Surface Complexes Formed during Simultaneous Catalytic Adsorption

J. Phys. Chem. C 2007, 111, 1417-1423

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Surface Complexes Formed during Simultaneous Catalytic Adsorption of NO and SO2 on Activated Carbons at Low Temperatures 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: June 7, 2006; In Final Form: August 31, 2006

Simultaneous adsorption of NO and SO2 on activated carbon provides an adequate alternative to control the low concentration emissions of these air pollutants. The surface complexes formed during the NO and SO2 adsorptions at 30 °C were studied by X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). The effects of the addition of copper supported on the carbon and O2 on the gas stream were studied. The presence of copper enhanced the chemical adsorption of both NO and SO2. In the simultaneous adsorption of NO and SO2, SO2 inhibited the adsorption of NO, while SO2 adsorption was notably improved. The reaction carried out under a NO/SO2/O2 gas mixture showed a synergic effect increasing the formation of surface complexes. The addition of copper assisted the oxygen transfer to the carbon matrix.

Introduction Nitrogen (NOx) and sulfur (SOx) oxides are considered among the most toxic gases emitted to the atmosphere during the combustion of fossil fuels in the sectors of industry and transport. As a consequence, the laws that regulate the emissions of these pollutants become more drastic and result in a demand for immediate solutions. These solutions may include substantial variations of the existent combustion processes, redesign of equipment, and application of new technologies that lead to the decrease of pollutants.1,2 In a practical process, SOx is removed mainly on the basis of solid-gas reactions using dolomites or calcite as sorbents,3 or in gas-liquid reactions.4 However, all of these processes generate considerable amounts of byproducts, require large space, and result in high capital costs.5,6 In the case of NOx, the most used commercial techniques are the selective catalytic reduction (SCR) by NH3 and the selective noncatalytic reduction (SNCR). However, these processes have a few problems. First, the SCR technique presents catalytic deactivation caused by catalyst poisoning by SO2 and high maintenance fees.7,8 On the other hand, SNCR requires high temperatures.9 Compared to the individual control of each one of these species, their simultaneous removal from flue gas may have some advantages. Currently, few technology options are available. One technique is the selective catalytic reduction of NO with NH3 in the presence of O2 followed by the adsorption or catalytic oxidation of SO2 with supported metal oxide catalysts.10 However, this technique is costly, complex, multistep, and suffers from many disadvantages such as deactivation of the catalyst by SO2 at temperatures lower than 300 °C.11,12 The Bergbau Forschung Co. in Germany proposed the simultaneous SO2 and NOx removal process by using a two-stage moving bed with activated coke.6,13 The simultaneous SO2 and NOx * Corresponding author. Phone: 574-210- 6613. Fax: 574-210-6565. E-mail [email protected]. † University of Antioquia. ‡ Universidad de Alicante.

removal in a single process and at the same temperature will be more desirable and economical, and the development of catalysts capable of simultaneously treating NOx and SO2 is very important. Several potential adsorbents have been investigated, including metal oxides, zeolites, and porous carbons in different forms. Activated carbon is an amorphous material used industrially as an adsorbent for the control of environmental pollution due to its high porosity and extensive surface area. It is obtained by pyrolysis, gasification, or chemical oxidation of any substance rich in organic carbon, with the purpose 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 capability for regeneration, it allows the removal of most of the impurities found in effluents such as SO2, NOx, particles, mercury, dioxins, furans, VOC, and heavy metals.13,14 Additionally, activated carbon has been used as a catalyst as well as a catalytic support,15-18 although it is necessary to minimize the catalyst tendency to behave as a reactant, especially in the presence of oxygen. Metals with catalytic activity can be exploited to reduce reaction temperature and to minimize carbon loss from gasification, and previous studies have demonstrated that when activated carbon is used as catalytic support the adsorption capacities increase considerably.17,19 In particular, copper has been reported as a good catalyst for the adsorption of SO2 or NOx at low temperatures, while in the absence of copper very low adsorption capacities have been reported.19,20 However, in the above-mentioned research, there is a lack of information concerning the interactions between SO2 and NOx with the surface functional groups on the activated carbon, or concerning the supported metal on the carbon materials.19,21 Tseng et al.22 found that the adsorption-catalytic reaction between SO2 and surface 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 description of the kinetics of the gas-phase catalytic process, it is necessary to determine the nature of the

10.1021/jp063544h CCC: $37.00 © 2007 American Chemical Society Published on Web 12/21/2006

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interaction between the gaseous molecules and the catalyst/ carbon surface to understand the elemental reactions that can describe the process. The objective of this work was to study the simultaneous adsorption of NO and SO2 on activated carbon, the catalytic effect of copper supported on this material, and the effect of the addition of oxygen to the gas stream. Experimental Section The char was prepared from a subbituminous coal (particle size, 75-150 µm) pyrolyzed at 900 °C for 1 h under flow of N2. The resultant coal char was then activated with a flow of steam at 850 °C for 3 h. The activated carbon contains 7% of mineral matter mainly composed of Na, K, Ca, and Fe, which could have a catalytic effect on the adsorption reaction of NO and SO2. Part of the activated carbon was demineralized with HCl-HF-HCl solutions using a technique adapted from Bishop and Ward.23 The demineralized samples were loaded with copper by impregnation with a Cu(NO3)2‚3H2O solution with the appropriate concentration to obtain a nominal loading of 10 wt % Cu on the activated carbon. The adsorption experiments were carried out in a fixed bed reactor using 0.100 g of the sample. The temperature was monitored with a thermocouple attached to the reactor external wall, closed 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 exposure of the sample to air while handling. The reactor was then cooled to 30 °C and He was switched to the reaction mixture: NO, SO2, NO/SO2, and NO/SO2/O2, with a flow of 100 mL/min for 2 h. The gas concentrations used in these experiments were the following: NO, 500 ppm; SO2, 500 ppm; and O2, 5% in the carrier He. After the adsorption, the samples were subjected to XPS analysis, in a VG-Microtech Multilab 3000 spectrometer equipped with a hemispherical electron analyzer and a Mg KR (15 kV, 20 mA) 300-W X-ray source. The C1s binding energy at 284.6 eV was taken as the reference. In order to obtain a reference point for comparison of the results, a char sample without Cu was heat-treated at 500 °C in He and then the surface complexes were characterized by XPS as described above. To establish the effects of the reversible chemisorption reactions of NO and SO2 and the formation of oxygen complexes independently, the adsorption experiments were repeated and the carbons were subsequently subjected to temperature-programmed desorption (TPD) under He at 15 °C/ min up to 950 °C. The evolved gases (NO, SO2, CO, and CO2) were monitored with a quadrupole mass spectrometer. Results and Discussion The usual method for the quantitative analysis in XPS is to determine the area of each photoelectronic signal, which is then corrected by the atomic sensibility factor. For carbon surfaces, the signal intensities are normalized with respect to the corresponding area below the C1s signal. To determine the relative changes in N-, O-, and S-complexes due to the reaction, the value of the relationship between the intensity of each complex and carbon intensity (i.e., N/C, O/C, and S/C) was subtracted from the relationship of the corresponding sample char used as reference. Figure 1 presents the most important nitrogen complexes observed on the char surface by XPS.24 They are the following: (1) nitro-type complexes (NO2, Figure 1(A)), (2) pyridinic complex (N-6, Figure 1(B)), (3) pyrrolic complexes (N-5, Figure 1 (C)), (4) pyridone (Figure 1 (D)) and nitroso complexes (-NO, Figure 1(E)), (5) pyridinic-N-oxide com-

Figure 1. Schematic representation of common nitrogen surface complexes found on carbon.

Figure 2. Schematic representation of common sulfur surface complexes formed during the SO2 adsorption on carbon.

Figure 3. Nitrogen and oxygen complexes formed during NO adsorption, represented as XPS data referred to the initial char.

plexes (N-X, Figure 1(F)), and (6) quaternary-N complexes (NQ, Figure 1(G, H, and I). Among these, pyridinic and pyrrolic complexes are the most abundant. The nitrate complexes (-NO3) are not shown in this figure, since they have been considered to be formed on the mineral matter present in the char.25,26 Figure 2 shows a representation of a graphene layer with some of the possible sulfur complexes formed during the adsorption of SO2 that have been reported in the literature.27,28 NO Adsorption. Figure 3 presents the nitrogen and oxygen complexes formed on the surface after NO adsorption on the activated carbon (AC) and on the Cu-loaded activated carbon (Cu/AC). The values of the y-axis (in arbitrary units) correspond to the subtraction of the N-complexes/C ratio of the initial char from the corresponding ratio of the char after reaction. The x-axis shows the complexes that underwent changes in their concentrations. The nitrogen complexes formed on the activated carbon surface (AC) were pyridinic-N-oxide (N-X) and nitrate (NO3) complexes, the last ones probably formed on the mineral matter, since the XPS spectra for the demineralized activated carbon showed the formation of quaternary-N (N-Q) and nitro

Complexes from Catalytic Adsorption of NO/SO2 on AC

Figure 4. NO evolution after the adsorption of 500 ppm NO.

(NO2) complexes. Nitrogen complexes were not detected on the impregnated sample (Cu/AC), maybe because they have been transferred to internal sites in the carbon support where the XPS technique cannot detect them, or another possibility could be that the surface complexes are removed by the vacuum prior to XPS analysis. In order to verify this possibility a DRIFT analysis was performed; however, they could not be clearly observed. The observed decrease in the amount of oxygen surface groups on the activated carbon (AC) indicates that gasification of the carbon material takes place, and the oxygen from NO is removed in the form of CO or CO2. The CO/CO2 product ratio from TPD after the adsorption of NO on AC was lower compared with the CO/CO2 product ratio from TPD of the same sample before the adsorption. On the other hand, Cu/ AC showed a considerable increase of the oxygen surface complexes, which can be explained by the dissociative NO chemisorption on the catalyst, the oxygen atom being transferred to the carbon support by a spillover mechanism. These oxygen surface complexes desorbed mainly as CO2 during the TPD. Before the adsorption process only Cu2+ was detected by XPS, but after the reaction a mixture of Cu2+and Cu+ was observed. An X-ray diffraction analysis confirmed the formation of CuO and Cu2O, thus suggesting the redox role of the catalyst during the adsorption of NO. A comparison of the NO TPD spectra from AC and Cu/AC following adsorption of 500 ppm NO is presented in Figure 4. The spectra are predominantly monomodal profiles, with small distribution shoulders at slightly higher temperatures. In the case of Cu/AC, the NO desorption peak is narrower and is centered at 180 °C, while the corresponding peak for AC is centered at about 300 °C. This shows that the active sites are very different in both samples and that, in the presence of copper, the NO reaction takes place preferably on the metal. Although nitrogen complexes were not observed by XPS for the Cu/ AC sample, they desorbed as NO during the TPD. NO desorption begins at temperatures as low as 50 °C (for Cu/AC) and is completed at temperatures lower than 400 °C; this means that several complexes with less thermal stability are being formed on the surface and Cu is catalyzing their decomposition.26,29,30 However, NO was the only species desorbed during the TPD in both the AC and the Cu/AC; the nitrates and nitrites complexes can be decomposed to NO2 which was further transformed to NO and an oxygen surface complex that desorbs as CO2. It has been reported that the reductive carbon surface captures oxygen from NO2 and NO3 to form oxygen functional groups which give CO and CO2 in TPD and release NO.31,32 However, when the adsorption reaction of NO/O2 on the activated carbon was performed for 70 h at 30 °C, NO and NO2 were the desorbed products.

J. Phys. Chem. C, Vol. 111, No. 3, 2007 1419 The activated carbon (AC) presents nitrogen complexes with large stability as observed in the XPS results, probably as nitrate complexes formed on the mineral matter. The desorbed amount of NO from the activated carbon (AC) was slightly higher than the amount desorbed from the catalyst (Cu/AC), due to the effect of the catalytic mineral material present in the activated carbon.33 Fluorescence X-ray analysis of the mineral matter in the char showed that the main inorganic compounds were CaO (25.2%) and Fe2O3 (11.1%) which have been reported to have catalytic activity for the reduction of NO and adsorption of SO2.34-40 SO2 Adsorption. The XPS analysis of AC after the adsorption of 500 ppm SO2 presented a S2p3/2 signal at 168.2 eV corresponding to the SO3 complex, accompanied by a decrease of the amount of oxygen complexes (O/C ratio). The CO/CO2 product ratio from TPD after the adsorption of SO2 on the activated carbon was lower compared with the CO/CO2 product ratio from TPD of the same sample before the adsorption. Once again, for the Cu/AC sample the sulfur complexes were not detected by XPS. However, the amount of oxygen complexes increased considerably, indicating oxygen transfer to the carbon support after the dissociative chemisorption of SO2 on the catalyst. Most of the proposed reaction mechanisms for this reaction involve the dissociation of the SO2 on different metals. For example, Rodrı´guez et al.41 studied it both from a theoretical and an experimental point of view; on the TiC(001) surface, and above 200 K, the SO2 adsorption and decomposition were found to occur as dissociative chemisorption to S(ads) and O(ads) on the surface. Sellers and Shustorovich42 studied the dissociation of SO2, the stability of the SO complex, and the oxidation of SO2 using ab initio 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 O(ads). On the other hand, Li and Henrich43 studied the SO2 interactions on NiO at room temperature. XPS and UPS analyses on the reduced NiO surface provided evidence of the dissociative chemisorption of SO2 on the oxygen vacancies of reduced NiO. Lu et al.44 studied high-resolution photoelectron spectroscopy evidence of the adsorption of SO2 at room temperature on copper surfaces. The following mechanism for the adsorption was proposed:

SO2 f O(ads) + SO(ads) SO(ads) + SO2 f S(ads) + SO3(ads) 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.45,46 As commented above, an analysis of the published literature shows that there is a lack of discussion about the most favorable way by which SO2 is dissociated on the catalyst surface. The comparison between the SO2 TPD spectra obtained from activated carbon (AC) and the supported catalyst (Cu/AC) following adsorption of 500 ppm SO2 is presented in Figure 5. Both profiles are bimodal, centered, respectively, at 180 and 400 °C, indicating the existence of at least two different adsorption sites. However, the contributions of the two signals at these temperatures are different. In the case of the AC sample,

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Figure 7. Nitrogen complexes formed during the simultaneous adsorption of NO/ SO2/O2 and after TPD on the activated carbon (AC).

Figure 5. SO2 evolution after the adsorption of 500 ppm SO2.

Figure 6. Nitrogen complexes formed during the simultaneous adsorption of NO and SO2 on the activated carbon (AC).

the low-temperature desorption is predominant (possibly SO2 is just physisorbed), while for the Cu/AC sample the contribution of the high-temperature desorption is more important, which could be related with the strongly adsorbed SO2 or being oxidized to SO3 or SO4,47,48 this indicating that copper has a catalytic effect toward the transformation of SO2 into other species that are strongly adsorbed in the carbon support. Sulfur complexes were not observed by XPS in the Cu/AC sample, although a significant amount of SO2 was desorbed during the TPD because they are not external and they are not accessible to the XPS. SO2 was the only species desorbed during the TPD in both the activated carbon and the impregnated activated carbon; therefore, the sulfites and sulfates complexes could be decomposed to SO2 and oxygen surface complexes that desorb as CO2 and CO.48,49 Raymundo et al.49 proposed that desorption of SO3 produces carbon oxidation and further carbon gasification, with evolution of SO2, CO, and CO2 during TPD. CO2 results from a secondary reaction between SO3 and CO-type oxygen groups. The proposed mechanism is the following:

SO3 ads f SO2 + C(O) SO3 + C(O) f SO2 + CO2 C(O) f CO Simultaneous Adsorption of NO/SO2 and the Effect of Adding O2. Figure 6 presents the relative amount of nitrogen complexes formed on the activated carbon (AC) after the adsorption of NO/SO2 in both the absence and presence of O2. The XPS results from the NO adsorption are included for comparison. It can be observed that during the simultaneous adsorption in the absence of O2 the pyridinic-N-oxide (N-X) complexes were formed in a smaller proportion than when the adsorption was made with just NO; N-5 complexes were also formed.

The total amount of nitrogen complexes was smaller in the presence of SO2; this suggests that SO2 partially inhibits the adsorption of NO and that it possibly occupies the active sites where NO was adsorbed in the absence of SO2. Some authors have reported a total inhibition on other types of adsorbent.5,6 However, there was a drastic increase of N-5 complexes in the presence of O2, and pyridinic-N (N-6) complexes were also formed. XPS analysis of the activated carbon after the adsorption of NO in the presence of O2 showed the formation of -NO2 and -NO3 complexes and a drastic increase of the amount of oxygen complexes (O/C ratio). It could suggest that O2 is adsorbed on the surface and then NO reacts with these active sites forming nitrites and nitrates complexes on the surface. Zhu et al.50 reported that in the presence of O2 the adsorbed NO is oxidized and subsequently migrates onto the adjacent sites by a mechanism similar to a spillover and/or by desorption and readsorption processes. Therefore, the addition of O2 favors both the adsorption of NO and SO249 by providing active sites for their adsorption. Interactions between the NO and SO2 on these active sites can also lead to the formation of [(NO2)(SO3)] complexes as reported by Das et al.48 Since upon TPD of these samples, only a small amount of complexes that desorb as NO was observed, one can deduce that N-5 and N-6 complexes of pyrrolic and pyridinic nature, respectively, are very stable at high temperature. With the purpose of verifying this hypothesis, XPS analysis was carried out on the sample that was obtained after the initial TPD analysis. The results are summarized in Figure 7. After TPD, the pyridinic-N complex (N-6) remains and the quaternary-N complex (N-Q) is formed, probably due to a transformation of the pyrrolic-N complex to the quaternary-N by the effect of temperature. Therefore, it can be concluded that during the simultaneous adsorption of NO/SO2/O2, a dissociative chemisorption of NO takes place and nitrogen is incorporated into the carbon support. Figure 8 summarizes the total complexes formed after the adsorption of NO, SO2, and the simultaneous adsorption of NO/ SO2 for the activated carbon. The nature of the sulfur complexes formed during the simultaneous adsorption, both in the absence and in the presence of O2, was the same as that observed during the adsorption with only SO2 (signal S2p3/2 at 168.2 eV). As shown, the concentration of sulfur complexes increased with the simultaneous adsorption of NO/SO2, this meaning that the presence of NO favors the adsorption of SO2. Bagreev et al.51 reported that nitrogen species such as pyrrolic-N, pyridinicN, and quaternary-N (N-Q) complexes favor the adsorption of SO2, this being in agreement with the results obtained in this investigation. In the presence of O2 the amount of sulfur complexes increased in a larger proportion, showing a synergistic effect where the O2 chemisorbs dissociatively, providing more active sites on the surface.49

Complexes from Catalytic Adsorption of NO/SO2 on AC

Figure 8. Total surface complexes formed during the adsorption on activated carbon (AC).

Figure 9. SO2 evolution after the adsorption of different reaction mixtures on the impregnated sample (Cu/AC).

For the Cu/AC sample, nitrogen complexes were not observed by XPS after treatment with any of the reaction mixtures. However, sulfur complexes were observed at 168.4 eV after simultaneous adsorption, which corresponded to SO3 complexes, and the amount of which increased in the presence of O2. The oxygen complexes increased more drastically than in the case of the carbon sample without catalyst. Figure 9 presents the SO2 TPD spectra for the different reaction mixtures on Cu/AC. The addition of NO favors desorption at lower temperatures, which can correspond to SO2 physisorbed or SO3 weakly adsorbed which is desorbed as SO2. In the presence of O2, the complexes of higher thermal stability that are favored could possibly be sulfates that were formed on copper or internal sites, where the XPS technique cannot detect them. The comparison of the surface sulfur complexes of the two samples during the simultaneous adsorption of NO and SO2 and the effect of O2 addition are shown in Figure 10. These results suggest that the simultaneous adsorption of NO and SO2 is not favored in the presence of copper, and that, on the other hand, the mineral matter in the activated carbon may have a significant effect on the SO2 adsorption. The main inorganic compounds found in the mineral matter which may be catalytically active for the adsorption of SO2 were CaO and Fe2O3. Several authors37,38,52 have reported that iron derivates favor the transformation of the adsorbed SO2 into forms having higher stabilities and also that impregnated activated carbons had better adsorption properties for SO2 and NOx. However, the TPD analysis of these samples showed the highest evolution of SO2 from the Cu-impregnated sample (Cu/AC), as can be seen in Figure 11. It appears that the effect of the catalyst is to facilitate the adsorption of SO2 or the transformation of this species into a more stable one that desorbs as SO2 at higher temperatures

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Figure 10. Sulfur surface complexes formed during the simultaneous adsorption of NO and SO2 on two different samples as determined from XPS data.

Figure 11. SO2 evolution after the simultaneous adsorption of NO/ SO2/O2 on different samples.

Figure 12. TPD spectra after the simultaneous adsorption of NO/SO2/ O2 on the activated carbon (AC).

(300-650 °C). These surface complexes may be located inside the pore structure of the carbon material where XPS cannot detect them. Figure 12 presents the TPD after the simultaneous adsorption of NO/SO2/O2 on the activated carbon. It can be observed that CO and CO2 are evolved in a very wide range of temperature, that is, from about 100 to more than 900 °C. CO2 is evolved in four main events centered at temperatures of around 180, 260, 500, and 700 °C. CO is evolved in much smaller quantities than CO2 at temperatures lower than 900 °C. In addition, NO and SO2 are also evolved at temperatures between 100 and 500 °C as a consequence of the decomposition of complexes that may

1422 J. Phys. Chem. C, Vol. 111, No. 3, 2007 be formed during desorption of NO2 and SO3. Most of the NO desorbs in the same temperature range as the first evolution event of CO and CO2. This can be explained if NO2 decomposes as presented in the following reactions, which have been proposed in the literature:50,53

2NO2 + C f 2NO + CO2 NO2 + C f NO + CO In the case of SO2, it desorbs in one main event centered at around 280 °C accompanied by the evolution of CO2 and CO. Raymundo et al.49 proposed that these carbon gasification reactions take place during desorption of SO3, which oxidizes the surface of the carbon desorbing SO2 forming oxygen complexes that decompose during a heat treatment giving CO and CO2. Possible Mechanistic Routes. Different reaction mechanisms have been proposed in the literature for the adsorption of NO and SO2. For SO2 adsorption, the majority of studies coincide in proposing a first step where SO2 is adsorbed on the surface of the activated carbon.6,54,55 Davini56 suggested the basic surface groups as the sites responsible for the dissociative chemisorption of SO2. Bagreev et al.51 showed that the nitrogen species present in the activated carbon, as pyridinic-N and quaternary-N, have a favorable influence on SO2 adsorption. In the presence of O2, Raymundo et al.49 proposed a mechanism based on an EleyRideal kinetic model to explain the oxidation of SO2; it is adsorbed on the surface and it then reacts with O2(g) to give SO3(ads). Mochida et al.5 proposed the reversible adsorption of O2 on the carbon surface, which then oxidizes the previously adsorbed SO2. All these mechanistic proposals have been mainly based on the results of the adsorption capacities and desorption of SO2 by the TPD technique. However, there is still scarce experimental evidence of the formation of the SO3 complex. The results found in this investigation for the adsorption of SO2 on activated carbon (AC) provide evidence of the formation of the SO3 complex, which was observed by XPS with a binding energy of 168.2 eV. This complex is very possibly formed via oxidation of SO2 by an oxygen complex present on the surface of the activated carbon. For the impregnated sample (Cu/AC), a considerable increase of the oxygen complexes was observed, possibly because SO2 is dissociatively chemisorbed on the catalyst, with the oxygen then being transferred to the carbon surface. TPD results seem to show evidence of the formation of two types of SO3 complexes. One type is weakly adsorbed and desorbs as SO2 at approximately 180 °C, this being mainly observed for the activated carbon (AC). The second type is more strongly adsorbed and desorbs at temperatures higher than 300 °C, leaving an oxygen atom on the surface of the carbon that desorbs at high temperatures as CO2 or CO. This second type is mainly observed in the activated-carbon-supported copper (Cu/AC). The mechanisms proposed in the literature for the adsorption of NO coincide mainly in the formation of an oxygenated nitrogen complex (for example, NO2 complex) in the presence of O2, suggesting that the oxygen surface groups improve the adsorption of the NO.5,6,57,58 In a previous work, Garcı´a et al.21,32 observed by XPS the formation of pyridinic-N (N-6) and nitro (NO2) complexes, during the adsorption of NO at 100 °C on a coal-based char. The NO2 complex was possibly formed from the dissociative chemisorption of NO on a surface with oxygen complexes.

Lo´pez et al. In this investigation, it was found that the nitrogen species formed during the adsorption of NO at 30 °C on activated carbon (AC) correspond to pyridinic-N-oxide (N-X) and nitrate (NO3) complexes, the last one possibly formed on the mineral matter. For the carbon-supported copper catalyst (Cu/AC), a considerable increase of the oxygen complexes was observed, thus suggesting that the NO is dissociatively chemisorbed on the catalyst and the oxygen is being transferred to the carbon surface. There is evidence from TPD results of the formation of at least three types of complexes that desorbed as NO. One of them is evident at low temperature, corresponding to NO weakly adsorbed on the Cu catalyst (Cu/AC). The activated carbon shows evidence of the second one, a strongly adsorbed complex, and of the third one that corresponds to a shoulder around 400 °C. This last type could correspond to the decomposition of the nitrates formed on the mineral matter. There are few works published on the simultaneous adsorption of NO and SO2, and experimental evidence of the proposed complexes is even scarcer. Rubel and Stencel59 proposed that the NO(ads) and SO2(ads) are the species formed during the simultaneous adsorption of these gases. Qiang et al.55 proposed that during the simultaneous adsorption of NO/SO2 a new intermediate complex is formed, [(NO2)(SO3)](ads). However, there is no experimental evidence of the formation of this complex. Davini56 proposed that NO helps to create or to stabilize basic surface oxides which improve the adsorption of acidic SO2. It was found in this investigation that formation of N-X and N-5 complexes took place during the simultaneous adsorption of NO/SO2. The N-5 type probably corresponds to pyrrolic-N complexes because it was not desorbed during TPD. In the presence of O2 the amount of nitrogen complexes increased drastically and pyrrolic-N (N-5) and pyridinic-N (N-6) complexes were formed, favoring the SO2 adsorption. These results are in agreement with some others reported in the literature.51 Regarding the sulfur complexes, the SO3 complex was formed on the carbon surface, both in the absence and the presence of O2. The catalytic active species was CuO, which reacts easily with SO2 and O2 to form sulfates.5 The TPD analysis of the carbon-supported Cu catalyst (Cu/ AC) shows that at least three complexes with different thermal stabilities were formed: (i) one (SO3) weakly adsorbed, (ii) a second one (SO3) more strongly adsorbed which desorbs at approximately 420 °C, and (iii) another one at around 580 °C, corresponding to the decomposition of sulfates probably formed on the copper surface. Conclusions • Pyridinic-N-oxide (N-X) and nitrate (NO3) complexes were formed upon NO adsorption on activated carbon. • The SO3 complex was the only species observed upon the adsorption of SO2. The amount of desorbed SO2 was higher for the carbon-supported copper catalyst. • There was a decrease of the amount of nitrogen surface complexes after the simultaneous adsorption of NO and SO2, due to partial inhibition of NO adsorption by SO2, which competes for the same active sites. However, the adsorption of SO2 was favored by the presence of nitrogen species. The amount of desorbed SO2 was higher for the activated-carbonsupported copper. • The presence of O2 in the simultaneous adsorption of NO and SO2 had a synergistic effect, favoring the formation of the

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