  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.
