In situ study of the thermal stability of supported Pt nanoparticles and their stabilization via atomic layer deposition overcoating.

Downscaling supported Pt structures to the nanoscale is motivated by the augmentation of the catalytic activity and selectivity, which depend on the particle size, shape and coverage. Harsh thermal and chemical conditions generally required during catalytic applications entail an undesirable particle coarsening, limiting as consequence the catalyst lifetime. Here we report an in situ synchrotron study on the stability of supported Pt nanoparticles using Atomic Layer Deposition (ALD) as stabilizing methodology against particle coarsening. Pt nanoparticles were thermally annealed up to 850 oC in an oxidizing environment while recording in situ synchrotron Grazing Incidence Small Angle X-ray Scattering (GISAXS) 2D patterns, obtaining continuous information about the particle radius evolution. Al2O3 overcoating as protective capping layer against coarsening via ALD was investigated. In situ data proved that only 1 cycle of Al2O3 ALD caused an augmentation of the onset temperature for particle coarsening. Moreover, results showed a dependence of the required overcoating thickness on the initial particle size and distribution, being more efficient (i.e. requiring lower thicknesses) when isolated particles are present on the sample surface. The Pt surface accessibility, which is decisive in catalytic applications, was analyzed via Low Energy Ion Scattering (LEIS) technique, revealing a larger Pt surface accessibility for a sample with Al2O3 overcoating than for a sample without protective layer after a long isothermal annealing.