Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting

Abstract

Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity—ideally obtained under realistic reaction conditions1,2,3. Transmission electron microscopy4,5,6 and scanning tunnelling microscopy7,8 have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases9, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy10,11,12 to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis13,14,15 and thus help with the rational design of new or more productive heterogeneous catalysts.

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Figure 1: Experimental set-up.
Figure 2: In situ wide field fluorescence micrographs of C-FDA transesterification on propyl amine-functionalized cover glasses.
Figure 3: Wide field images of catalytic reactions on individual LDH particles.
Figure 4: Analysis of single product molecules randomly diffusing on (0001) LDH surfaces after transesterification.

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Acknowledgements

M.B.J.R. thanks the Institute for the Promotion of Innovation through Science and Technology in Flanders (IWT-Vlaanderen) for a fellowship, B.F.S. thanks the FWO-Flanders for a post-doctoral fellowship and the KUL for a guest professor position. This work was performed within the framework of the IAP-V-03 programme ‘Supramolecular Chemistry and Catalysis’ of the Belgian Federal government and of GOA-2/01.

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Correspondence to Dirk E. De Vos or Johan Hofkens.

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Supplementary information

Supplementary Video 1

Wide field fluorescence images of C-FDA transesterification on a coverglass functionalized with a 1:10,000 DMAPTS:PTS mixture. (AVI 3484 kb)

Supplementary Video 2

Wide field fluorescence image of the catalytic transesterification of C-FDA on an individual LDH particle. (AVI 9700 kb)

Supplementary Video 3

Wide field fluorescence image of the catalytic transesterification of FDA on an individual LDH particle. (AVI 3420 kb)

Supplementary Video 4

Detail of the diffusion of single 5-carboxyfluorescein product molecules on the basal plane of an LDH. (AVI 2007 kb)

Supplementary Notes

This file contains Supplementary Data on the use of fluorescence microscopy for in situ study of organic transformations on mordenite zeolites. This file also contains a Supplementary Discussion and Supplementary Figures. (DOC 490 kb)

Supplementary Legends

Text to accompany the above Supplementary Videos. (DOC 30 kb)

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Roeffaers, M., Sels, B., Uji-i, H. et al. Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting. Nature 439, 572–575 (2006). https://doi.org/10.1038/nature04502

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