An in situ transmission electron microscopy study of sintering and redispersion phenomena over size-selected metal nanoparticles: environmental effects

Abstract

The thermal and chemical stability of micelle-synthesized size-selected Pt nanoparticles (NPs) supported on thin SiO2 (20 nm) films was monitored in situ via transmission electron microscopy (TEM) under pure hydrogen and pure oxygen environments. The coarsening treatment was performed for 30 min at each temperature (1 Torr of either O2 or H2), while the TEM measurements were carried out at 1 Torr of H2 and 0.5 Torr of O2. Surprisingly, the NPs were found to be stable against sintering under both gaseous atmospheres up to 650 °C. Nevertheless, drastic sintering via diffusion-coalescence was observed upon annealing in H2 at 800 °C. In contrast, an identically prepared sample demonstrated lack of agglomeration at the same temperature under O2. The latter observation is ascribed to a strengthened chemical bond at the NP/support interface due to the formation of PtOx species at low temperature. Subsequently, oxidative NP redispersion - associated with some loss of Pt due to the formation of volatile PtOx species - is inferred from the behavior in O2 at/above 650 °C. In contrast, SiO2 reduction catalyzed by the presence of the Pt NPs and Pt silicide formation was found in H2 at 800 °C, which might play a role in the enhanced coarsening observed. Subsequent exposure of the PtSi NPs to oxygen led to the formation of Pt-SiO2 core-shell structures. Our findings highlight the dynamic structural transformations that nanoscale materials experience under different environments and the important role played by their initial size, size distribution and dispersion on their stability against sintering.

Relating adatom emission to improved durability of Pt–Pd diesel oxidation catalysts

“…S4) showed only fcc-Pt. Previous work shows that Pt silicides tend to form when the samples are heated in H 2 [9]. We conclude that the presence of Pt particles smaller than those in the initial catalyst is likely due to condensation of the vapor phase of Pt present in equilibrium with the metal particles.…”

“…But many of the in-situ TEM studies are performed at low pressures which can sometimes result in different sintering behaviors. In a recent study, Behafarid et al [9] report that Pt is quite stable in O 2 up to 800°C when studied via in-situ TEM at 1 Torr total pressure showing negligible growth in particle size over 30 min. In contrast, Chen and Schmidt [10] found rapid sintering and also documented a loss in Pt content when model catalysts were heated in air at atmospheric pressure.…”

Revealing particle growth mechanisms by combining high-surface-area catalysts made with monodisperse particles and electron microscopy conducted at atmospheric pressure

“…Further, the validity of using the shape of the PSD to discriminate between the two main growth processes, has been questioned because similar log-normal distributions can satisfactorily fit both mechanisms of growth. 13 Efforts to characterize these processes on model supports using the environmental TEM, which allows for real-time observation, have been reported and greatly advanced the fundamental understanding, [14][15][16][17][18] but the material and pressure gaps inherent in such work still raise questions about whether the results hold true for conditions closer to realistic applications.…”

In Situ Microscopy and Spectroscopy Applied to Surfaces at Work

“…The time resolution also allowed scientists to witness the growth of nanocrystal and nanomaterial at the atomic scale . Other effects such as material reconstruction or particles sintering and ripening where successfully evidenced by environmental TEM.…”