Revealing the Dynamics of Platinum Nanoparticle Catalysts on Carbon in Oxygen and Water Using Environmental TEM

Deactivation of supported metal nanoparticle catalysts, especially under relevant gas conditions, is a critical challenge for many technological applications, including heterogeneous catalysis, electrocatalysis, and fuel cells. It has been commonly realized that deactivation of catalysts stems from surface area loss due to particle coarsening; however, the mechanism for this remains largely unclear. Herein, we use aberration-corrected environmental transmission electron microscopy, at an atomic level, to observe in situ the dynamics of Pt catalysts under fuel cell relevant gas and temperature conditions. Particle migration and coalescence is observed to be the dominant coarsening process. In comparison with the case of H2O, O-2 promotes Pt nanoparticle migration on the carbon surface. Surprisingly, coating Pt/carbon with a nanofilm of electrolyte (Nafion ionomer) leads to a faster migration of Pt in H2O than in O-2, a consequence of a Nafion carbon interface water "lubrication" effect. Atomically, the particle coalescence features reorientation of particles toward lattice matching, a process driven by orientation-dependent van der Waals forces. These results provide direct observations of the dynamics of metal nanoparticles at the critical surface/interface under relevant conditions and yield significant insights into the multiphase interaction in related technological processes.