H₂ Generation with (Mixed) Plasmonic Cu/Au-TiO₂ Photocatalysts: Structure–Reactivity Relationships Assessed by in situ Spectroscopy

Monometallic Cu and bimetallic Cu/Au-TiO₂ catalysts were prepared by impregnation (IM) and reductive precipitation (RP) methods in sequential (SP) and simultaneous mode (CP) and tested for photocatalytic H₂ generation from H₂O/methanol mixtures with visible (400–700 nm) and UV/Vis light (320–500 nm). Comprehensive studies by high-resolution (HR)-STEM, X-ray photoelectron spectroscopy (XPS), and different in situ methods (X-ray absorption near-edge structure (XANES), UV/Vis, and EPR spectroscopy) revealed that IM leads to dispersed surface Cu species with no clear particle formation, which is poorly active under visible light, whereas plasmonic Cu⁰ nanoparticles formed by RP are about three times more active under the same conditions. In Cu/Au-TiO₂ catalysts prepared by RP-SP, highly dispersed Cu surface species boost H₂ production under UV/Vis light, owing to the effective separation within TiO₂ and electron trapping by Cu, whereas small Cu⁰ and Au⁰ particles remain widely separated. When Cu/Au-TiO₂ catalysts are prepared by RP-CP, mixed Cu/Au particles of uniform size (4–8 nm) provide the highest H₂ evolution rates under visible light, owing to effective surface plasmon resonance absorption.