Fluorescence microscopy: Bridging the phase gap in catalysis Maarten B.J. Roeffaers a , Johan Hofkens b , Gert De Cremer a , Frans C. De Schryver b , Pierre A. Jacobs a , Dirk E. De Vos a , Bert F. Sels a, * a Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, 3001 Heverlee, Belgium b Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium Available online 30 April 2007 Abstract While many operando techniques focus on gas phase reactions, there is a growing need to look at working catalysts in the liquid phase. Fluorescence microscopy is a promising technique for bridging this phase gap. Like cellular biology, catalytic science may take advantage from the high spatiotemporal resolution and sensitivity of fluorescence microscopy. Earlier applications of fluorescence techniques in the study of diffusion or chemical transformation on inorganic solids are reviewed. The potential of fluorescence microscopy in catalysis is illustrated by data on acid- catalyzed transformation of fluorogenic organics on a mordenite zeolite. # 2007 Elsevier B.V. All rights reserved. Keywords: Fluorescence microscopy; Phase gap; Zeolite; Single molecule; Layered double hydroxides 1. Introduction In the early days of catalytic science, catalysts were treated as black boxes. Improvements of catalytic performance were largely based on trial-and-error. In a later, more scientifically founded approach, detailed kinetic studies were combined with physicochemical catalyst characterization. However, as these physical studies were typically performed ex situ, or using deactivated, ‘post mortem’ catalysts, it remained difficult in many cases to unambiguously relate molecular surface structure to catalytic performance. A more rational and efficient catalyst design is strongly dependent on insights in the molecular processes at the catalyst particle. This has been the major incentive for introducing in situ spectroscopic techniques in catalytic research. Instrumental and technological advances in the previous decade made it possible to study the molecular events that take place at a heterogeneous catalyst under reaction conditions. Initially use was made of techniques and instruments with a low sensitivity, and with limited spatial or temporal resolution, e.g. based on IR, Raman or UV–vis spectroscopy. Despite their merits in surface characterization, such studies leave several questions unanswered. First, in the study of complex catalytic materials, these techniques yield ensemble results, averaged over Avogadro numbers. It is not straightforward to extract from these results the precise contribution of the different surface groups or metal species one observes. Secondly, in many studies, attention is devoted to unravelling the spectro- scopic fingerprints of the inorganic species on the catalyst surface. While the observed surface species may be the actual active sites, one may also observe species or functional groups that are formed during the reaction but do not actively participate in the actual reaction, i.e. spectator species. Thirdly, the fate of adsorbed organics, such as reagents or reaction products, should be monitored simultaneously, but many techniques lack the specificity to follow transformations of single bonds in complex organic molecules. More specialized approaches, like sum frequency generation spectroscopy, are required to address reactivity of organics at catalytic surfaces. A final problem encountered with ensemble spectroscopic techniques is quantification, since it is hard to determine appropriate extinction coefficients. A major step forward was the introduction of high resolution microscopes, such as scanning probe microscopy and electron microscopy. These instruments can directly observe and quantify metal atoms and adatoms on surfaces. Scanning probe techniques are however typically limited to single, idealized crystals in ultrahigh-vacuum conditions (UHV), and www.elsevier.com/locate/cattod Catalysis Today 126 (2007) 44–53 * Corresponding author. Tel.: +32 16 321593; fax: +32 16 321998. E-mail address: bert.sels@biw.kuleuven.be (B.F. Sels). 0920-5861/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2007.03.007