Structure of highly dispersed metals and oxides: exploring the capabilities of high-resolution electron microscopy

Abstract

The potential applicability of high-resolution electron microscopy (HREM), in combination with image analysis and image simulation tools, to retrieve structural information from nanometre-sized particles present in oxide-supported metal and oxide catalysts is analysed. Specifically, the possibilities and limitations of this technique to determine features such as the size, morphology and chemical nature of the particles, their surface structure and their structural relationship with the support are considered through the discussion of several examples.The interpretation of a series of HREM images of Pt and Rh catalysts supported on cerium oxides after treatments under different redox environments illustrates the case of highly dispersed metals. In addition, the results obtained in this study provide an approximate picture of the evolution of metal-support interaction effects in this family of catalysts, which is closely related to three-way catalysts (TWCs). The results of a nanostructural investigation of two catalyst systems, one consisting of MgO-supported neodymia clusters and the second of vanadium-magnesium oxide also supported on MgO, provide the examples for supported oxide catalysts. These find application in oxidation reactions. For the former, the growth of neodymia in the form of rounded patches in a parallel orientation relationship with the support has been observed. For the vanadiacontaining catalysts, the formation of a weakly ordered MgV 2 O 4 spinel surface phase on the MgO support crystallites, after exposure to typical reaction conditions in the oxidative dehydrogenation of propane, has been confirmed. The structural relationship at the spinel/MgO interface has also been established. Copyright

Highly Dispersed Ce x Zr1−x O2 Nano-Oxides Over Alumina, Silica and Titania Supports for Catalytic Applications

“…The different spots that are present account for the existence of periodic contrasts in the original experimental micrographs, which correspond to different sets of atomic planes of the crystalline structure. The geometrical arrangement of these reflections is directly related to the structural aspects of the analyzed crystals [92,93]. The values of 4.8 and 3.6 Å measured on the big rounded particle in Fig.…”

Alumina Over-coating on Pd Nanoparticle Catalysts by Atomic Layer Deposition: Enhanced Stability and Reactivity

“…This problem is prominent for supported noble metal catalysts in which NP sintering at high temperature is a major contributor to catalyst deactivation [1][2][3][4]. Methods have been developed to encapsulate noble metal NPs in various types of porous materials using techniques such as chemical vapor deposition (CVD), grafting, micro-emulsion, dendrimer encapsulation, etc.…”

Techniques for the Study of the Structural Properties

“…), HRTEM quantitative analysis is usually based on matching experimental and calculated images, for which various computational methods have been developed. 85,87,91,92,98,99 Figure 7 shows an example of application of simulated images to resolve geometrical aspects in a supported oxide system. 91 Resolution limits of the TEM technique and schemes to achieve sub-angstrom resolution have been analyzed in detail by Spence.…”

“…85,87,91,92,98,99 Figure 7 shows an example of application of simulated images to resolve geometrical aspects in a supported oxide system. 91 Resolution limits of the TEM technique and schemes to achieve sub-angstrom resolution have been analyzed in detail by Spence. 87 The highest structural resolution possible (point resolution) when the obtained images can be directly related to the projected structure of the specimen is given by the Scherzer expression R s = 0.66 λ 3/4 C s 1/4 , where λ is the electron wavelength and C s is the spherical aberration coefficient.…”

“…Aberration correction has in turn allowed to achieve sub-angstrom resolution with microscopes operating at lower voltages (typically 200 KeV), allowing to resolve oxygen atoms in oxides materials. 87,91 On the other hand, considering that high-resolution is achieved in TEM as the result of electron wave interference among diffracted peaks, a limitation to structural resolution can also arise from decreasing the number of atoms in the nanoparticle. 99 This also can pose limitations to routinely employed determination of particle size distributions, particularly in cases when contrast differences between various sample components are not clearly resolved, as may occur for supported catalysts.…”