Isolating Clusters of Light Elements in Molecular Sieves with Atom Probe Tomography

Area of Science:

• Materials Science
• Catalysis
• Nanotechnology

Background:

• Understanding active site distribution in heterogeneous catalysts like SAPO-34 is key for structure-function relationships.
• Microporous materials pose challenges for traditional electron microscopy due to low z-contrast between framework elements (Al, O, P, Si).
• Silicon (Si) acts as the active site in SAPO-34, potentially existing as isolated species or clusters.

Purpose of the Study:

• To investigate the 3-D distribution and nature of silicon active sites in SAPO-34 catalysts.
• To characterize coke deposition and its interaction with active sites during the methanol-to-hydrocarbons (MTH) reaction.
• To determine the spatial resolution limits of atom probe tomography (APT) for molecular sieves.

Main Methods:

• Application of atom probe tomography (APT) for 3-D nanometer-scale imaging with light element contrast.
• Utilizing 29Si solid-state nuclear magnetic resonance (NMR) spectroscopy on isotopically enriched and natural abundance materials.
• Complementing experimental APT data with simulations to determine the smallest detectable cluster size.

Main Results:

• APT identified significant silicon-silicon (Si-Si) affinity, indicating clustering of active sites.
• Coke deposits from the MTH reaction were observed to cluster and show an affinity for Brønsted acid sites.
• Simulations confirmed APT's capability to resolve features down to 0.5-1 nm in molecular sieves.
• Observed 13C clusters align with hydrocarbon pool mechanism intermediates located near Brønsted acid sites.

Conclusions:

• APT is a powerful tool for visualizing 3-D active site distribution and coke formation in microporous catalysts.
• Silicon clustering and coke affinity with acid sites provide insights into the methanol-to-hydrocarbons reaction mechanism.
• The findings facilitate the rational design of improved heterogeneous catalysts by understanding nanoscale structural features.