(745h) Correlative Structure-Bonding and Stability Studies of Pt/?-Al2O3 Catalysts

Pt supported on gamma-alumina (γ-Al₂O₃) is one of the most important heterogeneous catalysts, with numerous technologically important applications including oil refining, catalytic converters, and fuel cells, making it the subject of numerous studies. Environmental transmission electron microscopy (ETEM) has been a significant boon to the field of heterogeneous catalysis, as it has rendered possible the real-time observation of supported nanoparticle catalysts while in the presence of relevant reaction conditions, allowing the observation of catalyst dynamics that are ordinarily missed in traditional post mortem analysis of spent catalysts.

Previous ETEM studies of supported Pt nanoparticles have shown that Pt nanoparticles can be altered from their initial shape and size during exposure to reactant gases. Gas adsorption on the surface of Pt nanoparticles not only modifies the shape of the nanoparticles, but also influences their stability. A previous study of Pt on CeO₂ found that finely dispersed Pt coalesced to form larger nanoparticles under H₂ gas at 250°C and redispersed under O₂ gas at 400°C. Our preliminary results showed that under H₂ gas at room temperature, ~1nm Pt nanoparticles rapidly coalesced into larger particles ranging up to as large as ~4nm in diameter. Under O₂ gas and at room temperature, ~1nm Pt nanoparticles appeared to disperse and are no longer visible. This is unexpected, as it has been previously suggested that redispersion should not be expected for Pt/γ-Al₂O₃, as the interaction between Pt and γ-Al₂O₃ is not as strong as that of Pt/CeO₂. However, it appears there could be a size threshold below which Pt nanoparticles on γ-Al₂O₃ do redisperse. This is being studied by preparing Pt/γ-Al₂O₃ samples with different Pt sizes and performing the same experiments. It has been noted previously that when Pt nanoparticles are oxidized to form PtO_x, they become raft-shaped. The formation of these PtO_x rafts would be more difficult to observe in the ETEM due to the reduced spatial dimensions and reduced contrast between PtO_x and γ-Al₂O₃. It was noted that the larger particles formed during the initial H₂ reduction did not redisperse under the O₂ oxidation conditions. It is also important to note the apparent effect of the support, in that under O₂ gas, the Pt nanoparticles on the γ-Al₂O₃ support dispersed whereas they coalesced on the amorphous ultrathin carbon backing film of the TEM grid. This suggests that the metal-support interaction also influences the NP agglomeration dynamics.

Having observed support-influenced Pt nanoparticle behavior in H₂ and O₂ environments, we will now study this behavior in atomic detail using a model catalyst of Pt on single crystal γ-Al₂O₃ support. Pt nanoparticles will be deposited on a single-crystal γ-Al₂O₃ thin film support synthesized by oxidation of single-crystal NiAl and cross-sectional TEM samples will be prepared using focused ion beam (FIB). With this much more well-defined system to study in the ETEM, we will be able to observe shape dynamics of the Pt nanoparticles as well as Pt/γ-Al₂O₃ interface-related dynamics in addition to the agglomeration/dispersion behavior of the Pt nanoparticles.