Water Vapor Adsorption by Some Manganese Oxide Forms

Tatyana Rakitskaya,Vitaliya Volkova,Ganna Dzhyga,Anna Nagaevs’Ka,Alla Truba

doi:10.3390/colloids2040061

Tatyana Rakitskaya, Vitaliya Volkova + Show 3 more

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https://doi.org/10.3390/colloids2040061

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Abstract

Manganese oxide forms prepared by different methods differ by their compositions, phase ratios in polyphase samples, and crystallite sizes (XRD and TEM characterization). Among the phases, tunnel-structured β-MnO2 (pyrolusite), α-MnO2 (cryptomelane), ε-MnO2 (akhtenskite), and β-Mn2O3 (bixbyite) have been identified. Water vapor sorption isotherms showed substantial differences in the affinities of water molecules to oxide surfaces of the manganese oxide forms under study. The parameters of the BET equation and pore size distribution curves have been calculated. The manganese oxide forms have mesoporous structures characterized by uniform and non-uniform pore sizes as well as by moderate hydrophilic behavior.

Highlights

Polymorphic forms of manganese dioxide are widely used as catalysts for such gaseous reactions as ozone decomposition [1,2,3], CO oxidation [4,5], NO reduction [6,7], and volatile organic compound oxidation [8,9]
When Type I, II, IV, or VI water vapor adsorption isotherms are observed, it means that the surface under study is hydrophilic, while hydrophobic surfaces are characterized by Type III or V adsorption isotherms [11]
By the appearances of Type III or V adsorption isotherms, one can judge about a change in the affinity of water molecules to the surface under investigation over the course of increasing coverage by water and about transition from hydrophobic to hydrophilic surface behavior at P/PS > 0.5–0.6

Summary

Introduction

Polymorphic forms of manganese dioxide are widely used as catalysts for such gaseous reactions as ozone decomposition [1,2,3], CO oxidation [4,5], NO reduction [6,7], and volatile organic compound oxidation [8,9]. Water vapor sorption isotherms are able to provide significant information about hydrophilic/hydrophobic behavior of these materials. For this purpose, values of water vapor adsorption at low P/PS would be compared [10,11,12,13,14]. It can be concluded that hydrophilic/hydrophobic properties of manganese dioxide polymorphic forms depend on their structures and compositions [14,16,17], the nature of cations and degrees of surface hydration [18], as well as on methods of surface modification [19]. In the case of manganese oxide forms, systematic investigations of the influence of synthesis methods and conditions on their structures (phase compositions) and crystallite sizes as well as on their water vapor adsorption properties are absent

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