ID-12-P-3158 Real-time Visualization of Phagocyte-like Migration and Coalescence of Si3N4 Supported Pd Nanoparticles under O2 Atmosphere
The stability of metal nanoparticles is pivotal for their extensive use as heterogeneous catalysts for many industrial applications. The high temperature and reactive gas environment encountered during catalysis, however, often exacerbate the tendency of nanoscale particles to coalesce into larger aggregates, which results in a reduction of active surface area and thus an undesired catalyst deactivation [1-4]. Here we use the differential pumping-based Hitachi H-9500 environmental TEM (Fig.1a) and the MEMS-based Protochips heating holder (Fig.1b-c) to reveal a real-time visualization of the oxygen-induced migration and aggregation behavior of Pd nanoparticles, which plays a crucial role in CO oxidation as well as a variety of catalytic reactions under oxidative atmosphere.
In this study, Pd nanocubes with the size of ~10 nm were dispersed in ethanol under sonication and dropped onto the Si3N4 membrane support (Fig.1c) for the in situ TEM experiments. The samples were then heated up to 300 °C followed by immersed in the oxygen environment. A typical example of the dynamic behavior of Pd nanoparticles is illustrated in Fig. 2. After oxygen injection, some of the particles were firstly activated and moved around, with dramatic shape variations during the migration. These pioneers tried to ingest other particles nearby (Particle IV in Fig.2a-c), like the phagocytes prefer to surround bacteria and swallow them. This led to a domino effect that an increasing number of particles served as active “phagocytes” after they contacted or coalesced with the original pioneers (Particle I-IV in Fig.2d-e, Particle I-II-IV in Fig.2e-f, Particle III-IV in Fig.2d-f). In response to this chain reaction, most of the small particles finally aggregated into larger ones with an average diameter of ~30 nm.
Although the sintering behavior of metal particles on carbon or hydrocarbon has been found as a catalytic gasification process [1,5,6], such phagocyte-like migration and coalescence dynamics in metal/Si3N4 model systems have not been reported before, especially considering that Si3N4 cannot be as easy to be oxidized as carbon at this temperature range. Detailed analysis of the trajectories of particle motion coupled with the inspection of the composition and microstructure of the samples would be presented further to clarify the mechanism behind this intriguing phenomenon.
References
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This work was supported by the Grants from NSFC (50925104 and 51231005). The authors would like to thank Dr. Mingshang Jin from XJTU for providing palladium nanoparticles samples, and also appreciate the fruitful discussion with Prof. Ju Li from MIT.
Fig. 1: Schematic of experimental setup. (a) Differential pumping-based Hitachi H-9500 ETEM. (b) Front end of Protochips heating holder with four electrical leads connecting a clamped MEMS heater chip. (c) Cross-section view of the chip shown in (b), illustrating Pd NPs were supported on the Si3N4 membrane in this study. |
Fig. 2: Time-lapsed TEM images of Pd NPs during exposure to oxygen after heating up to 300 °C, showing the phagocyte-like migration and coalescence dynamic behavior. The times relative to the start of the oxygen injection are indicated. |