For the production of novel or more efficient catalysts, the link between nanoparticle (NP) structure and catalytic performances need to be understood. Such an understanding requires tools that allow the “observation” with single-atom sensitivity of the surface of real catalysts in response to reaction conditions. Indeed, when nanocatalysts operate in gas environment and high temperature, they do not remain static but undergo dynamic atomic-scale processes (surface restructuration, oxidation…) which directly influence catalytic properties. In this work, we have studied the thermal stability of Au-Cu nanocatalysts under oxidative and reductive environments and/or at high temperature using environmental aberration corrected TEM (ETEM).
Au NPs with controlled sizes, compositions and morphology were synthesized by pulsed laser deposition. Environmental gas TEM of nanocatalysts at the atomic scale was undertaken in an aberration-corrected JEM-ARM200F TEM with in situ gas/temperature conditions achieved by using a MEMS-based nanoreactor (Protochips Inc.) and equipped with a high performance Gatan OneView camera enabling the capture of both high quality images and high-speed videos of in situ events (512 x 512 pixels at 300 fps).
Figure 1 compares TEM images of Au NPs (a) at room temperature in vacuum, (b) at 300°C under O2 atmosphere and (c) at 300°C under H2 atmosphere. At room temperature, in vacuum, NPs are stable but once a gas, O2 and/or H2 is injected, coalescence phenomena appear. The coalescence continues at 100°C and 200°C. An additional phenomenon appears at 200°C with O2 that does not with H2: particle faceting. At 300°C, under O2, all particles are well facetted (truncated octahedron). When we cool down the sample with O2, the faceting disappears. Thus, particles size and morphology are clearly dependent on temperature and gas nature with an increase in temperature favoring particle coarsening and exposure to an O2 atmosphere leading to more facetted Au NPs.
Figure2 shows TEM images of Au NPs at 300°C (a) which are all facetted at this temperature (truncated octahedron) and at constant temperature of 850 °C at different times (b,c,d). The NPs become more spherical at higher temperature and began to evaporate at 850°C. One can determine the surface energy of the evaporating particles by means of the Kelvin equation2. There is a size dependence of the surface energy of the NPs, indeed, the surface energy increases with the size. For a particle radius greater than 4nm, one cannot apply the Kelvin model. Thereby, in order to understand the underlying mechanism of the behavior of a nanoparticle in this size range, simulations using the tight binding model are in progress. Also, by alloying gold with copper, we are studying the phase reactivity of AuCu nanoparticles as well.
1) Prunier H. et al., Phys. Chem. Chem. Phys., 2015,17:28339-28346.
2) Sambles J.R et al., Proc. R. Soc. Long. 1970, A 318, 507.
To cite this abstract:Adrian Chmielewski, Hélène Prunier, Jaysen Nelayah, Hakim Amara, Jérôme Creuze, Damien Alloyeau, Guillaume Wang, Christian Ricolleau; In-situ thermal stability and reactivity monitoring of Au nanoparticles using Cs-Corrected environmental TEM. The 16th European Microscopy Congress, Lyon, France. https://emc-proceedings.com/abstract/in-situ-thermal-stability-and-reactivity-monitoring-of-au-nanoparticles-using-cs-corrected-environmental-tem/. Accessed: December 12, 2018
EMC Abstracts - https://emc-proceedings.com/abstract/in-situ-thermal-stability-and-reactivity-monitoring-of-au-nanoparticles-using-cs-corrected-environmental-tem/