Mechanistic study on adsorption and reduction of NO2 over activated carbon fibers

Adsorption and reduction of NO₂ over pitch-based ACFs, both as received and calcined at 1100 °C, were studied in a range of concentrations (NO₂, 250-1000 ppm; O₂, 0-10%) and temperatures (30-70 °C). Repeated adsorption after regeneration at 300 °C, temperature-programmed desorption (TPD) and diffuse reflectance fourier transformation infrared spectroscopy (DRIFTS) were also applied to analyze the adsorbed NO₂ species. Pitch-based ACFs showed rapid NO production and adsorption at 30 °C which stayed at similar conversions until the rapid breakthrough of NO₂. A higher reaction temperature of 70 °C decreased the ratio of NO₂ adsorption to reduction in the stationary state and shortened the breakthrough time. Higher NO₂ concentration increased the rates of both adsorption and reduction to shorten breakthrough time, whereas the presence of oxygen changed the NO₂ profiles by enhancing the NO₂ adsorption rate and decreasing both the rate and the capacity of reduction. It must be noted that 10% O₂ allowed still significant production of NO. The molar O/N ratio evolved from TPD decreased and converged to a constant value according to the NO₂ adsorption time, showing that NO_x species adsorbed on the ACF changed from NO₂ to NO₃ along with the time of NO₂ adsorption. Such a trend was confirmed by DRIFTS spectra of adsorbed NO₂. These results suggest two kinds of NO₂ adsorption sites. Site 1 adsorbs NO₂ molecules strongly, transferring one oxygen to another adsorbed molecule on a similar site to form NO₃ad. Although oxygen in the gas phase oxidized adsorbed NO₂ to some extent, especially in the initial stage, disproportionation is still dominant at 10% O₂. Such disproportionation produces gaseous NO, leaving NO₃ on the surface. Site 2 adsorbs NO₂ weakly. Saturation of both sites terminates the adsorption and reduction and results in the breakthrough of NO₂. Adsorbed NO₃ produces both NO and NO₂ when heated, leaving one or two oxygen atoms on the surface, which are evolved as CO and CO₂ at the same time, restoring a stationary ability for adsorption and reduction of NO₂ through carbon loss.     

Preparation and characterization of nano-sized CoMo/Al2O3 catalyst for hydrodesulfurization

CoMo on Al₂O₃ catalyst prepared by spray pyrolysis method was found in the form of spheres of 0.5–1.2 μm, which consisted of tiny primary particles of ca. 10–20 nm diameter. The materials shows comparable activity to those of commercial catalysts in HDS of straight run gas oil, in particular, refractory 4,6-dimethyldibenzothiophene (4,6-DMDBT). Weaker interactions between CoMo and Al₂O₃ are suggested by temperature-programmed reduction (TPR), Raman spectroscopy, to give more active species than those over the impregnated catalysts. This accounts for its comparable activity in spite of its smaller surface area.     

Facile ultra-deep desulfurization of gas oil through two-stage or -layer catalyst bed

Ultra-deep hydrodesulfurization (HDS) was attempted to achieve the sulfur level less than 15 ppm S by staged and layered catalyst beds over commercially available catalysts. It was confirmed that two-stage and two-layer reaction schemes can attain the targeted sulfur level. Two-stage reaction with hydrogen refreshment between stages could provide the sulfur content lower than 15 ppm S over most of the catalysts examined in the present study where the first stage desulfurized the feed oil to be of 300–400 ppm S. Two-layer reaction, where two layers were connected directly without gas renewal, suffered severe inhibition of H₂S and NH₃. Nevertheless, NiMo sulfides on alumina support with sufficient acidity or zeolite achieved the sulfur level lower than 15 ppm S by two-layer scheme. Nitrogen species survived in the first layer reaction were found to strongly affect the extent of desulfurization in the subsequent second layer reaction.     

Performance of spent sulfide catalysts in hydrodesulfurization of straight run and nitrogen-removed gas oils

After the test run of several months two kinds of commercial catalysts (NiMo/Al₂O₃ and CoMo/Al₂O₃) were examined in hydrodesulfurization (HDS) of straight run (SRGO) and nitrogen-removed gas oils, at 340 °C under 50 kg/cm² H₂. Hydrogen renewal between stages was attempted to show additional inhibition effects of the by-products such as H₂S and NH₃. Spent NiMo/Al₂O₃ and CoMo/Al₂O₃ catalysts showed contrasting activities in HDS and susceptibility to nitrogen species, according to their catalytic natures, compared to those of their virgin ones. HDS over spent NiMo/Al₂O₃ was significantly improved by removal of nitrogen species, while that over spent CoMo/Al₂O₃ was much improved by H₂ refreshment. The activity for refractory sulfur species such as 4,6-dimethyldibenzothiophene was reduced more severely than that for the reactive sulfur species such as benzothiophenes over spent catalysts. The effects of both two-stage hydrodesulfurization and nitrogen-removal were markedly reduced over the spent NiMo when compared with those over virgin NiMo one. The acidity of the catalysts was correlated with the inhibition susceptibility by nitrogen species as well as H₂S and NH₃. Spent catalysts apparently lost their activity due to the carbon deposition, which covered the active sites more preferentially. The spent NiMo catalyst carried more deposited carbon with larger C/H ratio and nitrogen content. Higher acidity was found to be present on the NiMo catalyst, but this was greatly decreased by the carbon deposition. Additionally, the reactivity of nitrogen species in HDS was briefly discussed in relation to the acidity of the catalyst and its deactivation by carbon deposition.     

Activation of coal tar derived needle coke with K2CO3 into an active carbon of low surface area and its performance as unique electrode of electric double-layer capacitor

Raw needle coke from coal tar pitch was activated with K₂CO₃ at a coke:carbonate weight ratio of 1:4, to prepare an electrode for an electric double-layer capacitor (EDLC). Although the surface area of the coke activated at 900 °C for 3 h was as small as 20 m²/g, with a very high yield, the coke achieved capacitances per weight and volume of 20 F/g and 20 F/ml, respectively, in the two-electrode system, by charging at 2.7 V. The surface area of KOH-activated coke with a similar ratio (coke:hydroxide = 1:4, wt:wt) was over 2300 m²/g, and it exhibited capacitance per weight and volume values of 42 F/g and 17 F/ml, respectively. The coke activated by K₂CO₃ was found to be further activated by the charging. This electrochemical activation, which has been reported as activation in an electric field, was investigated by cyclic voltammetry in order to clarify it. The graphitic and pore structures of the coke after the electrochemical activation were analyzed by XRD to confirm retention of the graphene structure. Xe-NMR showed that the formation of small new pores was induced in the cathode material, increasing the surface area from 6 m²/g to 18 m²/g before use, although the pore volume was around 0.015-0.017 m³/g both before and after the charging. This activation with K₂CO₃ and a deeper understanding of the activation on charging suggest future directions for the preparation of electrode carbon for EDLCs.     

Carbonization properties of carbonaceous substances oxidized by air blowing—II: Acid-catalyzed modification of oxidized residual oil for better anisotropic development

The catalytic carbonization and modification of oxidized residual oils were studied using AlCl₃. Although the catalyst increased the coke yield in the carbonization, the size of the anisotropic units developed in the resultant coke decreased. Modification at proper temperatures was found effective in restoring the potential of the pitch for the anisotropic development of the flow texture. The modified pitch remained essentially quinoline soluble after the above-mentioned modification with only a minor amount of it becoming insoluble. After the modification, some anisotropic units of small diameter were observable in the quinoline soluble phase after separation of the quinoline insoluble phase. These units were recrystallized into smaller numbers of larger units by the gentle evaporation of the solvent quinoline. The significance of the soluble mesophase was discussed.     

Effects of fullerene addition on the carbonization of synthetic naphthalene isotropic pitch

The toluene soluble fraction of fullerene soot, consisting of C₆₀ and C₇₀ and other fullerenes, was co-carbonized with synthesized isotropic pitch derived from naphthalene. Mixtures of fullerene and pitch gave carbons in higher yield than expected from their single carbonizations at fullerene contents <30 wt%. The fullerenes suppressed the expansion of the pitch during carbonizations, and changed the optical textures of resultant carbons. At levels of addition of fullerenes <30 wt%, no fullerenes could be detected in resultant carbons by spectroscopy, but were detected as spheres of ca. 10–20 nm diameter in the carbons by TEM. It is considered that fullerenes remove hydrogen from the naphthenic structures of the pitch and so alter carbonization characteristics. Hydrogenation breaks the spheroidal fullerene framework.     

Preparation of activated carbon fibers from polyvinyl chloride

Waste polyvinyl chloride (PVC) contains high content of chlorine, which is believed to liberate dioxine at its combustion. Efficient removal of chlorine from PVC achieved by selecting the heat-treatment conditions provided free-chlorine PVC based pitch by a two-stage heat-treatment process. The obtained pitch (softening point: 218 °C) was spun, stabilized, carbonized and activated to prepare activated carbon fibers (ACF) whose DeSO_x activity was tested preliminarily and found comparable to other ACF.     

Optimization of carbonization conditions for needle coke production from a low-sulphur petroleum vacuum residue

The optimum carbonization conditions for converting a low-sulphur petroleum vacuum residue in a tube bomb in terms of pressure and temperature into better needle coke of low CTE, with less production of poor bottom coke, were studied by observation of the resultant cokes and sequential analyses of carbonization intermediates by means of solvent fractionation and gas evolution. Carbonization at 460°C under 15 kg cm^–2 G produced the best needle coke. The quality of the resultant needle coke was strongly influenced by viscosity changes of the system, the solidification range and gas evolution in the carbonization progress. The first two characteristics reflect the rates of condensation (such as QI formation) and devolatilization of soluble fractions, strongly influencing the growth of anisotropic units. The last characteristic reflects the pyrolytic cracking reaction to define the timing of gas evolution during carbonization, influencing the axial orientation of anisotropic texture as well as the porosity in the resultant coke. Both profiles varied very much, depending on the carbonization temperature. Finally, the formation of a bottom coke of fine mosaic texture is discussed from the viewpoint of the co-carbonization concept.