Light–Heat Interactions in Photothermal Ammonia Synthesis: A Diagnostic Framework for Mechanistic Interpretation

Photothermal ammonia synthesis has emerged as a promising route to couple nitrogen fixation with solar energy, yet the mechanistic origin of light-assisted activity remains difficult to establish because illumination can contribute through heat generation, photoexcited carriers, or both simultaneously. This Review examines recent photothermal NH3 synthesis systems from a diagnostic perspective. Representative Ru- and Fe-based materials are discussed within four photothermal regimes: photo-driven thermocatalysis, photo-assisted thermocatalysis, photothermal co-catalysis, and thermally assisted photocatalysis; and are evaluated according to two complementary questions: what is the most likely dominant photothermal regime under the reported reaction conditions, and how strong is the mechanistic evidence supporting that assignment? Particular attention is given to temperature-normalized light/dark comparisons, apparent activation energy and reaction-order analysis, wavelength- and intensity-dependent activity, isotopic validation, operando spectroscopy, and methods for constraining local thermal gradients. Across the strongest cases, the clearest mechanistic effects emerge when light-responsive functionality overlaps with the kinetically relevant elementary step at a catalytically active interface. The analysis also shows that photothermal co-catalysis should be assigned more restrictively than the other regimes and only when simultaneous and cooperative thermo-photochemical contributions are supported by sufficiently robust mechanistic evidence. By comparing Ru- and Fe-based systems through this evidence-based framework, this Review identifies structure-, interface-, and kinetic-constraint-dependent trends, proposes practical criteria for mechanistic assignment, and outlines priorities for catalyst design, reporting standards, and reactor development in photothermal ammonia synthesis.