Glide shear defects

Figure 3 In situ studies using HRTEM (See text) reveal that so-called glide shear defects, identified by Gai play an important role in the catalytic action of (V02)P207 dnring the oxidation of butane to maleic anhydride. Pi and P2 are partial screw dislocations.

These findings, coupled with the results of detailed diffraction contrast experiments (85,89), show that the defects are formed by glide shear the lattice is... 

Fig. 21. (a) The nature of the glide shear plane defects in three-dimensional projection and (b) in one layer of idealized structure, showing the novel glide shear process and the formation of glide shear plane defects. Filled circles are anion vacancies, (c) Schematic of glide shear. Glide defects accommodate the misfit at the interface between catalyst surface layers with anion vacancies (filled circles) and the underlying bulk (85,89). 

To determine the character of the dislocations and the displacement vector b under dynamic reaction conditions, the g A criteria are used. The dislocation contrast is mapped in several reflections (g) by tilting the crystal, including the reflection in which the dislocation is invisible, i.e. = 0 when b is normal to the reflecting planes. Careful analysis (figure 3.25(f) and (g) of defects from the same crystal area (c)) shows that the displacement vector lies in the plane of shear (i.e. parallel to the shear plane), consistent with glide shear, with no lattice collapse. These criteria show the displacement vector to be 6 = (a/7, 0, c/4). 

Figure 3.25. In situ catalysis (a) fresh VPO catalyst (b) dynamic real-time formation of atomic scale catalyst restructuring in butane after 2 min at 400 °C (c) enlarged image of (b) showing two sets of partial dislocations and (d) dynamic image of two sets of extended defects along symmetry-related (201) in (010) VPO after reduction in butane for several hours (diffraction contrast). The inset shows the defect nucleation near the surface. Careful defect analysis shows them to be formed by novel glide shear, (e) One set of the defects in high resolution (f) and (g) show diffraction contrast images of defects in 201 and 201. (After Gai et al, Science, 1995 and 1997 Acta Cryst. B 53 346.)...

The EM studies show that the novel glide shear mechanism in the solid state heterogeneous catalytic process preserves active acid sites, accommodates non-stoichiometry without collapsing the catalyst bulk structure and allows oxide catalysts to continue to operate in selective oxidation reactions (Gai 1997, Gai et al 1995). This understanding of which defects make catalysts function may lead to the development of novel catalysts. Thus electron microscopy of VPO catalysts has provided new insights into the reaction mechanism of the butane oxidation catalysis, catalyst aging and regeneration. 

At the microscopic level, shear stresses cause the gliding of planes of atoms over each other. This is the most common and easiest way for a solid to change its shape. The hardness, or the force needed, is very dependent on the presence of crystal defects. Even a pure crystal in the process of being formed will contain... 

---