Nanoscale Hydration in Layered Manganese Oxides.

Wei Cheng,Michael Holmboe,Khalil Hanna,Andrey Shchukarev,Jerry Lindholm,Eugene S Ilton,Jean-François Boily,N Tan Luong

doi:10.1021/acs.langmuir.0c02592

Abstract

Birnessite is a layered MnO2 mineral capable of intercalating nanometric water films in its bulk. With its variable distributions of Mn oxidation states (MnIV, MnIII, and MnII), cationic vacancies, and interlayer cationic populations, birnessite plays key roles in catalysis, energy storage solutions, and environmental (geo)chemistry. We here report the molecular controls driving the nanoscale intercalation of water in potassium-exchanged birnessite nanoparticles. From microgravimetry, vibrational spectroscopy, and X-ray diffraction, we find that birnessite intercalates no more than one monolayer of water per interlayer when exposed to water vapor at 25 °C, even near the dew point. Molecular dynamics showed that a single monolayer is an energetically favorable hydration state that consists of 1.33 water molecules per unit cell. This monolayer is stabilized by concerted potassium–water and direct water–birnessite interactions, and involves negligible water–water interactions. Using our composite adsorption–condensation–intercalation model, we predicted humidity-dependent water loadings in terms of water intercalated in the internal and adsorbed at external basal faces, the proportions of which vary with particle size. The model also accounts for additional populations condensed on and between particles. By describing the nanoscale hydration of birnessite, our work secures a path for understanding the water-driven catalytic chemistry that this important layered manganese oxide mineral can host in natural and technological settings.

Highlights

Birnessite (MnO2) is a layered manganese oxide (Figure 1) occurring as fine-grained poorly crystalline nanoparticles or coatings in soils, sediments, and ferromanganese deposits.[1−3] This phyllomanganate has the notable quality of accommodating nanometric water films within its structure
The isotherms became nearly congruent when expressed on a mass per mass or unit cell (H2O/UC) basis (Figure 2) and reached ∼1.5−1.6 H2O/UC just below the dew point of water
Using a maximal water population of 1.33 H2O/UC (e.g., 121 mg of H2O per g MnO2 (0.17 K) for δ-MnO2) on the internal and external basal faces of birnessite, we modeled the microgravimetric data of Figure 2 with our adsorption−

Summary

Introduction

Birnessite (MnO2) is a layered manganese oxide (Figure 1) occurring as fine-grained poorly crystalline nanoparticles or coatings in soils, sediments, and ferromanganese deposits.[1−3] This phyllomanganate has the notable quality of accommodating nanometric water films within its structure. Manganese oxides with well-defined crystallographic sites host structured water layers, while those of lower crystallinity host relatively more disordered water.[31] Understanding the forms of water trapped in the interlayer region when birnessite is exposed to moist air is especially central for understanding how they mediate reactions of natural and technological importance. Of note, resolving this chemistry can be challenged by the coexistence of water condensed in pores between particles

Methods

Results

Conclusion