Purification of Wet‐Process Phosphoric Acid by Hydrogen Peroxide Oxidation,Activated Carbon Adsorption and Electrooxidation

In this work, three technologies are studied for the purification of phosphoric acid produced by the wet process: chemical oxidation with hydrogen peroxide, adsorption onto activated carbon, and electrochemical oxidation by boron‐doped diamond anodes. The treatment of wet‐process phosphoric acid by chemical oxidation with H₂O₂ as oxidizing agent can remove 75 % of the initial TOC as maximum, indicating that this wet‐process phosphoric acid contains an important amount of organics that cannot be oxidized by hydrogen peroxide under the operation conditions used. High temperatures and hydrogen peroxide/TOC ratios close to 150 g H₂O₂/g TOC allow obtaining the best chemical oxidation results. The adsorption onto activated carbon can remove between 40 and 60 % of the initial TOC as maximum. Adsorption times of 2 hours and activated carbon/WPA ratios close to 12 g AC/Kg WTP assure both steady state and maximum adsorption of organics. The electrochemical process is the only technique by which complete mineralization of WPA organics can be achieved. Operating at 60 mA cm^–2 and at room temperature, high current efficiencies are achieved which only seem to decrease by mass transport limitations.     

New Supported Pd and Pt Alloy Catalysts for Steam Reforming and Dehydrogenation of Methanol

The catalytic performances of supported Group 810 metal (Co, Ni, Ru, Pd, Ir and Pt) catalysts for steam reforming of methanol, CH₃OH + H₂O CO₂ + 3H₂, and dehydrogenation of methanol to methyl formate, 2CH₃ OH HCOOCH₃ + 2H₂, are markedly affected by the kinds of supports as well as the metals used. The selectivity for steam reforming and the formation of methyl formate was markedly improved when Pd or Pt were supported on ZnO, In₂O₃ and Ga₂O₃. The combined results of temperature-programmed reduction, XRD, XPS and AES revealed that Pd-Zn, Pd-In, Pd-Ga, Pt-Zn, Pt-In and Pt-Ga alloys were formed upon reduction. Over the catalysts having an alloy phase, the reactions proceeded selectively, whereas over the catalysts having a metallic phase, methanol was decomposed to carbon monoxide and hydrogen predominantly. It was shown that the reactivity of formaldehyde intermediate over the Pd and Pt alloys was markedly different from that over metallic Pd and Pt. Over Pd and Pt alloys, aldehyde species were stabilized and transformed into carbon dioxide and hydrogen or methyl formate by nucleophilic addition of water or methanol, respectively. By contrast, over metallic Pd and Pt, aldehyde species were rapidly decarbonylated to carbon monoxide and hydrogen.     

Analysis of sulfur poisoning on a PEM fuel cell electrode

The extent of irreversible deactivation of Pt towards hydrogen oxidation reaction (HOR) due to sulfur adsorption and subsequent electrochemical oxidation is quantified in a functional polymer electrolyte membrane (PEM) fuel cell. At 70 °C, sequential hydrogen sulfide (H₂S) exposure and electrochemical oxidation experiments indicate that as much as 6% of total Pt sites are deactivated per monolayer sulfur adsorption at open-circuit potential of a PEM fuel cell followed by its removal. The extent of such deactivation is much higher when the electrode is exposed to H₂S while the fuel cell is operating at a finite load, and is dependent on the local overpotential as well as the duration of exposure. Regardless of this deactivation, the H₂/O₂ polarization curves obtained on post-recovery electrodes do not show performance losses suggesting that such performance curves alone cannot be used to assess the extent of recovery due to sulfur poisoning. A concise mechanism for the adsorption and electro-oxidation of H₂S on Pt anode is presented. H₂S dissociatively adsorbs onto Pt as two different sulfur species and at intermediate oxidation potentials, undergoes electro-oxidation to sulfur and then to sulfur dioxide. This mechanism is validated by charge balances between hydrogen desorption and sulfur electro-oxidation on Pt. The ignition potential for sulfur oxidation decreases with increase in temperature, which coupled with faster electro-oxidation kinetics result in the easier removal of adsorbed sulfur at higher temperatures. Furthermore, the adsorption potential is found to influence sulfur coverage of an electrode exposed to H₂S. As an implication, the local potential of a PEM fuel cell anode exposed to H₂S contaminated fuel should be kept below the equilibrium potential for sulfur oxidation to prevent irreversible loss of Pt sites.     

Reaction pathways and poisons—II: The rate controlling step for electrochemical oxidation of hydrogen on Pt in acid and poisoning of the reaction by CO

The reaction mechanism for hydrogen molecule oxidation on platinum electrocatalysts in acid solutions is deduced by comparing kinetic rate parameters obtained electrochemically, to those rate parameters obtained in the gas phase from H₂—D₂ exchange. The electrochemical rate parameters were obtained from potentiodynamic scanning of smooth platinum and from polarisation curves of porous platinum black flooded electrode structures. The rate controlling step for hydrogen molecule oxidation on Pt is the dual site dissociative chemisorption of the hydrogen molecule H₂ → 2H (known as the Tafel reaction). Specific poisons for this reaction are chemisorbed carbon monoxide and adsorbed hydrogen, producing a simple site elimination for dissociative chemisorption of the hydrogen molecule and confirm the dual site mechanism. The electrochemical reaction rate parameters are the same on smooth platinum, unsupported platinum black and platinum crystallites supported on graphitised carbon, correlating with the H₂-H₂ exchange in the gas-phase, both with and without chemisorbed carbon monoxide and are dependent only upon the surface areas of platinum catalysts.     

The ability of Nb2O5 and Ta2O5 to generate active oxygen in contact with hydrogen peroxide

Amorphous and crystalline niobium(V) and tantalum(V) oxides were treated with hydrogen peroxide and studied by XRD, UV–vis, FTIR and ESR techniques to identify changes on their surface upon interaction with hydrogen peroxide. Differences between amorphous and crystalline materials in the interaction with H₂O₂ depending on the hydroxylation of the surface and the nature of OH groups were evident. The type of radical species formed on hydroxylated amorphous materials treated with H₂O₂ depended on the nature of metal oxide. It was proved that peroxo radical species formed in the interaction of H₂O₂ with amorphous Nb₂O₅ were the active intermediates in the oxidation of glycerol to glycolic acid with hydrogen peroxide. The radicals formed on amorphous Ta₂O₅ surface treated with hydrogen peroxide were poorly active in the oxidation of glycerol. Detailed study of the above mentioned radicals is in progress and will be a subject of a separate paper.     

Characterizing electrocatalytic surfaces: Electrochemical and NMR studies of methanol and carbon monoxide on Pt/C

We use cyclic voltammetry (CV) on fuel cell electrodes to elucidate the important differences between adsorbates resulting from carbon monoxide adsorption and methanol adsorption onto commercial Pt/C electrocatalysts in a sulfuric acid electrolyte. Under open circuit conditions, methanol was found to adsorb preferentially onto the Pt sites associated with “strongly bound” hydrogen. The sites associated with “weakly bound” hydrogen adsorbed methanol more slowly. In the case of CO adsorption, which requires no adsorbate dehydrogenation, all adsorption sites showed similar affinity towards the adsorbate. Electrochemical oxidation of the adsorbates derived from both methanol and CO exposure exhibit slower oxidation when the adsorbate is associated with cubic-packed-like sites than from close-packed-steps and other sites. NMR of a ¹³CO-adlayer prepared by electrochemical adsorption from low concentration ¹³CH₃OH shows a lower NMR shift and smaller linewidth than the previously reported values for electrochemically adsorbed ¹³CO gas. These results are interpreted in terms of adsorbate motion on the electrocatalyst surface.     

Influence of carbon monoxide, ethylene, acetylene, allyl alcohol and propargyl alcohol on nickel electrodissolution in aqueous sulfuric acid solution

The influence of carbon monoxide, ethylene, acetylene, allyl alcohol and propargyl alcohol on nickel electrodissolution in aqueous 0.5m H₂SO₄ was studied after the adsorption equilibrium of each of these species at a fixed potential and room temperature had been attained. The hindrance of nickel electrodissolution by carbon monoxide is greater than that resulting from double and triple C-C bond compounds at the same concentration in the solution. Corrosion current, corrosion potential, the amount of dissolved nickel at a constant potential and steady hydrogen evolution polarization data combined with voltammetric deconvolution data provide information about the nickel electrodissolution inhibition capability of these substances in acid solution.     

Study of the kinetics and the influence of Biirr on formic acid oxidation at Pt2Ru3/C

Electrochemical oxidation of HCOOH in H₂SO₄ and HClO₄ solutions was examined on thin film Pt₂Ru₃/C electrode. XRD pattern revealed that Pt₂Ru₃ alloy consisted of the solid solution of Ru in Pt and the small amount of Ru or solid solution of Pt in Ru. According to STM images, Pt₂Ru₃ particles size was between 2 and 6 nm. It was established that electrochemical oxidation of HCOOH commenced at −0.1 V versus SCE at Pt sites in the catalyst. Kinetic parameters indicated that dehydrogenation path was predominant. Dehydration occurs in parallel, but without significant poisoning by CO_ad owing to oxidative removal by OH species on Ru atoms. The coverage of Pt₂Ru₃ surface by CO preadsorbed from the solution was found to be 24% lower when the surface was modified by irreversibly adsorbed Bi. Modification by Bi also shifted the onset potential for HCOOH oxidation for about 50 mV towards more negative values and consequently, increased the reaction rate for a factor of two. It was proposed that Ru acts through bifunctional mechanism, i.e. OH species adsorbed on Ru oxidizes CO_ad from Pt sites, while Bi hinders the adsorption of CO on Pt sites via electronic and/or ensemble effects.     

The Peroxysilicate Question. 29Si-NMR Evidence for the Role of Silicates in Alkaline Peroxide Brightening of Mechanical Pulp

The influence of hydrogen peroxide on the chemistry of aqueous silicates was investigated by high-resolution ²⁹Si-NMR and electrochemical methods. The observations indicate that dissolved silicates form labile complexes with radical H₂O₂-decomposition products, possibly O₂·^?. An important role of soluble silicates in peroxide bleaching liquors thus might be to affect the activity of free radical species which mediate H₂O₂ bleaching and decomposition reactions. Attempts to prepare the widely reported peroxysilicates from silicate/H₂O₂/NaOH mixtures resulted in the isolation of Na₂O₂·8H₂O.     

Comparative study of different systems for adsorption and catalytic oxidation of hexamine in industrial wastewaters

This work reports adsorption and catalytic oxidation process for degradation of hexamine (HMT)-containing industrial wastewater based on the reduction in total organic carbon. The studied systems include hydrogen peroxide, MCM-41, phosphotungstic acid (PTA)/MCM-41 embedded via impregnation method, PTA/H₂O₂ and PTA/MCM-41 embedded via impregnation or direct synthesis methods in the presence of hydrogen peroxide. The TOC results indicated that the system including PTA embedded within MCM-41 via direct synthetic method in the presence of H₂O₂ has a higher performance for degradation of HMT-containing wastewater. The total organic carbon for the mineralization of HMT was obtained to be 57%, under optimum conditions.     

On adsorption and electro-oxidation of some compounds on tungsten carbide; their effect on hydrogen electro-oxidation

Adsorption and electro-oxidation of carbon monoxide, ethylene, acetylene, and hydrogen sulphide on tungsten carbide, in solutions of these compounds in 1 N H₂SO₄, have been investigated. It was found that CO, C₂H₄, and C₂H₂ do not undergo adsorption and oxidation and do not affect adsorption and electro-oxidation of hydrogen. H₂S does not oxidise as well, and it does not displace adsorbed hydrogen in any measurable amounts, though it does inhibit electro-oxidation of molecular hydrogen. Methanol is inert on tungsten carbide like carbon monoxide and hydrocarbons. Electro-oxidation of formaldehyde and formic acid proceeds without apparent WC-surface coverage by the adsorbed compound.     

Non-Faradaic catalysis: the case of CO oxidation over Ag-Pd alloy electrode in a solid oxide electrolyte cell

Studies of carbon monoxide oxidation on an Ag-25 at% Pd alloy electrode in a cell with a solid oxygen-conducting electrolyte - CO+O₂, Ag-Pd | 0.9 ZrO₂+0.1Y₂O₃ | Pt+PrO_2-x, air - were carried out. XRD, SEM and XPS techniques were used for characterisation of the Ag-Pd alloy electrode. The non-Faradaic effect of electrochemical oxygen pumping on the rate of carbon monoxide oxidation was demonstrated. The induced change in the reaction rate at cathodic polarization of the Ag-Pd alloy electrode was an order of magnitude higher than the rate of oxygen pumping from the reaction zone through the electrolyte. The observed phenomenon was qualitatively explained on the base of a chain reaction mechanism involving electrochemically generated oxygen species.     

Advanced Oxidation of Tannic Acid in Aqueous Solution

Decomposition of tannic acid in aqueous solution in advanced oxidation processes has been studied. Different oxidizing agents: ozone, hydrogen peroxide and UV radiation have been used both as single and mutually combined components of the system. The course of reaction was examined by the changes of chemical oxygen demand (COD) and total organic carbon (TOC) in aqueous solutions. Particular attention has been paid to determine optimal concentration of hydrogen peroxide, when it is used alone, together with O₃ and in H₂O₂+O₃+UV combination. The most effective optimal concentration of H₂O₂ was found. The entire mineralization of tannic acid into final products CO₂ and H₂O can be accomplished in all combinations of advanced oxidation with ozone. Bacteriological test ToxAlert® with luminescence bacteria Vibro fisheri proved that toxicity of solutions decreased considerably during advanced oxidation of tannic acid solution.     

Surface electrochemistry of UO2 in dilute alkaline hydrogen peroxide solutions: Part II. Effects of carbonate ions

The electrochemical reduction of hydrogen peroxide has been studied on uranium dioxide electrodes. The reduction kinetics are found to be influenced by dissolved carbonate/bicarbonate ions. The formation of hydrated U^VI species on the electrode surface is avoided in carbonate solutions, allowing H₂O₂ reduction to proceed at less cathodic potentials than in carbonate-free solutions. At more cathodic potentials, the adsorption of carbonate ions on the active reduction sites inhibits the H₂O₂ reduction reaction. Over a narrow potential region, the reduction of peroxide is catalyzed by coadsoption of H₂O₂ and HCO₃⁻/CO₃²⁻. The pH dependence of the H₂O₂ reduction reaction appears to be stronger in carbonate solutions than in solutions that do not contain carbonate. This can be attributed to the displacement of inhibiting CO₃²⁻/HCO₃⁻ adsorbed ions by OH⁻.     

Electro-reduction of oxygen and electro-oxidation of methanol at Pd monolayer-modified macroporous Pt electrode

With polystyrene latex spheres self-assembled on indium tin oxide-coated glass electrode as templates, highly ordered macroporous Pt was prepared by electrochemical deposition. Then, the macroporous Pt was modified by Pd monolayer involving the galvanic displacement of Cu monolayer formed by under-potential deposition on macroporous Pt. Electrocatalytic properties of the Pd-modified macroporous Pt electrode for oxygen reduction were investigated by cyclic voltammetry and chronoamperometry in O₂-saturated solution containing 0.1 M HClO₄. Methanol electro-oxidation on the Pd-modified macroporous Pt surfaces in 0.5 M H₂SO₄ containing 1 M CH₃OH was studied by cyclic voltammetry. The corresponding results showed that Pd-modified macroporous Pt electrode had negative catalytic activity for methanol oxidation in compared with macroporous Pt. However, Pd-modified macroporous Pt electrode had positive electrocatalytic activity to O₂ reduction.     

Sculpture preparation of crystalline mesoporous carbons from nanoshell-containing carbon

Crystalline mesoporous carbon was prepared from nanoshell-containing carbon (NSCC) by a sculpturing technique, i.e., the selective oxidation of amorphous carbon moieties (ACM) in NSCC using hydrogen peroxide (H₂O₂) treatment. The carbon structures of the NSCC and the H₂O₂-treated NSCC were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD) measurement and N₂ adsorption–desorption. Multiple repetitions of the treatment resulted in an increase of nanoshell fraction by the selective oxidation of the ACM intrinsically included in NSCC. This was confirmed by TEM, SEM and XRD measurements. N₂ adsorption–desorption measurement showed that the treatment caused remarkable increases in mesopore volumes and the mesopore ratios. In particular, when the treatment was repeated five times the highest mesopore ratio of approximately unity was observed.     

Oxidation of methane over platinum in a solid proton-conducting electrolyte cell

Studies of the complete oxidation of methane on a Pt electrode-catalyst in the cell with a solid proton-conducting electrolyte (CH₄ + O₂, Pt ¦ SrCe_0.92Dy_0.08O₃ ¦ Pt, H₂O + N₂) were carried out. The non-Faradaic effect of electrochemical hydrogen pumping on the rate of methane oxidation has been demonstrated. The induced change in the reaction rate at anodic polarization of a Pt electrode-catalyst was over two orders of magnitude higher than the rate of hydrogen pumping from the reaction zone through the electrolyte.     

Catalytic activity of ultrathin Pt films on aligned carbon nanotube arrays

Uniform ultrathin Pt films were electrodeposited onto an aligned array of carbon nanotubes (CNTs) for high-area chemically stable methanol fuel cell anodes. Electrochemical treatment of the graphitic CNT surfaces by diazoniumbenzoic acid allowed for uniform Pt electroplating. The mass activity of the Pt thin film can reach 400 A/g at a scan rate of 20 mV/s and in a solution of 1 M CH₃OH/0.5 M H₂SO₄. A programmed pulse potential at 0 V was also seen to nearly eliminate the effects of carbon monoxide poisoning. The mass activity of Pt for methanol oxidation can be maintained at 300 A/g for more than 3000 s, which is 19 times of that under a constant potential of 0.7 V (vs. Ag/AgCl).     

H2O2 Generation during Carbon Ozonation – Factors Influencing the Process and Their Implications

Hydrogen peroxide generation during contact of aqueous ozone with activated carbon surface is an established process. However, no systematic research concerning this phenomenon has been conducted. In this paper, factors affecting H₂O₂ generation are presented. Formation of hydrogen peroxide in contact of ozone with carbon is a surface phenomenon, strongly affected by the solution pH. Re-ozonation of the same carbon sample does not lead to H₂O₂ generation. Additionally, the amount of generated H₂O₂ is significant only in strongly acidic environment. It implies that hydrogen peroxide generated by surface of activated carbon cannot be ozone decomposition initiator in catalytic ozonation based on activated carbon as a catalyst.     

Electrochemical oxidation of hydrogen peroxide at nanoporous platinum electrodes and the application to glutamate microsensor

The electrochemical behavior of hydrogen peroxide (H₂O₂) at nanoporous platinum (Pt) thin film was investigated and the reaction of the enzymes immobilized on the electrode was examined. The nanoporous Pt underlying the enzyme layer was electrochemically deposited on Pt-Ir alloy microelectrodes in the solution consisting of hexachloroplatinic acid and a non-ionic surfactant, octaethylene glycol monohexadecyl ether (C₁₆EO₈). Glutamate oxidases were entrapped during electro-polymerization of 1,3-phenylenediamine on the nanoporous Pt microelectrodes to form a glutamate oxidase layer. The glutamate microsensors as made were compared with flat Pt-based one in terms of the performance and characteristics.