Complexity of ammonia interactions on activated carbons modified with V2O5

Complexity of ammonia interactions on activated carbons modified with V2O5

Journal of Colloid and Interface Science 325 (2008) 301–308 Contents lists available at ScienceDirect Journal of Colloid and Interface Science www.e...

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Journal of Colloid and Interface Science 325 (2008) 301–308

Contents lists available at ScienceDirect

Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Complexity of ammonia interactions on activated carbons modified with V2 O5 Camille Petit, Teresa J. Bandosz ∗ Department of Chemistry, The City College of New York and The Graduate School of CUNY, 160 Convent Ave, New York, NY 10031, USA

a r t i c l e

i n f o

a b s t r a c t

Article history: Received 4 April 2008 Accepted 2 June 2008 Available online 5 June 2008

A micro/mesoporous wood-based activated carbon was modified with different loadings of vanadium pentoxide via incipient impregnation with ammonium vanadate solution followed by heating in nitrogen at 500 ◦ C. The materials were used as adsorbents for ammonia. Both adsorption and desorption curves were recorded. The initial and exhausted samples were characterized by Fourier transform infrared spectroscopy (FTIR), potentiometric titration, thermal analysis and adsorption of nitrogen. An improvement in ammonia uptake compared to the virgin carbon was observed, and the adsorption capacity was found linearly dependent on the metal content. Water increases ammonia adsorption capacity via dissolution of the gas, but it also competes with ammonia because both of them are preferentially adsorbed on the same vanadium oxide sites (vanadyl oxygens). Even though an increase in the interactions strength between ammonia and the adsorbents’ surface has been reached compared to previous studies, some weakly adsorbed ammonia was still released from the surface during air purging. © 2008 Elsevier Inc. All rights reserved.

Keywords: Ammonia Activated carbon Adsorption Vanadium oxide Acidity Porosity

1. Introduction Environmental standards regarding the release of industrial toxic gases are constantly reinforced. Exposure to ammonia, a gas which belongs to that class of pollutants, has been limited by a time-weighted average (TWA) of 25 ppm and a short-term exposure limit (STEL) of 35 ppm ACGIH (American Conference of Governmental Industrial Hygienists) [1]. Those drastic measures are explained by the contribution of ammonia in particulate matter formation and thus air pollution [1], and also by its harmful potential for human beings [2]. To meet such standards, efficient adsorbents have to be designed. So far, researchers have explored different types of adsorbents such as zeolites, alumina and activated carbons [3–11], with a special interest granted to the latter ones. Indeed, activated carbons, due to their high surface area and pore volume, are considered as the most efficient adsorbents in pollutant removal [12]. It has to be noted that adsorbent efficiency in ammonia removal lies mainly in two different properties. Firstly, the adsorbent surface has to be acidic because ammonia gas is of basic nature. Secondly, the adsorbent must have pores similar in size to that of ammonia molecule, in order to provide strong adsorption forces able to retain this small gas molecule (about 3 Å [13]). The second factor depends mainly on the type of carbon selected as an adsorbent, and cannot be significantly modified by researchers, or at least it is not easily controlled. On the contrary, the surface acid-

*

Corresponding author. Fax: +1 (212) 650 6107. E-mail address: [email protected] (T.J. Bandosz).

0021-9797/$ – see front matter doi:10.1016/j.jcis.2008.06.001

©

2008 Elsevier Inc. All rights reserved.

ity of carbon can be modified and enhanced via carbon oxidation or impregnation with metal chlorides [4,14] or oxides [10,15,16]. In the latter case, ammonia adsorption is improved owing to the formation of new acid functional groups, coming from oxides, able to interact with ammonia via hydrogen bonding, and Lewis or Brønsted acid–base interactions. The latter ones were shown to be the most efficient for ammonia retention [6]. In our previous studies, we assessed the role of molybdenum and tungsten trioxides loaded carbons in ammonia retention [15, 16]. Improvement in ammonia adsorption was obtained with both oxides, but molybdenum trioxide was found more efficient. The ammonia uptake on those materials occurs either via its involvement in formation of ammonium salts by combination with metal oxides in the presence of water or via dissolution in water. Molybdenum, tungsten and vanadium oxides are often studied in oxidation processes due to their similar behavior [17,18]. In addition, vanadium pentoxide is widely used as a catalyst in selective catalytic reduction (SCR) of NO with NH3 because of its efficient interaction with ammonia molecule [19]. Yin and coworkers studied ammonia adsorption on vanadium pentoxide [6]. They showed that ammonia is preferentially adsorbed on Brønsted acid sites than on Lewis acid sites and especially on the vanadyl oxygen. Considering all of the above, vanadium pentoxide was introduced to the surface of meso/microporous carbons and the materials were tested as ammonia adsorbents. Changes in carbon porosity and acidity are analyzed and discussed considering their impact on ammonia uptake. Both the amount of ammonia adsorbed and the strength of the interactions between the gas and the adsor-

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bent surface are studied. To discuss the effect of modifications more generally, comparison with previous studies addressing the impregnation of the same carbon with other oxides is carried out. 2. Experimental 2.1. Materials Three different carbon samples loaded with vanadium oxide were prepared by impregnation on a wood-based carbon, BAX 1500 (Mead Westvaco), with a solution of ammonium metavanadate (NH4 VO3 ) containing oxalic acid. The metal contents chosen for impregnation are: 2.5, 5 and 6.25 wt%. This means, for instance, that in the case of the first sample, 25 mg of vanadium was present per gram of carbon adsorbent. This vanadium oxalate solution was prepared according to the method described by Economidis and co-workers [20]. The treatment applied involved incipient impregnation by adding to the carbon, the volume of solution equal to its pore volume. The samples were dried overnight at 120 ◦ C and then calcined at 500 ◦ C for three hours in a nitrogen atmosphere to remove ammonia and to form vanadium pentoxide as described in the equation 2NH4 VO3 → V2 O5 + 2NH3 + H2 O.

containing 0.4 g of carbon sample powder added to 20 ml of distilled water. 2.2.3. Thermal analysis Thermogravimetric (TG) curves were obtained using a TA instrument thermal analyzer. About 30 mg of carbon sample were submitted to a regular increase of temperature, from 30 to 1000 ◦ C, with a heating rate 10 ◦ C/min, while the nitrogen flow rate was 100 ml/min. Given the instrument sensitivity/precision, the errors made on the results are within a 2–6% range. From TG curves the DTG curves were calculated. 2.2.4. Structural parameters Nitrogen isotherms were measured at −196 ◦ C using an ASAP 2010 (Micromeritics). Prior to each measurement, samples were outgassed at 120 ◦ C. The surface area, S BET , the total pore volume, V t , the microporous volume, V mic (Dubinin–Radushkevich method [21]), and the mesoporous volume, V mes , were obtained from the isotherms. The pore size distributions (PSDs) were calculated using density functional theory (DFT) method [22]. Given the instrument sensitivity/precision, the errors made on the results are within a 5% range.

(1)

The three samples prepared as described are referred to as B-V1, B-V2 and B-V3 (“V” stands for vanadium, and 1, 2 and 3 represent increasing metal loadings being 2.5%, 5% and 6.25%, respectively). Another carbon sample, namely BAX-500, was prepared by calcination of the initial BAX carbon, at 500 ◦ C for three hours under a nitrogen atmosphere. This sample is expected to bring a distinction between the role of the inorganic oxides and the role of the heating process in ammonia adsorption. 2.2. Methods 2.2.1. Ammonia breakthrough capacity Adsorption capacity for removal of ammonia was measured in dynamic conditions, 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 the inlet gas was 450 ml/min with an ammonia concentration of 1000 ppm. 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 conditions were chosen to accelerate the test and limit the exposure of the sensor whose lifetime is relatively short. The ammonia concentration in the outlet gas was measured using an electrochemical sensor (Multi-Gas Monitor ITX system). The adsorption capacity of each sample was then calculated in mg per gram of adsorbent, 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. The experiments for all carbon samples were performed with a flow of ammonia gas diluted in moist air with and without a 2-hour prehumidification (70% humidity). On all samples, the desorption of ammonia was evaluated when exposed to 360 ml/min of dry air. In brief, for each sample, two different experiments were performed: one with ammonia diluted in moist air without a prehumidification step, and another one, in the same conditions but with an additional step of prehumidification. The references of the exhausted samples are respectively -EM and -EPM (M—moist, P— prehumidification). 2.2.2. Surface pH The pH of the initial carbon samples and the exhausted carbon samples was measured after an overnight stirring of a solution

2.2.5. 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, which means that during two minutes, no change in the pH measured was recorded. Subsamples of the materials studied of about 0.100 g in 50 ml 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 [23,24], assuming that the population of sites can be described by a continuous pK a distribution, f (pK a ). The experimental data can be transformed into a proton binding isotherm, Q, representing the total amount of protonated sites, which is related to the pK a distribution by the following integral equation:

∞ Q (pH) =

q(pH, pK a ) f (pK a ) dpK a .

(2)

−∞

Solving this adsorption integral equation using SAIEUS (solution of adsorption integral equation using splines) approach based on a regularization method [23,24] leads to the distributions of the pK a values of the species present on the surface of carbon. 2.2.6. FTIR FTIR spectroscopy was carried out using a Nicolet 380 spectrometer in a KBr medium, at room temperature, in the region of 4500–500 cm−1 . Samples were ground and then thoroughly mixed with KBr in an agate mortar before the spectrum was collected 84 times and corrected for the background noise. 3. Results and discussion Adsorbent surface acidity is an important parameter in ammonia uptake due to the basic character of this gas. As seen from Table 1, impregnation with vanadium oxide leads to a significant increase in the acidic character of the carbon surface (about 3 pH

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Table 1 Surface pH, tungsten and molybdenum content (M) for the virgin and modified carbons Sample

pH

Ash content (%)

Metal content (%)

BAX BAX-500 B-V1 B-V2 B-V3

6.01 6.38 3.17 3.15 3.01

3.47 4.18 7.99 13.45 14.63

– – 2.53 5.60 6.25

Fig. 2. pK a distributions for the virgin and modified carbons before exposure to ammonia.

Fig. 1. Proton binding curves for the virgin and modified carbons before exposure to ammonia.

unit decrease). This increase in acidity is almost evenly pronounced for each impregnated sample regardless its metal loading. This is likely the consequence of a clustering effect occurring for the most loaded samples. Unlike the impregnated samples, BAX-500 shows a slightly less acidic character than the virgin carbon owing to the decomposition of carboxylic groups, as a result of heat treatment [25]. The actual metal content of the impregnated samples was calculated based on their ash content. It was assumed that their ash contained the studied metal (vanadium) in addition to the compounds already present in the initial sample. Thus, taking into account the ash content of the initial carbon, and considering also that V2 O5 was the only specie formed during heating, the metal content was calculated in each case. The results, listed in Table 1, are in very good agreement with the expected values (2.5, 5 and 6.25 wt%). The biggest discrepancy is noticed for B-V2 and equals 6%. The amount of acidic groups for each sample can be assessed considering the proton binding curves plotted in Fig. 1. As seen from those curves, the amount of surface acidic groups for the loaded samples increases with their metal content. Indeed, deposited metal oxides provide additional groups on the surface of each loaded samples, and modify significantly the surface acidity. It has to be pointed out that oxidation of the carbon surface might happen due to the strong oxidizing character of vanadium pentoxide [26]. This effect might also increase the acidity of the carbon surface via formation of oxygen functional groups of acidic character [27]. Another way used to characterize surface acidity of the solid samples is their pK a distribution [23] (Fig. 2). The distributions of acidity constants of BAX and BAX-500 carbons show several peaks representing various oxygen-containing functional groups, corresponding mainly to phenolic (pK a > 8) and carboxylic groups

(pK a < 8) [28]. Slightly fewer groups are detected for BAX-500 in the range pK a < 8. This is expected owing to the decomposition of carboxylic groups during heat treatment [25]. Different pK a distributions are observed for the impregnated samples with apparition of 6 peaks (5 for B-V2). Those new peaks have pK a about 3.9, 4.5, 5.3, 6.9, 8.6 and 10.1. For B-V2, peaks at pK a = 4.5 and 5.3 are combined into one peak around pK a = 5.1. A precise attribution of each peak to a given functional group remains difficult because of the nature of the inorganic matter present, the influence of the carbon surface and the interactions between the different functional groups are still unknown. Nevertheless, these new pK a distributions are likely due to the combined effects of the functional groups initially present on the carbon, and the functional groups brought during the impregnation process. The latter are certainly the consequence of the various salts of vanadium oxides possibly formed, as for instance, the corresponding acids of orthovanadate (VO4 ), pyrovanadate (V2 O7 ), metavanadate (VO3 )n and decavanadate (V10 O68 ) [29]. DTG curves for the samples studied are plotted in Fig. 3. In the case of BAX sample, a continuous weight loss is observed between 150 and 500 ◦ C, as a result of carboxylic groups decomposition leading to the release of water and CO2 [25]. This behavior is not observed for BAX-500. Indeed, the derivative of weight loss is equal to zero for this sample in this range of temperature. This is because heating BAX at 500 ◦ C led to the decomposition of the carboxylic groups initially present. For both samples, the peak below 120 ◦ C is related to the removal of physically adsorbed water. We cannot interpret the samples’ behavior in the range above 600 ◦ C because the virgin BAX was not exposed to these temperatures (manufactured at about 600 ◦ C). For the impregnated samples, the most important feature of their DTG curves is the well-defined peak around 930 ◦ C, which might indicate the presence of V2 O5 [30]. Even though no certitude about the nature of that peak was found, it is likely that it corresponds to the reduction of V2 O5 by carbon upon heating under a flow of nitrogen, leading to the formation of carbon monoxide/carbon dioxide. Even though B-V3 has a higher metal content than BAX-V2, the intensity of the peak related to V2 O5 reduction is similar in those two cases. The reason for this can be the limitation in the sensitivity of our instrument and the relatively small difference in metal loading between B-V2 and B-V3 samples. Moreover, it is possible that samples are not perfectly homogeneous.

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Fig. 3. DTG curves for the virgin and impregnated carbons before exposure to ammonia.

Fig. 4. Pore size distributions (PSDs) curves in nitrogen for the virgin and modified carbons before exposure to ammonia.

Table 2 Structural parameters calculated from nitrogen adsorption isotherms at −196 ◦ C for the virgin and modified carbons Sample

S BET (m2 /g)

Vt (cm3 /g)

V meso (cm3 /g)

V mic (cm3 /g)

V mic / V t

BAX BAX-500 B-V1 B-V2 B-V3

2143 2062 1801 1513 1244

1.494 1.448 1.233 1.006 0.795

0.696 0.682 0.559 0.434 0.304

0.798 0.766 0.674 0.572 0.491

0.534 0.529 0.547 0.569 0.618

This would cause an apparent lower/higher loading than the calculated one (see above) which represents an average. The parameters of the porous structure also play an important role in retention of small gas molecules as ammonia. They are collected in Table 2, and the pore size distributions are plotted in Fig. 4. As seen in Table 2, the virgin carbon BAX has a high surface area and both micropores and mesopores are present. Heat treatment of that sample leads to a very slight decrease in the surface area and pore volume. This supports, once again, the fact that heating in nitrogen atmosphere, and at moderate temperatures, does not alter the surface of the sample to a large extent [16]. On the contrary, introduction of vanadium oxide to the surface leads to a significant decrease in all the structural parameters. These effects are the most pronounced for the mesoporous volume. Moreover, a higher metal content leads to greater changes as suggested by the gradual decrease in V mic / V t ratio. For B-V3, a decrease of about 75% is detected in the volume of mesopores compared to the virgin carbon. Those major alterations in the porosity of the impregnated samples are likely due to both a pore blocking effect via metal deposition and the strong oxidizing character of vanadium oxide. PSDs curves provide an additional insight into those wellmarked structural modifications and enable us to make a distinction between the different “zones” of porosity. Whereas pores with a width below 10 Å do not seem to be affected by the impregnation process, those with a larger width are significantly altered. Thus, it seems that the inorganic matter is deposited mainly in mesopores, blocking the pores of smaller diameter, and causes oxidation of the surface (causes some carbon from the pore walls to be consumed). The most pronounced changes are observed with B-V3 sample, which is expected based on its highest content of metal. Taking into account that vanadium pentoxide is formed from the impregnant salts and the size of those species, it is not

Fig. 5. FTIR spectra for the virgin and modified carbons before exposure to ammonia.

likely that they are present in pores smaller than 10 Å in their diameter. FTIR spectra of the initial samples before exposure to ammonia were also measured (Fig. 5), in order to get an insight of the nature of the functional groups and species present on the samples’ surface. Carboxylic groups are detected on the surface of the virgin carbon at 1710, 1595 and 1260 cm−1 [27]. Heat treatment of that sample in nitrogen is accompanied with slight changes in the FTIR spectra as for instance disappearance of the peak at 1710 cm−1 due to the decomposition of carboxylic groups. This is evidenced by a straight vertical line at 1710 cm−1 in Fig. 5 for BAX-500 spectrum. In the case of the impregnated samples, the intensity of this band also decreases.

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Table 3 Ammonia breakthrough capacity, amount of water preadsorbed, pH changes after ammonia adsorption, and molar ratio ammonia to metal Sample

NH3 breakthrough capacity (mg/g of carbon)

Molar ratio ammonia to metal

Water adsorbed (mg/g)

pH Initial

Exhausted

BAX-EM BAX-EPM BAX-500-EM BAX-500-EPM

8.3 ± 1.1 10.3 ± 1.3 6 .8 ± 0.9 9.8 ± 1.3

– – – –

– 332 – 407

6.01 6.01 6.38 6.38

7.23 7.51 6.70 7.54

B-V1-EM B-V1-EPM B-V2-EM B-V2-EPM B-V3-EM B-V3-EPM

29.1 ± 3.8 32.7 ± 4.3 38.0 ± 4.9 40.6 ± 5.3 44.4 ± 5.8 45.0 ± 5.6

1.345 1.510 0.877 0.938 0.821 0.832

– 393 – 460 – 306

3.17 3.17 3.15 3.15 3.01 3.01

6.32 6.43 6.64 6.75 6.70 6.59

Moreover, after impregnation, the wavelength range below 1000 cm−1 is modified and a new small peak is detected on B-V2 and B-V3 samples around 1000 cm−1 . It results from the stretching vibration of V=O [31]. Its intensity is higher for B-V3 due to its higher metal content. This peak is not observed for B-V1 likely because of the smaller metal loading of this sample and the high dispersion of the inorganic matter. It is also interesting to see that on the impregnated carbons, carboxylic groups are not present and no additional oxygen groups are detected. This indicates that acidic oxygen groups likely formed during impregnation with vanadium oxide via oxidation (noticeable heat release was observed) have been removed during heat treatment. An additional broad band is observed around 3200 cm−1 on the impregnated samples compared to the initial BAX and BAX-500. This new band is linked to O–H stretching vibrations, likely coming from water of hydration and hydrogen bonding created by the presence of new functional groups [32]. Based on the results reported above, heat treatment of BAX in nitrogen without impregnation does not significantly modify the acidity, structure and nature of the sample. Moreover, even though impregnation with vanadium oxide may have oxidized the carbon surface and created new acidic groups, those groups were removed during heating. That is why, in the following discussion, results on ammonia adsorption obtained on impregnated samples will only be attributed to the metal oxide deposited on the surface and won’t be related to heat treatment or oxidation effects. Adsorption capacities for all samples, measured at both conditions, are listed in Table 3 along with the amount of preadsorbed water and the surface pH values, before and after exposure to ammonia. A significant improvement in ammonia uptake is observed on the impregnated carbons even with the less loaded one. Without prehumidification, the adsorption capacities obtained are from 3 to more than 5 times higher than the ones measured for BAX. With prehumidification, this increase goes from 3 to more than 4 times the adsorption capacity of the virgin BAX. This improvement is comparable to the one obtained on BAX impregnated with molybdenum trioxide of similar metal loading discussed in a previous study [16]. Unlike what was found previously, prehumidification of carbons modified with vanadium species does not bring a significant improvement in ammonia retention. This might be a result of a competition between ammonia adsorption and water adsorption on vanadium oxide and differences in the nature of interactions and the chemistries of the reactions taking place on the surface. Indeed, even though water increases ammonia uptake via its dissolution in a water film, this enhancing effect can be diminished by a negative effect of water on ammonia adsorption caused by interactions of H2 O with vanadium sites which, otherwise, would be also sites for ammonia adsorption (competition). Yin and coworkers found that the most favorable site for ammonia retention on vanadium pentoxide is the vanadyl oxygen [6]. In an-

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other study, they showed that even though water can be adsorbed on different oxygen sites of vanadium oxide and also via hydrogen bonding with vanadium, the vanadyl oxygen appears also as the privileged site for water adsorption [33]. Moreover, they observed that in case of water dissociation, a proton (of water molecule) is transferred to an oxygen (of vanadium oxygen), and the remaining hydroxyl group (of water molecule) is attached to an exposed pentacoordinated vanadium site. This dissociation would be beneficial for ammonia adsorption as it leads to the formation of Brønsted sites (hydroxyl groups) able to interact with ammonia. Nevertheless, Yin and coworkers found this dissociation unlikely to occur. They justified this by the Coulombic repulsions existing between the oxygen atoms surrounding a vanadium atom and the oxygen from the hydroxyl group of dissociated water molecule. Consequently, it seems that most of the water molecules remain adsorbed on vanadyl oxygen and thus, might interfere with ammonia adsorption. However, the improvement in ammonia adsorption brought by water, via dissolution of the gas, seems to compensate the “competition effect” occurring and might explain that a slight improvement in the ammonia adsorption capacity is still observed after prehumidification. An increase in the metal loading, in both conditions (-EM and -EPM), results in a linear increase in the adsorption capacity with correlation coefficients over 0.99. It is interesting that the intercept for the experiments run without prehumidification is at 18.8 mg/g and, with prehumidification at 24.5 mg/g. Although the values are about 2.5 times greater than the corresponding values for BAX-500, the increase in the presence of water on the surface is similar and equal to about 30%. Even though one would consider that intercept as the capacity related to the carbon matrix without the inorganic matter, it cannot be directly compared to the virgin carbon owing to the observed changes in porosity (up to 75%) as a result of the deposition of the inorganic matter. These changes undoubtedly affect the performance of the materials as ammonia adsorbents. Another interesting parameter to consider is the molar ratio of ammonia adsorbed to metal. The average value of this ratio is 1 (Table 3). Moreover, two compounds involving ammonia and vanadium oxide are usually described in the literature: ammonium metavanadate and orthovanadate (NH4 VO3 and (NH4 )3 VO4 , respectively). The first one is the most often addressed [34]. Taking this into account and comparing the molar ratio of ammonia to metal in ammonium metavanadate and the one obtained in our experiment, it seems that NH4 VO3 was formed during ammonia exposure. This formation of ammonium metavanadate can be verified analyzing the FTIR spectra for samples run after exposure to ammonia (Fig. 6). For each exhausted sample, three new peaks are observed at about 1430, 940 and 840 cm−1 . Those three bands are related to the vibrations of NH+ 4 , V=O and V–O–V in ammonium vanadate [35,36]. The intensity of those three bands increases with the amount of ammonia adsorbed, which indicates that they represent a compound formed between the inorganic matter deposited on the carbons and NH3 . Besides this, the intensity of the broad band around 3200 cm−1 increases slightly. This can come from both the presence of more water adsorbed on the surface of the exhausted samples (since the experiment were run in moist air) and also the presence of ammonia via its N–H stretching vibration usually appearing in the range 3200–3600 cm−1 [32,37]. It is interesting to study the differences between the ratio of ammonia to metal, considering the metal content and the experimental conditions. Indeed, those two factors seem to have an influence on the amount of ammonia uptake. A decrease in that ratio with an increase in the metal content can be related to the clustering effect leading to less vanadium accessible for direct reaction with ammonia. Since prehumidification leads to an increase in the amount of adsorbed ammonia compared to experiments run

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Fig. 6. FTIR spectra after exposure to ammonia for (A) B-V1, (B) B-V2, and (C) B-V3.

Fig. 7. Ammonia breakthrough curves with their desorption part for (A) B-V1, (B) B-V2, and (C) B-V3.

without prehumidification, a part of the retained ammonia is likely adsorbed via its dissolution in water. The extent of this adsorption capacity enhancing effect decreases with the metal loading. This is likely because an increase in the metal content leads to a decrease in the volume of micropores (Table 2). Indeed, taking into account that water is mainly adsorbed in very small pores [38], a decrease in their volume results in a decrease in the amount of adsorbed

water and, consequently, less ammonia is adsorbed via dissolution in water for samples with the highest metal contents. Not only does the amount of ammonia adsorbed play an important role in the evaluation of an adsorbent’s efficiency, but the strength of the interactions between the gas and the adsorbent surface has to be considered as well. Indeed, weak interactions would result in a progressive release of the targeted gas, which is a significant “minus” of an adsorbent performance. The

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strength of those interactions can be assessed via analysis of desorption curves. In Fig. 7, adsorption and desorption curves for the impregnated samples have been plotted. It has to be noted that some parts of the desorption curves are not plotted since ammonia concentration in the outlet gas was higher than 100 ppm (sensor limit). This explains the gap observed between adsorption and desorption curves. Even though this gap is present for all samples, indicating weak interactions between the adsorbent surface and ammonia, it remains small and has been significantly shortened compared to the ones observed for the BAX carbon impregnated with molybdenum and tungsten oxides [16]. Thus the interactions in the case of vanadium oxide can be considered stronger. A faster decrease in ammonia concentration is found for samples run without prehumidification. Taking into account that the amount of adsorbed ammonia is similar with or without prehumidification, these results indicate that less ammonia was released during the desorption for samples run without prehumidification. This should be related to stronger interactions. A similar effect has already been observed for ammonia removal on the BAX carbon impregnated with molybdenum and tungsten oxides [16]. This is explained by the fact that, when prehumidification is performed, the amount of water present in large pores is bigger than without prehumidification. Consequently, more ammonia is dissolved in that water film. Since water located in large pores is easier to be removed by air purging than the one retained in small pores, more ammonia is detected as weakly adsorbed for samples run after prehumidification. An interesting behavior is observed for B-V1-EM sample for which, during the desorption step, the ammonia concentration decreases, increases and then decreases. This suggests that ammonia is adsorbed on centers of various strengths. Some of those centers are physically isolated because of the adsorbed species. Indeed, some ammonia or water adsorbed molecules can block the entrance of pores and thus the access to the centers present on their surface during the air purging. When the desorption proceeds, it is likely that the ammonia which is removed first is the one dissolved in water present in the relatively large pores (mesopores). This step corresponds to the first progressive decrease observed in ammonia concentration. With removal of that water (and consequently of that type of ammonia), other centers, initially “isolated,” get exposed to the desorption forces (via air purging). This, likely causes a release of ammonia molecules adsorbed on those sites, which might be Lewis sites of the vanadium oxide [6]. This step is related to the increase in ammonia concentration. Once those molecules have been released by air purging, the removal of water from smaller pores takes place. This causes a final decrease in ammonia concentration at the same rate as in the first step. This unusual pattern is not observed for the -EPM sample, likely because the prehumidification caused that only water was adsorbed on vanadium oxide Lewis centers. Lack of this phenomenon for samples with higher contents of vanadium oxide can be only apparent. This can be caused by the much higher amount of NH3 adsorbed than on B-V1 and the arbitrary time used for desorption run (desorption of ammonia was studied for 2 hours for each sample regardless its adsorption capacity). That time might be too short to see the addressed increased in concentration of desorbed ammonia. Indeed, as more ammonia was adsorbed on B-V2 and B-V3, it might take a longer time in those cases to “release” the initially “isolated” centers. Even though a part of ammonia adsorbed has been released during air purging, there is still a part of this pollutant retained on the adsorbent surface. This was first suggested by FTIR spectra of the exhausted samples where the presence of ammonium/vanadium salts is apparent. The significant increase in pH surface after exposure to ammonia (more than 3 pH units) also

307

Fig. 8. pK a distributions before and after exposure to ammonia for (A) B-V1, (B) B-V2, and (C) B-V3.

indicates the presence of these species on the samples surface (Table 3). Analysis of the pK a distributions for those exhausted samples provides an additional proof for the presence of ammonia on the adsorbent surface after desorption. As seen in Fig. 8, a new peak is detected after exposure to ammonia around pK a = 9.4. This peak is related to the presence of NH4 OH [30]. Its intensity increases with the amount of ammonia adsorbed.

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It is also interesting to notice that the intensity of the peak at pK a ∼ 8.5 in the initial samples decreased significantly after exposure to ammonia. This decrease is more pronounced for the samples having high adsorption capacities. This suggests that the species represented by that peak are likely involved in ammonia adsorption. Based on the work of Selling and coworkers [39], this 2− peak might represent H2 VO− 4 or H2 V2 O7 . Considering the molar ratio of ammonia to metal calculated from our experiments, the first compound is more likely to be the one involved in those strong interactions with ammonia. 4. Summary The results presented in this paper show an improvement in the amount of ammonia adsorbed and in the strength of interactions between ammonia and the adsorbent surface upon modification of a micro/mesoporous carbon with vanadium pentoxide. Compared to a previous analysis of the same carbon modified with molybdenum or tungsten oxide, the amount of ammonia released during the desorption step was significantly reduced, indicating stronger interactions between the gas and the adsorbent surface. Based on the desorption curves, the retention of ammonia on the surface is affected by the presence of adsorption sites of various strengths located in the pore system. The improvement in ammonia retention is partly due to the acidity increase caused by the presence of vanadium pentoxide and thus, formation of new acidic Brønsted sites in the presence of water being able to interact with ammonia. Moreover, water in the system enhances ammonia adsorption due to dissolution of the gas in a water film. Nevertheless, this enhancement seems to be affected by a competition between ammonia and water molecules for adsorption on vanadyl sites. Acknowledgments This work was supported by ARO grant W911NF-05-1-0537. The authors are grateful to Dr. Jacek Jagiello for SAIEUS software. References [1] [2] [3] [4] [5]

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