Catalytic oxidation of toluene and m-xylene by activated carbon fiber impregnated with transition metals

The catalytic oxidation of volatile organic compounds (VOCs) into mainly CO₂ and H₂O appears promising in the context of the abatement of atmospheric gaseous pollutants and is the subject of this paper. The catalytic oxidation of toluene and m-xylene was carried out in a tubular reactor on a phenolic resin based activated carbon fiber (ACF) impregnated with nitrates of Co, Cr, Ni, and Cu at the reaction temperatures below 300 °C. The extent of the removal (i.e. conversion) of toluene and m-xylene was examined through the breakthrough curves and was found to strongly depend on the types and loading of the metal precursors. The experimental results showed that the reaction rate was significantly affected by O₂ concentration below 3% (v/v). The performance of the ACFs impregnated with 5% (w/w) of Ni oxide was found to be superior to that of other metal oxides at reaction temperature between 170 and 290 °C. A surface kinetic mechanism for the catalytic oxidation of volatile organic compound was proposed and incorporated in a transport model developed to explain the experimental breakthrough data.     

The effect of cobalt precursors on NO oxidation over supported cobalt oxide catalysts

The effect of cobalt precursors such as cobalt acetate and cobalt nitrate on NO oxidation was examined over cobalt oxides supported on various supports such as SiO₂, ZrO₂, and CeO₂. The N₂ physisorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction with H₂ (H₂-TPR), NO chemisorptions, and temperature-programmed oxidation (TPO) with mass spectroscopy were conducted to characterize catalysts. The NO uptake as well as the catalytic activity for NO oxidation was dependent on the kinds of cobalt precursors and supports for supported cobalt oxides catalysts. Among tested catalysts, Co₃O₄/CeO₂ prepared from cobalt acetate showed the highest catalytic activity. The catalytic activity generally increased with the amount of chemisorbed NO. Reversible deactivation was observed over Co₃O₄/CeO₂ in the presence of H₂O. On the other hand, irreversible deactivation occurred over the same catalyst even in the presence of 5 ppm SO₂ in a feed. The strongly adsorbed SO₂ can prohibit NO from adsorbing on the active sites and also can prevent formed NO₂ from desorbing off the catalyst surface. The formation of SO₃ cannot be observed from the chemisorbed SO₂ on Co₃O₄/CeO₂ during TPO.     

The role of different nitrogen functional groups on the removal of SO2 from flue gases by N-doped activated carbon powders and fibres

SO₂ removal from flue gases by carbonaceous materials is determined by their behaviour as catalysts for SO₂ oxidation into SO₃ or H₂SO₄ in the presence of O₂ or O₂ and steam, respectively. Previous studies have demonstrated that nitrogen (N) functional groups are active sites for the adsorption and oxidation of SO₂, although the nature of the N groups with the higher activity had not been established yet. For this reason, in the present work a number of activated carbons (AC) and activated carbon fibres (ACF) doped with N atoms have been prepared using different methods. The number and nature of these N groups have been assessed by XPS. The materials prepared have a wide range of nitrogen content, which is distributed into different chemical species. In this way, we were able to determine the effect of the N content and the role of the different N-containing functional groups on the catalytic activity for SO₂ oxidation. The results confirm that, although the pore volume and the pore size distribution strongly influence the catalytic activity, the presence of N species at the surface increases the catalytic activity. They also demonstrate that, among the different N functional groups, pyridinic nitrogen is the most active for this reaction.     

Oxidative removal of 4-nitrophenol using activated carbon fiber and hydrogen peroxide to enhance reactivity of metallophthalocyanine

We have developed a novel heterogeneous metallophthalocyanine catalyst, Co-TDTAPc–ACF, by immobilizing cobalt tetra(2,4-dichloro-1,3,5-triazine)aminophthalocyanine (Co-TDTAPc) on activated carbon fiber (ACF) covalently. The oxidative removal of 4-nitrophenol (4-NP) in the Co-TDTAPc–ACF/H₂O₂ system, based on phase transfer catalytic oxidation, was investigated in aqueous solution by ultra-performance liquid chromatography (UPLC). The results indicated that 4-NP could be removed efficiently by catalytic oxidation in the presence of Co-TDTAPc–ACF and H₂O₂. In addition, the removal of total organic carbon of 4-NP accounted for about 90% in 300 min of reaction. Gas chromatography/mass spectrometry (GC–MS) analysis showed that the residue products were mainly small molecular compounds such as maleic acid and succinic acid, etc. This system exhibited high catalytic activity across a wide pH and temperature range. Importantly, compared with homogeneous Co-TDTAPc used alone, the introduction of ACF contributed specifically to the activity enhancement of Co-TDTAPc. Controlled experiments showed that the presence of 2-propanol, as hydroxyl radicals scavenger, has little influence on 4-NP oxidation. The further result of electron paramagnetic resonance (EPR) spin-trap experiments indicated that free radicals did not dominate the reaction in our system. This paper discusses a possible catalytic oxidation mechanism of the Co-TDTAPc–ACF/H₂O₂ system. Repetitive tests showed that Co-TDTAPc–ACF can maintain high catalytic activity over several cycles, and it has a better regeneration capability under mild conditions. We conclude that phase transfer catalytic oxidation has proven itself to be a feasible approach which may be potentially applied to the elimination of widely existing pollutants.     

掺杂元素对TiO2/SO2-4固体酸特性的影响

为了进行掺杂元素对超强酸特性的影响研究,采用均匀沉淀法合成了Fe、Si、Al和Zr氧化物掺杂的TiO2/SO2-4的固体酸(TiO2-MrOy),采用XRD、FTIR、NH3-TPD以及H2-FPR等分析方法对催化剂进行表征.以大豆油和甲醇的酯交换反应为活性评价反应,比较了各催化剂的催化活性.研究发现,除了Al外掺杂元...     

Measurement of the impedance change of impregnated activated carbon during exposure to SO2 vapors at ambient temperatures

The impedance of impregnated carbon is measured in real time during exposure to SO₂ vapors at ambient temperatures using a plug flow reactor and an impedance monitor. Impedance measurements are accomplished using a dual parallel plate mesh electrode configuration that measures the electrical conductivity of the carbon bed using an impedance analyzer operating at 10 Hz. It is observed for all impregnated carbon samples undergoing SO₂ exposure the carbon impedance undergoes an initial increase followed by a longer lasting characteristic decrease. The rate of the initial impedance increase appears to be proportional to the SO₂ partial pressure, while the impedance drops more quickly with increasing relative humidity and additive concentration. From these findings, along with XPS measurements to identify the chemical composition of the carbon bed, it is postulated that the SO₂ interaction with impregnated activated carbon is governed by an initial physical adsorption of SO₂ vapors onto the activated carbon followed by a catalytic oxidation of SO₂ to SO₃ and eventually a conductive sulfate-containing acid like H₂SO₄.     

Removal of SO2 by activated carbon fibers in the presence of O2 and H2O

This work describes the potential capability of activated carbon fibers (ACFs) in continuously removing SO₂ from inert atmosphere without requiring further regeneration. A tubular reactor packed with ACF was used to study the conversion of SO₂ into H₂SO₄ in the presence of O₂ and H₂O with varying concentrations of SO₂ (3000-10,000 ppm), O₂ (10-20%), and H₂O (10-70%) and temperatures (313-348 K). The experiments were carried out on two precursors (viscose rayon and phenolic resin) based ACFs. The breakthrough data revealed that the steady-state SO₂ concentration levels at the reactor exit increased with increasing inlet SO₂ concentration and decreased with increasing concentration levels of O₂ as well as H₂O. Increase in the reaction temperature was found to moderately enhance the steady-state exit concentration levels of SO₂. The viscose rayon-based ACF exhibited higher SO₂ removal activity in comparison to the phenolic resin-based ACF. A mathematical model was developed to predict the gas concentration profiles in the reactor, incorporating the mass transfer in the bed as well as within the ACF pores, along with the surface reactions on the ACF. The model predictions agreed reasonably well with the data.     

The Influence of O2, Hydrocarbons, CO, H2, NO x , SO2, and Water Vapor Molecules on Soot Combustion over LaCoO3 Perovskite

The effect of various gases (O₂, hydrocarbons, CO, H₂, NO _x , SO₂, and H₂O vapor) presenting in the diesel exhaust on soot combustion using LaCoO₃ as a catalytic material was investigated in this paper. A significant promotion of the combustion rate was found following a trend of 10% H₂O addition > 3,000 ppm NO _x  > 1% H₂ or 3,000 ppm C₃H₆ addition, while the improvement in soot oxidation due to the introduction of 3,000 ppm CO or 3,000 ppm CH₄ into the reactant gas is relatively less. The wet pretreatment of LaCoO₃ with 10% steam before soot oxidation hardly affects the combustion behavior. Interestingly, 10% water vapor in the reaction feed produced a significant promoting effect on combustion. In contrast, 30 ppm SO₂ treating led to an obvious deactivation likely owing to the coverage of active sites by sulfate compounds.     

Iron—molybdenum—oxide catalysts for selective oxidation of hydrogen sulfide to sulfur

The selective oxidation of hydrogen sulfide to sulfur was studied over iron-molybdenum oxides with various Fe-Mo ratios. Strong synergistic phenomenon in catalytic activity was observed for the Fe-Mo-O binary oxides. Under identical reaction conditions, the areal rates of the binary oxides were superior to those of the corresponding single oxide catalysts, which suggest that the new compound Fe₂(MO₄)₃ formed in the binary oxide is more active than Fe₂O₃ and MoO₃. The oxidation rates of H₂S were found to exhibit first-order dependence on the hydrogen sulfide concentration, which implies that the activation of H₂S is the rate-limiting step.     

Graft copolymerization onto cellulose by ceric ion initiator system: Effects of kind of acid and irradiation with ultraviolet light

The effects of kind of acid and irradiation of ultraviolet light on the graft copolymerization of methyl methacrylate onto cellulose with adsorbed ceric ion were investigated. Irrespective of ultraviolet light irradiation, the amount of reduced ceric ion in the reaction systems was increased in the order HCl > HClO₄ > HNO₃ > H₂SO₄, and the number of grafts formed was increased in the order HClO₄ > HNO₃ > HCl > H₂SO₄. Thus, it was definitely observed that the graft copolymerization is affected by the kind of acid. Ultraviolet light remarkably accelerated the reduction of ceric ion adsorbed on cellulose in the various acid mediums, but decreased the efficiency of graft formation. The most favorable results for the formation of grafts were obtained in the system in which HClO₄ and ultraviolet irradiation was employed. A combination of H₂SO₄ and ultraviolet irradiation resulted in the lowest per cent grafting and average molecular weight of grafts. It was found that H₂SO₄ characteristically dissolves out ceric ion adsorbed into an aqueous solution and accelerates the formation of homopolymer.     

Selective oxidation of h2s in the presence of ammonia and water using co3o4/sio2 catalyst

The catalytic performance of some metal oxides in the selective oxidation of H₂S in the stream containing water vapor and ammonia was investigated in this study. Among the catalysts tested, Co₃O₄SiO₂ was the most promising catalyst for practical application. It showed very small amount of SO₂ emission even in the presence of excess water and ammonia. The solid product formed in the reaction was a mixture of elemental sulfur, ammonium thiosulfate and ammonium sulfate.     

CO<Subscript>2</Subscript>/O<Subscript>2</Subscript>-oxidative dehydrogenation of ethane to ethylene over highly dispersed vanadium oxide on MgO-promoted sulfated-zirconia nanocatalyst: Effect of sulfation on catalytic properties and performance

The ZrO₂ was treated by various molarities of H₂SO₄ solution (0, 0.5, 1 and 2) then mixed by MgO and impregnated with 5 wt% of V₂O₅. The synthesized catalysts were characterized by XRD, FESEM, PSD, EDX, BET and FTIR techniques. According to the results obtained by characterization studies, the modification of MgO-ZrO₂ support by various molarities of H₂SO₄ solution had a great impact on the crystallinity, morphology and functional groups of prepared nanocatalysts. On the other hand, the catalytic activity of synthesized nanocatalysts in the oxidative dehydrogenation of ethane to ethylene is affected by the sulfur content on the support. The crystalline structures of MgO and ZrO₂ were confirmed by XRD analysis. The crystallinity of tetragonal ZrO₂ was decreased by increasing H₂SO₄ molarity used in ZrO₂ (Sx) synthesising. The highest catalytic performance and ethylene productivity (C₂H₄ yield of 48% and ethane conversion of 79% at 700 °C) were obtained on the V₂O₅/MgO-ZrO₂ (S1) nanocatalyst. This could be related to the superior acid-base property, smaller particles, better dispersion of active phase and uniform morphology of V₂O₅/MgO-ZrO₂ (S1).     

Ab initio molecular orbital study of the mechanism of SO2 oxidation catalyzed by carbon

Ab initio molecular orbital calculations were performed on the possible pathways of the carbon-catalyzed oxidation of SO₂ by O₂/H₂O to form sulfuric acid. Both zigzag and armchair edge sites of graphite, with and without surface oxide, were considered as the possible active sites. For the sites with oxide, both isolated and twin oxides were included. MO calculations at the B3LYP/6-31G(d)//HF/3-21G(d) level were used for calculating the energies of SO₂ adsorption, oxidation and hydration. Based on these calculations, three viable pathways emerged, and all three took place on the zigzag edge sites. Hence the armchair sites were not viable sites. On the bare surface, the only possible pathway involved the formation of a sulfurous acid intermediate. Thus, SO₂ was first adsorbed on the bare zigzag sites, followed by reaction with H₂O to form H₂SO₃, which was further oxidized by O₂ to form the end product. On the zigzag edge site with isolated oxide, both pathways with either SO₃ or H₂SO₃ as the intermediate are possible. Chemisorption on the edge sites containing twin oxides was not viable. This latter result explains the seemingly conflicting results in the literature regarding the dependence of SO₂ adsorption (and oxidation) on the amount of surface oxygen.     

The activated electro-oxidation of sulphur dioxide on smooth platinum

The electrochemical behaviour of sulphur dioxide in sulphuric acid solutions at a platinum rde is characterized. Cycling the potential in these solutions between about ?0.10 and 1.2 V sce results in activation of the electrode so that diffusion-controlled SO₂ oxidation currents can be observed in the double layer region of platinum. Without activation, SO₂ oxidation proceeds noticeably only in the potenial region of surface oxide formation. Evidence is presented which indicates that activation results from formation of a catalytic layer of sulphur species. The catalytic activity of this layer decays with time in the course of SO₂ oxidation. The formation of sulphur oxides through oxidation of adsorbed sulphur and the formation of platinum oxides complicate the voltammetric behaviour of the system.     

Activated carbon from sugar cane bagasse by carbonization in the presence of inorganic acids

Dried ground bagasse, impregnated with 50% inorganic acids and carbonized at 500°C, showed the sequence H₃PO₄ > H₂SO₄ > HCl > HNO₃, with respect to the efficiency of activation. Treatment with phosphoric acid of various concentrations (30–50 wt%) was followed by carbonization at 300–500°C for 3 h. Pore structure parameters were determined from the low-temperature adsorption of nitrogen, by applying the BET and α_s methods. Activated carbons obtained at low temperatures are essentially microporous with a low degree of mesoporosity. At higher temperatures products of higher surface area and total pore volume with developed mesoporosity and low microporosity are formed. An increase in the period of carbonization leads to a small decrease in both surface area and pore volume. Activated carbons with surface areas > 1000 m² g^?1 and mean pore dimensions around 2·0 nm, suitable for various purposes, are thus obtained.     

Catalytic Conversion of N2O to N2 over Metal-Based Catalysts in the Presence of Hydrocarbons and Oxygen

The catalytic conversion of N₂O to N₂ in the presence or the absence of propene and oxygen was studied. The catalysts examined in this work were synthesized impregnating metals (Rh, Ru, Pd, Co, Cu, Fe, In) on different supports (Al₂O₃, SiO₂, TiO₂, ZrO₂ and calcined hydrotalcite MgAl₂(OH)₈·H₂O). The experimental results varied both with the type of the active site and with the type of the support. Rh and Ru impregnated on -alumina exhibited the highest activity. The performance of the above most promising catalysts was studied using various hydrocarbons (CH₄, C₃H₆, C₃H₈) as reducing agents. These experimental results showed that the type of reducing agent does not affect the reaction yield. The temperature where complete conversion of N₂O to N₂ was measured was independent of the reductant type. The activity of the most active catalysts was also measured in the presence of SO₂ and H₂O in the feed. A shift of the N₂O conversion versus temperature curve to higher temperatures was observed when SO₂ and H₂O were added, separately or simultaneously, to the feed. The inhibition caused by SO₂ was attributed to the formation of sulfates and that caused by water to the competitive chemisorption of H₂O and N₂O on the same active sites.     

Activated carbons with metal containing bentonite binders as adsorbents of hydrogen sulfide

Wood-based activated carbon was ground and mixed with 10% bentonite binders containing either iron, zinc or copper cations adsorbed within the interlayer space and/or on the external surface of bentonite flakes. To better understand the role of transition metals, carbon was also impregnated with iron, zinc and copper salts. The structure of materials after modification was determined using nitrogen adsorption. The modification resulted in a decrease in porosity, especially in micropore volume, as a result of combined mass dilution effect and adsorption/re-adsorption of metals in small pores. Introduction of bentonite binders containing adsorbed metal increased the capacity of carbon for hydrogen sulfide only in the case of material containing copper. Copper also significantly increases the performance of carbon as an H₂S adsorbent when impregnation is applied whereas the effects of other metals used in this study are much less pronounced. It is likely that copper present in the small pores acts as a catalyst for oxygen activation causing hydrogen sulfide oxidation. As a result of this process, elemental sulfur is formed which, when present in small pores, is oxidized to weakly adsorbed SO₂. The SO₂ is removed from the surface when continuous reaction with hydrogen sulfide occurs. Thus, even though binding carbon with spent bentonites after copper adsorption increases the capacity of carbon toward H₂S removal, the formation of SO₂, another undesirable pollutant, does detract somewhat from the procedure.     

Effects of deep eutectic solvents on H2SO4-catalyzed alkylation: Combining experiment and molecular dynamics simulation

To enhance the catalytic performance of H₂SO₄-catalyzed alkylation, various catalytic additives have drawn considerable attentions. Herein, the effects of deep eutectic solvent (DES) additives on catalytic performance and the interfacial properties of H₂SO₄ alkylation were systematically investigated using experimental methods and molecular dynamics (MD) simulation. Experimental results indicate that DESs with the optimal concentration about 1.0 wt% can efficiently improve C₈ selectivity and research octane number of alkylate, especially at low temperature, but contribute less to the lifetime of H₂SO₄. MD results reveal that the phenyl molecules of DESs additives play a major role in enhancing interfacial properties of H₂SO₄ alkylation, including enlargement of interfacial thickness, promotion of isobutane relative solubility and diffusion to butene, which is probably the main reason for the better quality of alkylate. This work gives a good guideline for the design of novel DESs for H₂SO₄ alkylation.     

On the acidity of liquid and solid acid catalysts. Part 3. Esterification of benzoic and mesitoic acids

Methyl esters of benzoic and mesitoic acid have been prepared with high yields (>98 wt%) from the corresponding carboxylic acids + methanol in aprotic solvents over samples of H₂SO₄/SiO₂ at 60°C. The results show a high catalytic efficiency of the solids but also suggest an acid strength comparable to that observed in concentrated aqueous H₂SO₄ (range >90 wt%) when the acid requirements for the esterification of analogous compounds in aqueous acid solutions are taken into account. Indeed, different reacting species, i.e., ArC(OH) ₂ ⁺ from benzoic acid and 2,4,6‐triMe‐ArC=O⁺ from mesitoic acid are involved in the esterification, but the mesitoyl cation can be formed and esterified in the acidity ranges between 92 and 98 wt% H₂SO₄. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physicochemical properties,surface active species and formation of reverse micelles in the Cyanex 923‐n‐heptane/cerium(IV)‐H2SO4 extraction system

BACKGROUND: The Cyanex^® 923 (trialkylphosphine oxides, TRPO)‐n‐heptane/cerium(IV)‐H₂SO₄ extraction system has been investigated focusing on the physicochemical properties, surface active species and interfacial phenomena. The effects of H₂SO₄ and Ce(IV) extraction on them were considered. RESULTS: Results showed that the density and refractive index reflect the mass transfer by H₂SO₄ and Ce(IV) extraction and the change of refractive index was more sensitive than density. The interfacial tension decreased on extraction of H₂SO₄ but increased on extraction of Ce(IV). The viscosity of the equilibrium organic phase increased abruptly when the extracted H₂SO₄ concentration in the organic phase reached certain high values. The formation of reversed micelles, with mean diameter of about 10 nm, at high H₂SO₄ concentrations in the organic phase, is suggested by various measurements such as viscosity, interfacial tension and dynamic light‐scattering (DLS). CONCLUSION: It is suggested that TRPO‐H₂SO₄ complexes are more surface‐active than TRPO itself and tend to aggregate into reverse micelles by self‐assembling in the organic phase but the Ce(IV)‐TRPO complexes are neutral, less surface‐active than TRPO and not helpful for reverse micelle formation. Copyright © 2008 Society of Chemical Industry