Interactions of Ammonia with the Surface of Microporous Carbon

Aug 4, 2007 - Camille Petit,†,‡ Christopher Karwacki,§ Greg Peterson,§ and Teresa J. Bandosz*,†. Department of Chemistry, The City College of ...
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J. Phys. Chem. C 2007, 111, 12705-12714

12705

Interactions of Ammonia with the Surface of Microporous Carbon Impregnated with Transition Metal Chlorides Camille Petit,†,‡ Christopher Karwacki,§ Greg Peterson,§ and Teresa J. Bandosz*,† Department of Chemistry, The City College of New York, 160 ConVent AVenue, New York, New York 10038, Ecole Nationale Supe´ rieure de Chimie de Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France, and the U.S. Army, Edgewood Chemical Biological Center, 5183 Blackhawk Road, Aberdeen ProVing Ground, Maryland 21010-5424 ReceiVed: March 14, 2007; In Final Form: June 19, 2007

The BPL carbon samples impregnated with chlorides of copper, nickel, and zinc were used as ammonia adsorbents in dynamic conditions with various amounts of water present in the systems. The initial and exhausted samples were characterized using adsorption of nitrogen, XRF, XRD, FTIR, SEM, and thermal analysis. The results indicate that metal chlorides are active centers for ammonia adsorption via formation of complexes. Water enhances the amount of ammonia adsorbed, promoting its dissolving into the film of water adsorbed in micropores and formation of NH4+ ions. Even though the pore system of activated carbon provides spaces for dispersion of metals, it still contributes significantly to the amount of ammonia adsorbed (about 80%) via dispersive forces. The most efficient adsorbent for ammonia removal is the carbon impregnated with copper. In this material the efficiency is governed by the amount of copper introduced to the carbon and its dispersion on the surface.

Introduction

TABLE 1: Content of the Impregnants in the Samples Studied

One of the main concerns of our modern-day world is a growing contamination caused by the release of chemicals into the environment. One of those chemicals is ammonia, which can also be classified as a toxic industrial gas. It is usually emitted from various industries such as agriculture, fertilizer, food and beverage, and rubber industries. The toxic effects on the human body of this colorless and irritant gas are related to its high water solubility. Indeed, because of this chemical property, ammonia can readily react with skin, eyes, and the respiratory system to form ammonium hydroxide. This exothermic reaction is capable of causing thermal injury such as skin burns. The American Conference of Governmental Industrial Hygienists (ACGIH) has limited exposure to ammonia to a time-weighted average (TWA) of 25 ppm and a short-term exposure limit (STEL) of 35.1 In addition, ammonia contributes to air pollution with particulate matter via formation of salts, and to photochemical smog via its oxidation to nitric oxides.2 A continuous reinforcement of environmental measures driven by detrimental environmental effects has motivated research activities in the field of ammonia removal via adsorption processes. Indeed, several sorbents such as zeolites, alumina, and activated carbons have shown promising properties.3-15 Some of them are commonly used in the industrial processes, as for instance granulated activated carbons (GAC) or powdered activated carbons (PAC) which are employed to purify waste streams.16,17 Even though activated carbons are considered as good and efficient adsorbents during initial uptake of ammonia, their * To whom correspondence should be addressed: Telephone: (212)650-6017. Fax (212) 650-6107. E-mail: [email protected]. † The City College of New York. ‡ Ecole Nationale Supe ´ rieure de Chimie de Montpellier. § Edgewood Chemical Biological Center.

sample

C-ZnCl2

C-NiCl2

C-CuCl2A

C-CuCl2B

salt added to virgin carbon percentage of metal chloride (wt %) percentage of metal (wt %) amount of metal (mmol/g)

ZnCl2

NiCl2‚6H2O

CuCl2‚2H2O

CuCl2‚2H2O

20.4

16.9

19.2

10.9

9.81

7.67

9.08

5.17

1.50

1.31

1.43

0.813

applications for ammonia removal are limited. This is caused by weak adsorption forces, which are employed in the removal process. Ammonia is a small molecule with a width of about 3 Å,9 and at ambient conditions it can be strongly adsorbed only in pores similar in size to its diameter. Since the majority of an average activated carbon pore is in the range of 10-20 Å, only the small fraction of adsorbent surface is utilized. Moreover, due to the weak adsorption forces (isosteric heat of ammonia adsorption on graphitized carbon black reaches 30 kJ/mol and is only slightly larger than its heat of vaporization (25 kJ/mol)18,19) ammonia easily desorbs from the surface when, for instance, the adsorbent is purged by air. All of the above indicate that in order to remove ammonia, specific forces must be employed such as hydrogen bonding, acid/base interactions, complexation, precipitation, etc. Thus, many studies focus on texture and surface modifications of the solids to improve their adsorption capacity for ammonia retention. Among these modifications are oxidation11,13,14 and impregnation with metal oxides12 or metal chlorides.4 Since ammonia is a basic molecule, the most straightforward way of surface modifications is introduction of acidic groups.20,21 Following this direction, Guo and co-workers,13 Mangun and

10.1021/jp072066n CCC: $37.00 © 2007 American Chemical Society Published on Web 08/04/2007

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Petit et al. ing of the adsorption process, which may lead to the design of more efficient adsorbents. Experimental

Figure 1. NH3 breakthrough curves for the initial BPL carbon.

co-workers,14 and LeLeuch and Bandosz11 showed that oxidation of carbonaceous material (activated carbon and activated carbon fibers) leads to strong chemical interactions between ammonia and some oxygen functional groups via hydrogen bonding. These strong and specific interactions represent a way to more efficient adsorption performance. Yi and co-workers, by studying the interactions of ammonia with V2O5 surface, indicated that, although both Brønsted and Lewis sites are important for ammonia retention,8 the former ones are much more energetically favorable. On them hydrogen-bonding plays a crucial role. As a result of this, ammonium species are formed when ammonia is adsorbed at hydroxyl groups containing the vanadyl oxygen. The important role of surface acidity was also indicated by Helminen and co-workers in their extensive study of ammonia adsorption equilibria on various adsorbents including zeolites, alumina, activated carbons, polymer resins, and charcoal.3 The mechanism of ammonia adsorption was also investigated on single-walled carbon nanotubes (SWNTs).22 The results indicated that NH3 interacts with the carbon surface via both its lone pair of electrons and its hydrogen atom. The latter is based on interaction with π-electons of SWNTs. Sharanov and co-workers4 also managed to improve ammonia removal by modifying porous alumina modified with chlorides of alkaline-earth metals. The salts used to impregnate alumina were BaCl2, CaCl2, and MgCl2. As a conclusion to their work, it was assumed that the improvement in the adsorption capacity, compared to the results obtained with unmodified alumina, was due to salt-ammonia interactions that result in ammonia complexes. It was also noticed that the adsorption capacity depends on the metal, and their best results were obtained with alumina modified with MgCl2. Strong interactions of ammonia with Cu-ZSM-5 zeolites were studied by Valyon and coworkers.10 They detected the formation of copper (II)-diammine chloride complexes thermally stable up to 540 °C. The interaction of ammonia with an adsorbent’s surface can also be enhanced by the presence of water.23 Indeed, L. M. Le Leuch and T. J. Bandosz11 observed that, when ammonia gas is diluted in moist air, ammonium ions can be formed and then interact with the acidic groups of activated carbons. In the present work, the removal of ammonia by activated carbons impregnated with different metal chlorides (ZnCl2, CuCl2, and NiCl2) has been studied. The objective is to test the influence of several experimental factors, such as the type of metal, the metal loading, and the presence of water in the system. From this investigation, we attempt to reach a better understand-

Materials. Four different carbons were studied as adsorbents of NH3. All of them were prepared by impregnation of virgin BPL carbon (Calgon Carbon) with either zinc (ZnCl2,), nickel (NiCl2·6H2O), or copper (CuCl2·2H2O) chloride. More precisely, the method applied involved incipient impregnation by adding to the carbon a volume of metal chloride solution equal to its pore volume. In the case of copper salts, two different amounts of chloride were used. The carbon samples are referred to as C-ZnCl2, C-NiCl2, C-CuCl2A, and C-CuCl2B, respectively. The content of metals in samples (provided by the manufacturer) is listed in Table 1. Methods. Ammonia Breakthrough Capacity. Adsorption capacity for removal of ammonia was assessed by carrying out dynamic tests at room temperature. In this process, a flow of ammonia diluted in air went through a fixed bed of a carbon sample. The total flow rate of inlet gas was 900 mL/min with an ammonia concentration of 1000 ppm. These arbitrary conditions were chosen to accelerate the test. The adsorbent’s bed contained granules of carbons with a size between 1 and 2 mm packed into a glass column. The size of the bed was 80 mm (high) × 10 mm (diameter). The ammonia concentration in the outlet gas was measured using a Multi-Gas Monitor ITX system. The adsorption capacity of each sample was then calculated in milligrams per gram of sorbent, as the difference between the inlet and outlet concentrations multiplied by the inlet flow rate, the breakthrough time, and the ammonia molar mass in the experimental conditions. To evaluate the influence of water, the experiments for all carbon samples were performed with a flow of ammonia gas diluted either in dry air (ED) or in moist air (70% humidity) (EM). Moreover, the experiments were run with and without a 2-h prehumidification (70% humidity). On all samples the desorption of ammonia was evaluated when exposed to 720 mL/min of dry air. The combination of all these experimental parameters led to four different experiments for each carbon sample. In two experiments, carbon samples were exposed to a flow of ammonia diluted in dry air with or without prehumidification. In these cases, the references of the exhausted samples are respectively: C-MCl2-EPD and C-MCl2-ED (M, referring to the metal; P, to prehumidification). For the two other experiments, ammonia gas was diluted in moist air with and without prehumidification. In these cases, the references of the exhausted samples are respectively: C-MCl2-EPM and C-MCl2-EM. Here, M refers to the experiments run in the presence of moisture. pH. The pH of the initial carbon samples and the exhausted carbon samples was measured after an overnight stirring of a solution containing 0.4 g of carbon sample powder added to 20 mL of distilled water. Thermal Analysis. TG curves were obtained using a TA instrument thermal analyzer. About 30 mg of carbon sample (initial and exhausted) were submitted to a regular increase of temperature with a heating rate 10 °C/min while the nitrogen flow rate was 100 mL/min. XRF Analysis. To determine the content of metals, XRF analyses were carried out on a Spectro 300 T from ASOMA Instruments Inc., equipped with a Ti-target X-ray tube. The tube voltage was set at 24 kV with a current of 8 lA. The count and warm-up times were respectively 40 and 4 s. The region of

NH3 Adsorption by Carbon Impregnated with Metal Chlorides

J. Phys. Chem. C, Vol. 111, No. 34, 2007 12707

Figure 2. NH3 breakthrough curves for the impregnated carbons.

interest (ROI) was between 7 and 11 keV, while a background correction was done between 12 and 17 keV. Sorption of Nitrogen. Nitrogen isotherms were measured at -196 °C using an ASAP 2010 (Micromeritics). Prior to each measurement, all samples were outgassed at 120 °C for the initial samples and 100 °C for the exhausted ones to avoid extensive decomposition of ammonium-containing compounds. Approximately 0.20-0.25 g of sample was used for these analyses. The surface area, SBET, (BET method), the microporous volume, Vmic, (Dubinin-Radushkevitch method, D-R),24 the mesoporous volume, Vmes, the total pore volume, Vt, were calculated from the isotherms. The pore size distributions (PSDs) were obtained using a DFT method, which is described elsewhere.25 XRD. X-ray diffraction measurements were conducted using standard powder diffraction procedure. Adsorbents were ground with methanol in a small agate mortar. The mixture was smearmounted onto the zero-background quartz window of a Phillips specimen holder and allowed to air-dry. Samples were analyzed by Cu KR radiation generated in a Phillips XRG 300 X-ray diffractometer. A quartz standard slide was run to check for instrument wander and to obtain accurate location of 2θ peaks. SEM. Scanning electron microscopy was performed on a DSM 940 cold field emission instrument. The accelerating voltage was 2000 V. Scanning was performed in situ on a carbon powder. FTIR. Fourier transform infrared (FTIR) spectroscopy was carried out using a Nicolet Magna-IR 830 spectrometer.

Results and Discussions The ammonia breakthrough curves for the initial and metalmodified carbons are collected in Figures 1 and 2. All of them have a similar shape, indicating fast kinetics of the interactions of ammonia with the adsorbent’s surface. Although the effect is not so obvious in the case of initial (Figure 1) or coppermodified carbons (Figure 2), an introduction of moisture, either on carbon or in the air stream to the system has a positive effect on the performance of adsorbents. The performance of adsorbents is summarized in Table 2 where the breakthrough values are listed in unit mass per mass of carbon, in millimoles of ammonia per mass of adsorbent and in unit mass per unit volume of the bed. The latter quantity is important for real-life conditions where the volume of the adsorber is considered as one of the limiting factors. In the case of the as-received carbon, moisture in the air stream does not affect the adsorption capacity. On the other hand, when water is preadsorbed on the carbon, the capacity increases almost 3 times (see C-EPD in comparison with C-ED). This is related to the surface chemistry of this material. The base titration indicated that only 0.2 mmol/g acidic groups are present on the surface,26 and the pH is slightly basic, close to neutral (Table 2). Such a small amount of acidic groups is not able to change the adsorption process even though some ammonium ions can be present in the challenging gas.11 The situation changes after prehumidification. At 70% humidity micropores should be filled with water to a significant degree.27 In that film, the small

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TABLE 2: NH3 Breakthrough Capacity, Amount of Adsorbed Water, and Surface pH sample

(mg/g of carbon)

NH3 breakthrough capacity (mmol/g of carbon)

(mg/cm3 of carbon)

water adsorbed (mg/g)

initial

C-ED C-EM C-EPD C-EPM C-ZnCl2-ED C-ZnCl2-EM C-ZnCl2-EPD C-ZnCl2-EPM C-NiCl2-ED C-NiCl2-EM C-NiCl2-EPD C-NiCl2-EPM C-CuCl2A-ED C-CuCl2A-EM C-CuCl2A-EPD C-CuCl2A-EPM C-CuCl2B-ED C-CuCl2B-EM C-CuCl2B-EPD C-CuCl2B-EPM

0.8 0.9 2.2 2.0 51.7 58.4 55.3 65.0 58.9 80.8 59.2 73.9 67.7 62.8 62.7 70.6 29.1 38.7 33.7 34.2

0.05 0.05 0.13 0.12 3.0 3.4 3.3 3.9 3.5 4.8 3.5 4.3 4.0 3.7 3.7 4.2 1.71 2.28 1.98 2.01

0.36 0.41 0.99 0.86 31.5 36.7 33.6 39.3 37.7 51.7 35.9 44.1 43.3 38.6 38.6 45.7 15.6 21.7 20.4 18.0

236 245 246 247 286 273 256 231 269 286

7.87 7.87 7.87 7.87 6.15 6.15 6.15 6.15 6.98 6.98 6.98 6.98 4.19 4.19 4.19 4.19 4.69 4.69 4.69 4.69

amount of carboxylic groups can dissociate and strongly interact with ammonium ions dissolved there. Introduction of metals in the form of chlorides significantly improves the performance of materials, and in some cases, as for C-NiCl2-EM, the capacity is almost 2 orders of magnitude higher than that for the initial BPL carbon. For every sample, the NH3 breakthrough capacity follows the trend: ED < EM (except for C-CuCl2A) and EPD < EPM. Thus, as observed before,11 water has a positive effect on the performance of metalmodified carbons. As noticed previously,4 complexes with ammonium may be involved in the reactive adsorption process. In the cases of C-ZnCl2 and C-CuCl2A, the adsorption capacity is the higher when water is present on the surface of carbon than in the air stream. A reverse effect is observed for C-NiCl2 and C-CuCl2B. For the latter two carbons, the amount of water adsorbed on carbon’s surface during the prehumidification step is greater than for all other carbons. It is an interesting observation since C-CuCl2B contains much less copper than C-CuCl2A (Table 1). That enhancement in water adsorption can be linked to the creation of some small pores similar in size to water molecules as a result of copper salt deposition. Apparently those pores have no effect on ammonia adsorption, which is expected to be enhanced mainly via specific forces. High adsorption of water on nickel salts must be related to hygroscopic properties of nickel chloride. Twice as much ammonia is adsorbed on C-CuCl2A as compared to C-CuCl2B and can be directly correlated to the differences in the copper chloride content (19.2 wt % for C-CuCl2A and 10.9 wt % for C-CuCl2B). It is important to mention that for C-CuCl2A the influence of water on adsorption capacity is not as strong as it is for other samples. A difference of about 12% separates the highest and lowest values of adsorption capacity, whereas more than 23% is noted for other carbons. Besides the ammonia uptake, the desorption of NH3 gas was also studied to evaluate the strength of the adsorption forces promoting retention of NH3 on the surface of our carbons. This study showed that, when the fixed carbon bed is exposed to a pure air flow directly after the adsorption process, ammonia is continually released, and its concentration remains higher than 100 ppm (limit of sensor sensitivity) even after 3 h of the desorption process. This behavior indicates that a significant amount of ammonia is weakly adsorbed on the sorbent’s surface

pH exhausted 7.92 7.44 7.51 7.41 7.10 7.28 6.99 8.12 8.60 8.48 8.50 8.65 9.04 8.56 8.72 8.84 8.47 8.64 8.63 8.41

via nonspecific van der Waals interactions. In such a case, the carbon bed works as a “concentrator” of ammonia for the gas stream. This feature is not favorable for the use of these adsorbents in dynamic beds with a constant flow of air. Since ammonia is a basic gas, the presence of acidic groups on the surface and thus the pH of the carbon surface should affect the removal process.28 Indeed, as expected, the metal impregnation decreased the pH, especially for copper-containing samples.4 After ammonia adsorption the pH values significantly increased, especially for samples impregnated with copper chloride, which suggests formation of new basic species and thus a strong effect of reactive adsorption. It is interesting that, even though the pH of C-CuCl2B is quite low and acidic, its capacity is the lowest. This indicates that there are other factors than surface acidity that govern ammonia removal on those materials. Those factors certainly include the porosity and texture of adsorbents.29 The structural parameters calculated from nitrogen adsorption isotherms are listed in Table 3. It clearly shows that the impregnation, as expected, results in a decrease in the surface area and pore volume between 10 and 50%. The effect is the most pronounced for C-CuCl2A and the least for C-CuCl2B, which is likely related to the amount of salt and its dispersion on the surface. If only dispersive forces took part in the adsorption process, the changes in porosity would be directly related to the adsorption capacity. Since this is not the case for reactive ammonia adsorption, one has to look at the trends cautiously, taking into account the complex chemistry of the adsorption systems. The details of the changes in porosity are seen in pore size distributions presented in Figure 3. While the effects of impregnation on zinc- and nickel-modified samples are similar, mainly micropores are affected in the C-CuCl2A sample, which suggests that copper salts block their entrance. On the other hand, in the case of C-CuCl2B still a significant volume of micropores is open for nitrogen molecules. This suggests that, in the case of the latter sample, CuCl2 is deposited in larger pores, which might be related to bigger sizes of its particles. Figure 4 compares the changes in porosity caused by reactive adsorption of ammonia. To analyze these changes we have to realize that besides physical adsorption of ammonia in small pores, which in fact should not affect the porosity since ammonia

NH3 Adsorption by Carbon Impregnated with Metal Chlorides

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TABLE 3: Structural Parameters Calculated from Nitrogen Adsorption Isotherms sample

SBET (m2/g)

Vt (cm3/g)

Vmeso (cm3/g)

Vmic (cm3/g)

Vmic/Vt

sample

SBET (m2/g)

Vt (cm3/g)

Vmeso (cm3/g)

Vmic (cm3/g)

Vmic/Vt

C C-ZnCl2 C-ZnCl2-ED C-ZnCl2-EM C-ZnCl2-EPD C-ZnCl2-EPM C-NiCl2 C-NiCl2-ED C-NiCl2-EM C-NiCl2-EPD C-NiCl2-EPM

1033 726 562 593 603 555 646 732 681 616 650

0.614 0.437 0.353 0.370 0.373 0.349 0.395 0.447 0.412 0.375 0.392

0.139 0.095 0.096 0.102 0.102 0.104 0.096 0.117 0.112 0.090 0.096

0.475 0.342 0.257 0.268 0.271 0.245 0.299 0.330 0.310 0.285 0.296

0.774 0.783 0.728 0.724 0.726 0.702 0.757 0.738 0.752 0.760 0.755

C-CuCl2A C-CuCl2A-ED C-CuCl2A-EM C-CuCl2A-EPD C-CuCl2A-EPM C-CuCl2B C-CuCl2B-ED C-CuCl2B-EM C-CuCl2B-EPD C-CuCl2B-EPM

565 552 648 580 558 914 801 778 839 846

0.345 0.339 0.387 0.353 0.339 0.539 0.471 0.458 0.494 0.498

0.095 0.089 0.090 0.092 0.085 0.124 0.109 0.092 0.096 0.098

0.250 0.250 0.297 0.261 0.254 0.415 0.362 0.366 0.398 0.400

0.725 0.737 0.767 0.739 0.749 0.770 0.769 0.799 0.805 0.803

adsorbed in this way is removed during outgassing, a significant part of the adsorbate is expected to react with metal chlorides and form complexes which should be retained within the pore system. The new solids, besides blocking the pores, can also lead to the formation of secondary porosity when deposited in large pores. In fact, this is the case for the C-NiCl2 and C-CuCl2A samples. For the latter samples, besides an increase in the volume of mesopores, a significant increase in the volume of micropores is also noticed for all experimental runs. That increase in porosity must be caused by the deposition of salt on the surface and it is found for samples on which the highest capacities were measured. For the C-ZnCl2 and C-CuCl2A samples, a decrease in the volume of pores is noticed after ammonia adsorption. These results indicate that, even though similar mole amounts of metals were present in all samples, their location on the surface, dispersion, and reactivity with ammonia are also important factors for the removal process. The first two factors determine the accessibility of ammonia and thus the efficiency of impregnation. The example SEM images presented in Figure 5 show the texture of BPL carbon and its impregnated counterparts. The white particles represent the inorganic phase. BPL is a coalbased carbon with about 10% of ash, so some particles of an inorganic phase are seen on its surface. Since the images before and after impregnation do not show too many differences, we assume that salts are mainly deposited within the pore system of carbon. After exposure to ammonia, larger particles are seen on the outer surface, especially in the case of carbon modified with copper, C-CuCl2A-EM (Figure 6). In the XRF spectra presented in Figure 7 the sharp peaks representing metal species on the surface of some samples can be seen.

Figure 3. Comparison of pore size distributions for the initial carbon and the metal impregnated samples.

Additional evidence of the importance of accessibility and thus dispersion of metal salts are X-ray diffraction patterns presented in Figure 8. Besides the broad hub related to scattering from amorphous carbon material, some sharp diffraction peaks can be seen representing specific metal chlorides added to the samples. Sharper and better-defined peaks correlate to (or indicate) a less-dispersed metal chloride. These differences in the dispersion are particularly seen for copper-containing samples. While the copper chloride seems to be well dispersed in the case of the C-CuCl2A sample, the well-defined peaks of CuCl2 are seen for C-CuCl2B sample. This is consistent with the SEM observations and with the differences in the behavior of samples as ammonia adsorbents. For the nickel and zinc samples no peaks representing the inorganic phase can be distinguished. After exposure to ammonia in moist conditions, the well-defined peaks appear, especially for C-CuCl2A sample, on which the higher amount of ammonia was adsorbed. These peaks represent the surface reaction products. Thus, on the surface of copper-containing samples the peaks at 2θ 16.1, 32.5, and 33.9 represent CuCl2‚2NH4Cl‚2H2O. C-ZnCl2-EM, ZnCl2 and ZnCl2‚2NH4Cl can be detected as peaks at 2θ 15.1, 27.5 and 15.5, 31.2 respectively. For nickel-containing samples, only NiCl2‚6H2O is clearly seen at 2θ 15.2 and 25.0. This indicates high dispersion of ammonium complexes with nickel that must be present on the surface. It has to be mentioned here that lack of clear assignment of all peaks or lack of peaks representing ammonia compounds does not mean that those species are not present on the surface. Their dispersion can be high and the signal can be weakened by the amorphous nature of an activated carbon support. On the other hand, in the case of all samples a few sharp peaks are revealed that we cannot link to the most common metal-ammonium complexes and that might be the products of surface reactions. An important aspect of reactive adsorption on the surface of solids is the nature of the compounds it forms with the metal chlorides deposited on the surface. The changes in surface chemistry were obvious even by visual observation of carbon beds during exposure to ammonia. In the case of C-CuCl2A, some “blue-green” pale spots appeared for all the experiments except the one performed in dry air (ED). These spots were present in a random and heterogeneous way in the whole carbon bed. They were not noticed in the case of C-CuCl2B on which much less ammonia was adsorbed. On the other hand, the carbon-containing nickel does not change its color. For the zinccontaining carbon the changes were very complex and depended on experimental conditions. The color of C-ZnCl2-ED was not affected at all. When the same sample was run in moist conditions, a gradual change in the bed color from black to gray was observed with time, and at the end of experiment the whole bed was gray with the exception of the bottom 1 cm (the reaction zone). For the experiments with the prehumidification, the color

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Figure 4. Pore size distributions for the impregnated carbons after NH3 adsorption.

of the bed also gradually changed from black to gray. After 1 h of experimental time 10 mm of the carbon bed was gray, whereas at the end only 25 mm of carbon (from the bottom) changed color to gray. These observations prove the engagement of ammonia into the formation of new compounds deposited on the surface. To check the strength of ammonia retention on the surface of our carbons, the DTG curves were analyzed in comparison with the curves obtained for the initial samples (Figure 9). The peaks represent the weight loss, and their surface area is directly related to the magnitude of that weight loss. DTG curves for all initial samples show two peaks: one located between 50 and 130 °C, and another one between 400 and 850 °C. While the first one represents the removal of physically adsorbed water, the position of maximum for the second peak varies, since it represents decomposition of specific chlorides used for surface modification.30 After exposure to ammonia, additional peaks appear in the range between 150 and 400 °C. They must represent the decomposition of new species formed on the surface. Consistent with this is a decrease in the intensity of chloride decomposition peaks, indicating partial involvement of chloride/metal in a new compound. In the case of the carbon modified with zinc chloride exposed to ammonia the new peak located between 130 and 400 °C likely represents a complex of ammonium tetrachlorozincate, ZnCl2‚2NH4Cl.30 The observation of white spots on carbon samples after ammonia adsorption supports this hypothesis. In the case of C-CuCl2A and C-CuCl2B two additional broad peaks are revealed for the exhausted samples between 130 and 210 °C and between 210 and 310 °C. They might be related to the presence of two ammonium-containing complexes: am-

monium copper chloride, 2NH4Cl‚CuCl2‚2H2O, and copper diamine chloride, Cu(NH3)2Cl2.30 This latter complex was encountered by Vaylon and co-workers in their work on ammonia adsorption onto Cu-ZSM zeolites.10 Those two complexes might explain the presence of green spots on the carbon samples after ammonia adsorption. The assignment of ammonium-containing complexes does not seem to be clear in the case of nickel-impregnated carbon exposed to ammonia. A new peak for this series of sample appears between 130 and 400 °C. Concerning the species represented by that peak, several hypotheses might be plausible. The first one would assume that hexahydrated nickel chloride NiCl2‚6H2O dehydrates in two steps, releasing first 5 mol of water and then 1 mol of water at mean temperatures of 100 and 200 °C, respectively, as demonstrated by Charles and coworkers.31 This hypothesis is supported by the fact that nickelimpregnated carbon was prepared from NiCl2‚6H2O salt. Yet, it does not explain why this process does not occur for the initial sample. Another hypothesis is the formation of either nickel chloride amine Ni(NH3)nCl2 (with n ) 1,2 or 6) or ammonium nickel chloride NH4Cl‚NiCl2‚6H2O.30,32 Indeed, those complexes might be considered as the counterparts for C-NiCl2 of the complexes found for zinc- and copper-impregnated carbons. Yet, those nickel complexes are respectively blue and green, and no spots of these colors were observed after ammonia adsorption. It is possible that the complexes were very well dispersed on the surface, and thus the black color of the carbon matrix “masked” their appearance. The presence of ammonia in both NH3 and NH4+ forms is seen in the FTIR results obtained for copper-containing samples presented in Figure 10. The surfaces of exhausted samples run

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Figure 7. XRF spectra for the initial and impregnated carbons.

Figure 5. SEM images for (A) virgin BPL carbon and the impregnated counterparts: (B) C-CuCl2A, (C) C-CuCl2B, (D) C-ZnCl2, (E) C-NiCl2.

Figure 8. XRD spectra for the initial and impregnated carbons.

Figure 6. SEM images for the samples exhausted in moist air. (A) C-CuCl2A-EM. (B) C-CuCl2B-EM. (C) C-ZnCl2-EM. (D) C-NiCl2EM.

in moist air are compared to the initial ones. After exposure to ammonia, new peaks at about 3340, 3255, 1415, and 1250 cm-1 appear. The first one is related to the presence of NH3, and more specifically to the N-H stretching modes.33,34 The latter peak represents the deformation of coordinated ammonia.34-36 The peaks at 3255 and 1415 cm-1 are assigned to the presence of ammonium ion. It has to be noticed that those peaks are not as well defined in the case of C-CuCl2B as they are for C-CuCl2A. This can be readily explained by the fact that less ammonia is adsorbed on C-CuCl2B. Some of the ammonium-containing complexes, which have just been discussed, can be thermally stable up to 200, 300, or even 400 °C, depending on the kind of metal present.30 They represent the specific interactions or reactive adsorption of ammonia with the carbon surface.11 The weaker interaction involves hydrogen bonding,11,12 formation of salts involving carboxylic groups, or just van der Waals interactions in small pores. The latter are expected to be very weak. Ammonia interactions with ammonium ions involved in the complex with metal are also considered as weak. Thus, based on thermal analysis, the amount of ammonia strongly adsorbed on the surface was calculated and compared with ammonia breakthrough capacity. The results are given in Table 4. To calculate this amount, all the above-mentioned complexes were taken into

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Petit et al.

Figure 9. DTG curves in nitrogen.

TABLE 4: Amount of Weakly and Strongly Adsorbed NH3 and the Ratio of Strongly Adsorbed NH3 to the Amount of Metal on the Surface amount of weakly adsorbed NH3 sample C-ZnCl2-ED C-ZnCl2-EM C-ZnCl2-EPD C-ZnCl2-EPM C-CuCl2A-ED C-CuCl2A-EM C-CuCl2A-EPD C-CuCl2A-EPM C-CuCl2B-ED C-CuCl2B-EM C-CuCl2B-EPD C-CuCl2B-EPM

Figure 10. FTIR spectra for the initial carbon, C-CuCl2A, C-CuCl2B, and their exhausted counterparts.

account. The nickel-containing sample was not analyzed since great uncertainty exists about the compound present on the surface. As seen from Table 4, the amount of NH3 strongly adsorbed is noticeably smaller than the NH3 breakthrough capacity. Thus, a relevant amount of ammonia interacts with carbon’s surface via weak and nonspecific interactions. In fact, this is consistent with the high concentration of ammonia in the outlet gas during the desorption process and an increase in

amount of strongly adsorbed NH3

(mg/g of (mmol/mol of (mg/g of (mmol/mol of carbon) metal) carbon) metal) 45.5 54.1 47.5 60.2 51.6 48.5 46.8 57.6 24.5 31.8 26.7 29.3

1784 2122 1863 2361 2123 1995 1925 2369 1773 2301 1932 2120

6.2 4.3 7.8 4.8 16.1 14.3 15.9 13.0 4.6 6.9 7.0 4.9

243 169 306 188 662 588 654 535 333 499 506 355

the intensity of the first DTG peak for the exhausted samples which must include some contribution of retained ammonia (desorption was run only for 3 h). That ammonia must be adsorbed in small pores where metal compounds are not present. When the ratio of NH3 breakthrough capacity and strongly adsorbed NH3 is analyzed, the C-CuCl2A sample can be considered as the best, with an average ratio of 4.5, compared to 10.6 for C-ZnCl2, and 5.9 for C-CuCl2B. As seen from Table 1, the amount of metal in C-ZnCl2 and C-CuCl2A are similar. Thus, the results obtained demonstrate that complexes involved in copper-modified carbon are the most inclined to form strong specific interactions with ammonia. Even though

NH3 Adsorption by Carbon Impregnated with Metal Chlorides for C-CuCl2B the ratio is larger than for the “A” sample, it is still similar to that obtained for C-CuCl2A. This is consistent with the same chemistry involved with each copper containing sample. The discrepancy can be linked to the degree of dispersion that differs for those two samples as indicated from the X-ray diffraction experiments. The comparison of the number of moles of strongly adsorbed NH3 to the number of moles of metal introduced to the surface definitely shows that the copper chloride samples have the highest efficiency for reactive adsorption. Although the average ratio is about 0.5 for copper-containing samples as opposed to 0.2 for the zinc-containing sample, these results indicate that only a fraction of metal is involved in the surface reaction. As mentioned above, this is related to its dispersion on the surface. To check if the volume of the pore (or pore volume) affects the removal process, the relationships between the amount of ammonia adsorbed, weakly and strongly, and the volumes of the pores were analyzed. Even though for all cases smaller pore volumes resulted in higher adsorption, the linear dependence with R2 over 0.9 was found only for the experiments run in dry air. In fact, in this case the “disturbance” in the physical adsorption of ammonia relates to the formation of ammonium ions, or dissociation of residual surface acidic groups should be minimized. The fact that there is no direct relationship between the amount adsorbed and the volume of the pore supports the importance of the metal species dispersed on the surface and the reactive adsorption process. On the basis of the results obtained we determined that the following reactions can occur on zinc chloride-modified carbon surface when exposed to ammonia. For the formation of ZnCl2‚NH4Cl (reaction 1), ammonia is in the form of ammonium ions which are obtained via reaction with water. In the case of

copper-modified carbons two compounds can be involved in the ammonia uptake. The first one, which is Cu(NH3)2Cl2, can be formed via a reaction of substitution, involving series of associative and dissociative processes. This reaction is summarized in reaction 2.

CuCl2 + 2NH3 a Cu(NH3)2Cl2

(2)

The other complex, 2NH4Cl‚CuCl2‚2H2O, is formed by reaction of copper chloride with ammonium ions as seen from reactions 3 and 4

As copper chloride is a hydrophilic compound, it can utilize any lone pair of electrons, and especially those of water (from

J. Phys. Chem. C, Vol. 111, No. 34, 2007 12713 the oxygen) to satisfy Lewis acid/base interactions as in reaction 4. For nickel-modified carbon, even though the nature of the

complex that enables ammonia uptake is not clear, two types of complexes are proposed: Ni(NH3)nCl2 and NH4Cl‚NiCl2‚ 6H2O. The formation of the first one could be explained as for Cu(NH3)2Cl2 complex, by series of associative and dissociative processes leading to the progressive substitution of water molecule by ammonia. In that case, the reaction could be represented as in reaction 5.

NiCl2 + nNH3 a Ni(NH3)nCl2

(5)

The other complex seems to be the average formula of various complexes of nickel similar to those on which copper was involved. Conclusions The results presented in this paper demonstrate the role of metal chlorides in the reactive adsorption of ammonia on the activated carbon surface. Even though the carbon surface is able to provide the space for metal dispersion, a still significant amount of ammonia is adsorbed very weakly in the carbon pore system, which is considered as a negative factor in application of carbons for ammonia removal in dynamic conditions. The strong adsorption is based on formation of complexes with metal chlorides. From the metals studied, copper provides the most efficient adsorption centers. The efficiency of these centers depends on metal dispersion. Acknowledgment. This work was supported by ARO Grant W911NF-05-1-0537. References and Notes (1) http://www.emedicine.com/EMERG/topic846.htm and www.inrs.fr (ammonia toxicological card). (2) Manhan, S. E. EnVironmental Chemistry, 7th Edition; Lewis: Boca Raton, 1999. (3) Helminen, J.; Helenius, J.; Paatero, E. J. Chem. Eng. 2001, 46, 391-399. (4) Sharonov, V. E.; Aristov, Y. I.; React. Kinet. Catal. Lett. 2005, 85, 183-188. (5) Domingo-Garcı`a, M.; Groszek, A. J.; Lo´pez-Garzo´n, F. J.; Pe´rezMendoza, M. Appl. Catal., A 2002, 233, 141-150. (6) Mangun, C. L.; Benak, K. R.; Daley, M. A.; Economy, J. Chem. Mater. 1999, 11, 3476-3483. (7) Park, So.-J.; Kim, B.-J. J. Colloid Interface Sci. 2005, 291, 597599. (8) Yin, X.; Han, H.; Gunji, I.; Endou, A.; Ammal, S. S. C.; Kubo, M.; Miyamoto, A. J. Phys. Chem. B 1999, 103, 4701-4706. (9) Thompson, J. C. J. Phys. ReV. A 1971, 4, 801-804. (10) Valyon, J.; Onyestyak, G.; Rees, L. V. C. J. Phys. Chem. B 1998, 102, 8994-9001. (11) LeLeuch, L. M.; Bandosz, T. J. Carbon 2007, 45, 568-578. (12) Stoeckli, F.; Guillot, A.; Slasli, A. M. Carbon 2004, 42, 16191624.

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