Development of low-temperature desulfurization performance of a MnO₂/AC composite for a combined SO₂ trap for diesel exhaust

Abstract

Growing concern about the removal of sulfur dioxide (SO₂) from combustion exhaust has resulted in the development of desulfurization materials for a SO₂ trap. In this study, a manganese dioxide/activated carbon (MnO₂/AC) composite was proposed as a low temperature desulfurization material for a combined SO₂ trap. The MnO₂/AC composite was synthesized using a redox deposition method and characterized using scanning electron microscopy (SEM), nitrogen adsorption, X-ray fluorescence spectrometry (XRF) and Fourier transform infrared (FTIR) spectroscopy. The SO₂ adsorption capacity of the composites was measured using thermogravimetry and the SO₂ adsorption characteristics were also investigated. In the low temperature region (50–200 °C), the MnO₂/AC composite exhibits good SO₂ trap performance and the MnO₂ conversion of the composite is significantly improved. It was found that the SO₂ adsorption on the MnO₂/AC composite is a chemisorption process. The experimental data for SO₂ adsorption on the MnO₂/AC composite could fit the Freundlich model well. Changes in the thermodynamic parameters were determined. The calculated values of ΔG⁰ and ΔH⁰ indicate that the SO₂ adsorption on the MnO₂/AC composite is spontaneous and thermodynamically favorable.

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Introduction

The removal of sulfur dioxide (SO₂) from combustion exhaust (especially from diesel power plants and marine ships) is of particular interest for two main reasons: SO₂ has detrimental effects on human health and the environment, and SO₂ can deactivate NO_x removal catalysts.^1–4 To solve these issues caused by large amounts of SO₂ emissions from diesel engine exhausts, significant efforts have been contributed to removing SO₂ since the compact SO₂ trap device upstream of NO_x conversion device was proposed to improve the longevity of NO_x removal catalysts by working against SO₂ poisoning.^5–7 The temperature of diesel engine exhaust is over a wide range from 200 °C to 650 °C when the engine is working. So, most desulfurization materials are focused on the desulfurization performance at 200–650 °C for a compact SO₂ trap, such as calcined limestone,⁸ MgO⁹ and hydrotalcite-like compounds.¹⁰

At the beginning of ignition, as the diesel is incompletely combusted, the diesel engine exhaust is in a low temperature region from 30 °C to 200 °C. Therefore, the temperature of the diesel engine exhaust is under a very wide range from 30 °C to 650 °C. Based on fundamental studies, it has been found that carbonates have good reactivity with SO₂ at 650 °C, but the desulfurization rate declines below 400 °C for the reason that the reaction activity is limited by decarbonation.³ Metal oxides with the sulfate reaction path (M_xO_y + ySO₂ + 0.5yO₂ → M_x(SO₄)_y) exhibit good SO₂ capture performance over the temperature range of 200 °C to 450 °C.¹¹ Carbon-based materials have good desulfurization performance at 100 °C.¹² However, the investigated desulfurization materials do not have a high enough SO₂ capture capacity for their application in a compact SO₂ trap under the wide temperature range. Therefore, a combined SO₂ trap is proposed to completely capture SO₂ in the temperature range of 30 °C to 650 °C. The combined SO₂ trap has three parts: high-temperature materials (carbonates), middle-temperature materials (metal oxides) and low-temperature materials (carbon-based materials), as shown in the Fig. 1.

Fig. 1 Conceptual drawing of the combined SO₂ trap.

To enhance the performance of the combined SO₂ trap, the improvement of low temperature sulfate activity of desulfurization materials for compact SO₂ traps is needed. However, limited studies on desulfurization materials with low temperature activation have been reported. Nishioka⁷ aimed to improve the reaction activity of SO₂ trap catalysts with a noble metal under low temperature conditions. Kylhammar¹³ investigated the SO₂ trapping capacity of CeO₂-based materials at 250 °C and the fresh sample could store about 19 mg_SO2 g_CeO2⁻¹. Rubio¹² studied the SO₂ removal performance of coal fly ash based carbons under flue gas conditions and the amount of SO₂ removed was 13 mg g⁻¹ for the activated sample at 100 °C. Tseng¹⁴ investigated the SO₂ reaction activity of copper oxide supported on activated carbon in the low temperature region. Activated carbon has various outstanding advantages, such as high adsorption capacity, high stability, low cost, low density and easy accessibility, and activated carbon-based materials have promising prospects for their use as desulfurization materials under low temperature conditions. Furthermore, it is reported that highly dispersed metal oxide particles (especially particles in the nanoscale) on activated carbon can significantly improve the catalyst activity at low temperature.¹⁵

In our previous studies,^1,16 manganese dioxide (MnO₂) was focused on as a candidate for the material of a compact SO₂ trap in the low temperature region. However, in low temperature conditions, the sulfate ratios of the material are very small because of the aggregation of the MnO₂ materials. Furthermore, in the present work, manganese dioxide/activated carbon composites were prepared by translating potassium permanganate into MnO₂ which formed composites with residual carbon material using the redox deposition method and carbon as a reducer. The basic SO₂ capture capacity of the composites was measured using a thermogravimetry (TG) device in the low temperature region (50–200 °C) with a 2 L min⁻¹ gas flow containing 500 ppm SO₂ in nitrogen, and the adsorption characteristics of the MnO₂/AC composites toward SO₂ at low temperature were also investigated.

Experimental section

Materials

The chemicals, activated carbon (BET surface area, 598 m² g⁻¹) and potassium permanganate, were purchased from Beijing Chemical Co., Ltd., People’s Republic of China and were of analytical reagent grade.

In this experiment, MnO₂/AC composites were synthesized using a simple redox deposition method.¹⁷ A 250 mL three-necked round-bottomed flask was placed in a constant temperature oil bath as the reactor. 0.5 g of activated carbon was blended with 50 mL of 0.1 mol L⁻¹ potassium permanganate (KMnO₄) solution in the flask. The flask was kept at a constant temperature in the oil bath. Then the suspension containing activated carbon was refluxed at 140 °C with sustained magnetic stirring. After 10 h, the mixture was filtered, washed with distilled water several times to remove the residual KMnO₄, dried at 120 °C for 8 h, and then kept in the desiccator until use.

Characterization

In this study, the specific surface area, pore volume distribution, and surface structure were analyzed to assess the physical characteristics of the target materials. The specific surface area of these samples was measured using Brunauer–Emmett–Teller (BET) analysis with the nitrogen adsorption uptake at the boiling point of nitrogen which is 77 K using a capacitive measurement method. The pore diameter was measured using nitrogen physisorption under a normal relative pressure of 0.1–1.0 using the Barrett–Joyner–Halenda (BJH) method. Surface observation of the samples was conducted by scanning electron microscopy (SEM, Hitachi S-4800). The metal oxide yield of the composite was carried out utilizing a wavelength dispersive sequential X-ray fluorescence spectrometry (XRF, AxiosmAX Petro). Fourier transform infrared (FTIR) spectra were recorded using a Tensor 27 spectrometer with the KBr pellet method. The crystal structures were further determined using X-ray diffraction (XRD, X’Pert Pro MPD, Cu Kα radiation). X-ray photoelectron spectroscopy (XPS) was conducted to determine the chemical composition and functional groups using an XSAM-800 spectrometer (Kratos, UK) with Al (1486.6 eV) under ultrahigh vacuum (UHV) at 12 kV and 15 mA. Energy calibration was performed by recording the core level spectra of Au 4f_7/2 (84.0 eV) and Ag 3d_5/2 (368.30 eV).

SEM images of AC and the MnO₂/AC composite are shown in Fig. 2. It can be seen that a large number of MnO₂ nanoparticles are uniformly formed and highly dispersed on the AC surface (Fig. 2b). Fig. 2c shows a magnified SEM image of the MnO₂/AC composite. The diameters of the uniform MnO₂ nanoparticles are around 100 nm. The FTIR spectrum of AC is illustrated in Fig. 3a; the peaks around 1230 cm⁻¹ correspond to the C–H stretch vibration,¹⁸ bands at about 1570 cm⁻¹ denote O–H bending vibrations,¹⁹ the peaks around 2360 cm⁻¹ correspond to the C≡C stretching vibration,²⁰ and the bands around 3400 cm⁻¹ should be attributed to the O–H stretching vibration.²¹ From the FTIR spectra of the MnO₂/AC composite shown in Fig. 3a and b the relatively sharp peak around 525 cm⁻¹ should be ascribed to the Mn–O and Mn–O–Mn vibrations in octahedral MnO₂ (ref. 22). XRD analysis (Fig. 4) confirmed the MnO₂ nanoparticles. The diffraction peaks of as-synthesized MnO₂/AC were similar to those of ramsdellite phase MnO₂ (JCPDS 42-1316), where the reflection peak of layered AC had almost disappeared. This result indicated that homogeneous composites were formed on the AC surface, and were covered by MnO₂ nanoparticles. The surface area (calculated using Brunauer–Emmett–Teller (BET) theory) of the MnO₂/AC composite is 278 m² g⁻¹. The average pore diameter of the MnO₂/AC composite is 1.2 nm, and its pore volume (calculated using Barrett–Joyner–Halenda (BJH) theory and Horvath–Kawazoe (HK) theory) is 0.34 cm³ g⁻¹. TG/DSC profiles of parent AC and the MnO₂/AC composite in the air are shown in Fig. 5. It can be seen that, a sharp weightlessness of the parent AC around 500 °C can be clearly observed, which is mainly caused by the combustion of carbon. As the temperature of carbon combustion of the MnO₂/AC composite decreased to around 300 °C, the MnO₂/AC composite shows high oxidation capacity.

Fig. 2 (a) SEM image of parent AC, (b) SEM image of the MnO₂/AC composite, and (c) enlarged image of MnO₂ nanoparticles from panel (b).

Fig. 3 FTIR spectrum of (a) parent AC and (b) the MnO₂/AC composite.

Fig. 4 XRD patterns of pure AC and MnO₂/AC.

Fig. 5 TG/DSC profiles of AC and the MnO₂/AC composite.

Thermogravimetric measurements

Thermogravimetry (TG) was used in this study to measure the effectiveness of the sulfate reaction of the MnO₂/AC composite. Fig. 6 shows a schematic drawing of the TG analysis experiment. An amount of 50 mg of a sample on a quartz crucible was slowly (10 K min⁻¹) heated to the target temperature under an atmosphere of nitrogen, and maintained under this condition for about 2 hours. Reactant gas flow (SO₂ in base N₂) was controlled using a mass flow controller. The total flow gas rate was 2 L min⁻¹. During SO₂ adsorption, the gas stream was passed over the sample at the target temperature for 2 hours. The reaction temperature of the TG experiment ranged from 50 °C to 250 °C.

Fig. 6 Schematic drawing of TG analysis.

The SO₂ capture performance of the samples was measured. The SO₂ capture performance per unit mass P and the conversion of MnO₂ X_(t) are expressed by the following equations:

P is the SO₂ capture performance per unit mass [g_SO2 g_Material⁻¹], s₀ is the initial weight [mg], and s_t is the weight after t seconds [mg], M_MnO2 is the molar mass of MnO₂ [g mol⁻¹], M_SO2 is the molar mass of SO₂ [g mol⁻¹].

Results and discussion

Basic sulfate performance of the MnO₂/AC composite

Fig. 7 shows the influence of the sulfate reaction temperature on the SO₂ capture performance of the MnO₂/AC composite. The SO₂ capture performance of the MnO₂/AC composite was investigated from 50 °C to 200 °C. From the results shown in Fig. 7, the SO₂ capture performance of the MnO₂/AC composite increased with the experimental temperature. The SO₂ capture capacities (mg g⁻¹) of the MnO₂/AC composite and pure AC at different temperature conditions are listed in Table 1. At 200 °C, the composite exhibited good SO₂ capture performance with an absorbance of about 65.6 g_SO2 g_Composite⁻¹ and the SO₂ capture performance of AC was 2.1 g_SO2 g_AC⁻¹. As pure AC exhibited a higher SO₂ capture performance at a lower temperature and captured 15.4 g_SO2 g_AC⁻¹ at 50 °C, the MnO₂/AC composite had a very high SO₂ capture performance (21.7 g_SO2 g_Composite⁻¹) at 50 °C. In comparison with investigated low temperature desulfurization materials, such as CuO/AC (below 10 mg g⁻¹)¹⁴ and coal fly ash (13 mg g⁻¹),¹² the MnO₂/AC composite exhibited a high SO₂ capture performance over the low temperature range from 50 °C to 200 °C. It was found that manganese oxide impregnated on activated carbon could play a key role in SO₂ removal.²³ Based on the results shown in Fig. 2, it is evident that the oxidative activity of the MnO₂/AC composite was significantly improved because the MnO₂ nanoparticles are highly dispersed on activated carbon by the redox deposition method.²⁴

Fig. 7 Temperature dependence of the SO₂ capture performance of the MnO₂/AC composite.

Table 1 The SO₂ capture capacity (mg g⁻¹) of the MnO₂/AC composite and pure AC at 50 °C, 100 °C, 150 °C, and 200 °C, for 500 ppm SO₂ in base N₂

Sample	50 °C	100 °C	150 °C	200 °C
MnO2/AC composite	21.7	25.7	43.1	65.6
AC	15.4	8.6	3.3	2.1

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The surface functional groups of activated carbon are very important factors for SO₂ removal.²³ The XPS analyses were carried out to determine the functional groups on pure AC and the MnO₂/AC composite, as shown in Fig. 8. The C 1s pattern of all samples included graphitizing carbon (C–C/C–H), phenolic (C–O), carbonyl carbon (C=O) and transition (π–π*) with binding energies at around 285, 286, 288 and 290 eV, respectively.²³ The content of π–π* transition in the MnO₂/AC composite slightly increased compared to that in AC. It was reported that the functional group of π–π* transition with basic nature was more favorable for SO₂ removal.²⁵ Thus, the change of surface functional group was responsible for the better SO₂-capture performance of the MnO₂/AC composite.

Fig. 8 C 1s patterns of XPS spectra: (a) prepared AC, (b) MnO₂/AC composite before desulfurization, and (c) MnO₂/AC composite after desulfurization.

In order to further study the chemical composition information of the MnO₂/AC composite after SO₂ removal, XPS analysis was carried out to clarify the Mn species on activated carbon; the Mn 2p XPS patterns of the MnO₂/AC composite are shown in Fig. 9. Before desulfurization, the Mn 2p_3/2 region of the MnO₂/AC composite consisted of one sharp and strong peak at 641.90 eV (Fig. 9a), attributed to Mn⁴⁺ (641.1–642.4 eV) in MnO₂,²⁶ indicating that MnO₂ was formed on the surface of activated carbon. After desulfurization, the peak at 642.60 eV (Fig. 9b) is characteristic of Mn²⁺.²⁷ Fig. 9c shows S 2p XPS spectra of the MnO₂/AC composite after desulfurization. The S 2p_3/2 binding energy of the MnO₂/AC composite was 168.67 eV and could be assigned to SO₄²⁻, indicating that MnO₂ on the surface of the activated carbon was transformed to MnSO₄ during the desulfurization process.²⁸

Fig. 9 Mn 2p patterns of MnO₂/AC: (a) before desulfurization, and (b) after desulfurization. (c) S 2p pattern of MnO₂/AC after desulfurization.

Fig. 10 shows a comparison of the sulfate ratio of the MnO₂/AC composite excluding the SO₂ capture amount of the pure AC with that of MnO₂ at 50 °C, 100 °C, 150 °C and 200 °C. The sulfate ratio of the composite was significantly improved at low temperature conditions compared with that of pure MnO₂. After desulfurization at the low temperatures of 50 °C and 100 °C, the sulfate ratio of the MnO₂/AC composite was 80% and 65% higher than that of MnO₂, respectively. It was indicated that the deposition of MnO₂ on the surface of activated carbon by a simple redox deposition method was effective in enhancing SO₂ removal at low temperatures for a combined SO₂ trap for a diesel exhaust system.

Fig. 10 Comparison of the sulfate ratio of the MnO₂/AC composite with that of MnO₂ at low temperatures from 50 °C to 200 °C.

Effect of the precursor concentration on SO₂ adsorption capacity

The concentration of KMnO₄ was varied from 0.01 mol L⁻¹ to 0.5 mol L⁻¹. Table 2 displays the textural properties of MnO₂/AC composites. After the redox deposition process, the pore volume of the product MnO₂/AC composite was much smaller than the pure AC (0.79 cm³ g⁻¹) and the BET surface area as well as the pore volume of the MnO₂/AC composites decreased along with the rise in the concentration of KMnO₄ because the uniform MnO₂ nanoparticles plugged the pores of the AC surface, as is shown in Table 2.

Table 2 Textural properties of MnO₂/AC composites

Sample	KMnO4 concentration (mol L^−1)	BET surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)
AC0	0	598	0.79
AC1	0.01	473	0.64
AC2	0.05	345	0.41
AC3	0.1	278	0.34
AC4	0.2	186	0.15
AC5	0.3	91	0.09
AC6	0.5	45	0.03

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The MnO₂ loading amount of the prepared composites was calculated utilizing Wavelength Dispersive Sequential X-ray Fluorescence Spectrometry (XRF, AxiosmAX Petro). In the study, the loading amount of MnO₂ ranged from 5% to 40%. In Fig. 11, the loading amount of MnO₂ increased along with the rise of the concentration of KMnO₄ as expected. From the results in Fig. 11, it is found that at high concentrations the increasing rate was much lower than at low concentrations, the reason of which could be that, at lower concentrations, the degree of supersaturation was not adequate to generate abundant growth units on the AC surface. At higher concentrations the degree of supersaturation was adequate, so the loading amount of MnO₂ grew much slower with the increase of KMnO₄ concentration.¹⁵ It can be concluded that, the supersaturation was intensified with the increase of the KMnO₄ concentration, leading to quick nucleation and a high growth rate of the loading amount of MnO₂ and a high density of MnO₂ nanoparticles was dispersed on the AC surface.

Fig. 11 Effect of the KMnO₄ concentration on SO₂ adsorption capacity of the as-prepared MnO₂/AC composites at 100 °C, for 500 ppm SO₂ in nitrogen.

Fig. 11 also displays the SO₂ capture capacity of the as-prepared MnO₂/AC composites with a variation of precursor concentrations at 100 °C, for 500 ppm SO₂ in nitrogen. The AC composites reveal a tremendously improved adsorption capacity in comparison to the parent AC. The SO₂ capture capacity of the composites initially increased with the rising KMnO₄ concentration and reached a maximum value when the KMnO₄ concentration was 0.2 mol L⁻¹. The maximum SO₂ capture capacity of the as-prepared MnO₂/AC composites was 29 mg_SO2 g_Material⁻¹, which was significantly higher than for other low temperature desulfurization materials (such as vanadium/activated carbon reported by Carabineiro,²⁹ coal fly ash reported by Rubio¹² and CuO/AC reported by Tseng¹⁴). However, when the KMnO₄ concentration further increased, the desulfurization capacity decreased instead. A similar tendency was also observed by Carabineiro²⁹ and Sumathi.³⁰ The SO₂ capture capacity presented an obvious decrease at the high KMnO₄ concentration, mostly because the accumulation of MnO₂ nanoparticles greatly reduced the surface area, pore volume and the amount of effective active sites.¹⁵

Equilibrium adsorption isotherm

For heterogeneous adsorption on the surface of a material, the three most conventional sorption isotherm models used to represent the collected equilibrium data are the Langmuir, Freundlich and BET models. This study employed these three models to describe the equilibrium adsorption. The Langmuir model assumes that equilibrium is obtained when a monolayer of the adsorbate-molecules (SO₂) saturates the adsorbent (MnO₂/AC composite). The expression of the Langmuir model can be written as:where, q is the amount of SO₂ adsorbed per unit mass of MnO₂/AC composite (mg g⁻¹), q_m is the Langmuir constant related to the adsorption capacity (mg g⁻¹), b (L mg⁻¹) is a constant related to the affinity between the MnO₂/AC composite and SO₂, and C_e is the equilibrium concentration of SO₂ (mg L⁻¹). The linear form of the Langmuir model can be expressed as:

The values of q_m and b can be determined from the slope and intercept of a linear plot of C_e/q versus C_e.

The Freundlich model is an empirical equation that assumes heterogeneous adsorption owing to the diversity of adsorption sites. The Freundlich model can be described by the following equation:

q = K_fC_e^(1/n) (5)

where, K_f ((L mg⁻¹)^1/n) and n are Freundlich constants related to the sorption capacity and sorption intensity, respectively.

The linear form of the Freundlich model takes the form:

ln q = ln K_f + (1/n)ln C_e (6)

where the values of K_f and n can be obtained from the intercept and the slope of a linear plot of ln q versus ln C_e, respectively.

The Brunauer–Emmett–Teller (BET) model was used to fit data that follow multilayer adsorption. The expression of the BET model can be written as:

where q is the SO₂ capture amount per gram of MnO₂/AC composite at the equilibrium value (mg g⁻¹), c is a constant related to surface energy, q_b is the monolayer coverage capacity (mg g⁻¹), C_e is the equilibrium concentration of SO₂ (mg L⁻¹), and C₀ is maximum equilibrium saturation concentration of SO₂ (mg L⁻¹).

Among these three conventional sorption isotherm models, the Freundlich model is very applicable when the adsorbate is strongly adsorbed on the surface of the adsorbent where a logarithmic fall in the enthalpy of adsorption with surface coverage has occurred. To determine the shape of the adsorption equilibrium for this system, the type of interaction between SO₂ gas and the MnO₂/AC composite should be understood.³¹ To obtain further insight into the SO₂ adsorption by the MnO₂/AC composite, Fourier transform infrared (FTIR) spectra were conducted in this study. The FTIR spectrum of the MnO₂/AC composite after desulfurization is illustrated in Fig. 12b, in which it can be seen that the intensity of the band at 1102 cm⁻¹ is due to the stretching motion of adsorbed sulfate on the surface of desulfurization materials.³² It can be concluded that SO₂ adsorption by the MnO₂/AC composite is a chemisorption process and the sulfate is formed. Based on the results of Fig. 2b and 12, as the uniformly MnO₂ nanoparticles are highly dispersed on the AC surface, the SO₂ gas can easily cover the surface of the MnO₂/AC composite and it can be strongly chemisorbed by the absorbent to form sulfate. Therefore, the Freundlich model could be applicable for the SO₂ adsorption by the MnO₂/AC composite.

Fig. 12 FTIR spectra of (a) the MnO₂/AC composite and (b) the MnO₂/AC composite after desulfurization.

In order to determine the constants of the Freundlich model for the SO₂ adsorption by the MnO₂/AC composite, the equilibrium adsorption capacity was measured under different SO₂ gas concentration conditions (200 ppm, 300 ppm, 500 ppm and 700 ppm) at 373 K. The corresponding Freundlich plot of the SO₂ adsorption capacity of the MnO₂/AC composite is shown in Fig. 13. From the results of Fig. 13, it can be seen that the Freundlich model fits the experimental data reasonably well, and the value of R-square is as high as 0.9936. A Freundlich constant, n related to the sorption intensity, of 1.170 was calculated from the intercept and the slope of the Freundlich model. Compared with the value (1.059) of the Freundlich constant n for zeolitic tuff reported by Al-Harahsheh,³¹ it can be concluded that the MnO₂/AC composite exhibits high activity for SO₂ adsorption.

Fig. 13 Freundlich plot of the SO₂ adsorption capacity of the sample.

Adsorption thermodynamics

The thermodynamic parameters could provide further information of inherent energetic changes of the SO₂ adsorption by the MnO₂/AC composite. To determine the thermodynamic parameters such as heat of adsorption (ΔH⁰), entropy (ΔS⁰) changes and the free energy of the process (ΔG⁰), the following equations were used:

ΔG⁰ = −RT ln K_f (8)

where K_f is the Freundlich equilibrium constant (L mg⁻¹), R is the gas constant (8.314 J mol⁻¹ K⁻¹) and T is the temperature (K). ΔH⁰ and ΔS⁰ were determined from the slope and intercept of the Van’t Hoff plots of ln(K_f) versus 1/T. Fig. 14 shows regressions of the Van’t Hoff plot about the SO₂ adsorption by the MnO₂/AC composite, and Table 3 displays the thermodynamic parameters (ΔG⁰, ΔH⁰ and ΔS⁰) at various temperatures (325 K, 373 K, 423 K, 473 K and 523 K). The calculated values of ΔH⁰ and ΔS⁰ were 14.30 kJ mol⁻¹ and 62.97 J mol⁻¹ K⁻¹, respectively. Positive ΔH⁰ and ΔS⁰ values suggest that the SO₂ adsorption by the MnO₂/AC composite is endothermic, which in fact is supported by the increase in the SO₂ adsorption with temperature. Moreover, the positive ΔS⁰ indicates that the degrees of freedom increased at the solid–gas interface during the SO₂ adsorption process.³³ The ΔG⁰ values are negative at all of the tested temperatures, indicating that the SO₂ adsorption on the MnO₂/AC composite is spontaneous and thermodynamically favorable. It is well known that a more negative ΔG⁰ implies a greater driving force of SO₂ adsorption, leading to a higher adsorption capacity. The value of ΔG⁰ decreases with the increase of temperature, suggesting that the adsorption of SO₂ on the MnO₂/AC composite was more spontaneous at high temperature.³¹

Fig. 14 Regressions of the Van’t Hoff plot for thermodynamic parameters.

Table 3 Thermodynamic parameters at various temperatures

Temperature (K)	ΔG^0 (kJ mol^−1)	ΔH^0 (kJ mol^−1)	ΔS^0 [J mol^−1 K^−1]
323	−8.01
373	−9.25
423	−12.23	14.30	62.97
473	−15.43
523	−18.73

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Conclusions

In this study, MnO₂/AC composites have been successfully synthesized using the redox deposition method. The uniform MnO₂ nanoparticles are highly dispersed on the AC surface. The MnO₂/AC composite exhibits good SO₂ trap performance and the MnO₂ conversion is improved over a low temperature range from 50 °C to 200 °C. The deposition of MnO₂ on the surface of activated carbon was effective in enhancing SO₂ removal at low temperature conditions for a combined SO₂ trap for a diesel exhaust system. The SO₂ capture capacity of the MnO₂/AC composite initially increased and then decreased with the precursor concentration rising. Furthermore, the type of interaction between SO₂ gas and the MnO₂/AC composite was determined, and the results indicated that the SO₂ adsorption on the MnO₂/AC composite is a chemisorption process. The Freundlich model fit the experimental data very well. The calculated value of the Freundlich constant (n) is 1.170, indicating high activity for SO₂ adsorption on the MnO₂/AC composite. The calculated values of ΔH⁰ and ΔS⁰ are 14.30 kJ mol⁻¹ and 62.97 J mol⁻¹ K⁻¹, respectively. Positive ΔH⁰ and ΔS⁰ values suggest that the SO₂ adsorption on the MnO₂/AC composite is endothermic and the ΔG⁰ values are negative, indicating that the SO₂ adsorption on the MnO₂/AC composite is spontaneous and favorable.

Acknowledgements

This research was supported by National Natural Science Foundation of China (NSFC) through International (Regional) Cooperation and Exchange Projects (21550110494).

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