Mn3O4/Vulcan XC-72R composites: preparation, physicochemical characterization and electrochemical study for purification of organic contaminants

The microwave-assisted technique was applied to the preparation of composites from manganese(II) acetate and Carbon-supported (Vulcan XC-72R). Composites were prepared by chemical deposition of manganese(II) acetate nanoparticles onto Vulcan XC-72R carbon black (catalytic mass) using microwave irradiation. The new composites were physicochemically characterized by Energy-dispersive X-ray spectroscopy (EDX) with scanning electron microscopy (SEM), BET surface area, pore size distribution and X-ray diffraction (XRD). BET analysis of Mn3O4/Vulcan XC-72R composites obtained by alkali method has shown specific surface area of 1135 m2/g. Novel composites were electrochemically studied as catalysts for the remediation of organic contaminants in industrial waters. From novel catalytic mass, double-sided coated electrodes on a Nickel mesh conductor were made by pressing and heating at 250 °C. The prepared electrodes have geometric area of 2.5 cm2. The electrodes were fabricated and studied regarding the amount of catalyst and 35 % Teflonized Vulcan XC-72R. Electrochemical studies were carried out for phenol oxidation in an aqueous solution in model electrolyte: phenol solution 200 mg.L-1 and supporting electrolyte: 18 g.L-1 NaCl and 2.6 mg.L-1 Na2SO4.


Introduction
People's changing living standards and growing industry have an increasing impact on water resources due to the release of emerging pollutants into aquatic habitats [1].
These emerging pollutants are found in pesticides, pharmaceuticals, phenols, surfactants, and many others and organics are common with trace concentrations (µg L −1 ) [1].Emerging pollutants are toxic to animals and humans and are complex structural compounds.Finding ways to dispose of them in contaminated water is of significant importance.
The studied advanced technologies such as adsorption, ozonation, plasma, membrane bioreactor, UV-based treatment and many others have been used to remediate emerging pollutants from polluted water.These technologies have their drawbacks related to high treatment costs in the ozonation process, membrane fouling in the case of membrane bioreactor treatment, high cost of activated carbon for adsorption, need for skilled labor to operate plasma reactors and high sludge generation in the case of chemical coagulation leads to additional costs for their operation [2].Following these drawbacks, electrochemical technologies and microbial electrochemical technologies, such as electrochemical oxidation, microbial fuel cells, and microbial electrolysis cells have great attention of researchers due to their various advantages compared to their traditional counterparts.
Water pollution is one of the most serious environmental problems.Fresh water represents only 2.5% of the planet's water and its limited availability threatens water security, potentially affecting 80% of the earth's population [3].
Due to the poor management of industrial wastewater, there are increases in emerging pollutants such as organic pollutants, pesticides and pharmaceuticals.Phenol-containing substances are watersoluble organic compounds, which is why they are widely distributed, with approximately three million tons identified in the environment worldwide [4].Phenolic wastewater comes mainly from the production of phenolic resins, the production of pharmaceuticals, petrochemicals, pulp and paper, and herbicides [4,5].Phenolic compounds are extremely harmful to humans due to their bio-sustainability, mutagenicity and acute toxicity [4,6].
Various chemical, physical and biological processes for cleaning organic pollutants from water, such as coagulation-flocculation, adsorption, ion exchange, membrane electrodialysis, photocatalytic, filtration and various types of extraction, have been studied and applied.These studied methods are not cost-effective and are not able to completely remove phenolic compounds [4,[7][8][9][10][11].
A promising alternative for water treatment are oxidative processes due to their potential for mineralization of organic pollutants [12].They generate radical species, such as the hydroxyl radical ( • OH) and the sulfate radical (SO4 •− ) [13].The SO4 •− radical has a higher reduction potential (E0 = 2.5-3.1 V vs • OH radical with E0 = 1.8-2.7 V), has a better half-life, better reaction stoichiometric efficiency of • OH, and can be used over a wider pH range [14,15].
The aim of the present work is Mn nanoparticles and Vulcan XC-72 electrocatalyst synthesis using microwave-assisted irradiation technique and physicochemical characterization for application in electrochemical technologies and microbial electrochemical technologies at organic pollutants remediation in particular phenol oxidation by electrolysis and microbial electrolysis cell.

Synthesis of electrocatalysts
The synthesis of electrocatalysts was performed using microwave irradiation.Vulcan XC72 1 g, manganese(II) acetate 500 mg and polypyrrole (Ppy) 200 mg were mixed with 40 ml DI water by ultrasound 180 min.The reaction was carried out under microwave irradiation in an open vessel (170 W) at a hold temperature of 130 o C read by an infrared thermometer.The reaction was finished for 210 min.The reaction product is rinsed 3 times with DI water, collected by simple filtration and dried at 100 o C in a vacuum dryer.

XRD analysis
Depending on the synthesis methodology, the obtained Mn3O4/Vulcan XC-72R nanocomposites showed different degrees of crystallinity (figure 2).An amorphous halo of Vulcan XC-72R and a phase in the initial stage of crystallization was observed in the X-ray diffraction pattern of sample ID-20.The presence of a crystalline phase in the structure is evidenced by the appearance of low-intensity peaks at 36.09, 32.34, 28.90, 58.52, 59.88 2θ corresponding to the symmetry planes (211), ( 103), ( 112), ( 321) and ( 224) of the tetragonal structure of Mn3O4.
Upon changing the synthesis conditions of sample ID-40 (alkaline medium), the main phase is crystalline and the amorphous halo almost disappears.The recorded peaks in the X-ray diffraction pattern are fully consistent with tetragonal Mn3O4.The increase in the intensity of the main peaks relative to ID20 indicates an increase in the size of the crystallites: from 4 nm (ID-20) to 15 nm (ID-40).When the synthesis was carried out in an acidic medium, the material was completely amorphous (ID-41).

BET Surface area and pore size analysis
Through nitrogen gas physisorption, the adsorption and desorption isotherms of volcano-manganese polypyrrole samples synthesized under different conditions (figure 3) were measured, marked as ID-20, ID-40, ID-41.The complete isotherms are presented in figure 3.
The measured sorption isotherms are of type IV(a), according to the IUPAC classification, with a well-pronounced hysteresis of H2(b) type.Isotherms of this type are characteristic of mesoporous adsorbents, and the hysteresis loop is associated with the appearance of capillary condensation of nitrogen in the pores.At high p/p0, limited absorption is observed, indicating pore filling.
The structural characteristics were determined from the adsorption isotherms, which are summarized in table 1 The BET method is used to determine the surface area.Multipoint BET is determined at comparative pressure within the range p/p0=0.1 -0.3.The pore volume is accounted for by a comparative pressure measuring near to 1 (p/p0=0.99The average pore diameter is determined supposing that the pores have a cylindrical geometry at p/p0=0.99.The Pore Diameter was calculated by the BJH method (Barrett, Joyner, Halenda), a desorption branch.The measured structural characteristics of the studied samples show high surface areas, well-developed pore volume /Pore Volume/ and close average pore size /Average Pore Diameter/.Samples ID-20 and ID-41 have similar surface areas and pore volumes.While on ID-40 the specific surface is the highest is 1128 m 2 /g, correspondingly the most developed pore volume is 1.388 cm3 /g.On sample ID-40, the measurement of the specific surface by the BET method was repeated -1135 m 2 /g.Pore size distribution is the ratio of pore volume to pore size.Having that in mind, the distribution of pores by diameter is calculated from the desorption branch of the isotherms in agreement with the Barrett, Joyner, Halenda method (BJH method, Desorption).The differential pore diameter distribution curves of ID-20, ID-40, and ID-41 are given in figure 4. The type of distribution curves is similar and for the three samples mainly mesopores in the range from 3 nm to 28 nm are observed, with the prevailing sizes (peaks) for the respective samples being: for ID-20 it is at 9.4 nm, for ID-41 it is at 7.7 nm, and for ID-40 the peak is shifted to smaller pore diameters -3.4 nm (see table 1).The research was carried out on a NOVA touch-Quantachrome instrument (USA), which measures adsorbed or desorbed volumes of gas at a relative pressure of less than one.The obtained data after computer processing are presented as adsorption and/or desorption isotherms, from which the specific surface area, pore volume, pore size and pore size distribution of solid and powder samples are calculated.

SEM analysis
The micrographs from the SEM analysis shown in figure 5

Electrochemical study
The electrodes studied have geometrical areas of 2.5 cm 2 .The electrodes were prepared from a mixture of the catalyst mass (constant for all electrodes -40 mg/cm 2 ) and teflonized carbon Vulcan XC-72R (35% Teflon) as a binder content of 50 mg/cm 2 .
The studies investigate at constant current density -6 mA.cm -2 in three-electrode cell.The volume of the solution in the electrolysis cell is 150 ml.
The working electrode is a geometrical area 2.5 mA/cm 2 Mn3O4/Vulcan XC-72R nanocomposite electrode preparation from ID-40.The counter electrode is the gas diffusion electrode (GDE) [22].Phenol solution 200 mg.L -1 and supporting electrolyte: 0.5 M NaCl and 0.2 M Na2SO4.Anode potential after 3 hours is changed from 580 mV versus RHE at PCV to 1500 mV versus RHE.Model solutions of phenol-contaminated waters were prepared by dissolving phenol (p.a.) in distilled water with 18 g.l - NaCl and 2.6 mg.L -1 Na2SO4 used as supporting electrolytes.Phenol concentration was determined by measuring the absorbance of the sample (triplicated and averaged) on UV/Vis spectrophotometer (VWR ® UV/Vis -1600 PC Spectrophotometer, USA) at λ = 270 nm and calculated based on calibration the curve prepared in advance.Analytical studies showed a 35% reduction of phenol in the tested solution (figure 6).

Conclusion
New Mn3O4/Vulcan XC-72R nanocomposites have been successfully developed to improve the activity of catalysts for various electrochemical applications such as purification of organic pollutants and electrolysis of seawater.
The new nanocomposite synthesis technology by alkaline method combined with microwave irradiation resulted in a very high specific surface area of 1135 m 2 /g, shown by BET analysis.
Here, we reported the microwave synthesis, physicochemical characterization, and electrochemical investigation of novel Mn3O4/Vulcan XC72R nanocomposites as a promising candidate for the purification of phenolic pollutants.

Figure 6 .
Figure 6.Chemical analysis showed a decrease in the concentration of phenol for 3 h.