15596
J. Phys. Chem. C 2007, 111, 15596-15604
Mechanism of Ammonia Retention on Graphite Oxides: Role of Surface Chemistry and Structure† Mykola Seredych and Teresa J. Bandosz* Department of Chemistry, The City College of New York, 138th Street and ConVent AVenue, New York, New York 10031 ReceiVed: May 10, 2007; In Final Form: July 7, 2007
Graphite oxide (GO) was synthesized from commercial graphite and modified by calcinations at 350 °C. The samples were used as adsorbents of ammonia at dry and moist conditions. Their surface before and after exposure to ammonia was characterized using adsorption of nitrogen, XRD, SEM, FTIR, TA, CHN analysis, and potentiometric titration. The results showed that oxidation results in the incorporation of a significant amount of epoxy, phenolic, and carboxylic groups, which is directly manifested by a decrease in surface pH. The majority of the latter is removed by calcinations. During this process, the exfoliation of graphite occurs, resulting in the formation of some micro- and mesporosity. The materials obtained have proven to be excellent adsorbents of ammonia. The NH3 reacts with carboxylic acids and is intercalated between layers or dissolved in pores with adsorbed water. The mechanism depends on the conditions of experiments and surface features of the samples. The highest capacity is obtained on GO on which a significant amount of water is adsorbed, likely on functional groups. This helps in the dissociation of carboxylic groups and their acid-base reactions with ammonia. Nevertheless, a significant amount of ammonia is intercalated between layers. Those two interactions are the strongest. When water is present in the challenge gas, the amount adsorbed decreases because of the competition between water and ammonia for active sites.
Introduction Graphite oxide (GO) has a layered structure and various nonstochiometric chemical compositions, which depend on the level of oxidation. Its graphitic layers lose their polyaromatic character because of the incorporation of various oxygencontaining functional groups.1 It is generally accepted that the graphene layers have epoxy and -OH groups attached to carbons.1-5 At the edges of the graphene layers, the carboxylic groups with strong acidic character are located.1,3 The interlayer distance, d001, in GO varies from 6 to 12 Å, which is caused by variations in the level of hydration. Because of the epoxy and -OH groups existing within the interlayer space, the water molecules are attracted there via hydrogen bonding.1 The water is either embedded or distributed in interlayer voids depending on its location. A very popular way of GO modification is the introduction of amines or conduction polymers within the interlayer space6-10 with goals toward production of functional nanometer-scale structures, devices, and particularly cathode materials for Li secondary batteries. Besides the intercalation of organic species, modifications with inorganic materials were also proposed.11-18 Such chemicals include Cu(OH)2-poly(vinyl alcohol).11 The structure of that complex depends upon the solution pH. It was proposed that the selective absorption of small molecules can be a possible application of this kind of material. Another modification was based on the introduction of dibutyltin oxide,12 whose SndO groups reacted with the hydroxyl groups of GO resulting in formation of Sn-O bonds leading to the stable materials under ambient environment. The †
Part of the “Keith E. Gubbins Festschrift”. * To whom correspondence should be addressed. Tel: (212) 650-6017. Fax: (212) 650-6107. E-mail:
[email protected].
butyl groups prevented the penetration of water within the interlayer space. The layered graphite oxide-birnessite manganese oxide nanocomposite13 was synthesized via coagulation of GO and BirMO, which overcome the electric repulsive force between negatively charged graphite oxide layers and birnessite manganese oxide sheets. In pillared graphite,15 the pillars consisting of stable oxides are introduced between the sheets of layered hosts.16 Examples are R-Fe2O3 and Fe3O4; pillared graphite is prepared by pillaring graphite oxide with the trinuclear iron acetato complex ion.15 On these materials, analogous to pillared clays, adsorption of nitrogen, water, oxygen, NO, and NO2 was investigated. The materials had a mesoporosity reaching 0.15 cm3/g and a surface area up to 130 m2/g. It was proposed that although water is adsorbed in the limited space within the mesopores O2, O, and NO2 are chemisorbed on FeO-like pillars.15 Materials used as adsorbents of small-molecule gases should have small pore sizes to enhance to dispersive interactions. Moreover, their surface should be active toward the processes expected to occur with the adsorbates. The most commonly used adsorption media are activated carbons or natural or synthetic zeolites. The former are known to have high surface area and pore volume reaching more than 1 cm3/g. Their hydrophobic surface built of randomly oriented graphene layers can be modified by oxidation,19-21 introduction of nitrogen,22,23 halogens,24 or impregnation with various redox chemicals or metals.25 Even though the potential for molecules to be adsorbed in small pores exists, very often those spaces are not used efficiently because of imperfections in the impregnation process (low surface dispersion) or weak adsorption forces at temperatures close to ambient. So far, there is a challenge to produce activated carbon with uniform pore sizes. The efforts include chemical vapor deposition or template carbonization within the
10.1021/jp0735785 CCC: $37.00 © 2007 American Chemical Society Published on Web 08/15/2007
Mechanism of Ammonia Retention on Graphite Oxides uniformed pore sizes of zeolites, silicas, or synthesized aluminosilicates.26,27 The latter method, although leading to ordered carbons, seems to be technologically complex and expensive. All of these things cause graphite oxide (GO), even though studied extensively in the 1950s, to reappear, but this time as an adsorbent of gases. The objective of this paper is to investigate the mechanism of ammonia retention on the GO surface. Although the application of GO for this purpose has been investigated previously, the process was studied at low temperatures, which enhance the dispersive interaction.28 In this study, we attempt to investigate the GO as adsorbents of ammonia at ambient conditions. The mechanism of adsorption is derived based on the evaluation of ammonia retention on GO with various degrees of surface oxidation and in dry and moist conditions. Extensive surface characterization helps to evaluate the role of GO surface chemistry and structure in the attraction of ammonia molecules via chemical and physical forces. Experimental Section Materials. The starting material, GO, was synthesized from commercial graphite with a particle size GO-EM > GO-ED > GO-C-ED > GO-CEM and is 0.72, 0.67, 0.58, 0.20, and 0.17, respectively. This suggests that when graphite oxide is completely dry adsorption is the strongest. Then, when the adsorbed water is not released, the presence of moisture increases the adsorption strength. The changes in the pH values reflect the changes in the surface chemistry related to the modification of graphite and the adsorption/reaction of ammonia on the graphite oxide surface. Taking into account that the surface pH of the initial
Mechanism of Ammonia Retention on Graphite Oxides
J. Phys. Chem. C, Vol. 111, No. 43, 2007 15599
Figure 3
TABLE 3: d001 and the Weight Loss during Heating in Nitrogen at Temperatures 8) groups.32 After calcinations, the number of carboxylic acids decreased significantly, as expected.36 Exposure to ammonia followed by desorption of weakly adsorbed species resulted in an increase in the intensity of the peaks at pKa between 9 and 10, which represent the ammonium hydroxide (pKa 9.4). That peak is much more intense on GO than it is on its calcined counterpart, and its intensity can be linked to the amount of ammonia adsorbed. Table 5 compares the amount of ammonia left on the graphite oxide after desorption to the amount of carboxylic groups with pKa less than 8. The results indicate that for GO samples from 1.6 to 2 times more ammonia is revealed on pKa distribution then is the number of carboxylic groups. It means that either other groups are active in chemical interactions with ammonia or some physically adsorbed ammonia present on the surface is converted in water in NH4+. In fact, that ammonia was detected from FTIR analysis. Moreover, the comparison of the total amount of ammonia adsorbed to that detected from potentiometric titration indicates that in the case of the GO
pK 7-8
7.23 (0.288) 7.16 (0.397) 7.13 (0.546) 7.14 (0.287) 7.08 (0.217) 7.13 (0.062) 7.42 (0.044) 7.51 (0.044)
pK 8-9
pK 9-10
8.25 (0.041) 7.90 (0.280) 8.20 (0.214) 8.14 (0.611) 8.28 (0.374) 7.99 (0.250) 8.62 (0.069) 8.84 (0.122)
9.66 (0.012) 8.96 (0.580) 9.09 (0.463) 9.19 (1.568) 9.33 (2.640) 9.26 (2.204) 9.38 (0.065) 9.68 (0.208) 9.10 (0.152)
pK 10-11
all 0.149
10.20 (0.811) 10.12 (0.774) 10.10 (0.786) 10.43 (0.873) 9.92 (0.746) 10.33 (0.193) 10.30 (0.050) 10.02 (0.094)
2.705 2.714 4.660 5.032 4.214 0.534 0.597 0.402
samples about 60% ammonia is strongly adsorbed and retained on the surface. In the case of calcined materials when the experiment is run in dry conditions, not all carboxylic groups seem to be involved in retention on ammonia as NH4 + ions. This ammonia consists of only about 10-14% of all adsorbed ammonia, which is in rough agreement with the trend in the strength of adsorption. When the experiment is run in moist air, those carboxylic groups have less influence on the amount of ammonia strongly adsorbed, although the numbers are comparable. This suggests that in the case of graphite oxidebased adsorbents water in the challenge gas hinders ammonia adsorption not only from the point of view of the capacity but also from the point of view of the strength of the adsorption. It is likely that it competes with ammonia to be hydrogen bonded to epoxy and hydroxyl groups present between layers. The similar size of both molecules (σH2O ) 2.8 Å,38 σNH3 ) 2.9 Å39) can also play a role in this competition. When water is not present in the challenge gas, a significant amount of ammonia is intercalated between layers. Alternatively, drying graphite oxide decreases the capacity about 50%. This can be explained by the influence of water on the interaction of ammonia with carboxylic groups. These groups can dissociate when the film of adsorbed water is present. In fact, 27 wt % water adsorbed on the surface of GO equals to about 15 mmol/g, which is about 5 times more than the amount of acidic groups. Lack of that water adsorbed due to an increased surface hydrophobicity of calcined/exfoliated samples likely contributes to their low capacity. The dynamic and nonequilibrium conditions regarding water adsorption can also contribute to the lower capacity. Ammonia can interact with carboxylic groups as either Bronsted:
or Lewis acids:
The second reaction is more likely to happen in dry conditions where bands from NH3 on FTIR spectra are more intense. Also in dry conditions, more ammonia must be present between the graphitic layers interacting with epoxy and hydroxyl
15602 J. Phys. Chem. C, Vol. 111, No. 43, 2007
Figure 7. Proton binding curves for the GO (A), GO-D (B), and GO-C (C) series of samples
groups via hydrogen bonding. It should be difficult to remove that ammonia from the surface using air purging, which explains the stronger adsorption on the samples, which were not calcined. In the case of calcined samples, the adsorption is weak because the porosity and this weak dispersive force contribute to the retention of ammonia on the surface of these materials. Some carboxylic groups of those materials are not active in ammonia retention, which can be explained by their location in small pores excluded from further adsorption. Although the interactions of ammonia with surface phenolic groups are not expected to be only hydrogen-bond-type, the
Seredych and Bandosz
Figure 8. DTG curves in nitrogen for the GO (A), GO-D (B), and GO-C (C) series of samples. For A and B, only the curves up to 350 °C are shown because at higher temperature no changes in the mass are revealed in this scale.
reaction with the epoxy groups is also possible as a nucleophilic substitution:
Mechanism of Ammonia Retention on Graphite Oxides
J. Phys. Chem. C, Vol. 111, No. 43, 2007 15603 TABLE 5: Comparison on the Numbers of Carboxylic Groups to the Amount of Ammonia Adsorbed Detected as NH4OH (mmol/g): A, Number of Carboxylic Groups on the Surface of Initial Materials; B, Total Number of Groups; C, Amount of Ammonia Adsorbed from Breakthrough Capacity Experiments; D, Amount of Ammonia Retained as NH4+ Determined from Potentiometric Titration sample
A
B
GO GO-ED GO-EM GO-C GO-C-EM GO-C-ED
1.034
2.705
0.207
C
D
3.58 2.25
2.06 1.62
0.980 0.910
0.143 0.087
2.705
TABLE 6: Elemental Analysis and Calculated Chemical Formulas sample
C (%)
H (%)
N (%)
O (%)
formula
GO GO-EM GO-C GO-C-EM
46.9 47.1 80.7 79.5
2.5 2.7 0.1 0.2
