M. Bückins

Super‐Resolution Reactivity Mapping of Nanostructured Catalyst Particles

For almost a century, heterogeneous catalysts have been at the heart of countless industrial chemical processes, but their operation at the molecular level is generally much less understood than that of homogeneous catalysts or enzymes. The principal reason is that despite the macroscopic dimensions of solid catalyst particles, their activity seems to be governed by compositional heterogeneities and structural features at the nanoscale. Progress in understanding heterogeneous catalysis thus requires that the nanoscale compositional and structural data be linked with local catalytic activity data, recorded in the same small spatial domains and under in situ reaction conditions. Light microscopy is a recent addition to the toolbox for in situ study of solid catalytic materials. It combines high temporal resolution and sensitivity with considerable specificity in distinguishing reaction products from reagents. However, lens-based microscopes are subjected to light diffraction which limits the optical resolution to 250 nm in the image plane. This resolution is far too limited to resolve the nanosized domains on solid catalysts. Nanometer-accurate localization of single emitters can be achieved by fitting a Gaussian distribution function to the intensity of the observed fluorescence spot (point-spread function, PSF). This method has been used to map out diffusion pathways in mesoporous or clay materials under highly dilute conditions. However, for more concentrated systems, several molecules simultaneously located within a diffraction-limited area cannot be distinguished. Separating the emission of the different fluorescent labels in time, for example by selective photoactivation, solves the problem for imaging of static systems, 13–18] but not when looking at the dynamics of a working catalyst. Herein, we used single catalytic conversions of small fluorogenic reactants, which occurred stochastically on the densely packed active sites of the catalyst, to reconstruct diffraction-unlimited reactivity maps of catalyst particles. As successive catalytic reactions do not overlap in time, one can precisely determine the location of reaction sites that show turnovers at different moments in time, even if the distance between them is only 10 nm (or less, depending on the signal-to-noise ratio), and reconstruct images of catalytically active zones with super-resolution. Although fluorogenic substrates are widely used in singlemolecule enzymology, so far only a few studies have reported single-turnover counting using fluorescence microscopy on solid chemocatalysts. 24, 25] Such studies typically use large polycyclic substrates, which cannot enter the micropores of many heterogeneous catalysts. Hence, similar experiments on microporous materials critically depend on identifying a small reagent that is converted to a product detectable at the single-molecule level. Surprisingly, furfuryl alcohol is such a reagent, and it appears that after acid-catalyzed reaction (see the Supporting Information), the pore-entrapped products are sufficiently fluorescent to be individually observed using a standard microscope equipped with a single excitation source (532 nm diode laser) and sensitive CCD camera (for experimental details, see the Supporting Information). We refer to this novel high-resolution reconstruction method based on catalytic conversion of fluorogenic substrates as NASCA microscopy, or nanometer accuracy by stochastic catalytic reactions microscopy. Figure 1a and b show the concept of NASCA microscopy and a 2D fluorescence intensity image of individual product molecules formed by an acid zeolite crystal, respectively. The fluorescence intensity plot of Figure 1c proves how well the intensity of the individual product molecules allows them to be distinguished from background signals, caused by scatter[*] Dr. M. B. J. Roeffaers, Dr. P. Dedecker, Prof. Dr. J. Hofkens Department of Chemistry, Katholieke Universiteit Leuven Celestijnenlaan 200F, 3001 Heverlee (Belgium) Fax: (+ 32)163-2799 E-mail: johan.hofkens@chem.kuleuven.be

Nanomechanical and analytical investigations of tribological layers for wear protection in slow-running roller bearings

In a tribological system consisting of roller thrust bearings and various lubricants, we investigated reaction layers formed in the presence of lubricants with low wear protection, high wear and fatigue protection as well as high wear but low fatigue protection. The bearings were tested under heavy-duty conditions in an FE-8 test rig in order to rapidly asses the efficiency of different reaction layer systems. Chemical composition and microstructure analysis of the layers was subsequently carried out by transmission electron microscopy on thin cross-sections prepared by the focused ion beam technique. The nanomechanical properties of the different tribological layers were analyzed by nanoindentation. The formation and structure of the layered system, and thus the ability to protect against wear and fatigue, depends on the lubricants and additives, respectively. Our results indicate that wear protection not only relies on the reaction layer itself but also on the properties of the combined system of the reaction layer and an underlying tribomutation layer.

Novel applications of a focused ion beam workstation for specimen preparation and nanostructuring

For the quality of materials science investigations with SEM or TEM, it is mandatory to have a very well prepared sample, with flat surfaces. The preparation can be achieved with the FIB by using the Gallium beam almost parallel to the surface, thus limiting the amount of mechanically disturbed areas or obstacles on the surface. As a first example, the analysis of surfaces and thin layer systems with an electron microprobe (EPMA) will be discussed. The depth resolution can be advanced by milling a shallow bevel into the sample in a way that the inner layers of the specimen appear layer by layer on the surface. In order to spread a sequence of thin films of a layer system for achieving better results during analysis with the EPMA, we developed a technique in which we cut a shallow bevel with the Gallium beam with a small angle Novel Applications of a Focused Ion Beam Workstation for Specimen Preparation and Nanostructuring