  Gas-solid reactions are cornerstones of many catalytic and redox processes
that will underpin the energy and sustainability transition. The specific case
of hydrogen-based iron oxide reduction is the foundation to render the global
steel industry fossil-free, an essential target as iron production is the
largest single industrial emitter of carbon dioxide. Our perception of
gas-solid reactions has not only been limited by the availability of
state-of-the-art techniques which can delve into the reacted solids in great
structural and chemical detail, but we continue to miss an important reaction
partner that defines the thermodynamics and kinetics of gas phase reactions:
the gas molecules. In this investigation, we use the latest development in
cryogenic atom probe tomography to study the quasi in-situ evolution of gas
phase heavy water at iron-iron oxide interfaces resulting from the direct
reduction of iron oxide by deuterium gas at 700{\deg}C. The findings provide
new insights into the formation kinetics and location of water formed during
hydrogen-based reduction of FeO, an its interaction with the ongoing redox
reaction.
