Recent Advances in Metal-Organic Framework Photo-Electrocatalysts for the Synthesis of Ammonia

Abstract

Ammonia (NH₃) is a key precursor for fertilizers and bulk chemicals, with global production reaching approximately 200 million tons in 2018. It is also regarded as a highly promising hydrogen carrier thanks to its high hydrogen content and the ease with which it can be stored and transported. However, the traditional Haber process for synthesizing ammonia is energy-intensive and produces significant carbon emissions, making the development of sustainable new synthesis routes imperative. In recent years, nitrogen reduction reactions (NRR/NO_xRR) involving photocatalysis, electrocatalysis, or both (photo-electrocatalysis) have received significant attention for their potential to drive ammonia synthesis under mild conditions using renewable electrical or solar energy. The key advancement in this field is designing high-performance catalysts. Metal-organic frameworks (MOFs), in particular, have emerged as an ideal platform for elucidating reaction mechanisms and enhancing catalytic performance, thanks to their precisely tunable structures, adjustable porosity, well-defined active sites, and ease of modification. This review outlines the application and research advances of MOF-based materials in photocatalytic, electrocatalytic, and photo-electrocatalytic nitrogen and NO₃^- reduction for ammonia synthesis in a systematic way. The focus is on how the rational design of MOF metal nodes, organic ligands, and pore environments can regulate key steps, including light absorption, charge separation/transport, nitrogen adsorption activation, and proton transfer, to optimize catalytic performance. The paper thoroughly analyses effective strategies, including defect engineering, structural regulation, and morphology design, to overcome current challenges associated with catalysts, such as low selectivity, insufficient activity, and poor stability. Leveraging the programmable nature of MOF materials, the paper envisages their future role as models for studying mechanisms and as high-performance catalysts in advancing green ammonia synthesis technology toward practical application.