Dissertation

Novel Electrocatalysts for Ammonia Synthesis and Hydrogen Evolution

Sudmeier, Tim

2020

Overview

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). 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-N₄ environment caused by axial oxygen ligands leads to decreased HER activity. In the second part of this thesis Co₃Mo₃N was identified as a potential electrocatalyst for the direct reduction of N₂ to N³- 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⁺ ions facilitate the activation of dinitrogen, as well as enable participation of the lattice nitrogen of Co₃Mo₃N.

• Publisher: ProQuest Dissertations & Theses
• Subject: Graphene / Nitrogen