Size-dependent phase transformation of catalytically active nanoparticles captured in situ. 
The utilization of metal nanoparticles traverses across disciplines and we continue to explore the intrinsic size-dependent properties that make them so unique.  Ideal nanoparticle formulation to improve a process's efficiency is classically presented as exposing a greater surface area to volume ratio through decreasing the nanoparticle size.  Although, the physiochemical characteristics of the nanoparticles, such as phase, structure, or behavior, may be influenced by the nature of the environment in which the nanoparticles are subjected1, 2 and, in some cases, could potentially lead to unwanted side effects.  The degree of this influence on the particle properties can be size-dependent, which is seldom highlighted in research.  Herein we reveal such an effect in an industrially valuable cobalt Fischer-Tropsch synthesis (FTS) catalyst using novel in situ characterization.  We expose a direct correlation that exists between the cobalt nanoparticle's size and a phase transformation, which ultimately leads to catalyst deactivation.