Synthesis and activation of metal cluster-based electrocatalysts for CO₂ reduction.

Type of content
Theses / Dissertations
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Thesis discipline
Chemistry
Degree name
Doctor of Philosophy
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Journal Title
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Volume Title
Language
English
Date
2021
Authors
Sharma, Shailendra Kumar
Abstract

Today, most of the global energy demand is supplemented by burning fossil fuels, which leads to the emission of environmentally unfriendly greenhouse gases that adversely affect climate and human health. Electrochemical reduction of CO₂ into chemical or fuel can be a promising way to store renewable energy sources such as solar and wind energies into the chemical form along with alleviating the environmental concerns. To make the process economically feasible, the electrocatalyst should convert the CO₂ to fuel at a low energy input (or low overpotential).

Metal clusters have been extensively explored in green catalysis due to their superior geometric, electronic, and optical properties compared to bulk-like nanoparticles. The clusters have size-dependent catalytic properties; thus, their catalytic activity and selectivity can be fine-tuned by changing the number of atoms in the metal cluster core. Moreover, the well- defined molecular structure of metal clusters makes them model catalysts for the fundamental studies on catalysis at the atomic level. In the first part of the thesis, a series of phosphine capped gold clusters were synthesised and characterised using X-ray absorption spectroscopy.

In the second part, a range of atomically precise Au clusters (Au₆, Au₉, Au₁₃ and Au₁0₁) were dropcasted onto a carbon paper to prepare the cathode, and the electrochemical reduction of CO₂ (CO₂RR) was studied in 0.2 M KHCO₃ electrolyte. Out of all the clusters studied here, Au₁₃ clusters exhibited superior selectivity for CO₂ to CO conversion. The thermal removal of ligands to avail maximum metal surface area to reactant had adverse effects on the catalytic activity and the selectivity. The characterisation of catalyst using XPS, XAS and SEM suggested that the clusters agglomerates into bulk-like nanoparticles upon thermal annealing at 200 °C.

To make electrode preparation easy and scalable, the catalysts were prepared by depositing the clusters onto carbon black (Au₉/C), and the cathode layer was prepared by spraying the active catalyst with Nafion ionomer binder onto a carbon felt. The potentiostatic (-1.3 V vs. Ag|AgCl) electrochemical CO₂ reduction on Au₉/C catalyst shows an increase in current density and selectivity towards CO2RR during the catalytic run suggesting activation of the catalyst. Furthermore, the activation of the electrode at higher potential (-1.7 V vs. Ag|AgCl) before the experiment resulted in improvement in the catalytic activity at -1.3 V vs. Ag|AgCl. The physical mixture of Au₉ clusters and Nafion ionomer in the methanol results in dilution of solution colour and eventual precipitation suggesting interactions between clusters and ionomer. These interactions were further investigated using UV-vis spectroscopy and X- ray absorption spectroscopy; the results suggest the formation of a bond between metallic cluster core and Nafion molecule. Ex-situ XAS studies on Au₉/C based electrodes before and after electrolysis suggest that the Au₉-Nafion matrix realign itself upon applying external bias, resulting in improved activity. In addition, another method for electrode preparation was proposed to minimise the interaction between clusters and Nafion by first spraying a layer of carbon black followed by drop-casting the active catalyst (Au₉ clusters).

Finally, we investigated the electronic and geometric structure of a series of Au and AuPd clusters deposited on a range of supports using XPS and XAS and studied the effect of thermal annealing on the stability. Our results suggest that the metal-support interaction plays a crucial role in determining the structure of as-deposited clusters. For example, Au₉ clusters, when deposited on Vulcan carbon, undergo substantial structural deformation but remain nearly intact on RuO₂. The thermal stability of various clusters on Vulcan carbon have a few trends as follows: (i) bidentate ligands usually reinforce the stability of supported clusters, (ii) clusters with bulkier ligands show high resistance for sintering, and (iii) clusters with heteroatoms show resistance at low temperature but when the agglomeration start, the process is much faster.

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