Spatial resolution catalyst requirements

Another approach is to use focused X-ray beams in the scanning mode. This technique will require specialized focusing optics and a fast monochromator. Such a nanoprobe beam line is currently being developed at the APS and is expected to have the spatial resolution of several tens of nanometers. At the proposed NSLS-II synchrotron, a beam line with a spatial resolution of ten nanometers is planned. With the development of these beam lines, it is expected that spatial imaging will be available for characterization of catalysts, and of course the hope is to do this with catalysts in reactive atmospheres. 

A prerequisite for the development indicated above to occur, is a parallel development in instrumentation to facilitate both physical and chemical characterization. TEM and SPM based methods will continue to play a central role in this development, since they possess the required nanometer (and subnanometer) spatial resolution. Optical spectroscopy using reflection adsorption infrared spectroscopy (RAIRS), polarization modulation infrared adsorption reflection spectroscopy (PM-IRRAS), second harmonic generation (SFIG), sum frequency generation (SFG), various in situ X-ray absorption (XAFS) and X-ray diffraction spectroscopies (XRD), and maybe also surface enhanced Raman scattering (SERS), etc., will play an important role when characterizing adsorbates on catalyst surfaces under reaction conditions. Few other methods fulfill the requirements of being able to operate over a wide pressure gap (to several atmospheres) and to be nondestructive. 

Going from planar to porous electrode introduces another length scale, the electrode thickness. In the case of a PEM fuel cell catalyst layer, the thickness lies in the range of IcL — 5-10 pm. The objective of porous electrode theory is to describe distributions of electrostatic potentials, concentrations of reactant and product species, and rates of electrochemical reactions at this scale. An accurate description of a potential distribution that accounts explicitly for the potential drop at the metal/electrolyte interface would require spatial resolution in the order of 1 A. This resolution is hardly feasible (and in most cases not necessary) in electrode modeling because of the huge disparity of length scales. The simplified description of a porous electrode as an effective medium with two continuous potential distributions for the metal and electrolyte phases appears to be a consistent and practicable option for modeling these stmctures. 

In contrast to the facile in-situ racemization of sec-alcohols via Ru-catalysts (Schemes 3.14 and 3.17), which allows dynamic resolution, the isomerization of ot-chiral amines requires more drastic conditions. Hydrogen transfer catalyzed by Pd [283, 284], Ru [285, 286] Ni, or Co [287] is slow and requires elevated temperatures close to 100°C, which still requires the spatial separation of (metal-catalyzed) racemization from the lipase aminolysis [288]. 

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