X-ray powder diffraction is also frequently used for the characterisation of mesoporous materials. Using this method, the markedly complex 10-ring channel system forming the IM-5 zeolite, an active catalyst for hydrocarbon cracking and related reactions, was determined. An interesting example is an algorithmic advance that facilitates combined analysis of powder diffraction and electron microscopy data to solve particularly insolvable zeolite structures. 42,43 Maybe even more important is a constant development of powder X-ray diffraction methods that combine experimental data with crystal chemistry-based modelling. 41 We have recently written two review papers about the principles of X-ray anomalous dispersion and about the use of X-ray diffraction and anomalous dispersion method in the structure elucidation of microporous materials. 40 Synchrotron radiation has also enabled in situ diffraction studies of the structural changes during crystallisation and phase transitions as a function of temperature or pressure and in situ studies of reaction kinetics by following the structural changes during the catalysis and other processes that are taking place on the microporous surfaces. Rapid development of synchrotron radiation sources enabled the development of so-called microcrystallography, dealing with a few micron-sized single-crystals, and development of anomalous dispersion methods for the localization of active metal sites in the structures. In the recent past, structure determination of microporous materials using X-ray diffraction techniques has experienced considerable developments in techniques and methodology. This is the case, when the synthesized crystals or crystallites are too small with highly polycrystalline morphologies, when there is a low concentration and random distribution of metal active sites over the framework or extra-framework positions or simply when the material is not fully crystalline. For many nanoporous materials, however, it cannot provide reliable structural information. The conventional single-crystal X-ray diffraction gives the most complete answers about the structure properties of ordered crystalline materials. Venčeslav Kaučič, in Ordered Porous Solids, 2009 3.1 X-ray diffraction Similar structural transitions have been reported for PET-protected Au clusters smaller Au 103-105(PET) 45-46 and Au 144(PET) 60 have nonbulk-like core structures, whereas larger Au 333(PET) 79 and Au ∼940(PET) ∼160 have fcc Au cores. 138 These comparisons lead us to conclude that clusters smaller than Au 144(SC 12H 25) 60 have nonbulk-like Au cores. The XRD patterns of Au 104(SC12) 45, Au 130(SC12) 50, and Au 144(SC12) 60 are, however, reproduced well with computational models of Au 104(SH) 45 with a Mark's decahedral (M-Dh) core, Au 130(SH) 50 with an M-Dh core, and Au 144(SH) 60 with an icosahedral core, respectively. 138 The XRD patterns of Au 187(SC12) 68, Au 329(SC12) 84, and Au ∼520(SC12) ∼130 are similar to that of Au bulk, suggesting that they have fcc-based cores, although each peak is significantly broadened because of the small size effect. The powder XRD patterns of a series of Au n(SC12) m are presented in Figure 16. Thus, structural information has conventionally been obtained by powder XRD. However, single crystals have been obtained only for a limited number of clusters ( Table 1). These results suggest that the transition between fcc and non-fcc structures occurs in a certain critical size region. For example, Au 25(SR) 18 ( Figure 10) and Au 38(SR) 24 with RS = PET and C xSH have icosahedral Au 13 and bi-icosahedral Au 23 cores, respectively. Single crystal XRD analyses of small Au n(SR) m revealed that their atomic packing structures are different from the fcc motif of bulk gold.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |