Abstract
The present report describes the structural and physical–chemical variations of the potassium manganese oxide mineral, α–MnO2, which is a specific manganese octahedral molecular sieve (OMS) named cryptomelane (K–OMS–2), with different transition metal cations. We will describe some frequently used synthesis methods to obtain isomorphic substituted materials [M]–K–OMS–2 by replacing the original manganese cationic species in a controlled way. It is important to note that one of the main effects of doping is related to electronic environmental changes, as well as to an increase of oxygen species mobility, which is ultimately related to the creation of new vacancies. Given the interest and the importance of these materials, here, we collect the most recent advances in [M]–K–OMS–2 oxides (M = Ag, Ce, Mo, V, Nb, W, In, Zr and Ru) that have appeared in the literature during the last ten years, leaving aside other metal–doped [M]–K–OMS–2 oxides that have already been treated in previous reviews. Besides showing the most important structural and physic-chemical features of these oxides, we will highlight their applications in the field of degradation of pollutants, fine chemistry and electrocatalysis, and will suggest potential alternative applications.
Highlights
Manganese is the third most abundant transition metal in the Earth’s crust
We focus on cryptomelane, and on this material will be named as K–octahedral molecular sieves (OMS)–2
This review discussed the metal isomorphic substitution of cryptomelane [M]–K–OMS– 2 oxides that have appeared recently in the literature. We have addressed their syntheses on the basis of the replacement of manganese species in the framework by other transition metal cations with the proper size and coordination ability for this aim
Summary
Manganese is the third most abundant transition metal in the Earth’s crust. It is commonly found in a wide variety of minerals, including carbonates (rhodocrosite, kutnahorite), oxides (birnessite, cryptomelane, hollandite, etc.), silicates (braunite, rhodonite) and sulfides (alabandite) [1,2]. Manganese oxide-based materials are the subject of intense research, in particular layered and porous tunneled structures have received significant attention due to their excellent catalytic activity and especially from their potential implementation in energy-related topics In view of these precedents, our aim has been to offer the reader a comprehensive survey on design, synthesis, characterization, and applications of nanostructured MnO2, in particular octahedral molecular sieves (OMS), as catalytic functional materials during the last ten years. There are octahedral molecular sieves (OMS), which consist of the assembly of octahedral MnO6 building units interconnected through oxygen atoms [70], showing an unprecedented versatility in terms of redox properties, arrangement and doping possibilities with respect to other materials or building systems [63,71,72]
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