Atomic scale insight into the formation, size and location of plati- num nanoparticles supported on γ-alumina

Abstract : The clear description of the morphology and location, with respect to the support, of metallic sub-nanometric particles remains a current experimental strenuous challenge in numerous catalytic applications. High resolution-HAADF-STEM coupled with in situ and tomographic analyses are undertaken on platinum (Pt) active phase supported on chlorinated alumina (γ-Al 2 O 3) with 0.3 and 1% w/w Pt loadings highlighting the formation of flat nanoparticles (NPs) of 0.9 nm diameter and Pt single atoms (SAs) in the reduced state. While SAs and weakly cohesive clusters are predominantly observed in the oxide state, with a coordination sphere of Pt composed of O and Cl as revealed by EXAFS, the ratio between SAs and Pt NPs in the reduced state is found to be about 2.8. This ratio is not affected by metal loading which increases both the total number of NPs and SA. Electron tomography reveals that the vast majority of NPs are located on the edges or defects (steps, kinks) of the alumina support crystallites. Density functional theory calculations further highlight the optimized structures of NPs located at the γ-Al 2 O 3 (110)(100) edge and near-edge with a stability competing with NPs located either on the (110) or on the (100) γ-Al 2 O 3 facet. A mathematical analysis of the segmented volumes shows that the average geodesic distances between NPs is linked to Pt loading: 9 nm for 1% w/w Pt, and 16 nm for 0.3% w/w Pt. Evaluation of support tortuosity descriptors using the nanoparticles positions confirms a uniform distribution on the support. A square network geometric model compatible with the geodesic distances between NPs reveals that 1 to 5 NPs can be present at the same time on each alumina crystallite depending on Pt loading.