Catalytic Oxidation Of SO2 Research Articles

In-situ inhibition mechanism of SO3 formation over MoS2 modified V2O5/TiO2 catalyst within selective catalytic reduction process

The utilization of commercial vanadium-titanium based catalysts (V2O5/TiO2) within the selective catalytic reduction (SCR) process is a prominent contributor to the emergence of sulfur trioxide (SO3) in coal-fired boiler environments. The direct modification of vanadium-titanium based catalysts has big potential to be an efficacious strategy for in-situ inhibiting SO3 formation. This research evaluates the effectiveness of MoS2 modified V2O5/TiO2 catalyst on SO2 oxidation under diverse conditions, using controlled condensation for SO3 collection. The results showed that V2O5-MoS2/TiO2 catalysts outperformed conventional V2O5/TiO2 catalysts in inhibiting the oxidation from SO2 to SO3 between 200 and 400 °C, with optimal performance at 350 °C. No significant effect was observed on the catalytic oxidation of SO2 when the O2 concentration was lower than that of SO2, whereas the introduction of appropriate quantities of NO promoted the generation of SO3. The excessive inclusion of NH3 nearly doubled the SO3 generation rate, increasing it from 0.284 % to 0.434 %. The NO and NH3 mixture markedly reduced SO3 generation to 0.085 % at a 62 mL/min mixture concentration, just 30 % of the rate without the mixture. Following the reaction of SO2 and O2, the reacted catalyst exhibited reduced pore volume, enhanced thermochemical stability, and lower susceptibility to SO2 catalytic oxidation. The addition of low-valence sulfur improves catalyst reducibility, reduces V5+–OH reactions in various atmospheres, and decreases the formation of sulphate and gaseous SO3. The Mo-S-Mo group also effectively prevents the formation of HSO4− intermediates. Additionally, MoS2 reacts with SO3 via a reduction reaction, significantly lowering the SO3 levels. Finally, in diverse atmospheric conditions, the catalysts maintained their V5+ content and catalytic efficiency, thus improving SCR performance stability.

Seasonal simulation and source apportionment of SO42− with integration of major highlighted chemical pathways in WRF-Chem model in the NCP

Particulate sulfate (SO42−) is the major component of fine particles in the North China Plain (NCP). It plays an essential role in the entire atmosphere and climate system. Accurately reproducing the SO42− levels is challenging for chemical transport model. Here, the major highlighted multiple phase SO42− formation pathways are integrated into the WRF-Chem model and seasonal model performance of SO42− are assessed by observed SO42− at multiple sampling sites located in the NCP. The results show that the integration of these SO42− formation pathways obviously narrow the gap between simulation and observation in autumn, winter, and summer at three sampling sites in the NCP, for high RH conditions facilitate the hygroscopic growth of PM2.5 and promote the multiple phase reaction to form SO42−. The underestimation in autumn, winter, and summer may ascribe to the missed SO42− source from photo-induced chemical route and missed hygroscopicity of organic aerosols. The obviously discrepancy between simulation and observation in spring may ascribe to the large underestimation of SO2 levels and lack consideration of dust scheme. Source apportionment results show that the gas phase reaction played a vital role in the formation of SO42− in each season. The contribution of aqueous phase SO2 oxidation by H2O2 is higher in autumn and summer due to simultaneously high RH levels and stronger photochemical reaction activity. The Mn(II) catalytic SO2 oxidation pathway at the particle interface is important in autumn and make greater contribution to SO42− formation during winter severe haze events under extremely high RH levels.

Catalytic oxidation of SO2 in O2-aerated aqueous Solution: superior catalytic capability of KOH/urea-treated biochars and catalytic mechanism

The catalytic oxidation of SO2 (COS) by dissolved oxygen in aqueous solution allows to recycle sulfate salt/sulfuric acid from the SO2-containing flue gas, and high-performance catalysts are desired for an efficient COS process. In this study, the chestnut leaves-derived biochars treated by the co-pyrolysis process with KOH/urea exhibited a favorable capability to catalyze the oxidation of SO2 in aqueous solution. The BK-N-8 biochar catalyst was the optimal one due to its moderate nitrogen content and appropriate surface functional groups. It exhibited a fast initial conversion rate from SO2 to SO42− (6.50 μmol g−1 s−1), and an excellent cycling durability for the COS process. The material showed a catalytic activity that surpassed all the reported carbonaceous materials. The surface functionalities were revealed to be the decisive factor for the conversion rate and durability of catalytic capacity. The superiority of the BK-N-8 with pyridinic N-doped aromatic cluster (AC) was further investigated via density-functional-theory calculations. The epoxy groups from O2 dissociation at the edge sites of ACs played a pivotal role for COS process onto biochars. This study provides insightful information for an in-depth understanding of the mechanism of COS process catalyzed by carbonaceous materials.

SO2 Mitigation via Catalytic Oxidation using Carbonaceous Materials and Metal Oxides for Environmental Sustainability

The high concentration of sulfur dioxide (SO2) in the air that contributes to increasing health and environmental issues has caught the attention of all countries. Numerous tactics to regulate and lower the SO2 levels in the environment that have been applied through regulations and promising technology, progress has been obtained to decrease the SO2 concentration. Among methods for SO2 removal, one of the promising techniques used is the catalytic oxidation of SO2 to SO3, which not only reduces the SO2 concentration in the environment but also produces sulfuric acid (H2SO4). Thus, the performance of the catalysts that can promote the catalytic oxidation of SO2 to SO3 for environmental sustainability is reviewed in this study. The types of catalysts evaluated in this study are carbon-based materials and metal oxides. Worth noting that these catalysts are feasible to catalytically converting SO2 hazardous material to resources, viz. SO3 and H2SO4 for industrial use. The findings of this study can serve as a foundation for devising an innovative method for SO2 mitigation through catalytic oxidation. Copyright © 2023 by Authors, Published by BCREC Group. This is an open access article under the CC BY-SA License (https://creativecommons.org/licenses/by-sa/4.0).

On the Mechanism of Sulfur Dioxide Oxidation in Cloud Droplets

Data from field experiments on the dynamics of SO2 oxidation in cloud droplets are presented. The rapid oxidation of SO2 by molecular oxygen observed in experiments is attributed here to the catalytic action of a pair of manganese and iron ions in droplets. Their inhomogeneous effect by the drop-size distribution, attributed in experiments only to the leaching of ions of these metals from coarse particles of mineral dust is also due to the transition of the oxidation reaction into a branched mode. The results obtained indicate that the branched regime of catalytic oxidation of SO2 detected in cloud droplets should be considered as a new and significant source of sulfates in the atmosphere. This process must be taken into account when considering both the budget of sulfates in the global atmosphere and their impact on the climate.

Microstructure and Catalytic Activity of MnSbOx Catalysts for Selective Catalytic Reduction of NO with NH3

AbstractManganese‐antimony composite oxide catalysts were prepared for use in low‐temperature selective catalytic reduction of flue gas, by adopting the strategy of passivation to regulate the valence state of the active component and control catalytic activity. Activity evaluation results found that MnSb0.36Oy delivered 80 % NO conversion in the presence of SO2 at 200 °C, and nearly 90 % conversion at 250 °C. Doping with Sb changed the surface micromorphology, resulting in a perforated porousness layered foam with a porous structure of tens of nanometers, which was conducive to molecular mass transfer of the reaction gas. Doping with Sb regulated the valence state of the active MnOx component, which diminished catalytic oxidation of SO2, thus promoting catalyst stability and limiting the toxic effect of SO2.

Process compatible desulfurization of new suspension preheater cement production: Rapid catalytic oxidation of trace SO2 over V2O5-based catalysts in preheater environment

Efficient capture of sulfur dioxide (SO2) in the preheater of the cement production system is the key to achieving process compatible flue gas desulfurization (FGD). Herein, the rapid oxidation of trace SO2 over V2O5-based catalysts in the preheater environment was realized to enhance the SO2 capture capacity of limestone in raw meal, and the catalytic efficiency was further improved by 45.2%, 56.1% and 61.9% after introducing TiO2/MnO2/CeO2 as cocatalysts, respectively. The V2O5-based catalysts had superior catalytic behavior only when the flue gas temperature exceeded 500 °C, while the composition variation of flue gas had a minor influence on SO2 oxidation. Finally, a maximum SO2 oxidation efficiency of 68.1% was achieved in the presence of 4.0% O2, 25.0% CO2 and 15.0% H2O at 600 °C. Furthermore, the mechanism of SO2 catalytic oxidation over V2O5-based catalysts was deduced based on the results of XRD, BET, XPS and Raman, and V5+ was identified as the primary vanadium species for SO2 oxidation. The results provide a deeper insight into the catalytic oxidation of SO2 in the preheater environment, and then facilitate the improvement of process compatible FGD and available dry FGD technologies for cement industry.

Diesel soot photooxidation enhances the heterogeneous formation of H2SO4

Both field observation and experimental simulation have implied that black carbon or soot plays a remarkable role in the catalytic oxidation of SO2 for the formation of atmospheric sulfate. However, the catalytic mechanism remains ambiguous, especially that under light irradiation. Here we systematically investigate the heterogeneous conversion of SO2 on diesel soot or black carbon (DBC) under light irradiation. The experimental results show that the presence of DBC under light irradiation can significantly promote the heterogeneous conversion of SO2 to H2SO4, mainly through the heterogeneous reaction between SO2 and photo-induced OH radicals. The detected photo-chemical behaviors on DBC suggest that OH radical formation is closely related to the abstraction and transfer of electrons in DBC and the formation of reactive superoxide radical (•O2−) as an intermediate. Our results extend the known sources of atmospheric H2SO4 and provide insight into the internal photochemical oxidation mechanism of SO2 on DBC.

Combined method for production of reducing gases and processing galena-containing concentrate

A combined method for production of reducing gases by gasification of coal and pyrometallurgical processing of galena-containing concentrate is proposed. The coal is preliminarily cleaned from the components accompanying carbon in the reactor, which is heated through the inner walls of the double vessel by the heat of the stream of heated gases of metallurgical production. When coal is heated to 300–350 оC, moisture evaporates from it, CO and CO2 are desorbed and volatile components are released, and when the temperature rises to 500–550 оC, the graphitization of coal begins with the release of resinous substances. As the temperature rises to 700 оC, coke is formed, which is subjected to gasification with water vapor in the ratio C:H2O = 1:1 in a gas generator at a temperature of 1000–1050 оC. The resulting generator gas of equimolar composition H2:CO = 1:1 with a temperature of up to 1050 оC initially heats the reaction mixture from the galena-containing concentrate and air through the reactor wall. Part of the heat is used to obtain water vapor, and at a temperature of 350–380 оC as a reducing gas to obtain crude lead. Oxidation gases of sulphide minerals with a temperature of 1050 оC are used to heat coal in a double-vessel reactor to a temperature of 700 оC, then they give off part of their heat to produce water vapor and, with a temperature of 400 оC, enter the sulfur dioxide oxidation reactor, where the catalytic oxidation of SO2 to sulfuric anhydride, used to produce sulfuric acid, takes place. The remainder of the gas mixture, consisting of inert nitrogen and argon, is separated into individual components by a membrane method. The proposed method is implemented using a waste-free technology: incidentally formed water vapor and condensate, CO2, sulfuric acid and inert gases are used as useful products. Heat supply of the method is realized due to heat exchange between flows of substances within the technological scheme. The method is low-cost and environmentally friendly, emissions into the environment are excluded.

A chamber study of catalytic oxidation of SO2 by Mn2+/Fe3+ in aerosol water

The catalytic oxidation of SO2 by metal ions in aerosol water may be one of the important sources of sulfate in the atmospheric environment. In order to better understand the catalytic oxidation in aerosol water, a series of experiments were carried out in a smog chamber system under the conditions of relatively low concentrations of SO2 with Mn2+/Fe3+ as catalysts in NaNO3 aerosol water. In the Mn2+-catalytic oxidation process, the reaction rate (R) in aerosol water is expressed as R = 2.0 × 104 [Mn2+][S(IV)]1.3 at the initial pH range 4–5, which is about 2 orders of magnitude higher than that in bulk water. When the pH of aerosol water and relative humidity (RH) are high, the catalytic oxidation of SO2 by Mn2+ in aerosol water is probably an important source of sulfate. In the Fe3+-catalytic oxidation process, the reaction rate is significantly lower than that of Mn2+-catalytic oxidation, and the rate hardly exceeds 2 orders of magnitude larger than that in bulk water. The role of synergistic catalysis between Fe3+ and Mn2+ for oxidation of SO2 in aerosol water is very limited.

Unprecedented Ambient Sulfur Trioxide (SO3) Detection: Possible Formation Mechanism and Atmospheric Implications.

Sulfur trioxide (SO3) is a crucial compound for atmospheric sulfuric acid (H2SO4) formation, acid rain formation, and other atmospheric physicochemical processes. During the daytime, SO3 is mainly produced from the photo-oxidation of SO2 by OH radicals. However, the sources of SO3 during the early morning and night, when OH radicals are scarce, are not fully understood. We report results from two field measurements in urban Beijing during winter and summer 2019, using a nitrate-CI-APi-LTOF (chemical ionization-atmospheric pressure interface-long-time-of-flight) mass spectrometer to detect atmospheric SO3 and H2SO4. Our results show the level of SO3 was higher during the winter than during the summer, with high SO3 levels observed especially during the early morning (∼05:00 to ∼08:30) and night (∼18:00 to ∼05:00 the next day). On the basis of analysis of SO2, NOx, black carbon, traffic flow, and atmospheric ions, we suggest SO3 could be formed from the catalytic oxidation of SO2 on the surface of traffic-related black carbon. This previously unidentified SO3 source results in significant H2SO4 formation in the early morning and thus promotes sub-2.5 nm particle formation. These findings will help in understanding urban SO3 and formulating policies to mitigate secondary particle formation in Chinese megacities.

Catalytic combustion of sulfur-containing liquid fuels in the fluidized bed: Experiment and modeling

Environmentally safe fuel combustion is an actual worldwide problem. The alternative technology of combustion in fluidized bed of catalyst was developed in the Boreskov Institute of Catalysis. The technology allows one to conduct the process at 400–700 оC and significantly reduce outlet concentrations of toxic compounds. However, the use of sulfur oils as fuel demands solving a problem of the decreasing the outlet SO2 concentration because the oils contain small amount of ash to absorb SO2. The work is devoted to study features of sulfur light oil and high-sulfur heavy oil combustion in the fluidized bed of the Al–Cu–Cr-oxide catalyst. Reduction of SO2 emission at conserving oxidation degree of oils within range of 99.5–99.8% might be reached by using a non-isothermal temperature profile in the reactor as well as a use of CaCO3 or CaO at Ca/S > 2 to absorb sulfur oxides. The non-isothermal temperature profile in the reactor consisted of an oxidation zone with the temperature in the fluidized bed of 700 оC and a catalytic oxidation of SO2 to SO3 zone with equilibrium shift to reducing SO2 concentration because of the temperature change from 700 оC to 585 оC.

Catalyst design using artificial intelligence: SO2 to SO3 case study

AbstractCatalyst design is key to the improvement of chemical process efficiency. The required work for the development of new catalysts can be supported through the proper application of artificial intelligence to identify optimal compositions. A generic methodology for the application of machine learning to catalysis research is therefore outlined in this work. The catalytic oxidation of SO2 was used to exemplify the first iteration of this methodology. 1784 data points from 31 published papers were compiled into a databank. The inlet SO2 concentration ranged from 0 to 66 mol%. An artificial neural network (ANN) was trained on the databank in order to predict SO2 conversion based on the catalyst composition and the reactor operating conditions (temperature, pressure, catalyst mass: volumetric flowrate ratio (w/v), and feed composition). The model achieved a root‐mean‐square error of 6.6%. A preliminary screening step identified 3:1 V‐Mg/SiO2 catalysts as exhibiting high conversion at 648 K. A multi‐objective optimization was then performed on a single catalyst to identify solutions exhibiting high conversion and high productivity at 648 K while minimizing the catalyst cost. The optimal solution was predicted to be a 2.9 wt% V/0.2 wt% Mg/SiO2 catalyst operating at a w/v of 7.49 kg‐cat · s/m3 STP, achieving 100% SO2 conversion with a material cost among the bottom third of cost values. Artificial intelligence can then be employed to extract useful knowledge from published catalytic data and orient future search for novel catalyst development.

Synthesis and characterization of vanadium pentoxide on different metal oxides by the sol-gel process for application in the conversion of the SO2 to SO3

Sulfuric acid is the largest volume chemical currently produced in the world. Is manufactured by the contact process, it involves three stages: combustion, conversion and absorption. The SO 2 conversion reaction is the key step in the process, it uses catalysis. The objective of this work is to synthesize a series of mixed vanadium oxides X% / MO 2 with M (Si, Al and Ti) by sol-gel process followed by calcination at 400 ° C, in order to study their reactivity in the catalytic oxidation of SO 2 to SO 3 . Characterization of those materials was carried out by Fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM) with energy dispersive X-ray (EDX), X-ray diffraction, thermal analysis (TDA/TGA) and N 2 adsorption at 77 K. Their acid-base properties are studied by the decomposition reaction of isopropanol (propan-2-ol). In this work, we have studied the reactivity of the catalysts prepared in the conversion of SO 2 to SO 3 by an iodometric as dosage which consists in assaying the iode with sodium thiosulfate.

Catalytic function of ferric oxide and effect of water on the formation of sulfur trioxide

Sulfur trioxide (SO3) is not only environmentally harmful but also highly corrosive, taking a great threat to the safe operation of coal-fired power plants. A dominant pathway of SO3 formation in coal-fired power plant is through the catalytic oxidation of SO2 (SO2+1/2O2→SO3) on the surfaces of ash particles containing Fe2O3. The catalytic formation of SO3 could be affected by complex atmosphere, where the effect from H2O is still debatable. In this paper, density functional theory (DFT) is employed to explore the reaction pathway of SO3 formation catalyzed by α-Fe2O3 in complex atmosphere containing O, O2, SO2 and H2O. In order to get the stable adsorption sites of these species, the adsorption energy of potential adsorption configurations on the α-Fe2O3 (001) surface is calculated. The dissociations of O2 molecule on complete and defect α-Fe2O3 (001) surfaces with O vacancy are calculated, and the Langmuir-Hinshelwood and Eley-Rideal mechanisms for the O(ads) reaction with SO2(ads) or SO2 are compared. The effect of H2O besides of SO2 and O2 on the formation of SO3 is especially discussed. The DFT calculation results show that for the formation of SO3 in gas phase, the energy barrier of ‘SO2+1/2O2→SO3’ is 436.75 kJ mol−1, in contrast, for the catalytic formation of SO3 on α-Fe2O3 surfaces, this energy barrier becomes an order of magnitude smaller, 24.82 kJ mol−1. O2 molecules can dissociate on the defect α-Fe2O3 (001) surface with O vacancy spontaneously, indicating that the defect α-Fe2O3 is favorable for the dissociation of O2, thereby promotes the formation of SO3. The energy barrier of ‘SO2(ads)+O(ads)→SO3(ads)’ through Langmuir-Hinshelwood mechanism is much higher than that of ‘SO2+O(ads)→SO3(ads)’ through Eley-Rideal mechanism. The adsorption energy on the α-Fe2O3 (001) surface of H2O is much smaller than that of SO2 and O2, indicating that H2O has little effect on the adsorption of O, O2, SO2 and eventually the heterogeneous formation of SO3. The DFT analysis results in this study provide a deep understanding on the reaction pathway of SO3 catalytic formation by Fe2O3.

Ru@Pt/C core-shell catalyst for SO2 electrocatalytic oxidation in electrochemical Bunsen reaction

Although SO2 is a highly toxic industrial pollutant, it can be used as a potential source of hydrogen and H2SO4 in the presence of water. This characteristic of SO2 is widely applied in desulfurization of industrial flue gas, hybrid-sulfur cycle and sulfur-iodine cycle. The main way to advance this technology is to develop a suitable electrocatalyst. So far researches on electrocatalysts for liquid phase oxidation of SO2 mainly focus on noble metal, which have excellent corrosion resistance and catalytic activity in acidic environment. In order to improve the performance of Pt-catalysts in SO2 electrocatalytic oxidation and reduce the use of Pt, a series of Ru@Pt/C core-shell catalysts with different Ru/Pt molar ratios were synthesized by microwave-assisted alcohol reduction. Reduction in the size of Pt shell is beneficial to increase the catalytic activity brought by the electronic effect. The bi-functional effect of Ru and Pt is more pronounced in the case of low Pt coverage. Ru only plays an auxiliary role and does not have the property of direct catalytic oxidation of SO2. The results showed that the optimal mass activity was achieved when Ru/Pt was 1:1. While when Ru/Pt was 3:1, the unit mass activity of Pt was the highest, which was 3.97 times of Pt/C.

Influenced factors study and evaluation for SO2/SO3 conversion rate in SCR process

SO3 generated in coal-fired boiler and selective catalytic reduction (SCR) process has become a significant problem due to its adverse effects on boiler facilities and air environment. The factors influenced conversion rate of SO2 to SO3 (SO2/SO3) during SCR process were studied and evaluated, which included the effects of flue gas composition and SCR catalyst components on the conversion of SO2/SO3. SO3 was trapped by the condensation and isopropanol absorption method, and SO42− concentration was detected by spectrophotometer. The results showed that a certain concentration of NH3 in the flue gas significantly promoted the conversion rate, but when NH3 and NO coexisted, NO inhibited the catalytic action of NH3, while NO itself and O2 had small effect on the conversion rate. Besides, the conversion rate of SO2 increased with the increase of Ti and V content in the catalyst, and SiO2 and BaO abated the catalytic oxidation of SO2, while WO3 promoted the formation of SO3.

Oxidation of SO2 over C-doped boron nitride nanosheets: The role of C-doping, and solvent effects

In this work, density functional theory calculations are performed to examine the catalytic oxidation of SO2 in the presence of O2 molecule over carbon-doped hexagonal boron nitride nanosheets (h-BNNSs). The SO2 oxidation over these surfaces is characterized as a two-step mechanism; (a) SO2 + O2 → SO3 + O* and (b) SO2 + O* → SO3. According to the obtained results, the activation energies and reaction mechanism depend greatly on the substitution site of the C-doped h-BNNS. That is, the catalytic activity of C atom located on top of the B-vacancy site of h-BNNS is larger than that of on top of the N-vacancy. Moreover, it is found that the energy barriers for the oxidation of SO2 are considerably decreased in an aqueous solution. For a given substrate, the activation energy for the oxidation of H2SO3 is much larger than that of SO2, suggesting that the direct conversion of SO2 to SO3 should be the main reaction pathway for the oxidation of SO2. The results of present study could contribute to design highly active BN-based catalysts to oxidize SO2 molecule.

Catalytic oxidation of SO2 by novel Mn/copper slag nanocatalyst and optimization by Box-Behnken design

In this research, the oxidation of sulfur dioxide (SO2) gases is investigated by Mn/copper slag nanocatalyst (Mn/CS) as a novel catalyst at low temperatures. The removal of SO2 gas from industrial exhaust is important to reduce environmental pollution. The SO2 gas in aqueous solution was oxidized and converted to sulfuric acid as an energy source by Mn/CS in the semi-batch reactor (SBR). The characterization of the catalyst was studied using X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), field emission scanning electron microscopy (FESEM), and Fourier transform infrared spectroscopy (FTIR), simultaneous thermal analysis (thermogravimetry/differential thermal analysis) STA (TG/DTA) techniques, X-ray fluorescence microscopy (XRF) and BET surface area. A Box-Behnken design (BBD) was used for the optimization of influencing factors such as the amount of nanocatalyst, the temperature and the reaction time in the oxidation of SO2. The graphical counter plots and response surface were used to determine the optimum conditions. The results showed that the nanocatalyst had the most significant effect on SO2 oxidation compared with the other two variables. Temperature = 283 K, Mn/CS amount = 6 g/L and Time = 60 min were determined as maximum efficiency for oxidation of SO2.

Heterogeneous Reaction of SO2 on Manganese Oxides: the Effect of Crystal Structure and Relative Humidity

Manganese oxides from anthropogenic sources can promote the formation of sulfate through catalytic oxidation of SO2. In this study, the kinetics of SO2 reactions on MnO2 with different morphologies (α, β, γ and δ) was investigated using flow tube reactor and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Under dry conditions, the reactivity towards SO2 uptake was highest on δ-MnO2 but lowest on β-MnO2, with a geometric uptake coefficient (γobs) of (2.42 ± 0.13) ×10–2 and a corrected uptake coefficient (γc) of (1.48 ± 0.21) ×10−6 for the former while γobs of (3.35 ± 0.43) ×10−3 and γc of (7.46 ± 2.97) ×10−7 for the latter. Under wet conditions, the presence of water altered the chemical form of sulfate and was in favor for the heterogeneous oxidation of SO2. The maximum sulfate formation rate was reached at 25% RH and 45% for δ-MnO2 and γ-MnO2, respectively, possibly due to their different crystal structures. The results suggest that morphologies and RH are important factors influencing the heterogeneous reaction of SO2 on mineral aerosols, and that aqueous oxidation process involving transition metals of Mn might be a potential important pathway for SO2 oxidation in the atmosphere.