Fundamentals and catalyst designs of solar photo-thermo-catalytic ammonia synthesis under mild conditions: A critical review

Ammonia is an indispensable chemical feedstock and an emerging carbon-free hydrogen carrier. Solar photo-thermo-catalytic ammonia synthesis, through the dual activation of photoelectric and photothermal effects, overcomes the trade-off between the high energy consumption of thermocatalysis and the low efficiency of photocatalysis, becoming a groundbreaking approach for efficient ammonia synthesis under mild conditions. This approach utilizes both short-wavelength light-excited hot electrons to reduce the activation energy of N₂ dissociation and hydrogenation, and long-wavelength light-induced thermal energy to promote charge separation and accelerate the adsorption/desorption of reactants/products. Based on different synergistic modes between photoelectric and photothermal effects, this review systematically classifies the photo-thermo-catalytic ammonia synthesis into four categories: photo-assisted thermocatalysis (PATC), thermo-assisted photocatalysis (TAPC), photo-driven thermocatalysis (PDTC), and photo-thermal co-catalysis (PTCC). It elaborates on their unique mechanistic pathways, catalyst design strategies, and fundamentals of performance enhancement, and analyzes current challenges and future development directions. This review aims to provide a guideline for future investigations into mechanisms and catalyst design of photo-thermo-catalytic ammonia synthesis.