Materials gap

Assmann J, Narkhede V, Bteuer A, Muhler M, Seitsonen AP, Knapp M, Crihan D, Farkas A, Mellau G, Over H. 2003. Heterogeneous oxidation catalysis on mthenium Bridging the pressure and materials gaps and beyond. J Phys Cond Matt 20 184017. 

In addition to performing experiments under pressures similar to those encountered in real processes to bridge the pressure gap , surface scientists have also been increasing the level of complexity of the model surfaces they use to better mimic real supported catalysts, thus bridging the materials gap . A few groups, including those of Professors Freund and Henry, have extended this approach to address the catalytic reduction of NO. The former has published a fairly comprehensive review on the subject [23], Here we will just highlight the information obtained on the reactivity of NO + CO mixtures on these model supported catalysts. 

Like most surface science techniques, conventional in situ STM studies have been carried out in UHV on model catalysts consisting of extended planar surfaces. When extrapolating the information obtained in UHV surface science studies to real-world catalysis, two issues have generally concerned the catalysis community, namely, the pressure and material gaps. 

As mentioned above, most modem surface-sensitive techniques operate under vacuum, and are often used for studies in model systems. Nevertheless, there have been recent attempts to extend that work to more relevant catalytic problems. Great advances have already been made to bridge the so-called pressure and materials gaps, that is, to address the issues related to the differences in catalytic behavior between small simple samples (often single crystals) in vacuum, and supported catalysts under higher (atmospheric) pressures [155-157], Nevertheless, more work is still needed. 

While the development of planar model catalysts has largely led to the closing of the material gap, the pressure gap still remains. Rupprechter et al.109 nicely cover some of the concerns of the pressure... 

The nano-architecture is thus an important aspect to consider for the design of novel catalysts and a critical element to consider also in analyzing how to bridge the gap between model and real catalysts. In fact, in addition to the issues of pressure and material gap , the complexity gap exists." Goodman " over ten years ago pointed out that despite the successes in modelling catalysts with single crystals, there is a clear need to develop models with higher levels of complexity and which take into account the 3D nanoarchitecture. 

In recent years, physical-chemical measurements on supported ILs have been performed by using many techniques that have appeared to be a performing tool to derive physical-chemical properties and the reaction mechanism of catalytic reaction on ILs. These studies have been motivated by the necessity to bridge the material gap in many fields of sciences with novel compounds. 

Freund, H.J., Kuhlenbeck, H., Libuda, J., Rupprechter, G., Baumer, M., Hamann, H. (2001) Bridging the pressure and materials gaps between catalysis and surface science clean and modified oxide surfaces. Top. Catal, 15, 201. 

Bron M, Teschner D, Knop-Gericke A, Steinhauer B, Scheybal A, Havecker M, Wang D, Fodisch R, Honicke D, Wootsch A, Schlogl R, Claus P. Bridging the pressure and materials gap in-depth characterisation and reaction studies of silver-catalysed acrolein hydrogenation. Journal of Catalysis. 2005 234(l) 37-47. 

Apparently, the same scenario holds true for the encapsulated Pt particles. Figure 4.4.7B shows STM images of the encapsulated Pt particle on FesO lll) and, for comparison, of the FeO(l 11)/Pt(l 11) film, both exposed to 20 mbar O2 at 450 K. The close similarities between these two systems with respect to the surface morphology and reactivity indicate the absence of the material gap, suggesting that the results and conclusions drawn for extended surfaces can be transferred to the supported nanoparticles. 

Centi G, Passalacqua R, Perathoner S, Su DS, Weinberg G, Schlogl R. Oxide thin films based on ordered arrays of one-dimensional nanostructure. A possible approach toward bridging material gap in catalysis. Phys Chem Chem Phys. 2007 9 4930-8. 

Girgsdies F, et al. Strained thin copper films as model catalysts in the materials gap. Catal Lett. 2005 102(l-2) 91-7. 

The results of the studies presented above clearly demonstrate that SFG surface vibrational spectroscopy, when combined with appropriate calibration measurements, is a promising experimental method for bridging the pressure gap as well as the materials gap which separate the UHV single-crystal model studies from technical catalytic investigations. Further experimental work is under way in which the method developed in the present work will be applied to investigate the influence of surface structure, mixture composition and pressure on the ignition behavior of the CO/02/Pt-system and other reactants/surface-systems under technically relevant conditions. 

These difficulties have stimulated the development of defined model catalysts better suited for fundamental studies (Fig. 15.2). Single crystals are the most well-defined model systems, and studies of their structure and interaction with gas molecules have explained the elementary steps of catalytic reactions, including surface relaxation/reconstruction, adsorbate bonding, structure sensitivity, defect reactivity, surface dynamics, etc. [2, 5-7]. Single crystals were also modified by overlayers of oxides ( inverse catalysts ) [8], metals, alkali, and carbon (Fig. 15.2). However, macroscopic (cm size) single crystals cannot mimic catalyst properties that are related to nanosized metal particles. The structural difference between a single-crystal surface and supported metal nanoparticles ( 1-10 nm in diameter) is typically referred to as a materials gap. Provided that nanoparticles exhibit only low Miller index facets (such as the cuboctahedral particles in Fig. 15.1 and 15.2), and assuming that the support material is inert, one could assume that the catalytic properties of a... 

Dellwig T, Rupprechter G, Unterhalt H, Freund H-J (2000) Bridging the pressure and materials gaps High pressure sum frequency generation study on supported Pd nanoparticles. Phys Rev Lett 85 776... 

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