Microstrain distortion

Figure 9 The production of local microstrains and distorted lattice cells by the inclusion of aluminate groups in the nickel lattice (Reproduced by permission from Preparation of Catalysts III, 1983, p. 256...

Ceria-zirconia nanophases were synthesied by a surfactant-assisted method. The refined structural data concerning the crystallite size, lattice parameters, structural microstrain, cationic occupy number and cationic defect concentration are reported. Zirconium addition into the cubic structure of ceria inhibits crystal sintering but leads to structure distortion. Different CO-metal bonds are formed when CO chemisorbs on Pd-loaded CesZr. x02 catalysts. Catalytic tests reveal that the lower zirconium content benefits the CO oxidation. 

The Rietveld refinements show that the crystalline structure contains microstrain that is mainly caused by the crystal distortion related to zirconium replacement. It is also found that cationic occupancy number in the crystalline structure is smaller than their normal value 0.02083 present in an ideal crystal, indicating that the crystalline structure is of cationic deficient. As far as the local environment of the cationic defect is concerned, the lattice oxygen ions around it are not fully bonded, which are mobile and more active under the reaction condition compared to the normal ones. Therefore, the creation of cationic defects in the structure is a possible origin of the unusual reducibility and high mobility of oxygen species from bulk to surface exhibited on the ceria-zircnia solids, that are reported by other groups [5, 6]. 

Nanosized ceria-zirconia materials with improved thermal stability can be prepared by using the surfactant-assisted method. Structural refinements confirm that the nanocrystals contain structural microstrain and cationic lattice defects. Zirconium addition to ceria supresses the crystal sintering and imporves the thermal stability but leads to structure distortion. Both catalytic tests and CO-chemisorption show that Pd supported ceria-zirconia nanoparticles are active for CO oxidation. 

The cosine coefficient of this series, A , is a product of the two cpiantities N /N3 and cos27tlZ ). N /N3 only depends on the length of the colnnms and therefore corresponds to a size. The other term depends on Z , which means that it is related to the distortions of the lattice. It represents the contributions from the microstrains. This means that the cosine term is the product of the size and distortion coefficients. If we denote these two terms by A and A , respectively, we get ... 

By differentiating the XANES spectra, it was identified that the intensity of the edge absorption for the catalyst was lower than that for copper foil, consistent with the nanosized dimension of the copper particles. Moreover, the positive shift of the derivative peaks at ca. 8990 and 8984 eV relative to those of copper foil suggested some alterations in the chemical environment around the copper species. Combined with the XRD results, the positive shift was tentatively assigned to a distortion of the copper lattice due to the presence of microstrain at the interface between Cu NPs and ZnO NRs. Consequently, the superior catalytic performance of the ZnO NR Cu NPs catalyst in methanol reforming was attributed to the enhanced dispersion of Cu NPs and the existence of the SMSl effect. 

Microstructural imperfections (lattice distortions, stacking faults) and the small size of crystallites (i.e. domains over which diffraction is coherent) are usually extracted from the integral breadth or a Fourier analysis of individual diffraction line profiles. Lattice distortion (microstrain) represents departure of atom position from an ideal structure. Crystallite sizes covered in line-broadening analysis are in the approximate range 20-1000 A. Stacking faults may occur in close-packed or layer structures, e.g. hexagonal Co and ZnO. The effect on line breadths is similar to that due to crystallite size, but there is usually a marked / fe/-dependence. Fourier coefficients for a reflection of order /, C( ,/), corrected from the instrumental contribution, are expressed as the product of real, order-independent, size coefficients A n) and complex, order-dependent, distortion coefficients C (n,l) [=A n,l)+iB n,l)]. Considering only the cosine coefficients A(n,l) [=A ( ).AD( ,/)] and a series expansion oiAP(n,l), A (n) and the microstrain e (n)) can be readily separated, if at least two orders of a reflection are available, e.g. from the equation... 

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