Showing papers on "Wet oxidation published in 2014"

Ruthenium nanoparticles supported on nitrogen-doped carbon nanofibers for the catalytic wet air oxidation of phenol

Abstract: The effect of nitrogen content in N-doped carbon nanofibers (N-CNFs) on the catalytic activity of Ru/N-CNFs in the wet air oxidation of phenol has been studied. The N-CNFs, irrespective of nitrogen content and Sibunit, are shown to be low active. In the case of Ru-containing catalysts, nitrogen in N-CNFs was found to be responsible for both the increased activity and stability of the catalysts toward deactivation. The XPS showed the formation of carbon-oxygen structures with hydroxyl (carbonyl) end groups blocking ruthenium on the surface of the catalysts without nitrogen. For the catalysts with nitrogen, the ruthenium nanoparticles were not blocked in the course of the reaction and mainly the carboxyl (carbonate) surface groups were formed. The nature of this effect is discussed.

Oxidation of Rhodamine B in aqueous medium in ambient conditions with raw and acid-activated MnO2, NiO, ZnO as catalysts

Abstract: Various catalysts have been utilized for wet oxidation of organic compounds in water. Rhodamine B is a cationic xanthene dye, used in a large number of industries and is considered as an undesirable chemical in water. In the present work, commercially available metal oxides, MnO 2 , NiO and ZnO, and those activated by treating with 1.0 N H 2 SO 4 were used to oxidize the dye in water to innocuous compounds. The catalysts were characterized with FTIR, XRD, SEM, cation exchange capacity and BET surface area, pore volume and pore size distribution measurements. Oxidation was carried out in a batch reactor at ambient temperature and pressure under different conditions of pH, reaction time, dye concentration, catalyst loading, and temperature. Acid-activated MnO 2 was the best catalyst with almost 100% Rhodamine B oxidation (dye 1.0 mg/L, catalyst loading 2.5 g/L). The catalysts could be recovered and reused. Oxidation followed first order kinetics and a reaction mechanism was proposed based on analysis of the products.

Degradation of phenol via wet-air oxidation over CuO/CeO2-ZrO2 nanocatalyst synthesized employing ultrasound energy: physicochemical characterization and catalytic performance.

Abstract: Catalytic wet air oxidation (CWAO) of phenol was carried out under atmospheric pressure of oxygen at 160°C in a stirred batch reactor over copper catalysts supported by CeO2–ZrO2. The copper with different loadings were impregnated over the composite support by a sonication process. The catalysts were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray analysis (EDX), Brunauer-Emmett-Teller (BET) specific surface area and Fourier-transformed infrared analyses. Characteristic peaks attributed to copper were not found in XRD patterns even at high loadings, but based on EDX results, the existence of copper particles was confirmed. It means that sonochemical synthesis method even at high loadings produced small copper particles with low crystallinity and excellent dispersion over the CeO2–ZrO2 composite. FESEM micrographs indicated just slight enhancement in particle size at high loadings of Cu. Blank CWAO experiments illustrated low conversio...

The role of O- and S-containing surface groups on carbon nanotubes for the elimination of organic pollutants by catalytic wet air oxidation

Abstract: Multi-walled carbon nanotubes (CNTs) were subjected to several liquid-phase chemical treatments (using HNO 3 , H 2 SO 4 , a mixture of HNO 3 /H 2 SO 4 , HCl or bubbling O 3 in water) as well as gas-phase thermal treatments under nitrogen atmosphere (at 200, 400 and 600 °C) in order to obtain CNTs with different chemical properties. The modified CNTs were characterized by common techniques including nitrogen adsorption at −196 °C, temperature programmed desorption, thermogravimetry, X-ray photoelectron spectroscopy and point of zero charge (pH pzc ). The HNO 3 and HNO 3 /H 2 SO 4 treatments induced a pronounced acidic character to the pristine CNTs (originally with a neutral pH pzc of 6.8), creating a large amount of carboxylic acids, anhydrides and phenol surface groups, and also lactones and carbonyl/quinone surface groups. The treatment with H 2 SO 4 alone, or with the HNO 3 /H 2 SO 4 mixture, led to the additional introduction of S-containing groups (such as sulphonic groups), while O 3 and HCl treatments did not affect significantly the surface chemistry. The pristine and modified CNTs were studied as catalysts in the catalytic wet air oxidation (CWAO) process by using oxalic acid and phenol as model pollutants (at 140 and 160 °C, respectively, and 40 bar of total pressure). At the selected operating conditions, these pollutants are quite stable in the absence of a catalyst. However, a marked degradation of both compounds was observed in the presence of CNTs. The O-containing surface groups in CNTs (carboxylic acids, phenols, anhydrides) contribute to the acidic character of the surface, and, simultaneously, decrease the catalytic activity for degradation of the tested pollutants. The presence of S-containing groups also increases the acidity of the CNTs, but a marked increase of the catalytic activity was observed in this particular case (complete degradation of the pollutants, 56% of TOC reduction in 120 min when phenol was the pollutant, and complete mineralization of oxalic acid). Therefore, the presence of S-containing groups and the absence of carboxylic groups (such as carboxylic acids and anhydrides) seem to improve the catalytic performance of CNTs; however, the S-containing materials are not stable in consecutive runs, probably as a result of the high temperatures and pressures employed in the CWAO process.

Wet Air Oxidation of phenol over Pt and Ru catalysts supported on cerium-based oxides: Resistance to fouling and kinetic modelling

Abstract: Ceria and doped ceria supported Pt and Ru catalysts were tested at 160 °C in the Catalytic Wet Air Oxidation (CWAO) of phenol. Catalysts were compared in terms of activity, selectivity and resistance towards fouling. The respective influences of metal phase and support were studied. Under the selected operating conditions, 100% phenol conversion could be reached. Contrary to what was expected, improved Oxygen Storage Capacities (OSC) accelerated the accumulation of adsorbed species on the catalyst surface, therefore limiting the catalytic performance. By contrast, high metal dispersions enhanced both the elimination of aqueous organic compounds and the degradation of heavy molecules involved in the catalyst fouling. The progressive decrease in activity induced by carbonaceous deposits could be kinetically modelled using a simple reaction scheme.

Catalytic wet air oxidation of phenol with functionalized carbon materials as catalysts: reaction mechanism and pathway.

Abstract: The development of highly active carbon material catalysts in catalytic wet air oxidation (CWAO) has attracted a great deal of attention. In this study different carbon material catalysts (multi-walled carbon nanotubes, carbon fibers and graphite) were developed to enhance the CWAO of phenol in aqueous solution. The functionalized carbon materials exhibited excellent catalytic activity in the CWAO of phenol. After 60 min reaction, the removal of phenol was nearly 100% over the functionalized multi-walled carbon, while it was only 14% over the purified multi-walled carbon under the same reaction conditions. Carboxylic acid groups introduced on the surface of the functionalized carbon materials play an important role in the catalytic activity in CWAO. They can promote the production of free radicals, which act as strong oxidants in CWAO. Based on the analysis of the intermediates produced in the CWAO reactions, a new reaction pathway for the CWAO of phenol was proposed in this study. There are some differences between the proposed reaction pathway and that reported in the literature. First, maleic acid is transformed directly into malonic acid. Second, acetic acid is oxidized into an unknown intermediate, which is then oxidized into CO2 and H2O. Finally, formic acid and oxalic acid can mutually interconvert when conditions are favorable.

A mechanistic assessment of the wet air oxidation activity of MnCeOx catalyst toward toxic and refractory organic pollutants

Abstract: The reactivity pattern of a model MnCeOx catalyst (Mnat:Ceat, 1) in the wet air oxidation (CWAO) of toxic (phenol) and refractory (acetic, oxalic and formic acids) organic pollutants has been probed, using a stirred batch reactor with continuous oxygen feeding ( P O 2 , 0.9 MPa). In the range of 110–150 °C the MnCeOx catalyst (wcat/wsub, 5) shows high abatement and mineralization efficiency toward all the substrates. Parallel trends of substrate and total organic carbon (TOC) conversion prove that adsorption is the primary reaction step, while slower mineralization rates signal that surface oxidation is rate determining step (r.d.s.). Activity data in the pH range of 3–10 and straight relationships between conversion and dissociation constant (Ka) signal that acids adsorption is driven by electrostatic interactions with acid sites (EAds ≈ 80 kJ/mol), while a low energetic barrier (EAds ≈ 16 kJ/mol) discloses the physical nature of phenol adsorption. A kinetic analysis of conversion-selectivity data, based on a dual-site Langmuir–Hinshelwood (L–H) mechanism, sheds light into the CWAO pattern of MnCeOx catalyst toward different classes of organic pollutants.

Near- and Supercritical Water

Abstract: Water is a benign media for processing, and possesses several interesting properties when used in a processing context. When water is subjected to high pressure and temperature its properties change significantly. Approaching the critical point, where gas and liquid phases can no longer be distinguished, brings changes to ion dissociation and dielectric constants due to factors such as changes in the amount of hydrogen bonding. The changes occurring at near- and supercritical conditions may be utilised in processing for degradation and conversion reactions. However, at these conditions several issues must be considered as well. Especially, the increased risk of corrosion due to a larger degree of auto-dissociation of water is a factor that must be considered when using near-critical water in processing plants. Also, the decreased dielectric constant may lead to precipitation and possible deposition of mineral salts in processing equipment, which in turn may lead to clogging of valves, etc. Furthermore, the application of near- and supercritical water in processes such as wet air oxidation of wastewater and liquefaction of biomass is discussed.

Activation of a Carbon Support Through a Two‐Step Wet Oxidation and Highly Active Ruthenium–Activated Carbon Catalysts for the Hydrogenation of Benzene

Abstract: A two-step liquid oxidation approach was developed for the activation of carbon materials. Following nitric acid treatment and subsequent liquid oxidation by a mild oxidant such as H2O2, the number of surface acidic functional groups was increased without destroying the physical structures of the carbon materials. Ruthenium catalysts supported on activated carbon prepared by this two-step liquid oxidation method show significantly improved Ru dispersion and excellent catalytic performance in the hydrogenation of benzene. The dispersion of ruthenium and the catalytic performance of Ru/activated carbon increases monotonically with the amount of surface functional groups.

Pulsed corona discharge oxidation of aqueous lignin: decomposition and aldehydes formation.

Abstract: Lignin is the mass waste product of pulp and paper industry mostly incinerated for energy recovery. Lignin is, however, a substantial source of raw material for derivatives currently produced in costly wet oxidation processes. The pulsed corona discharge (PCD) for the first time was applied to lignin oxidation aiming a cost-effective environmentally friendly lignin removal and transformation to aldehydes. The experimental research into treatment of coniferous kraft lignin aqueous solutions was undertaken to establish the dependence of lignin oxidation and aldehyde formation on the discharge parameters, initial concentration of lignin and gas phase composition. The rate and the energy efficiency of lignin oxidation increased with increasing oxygen concentration reaching up to 82 g kW−1 h−1 in 89% vol. oxygen. Oxidation energy efficiency in PCD treatment exceeds the one for conventional ozonation by the factor of two under the experimental conditions. Oxidation at low oxygen concentrations showed a tendency...

Wet Air Oxidation and Kinetics of Cerium(III) of Rare Earth Hydroxides

Abstract: The investigation of wet air oxidation of cerium(III) (Ce(III)) of rare earth (RE) hydroxides was carried out in this work. The effects of air flow rate, reaction temperature, reaction time, soluti...

Ceria promoted γ-Al2O3 supported platinum catalyst for catalytic wet air oxidation of oxalic acid: kinetics and catalyst deactivation

Abstract: Alumina-supported platinum catalysts were prepared for catalytic wet air oxidation of oxalic acid. Addition of ceria as a promoter to alumina supported catalyst enhanced the catalytic activity, and significant conversions of about 74% were obtained at 363 K and atmospheric pressure. The long-term deactivation resistance of the ceria promoted Pt/Al2O3 catalyst is higher than that of the Pt/Al2O3 catalyst. A deactivation study of the catalysts was carried out using selective chemisorption, X-ray powder diffraction (XRD) patterns and total organic carbon (TOC) analysis. The deactivation observed with the Pt/Al2O3 catalyst was attributed to the over oxidation of the surface of platinum by molecular oxygen and coke deposition on the catalyst surface.

Performance of the wet oxidation unit of the HPLC isotope ratio mass spectrometry system for halogenated compounds.

Abstract: The performance of liquid chromatography-isotope ratio mass spectrometry (LC-IRMS) for polar halogenated compounds was evaluated. Oxidation capacity of the system was tested with halogenated acetic acids and halogenated aromatic compounds. Acetic acid (AA) was selected as a reference compound for complete oxidation and compared on the molar basis to the oxidation of other analytes. The isotope values were proofed with calibrated δ(13)C values obtained with an elemental analyzer (EA). Correct isotope values were obtained for mono- and dichlorinated, fluorinated, and tribrominated acetic acids and also for aniline, phenol, benzene, bromobenzene, chlorobenzene, 1,2-dichlorobenzene, 2,4,6-trichlorophenol, pentafluorophenol, and nitrobenzene. Incomplete oxidation of trichloroacetic acid (TCA) and trifluoroacetic acid (TFA) resulted in lower recovery compared to AA (37% and 24%, respectively) and in isotopic shift compared to values obtained with EA (TCA Δδ(13)C(EA/LC-IRMS) = 8.8‰, TFA Δδ(13)C(EA/LC-IRMS) = 6.0‰). Improvement of oxidation by longer reaction time in the reactor and increase in the concentration of sulfate radicals did not lead to complete combustion of TCA and TFA needed for δ(13)C analysis. To the best of our knowledge, this is the first time such highly chlorinated compounds were studied with the LC-IRMS system. This work provides information for method development of LC-IRMS methods for halogenated contaminants that are known as potential threats to public health and the environment.

Improvement of Channel Mobility in 4H-SiC C-Face MOSFETs by H2 Rich Wet Re-Oxidation

Abstract: 4H-SiC(000-1) C-face was oxidized in H2O and H2 mixture gas (H2 rich wet ambient) for the first time. H2 rich wet ambient was formed by the catalytic water vapor generator (WVG) system, where the catalytic action instantaneously enhances the reactivity between H2 and O2 to produce H2O. The dependence of SiC oxidation rate on the H2O partial pressure was investigated. We fabricated 4H-SiC C-face MOS capacitor and MOSFET by the H2 rich wet re-oxidation following the dry O2 oxidation. The density of interface traps was reduced and the channel mobility was improved in comparison with the conventional O2 rich wet oxidation.

Wet oxidation of glycerol into fine organic acids: catalyst selection and kinetic evaluation

Abstract: The liquid phase oxidation of glycerol was performed producing fine organic acids. Catalysts based on Pt, Pd and Bi supported on activated carbon were employed to perform the conversion of glycerol into organic acids at 313 K, 323 K and 333 K, under atmospheric pressure (1.0 bar), in a mechanically agitated slurry reactor (MASR). The experimental results indicated glycerol conversions of 98% with production of glyceric, tartronic and glycolic acids, and dihydroxyacetone. A yield of glyceric acid of 69.8%, and a selectivity of this compound of 70.6% were reached after 4 h of operation. Surface mechanisms were proposed and rate equations were formulated to represent the kinetic behavior of the process. Selective formation of glyceric acid was observed, and the kinetic parameter values indicated the lowest activation energy (38.5 kJ/mol) for its production reaction step, and the highest value of the adsorption equilibrium constant of the reactant glycerol (10-4 dm3/mol).