Catalyst development and mechanistic study of ammonia oxidation reaction

Abstract

Ammonia (NH₃) electro-oxidation reaction (AOR) is an important reaction in direct NH₃ fuel cells, NH₃ electrolyzer, NH₃-based electrochemical sensors and for understanding the nitrogen cycle. However, its slow kinetics and structure-sensitive properties require specific electrocatalyst design. Development of efficient electrocatalysts for AOR also needs comprehensive understanding of mechanism and intermediates involved. The widespread commercial application of this technique is hindered by the unsatisfactory performance of the electrocatalysts. Therefore, design of novel catalytic materials and supports along with insightful fundamental understanding of the reaction through different techniques is required for accelerating AOR practical application. In this thesis, one of the major efforts has been made towards design of unique high-performance surface engineered model catalysts. Ir-decorated Pt nanocubes and Ir and Ni(OH)₂-decorated Pt nanocubes are prepared by wet chemistry method with CO as selective capping agent. For the first time, it is found that a trace amount of Ir (less than 2%) could increase the AOR activity of Pt nanocubes by more than two times. Theoretical simulation results also illustrate that the surface-decorated Ir could lower the energy barrier in the rate determining *NH formation step on Pt surfaces, thus increasing activity. Interestingly, Ir and Ni(OH)₂-decorated Pt nanocubes could significantly improve the durability. In addition, it is observed that Pt/N-doped graphene (NDG) exhibit a specific activity of 0.472 mA cm^-2, which is 44% higher than commercial Pt/C, thus establishing NDG as a more efficient support for AOR than carbon. Furthermore, tungsten monocarbide (WC) as support also shows promise, with Pt/WC/NDG having 30 % increase in AOR activity in comparison to Pt/NDG. Surface modification of this catalyst with Ir results in the best performance, with Pt-Ir/WC/NDG (specific activity 0.94 mA cm^-2) having almost three times the current density of commercial Pt/C and much lower onset potential of 0.48 V vs RHE. Furthermore, aggregation-induced emission (AIE), a robust fluorescence sensing platform is employed for the sensitive and qualitative detection of hydrazine (N₂H₄). N₂H₄ is successfully identified during the AOR on the model Pt/C electrocatalyst using TPE-CHO, an aggregation-induced emission luminogen (AIEgen). AOR mechanism for Pt with N₂H₄ being formed during the dimerization process (NH₂ coupling) is proposed within the framework of Gerischer and Mauerer mechanism. The unique chemodosimeter approach demonstrated in this study opens a novel pathway for understanding electrochemical reactions in-depth. Moreover, in-situ FTIR studies on Pt nanofilm also illustrate N₂H_y as an intermediate, thus establishing dehydrogenation and dimerization steps in AOR. Interestingly, in a neutral PBS electrolytic solution, azide as an intermediate is observed for Pt and a parallel pathway for nitrogen production at pH ~7 is proposed.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology