Low Temperature Catalytic Adsorption of SO2 on Activated Carbon

J. Phys. Chem. C 2008, 112, 15335–15340

15335

Low Temperature Catalytic Adsorption of SO2 on Activated Carbon Diana Lo´pez,*,† Robison Buitrago,† Antonio Sepu´lveda-Escribano,‡ Francisco Rodrı´guez-Reinoso,‡ and Fanor Mondrago´n† Institute of Chemistry, UniVersity of Antioquia, A.A. 1226, Medellı´n, Colombia, and Laboratorio de Materiales AVanzados, UniVersidad de Alicante, Apartado 99, E-03080, Alicante, Spain ReceiVed: September 6, 2007; ReVised Manuscript ReceiVed: July 25, 2008

Catalytic adsorption of SO2 on activated carbon materials provides an appropriate alternative for the control of low concentration emissions of this air pollutant. The surface complexes formed upon SO2 adsorption at 30 °C were studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The effect of the addition of O2 and the presence of copper as catalyst were studied. Copper assisted the oxygen transfer to the carbon matrix. For the Cu-impregnated carbon sample, the presence of O2 favored SO2 adsorption by increasing the breakthrough time, the adsorption capacity and the formation of sulfur and oxygen complexes of higher thermal stabilities, which mainly desorb as SO2 and CO2. Introduction

TABLE 1: Ash Composition Analysis of AC by XRFa

Sulfur oxides (SOx) are considered among the most toxic gases emitted to the atmosphere during the combustion of fossil fuels in the industry and transport sectors. This requires tightening a stricter control of the emissions especially in electric power stations because the projections show that fossil fuels will continue constituting the main source of energy during the next decades. As a consequence, the laws that regulate the emissions of pollutants are becoming more drastic and have encouraged research toward the development of immediate solutions. These solutions may include substantial variations of the actual combustion processes, redesign of equipment and application of new technologies leading to the decrease of the emission of pollutants to the atmosphere. In a practical process, SOx is removed mainly by solid-gas reactions using dolomites or calcite as sorbents,1 or in gas-liquid reactions with basic solutions.2 However, all of these processes generate considerable amounts of byproduct, require large space and high capital costs for their handling.3,4 SOx removal by adsorption provides an appropriate alternative to control the emissions of low concentrations of this pollutant. Several potential adsorbents have been investigated, including metal oxides, zeolites and porous carbons under different forms. Activated carbon is an amorphous material used industrially as adsorbent that exhibits high porosity and an extensive surface area. It is obtained by pyrolysis, gasification or chemical oxidation of any substance rich in organic carbon, with the objective of increasing the volume and diameter of pores that were created during the carbonization process. Adsorption on activated carbons is one of the most investigated methodologies; besides its regenerative capability, it allows the removal of most of the impurities found in the effluent such as SO2, NOx, particles, mercury, dioxins, furans, VOCs, and heavy metals.5,6 Additionally, activated carbon has been used as catalyst as well as catalyst support.7-10 To promote the activity of these catalytic systems it is necessary to minimize its tendency to behave as a reactant, especially in the presence

compound Na2O SiO2 Al2O3 MgO Fe2O3 CaO SO3 others

* To whom correspondence should be addressed. E-mail: diana_lopez@ yahoo.com. Phone: 574-210- 6613. Fax: 574-210-6565. † University of Antioquia. ‡ Universidad de Alicante.

% a

12.3

9.7

13.4

3.1

11.1

25.2 22.4

2.8

Others: K2O, TiO2, CuO, Cl, P2O5.

of oxygen. Metals with catalytic activity can be used to reduce the reaction temperatures and to minimize carbon loss by gasification. Previous studies have demonstrated that when this material is used as catalyst support, the adsorption capacity increases considerably in comparison with other materials.8,11 Carabineiro et al.12 studied the adsorption of SO2 on activated carbon impregnated with V, Cu, Fe, Pb, Mn, Co, Ni, Ba, Mg and their binary mixtures. It was found that the best additives were Cu, V and Fe, mainly in their binary mixtures, for which a synergetic effect was evidenced. In particular, copper has been reported to be a good catalyst for the adsorption of SO2 at low temperatures, and very low adsorption capacities have been reported in the absence of copper.13-15 Tseng et al.16 found that the catalytic reaction between SO2 and supported copper species was affected by the surface functional groups that are generated during the synthesis of metal-loaded activated carbon supports. In order to have a kinetic description of the gas-phase catalytic process it is necessary to determine the nature of the interaction between the gas molecules and the catalyst/carbon surface, thus helping to understand the elemental reactions that describe the process. The objective of this investigation was to study the adsorption of SO2 on activated carbons, the catalytic effect of copper, and the effect of the addition of oxygen. Experimental Section A sub-bituminous coal (mesh 100/200) was pyrolyzed at 900 °C for 1 h under a N2 atmosphere to produce the char that was subsequently activated with steam at 850 °C for 3 h. The activated carbon (AC) contains 7 wt % of mineral matter (ash), mainly composed of Na, K, Ca and Fe, which could have a catalytic effect on the SO2 adsorption reaction. Table 1 presents the ash composition analysis of the activated carbon by X-ray fluorescence in a Philips-PW1480 (XRF). Thus, part of the activated carbon was demineralized with HCl-HF-HCl-HNO3

10.1021/jp802809c CCC: $40.75  2008 American Chemical Society Published on Web 09/09/2008

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TABLE 2: Surface Area and Microporosity sample

SN2 (BET) (m2/g) ( 26

SCO2 (DR) (m2/g) ( 30

VCO2 (DA) (cm3/g) ( 1 × 10-2

AC DAC Cu/DAC

228 985 711

598 1286 937

0.23 0.57 0.40

TABLE 3: Pore Size Distribution sample

% micropores 500 Å

AC DAC Cu/DAC

2.6 5.4 8.3

56.6 48.6 51.1

40.7 46.0 40.6

solutions, using a technique adapted from Bishop and Ward.17 The demineralized activated carbon (DAC) was loaded with copper by immersion into an aqueous solution of Cu(NO3)2 · 3H2O using the method of incipient wetness, the copper content was 7.2 wt % (Cu/DAC). Adsorption-desorption isotherm measurements were performed with an automated volumetric gas apparatus (ASAP 2000). Nitrogen at -196 °C and carbon dioxide at 0 °C adsorptions were used to obtain structural information such as surface area, micropore volume, and pore size distribution (PSD), which are summarized in Tables 2 and 3. The SO2 adsorption experiments were carried out in a fixed bed reactor using 0.100 g of sample. The temperature was monitored with a thermocouple attached to the reactor’s external wall, close to the sample. Before each experiment the sample was heat treated at 500 °C under a helium flow of 100 mL/min for 30 min to partially clean the surface oxides that may be formed during the exposure of the sample to air while handling. The reactor was then cooled down to 30 °C and the He flow was switched to the reaction mixture (SO2 or SO2/O2), with a total flow of 100 mL/min until saturation of the activated carbon. The gas concentrations used in these experiments were: SO2, 500 ppm; O2, 5%; He, balance. The SO2 concentration was continuously monitored with a quadrupole mass spectrometer. After the adsorption experiment, the samples were subjected to X-Ray Photoelectron Spectroscopy (XPS) analysis with a VGMicrotech Multilab 3000 apparatus equipped with a hemispherical electron analyzer and a Mg KR (15kV, 20mA) X-ray source. The C1s binding energy at 284.6 eV signal was taken as the reference. In order to obtain a reference point for comparison of the results, an activated carbon sample without Cu was heattreated at 500 °C under He, and then the surface complexes were characterized by XPS. The nature of the copper species on the Cu/DAC sample before and after the SO2 adsorption were determined by X-ray diffraction (XRD) in a 3000P Seifert powder diffractometer, using a Cu R radiation. The scanning rate was 2°/min. To establish the effects of the reversible chemisorption reactions of SO2 and the formation of oxygen complexes, the adsorption experiments were run for 2 h and then flushed with He for 30 min to remove the SO2 remaining on the reactor or weakly adsorbed on the surface, after that the samples were subsequently subjected to temperature-programmed desorption (TPD) under a He flow, with a heating rate of 15 °C/min up to 900 °C. The evolved gases (SO2, CO and CO2) were monitored with a quadrupole mass spectrometer. Results and Discussion Adsorption Profiles. Figure 1 shows the profiles for the SO2 concentration at the exit of the reactor (flow of 100 mL/min

Figure 1. SO2 outlet concentration profiles for the adsorption of SO2 at 30 °C on three different samples.

Figure 2. SO2 concentration profiles during the adsorption of SO2 and SO2/O2 on Cu/DAC.

containing 500 ppm SO2 in He at 30 °C) on the different samples, as a function of time. All the adsorption profiles show a similar behavior in which three stages of adsorption are clearly differentiated. In the first stage, the diffusion and adsorption of SO2 mainly occurs at the internal surface of the activated carbons; in the second stage, just after the breakthrough point, the adsorption takes place at the active sites of the surface; and, finally, the outlet concentration of SO2 reaches the 500 ppm for the AC and DAC samples, this indicating that active sites for adsorption have been saturated. However, for the Cu/DAC sample the SO2 adsorption continues for more than 7 h, which could indicate that there is a favorable effect of Cu catalyzing the adsorption of SO2 on the carbonaceous material. Figure 2 shows the concentration profiles of the gas stream exiting the reactor during the adsorption of SO2 and SO2/O2 on the impregnated activated carbon, as a function of time. The addition of oxygen drastically favored SO2 adsorption, what can be explained on the basis of new active sites to further adsorption provided by oxygen. As a result of this, both the breakthrough time and the adsorption capacity of this adsorbent are increased. CO2 formation was also observed during the adsorption of SO2 in the presence of O2. XPS Results. Figure 3 shows a schematic representation of a graphene layer with some of the possible sulfur complexes that can be formed during the adsorption of SO2, as reported in the literature.18,19 The usual method for quantitative analysis in XPS is to determine the area of each photoelectronic signal, which is then

Adsorption of SO2 on Activated Carbon

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Figure 3. Schematic representation of common sulfur surface complexes formed during the SO2 adsorption on carbon. (a) When the surface is not oxidized and there just one active site, (b) when the surface is not oxidized and there are two neighboring zigzag active sites, (c) when there is one carbonyl group and an active site next to it, (d) when there are two neighboring zigzag carbonyl groups, and (e) when there is one carbonyl group in an arm chair position with an active site next to it.

Figure 5. Surface concentration modifications after SO2 and SO2/O2 adsorption on different activated carbons, represented as XPS data referred to the parent activated carbon, (a) sulfur and (b) oxygen complexes.

Figure 4. S 2p spectrum after SO2 adsorption on activated carbon (AC).

corrected by the atomic sensitivity factor. Figure 4 shows the XPS spectrum of the activated carbon (AC) in the S2p level, after SO2 adsorption at 30 °C. The main signal can be deconvoluted into two contributions, a doublet corresponding to the S2p3/2 and S2p1/2 levels (spin-orbit coupling), with an intensity ratio of 2:1. The most intense S2p3/2 signal appears at a binding energy of 168.2 eV, which is characteristic of sulfur atom surrounded by three oxygen atoms (sulfite type), for carbon surfaces, the signal intensities are normalized with respect to the corresponding area below the C1s signal. To determine the relative changes in oxygen and sulfur complexes due to the reaction, the value of the ratio between the intensity of the peak corresponding to each heteroatom (S or O) and the C1s peak intensity (i.e., O/C and S/C) was subtracted from the ratio in the heat-treated sample used as reference. Modifications in the concentration of sulfur and oxygen complexes on the different activated carbons after adsorption of SO2 in the absence and presence of O2, obtained from XPS data, are presented in Figures 5a and b, respectively. The values of the y axis (in arbitrary units) correspond to the subtraction of the O- or S-complexes/C ratio of the heat-treated activated carbon from the corresponding ratio for the activated carbon after reaction. The x axis shows the complexes whose concentration has changed after SO2 adsorption. The only sulfur species detected by XPS in both the absence and presence of O2, was the SO3 complex. Sulfur complexes were not detected on the impregnated sample (Cu/DAC) in the absence of O2, very probably because they have been transferred to internal sites in

the carbon support, where the XPS technique cannot detect them, or because they have been removed upon the vacuum treatment previous to XPS analysis. It can be seen in Figure 5a that the concentration of sulfur complexes increased in the presence of O2 in a larger extent, showing a synergistic effect for all materials. One tentative explanation is that O2 dissociatively chemisorbs, thus increasing the number of active sites on the surface. The Cu-impregnated activated carbon showed the highest SO2 adsorption capacity. These observations suggest that copper catalyzes the SO2 adsorption, the presence of O2 providing more oxidized active sites for its anchoring. Figure 5b shows that there is a decrease in the concentration of oxygen complexes for activated carbon AC, this indicating that gasification of the carbon material takes place and that oxygen from SO2 is removed in the form of CO or CO2. The CO/CO2 peak ratio obtained from TPD analysis after the adsorption of SO2 on the activated carbon was lower than before the adsorption. Moreover, the XPS analysis showed a decrease in the intensity of the O1s signal with binding energies between 532 and 534.8 eV, which correspond to oxygen in complexes such as C-O, CdO, -COO-, formed after SO2 adsorption on the AC sample. However, for the DAC and Cu/DAC samples, the amount of oxygen complexes increased considerably. Particularly, the presence of O2 favored the formation of oxygen complexes in the impregnated activated carbon, this indicating that the oxygen atom is being transferred to the carbon support. These oxygen surface complexes desorbed mainly as CO2 during the TPD experiments. Before the adsorption process only Cu2+ was detected by XPS with a binding energy of 932 eV, it was confirmed by the diffractogram recorded for the Cu/DAC sample after the heattreatment, which showed peaks at 35.4° and 38.6°, but after

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Figure 6. XRD characterization of Cu/DAC sample after SO2 adsorption.

Figure 8. SO2 evolution after the adsorption of SO2 and SO2/O2 on the Cu-impregnated activated carbon (Cu/DAC).

Figure 7. SO2 evolution after the adsorption of (a) SO2 and (b) SO2/ O2 on the different samples.

the reaction a mixture of Cu2+ and Cu+ was observed. Figure 6 presents the diffractogram for the Cu/DAC sample after the SO2 adsorption; this analysis confirms the formation of CuO and Cu2O (2θ ) 36.6) phases, thus suggesting the redox role of the catalyst during the adsorption of SO2. TPD Results. SO2-TPD spectra obtained with the three activated carbons following adsorption of SO2 and SO2/O2 are presented in panels a and b of Figure 7, respectively. The desorption profiles are bimodal, thus indicating that at least two different types of adsorption site exist. However, the contributions of the two signals are different. In Figure 7a, the low temperature desorption peak takes place at different temperatures increasing from DAC to AC, it could be explained that on DAC this low temperature adsorption (probably just SO3 weakly adsorbed) takes place on C sites, while on AC it takes place on the different metals (like Fe or Ca) present on mineral matter, whereas for the Cu/DAC sample the contribution of the high

temperature desorption peak is more important. This peak could be related to SO2 that has been oxidized to SO3 or SO4 species,20,21 indicating that copper has a catalytic effect toward the transformation of SO2 into other species that are strongly adsorbed on the carbon support. Although sulfur complexes were not observed by XPS in the Cu/DAC sample in the absence of O2, a significant amount of SO2 was desorbed during the TPD. The XPS technique can analyze only the most external surface, while the TPD analysis monitors species desorbed not only from the most external surface but also form the inner pore surface. In Figure 7b, the adsorption of SO2 in the presence of O2 showed an increase of the complexes that desorb as SO2 for the AC and Cu/DAC samples. The impregnated sample (Cu/ DAC) presented the largest SO2 evolution, which is in agreement with the results obtained from the XPS analysis and it confirms the beneficial effect of Cu in the SO2 adsorption. However, the low temperature desorption peak for the DAC sample presented lower intensity in the presence of O2, probably the oxygen is blocking some active sites. It is also observed in this figure that the low temperature desorption peak desorbs at the same temperature for all the activated carbons; it means that the same type of complex is formed on the surface of the three samples. Therefore, it could suggest that during the SO2/O2 adsorption, oxygen is dissociative chemisorbed on the surface creating a more homogeneous surface and then SO2 is adsorbed on these complexes, thus probably forming a SO3 complex as observed by XPS. Figure 8 compares the TPD spectra after SO2 adsorption in both the absence and presence of O2 on the Cu/DAC sample. There was a drastic increase in the concentration of complexes of low thermal stability in the presence of SO2, the desorption of which was shifted to lower temperatures, very likely corresponding to SO3 weakly adsorbed, as observed by XPS. An increase in the concentration of complexes of high thermal stability could possibly be due to the presence of sulfates, which could be formed on the copper surface or on the internal sites of the carbon particles. The active catalytic species is CuO, which reacts easily with SO2 and O2 to form sulfates.22 SO2 was the only desorbed species during the TPD of the different carbon samples. Therefore, the sulfites and sulfates complexes could be decomposed to SO2 and oxygen surface complexes that desorbs as CO2 and CO.21,23 Raymundo et al.23 proposed that desorption of SO3 produces carbon oxidation and further carbon gasification, with evolution of SO2, CO and CO2

Adsorption of SO2 on Activated Carbon

J. Phys. Chem. C, Vol. 112, No. 39, 2008 15339 calculations of the heats of adsorption of SO2 on copper and nickel surfaces. It was found that (i) the dissociation, SO2 f S + O + O, was the most favorable pathway; (ii) the formation of SO on the surfaces of Cu and Ni was not favorable; and (iii) SO3 is formed by the oxidation of SO2 with a previously adsorbed oxygen atom (O(ads)). On the other hand, Li and Henrich26 studied the interactions of SO2 with NiO at room temperature. XPS and UPS characterization of the reduced NiO surface provided evidence of the dissociative chemisorption of SO2 on the oxygen vacancies of reduced NiO. Lu et al.27 studied by high-resolution photoelectron spectroscopy the adsorption of SO2 at room temperature on copper surfaces. The following mechanism was proposed

SO3 + C(O) f SO2 + CO2 SO3 + C(O) f SO2 + CO2 Figure 9. SO2, CO2 and CO desorption profiles after the adsorption of SO2/O2 on Cu/DAC.

TABLE 4: Total Amount of Desorbed Species from the Cu-Impregnated Activated Carbon (Cu/DAC) after Adsorption of SO2 and SO2/O2 adsorptive SO2 SO2/O2

SO2 (µmol/gC)

CO (µmol/gC)

CO2 (µmol/gC)

72 138

1476 883

1274 1418

during TPD. CO2 results from secondary reaction between SO3 and CO-type oxygen groups. The proposed mechanism is the following

SO3ads + C f SO2 + C(O) SO3 + C(O) f SO2 + CO2 SO3 + C(O) f SO2 + CO2 Figure 9 presents the SO2, CO2 and CO desorption profiles obtained after the adsorption of SO2/O2 on the impregnated activated carbon. A broad peak of SO2 desorption is observed in the same temperature range that the evolution peak of CO2 and the second desorption peak of CO at low temperature (i.e., between 250 and 400 °C), which is in agreement with the mechanism mentioned above. Table 4 summarizes the total amount of desorbed surface complexes from the impregnated activated carbon after the adsorption of SO2 and SO2/O2. The presence of O2 has an important effect on both the formation of sulfur complexes that desorb as SO2, providing more active sites for SO2 adsorption, and the formation of oxygen complexes that desorb mainly as CO2, some of them formed by the reaction of SO3 with an oxygen surface complexes, as observed in Figure 9. The adsorption of SO2 in the absence of O2 mainly favored the formation of CO complexes of high thermal stability. Similar results were observed for the other carbon samples, but with lower amounts. Possible Mechanistic Routes. Most of the proposed reaction mechanisms for this reaction involve the dissociation of SO2 on different metals. For example, Rodrı´guez et al. 24 studied the mechanism using both theoretical and experimental approaches; on the TiC (001) surface above 200 K, SO2 adsorption and decomposition was found to occur as dissociative chemisorption of SO2 to S(ads) and O(ads) on the surface. Sellers and Shustorovich25 studied the dissociation of SO2, the stability of the SO complex and the oxidation of SO2 using ab initio

SO2 + O(ads) f SO3(ads) In the case of an oxidized copper surface, the decomposition of SO2 was inhibited and SO3 was the only species formed. Similar results were observed by XPS for the adsorption of SO2 on Cu (111) and Ni (111) surfaces at room temperature.22,28 Most of the SO2 adsorption studies on carbonaceous materials coincide in proposing that there is a first step where SO2 is adsorbed on the surface of the activated carbon.3,29 Davini30 suggested the basic surface groups as the sites responsible for the dissociative chemisorption of SO2. In the presence of O2, Raymundo et al.23 proposed a mechanism based on an Eley-Rideal kinetic model to explain the oxidation of SO2; it is adsorbed on the surface and then reacts with O2(g) to give SO3(ads). Mochida et al.4 proposed the reversible adsorption of O2 on the carbon surface, which then oxidizes the previously adsorbed SO2. All of these mechanistic proposals have been mainly based on the results of the adsorption capacities and desorption of SO2 studied by the TPD technique. However, there is not a clear evidence of the formation of the SO3 complex. The results found in this investigation for the adsorption of SO2 on activated carbons provide evidence of the formation of the SO3 complex, which was observed by XPS (S 2p3/2 binding energy between 167.8 and 168.2 eV). This complex is likely formed via oxidation of SO2 by an oxygen complex present on the surface of the carbon material, as observed in Figure 7b. For the impregnated sample (Cu/DAC), a considerable increase of the oxygen complexes concentration was observed, possibly because SO2 is dissociative chemisorbed on the catalyst, with the oxygen being then transferred to the carbon surface. From the TPD results, there is clear evidence of the formation of at least two types of SO3 complexes. One type is weakly adsorbed, and it desorbs as SO2 at approximately 200 °C. It is mainly found on the activated (AC) and demineralized (DAC) samples. The second type is more strongly adsorbed, and it desorbs at temperatures higher than 300 °C, leaving oxygen on the surface that then desorbs at high temperatures as CO2 or CO. This second type is mainly observed in the Cu-impregnated activated carbon (Cu/DAC). Conclusions The SO3 complex was the only species observed upon the adsorption of SO2 by XPS. The amount of desorbed SO2 was higher for the carbon-supported copper catalyst, indicating that copper has a catalytic effect toward the transformation of SO2 into other species that are strongly adsorbed on the carbon support.

15340 J. Phys. Chem. C, Vol. 112, No. 39, 2008 From the adsorption experiments, the presence of O2 on the Cu/DAC catalyst favored SO2 adsorption, this increasing the breakthrough time, the adsorption capacity and favoring the formation of sulfur and oxygen complexes of higher thermal stabilities that mainly desorb as SO2 and CO2. Acknowledgment. We thank the Banco de la Republica for the financing of Project No. 1867, to the University of Antioquia for the financing of the Sostenibilidad program, and to CYTED and the University of Alicante, Spain, for the research internship of D.L. R.B. thanks the Young Research Program for the financing support. References and Notes (1) Sakai, M.; Su, C.; Sasaoka, E. Ind. Eng. Chem. Res. 2002, 41, 5029. (2) Kaminski, J. Appl. Energy 2003, 75, 165. (3) Wilde, J. D.; Marin, G. B. Catal. Today 2000, 62, 319. (4) Mochida, I.; Korai, Y.; Shirahama, M.; Kawano, S.; Hada, T.; Seo, Y.; Yoshikawa, M.; Yasutake, A. Carbon 2000, 38, 227–239. (5) Farrauto, R. J.; Bartholomew, C. H. Fundamentals of Industrial Catalytic Processes; Blakie Academic and Professional: London, 1997. (6) Lizzio, A. A.; DeBarr, J. A. Fuel 1996, 76, 1515. (7) Rodrı´guez-Reinoso, F. Carbon 1998, 36, 159–175. (8) Radovic, L. R.; Rodriguez-Reinoso, F. Chem. Phys. Carbon 1997, 25, 243. (9) Tomita, A. Fuel. Proc. Tech. 2001, 71, 53. (10) Marsh, H.; Rodrı´guez-Reinoso, F. ActiVated Carbon; Elsevier: Oxford, 2006.

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