Aberration-Corrected STEM
Figure 4 : Atomic-resolution HAADF STEM images sampling WS 2 nanostructure development during the ex situ sulfi dation process at various temperatures. Arrows within images indicate locations of WS 2 sheet-like structures.
electron energy loss spectrometry (EELS) work done by others [ 11 ] and can serve as indirect confi rmation that the promoter species also modifi es the sheet edge-structure, which in turn gives rise to enhanced catalytic behavior.
To quantitatively understand the change in dispersion, the perimeter (P) and area (A) were measured for a series of individual WS 2 sheets. Presumably, if edge-sites are the active component of these nanomaterials [ 8 , 9 ], then it follows that an increased dispersion (a higher P/A ratio) would be benefi cial to performance. Figure 5 shows that the Ni-promoted sample has a higher P/A ratio indicative of more edge-sites. Literature reports observing the same structural phenomena [ 8 , 9 ], coupled with internal testing results for these two catalysts, show clear improvements in yield and activity with promoter addition. T e next question is whether the performance benefi ts are chemical (associated more with the presence of a diff erent elemental species) or physical (related to the structural modifi - cation) in nature. Toward this end, an attempt was made to understand whether a W-only sample with comparable P/A ratios to the W-Ni system could achieve similar performance. T e idea would be that oxidation of the existing nanostructures would commence at the edge-sites, where bonding is most fragile, and gradually work inward. Oxidation using a O 2 plasma treatment (PT) was employed. Figure 6 shows a side-by-side comparison of the sample (a) sulfi ded at 413°C and the same sample then
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(b) oxidized with a plasma treatment. Examination of the two images suggests that in Figure 6a the WS 2 edge-structures formed facets with well-defi ned edges; whereas, in Figure 6b the subsequent plasma treatment resulted in several discon- tinuities in the edge-structure. Figure 6c shows a quantitative account of these fi ndings, where P is plotted versus A for the two samples. Based on the diff erent trajectories of the scatter, it can be surmised that the PT was eff ective at creating more under- coordinated edge-sites. T is result supports the assumption that subjecting metal sulfi des to PT, resulting in edge modifi cations similar to the Ni-promoted samples, could lead to performance gains similar to that of the promoted catalyst.
Conclusions T e fi ndings in this article demonstrate important advances in nanomaterial processing and characterization. Multiple methods were shown to produce quantifi able modifi cations of nanomaterials at atomic-length scales. Documentation of these subtleties mandates that analyses be carried out with a probe- corrected fi eld-emission STEM at a minimum. Collectively the data show how previously unperceivable, atomic-scale changes can lead to dramatic large-scale changes in catalyst performance. Furthermore we show several examples of nanostructure modifi cations that previously would have been unobserved and undocumented if not for advances in electron microscopy.
www.microscopy-today.com • 2018 May
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