J. Phys. Chem. C 2007, 111, 16445-16452
16445
Role of Aluminum Oxycations in Retention of Ammonia on Modified Activated Carbons Camille Petit‡ and Teresa J. Bandosz*,† Department of Chemistry, The City College of New York, 160 ConVent AVenue, New York, New York 10038, and Ecole Nationale Supe´ rieure de Chimie de Montpellier, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France ReceiVed: May 28, 2007; In Final Form: August 13, 2007
Highly porous wood-based activated carbon was modified by impregnation with Keggin Al13 oxycations and calcined at 300 °C. This resulted in the formation of new acidic centers highly dispersed on the surface. The carbons were used as adsorbents of ammonia from the gas phase with different contents of water either on the carbon surface or in challenge air. The materials before and after exposure to ammonia were characterized using adsorption of nitrogen, potentiometric titration, XRF, XRD, FTIR, and thermal analysis. Even though modification with oxycations significantly increased the ammonia removal capacity of the carbons, a significant part of ammonia was weakly adsorbed and easily desorbed from the surface by air purging. The presence of water and Brønsted acidic centers favored ammonia binding on the surface as NH4+ ions. Alternatively, on the surfaces of calcined samples, where Lewis centers were formed, the interaction of ammonia via lone pairs of electrons with these centers seems to be the important mechanism of adsorption. This kind of interaction is apparently weaker than the Brønsted interaction.
Introduction Removal of ammonia from air is an important issue not only related to environmental problems caused by the role of ammonia in particulate matter formation1 but also related to toxic effects of ammonia on human beings.2 It is well known that these toxic effects are linked to its high water solubility. As a strong base, ammonium affects the respiratory system and skin via exothermic reaction leading to severe burning. ACGIH (American Conference of Governmental Industrial Hygienists) has limited exposure to ammonia to a time-weighted average (TWA) of 25 ppm and a short-term exposure limit (STEL) of 35 ppm.2 Besides this, being one of the major chemicals used in industry makes ammonia relatively easily available. Used in a confined space without proper protection, it can create a lethal threat to human beings. All of this has caused a continuous search for effective adsorbents that can be used in personal protection equipment, gas masks, and also applied as air filters in HVAC systems (heating, ventilation, and air conditioning).3 Sorbents such as zeolites, alumina, and activated carbons are described in the literature as having promising properties in air decontamination.4-16 Adsorbent surface features required to effectively remove ammonia at ambient conditions include very small pores and acidic nature of the surface. Activated carbons are adsorbents that are commonly used in personal protection equipment17 and HVAC filters.3 Their high surface area and pore volume18 are considered as valuable assets for the removal of pollutants. Although the volume of small pores is usually close to 1 cm3/g in the majority of activated carbons, their surface, as it is obtained after activation processes, does not have favorable features to retain ammonia. This is * To whom correspondence should be addressed. Tel: (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.
owing to weak physical forces, which are employed in the adsorption of this molecule. Because ammonia is a small molecule with a width of about 3.0 Å,19 at ambient conditions it is strongly adsorbed only in pores similar in size to its diameter. Taking into account that the majority of average activated carbon pores is in the range of 10-20 Å, the suitability of this surface to adsorb ammonia does not look very promising. The fact that the isosteric heat of ammonia adsorption on graphitized carbon black is 30 kJ/mol (slightly larger than its heat of vaporization (25 kJ/mol)20,21) contributes to the ease of ammonia desorption from the surface when the adsorbent is purged with air.22 All of the above indicate that in order to remove ammonia specific adsorption forces such as hydrogen bonding, acidbase interactions, or even surface chemical reactions such as complexation, precipitation, oxidation, and so forth must be considered. The only way to employ those forces for the removal of ammonia on carbonaceous adsorbents is via surface modifications of activated carbons. So far, these modifications have included oxidation12,14,15 and impregnation with metal oxides13 or metal chlorides.5,22 Oxidation of the carbon surface is the most-common way of surface modifications leading to the introduction of acidic groups.23,24 Some chemical interactions between ammonia and surface oxygen functional groups are formed,12,14,15 but they are not strong enough to prevent slow desorption of NH3 from the surface when the filter is purged with air.22 The presence of water was found beneficial in ammonia retention because it leads to acid-base interactions via the formation of ammonium ions.12 Studying the importance of surface acidity for ammonia adsorption, Helminen and co-workers4 found that the adsorption capacity is linked not only to the surface area but also to the chemical character of the surface. Another way of effective improvement of ammonia retention on the carbon surface is impregnation with metal chlorides,22
10.1021/jp074118e CCC: $37.00 © 2007 American Chemical Society Published on Web 10/06/2007
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Figure 1. XRF spectra for the BAX carbon and its modified counterparts.
TABLE 1: Surface pH Values and Ash Content for the Carbons Studied sample
pH
ash content (%)
BAX BAX-C BAX-C300
6.01 4.15 4.36
3.96 13.35 11.72
which leads to the formation of complexes of ammonia with metal chlorides and water, as observed previously on porous alumina modified with chlorides of alkaline-earth metals.5 The acid-base interactions involved in the retention of ammonia on adsorbents’ surfaces were investigated by Yin and co-workers.9 Even though they found that on the surface V2O5 both Brønsted and Lewis sites are important for ammonia adsorption, the former ones seemed to be much more energetically favorable for hydrogen-bonding interactions. When the mechanism of ammonia adsorption was investigated on singlewalled carbon nanotubes (SWNTs), the specific interactions were also indicated as a predominant mechanism of adsorption.10 The NH3 retention on the SWCNTs surface occurs via interactions of both a lone pair of electrons of ammonia and its hydrogen atom. The objective of this paper is to demonstrate the improvement in ammonia retention on the surface of activated carbons modified with Keggin Al13 oxyaluminium cations. The acidity of those species was studied extensively when they were used as pillars to improve the catalytic properties of clay minerals.25-27 Because the size of the Keggin cation is 9.8 Å,28 the highly porous wood-based carbon with the predominant volume of pores with sizes between 20 and 30 Å was used for this study. Introduction of Keggin cations not only decreases the hydrophobicity of activated carbon but also significantly increases its acidity. Moreover, that acidity can even be altered when Keggin cations are calcined at 300 °C. This process leads to the deposition of aluminum oxide-based species. It is expected that their high Lewis acidity25,26 should improve the retention of ammonia. The effects of oxycations’ loading, distribution, and acidity on the interactions with ammonia in dry and wet conditions are discussed. Experimental Section Materials. Two carbon samples containing oxyaluminum species were prepared by impregnation of BAX 1500 carbon
Petit and Bandosz (MeadWestvaco) with a solution of Chlorhydrol supplied by Reheis Chemical Company. Chlorhydrol is the trade name of solutions of hydroxyaluminium polycations of the following formula: Al2(OH)5Cl 2.5H2O. For the experiments, 10% solution of Chlorhydrol was used aged for 3 months. Upon aging, the Keggin Al13 cations of the following chemistry are expected to be formed: [Al13O4(OH)24(H2O)12]+7.26 The treatment applied was based on incipient impregnation by adding to the carbon the volume of Chlorhydrol solution equal to its pore volume. The sample was dried overnight at 120 °C and then calcined at 300 °C for 3 h in air atmosphere. The dry sample is referred to as a BAX-C, and the calcined one, as BAX-C300. Methods. Ammonia Breakthrough Capacity. The adsorption capacity for the 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 450 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 carbon 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 by purging the carbon’s bed with dry air (at 360 mL/min). 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, -EPD and -ED (P- 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, -EPM and -EM. 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) was submitted to a regular increase of temperature with a heating rate of 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 interest (ROI) was between 1 and 20 keV, with a background correction done in the same region. 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 to avoid
Role of Aluminum Oxycations in Ammonia Retention
Figure 2. DTG curves in nitrogen for the BAX carbon and its modified counterparts.
Figure 3. Proton binding curves for BAX, BAX-C, and BAX-C300.
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16447
Figure 5. FTIR spectra for BAX, BAX-C, and BAX-C300.
calculated from the isotherms. The pore size distributions (PSDs) were obtained using the DFT method, which is described elsewhere.30 Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode when the equilibrium pH was collected. Subsamples of the materials studied of about 0.100 g in 50 mL of 0.01 M NaNO3 were placed in a container thermostatted at 298 K and equilibrated overnight with the electrolyte solution. To eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The carbon suspension was stirred throughout the measurements. Volumetric standard NaOH (0.1 M) was used as the titrant. The experiments were done in the pH range of 3-10. Each sample was titrated with base after acidifying the sample suspension. The surface properties were evaluated first using potentiometric titration experiments.31,32 Here, it is assumed that the population of sites can be described by a continuous pKa distribution, f(pKa). The experimental data can be transformed into a proton binding isotherm, Q, representing the total amount of protonated sites, which is related to the pKa distribution by the following integral equation:
Q(pH) )
Figure 4. pKa distributions for BAX, BAX-C, and BAX-C300.
extensive decomposition of ammonia containing compounds. Approximately 0.20 to 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),29 the mesoporous volume, Vmes, and the total pore volume, Vt, were
∫-∞∞ q(pH,pKa) f(pKa) dpKa
(1)
The solution of this equation is obtained using the numerical procedure,32 which applies regularization combined with nonnegativity constraints. On the basis of the spectrum of acidity constants and the history of the samples, the detailed surface chemistry was evaluated. 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 allow 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. FTIR. Fourier transform infrared (FTIR) spectroscopy was carried out using a Nicolet Magna-IR 830 spectrometer using the attenuated total reflectance method (ATR). The spectrum was collected 16 times and corrected for the background noise. The experiments were done on the powdered samples, without
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Figure 6. Pore size distributions for BAX, BAX-C, and BAX-C300.
TABLE 2: Structural Parameters Calculated from Nitrogen Adsorption Isotherms sample
SBET (m2/g)
Vt (cm3/g)
Vmeso (cm3/g)
Vmic (cm3/g)
Vmic/Vt
BAX BAX-C BAX-C300
2176 1929 1566
1.519 1.324 1.036
0.701 0.598 0.438
0.818 0.726 0.598
0.539 0.548 0.577
KBr addition. Results and Discussion Introduction of hydroxyaluminium cations to the surface of BAX carbon has a pronounced effect on its acidity. As listed in Table 1, the surface pH decreased of almost two units from 6 to 4. This is accompanied by a significant increase in the ash content from 4% to over 13% as a result of deposition of aluminum oxycations. Calcination at 300 °C is expected to dehydrate and dehydroxylate the aluminum species,27 and this leads to a slight decrease in the load of inorganic matter. These changes in the chemistry of carbon are seen on XRF spectra presented in Figure 1. Although for the initial carbon iron and nickel seem to be the main ash components, after impregnation the intensities of their peaks decreased with an increase in the intensity of the peaks representing aluminum and chlorine. It has to be pointed out here that Chlorhydrol, which is expected to form Keggin cation upon aging,25-27,33-36 has a significant amount of chlorine in its chemical formula (Al2(OH)5Cl· 2.5H2O). The DTG curves collected in Figure 2 show the presence of acidic groups on the surface of BAX carbon revealed by a continuous weight loss between 150 and 500 °C representing the decomposition of carboxylic groups associated by a release of water and CO2.37 We do not interpret the peak over 500 °C because the original carbon was not exposed to these temperatures (obtained at about 600 °C). After impregnation with Chlorhydrol, a new intense peak located between 120 and 450 °C is noticed. It represents dehydration and dehydroxylation of Keggin Al13 oxycations.27 After calcination at 300 °C, this peak disappears. It is important to mention that the dehydroxylation process does not seem to be complete at 300 °C and still the decomposition of Keggin structure continues to higher temperatures.27 In this work, 300 °C was chosen to avoid additional oxidation of the carbon surface. The aging of Chlorhydrol was shown to lead to the development of the Keggin cation where 1 octahedral aluminum is
Figure 7. Ammonia breakthrough curves with the desorption parts for BAX (A), BAX-C (B), and BAX-C300 (C).
surrounded by 12 aluminum atoms in tetrahedral coordination.25,26 The OH groups present at various surroundings are expected to have different acid dissociation constants.38,39 The proton binding curves presented in Figure 3 show changes in the acidity of carbon upon modification. Although for the initial BAX carbon both proton uptake, related to basicity, and proton release, related to acidity, can be noticed, after impregnation with Chlohydrol, the surface becomes more acidic. The chemical character changes upon calcination. When compared to BAXC, a steep increase in the proton uptake at pH less than 5 and in the proton release at pH more than 9 can be clearly seen. These changes must be linked to the formation of aluminum oxide.26,27 From the proton binding curves, the pKa distributions of the species present on the carbon surface were calculated using the
Role of Aluminum Oxycations in Ammonia Retention
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TABLE 3: NH3 Breakthrough Capacity, Amount of Water Preadsorbed, and the Surface pH Values for the Samples Exposed to Ammonia at Various Conditions NH3 breakthrough capacity
sample BAX -ED BAX -EM BAX -EPD BAX -EPM BAX -C-ED BAX -C-EM BAX -C-EPD BAX -C-EPM BAX -C-300-ED BAX -C-300-EM BAX -C-300-EPD BAX -C-300-EPM
pH water (mg/g of (mg/cm3 adsorbed carbon) of carbon) (mg/g) initial exhausted 6.5 8.3 10.6 10.3 24.8 18.4 24.4 23.0 19.9 25.5 27.5 20.4
1.74 2.20 2.77 2.75 7.9 5.9 7.9 7.8 6.0 7.8 8.3 6.6
401 332 468 467 427 463
6.01 6.01 6.01 6.01 4.15 4.15 4.15 4.15 4.36 4.36 4.36 4.36
6.65 7.23 7.43 7.51 7.29 5.70 7.66 6.17 6.53 7.25 7.30 7.24
SAIEUS approach.32 The results are summarized in Figure 4. The surface of the initial BAX carbon is typical for the surface of a wood-based carbon obtained by chemical activation at low temperature.40 Various peaks in the pKa range between 4 and 11 represent carboxylic (pKa< 8) and phenolic (pKa > 8) groups.41 After the introduction of Keggin Al13 cations, the predominant peaks are those at pKa 4.9, 6.6, and 10. The intensities of the first (shifted to pKa 4.5) and last peaks increase significantly upon calcination. This is the result of the presence of acidic groups represented by (OH)-AlIV and (OH)-AlVI, respectively.27 Changes in the chemical character of the BAX carbon surface are also seen on FTIR spectra presented in Figure 5. For the BAX initial carbon, the bands at 1703, 1571, and 1150 cm-1 represent the vibration of oxygen in the carboxylic groups’ arrangements.24 After impregnation, the band representing vibration of OH groups between 3000 and 3500 cm-1 increased and decreased slightly after calcination, as expected. Besides this, the intensity of the band at about 1050 cm-1 increased after calcination, which is related to Al-O stretching in alumina (γ-Al2O3).42 Although impregnation with Keggin Al13 cations caused visible changes in the surface acidity of the BAX carbon, the textural changes are minimal. The pore size distributions calculated from the nitrogen isotherms are presented in Figure 6. The structural parameters are collected in Table 2. The presence of additional 10% of inorganic matter on the surface resulted in only about 10% decrease in the surface area and volume of micropores, which can be considered as a “dilution effect”. The most-pronounced change, a loss of about 15%, is seen in the volume of mesopores. As seen on PSD curves, pores smaller than 10 Å are apparently not affected. This is related to the 9.8 Å size of the Keggin Al13 cation,28 which is not able to enter those pores. The oxyaluminum species, highly dispersed, are likely located in larger mesopores. The pore blocking effect is not observed on the pore size distributions curves, yet we are aware that it could happen. In fact, as Al13 Keggin units deposit on mesopores, they may form simultaneously an additional microporosity. The presence of this additional microporosity must balance the weak pore blocking effect. That is why that effect is not detected even though it probably occurs. The inorganic phase must also be highly dispersed because no changes in X-ray diffraction patterns were observed for carbon before and after impregnation. The ammonia breakthrough curves along with the desorption curves measured at various conditions on the BAX carbon and its modified counterparts are presented in Figure 7. The
Figure 8. Comparison of the DTG curves for the initial and exhausted samples: BAX (A), BAX-C (B), and BAX-C300 (C).
calculated capacities along with the amount of water preadsorbed (depending on the experimental conditions) and the changes in the surface pH after exposure to ammonia are summarized in Table 3. Although for the initial BAX carbon, the presence of moisture, especially water preadsorbed on the surface, visibly improves the capacity with an about 1.5 pH unit increase in the surface pH as a result of ammonium hydroxide formation and its reaction with surface functional groups,12,22 after modification with Keggin Al13 oxycation water supplied to the system does not have an enhancing effect on the capacity. This, along with the high water content in the oxycation, suggest that water involved in the Keggin Al13 structures provides enough Brønsted acidic centers for reactions with ammonia and its binding via acid-base interactions and the acceptance of a
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Figure 10. Comparison of the pKa distributions for the initial and exhausted samples: BAX (A), BAX-C (B), and BAX-C300 (C). Figure 9. Comparison of the FTIR spectra for the initial and exhausted samples: BAX (A), BAX-C (B), and BAX-C300 (C).
proton from water by ammonia. Nevertheless, the amount of water preadsorbed during prehumidification increases slightly for the modified samples as a result of increased surface hydrophilicity. On all modified samples, the pH increases from 2 to 3 pH units after exposure to ammonia. Calcination of surface and partial conversion of Keggin Al13 oxycations to aluminum oxide species result in a slight increase in the capacity when moisture is present in the system. When the experiment is run in dry conditions, the lowest capacity is measured for the calcined samples, even though it is still three times higher than that on the unmodified sample. Once again, this indicates the importance of water and Brønsted centers for ammonia
retention. In dry conditions, the main adsorption mechanism can be linked to the interaction of lone pairs of electrons from ammonia with surface Lewis acidic centers and the hydrogen bonding of ammonia with surface oxygen associated with aluminum oxides. An increase in the ammonia adsorption after calcination must be related to an increase in the number and strength surface acidic groups linked to (OH)-AlIV and (OH)AlVI. The differences in the mechanism of ammonia adsorption are also seen on the shape of the breakthrough curves. Although for BAX and BAX-C the curves are rather steep and the concentration reaches 100 ppm almost immediately after ammonia is detected in the outlet gas, in the case of the calcined samples the slope of the curves decreases gradually, suggesting a more-complex chemistry involved in surface reactions.
Role of Aluminum Oxycations in Ammonia Retention Although in all cases the desorption curves indicate rather weak adsorption, some differences in the strength of this process can be deducted from the experimental results. For all BAX-C runs, the concentration is over 30 ppm for over 200 min, whereas for BAX-C300, less than 30 ppm of ammonia is detected after about 100 min. This suggests that more ammonia is strongly adsorbed in the case of samples where aluminum oxide provides strong surface acidity. Comparison of DTG curves for the initial samples and those exposed to ammonia presented in Figure 8 can provide some insight into the mechanism of adsorption. It has to pointed out here that the curves are measured on the samples on which desorption was already run, so the ammonia present on the surface is only the one that can be considered as strongly adsorbed. The first peak centered at about 100 °C represents the removal of physically adsorbed water, and its intensity is high on samples on which prehumidification was performed. The second peak located between 150 and 250 °C must be related to the removal of ammonia because its intensity follows the trends in the breakthrough capacity listed in Table 3. For the BAX samples, the peak related to ammonia is not seen. The FTIR spectra presented in Figure 9 show the presence of ammonium ions on the exhausted samples after desorption as a small peak at 1415 cm-1 (deformation mode) along with a broad band at 3255 cm-1 (N-H stretching).43 On the spectra of samples run after prehumidification, a significant increase in the OH stretching vibration is seen between 3000 and 3500 cm-1. No marked changes are seen on the bands representing oxygen-containing groups of carbons or alumina. The pKa distribution for the species present on the surface of carbons after exposure to ammonia and desorption of weakly adsorbed species are collected in Figure 10. In the case of BAX, heterogeneous surface chemistry can be seen with peaks at pKa less than 8 associated to carboxylic acid and with pKa greater than 8, to phenolic groups.41 After ammonia adsorption and desorption, owing to the small amount adsorbed, the surface does not change significantly. For the BAX-C series of samples, the amounts of strongly acidic groups with pKa less than 7 decrease suggesting their involvement in reaction with ammonia. Alternatively, the intensity of the peak at pKa about 9 increases significantly. These groups are associated with the presence of ammonium hydroxide.44 For one sample, BAX-C-ED, the peak at pKa about 10 also increases, which we cannot explain at this stage of our study. In the case of the calcined samples exposed to ammonia, a decrease in the amount of acidic groups having pKa < 8 is also found. This is accompanied by only a slight increase in the intensity of the peak at about 10, especially for the samples run in dry conditions. Although some amount of NH4+ ions is detected on the surface, their quantity is definitely smaller than that in the case of the sample with Keggin Al13 oxycations without any dehydration/dehydroxylation stage. This suggests that significant amount of ammonia interacts with Lewis centers of alumina without protonation involved. On the basis of the results presented and changes in the chemical nature of the deposit, the following reactions are proposed to occur in dry conditions: For BAX-C:
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16451 For BAX-C300: At first, dehydration and formation of Lewis sites occurs due to the calcination step:
Then, ammonia reacts with those sites:
The following reactions are proposed between ammonia and the carbon surface in wet conditions: For BAX-C: If we assume that Al13 Keggin units can be compared to molybdenum or tungsten Keggin heteropolyacids, then some protons should reside in the bridging water moieties forming H5O2+ species.45 In that case, the interactions between ammonia and OH groups from Al13 Keggin units would be similar to the previous ones (in dry conditions). Yet, they would be fewer because some of them would be involved in other interactions (formation of H5O2+). For BAX-C300:
The main observations that let us believe that Brønsted centers form stronger interactions with ammonia are the potentiometric titrations and the DTG curves. First, regarding the potentiometric titration, the BAX-C300 sample shows a higher amount of functional groups than the noncalcined sample. Yet, this sample does not have a better adsorption capacity than the BAX-C sample. This could mean that a notable number of functional groups are not used efficiently in ammonia retention on this sample. In other words, they do not interact strongly with the gas studied. Moreover, if we take a look at the DTG curves then the peak attributed to the release of ammonia is better defined in the case of the BAX-C sample (greater weight loss). Thus, ammonia interacts more strongly with the surface of this sample, which is supposed to provide only Brønsted acidic centers (compared to BAX-C300, which has also Lewis acidic centers). Conclusions The results presented in this paper show the effectiveness of acidic centers associated with aluminum oxycations in retention of ammonia on modified activated carbons. Although both Brønsted and Lewis centers are active in ammonia adsorption, the former ones seem to provide stronger adsorption centers. When they are present, ammonia is adsorbed on the surface in its protonated form. On Lewis acidic centers, a lone pair of electrons is likely involved in interactions of ammonia with (OH)-AlVI and (OH)-AlIV. This results in weaker adsorption forces. In the case of activated carbons with relatively large micropores and small mesopores, the Keggin Al13 cations can
16452 J. Phys. Chem. C, Vol. 111, No. 44, 2007 be evenly dispersed on the surface. They increase the acidity and thus the potential for specific interactions. Calcination of Al13 polycations leads to dehydration and dehydroxylation processes, which result in the formation of more Lewis adsorption centers associated with alumina. Acknowledgment. This work was supported by ARO grant W911NF-05-1-0537. We are grateful to Dr. Jacek Jagiello for SAIEUS software. References and Notes (1) Manahan, S. E. EnVironmental Chemistry, 7th ed.; Lewis: Boca Raton, FL, 1999. (2) http://www.emedicine.com/EMERG/topic846.htm and www.inrs.fr (ammonia toxicological card). (3) Przepiorski, J. Activated Carbon Fibers and Their Industrial Applications. In ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T.J., Ed.; Elsevier: Amsterdam, 2007; pp 421-474. (4) Helminen, J.; Helenius, J.; Paatero, E. J. Chem. Eng. 2001, 46, 391. (5) Sharonov, V. E.; Aristov, Y. I. React. Kinet. Catal. Lett. 2005, 85, 183. (6) Domingo-Garcı`a, M.; Groszek, A. J.; Lo´pez-Garzo´n, F. J.; Pe´rezMendoza, M. App. Catal., A 2002, 233, 141. (7) Mangun, C. L.; Benak, K. R.; Daley, M. A.; Economy, J. Chem. Mater. 1999, 11, 3476. (8) Park, So.-J.; Kim, B.-J. J. Colloid Interface Sci. 2005, 291, 597. (9) Yin, X.; Han, H.; Gunji, I.; Endou, A.; Ammal, S. S. C.; Kubo, M.; Miyamoto, A. J. Phys. Chem. B 1999, 103, 4701. (10) Ellison, M. D.; Crotty, M. J.; Koh, D. K.; Spray, R. L.; Tate, K. E. J. Phys. Chem. B 2004, 108, 7938. (11) Valyon, J.; Onyestyak, G.; Rees, L. V. C. J. Phys. Chem. B 1998, 102, 8994. (12) LeLeuch, L. M.; Bandosz, T. J. Carbon 2007, 45, 568. (13) Stoeckli, F.; Guillot, A.; Slasli, A. M. Carbon 2004, 42, 1619. (14) Guo, J.; Xu, W. S.; Chen, Y. L.; Lua, A. C. J. Colloid Interface Sci. 2005, 281, 285. (15) Mangun, C. L.; Braatz, R. D.; Economy, J.; Hall, A. J. Ind. Eng. Chem. Res. 1999, 38, 3499. (16) Le Leuch, L. M.; Subrenat, A.; Le Cloirec, P. EnViron. Technol. 2005, 26, 1243. (17) Lodewyckx, P. Adsorption of Chemical Warfare Agents. In ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz, T. J., Ed.; Elsevier: Amsterdam, 2006. (18) Bandosz, T. J. ActiVated Carbon Surfaces in EnVironmental Remediation; Bandosz T. J., Ed.; Elsevier: Oxford, 2006; p 231.
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