Size Effects in Structural Stabilization of Aluminum Oxide for High-Temperature Applications
Статья (Full article),
||Aluminium Oxide: Structure, Production and Applications
Nova Science Publishers, Inc. NY, USA.2020.
||alumina, phase composition, textural properties, phase transformations, thermal stability, acidic sites, size effect, textural promoters, carbon nanoreactor
Vedyagin Aleksey A.
Volodin Alexander M.
Department of Material Sciences and Functional Materials,
Boreskov Institute of Catalysis SB RAS
Aluminum oxide is widely used in the modern science and industry as a catalyst’s support for a variety of catalytic processes. In some cases, it can be considered as a bulk catalyst or can be applied as a binder for shaping the oxide materials of high crystallinity (zeolites, for instance). By varying the phase composition and using the various modifiers, it is possible to affect controllably the concentration of the surface acidic sites (Lewis and Brønsted), desired specific surface area, and degree of crystallinity. The phase transformations of the aluminum hydroxide into the final state of a low-surface, well-crystallized phase of corundum (αAl2O3) are widely and precisely studied. Such transformations take place during the calcination of the starting material, for example, boehmite, in a temperature range of 300-1200°C, and include the formation of eta, gamma, delta, and theta intermediate phases. Among the effective tools for the determination of the phase composition at different stages, such methods as X-ray diffraction analysis, transmission electron microscopy, nuclear magnetic resonance spectroscopy, and laser-induced luminescence spectroscopy should be mentioned. The genesis of alumina’s surface properties such as the concentration of acidic sites of different strengths as well as the electron-donor and electron-acceptor sites can be qualitatively and quantitatively followed by a number of techniques including an electron paramagnetic resonance of spin probes, a variety of temperature-programmed desorption methods, and Fourier-transform infrared spectroscopy. One of the main challenges of the aluminum oxide usage is to prevent the undesired phase transformations into corundum, which is accompanied with a significant drop in the specific surface area, and to keep the appropriate surface properties even at elevated temperatures. As it became evident not a long time ago, the different alumina phases are thermodynamically stable only at the specified size of the particles. The enlargement of the particles resulted from their agglomeration and was found to be accompanied by the corundum phase formation. For instance, if the amount of the neighboring alumina fragments within such particle or agglomerate is not large enough, the corundum phase cannot be formed. Thereby, the task to solve the problem mentioned above appears to involve the diminishing contacts between the alumina fragments. The so-called “carbon nanoreactor” concept was recently shown to be applicable for this purpose. In this case, a carbon coating obtained via the pyrolysis of the carbon-containing precursor over the alumina with developed surface area serves as a textural promoter, preventing the direct contact and agglomeration of the particles, and their further phase transformation. The carbon nanoreactor approach is quite versatile, easily applicable and restricted only by the limitation to use an inert atmosphere for the high temperature treatment, and by the temperature of the carbothermal reduction of alumina, which exceeds 1400°C. In this term, the search for other coatings, which are inert with respect to alumina, thermally stable and resistant towards any reductive or oxidative media, are of great importance.