Visible-Light Acceleration of H2 Evolution from Aqueous Solutions of Inorganic Hydrides Catalyzed by Gold-Transition-Metal Nanoalloys

Abstract

Production of hydrogen (H₂) upon hydrolysis of inorganic hydrides potentially is a key step in green energy production. We find that visible-light irradiation of aqueous solutions of ammonia-borane (AB) or NaBH₄ containing "click"-dendrimer-stabilized alloyed nanocatalysts composed of nanogold and another late transition-metal nanoparticle (LTMNP) highly enhances catalytic activity for H₂ generation while also inducing alloy to Au core@M shell nanocatalyst restructuration. In terms of visible-light-induced acceleration of H₂ production from both AB and NaBH₄, the Au₁Ru₁ alloy catalysts show the most significant light-boosting effect. Au-Rh and Au-PtNPs are also remarkable with total H₂ release time from AB and NaBH₄ down to 1.3 min at 25 °C (AuRh), 3 times less than in the dark, and Co is the best earth-abundant metal alloyed with nanogold. This boosting effect is explained by the transfer of plasmon-induced hot electron from the Au atoms to the LTMNP atoms facilitating water O-H oxidative addition on the LTMNP surface, as shown by the large primary kinetic isotope effect k_H/k_D upon using D₂O obtained for both AB and NaBH₄. The second simultaneous and progressive effect of visible-light irradiation during these reactions, alloy to Au core@M shell restructuration, enhances the catalytic activity in the recycling, because, in the resulting Au core@M shell, the surface metal (such as Ru) is much more active than the original Au-containing alloy surface in dark reactions. There is no light effect on the rate of hydrogen production for the recycled nanocatalyst because of the absence of Au on the NP surface, but it is still very efficient in hydrogen release during four cycles because of the initial light-induced restructuration, although it is slightly less efficient than the original nanoalloy in the presence of light. The dendritic triazole coordination on each LTMNP surface appears to play a key role in these remarkable light-induced processes.

Keywords: bimetallic nanoparticles; boron hydride; hydrogen production; nanogold; plasmon.