An inside look into a catalyst

AMOLF spectroscopists together with catalysis researchers from Utrecht University have managed to push the frontiers of catalyst characterization in both the space and time domains by a combined approach of coherent anti-Stokes Raman scattering (CARS) and synchrotron-based infrared spectro-microscopy.

These state-of-the-art experimental techniques were introduced to the field of heterogeneous catalysis to investigate the conversion of thiophene derivates on acidic sites in zeolite crystals, providing unprecedented insight into a catalytic reaction of significant technological and environmental relevance.

Zeolite catalysts are porous SiAl-oxide networks that contain catalytically active adsorption sites. Such molecular sieves have proven to be effective additives for the removal of sulfur-containing species during petroleum refinement, a process that is essential to comply with the increasing environmental restrictions on sulfur levels in gasoline.

Whereas the structural building blocks of individual zeolite crystals are well understood and detailed reaction pathways on the molecular level have been proposed theoretically, the link between the macroscopic and molecular worlds, namely, the correlation of the crystal's internal architecture and its catalytic activity on the micrometer scale, has remained elusive.

CARS and infrared spectra probe vibrational fingerprints that reveal the local chemical composition of the sample and inform upon interactions between analyte molecules and the catalyst environment without the need for labelling. Owing to the inherently small CARS focal volume, 3D imaging at submicrometer spatial resolution is feasible, including true depth profiling.

Detailed 3D chemical maps of zeolite catalysts loaded with thiophene derivates reveal a non-homogeneous, diffusion-limited analyte distribution throughout the crystal where molecules accumulate in the center of the crystal and along defect sites. The diffusion of the reactant molecules across crystal phase boundaries appears to be strongly hindered. At elevated temperatures, product oligomers form that are aligned inside the zeolite pores.

This novel combination of two spectro-microscopic approaches provides an advanced method for the characterization of catalysts during reaction cycles and promises to deliver fundamental insights into industrially important heterogeneous catalysis processes.

Reference
Label-Free Chemical Imaging of Catalytic Solids by Coherent Anti-Stokes Raman Scattering and Synchrotron-Based Infrared Microscopy
Marianne H. F. Kox, Katrin F. Domke, James P. R. Day, Gianluca Rago, Eli Stavitski, Mischa Bonn, and Bert M. Weckhuysen.
The results are published as a Very Important Paper (VIP) in Angew. Chem. Int. Ed. 48 (2009), DOI: 10.1002/anie.200904282.

Press contact:
Katrin Domke
(Tel. (00 31) (0)20 7547100)

Left: Probing zeolite crystals with CARS and infrared spectro-microscopy allows label-free imaging through the vibrational fingerprints of reactants and products adsorbed at catalytically active sites. Right: Label-free 3D chemical maps with submicrometer resolution reveal a non-homogeneous, diffusion-limited distribution of reactants in the crystal.