Alloying platinum with other transition metals is a well-known approach to improve catalytic performance for oxygen reduction, a key reaction in hydrogen fuel cells and metal–air batteries. The improved performance of alloys is often attributed to so-called ligand effects (that are due to the presence of dissimilar neighbouring atoms with different electronic structure) and/or strain effects (that are due to the changes in interatomic distance). Despite progress in elucidating these effects, which atomic arrangements at the alloy surface produce the most active sites for catalysis is still not well-understood. Now, Regina Kluge, Federico Calle-Vallejo, Aliaksandr Bandarenka and colleagues, based at the Technical University of Munich and the University of the Basque Country, use electrochemical scanning tunnelling microscopy (STM) under reaction conditions to probe the active sites of Pt3Ni with nanometre-scale resolution.
The researchers use a technique developed in earlier works, which exploits changes in the noise level of the electrochemical STM measurements to assess the catalytic activity occurring at a particular point on the surface. They find that on the (111) terrace of a Pt3Ni single crystal the noise level is constant, implying that the surface is composed of equally active sites; step sites have similar or poorer activity. This contrasts with pure platinum catalysts, where the most active regions are found at concave sites near to steps. The researchers also combine the STM data with a computational model, allowing them to quantify strain and ligand effects and extend their analysis to other alloys, shedding light on catalyst design guidelines.
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