(230c) Understanding the Selectivity of Single Atom Alloys in Complex Reaction Pathways: Examining Electrochemical Nitrate Reduction Reaction through Ab-Initio Calculations.

The constant recycling of nitrogen between earth, soil, water and atmosphere is essential for sustaining life on this planet. However, anthropogenic activities have disturbed the balance of this delicate cycle which has depreciated environmental and human health. Electrocatalytic nitrate reduction (NO₃RR) is a potential remediation process for rebuilding a circular economy by reducing NO₃^- to NH₃ or N₂. However, NO₃RR is currently not commercially viable because of low selectivity towards single product, competition with other reactions such as hydrogen evolution reaction (HER) and high cost of currently available electrocatalysts. Nitrate reduction on Single Atom alloy (SAA) dispersed in cheap metal (for e.g., Cu) can significantly reduce the cost of electrocatalyst while improving the selectivity of Nitrate reduction.

Here we investigate the mechanism of NO₃RR on SAA using Density Functional theory to understand the critical effects of dispersed atoms on the activity of the electrode surface. The potential and pH effects are included by considering Chemical Hydrogen electrode and protonation energies of aqueous species. DFT results predict that the activity of NO₃^- reduction increases as we increase the pH and potential as it increases energy required to desorb NO_x^- species and activate H₂ evolution near SAA. The selectivity of NO₃RR is dependent upon the energy difference between N^* hopping from SAA site to encapsulating metal sites and NH^* formation. For Ru-SAA, the N^* are localized near single atom, which results in highly selective NH₃ formation. For Pd-SAA, the N^* hopping has lower activation barrier, which leads to final reaction occurring on Cu sites, which is dependent upon reaction potential (selective towards N₂ at low potential). We use the energy of single species (H^*, O* and N*) as descriptors for building volcano plots to predict ideal single atom catalyst that can be used for highly selective NO₃RR near neutral pH.