Designing Titanium Dioxide-based and Copper-based Catalysts for Enhanced Electrocatalytic Ammonia Synthesis from Nitrogen/Nitrate Reduction Reaction

Abstract

Ammonia (NH₃) is an important chemical, mainly produced from the energy- and cost-extensive Haber-Bosch process. Electrocatalytic nitrogen reduction reaction (NRR) and nitrate reduction reaction (NO₃^-RR) are considered promising technologies for NH3 synthesis since they can be performed in ambient conditions. This Ph.D. thesis aims to design and investigate catalyst materials based on titanium dioxide (TiO₂) and copper (Cu) materials for electrocatalytic NRR and NO₃^-RR, emphasizing the relationship between the modulated electronic structure of catalysts to the exhibited catalytic performance and elucidating the underlying process. To achieve these objectives, the modifications of TiO₂-based and Cu-based materials were performed by defects generation, co-catalyst loading, heteroatom doping, and oxidation state regulation.

In TiO₂-based materials, oxygen vacancies generated on TiO₂ serve as the active sites for N₂ and NO₃^- activation. Moreover, Cu loaded on the surface of defected TiO2 serves as a co-catalyst, which provides additional active sites. In NRR, Cu loaded on oxygen-deficient TiO₂ exhibits an NH₃ yield rate of 13.6 µg mg_cat^-1 h^-1 at -0.5 V vs. reversible hydrogen electrode (RHE) and Faradaic efficiency (FE) of 17.9% at -0.4 V vs. RHE, which are higher than the pristine TiO₂. While in NO₃^-RR, Cu loaded on faceted TiO₂ with dominant (101) facets results in an enhanced NH₃ yield rate of 417.5 µg mg_cat^-1 h^-1 at -0.9 V vs. RHE and FE of 69.9% at -0.8 V vs. RHE, which are higher compared to the Cu loaded-(001)-dominant TiO₂. In both cases, the strong metal-support interaction between Cu nanoparticles and oxygen-deficient TiO₂ or (101)-dominant TiO₂ leads to the electron-deficient Cu, which polarizes the adsorbed N₂ molecules for better activation in NRR and facilitates the accumulation of NO₃^- molecules in NO₃^-RR. Additionally, the strong metal-support interaction also promotes electron transfer, increases electron density, and strengthens the binding energy of *NO₂ (in NO₃^-RR).

Cu-based catalysts were reported to have superior activity and selectivity compared to the other transition metal catalysts for NO₃^-RR. In this work, phosphorus (P)-doped Cu/Cu₂O catalyst formed by in-situ electroreduction of its P-doped CuO/Cu₂O precursor achieves an NH₃ yield rate of 0.27 mmol h^-1 cm^-2 (4590 μg cm^-2 h^-1), NH₃ FE of 97.0%, and NH₃ selectivity of 97.0% at -0.8 V vs. RHE with the current density reaching ~120 mA cm^-2. The electrochemical analyses suggest the promoted electron transfer and increased electron density in Cu/Cu₂O upon the P-doping. Time-dependent activity experiments indicate that the high activity is ascribed to the higher binding strength of *NO₂ on the catalyst surface. Moreover, the P-doping retards the depletion of Cu⁺, which is favorable for NH3 formation. Following this study, oxidation-state regulation over Cu/Cu₂O catalysts further verifies that the Cu⁺ species is the determining species for enhanced NH3 production and selectivity. The higher proportion of Cu⁺/Cu⁰ strengthens the binding energy of *NO₂ on the catalyst surface, which suppresses the desorption of *NO₂ and expedites further hydrogenation to NH₃.

Date of Award	14 Aug 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Yun Hau NG (Supervisor)