Dynamic Potassium Segregation Drives Cu²⁺ → Cu⁺ Transition and Redefines Alkali-Metal Promotion in Selective...

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.