Surface-enhanced Raman spectroscopy (SERS) and tip-enhanced Raman spectroscopy (TERS) were until recently limited in their applicability to the majority of heterogeneous catalytic reactions. Recent developments begin to resolve the conflicting experimental requirements for SERS and TERS on the one hand, and heterogeneous catalysis on the other hand. This article discusses the development and use of SERS and TERS to study heterogeneous catalytic reactions, and the exciting possibilities that may now be within reach thanks to the latest technical developments. This will be illustrated with showcase examples from photo- and electrocatalysis. Graphical Abstract
Impact of titanium addition on film characteristics of Hf O 2 gate dielectrics deposited by atomic layer deposition J. Appl. Phys. 98, 054104 (2005); 10.1063/1.2030407Microstructure characterization of sol-gel prepared MoO 3 -TiO 2 thin films for oxygen gas sensors Thin films of MoO 3 deposited on Si(111) and Al 2 O 3 (001) substrates by atomic layer deposition have been investigated by x-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Raman spectroscopy for detailed characterization of composition and morphology. Comparison of angle resolved x-ray photoelectron spectroscopy (ARXPS) and XPS depth profiles based on Ar þ sputtering is reported. Sputtering induces a reduction of molybdenum in MoO 3 from þIV to metallic Mo as the interface toward Si is approached, whereas ARXPS on a 10 nm thin film shows that Mo(VI) remains outside the interface toward Si where lower valent molybdenum compounds are formed. Upon annealing, the as-deposited amorphous thin films of MoO 3 crystallize into bor a-MoO 3 as identified by x-ray diffraction. The current study provides a convenient route toward formation of metastable b-MoO 3 and a full crystallization pathway from amorphous to crystalline a-MoO 3 . Combined AFM and Raman analysis have been performed on thin films of a-MoO 3 deposited on Al 2 O 3 (001) and prove that the crystallization proceeds via island growth at 600 C. The Raman intensity ratios between different bands depend strongly on morphology and size of crystalites.
The integration of Atomic Force Microscopy and Raman spectroscopy is tested for use in heterogeneous catalysis research by a preliminary investigation, the photo-oxidation of rhodamine-6G. Temperature and atmosphere were varied in an in situ cell to show compatibility with realistic reaction conditions. Supported metal nanoparticles (NPs) are important heterogeneous catalysts in many industrial processes.1-3 A thorough understanding of which specific surfaces have the highest catalytic activity is required in order to tune the shape and size of NPs to achieve maximum catalytic activity. [4][5][6][7][8] A variety of spectroscopic techniques are employed for the study of heterogeneous catalysts, both ex situ and in situ. 9,10Vibrational spectroscopy is one of the most valuable methods for obtaining chemical information about a catalytic system. However, IR spectroscopy is severely limited spatially, while Raman spectroscopy has a better spatial resolution, but lacks sensitivity under normal measurement circumstances. The use of surface enhancement makes Raman spectroscopy a more sensitive and versatile tool for studying chemical reactions. 11-14Surface Enhanced Raman Scattering (SERS) occurs principally on roughened noble metal surfaces or noble metal NPs, and allows chemical imaging of adsorbate-surface interactions with high sensitivity. [15][16][17] This makes it uniquely suited for investigations of reactions at a catalytic surface. 18,19 The integration of SERS and Atomic Force Microscopy (AFM) forms an even more powerful tool for in situ chemical imaging of catalytic solids, allowing nanoscale morphological features to be correlated directly with chemical information. 20-22Here we demonstrate the potential and limitations of this integrated approach for identifying exactly which NP morphologies and sizes are the most active through a study of the photo-oxidation of rhodamine-6G (Rh6G) over Al 2 O 3 -supported Ag NPs. We show that it is possible to follow the reactants or products of a reaction under in situ conditions (i.e. controlled temperature and atmosphere), and pinpoint the active section of the substrate on the nanoscale. Using the integrated setup, Raman maps can be measured in combination with AFM. The resolution of Raman measurements is, however, still dependent on the excitation wavelength that is used, and typically not better than 300 nm. With AFM it is possible to obtain topography images of the surface that is being measured with Raman spectroscopy with a resolution of less than a nanometre. A top-view set-up is required as in general the support oxide material for dispersing the catalytic particles is not transparent. In the integrated setup an AFM cantilever is used with a tip that protrudes at the end of the cantilever, so that it is possible to focus the laser beam onto the tip itself. The optical focus on the AFM tip, as shown in Fig. 1, means that the focus stays with the surface during sample scanning. At every measurement point, an AFM height point is recorded along with a Raman spectr...
Dedicated to Professor Rüdiger Lange on the occasion of his 65th birthday Small-scale parallel trickle-bed reactors were used to evaluate the performance of a commercial hydrodesulfurization catalyst under industrially relevant conditions. Catalyst extrudates were loaded as a single string in reactor tubes. It is demonstrated that product sulfur levels and densities obtained with the single-pelletstring reactor are close to the results obtained in a bench-scale fixed-bed reactor operated under the same conditions. Moreover, parallel single-pellet-string reactors show high reproducibility. To study the hydrodynamic effects of the catalystbed packing, the catalyst-bed length was varied by loading different amounts of catalysts, and crushed catalyst was also loaded.
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