Periodic Density Functional Theory and Atomistic Thermodynamic Studies of Cobalt Spinel Nanocrystals in Wet Environment: Molecular Interpretation of Water Adsorption Equilibria

Abstract

In this paper we present a theoretical study of water sorption on cobalt spinel nanocrystals by means of plane-wave periodic density functional theory (DFT) calculations jointly with statistical thermodynamics. The three most stable (100), (110), and (111) planes exposed by Co 3 O 4 were considered, and their stabilization upon water adsorption is discussed in detail. The calculated changes in free enthalpy of the investigated system under different hydration conditions along with the Wulff construction were used to predict the rhombicuboctahedral equilibrium morphology of cobalt spinel nanocrystals in different conditions, which corresponds very well to the experimental transmission electron microscopic (TEM) images. Two-dimensional surface coverage versus temperature and pressure diagrams were constructed for each of the examined (100), (110), and (111) planes to illustrate water adsorption processes in a concise way.

Water interaction with perfect and fluorine-doped Co3O4 (100) surface

“…Octahedral cobalt-oxygen and tetrahedral cobalt-oxygen bond lengths were equal to d Co − calculations we consider the energetically most stable Co 3 O 4 (100) surface. As was shown by Zasada et al[24], the surface energies (under vacuum conditions) for (100), (110) and (111) surfaces are 1.39, 1.65 and 1.48 J ⋅ m −2 that is, the most stable is the (100) surface.…”

“…The calculated GGA + U (100) surface energy is equal to 1.23 J ⋅ m −2 smaller than 1.39 J ⋅ m −2 , obtained in the GGA calculations [24].…”

Surface oxygen dynamics and H2 oxidation on cobalt spinel surface probed by 18O/16O isotopic exchange and accounted for by DFT molecular modeling: facile interfacial oxygen atoms flipping through transient peroxy intermediate

“…Following our previous investigations [38] adsorbed water is thermodynamically stable on the cobalt spinel surface at p H2O * 0.005 atm. until T * 150°C, thus part of the produced H 2 O molecules may be retained in dissociative and associative forms.…”

“…Dissociative forms of water exhibit DE ads ranging from 1.18 eV to 0.89 eV, whereas adsorption energies of associatively attached H 2 O are equal to 0.7 eV. From the thermodynamic diagram published previously [38] it may be thus expected that below 150°C the most stable water forms are retained on the surface during catalytic reaction, leading to partial self-poisoning of the spinel surface by blocking the octahedral cobalt active sites. This gives rise to temporal stagnation of the catalytic activity around 180-200°C, clearly seen in Fig.…”

Defects in Metal Oxide Nanoparticle Powders

“…9.2d, however, covers only 50 of them. There is a growing number of publications-predominantly on colloidal particle systems-which underline the potential of high resolution transmission electron microscopy (HR-TEM) for the determination of the geometrical shape of crystalline polyhedra and for the estimation of relative and absolute surface areas of different crystal facets [13][14][15][16]. Technically used powders of functional metal oxides (e.g.…”