Operando Analysis of Adsorption-Limited Hydrogen Oxidation Reaction at Palladium Surfaces

TL;DR

This study uses in situ transmission electron microscopy to reveal that hydrogen oxidation on palladium is limited by adsorption processes, with reaction rates influenced by gas introduction sequence, providing nanoscale insights into catalytic mechanisms.

Contribution

It offers the first real-time nanoscale visualization of intermediate stages in Pd-catalyzed hydrogen oxidation, linking adsorption dynamics to reaction kinetics.

Findings

Hydrogen oxidation rate is affected by gas introduction sequence.

Water formation involves reversible palladium hydride formation.

Adsorption processes limit the overall reaction rate.

Abstract

Palladium (Pd) catalysts have been extensively studied for the direct synthesis of H2O through the hydrogen oxidation reaction at ambient conditions. This heterogeneous catalytic reaction not only holds considerable practical significance but also serves as a classical model for investigating fundamental mechanisms, including adsorption and reactions between adsorbates. Nonetheless, the governing mechanisms and kinetics of its intermediate reaction stages under varying gas conditions remains elusive. This is attributed to the intricate interplay between adsorption, atomic diffusion, and concurrent phase transformation of catalyst. Herein, the Pd-catalyzed, water-forming hydrogen oxidation is studied, in situ, to investigate intermediate reaction stages via fluid cell transmission electron microscopy. The dynamic behaviors of water generation, associated with reversible palladium hydride formation, are captured in real time with a nanoscale spatial resolution. Our findings suggest that the hydrogen oxidation rate catalyzed by Pd is significantly affected by the sequence in which gases are introduced. Through direct evidence of electron diffraction and density functional theory calculation, we demonstrate that the hydrogen oxidation rate is limited by adsorption processes of gas precursors. These nanoscale insights help identify the optimal reaction conditions for Pd-catalyzed hydrogen oxidation, which has substantial implications for water production technologies. The developed understanding also advocates a broader exploration of analogous mechanisms in other metal-catalyzed reactions.