Novel electrocatalysts for ammonia synthesis and hydrogen evolution
<p>This thesis explores the activity of single atom metal catalysts on graphene-like support (SACs) and metal nitrides for the nitrogen reduction reaction (NRR). Furthermore, extensive analysis of HER activity and in-situ characterisation are carried out to derive a structure-activity relation...
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| Format: | Thesis |
| Language: | English |
| Published: |
2019
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| Summary: | <p>This thesis explores the activity of single atom metal catalysts on graphene-like support (SACs) and metal nitrides for the nitrogen reduction reaction (NRR). Furthermore, extensive analysis of HER activity and in-situ characterisation are carried out to derive a structure-activity relationship of SACs in the hydrogen evolution reaction (HER).</p>
<p>To that end, a library of single atom transition metals on nitrogen-doped graphene (M-NGs) with comparable compositions is synthesised by a carbon backbone method. Their HER activity is assessed showing Ru-NG and Co-NG to be highly active. The data is correlated with the φ factor, a computationally suggested descriptor for electron density on the metal site constituting the first experimental substantiation of this design principle. Additionally it is found that HER activity on cobalt-containing SACs can be further tuned by changing the coordination environment of the metal via the inclusion of oxygen into the first coordination shell. In in-situ XANES and IR spectroelectrochemical thiocyanate poisoning experiments three distinct active sites are observed and it is suggested that distortion from the square planar Co-N<sub>4</sub> environment caused by axial oxygen ligands leads to decreased HER activity.</p>
<p>In the second part of this thesis Co<sub>3</sub>Mo<sub>3</sub>N was identified as a potential electrocatalyst for the direct reduction of N<sub>2</sub> to N<sup>3</sup>- in molten chloride eutectics. Based on experimental evidence and previous literature reports, a Mars van Krevelen-type mechanism is suggested for the reaction in which Li<sup>+</sup> ions facilitate the activation of dinitrogen, as well as enable participation of the lattice nitrogen of Co<sub>3</sub>Mo<sub>3</sub>N.</p> |
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