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Isolating Clusters of Light Elements in Molecular Sieves with Atom Probe Tomography
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posted on 2018-07-13, 16:54 authored by Joel E. Schmidt, Linqing Peng, Alessandra Lucini Paioni, Helena Leona Ehren, Wei Guo, Baishakhi Mazumder, D. A. Matthijs de Winter, Özgün Attila, Donglong Fu, Abhishek Dutta Chowdhury, Klaartje Houben, Marc Baldus, Jonathan D. Poplawsky, Bert M. WeckhuysenUnderstanding the
3-D distribution and nature of active sites in
heterogeneous catalysts is critical to developing structure–function
relationships. However, this is difficult to achieve in microporous
materials as there is little relative z-contrast between active and
inactive framework elements (e.g., Al, O, P, and Si), making them
difficult to differentiate with electron microscopies. We have applied
atom probe tomography (APT), currently the only nanometer-scale 3-D
microscopy to offer routine light element contrast, to the methanol-to-hydrocarbons
(MTH) catalyst SAPO-34, with Si as the active site, which may be present
in the framework as either isolated Si species or clusters (islands)
of Si atoms. 29Si solid-state NMR data on isotopically
enriched and natural abundance materials are consistent with the presence
of Si islands, and the APT results have been complemented with simulations
to show the smallest detectable cluster size as a function of instrument
spatial resolution and detector efficiency. We have identified significant
Si–Si affinity in the materials, as well as clustering of coke
deposited by the MTH reaction (13CH3OH used)
and an affinity between Brønsted acid sites and coke. A comparison
with simulations shows that the ultimate spatial resolution that can
be attained by APT applied to molecular sieves is 0.5–1 nm.
Finally, the observed 13C clusters are consistent with
hydrocarbon pool mechanism intermediates that are preferentially located
in regions of increased Brønsted acidity.
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13 C clustersSi specieslight element contrastelectron microscopiesMTH reactionatom probe tomographyBr ønsted aciditySi islandsAtom Probe TomographyNMR dataabundance materialsSAPOmicroporous materialsdetector efficiencycluster size29 SiAPT resultsframework elements13 CH 3 OHhydrocarbon pool mechanism intermediatesSi atomsnanometer-scale 3- D microscopy3- D distributionBr ønsted acid sitesMolecular SievesLight Elements
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