Catalytic oxidation of hydrogen sulfide by dioxygen on CoN4 type catalyst

A new catalyst based on the formation of carbon supported “CoN₄” structures by heat treatment of Co(CH₃COO)₂ and imidazole impregnated carbon black is introduced. The performance of the new catalyst is compared to the performance of other catalysts including activated carbon powder, granular activated carbon, and pyrolysed cobalt(II) mesotetra-4-methoxyphenylporphyrin. The optimized form of the new catalyst outperforms all these catalysts. The underlying concept is borrowed from fuel cell technology, which uses electrocatalysts that activate triplet dioxygen and facilitate its reduction. The same concept is used to reduce oxygen by hydrogen sulfide. In electrochemistry oxygen reduction is carried out by heterogeneous electron transfer, whereas in dehydrosulfurization the reduced sulfur moieties serve as the electron donors. We postulate that the reason for the improved performance of the new catalyst compared to pyrolysed carbon supported cobalt porphyrin, which is also a Me–N₄ type catalyst, stems from the ability to load a larger amount of cobalt chelates rather than from a change of oxidation mechanism.     

New effective catalysts based on mesoporous nanofibrous carbon for selective oxidation of hydrogen sulfide

The systems based on granular mesoporous nanofibrous carbonaceous (NFC) materials synthesized by decomposition of hydrocarbons over nickel-containing catalysts are promising catalysts for selective oxidation of hydrogen sulfide. Sample series of nanofibrous carbon with three main types of their fiber structures and different contents of metal catalysts inherited from the catalysts for their synthesis were studied in this reaction. The correlation between NFC structure and its activity and selectivity in hydrogen sulfide oxidation was determined. The metal inherited from the initial catalysts for the synthesis of NFC influences the activity and selectivity of the resulting carbon catalysts. A particular influence is observed in the case of the catalyst withdrawn from the synthesis reactor at the stage of stationary operation of the metal catalyst (low specific carbon yields per unit weight of the catalyst). The presence of the metal phase results in an increase in the carbon catalyst activity and in a decrease in the selectivity to sulfur. NFC samples with the highest activity and selectivity are nanotubes and those with graphite planes perpendicular to the axis of the fibers. Carbon nanotubes have high selectivity, while samples obtained on copper–nickel catalysts also possess high activity. The promising NFC catalysts provide high conversion and selectivity (almost independent of the molar oxygen/hydrogen sulfide ratio) when a large excess of oxygen is contained in the reaction mixture.     

Effects of preparation methods on the properties of cobalt/carbon catalyst for methane reforming with carbon dioxide to syngas

The effect of different preparation methods on the physicochemical property, reforming reactivity, stability and carbon deposition resistance of cobalt/carbon catalyst was investigated through fixed bed flow reaction. The catalysts were prepared by the impregnation and characterized by the XRD and scanning electron microscopy (SEM). The result indicated that the active components of cobalt/carbon catalyst prepared by using ultrasonic wave distributed evenly, activity was high and the loading time was short. The Co/Carbon catalyst prepared by incipient-wetness impregnation, 10 wt% loading and 300 °C calcination, achieved the best activity. Furthermore, the effect of reaction temperature, air speed and CH₄/CO₂ ratio on the catalyst activity and CO/H₂ ratio in products was investigated. It was found that the conversion of CO₂ and CH₄ increased with the increasing of reaction temperature. However, the conversion of CO₂ and CH₄ increased first and then decreased with the increasing of air speed. With the increasing of CH₄/CO₂ in feed gas, both the catalyst activity and the CO/H₂ ratio in products decreased.     

Relationship among the physicochemical properties, electrocatalytic performance and kinetics of carbon supported Pt catalyst for ethanol oxidation

A new carbon supported Pt (Pt/C(b)) catalyst was prepared by reducing H₂PtCl₆ in glycol solution using formic acid as a reducing agent, and has been found in this work to be highly active and stable for the electrochemical oxidation of ethanol. The preparation produces highly dispersed Pt particles, of 2.6 nm average size, and with high electrochemical surface area, 98 m²/g. The apparent activation energy of ethanol oxidation over the Pt/C(b) catalyst electrode is low, 10–14 kJ/mol, over the range of potentials from 0.3 to 0.6 V.     

Wet air oxidation of phenol using active carbon as catalyst

Catalytic wet air oxidation is a promising alternative for the treatment of phenolic waste water which cannot be treated in conventional sewage plants. Catalytic wet air oxidation of an aqueous phenol solution was conducted in a fixed bed reactor operating in trickle flow regime. Either active carbon or a commercial copper oxide supported over γ-alumina was used as catalyst. The performance of both materials was compared in terms of phenol conversion in 240 h tests. The results showed that the active carbon, without any active metal supported, gives the highest phenol conversion. The supported copper catalyst undergoes a rapid deactivation due to the dissolution of the metal active species in the acidic medium in which the reaction takes place. On the other hand, the active carbon maintains a higher activity throughout the test, although a decrease of the phenol conversion was also observed due to both the loss of active carbon by combustion and the reduction of its surface area. The phenol oxidation was proved to occur through a first order mechanism with respect to phenol. After the ten-day run, the catalytic activity of the active carbon was found to be about eight times higher than that of the commercial catalyst, also showing high selectivity to the production of carbon dioxide.     

Catalytic wet air oxidation of low molecular weight carboxylic acids using a carbon supported platinum catalyst

Aqueous solutions of low molecular weight carboxylic acids, such as acetic, propionic and butyric acids, were treated by catalytic wet air oxidation (CWAO) using a carbon supported platinum catalyst. Oxidation in the presence of the catalyst, in a stirred reactor, was carried out at 200°C and 6.9 bar of oxygen partial pressure, with conversions (after 2 h) ranging from 59.4 to 75%, and selectivities to gaseous products of up to 100%. Initial rates for conversion varied from 184 (butyric acid) to 260 mmol h⁻¹ g_Pt⁻¹ (propionic acid). The activation energy for butyric acid conversion was found to be 56.7 kJ mol⁻¹.     

Effect of temperature on activity and selectivity of carbon supported Mo sulfide in simultaneous hydrodenitrogenation of pyridine and hydrodesulfurization of thiophene

Activity and selectivity of carbon supported Mo catalyst was tested in parallel hydrodenitrogenation (HDN) of pyridine and hydrodesulfurization (HDS) of thiophene in the temperature range 260–350 °C at 2 MPa of hydrogen pressure and compared with that of commercial NiMo-alumina catalyst Shell 324. The main advantages of carbon supported Mo sulfide over commercial NiMo catalyst can be summarized as follows: the markedly higher HDN and better HDS activities normalized to moles of active metals, the lower content of piperidine in the reaction products and the distinctly better selectivity towards HDN reaction.     

氧化铁/活性炭负载型硫化氢脱除剂制备及性能评价

为提高硫化氢脱除剂性能,利用活性炭吸附表面积大的特点,合成具有吸附性能的氧化铁负载型硫化氢脱除剂。为便于不同硫化氢脱除剂性能评价,设计和搭建了一种用于快速评价气体硫化氢脱除剂脱除效果的评价方法与实验装置。结果表明,吸附型脱硫剂加入反应型脱硫组分后,脱硫速率的持续性优于吸附型脱硫组分,氧化铁负载质量分数为5%时,脱除剂初始脱硫速率较高。     

Catalytic wet peroxide oxidation of phenol with a Fe/active carbon catalyst

A Fe on activated carbon catalyst has been prepared and tested for phenol oxidation with H₂O₂ in aqueous solution at low concentration (100 mg/L). Working at 50 °C, initial pH 3 and a dose of H₂O₂ corresponding to the stoichiometric amount (500 mg/L) complete removal of phenol and a high TOC reduction (around 85%) has been reached. Oxidation of phenol gives rise to highly toxic aromatic intermediates which finally disappear completely evolving to short-chain organic acids. Some of these last showed to be fairly resistant to oxidation being responsible for the residual TOC. In long-term continuous experiments the catalyst undergoes a significant loss of activity in a relatively short term (20–25 h) due to Fe leaching, this being related with the amount of oxalic acid produced. Deactivation may also be caused by active sites blockage due to polymeric deposits on whose formation some evidences were found. Washing with 1N NaOH solution allows to recover the activity although complete restoration was not achieved.     

Three-phase reactors for environmental remediation: catalytic wet oxidation of phenol using active carbon

Wet oxidation of phenol aqueous solutions was carried out in a fixed bed reactor operating in trickle flow regime. Mild conditions of temperature (140°C) and oxygen partial pressure (1–9 bar) were used. Three active carbons and one commercially available supported copper catalyst were tested as catalytic material. Previous studies demonstrated that active carbon gives higher phenol conversion than conventional oxidation catalysts, although significant loss of active carbon due to combustion was also found. In the present study, the combustion of the active carbon during the process is highly reduced by lowering the oxygen partial pressure from 9 to 2 bar, maintaining an acceptable phenol conversion. The comparison of the performance of three different active carbons shows that their physical and chemical characteristics largely influence on the phenol conversion achieved.     

Enhancement of bio-electrocatalytic oxygen reduction at the composite film of cobalt porphyrin immobilized within the carbon nanotube-supported peroxidase enzyme

Combination of multi-walled carbon nanotubes, cobalt porphyrin, and peroxidase (horseradish, cabbage) enzyme in the film (deposited onto glassy carbon electrode substrate) produces a bio-electrocatalytic system capable of effective reduction of oxygen in such neutral media as 0.1 mol dm⁻³ KCl and 0.1 mol dm⁻³ KCl + 0.01 citrate buffer (pH 6).Carbon nanotubes have been modified with ultra-thin layers of 4-(pyrrole-1-yl) benzoic acid, or phosphododecamolybdate, to form stable colloidal suspensions of carbon nanostructures. The resulting inks have been utilized during sequential deposition of components. Co-existence of cobalt porphyrin, peroxidase enzyme together with dispersed carbon nanotubes leads to synergistic effect that is evident from some positive shift of the oxygen reduction voltammetric potentials (more than 50 mV in citrate buffer) and significant (ca. twice) increase of voltammetric currents (relative to those of the enzyme-free system). The multi-component bio-electrocatalytic film has also exhibited relatively higher activity towards reduction of hydrogen peroxide. It is reasonable to expect that the reduction of oxygen is initiated at cobalt porphyrin redox centers, and the undesirable hydrogen peroxide intermediate is further reduced at the horseradish or cabbage peroxidase enzymatic sites. An important function of carbon nanotubes is to improve transport of electrons within the bio-electrocatalytic multi-component film.     

Effect of sintering on the catalytic activity of a Pt based catalyst for CO oxidation: Experiments and modeling

The goal of this paper was to make the link between sintering of a 1.6% Pt/Al₂O₃ catalyst and its activity for CO oxidation reaction. Thermal aging of this catalyst for different durations ranging from 15 min to 16 h, at 600 and 700 °C, under 7% O₂, led to a shift of the platinum particle size distributions towards larger diameters, due to sintering. These distributions were studied by transmission electron microscopy. The number and the surface average diameters of platinum particles increase from 1.3 to 8.9 nm and 2.1 to 12.8 nm, respectively, after 16 h aging at 600 °C. The catalytic activity for CO oxidation under different CO and O₂ inlet concentrations decreases after aging the catalyst. The light-off temperature increased by 48 °C when the catalyst was aged for 16 h at 600 °C. The CO oxidation reaction is structure sensitive with a catalytic activity increasing with the platinum particle size. To account for this size effect, two intrinsic kinetic constants, related either to platinum atoms on planar faces or atoms on edges and corners were defined. A platinum site located on a planar face was found to be 2.5 more active than a platinum site on edges or corners, whatever the temperature. The global kinetic law {r (mol m⁻² s⁻¹) = 103 × exp(−64,500/RT)[O₂]^0.74[CO]^−0.5)} related to a reaction occurring on a platinum atom located on planar faces allows a simulation of the CO conversion curves during a temperature ramp. Modeling of the catalytic CO conversion during a temperature ramp, using the different aged catalysts, allows prediction of the CO conversion curves over a wide range of experimental conditions.     

负载型Co基F-T合成催化剂中贵金属助剂促进作用的研究进展

在负载型Co基催化剂中添加少量贵金属助剂能显著影响催化剂活性、选择性与稳定性。贵金属助剂的添加提高了Co活性组分的还原度与分散度,增加了催化剂表面的活性位数目,改变了催化剂的几何结构和电子结构,影响CO、H₂或中间产物的吸附活化行为,抑制催化剂积炭和金属Co小粒子的再氧化。综述贵金属助剂对催化剂活性中心性质、反应物的吸附活化行为和催化剂反应稳定性的影响。     

负载型磷钼钒钴杂多酸盐催化氧化乙苯合成苯乙酮

采用浸渍法制备了系列负载型杂多酸盐催化剂,并将其应用于乙苯氧化合成苯乙酮的反应中。考察了载体类型、负载量、焙烧温度、催化剂用量、反应时间等因素对催化活性的影响。结果表明,以HZSM-5为载体,杂多酸盐Co_4HP_2Mo_（15）V_3O_（62）负载量30%,固定乙苯用量25mmol,催化剂用量0.4g,m（催化剂）：m（KBr）=2：3,n（乙苯）：n（过氧化氢）=0.125：1,反应温度80℃.反应时间1h,乙苯转化率达到82.21%,苯乙酮收率为55.75%。     

过渡金属硫化物对Claus尾气催化加氢制备硫化氢的影响

以γ-Al2O3为载体,考察了第一过渡金属亲硫元素和Cr系元素及其负载量对Claus尾气加氢生成H2S的催化活性。结果表明,经预硫化后上述负载量为10%质量分数的各元素,其催化活性按大小可以划分为4组:Ni、Co、Fe>Mo、W、Cu>Cr>Zn、Mn。针对第一组的3种元素进行物相分析表明,在反应气体积比为SO2/H2/N2=0.5/3/96.5,重量时间空速(WHSV)=6 000mL/(h.g)的强还原气氛下,Ni的催化活性相为稳定的NiS2,但Co和Fe除主晶相CoS2和FeS2外,还分别具有Co3S4和Co9S8或FeS和Fe7S8的贫硫相。     

Effect of carbon surface oxidation on platinum supported carbon particles on the performance of gas diffusion electrodes for the oxygen reduction reaction

The effect of carbon surface oxidation on platinum supported carbon particles (Pt/C) with nitric acid was investigated by cyclic voltammetry, electrochemical impedance spectroscopy, polarization experiments and chronoamperometry. Cyclic voltammograms, polarization curves and electrochemical impedance spectra showed that the treated catalyst had much larger active surface area and higher ionic conductivity than the untreated catalyst, and provided enhanced performance for oxygen reduction. The formation of acidic groups was examined by IR spectra. The Pt/C surface oxidation had a large effect on the performance of a gas diffusion electrode for oxygen reduction reaction.     

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     

In situ hydrogen generation from cycloalkanes using a Pt/CNF catalyst

Pt catalyst supported on carbon nanofibers (CNFs) has been prepared via ion-exchange and it was characterized by XRD, TEM, N₂ physisorption and CO chemisorption. The Pt/CNF catalyst has a small Pt crystallite size in the range of 2–3 nm. This catalyst has been tested in the dehydrogenation of decalin, which is a cycloalkane proposed in the literature as H₂ storage media for vehicles and portable devices. The objective is finding a Pt catalyst suitable for in situ generation of H₂ from chemical storage in decalin. The results revealed that Pt supported on CNF outperforms a Pt catalyst supported on micro–mesoporous activated carbon. Finally, we propose a reactor configuration aiming at the intensification of H₂ production in continuous.     

Catalytic synthesis of methanethiol from hydrogen sulfide and carbon monoxide over vanadium-based catalysts

The direct synthesis of methanethiol, CH₃SH, from CO and H₂S was investigated using sulfided vanadium catalysts based on TiO₂ and Al₂O₃. These catalysts yield high activity and selectivity to methanethiol at an optimized temperature of 615 K. Carbonyl sulfide and hydrogen are predominant products below 615 K, whereas above this temperature methane becomes the preferred product. Methanethiol is formed by hydrogenation of COS, via surface thioformic acid and methylthiolate intermediates. Water produced in this reaction step is rapidly converted into CO₂ and H₂S by COS hydrolysis.

Titania was found to be a good catalyst for methanethiol formation. The effect of vanadium addition was to increase CO and H₂S conversion at the expense of methanethiol selectivity. High activities and selectivities to methanethiol were obtained using a sulfided vanadium catalyst supported on Al₂O₃. The TiO₂, V₂O₅/TiO₂ and V₂O₅/Al₂O₃ catalysts have been characterized by temperature programmed sulfidation (TPS). TPS profiles suggest a role of V₂O₅ in the sulfur exchange reactions taking place in the reaction network of H₂S and CO.     

Study on the reaction rate of sulfite oxidation with cobalt ion catalyst

Wet limestone scrubbing is the most common flue gas desulfurization process for control of sulfur dioxide emissions from the combustion of fossil fuels, and forced oxidation is a key past of the reaction. In the present work the oxidation rate was experimentally studied by contacting pure oxygen with a sodium sulfite solution. The sulfite oxidation reaction rate was then measured photographically for various sodium sulfite and Co²⁺ concentrations. The sulfite oxidation reaction rate was affected by the mass transfer characteristics of the gas-liquid absorbers with the cobalt ion catalyst as well as the reaction kinetics. The sulfite oxidation reaction rate controlled by the reaction kinetics was first order with respect to the sulfite. When the sulfite oxidation reaction rate was controlled by the gas-liquid rate of mass transfer characteristics, the reaction was zeroth order with respect to the sulfite.