Ultra-small Rh nanoparticles supported on WO_3−x nanowires as efficient catalysts for visible-light-enhanced hydrogen evolution from ammonia borane

Abstract

Hydrolysis of ammonia borane (AB) is a safe and convenient means of H₂ production when efficient catalysts are used. Here we report a facile one-pot solvothermal method to synthesize Rh/WO_3-<i>x</i> hybrid nanowires. Ultra-small Rh nanoparticles with an average size of ∼1.7 nm were tightly anchored on WO_3-<i>x</i> nanowires. Rh/WO_3-<i>x</i> catalysts exhibited substantially enhanced activity for hydrolytic dehydrogenation of AB under both dark and visible light irradiation conditions relative to mixed Rh nanoparticles and WO_3-<i>x</i> nanowires (Rh + WO_3-<i>x</i> ), and Rh/C and WO_3-<i>x</i> nanowires. X-ray photoelectron spectroscopy (XPS) analysis indicated that the synergistic effect between Rh nanoparticles and WO_3-<i>x</i> nanowires was responsible for such an enhancement in activity. Specifically, Rh/WO_3-<i>x</i> achieved the highest turnover frequency (TOF) with a value of 805.0 mol_H₂ mol_Rh ^-1 min^-1 at room temperature under visible light irradiation. The H₂ release rate as a function of reaction time exhibited a volcano plot under visible light irradiation, indicating that a self-activation process occurred in the hydrolytic dehydrogenation of AB due to additional oxygen vacancies arising from <i>in situ</i> reduction of WO_3-<i>x</i> nanowires by AB, and thus an enhanced localized surface plasmon resonance (LSPR). Such a self-activation process was responsible for the enhanced catalytic activity under visible light irradiation relative to that under dark conditions, which was supported by the lower activation energy (45.2 <i>vs.</i> 50.5 kJ mol^-1). In addition, Rh/WO_3-<i>x</i> catalysts were relatively stable with only little loss in activity after five cycles due to the tight attachment between two components.