Dynamic Potassium Segregation Drives Cu 2+ → Cu + Transition and Redefines Alkali-Metal Promotion in Selective Propylene Epoxidation

Alkali-metal promoters, particularly potassium, are ubiquitous in heterogeneous catalysis and are commonly rationalized as electron donors to generate low-valence active sites. Yet potassium is typically introduced as electron-deficient K+ species under realistic conditions, obscuring the true origin of alkali-metal promotion. A striking example is that potassium salts, rather than metallic K, induce Cu+ formation in CuO even under O2-rich conditions, enhancing propylene epoxidation and directly challenging the conventional electronic picture. This long-standing paradox arises from the lack of atomic-scale insight into potassium’s operando structural state, which spans a vast configurational space inaccessible to conventional first-principles calculations. Here, by developing a genetic-algorithm-driven active-learning workflow integrating neural network potentials and large-scale molecular dynamics simulations, we uncover the atomic-level origin of K+ promotion in driving the Cu2+ → Cu+ transition in the Cu–O–K system. We show that K+ incorporated into the CuO lattice or statically embedded in the surface is electronically inert. Instead, Cu2+ → Cu+ reduction emerges exclusively from the dynamic surface segregation of undercoordinated K+, where local lattice distortions weaken Cu–O bonds and stabilize reduced copper centers. Dynamic simulations of realistic 3 nm K+-modified CuO nanoparticles confirm K+ surface segregation and concurrent formation of Cu+-rich domains that cooperatively activate propylene and O2 to generate reactive O2– species essential for selective propylene oxide production. In parallel, potassium segregation reshapes the catalyst morphology and stabilizes previously unrecognized square-planar CuO surface motifs with enhanced catalytic performance. These findings overturn the long-standing electron-donor paradigm of alkali-metal promotion and establish a general structural origin driven by dynamic ionic segregation and surface restructuring under realistic catalytic conditions.