Photo-stimulated hydrogen atom transfer quantum reactions in WO 3 and MoO 3 thin films

Abstract

WO₃ and MoO₃ are important hydrogen storage and transport materials capable of accommodating large quantities of hydrogen. Upon insertion of hydrogen, their properties change dramatically, giving rise to electrochromic and photochromic transformations that have been intensively investigated [Handbook of Inorganic Electrochromic Materials (Elsevier, Amsterdam, 1995); Ionics 4 372 (1998); Electrochim. Acta 44 3027 (1999)]. On the other hand, the same materials are effective catalysts for hydrogen production via photochemical surface reactions. In this work, we investigate the kinetics of the photoinjection of hydrogen into highly disordered WO₃ and MoO₃ thin films over a wide temperature range and discover a low-temperature non-zero reaction rate limit for photo-stimulated hydrogen atom transfer between adsorbed molecules and the oxide surface, which provides unambiguous evidence for a quantum mechanism reaction. The absence of a kinetic isotope effect on the reaction rate is demonstrated. The reaction model shows an amazing similarity to proton-coupled electron transfer, which is a basic mechanism of bioenergetic conversion.