A Perspective on Dynamic Active Sites in Catalysis: Linking Operando X‐Ray Probes With Transient Kinetics

ABSTRACT Catalytic active sites are essentially dynamic and restructure, change oxidation state, undergo local thermal and compositional gradients under reaction conditions, decoupling “what we think is present” from “what is actually active.” Resolving this gap requires workflows combining operando structural probes with kinetic discrimination. X‐ray diffraction and absorption spectroscopy provide phase‐ and element‐specific views under realistic environments, while modulation‐excitation with phase‐sensitive detection isolates weak, responsive signatures of minority—yet decisive—species. Further, local temperature is an under‐measured variable: EXAFS‐based nanothermometry offers a practical route to quantify nanoparticle temperatures and prevent misassignment of apparent kinetics and stability. Because kinetics remains the most stringent test of active versus spectator species, we highlight transient methodologies (TAP, SSITKA) to extract time scales, surface inventories, and mechanistic constraints that spectroscopy alone cannot supply. Using Ni‐ and Fe‐containing nanomaterials for dry reforming of methane as illustrative cases, we show how integrating operando spectroscopy with transient kinetics links site evolution directly to activity, selectivity, and deactivation pathways (including carbon formation and removal). We close by outlining priorities for the field: operando –transient protocols, tighter coupling to microkinetic theory, and physics‐informed machine learning, respecting transport‐reactor realities, to enable active catalysts that are also predictably selective, efficient and stable.