Texture and Surface Heterogeneity Changes During Topochemical Transformations in a Scope of the Periodic Table
Ninth International Symposium “Effects of Surface Heterogeneity in Adsorption and Catalysis on Solids” (ISSHAC-9)
17-23 Jul 2015
|| Mel'gunov Maksim Sergeevich
Boreskov Institute of Catalysis SB RAS
The life cycle of porous and particulate materials (catalysts, adsorbents, and others.) contains a set of phase transitions and topochemical reactions. The result of any of such transformation is the conversion or removal of a part or all of the solid phase material. Any topochemical reaction proceeds with the change in the volume of the solid phase, which can be accounted by the ratio of the molar volume of the solid product and the molar volume of the solid precursor. Ease estimation of this relationship allows prediction of possible changes in porous structure of the material, explaination of the behavior of a porous material in a topochemical reactions, etc. Novadays, the discussed analysis of changes in the volume of the solid phase is used in the literature only for a narrow range of topochemical reactions, and it is not considered from general physico-chemical positions, for example, in terms of the Periodic Table.
Usually, the topochemical transformations occur with a change in the number of atoms. The features of the interaction between atoms and the changes of their spatial arrangement can lead to both an increase and a decrease in the volume of the solid phase with an increase in the number of atoms. The direction of changes in the volume of the solid phase is not difficult to predict from the reference data on the true density or from X-ray data for the volumes of the unit cells of the product and reactant. For example, the analysis of the reference data shows a decrease of volume of the solid phase during the formation of oxides and hydrides of alkali and some other metals, whereas for the other elements there is an increase of the volume of the solid phase in analogous reactions. For certain metals, which volumes change antibatically during oxidation or hydrogenation, one can predict a combination, when the volume of one component is increased at expence of the volume of another component in a way that the total volume of the material remains unchanged. An additional factor may be the presence of the porous system capable of "damping" during cyclical changes in volume. This opens up the possibilities for the design of products, the volume of which does not change in the cyclic tests, which should positively affect their performance.
In this contribution we will show the particular examples of estimating of textural and surface heterogeneity changes in various porous systems, which are of practical interest.
This work was financially supported by RFBR (grant 14-08-00251).