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Low-temperature NO removal over copper-containing activated carbon
Colloids and Surfaces A: Physicochem. Eng. Aspects 295 (2007) 239–245
Low-temperature NO removal over copper-containing activated carbon P. Nikolov ∗ , M. Khristova, D. Mehandjiev Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev” str., bl. 11, 1113 Sofia, Bulgaria Received 25 January 2006; received in revised form 1 September 2006; accepted 5 September 2006 Available online 8 September 2006
Abstract Adsorbents based on a copper modified activated carbon were synthesized and investigated in respect to NO chemisorption at ambient temperature in both inert and oxidative media. To correlate the adsorption ability of the adsorbents with the preparation parameters the materials were characterized by N2 -physisorption, AAA, NO-TPD, XRD, XPS, EPR, IR and magnetic measurements. In the presence of oxygen, the NO chemisorption is higher, proceeds through two stages of oxidation on the carbon surface and forms three N-containing species, which decompose to produce either NO and CO2 or NO and CO. The presence of the copper phase does not change the mechanism of adsorption, but can increase the adsorption capacity. On the surface of the most active adsorbent obtained in vacuum – CuACM-v, small clusters of Cu2+ ions were observed related to the exchange interaction. © 2006 Elsevier B.V. All rights reserved. Keywords: Activated carbon; Chemically modified carbons; Chemisorption; Temperature programmed desorption
1. Introduction Nitrogen oxide is a widespread pollutant originating from internal combustion engines and industrial sources. The selective reduction of NO with different reducing agents is the main method for NO removal used nowadays. However, when NO is present in a low concentration in waste gases containing oxygen, this method yields unsatisfactory results. For these particular cases the chemosorption method seems more appropriate. Adsorbents for NO must possess a high specific adsorption capacity towards NO at temperatures close to the ambient. Activated carbon seems to respond to this requirement due to its very large specific surface area. It should be noted that activated carbon itself can reduce nitrogen oxides [1,2]. Yamashita et al. [1] have found that low nitrogen oxide concentrations can be efficiently removed with activated carbon as reducing agent. They investigated the C + NO reaction at 300–500 ◦ C on activated carbon modified with different metals (Cu, Ca and Ni). The results obtained have shown unambiguously that coppercontaining samples are the most active for NO removal. Teng and Suuberg [3] have investigated NO chemisorption on acti-
vated carbon at 50–200 ◦ C and have found carbon gasification to be negligibly small. According to these authors nitrogen oxide chemisorption occurs at relatively high temperatures and is accompanied by nitrogen evolution and formation of surface oxides. In a previous work [4] we have investigated the conversion of NO in the absence of a reducing agent in the gas phase over an activated carbon impregnated with nickel ions. The results revealed that in the presence of oxygen in the gas phase, the NO conversion efficiency was twice as high as that in the absence of oxygen. The catalytic reduction of NO with CO occurs at relatively low temperatures over Cu, Mn and Cu-Mn phases supported on activated carbon [5]. Therefore, it may be concluded that copper plays a promoting role with respect to this reaction. The present paper is aimed at studying the adsorption ability towards NO of copper-modified activated carbon at room temperature. Different preparation procedures were tested in order to increase the adsorption efficiency. 2. Experimental 2.1. Sample preparation
∗
Corresponding author. Tel.: +3592 9796312; fax: +3592 8705024. E-mail address: plamen [email protected] (P. Nikolov).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.09.004
The samples investigated were prepared by impregnation of activated carbon from apricot shells with various solutions
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and subsequent thermal treatment. Two types of samples were prepared: (i) CuACM-v and CuACM-a, representing carbon promoted by a supported active copper phase and (ii) ACM-v and ACM-a consisting of modified carbon without a supported active copper phase. Samples CuACM-v and CuACM-a were impregnated with aqueous ammonia solution of malachite buffered with ammonium carbonate. The concentration of copper ions in the impregnating solution was 0.22 mol/l for CuACM-v and CuACM-a. Samples ACM-v and ACM-a were reference samples impregnated with aqueous solution of ammonium carbonate containing no copper ions. After impregnation the samples were calcined up to 150 ◦ C in two different regimes: in air (ACM-a and CuACM-a) and under vacuum of 10−3 Torr (ACM-v and CuACM-v). The letter “-v” or “-a” in the end of the sample’s name represents abbreviation of the media of calcinations (vacuum or air, respectively).
containing 1200 ± 30 ppm NO and Ar or air as a balance, respectively. After the adsorption, the samples were blown through isothermally for 10 min under Ar or air in order to eliminate physical adsorption at room temperature. For comparison, initial unmodified active carbon (AC) was investigated under the same conditions. The TPD spectra of NO, CO and CO2 were registered in argon or air flow (flow rate 440 ml/min). The composition of the carrier was chosen to be the same as in the adsorption process. The study proceeded at 25–200 ◦ C with a heating rate of 13 ◦ C/min. The CO and NO concentrations were measured using a UNOR 5N (Maihak, Germany) gas analyser. The concentration of CO2 was measured with an Infralyt 2106 (Germany) gas analyser. The data from the gas analysers were collected and processed using a CSY-10 microprocessor computer system. 3. Results
2.2. Sample characterization The specific surface areas of the samples were determined by the BET method using low-temperature nitrogen adsorption. The mesopore size distribution was obtained from the adsorption curve of the isotherm, using the procedure developed by Orr and Dalla Valle [6]. The copper content was determined by atomic absorption technique with Pye Unicam SP-90B. The infrared spectra in the region 4000–400 cm−1 were recorded on a Br˝uker model IFS 25 Fourier transform interferometer (resolution <2 cm−1 ) using KBr discs as matrices. The XPS studies were performed in an ESCALAB MkII (VG Scientific) spectrometer using an Al K␣ source with energy 1486.6 eV. The residual gas pressure in the analysis chamber was 10−7 Pa. The C1s peak (284.6 eV) was used as internal standard for calibrating the binding energies (BE ± 0.2 eV). Magnetic measurements were carried out in air with a magnetic apparatus constructed according to the Faraday method. The magnetic susceptibility (χ) was measured in the temperature range of 20–220 ◦ C with a magnetic field intensity varying from 2 × 103 to 10 × 103 Oe. X-ray powder diffraction technique (XRD) measurements were carried out using a DRON-3 apparatus with Cu K␣ radiation. EPR spectra were registered as a first derivative of the absorption signal in the temperature interval from −160 to 80 ◦ C using an ERS 220/Q instrument.
Table 1 presents the copper content in the samples and the specific surface areas as obtained by the BET method. The adsorption isotherms of the samples obtained by nitrogen adsorption at −196 ◦ C are shown in Fig. 1. According to BDDT classification, they correspond to type IV. The hysteresis loop is of H-3 type according to the IUPAC classification (type B according to de Boer), which is typical of activated carbons due to the presence of slit-shaped pores. The mesopore size distribution obtained Table 1 Specific surface area and copper concentration of the prepared samples and initial active carbon (AC) Samples
Cu (%)
S (m2 /g)
ACM-a ACM-v CuACM-v CuACM-a AC
0 0 0.54 0.59 0
604 606 581 590 579
2.3. Activity measurements The NO sorption activity of the samples was investigated by a catalytic flow apparatus in an isothermal flow reactor (quartz tube, i.d. 8 mm). The samples investigated were 0.3–0.8 mm fractions with an adsorption volume of 1 cm3 and mass of 0.35 g, The carrier gas was argon (purity 99.99 vol.%) or air (purity 99.9 vol.%) with a flow rate of 440 ml/min and a volume rate of 26 000 h−1 . The preliminary treatment of the samples included heating up to 150 ◦ C for 30 min in an argon or air flow depending on the conditions of the experiment. After that the reactor was cooled to room temperature and the samples were subjected to 1 h adsorption in two gas mixtures: A = NO + Ar and B = NO + air,
Fig. 1. Adsorption isotherms of the samples.
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Fig. 2. Mesopore size distribution of the samples.
from the adsorption branch of the isotherm is presented in Fig. 2. Fig. 3 shows the micropore size distribution calculated by the SE method [7]. The copper-containing samples (CuACM-v and CuACM-a) proved amorphous as shown by XRD analysis, which could be ascribed to the low copper content in them. XPS spectra of CuACM-v and CuACM-a samples in the Cu (2p) photoelectron region show the binding energies (BE) of the Cu (2p3/2 ) peaks characteristic of the Cu2+ oxidation state. The XPS studies do not evidence substantial phase composition difference between samples CuACM-v and CuACM-a and the BE values are close to that for CuO phase: Cu (2p3/2 ) BE = 934.1 eV and O(1s) BE = 529.8 eV. The magnetic measurements characterize the copper-containing samples as paramagnetic. The calculated magnetic moments are lower than expected from the amount of supported copper. This fact suggests that weak interaction between Cu2+ ions exists.
Fig. 3. Micropore size distribution of the samples.
Fig. 4. EPR spectra of the samples at different temperatures: (a) obtained in vacuum ACM-v and CuACM-v; (b) obtained in air ACM-a and CuACM-a, as well as the copper-ammonia precursor.
The EPR spectra of the samples at different temperatures are presented in Fig. 4(a and b). For the sake of comparison, the EPR spectrum of the frozen at −160 ◦ C aqueous solution of the copper complex used as precursor is also shown. A narrow symmetrical signal with g = 2.002 is observed in the EPR spectra of all samples at 80 ◦ C. According to the EPR study on carbonaceous materials, this signal can be assigned to paramagnetic radicals in the activated carbon. The EPR spectra of CuACM-v and CuACM-a samples display a nearly symmetrical signal with effective g = 2.05 which becomes wider with the decrease of the registration temperature. The latter fact shows that the signal originates from antiferromagnetically interacting non-isolated Cu2+ ions. At a lower registration temperature, another signal due to isolated Cu2+ ions is clearly resolved in the EPR spectra of sample CuACM-a. A new signal, shifted towards lower magnetic fields (g = 5.25), is visible in the EPR spectrum of CuACM-v only. This signal can be attributed to exchange coupled Cu2+ ions with an effective spin state S > 1/2. Table 2 presents the adsorption capacities of the samples with respect to NO at room temperature in both inert and oxidation environment. Fig. 5 illustrates the NO, CO and CO2 concentration TPD profiles obtained after adsorption of NO at room temperature in an argon carrier. The TPD presented spectra are obtained in an argon flow with raising the temperature from
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Table 2 Sorption capacity of the prepared samples towards NO Samples
NO adsorbed in NO + Ar flow (mol/g)
NO adsorbed in NO + air flow (mol/g)
CuACM-v CuACM-a ACM-v ACM-a
321.5 66.5 87.5 59.3
862.8 1349.0 477.4 781.0
ambient to 200 ◦ C. Three of the samples, i.e. CuACM-v (Fig. 5a) ACM-a (Fig. 5c) and ACM-v (Fig. 5d) exhibit NO TPD peaks at T = 95 ◦ C. Sample CuACM-v (obtained in vacuum) displays two more desorption peaks: at 110 and 130 ◦ C. At the latter temperature (about 130 ◦ C), the NO desorption peak for CuACM-a is also observed. The samples do not show TPD peaks for CO and CO2 . The evolution of small amounts of CO2 from all of the samples commences at temperatures above 120 ◦ C and is not connected with the process of NO desorption. Fig. 6 presents the TPD spectra (taken in air) after adsorption of NO in oxidative media (NO + air) at room temperature. The
evolution of NO, CO2 and CO for all four samples started at approximately 70 ◦ C. The maxima of the NO peaks observed are situated within the temperature interval 120–130 ◦ C, and two shoulders on both branches of the peaks are also seen at 100–110 ◦ C and 145–155 ◦ C, respectively. The NO desorption is accompanied by the evolution of significant amounts of CO2 and CO. The TPD spectra of CO2 have two desorption peaks with maxima within the ranges of 100–110 ◦ C and 140–160 ◦ C, respectively. The maxima of CO evolution are situated at 120–125 ◦ C. The IR spectra of the samples synthesized in vacuum and in air are shown on Fig. 7a and b, respectively. As can be seen, a large number of IR bands are registered, due to the presence of different chemical groups on the activated carbon surface. The most pronounced IR bands of the fresh samples are shown in Table 3 [8–11]. After NO adsorption some new bands appear in the IR spectra: (i) a very narrow and intense band centred at 1384 cm−1 (Fig. 7a and b), which is due to asymmetric stretching vibrations of free-like NO3 ions [11,12]; (ii) two bands centred at 1562 cm−1 and 1240 cm−1 , which we ascribe to chelated NO3 stretching [13].
Fig. 5. TPD spectra of NO, CO and CO2 for the worked samples (adsorption of NO with Ar): CuACM-v (a), CuACM-a (b), ACM-a (c) and ACM-v (d). Conditions: 40 ml/min Ar flow, temperature range 25–200 ◦ C, heating rate of 13 ◦ C/min.
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Fig. 6. TPD spectra of NO, CO and CO2 for the worked samples (adsorption of NO with air): CuACM-v (a), CuACM-a (b), ACM-v (c) and ACM-a (d). Conditions: 40 ml/min air flow, temperature range 25–200 ◦ C, heating rate of 13 ◦ C/min.
4. Discussion Table 3 Assignment of some IR bands, present in the spectra of the samples investigated IR bands (cm−1 )
Assignment
Ref.
3450 2955–2958 2921–2924 2851–2853 2349 1749 1733 1684 1652 1645 1576 1558–1560 1540 1463 1456 1260 1124 667
H-bonded OH stretching Aliphatic CHn stretching Aliphatic CHn stretching Aliphatic CHn stretching Atmospheric CO2 Carboxyl band Carboxyl band Ketone carboxyl band Carboxyls in a resonant system Bending vibrations of water in liquid phase Quinone structure Carboxylate functionalities Carboxyle groups in ionized configuration Aliphatic CH deformation modes Carbonate-like structures Single bond C–O structures of the ether-like type C–O stretching and O–H bending modes Atmospheric CO2
[8] [11] [11] [11] – [8] [9] [8] [8] – [9] [9] [8] [10] [10] [10] [9] –
The copper content on the activated carbon (Table 1) is relatively low. The specific surface area values of the modified and copper supported samples are very close. The mesopore size distributions for all the samples are also very similar (see Fig. 2), which indicates that copper promotion leads to no change in mesopore size distribution. This result is not unexpected in view of the low concentration of copper ions in samples CuACM-v and CuACM-a. The micropore size distributions (Fig. 3) for samples ACM-a, ACM-v and CuACM-a are shifted to the narrower micropores as compared to that in the activated carbon used as a precursor. Most probably, a great number of micropores have become accessible to the adsorptive as a result of carbon modification. The samples CuACM-v and ACM-v obtained under the same conditions (temperature regime, pH and vacuum) are expected to have similar micropore distributions. However, the micropore size distribution of CuACM-v is more similar to that of the unmodified activated carbon. This fact should be ascribed to blocking of the small-size micropores by the surface compounds
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Three main differences can be seen comparing the TPD spectra obtained under inert (Fig. 5) and oxidative (Fig. 6) conditions: (i) the desorbed amount of NO is much larger, when the adsorption is carried out in air flow; (ii) the maxima for all TPD peaks in argon flow (Fig. 5) are at 95–100 ◦ C, whereas those obtained in air (Fig. 6) have a maxima in the temperature range of 120–130 ◦ C; (iii) NO desorption under oxidative conditions is accompanied by desorption of significant amounts of CO2 and CO. When TPD spectra are obtained in argon flow, the release of a small quantity of carbon oxides commences just when almost the whole amount of NO is desorbed. When adsorption is carried out in the presence of oxygen, NO is adsorbed as three distinct surface species (see Fig. 6) with different thermal stabilities. The IR spectra obtained after NO adsorption in air (see Fig. 7) show intense bands of free-like nitrates (centered at 1384 cm−1 ) and chelated nitrates (centered at 1240 and 1562 cm−1 ). When NO adsorption is carried out in an argon medium, the IR spectra differ considerably. The band centered at 1384 cm−1 is very weak and appears in the spectra of ACM-v and ACMa samples only. Hence, it may be concluded that the presence of oxygen in the gas phase is very important for the process of nitrate formation. More likely, their formation follows a twostage NO oxidation pathway [12,14]. The first step involves the formation of adsorbed NO2 (a) species according to the reaction: NO + O(a) → NO2 (a) where O(a) is the adsorbed active oxygen upon the carbon surface. The second step is the transformation of NO2 (a) to NO3 groups by oxidation or disproportionation [12]: Fig. 7. IR spectra of the fresh and worked in different conditions (NO + Ar and NO + Air) samples: (a) obtained in vacuum ACM-v and CuACM-v; (b) obtained in air ACM-a and CuACM-a.
formed on the carbon. Although the sample CuACM-a contains the same amount of copper as the CuACM-v does, the distribution of the micropores suggests that blocking of the micropores occurs in the case of latter sample only. This explanation is supported by the values of the specific surface areas presented in Table 1. The modified samples ACM-v and ACM-a have the largest specific surface areas (larger than that of the original carbon). The specific surface area of CuACM-a has an intermediate value while sample CuACM-v has a specific surface area equal to that of the original carbon. As is evident from Table 2, the sorption capacities for all samples are higher when the adsorption of NO is carried out in air than is the case when argon is used as a carrier. This observation is in agreement with the results of Shirahama et al. [12]. Obviously, NO is adsorbed in an oxidized form and oxygen from the air takes part in the adsorption process. The adsorbed amounts of NO are equal to those obtained by integration from TPD concentration profiles for all samples. This evidences that there is no evolution of nitrogen oxides different from NO.
NO2 (a) + O(a) → NO3 2NO2 (a) → NO3 + NO, The maximum of the low temperature CO2 peak, as seen from Fig. 6, appears at approximately 110 ◦ C and coincides with the position of the shoulder on the left branch of the NO peak. The fact that the molar ratio between desorbed NO and CO2 is roughly 1:1 in the temperature range 70–100 ◦ C indicates that the low temperature TPD peak of CO2 results from the destruction of N-containing surface complexes (more likely nitrates), by the reaction: [C(NO3 )]type1 → NO + CO2 . A similar mechanism can also be proposed to explain the observed coincidence of the high temperature CO2 desorption peak with the shoulder of the NO desorption curve at 145–155 ◦ C: [C(NO3 )]type3 → NO + CO2 . Obviously, these [C(NO3 )]type3 species are thermally more stable. The main NO TPD peak observed at 120–130 ◦ C (see Fig. 6) for all the samples is accompanied by a CO desorption peak. It must be emphasized that no CO2 evolution has been detected
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in this particular case. The molar ratio between NO and CO is higher than 1:1, which suggests that the second TPD peak for NO is due to thermal destruction of different N-containing species such as: [C(NO3 )]type2 → NO + CO + CO∗ [C(NO3 )]type2 → NO + 2CO∗ , where CO* denotes surface oxygen-containing chemical groups. The presence of a copper phase on the carbon surface, as is evident from Fig. 6 and Table 2, does not change the adsorption mechanism but results in the increase of the adsorption capacity for NO when adsorption is carried out in air. In this case sample CuACM-a has the highest adsorption capacity towards NO. The adsorption of NO in an inert carrier (argon) proceeds through the formation of N-containing surface species which decompose at relatively low temperature (95 ◦ C) without producing carbon oxides (see Fig. 5). Because these species are formed by interaction of NO with O-containing sites of the carbon surface, the lack of carbon oxides in the products of the thermal decomposition is an indirect indication of the nature of these sites. More likely, they are stable O-containing chemical groups strongly bound to the carbon surface and interacting weakly with NO. The presence of a supported copper phase changes the adsorption capacity for NO adsorption in an inert medium only when the adsorbent is synthesized in vacuum. In this case nanosize clusters of Cu2+ ions connected with exchange interaction are formed on the carbon surface, as is evident from the EPR data (see Fig. 4a). More likely, the highest adsorption capacity in vacuum for sample CuACM-v is due to interaction of these species with NO. Elucidation of the role of these clusters in NO adsorption will be the subject of our future investigations. 5. Conclusion The adsorbed amount of NO is much larger when adsorption is carried out with air as a carrier, because NO chemisorption proceeds through a two-stage oxidation on the carbon surface. When adsorption is carried out in the presence of oxygen, NO is chemisorbed as three distinct N-containing surface species, which decompose to produce either NO and CO2 or NO and CO. Two types of nitrate groups (free-like and chelated) are formed on the carbon surface after NO adsorption in air, as is registered by IR spectroscopy. The desorption of CO and CO2 is a result of thermal decomposition of different N-containing surface species. Their chemical nature is not completely clarified.
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The presence of a supported copper phase does not change the mechanism of adsorption but can increase the adsorption capacity of the samples when adsorption proceeds in air. The technique of sample preparation is very important with a view to achieving a high sorption capacity towards NO. The copper-containing adsorbent prepared in vacuum has the highest adsorption ability among all samples when the carrier gas is inert (argon). The copper-containing sample prepared in air has the highest adsorption capacity in presence of oxygen. The special behaviour of sample CuACM-v towards NO adsorption in argon is probably due to the presence of small clusters of Cu2+ ions connected with exchange interaction. Their presence leads to the appearance of different N-containing species and enhance the adsorption. Acknowledgement The authors gratefully acknowledge Dr. R. Stoyanova from IGIC, BAS for providing the EPR data. References [1] H. Yamashita, H. Yamada, A. Tomita, Reaction of nitric oxide with metalloaded carbon in the presence of oxygen, Appl. Catal. 78 (1991) L1. [2] M.J. Illan-Gomez, A. Linares-Solano, L.R. Radovic, C. Salinas-Martinez de Lecea, NO reduction by activated carbons. 7. Some mechanistic aspects of uncatalyzed and catalyzed reaction, Energy Fuels 10 (1996) 158–168. [3] H. Teng, E. Suuberg, Chemisorption of nitric oxide on char. 1. Reversible nitric oxide sorption, J. Phys. Chem. 97 (2) (1993) 478–483. [4] M. Khristova, D. Mehandjiev, Conversion of NO on a Ni impregnatedactive carbon catalyst in the presence of oxygen, Carbon 36 (9) (1998) 1379–1385. [5] N. Stankova, M. Khristova, D. Mehandjiev, Catalytic reduction of NO with CO on active carbon-supported copper, manganese, and copper-manganese oxides, J. Coll. Interf. Sci. 241 (2) (2001) 439–447. [6] C. Orr, J.M. Dalla Valle, Fine Particle Measurements, Macmillan, New York, 1959. [7] D. Mehandjiev, E. Bekyarova, R. Nickolov, Micropore size distribution by a simplified equation, Carbon 32 (2) (1994) 372–375. [8] C. Morterra, M. Low, IR studies of carbons - IV. The vacuum pyrolysis of oxidized cellulose and the characterization of the chars, Carbon 23 (3) (1985) 301–310. [9] C. Sellitti, J. Koenig, H. Ishida, Surface characterization of graphitized carbon fibers by attenuated total reflection fourier transform infrared spectroscopy, Carbon 28 (1) (1990) 221–228. [10] M. Low, C. Morterra, IR studies of carbons - V. Effects of NaCl on cellulose pyrolysis and char oxidation, Carbon 23 (3) (1985) 311–316. [11] U. Zielke, K. Huttinger, W. Hoffman, Surface-oxidized carbon fibers: I. Surface structure and chemistry, Carbon 34 (8) (1996) 983–998. [12] N. Shirahama, S.H. Moon, K.-H. Choi, T. Enjoji, S. Kawano, Y. Korai, et al., Mechanistic study on adsorption and reduction of NO2 over activated carbon fibers, Carbon 40 (14) (2002) 2605–2611. [13] K. Hajiivanov, Identification of neutral and charged Nx Oy surface species by IR spectroscopy, Catal. Rev. -Sci. Eng. 42 (1 & 2) (2000) 71–144. [14] Y. Kong, C.Y. Cha, NOx adsorption on char in presence of oxygen and moisture, Carbon 34 (8) (1996) 1027–1033.
Low-temperature NO removal over copper-containing activated carbon P. Nikolov ∗ , M. Khristova, D. Mehandjiev Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, “Acad. G. Bonchev” str., bl. 11, 1113 Sofia, Bulgaria Received 25 January 2006; received in revised form 1 September 2006; accepted 5 September 2006 Available online 8 September 2006
Abstract Adsorbents based on a copper modified activated carbon were synthesized and investigated in respect to NO chemisorption at ambient temperature in both inert and oxidative media. To correlate the adsorption ability of the adsorbents with the preparation parameters the materials were characterized by N2 -physisorption, AAA, NO-TPD, XRD, XPS, EPR, IR and magnetic measurements. In the presence of oxygen, the NO chemisorption is higher, proceeds through two stages of oxidation on the carbon surface and forms three N-containing species, which decompose to produce either NO and CO2 or NO and CO. The presence of the copper phase does not change the mechanism of adsorption, but can increase the adsorption capacity. On the surface of the most active adsorbent obtained in vacuum – CuACM-v, small clusters of Cu2+ ions were observed related to the exchange interaction. © 2006 Elsevier B.V. All rights reserved. Keywords: Activated carbon; Chemically modified carbons; Chemisorption; Temperature programmed desorption
1. Introduction Nitrogen oxide is a widespread pollutant originating from internal combustion engines and industrial sources. The selective reduction of NO with different reducing agents is the main method for NO removal used nowadays. However, when NO is present in a low concentration in waste gases containing oxygen, this method yields unsatisfactory results. For these particular cases the chemosorption method seems more appropriate. Adsorbents for NO must possess a high specific adsorption capacity towards NO at temperatures close to the ambient. Activated carbon seems to respond to this requirement due to its very large specific surface area. It should be noted that activated carbon itself can reduce nitrogen oxides [1,2]. Yamashita et al. [1] have found that low nitrogen oxide concentrations can be efficiently removed with activated carbon as reducing agent. They investigated the C + NO reaction at 300–500 ◦ C on activated carbon modified with different metals (Cu, Ca and Ni). The results obtained have shown unambiguously that coppercontaining samples are the most active for NO removal. Teng and Suuberg [3] have investigated NO chemisorption on acti-
vated carbon at 50–200 ◦ C and have found carbon gasification to be negligibly small. According to these authors nitrogen oxide chemisorption occurs at relatively high temperatures and is accompanied by nitrogen evolution and formation of surface oxides. In a previous work [4] we have investigated the conversion of NO in the absence of a reducing agent in the gas phase over an activated carbon impregnated with nickel ions. The results revealed that in the presence of oxygen in the gas phase, the NO conversion efficiency was twice as high as that in the absence of oxygen. The catalytic reduction of NO with CO occurs at relatively low temperatures over Cu, Mn and Cu-Mn phases supported on activated carbon [5]. Therefore, it may be concluded that copper plays a promoting role with respect to this reaction. The present paper is aimed at studying the adsorption ability towards NO of copper-modified activated carbon at room temperature. Different preparation procedures were tested in order to increase the adsorption efficiency. 2. Experimental 2.1. Sample preparation
∗
Corresponding author. Tel.: +3592 9796312; fax: +3592 8705024. E-mail address: plamen [email protected] (P. Nikolov).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.09.004
The samples investigated were prepared by impregnation of activated carbon from apricot shells with various solutions
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P. Nikolov et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 295 (2007) 239–245
and subsequent thermal treatment. Two types of samples were prepared: (i) CuACM-v and CuACM-a, representing carbon promoted by a supported active copper phase and (ii) ACM-v and ACM-a consisting of modified carbon without a supported active copper phase. Samples CuACM-v and CuACM-a were impregnated with aqueous ammonia solution of malachite buffered with ammonium carbonate. The concentration of copper ions in the impregnating solution was 0.22 mol/l for CuACM-v and CuACM-a. Samples ACM-v and ACM-a were reference samples impregnated with aqueous solution of ammonium carbonate containing no copper ions. After impregnation the samples were calcined up to 150 ◦ C in two different regimes: in air (ACM-a and CuACM-a) and under vacuum of 10−3 Torr (ACM-v and CuACM-v). The letter “-v” or “-a” in the end of the sample’s name represents abbreviation of the media of calcinations (vacuum or air, respectively).
containing 1200 ± 30 ppm NO and Ar or air as a balance, respectively. After the adsorption, the samples were blown through isothermally for 10 min under Ar or air in order to eliminate physical adsorption at room temperature. For comparison, initial unmodified active carbon (AC) was investigated under the same conditions. The TPD spectra of NO, CO and CO2 were registered in argon or air flow (flow rate 440 ml/min). The composition of the carrier was chosen to be the same as in the adsorption process. The study proceeded at 25–200 ◦ C with a heating rate of 13 ◦ C/min. The CO and NO concentrations were measured using a UNOR 5N (Maihak, Germany) gas analyser. The concentration of CO2 was measured with an Infralyt 2106 (Germany) gas analyser. The data from the gas analysers were collected and processed using a CSY-10 microprocessor computer system. 3. Results
2.2. Sample characterization The specific surface areas of the samples were determined by the BET method using low-temperature nitrogen adsorption. The mesopore size distribution was obtained from the adsorption curve of the isotherm, using the procedure developed by Orr and Dalla Valle [6]. The copper content was determined by atomic absorption technique with Pye Unicam SP-90B. The infrared spectra in the region 4000–400 cm−1 were recorded on a Br˝uker model IFS 25 Fourier transform interferometer (resolution <2 cm−1 ) using KBr discs as matrices. The XPS studies were performed in an ESCALAB MkII (VG Scientific) spectrometer using an Al K␣ source with energy 1486.6 eV. The residual gas pressure in the analysis chamber was 10−7 Pa. The C1s peak (284.6 eV) was used as internal standard for calibrating the binding energies (BE ± 0.2 eV). Magnetic measurements were carried out in air with a magnetic apparatus constructed according to the Faraday method. The magnetic susceptibility (χ) was measured in the temperature range of 20–220 ◦ C with a magnetic field intensity varying from 2 × 103 to 10 × 103 Oe. X-ray powder diffraction technique (XRD) measurements were carried out using a DRON-3 apparatus with Cu K␣ radiation. EPR spectra were registered as a first derivative of the absorption signal in the temperature interval from −160 to 80 ◦ C using an ERS 220/Q instrument.
Table 1 presents the copper content in the samples and the specific surface areas as obtained by the BET method. The adsorption isotherms of the samples obtained by nitrogen adsorption at −196 ◦ C are shown in Fig. 1. According to BDDT classification, they correspond to type IV. The hysteresis loop is of H-3 type according to the IUPAC classification (type B according to de Boer), which is typical of activated carbons due to the presence of slit-shaped pores. The mesopore size distribution obtained Table 1 Specific surface area and copper concentration of the prepared samples and initial active carbon (AC) Samples
Cu (%)
S (m2 /g)
ACM-a ACM-v CuACM-v CuACM-a AC
0 0 0.54 0.59 0
604 606 581 590 579
2.3. Activity measurements The NO sorption activity of the samples was investigated by a catalytic flow apparatus in an isothermal flow reactor (quartz tube, i.d. 8 mm). The samples investigated were 0.3–0.8 mm fractions with an adsorption volume of 1 cm3 and mass of 0.35 g, The carrier gas was argon (purity 99.99 vol.%) or air (purity 99.9 vol.%) with a flow rate of 440 ml/min and a volume rate of 26 000 h−1 . The preliminary treatment of the samples included heating up to 150 ◦ C for 30 min in an argon or air flow depending on the conditions of the experiment. After that the reactor was cooled to room temperature and the samples were subjected to 1 h adsorption in two gas mixtures: A = NO + Ar and B = NO + air,
Fig. 1. Adsorption isotherms of the samples.
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Fig. 2. Mesopore size distribution of the samples.
from the adsorption branch of the isotherm is presented in Fig. 2. Fig. 3 shows the micropore size distribution calculated by the SE method [7]. The copper-containing samples (CuACM-v and CuACM-a) proved amorphous as shown by XRD analysis, which could be ascribed to the low copper content in them. XPS spectra of CuACM-v and CuACM-a samples in the Cu (2p) photoelectron region show the binding energies (BE) of the Cu (2p3/2 ) peaks characteristic of the Cu2+ oxidation state. The XPS studies do not evidence substantial phase composition difference between samples CuACM-v and CuACM-a and the BE values are close to that for CuO phase: Cu (2p3/2 ) BE = 934.1 eV and O(1s) BE = 529.8 eV. The magnetic measurements characterize the copper-containing samples as paramagnetic. The calculated magnetic moments are lower than expected from the amount of supported copper. This fact suggests that weak interaction between Cu2+ ions exists.
Fig. 3. Micropore size distribution of the samples.
Fig. 4. EPR spectra of the samples at different temperatures: (a) obtained in vacuum ACM-v and CuACM-v; (b) obtained in air ACM-a and CuACM-a, as well as the copper-ammonia precursor.
The EPR spectra of the samples at different temperatures are presented in Fig. 4(a and b). For the sake of comparison, the EPR spectrum of the frozen at −160 ◦ C aqueous solution of the copper complex used as precursor is also shown. A narrow symmetrical signal with g = 2.002 is observed in the EPR spectra of all samples at 80 ◦ C. According to the EPR study on carbonaceous materials, this signal can be assigned to paramagnetic radicals in the activated carbon. The EPR spectra of CuACM-v and CuACM-a samples display a nearly symmetrical signal with effective g = 2.05 which becomes wider with the decrease of the registration temperature. The latter fact shows that the signal originates from antiferromagnetically interacting non-isolated Cu2+ ions. At a lower registration temperature, another signal due to isolated Cu2+ ions is clearly resolved in the EPR spectra of sample CuACM-a. A new signal, shifted towards lower magnetic fields (g = 5.25), is visible in the EPR spectrum of CuACM-v only. This signal can be attributed to exchange coupled Cu2+ ions with an effective spin state S > 1/2. Table 2 presents the adsorption capacities of the samples with respect to NO at room temperature in both inert and oxidation environment. Fig. 5 illustrates the NO, CO and CO2 concentration TPD profiles obtained after adsorption of NO at room temperature in an argon carrier. The TPD presented spectra are obtained in an argon flow with raising the temperature from
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Table 2 Sorption capacity of the prepared samples towards NO Samples
NO adsorbed in NO + Ar flow (mol/g)
NO adsorbed in NO + air flow (mol/g)
CuACM-v CuACM-a ACM-v ACM-a
321.5 66.5 87.5 59.3
862.8 1349.0 477.4 781.0
ambient to 200 ◦ C. Three of the samples, i.e. CuACM-v (Fig. 5a) ACM-a (Fig. 5c) and ACM-v (Fig. 5d) exhibit NO TPD peaks at T = 95 ◦ C. Sample CuACM-v (obtained in vacuum) displays two more desorption peaks: at 110 and 130 ◦ C. At the latter temperature (about 130 ◦ C), the NO desorption peak for CuACM-a is also observed. The samples do not show TPD peaks for CO and CO2 . The evolution of small amounts of CO2 from all of the samples commences at temperatures above 120 ◦ C and is not connected with the process of NO desorption. Fig. 6 presents the TPD spectra (taken in air) after adsorption of NO in oxidative media (NO + air) at room temperature. The
evolution of NO, CO2 and CO for all four samples started at approximately 70 ◦ C. The maxima of the NO peaks observed are situated within the temperature interval 120–130 ◦ C, and two shoulders on both branches of the peaks are also seen at 100–110 ◦ C and 145–155 ◦ C, respectively. The NO desorption is accompanied by the evolution of significant amounts of CO2 and CO. The TPD spectra of CO2 have two desorption peaks with maxima within the ranges of 100–110 ◦ C and 140–160 ◦ C, respectively. The maxima of CO evolution are situated at 120–125 ◦ C. The IR spectra of the samples synthesized in vacuum and in air are shown on Fig. 7a and b, respectively. As can be seen, a large number of IR bands are registered, due to the presence of different chemical groups on the activated carbon surface. The most pronounced IR bands of the fresh samples are shown in Table 3 [8–11]. After NO adsorption some new bands appear in the IR spectra: (i) a very narrow and intense band centred at 1384 cm−1 (Fig. 7a and b), which is due to asymmetric stretching vibrations of free-like NO3 ions [11,12]; (ii) two bands centred at 1562 cm−1 and 1240 cm−1 , which we ascribe to chelated NO3 stretching [13].
Fig. 5. TPD spectra of NO, CO and CO2 for the worked samples (adsorption of NO with Ar): CuACM-v (a), CuACM-a (b), ACM-a (c) and ACM-v (d). Conditions: 40 ml/min Ar flow, temperature range 25–200 ◦ C, heating rate of 13 ◦ C/min.
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Fig. 6. TPD spectra of NO, CO and CO2 for the worked samples (adsorption of NO with air): CuACM-v (a), CuACM-a (b), ACM-v (c) and ACM-a (d). Conditions: 40 ml/min air flow, temperature range 25–200 ◦ C, heating rate of 13 ◦ C/min.
4. Discussion Table 3 Assignment of some IR bands, present in the spectra of the samples investigated IR bands (cm−1 )
Assignment
Ref.
3450 2955–2958 2921–2924 2851–2853 2349 1749 1733 1684 1652 1645 1576 1558–1560 1540 1463 1456 1260 1124 667
H-bonded OH stretching Aliphatic CHn stretching Aliphatic CHn stretching Aliphatic CHn stretching Atmospheric CO2 Carboxyl band Carboxyl band Ketone carboxyl band Carboxyls in a resonant system Bending vibrations of water in liquid phase Quinone structure Carboxylate functionalities Carboxyle groups in ionized configuration Aliphatic CH deformation modes Carbonate-like structures Single bond C–O structures of the ether-like type C–O stretching and O–H bending modes Atmospheric CO2
[8] [11] [11] [11] – [8] [9] [8] [8] – [9] [9] [8] [10] [10] [10] [9] –
The copper content on the activated carbon (Table 1) is relatively low. The specific surface area values of the modified and copper supported samples are very close. The mesopore size distributions for all the samples are also very similar (see Fig. 2), which indicates that copper promotion leads to no change in mesopore size distribution. This result is not unexpected in view of the low concentration of copper ions in samples CuACM-v and CuACM-a. The micropore size distributions (Fig. 3) for samples ACM-a, ACM-v and CuACM-a are shifted to the narrower micropores as compared to that in the activated carbon used as a precursor. Most probably, a great number of micropores have become accessible to the adsorptive as a result of carbon modification. The samples CuACM-v and ACM-v obtained under the same conditions (temperature regime, pH and vacuum) are expected to have similar micropore distributions. However, the micropore size distribution of CuACM-v is more similar to that of the unmodified activated carbon. This fact should be ascribed to blocking of the small-size micropores by the surface compounds
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Three main differences can be seen comparing the TPD spectra obtained under inert (Fig. 5) and oxidative (Fig. 6) conditions: (i) the desorbed amount of NO is much larger, when the adsorption is carried out in air flow; (ii) the maxima for all TPD peaks in argon flow (Fig. 5) are at 95–100 ◦ C, whereas those obtained in air (Fig. 6) have a maxima in the temperature range of 120–130 ◦ C; (iii) NO desorption under oxidative conditions is accompanied by desorption of significant amounts of CO2 and CO. When TPD spectra are obtained in argon flow, the release of a small quantity of carbon oxides commences just when almost the whole amount of NO is desorbed. When adsorption is carried out in the presence of oxygen, NO is adsorbed as three distinct surface species (see Fig. 6) with different thermal stabilities. The IR spectra obtained after NO adsorption in air (see Fig. 7) show intense bands of free-like nitrates (centered at 1384 cm−1 ) and chelated nitrates (centered at 1240 and 1562 cm−1 ). When NO adsorption is carried out in an argon medium, the IR spectra differ considerably. The band centered at 1384 cm−1 is very weak and appears in the spectra of ACM-v and ACMa samples only. Hence, it may be concluded that the presence of oxygen in the gas phase is very important for the process of nitrate formation. More likely, their formation follows a twostage NO oxidation pathway [12,14]. The first step involves the formation of adsorbed NO2 (a) species according to the reaction: NO + O(a) → NO2 (a) where O(a) is the adsorbed active oxygen upon the carbon surface. The second step is the transformation of NO2 (a) to NO3 groups by oxidation or disproportionation [12]: Fig. 7. IR spectra of the fresh and worked in different conditions (NO + Ar and NO + Air) samples: (a) obtained in vacuum ACM-v and CuACM-v; (b) obtained in air ACM-a and CuACM-a.
formed on the carbon. Although the sample CuACM-a contains the same amount of copper as the CuACM-v does, the distribution of the micropores suggests that blocking of the micropores occurs in the case of latter sample only. This explanation is supported by the values of the specific surface areas presented in Table 1. The modified samples ACM-v and ACM-a have the largest specific surface areas (larger than that of the original carbon). The specific surface area of CuACM-a has an intermediate value while sample CuACM-v has a specific surface area equal to that of the original carbon. As is evident from Table 2, the sorption capacities for all samples are higher when the adsorption of NO is carried out in air than is the case when argon is used as a carrier. This observation is in agreement with the results of Shirahama et al. [12]. Obviously, NO is adsorbed in an oxidized form and oxygen from the air takes part in the adsorption process. The adsorbed amounts of NO are equal to those obtained by integration from TPD concentration profiles for all samples. This evidences that there is no evolution of nitrogen oxides different from NO.
NO2 (a) + O(a) → NO3 2NO2 (a) → NO3 + NO, The maximum of the low temperature CO2 peak, as seen from Fig. 6, appears at approximately 110 ◦ C and coincides with the position of the shoulder on the left branch of the NO peak. The fact that the molar ratio between desorbed NO and CO2 is roughly 1:1 in the temperature range 70–100 ◦ C indicates that the low temperature TPD peak of CO2 results from the destruction of N-containing surface complexes (more likely nitrates), by the reaction: [C(NO3 )]type1 → NO + CO2 . A similar mechanism can also be proposed to explain the observed coincidence of the high temperature CO2 desorption peak with the shoulder of the NO desorption curve at 145–155 ◦ C: [C(NO3 )]type3 → NO + CO2 . Obviously, these [C(NO3 )]type3 species are thermally more stable. The main NO TPD peak observed at 120–130 ◦ C (see Fig. 6) for all the samples is accompanied by a CO desorption peak. It must be emphasized that no CO2 evolution has been detected
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in this particular case. The molar ratio between NO and CO is higher than 1:1, which suggests that the second TPD peak for NO is due to thermal destruction of different N-containing species such as: [C(NO3 )]type2 → NO + CO + CO∗ [C(NO3 )]type2 → NO + 2CO∗ , where CO* denotes surface oxygen-containing chemical groups. The presence of a copper phase on the carbon surface, as is evident from Fig. 6 and Table 2, does not change the adsorption mechanism but results in the increase of the adsorption capacity for NO when adsorption is carried out in air. In this case sample CuACM-a has the highest adsorption capacity towards NO. The adsorption of NO in an inert carrier (argon) proceeds through the formation of N-containing surface species which decompose at relatively low temperature (95 ◦ C) without producing carbon oxides (see Fig. 5). Because these species are formed by interaction of NO with O-containing sites of the carbon surface, the lack of carbon oxides in the products of the thermal decomposition is an indirect indication of the nature of these sites. More likely, they are stable O-containing chemical groups strongly bound to the carbon surface and interacting weakly with NO. The presence of a supported copper phase changes the adsorption capacity for NO adsorption in an inert medium only when the adsorbent is synthesized in vacuum. In this case nanosize clusters of Cu2+ ions connected with exchange interaction are formed on the carbon surface, as is evident from the EPR data (see Fig. 4a). More likely, the highest adsorption capacity in vacuum for sample CuACM-v is due to interaction of these species with NO. Elucidation of the role of these clusters in NO adsorption will be the subject of our future investigations. 5. Conclusion The adsorbed amount of NO is much larger when adsorption is carried out with air as a carrier, because NO chemisorption proceeds through a two-stage oxidation on the carbon surface. When adsorption is carried out in the presence of oxygen, NO is chemisorbed as three distinct N-containing surface species, which decompose to produce either NO and CO2 or NO and CO. Two types of nitrate groups (free-like and chelated) are formed on the carbon surface after NO adsorption in air, as is registered by IR spectroscopy. The desorption of CO and CO2 is a result of thermal decomposition of different N-containing surface species. Their chemical nature is not completely clarified.
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The presence of a supported copper phase does not change the mechanism of adsorption but can increase the adsorption capacity of the samples when adsorption proceeds in air. The technique of sample preparation is very important with a view to achieving a high sorption capacity towards NO. The copper-containing adsorbent prepared in vacuum has the highest adsorption ability among all samples when the carrier gas is inert (argon). The copper-containing sample prepared in air has the highest adsorption capacity in presence of oxygen. The special behaviour of sample CuACM-v towards NO adsorption in argon is probably due to the presence of small clusters of Cu2+ ions connected with exchange interaction. Their presence leads to the appearance of different N-containing species and enhance the adsorption. Acknowledgement The authors gratefully acknowledge Dr. R. Stoyanova from IGIC, BAS for providing the EPR data. References [1] H. Yamashita, H. Yamada, A. Tomita, Reaction of nitric oxide with metalloaded carbon in the presence of oxygen, Appl. Catal. 78 (1991) L1. [2] M.J. Illan-Gomez, A. Linares-Solano, L.R. Radovic, C. Salinas-Martinez de Lecea, NO reduction by activated carbons. 7. Some mechanistic aspects of uncatalyzed and catalyzed reaction, Energy Fuels 10 (1996) 158–168. [3] H. Teng, E. Suuberg, Chemisorption of nitric oxide on char. 1. Reversible nitric oxide sorption, J. Phys. Chem. 97 (2) (1993) 478–483. [4] M. Khristova, D. Mehandjiev, Conversion of NO on a Ni impregnatedactive carbon catalyst in the presence of oxygen, Carbon 36 (9) (1998) 1379–1385. [5] N. Stankova, M. Khristova, D. Mehandjiev, Catalytic reduction of NO with CO on active carbon-supported copper, manganese, and copper-manganese oxides, J. Coll. Interf. Sci. 241 (2) (2001) 439–447. [6] C. Orr, J.M. Dalla Valle, Fine Particle Measurements, Macmillan, New York, 1959. [7] D. Mehandjiev, E. Bekyarova, R. Nickolov, Micropore size distribution by a simplified equation, Carbon 32 (2) (1994) 372–375. [8] C. Morterra, M. Low, IR studies of carbons - IV. The vacuum pyrolysis of oxidized cellulose and the characterization of the chars, Carbon 23 (3) (1985) 301–310. [9] C. Sellitti, J. Koenig, H. Ishida, Surface characterization of graphitized carbon fibers by attenuated total reflection fourier transform infrared spectroscopy, Carbon 28 (1) (1990) 221–228. [10] M. Low, C. Morterra, IR studies of carbons - V. Effects of NaCl on cellulose pyrolysis and char oxidation, Carbon 23 (3) (1985) 311–316. [11] U. Zielke, K. Huttinger, W. Hoffman, Surface-oxidized carbon fibers: I. Surface structure and chemistry, Carbon 34 (8) (1996) 983–998. [12] N. Shirahama, S.H. Moon, K.-H. Choi, T. Enjoji, S. Kawano, Y. Korai, et al., Mechanistic study on adsorption and reduction of NO2 over activated carbon fibers, Carbon 40 (14) (2002) 2605–2611. [13] K. Hajiivanov, Identification of neutral and charged Nx Oy surface species by IR spectroscopy, Catal. Rev. -Sci. Eng. 42 (1 & 2) (2000) 71–144. [14] Y. Kong, C.Y. Cha, NOx adsorption on char in presence of oxygen and moisture, Carbon 34 (8) (1996) 1027–1033.