Crystallographic phase

Because x-rays are particularly penetrating, they are very usefiil in probing solids, but are not as well suited for the analysis of surfaces. X-ray diffraction (XRD) methods are nevertheless used routinely in the characterization of powders and of supported catalysts to extract infomration about the degree of crystallinity and the nature and crystallographic phases of oxides, nitrides and carbides [, ]. Particle size and dispersion data are often acquired with XRD as well. 

One must be sure of the purity of the model compound. It may have deteriorated (for example, by reaction or water absorption), its surface may not have the same composition as the bulk, or it may not be of the correct crystallographic phase. It is tempting to use single crystals to be sure of the geometric structure, but noncubic crystals give angle-dependent spectra. The crystallography of any compound should be checked with XRD. 

In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst. Figure 4.5 gives an example where XRD readily reveals the Pd reflections in two Pd/Si02 catalysts. 

As illustrated by Eig. 4.13, an electron microscope offers additional possibilities for analyzing the sample. Diffraction patterns (spots from a single-crystal particle and rings from a collection of randomly oriented particles) enable one to identify crystallographic phases as in XRD. Emitted X-rays are characteristic for an element and allow for a determination of the chemical composition of a selected part of the sample. This technique is referred to as energy-dispersive X-ray analysis (EDX). 

All the catalysts were crushed, pelletized then sieved between 0.2 and 0.4 mm. Several crystallographic phases of alumina, r, y, 5 and 9 provided by IFP were characterized. For the test reaction, all catalysts were activated overnight in situ at 530°C under dry air flow (25 mL/min). Cyclopentanol and cyclohexanone (Alfa Easer, >99%) were used without any purification. 

Particles can be characterized by their composition and crystallographic phase, as well as by their size, density, and shape. The particle composition can have a dramatic impact on the amount of incorporation obtained for a particular bath composition. For instance three times more TiC>2 than AI2O3 has reportedly [54] been incorporated into a Ni matrix, under the same deposition conditions. 

Perhaps even more noteworthy is the effect of crystallographic phase. While one phase of a specific composition may readily incorporate from a particular bath composition, another phase of the same composition may incorporate to a much lower extent or not at all. For instance, in the alumina particle system, the alpha phase has been found to readily incorporate from an acidic copper bath while the gamma phase incorporates at less than one tenth the amount of alpha, if at all, as shown in Table 1 [2, 11, 27, 31, 33],... 

The long-range cooperative nature of the electronic spin-state crossover in [Fe(HB(pz)3)2] and the accompanying crystallographic phase transition is... 

Fe(btr)2(NCS)2]-H20 undergoes a complete ST centred at 134 Kwith a hysteresis loop of width 21 K. This derivative represents the first example of a 2D ST compound and has become a model material in SCO research. The presence of a crystallographic phase transition to account for the observed hysteresis was first proposed since crystal cracking was regularly observed when the sample was cooled through the temperature region of the spin transition [59]. Recent X-ray data recorded at 95 K, where the com-... 

In cases where the wavelength of the modulation fits exactly with the dimensions of the underlying structure, a commensurately modulated crystallographic phase forms. In cases where the dimensions of the modulation are incommensurate (i.e., do not fit) with the underlying (generally small) unit cell of the parent structure, the phase is an incommensurately modulated phase. 

In catalyst characterization, diffraction patterns are mainly used to identify the crystallographic phases that are present in the catalyst. Figure 6.2 gives an example where XRD readily reveals the phases in an Fe-MnO Fischer-Tropsch catalyst [7], The pattern at the top is that of an MnO reference sample. The diffraction pattern of the reduced Fe-MnO catalyst shows a peak at an angle 29 of 57°, corresponding to metallic iron, and two peaks which are slightly shifted and broadened in comparison with the ones obtained from the bulk MnO reference. The Mossbauer spectrum of the reduced catalyst contains evidence for the presence of Fe2+ ions in a mixed (Fe,Mn)0 oxide [7], and thus it appears justified to attribute the distortion of the XRD peaks to the incorporation of Fe into the MnO lattice. Small particle size is another possible reason why diffraction lines can be broad, as we discuss below. 

The examples illustrate the strong points of XRD for catalyst studies XRD identifies crystallographic phases, if desired under in situ conditions, and can be used to monitor the kinetics of solid state reactions such as reduction, oxidation, sulfidation, carburization or nitridation that are used in the activation of catalysts. In addition, careful analysis of diffraction line shapes or - more common but less accurate-simple determination of the line broadening gives information on particle size. 

Strub, M. R, et al. (2003). Selenomethionine and seleno-cysteine double labeling strategy for crystallographic phasing. Structure (Camb) 11,1359-1367. 

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