Atomic scale characterization

Klie, R.F., and N.D. Browning. 2000. Atomic scale characterization of a temperature dependence to oxygen vacancy segregation at SrTiOj grain boundaries. Appl. Phys. Lett. 77 3737-3739. 

Keywords Atomic scale characterization surface structure epoxidation reaction 111 cleaved silver surface oxide STM simulations DFT slab calculations ab initio phase diagram free energy non-stoichiometry oxygen adatoms site specificity epoxidation mechanism catalytic reactivity oxametallacycle intermediate transition state catalytic cycle. 

Before the introduction of STM, high-resolution (HR-)TEM was the primary technique for determination of the surface structures of nanoparticle model catalysts (14,54,74,77,197,198,211,226-230). For technological catalysts, it is still the only method that provides a direct atomic-scale characterization of metal nanoparticles and of the oxide support (211,231-238). Although TEM is unable to detect adsorbed molecules (in contrast to the methods discussed above), it is briefly mentioned here because HR-TEM was sometimes employed to corroborate STM data characterizing model catalysts and, in particular, to demonstrate the internal... 

Appropriately tailoring and exploiting these functionalities requires an atomic-scale characterization and understanding of the oxidation process. 

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

We have already mentioned that fundamental studies in catalysis often require the use of single crystals or other model systems. As catalyst characterization in academic research aims to determine the surface composition on the molecular level under the conditions where the catalyst does its work, one can in principle adopt two approaches. The first is to model the catalytic surface, for example with that of a single crystal. By using the appropriate combination of surface science tools, the desired characterization on the atomic scale is certainly possible in favorable cases. However, although one may be able to study the catalytic properties of such samples under realistic conditions (pressures of 1 atm or higher), most of the characterization is necessarily carried out in ultrahigh vacuum, and not under reaction conditions. 

The catalytic properties of a surface are determined by its composition and structure on the atomic scale. Hence, it is not sufficient to know that a surface consists of a metal and a promoter, say iron and potassium, but it is essential to know the exact structure of the iron surface, including defects, steps, etc., as well as the exact locations of the promoter atoms. Thus, from a fundamental point of view, the ultimate goal of catalyst characterization should be to look at the surface atom by atom, and under reaction conditions. The well-defined surfaces of single crystals offer the best likelihood of atom-by-atom characterization, although occasionally atomic scale information can be obtained from real catalysts under in situ conditions as well, as the examples in Chapter 9 show. 

Simplifying, one could say that catalyst characterization in industrial research deals with the materials science of catalysts on a more or less mesoscopic scale, whereas the ultimate goal of fundamental catalytic research is to characterize the surface of a catalyst at the microscopic level, i.e. on the atomic scale. 

Classical relaxors [22,23] are perovskite soUd solutions like PbMgi/3Nb2/303 (PMN), which exhibit both site and charge disorder resulting in random fields in addition to random bonds. In contrast to dipolar glasses where the elementary dipole moments exist on the atomic scale, the relaxor state is characterized by the presence of polar clusters of nanometric size. The dynamical properties of relaxor ferroelectrics are determined by the presence of these polar nanoclusters [24]. PMN remains cubic to the lowest temperatures measured. One expects that the disorder -type dynamics found in the cubic phase of BaTiOs, characterized by two timescales, is somehow translated into the... 

Sample surfaces prepared by various macroscopic techniques are not well characterized on an atomic scale over the extended area of the macroscopic surfaces. It is now well known that even the most carefully processed surfaces contain only domains of well defined atomic structures of sizes of the order of 100 to 1000 A. 

The global thermodynamic approach used in the above sections is insensitive to details at the atomic level and can only yield a gross characterization of the surface. Properties such as the specific surface area and the presence or absence of pores can be determined using the above approach since only the average surface —not atomic details —is involved. The existence of a distribution of surface energy sites can also be inferred from adsorption data, but the method falls short when it comes to specifics about this distribution. Observations on an atomic scale are needed to learn more about the details of the surface structure. Such observations comprise the subject matter of the last two sections of the chapter. 

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