Factors controlling the catalytic activities of layered two-dimensional semiconducting fe and mn minerals

Wang, Chenying
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Shi, Sufei
Zha, Helen
Bonitatibus, Peter
Chakrapani, Vidhya
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Chemical engineering
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Oxides and oxyhydroxides of iron and manganese, two of the most abundant elements on Earth, are ubiquitous in many geological settings such as oceans, lakes, soils, and sediments, where they perform many important biogeochemical functions. Among these various oxides, naturally formed two-dimensional layered minerals, such as birnessite (MnO2) and green rust (Fe(II)Fe(III)OOH), are powerful environmental catalysts for many species such as water, metal cations and organic pollutants, and play an important role in many biogeochemical cycles. Although the reactivity of these minerals has been well studied from the perspectives of crystal structure and composition, the electronic properties of these semiconducting minerals and their role in affecting the catalytic reactivity is not been well explored, which is the overarching goal of this dissertation. Studies were carried out with a wide variety of natural and synthetic birnessite samples. In the first study, the factors responsible for its high oxidation activity are evaluated across a series of birnessites with H, Li, Na, K, Cs, Ca, and Mg interlayer cations as well as other MnOx polymorphs. The results show that the oxidation activity of the cation-exchanged birnessite decreases in the order Mg > Cs = K > Na > Ca > Li > H. Their high reactivity is due to their unusually high electron affinity (∼5.9 eV), which is the highest among all known functional oxides and sulfide in aqueous solution. Both the band gap (Cs > Mg > K > Na > Ca > Li > H) and the electron affinity (Na > Cs > K > Mg > Ca > Li > H) are strongly affected by the nature of the interlayer cations and their coordinating water molecules. Analysis shows that the band gap increases with increase in the interlayer spacing, while the electron affinity increases with the relative Mn(III)/Mn(II) concentrations within birnessite. The high electron affinity values place the band edges of the birnessite-type structure well below the redox potential of most thermodynamically stable cations and water, thereby enabling spontaneous oxidation. Other tunnel-like and spinel MnOx polymorphs undergo slow phase transitions driven by Mn(II) lattice dissolution to form layered birnessite structures. The results reveal another unusual feature of manganese compounds specifically selected by nature for important biogeochemical functions. Birnessite, the closest natural analogue of Mn4CaO5 in the photosystem-II cluster, is also an important model compound for the development of biomimetic electrocatalysts for water oxidation reactions. Our second work reports the mechanism of formation of key Mn(III) intermediates achieved by studying the effect of several electrolyte anions and cations on the catalytic efficiency of birnessite. In situ Spectro-electrochemical measurements indicate that the activity is controlled by a dynamic solution-oxidation process in which Mn(III) is formed by oxidation of unstable uncomplexed Mn(II) in a manner similar to that of reversible shuttle between birnessite lattice and electrolyte: Photoactivation in Photosystem II. The role of electrolyte cations with different ionic radii and hydration strengths is to control the interlayer spacing, while electrolyte anions control the degree of deprotonation of complexed Mn(II) in the lattice. Both in turn control the shuttling efficiency of uncomplexed Mn(II) and its subsequent electrooxidation to Mn(III). The result of our third study shows that the high oxidation activity of the various birnessites is linked to their high electron affinity, which in turn is modulated by various aqueous anions and cations through their dynamic influence on the dissolution-controlled structure Mn(III)/Mn(II). Inorganic cations with different ionic radii and hydration strengths control the interlayer spacing of the base layer, while inorganic and organic aqueous anions control the degree of Mn(II) and Mn(III) complexation. Their combined effect is thought to control the efficiency of manganese dissolution from the crystal, thereby modulating the activity and band structure of birnessite. Finally, we evaluated the nature of ROS generated by photocatalytic excitation of various Fe and Mn oxides/hydroxides compared to photoactive anatase (TiO2) and clay mineral. Results show that Fe and Mn oxides have contrasting properties. Fe minerals, especially Fe(II)Fe(III) oxyhydroxide (green rust), have the highest activity for H2O2 generation but only a low to negligible activity for OH*, while Mn minerals (triclinic birnessite) have the highest activity of OH* and a low to moderate activity for H2O2. The highest concentrations of ROS seen with these minerals are nearly 5 to 7 times higher than that seen with TiO2, which is due to their unique electronic band alignments of birnessite and green rust minerals with respect to other common semiconductors.
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