China
Africa
Explore chapters and articles related to this topic
Foams and Bubbles
Published in K.S. Birdi, Surface Chemistry and Geochemistry of Hydraulic Fracturing, 2016
The most common application of microscopy is the study of molecules at surfaces. Generally, the study of surfaces is dependent on understanding not only the reactivity of the surface but also the underlying structures that determine that reactivity. Understanding the effects of different morphologies may lead to a process for enhancement of a given morphology, and hence to improved reaction selectivities and product yields. Atoms or molecules at the surface of a solid have fewer neighbors as compared with atoms in the bulk phase, which is analogous to the liquid surface; therefore, surface atoms are characterized by an unsaturated, bond-forming capability and accordingly are quite reactive. Until a decade ago, electron microscopy and some other similarly sensitive methods provided some information about the interfaces, but there were always some limitations inherent in all these techniques. Now, however, due to relentless technical advances, electron crystallography is capable of producing images at resolutions close to those attained by x-ray crystallography or multidimensional nuclear magnetic resonance (NMR). In order to improve on some of the limitations of the electron microscope, newer methods and procedures were needed. The recent scanning probe microscopes (SPM) not only provide a new kind of information, as known from x-ray diffraction, for example, but they also open up new areas of research, in nanoscience and nanotechnology, for example.
Synthesis and Characterization of Phosphors
Published in Vijay B. Pawade, Sanjay J. Dhoble, Phosphors for Energy Saving and Conversion Technology, 2018
Vijay B. Pawade, Sanjay J. Dhoble
HRTEM is a technique for high-magnification studies of nanostructured materials. High resolution makes it perfect for imaging materials on the atomic scale [15, 16]. It is a powerful technique to study the properties of materials on the atomic scale of a crystalline material, such as semiconductors, metals, nanoparticles, graphene, and carbon nanotubes. HRTEM is also referred to as high-resolution scanning TEM and phase contrast TEM. A resolution of around 0.5 Å (0.050 nm) can be achieved in phase contrast TEM [17]. In this instrument, we can characterize small-scale nanoparticles, individual atoms of a crystal, and the defects present inside the materials. This technique is also useful to know the exact structure of atoms, and hence, it is also called electron crystallography. The contrast of an HRTEM image arises from the phenomenon of interference in the image plane of the electron wave with itself, and the amplitude in the image plane is recorded. This gives structural information on the sample as well as d-spacing between the planes. To detect the wave, the aberrations of the microscope have to be tuned in such a way that it will convert the amplitudes in the image plane. Thus, the interaction of the electron wave with the crystallographic structure of the sample is complex, but a qualitative idea of the interaction can be readily obtained. The imaging electron from the wave interacts independently with the sample surface. While incident on the sample surface, it penetrates through it and is attracted by the positive atomic potentials of the atom cores in channels along the atom columns of the crystallographic lattice [18]. The interaction between the electron wave in different atom columns leads to Bragg’s law of diffraction (2dsinθ = nλ). Thus, based on the dynamical theory of electron scattering, image formation by the electron microscope is sufficiently well known to allow accurate simulation of electron microscope images [18,19]. Therefore, HRTEM is an important tool used for nanomaterials to characterize the lattice structure and other useful information related to shape, size, defects, and so on. Figure 3.7a to f shows the HRTEM image of a KSr(PO4) phosphor material synthesized by wet chemical methods [20]. It is first characterized by TEM to determine the exact structure of KSr(PO4) phosphor crystallites, as shown in Figure 3.7a; this shows a small core-shell structure with different crystallite sizes. For further investigation, it is characterized by HRTEM as indicated in Figure 3.8b through f. Figure 3.7b and c show the HRTEM image observed under 20 nm resolution, which clearly indicates the spherical shape of phosphor nanoparticles. With 1, 2, and 5 nm resolution, as shown in Figure 3.7d, it is seen that particles have a spherical shape with the separation of equidistant lattice planes on the surface as depicted in Figure 3.7e. Here, we have estimated the spacing between the crystallographic planes, which was found to be 2.2 Å, showing good agreement with the indexed plane of (212) in the JCPDS pattern. Figure 3.7f shows the electron diffraction images of the lattice point. Hence, this is a more qualitative technique to determine the structure of the material.
Hydrotreating of gasoline and diesel oil fractions over modified alumina/zeolite catalysts
Published in Petroleum Science and Technology, 2019
Balga Tuktin, Erbolat Zhandarov, Nurzhan Nurgaliyev, Aliya Tenizbayeva, Anatoli Shapovalov
Combination of electron crystallography with X-ray diffraction revealed that developed catalysts are highly dispersed, metal components of the active phase are predominantly in an oxidized state and forming cluster associates on the surface. For example, CoO-MoO3-CAR has small aggregates of Mo3Si and СеP on its surface with d = 3–4 nm, and structures with d ≈ 5–10 nm formed by Се2о3, MoSi2, and MoP. Structures such as AlСе3, Co2O3, Co2SiO3, and Се6O11 with d ≈ 4–6 nm are also presented. Nanosized particles of NiO and MoNiSi (d ≈ 4–5 nm) and an aggregate of Ni2O3 (d = 7–10 nm) have been found on the surface of the NiO-MoO3-CAR. In addition, Ni/Mo catalyst contains semitransparent aggregates (d ≈ 20 nm) consisting of smaller particles of Сео2, Се2O3, Се(МоO4), МоР, SiР, and NiOон. It should be noted that the presence of the MoSi, SiP and MoNiSi clusters indicate a direct interaction of matrix atoms with modifying metals. Moreover, these structures can function as Lewis acid sites (Asaftei et al. 2016; Rao, Zahidi, and Sayari 2009; Le and Valla 2017). On the surface of the NiO-WO3-CAR, clusters with d ≈ 4.5–5 nm consisting of Ni2о3, Се6о11, Ni3Si, Ni3Si2, Сео2, and AlP prevail as well as aggregates with d ≈ 20–40 nm, formed by NiWO4, СеP2, and SiP2O7. Complex nickel oxide Ni5о2 forms large particles with d ≈ 150–300 nm is also presented on the surface of this catalyst. The CoO-WO3-CAR catalyst contains Се2о3 (d ≈ 2.5–3 nm) and highly dispersed particles consisting of Се4W9о33 and Wо3 (d ≈ 5 and 20 nm). In addition, regions of small, low-density clusters attributed to Сео2, CoSi, AlP and Co2Si (d ≈ 5–7 nm) and aggregates (d ≈ 20–40 nm), identified as a mixture of Al5Co2 and Al3Се were found. Extensive accumulations of particles contained Се2W2о9 and AlСе3 with d ≈ 4 and 70 nm respectively, have been also indicated.