Redox properties of birnessite from a defect perspective.

Birnessite, a layered-structure MnO2, is an earth-abundant functional material with potential for various energy and environmental applications, such as water oxidation. An important feature of birnessite is the existence of Mn(III) within the MnO2 layers, accompanied by interlayer charge-neutralizing cations. Using first-principles calculations, we reveal the nature of Mn(III) in birnessite with the concept of the small polaron, a special kind of point defect. Further taking into account the effect of the spatial distribution of Mn(III), we propose a theoretical model to explain the structure-performance dependence of birnessite as an oxygen evolution catalyst. We find an internal potential step which leads to the easy switching of the oxidation state between Mn(III) and Mn(IV) that is critical for enhancing the catalytic activity of birnessite. Finally, we conduct a series of comparative experiments which support our model.

Antimicrobial Properties of 2D MnO2 and MoS2 Nanomaterials Vertically Aligned on Graphene Materials and Ti3C2 MXene.

Two-dimensional (2D) nanomaterials have attracted considerable attention in biomedical and environmental applications due to their antimicrobial activity. In the interest of investigating the primary antimicrobial mode-of-action of 2D nanomaterials, we studied the antimicrobial properties of MnO2 and MoS2, toward Gram-positive and Gram-negative bacteria. Bacillus subtilis and Escherichia coli bacteria were treated individually with 100 µg/mL of randomly oriented and vertically aligned nanomaterials for ∼3 h in the dark. The vertically aligned 2D MnO2 and MoS2 were grown on 2D sheets of graphene oxide, reduced graphene oxide, and Ti3C2 MXene. Measurements to determine the viability of bacteria in the presence of the 2D nanomaterials performed by using two complementary techniques, flow cytometry, and fluorescence imaging showed that, while MnO2 and MoS2 nanosheets show different antibacterial activities, in both cases, Gram-positive bacteria show a higher loss in membrane integrity. Scanning electron microscopy images suggest that the 2D nanomaterials, which have a detrimental effect on bacteria viability, compromise the cell wall, leading to significant morphological changes. We propose that the peptidoglycan mesh (PM) in the bacterial wall is likely the primary target of the 2D nanomaterials. Vertically aligned 2D MnO2 nanosheets showed the highest antimicrobial activity, suggesting that the edges of the nanosheets were likely compromising the cell walls upon contact.

Systematic Doping of Cobalt into Layered Manganese Oxide Sheets Substantially Enhances Water Oxidation Catalysis.

The effect on the electrocatalytic oxygen evolution reaction (OER) of cobalt incorporation into the metal oxide sheets of the layered manganese oxide birnessite was investigated. Birnessite and cobalt-doped birnessite were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and conductivity measurements. A cobalt:manganese ratio of 1:2 resulted in the most active catalyst for the OER. In particular, the overpotential (η) for the OER was 420 mV, significantly lower than the η = 780 mV associated with birnessite in the absence of Co. Furthermore, the Tafel slope for Co/birnessite was 81 mV/dec, in comparison to a Tafel slope of greater than 200 mV/dec for birnessite. For chemical water oxidation catalysis, an 8-fold turnover number (TON) was achieved (h = 70 mmol of O2/mol of metal). Density functional theory (DFT) calculations predict that cobalt modification of birnessite resulted in a raising of the valence band edge and occupation of that edge by holes with enhanced mobility during catalysis. Inclusion of extra cobalt beyond the ideal 1:2 ratio was detrimental to catalysis due to disruption of the layered structure of the birnessite phase.

Effect of Interlayer Spacing on the Activity of Layered Manganese Oxide Bilayer Catalysts for the Oxygen Evolution Reaction.

We investigated the dependence of the electrocatalytic activity for the oxygen evolution reaction (OER) on the interlayer distance of five compositionally distinct layered manganese oxide nanostructures. Each individual electrocatalyst was assembled with a different alkali metal intercalated between two nanosheets (NS) of manganese oxide to form a bilayer structure. Manganese oxide NS were synthesized via the exfoliation of a layered material, birnessite. Atomic force microscopy was used to determine the heights of the bilayer catalysts. The interlayer spacing of the supported bilayers positively correlates with the size of the alkali cation: NS/Cs+/NS > NS/Rb+/NS > NS/K+/NS > NS/Na+/NS > NS/Li+/NS. The thermodynamic origins of these bilayer heights were investigated using molecular dynamics simulations. The overpotential (η) for the OER correlates with the interlayer spacing; NS/Cs+/NS has the lowest η (0.45 V), while NS/Li+/NS exhibits the highest η (0.68 V) for OER at a current density of 1 mA/cm2. Kinetic parameters (η and Tafel slope) associated with NS/Cs+/NS for the OER were superior to that of the bulk birnessite phase, highlighting the structural uniqueness of these nanoscale assemblies.

Oxidation of arsenite to arsenate on birnessite in the presence of light.

The effect of simulated solar radiation on the oxidation of arsenite [As(III)] to arsenate [As(V)] on the layered manganese oxide, birnessite, was investigated. Experiments were conducted where birnessite suspensions, under both anoxic and oxic conditions, were irradiated with simulated solar radiation in the presence of As(III) at pH 5, 7, and 9. X-ray absorption spectroscopy (XAS) was used to determine the nature of the adsorbed product on the surface of the birnessite. The oxidation of As(III) in the presence of birnessite under simulated solar light irradiation occurred at a rate that was faster than in the absence of light at pH 5. At pH 7 and 9, As(V) production was significantly less than at pH 5 and the amount of As(V) production for a given reaction time was the same under dark and light conditions. The first order rate constant (kobs) for As(III) oxidation in the presence of light and in the dark at pH 5 were determined to be 0.07 and 0.04 h-1, respectively. The As(V) product was released into solution along with Mn(II), with the latter product resulting from the reduction of Mn(IV) and/or Mn(III) during the As(III) oxidation process. Post-reaction XAS analysis of As(III) exposed birnessite showed that arsenic was present on the surface as As(V). Experimental results also showed no evidence that reactive oxygen species played a role in the As(III) oxidation process.

Nickel Confined in the Interlayer Region of Birnessite: an Active Electrocatalyst for Water Oxidation.

We report a synthetic method to enhance the electrocatalytic activity of birnessite for the oxygen evolution reaction (OER) by intercalating Ni(2+) ions into the interlayer region. Electrocatalytic studies showed that nickel (7.7 atomic %)-intercalated birnessite exhibits an overpotential (η) of 400 mV for OER at an anodic current of 10 mA cm(-2) . This η is significantly lower than the η values for birnessite (η≈700 mV) and the active OER catalyst ß-Ni(OH)2 (η≈550 mV). Molecular dynamics simulations suggest that a competition among the interactions between the nickel cation, water, and birnessite promote redox chemistry in the spatially confined interlayer region.

Copper-Intercalated Birnessite as a Water Oxidation Catalyst.

We report a synthetic method to increase the catalytic activity of birnessite toward water oxidation by intercalating copper in the interlayer region of the layered manganese oxide. Intercalation of copper, verified by XRD, XPS, ICP, and Raman spectroscopy, was accomplished by exposing a suspension of birnessite to a Cu(+)-bearing precursor molecule that underwent disproportionation in solution to yield Cu(0) and Cu(2+). Electrocatalytic studies showed that the Cu-modified birnessite exhibited an overpotential for water oxidation of ∼490 mV (at 10 mA/cm(2)) and a Tafel slope of 126 mV/decade compared to ∼700 mV (at 10 mA/cm(2)) and 240 mV/decade, respectively, for birnessite without copper. Impedance spectroscopy results suggested that the charge transfer resistivity of the Cu-modified sample was significantly lower than Cu-free birnessite, suggesting that Cu in the interlayer increased the conductivity of birnessite leading to an enhancement of water oxidation kinetics. Density functional theory calculations show that the intercalation of Cu(0) into a layered MnO2 model structure led to a change of the electronic properties of the material from a semiconductor to a metallic-like structure. This conclusion from computation is in general agreement with the aforementioned impedance spectroscopy results. X-ray photoelectron spectroscopy (XPS) showed that Cu(0) coexisted with Cu(2+) in the prepared Cu-modified birnessite. Control experiments using birnessite that was decorated with only Cu(2+) showed a reduction in water oxidation kinetics, further emphasizing the importance of Cu(0) for the increased activity of birnessite. The introduction of Cu(0) into the birnessite structure also increased the stability of the electrocatalyst. At a working current of 2 mA, the Cu-modified birnessite took ∼3 times longer for the overpotential for water oxdiation to increase by 100 mV compared to when Cu was not present in the birnessite.

Coupled redox transformation of chromate and arsenite on ferrihydrite.

The redox chemistry of chromate (Cr(VI)) and arsenite (As(III)) on the iron oxyhydroxide, ferrihydrite (Fh), was investigated. Attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), X-ray absorption spectroscopy (XAS), and X-ray photoelectron spectroscopy (XPS) were used to determine the composition of the adsorbed layer on Fh during and after exposure to solution-phase Cr(VI) and As(III). The individual exposure of Cr(VI) or As(III) on Fh resulted in the adsorption of the respective species, and there was no change in the oxidation state of either species. In contrast, exposure of Fh simultaneously to Cr(VI) and As(III) led to an adsorbed layer that was primarily Cr(III) and As(V). This redox transformation occurred over various experimental conditions at pH 3, 5, and 7 and in the presence or absence of O2, as demonstrated by in situ ATR-FTIR results. A similar redox transformation was not observed at a solution of pH 9, due to minimal Cr(VI) adsorption. Postreaction XPS showed that the majority of adsorbed arsenic existed as As(V) at pH 3, 5, and 7, while As(III) was the main species detected at pH 9. At pH 3 the redox chemistry between Cr(VI) and As(III) led to a As(V) product surface loading of ∼600 mmol/kg. Experiments performed in the absence of dissolved O2 resulted in less As(V) on the surface compared to experiments in which O2 was present for equivalent reaction times.

Effect of Phospholipid on Pyrite Oxidation and Microbial Communities under Simulated Acid Mine Drainage (AMD) Conditions.

The effect of phospholipid on the biogeochemistry of pyrite oxidation, which leads to acid mine drainage (AMD) chemistry in the environment, was investigated. Metagenomic analyses were carried out to understand how the microbial community structure, which developed during the oxidation of pyrite-containing coal mining overburden/waste rock (OWR), was affected by the presence of adsorbed phospholipid. Using columns packed with OWR (with and without lipid adsorption), the release of sulfate (SO4(2-)) and soluble iron (FeTot) was investigated. Exposure of lipid-free OWR to flowing pH-neutral water resulted in an acidic effluent with a pH range of 2-4.5 over a 3-year period. The average concentration of FeTot and SO4(2-) in the effluent was ≥20 and ≥30 mg/L, respectively. In contrast, in packed-column experiments where OWR was first treated with phospholipid, the effluent pH remained at ∼6.5 and the average concentrations of FeTot and SO4(2-) were ≤2 and l.6 mg/L, respectively. 16S rDNA metagenomic pyrosequencing analysis of the microbial communities associated with OWR samples revealed the development of AMD-like communities dominated by acidophilic sulfide-oxidizing bacteria on untreated OWR samples, but not on refuse pretreated with phospholipid.

Reduction of nitrite and nitrate on nano-dimensioned FeS.

The reaction of nitrite (NO2(-)) and nitrate (NO3(-)) on nanometer-sized FeS particles was investigated in alkaline (initial pH = 10.3) solutions at reaction temperatures of 22, 70, and 120 °C using in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) and fluorescence spectroscopy that allowed an analysis of adsorbate complexation on the FeS and reaction product in the aqueous phase, respectively. ATR-FTIR showed that NO was a surface-bound intermediate on FeS during its exposure to NO2(-) at all three reaction temperatures. Ammonia/ammonium (NH3/NH4(+)) product was also produced when FeS was exposed to NO2(-) at the 70 °C and 120 °C reaction temperatures. Activation of NO3(-) to form surface-bound NO was experimentally observed to occur at 120 °C on FeS, but not at the lower reaction temperatures. Furthermore, NH3/NH4(+) product in the aqueous phase was only present during the reaction of FeS with NO3(-) at the highest temperature used in this study.

ATR-FTIR and density functional theory study of the structures, energetics, and vibrational spectra of phosphate adsorbed onto goethite.

Periodic plane-wave density functional theory (DFT) and molecular cluster hybrid molecular orbital-DFT (MO-DFT) calculations were performed on models of phosphate surface complexes on the (100), (010), (001), (101), and (210) surfaces of α-FeOOH (goethite). Binding energies of monodentate and bidentate HPO(4)(2-) surface complexes were compared to H(2)PO(4)(-) outer-sphere complexes. Both the average potential energies from DFT molecular dynamics (DFT-MD) simulations and energy minimizations were used to estimate adsorption energies for each configuration. Molecular clusters were extracted from the energy-minimized structures of the periodic systems and subjected to energy reminimization and frequency analysis with MO-DFT. The modeled P-O and P---Fe distances were consistent with EXAFS data for the arsenate oxyanion that is an analog of phosphate, and the interatomic distances predicted by the clusters were similar to those of the periodic models. Calculated vibrational frequencies from these clusters were then correlated with observed infrared bands. Configurations that resulted in favorable adsorption energies were also found to produce theoretical vibrational frequencies that correlated well with experiment. The relative stability of monodentate versus bidentate configurations was a function of the goethite surface under consideration. Overall, our results show that phosphate adsorption onto goethite occurs as a variety of surface complexes depending on the habit of the mineral (i.e., surfaces present) and solution pH. Previous IR spectroscopic studies may have been difficult to interpret because the observed spectra averaged the structural properties of three or more configurations on any given sample with multiple surfaces.

Photoinduced oxidation of arsenite to arsenate in the presence of goethite.

The photochemistry of an aqueous suspension of goethite in the presence of arsenite (As(III)) was investigated with X-ray absorption near edge structure (XANES) spectroscopy and solution-phase analysis. Irradiation of the arsenite/goethite under conditions where dissolved oxygen was present in solution led to the presence of arsenate (As(V)) product adsorbed on goethite and in solution. Under anoxic conditions (absence of dissolved oxygen), As(III) oxidation occurred, but the As(V) product was largely restricted to the goethite surface. In this circumstance, however, there was a significant amount of ferrous iron release, in stark contrast to the As(III) oxidation reaction in the presence of dissolved oxygen. Results suggested that in the oxic environment ferrous iron, which formed via the photoinduced oxidation of As(III) in the presence of goethite, was heterogeneously oxidized to ferric iron by dissolved oxygen. It is likely that aqueous reactive oxygen species formed during this process led to the further oxidation of As(III) in solution. Results from the current study for As(III)/goethite also were compared to results from a prior study of the photochemistry of As(III) in the presence of another iron oxyhydroxide, ferrihydrite. The comparison showed that at pH 5 and 2 h of light exposure the instantaneous rate of aqueous-phase As(V) formation in the presence of goethite (12.4 × 10(-5) M s(-1) m(-2)) was significantly faster than in the presence of ferrihydrite (6.73 × 10(-6) M s(-1) m(-2)). It was proposed that this increased rate of ferrous iron oxidation in the presence of goethite and dissolved oxygen was the primary reason for the higher As(III) oxidation rate when compared to the As(III)/ferrihydrite system. The surface area-normalized pseudo-first-order rate constant, for example, associated with the heterogeneous oxidation of Fe(II) by dissolved oxygen in the presence of goethite (1.9 × 10(-6) L s(-1) m(-2)) was experimentally determined to be considerably higher than if ferrihydrite was present (2.0 × 10(-7) L s(-1) m(-2)) at a solution pH of 5.

Reduction of nitrite and nitrate to ammonium on pyrite.

An important constraint on the formation of the building blocks of life in the Hadean is the availability of small, activated compounds such as ammonia (NH(3)) relative to its inert dinitrogen source. Iron-sulfur particles and/or mineral surfaces have been implicated to provide the catalytic active sites for the reduction of dinitrogen. Here we provide a combined kinetic, spectroscopic, and computational modeling study for an alternative source of ammonia from water soluble nitrogen oxide ions. The adsorption of aqueous nitrite (NO(2)(-)) and nitrate (NO(3)(-)) on pyrite (FeS(2)) and subsequent reduction chemistry to ammonia was investigated at 22°C, 70°C, and 120°C. Batch geochemical and in situ Attenuated Total Reflection - Fourier Transform Infrared (ATR-FTIR) spectroscopy experiments were used to determine the reduction kinetics to NH(3) and to elucidate the identity of the surface complexes, respectively, during the reaction chemistry of NO(2)(-) and NO(3)(-). Density functional theory (DFT) calculations aided the interpretation of the vibrational data for a representative set of surface species. Under the experimental conditions used in this study, we detected the adsorption of nitric oxide (NO) intermediate on the pyrite surface. NH(3) production from NO(2)(-) occurred at 70 and 120°C and from NO(3)(-) occurred only at 120°C.

Implementing the donor-acceptor approach in electronically conducting copolymers via electropolymerization.

Electropolymerization has become a convenient method for synthesizing and characterizing complex organic copolymers having intrinsic electronic conductivity, including the donor (D)-acceptor (A) class of electronically conducting polymers (ECPs). This review begins with an introduction to the electrosynthesis of common second-generation ECPs. The information obtainable from electroanalytical studies, charge carriers such as polarons (positive and negative) and bipolarons (positive and negative) and doping will be discussed. The evolutionary chain of ECPs is then presented. ECPs comprising electron-rich D and electron-deficient A moieties have been shown to possess intrinsic electronic conductivity and unique optical and electronic properties. They are third generation ECPs and electropolymerization of mixtures of D and A leads to stoichiometrically controlled block copolymers. These D-A type ECPs are discussed on the basis of selected representative materials. Since the discovery of electropolymerization as a powerful tool to synthesize copolymers of conjugated monomers with a pre-determined ratio of D and A repeat units present in the polymer, the field of D-A type ECPs has grown considerably and the literature available since 2004 to 2021 is summarized and tabulated. Electronic and optical properties of the materials determined by computational chemistry are presented. The data obtained from electrochemical and optical methods are compared with those obtained from computational methods and reasons for discrepancies are given. The literature on the concept of electropolymerization extended to synthesizing triblock and many-block copolymers is reviewed. Finally, applications of D-A polymers in optoelectronic devices (organic solar cells and field-effect transistors) and in bio-imaging are explained quoting appropriate examples.

Reactivity of ferritin and the structure of ferritin-derived ferrihydrite.

BACKGROUND: In nature or in the laboratory, the roughly spherical interior of the ferritin protein is well suited for the formation and storage of a variety of nanosized metal oxy-hydroxide compounds which hold promise for a range of applications. However, the linkages between ferritin reactivity and the structure and physicochemical properties of the nanoparticle core, either native or reconstituted, remain only partly understood. SCOPE OF REVIEW: Here we review studies, including those from our laboratory, which have investigated the structure of ferritin-derived ferrihydrite and reactivity of ferritin, both native and reconstituted. Selected proposed structure models for ferrihydrite are discussed along with the structural and genetic relationships that exist among several different forms of ferrihydrite. With regard to reactivity, the review will emphasize studies that have investigated the (photo)reactivity of ferritin and ferritin-derived materials with environmentally relevant gaseous and aqueous species. MAJOR CONCLUSIONS: The inorganic core formed from apoferritin reconstituted with varied amounts of Fe has the same structural topology as the inorganically derived ferrihydrite that is an important component of many environmental and soil systems. Reactivity of ferritin toward aqueous species resulting from the photoexcitation of the inorganic core of the protein shows promise for driving redox reactions relevant to environmental chemistry. GENERAL SIGNIFICANCE: Ferritin-derived ferrihydrite is effectively maintained in a relatively unaggregated state, which improves reactivity and opens the possibility of future applications in environmental remediation. Advances in our understanding of the structure, composition, and disorder in synthetic, inorganically derived ferrihydrite are shedding new light on the reactivity and stability of ferrihydrite derived artificially from ferritin.

Photoinduced oxidation of arsenite to arsenate on ferrihydrite.

The photochemistry of an aqueous suspension of the iron oxyhydroxide, ferrihydrite, in the presence of arsenite has been investigated using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray absorption near edge structure (XANES), and solution phase analysis. Both ATR-FTIR and XANES show that the exposure of ferrihydrite to arsenite in the dark leads to no change in the As oxidation state, but the exposure of this arsenite-bearing surface, which is in contact with pH 5 water, to light leads to the conversion of the majority of the adsorbed arsenite to the As(V) bearing species, arsenate. Analysis of the solution phase shows that ferrous iron is released into solution during the oxidation of arsenite. The photochemical reaction, however, shows the characteristics of a self-terminating reaction in that there is a significant suppression of this redox chemistry before 10% of the total iron making up the ferrihydrite partitions into solution as ferrous iron. The self-terminating behavior exhibited by this photochemical arsenite/ferrihydrite system is likely due to the passivation of the ferrihydrite surface by the strongly bound arsenate product.

CO2 sequestration through mineral carbonation of iron oxyhydroxides.

Carbon dioxide sequestration via the use of sulfide reductants and mineral carbonation of the iron oxyhydroxide polymorphs lepidocrocite, goethite, and akaganeite with supercritical CO(2) (scCO(2)) was investigated using in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The exposure of the different iron oxyhydroxides to aqueous sulfide in contact with scCO(2) at ∼70-100 °C resulted in the partial transformation of the minerals to siderite (FeCO(3)) and sulfide phases such as pyrite (FeS(2)). The relative yield of siderite to iron sulfide bearing mineral product was a strong function of the initial sulfide concentration. The order of mineral reactivity with regard to the amount of siderite formation in the scCO(2)/sulfide environment for a specific reaction time was goethite < lepidocrocite ≤ akaganeite. Given the presence of goethite in sedimentary formations, this conversion reaction may have relevance to the subsurface sequestration and geologic storage of carbon dioxide.

Neutron pair distribution function study of two-line ferrihydrite.

Pair distribution function (PDF) analysis of neutron total scattering data from deuterated two-line ferrihydrite is consistent with the Keggin-related structural model for ferrihydrite published by Michel et al. (2007). Other models proposed in the literature, such as that of Drits et al. (1993), lead to inferior fits. Bond valence sums indicate that O(1) is bonded to a hydrogen atom, but the quality of the data is such that the exact position of the hydrogen could not be elucidated with confidence.

Electrocatalytic CO2 reduction on earth abundant 2D Mo2C and Ti3C2 MXenes.

Mo2C and Ti3C2 MXenes were investigated as earth-abundant electrocatalyts for the CO2 reduction reaction (CO2RR). Mo2C and Ti3C2 exhibited faradaic efficiencies of 90% (250 mV overpotential) and 65% (650 mV overpotential), respectively, for the reduction of CO2 to CO in acetonitrile using an ionic liquid electrolyte. The use of ionic liquid 1-ethyl-2-methylimidazolium tetrafluoroborate as an electrolyte in organic solvent suppressed the competing hydrogen evolution reaction. Density functional theory (DFT) calculations suggested that the catalytic active sites are oxygen vacancy sites on both MXene surfaces. Also, a spontaneous dissociation of adsorbed COOH species to a water molecule and adsorbed CO on Mo2C promote the CO2RR.

Photodissolution of ferrihydrite in the presence of oxalic acid: an in situ ATR-FTIR/DFT study.

The photodissolution of the iron oxyhydroxide, ferrihydrite, in the presence of oxalic acid was investigated with vibrational spectroscopy, density functional theory (DFT) calculations, and batch geochemical techniques that determined the composition of the solution phase during the dissolution process. Specifically, in situ attenuated total reflection Fourier transform infrared spectroscopy (ATR- FTIR) was used to determine the structure of the adsorbed layer during the dissolution process at a solution pH of 4.5. DFT based computations were used to interpret the vibrational data associated with the surface monolayer in order to help determine the structure of the adsorbed complexes. Results showed that at pH 4.5, oxalate adsorbed on ferrihydrite adopted a mononuclear bidentate (MNBD) binding geometry. Photodissolution at pH 4.5 exhibited an induction period where the rate of Fe(II) release was limited by a low concentration of adsorbed oxalate due to the site-blocking of carbonate that was intrinsic to the surface of the ferrihydrite starting material. Oxalate displaced this initial carbonate over time, and the dissolution rate showed a corresponding increase. Irradiation of oxalate/ferrihydrite at pH 4.5 also ultimately led to the appearance of carbonate reaction product (distinct from carbonate intrinsic to the starting material) on the surface.