Evolution of atomically dispersed co-catalysts during solar or UV photocatalysis for efficient and sustained H2 production

The evolution of metal/titania photocatalysts during photocatalytic H 2 evolution is herein studied. Samples containing atomically dispersed Pt co-catalysts (single atoms, clusters and sub-nanoparticles) formed after calcination were compared to pre-reduced analogues mostly having metallic nanoparticles (diameters >1 nm) during ethanol photoreforming under either UV-rich irradiation or natural sunlight. Aggregation of ultra-dispersed oxidised platinum entities (Pt δ+ ) with concomitant reduction into Pt 0 nanoparticles (1–2 nm) was observed after UV irradiation by transmission electron microscopy (TEM), and diffuse reflectance UV–visible (DRUV-vis) and X-ray photoelectron (XPS) spectroscopies. A parallel, albeit slower, evolution trend was evidenced during solar photocatalysis. Conversely, atomically dispersed Cu co-catalyst species did not grow and became in-situ reduced into sub-nanometric Cu 0 under irradiation. Hydrogen production rates were remarkably high during initial stages of UV irradiation, and then declined to a sustained regime (≈50 and 8 mmol g −1 h −1 for Pt/TiO 2 or Cu/TiO 2 , respectively, for up to 24 h of irradiation). Steadier solar photoreforming was observed in experiments performed in a compound parabolic collector tubular reactor (≈7.6 and 1.7 mmol g −1 h −1 for Pt/TiO 2 or Cu/TiO 2 , respectively). Despite the non-negligible effect of co-catalyst rearrangement on activity rationalised herein, attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy measurements pre- and post-photocatalysis suggest that accumulation of strongly adsorbed degradation intermediates, chiefly acetate, is a major cause for rate decreases. Notwithstanding, this phenomenon did not result in total deactivation, so that sustained hydrogen production upon long-term irradiation was not compromised. • Ultra-dispersed Pt species on TiO 2 evolve into nanoparticles under irradiation. • H 2 production activity is steady under either natural sunlight or UV. • Solar H 2 production activity is slightly lower but considerable using Cu cocatalyst. • Adsorbed intermediate oxygenates inhibit but do not lead to deactivation.