Catalysis forms the cornerstone of the chemical industry, with over 90% of the industrial-scale processes involving at least one catalytic step, mainly carried out using heterogeneous catalysts. These solid materials lower the activation energy of chemical reactions occurring on their surface, enhancing the reaction rate and thus enabling processes at milder operational conditions and with enhanced product selectivity. Furthermore, the material structure's geometrical constraints can strongly influence the nature of the chemicals formed, a feature that can be further exploited to improve the reaction selectivity.
The relevant length scales for the catalytic process taking place at the surface of solid materials span from the angstrom-scale with the molecular bond breaking and formation at the active sites to the size of catalyst particles (nm-μm) and catalyst bodies (mm-cm) used in industrial reactors (m). Most of the actual catalyst bodies used in chemical processes are complex agglomerates consisting of the catalytically active solid held together and shaped with binder materials. The catalyst particles are rarely uniform and often feature local structural and compositional variations. While typical catalyst characterization aims to generate averaged pictures generated through bulk techniques looking at the gram or milligram scale, microscopy methodologies allow identifying material heterogeneities. Shedding light on local structural and compositional variability and understanding their origin enables rationalizing the catalytic performance.
Vibrational spectroscopy offers the possibility to target specific molecules of interest without the need of specific probe molecules such as required for fluorescence-based techniques. Spatially resolving the vibrational modes of interest even enables to spatially resolved molecular processes inside a sample using infrared absorption or Raman scattering processes. Spontaneous Raman scattering is an attractive contrast mechanism compared to infrared absorption as it offers a sub-micrometer resolution and 3D sectioning. However, the intrinsically weak signals generated by this phenomenon limit its use in imaging studies. This limitation is overcome by stimulated Raman scattering (SRS) microscopy, an advanced vibrational technique developed in 2008 as imaging contrast. However, although it is a powerful tool, the application of SRS microscopy has been mostly limited to the study of biological samples up to now. In this work, SRS microscopy assays were developed to spatially resolve the fine details of catalytic materials in individual zeolite crystals. In particular, the orientation of templating agents occluded in the pores of a hierarchically intergrown silicalite-1, and the acid properties of mordenite and ZSM-5 were investigated at the sub-micrometer scale using the Raman signature of organic molecules located in their porous structure.
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