page_title: Electrocatalyst
section_title: Particle size effect
hierarchical_section_title: Electrocatalyst / Heterogeneous electrocatalysis / Particle size effect
caption_attribution_description: Italiano: Nanoparticelle in platino di diverse dimensioni; per i metalli CFC la forma termodinamicamente stabile delle nanoparticelle (alla temperatura di 0K) è l'ottaedro troncato. Sono messi in evidenza (color oro) gli atomi di superficie che occupano posizioni kink tra i piani (111) e (100); in tutte e quattro le nanoparticelle il numero di siti kink (corrispondenti ad atomi con numero di coordinazione 6) vale 24. English: Platinum nanoparticles of different sizes; for FCC metals the equilibrium shape (at the temperature of 0K) is the truncated octahedron. Surface atoms at kink positions, between (111) and (100) planes, are highlighted (in gold colour); for all of the four nanoparticles, the number of kink sites (corresponding to the atoms having coordination number 6) is 24.
context_page_description: An electrocatalyst is a catalyst that participates in electrochemical reactions. Catalyst materials modify and increase the rate of chemical reactions without being consumed in the process. Electrocatalysts are a specific form of catalysts that function at electrode surfaces or may be the electrode surface itself. An electrocatalyst can be heterogeneous such as a platinum surface or nanoparticles, or homogeneous like a coordination complex or enzyme. The electrocatalyst assists in transferring electrons between the electrode and reactants, and/or facilitates an intermediate chemical transformation described by an overall half-reaction.
context_section_description: The interest in reducing as much as possible the costs of the catalyst for electrochemical processes led to the use of fine catalyst powders since the specific surface area increases as the average particle size decreases. For instance, most common PEM fuel cells and electrolyzers design is based on a polymeric membrane charged in platinum nanoparticles as an electrocatalyst (the so-called platinum black).
  Although the surface area to volume ratio is commonly considered to be the main parameter relating electrocatalyst size with its activity, to understand the particle-size effect several more phenomena need to be taken into account:
  Equilibrium shape: for any given size of a nanoparticle there is an equilibrium shape which exactly determines its crystal planes
  Reaction sites relative number: a given size for a nanoparticle corresponds to a certain number of surface atoms and only some of them host a reaction site
  Electronic structure: below a certain size, the work function of a nanoparticle changes and its band structure fades away
  Defects: the crystal lattice of a small nanoparticle is perfect; thus, reactions enhanced by defects as reaction sites get slowed down as the particle size decreases
  Stability: small nanoparticles have the tendency to lose mass due to the diffusion of their atoms towards bigger particles, according to the Ostwald ripening phenomenon
  Capping agents: in order to stabilize nanoparticles it is necessary a capping layer, therefore part of their surface is unavailable for reactants
  Support: nanoparticles are often fixed onto a support in order to stay in place, therefore part of their surface is unavailable for reactants
caption: An example of a particle-size effect: the number of reaction sites of different kinds depends on the size of the particle. In this four FCC nanoparticles model, the kink site between (111) and (100) planes (coordination number 6, represented by golden spheres) is 24 for all of the four different nanoparticles, while the number of other surface sites varies.
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