Catalysts from single crystals

Oj-rich conditions, CO oxidation is reported to be demanding or structure sensitive on various catalysts from single crystals - to model catalysts with small Pd or Pt clusters supported on flat supports,to supported catalysts, - similar to the results presented in this section. 

Schiile, A., Nieken, U., Shekhah, O., Ranke, W., Schlogl, R. Kolios, G. (2007. )Styrene synthesis over iron oxide catalysts from single crystal model system to real catalysts. Phys. Chem. Chem. Phys., 9, 3619-3634. 

Temperature programmed reaction methods form a class of techniques in which a chemical reaction is monitored while the temperature increases linearly with time [1,2]. Several forms are in use. All these techniques are applicable to real catalysts and single crystals and have the advantage that they are experimentally simple and inexpensive in comparison to many other spectroscopies. Interpretation on a qualitative basis is fairly straightforward. However, obtaining reaction parameters such as activation energies or preexponential factors from temperature programmed methods is a complicated matter. 

Temperature programmed desorption (TPD) or thermal desorption spectroscopy (TDS), as it is also called, can be used on technical catalysts, but is particularly useful in surface science, where one studies the desorption of gases from single crystals and polycrystalline foils into vacuum [2]. Figure 2.9 shows a set of desorption spectra of CO from two rhodium surfaces [14]. Because TDS offers interesting opportunities to interpret desorption in terms of reaction kinetic theories, such as the transition state formalism, we will discuss TDS in somewhat more detail than would be justified from the point of view of practical catalyst characterization alone. 

Idriss, H. Barteau, M.A. Active sites on oxides From single crystals to catalysts. Adv. Catal. 2000, 45, 261-331. 

The six sections following the overview chapter deal with aspects of selective oxidation that range from theories and concepts to state-of-the-art engineering applications. Several chapters describe the synthesis, characterization, and performance of potentially attractive new catalytic materials. These catalysts range from single crystals with well-defined crystal faces to highly dispersed or amorphous solids. Most of the actual catalytic reactions studied involve the oxidation of hydrocarbons in the range from to C. ... 

The phase transformations in the catalyst play an important role in determining the activity, attrition resistance, and deactivation of this catalyst. Activation of this precipitated catalyst transforms single crystals of hematite to smaller crystallites of carbide. While the transformation from hematite to magnetite is extremely rapid, the magnetite to carbide transition is much slower under the conditions of temperature and pressure employed in this study. As carbon deposits on the carbide particles, it serves to further prise the carbide particles apart. In a commercial slurry phase reactor the carbide particles break away leading to catalyst attrition. The implication of this work for the attrition resistance of iron FT catalysts is explored in detail elsewhere.18... 

Second, apart from single crystals, nanoparticle model catalysts should be employed to better mimic the complex properties of supported metals. Nevertheless, the metal nanoparticles should still exhibit well-defined surface facets to allow more reliable data interpretation and a comparison with single-crystal results. 

Surface science offers many opportunities in catalysis research because a variety of techniques are available to characterize in detail the composition and structure of the catalyst surface and to identify the adsorbed species. A frequent criticism of the surface science approach is that it is far removed from real catalysis since most of the surface science techniques can only be applied at low pressures and with model catalysts, often single-crystal surfaces. The so-called pressure gap has been bridged by combining, in the same apparatus, the techniques needed for surface analysis and characterization with the ability to measure reaction rates at elevated pressures. In addition, many techniques can also be apphed in situ at elevated pressures. 

Figure 19 CO + NO reaction Arrhenius plots for single-crystal, model planner-supported, and Pd/Al203 powder catalysts. The powder catalyst data were taken in the flow reaction mode (4.4/5.2 CO/NO ratio, steady state), and the model catalyst and single-crystal data were acquired for a batch reaction mode in 1 Torr of each reactant. (From Ref. 32.)...

Simultaneously, as articulated above, it is realized that intrinsically new phenomena may occur on the nanoparticles, which cannot be deduced from the knowledge obtained from single crystals, due to changes in the electronic structure [3], morphology [3,6], and kinetics of nanometer scale particles [4] (Fig. 4.3). This is in essence what the structure gap in catalysis is about. Clearly, it is desirable to devise better methods to prepare model catalysts, which not only maintain the critical aspects of their original function but also provide a well-defined structure that allows for detailed scrutiny with a large range of analytical techniques (experimental and theoretical). 

Unfortunately for the individual investigator, it appears that the way to progress in this field is through complicated preparation methods, the use of heavy techniques such as TEM and EXAFS, and detailed kinetic (mechanistic) studies on each catalyst. Ideally, single-crystal and metal cluster studies would be done on the same systems. The holders of purse strings will have to decide whether the added understanding resulting from these expensive studies is justified in a field in which empiricism has... 

The microkinetic analysis of the H2 TPD data from Cu catalysts revealed that the influence of readsorption was negligible rendering the mathematically easier CSTR model appropriate. In this case, the proper kinetic desorption parameters can be extracted from the shifting TPD peak maximum temepratures as function of the heating rate. However, the example illustrated that deriving the coverage dependence from single crystal data is not without pitfalls. 

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