DFT guided catalyst design for the electrochemical ammonia and hydroxylamine synthesis

Abstract

Electrochemical reactions are globally acknowledged as a preeminent approach to solving today's critical problems on which the sustainability of modern society heavily relies, such as energy shortage, efficient product synthesis, and environmental issues. Electrochemical reactions occur at the cathode surface through a consecutive electron transfer process. Therefore, the conductivity and the ability to interact with reactants and electrolytes of the electrode material play a vital role in developing an efficient electrochemical system. The progress in theoretical calculations played a critical role in designing heterogeneous catalysts and provided clear mechanistic insights to understand the electrochemical reactions better at the atomistic level. Specifically, the first principles density functional theory (DFT) simulations offer a splendid platform to evaluate the intrinsic catalytic activity of the heterogeneous materials toward electrochemical reactions. This PhD work aims to provide design guidelines for the NOx reduction to a specific product on the metal-based single and dual-atom catalyst anchored in the two-dimensional (2D) substrates and to probe the potential-dependent reaction mechanism and kinetics of the N₂ reduction to ammonia. In the first part of this dissertation, we demonstrated the grand canonical potential kinetics approach to study the potential-dependent reaction mechanism and kinetics for the N₂ reduction to ammonia on the Fe-Ru-based dual atom catalyst. Electrochemical ammonia synthesis from NRR is an energy-efficient and green process but suffers from sluggish reaction kinetics and poor selectivity mainly due to the competing HER. To understand the interplay between NRR and HER, a potential-dependent mechanistic understanding is required. Here, we construct a DAC containing 1 Fe and 1 Ru atom embedded in the novel 2D C₃N-C₂N heterostructure. We calculated the reaction kinetics under acidic conditions of NRR and HER and found that the onset potential for NH₃ synthesis is -0.22 V with a turn of frequency (TOF) 434 h^-1. While the onset potential for HER is -0.14 V, leading to the Faradaic efficiency (F.E.) of 10.53% for NH₃ synthesis. In the second part of the thesis, we applied the DFT calculations to evaluate the electrocatalytic performance of the metal-based DAC embedded in the 2D phthalocyanine (Pc) substrate for nitrate reduction toward ammonia synthesis. The calculated formation energies and dissociation potential of dual-atom catalysts anchored on 2D Pc predict the high possibility of experimental synthesis and electrochemical stability. We found that ammonia can efficiently be produced from nitrate reduction on the Cr₂-Pc, V₂-Pc, Ti₂-Pc, and Mn₂-Pc surfaces. Moreover, the DAC effectively suppressed the competing hydrogen evolution reaction and favors the NO3RR to ammonia synthesis. In the third part, we evaluate the performance of transition metal-based single-atom catalyst anchored on nitrogen-doped graphene (TM/N_xC_y) for the selective NO reduction to hydroxylamine. Our theoretical results revealed that the Cu and Ni-based SAC (Cu-N_xC_y and Ni-N_xC_y) possess excellent electrocatalytic performance towards NORR. The electrocatalytic NORR preferably occurs at the low limiting potential of -0.02 V for Ni-N₄, -0.08 V for Ni-N₃C, -0.13 V for Ni-N₂C₂, and -0.28 V for Ni-NC₃. For Cu-N_xC_y, the U_L required for NH₂OH is zero, as all the reaction steps downhill. The NO conversion to NH₂OH preferably follows the NO→NHO→NH₂O→NH₂OH pathway for the Ni and Cu surfaces.

Date of Award	2023
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology
Supervisor	Zhengtang LUO (Supervisor)