In Situ Scanning Transmission Electron Microscopy of Ni Nanoparticle Redispersion via the Reduction of Hollow NiO

Abstract

Oxidation and reduction cycles are used in the regeneration of nanoparticle catalysts that have deactivated due to sintering and poisoning. We examine the case of nickel oxidation and reduction on the regeneration of the smallest end of the particle size distribution via in-situ high angle annular dark field environmental scanning transmission electron microscopy (HAADF-ESTEM). Cycling the Ni/NiO system through successive redox cycles shows that the particles retain the same general size distributions even though Ostwald ripening and particle migration and coalescence is occurring. The regeneration of the smallest nanoparticle sizes which disappear due to sintering processes occurs by the ejection of small (2-3 nm) nickel particles during the reduction of the hollow nickel oxide nanostructures. The nickel nanoparticles above ~3.5 nm in size, form hollow polycrystalline nickel oxide upon oxidation. Upon reduction, the grains making up the shell of the hollow nickel oxide reduce separately at the grains surface and at the grain boundaries between the polycrystalline grains. The contraction in particle size upon reduction destabilizes the hollow nanostructure and causes the particle to rearrange and collapse. As this process occurs, some parts of the particle are ejected from the reducing particle and form small particles of nickel, which regenerate the smallest parts of the size distribution. Once the particle collapses, the nickel rearranges reforming solid nickel nanoparticles enclosed by their low index facets.

Atom-by-atom analysis of sintering dynamics and stability of Pt nanoparticle catalysts in chemical reactions

“…As the nature of the active reaction site of the metal nanoparticle, along with the processes by which the particle sinters and deactivates, have been shown to change both in magnitude and mechanism with the reaction environment [4,22,[27][28][29][30], it is of fundamental importance to activate, react, observe and analyse nanoparticle systems in-situ, in real time, under controlled continuous gas flow reaction conditions at operating temperatures. Reacting supported single platinum atoms can now be resolved and analysed in controlled flowing reducing gas environments at operating temperatures in the ESTEM using HAADF imaging (ESTEM-HAADF) [3,31].…”

Relationship between reaction and diffusion during ultrafine NiO reduction toward agglomeration fluidization

“…It has been reported that the NiO is reduced from the grain boundaries between the polycrystalline grains to form new Ni nuclei at the grain boundary 40 . As the reduction proceeds, the formed Ni nuclei at the grain boundary derived by diffusion/migration tend to coalesce (sintering) rapidly to form larger Ni grains, in addition to form Ni layer around each grain 5,11,25 .…”

Visualizing single atom dynamics in heterogeneous catalysis using analytical in situ environmental scanning transmission electron microscopy

“…Despite the in situ modifications, single-atom imaging has been possible in practical catalysts under reaction conditions [4,[23][24][25][26][27][28][29][30][31][32]. The permanently mounted ECELL developments retain or enhance the full performance of the original core instrument with proven less than 0.1 nm resolution and single-atom sensitivity in imaging.…”

“…Minimally invasive low-dose electron beam techniques are used throughout [4,9,[21][22][23][24][25][26][27][28][29][30][31][32]. Data on in situ experiments under low electron dose conditions are checked in parallel with in situ data from 'blank' calibration experiments in which the dynamic experiments are conducted without the electron beam (by blanking the beam) and the beam is switched on only to record the final reaction endpoint [4,9,[21][22][23][24][25][26][27][28][29][30][31][32]. In addition to providing direct and unparalleled insights into the dynamic evolution of structural changes and reaction mechanisms at the atomic level, the ESTEM and ETEM allow the detection, in real time, of surface as well as subsurface structural phenomena.…”