Electrochemical synthesis of hydrogen peroxide: Rotating disk electrode and fuel cell studies

The electrochemical reduction of oxygen on various catalysts was studied using the thin-layer rotating disk electrode (RDE) method. High-surface-area carbon was modified with an anthraquinone derivative and gold nanoparticles. Polytetrafluoroethylene (PTFE) and cationic polyelectrolyte (FAA) were used as binders in the preparation of thin-film electrodes. Our primary goal was to find a good electrocatalyst for the two-electron reduction of oxygen to hydrogen peroxide. All electrochemical measurements were carried out in 0.1 M KOH. Cyclic voltammetry was used in order to characterise the surface processes of the modified electrodes in O₂-free electrolyte. The RDE results revealed that the carbon-supported gold nanoparticles are active catalysts for the four-electron reduction of oxygen in alkaline solution. Anthraquinone-modified high-area carbon catalyses the two-electron reduction at low overpotentials, which is advantageous for hydrogen peroxide production.In addition, the polymer electrolyte fuel cell technology was used for the generation of hydrogen peroxide. The cell was equipped with a bipolar membrane which consisted of commercial Nafion 117 as a cation-exchange layer and FT-FAA as an anion-exchange layer. The bipolar membranes were prepared by a hot pressing method. Use of the FAA ionomer as a binder for the anthraquinone-modified carbon catalyst resulted in production of hydrogen peroxide.     

Azobenzene-modified oxygen-fed graphite/PTFE electrodes for hydrogen peroxide synthesis

The performance of a catalyzed H₂O₂ electrogeneration process using a modified oxygen-fed graphite/PTFE electrode is reported. The organic redox catalyst chosen for incorporation into the graphitic mass was azobenzene. The yield of the hydrogen peroxide is related to the applied potential and to azobenzene concentration. Modification of the gas diffusion electrode with azobenzene improved hydrogen peroxide production, and the overpotential for oxygen reduction was shifted to less negative values compared to the performance of a non-catalyzed electrode, indicating that these modified electrodes have good electro-activity.     

Catalytic activity of dual catalysts system based on nano-manganese oxide and cobalt octacyanophthalocyanine toward four-electron reduction of oxygen in alkaline media

The electrocatalysis of the dual functional catalysts system composed of electrolytic nano-manganese oxide (nano-MnOx) and cobalt octacyanophthalocyanine (CoPcCN) toward 4-electron reduction of oxygen (O₂) in alkaline media was studied. Nano-MnOx electrodeposited on the CoPcCN monolayer-modified glassy carbon (GC) electrode was clarified as the nano-rods with ca. 10-20 nm diameter by scanning electron microscopy. The peak current for O₂ reduction at the dual catalysts-modified GC electrode increases largely and the peak potential shifts by ca. 160 mV to the positive direction in cyclic voltammograms compared with those obtained at the bare GC electrode. The Koutecký-Levich plots indicate that the O₂ reduction at the dual catalysts-modified GC electrode is an apparent 4-electron process. Collection efficiencies obtained at the dual catalysts-modified GC electrode are much lower than those at the GC electrode and are almost similar to those at the Pt nano-particles modified GC electrode. The obtained results demonstrate that the dual catalysts system possesses a bifuctional catalytic activity for redox-mediating 2-electron reduction of O₂ to HO₂⁻ by CoPcCN as well as catalyzing the disproportionation of HO₂⁻ to OH⁻ and O₂ by nano-MnOx, and enables an apparent 4-electron reduction of O₂ at a relatively low overpotential in alkaline media. In addition, it has been found that the cleaning of the dual catalysts-modified electrode by soaking in 0.1 M sulfuric acid solution enhances its catalytic activity toward the reduction of O₂.     

Direct synthesis of hydrogen peroxide from H2/O2 and oxidation of thiophene over supported gold catalysts

Direct synthesis of hydrogen peroxide from H₂ and O₂ was performed over supported gold catalysts. The catalysts were characterized by means of UV–vis, H₂-TPR, TEM and XPS. Based on the results we conclude that metallic Au is the active species in the direct synthesis of hydrogen peroxide from H₂ and O₂. During preparation process of catalyst by deposition–precipitation with urea, the pH value increased and the gold particle size decreased with increasing the urea concentration. The catalyst prepared with higher urea concentration showed a higher activity and its stability also was efficiently improved. Gold nanoparticles, supported on TiO₂ or Ti contained supports, gave a higher catalytic activity. Thiophene can be efficiently oxidized by hydrogen peroxide synthesized in situ from H₂ and O₂ over Au/TS-1.     

Non-platinum electrocatalysts: Manganese oxide nanoparticle-cobaltporphyrin binary catalysts for oxygen reduction

This study is concerned with the development of non-platinum electrocatalysts for the efficient 4-electron reduction of molecular oxygen to water in acidic media. A binary catalyst composed of electrodeposited manganese oxide nanoparticles (nano-MnO _x ) and cobalt porphyrin macro complex (CoP) has been proposed in. The modification of glassy carbon (GC) electrode with CoP alone resulted in a significant positive shift of the oxygen reduction reaction (ORR) compared to the unmodified GC electrode while maintining a 2-electron reduction. That is a positive shift of the onset potential of the ORR of ca. 450 mV was achieved at the former electrode. The modification of the GC electrode with nano-MnO _x alone did not affect the ORR peak potential, but caused a remarkable increase in the reduction peak current due to the catalytic disproportionation of the electrogenerated hydrogen peroxide into water and oxygen. The modification of a GC electrode with CoP and nano-MnO _x (utilizing the advantages of the individual catalysts) resulted in the occurrence of the ORR at a significantly positive potential with almost double peak current compared to the unmodified GC electrode, suggesting a promising procedure for developing electrocatalysts for oxygen reduction in replacement of costly Pt. XPS and SEM techniques were employed to probe the structural and morphological characterization of the proposed binary catalysts.     

Importance of catalyst stability vis-à-vis hydrogen peroxide formation rates in PEM fuel cell electrodes

The role of catalyst stability on the adverse effects of hydrogen peroxide (H₂O₂) formation rates in a proton exchange membrane fuel cell (PEMFC) is investigated for Pt, Pt binary (PtX, X = Co, Ru, Rh, V, Ni) and ternary (PtCoX, X = Ir, Rh) catalysts supported on ketjen black (KB) carbon. The selectivity of these catalysts towards H₂O₂ formation in the oxygen reduction reaction (ORR) was measured on a rotating ring disc electrode. These measured values were used in conjunction with local oxygen and proton concentrations to estimate local H₂O₂ formation rates in a PEMFC anode and cathode. The effect of H₂O₂ formation rates on the most active and durable of these catalysts (PtCo and PtIrCo) on Nafion membrane durability was studied using a single-sided membrane electrode assembly (MEA) with a built-in reference electrode. Fluoride ion concentration in the effluent water was used as an indicator of the membrane degradation rate. PtIrCo had the least fluorine emission rate (FER) followed by PtCo/KB and Pt/KB. Though PtCo and PtIrCo show higher selectivity for H₂O₂ formation than unalloyed Pt, they did not contribute to membrane degradation. This result is explained in terms of catalyst stability as measured in potential cycling tests in liquid electrolyte as well as in a functional PEM fuel cell.     

Oxygen evolution over Ag/FexAl2−xO3 (0.0 ≤ x ≤ 2.0) catalysts via N2O and H2O2 decomposition

In this paper, a comparative study between nitrous oxide and hydrogen peroxide decomposition over a series of catalysts prepared via the combustion of silver, aluminum, and iron nitrates (with different aluminum: iron ratios). Urea was used as a combustion fuel. The calcinations were affected at the 400–700 °C temperature range. The produced catalysts were characterized by using XRD and SEM analyses. The obtained results revealed that silver metal supported on Al₂O₃ and/or Fe₂O₃ represent the major constituents of all the calcinations products, i.e. Ag/Fe_xAl_2−xO₃. However, two different interfaces are involved in the two test reactions, all the catalysts were able to decompose both reactants yielding oxygen as a joint product. Meanwhile, it was found nitrous oxide destruction activity increases with decreasing both silver particles size and iron content in the catalysts substrate. On the contrary, increasing iron content in the different catalyst was found to enhance hydrogen peroxide decomposition activity. Moreover, a synergic effect was observed for the catalysts having Al:Fe ratio of 0.5:1.5.     

Oxygen reduction at Au nanoparticles electrodeposited on different carbon substrates

The electrocatalytic reduction of molecular oxygen (O₂) has been performed in O₂-saturated 0.5 M KOH solution at Au nanoparticles electrodeposited onto two different carbon substrates, namely glassy carbon (GC) and highly oriented pyrolytic graphite (HOPG). Cyclic voltammetry (CV) technique has been used in this investigation. The electrocatalytic activity of the Au nanoparticle-based electrodes is inherently related to its electrodeposition conditions (i.e., the absence or presence of some additives) as well as the nature of the substrate. For instance, Au nanoparticles electrodeposited onto GC (nano-Au/GC) from K[AuBr₄] in the presence of 25 μM cysteine showed a high electrocatalytic activity towards the oxygen reduction reaction (ORR) as demonstrated by the largest positive shift of the cathodic peak potential (at ca. −0.165 V versus Ag/AgCl/KCl (sat)). On the other hand, two well-separated successive reduction peaks corresponding to the 2-step 4-electron reduction of oxygen were observed at the different nano-Au/HOPG electrodes. The relative ratio of the two peak current heights changed significantly depending on the electrodeposition conditions of the Au nanoparticles. The morphology of the different Au nanoparticles electrodeposited onto the different substrates was depicted by scanning electron microscope (SEM) technique.     

RRDE study of oxygen reduction on Pt nanoparticles inside Nafion®: H2O2 production in PEMFC cathode conditions

Dihydrogen peroxide production on platinum particles supported on carbon inside a proton exchange membrane (Nafion^®), that is, under the working conditions of PEMFC cathodes, is rather small at the usual oxygen reduction potentials. As on bulk platinum, a four-electron mechanism appears to be the main pathway, with particle size and carbon substrate effects on the H₂O₂ production. A large increase in the H₂O₂ contribution takes place at low potentials, that is, at the working potentials of PEMFC anodes.     

Reconsideration of the quantitative characterization of the reaction intermediate on electrocatalysts by a rotating ring-disk electrode: The intrinsic yield of H2O2 on Pt/C

To solve the problem of the catalyst-loading-effect on quantifying the reaction intermediates on the surface of electrocatalysts with a rotating ring-disk electrode, we studied the formation of hydrogen peroxide in the oxygen reduction reaction on Pt/C with various sample loadings and then proposed an extrapolation model for measuring the intrinsic yield of H₂O₂, which can quantitatively reflect the characteristics of the surface of a given catalyst. In the extrapolation model, the catalyst loading effect can be compensated by taking the catalyst loading-dependent probability of the re-adsorption + further reaction of the desorbed H₂O₂ into consideration. The core concept in this extrapolation model is that the probability of the re-adsorption + reaction of the desorbed H₂O₂ becomes zero if there is no other active site available (i.e., at the extrapolated hypothetical point of zero catalyst loading) for re-adsorption of the desorbed H₂O₂. The intrinsic yield of H₂O₂ by extrapolation was much higher than that measured by the conventional model, in which the re-adsorption + reaction of the desorbed H₂O₂ is not considered, and thus the catalyst loading-dependent apparent yield of H₂O₂ does not properly reflect the intrinsic characteristics of the surface of a given catalyst.     

Synthesis and Dispersion of Dendrimer-Encapsulated Pt Nanoparticles on γ-Al2O3 for the Reduction of NO x by Methane

Dendrimer encapsulated Pt nanoparticles were prepared by using hydroxyl terminated generation four (G4OH) PAMAM dendrimers (DEN) as the templating agents. The encapsulated Pt nanoparticles were dispersed on γ-Al₂O₃ at room temperature by impregnation. Pt/Al₂O₃ (DEN) catalysts were then subjected to thermal treatments in oxidizing and reducing atmospheres at different temperatures. These catalysts were characterized by Transmission Electron microscopy (TEM) and In situ Fourier-Transform Infrared (FTIR) spectroscopy. The TEM analysis of the as synthesized catalysts revealed that the Pt nanoparticles were found to be 2–4 nm in size. It is observed that the Pt particle size in 0.5% Pt/Al₂O₃ (DEN) catalyst increased upon thermal decomposition of the dendrimer. The in situ FTIR results suggested that the presence of oxygen and the Pt nanoparticles in the Pt-dendrimer nanocomposite accelerate the dendrimer decomposition at low temperatures. All the catalysts were tested for the reduction of NO _x with CH₄ in the temperature range of 250–500 °C. NO _x reduction efficiency of Pt/Al₂O₃ (DEN) catalysts were compared with the Pt/Al₂O₃ (CON; conventional) catalyst. The conversion of NO _x was started from the low temperatures over Pt/Al₂O₃ (DEN) catalysts. The high selectivity of NO _x to N₂ of 74% was obtained over 0.5% Pt/Al₂O₃ (DEN) catalyst at low temperatures around 350 °C.     

Noncovalent hybrid of CoMn2O4 spinel nanocrystals and poly (diallyldimethylammonium chloride) functionalized carbon nanotubes as efficient electrocatalysts for oxygen reduction reaction

We have grown CoMn₂O₄ spinel nanocrystals on poly (diallyldimethylammonium chloride) functionalized carbon nanotubes (PDDA-CNTs) by noncovalent functionalization and solvothermal techniques. PDDA plays an important role in homogeneously increasing the surface density of available functional groups, which can provide active sites for decoration of CoMn₂O₄ on CNTs. In addition, PDDA preserves the intrinsic properties of CNTs, increases the active sites of catalysts, and enhances the durability of the catalysts. Here, CoMn₂O₄ nanocrystals were uniformly deposited on PDDA-CNTs with loading amounts from 36% to 83%. The as-prepared CoMn₂O₄/PDDA-CNT catalyst showed high current densities for the oxygen reduction reaction (ORR) in alkaline and neutral conditions, which outperformed the Co₃O₄/PDDA-CNT and Pt/C catalysts at medium overpotential, mainly through a 4e reduction pathway. The obtained CoMn₂O₄/PDDA-CNT hybrid exhibited excellent activity and durability when subjected to an oxygen evolution reaction. These results indicate that the CoMn₂O₄/PDDA-CNT hybrid represents a promising alternative to Pt for ORR electrocatalysis, and this non-precious bifunctional electrocatalyst provides a corrosion resistant and protective cathode layer to fuel cells. The excellent activity and stability of the hybrid materials demonstrate the potential of noncovalent coupling inorganic/carbon composites as novel catalytic systems for lithium–air batteries and chlor-alkali production.     

Direct synthesis of hydrogen peroxide from H2 and O2 and in situ oxidation using zeolite-supported catalysts

Four microporous materials, zeolites HZSM-5, Y, Beta and TS-1, were used as the supports to prepare supported gold catalysts using impregnation or deposition precipitation. The gold catalysts were tested in the direct synthesis of hydrogen peroxide from H₂ and O₂ and for CO oxidation. The effect on the catalytic activity of different metal (e.g., Pd, Pt, Cu, Ag, Rh or Ru) on the synthesis of hydrogen peroxide was also tested. Organic substrates, such as cyclohexane or cyclooctene, were introduced to investigate the possibility of in situ H₂O₂ oxidation with these catalysts.     

Phenol degradation by Fenton's process using catalytic in situ generated hydrogen peroxide

The recent reported pathway using oxygen and formic acid at ambient conditions has been utilized to generate hydrogen peroxide in situ for the degradation of phenol. An alumina supported palladium catalyst prepared via impregnation was used for this purpose. Almost full destruction of phenol was carried out within 6 h corresponding to the termination of 100 mM formic acid at the same time. In addition, a significant mineralization (60%) was attained. A simulated conventional Fenton process (CFP) using continuous addition of 300 ppm H₂O₂ displayed maximum 48% mineralization. Study of different doses of formic acid showed that decreasing the initial concentration of formic acid caused faster destruction of phenol and its toxic intermediates. The catalytic in situ generation of hydrogen peroxide system demonstrated interesting ability to oxidize phenol without the addition of Fenton's catalyst (ferrous ion). Lower Pd content catalysts (Pd1/Al and Pd0.5/Al) despite of producing higher hydrogen peroxide amount for bulk purposes, did not reach the same efficiency as the Pd5/Al catalyst in phenol degradation. The later catalyst showed a remarkable repeatability so that more than 90% phenol degradation along with 57% mineralization was attained by the used catalyst after twice recovery. Higher temperature (45 °C) gave rise to faster degradation of phenol resulting to almost the same mineralization degree as obtained at ambient temperature. Meanwhile, Pd leaching studied by atomic adsorption proved excellent stability of the catalysts.     

Hydrogen production by oxidative methanol reforming with various oxidants over Cu-based catalysts

In order to improve the start-up property of a small hydrogen producer with a micro methanol reformer, oxidative methanol reforming (OMR) with various oxidants over copper-based catalysts was examined. The addition of Fe to a Cu/ZnO/Al₂O₃ catalyst resulted in higher catalyst durability, with a slight improvement in catalytic activity, for OMR with air. However, the addition of larger amounts of Fe inhibited further improvement of catalytic performance, possibly due to the formation of less active CuFe₂O₄ spinel in the catalyst. The production of hydrogen by OMR with hydrogen peroxide as an alternative oxidant, which has the potential to provide concentrated hydrogen without nitrogen dilution, was also considered. It was found that hydrogen peroxide is an effective oxidant for OMR over copper-based catalysts due to its ability to suppress CO formation and its improving effect on methanol conversion.     

A novel alkaline air electrode based on a combined use of cobalt hexadecafluoro-phthalocyanine and manganese oxide

Electrocatalytic reduction of O₂ with dual catalysts of cobalt 1, 2, 3, 4, 8, 9, 10, 11, 15, 16, 17, 18, 22, 23, 24, 25-hexadecafluoro-29 H, 31H-phthalocyanine (CoPcF₁₆) and MnOOH was studied in alkaline media with cyclic and rotating ring-disk electrode (RRDE) voltammetry. Cyclic voltammetric results show that CoPcF₁₆ possesses a good catalytic activity for redox-catalyzing an apparent two-electron reduction of O₂ with superoxide (O₂⁻) as an intermediate. The combined use of CoPcF₁₆ with MnOOH which shows a bifunctional catalytic activity toward the sequential disproportionations of the reduction intermediate and product, i.e. O₂⁻ and peroxide (HO₂⁻), eventually enables an apparent four-electron reduction of O₂ to be achieved at a positively-shifted potential in alkaline media. The possibility of utilizing the dual catalysts for the development of practical alkaline air electrodes was further explored by confining the catalysts in active carbon (AC) and carbon black (CB) matrices that are generally used as the substrate for constructing air electrodes. The RRDE voltammetric results suggest that an apparent four-electron reduction of O₂ reduction can be obtained at the as-prepared carbon-based air electrode at a potential close to that at the Pt-based air electrode, and that the as-prepared electrode shows a high tolerance against methanol and glucose crossover.     

Electroreduction of oxygen on Vulcan carbon supported Pd nanoparticles and Pd–M nanoalloys in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied using the rotating disk electrode (RDE) technique. Palladium nanoparticles and Pd–M (PdCo and PdFe) nanoalloys supported on Vulcan XC-72R were prepared using two different synthetic routes. The catalyst samples were examined by transmission electron microscopy (TEM) and the average size of metal nanoparticles was determined. Electrochemical measurements were performed in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions. The influence of different synthetic conditions on the values of specific activity and other kinetic parameters was investigated. These parameters were determined from the Tafel plots taking into account the real electroactive area for each electrode. Pd nanoparticles and Pd–M nanoalloys exhibit significantly high electrocatalytic activity for the four-electron reduction of oxygen to water.     

Oxygen reduction on silver catalysts in solutions containing various concentrations of sodium hydroxide – comparison with platinum

In air cathodes for chlorine–sodium hydroxide production, silver is a suitable catalyst for the oxygen reduction reaction (ORR), as is platinum. The ORR mechanism, studied with both rotating disc and ring-disc electrodes and impedance spectroscopy, is first order towards O₂. However, the reaction can involve a direct four-electron or two-electron pathway. Although the latter involves different chemistry, including decomposition of hydrogen peroxide, the two pathways are difficult to distinguish, probably because they have the same rate-determining step. Considering kinetics/solubility ratios, temperature increase favours the ORR kinetics on both metals, whereas an increase in sodium hydroxide concentration is only positive for silver: for high sodium hydroxide concentration, platinum properties are hindered by greater adsorbed oxygenated species coverage. Thus, silver becomes competitive to platinum in high concentration alkaline media.     

Direct synthesis of hydrogen peroxide from hydrogen and oxygen over palladium catalyst supported on SO<Subscript>3</Subscript>H-functionalized MCF silica: Effect of calcination temperature of mesostructured cellular foam silica

Palladium catalysts supported on SO₃H-functionalized MCF silica (Pd/SO₃H-MCF-T (T=450, 550, 650, 750, 850, and 950)) were prepared with a variation of calcination temperature (T, °C) of MCF silica. They were then applied to the direct synthesis of hydrogen peroxide from hydrogen and oxygen. Conversion of hydrogen, selectivity for hydrogen peroxide, and yield for hydrogen peroxide showed volcano-shaped curves with respect to calcination temperature of MCF silica. Yield for hydrogen peroxide increased with increasing acid density of Pd/SO₃H-MCF-T catalysts. Thus, acid density of Pd/SO₃H-MCF-T catalysts played an important role in determining the catalytic performance in the direct synthesis of hydrogen peroxide. Pd/SO₃H-MCF-T catalysts efficiently served as an acid source and as an active metal catalyst in the direct synthesis of hydrogen peroxide.     

Electroreduction of oxygen on Co-based catalysts: determination of the parameters affecting the two-electron transfer reaction in an acid medium

Co-based catalysts for the oxygen reduction reaction (ORR) in an acid medium have been prepared from cobalt acetate (CoAc) adsorbed on nine different carbons (previously enriched in surface nitrogen or not). The catalysts were obtained by heat-treating these materials at 900 °C in a reducing environment rich in NH₃. In this work, the emphasis was mainly placed on the electrochemical production of H₂O₂ as measured by the rotating ring-disk electrode (RRDE) technique. It is shown that all Co-based catalysts are active for ORR. The activity and specificity of the catalysts for peroxide production depend essentially on three factors: (i) the potential applied to the disk, (ii) the type of carbon support; and (iii) the concentration of the cobalt precursor. At identical Co loadings (2000 ppm), the percentage of peroxide produced at the disk (%H₂O₂) reaches a maximum in the 0.3-0.1 V versus SCE potential range and decreases for more negative potentials. When the potential is set at a constant value (100 mV versus SCE for instance), a strong effect of the carbon support on %H₂O₂ and on the ring current I_R is noticed, with lower values of %H₂O₂ and I_R corresponding to higher nitrogen content at the surface of the catalysts, while higher values of disk current I_D are obtained under the same conditions. A figure of merit for the electroreduction of oxygen to hydrogen peroxide was obtained for each catalyst by multiplying I_D (representing their activity for ORR) by %H₂O₂ (representing their specificity for H₂O₂ production). According to this figure of merit, the best catalysts for peroxide production are made with Ketjenblack, Black Pearls, Vulcan, and Norit carbon supports. For Co loadings higher than 2000 ppm, it is shown that increasing the loading by more than one order of magnitude (from 2000 to 50,000 ppm) has practically no effect on %H₂O₂ and I_R, while I_D decreases.