Gallium-based nascent electrode materials towards promising supercapacitor applications: a review

To meet the energy requirement of the modern era, supercapacitors are promising candidates for energy storage devices, which possess the potential to compete with the future battery technology. To accomplish this pivotal task, it is vital to choose electrode materials that have high power and energy density as well as superb electrochemical stability. For the past few years, the use of gallium-based materials for energy storage applications has attracted attention because of their excellent activity towards electrochemical energy storage applications despite the single oxidation state (i.e., +3 which is redox inactive and does not contribute towards pseudo capacitance). Recently, research on gallium-based materials has started and will be continued further owing to the fact that gallium-based materials possess numerous excellent properties such as fast charge and discharge rate, high power density, long cycle life, stability over a wide range of temperatures, excellent electron velocity, superior chemical and physical stabilities and high voltage application capability, which make them a potential class of electrode materials for supercapacitors. The enhancement in the electrochemical performance upon the introduction of gallium into the system can make it a futuristic candidate for electrochemical energy storage devices. Herein, we systematically outline the synthesis and characterization of gallium-based materials and their composites as explored by esteemed researchers focusing only on their supercapacitive performance via electrochemical techniques. For a better understanding, the underlying charge storage mechanism and identified characteristics are presented to give a crystal-clear idea about the field. In addition, the key challenges and impending perspectives of gallium-based electrodes for supercapacitor applications are debated.

Photocatalytic and photo-electrochemical ammonia synthesis over dimensional oriented cobalt titanate/nitrogen-doped reduced graphene oxide junction interface catalyst

Nitrogen reduction to ammonia is vital for chemical industries and renewable clean energy. Denying the harsh reaction conditions adopted in the Haber-Bosch process and stimulation research for ammonia production through sustainable technologies is a smart approach. Hitherto, photocatalyst acquiring the potential to attain high nitrogen reduction reaction (NRR) efficiency is a challenging task. Here, this study demonstrated cobalt titanate (CoTiO₃) rods (p-type) straddled with two-dimensional (2D) sheets of nitrogen-doped reduced graphene oxide (N-rGO, n-type) via, reflux method; realizing the advantages of dissimilar dimensionalities and strong interfacial junction coupling for efficient NRR under visible light irradiation. The successful interface junction establishment between CoTiO₃ and N-rGO has been witnessed from Raman, x-ray photoelectron spectroscopy (XPS), and Mott-Schottky analysis. Moreover, a well-defined type-II band structure is capable to curl the charge anti-recombination process; reflected in upgraded photo-catalytic/electrocatalytic upshots. The CoTiO₃ modified with an optimized concentration of N-rGO exhibits high stability with an improved photocatalytic (1722.22 μmolL^-1h^-1) and photo-electrocatalytic (16.8 µg cm^-1h^-1) nitrogen reduction to ammonia production; multiple times higher than counterparts. This improved photo-activity of CoTiO₃/N-rGO junction hybrid stems from the built-in electric field existing across the dissimilar junction interface, triggering charge transfer channels for reduction reaction in mild reaction conditions. The result of these materials might strategies the way for future development of new functionalities bearing highly active catalyst materials for sustainable energy-related conversion.

Robust direct Z-scheme exciton transfer dynamics by architecting 3D BiOI MF-supported non-stoichiometric Cu0.75In0.25S NC nanocomposite for co-catalyst-free photocatalytic hydrogen evolution

Designing promising photocatalytic systems with wide photon absorption and better exciton separation ability is a cutting-edge technology for enhanced solar-light-driven hydrogen production. In this context, non-stoichiometric Cu0.75In0.25S nanocrystals (CIS NCs) coupled with three-dimensional (3D) BiOI micro-flowers (BOI MFs) were synthesized through an ultra-sonication strategy forming a CIS-BOI heterojunction, which was well supported by XRD, photocurrent, XPS and Mott-Schottky analyses. Further, the co-catalyst-free CIS-BOI binary hybrid shows improved hydrogen evolution, i.e., 588.72 μmol h-1, which is 3.2 times greater than the pristine CIS NC (183.97 μmol h-1). Additionally, the binary composite confers an apparent conversion efficiency (ACE) of 9.44% (8.90 × 1016 number of H2 molecule per sec), which is extensively attributed to the robust charge carrier separation and transfer efficiency via the direct Z-scheme mechanism (proved through superoxide and H2 evolution activity). Moreover, the broad photon absorption range and productive exciton separation over the CIS-BOI composite are substantially justified by UV-Vis DRS, PL, EIS and photocurrent measurements.

An energy band compactable B-rGO/PbTiO3 p-n junction: a highly dynamic and durable photocatalyst for enhanced photocatalytic H2 evolution

Reduced graphene oxide (rGO) intentionally doped with boron atoms is a promising tactic to extract bandgap energy and p-type semiconducting behavior from graphene-based materials. Moreover, the integration of p-type boron-doped rGO with an n-type material through a heterojunction interface exhibits complementary properties to restrict the fast recombination of charge carriers and enhance the photoreaction towards energy applications. Herein, we have prepared boron-doped rGO/PbTiO3 p-n heterojunctions via a hydrothermal method. The successful formation of an excellent p-n heterojunction was demonstrated by TEM, XPS and Raman analysis. The constructed boron-doped rGO/PbTiO3 p-n heterojunctions exhibit dramatic increases in photoelectrochemical and photocatalytic performance due to the presence of a space charge region at the interface of the two materials. The scenario also revealed the double-edge sword functions of B-rGO: the material itself (i) acts as a visible light active photocatalyst with a band gap energy of 2.7 eV and (ii) participates in enhanced charge transfer via the band edge alignment between B-rGO and PbTiO3, as elucidated from photoluminescence and electrochemical impedance studies. Furthermore, the optimal 2B-rGO/PT p-n heterojunction possesses outstanding repeatability and exhibited the highest rate of hydrogen evolution, i.e. 293.79 μmol h-1 under visible light irradiation. The results for these materials may provide advanced insight into the design of next-generation high-efficiency black graphene-based heterojunctions.

Deciphering Z-scheme Charge Transfer Dynamics in Heterostructure NiFe-LDH/N-rGO/g-C3N4 Nanocomposite for Photocatalytic Pollutant Removal and Water Splitting Reactions

A series of heterostructure NiFe LDH/N-rGO/g-C3N4 nanocomposite were fabricated by combining calcinations-electrostatic self-assembly and hydrothermal steps. In this method, negatively charged N-rGO was electrostaticaly bonded to the self-assembled interface of n-n type g-C3N4/NiFe LDH hybrid. XRD and AFM results revealed successful formation of heterostructure nanocomposite due to the coupling effect of exfoliated NiFe LDH nanosheets with N-rGO and g-C3N4. Among the as synthesized heterostructure, CNNG3LDH performed superior photocatalytic activities towards 95 and 72% mineralization of RhB and phenol. Furthermore, CNNG3LDH could achieve the highest photocatalytic H2 evolution rate of 2508 μmolg-12h-1 and O2 evolution rate of 1280 μmolg-12h-1 under visible light irradiation. The CNNG3LDH possess lowest PL intensity, reduced arc of the Nyquist plot (43.8 Ώ) and highest photocurrent density (-0.97 mA cm-2) which revealed effective charge separation for superior photocatalytic activities. TRPL spectral results reveal the synergistic effect of layered component in CNNG3LDH for achievable higher life time of excitons of ~16.52 ns. In addition, N-rGO mediator based Z-scheme charge transfer mechanisms in CNNG3LDH were verified by the ESR and TA-PL studies. Enriched oxygen vacancy type defects in NiFe LDH and N-rGO mediated Z-scheme charge transfer mechanistic path strongly manifest the superior photocatalytic activities of the heterostructure materials.

Erratic charge transfer dynamics of Au/ZnTiO3 nanocomposites under UV and visible light irradiation and their related photocatalytic activities

The present article presents an in-depth discussion on the state-of-the-art multifarious roles of Au nanoparticles and their associated charge anti-recombination process in burgeoning photocatalysis research. Hexagonal-phase ZnTiO₃ was fabricated through a sol-gel auto-combustion method by optimizing the calcination temperatures. To further improve the charge separation efficiency and visible light induced photocatalytic activity of pristine ZnTiO₃, we designed a new type of Au/ZnTiO₃ nanocomposite by a precipitation-deposition method. The photocatalytic activities of the Au/ZnTiO₃ nanocomposites were substantiated by evaluating the rate of hydrogen evolution under both UV and visible light illumination. The photocatalytic activity of the Au/ZnTiO₃ nanocomposites rises proportionally with an increase in Au content up to 1.5 wt% under UV light illumination and it produce around 285 μmol h^-1 of H₂ which is approximately 2.6 times higher than that produced by pristine ZnTiO₃. Therefore, the Au nanoparticles present on the surface of ZnTiO₃ act as electron acceptors, leading to an increase in the rate of generation and separation of charge carriers. This process helps to enhance the congregation of electrons on Au nanoparticles through the Schottky junction. The obtained results are very consistent with steady-state PL and UV light induced photocurrent measurements. Conversely, such a trend was not detected under visible light illumination. The visible light induced photocatalytic activity of Au/ZnTiO₃ nanocomposites increases with a rise in Au content up to 1 wt% and thereafter decreases with further Au loading. Therefore, the initial increment in photocatalytic activity is due to the generation, separation and participation of a large number of SPR-induced charge carriers and thereafter decreases due to the recombination of SPR-generated charge carriers because of the formation of defect sites at the Au and ZnTiO₃ interface. That the excess Au loading causes the recombination of SPR charge carriers was well explained by undertaking SPR-induced TRPL analysis and this result is directly followed up with the results of visible light induced photocurrent and EIS measurements. The Au/ZnTiO₃ nanocomposites with optimal Au loading (1 wt%) delivered an amazingly high rate of hydrogen evolution i.e. 108 μmol h^-1 with an energy conversion efficiency of 7.14%, whereas pristine ZnTiO₃ shows negligible activity under visible light illumination.

Topotactic Transformation of Solvated MgCr-LDH Nanosheets to Highly Efficient Porous MgO/MgCr2O4 Nanocomposite for Photocatalytic H2 Evolution

The hybrid structure of nanoparticles (NPs) with nanosheets has the advantage of both anisotropic properties of NPs and large specific surface areas of nanosheets, which is desirable for many technological applications. In this study, MgCr₂O₄ spinel NPs decorated on highly porous MgO nanosheets forming MgO/MgCr₂ O₄( x) nanocomposites were synthesized by a one pot coprecipitation method followed by a heat treatment process of the solvated wet gel of MgCr-LDH with polar solvent N, N-dimethylformamide (DMF) at 400 °C. This novel synthetic methodology generates materials consisting of porous metal oxides nanosheets adhered with spinel phase NPs due to the slow generation of gases such as H₂O, CO₂, and NH₃ under moderate temperature during the heat treatment process. The synergistic effect of much wider band gap MgO nanosheets and narrow band gap MgCr₂O₄ NPs added increased stability due to the stronger bonding coordination of MgCr₂O₄ NPs with MgO nanosheets. The obtained MgO/MgCr₂ O₄( x) nanocomposites possess large specific surface areas, highly porous structure, and excellent interface between MgCr₂O₄ NPs and MgO nanosheets, which proved from N₂ sorption isotherm, TEM, HR-TEM study. With metallic ratio of MgCr3:1, MgO/MgCr₂O₄(MgCr3:1) nanocomposites exhibit highest H₂ evolution rate of 840 μmolg^-12h^-1, which was 2 times higher than that of pure MgCr₂O₄(420 μmolg^-12h^-1). The LSV measurement study of MgO/MgCr₂O₄ (MgCr3:1) nanocomposite shows an enhancement of light current density of 0.22 μA/cm² at potential bias of -1.1 V. The Mott-Schottky analysis suggested the band edge positions of the n-type constituents and formation of n-n type heterojunctions in MgO/MgCr₂O₄ (MgCr3:1) nanocomposite, which facilitates the flow of charge carriers. The EIS and Bode phase plot of MgO/MgCr₂O₄ (MgCr3:1) nanocomposite signifies the lower interfacial charge transfer resistance and higher lifetime of electrons (2.7 ms) for enhanced H₂ production. Lastly, the enhanced photocatalytic H₂ production activity and long-term stability of MgO/MgCr₂O₄(MgCr3:1) could be attributed to maximum specific surface area, porous structure, close intimacy contact angle between two cubic phases of MgCr₂O₄ NPs and MgO nanosheets, abundant oxygen vacancies sites, reduced charge transfer resistance and suitable band edge potential to drive the thermodynamic energy for H₂ production. This work highlighted an effective strategy for the synthesis of cost-effective 2D porous heterojunctions nanocomposite photocatalyst for promising applications in the field of clean H₂ production utilizing abundant solar energy.

Highly efficient charge transfer through a double Z-scheme mechanism by a Cu-promoted MoO3/g-C3N4 hybrid nanocomposite with superior electrochemical and photocatalytic performance

Herein, a novel Cu-MoO3/g-C3N4 hybrid nanocomposite was successfully synthesized by a two-step strategy of one-pot pyrolysis followed by the impregnation method. The structure, phase, morphology and electronic environment of MoO3, g-C3N4 and Cu in the composite were determined by various characterization methods. The oxygen vacancies of MoO3 were ascertained by UV-DRS, Raman, and XPS analysis. The formation of the heterostructure was characterised by electrochemical measurements. The photocatalytic performance of the composite was investigated by the water reduction reaction and the reduction of an important inorganic pollutant, Cr(vi). In the presence of Cu NPs, the H2 evolution of the MoO3/g-C3N4 hybrid was found to be 652 μmol h-1 with an apparent energy conversion efficiency of 13.46%, and up to 95% of Cr(vi) was reduced using citric acid as a hole scavenger. The remarkably enhanced photocatalytic performance was attributed to the combined effect of the double Z-scheme mechanism and defective MoO3. The in situ formation process of the MoO3/g-C3N4 hybrid followed a direct Z-scheme charge transfer by generating a great number of defects at the solid-solid interface, similar to that of a conductor, and offered low electrical resistance, whereas loading of Cu NPs built up an indirect Z-scheme charge transfer to establish the double Z-scheme charge transfer mechanism. This hybrid material produces a photocurrent density of 12.1 mA cm-2, in good agreement with the photocatalytic activity. This study highlights the facilitation effect of MoO3 due to oxygen vacancies and charge transfer through the double Z-scheme mechanism to open up a new window in the field of 2D nanostructured photocatalytic materials.

Controlled Synthesis of CeO2NS-Au-CdSQDs Ternary Nanoheterostructure: A Promising Visible Light Responsive Photocatalyst for H2 Evolution

With the advancement of promising multifaceted powdered photocatalytic systems, problems related to environmental pollution and energy requirements have been addressed to a significant extent. The major reason for this great achievement lies in the combined effect of both structure modification and integration of different functional materials. Here, we report a ternary hybrid containing wide band gap CeO₂ nanosheets with CdSQDs and Au nanoparticles, incorporated between this type II heterostructure through simple chemical reduction methods. Structural and morphological characterization of the fabricated samples was carried out by XRD, XPS, and TEM analysis. From a series of optical and photoelectrochemical measurements, it was found that the incorporation of Au nanoparticles into the interfaces of CeO₂ and CdSQDs was the major cause of the enhancement in the photocatalytic activity. Au nanoparticles play a dual character by acting as a mediator and also inject hot electrons through LSPR (light-induced surface plasmon resonance) effects in the ternary hybrid. The photocatalytic activity of the fabricated samples was tested toward H₂ evolution, where the ternary hybrid CeO₂NS-Au-CdSQDs lead the activity sequence with 499 μmol/2 h followed by the binary and neat counterparts. From the Mott-Shottky and linear sweep voltammetry measurements, a heterostructure relay mechanism was predicted where electrons from CdSQDs flow to the surface of CeO₂ via Au. The novelty of this work is that it provides useful information about the synergistic effect among three functional components, integrated in a nanosheet structured system, as the basic requirement for constructing good heterostructures in powdered photocatalytic systems.

Dramatic activities of vanadate intercalated bismuth doped LDH for solar light photocatalysis

To harvest solar energy efficiently, a series of Zn/Bi layered double hydroxide (LDH) photocatalysts with different molar ratios of Zn/Bi (2 : 1, 3 : 1, 4 : 1) has been synthesized by a coprecipitation method at constant pH. All the Bi doped LDH samples displayed hydrotalcite-like structure with interlayer carbonate, in which crystallinity decreases as the bismuth content increases. The Zn/Bi (4 : 1) LDH with a small amount of bismuth in the brucite layer and possessing high crystallinity was further modified hydrothermally by intercalating decavanadate and it showed high photochemical stability and photocatalytic activity for the degradation of different organic pollutants for practical applications under solar light irradiation. The structural integrity of the materials has been successfully characterized by studying their structural, morphological, electronic and optical properties by various physico-chemical techniques. The present study provided an insight into oxo-bridged MMCT of the LDH and established that the Zn(II)-O-Bi(III) units resulted in the generation of superoxide radicals which is clearly observed by the EPR technique. The ˙OH radicals formed during photocatalysis were revealed by means of the terephthalic acid fluorescence probe method. The photoelectrochemical measurement confirmed that the intercalated vanadate anion was crucial to obtain an optimal synergistic effect for the degradation of organic pollutants. The prolonged lifetime of photogenerated charges and improved charge transfer capability were confirmed by time-resolved fluorescence emission spectra. Furthermore, a detailed mechanism for the enhanced photocatalytic activity was discussed.

Fabrication of α-Fe2O3 nanorod/RGO composite: a novel hybrid photocatalyst for phenol degradation

We report herein the fabrication of a hematite nanorod-graphene composite (α-Fe2O3 nanorod/RGO) via a facile template-free hydrothermal route with an aim to improve the photocatalytic efficiency of the α-Fe2O3 nanorod. The structural and morphological characterizations of the as-prepared composites were carried out using X-ray diffraction, Raman spectra, X-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscopy, N2 adsorption-desorption, etc. The α-Fe2O3 nanorods were well-decorated on the surface of the graphene sheets, which helps in electron transfer from α-Fe2O3 to graphene and hence can delay the recombination process, leading to the improvement in photocatalytic activity. The composite containing 5 wt % RGO and α-Fe2O3 nanorods shows a 4-fold enhancement in the photocatalytic activity. The performance of photocatalytic activity was discussed in light of surface area, interaction between nanorods and graphene nanosheets, synergism between α-Fe2O3 nanorods and RGO sheets, light-harvesting properties of the composites, photoluminescence spectra, photocurrent measurement, and hydroxyl radical formation.

Quick photo-Fenton degradation of phenolic compounds by Cu/Al2O3-MCM-41 under visible light irradiation: small particle size, stabilization of copper, easy reducibility of Cu and visible light active material

The present study reports the photo-Fenton degradation of phenolic compounds (phenol, 2-chloro-4-nitrophenol and 4-chloro-2-nitrophenol) in aqueous solution using mesoporous Cu/Al(2)O(3)-MCM-41 nanocomposite as a heterogeneous photo-Fenton-like catalyst. The in situ incorporation of mesoporous Al(2)O(3) (MA) into the framework of MCM-41 (sol-gel method) forms Al(2)O(3)-MCM-41 and wetness impregnation of Cu(II) on Al(2)O(3)-MCM-41 generates mesoporous Cu/Al(2)O(3)-MCM-41 composite. The effects of pH and H(2)O(2) concentration on degradation of phenol, 2-chloro-4-nitrophenol and 4-chloro-2-nitrophenol are studied. Kinetics analysis shows that the photocatalytic degradation reaction follows a first-order rate equation. Mesoporous 5 Cu/Al(2)O(3)-MCM-41 is found to be an efficient photo-Fenton-like catalyst for the degradation of phenolic compounds. It shows nearly 100% degradation in 45 min at pH 4. The combined effect of small particle size, stabilization of Cu(2+) on the support Al(2)O(3)-MCM-41, ease reducibility of Cu(2+) and visible light activeness are the key factors for quick degradation of phenolic compounds by Cu/Al(2)O(3)-MCM-41.

Synthesis of multifunctional nanostructured zinc-iron mixed oxide photocatalyst by a simple solution-combustion technique

A series of nanostructure zinc-iron mixed oxide photocatalysts have been fabricated by solution-combustion method using urea as the fuel, and nitrate salts of both iron and zinc as the metal source. Different characterization tools, such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), diffuse reflectance UV-visible spectra (DRUV-vis), electron microscopy, and photoelectrochemical measurement were employed to establish the structural, electronic, and optical properties of the material. Electron microscopy confirmed the nanostructure of the photocatalyst. The synthesized photocatalysts were examined towards photodegradation of 4-chloro-2-nitro phenol (CNP), rhodamine 6G (R6G), and photocatalytic hydrogen production under visible light (λ ≥ 400 nm). The photocatalyst having zinc to iron ratio of 50:50 showed best photocatalytic activity among all the synthesized photocatalysts.

Fabrication, growth mechanism, and characterization of α-Fe(2)O(3) nanorods

This work reports the facile synthesis of α-Fe(2)O(3) nanorods and nano-hexagons and its application as sunlight-driven photocatalysis. The obtained products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), scanning electron microscopy (SEM), diffused reflectance spectroscopy (DRUV-vis), Fourier transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The phase and crystallinity were confirmed from the XRD study. Electron microscopy study clearly indicates the formation of different morphologies of nanocrystals. These hematite nanostructures were used as a model system for studying the shape-dependent photocatalytic degradation of phenol, methylene blue, and congo red. Amongst all the nanostructured semiconductors, Pt-doped hematite nanorod showed 55% efficiency towards the decolonization of methylene blue and 63% toward congo red under sun light illumination. The difference in photocatalytic activity is discussed in terms of their crystallize size and morphological ordering.

Adsorption of Cu2+ on spherical Fe-MCM-41 and its application for oxidation of adamantane

Fe-MCM-41 with varying Si/Fe ratios (20, 50, 70 and 90) were prepared by using cetyltrimethylammoniumbromide (CTAB) as the structure-directing agent (SDA), tetraethylorthosilicate (TEOS) as the silica source and ethanol as co-surfactant in alkaline medium. The characterization was done by SEM; UV-vis diffused reflectance and FT-IR. Adsorption of copper solutions with varying parameters such as concentration, temperature and pH were performed over Fe-MCM-41 samples. The experimental data fitted well to the Langmuir and Freundlich adsorption isotherm. Fe-MCM-41 having Si/Fe ratio (90) showed highest copper adsorption capacity at pH 5.5, Cu(2+) concentration of 59.6 ppm and temperature 323 K. Fe-MCM-41(90)-Cu(59.6) was tested as a catalyst for oxidation of adamantane using hydrogen peroxide as the oxidant. The GC analyses revealed significant conversion of 32.5% and selectivity of 54% towards formation of 1-adamantanol.

Gold promoted S,N-doped TiO(2): an efficient catalyst for CO adsorption and oxidation

Mesoporous S,N-TiO(2) nanocomposite was prepared by a one-pot template free homogeneous coprecipitation technique using titanium oxysulfate sulfuric acid complex hydrate, thiourea, ethanol, and water. Nano gold deposition on mesoporous S,N-TiO(2) was preformed by a borohydrate reduction method. To evaluate the structural and electronic properties, these catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HR-TEM), UV-vis DRS, photoluminescent (PL) spectra, Fourier transform infrared (FTIR), and TPO/TPD. CO adsorption and CO + O(2) interaction over these catalysts were investigated by in situ FTIR. Sulfur and nitrogen doping enhances the catalytic activity of Au/TiO(2.) Higher catalytic activity of Au/S,N-TiO(2) compared to Au/TiO(2) was attributed to the presence of oxygen vacancy and creation of new adsorption sites at Au/TiO(2) interfaces for the adsorption and activation of O(2) molecules.

Removal of phenolic compounds from aqueous solutions by adsorption onto manganese nodule leached residue

Manganese nodule leached residue is a good adsorbent. Adsorption of phenol (PhOH) on water washed manganese nodule leached residue (WMNLR), waste materials from manganese nodules processing plant, has been investigated. The adsorbent (WMNLR) used for the removal of phenolic compounds were characterized by EDX, FTIR, SEM and BET surface area measurement before and after the adsorption process. Adsorption experiments were carried out to study the effect of various parameters like adsorbent dose, pH, adsorbate concentration, reaction time, temperature and calcination temperature. WMNLR calcined at 400 degrees C showed highest adsorption capacity. The equilibrium adsorption data for phenol was analyzed by using Langmuir isotherm model. The maximum adsorption of phenol was obtained at pH 3 (about 95% for adsorbent dose 1g/L and 30 mg/L adsorbate concentration). The increase in percentage of adsorption with increase in temperature indicates that adsorption is endothermic in nature. The pseudo-second-order kinetics was followed in the adsorption process.

Surface characterization and catalytic evaluation of manganese nodule leached residue toward oxidation of benzene to phenol

Water washed manganese nodule leached residue (WMNLR) calcined at different temperatures was characterized by XRD, FTIR, TG-DTA, surface area, surface oxygen, and surface acid sites. Surface area, surface oxygen, surface hydroxyl group, and surface acid sites increase up to 400 degrees C and then decrease with further rise in calcination temperature up to 700 degrees C. The catalytic activity of the calcined samples was tested for single-step oxidation of benzene to phenol using hydrogen peroxide as the oxidant and acetic acid as the solvent at room temperature. The influence of various reaction parameters such as solvent, concentration of solvent, oxidant amount, time, temperature, and catalyst amount was studied to optimize the reaction conditions. WMNLR calcined at 400 degrees C showed the highest catalytic activity towards oxidation of benzene with 12.7% conversion and 98% selectivity.

Calcined Mg-Fe-CO(3) LDH as an adsorbent for the removal of selenite

Mg-Fe-CO(3) layered double hydroxide (LDH) with a Mg/Fe molar ratio of 2.0 was synthesized by co-precipitation method and its calcined product (CLDH) was obtained by heating Mg/Fe-LDH at 500 degrees C. Sorption of SeO(2-)(3) on CLDHs was studied and the results indicate that the sorption capacity of CLDHs was higher than that of uncalcined LDHs. Isotherms for SeO(2-)(3) sorption by CLDHs were well described using the Freundlich and Langmuir equations. The thermodynamic parameters, viz. DeltaG*, DeltaH*, DeltaS* were calculated to predict the nature of adsorption. The negative and positive values of DeltaG* and DeltaH* indicate that the adsorption process is spontaneous and endothermic in nature, respectively. The adsorption process followed first-order kinetics.

Studies on selenite adsorption using manganese nodule leached residues

Selenite adsorption on water-washed manganese nodule leached residues (WMNLR) was studied with the aim of detoxifying industrial effluents before their safe disposal into the environment. WMNLR, a solid waste material, was characterized with the help of XRD, FTIR, TG-DTA, pH(pzc), BET surface area, surface oxygen, surface hydroxyl group, and chemical analyses. The adsorption behavior of WMNLR toward selenite was studied as a function of time, pH, temperature, and concentration of adsorbate and adsorbent. Increased adsorption capacity with rise in temperature indicates that the adsorption process was endothermic in nature. Based on the thermodynamic parameters such as the Gibbs free energy change, standard enthalpy change, and standard entropy change, the adsorption process was found to be spontaneous and endothermic in nature. Adsorption followed second-order kinetics. The adsorption capacity of the material was found to be 54.6 mg g(-1) at 60 mg L(-1) of selenite concentration at pH 5 in 3 h contact time.

Dramatic promotion of gold/titania for CO oxidation by sulfate ions

The activity of gold/titania catalysts for the room-temperature oxidation of CO can be dramatically enhanced by the addition of sulfate ions to the support; it appears that anion promotion of gold may be a general phenomenon and may be related to the direct modification of active gold sites in the case of sulfate ions, as evidenced by secondary ion mass spectrometry.

Preparation and characterization of Mg–Al hydrotalcite-like compounds containing cerium

Hydrotalcite-like compounds (HTlcs) containing Mg(2+), Al(3+), and Ce(3+) in the hydroxide layer and with carbonate as charge-balancing anion have been prepared by a co-precipitation (at constant pH) method. The Al/Ce ratio in the final product depends on the concentration in the initial solution. The crystallinity of the layered materials decreases with the increase in cerium content, probably, due to the distortions introduced by the large difference in the ionic radii of the cations. All the synthesized materials were characterized by XRD, FT-IR spectroscopy and TG/DTA. The textural properties were determined from low-temperature nitrogen adsorption-desorption measurements.

Effect of sulfate on the surface and catalytic properties of iron–chromium mixed oxide pillared clay

Sulfate-supported iron-chromium mixed oxide pillared clay was prepared varying the sulfate loading from 1 to 5 wt% by the incipient wetness method and characterized by low-angle XRD, BET surface area, and ammonia TPD. All the samples were found to be stable up to 500 degrees C having the basal spacing 17.7 A even after sulfate impregnation. Formation of strong Lewis acid sites and decrease in the number of Brønsted acid sites due to the sulfate loading were observed from the ammonia TPD curve. Catalytic properties of the sulfated materials were evaluated with the help of methanol conversion and aromatic alkylation reactions and correlated with the surface area and TPD results. For methanol conversion, decomposition product selectivity increases due to the sulfate addition. A negligible decrease in the surface area and a substantial increase in the catalytic activity were observed due to the sulfate loading of 1-2 wt%. However, a significant decrease in the surface area as well as catalytic activity was observed for the 3 wt% and above sulfate loaded samples which may be due to the partial blockage of pores by excess sulfate. Results shows the importance of acidity of the material due to sulfate loading (up to 2 wt%) and thus can be used as a better acid catalyst.

Physico-chemical characterization and photocatalytic activity of zinc oxide prepared by various methods

Zinc oxide was prepared by different methods by varying precipitating agents, the source of the salt precursors and the microwave irradiation time and was characterized by XRD, BET-surface area, surface acidity and crystallite sizes. The photocatalytic reactions were carried out under solar radiation in batch reactors towards oxidation of 4-nitrophenol and reduction of Cr(VI) by varying different parameters such as irradiation time, pH of the solution, catalyst amount and substrate concentration and the activities were correlated with the physico-chemical parameters. Zinc oxide samples prepared by microwave irradiation and calcined at 300 degrees C exhibit highest surface area, acid sites and lowest crystallite sizes and show highest activity towards photocatalytic reactions.

Adsorption of hexavalent chromium on manganese nodule leached residue obtained from NH3–SO2 leaching

Adsorption of hexavalent chromium onto manganese nodule leached residues was investigated as a possible alternative to the conventional methods of its removal from aqueous synthetic solutions. Adsorption behavior was studied as a function of time, pH, temperature, and concentration of adsorbate and adsorbent in acetic acid-sodium acetate buffer medium. Cr (VI) removal was pH dependent and was found to be of a maximum at pH 3. The applicability of the Langmuir isotherm to the present system was tested. Increased adsorption capacity with increased temperature indicates that the adsorption reaction was endothermic in nature. Based on these studies, thermodynamic parameters such as Gibbs free energy change (DeltaG0), standard enthalpy change (DeltaH0), and standard entropy change (DeltaS0) were calculated.

Studies on manganese nodule leached residue 4. Physicochemical characterization and catalytic activity of acetic acid treated manganese nodule leached residue

The catalytic activity of water-washed manganese nodule leached residue (WMNLR) samples improved by treating with acetic acid. The effects of acetic acid treatment on the physicochemical properties and catalytic activity of manganese nodule leached residue have been studied. The surface area, surface oxygen, surface hydroxyl groups, surface acidity, electron donating properties, etc., increase gradually with acid treatment up to 0.5 M and thereafter show a decreasing trend. The rate constant of H2O2 decomposition, catalytic activity of CO oxidation, and esterification of acetic acid also show a similar trend to that of surface properties.

Effect of heat treatment on the physico-chemical properties and catalytic activity of manganese nodules leached residue towards decomposition of hydrogen peroxide

The effect of calcination temperature on the physico-chemical characterization of manganese nodule leached residue (MNLR) and water-washed manganese nodule leached residue (WMNLR) has been investigated on the basis of chemical analysis, XRD, TG-DTA, FTIR, surface hydroxyl groups, surface oxygen, reducing and oxidizing sites, surface area. XRD and IR confirm the presence of amorphous iron oxyhydroxides, delta-MnO2, which are converted to alpha-Fe2O3 and gamma-Mn2O3 phases above 400 degrees C of calcination, respectively. A solid solution of Fe2O3 and Mn2O3 is formed above 700 degrees C. The surface area, surface hydroxyl group, surface oxygen, reducing and oxidizing sites increase with the increase in calcination temperature up to 400 degrees C and then decrease with further rise in calcination temperature up to 700 degrees C. The catalytic activity of the sample towards H2O2 decomposition shows the similar trend as surface properties. A suitable Mn(3+)Mn4+ couple favours H2O2 decomposition reaction. The activity has been correlated with various physico-chemical properties.

Studies on manganese nodule leached residues

Adsorption of phosphate onto manganese nodule leached residues was investigated as a possible alternative to conventional methods of phosphate removal from industrial effluents. Adsorption behaviors were studied as a function of time, temperature, pH, and concentration level of adsorbate and adsorbent in acetic acid-sodium acetate buffer medium. The adsorption of phosphate follows the Langmuir adsorption isotherms. The magnitude of adsorption of phosphate in manganese nodule leached residues was compared with that in naturally occurring Mn nodule. Manganese nodule leached residues show better affinity toward phosphate adsorption.

Studies on manganese-nodule leached residues

Physicochemical characterization of manganese-nodule leached residues was carried out by chemical analyses, XRD, TG-DTA, surface area measurement, and FTIR techniques. The material is very fine-grained (<75 microm), is cryptocrystalline to amorphous in nature, and contains mainly of delta-MnO(2), quartz (alpha-SiO(2)), and zeolite/feldspar minerals. Physically adsorbed sulfates in the leached residue are removed by repeated water washing and the washed sample shows an appreciable increase in surface area. This is indicated by the absence of 1387 and 1099 cm(-1) peaks in the IR spectrum of the washed sample. The adsorption behavior of the washed sample toward Ni(2+) was recorded as a function of time, pH, temperature, and concentrations of adsorbent and adsorbate.

Sulfated nanozirconia: an investigation on acid–base properties and n-butane isomerization activity

Hydrated zirconia was synthesized by an organo-inorganic route employing surfactant and was sulfated using aqueous ammonium persulfate, followed by drying at 110 degrees C. The sample thus obtained was calcined at 600 degrees C to obtain sulfated zirconia and was characterized by several physicochemical methods. Crystallite sizes of sulfated zirconia were calculated from X-ray line broadening using the Debye-Scherer equation and were found to be in the range of 25 nm. When pretreated in air, the catalyst was found to exhibit butane isomerization activity at a temperature as low as 35 degrees C under atmospheric pressure. It showed conversion as high as 37% at 100 degrees C under normal pressure when pretreated in air, whereas nitrogen-pretreated catalyst showed zero activity under similar conditions. NH(3) and CO(2) temperature-programmed desorption studies on air- and helium-pretreated samples indicated that the catalyst surface changes appreciably during air pretreatment. Results on butane isomerization in conjunction with TPD studies suggest that zirconium-oxy sites play an important role in butane activation during the reaction.

A comparative study on textural characterization: cation-exchange and sorption properties of crystalline α-zirconium(IV), tin(IV), and titanium(IV) phosphates

Tetravalent metal phosphates (M=Zr, Ti, and Sn) were prepared and characterized by XRD, surface properties, and TG-DTA. The cation exchange and sorption behavior of these metal phosphates toward transition metal ions such as Cu(2+), Co(2+), and Ni(2+) have been studied comparatively as a function of temperature and concentration. The adsorption process was found to increases with increase in temperature and concentration. The selectivity order for alpha-titanium and alpha-tin phosphates is Cu(2+)>Co(2+)>Ni(2+), whereas for alpha-zirconium phosphate it is Cu(2+)>Ni(2+)>Co(2+). The ion exchange capacity of alpha-titanium phosphate is greater than those of other phosphates, which is explained on the basis of the surface behavior, disorderness of the system, degree of hydrolysis of incoming guest adsorbate metal ions, and structural steric hindrance of the exchangers during adsorption and sorption. The distribution coefficient, Gibbs free energy, enthalpy, and entropy values indicate that the ion-exchange processes are spontaneous.

Studies on Mg/Fe Hydrotalcite-Like-Compound (HTlc)

A Mg/Fe hydrotalcite-like-compound (HTlc) was prepared and its affinity toward the removal of SeO(3)(2-) from an aqueous medium was studied as a function of pH, time, temperature, particle dose, and SeO(3)(2-) concentration. The fraction of SeO(3)(2-) removal increases with decrease in both pH and temperature. The adsorption data are fitted to the Langmuir adsorption isotherm in the temperature range 303-333 K, and the thermodynamic parameters viz. standard Gibbs' free energy change (DeltaG degrees ), enthalpy change (DeltaH degrees ), and entropy change (DeltaS degrees ) are calculated. The negative value of DeltaH degrees indicates that the adsorption process is exothermic. The apparent equilibrium constants (K(a)) are also calculated and found to decrease with increase in temperature.

Studies on Anion Promoted Titania.1: Preparation, Characterization, and Catalytic Activity toward Alcohol and Cumene Conversion Reactions of Phosphated Titania

Phosphate impregnated titania samples with varying amount of phosphate have been prepared by solid-solid kneading as well as aqueous impregnation method. All the samples are characterized by XRD, TG-DTA, and N(2) adsorption-desorption isotherm. Surface area is found to increase with the increase in phosphate content up to 7.5 wt% loading and thereafter decreases. The average pore diameter and crystallite size of titania decreases with the addition of phosphate. However, total acidity (determined by base adsorption method) and the catalytic activity increases with the increase in phosphate content up to 10 wt%. Phosphated samples prepared using phosphoric acid as the source of phosphate exhibit higher acidity compared to the samples prepared using (NH(4))(3)PO(4). However, the sample prepared from (NH(4))(3)PO(4) shows the presence of both acid and basic sites. Though from the cumene conversion study it is understood that phosphated samples contain both Lewis and Brønsted acid sites, the latter predominates over the former. Copyright 1999 Academic Press.

SO2-4/TiO2-SiO2 Mixed Oxide Catalyst, I: Synthesis, Characterization, and Acidic Properties

A series of sulfate-doped titania-silica mixed oxides have been prepared by immersing titania-silica gel in the required volume of sulfuric acid, followed by drying. The mixed oxide gel is obtained by hydrolyzing an equimolar mixture of tetraethylorthosilicate (TEOS) and tetrabutylorthotitanate (TBOT) at pH 3. The materials, after calcining at 723 K for 4 h, are characterized by XRD, FT-IR, the BET method, and surface acid strength by the Hammett indicator method. The catalytic activity tests are carried in a fixed bed catalytic reactor (i.d. = 10 mm) for alcohol conversion, whereas cumene cracking/dehydrogenation reactions are carried out in a micropulse reactor. XRD results shows that the titania-silica mixture is amorphous and the crystallization starts with sulfation. The surface of the mixed oxide contains both bridged and normal hydroxyl groups, as observed from FT-IR data. The surface area of the material is not much altered by sulfation and lies within 50 m2/g. The acid strength of 4 wt% SO2-4/TiO2-SiO2 is found to be stronger than that of 100% concentrated H2SO4. In the case of 2-propanol conversion, low acetone selectivity indicates the presence of weak basic sites, whereas methanol conversion over all solids shows that dehydration follows a parallel and consecutive pathway. A good correlation is found between the cumene cracking and the acidity of the catalysts. Copyright 1999 Academic Press.

Studies on Indian Ocean Manganese Nodules

The effects of alkali (sodium hydroxide) treatment on the physico-chemical properties and catalytic activity for H2O2 decomposition and CO oxidation of Indian Ocean manganese nodules have been studied. The surface area, surface oxygen, surface hydroxyl groups, etc. increase with alkali treatment up to 0.05 M and there after show a decreasing trend. The high catalytic activity of 0.05 M NaOH treated samples are correlated with the surface properties. Copyright 1999 Academic Press.

Studies on Indian Ocean Manganese Nodules

The oxidation of thiols to corresponding disulfides by Indian Ocean ferromanganese nodules has been studied under varying experimental conditions. More than 90% conversion of thiols (2.5 x 10(-3) mol) was achieved at 35&deg;C using 0.1 g nodules. The oxides of Mn, Fe, Ca, Mg, and Al and surface oxygen in the nodules are most likely responsible for the oxidation of thiols. Under identical conditions the oxidative conversion of thiols decreases in the order 1-dodecanethiol < 1-hexanethiol < 1,4-butanedithiol < alpha-toluenethiol. Copyright 1998 Academic Press. Copyright 1998Academic Press

Studies on Indian Ocean Manganese Nodules

Adsorption of aqueous selenite (SeO32-) on Indian Ocean manganese nodules was studied as a function of time, temperature, pH, and concentrations of adsorbate and adsorbent in acetic acid-sodium acetate buffer medium. Analysis of adsorption data supports a heterogeneous nature for the surface of manganese nodules. The adsorption capacity of various manganese nodules for selenite was correlated with their chemical composition and surface properties.

Studies on Indian Ocean Manganese Nodules

A series of strontium-lanthanum oxide mixed manganese nodules were prepared and characterized by X-ray diffraction, nitrogen adsorption-desorption, electrical conductivity, and surface excess oxygen. X-ray diffraction patterns confirm the formation of perovskite-type oxides at low strontium content (x </= 0.5, in La1-xSrxMO3, where M denotes the transition metal present in nodules), whereas at 0.6 </= x </= 0.8, La2O3 and SrO are detected in addition to the perovskite-type oxides. The catalytic activities for hydrogen peroxide decomposition and carbon monoxide oxidation increase with an increase in Sr substitution for La, attain a maximum at x = 0.4, and then decrease with further increase in Sr. The activities have been correlated with various physicochemical properties.

Studies on Ferric Oxide Hydroxides

Adsorption of selenite (SeO2-3) on different polymorphic forms of iron oxyhydroxides and amorphous ferrihydrite was studied as a function of time, temperature, pH, and concentration of adsorbate(s) and adsorbent(s). Analysis of adsorption data indicates that the surfaces of all the forms of oxyhydroxides and ferrihydrite are heterogeneous in nature and that adsorption fits into a heterogeneous site binding model. The adsorption capacity of oxyhydroxides for SeO2-3 follows the order beta-FeOOH < alpha-FeOOH < gamma-FeOOH < delta-FeOOH < ferrihydrite.

Studies on Indian Ocean Manganese Nodules

A series of rare earth oxide mixed manganese nodules (perovskite type mixed oxides) have been prepared by coprecipitation method followed by calcination at 900&deg;C and characterized by different techniques. The effect of rare earth ions on the surface, textural and catalytic activity for the oxidation of CO, and decomposition of H2O2 have been discussed. The activity for CO oxidation followed an order: Pr >/= La > Nd > Yb > Dy > Tb > Sm > Tm > Gd > Eu > Ce.