Dynamics of inverse metal oxides on metal catalysts using spectro-kinetics: reversible Brønsted acid site formation and irreversible reduction

Abstract

Brønsted acid sites (BASs) in inverse catalysts are vital for the selective hydrogenolysis of polyols, specifically cleaving secondary C–O bonds. These BASs form dynamically in situ in an H₂ environment. While H₂ enables rapid BAS generation on short timescales, it reduces the catalyst at prolonged exposures. The active center for BAS generation, the kinetics of BAS formation, its reverse decomposition, and the irreversible oxide reduction have lacked direct experimental evidence. Here, aided by advanced spectro-kinetic studies, we identify trimeric W₃O_x sites on Pt as the active centers for BAS generation, whereas isolated WO_x species on SiO₂ act merely as spectator species, demonstrated using an inverse WO_x/Pt catalyst as a representative system. A detailed kinetic profile capturing the dynamics of W₃O_x sites on Pt is also established. The rate constant for BAS formation is two orders of magnitude higher than for its decomposition, which is one order of magnitude faster than the irreversible site reduction. Co-fed H₂O suppresses the site reduction by ∼50%. Furthermore, the H₂ partial pressure plays an important role. While lower gas-phase H₂ partial pressure does not influence the reversible BAS formation, it can significantly (∼3×) suppress catalyst reduction. These findings offer critical insights into optimizing reaction conditions through periodic H₂ pulsing, enhancing catalyst stability and performance in hydrogenolysis reactions.