Metal catalysts play an important role in industrial redox reactions. Although extensively studied, the state of these catalysts under operating conditions is largely unknown and assignments of active sites remain speculative. Herein, we present an operando transmission electron microscopy study that interrelates structural dynamics of redox metal catalysts to their activity. Using hydrogen oxidation on copper as an elementary redox reaction, we reveal how the interaction between metal and surrounding gas phase induces complex structural transformations and drives the system from a thermodynamic equilibrium towards a state controlled by chemical dynamics. Direct imaging combined with the simultaneous detection of catalytic activity provides unparalleled structureactivity insights that identify distinct mechanisms for water formation and reveals the means by which the system self-adjusts to changes of the gas phase chemical potential. Density function theory calculations show that surface phase transitions are driven by chemical dynamics even when the system is far from a thermodynamic phase boundary. In a bottom-up approach, the dynamic behavior observed here for an elementary reaction is finally extended to more relevant redox reactions and other metal catalysts, which underlines the importance of chemical dynamics for the formation and constant re-generation of transient active sites during catalysis.