Hydrothermal Synthesis of CeO2 Nanoparticles on Activated Carbon

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Hydrothermal Synthesis of CeO2 Nanoparticles on Activated Carbon with Enhanced Desulfurization Activity Zheng Yan,† Jinping Wang,† Ruqiang Zou,‡ Lili Liu,† Zuotai Zhang,† and Xidong Wang*,† †

Beijing Key Laboratory for Solid Waste Utilization and Management, and ‡Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China ABSTRACT: Nanosized CeO2 particles with a diameter of ∼200 nm were successfully loaded on activated carbon (AC) via a single-step hydrothermal process. The synthetic parameters, including hydrothermal temperature, precursor concentration, and reaction time, were regulated to control the size of the as-prepared CeO2 nanoparticles. The catalytic activity of CeO2 nanoparticles/AC was investigated by dynamic adsorption of SO2 from simulated flue gas, and the results exhibited remarkably enhanced SO2 adsorption capacity. The reaction mechanism of adsorbing and trapping SO2 by CeO2 nanoparticles/AC was discussed by laser Raman spectroscopy, X-ray powder diffraction, and thermodynamic calculations, which demonstrated that, in the presence of SO2 and O2, the generation of sulfate, accompanied by partial reduction of Ce4+ to Ce3+, was the key process during SO2 removal in our desulfurization experiments.

1. INTRODUCTION It is well-known that SO2 emissions can cause detrimental effects on either human health or the environment. Combustion of fossil fuel, especially from coal-fired power plants, accounts for the majority of generated SO2. To tackle the health and environmental issues caused by the large amount of SO2 emissions, great efforts have been devoted to SO2 removal since the first flue gas desulfurization (FGD) system was established in the 1900s.1 By now, a number of desulfurization techniques have already been employed in SO2 removal, such as the lime/limestone-based process, ionic liquid absorption, hydrotalcite-like material adsorption, and carbonaceous material selective removal process.1−12 Among the techniques mentioned above, adsorption and catalytic oxidation of SO2 over carbon-based materials, especially activated carbons (ACs), have a promising prospect, because ACs exhibit various outstanding advantages, such as being costeffective, having low density, being easy accessibility, having high adsorption capacity, and having high stability in acidic or basic media.5,11−15 Furthermore, during the last few decades, ACs are not only used in desulfurization but also widely used in heterogeneous catalysis, as either catalysts directly or catalyst supports.14−17 In the case of FGD, metals or metal oxides are usually loaded on carbon-based materials to promote the activity and service time. Catalysts with oxides or binary oxides supported on ACs, such as Ba, Co, Cu, Fe, Mg, Mn, Ni, Pb, V, and Ce with improved performance, have been successfully synthesized.18−20 In the research by Sumathi et al., the palm shell ACs carrying 10% CeO2 yielded the best removal capacity of SO2 and NOx simultaneously. They also suggested that CeO2loaded materials were potential candidates for flue gas purification because of their high affinity to SO2.19,20 It has been acknowledged that highly dispersed metal oxide particles, especially the particles in nanoscale, can tremendously enhance the catalyst performance with a low loading amount.21 There are several processes to synthesize the sorbents, which are able to directly determine the configuration and structure of © 2012 American Chemical Society

catalysts, and, hence, affect the performance of catalysts. However, the impregnation approach of loading metal oxide particles, employed in most previous studies, lacks precise control in the size and dispersion state of the particles, because most ACs possess abundant complex surface functional groups. In addition, a following step of annealing treatment at high temperature under an inert atmosphere is usually essential to improve the crystallinity of metal oxides. In contrast, a hydrothermal technique, with advantages of morphology control and high crystallinity in synthesizing materials, is considered to be one of the most prominent and welldeveloped methods in preparing nanomaterials, in either laboratory-scale or industrial production,22,23 which might also be employed in the preparation of uniform metal oxides composited with ACs for catalysis reactions. Nevertheless, by now, there is little information available in the literature on hydrothermal preparation of nanoscale metal oxides on ACs. The present work involves a relatively mild single-step hydrothermal process for in situ synthesis of CeO2 nanoparticles on coconut shell AC. To better regulate and control the loading state of CeO2 nanoparticles, preparation conditions will be systematically investigated, including hydrothermal temperature, precursor concentration, and reaction time. With regard to the measurement of catalytic activity, samples are employed in dynamic adsorption of SO2 from simulated flue gas. Influences of loading amount and texture properties on the catalytic capacity will be discussed in detail. Besides, further analysis of samples after desulfurization will demonstrate the interactions of SO2 with CeO2.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were of analytical reagent grade (China, Beijing Chemical Co., Ltd., purity ≥ 99.0%) and used without further purification. All of the aqueous solutions were prepared using Received: June 27, 2012 Revised: August 10, 2012 Published: August 10, 2012 5879

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deionized water (electrical resistivity at 25 °C of 18.2 MΩ cm). The AC was a commercial coconut-shell-derived carbon (Tangshan Hua Neng Technology Carbon Co., Ltd., China) and used as a catalyst support. To remove the impurities on the surface, the AC was cleaned ultrasonically in deionized water at 60 °C for 15 min. After filtrating, the AC was then dried in air at 120 °C for 5 h. 2.2. Synthesis Process. In previous research, CeO2 nanomaterials were successfully prepared in a wide range of concentrations and temperatures.22−25 The temperature was optimized to 140−180 °C, with the concentration ranging from 30 to 50 mM in this study. During a typical procedure to obtain the CeO2 nanoparticles/AC composites, Ce(NO3)3·6H2O was first dissolved into deionized water to prepare aqueous Ce(NO3)3 solutions as precursors. The hydrothermal growth was then carried out in autoclaves by immersing 1 g of AC with 10 mL of Ce(NO3)3 solution. After the hydrothermal reaction, samples were rinsed with distilled water, dried at 120 °C for 5 h, and then kept in the desiccator until use. Details of the preparation parameters are presented in Table 1.

2.3. Characterization. The textural properties of as-prepared samples were determined by nitrogen adsorption at 77 K on an accelerated surface area and porosimetry analyzer (Micromeritics, ASAP 2020). Before measurements, the samples were outgassed at 300 °C overnight under vacuum. Scanning electron microscopy (SEM, Hitachi S-4800) equipped with energy-dispersive X-ray spectroscopy (EDX, Bruker) was carried out to observe the microstructure and examine the elemental composition of the products. The crystal structures were further identified by X-ray diffraction (XRD, Rigaku Dmax/2400, Cu Kα radiation). The molecular structures of the products before and after desulfurization were detected by the laser Raman spectrometer at room temperature (Renishaw, RM1000). To accurately ascertain the loading amount of CeO2, 1.0000 ± 0.005 g of the sample was ground to 200 mesh, which was then heated to 900 °C in air, and kept for 3 h in a muffle furnace. The residue of original AC was considered to be ash content. The amount of CeO2 loading was obtained by subtracting the corresponding ash content from the residue.26 Measurement for each sample was carried out 3 times, and the final result was achieved according to the average value. To acquire the yield of CeO2, the loading amount of CeO2 achieved in the experiment was divided by the theoretical CeO2 quantity, entirely transformed from the original Ce(NO3)3 precursor. 2.4. Catalytic Activity Measurements. Flue gas desulfurization capacity was detected in a fixed-bed reactor maintained at 100 °C. A 2.0 g sample was placed in a quartz tube with an internal diameter of 25 mm. A stream of simulated flue gas, containing SO2 (3000 ppm), O2 (6%), H2O (8%), and N2 as the balance, was injected at the speed of 100 mL/min. H2O was introduced by administrating N2 through a water bath. Thermodynamic saturated N2 was then well-mixed with other gases prior to feeding into the reactor. According to the dew point of air, the temperature of the water bath was adjusted to control the vapor content in the mixed gas flow. The temperature of the whole set equipment was maintained at 100 °C to keep the vapor in flue gas. Before the measurement, samples were first flushed with pure N2 at 100 mL/min for 1 h and then switched to flue gas. Concentrations of SO2 at the inlet and outlet were recorded in real time through a flue

Table 1. Conditions during the Hydrothermal Process sample number

temperature (T, °C)

Ce(NO3)3 concentration (C, mM)

time (t, h)

A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11

0 140 160 180 160 160 160 160 160 160 160 160

0 40 40 40 30 35 45 50 40 40 40 40

0 8 8 8 8 8 8 8 2 4 6 10

Figure 1. (a) SEM image of parent AC (A0), (b) SEM image of CeO2 nanoparticles/AC (A2), (c) enlarged image of CeO2 nanoparticles from panel b, (d) EDX spectra of the CeO2 nanoparticles in panel c, (e) XRD patterns of parent AC (A0) and CeO2 nanoparticles/AC (A2), and (f) comparison of DeSO2 efficiency with and without CeO2 nanoparticles (A2 and A0). 5880

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gas analyzer (Testo, Pro 350) equipped with a SO2 electrochemical sensor. The dynamic adsorption capacity is commonly defined as the accumulated quantity of the gas removed by the absorbent before the time at which the concentration of the gas in the outlet reaches an arbitrary “breakthrough” value of the inlet, 50% in this study. Herein, SO2 removal capacities of different samples were compared according to the calculated dynamic adsorption capacity.

Table 2. Textural Properties and CeO2 Loading Amounts of Samples surface area (m2/g)

3. RESULTS AND DISCUSSION 3.1. Characterizations of CeO2 Nanoparticles/AC Composites. In this work, we have successfully synthesized CeO2 nanoparticles on parent AC, and the composite materials presented improved desulfurization capacity, which will be demonstrated in detail at section 3.3. For clarity, sample A2, uniformly dispersed and a proper loading amount of nanoparticles, is chosen as the representative sample to illustrate the comparison to parent AC (A0 in Table 1). SEM images of A0 and A2 are shown in panels a and b of Figure 1, separately. It can be seen that, after 8 h of the hydrothermal reaction, a large quantity of nanoparticles are uniformly formed on the AC surface (Figure 1b). The magnified SEM image (Figure 1c) shows that the diameters of the synthesized nanoparticles are around 100 nm. Figure 1d gives the EDX spectrum of the synthesized nanoparticles, which demonstrates that the particles are composed of Ce and O. XRD results of A0 and A2 (Figure 1e) confirm that the synthesized nanoparticles are CeO2. Figure 1f presents the desulfurization (DeSO2) efficiency recorded in real time, which is obtained from the equation as follows: DeSO2 efficiency =

pore volume (cm3/g)

sample

BET

Langmuir

BJH desorption (mesopore)

HK

t-plot

A0 A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 A11

764.741 768.088 745.876 728.619 710.517 722.405 725.131 709.968 771.406 707.522 745.407 728.39

1057.62 1072.58 1030.77 1013.661 944.042 1013.704 1011.813 986.346 1077.011 983.095 1037.11 1012.72

0.022 0.063 0.015 0.066 0.064 0.056 0.056 0.053 0.062 0.057 0.063 0.056

0.3627 0.3628 0.3568 0.3431 0.3182 0.3419 0.3429 0.3347 0.3647 0.3342 0.3517 0.3430

0.364 0.363 0.351 0.343 0.318 0.343 0.343 0.336 0.365 0.334 0.358 0.344

adsorption capacity. However, an appropriate loading of nanoparticles can create micropores to a certain extent and, thus, compensates for the loss on texture properties, such as in sample A8. While the AC is overloaded, the nanoparticles adhere to each other, which can reduce the surface area and micropore volume of the CeO2 nanoparticles/AC in contrast. On the basis of the analysis of texture properties, it can be seen that the preparation conditions of the hydrothermal process can affect the dispersion of nanoparticles and further influence the performance of catalysts. Because the adsorbate can be chemisorbed onto the active sites through chemical bondings and the chemisorptions are restricted to monolayer surface coverage, a large quantity of highly dispersed nanoparticles can create more active sites and, therefore, enhance the uptake capacity of adsorbents. During the hydrothermal reaction, the degree of supersaturation is the key factor in controlling the growth habit of the crystal. When the precursor is supersaturated, surface tension of the solution decreases and the wetting effect of the aqueous precursor to AC is greatly promoted, meaning that the bulk diffusion of the precursor on the interface is adequate to diffuse into the micropores. Thus, the probability of three-dimensional nucleation is largely enhanced, giving rise to the rapid growth of CeO2 on both lateral and vertical directions in particles. When the supersaturation is too high, the nucleation process is further promoted. As a result, both the density and diameter of nanoparticles simultaneously increase quickly, which can result in particles adhering to each other and, hence, the decrease in the amount of active sites. To regulate the loading status of CeO2 nanoparticles on AC, effects of different synthesis conditions, including hydrothermal temperature, precursor concentration, and reaction time, are discussed in this study (the experimental details are shown in Table 1). 3.2.1. Effect of the Hydrothermal Temperature. The temperature plays a significant role in affecting the growth habit of the crystal. In this paper, three temperatures [140 °C (A1), 160 °C (A2), and 180 °C (A3)] were employed to investigate the influence of the temperature on the growth of CeO2 nanoparticles on AC. The corresponding XRD patterns of as-prepared samples are displayed in Figure 2a. All of the three samples present relatively a high intensity of CeO2 peaks. Figure 2b shows that the loading amounts and CeO2 yields are enhanced with the rise of the temperature. The SEM images

SO2 (inlet) − SO2 (outlet) × 100% SO2 (inlet)

It can be seen that the breakthrough time of A2 is nearly 3 times that of A0, indicating improved desulfurization capacity mainly because of the introduction of CeO2 nanoparticles. The DeSO2 efficiency of A2 drops sharply to 60% at about 140 min but presents a slow decrease afterward, which is of great difference to the tendency exhibited by A0. The possible reason might be that the physical and chemical adsorption take place simultaneously at the initial stage (0−138 min), during which the physical adsorption plays a major role. However, after the physical adsorption becomes defunct, the adsorption of SO2 mainly depends upon the chemical reaction, implying the high desulfurization activity of CeO2 nanoparticles. The desulfurization mechanism will be further discussed in section 3.4. 3.2. Synthesis Parameter Investigation. The synthesis conditions can affect the catalysts on texture properties and active site dispersion, both of which are able to further influence the catalytic activities of sorbents. The surface area [calculated by the theory of Brunauer−Emmet−Teller (BET) and Langmuir]27 and pore volume [calculated by the theory of Barrett−Joiner−Halenda (BJH), Horvath−Kawazoe (HK), and t-plot]28 of CeO2 nanoparticles/AC composites under different synthesis conditions (Table 1) are shown in Table 2. In the physical adsorption of small gas molecules by AC, mircopores are dominated because of the narrow pore width and high adsorption potential, whereas the meso- and macropores mainly act as channels. Therefore, the physisorption capacity is governed by an accessible micropore volume. As shown in Table 2, with the growth of nanoparticles, part of the micropores in AC is occupied, leading to the decrease of the surface area and micropore volume and, hence, the reduction of 5881

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Figure 2. Characterizations of the samples prepared at different hydrothermal temperatures: (a) XRD patterns, (b) CeO2 loading amount and yield, and SEM images of (c) A1, (d) A2, and (e) A3.

Figure 3. Characterizations of the samples prepared at different precursor concentrations: (a) XRD patterns, (b) CeO2 loading amount and yield, and SEM images of (c) A4, (d) A5, (e) A2, (f) A6, and (g) A7.

of the as-prepared samples, in which the XRD peaks of CeO2 are not available for samples A4 and A5, implying the low quantity and high dispersion of CeO2 particles at low Ce(NO3)3 concentrations, which can be evidenced by Figure 3b and the SEM images (panels c and d of Figure 3). In Figure 3b, the plot of the CeO2 yield exhibits almost a linear increase along with the rise of the concentration as expected but the increasing rate from A4 (30 mM) to A5 (35 mM) is lower than that from A5 (35 mM) to A7 (50 mM), the reason of which might be that, at lower concentrations, the degree of supersaturation is not adequate to generate abundant growth units on the AC surface, as illustrated in panels c and d of Figure 3. Nevertheless, a significant accumulation of nanoparticles can be observed in panels f and g of Figure 3. It can be concluded that, with the increase of the precursor concen-

presented in panels c−e of Figure 2 indicate that the diameter of the crystal increases from about 50 to 100 nm with the temperature elevated from 140 to 160 °C. However, after the temperature increases to 180 °C, little increment on the diameter can be noticed and partial nanoparticles begin to adhere, mainly caused by the high density. It can be concluded that, in the temperature ranging from 140 to 180 °C, the precursor is highly supersaturated, thus enabling CeO2 to crystallize into nanoparticles evenly on the surface of AC. Moreover, the degree of supersaturation is also promoted along with the increase of the temperature, which is beneficial to the rapid crystallization, giving rise to particles with large size. 3.2.2. Effect of the Precursor Concentration. The concentration of Ce(NO3)3 was varied from 30 to 50 mM (A4−A5, A2, and A6−A7). Figure 3a shows the XRD patterns 5882

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Figure 4. Characterizations of samples prepared in different reaction times: (a) XRD patterns, (b) CeO2 loading amount and yield, and SEM images of (c) A8, (d) A9, (e) A10, (f) A2, and (g) A11.

reaction. At the time of 10 h (A11, Figure 4g), the particles begin to accumulate and adhere to each other, leading to a thick film covered on the surface consequently. 3.3. SO2 Dynamic Adsorption Capacity. In previous literature, it is well-demonstrated that CeO2, either pure CeO229−31 or CeO2 composite materials,20,21,32−35 offers a positive effect in trapping SO2. In this work, the removal of SO2 from flue gas was simulated on a laboratory scale. Figure 5

tration, the supersaturation is enhanced as well, leading to rapid nucleation and a high growth rate and, thus, a high density of CeO2 nanoparticles. 3.2.3. Effect of the Reaction Time. Characterizations of nanoparticles loaded on the AC along with prolonging the reaction time (A8−A10, A2, and A11) are displayed in Figure 4 to investigate the growth procedure of CeO2 nanoparticles on AC. The XRD patterns shown in Figure 4a indicate that peaks of CeO2 in A8 and A9 are undetectable because of low loading amounts. For sample A10, even though the loading amount is approximate 4%, there are no CeO2 peaks shown in XRD patterns either. This can be explained by the high dispersion and the small size of the synthesized nanocrystal (less than 50 nm), as demonstrated in Figure 4e. As seen from the SEM images in Figure 4c, only a small amount of nanoparticles on the surface of edges and pores can be observed at the initial 2 h (A8). The reason for uneven growth is deduced as follows: First, on the surface of AC, in comparison to the carbon atoms on the basal planes, the carbon atoms at the edges of the planes are highly unsaturated with lattice defects and relative free valences, which are more active and, consequently, result in a much higher density of CeO2 particles on the edges than that on the basal planes. Second, the unsaturated carbon atoms usually bond to the large quantities of heteroatoms, giving rise to various surface groups. Because of the diversity on nature and amount of surface functional groups, the locations of growth units on the AC surface reveal a difference, resulting in the heterogeneous distribution of active sites. Furthermore, on the basis of the previous analysis, with the precursor concentration of 40 mM and hydrothermal temperature of 160 °C, the degree of supersaturation is not the rate control factor. Therefore, the generation of CeO2 growth units at the regions with active sites can take place quickly. With a prolonged reaction time, the equilibrium proceeds more sufficiently and both the density and grain size increase, which can be evidenced by panels b−g of Figure 4. Nevertheless, it should be noted in Figure 4b that, after the time increases to 6 h, the loading amount and yield of CeO2 increase rather slightly, which implies that the crystal particles grow up slowly. As illustrated in the SEM images, it can be speculated that the amount of active sites on the surface arrives at the maximum at 6 h; thus, the growth of the crystal size turns into the main

Figure 5. SO2 dynamic adsorption capacities of the as-prepared CeO2 nanoparticles/AC composites.

displays the SO2 dynamic adsorption capacities of the asprepared samples with the variation of loading amounts. For better comparison, the corresponding BET surface area and micropore volume obtained by the t-plot method are presented in Figure 5 as well. On the basis of Figure 5, it is found that CeO2 nanoparticles/ AC composites reveal tremendously improved adsorption capacity in comparison to the parent AC. The desulfurization capacity of composites initially increases with the loading amount rising and reaches the maximum value when the loading amount of CeO2 is 4.164% (A2), much lower than that of other research (mostly approximate 10%),18−20 which could be attributed to the more reactive surface revealed by CeO2 nanoparticles. However, when the loading amount further increases, the desulfurization capacity decreases instead. A similar tendency is also observed in refs 18−20. 5883

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spectroscopy, XRD, and thermodynamic analysis were conducted in this study. The laser Raman spectrum of AC is illustrated in Figure 6a, which exhibits a pair of relatively sharp

According to Table 2 and Figure 5, A0 and A8 present nearly equal surface area and micropore volume but the desulfurization capacity of A8 is much higher than A0, demonstrating the adsorption of SO2 by CeO2 nanoparticles. Afterward, the micropore volume drops greatly from A8 to A4, demonstrating a large amount of nanoparticles successfully locating in the micropores of AC. Nevertheless, it should be noted that the BET surface area of A4 is slightly higher than A9, although the micropore volume of A4 is lower than A9, indicating that the chemisorption of SO2 by CeO2 active sites generated from CeO2 nanoparticles starts to play a dominant role compared to the texture properties. The desulfurization capacity begins to rapidly increase from A4 to A2, despite the decrease of the micropore volume and BET surface area from A1 to A2, which further proves the crucial effect of CeO2 nanoparticles in the adsorption of SO2. For samples A2−A7 in Figure 5, it can be seen that, the micropore volume and surface area decrease slightly along with the loading amount increasing; meanwhile, the desulfurization capacity presents an obvious reduction, mostly because the accumulation of CeO2 nanoparticles greatly reduces the amount of effective active sites. In view of the foregoing analysis, the SO2 dynamic adsorption capacity of the synthesized catalysts mainly involves the factors below: (1) the pores, especially micropores, play a primary role in the physisorption of SO2, but the location of CeO2 nanoparticles on the AC surface block partial micropores; (2) CeO2 nanoparticles are responsible for the chemisorption of SO2, and therefore, the more effective the active sites generating on the surface, the higher activity the sample attains; (3) after the loading amount comes to a certain value, CeO2 nanoparticles prefer to adhere together, giving rise to the decrease of the effective reaction interface; meanwhile, the reduction of the micropore volume accelerates the decrease of the desulfurization capacity. The SO2 dynamic adsorption capacity depends upon the comprehensive impact of both the texture and surface chemical properties. An optimal loading amount should simultaneously guarantee the high dispersion of CeO2 nanoparticles and the adequate effective active sites. In this study, the optimum loading amount of CeO2 is 4.164%, achieved at a hydrothermal reaction at 160 °C for 8 h, with a precursor concentration of 40 mM (A2). 3.4. Desulfurization Mechanism of CeO2/AC. In the research by Sumathi et al., the oxidizing and oxygen storage property of CeO2 was responsible for the chemisorption of SO2. They proposed a simplified possible mechanism as follows:20 CeO2 absorbs and releases oxygen according to the reactions

Figure 6. Raman spectrum of (a) parent AC (A0), (b) CeO2 nanoparticles/AC composite (A2), and (c) CeO2 nanoparticles/AC composite after desulfurization (A2S).

Raman peaks at about 1340 and 1600 cm−1 denoting graphite (G) and disordered carbon (D) bands, respectively.40 From the Raman spectrum of CeO2 nanoparticles/AC (A2) shown in Figure 6b, an intense peak at 458 cm−1, unambiguously corresponding to the F2g mode characteristic of CeO2, can be detected. The weak and broad band at about 616 cm−1 is attributed to the existence of oxygen vacancies,37,39−41 which might associate with the concomitant reduction of Ce4+ to Ce3+ during the hydrothermal treatment, giving rise to nonstoichiometric oxides (CeO2−x, where 0 < x < 0.5). The weak peak of the Raman shift at 266 cm−1 arising from size effects when the crystallite dimension is below 20 nm42 provides another evidence for the successful loading of CeO2 nanoparticles onto AC. Figure 6c illustrates the Raman spectrum of the catalyst after desulfurization (noted as A2S) in detail. It can be seen that the intensity of the band at 616 cm−1 is stronger, revealing the newly formed oxygen vacancies. Three sharp bands separately at 1000, 1033, and 1112 cm−1 are also displayed in the spectrum. It is known that bands between 965 and 1035 cm−1 and between 1100 and 1180 cm−1 are due to the S−O and SO stretching vibrations of SO42− species, meaning the formation of SO42−. However, no peaks appear in the range of 950−975 and 1050−1100 cm−1, indicating that sulfite and disulfide are not able to be formed in our system.39,43−45 The XRD measurement was carried out to further determine the existence of sulfated ceria, which is shown in Figure 7. It can be seen that, after desulfurization, sulfur features are shown in the corresponding XRD patterns (Figure 7b). Although some discrepancies exist on the elaborated physicochemical adsorption mechanisms, a common view has been reached that, during the removal of SO2 by AC, sulfuric acid is produced in the presence of water and oxygen, involving the following simplified reactions:46,47

xSO2 + CeO2 ⇔ xSO3 + CeO2 − x CeO2 − x + (1/2)xO2 ⇔ CeO2

However, to the authors’ knowledge, the reactions above are not able to state the detailed mechanism. In most foregoing studies working on the interaction of CeO2 and SO2 under oxidized or reduced conditions, it is documented by techniques of X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR) spectroscopy, electron paramagnetic resonance (EPR), temperature-programmed desorption (TPD), X-ray absorption near edge structure (XANES), and laser Raman spectra that CeO2 is efficient in both trapping and oxidizing SO2, leading to the formation of sulfate or sulfite at a wide range of temperatures.31−39 To give further insight into the SO2 adsorption by the CeO2 nanoparticles/AC, laser Raman

2SO2 + O2 → 2SO3 SO3 + H 2O → H 2SO4

At the temperature of 100 °C, carbon is able to react with H2SO4, accompanied by the generation of CO2 and sulfur. 5884

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Figure 7. XRD patterns of (a) parent AC (A0), (b) AC after desulfurization (A0S), and (c) CeO 2 nanoparticles/AC after desulfurization (A2S).

Figure 8. Comparison of ΔrGm for reactions 1 and 2.

4. CONCLUSION In this work, we have successfully in situ synthesized CeO2 particles onto the AC surface via a mild hydrothermal process. When the hydrothermal temperature, precursor concentration, and reaction time are varied, the diameter of CeO2 nanoparticles can be controlled in the range of ∼200 nm. The CeO2 nanoparticles could be uniformly formed on the AC surface under the following conditions: temperature range of 140−180 °C, Ce(NO3)3 concentration of 30−50 mM, and reaction time of 2−10 h. With the purpose of examining the activities of CeO2 nanoparticles/AC composites, the dynamic adsorption capacity of SO2 from simulated flue gas was conducted and the best performance was achieved at the CeO2 loading of 4.164% (A2, hydrothermal temperature T = 160 °C, precursor concentration C = 40 mM, and reaction time t = 8 h), much lower than other research, demonstrating the high activity of CeO2 nanoparticles. Furthermore, laser Raman spectroscopy and XRD were carried out to elucidate the possible mechanism of CeO2 nanoparticles/AC composites reacting with SO2 in the presence of O2, the results of which indicated the simultaneous generation of Ce(SO4)2 and Ce2(SO4)3 after the samples went through the desulfurization process. Thermodynamic calculations confirmed the formation of both products, accompanied with partial reduction of Ce4+ to Ce3+.

100 ° C

H 2SO4 + (3/2)C ⎯⎯⎯⎯⎯⎯→ S + (3/2)CO2 + H 2O(g) ΔGr = −37.775 kJ mol−1

In the case of CeO2 reacting with SO2, the XRD patterns of A2S prove the formation of both Ce(SO4)2 and Ce2(SO4)3, which is consistent with the Raman analysis results. Many experimental and theoretical studies of SO2 adsorbed onto CeO2 have been performed. For example, through the corresponding XPS, TPD, and XANES analyses, Rodriguez et al. demonstrated that heating of pure CeO2 in the presence of SO2 above 50 °C could lead to the formation of SO42−.33 On the other hand, from the introduction of O2 + SO2 (1:1) onto CeO2 film heating from 25 to 550 °C, Ferrizz et al. proposed that sulfate formation was accompanied by a fraction of Ce4+ reduced to Ce3+.31 In a study reported by Lu et al., the theoretical calculation offered proof that SO2 adsorbed via its S atom to one or two surface oxygen atoms to form sulfite (SO32−) or sulfate (SO42−), together with Ce3+/Ce4+ redox, and sulfate formation occurred with the creation of an oxygen vacancy nearby.48 The XPS analysis on the reaction between CeO2 and SO2 at various temperatures (30−400 °C) with or without O2 examined by Sminov et al. identified that the addition of O2 suppressed the formation of sulfite species, involving reduction of Ce4+ to Ce3+.35 They also proposed that sulfation of CeO2 in the presence of O2 could be described as CeO2 + (1/2)O2 + (3/2)SO2 → (1/2)Ce(SO4 )3

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*Telephone: +86-10-8252-9083. Fax: +86-10-8252-9010. Email: [email protected].

(1)

Notes

To investigate the feasibility of the reaction above, HSC 5.1 chemistry software was employed to calculate the delta Gibbs free energy (ΔrGm) over the temperature range of 0−200 °C. According to the forgoing results, another possible reaction equation is proposed in attempt to clarify the production of Ce(SO4)2 in our system. CeO2 + O2 + 2SO2 → Ce(SO4 )2

AUTHOR INFORMATION

Corresponding Author

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support by the Common Development Fund of Beijing and the National Natural Science Foundation of China (Grants 51172001, 51074009, and 51172003).

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(2)

The details of ΔrGm for reactions 1 and 2 are presented in Figure 8, suggesting that the existence of both Ce2(SO4)3 and Ce(SO4)2 are rational, which agrees well with the conclusions obtained by other researchers.31,35,48 It also can be seen in Figure 8 that the reaction generating Ce2(SO4)3 is superior to that of Ce(SO4)2 and carbon might affect the reduction process as well.

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