Enfoque de Motivación y Valores

More analysis is required for the η2 _{intermediate species of EP, to ascertain the role it plays}

in the reaction. Liquid chromatography or NMR analysis of the electrolyte solution after the intermediate is formed on the low index platinum surfaces may help to elucidate if the formation of the η2 _{intermediate is beneficial for the enantiomeric excess of the reaction.}

Analysis of the reaction rate when either the η2 _{intermediate or HHS is dominant on the}

surface may also give so useful mechanistic insights.

A different approach to the analysis of the intermediate species may help to acquire more information on the nature of the intermediate species. A combination of XPS and SHINERS could be a powerful technique to identify the species than gather molecular information at working conditions. Tian et al have employed a similar combinative technique to great effect for the investigation of CO oxidation over PtFe bimetallic nanoparticles (6). Another applicable technique is electrochemical tip enhanced Raman spectroscopy in conjunction with electrochemical scanning tunnelling microscopy, presented by Ren et al (7). This technique would be capable of detecting the outlined surface species, while characterising the surface environment in detail. If this spectroelectrochemical method was successfully implemented to the asymmetric systems probed in this thesis, the postulations of

intermediate surface species being specific to terrace or defect sites can be properly elucidated. In general, integrating further techniques into a SEC approach to each system would be a worthwhile endeavour to garner as much information as possible on the systems in question. Expanding the single crystal substrates utilised in the reaction would also be beneficial, as presented in the research there is still a theoretical and technological shortfall when trying to recreate phenomena present on single crystal substrates on practical

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surfaces with an emphasis on pairs of chiral SC surfaces could be a fruitful further investigation.

Regarding electrochemical reactions in combination with SHINERS, it can sometimes be beneficial to be able to switch from acidic to alkaline solutions. An example would be the cleaning process for platinum shaped nanoparticles. The Au@SiO2 nanoparticles degrade in

alkaline solutions, but an aluminium coated gold nanoparticle can be used. This core-shell nanoparticle however, degrades in acidic solutions(8). A nanoparticle capable of SHINERS that could transition between acidic and aqueous media would be very beneficial, when coupled with electrochemistry. With the SHINERS technique still being a rather niche analytical technique few well established alternatives to the Au@SiO2 or Au@Al2O3

nanoparticles are available. However, it has been reported that Ag@ZrO2 nanoparticles have

shown remarkably improved stability in both acidic and alkaline media when compared to the Au@SiO2 nanoparticles (9, 10).

Finally, regarding the SHINERS technique as a powerful modern analytical tool. This thesis has demonstrated the influence SHINERS can have on developing the theory of interfacial catalytic interactions. The SHINERS method is enjoying widespread applications in significant catalytic, electrochemical and biological investigations (11). However, the future success of the SHINERS technique will be dependent on the theoretical and practical development of the method. From experience, a key area to address is the synthesis and application of the Au@SiO2 nanoparticles. The wide size distribution of the gold nanoparticle cores and

differences in silica coating thicknesses can lead to inconsistences with the SERS hot spots. The drop casting method of applying the SHINERS particles to the nanoparticle surface can further compound the inconsistencies, thus, an alternative method that utilises printing or moulding would be preferential. The technique is constantly developing and expanding at a significant rate and will be a valuable analytical resource in the coming years.

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7.4 References

1. S. Guan et al., Structure Sensitivity in Catalytic Hydrogenation at Platinum Surfaces Measured by Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS). ACS

Catalysis, (2016).

2. E. Schmidt, A. Vargas, T. Mallat, A. Baiker, Shape-selective enantioselective hydrogenation on Pt nanoparticles. Journal of the American Chemical Society 131, 12358-12367 (2009). 3. D. J. Jenkins et al., Enantioselectivity and catalyst morphology: step and terrace site

contributions to rate and enantiomeric excess in Pt-catalysed ethyl pyruvate hydrogenation.

Journal of Catalysis 234, 230-239 (2005).

4. S. C. Tsang et al., Engineering preformed cobalt-doped platinum nanocatalysts for ultraselective hydrogenation. Acs Nano 2, 2547-2553 (2008).

5. F. Delbecq, P. Sautet, Competitive C C and C O Adsorption of α-β-Unsaturated

Aldehydes on Pt and Pd Surfaces in Relation with the Selectivity of Hydrogenation Reactions: A Theoretical Approach. Journal of Catalysis 152, 217-236 (1995).

6. H. Zhang et al., In situ dynamic tracking of heterogeneous nanocatalytic processes by shell- isolated nanoparticle-enhanced Raman spectroscopy. Nature communications 8, 15447 (2017).

7. Z.-C. Zeng et al., Electrochemical Tip-Enhanced Raman Spectroscopy. Journal of the

American Chemical Society 137, 11928-11931 (2015).

8. J.-F. Li et al., Synthesis and characterization of gold nanoparticles coated with ultrathin and chemically inert dielectric shells for SHINERS applications. Applied spectroscopy 65, 620-626 (2011).

9. H. Abdulrahman, Synthesis and characterisation of selected nanoresonators for Raman analysis of surfaces. (2017).

10. R. T. Tom et al., Freely dispersible Au@ TiO2, Au@ ZrO2, Ag@ TiO2, and Ag@ ZrO2 core− shell nanoparticles: one-step synthesis, characterization, spectroscopy, and optical limiting properties. Langmuir 19, 3439-3445 (2003).

11. P.-P. Fang, X. Lu, H. Liu, Y. Tong, Applications of shell-isolated nanoparticles in surface- enhanced Raman spectroscopy and fluorescence. TrAC Trends in Analytical Chemistry 66, 103-117 (2015).