Electroless oxidation of boron-doped diamond surfaces: comparison between four oxidizing agents; Ce, MnO4, H2O2 and S2O8

The electrochemical properties of diamond are very sensitive to the surface terminations. It is still a challenge to successfully produce well-defined “C-O” functions. In this paper, we describe and compare the oxidation of as-grown polycrystalline boron-doped diamond (BDD) films using four different oxidizing agents in aqueous media: Ce⁴⁺, MnO₄⁻, H₂O₂ and S₂O₈²⁻. The different treatments lead to the formation of oxygenated functions at the diamond surface, mainly singly oxidized “C-O” groups such as “C-OH” or “C-O-C”. Processes with Ce⁴⁺ and MnO₄⁻ seem to be particularly interesting as they both lead to the creation of a high amount of oxygenated functions and an improvement of the charge transfer at BDD surfaces.     

The importance of electrode material in environmental electrochemistry: Formation and reactivity of free hydroxyl radicals on boron-doped diamond electrodes

A model describing the hydroxyl radical (HO) concentration profile at the boron-doped diamond (BDD) electrode, in the presence and absence of organic compounds, is presented. It is shown that this profile depends strongly on the reaction rate constant between the HO and the organic compound. Furthermore, it is shown that the presence of organics affects the current-potential (I-V) curves. In fact, the higher the reaction rate between organics and HO, the higher is the shift of the I-V curves toward lower potential with respect to oxygen evolution. Supposing that water discharge to free hydroxyl radicals on BDD is governed by Nernst equation, this shift of the I-V curves toward lower potentials has been calculated and compared with the experimental data obtained on BDD using two model compounds: methanol and formic acid.     

Conductive‐diamond electrochemical advanced oxidation of naproxen in aqueous solution: optimizing the process

BACKGROUND: Electrochemical advanced oxidation treatment using boron‐doped diamond (BDD) electrodes is a promising technology to treat small amounts of toxic and biorefractory pollutants in water. This process has been tested on the degradation of naproxen, a common pollutant drug present in surface waters. To optimize the process a series of experiments have been designed to study the interaction between four variables: pH (over the range 5–11); current (0–320 mA cm^?2); supporting Na₂SO₄ electrolyte concentration (0–0.375 mol L^?1); and solution flow rate (Q_v) between 3.64 and 10.8 cm³ min^?1. RESULTS: Among these variables the influence of current was the greatest, the second was the salt concentration, the third flow rate, and the fourth pH. An ANOVA test reported significance for seven of the fourteen variables involved and the degradation of naproxen was optimized using response surface methodology. CONCLUSIONS: Optimum conditions for naproxen removal (100%) were found to be pH = 10.70, Q_v = 4.10 cm³ min^?1, current density = 194 mA cm^?2 using a supporting electrolyte concentration of 0.392 mol L^?1. Copyright © 2010 Society of Chemical Industry     

Electrochemical degradation of clofibric acid in water by anodic oxidation: Comparative study with platinum and boron-doped diamond electrodes

Aqueous solutions containing the metabolite clofibric acid (2-(4-chlorophenoxy)-2-methylpropionic acid) up to close to saturation in the pH range 2.0-12.0 have been degraded by anodic oxidation with Pt and boron-doped diamond (BDD) as anodes. The use of BDD leads to total mineralization in all media due to the efficient production of oxidant hydroxyl radical (OH). This procedure is then viable for the treatment of wastewaters containing this compound. The effect of pH, apparent current density, temperature and metabolite concentration on the degradation rate, consumed specific charge and mineralization current efficiency has been investigated. Comparative treatment with Pt yields poor decontamination with complete release of stable chloride ion. When BDD is used, this ion is oxidized to Cl₂. Clofibric acid is more rapidly destroyed on Pt than on BDD, indicating that it is more strongly adsorbed on the Pt surface enhancing its reaction with OH. Its decay kinetics always follows a pseudo-first-order reaction and the rate constant for each anode increases with increasing apparent current density, being practically independent of pH and metabolite concentration. Aromatic products such as 4-chlorophenol, 4-chlorocatechol, 4-chlororesorcinol, hydroquinone, p-benzoquinone and 1,2,4-benzenetriol are detected by gas chromatography-mass spectrometry (GC-MS) and reversed-phase chromatography. Tartronic, maleic, fumaric, formic, 2-hydroxyisobutyric, pyruvic and oxalic acids are identified as generated carboxylic acids by ion-exclusion chromatography. These acids remain stable in solution using Pt, but they are completely converted into CO₂ with BDD. A reaction pathway for clofibric acid degradation involving all these intermediates is proposed.     

Electrochemical oxidation of acid black 210 dye on the boron-doped diamond electrode in the presence of phosphate ions: Effect of current density, pH, and chloride ions

The electrochemical oxidation of acid black 210 dye (AB-210) on the boron-doped diamond (BDD) was investigated under different pH conditions. The best performance for the AB-210 oxidation occurred in alkaline phosphate solution. This is probably due to oxidizing agents such as phosphate radicals and peroxodiphosphate ions, which can be electrochemically produced with good yields on the BDD anode, mainly in alkaline solution. Under this condition, the COD (chemical oxygen demand) removal was higher than that obtained from the model proposed by Comninellis. Electrolyses performed in phosphate buffer and in the presence of chloride ions resulted in faster COD and color removals in acid and neutral solutions, but in alkaline phosphate solution, a better performance in terms of TOC removal was obtained in the absence of chloride. Moreover, organochloride compounds were detected in all electrolyses performed in the presence of chloride. The AB-210 electrooxidation on BDD using phosphate as supporting electrolyte proved to be interesting since oxidizing species generated from phosphate ions were able to completely degrade the dye without producing organochloride compounds.     

The high catalytic activity and strong stability of 3%Fe/AC catalysts for catalytic wet peroxide oxidation of m-cresol: The role of surface functional groups and FeOx particles

FeO supported on activated carbon (AC) has been shown to be an ideal catalyst for catalytic wet peroxide oxidation (CWPO) due to its high CWPO reaction activity and stability. Although there have been some studies on the mechanism of Fe/AC catalysis in CWPO, the specific contribution of each component (surface oxygen groups and FeO_x on AC) inside an Fe/AC catalyst and their corresponding reaction mechanism remain unclear, and the reaction stability of CWPO catalysts has rarely been discussed. Then the optimal CWPO catalyst in our laboratory, 3%Fe/AC, was selected. (1) By removing certain components on the AC through heat treatment, its contribution to the reaction and the corresponding reaction mechanism were investigated. With the aid of temperature-programmed desorption–mass spectrometry (TPD–MS) and the CWPO reaction, the normalized catalytic contributions of components were shown to be: 37.3% (carboxylic groups), 5.3% (anhydride), 19.3% (ether/hydroxyl), -71.4% (carbonyl groups) and 100% (FeO_x), respectively. DFT calculation and EPR analysis confirmed that carboxylic groups and Fe₂O₃ are able to activate the H₂O₂ to generate ·OH. (2) The catalysts at were characterized at different reaction times (0 h, 450 h, 900 h, 1350 h, and 1800 h) by TPD–MS and Mössbauer spectroscopy. Results suggested that the number of carboxylic goups gradually increased and the size of paramagnetic Fe₂O₃ particle crystallites gradually increased as the reactions progressed. The occurrence of strong interactions between metal oxides and AC was also confirmed. Due to these effects, the strong stability of 3%Fe/AC was further improved. Therefore, the reasons for the high activity and strong stability of 3%Fe/AC in CWPO were clearly shown. We believe that this work provides an idea of the removal of cresols from wastewater into the introduction to show the potential applications of CWPO.     

Hreels of methanol oxidation on polycrystalline silver: Spectroscopic evidence for adsorbed intermediates in the conversion of methoxide to formaldehyde and water

The adsorption and the decomposition of methanol on an oxygen precovered polycrystalline silver surface were characterized by high resolution electron energy loss spectroscopy. A stable methoxy species and subsequently two new intermediates related to the decomposition of methoxy have been isolated and identified by temperature profiled ELS.     

Electrocatalytic reduction of chromium by poly(aniline-co-o-aminophenol): An efficient and recyclable way to remove Cr(VI) in wastewater

Poly(aniline-co-o-aminophenol) (PANOA)-modified glassy carbon electrode (GCE) was first used to investigate the electrocatalytic reduction of dichromate in a NaCl solution of pH 5.0. The results of cyclic voltammograms and UV–vis spectra demonstrated that the reduction of Cr(VI) occurred at PANOA-modified GCE. The FT-IR, ESR and XPS results showed that the Cr(VI) can be doped in the PANOA films and can convert to less-toxic Cr(III). The doping level Cr/N was 78.2% and trace amount of Cl (0.42%) was detected in the doped PANOA, which indicated that the doping process is effective and PANOA had rather good ion selectivity in the 0.10 M NaCl supporting electrolyte. The factors influenced the reduction were also considered. Various initial concentrations of Cr(VI) had different removal rates. The maximum removal rate of Cr(VI) at 20 mg L⁻¹ (32.3%) was better than that of at 5 mg L⁻¹ (22.9%). The solution pH had little effect on Cr(VI) reduction and doping process of the PANOA because PANOA had good electrochemical activity and stability in a wide range of pHs (from pH 4 to pH 8).     

From single crystals to supported nanoparticles in experimental and theoretical studies of H2 oxidation over platinum metals (Pt, Pd): Intermediates, surface waves and spillover

The present paper reviews our investigations concerning the mechanism of H₂ + O₂ reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti³⁺/TiO₂). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H₂ oxidation on a nanolevel. The adsorption of both O₂ and H₂ and several dissociative products (H_ads, O_ads, OH_ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H₂O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti³⁺-□_O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H₂ + O₂ steady-state reaction over Pd–Ti³⁺/TiO₂ surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H⁺/TiO_x) as a result of spillover effect: diffusion of H_ads atoms from Pd particles on a TiO_x surface. The comprehensive study of H₂, O₂ adsorption and H₂ + O₂ reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces.