Does Turnover Number Represent a Single Value or a Distribution?

Abstract Catalyst turnover number is traditionally measured at the macroscopic scale, yielding a single numerical value under specific reaction conditions. This bulk-averaged approach assumes uniform catalyst behavior, masking intrinsic heterogeneity within a sample and limiting meaningful comparisons across different systems. Here, we present a novel method to simultaneously quantify the number of active sites and total catalytic charge of individual platinum nanoparticles during electrochemical hydrogen evolution at high current densities (>0.4 A/cm²). This enables direct calculation of single-particle turnover number, one at a time, which reveals orders of magnitude of variation among monodisperse particles. We further show that the observed turnover number heterogeneity cannot be primarily attributed to size variation within the monodisperse particle population, but is instead governed by applied potential and particle size. This work redefines our understanding of catalytic turnover number, establishing a framework for assessing catalyst longevity distribution at the single-particle level. Understanding the origins of turnover number heterogeneity and the underlying molecular nuances within electrocatalysts can inform strategies to enhance durability and optimize catalytic performance.