Application of carbon nanostructures toward SO2 and SO3 adsorption: a comparison between pristine graphene and N-doped graphene by DFT calculations

We studied the adsorption of SO_x (x?=?2,3) molecules on the surface of pristine graphene (PG) and N-doped graphene (NDG) by density functional theory (DFT) calculations at the B3LYP/6-31G(d) level. We used Mulliken and NBO charge analysis to calculate the net charge transfer of adsorbed SO_x on pristine and defected graphene systems. Our calculations reveal much higher adsorption energy and higher net charge transfer by using NDG instead of pristine graphene. Furthermore, the density of state (DOS) graphs point to major orbital hybridization between the SO_x and NDG, while there is no evidence of hybridization by using pristine graphene. Based on our results, it is found that SO₂ and SO₃ molecules can be adsorbed on the surface of NDG physically and chemically with adsorption energies (E_ads) of ?27.5 and 65.2?kJ?mol^?1 (19.6 and 51.4?kJ?mol^?1 BSSE), respectively, while low adsorption energies were calculated in the case of using pristine graphene. So we introduced NDG as a sensitive adsorbent/sensor for detection of SO₂ and SO₃.     

Prediction of a highly sensitive molecule sensor for SOx detection based on TiO2/MoS2 nanocomposites: a DFT study

First principles calculations within density functional theory have been carried out to investigate the adsorptions of SO_x (x?=?1, 2) molecules on TiO₂/MoS₂ nanocomposites in order to fully discover the gas sensing capabilities of TiO₂/MoS₂ composite systems. The van der Waals interactions were included to obtain the most stable geometrical structures of TiO₂/MoS₂ nanocomposites with adsorbed SO_x molecules. SO_x molecules preferentially interact with the doped nitrogen and fivefold coordinated titanium sites of the TiO₂ anatase nanoparticles because of their higher activities in comparison with the other sites. The results presented include structural parameters such as bond lengths and bond angles and energetics of the systems such as adsorption energies. The variation of electronic structures are discussed in view of the density of states and molecular orbitals of the SO_x molecules adsorbed on the nanocomposites. The results show that the adsorption of the SO_x molecule on the N-doped TiO₂/MoS₂ nanocomposite is energetically more favorable than the adsorption on the undoped one, implying that the nitrogen doping helps to strengthen the interaction of SO_x molecules with TiO₂/MoS₂ nanocomposites. These calculated results thus provide a theoretical basis for the potential applications of TiO₂/MoS₂ nanocomposites in the removal and sensing of harmful SO_x molecules.     

Reduction of NOx over a NOx-Trap Catalyst and the Regeneration Behaviour of Adsorbed SO2

A conventional NO _x -trap catalyst containing platinum, rhodium, barium and lanthanum was conditioned with oxygen at 500°C, preloaded with NO under standard oxidising conditions and then subjected to regeneration with the reductants H₂, CO and C₃H₆, either alone or as a mixture. Hydrogen is the most efficient reductant in terms of NO _x conversion efficiency and reductant usage efficiency. There is a temperature optimum for CO between 300 and 400°C and a catalyst loading optimum (mols reductant added)/(mols NO _x adsorbed) between 1.5 and 3.0. The behaviour of the catalyst towards sulphur poisoning was examined in supplementary trials with the adsorption of SO₂ in the presence or absence of water vapour. When water is not present in both adsorption and reduction steps, very stable sulphates are formed, unattacked by reductants even at 1000°C. Sulfates are more easily reduced when water is present in the reductant mixture.     

SOx storage and release kinetics for ceria-supported platinum

The SO_x storage and release kinetics on CeO₂ have been studied by lean SO_x adsorption and temperature programmed desorption for different pairwise configurations of individual monolith samples, i.e., Pt/CeO₂ + SiO₂, Pt/SiO₂ + CeO₂, CeO₂ + Pt/SiO₂ and CeO₂ + SiO₂. In the case of sole ceria, SO_x adsorption proceeds both via SO₂ and SO₃ adsorption although the latter channel is kinetically favored. Hence, the rate of SO₂ oxidation is crucial for the overall SO_x storage kinetics. It is also found that physical contact between Pt and ceria is important for the storage process. This is attributed to efficient transport routes for SO_x (surface diffusion and spill-over processes) and/or specific adsorption sites at the platinum–ceria interface. The main route for SO_x release is found to be thermal decomposition where the effect of platinum is minor, although an indirect effect cannot be ruled out. Different mechanistic scenarios for SO_x adsorption are discussed, which may serve as a guide for future experiments.     

Simultaneous removal of SO2, NO and particulate by pilot-scale scrubber system

SO _x and NO _x have both previously been identified as primary precursors of acid rain, and thus the abatement of SO _x and NO _x emissions constitutes a major target in the field of air pollution control. In this study, the efficacy of a pilot-scale scrubber was evaluated with regard to the simultaneous removal of SO₂, NO and particulate with wet catalysts. The removal efficiencies of particulate were measured to be 83, 92 and 97% with catalyst flux of 0.5, 0.8 and 1.5 L/min, respectively. The average removal efficiencies of particulate with different nozzles were approximately 94 and 90% with FF6.5 (5/8 in.) and 14 W (1.0 in.) nozzles, respectively. At least 96–98% of particulate and SO₂ were removed, regardless of the stage number of reactor. In a one-stage scrubber, 83.3% removal efficiency of NO was achieved after 48 hours; however, the two-stage scrubber achieved an NO removal efficiency of 95.7%. Regardless of the liquid-gas ratio, SO₂ and particulate were removed effectively, whereas NO was removed about 84% and 74% under liquid-gas ratio conditions of 39.32 L/m³ and 27.52 L/m³, respectively. In experiments using STS and P.P. pall ring as packing material, particulate and SO₂ removal efficiency values in excess of 98% were achieved; however, NO removal was correlated with the different packing materials tested in this study. With the above optimum operation conditions, even after 20 hours, the removal efficiency for NO stayed at 95% or higher, the removal efficiency for SO₂ stayed at 97% or higher, and the removal efficiency for particulate stayed at 92% or higher. In accordance, then, with the above results, it appears that this process might be utilized in scrubber systems, as well as systems designed to simultaneously remove particulate, SO₂ and NO from flue gas.     

Development of Catalyst-Sorbents for Simultaneous Removal of SO2 From Flue Gas by Low Temperature Ozone Oxidation

One technological process employing ozone and heterogeneous catalyst-sorbents was proposed for removal of SO₂ from flue gas. The catalyst-sorbents were developed and tested especially for adsorption and oxidation of SO₂. Alternative catalyst-supporters including γ-Al₂O₃, permutite, silica gel, activated carbon and diatomite combined with different metal oxides (MnO₂, Cr₂O₃, Fe₂O₃, CuO, CoO and NiO) were evaluated and tested. It was found that γ-Al₂O₃ doped with MnO₂ can be considered as removal-effective sorbent for adsorption and oxidation of SO₂. The synergetic effect between ozone and catalyst was found to be dominated. Effects of catalyst preparation parameters like calcination temperature, metal loaded and reaction temperature, etc. were investigated based on the MnO₂/Al₂O₃ catalyst-sorbents. Results show that γ-Al₂O₃ combined with 8% Mn, calcinated under 573 K and reacted at 413 K are the optimal parameters for removal of SO₂. Extra NO in flue gas can slightly enhance the capture efficiency of SO₂.     

Effect of SO2 on a cordierite honeycomb supported CuO catalyst for NO reduction by NH3

Supporting CuO on a Al₂O₃-coated cordierite honeycomb yields a good catalyst (CuO/HC–Al) for selective catalytic reduction (SCR) of NO with NH₃ at 350–500 °C. SO₂ has complex effects on the catalysts activity. It significantly promotes the SCR activity through conversion of CuO to CuSO₄, however, when a certain amount of CuO is converted, it slightly decreases the SCR activity through competitive adsorption with NH₃. This competitive adsorption reduces the amount of NH₃ adsorbed on the catalyst surface, especially on the sites highly active to the SCR. It also prevents transformation of CuO to CuSO₄ and as a result, the catalysts subjected to pre-sulfation and in situ sulfation show different SCR behaviors.     

Influence of the Chemical Composition of MgAlFeCu Mixed Oxides as Catalysts for SO2 Removal

A series of MgAlFeCu mixed oxides with different chemical compositions derived from hydrotalcite-like compounds were prepared by the NaAlO₂-coprecipitation method and characterized by XRF, XRD, and N₂ adsorption analysis. These sulfur-transfer catalysts were evaluated as SO_x removal catalysts under the conditions similar to those of FCC units. The results showed that the total SO₂ adsorption amounts increased with increasing magnesium content. And the redox properties of Fe₂O₃ enhanced significantly the adsorption-reduction capacities of the catalysts, while CuO can be used as an effective oxidation promoter. Furthermore, the catalyst LDOs-6 (Mg/Al = 6, Fe₂O₃% = 8% and CuO% = 1%) presented the excellent SO₂ removal capacity, its total amounts of adsorbed SO₂ reached 1.74 g /g in 7 min, which also remained constant after eight cycles. These results indicated that the performances of the catalysts were closely related to the chemical composition.     

Influence of low concentration of SO2 for selective reduction of NO by C3H8 in lean-exhaust conditions on the activity of Ag/Al2O3 catalyst

The effect of SO₂ for the selective reduction of NO by C₃H₈ on Ag/Al₂O₃ was investigated in the presence of excess oxygen and water vapor. The NO_x conversion decreased permanently even in the presence of a low concentration of SO₂ (0.5–10 ppm) at <773 K. The increase in SO₂ concentration resulted in a large decrease in NO_x conversion at 773 K. However, when the reaction temperature was more than 823 K, the activity of Ag/Al₂O₃ remained constant even in the presence of 10 ppm of SO₂. The sulfate species formed on the used Ag/Al₂O₃ were characterized by a temperature programmed desorption method. The sulfated species formed on silver should mainly decrease the deNO_x activity on the Ag/Al₂O₃. The sulfated Ag/Al₂O₃ was appreciably regenerated by thermal treatment in the deNO_x feed at 873 K. The moderate activity remains at 773 K in the presence of 1 ppm SO₂ for long time by the heat treatment at every 20 h intervals.     

Enhanced electrochemical stability and charge storage of MnO2/carbon nanotubes composite modified by polyaniline coating layer in acidic electrolytes

Manganese dioxide/multiwalled carbon nanotubes (MnO₂/MWCNTs) were synthesized by chemically depositing MnO₂ onto the surface of MWCNTs wrapped with poly(sodium-p-styrenesulfonate). Then, polyaniline (PANI) with good supercapacitive performance was further coated onto the MnO₂/MWCNTs composite to form PANI/MnO₂/MWCNTs organic-inorganic hybrid nanoarchitecture. Electrochemical performance of the hybrid in Na₂SO₄-H₂SO₄ mixed acidic electrolytes was evaluated by cyclic voltammetry (CV) and chronopotentiometry (CP) in detail. Comparative electrochemical tests revealed that the hybrid nanoarchitecture could operate in the acidic medium due to the protective modification of PANI coating layer onto the MnO₂/MWCNTs composite, and that its electrochemical behavior was greatly dependent upon the concentration of protons in the acidic electrolytes. Here, PANI not only served as a physical barrier to restrain the underlying MnO₂/MWCNTs composite from reductive-dissolution process so as to make the novel ternary hybrid material work in acidic medium to enhance the utilization of manganese oxide as much as possible, but also was another electroactive material for energy storage in the acidic mixed electrolytes. It was due to the existence of PNAI layer that an even larger specific capacitance (SC) of 384 F g⁻¹ and a much better SC retention of 79.9% over 1000 continuous charge/discharge cycles than those for the MnO₂/MWCNTs nanocomposite were delivered for the hybrid in the optimum 0.5 M Na₂SO₄-0.5 M H₂SO₄ mixed acidic electrolyte.     

Effect of SO2 on the catalytic activity of Ga2O3–Al2O3 for the selective reduction of NO with propene in the presence of oxygen

The effect of coexisting SO₂ on the catalytic activity of Ga₂O₃–Al₂O₃ prepared by impregnation, coprecipitation and sol–gel method for NO reduction by propene in the presence of oxygen was studied. Although the activity of Al₂O₃ and Ga₂O₃–Al₂O₃ prepared by impregnation (Ga₂O₃/Al₂O₃(I)) and coprecipitation (Ga₂O₃–Al₂O₃(CP)) was depressed considerably by the presence of SO₂, NO conversion on Ga₂O₃–Al₂O₃ prepared by sol–gel method (Ga₂O₃–Al₂O₃(S)) was not decreased but increased slightly by SO₂ at temperatures below 723 K. From catalyst characterization, SO₂ treatment was found to cause two important effects on the surface properties: one is the creation of Brønsted acid sites on which propene activation is promoted (positive effect), and the other is the poisoning of NO_x adsorption sites on which NO reduction proceeds (negative effect). It was presumed that the influence of SO₂ treatment on the catalytic activity is strongly related to the balance between the negative and positive. The activity enhancement of Ga₂O₃–Al₂O₃(S) by SO₂ was accounted for by the following consideration: (1) increase of the propene activation ability by SO₂, (2) incomplete inhibition of NO_x adsorption sites by SO₂.     

SO2 oxidation over the V2O5/TiO2 SCR catalyst

The effects of V₂O₅ loading of the V₂O₅/TiO₂ SCR catalyst on SO₂ oxidation activity were examined by infrared spectroscopy (DRIFT) and SO₂ oxidation measurement. Vanadium oxide added to the catalyst was found to be well dispersed over the TiO₂ carrier until covered with monolayer V₂O₅. The rate of SO₂ oxidation increased almost linearly with V₂O₅ loading below the monolayer capacity and attained saturation with further increase. The hydroxyl groups bonded to vanadium atoms, V–OH, might be altered by SO₂ oxidation. Both V=O and V–OH groups are likely involved in the adsorption and desorption of SO₂ and SO₃.     

Adsorption of gaseous SO2 and structural changes of montmorillonite

Several montmorillonite samples after adsorption of gaseous SO₂ were analyzed to evaluate structural and textural changes. The equilibrium adsorption of the SO₂ gas was measured at 25 °C and 0.1 MPa. The samples were characterized by X-ray diffraction (XRD), infrared spectroscopy (IR), swelling index (SI), pH measurements, and N₂ adsorption–desorption isotherms. SO₂ adsorption increased with the specific surface area of montmorillonite. SO₂ retention decreased pH of the dispersed samples from 6 to 1 and released interlayer and octahedral cations from the structure, which increased the specific BET surface area and specific micropore surface similar to that of acid-activated montmorillonite.     

SO2 and NO selective adsorption properties of coal-based activated carbons

The aim of this paper is to study binary gas adsorption on the activated carbon in the fixed-bed reactor. Coal-based granular activated carbons can selectively adsorb SO₂ and NO. Physically adsorbed NO is replaced and desorbed by SO_2. Chemically adsorbed NO can promote the absorption of SO₂. The presence of SO₂ and NO can enhance the chemical adsorption of NO and SO₂, respectively. When the diameter of granular activated carbon decreases and the specific surface area increases, both the penetration time of the activated carbon bed and SO₂ removal efficiency increase. The whole removal efficiency of SO₂ is more than 99% in the penetration time, but the whole removal efficiency of NO is only 55% in the coexistence of SO₂ and NO. SO₂ adsorption capacity of HNO₃ dipped granular activated carbon is higher than that of non-treated one. The two experimental results are agree with each other.     

Molten V2O5/Cs0.9K0.9Na0.2S2O7 and V2O5/K2S2O7 catalysts as electrolytesin an electrocatalytic membrane separation device for SO2 removal

Bench scale fuel cell tests have been carried out on the SO₂ oxidation catalyst systems V₂O₅/M₂S₂O₇ (M = alkali) used as electrolytes in a standard molten carbonate fuel cell (MCFC) fuel cell setup for removal of SO₂ from power plant flue gases. Porous Li _x Ni_(1–x)O electrodes were used both as anode and cathode. The cleaning cell removes SO₂ when a potential is applied across the membrane, potentially providing cheap and ecological viable means for regeneration of SO₂ from off-gases into high quality H₂SO₄. Results show that successful removal of up to 80% SO₂ at 450 °C can be achieved at approximately 5 mAcm^–2. However, the data obtained during the experiments explain the current limitations of the process, especially in terms of electrolyte wetting capability and acid/base chemistry of the electrolyte.     

壳聚糖基大孔炭质整体材料的制备及其对痕量二氧化硫的吸附性能

以壳聚糖为碳源,综合采用冰模板技术和低温热解炭化技术,制备出具有蜂窝状孔结构特征的大孔炭质整体材料,研究了此类新材料对痕量二氧化硫的吸附性能及再生能力。结果表明,大孔炭质整体材料蜂窝状孔结构的形成及其规整程度与冰模板过程的冷冻时间等因素密切相关;200℃下低温热处理可得到具有丰富表面含氮官能团的大孔炭质整体材料;经氨水溶液中的离子交换功能化处理后,该大孔炭质整体材料对低浓度SO₂的吸附容量显著提高,可达到57 mg·g^-1;吸附饱和后,经空气简单吹扫处理,大孔炭质整体材料即可大部分再生,少数不可再生部分是由于质子化的氨基与亚硫酸根和硫酸根在吸附过程中形成了不可逆化学吸附产物季铵盐所致。壳聚糖基大孔炭质整体吸附剂材料有望在污染空气的脱硫净化,特别是在质子交换膜燃料电池的阴极空气脱硫净化方面发挥重要作用。     

Removal of SO2 at low temperature using dead Bacillus licheniformis

In this paper we studied the adsorption and desorption behavior of SO₂ by the dead Bacillus licheniformis R08 biomass. The effects of water vapor, temperature and O₂ on the removal of SO₂ by the biomass were studied. FTIR and XPS were used to characterize the mechanism of the SO₂ adsorption on the biomass. The experimental results showed that water vapor and temperature deeply influenced the adsorption of SO₂ by the biomass. However, O₂ cannot oxidize SO₂ to SO₃ on the biomass. FTIR and XPS results showed that oxygenous and nitrogenous functional groups on the cell walls of biomass may be related to the SO₂ adsorption and three sulfur species were formed on the biomass in adsorption process. In the desorption process, weakly adsorbed SO₂ could be desorbed by increasing temperature and the biomass can be reused for 10 cycles.     

SO2 absorption/desorption performance of renewable phenol-based deep eutectic solvents

Six renewable DESs prepared from choline chloride (CC) and phenols (guaiacol, GC; cardanol, CL) with molar ratios of phenols to CC of 3:1, 4:1 and 5:1 were investigated to absorb SO₂ at 293.15–323.15 K and 0–1.0 bar. Results showed that DESs demonstrated satisfactory SO₂ absorption performance, and GC–CC (3:1) exhibited the maximum absorption capacity of 0.528 g SO₂ per g DES. The SO₂ enriched DESs could be easily regenerated and recycled five times. Moreover, the DESs exhibited high selectivity of SO₂/CO₂. Present DESs seem to be promising absorbents for SO₂ due to good performance.     

Preparation and characterization of SO42-/TiO2 and S2O82-/TiO2 catalysts

Nanosized solid superacids SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂, as well as MCM-41-supported SO₄ ²⁻/ZrO₂, were prepared. Their structures, acidities, and catalytic activities were investigated and compared using XRD, N₂ adsorption-desorption, and in situ FTIR-pyridine adsorption, as well as an evaluation reaction with pseudoionone cyclization. The results showed that SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ possess not only nanosized particles with diameters < 7.0 nm, a BET surface greater than 140 cm²/g and relatively regular mesostructures with pores around 4.0 nm, but also a pure anatase phase and strong acidity. Different from the Lewis acid nature of SO₄ ²⁻/ZrO₂/MCM-41, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibit mainly Bronsted acidities. The strongest Bronsted acid sites were produced on SO₄ ²⁻/TiO₂ promoted with H₂SO₄, while Lewis acid sites on S₂O₈ ²⁻/TiO₂ even stronger than those on SO₄ ²⁻/ZrO₂/MCM-41 were generated when persulfate solution was used as sulfating agent. Because of their distinct acid natures, SO₄ ²⁻/TiO₂ and S₂O₈ ²⁻/TiO₂ exhibited catalytic activities for the cyclization of pseudoionone that were much higher than that of SO₄ ²⁻/ZrO₂/MCM-41. It can be concluded that the existence of more Br?nsted acid sites was favorable for proton participation in the cyclization reaction. Translated from Journal of Chemical Engineering of Chinese Universities, 2006, 20(2): 239–244 [译自: 高校化学工程学报]     

Advanced De-SOx catalyst: Mixed solid solution spinels with cerium oxide

Mixed solid solution spinels impregnated with cerium, Ce/MgO·MgAl_2-xM_xO₄ (M=Fe, V, Cr, x≤0.4), were studied for controlling the SO_x emission from the fluid catalytic cracking (FCC) regenerator. An insufficient sulfur release problem inherent to the earlier De---SO_x catalyst, Ce/MgO·MgAl₂O₄, was effectively overcome by incorporating a transition metal into the spinel structure. Studies of the SO_x pick-up, temperature profile for the sulfate reduction, the thermal analysis, and the De---SO_x cycle test in the batch as well as the automated continuous reactor are discussed to define the role of a transition metal in the mixed spinels for the De---SO_x performance. These advanced De---SO_x catalysts have led to a commercial success for the simultaneous control of SO_x and NO_x emissions from the FCC regenerator.