Removal of geosmin (trans-1,10-dimethyl-trans-9-decalol) from aqueous solution using an indirect electrochemical method

Geosmin was effectively removed using an electrochemical method at the Ti/RuO₂-Pt anode in the presence of NaCl as a reactant. At a current density of 40 mA cm⁻², the geosmin concentration decreased from 600 ng L⁻¹ to 6 ng L⁻¹ in 60 min in the presence of 3.0 g L⁻¹ NaCl. HOCl formed during electrolysis would likely play an important role in the geosmin oxidation. The electrochemical method exhibited good performance for geosmin removal under various conditions. The geosmin removal rate increased with increasing current density, while the geosmin removal rates were similar at different initial pH values. The Ti/RuO₂-Pt anode also performed well for electrochemical degradation of geosmin at both low and high concentrations. According to the present experimental results, the electrochemical method should be a promising alternative for the efficient and rapid removal of musty odor compounds in a water treatment process.     

Electrochemical oxidation of organics at metal oxide electrodes: The incineration of oxalic acid at IrO2-Ta2O5 (DSA-O2) anode

The electrochemical incineration of oxalic acid (OA) at Ti/IrO₂-Ta₂O₅ (DSA-O₂) anode was investigated to find the influence of the operative parameters on the performances of the process. Polarization curves and chronoamperometric measurements indicate the probable occurrence of a direct electrochemical oxidation of OA at the surface of the DSA anode. In incineration electrolyses, the performances of the process in terms of OA conversion and current efficiency dramatically depend on the adopted operative conditions. Interestingly, very high OA removal and current efficiency were obtained when the process was performed at relatively high temperatures (50 °C) or in the presence of NaCl. The experimental results are in good agreement with the previsions of a simple theoretical model previously developed.     

Oxygen reduction behavior of rutile-type iridium oxide in sulfuric acid solution

Two different forms of rutile-type iridium oxide catalysts were prepared: IrO₂-coated titanium plate electrocatalysts prepared by a dip-coating method (IrO₂/Ti) and iridium oxide nanoparticles (IrO₂) prepared by a wet method, the Adams fusion method. The catalytic behavior of the oxygen reduction reaction (ORR) was evaluated by cyclic voltammetry in 0.5 M H₂SO₄ at 60 °C. Both catalysts were found to exhibit considerable activity for the ORR; however, the former oxide electrodes showed higher activity than the latter ones. All the IrO₂/Ti catalyst electrodes heat-treated at a temperature between 400 °C and 550 °C showed ca. 0.84 V (vs. RHE) of the onset potential for the ORR, E_ORR, where the reduction current of oxygen had begun to be observed during the cathodic potential sweep of the test electrodes. It has been confirmed clearly that IrO₂, but neither metallic Ir nor the hydrated IrO₂, behaves as an active catalyst for the ORR in an acidic solution. It was also demonstrated that the enlargement of the surface area of the IrO₂/Ti with the help of lanthanum is effective for the enhancement of the catalytic activity in the reaction.     

DSA electrochemical treatment of olive mill wastewater on Ti/RuO<Subscript>2</Subscript> anode

The electrochemical oxidation of olive mill wastewater (OMW) over a Ti/RuO₂ anode was studied by means of cyclic voltammetry and bulk electrolysis and compared with previous results over a Ti/IrO₂ anode. Experiments were conducted at 300–1,220 mg L⁻¹ initial chemical oxygen demand (COD) concentrations, 0.05–1.35 V versus SHE and 1.39–1.48 V versus SHE potential windows, 15–50 mA cm⁻² current densities, 0–20 mM NaCl, Na₂SO₄, or FeCl₃ concentrations, 80 °C temperature, and acidic conditions. Partial and total oxidation reactions occur with the overall rate being near first-order kinetics with respect to COD. Oxidation at 28 Ah L⁻¹ and 50 mA cm⁻² leads to quite high color and phenols removal (86 and 84%, respectively), elimination of ecotoxicity, and a satisfactory COD and total organic carbon reduction (52 and 38%, respectively). Similar performance can be achieved at the same charge (28 Ah L⁻¹) using lower current densities (15 mA cm⁻²) but in the presence of various salts. For example, COD removal is less than 7% at 28 Ah L⁻¹ in a salt-free sample, while addition of 20 mM NaCl results in 54% COD reduction. Decolorization of OMW using Ti/RuO₂ anode seems to be independent of the presence of salts in contrast with Ti/IrO₂ where addition of NaCl has a beneficial effect on decolorization.     

Investigation of the properties of Ti/[IrO2-Nb2O5] electrodes for simultaneous oxygen evolution and electrochemical ozone production, EOP

A systematic investigation of the influence of Ti/[IrO₂-Nb₂O₅] electrode composition ([IrO₂]=40, 45 and 50 mol%) on electrochemical ozone production (EOP), was conducted in 3.0 mol dm⁻³ H₂SO₄ in the presence and absence of 0.03 mol dm⁻³ KPF₆. “In situ” characterisation revealed all oxide layer presented similar structures, except for the 50 mol% IrO₂ nominal composition which showed a higher porosity/roughness. The introduction of KPF₆ in the electrolyte resulted in an inhibition of the oxygen evolution reaction (OER) at high current densities, improving ozone generation efficiency at i > 0.4 A cm⁻², while reducing overpotential for OER. When normalised for the area, the ozone current efficiency presented a good performance of the system. However, improvement of the electrode service life is necessary in order to support the drastic conditions observed during EOP.     

Optimization of process parameters for electrochemical nitrate removal using Box-Behnken design

Electrochemical reduction of nitrate in an undivided cell was studied in the present experiments. The optimization of the influencing factors on electrochemical reduction of nitrate by response surface methodology (RSM) was also studied. An ideal condition of performing both cathodic reduction of nitrate and anodic oxidation of the formed by-product in the presence of NaCl was achieved in the present experiment. The Box-Behnken design can be employed to develop mathematical models for predicting electrochemical nitrate removal geometry. The removal is sensitive to the current density and time in the present study. The value of R² > 0.99 for the present mathematical model indicates the high correlation between observed and predicted values. The optimal NaCl dosage, current density and electrolysis time for nitrate removal in the present experiment are 0.47 g L⁻¹, 26.06 mA cm⁻², and 111.88 min, respectively, at which the nitrate nitrogen (nitrate-N) and ammonia nitrogen (ammonia-N) concentration in the treated solution are 9.80 and 0 mg L⁻¹, respectively, which will meet the standards for drinking water.     

Synthesis and characterization of MoOx-Pt/C and TiOx-Pt/C nano-catalysts for oxygen reduction

The oxygen reduction reaction (ORR) was studied at carbon supported MoO_x-Pt/C and TiO_x-Pt nanocatalysts in 0.5 mol dm⁻³ HClO₄ solution, at 25 °C. The MoO_x-Pt/C and TiO_x-Pt/C catalysts were prepared by the polyole method combined by MoO_x or TiO_x post-deposition. Home made catalysts were characterized by TEM and EDX techniques. It was found that catalyst nanoparticles were homogenously distributed over the carbon support with a mean particle size about 2.5 nm. Quite similar distribution and particle size was previously obtained for Pt/C catalyst. Results confirmed that MoO_x and TiO_x post-deposition did not lead to a significant growth of the Pt nanoparticles.The ORR kinetics was investigated by cyclic voltammetry and linear sweep voltammetry at the rotating disc electrode. These results showed the existence of two E − log j regions, usually observed with polycrystalline Pt in acid solution. It was proposed that the main path in the ORR mechanism on MoO_x-Pt/C and TiO_x-Pt/C catalysts was the direct four-electron process with the transfer of the first electron as the rate-determining step. The increase in catalytic activity for ORR on MoO_x-Pt/C and TiO_x-Pt/C catalysts, in comparison with Pt/C catalyst, was explained by synergetic effects due to the formation of the interface between the platinum and oxide materials and by spillover due to the surface diffusion of oxygen reaction intermediates.     

Carbothermic reduction synthesis of Ti(C,N) powder in the presence of molten salt

In the presence of sodium chloride (NaCl), Ti(C, N) powder was successfully obtained by the carbothermal reduction of TiO₂ in lab-scale experiments. The effects of NaCl addition and reaction temperature on the formation of the powder were studied in the temperature range of 1100–1600 °C, the reaction time used in all cases was 3 h. The final powder was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The results indicated that addition of NaCl played a facilitating role in the formation process of Ti(C, N). Ti(C, N) was detected at 1100–1200 °C, and the yield of powder was purer at about 1300 °C when 10 wt.% NaCl was added. The as-prepared Ti(C, N) was uniform in shape and the particle size was about 5–8 μm. With increasing temperature, the residual carbon content in the products decreased but the degree of oxidation increased at temperatures above 1300 °C. The possible mechanism involved in the reactions was discussed.     

Electroreduction of nitrate ions on Pt(1 1 1) electrodes modified by copper adatoms

Kinetics and mechanism of nitrate ion reduction on Pt(1 1 1) and Cu-modified Pt(1 1 1) electrodes have been studied by means of cyclic voltammetry, potentiostatic current transient technique and in situ FTIRS in solutions of perchloric and sulphuric acids to elucidate the role of the background anion. Modification of platinum surface with copper adatoms or small amount of 3D-Cu crystallites was performed using potential cycling between 0.05 and 0.3 V in solutions with low concentration of copper ions, this allowed us to vary coverage θ_Cu smoothly. Following desorption of copper during the potential sweep from 0.3 to 1.0 V allowed us to estimate actual coverage of Pt surface with Cu adatoms. Another manner of the modification was also applied: copper was electrochemically deposited at several constant potentials in solutions containing 10⁻⁵ or 10⁻⁴ M Cu²⁺ and 5 mM NaNO₃ with registration of current transients of copper deposition and nitrate reduction.It has been found that nitrate reduction at the Pt(1 1 1) surface modified by copper adatoms in sulphuric acid solutions is hindered as compared to pure platinum due to induced sulphate adsorption at E < 0.3 V. Sulphate blocks the adsorption sites on the platinum surface and/or islands of epitaxial Cu(1 × 1) monolayer thus hindering the adsorption of nitrate anions and their reduction. The extent of inhibition weakly depends on the copper adatom coverage. Deposition of a small amount of bulk copper does not affect noticeably the rate of nitrate reduction.Nitrate reduction on copper-modified Pt(1 1 1) electrodes in perchloric acid solutions occurs much faster as compared to pure platinum. The steady-state currents are higher by 4 and 2 orders of magnitude at the potentials of 0.12 and 0.3 V, respectively. The catalytic effect of copper adatoms is largely caused by the facilitation of nitrate adsorption on the platinum surface near Cu_ad and/or on the islands of the Cu(1 × 1) monolayer (induced nitrate adsorption).Hydrogen adatoms block the adsorption sites on platinum for NO₃⁻ anion adsorption and inhibit reactions of nitrate reduction even at moderate surface coverage.The products of nitrate reduction in sulphuric and perchloric acids are essentially the same (NO and ammonia) irrespective of the presence or absence of Cu on the platinum surface.     

Electrochemical removal of phenol in alkaline solution. Contribution of the anodic polymerization on different electrode materials

The removal of organic pollutants based on electropolymerization on an anode was performed in the case of phenol in alkaline solution. The polymer formed by a process involving less than two electrons per molecule of phenol, is then precipitated by decreasing the pH and finally filtered and disposed. The electrochemical polymerization of phenol (C₀ = 0.105 M) in alkaline solution (pH = 13) at 86 °C has been studied by galvanostatic electrolysis, using a range of anode materials characterized by different O₂-overpotentials (IrO₂, Pt and β-PbO₂). Measurements of total organic carbon and HPLC have been used to follow phenol oxidation; the morphology of the polymer deposited on the electrode surface has been examined by SEM. Experimental data indicate that phenol concentration decreases by oxidation according to a first order reaction suggesting a mass transport limitation process. Polymeric films formed in alkaline solution did not cause the complete deactivation of the anodes. SEM results show that the polymeric films formed on Ti/IrO₂ and Pt anodes cannot be mineralized. On the other hand, complex oxidation reactions leading to the partial incineration of polymeric materials can take place on the Ta/β-PbO₂ surface due to electrogenerated HO radicals which have an oxidizing power much higher than that of intermediaries formed respectively on IrO₂ and Pt. It is assumed that the polymer films formed on these anodes have different permeability characteristics which determine the rate of mass transfer of the phenol. The fractions of phenol converted in polymers were 25, 32 and 39% respectively with Ti/IrO₂, Pt and Ta/β-PbO₂, a series of materials in which the O₂-overvoltage increases.     

Influence of the concentration and the nature of the supporting electrolyte on the electrochemical reduction of nitrate on tin cathode

The influences of the potential, the concentration and the nature of the supporting electrolyte on the rate of the reduction of nitrate on tin were studied by both voltammetry and constant potential electrolytic experiments.Both the rate of the reduction of nitrate and the yield of nitrogen increase as the negative potential increases from −1.8 to −2.8 V versus Ag/AgCl, while the yield of nitrite decreases. The yield of ammonia displays a maximum at −2.4 V and consequently decreases.The rate of the reduction at −1.8 V versus Ag/AgCl increases significantly as the concentration of NaCl increases. The cation of the supporting electrolyte increases the rate of the reduction along the series Li⁺ < Na⁺ < K⁺ < Cs⁺. Higher rates than that of the alkalimetals have been obtained in the presence of ammonium as well as of multivalent cations such as Ca²⁺ and La³⁺. The anion of the supporting electrolyte decreases the rate of the reduction in the order I⁻ > Br⁻ > Cl⁻ > F⁻ at −1.8 V.The experimental results were qualitatively explained by the Frumkin theory and additionally by the theory of the formation of ion pairs between the cation of the supporting electrolyte and the reacting nitrate.     

A study of RhxSy/C and RuSex/C as methanol-tolerant oxygen reduction catalysts for mixed-reactant fuel cell applications

For efficient operation, mixed-reactant fuel cells utilise highly selective anode and cathode electrocatalysts. While platinum and its alloys are the most widely used ORR electrocatalysts in conventional DMFCs, they suffer from both their very high activity for methanol oxidation and their inherent cost. Platinum-free precious metal chalcogenides have been suggested as alternatives with comparable oxygen reduction activity in the presence of methanol. Of these, commercially available carbon supported rhodium sulphide and developmental ruthenium selenium were electrochemically tested and assessed for their potential as selective ORR cathode catalysts. Both materials exhibited oxygen reduction activity approaching that of platinum, albeit at potentials 150 and 80 mV more negative. The three materials’ ability to maintain their oxygen reduction activity in the presence of methanol ranks ruthenium selenium > rhodium sulphide ? platinum.     

Anodic oxidation of phenol in the presence of NaCl for wastewater treatment

The electrochemical oxidation of phenol in the presence of NaCl for wastewater treatment was studied at Ti/SnO₂ and Ti/IrO₂ anodes. The experimental results have shown that the presence of NaCl catalyses the anodic oxidation of phenol only at Ti/IrO₂ anodes due to the participation of electro-generated ClO^– in the oxidation. Analysis of the oxidation products has shown that initially organo-chlorinated compounds are formed in the electrolyte which are further oxidized to volatile organics (CHCl₃).     

Determination of the kinetic parameters of oxygen reduction on copper using a rotating ring single crystal disk assembly (RRDCu(h k l)E)

The kinetics of the oxygen reduction reaction (orr) on Cu(h k l) surfaces are investigated in perchloric acid and sulfuric acid solutions using rotating ring disk electrode (RRD_Cu(h k l)E). Parameters, such as reaction order, kinetic current, rate constant, Tafel slopes as well as the number of electrons transferred are determined. The variation in the activity and reaction pathway with the crystal faces in different electrolytes is related to the surface characteristics of Cu(h k l) and the structure-sensitive inhibiting effect of the adsorbed anions on their surfaces. In 0.1 M HClO₄, the difference in activity is clearly observed on Cu(h k l) surfaces (Cu(1 0 0) > Cu(1 1 1) although it is relatively small). The higher activity of Cu(1 0 0) arises from its more open characteristics which may facilitate the co-adsorption of O₂. On the other hand, the adsorption of oxygenated species on Cu(1 1 1) at E > −0.35 V induces a 2 e⁻ pathway; while a 4 e⁻ reduction is observed on Cu(1 0 0) in the entire potential region (−0.70 V < E < −0.10 V). In 0.5 M H₂SO₄, the sequence in activity between Cu(1 1 1) and Cu(1 0 0) varies with the potentials, i.e., Cu(1 0 0) is initially more active than Cu(1 1 1) at −0.35 V < E < −0.15 V, however, the reversal in the activity between Cu(1 1 1) and Cu(1 0 0) is observed at more negative potentials (−0.45 V < E < −0.35 V). The desorption of strongly adsorbed (bi)sulfate anions on Cu(1 1 1) induces the 2 e⁻ reduction via peroxide formation, however, a 4 e⁻ reduction is dominant on the Cu(1 0 0) surfaces. The major effect of (bi)sulfate anions and oxygenated species on the orr kinetics and reaction pathway on Cu(h k l) surfaces is the blocking of active copper sites for the adsorption of O₂ molecules.     

Electrocatalytic oxidation of coal on Ti-supported metal oxides coupled with liquid catalysts for H2 production

Electrocatalytic oxidation of coal on Ti-supported metal/metal oxides coupled with liquid catalysts is systematically investigated as a method of producing hydrogen at the cathode. The composition of the liquid catalyst was varied to determine its effect on the coal electrolysis. A spectrum of byproducts from the coal oxidation at the anode was analyzed. The Ti-supported metal oxide electrodes were prepared by thermal decomposition and characterized by scanning electron microscopy (SEM). X-ray diffraction results show that the composition of the electrodes was Ti/Pt, Ti/RuO₂, Ti/IrO₂ and Ti/IrO₂–RuO₂. Coal oxidation tests on these electrodes indicate that Ti/IrO₂ has the best electrocatalytic activity. Polarization curves reveal that redox catalysts, such as Fe³⁺, K₃Fe(CN)₆, KBr and V₂O₅, bridge the coal particles and the solid electrode surface, thus increasing the rates of coal oxidation. The dynamic transition of Fe³⁺/Fe²⁺ is proven by a KMnO₄ titration experiment, and the possible catalytic mechanism is discussed. Product analysis shows that pure H₂ is generated at the cathode and that CO₂ is the main product at the anode.     

Study of carbon-supported IrO2 and RuO2 for use in the hydrogen evolution reaction in a solid polymer electrolyte electrolyzer

Carbon-supported IrO₂ and RuO₂ were prepared using an incipient wetness method and were then calcinated at various temperatures. IrO₂/C and RuO₂/C are less expensive than the conventional Pt/C material and more stable than metal Ni in an acidic electrolyte. Moreover, IrO₂/C and RuO₂/C are not influenced by under potential deposition (UPD) and show lower sensitivity to poisoning by Ni or Fe impurities. The physical properties of IrO₂/C and RuO₂/C were investigated via XRD and TEM. Cyclic voltammograms (CV) and Tafel plots were used to provide information regarding surface redox reaction and electrocatalytic activity. The activity and durability of IrO₂/C and RuO₂/C were studied after prolonged potential cycling between −0.3 and 0.3 V_SCE. After comparison of Tafel plots of Pt/C and IrO₂/C after activation, it was observed that they have similar electrocatalytic activities in a hydrogen evolution reaction (HER). A single cell test with solid polymer electrolyte (SPE) proved that the performance of IrO₂/C (0.5 mg cm⁻²) was similar to that of Pt/C (0.5 mg cm⁻²).     

Influence of nitrate concentration on its electrochemical reduction on tin cathode: Identification of reaction intermediates

The influence of the concentration of nitrate in the range between 100 and 62,000 mg L⁻¹ NaNO₃ in NaCl solutions was studied under constant potential electrolysis at −2.8 V vs. Ag/AgCl. The rate of the reduction follows Langmuir-Hinshelwood kinetics, according to which zero order kinetics is followed at concentrations higher than 0.3 M whereas first order at lower concentrations.The selectivity to nitrogen increases from 70 to 83% as the concentration of nitrate increases from 100 to 1500 mg L⁻¹ and it remains almost constant for higher nitrate concentrations, whereas that of ammonia exhibits the opposite trend decreasing from 25 to 11%. The % Faradaic Efficiency (%FE) increased with the increase of the concentration of nitrate from 25% at 0.1 M to 78% at 1 M when 95% of nitrate was reduced in both cases. At high concentrations of nitrate, hyponitrite and hydroxylamine were detected as intermediates of the reduction and a reaction scheme which is in agreement with the experimental results has been proposed.The hydrogen evolution in our conditions probably takes place through the discharge of the cation of the supporting electrolyte instead of the Volmer-Tafel mechanism and the reduction of nitrate proceeds through electrochemical hydrogenation.     

Electrochemical activity, stability and degradation characteristics of IrO2-based electrodes in aqueous solutions containing C1 compounds

In this paper, we investigate the electrocatalytic behavior and degradation characteristics of IrO₂-based electrodes in Na₂SO₄ solutions containing C₁ compounds (CH₃OH, HCHO and HCOOH). Decreases are generally observed in the electrochemically active area, electrochemical stability and durability of the electrodes in aqueous solutions in the presence of these organic substrates. The following sequence holds for the influence of C₁ compounds on the electrode properties (i.e. activity and stability): CH₃OH > HCHO > HCOOH. The corrosion characteristics of electrode are studied by X-ray diffraction measurements. For the first time, the decomposition and dissolution of active oxide layers are quantitatively characterized from the decreases in cell volume of rutile-structured IrO₂ crystallite and from the increases in texture coefficient of (0 0 2) planes, respectively, as a result of the accelerated lifetime tests.     

Comparative studies on the electrocatalytic properties of modified PbO2 anodes

A set of modified PbO₂ anodes doped with the oxides of bismuth and cobalt were prepared by means of electrodeposition in nitrate solutions. Of them, Ti/Bi-PbO₂ anode displayed excellent electrocatalytic performance. Cyclic voltammetric experiments were carried out to get a better understanding of the electrocatalytic properties of these anodes. The Ti/Bi-PbO₂ anode had the highest overpotential of 1.75 V (vs. SCE) for oxygen evolution. Both XRD patterns and scanning electronic microscopy (SEM) images demonstrated that the incorporation of Bi could diminish the size of the crystal particles. Oxidants such as hydroxyl radical, hydrogen peroxide were determined, and their amount was proportional to the electrocatalyitc activities of modified PbO₂ anodes. Electrocatalytic oxidation of o-nitrophenol was conducted by using these anodes as anode and stainless steel sheet as cathode. Ti/Bi-PbO₂ anode displayed not only excellent electrocatalytic performance but also low energy consumption. The Ti/Bi-PbO₂ anode is a promising anode for the treatment of organic pollutants.     

Electrochemical removal of nitrate ions in waste solutions after regeneration of ion exchange columns

Electrochemical reduction of nitrate ions in synthetic regenerating solutions after ion exchange column regeneration was studied. The influence of current density on the current efficiency was determined in the range 2.8–7.6 mA cm–2 in a diaphragmless flow-through electrolyser in a batch recirculation mode. A Cu cathode and a Ti/Pt anode was used, the temperature being maintained at 25 C. Highest integral current efficiency occurred at 2.8 mA cm–2. The presence of about 6 mg dm–3 Cu ions in treated solution was found to prevent a decrease in cathode activity and, consequently, in electrolysis efficiency. The catalytic influence of Cu ions was verified by potentiodynamic polarisation experiments on a copper rotating disc electrode and by chronopotentiometry performed during the course of electrolysis.