(533g) Surface Phenomenon Affecting Removal Efficiency of Nitrate from Water on Dispersed Single Atoms in Cu Metal Catalyst: An Ab-Initio Study

Anthropogenic activities have disturbed the balance of the delicate nitrogen cycle. Overuse of nitrate (NO₃^-) rich fertilizers have led to the leaching of nitrate in our water system. If nitrogen-rich water is consumed by humans at concentrations higher than 10 mg/L, it can cause methemoglobinemia, or “blue baby syndrome”, thyroid disease, fatigue, weight gain, hair loss, goiters and colon cancer. Therefore, Nitrate remediation is an important step to re-balance the nitrogen cycle. Thermochemical and Electrocatalytic nitrate reduction is a potential remediation process for rebuilding a circular economy by reducing NO₃^- to NH₃ or N₂. However, both are currently not commercially viable because of low selectivity towards safe (N₂) or reusable (NH₃) products. Nitrate reduction reaction often competes with other electrochemical and thermochemical reactions such as hydrogen evolution reaction (HER). Moreover, the high cost of the best catalyst (like Pt, Pd, In etc.) makes nitrate remediation an expensive process. One method to improve the activity and selectivity of Nitrate reduction and decrease the cost of raw material is to opt for cheaper metal (e.g., Cu, $0.43 per mol) catalyst impregnated with Single-Atom Alloy (SAA).

Here we investigate the mechanism of electrochemical and thermochemical nitrate reduction using Density Functional theory over 6 different transition single atom alloys dispersed in Cu[111] to understand the critical effects of dispersed atoms on the activity and selectivity of the electrode surface. The potential and pH effects are included by considering the Chemical Hydrogen electrode and protonation energies of aqueous species. We find that the energy required to dissociate NO₃ Ru and Pd-SAA by thermochemical Nitrate Reduction pathway is higher than desorption on NO₃^-. The difference in energies is reversed by increasing the potential and pH of the solution in the electrochemical system. The selectivity is dependent upon the energy difference between N^* hopping from the SAA site to Cu sites and NH^* formation. For Ru-SAA, the N^* is localized near a single atom, which results in highly selective NH₃ formation. For Pd-SAA, the N^* hopping has a lower activation barrier, which leads to the final reaction occurring on Cu sites, which is dependent upon reaction potential (selective towards N₂ at potential < -0.2 V vs. HER). We use the energy of single species (H^*, O* and N*) as descriptors for building volcano plots to predict an ideal single-atom catalyst that can be used for highly selective NO₃RR near-neutral pH at a low cost.