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Electrochemical and shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) approaches to probing novel catalytic nanomaterials

Donovan-Sheppard, Oliver 2018. Electrochemical and shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) approaches to probing novel catalytic nanomaterials. PhD Thesis, Cardiff University.
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Abstract

The surface sensitivity of asymmetric catalytic hydrogenation was investigated by utilising platinum single crystal surfaces and a novel in-situ analytical combination of cyclic voltammetry and shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS). The adsorption behaviour of aqueous Ethyl Pyruvate (EP) on a range of well-defined platinum surfaces, both with and without the presence of electrolytically evolved hydrogen and the chiral modifier Cinchonidine (CD), was monitored by both cyclic voltammetry and Raman spectroscopy. Two key surface intermediates of Ethyl Pyruvate (EP) were identified, the first being the half-hydrogenated state (HHS), reported previously by Attard et al. This intermediate is formed by the addition of a hydrogen atom to the keto-carbonyl group. The presence of second intermediate was also detected, this new species was not present in previous surface enhanced Raman spectroscopy (SERS) spectra and is attributed to chemisorbed EP bound in the μ2 (C, O) configuration. The degree of surface occupation of each intermediate was shown to be sensitive to the structure of the catalyst surface. The new μ2 (C, O) adsorbate was solely present in large quantities on pristine Pt{111} and Pt{100} surfaces. The HHS was only detected in the spectra on these surfaces after the introduction of low coordination defect sites through electrochemical roughening. Similar spectra were produced for intrinsically defected surfaces Pt{110}, Pt{321} and Pt{721}. This was an indicator that for the HHS to form, low coordination sites on the catalyst surface are required. The differing populations of both the intermediate states was further investigated by density functional theory (DFT) calculations, which supports the findings. The μ2 (C, O) state was found to have an appreciably higher stability on the Pt{111} in comparison to the Pt{221} surface. A mechanism was proposed that the μ2 (C, O) EP adsorbate was a precursor to the HHS, with the rates of hydrogen addition on the different surfaces impacting on the population of each adsorption state. With a slow rate of addition of the first hydrogen leading to a low steady-state population of the HHS on terrace sites. The impact of these findings was discussed with regards to enantioselective catalysis and catalyst design. The in-situ combination of cyclic voltammetry and SHINERS was applied to shape-controlled platinum nanoparticles, a more realistic catalytic system and a new asymmetric hydrogenation of α,β-unsaturated aldehydes with limited success. The potential and issues with this analytical approach are also discussed in depth.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Chemistry
Subjects: Q Science > QD Chemistry
Date of First Compliant Deposit: 9 May 2019
Last Modified: 10 May 2019 08:53
URI: http://orca-mwe.cf.ac.uk/id/eprint/122284

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