understand and control such complex transformations at the microscopic level, we need to characterise the underlying, high-dimensional potential energy landscape and sample the rare events directly. Based on this knowledge, we aim to predict the relevant transformation pathways and rates and, more ambitiously, to understand how we can influence these rates.

The project is comprised of four inter-linked projects which are aligned with the aims and objectives set out above:

P1. Construction of the Nanoalloy Database

P2. Determination of Thermal Stabilities of Nanoalloy Isomers

P3. Chemisorption Maps

P4. Reaction Rates for Molecular Dissociation on Nanoalloys

The relevance of our proposal can be appreciated by reference to the 2009 International Review of UK Chemistry Research. A key recommendation in the Executive Summary is for "Integration of Computational Chemistry" and the "Need to enhance the participation of theory and computation, especially in areas that involve energy, materials and health applications". This participation clearly depends upon the development of novel simulation methods to treat dynamics on relevant time scales in complex systems, which is the focus of our proposal. The systems we propose to study are directly relevant to both materials and health applications in the future.

Nanocatalysis is becoming a massive and strategic worldwide industry, aimed at the replacement of precious metals by catalysts tailored at the nanoscale with improved catalytic activity, selectivity, and lifetime and reduced process costs. Thus, industrial companies such as Johnson-Matthey and Shell could benefit from the outcomes of the project.

During the course of this project, new simulation tools will be developed to address previously inaccessible time and length scales, as well as to couple global optimisation and energy landscape analysis. The resulting codes will be made freely available to the academic community. These will be of great interest to the wider modelling community (including in industry), since computer simulations are used to investigate and predict the properties of systems including surfaces, biomolecules and condensed matter phases.

In addition to regular submission of high quality research articles in high impact, peer-reviewed journals, during the course of the grant, several articles will be written for publication in general science journals. All members of the project will contribute to appropriate meetings of interest groups in simulation and modelling, which are key multidisciplinary themes within the UK research community We have also requested funding to attend important national and international conferences where our results will be presented -- e.g. conferences on nanoparticles and catalysis. We also propose to organise a 2-day symposium on nanoalloy catalysis towards the end of the research programme (early in year 5), in order to showcase our research and to aid the development of future research directions and collaborations. Industrial, as well as academic participants will be invited. We will also undertake to increase the public understanding of science in the UK and internationally through giving public lectures.

We will create a network among our institutions in order to share and improve our expertise (e.g. by regular visits of PDRAs to collaborating groups). We have also allowed for one annual meeting of all the PIs and PDRAs.This programme of research and the associated collaborative visits and meetings will also benefit the younger PhD researchers in each of the groups involved. Annual meetings with an Advisory Panel will guarantee the progress and success of our collaborations.

The exploitation and protection of any results with commercia lpotential deriving from the project will be handled through the Research Services Divisions of the relevant university, all of which have considerable experience in this area.