BINARY AND DOPED MANGANESE OXIDES: SYNTHESIS AND THEIR APPLICATIONS IN ENVIRONMENTAL REMEDIATION

Abstract

The role of transition metal oxides is well known in a range of applications and the study of their structure, and properties in various forms has increased over the years. Transition metals exist as minerals in the soil as oxides, sulphides, hydroxides, oxyhydroxides, or hydrated oxides. Metal oxides in the form of nanostructures or engineered nanomaterials are widely used as catalysts, photocatalysts, adsorbents, semiconductors, gas sensors, or medicines. Oxides of transition metals, such as Mn, Fe, Co, Ni, Cu, Mg, Ce, etc. have been synthesized with modified features, i.e., surface area, morphology, size, and shape, which endow them with enhanced chemical activity. These oxides work efficiently as a catalyst in many reactions, such as degradation of pollutants, oxidation of metals, and synthesis of numerous organic compounds. Sometimes, to tune up the activity of a catalyst, metal oxides are doped with metal ions or form a composite with carbon nanotubes, graphene, graphitic carbon nitrides (g-C4N4), etc. It often helps overcome their limitation and achieve desired activities for potential practical applications. The oxides of manganese are one of the important binary oxides that are easy to employ in various fields due to its less toxicity and natural abundance. Manganese form oxides in many oxidation states, such as MnO2 (IV), Mn2O3 (III), Mn3O4 (II and III) etc., which have been used in different applications including environmental remediation. More than 30 different crystal structures of manganese oxides generated by various arrangements of MnO6 octahedra have been found, and these crystal structures are responsible for the different activities of manganese oxides. The preparation method that is usually employed for the synthesis of manganese oxides is the thermal decomposition of MnCO3, which is initially prepared by a hydrothermal method. Nowadays, organic and inorganic pollutants have become a major concern for humankind as well as other living organisms due to their increased presence in the environment. As a solution to alleviate the environmental problems, various methods have been employed for its remediation, such as filtration, ion-exchange, reverse osmosis, adsorption, electrolysis, catalysis, and photocatalysis. The last two methods are best suited for the degradation of different pollutants due to their low cost and simplicity. There are numerous reports that manganese oxides are able to remove or degrade pollutants by catalysis or photocatalysis from aqueous, gaseous, or even solid medium. For example, MnO2 is a typical binary oxide of ii manganese, which is a strong oxidant and can oxidize or degrade organic and inorganic contaminants. Moreover, the activity of MnO2 may also vary depending on the size and microstructure of the oxide. Therefore, the synthesis of manganese oxides has been studied extensively by varying different parameters to achieve different kinds of manganese oxides with varied size and microstructure to enhance their activity. Mostly, the hydrothermal synthesis method has been employed to introduce new micro- / nanostructures in them. The hydrothermal method has allowed the formation and morphology modification of manganese oxides along with changing surface areas and varied particle sizes. It has been found that the catalytic and photocatalytic activity can be tuned by changing the morphology of manganese oxides. Many variations have been introduced in the morphology of metal oxides, which has shown significant changes in the rates of activity toward pollutant degradation or removal. On the other hand, doping of metal ions (e. g., Fe, Cu) or combining it with other metal oxides/graphene/graphitic carbon nitrides has also increased the efficiency of manganese oxides in many applications. MnO2, Mn2O3, and Mn3O4 with different morphologies have been used in degradation/adsorption of toxic metal ions, organic dyes, or drugs. It has been found that MnO2 is highly active in the oxidation of toxic gases, such as CO, NOx, VOCs, and sometimes supported/composite manganese oxides have shown enhanced catalytic activity. Apart from this, MnO2 is also used in the degradation of drugs (such as Triclosan, Chlorophene, Quinoline N-oxides, etc.), endocrine disruptors (Bisphenol A, Ethinylestradiol, Estradiol, etc.), phenols and aldehydes, organic dyes (Methylene blue, Rhodamine B), or in the adsorptive removal of heavy metals (e.g., Cd, Hg, Pb, As etc.). Mn2O3 and Mn3O4 have also been used in the degradation of phenols and dyes, water oxidation, CO oxidation, by either a catalytic or a photocatalytic process. It has been realized that manganese oxides constitute an important class of metal oxides that can be prepared with an easy synthesis method and employed in a range of applications. The properties of manganese oxides are easy to tune and can be introduced by the changing reaction parameters such as precursor salt, temperature, and reaction time. In the present work, attempts are being made to develop easy and template free hydro-/solvothermal methods for the synthesis of binary and doped manganese oxides with different morphologies, which have been applied in the degradation of pollutants in the aqueous system via catalysis or photocatalysis. The details of these investigations are presented in subsequent chapters. The introductory iii chapter (Chapter-I) gives a brief overview of the synthesis, properties, and applications of some simple binary oxides with special emphasis on various types of manganese oxides. Chapter-II contains the details of the characterization techniques used in this study. For the synthesis of simple binary and doped manganese oxides, the hydrothermal method is employed. The phase formation of the binary and doped oxides is studied by powder X-ray diffraction (P-XRD). Field-Emission Scanning Electron Microscopy (FE-SEM) and Energy Dispersive X-Ray Spectroscopy (EDS) are employed for the morphology and compositional studies, respectively. A BET surface analyzer is used to study the sorption isotherms, analyze the surface area, and pore size distribution of the oxides. Catalytic/photocatalytic activities are studied by way of degradation of organic dyes and endocrine disruptors. To monitor the progress of the degradation reaction, the solution aliquots were analyzed by UV-Vis spectroscopy. To monitor the reaction course, samples are analyzed by HPLC, and reaction intermediates are identified by GCMS and LCMS. The photocatalytic reactions are performed under sunlight-irradiation by following standard procedures. UV-Vis Diffuse Reflectance Spectroscopy (UV-Vis DRS) is used for band gap evaluation. To ascertain the efficiency of removal/degradation, estimation of Chemical Oxygen Demand (COD) is carried out. The details of instrumentation used and procedures followed are described in this chapter. Chapter-III presents the synthesis of manganese trioxide with various morphologies and their applications in catalytic dye degradation. The hierarchical manganese trioxides (Mn2O3) are prepared by thermal treatment of carbonate precursors synthesized using a facile and template-free hydro-solvothermal method. Various morphologies of Mn2O3 have evolved as a function of precursor salts keeping the solvent system and temperature of synthesis intact. On hydro-solvothermal treatment, manganese carbonate with different microstructures is formed first, and on subsequent calcination at 500C, the carbonates are converted into manganese trioxide with the retention of the initial microstructure. While the formation of Mn2O3 microcubes is achieved with MnCl2 and Mn(NO3)2 as precursor salts, microspheres, and microflakes are obtained with Mn(OAc)2 and MnC2O4 precursors, respectively. The average microcrystal size varied from ~ 10 – 20 (±5) μm with somewhat high surface areas. The study of morphology evolution by varying the reaction temperature and reaction time through FE-SEM imaging revealed that the cube-shaped morphology is affected by temperature. At the same time, the sphere-shaped Mn2O3 remains unaffected by the reaction conditions. The COD iv removal efficiency showed near-complete mineralization of RhB and MB dyes at room temperature in the dark, while in the case of MO, rapid fragmentation of the dye took place. Complete degradation and decolorization time varies with the type of dyes. While RhB and MB degrade within 45-50 and 60 min, respectively, MO decolorizes within 2 min with both cubes- and sphere-shaped Mn2O3. The activity of the catalysts seems to be independent of the morphology of the Mn2O3 reported here and presumably supports for a catalytic process instead of adsorptive removal, also supported by XPS studies. Both the catalyst has shown good stability towards dye degradation over multiple catalytic cycles. The dye degradation is sensitive to the pH of the medium, and it works under the acidic conditions only. The ability of the catalysts to degrade dye pollutants in the dark in the absence of any chemical additives will enable the operation of waste treatment plants throughout the day without incurring additional energy or added chemical cost. Chapter-IV deals with the synthesis of nanostructured manganese dioxide and its application in oxidative removal of Bisphenol A (BPA). Nanostructured MnO2 is synthesized by the hydrothermal method. Synthesis of both the - and -MnO2 is achieved by merely varying the reaction temperature of the hydrothermal synthesis. While -MnO2 is evolved as spherical shaped and spiked particles, -MnO2 is formed with the nanorod morphology. The BET surface area for -MnO2 (44.24 m2/g) is nearly four times larger than that of -MnO2 (11.37 m2/g). Both the catalysts are tested for catalytic degradation of BPA in the aqueous medium at natural pH of BPA, in the absence of light and any additives. It is found that 100 mg of -MnO2 is capable of degrading 100 mL of 0.12 mM BPA within 90 minutes, comparatively higher than that of -MnO2, 100 g of which is able to degrade the same amount of 0.04 mM BPA in the same reaction time. The rate of degradation is evaluated as a function of initial doses of catalyst, BPA concentration, temperatures, and pH. To identify the intermediate species formed during degradation, sample aliquots are collected at regular intervals and analyzed by HPLC. The intermediates are identified by GCMS analysis, and a reaction pathway is devised. Complete mineralization of the BPA at the end of the reaction is confirmed by the COD results. The differences in the activity of - and -MnO2 are explained based on the crystal structure, morphology, surface area, and chemical composition. It appears that the high surface area and high oxygen content of -MnO2 are responsible for the high rate of BPA degradation and mineralization. v Synthesis of iron-doped nanostructured manganese dioxides, Mn1-xFexO2 (x = 0.1, 0.2), and their applications in photocatalytic removal of carbamazepine (CBZ) is described in this chapter (Chapter-V). The synthesis of Mn1-xFexO2 (x = 0.1, 0.2) is carried out by one-step hydrothermal method. The oxides form with the -MnO2 structure for both the compositions under the same synthetic condition. While the FE-SEM results indicate the formation of uniform sized microspheres with ~1-5 m particle size, the EDS analysis confirmed the incorporation of desired amounts of iron into the -MnO2 structure. The surface area of the oxides (12.3 and 15.9 m2/g for Mn0.9Fe0.1O2 and Mn0.8Fe0.2O2, respectively) measured by BET analysis do not vary significantly with the extent of iron doping, although a slight increase is observed in the sample with higher iron content. UV-vis DRS data showed a broad absorbance in the visible region and band gaps of 1.93 and 1.96 eV for Mn0.9Fe0.1O2 and Mn0.8Fe0.2O2, respectively, are estimated using Tauc plots and considering them as indirect band gap semiconductors. The zeta potential indicates the isoelectric point of iron-doped manganese dioxides at around ~ pH 5.2. The iron-doped catalysts are employed in the photocatalytic degradation of CBZ (at its natural pH of 6.2), and the complete removal of CBZ is observed in 3 h. The effect of pH on the photocatalytic degradation of CBZ is investigated, and pH 5 is found to favor the degradation, while a higher and lower pH than 5 slows down the photocatalytic degradation. Superoxide radical (O2) and holes (h+) are identified as the dominant reactive species in the photocatalytic degradation of CBZ, while no role is played by the OH radical. The COD results support complete mineralization of CBZ, and post-catalytic P-XRD analysis confirms the catalyst stability and usability up to four tested cycles. Further, by HPLC and LCMS analysis, intermediates are identified, and a probable mechanistic pathway is proposed. Chapter-VI gives overall conclusion and future scope of the present work. The current work makes an effort to understand the effect of precursor salt and reaction temperature, two simple variables in hydro-/solvothermal method, as a guide to develop varied morphologies of binary and doped manganese oxides and study their applications in environmental remediation. In addition to various colored organic dye pollutants (such as MO, MB, and RhB), catalytic and photocatalytic degradation studies are conducted with the colorless environmental pollutants such as bisphenol A and carbamazepine. The study enabled the formation of Mn2O3 with four different morphologies by a mere change of precursor salts, while - and -MnO2 with two different morphologies are made by changing the reaction temperature. Sphere and cube-shaped vi Mn2O3 have shown catalytic degradation of MO, MB, and RhB, while - and -MnO2 have shown efficient oxidative degradation of BPA at natural pH in the aqueous medium. The Fe-doped -MnO2 has shown efficient photocatalytic degradation of CBZ in the aqueous medium. The BPA and CBZ degradation mechanisms are elucidated by GCMS, and LCMS analysis and plausible degradation mechanisms are proposed. The binary and doped manganese oxides reported in this work are found as good catalysts for catalytic and photocatalytic degradation of pollutants in the aqueous medium. The work also opens up new possibilities for future investigations. The manganese oxides may well be tested for the degradation of other colored and colorless pollutants. Many other structural types of simple binary manganese oxides and the effect of doping other transition elements in them may also be investigated. One of the synthesized MnO2 showing spiked nanospheres may be investigated for its antiviral, antifungal, and antibacterial properties. The work may also encourage the development of several other simple binary transition metal oxides with varied morphology, size, and shape as a function of metal precursors, reaction temperatures and reaction time, etc. and their catalytic activities toward many other environmental pollutants.