Heterogeneous catalysis has a key role in many clean energy technology applications including soot emissions reduction. During the process, reactants are transformed into products on the surface of a solid catalyst material. Nanoparticles (NP) are the focus of much development of catalyst materials as they have a large surface-to-volume ratio. Silver (Ag) has been the subject of important research for oxidation of soot and other carbonaceous materials.
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This work explores the atomic structure of Ag nanoparticle active sites for carbon oxidation using a model carbonaceous material: multi-walled carbon nanotubes (MW-CNT).
Materials and Methods
The model 628 heating holder from Gatan is a furnace-type holder intended to enable the direct observation of microstructural phase changes, catalysis, nucleation, growth, and dissolution processes in 3 mm TEM samples at raised temperatures. It was utilized within an aberration-corrected environmental TEM (AC-TEM) to attain an elevated temperature in an O2 environment.
Video recording was avoided, to ensure the electron beam exposure as low as practically possible. Single frame images were attained using an UltraScan camera with the sample at 250 °C, but the oxygen removed to avoid ionization damage. A series of these images is shown in Figure 1.
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Figure 1. Series of AC-ETEM images during nanotube oxidation. The oxidation reaction was conducted at 250 °C in 2 mbar O2 images were acquired after: (a) 0 s, (b) 300 s, (c) 600 s, (d) 900 s, and (e) 1,800 s. The two dashed black lines in a) mark the angle between the Ag NP/MW-CNT system, which was used to monitor the rotation of the Ag NP. The curved red arrow denotes the rotation direction, and the yellow dashed line marks the contact interface between the Ag NP and the MW-CNT. Scale bars are 2 nm. Small red arrows indicate the depth to which the oxidation has reached. The turnover frequency of the Ag NP sites on this single particle could be directly determined from these images.
Yue, Y., Yuchi, D., Guan, P., Xu, J., Guo, L., Liu, J., 2016. Atomic scale observation of oxygen delivery during silver-oxygen nanoparticle catalyzed oxidation of carbon nanotubes. Nature Communications 7, 12251. doi:10.1038/ncomms12251
Summary
The use of a model 628 heating holder in an aberration-corrected ETEM enabled the careful in-situ observation of Ag nanoparticles catalyzing the oxidation of carbon nanotubes. The high-resolution images of the catalyst particle at various points in the tube oxidation enabled the quantification of the turnover frequency for this single nanoparticle. First-principles simulations directed by these observations suggested a Mars-van Krevelen type mechanism, in which oxygen is dissociated on the Ag surface and diffuses through the Ag NP to reach the MW-CNT surface.
This information has been sourced, reviewed and adapted from materials provided by Gatan, Inc.
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