Combined Role of Water and Surface Chemistry in Reactive

Dec 23, 2009 - On the other hand, water screens the accessibility of epoxy and −COOH .... (POP) as a high-capacity sorbent for representative chemic...
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Combined Role of Water and Surface Chemistry in Reactive Adsorption of Ammonia on Graphite Oxides Mykola Seredych and Teresa J. Bandosz* Department of Chemistry, The City University of New York, 160 Convent Avenue, New York, New York 10031 Received October 1, 2009. Revised Manuscript Received November 8, 2009 Graphite oxide synthesized using the Brodie method was tested for ammonia adsorption after two different levels of drying in dynamic conditions at the ambient temperature. Surface characterization before and after exposure to ammonia was done using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, and potentiometric titration. On the surface of the initial materials, besides epoxy, hydroxyl, and carboxylic groups, various amounts of water within the interlayer space are present. The results showed that ammonia is intercalated within the interlayer space of graphite oxides. Water enhances the amount of ammonia adsorbed via the dissolution and promotes the dissociation of surface functional groups. This enhances formation of ammonium ions. On the other hand, water screens the accessibility of epoxy and -COOH groups for reactions with ammonia and thus limits the amount adsorbed. The retention of ammonia on a partially dried graphite oxide is enhanced not only owing to those reactions but also because of the formation of new adsorption centers as a result of an incorporation of ammonia to the graphene layers.

Introduction Recently, graphite oxide (GO) has attracted the attention of many researchers owing to its promising applications as an adsorbent,1,2 a component in composite materials with photochemical, conductive, electric, or adsorptive properties,3-6 or a precursor in the formation of graphene layers.7-9 As adsorbent, GO has been tested in the retention of NO, NO2, and NH31,2,10 and it showed an especially good performance in the adsorption of the ammonia due to its acidic character and its ability to provide an interlayer space where molecules of ammonia can be stored.1,2 Oxidation of graphite results in an incorporation of oxygen atoms to the basal planes and edges of graphene layers in the form of functional groups, such as epoxide, keto, and hydroxylic, to the basal planes.11-13 This process is accompanied by an increase in the distance between the graphene layers from about 3.4 to 6-12 A˚. *To whom correspondence should be addressed: E-mail: tbandosz@ccny. cuny.edu. Telephone: (212) 650-6017. Fax: (212) 650-6107. (1) Seredych, M.; Bandosz, T. J. J. Phys. Chem. C 2007, 111, 15596–15604. (2) Seredych, M.; Petit, C.; Tamashausky, A. V.; Bandosz, T. J. Carbon 2009, 47, 445–456. (3) Matsuo, Y.; Tabata, T.; Fukunaga, T.; Fukutsuka, T.; Sugie, Y. Carbon 2005, 43, 2875–2882. (4) Bissessur, R.; Liu, P. K. Y.; White, W.; Scully, S. F. Langmuir 2006, 22, 1729– 1734. (5) Morishige, K.; Hamada, T. Langmuir 2005, 21, 6277–6281. (6) Petit, C.; Bandosz, T. J. Adv. Mater. 2009, 21, 4753–4757. (7) Dikin, D. A.; Stankovich, S.; Zimney, E. J.; Piner, R. D.; Dommett, G. H. B.; Evmenenko, G.; Nguyen, S.-B. T.; Ruoff, R. S. Nature 2007, 448, 457–460. (8) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlaas, K. M.; Zimney, E. J.; Stach, E. A.; Piner, R. D.; Nguyen, S.-B. T.; Ruoff, R. S. Nature 2006, 442, 282–286. (9) Schniepp, H. C.; Li, J.-L.; McAllister, M. J.; Sai, H.; Herrera-Alonso, M.; Adamson, D. H.; Prud’homme, R. K.; Car, R.; Saville, D. A.; Aksay, I. A. J. Phys. Chem. B 2006, 110, 8535–8539. (10) Seredych, M.; Pietrzak, R.; Bandosz, T. J. Ind. Eng. Chem. Res. 2007, 46, 6925–6935. (11) Szabo, T.; Berkesi, O.; Dekany, I. Carbon 2005, 43, 3186–3189. (12) Hontoria-Lucas, C.; Lopez-Peinado, A. J.; Lopez-Gonzalez, J.; de, D.; Rojas-Cervantes, M. L.; Martı´ n-Aranda, R. M. Carbon 1995, 33, 1585–1592. (13) Li, J.-L.; Kudin, K. N.; McAllister, M. J.; Prud’homme, R. K.; Aksay, I. A.; Car, R. Phys. Rev. Lett. 2006, 96, 176101–176104. (14) Buchsteiner, A.; Lerf, A.; Pieper, J. J. Phys. Chem. B 2006, 110, 22328– 22338.

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The size of the interlayer distance is related to the degree of oxidation and the hydration level.14,15 All of this determines the hydrophilic character of GO, the easiness for the water molecules to be intercalated between the graphene layers, and thus an easy dispersion of GO in water, alkaline solutions, or alcoholic media.16,17 It is generally accepted that in GO distorted/corrugated graphene layers are stacked in a more or less ordered fashion.18,19 In spite of the increased attention of researchers, there are still different views on the structure of GO.20-27 Lerf and co-workers described GO as an agglomerate of pseudoflat oxidized graphene layers with a carbon grid formed by a random distribution of benzene and aliphatic rings. In this model, the oxygen functional groups are 1,2-ethers and hydroxyl groups randomly distributed on the basal planes. Szabo and co-workers27 see the graphite oxide with a wrinkled carbon grid made of linked cyclohexane chairs connected to a benzene ring network: 1,3-ethers, hydroxyl groups, and keto groups are attached to the graphene layers. Recently, evaluating the differences in the mechanisms of ammonia adsorption in GO obtained using Hummers28 and Brodie methods,29 we have found30 that sulfonic groups in the (15) Jeong, H.-K.; Lee, Y. P.; Lahaye, R. J. W. E.; Park, M.-H.; An, K. H.; Kim, I. J.; Yang, C.-W.; Park, C. Y.; Ruoff, R. S.; Lee, Y. H. J. Am. Chem. Soc. 2008, 130, 1362–1366. (16) Szabo, T.; Tombacz, E.; Illes, E.; Dekany, I. Carbon 2006, 44, 537–545. (17) Hirata, M.; Gotou, T.; Horiuchi, S.; Fujiwara, M.; Ohba, M. Carbon 2004, 42, 2929–2937. (18) Hashimoto, A.; Gloter, A.; Urita, K.; Iijima, S.; Suenaga, K. Nature 2004, 430, 870–873. (19) Mkhoyan, K. A; Contryman, A. W.; Silcox, J.; Stewart, D. A.; Eda, G.; Mattevi, C.; Miller, S.; Chhowalla, M. Nano Lett. 2009, 9, 1058–1063. (20) Hofmann, U.; Holst, R. Ber. Dtsch. Chem. Ges. 1939, 72, 754–771. (21) Ruess, G. Monatsh. Chem. 1946, 76, 381–417. (22) Clauss, A.; Plass, R.; Boehm, H.-P.; Hofmann, U. Z. Anorg. Allg. Chem. 1957, 291, 205–220. (23) Scholz, W.; Boehm, H.-P. Z. Anorg. Allg. Chem. 1969, 369, 327–340. (24) Mermoux, M.; Chabre, Y.; Rousseau, A. Carbon 1991, 29, 469–474. (25) Nakajima, T.; Mabuchi, A.; Hagiwara, R. Carbon 1988, 26, 357–361. (26) Lerf, A.; He, H.; Forster, M.; Klinowski, J. J. Phys. Chem. B 1998, 102, 4477–4482. (27) Szabo, T.; Berkesi, O.; Forgo, P.; Josepovits, K.; Sanakis, Y.; Petridis, D.; Dekany, I. Chem. Mater. 2006, 18, 2740–2749. (28) Hummers, W. S.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. (29) Brodie, M. B. C. Ann. Chim. Phys. 1860, 59, 466–472. (30) Petit, C.; Seredych, M.; Bandosz, T. J. J. Mater. Chem. 2009, 19, 9176–9185.

Published on Web 12/23/2009

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former materials are responsible for enhanced reactions of ammonia with sulfate ions formed as a result of HSO3- oxidation by an active oxygen. Besides this, X-ray photoelectron (XPS) and Fourier transform infrared (FTIR) spectroscopy analyses showed the formation of amines and amides when ammonia reacted with oxygen groups. Since in an as-received graphite oxide a large quantity of water (about 20 wt %) is always present between the graphene layers, we hypothesize that that water can affect the mechanism of adsorption and the quantity of adsorbed ammonia. Our previous studies showed that the water present in a gas phase, on one hand, competes with ammonia for adsorption centers and, on the other hand, allows dissolution of ammonia in a water film, resulting in increased adsorption capacity.2 Therefore, the objective of this paper is an evaluation of the mechanism of ammonia adsorption on GO with two different amounts of water within the interlayer species. For this purpose, GO synthesized using the Brodie method was chosen. This material was shown to have a relatively small quantity of carboxylic groups.27,30 A significant adsorption/reaction of ammonia on those groups would complicate the analysis of the adsorption mechanism.

Experimental Section Materials. GO was synthesized from commercial graphite (Sigma-Aldrich) by the Brodie method.29 Graphite powder (10 g) was thoroughly mixed with potassium chlorate (50 g) in a flask placed into an ice-bath. Then fuming nitric acid (100 mL) was slowly added to liquefy the mixture and as an oxidant. After removal of the ice-bath, the mixture was left at a room temperature for 24 h. Another portion of nitric acid (60 mL) was then added to the reaction vessel. Following this, the slurry was placed in a water bath at 60 C for 4 days (until no more emission of yellow vapors) and then further diluted to 6 L. Then the GO particles settled at the bottom were separated from the excess liquid by decantation and washed with distilled water until all acids and salts were removed (detected by XRF analysis). The wet form of GO was centrifuged, and the resulting material was freeze-dried. The fine brown powder obtained is referred to as GO1. A subsample of GO was dried at 60 C, and it is referred to as GO2. This treatment, as indicated by Brodie, is supposed to decrease the amount of water present within interlayer space. Methods. NH3 Breakthrough Dynamic Test. The laboratory designed dynamic test was used to evaluate NH3 adsorption.31 The GO samples (2 cm3) were packed into a glass column and exposed to a flow of ammonia diluted in dry or moist air (70% humidity) at room temperature. The concentration of ammonia in the inlet stream was 1000 ppm, and the total flow rate 450 mL/min. The breakthrough of NH3 was monitored using an electrochemical sensor (Multi-Gas Monitor ITX system). The adsorption tests were arbitrarily stopped at a breakthrough concentration of 100 ppm, and then the desorption process was studied by purging the bed with dry air only (360 mL/min) and recording the ammonia concentration. The adsorption capacity of each GO, in terms of mg of ammonia per g of adsorbent, was calculated by integration of the area above the breakthrough curve, and considering the NH3 concentration in the inlet gas, flow rate, breakthrough time, and mass of adsorbent. In a similar way, the amount of ammonia desorbed was determined by an integration of the area under the desorption curve balanced with the experiment parameters. To increase the pool of testing conditions, the samples were also prehumidified for 2 h with moist air (70%) before the test at humidity was carried out. The samples obtained after exposure to ammonia have ED, EM, and EPM added to their names. ED refers to the experiments run in dry air, EM in moist air, EPD in dry air after prehumidification, and EPM in moist air after 2 h of prehumidification. (31) Le Leuch, L. M.; Bandosz, T. J. Carbon 2007, 45, 568–578.

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The GO2-EPM sample was analyzed immediately after adsorption and also after desorption (purging with the air). The subsamples are referred to as GO2-EPM-ads and GO2-EPM-des, respectively. pH. The pH of the initial and the exhausted samples was measured. An amount of 0.1 g of initial or exhausted GO powder was stirred overnight with 5 mL of distilled water, and then the pH of the suspension was recorded. XRD. X-ray diffraction (XRD) measurements were conducted using standard powder diffraction procedures. Adsorbents were ground with methanol in a small agate mortar. The mixture was smear-mounted and then analyzed by Cu KR radiation generated in a Phillips X’Pert X-ray diffractometer. A standard glass slide was run for the background. FTIR. Fourier transform infrared (FTIR) spectroscopy was carried out using a Nicolet Magna-IR 830 spectrometer using the attenuated total reflectance (ATR) method. The spectrum was generated and collected 16 times and corrected for the background noise. The experiments were done on the powdered samples, without KBr addition. Potentiometric Titration. Potentiometric titration measurements were performed with a DMS Titrino 716 automatic titrator (Metrohm). The instrument was set at the mode where the equilibrium pH is collected. Subsamples of the initial and exhausted materials (∼0.100 g) were added to NaNO3 (0.01 M, 50 mL) and placed in a container maintained at 25 C overnight for equilibrium. During the titration, to eliminate the influence of atmospheric CO2, the suspension was continuously saturated with N2. The 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.32,33 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: Z QðpHÞ ¼

¥ -¥

qðpH, pKa Þ f ðpKa Þ d pKa

The solution of this equation is obtained using the numerical procedure,32,33 which applies regularization combined with nonnegativity constraints. Based on the spectrum of acidity constants and the history of the samples, the detailed surface chemistry was evaluated. XPS. The elements present in the two GOs studied as well as their chemical state were identified by XPS analyses. These analyses were performed by Evans Analytical Group laboratories with a PHI 5701 LSci instrument, a monochrome Al KR source (1486.6 eV), and an analysis area of about 2.0 mm  0.8 mm.

Results and Discussion Ammonia breakthrough curves are presented in Figure 1. Generally speaking and based only on the breakthrough times, the performance of the GO2 series of samples is much better than that of GO1. Another important difference is that the presence of moisture in the gas phase decreases the capacity of GO1 in comparison with the run in dry air. When the sample is prehumidified, the opposite relationship is found. On the other hand, for GO2, moisture in the gas phase always increases the ammonia removal capacity. The areas under the desorption (32) Jagiello, J.; Bandosz, T. J.; Schwarz, J. A. Carbon 1994, 32, 1026–1028. (33) Jagiello, J. Langmuir 1994, 10, 2778–2785.

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Figure 1. Ammonia adsorption and desorption curves.

curves represent the weakly adsorbed ammonia removed during air purging. The similarities in the magnitude of those areas suggest a similar adsorption strength. The slightly weaker adsorption is found on GO1-EPD. On GO2-EPM, a gap between adsorption and desorption exists. It is linked to the limitation of the sensor (more than 100 ppm of ammonia was released) and indicates the highest contribution of weakly adsorbed NH3 on this sample. It has to be pointed out that the performance of this sample in the adsorption run is the best. A comparison of the calculated capacities is presented in Table 1. As seen, after the experiments run in dry air after prehumidification, almost twice more ammonia is adsorbed on GO2 than on GO1. It is interesting that the similar capacities are measured on GO2 either in moist air or after prehumidification and the run in dry air. On the other hand, the visible differences in the breakthrough times between the runs in dry and moist air on GO1 are found. Drying GO at 60 C decreases the capacity of about 50% when run in dry air. This is consistent with the importance of water for the ammonia adsorption indicated in the literature.1,2 In fact, the removal of water by drying is compensated by an almost twice higher amount of water adsorbed during prehumidification on GO2 compared to that on GO1. Nevertheless, despite that some water was present on the sample GO1 before prehumidification, its capacity is much less than that of GO2. To understand the observed peculiarities in the ammonia adsorption described above, the initial samples and those exhausted ones run at different conditions were analyzed. Since, as reported previously,1 GOs are the nonporous materials from the point of view of nitrogen molecules, the stress of the analyses is Langmuir 2010, 26(8), 5491–5498

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placed on the structure and surface chemistry. The X-ray diffraction patterns are presented in Figure 2. For all initial and exhausted samples, the well-defined diffraction peaks are preserved, indicating the organized layered structure. The d002 parameters and the weight loss during heating up to 150 C are collected in Table 2. As seen, d002 of graphite increases by 4.4 A˚ after oxidation as a result of an incorporation of oxygen groups and water adsorbed on the surface. After drying at 60 C, d002 slightly decreases by about 0.5 A˚. The adsorption of ammonia visibly increases d002 for both series of samples, even when measured after the removal of weakly adsorbed species. A comparison of the interlayer space for GO2-EPM analyzed before and after desorption reveals an expected decrease in d002. When the dependence of the increase in d002 (Δd002) after exposure to NH3 on the amount of ammonia adsorbed was analyzed, the linear trends were found for both series of samples (Figure 3). Nevertheless, the differences in the correlation coefficients are visible. In the case of GO1, almost a perfect linear correlation is found which suggests the adsorption is governed mainly by intercalation leading to the expansion of the interlayer space. The less perfect linear trend for GO2 indicates the more complex adsorption mechanism. Since the only difference is in the drying level, those variations must be related to the amount/location/ interactions of water. The weight losses listed in Table 2 for the unexposed GO1 and GO2 represent the water located mainly within the interlayer space. For the exhausted samples, they can also include the ammonia dissolved in that water or weakly physically adsorbed on the surface at dry conditions. During heating of GO1 at 60 C, about 70% of water was removed. Then after prehumidification the same weight losses were found for both samples, GO1-EPM and GO2-EPM, even though the latter adsorbed twice more ammonia. For other samples from both series run in parallel conditions, comparable weigh losses are found. This must be related to the capacity of an interlayer space and the sensitivity of the methods used for analyses. Even if the centers on which water could be adsorbed are occupied by ammonia, this process cannot be determined owing to the similarity between water and ammonia in molecular masses. The most visible difference between samples is observed in the dry conditions without any water introduced to them. In this case, the weight loss of GO2-ED is twice less than that of GO1-ED. The former sample has also twice less ammonia adsorbed. A decrease in the weight loss between GO and GO-ED is caused by the removal of water with dissolved ammonia from the interlayer space by dry air purging. The results discussed above suggest that not only the amount of water is important for ammonia adsorption but also water localization and its covering of surface groups can play an important role in affecting the mechanism of ammonia removal. Although detailed XPS analyses of the GO was presented elsewhere,30 for the sake of discussion, we would like to mention that deconvolution of O1s spectra showed the peak at 533.0 eV related to C-O in epoxy, phenol, or carboxylic groups and the one at 535.0 eV to oxygen atoms in water or chemisorbed oxygen species (COOH).34,35 The content of each species was 26.5% and 73.5%, respectively. The high contribution of the latter species, based on the analysis performed, was rather linked to the water and chemisorbed oxygen than to the carboxylic groups.30 It was interesting that the contributions of C-C, C-O, and O-CdO from the deconvolution of the C1s spectrum (34) Desimoni, E.; Casella, G. I.; Salvi, A. M. Carbon 1992, 30, 521–538. (35) Biniak, S.; Szymanski, G.; Siedlewski, J.; Swiatkowski, A. Carbon 1997, 35, 1799–1810.

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Seredych and Bandosz Table 1. NH3 Breakthrough Capacity per Gram or per Volume of Adsorbent, Amount of Water Adsorbed, and pH Surface NH3 breakthrough capacity 3

sample

[mg/g of ads]

[mg/cm of ads]

GO1-ED GO1-EM GO1-EPD GO1-EPM GO2-ED GO2-EM GO2-EPD GO2-EPM

18.3 11.7 26.2 33.7 9.2 32.6 29.8 60.8

9.95 6.35 14.2 18.2 4.8 17.0 15.5 31.6

pH water adsorbed [mg/g of ads]

89.1 89.4 170 218

initial

exhausted

2.91 2.91 2.91 2.91 3.06 3.06 3.06 3.06

6.35 6.10 7.01 7.78 5.63 7.75 7.62 8.19

Figure 2. X-ray diffraction patterns. Table 2. d002 and Weight Loss during Heating in Nitrogen at Temperatures < 150 C for GOs before and after Exposure to Ammonia sample

d002 [A˚]

H2Ocont (30-150 C) [%]

G GO1 GO1-ED GO1-EM GO1-EPD GO1-EPM GO2 GO2-ED GO2-EM GO2-EPD GO2-EPM-ads GO2-EPM-des

3.37 7.77 7.92 7.86 8.00 8.07 7.25 7.48 7.77 7.70 7.89 7.80

14.7 7.2 15.5 6.9 18.7 4.4 3.4 16.4 5.4 18.0 19.3

were almost equal. The various elemental analyses were consistent and indicated about 60% of carbon and 37% of oxygen in this materials. All analyses were done on as-received GO1 without predrying. The FTIR spectra for the initial and exhausted samples are summarized in Figure 4. As expected, there are no significant differences between GO1 and GO2. The bands at 1040 and 1570 cm-1 represent the vibrations of C-O and aromatic CdC bonds, respectively.11,12 The band at 1630 cm-1 is assigned to O-H vibration in water and/or to the presence of oxygen surface compounds (cyclic ethers),11 and that at 1720 cm-1 corresponds to the vibration of CdO in carboxylic or carbonyl groups.11,12 The band 930 cm-1 can be assigned to epoxy/peroxide groups.12 5494 DOI: 10.1021/la9037217

In the range of 3000-3700 cm-1, the vibration of O-H in C-OH or water is observed.11 After the ammonia adsorption, no band related to ammonia or ammonium ion is observed on the spectrum for the samples run in dry conditions. This can be explained by their small adsorption capacity and limitations in FTIR detection. The observed decrease in the intensity of the band at 1630 cm-1 is due to the removal of water, since the adsorption test was run in the dry conditions. This is consistent with the decrease in the intensity of the overlapping bands between 3000 and 3700 cm-1. After the exposure to ammonia in the moist conditions, a new band at 1430 cm-1 which is assigned to the vibration of N-H in NH4þ36 is visible. This indicates either an acid-base reaction with the carboxylic groups or the formation of ammonium hydroxide by the reaction of ammonia with water. Another possibility is the formation of amines or amides, which was evidenced previously by XPS results.30 Support for this is a decrease in the intensity of the band at ∼1040 cm-1 for GO2-EPM, which might be related to the reaction of ammonia with epoxy groups leading to the formation of amine.1,2,30 A more visible band at 1630 cm-1 for the GO2 series than that for GO1 indicates the formation of O-H groups and/or N-H vibration in adsorbed ammonia or amine.36-38 The broad overlapping bands at a high wavelength after the adsorption of ammonia represent the vibrations of O-H (36) Zawadszki, J.; Wisniewski, M. Carbon 2003, 41, 2257–2267. (37) Onida, B.; Gabelica, Z.; Lourenco, J.; Garrone, E. J. Phys. Chem. 1996, 100, 11072–11079. (38) Qi, G.; Yang, R. T. J. Phys. Chem. B 2004, 108, 15738–15747.

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Figure 3. Dependence of the increase in Δd002 on the amount of ammonia adsorbed.

Figure 4. FTIR spectra.

(phenol) and N-H (NH4þ, NH3, NH2).11,36,37 The large increase in the intensity of the overlapping bands between 3100 and 3700 cm-1 even though the experiment was run in dry conditions supports the reaction of ammonia with epoxy groups leading to the formation of hydroxyl groups whose vibrations are in this range. The surface chemistry of GO1 and GO2 is also seen on pKa distributions presented in Figure 5. After ammonia adsorption (Figure 6), some differences in the distribution of acidity constants of the species present on the surface area are seen. They represent not only oxygen containing species but also those formed as a result of the reaction with ammonia. Generally, for GO2, the surface after the reactive adsorption is more Langmuir 2010, 26(8), 5491–5498

heterogeneous, which is represented by an additional peak with pKa of about 10. This peak is not visible on the GO1 series. The complexity of surface chemistry increases with an increase in the water content in the system. When water is present, the peaks with pKa between 9 and 10 are revealed. They are assigned to ammonium hydroxide and supported by FTIR results and also by an increase in the pH toward the basic range (Table 1). Table 3 compares the number of moles of ammonia adsorbed to those detected as NH4þ ions. One has to remember that, owing to desorption, the amount of ammonia left on the surface is expected to be smaller than the desorbed one. When ratios of those two numbers are analyzed, an interesting trend is revealed (Figure 7). For GO1, with an increase in the content of water DOI: 10.1021/la9037217

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prehumidification is higher than that when water is only present in the gas phase. The smaller conversion ratio for GO1-ED and GO1-EM is caused by the limiting reaction of NH3 with water, for which content is not sufficient to promote the dissociation of acidic groups. Therefore, adsorption at these conditions is governed mainly by the dissolution in water within the interlayer space. Apparently, less water is present at EM than after prehumidification, and thus, the adsorption is smaller. Moreover, water within the interlayer space associated around the functional groups screens the functional groups and does not permit them to participate to the full extent in the reaction

Figure 5. Comparison of pKa distributions for GO1 and GO2.

supplied to the system, a higher percentage of ammonia is adsorbed as NH4þ, whereas an opposite trend is found for GO2. To understand it, we have to consider the expected reactions. The ammonium ion determined in our analyses can be the result of the reactions of ammonia with carboxylic groups, even in totally dry conditions:

which is accompanied by the formation of water, and

That ammonium ions can be also formed just in the acidic environment: Hþ þ : NH3 f NH4 þ

ð2Þ

Moreover, when water is present in the system, the dissolution of ammonia occurs (702 g/100 mL at 20 C39) and a significant part of the ammonia may be converted to ammonium since the acidic pH shifts reaction 2 to the right. Water, besides helping in the ammonia dissolution and formation of the ammonium ions, can also compete with it for the adsorption centers by forming aggregates close to surface functional groups.1 This happens owing to the fact that the hydrophobic character of the graphene layers is not a welcoming environment for water adsorption. More water within the interlayer spaces results in the formation of a film, which can promote the dissociation of acidic groups and thus can help in the formation of the ammonium ions. That film can be formed when our materials are prehumidified, especially GO1, which already had water within the interlayer space. This explains why the percentage of ammonia adsorbed as ammonium for GO1 after (39) Weast, R. C.; Melvin, J. A. Handbook of Chemistry and Physics, 62nd ed.; CRC Press: Boca Raton FL, 1981-1982; D-201.

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That relatively simple dissolution/ammonium formation mechanism may explain the almost perfect correlation between the increase in the interlayer space and the amount of ammonia adsorbed (Figure 3). The scenario seems to be more complex for the GO2 series of samples. As seen from Figure 7, for them a smaller fraction of the adsorbed ammonia is converted to ammonium with an increase in the amount of water in the system. For GO2-ED, the ratio is almost one. This suggests that reaction 1 is the major mechanism. Since much less water is present on this material, the carboxylic groups are not screened and the ammonium salts can be formed in reaction with gaseous ammonia. When water is introduced to the system, its adsorption enables the dissolution of ammonia and thus the amount adsorbed significantly increases with a slightly higher fraction of ammonium formed. Prehumidification introduces a significant amount of water to the system. That amount is similar to that retained on the surface of GO2-EM (Table 1), and thus, running the experiment in dry gas results in a comparable adsorption to that on GO2-EM (where water screened the active surface groups and dissolution was responsible for a significant fraction of ammonia adsorbed), even though the degree of conversion Langmuir 2010, 26(8), 5491–5498

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Figure 6. Comparison of pKa distributions for GO1 and GO2 exhausted samples run at various conditions. Table 3. Numbers of Strong and Weak Acidic Groups and Amount of Ammonium Detected Using Potentiometric Titration samples

pKa < 7 [mmol/g]

pKa > 7 [mmol/g]

GO1 GO1-ED GO1-EM GO1-EPD GO1-EPM GO2 GO2-ED GO2-EM GO2-EPD GO2-EPM

0.301 0.336 0.299 0.406 0.321 0.282 0.396 0.347 0.342 0.349

1.610 2.228 1.895 2.753 2.585 1.354 1.964 2.031 2.605 2.222

to ammonium slightly decreases. When after prehumidification the experiment is run in the presence of moisture (EPM), the conversion ratio to ammonium ions decreases with a significant increase in the amount adsorbed, likely via dissolution. A closer look at the amount of ammonium ions formed in our system indicates that there is a limitation in the amount of this species and more than about 1.1 mmol/g is not formed. This must be governed by the chemical equilibrium process. A plausible explanation is that the amount of ammonium ions is determined both by the number of strong acidic groups (assuming carboxylic) and by the extent of ammonium ion formation (reaction 2), which are fixed in our systems. More ammonia dissolved in water can react with epoxy and OH groups forming amines and amides in reactions 3 and 4. This likely happens to Langmuir 2010, 26(8), 5491–5498

NH3 ads [mmol/g]

NH4þ PT [mmol/g]

1.076 0.688 1.541 1.982

0.277 0.179 1.130 1.160

0.541 1.918 1.753 3.576

0.518 1.070 0.713 0.947

greater extent in the case of GO2 samples, as shown in the heterogeneity of the pKa distribution. The observed lack of perfect correlation between Δd002 and the amount of ammonia adsorbed can be caused by the formation of amines and amides within the interlayer space. This disturbs the perfect expansion of the layers caused when only a film of water with dissolved ammonia/ammonium is present within the interlayer space. On GO1, the extent of reactions 3 and 4 is smaller due to more water within the interlayer place screening the groups and limiting access of ammonia to those centers. Nevertheless, for GO1, it was found using XPS that about 30% of ammonia was introduced as amides and amines.30 The much higher adsorption of ammonia on GO2-EPM than on the corresponding GO1 counterpart can be explained by the interaction of ammonia via DOI: 10.1021/la9037217

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Article

Seredych and Bandosz

Conclusions The results presented in this paper show differences in the performance of graphite oxides as the ammonia adsorbent caused by the different amounts of water present within the interlayer space. That water enhances the amount of ammonia adsorbed via dissolution and promotes dissociation of surface functional groups. This enhances formation of ammonium ions. On the other hand, water also screens the accessibility of epoxy and COOH groups for reaction with ammonia and thus limits the amount adsorbed. The retention of ammonia is enhanced not only owing to those reactions but also as a result of the formation of new adsorption centers as a result of an incorporation of ammonia to the GO layers. Figure 7. Comparison of the fraction of ammonia converted to ammonium ions.

additional hydrogen bonding with amines and amides formed during the dynamic breakthrough experiments.

5498 DOI: 10.1021/la9037217

Acknowledgment. This work was supported by ARO Grant W911NF-05-1-0537 and NSF collaborative Grant 0754945/ 0754979. The authors are grateful to Dr. Jacek Jagiello for SAIEUS software.

Langmuir 2010, 26(8), 5491–5498