Hydrogen peroxide promoted wet air oxidation of phenol: influence of operating conditions and homogeneous metal catalysts

The promoted wet air oxidation of phenol has been investigated through the addition of hydrogen peroxide as a source of free radicals. The reaction has been shown to proceed in two stages, an initial fast reaction associated with hydrogen peroxide consumption and a second slower step that occurs at a rate comparable with conventional wet air oxidation. An increase in temperature has a positive effect on both stages, while oxygen partial pressure only influences the second slower stage. The influence of pH on phenol oxidation is shown to be significant with the highest efficiency achieved at very alkaline conditions when phenol is completely dissociated. The catalytic activity of homogeneous metal salts was investigated in both the presence and absence of hydrogen peroxide. The combined addition of hydrogen peroxide and a bivalent metal (ie copper, cobalt or manganese) is shown to enhance the rate of phenol removal. However, in the absence of hydrogen peroxide only copper exhibited catalytic activity. Finally, a reaction mechanism involving different radical species has been proposed. From the experimental results the apparent activation energy (96.9 ± 3.5 kJ mol⁻¹) and pre‐exponential factor (1.6 ± 0.2 10¹⁰ s⁻¹) were calculated for hydrogen peroxide decomposition into hydroxyl radicals. © 1999 Society of Chemical Industry     

Peroxide Formation in a Zero-Gap Chlor-Alkali Cell with an Oxygen-Depolarized Cathode

The effects of various factors on the undesired generation of hydrogen peroxide in a zero-gap oxygen-depolarized chlor-alkali cell employing carbon-supported platinum catalysts were studied. The rate of peroxide generation was found to decrease with platinum loading and increase with current density. The quantity of peroxide generated also increased with electrolysis time, and reached a steady state value after a few 100 h of cell operation at 10 kA m⁻². The steady-state peroxide to hydroxide molar ratio was found to increase with brine concentration. This phenomenon is believed to originate from a decrease of water activity at the reaction site that accompanies the brine concentration increase. No correlation between chloride crossover and the concentration of peroxide generated was detected. It is postulated that carbon particles are predominantly responsible for the partial oxygen reduction and that their contribution increases with electrolysis time as a result of processes that render the carbon surface more hydrophilic.     

Kinetics of the reaction between steam and the basal plane of graphite

The gold decoration/transmission electron microscopy technique has been used to measure the rate of removal of carbon atoms from the basal plane of gra processes have been determined: removal of edge atoms surrounding the monolayer etch pits, and abstraction of carbon atoms within the basal plane which for edge carbon removal ranges from 0.02 at 600°C to 26 atom/atom active site/s at 900°C. The activation energy of 52 kcal/mole is close to the val of the saturated basal atoms (with 3 sp² bonds) may be expressed in terms of the probabilities of successful collisions of O atoms which are fo for both CO₂ and CH₂O reactions, are of the order of 10^?10 in the above temperature range. Comparison of the two reaction oxygen and water is the species responsible for basal atom abstraction.The gasification of graphite by steam is immensely anisotropic. At 23 torr H₂O the turnover frequency in the prismatic or edge directions, {10$\text{1}$0} and {11$\text{2}$0} faces, is higher than that on the basal {0001} plane by a factor of 4 × 10¹⁰ at 900°C and 4 × 10¹² at 700°C.     

The reaction of hydroxyl radicals with carbon at 298 K

The reaction of free OH radicals with graphite was studied in a flow system by mass spectrometry, the OH being produced by the reaction H + NO₂ → OH + NO. The OH radicals react rapidly at 298 K to produce approximately equal amounts of CO and CO₂. The collision efficiency (γ) for gasification of the carbon is>5 × 10^?3. OH radicals are much more reactive than free oxygen atoms towards graphite at 298 K. Carbon is an efficient heterogeneous catalyst for the reaction H + OH → H₂O, and when free hydrogen atoms are present, this reaction is several times faster than the gasification of the carbon by OH. Carbon is also an efficient catalyst for the recombination of H atoms: 2H → H₂.     

Catalytic oxidation of Fe(II) by activated carbon in the presence of oxygen.: Effect of the surface oxidation degree on the catalytic activity

The catalytic oxidation of Fe(II) species in aqueous solution by activated carbons with different degrees of surface oxidation is described. The parent activated carbon was oxidized with aqueous solutions of nitric acid or hydrogen peroxide, and submitted to thermal treatment at 373, 523 and 773 K. The activated carbons prepared were characterized by N₂ adsorption and temperature-programmed desorption, and their catalytic behavior was determined by measuring the oxidation rate of Fe(II) to Fe(III) and the generation of hydrogen peroxide. Catalytic activity is a function of the nature of oxygen surface groups generated by oxidation.     

Electrochemical generation of hydrogen peroxide using surface area-enhanced Ti-mesh electrodes

Ti-mesh electrodes coated with Ti were obtained by using an electrophoretic deposition (EPD) method. The Ti coating was porous and showed a good adherence to the Ti-mesh surface, due to sintering of Ti particles during thermal treatment at 900 °C. The Ti-coated mesh electrode has a BET surface area of 3.5 m²/g, about four times larger than that of the bare electrode. The surface area-enhanced Ti-mesh electrode was applied in electrical generation of hydrogen peroxide. It was shown that the rate of hydrogen peroxide generation increased drastically compared to the fresh electrode, since the larger electrode surface area enhanced not only current density, but also the oxygen mass transfer rate.     

Catalytic oxidation of Fe(II) by activated carbon in the presence of oxygen.: Effect of the surface oxidation degree on the catalytic activity

The catalytic oxidation of Fe(II) species in aqueous solution by activated carbons with different degrees of surface oxidation is described. The parent activated carbon was oxidized with aqueous solutions of nitric acid or hydrogen peroxide, and submitted to thermal treatment at 373, 523 and 773 K. The activated carbons prepared were characterized by N₂ adsorption and temperature-programmed desorption, and their catalytic behavior was determined by measuring the oxidation rate of Fe(II) to Fe(III) and the generation of hydrogen peroxide. Catalytic activity is a function of the nature of oxygen surface groups generated by oxidation.     

Studies on the potential of coal fly ash as a heterogeneous catalyst in oxidation of aqueous sodium sulfide solutions with hydrogen peroxide

The potential of fly ash procured form coal-fired thermal power plants was studied as a heterogeneous catalyst in the oxidation of aqueous sodium sulfide solutions with hydrogen peroxide in the temperature range of 303–323 K. The effects of various parameters (source of fly ash, fly ash loading, initial concentrations of sodium sulfide and hydrogen peroxide, electrolyte and deactivation of catalytic effect of fly ash) were studied. For an initial sodium sulfide and hydrogen peroxide concentration of 26·98×10⁻² kmol m⁻³ and 24·28×10⁻² kmol m⁻³ respectively, only 4% (w/v) fly ash loading intensified the rate of oxidation by a factor of 4·52 over that without fly ash at 303 K. The deactivation of the catalytic effect of fly ash was found to be less than 20% even after six repeated uses. The kinetics of aqueous phase decomposition of hydrogen peroxide was also studied in the presence of fly ash in alkaline medium. ©1997 SCI     

Kinetics study of hydrogen adsorption over Pt/MoO3

The rate controlling step and the energy barrier involved in the hydrogen adsorption over Pt/MoO₃ were studied. Rates of hydrogen adsorption on Pt/MoO₃ were measured at the adsorption temperature range of 323–573 K and at the initial hydrogen pressure of 6.7 kPa. The rate of hydrogen uptake was very high for the initial few minutes for adsorption at and above 473 K, and reached equilibrium within 2 h. At and below 423 K, the hydrogen uptake still continued and did not reach equilibrium after 10 h. The hydrogen uptake exceeded the H/Pt ratio of unity for adsorption at and above 423 K, indicating that hydrogen adsorption involves hydrogen atom spillover and surface diffusion of the spiltover hydrogen atom over the bulk surface of MoO₃ followed by formation of H_xMoO₃. The hydrogen uptake was scarcely appreciable for Pt-free MoO₃. The rate controlling step of the hydrogen adsorption on Pt/MoO₃ was the surface diffusion of the spiltover hydrogen with the activation energy of 83.1 kJ/mol. The isosteric heats of hydrogen adsorption on Pt/MoO₃ were 18.1–16.9 kJ/mol for the hydrogen uptake range 2.4–2.8 × 10¹⁹ H-atom/g-cat. Similarities and differences in hydrogen adsorption on Pt/SO₄^2?–ZrO₂, Pt/WO₃–ZrO₂ and Pt/MoO₃ catalysts are discussed.     

Thermal effect optimization endows a selective and stable PdCu single atom alloy catalyst for acetylene hydrogenation

Here, we proposed a universal approach to solve a seesaw relationship between thermal effect control and conversion efficiency in strong exothermic acetylene hydrogenation by constructing Pd-based single atom alloy (SAA), in which Cu was chosen as host atom and heat transfer agent. Lower desorption energy of *C₂H₄ intermediate on isolated Pd atoms greatly improved ethylene selectivity. Notably, this excellent performance was well maintained, even at extremely low space velocity, implying the thermal effect was effectively controlled. On one hand, the heat generation rate over single active site was largely decreased. On another hand, host Cu atoms with proper continuity degree endowed the SAA with attractive lattice heat capacity and phonon scattering rate, which ensured the instant heat transfer from Pd atoms to the surroundings. The optimized performance (conversion 90.7% and selectivity 95.8%) was stably operated for 270 h under GHSV = 2400 h⁻¹.     

Water disinfection by pulsed atmospheric air plasma along water surface

An above water‐pulsed nonthermal plasma technique for water disinfection is reported. Experiments are performed with a 2.5 L plasma reactor and Escherichia coli. When its initial number density is less than 10⁶ cfu/mL, up to six orders reduction can be achieved within 0.5–2.5 min and at a plasma energy density of less than 2 J/mL. The disinfection performance is always effective by controlling water conductivity of below 1.5 mS/cm, which significantly affects plasma generation. Aqueous hydrogen peroxide, ozone, and nitric acid are continuously accumulated inside the reactor by the processing. Plasma generated UV radiation plays an important role in the cell inactivation. Inactivated cell morphology almost remains the same shape, their intracellular protein like green fluorescent protein, however, is destructed according to fluorescence observation. © 2012 American Institute of Chemical Engineers AIChE J, 59: 1458–1467, 2013     

Molecular dynamics simulations for the growth of diamond-like carbon films from low kinetic energy species

In this study, molecular dynamics simulations using the Brenner potential for hydrocarbons have been used to simulate the formation of diamond-like carbon (DLC) films grown from low-energy hydrocarbon radicals (<2 eV). With these simulations, insight is gained in the processes occurring in this type of deposition. The initial surface is a previously deposited DLC surface; impinging particles include Ar⁺ ions, with an energy of 2 eV, as well as several carbon radicals and molecules, and hydrogen atoms, with an energy of 1 eV. Two different radical flux compositions were examined: in the first condition, only C, C₂, and CH were used as growth species, as well as a large flux of H atoms. In the second condition, the same carbon radicals were considered, as well as the C₂H radical and C₂H₂, C₄H₂, and C₆H₂ molecules, but without the H atom flux. These fluxes are similar to different experimental conditions in an expanding thermal Ar/C₂H₂ plasma (expanding thermal plasma, or ETP), using different influxes of acetylene. Several properties of the resulting films will be presented, focusing mainly on the carbon coordination and the bonding network. The simulations suggest that lowering the acetylene influx results in films having a more extensive bonding network, but with more H incorporated. This leads to more polymeric films having a less diamond-like character, as is expected also from experiments. The aim of this work is twofold. The first objective is to compare the structural composition of the simulated films to the structure of the experimentally deposited films by applying similar conditions. Second, the simulations can give us valuable information about the key mechanisms in the deposition process.     

Kinetics of the thermal decomposition of alkyl hydrogen sulphates

First order rate constants and Arrhenius parameters have been obtained for the thermal decomposition of 1-hexadecyl hydrogen sulphate. From published data on thermal decomposition of lauryl hydrogen sulphate and lauryl ether hydrogen sulphate, first order rate constants and Arrhenius parameters have been obtained. The agreement between the two sets of data for the two alkyl hydrogen sulphates is within the 95% confidence limits, a combined Arrhenius plot giving an activation energy of 12.69 ± 1.97 K cal mol⁻¹ and pre-exponential factor of 10^{(4.68 ± 1.24)} sec⁻¹. For lauryl ether hydrogen sulphate, a 3-point Arrhenius plot gives an activation energy of 9.4 K cal. mol⁻¹ and a pre-exponential factor of 10² sec⁻¹.     

Restructuring of Hydrogenation Metal Catalysts Under the Influence of CO and H2

CO and H₂ structure and reactivity on single-crystal transition metal surfaces (platinum, rhodium, and palladium) were examined by surface-sensitive techniques including scanning tunneling microscopy (STM) and sum frequency generation (SFG) in high-pressure surface science studies. The studies indicated that ordered CO structures not observed in ultrahigh vacuum (UHV) can form at high pressure (10^-6–10³ torr). In addition, CO and H₂ induce metal atom mobility and restructure the surface. On platinum, CO dissociates at high temperature (≥ 500 K), and a platinum carbonyl precursor is implicated. Concerning catalytic reactions, structure sensitive CO dissociation plays an important role in the ignition of CO oxidation, whereas CO poisons olefin hydrogenation, which becomes CO desorption limited. Lastly, solid-state hydrogen atoms are more active for hydrogenation than surface hydrogen atoms. These results suggest that spatially and temporally resolved techniques would permit molecular studies of reaction intermediates of CO and H₂ in the future.     

Ethene Oxidation on Pd(111): Kinetic Hysteresis Induced by Carbon Dissolution

The catalytic oxidation of ethene was studied on Pd(111) in the 10⁻⁷–10⁻⁶ mbar pressure range by a molecular beam TPR hysteresis experiment between 400 K and 1,000 K. Two important effects were identified: the reaction-blocking effect of a dense chemisorbed adlayer of oxygen and the promotional effect of dissolved carbon segregating back to the surface and efficiently reducing the adsorbed oxygen. A strong dependence of the catalytic activity on the oxygen partial pressure is explained by the inhibiting effect of oxygen adsorption; high oxygen pressures in fact extinguish the reaction. The presence of oxygen-free metal surface area, where ethene can dissociate, is necessary for high activity. During heating the highest activity is observed at T ∼ 620 K, where a combination of oxygen clean-off by carbon segregating back to the surface is combined with a high ethene adsorption rate, thus forming additional reaction sites and additional reaction products. During heating this carbon-induced clean-off of O(ad) is very efficient because the dissolved C atoms rather accumulate in the surface-near region and largely segregate back to the surface at T > 600 K. In contrast, during cooling from higher temperatures a high surface-near carbon bulk concentration does not build up because the bulk mobility of C atoms is also high and the faster diffusion of C into deeper layers counteracts carbon enrichment in the surface-near metal bulk. This effect favours a higher oxygen surface coverage and a stronger deactivation during cooling. If the carbon loading of the surface-near region was increased by decomposition of clean ethene prior to the reaction experiment, the promotional effect during the heating cycle was strongly enhanced, but the cooling cycle showed no memory of the C presaturation. Generally, the observed hysteresis effects stem from an interplay of combined oxygen site blocking and carbon diffusion effects. Konrad Hayek—deceased.     

高频氢等离子体增强还原制备超细铜粉

利用高频感应热氢等离子体强化还原制备超细铜粉,考察了加料速率、还原氢气流量、氢气分布位置、反应区空间、冷却温度等因素对铜粉颗粒性能的影响,对制备的铜粉颗粒进行氧含量、XRD晶体结构、松装密度、粒度分布和比表面积的表征。结果表明,优化的工艺条件为反应区内径100 mm,加料速率4 g/min,淬火气氩气气量500 L/h,氢气气量500 L/h并通入少量载气,由氢等离子电离产生的氢自由基可强化反应实现瞬时还原,不仅可控制铜粉形貌,还能有效控制铜粉颗粒大小;利用该方法制备出粒径分布100?200 nm、分散性好的超细球形铜粉颗粒。该方法操作简便、产品纯度高、气氛可控、对环境污染小。     

Chemical-looping hydrogen generation system: Performance estimation and process selection

To find a suitable metal component in an oxygen carrier particle for a chemical-looping hydrogen generation system (CLH), oxygen transfer capacities of metal components were investigated, and Ni was selected as the best metal component. The optimum operating conditions to have maximum hydrogen generation rate have been determined based on the chemical-equilibrium composition analysis in a water splitting reactor. Moreover, suitable compositions of syngas from a gasifier of heavy residue oil to provide high energy efficiency have been determined based on the heat of reaction. With the selected operating conditions, the best configuration of two interconnected fluidized beds system for the chemical-looping hydrogen generator has been devised. This work was presented at the 6 ^th Korea-China Workshop on Clean Energy Technology held at Busan, Korea, July 4–7, 2006.     

Time-of-flight SIMS and in-situ XPS study of O2 and O2-N2 post-discharge microwave plasma-modified high-density polyethylene and hexatriacontane surfaces

The O₂ and O₂-N₂ ([N₂] < 15%) post-discharge microwave plasma modifications of high-density polyethylene (HDPE) and hexatriacontane (HTC) surfaces have been studied as a function of the distance from the discharge and the gas composition. They are compared in terms of the in-situ XPS O/C ratios and C 1s components, and the ex-situ ToF-SIMS O^-/CH^- ratios and relative intensities of series of peaks. The results on the effect of the distance from the discharge showed a clear correlation between the in-situ XPS results and the O₂ post-discharge modeling, exhibiting the double role of oxygen atoms: functionalization initialization by creating radicals (which react with molecular oxygen) and degradation due to the energy released by the oxygen atom recombination process. Specific in-situ XPS and ex-situ ToF-SIMS signatures of this in-situ degradation related to the oxygen atom recombination process were exhibited. When N₂ was introduced in the plasma gas, the in-situ XPS results and the ex-situ ToF-SIMS results were very different. The in-situ functionalization decreased as a function of the N₂ addition and the ex-situ functionalization exhibited a high maximum for the 5% N₂-95% O₂ post-discharge plasma and then decreased. Despite the absence of a complete O₂-N₂ post-discharge modeling, it can be assumed that there is also a maximum of the oxygen atom content for the 5% N₂-95% O₂ post-discharge. Thanks to the in-situ XPS and ex-situ ToF-SIMS specific signatures, it was also shown that this maximum corresponds to a low in-situ degradation effect. Nitrogen introduction could influence the role of oxygen atoms in such a way that there is a decrease in oxygen atom recombination processes (thus in degradation) for small N₂ addition and even a decrease in oxygen functionalization initialization for further N₂ incorporation in the plasma gas. No nitrogen was observed in the in-situ XPS results, whereas some ex-situ ToF-SIMS nitrogen-containing ions were observed for the O₂ and O₂-N₂ post-discharge. However, their relative intensities followed the variation in oxidation and not the variation in N₂ concentration in the plasma gas. They could be related to a post-treatment functionalization effect. Differences observed between HDPE and HTC are explained in terms of their structural differences (desorption of low molecular weight oxygen-containing fragments for HTC).     

New continuous gas-phase synthesis of high purity carbon nanotubes by a thermal plasma jet

We have developed a new gas-phase synthesis technique to produce carbon nanotubes (CNTs) with a continuous process and at high temperature, by using a thermal plasma jet. A thermal plasma jet was generated by applying a direct current of 100-300 A, using Ar as the plasma gas with a flow rate of ∼6 ksccm. The temperature of the thermal plasma jet was very high (∼10⁴ K) and the velocity was very fast (∼100 m/s). Fe(CO)₅ and CO were used as a catalyst precursor and carbon source, respectively. The yield of CNTs was dramatically increased by attaching a helical extension reactor at the end of the plasma nozzle. High purity (∼80%) CNTs were produced with a continuous process by using a thermal plasma jet with helical extension reactor equipment. The number of CNT walls produced was critically affected by the hydrogen gas injected as an auxiliary plasma gas. Without hydrogen gas, single-walled carbon nanotubes whose diameter was about 1 nm were mostly produced while with hydrogen gas double-walled carbon nanotubes (about 4 nm in diameter) were predominantly produced, with small amount of 3- and 4-walled carbon nanotubes.     

Formation of Hydroxyl Radicals from Hydrogen Peroxide and their Effect on Bleaching of Mechanical Pulps

The formation of hydroxyl radicals from hydrogen peroxide in alkaline solutions and under the conditions of hydrogen peroxide bleaching of pulps was investigated. The results lend support to the generally accepted view that the decomposition of alkaline hydrogen peroxide is catalyzed by redox processes involving transition metal ion species. The formation of hydroxyl radicals by one-electron reduction of hydrogen peroxide in this process is believed to be catalyzed primarily by mononuclear transition metal ion complexes, polynuclear species being far less efficient in this respect. It was found that a certain formation of hydroxyl radicals during alkaline hydrogen peroxide bleaching of pulp may have a beneficial effect on the final brightness. This finding is ascribed to the cleavage of crosslinks in the rigid lignin matrix which facilitates penetration of the bleaching reagent(s). Silicate does not appreciably suppress the formation of hydroxyl radicals in alkaline hydrogen peroxide solution. The stabilizing effect of this additive is probably due to deactivation of finely dispersed colloidal particles of metal hydroxides and hydrated oxides which decompose hydrogen peroxide to give mainly molecular oxygen and water.