Mechanistic understanding and improvement of photochemical proton reduction catalyzed by iron carbonyl complexes for sustainable hydrogen production Mechanistic investigation of three homogeneous photochemical proton reduction systems that comprise Ir or Cu based photosensitizers and iron carbonyl catalysts to produce hydrogen. Operando IR spectroscopy coupled with gas volumetry was applied supported by other spectroscopic techniques. The rate determining steps deactivation mechanisms and catalytic key species were determined with the latter being a self assembling [FeFe] hydrogenase mimic in case phosphines are present as co catalysts in the reaction mixture. These results lead to a better understanding and improvement of the systems.<eng>