Exploring copper β-diketiminate complexes for heterogeneous ammonia oxidation anchored on graphitic surfaces via CH–π and π–π interactions

Abstract

Ammonia is a compelling candidate for a carbon-free energy carrier, offering potential applications as both a hydrogen vector and a fuel. Unlocking these applications requires the development of efficient ammonia oxidation catalysts. Building on recent work with a fluorinated Cu(I) β-diketiminate catalyst for homogeneous ammonia oxidation, we present the first examples of hybrid electrodes employing first-row metal complexes and supramolecular anchoring for heterogeneous ammonia oxidation. In this study, we developed innovative synthetic methodologies to produce a family of β-diketiminate ligands and complexes. Some analogues clarified the role of each functional group in the original ligand, while others incorporated moieties designed to facilitate immobilization on graphitic surfaces via CH–π and π–π interactions. These structural modifications not only broadened the scope of ligand design but also advanced our understanding of their roles in catalysis and immobilization. Unexpectedly, all synthesized Cu complexes, including the previously reported catalyst, formed an equilibrium with [Cu^I(NH₃)₄]⁺ or [Cu^II(NH₃)₄]²⁺ in the presence of ammonia, leading to ligand dissociation and subsequent catalyst deactivation. Despite these challenges, we demonstrated that anchoring these complexes is feasible and confirmed that heterogeneous ammonia oxidation is possible with this system. This work represents a pivotal step toward the construction of ammonia oxidation systems based on earth-abundant metals, highlighting the promise of Cu β-diketiminate complexes. Addressing the challenges of ligand loss and catalyst stability will pave the way for robust and efficient systems, driving advancements in sustainable ammonia-based energy technologies.