Surface chemistry of a viscose-based activated carbon cloth modified by treatment with ammonia and steam

The influence of ammonia treatment at 800 °C on the catalytic activity of a viscose-based activated carbon cloth (ACC) was evaluated for the oxidative retention of H₂S or SO₂ at room temperature. Change in the surface chemistry was observed by X-ray spectroscopy of nitrogen (N1s) and by temperature programmed desorption (TPD). Dynamic adsorption of H₂S or SO₂ in moist air onto a packed bed of activated carbon cloth was monitored by measurement of the breakthrough curves at room temperature. ACC modified by ammonia showed noteworthy enhanced SO₂ and H₂S loading relative to the untreated ACC. Improved SO₂ retention rate could be replicated several times after regeneration by washing at room temperature, in contrast to the case with H₂S. The likely reasons for the behavior of H₂S and SO₂ on the ammonia-treated ACC are discussed with reference to the recent literature.     

SO2 removal from flue gas by activated semi-cokes: 2. Effects of physical structures and chemical properties on SO2 removal activity

This paper deals with the effects of physical structures and chemical properties of the catalysts, activated semi-cokes, on SO₂ removal activity. The catalysts were characterized in terms of physical structures—specific surface area, pore volume and pore size—and chemical properties—acidity and basicity. Results show that the presence of basic groups on the catalyst surface is a precondition for SO₂ removal at 90 °C. For catalysts containing copper species, there is no relation between chemical properties and sulfur retention. For catalysts without copper species, sulfur retention shows no correlation with the content of acidic groups, but it nonlinearly increases with increasing the content of basic groups, and there is a good linear relationship between SO₂ capture capacity and Brunauer-Emmett-Teller surface area and pore volume. These results indicate that for catalysts without copper species in this work, physical structure dominates SO₂ capture capacity while chemical properties have a smaller influence on SO₂ removal efficiency.     

The effect of H2 treatment on the activity of activated carbon for the oxidation of organic contaminants in water and the H2O2 decomposition

In this work, hydrogen peroxide reactions, i.e. H₂O₂ decomposition and oxidation of organics in aqueous medium, were studied in the presence of activated carbon. It was observed that the carbon pre-treatment with H₂ at 300, 500, 700 and 800 °C resulted in an increase in activity for both reactions. The carbons were characterized by BET nitrogen adsorption, thermogravimetric analyses (TG), temperature programmed reduction (TPR), electron paramagnetic resonance (EPR), iodometric titration and determination of the acid/basic sites. TPR experiments showed that activated carbon reacts with H₂ at temperatures higher than 400 °C. The treatment produces a slight increase in the surface area. EPR analyses indicate the absence of unpaired electrons in the carbon. Iodometric titrations and TG analyses suggested that the treatment with H₂ generates reduction sites in the carbon structure, with concentration of approximately 0.33, 0.53, 0.59, 0.65 and 0.60 mmol/g for carbons treated at 25, 300, 500, 700 and 800 °C, respectively. It was also observed the appearance of basic sites which might be related to the reduction sites. It is proposed that these reducing sites in the carbon can activate H₂O₂ to generate HO^* radicals which can lead to two competitive reactions, i.e. the hydrogen peroxide decomposition or the oxidation of organics in water.     

SO2 removal from flue gas by activated semi-cokes: 1. The preparation of catalysts and determination of operating conditions

The objective of this work was to use waste semi-coke as the raw material to prepare catalysts of industrial-scale size for SO₂ removal from flue gas and to find the optimal preparation methods. Results showed that lignite semi-coke was a suitable raw material, and that the catalyst, prepared by pre-activating in an autoclave, oxidizing with HNO₃, loading with CuSO₄ and finally calcining at 700 °C, exhibited the best desulfurizing property with a sulfur retention of about 9.6% SO₂/100 gC at a reaction temperature of 90 °C. Also, the effects of H₂O content in the flue gas, reaction temperature and space velocity on the desulfurizing property were investigated to determine optimum operating conditions. An H₂O content of 7% was appropriate for catalysts in this work. In the temperature range 80-120 °C, the catalyst showed good performance for SO₂ removal and was gradually deactivated at temperatures above 120 °C. Space velocity exhibited an optimal value of 830 h⁻¹. The kinetic behavior varied with space velocity and the desulfurizing property was controlled by diffusion at space velocities below 830 h⁻¹, and controlled by adsorption or catalytic reaction at space velocities above 830 h⁻¹.     

Enhancement of nitric oxide removal by ammonia on a low-rank coal based carbon by sulphuric acid treatment

This work presents a study of the effect of wet sulphuric acid treatment and gas-phase treatment with SO₂ + O₂ + H₂O on the catalytic activity of a low-rank coal-based carbon for the nitric oxide reduction with ammonia. Carbons were characterized by N₂ adsorption, TPD, and FTIR in order to assess how the surface chemistry and the texture of carbons change after the treatments. A great amount of oxygenated functional groups both CO₂ and CO evolving ones are produced by liquid-phase sulphuric acid treatment. However, the amount of those groups after gas-phase treatment with SO₂ + O₂ + H₂O is lower, in particular the CO₂ evolving groups. The catalytic activity of carbons was examined in a fixed bed reactor at 150 °C in a gas flow containing NO, O₂, N₂ and NH₃, the effluent concentration being monitored continuously during the reaction. The obtained results indicate that an appropriate balance between the type of oxygen functional groups and surface area available to the reactant gas are required to reach high levels of NO conversion.     

Flue gas desulphurization by activated carbon fibers obtained from polyacrylonitrile by-product

By pyrolysis of a polyacrylonitrile textile by-product, subsequent activation by CO₂ and treatment (at high temperature) with a N₂ flow containing a low percentage of O₂ or of NH₃, three carbonaceous matrices are obtained having a high surface area and surface sites with basic characteristics. The SO₂ sorption properties of these carbon samples (in the temperature range between 100 and 160 °C) from gaseous mixtures having a similar composition to flue gases, seems to be promoted by nitrogen bonded to carbon. The SO₂ adsorbed by the carbons can be divided, by suitable extraction with distilled water, into: (i) desorbable, such as SO₂ or H₂SO₃, (ii) desorbable, such as SO₃ or H₂SO₄, (iii) non-desorbable. Following 10 SO₂ adsorption and desorption cycles, the surface area values of the activated carbons remain practically constant, while both the content of the acidic surface sites and the amount of non-desorbable SO₂ increase; this results in the decrease in the SO₂ carbon sorption property seeming to be even more marked for the carbon sample containing nitrogen.     

Electrochemical behaviour of titanium in H2SO4-MnSO4 electrolytes

The corrosion of titanium in H₂SO₄ electrolytes (0.001-1.0 M) at temperatures from ambient to 98 °C has been investigated using steady-state polarization measurements. Four distinct regions of behaviour were identified, namely active corrosion, the active-passive transition, passive region and the dielectric breakdown region. The active corrosion and active-passive transition were characterized by anodic peak current (i_m) and voltage (E_m), which in turn were found to vary with the experimental conditions, i.e., d(log?(im))/dpH=−0.8±0.1 and dE_m/dpH which was −71 mV at 98 °C, −58 mV at 80 °C and −28 mV at 60 °C. The activation energy for titanium corrosion, determined from temperature studies, was found to be 67.7 kJ mol⁻¹ in 0.1 M H₂SO₄ and 56.7 kJ mol⁻¹ in 1.0 M H₂SO₄. The dielectric breakdown voltage (E_d) of the passive TiO₂ film was found to vary depending on how much TiO₂ was present. The inclusion of Mn²⁺ into the H₂SO₄ electrolyte, as is done during the commercial electrodeposition of manganese dioxide, resulted in a decrease in titanium corrosion current, possibly due to Mn²⁺ adsorption limiting electrolyte access to the substrate.     

Low cost coal-based carbons for combined SO2 and NO removal from exhaust gas

The aim of this paper is to show how a cheap carbonaceous material such as low rank coal-based carbon (or char) can be used in the combined SO₂/NO removal from exhaust gas at the linear gas velocity used in commercial systems (0.12 m s⁻¹). Char is produced from carbonization and optionally activated with steam. This char is used in a first step to abate the SO₂ concentration at the following conditions: 100 °C, space velocity of 3600 h⁻¹, 6% O₂, 10% H₂O, 1000 ppmv SO₂, 1000 ppmv NO and N₂ as remainder. In a second step, when the SO₂ concentration in the flue gas is low, NO is reduced to N₂ and steam at the following experimental conditions: 150 °C, space velocity of 900 h⁻¹, 6% O₂, 10% H₂O, 0-500 ppmv SO₂, 1000 ppmv NO, 1000 ppmv NH₃ and N₂ as remainder.It has been shown that the presence of NO has no effect on SO₂ abatement during the first step of combined SO₂/NO removal system and that low SO₂ inlet concentration has a negligible effect on NO reduction in the second step. Moreover, this char can be thermally regenerated after use for various cycles without loss of activity. On the other hand, this regenerated char shows the highest NO removal activity (compared to parent chars, either carbonized or steam activated) which can be attributed to the activating effect of the sulfuric acid formed during the first step of the combined SO₂/NO removal system.     

RuO2·xH2O/NiO composites as electrodes for electrochemical capacitors: Effect of the RuO2 content and the thermal treatment on the specific capacitance

RuO₂·xH₂O/NiO composites having RuO₂ contents in the range 0-100 wt.% have been prepared by a co-precipitation method. Structural, microstructural and textural transformations after heating the as-prepared composites at 200 and 600 °C have been followed by X-ray diffraction, scanning electron microscopy (SEM) and nitrogen adsorption/desorption isotherms. At 200 °C the composites are made of micrometric particles in which nanometric crystallites of the two oxides are aggregated. The composites show microporosity (0.02-0.10 cm³/g), mesoporosity (0.07-0.12 cm³/g) and relatively high specific surface area (62-309 m²/g). At 600 °C the composites are fully dehydrated and RuO₂ has crystallized and segregated. Microporosity and mesoporosity as well as specific surface area are strongly decreased. Specific capacitance and specific surface area of the composites heated at 200 and 600 °C have been measured and discussed on the basis of the RuO₂ content. For comparison the specific capacitance and specific surface area of mixtures of NiO and RuO₂·xH₂O (or RuO₂) have been taken as references. The higher specific capacitance of the 200 °C-heated composites compared to the 600 °C-heated ones is due to the higher specific surface area of the former and the higher pseudocapacitance of RuO₂·xH₂O compared to RuO₂. The discussion reported in this work can be applied to other composites such as RuO₂·xH₂O/carbon and RuO₂·xH₂O/other oxides.     

Decomposition of MgSiN2 in nitrogen atmosphere

Magnesium silicon nitride MgSiN₂ was prepared by direct nitridation of Si/Mg₂Si/Mg/Si₃N₄ powder compact in a temperature range 1350-1420 °C. The thermal stability examination showed that MgSiN₂ is stable up to 1400 °C at 0.1 MPa N₂ pressure. The activation energy of decomposition of MgSiN₂ calculated from the temperature dependence of mass loss in the range of 1400-1650 °C is ΔH = 501 kJ mol⁻¹. The time dependence and nitrogen pressure dependence of MgSiN₂ decomposition was also investigated at constant temperature. MgSiN₂ is stable at 1560 °C in 0.6 MPa nitrogen atmosphere. Using these experimental data together with the heat capacity published in a literature the Gibbs energy of formation of MgSiN₂ was calculated in a temperature range 25-2200 °C.     

Ammonia-tailoring of GAC to enhance perchlorate removal. I: Characterization of NH3 thermally tailored GACs

A commercially available bituminous granular activated carbon (GAC) was subjected to thermal treatment with ammonia for 60 min at temperatures of 500-800 °C. The effect of this treatment on the physical and chemical characteristics of carbon was studied in detail. Comparison of virgin and NH₃-tailored carbons relative to pore volume distribution and argon-BET surface area indicated that ammonia acted as an activating agent under these conditions. Micropore volume increased with increasing treatment temperature up to 700 °C; and then declined at 800 °C. Elemental analysis results showed that nitrogen was incorporated into the carbon matrix during the thermal treatment as proven by an increase in N-content from 0.64% for virgin GAC up to 1.61-1.63% for all the tailored samples. The ammonia treatment caused an increase in positive surface charge density over a broad pH range; and this was reflected in a higher pHpzc and pH_IEP. This was attributed to the more basic nature of the carbon surface after its treatment in ammonia.     

Effect of Na2CO3 on hexagonal boron nitride prepared from urea and boric acid

Formation of hexagonal boron nitride (hBN) from a precursor obtained by the reaction of urea and boric acid was studied in nitrogen, ammonia and argon atmospheres in 700-1200 °C temperature range. Effect of sodium carbonate (Na₂CO₃) addition on this process was investigated. Reaction products were subjected to X-ray diffraction, particle size distribution, gravimetric and Fourier transformed infrared spectroscopy analyses. Particle size and crystallite thickness of the formed hBN were seen to increase from about 60 nm and 5 nm at 700 °C to 230 nm and 19 nm at 1200 °C, respectively in NH₃ atmosphere with Na₂CO₃ addition. Highest conversion of boron in the precursor into hBN was achieved as 73.6% when Na₂CO₃ added precursor was reacted at 1200 °C in NH₃. hBN powder with high yield and relatively large particle size was obtained at low temperature such as 1200 °C with Na₂CO₃ addition. Role of Na₂CO₃ addition was suggested to be formation of a sodium borate melt from which hBN crystallized via the reaction of borate and nitrogen ions in the melt. Obtained hBN has the potential for utilization as a clean starting material for synthesis of B or N containing compounds.     

Effect of Bi2O3 additives on sintering and microwave dielectric behavior of La(Mg0.5Ti0.5)O3 ceramics

Bi₂O₃ was selected as liquid phase sintering aid to lower the sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics. The sintering temperature of La(Mg_0.5Ti_0.5)O₃ ceramics is generally high, about 1600 °C. However, the sintering temperature was significantly lowered about 275 °C from 1600 °C to 1325 °C by incorporating in 15 mol% Bi₂O₃ and revealed the optimum microwave dielectric properties of dielectric constant (?_r) value of 40.1, a quality factor (Q × f) value of 60,231 GHz, and the temperature coefficient (τ_f) value of 70.1 ppm/°C. During all addition ranges, the relative dielectric constants (?_r) were different and ranged from 32.0 to 41.9, the quality factors (Q × f) were distributed in the range of 928–60,231 GHz, and the temperature coefficient (τ_f) varies from 0.3 ppm/°C to 70.3 ppm/°C. Noticeably, a nearly zero τ_f can be found for doping 5 mol% Bi₂O₃ sintering at 1325 °C. It implies that nearly zero τ_f can be achieved by appropriately adjusting the amount of Bi₂O₃ additions and sintering temperature for La(Mg_0.5Ti_0.5)O₃ ceramics.     

SO2 retention on CaO/activated carbon sorbents. Part II: Effect of the activated carbon support

The present paper analyses the role of the activated carbon (AC) properties on the SO₂ uptake capacity of CaO/AC sorbents prepared by AC impregnation or ionic exchange with calcium acetate water solutions. Gas adsorption and mercury porosimetry have been used for textural characterization of the AC and surface oxygen groups have been characterized by temperature programmed desorption (TPD). Thermogravimetry has been used for SO₂ retention tests and CO₂ chemisorption at 300 °C for CaO dispersion (d) determinations. The results show that the surface calcium on CaO/AC samples, determined as “Ca loading · CaO dispersion” (parameter Ca(%) · d), governs the SO₂ uptake. The surface oxygen content is the AC property that mainly controls both the calcium loading and surface calcium on CaO/AC samples, which could be explained by the fact that the surface oxygen lowers the hydrophobic character of the AC supports therefore favouring the interaction with the calcium acetate water solutions. The combination of high calcium loading and dispersion leads to SO₂ uptakes up to 123 mg SO₂/g. The textural properties of the supports have some influence in the calcium loading. However, the effect is masked by the blockage of AC porosity by the calcium loaded.     

Synthesis and microstructure of the Ti3SiC2 in SiC matrix grown by chemical vapor deposition

Ti₃SiC₂ + SiC and TiC + SiC were deposited on graphite substrate at 1300–1600 °C by chemical vapor deposition with TiCl₄, SiCl₄, C₃H₈, H₂ as reactive gases. Process parameters such as temperature, pressure, concentration of C₃H₈ were varied to study their effects on the phases and microstructure of the deposited layers. The results show that binary phases of Ti₃SiC₂ + SiC are formed at temperature less than 1400 °C. For temperature above 1500 °C, TiC + SiC phases are formed. Increase of the process pressure causes the disappearance of Ti₃SiC₂ and the formation of TiC. The surface morphology of Ti₃SiC₂ shows a plate-like structure. The hardness of Ti₃SiC₂ + SiC and TiC + SiC is HV4251 and HV4612 respectively for a load of 10 g.     

Effect of heat treatment on 7Na2O-23B2O3-70SiO2 glass

Porous 7Na₂O-23B₂O₃-70SiO₂ glass was successfully fabricated by acid leaching treatment and phase-separation. The 2 mol/l hydrochloric acid (HCl) solution treatment was used for 24 h. Thermal analysis and X-ray diffraction were used to identify the temperature range of heat-treatment. The average pore size and the pore volume were investigated by a nitrogen adsorption instrument, and SEM was used to characterize the appearance of the porous glass. The results show that the average size of pores changed from 3.75 nm to 3.03 nm when heat treated at 640-680 °C for 6 h. In addition, when heat treated at 640 °C for 6-24 h, the pore size fell from 3.75 nm to 3.66 nm. The surface area and pore volume become larger with the increase in both temperature and heat treatment time.     

V-doped In2O3 nanofibers for H2S detection at low temperature

Pristine and vanadium-doped In₂O₃ nanofibers were fabricated by electrospinning and their sensing properties to H₂S gas were studied. X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to investigate the inner structure and the surface morphology. The H₂S-sensing performances were characterized at different temperatures ranging from 50 to 170 °C. The sensor based on 6 mol% V-doped In₂O₃ nanofibers exhibit the highest response, i.e. 13.9–50 ppm H₂S at the relatively low temperature of 90 °C. In addition, the fast response (15 s) and recovery (18 s) time, and good selectivity were observed.     

ZnLi2/3Ti4/3O4: A new low loss spinel microwave dielectric ceramic

A new low loss spinel microwave dielectric ceramic with composition of ZnLi_2/3Ti_4/3O₄ was synthesized by the conventional solid-state ceramic route. The ceramic can be well densified after sintering above 1075 °C for 2 h in air. X-ray diffraction data show that ZnLi_2/3Ti_4/3O₄ ceramic has a cubic structure [Fd-3m (227)] similar to MgFe₂O₄ with lattice parameters of a = 8.40172 Å, V = 593.07 Å³, Z = 8 and ρ = 4.43 g/cm³. The best microwave dielectric properties can be obtained in ceramic with relative permittivity of 20.6, Q × f value of 106,700 GHz and τ_f value of −48 ppm/°C. The addition of BaCu(B₂O₅) (BCB) can effectively lower the sintering temperature from 1075 °C to 900 °C and does not induce much degradation of the microwave dielectric properties. Compatibility with Ag electrode indicates that the BCB added ZnLi_2/3Ti_4/3O₄ ceramics are good candidates for LTCC applications.     

Oxidation behavior of fine-grained SiC-B4C/C composites up to 1400 °C

Fine-grained B₄C-SiC/C composites were fabricated using a ball-milling dispersion process. The oxidation behaviors of both fine-grained B₄C-SiC/C composites and coarse-grained B₄C-SiC/C composites at temperatures of up to 1400 °C were analyzed by the differential thermal analysis technique, and the surface morphology of the composites after isothermal oxidation at 800, 1200 and 1400 °C was examined by scanning electron microscopy (SEM). The results indicated that fine-grained B₄C-SiC/C composites had excellent oxidation resistance with self-healing properties at 1400 °C. A general model and mechanism for self-protection against oxidation of carbon materials were proposed.     

Effect of processing method on physical properties of Nb2O5

Samples of Nb₂O₅ were prepared by laser floating zone (LFZ) technique and by solid-state reaction in order to study some of their physical properties as a function of synthesis conditions. Single crystals fibres were obtained by LFZ, while a structural orthorhombic to monoclinic phase transition was observed in samples sintered at temperature higher than 800 °C. Transmission optical spectroscopy and photoconductivity measurements allowed identifying a ∼3.2 eV bandgap energy for the H-Nb₂O₅ monoclinic crystalline phase. Band gap shrinkage of ∼100 meV was observed from 14 K to RT. For the orthorhombic phase (T-Nb₂O₅), the photoconductivity measurements evidence a higher energy bandgap. The sintered samples have shown a broad recombination luminescence band at the orange/red spectral region while no luminescence was detected from the LFZ grown fibres. A dielectric constant of ∼40 was found for the 800 °C and 1200 °C sintered pellets while that of 1000 °C reached four times that value.