Article pubs.acs.org/JPCC
Water Adsorption in Nanoporous Carbon Characterized by in Situ NMR: Measurements of Pore Size and Pore Size Distribution Hai-Jing Wang,†,§ Alfred Kleinhammes,† Thomas P. McNicholas,‡ Jie Liu,‡ and Yue Wu*,† †
Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 27599-3255, United States Department of Chemistry, Duke University, Durham, North Carolina 27708, United States
‡
S Supporting Information *
ABSTRACT: We report an in situ nuclear magnetic resonance (NMR) study of water adsorption in a series of activated carbon samples with pore sizes of a few nanometers down to the subnanometer scale (nanoporous carbon). Water adsorption exhibits Sshaped type V isotherms with a steep increase near a certain vapor pressure. Using a previously proposed water isotherm model, pore size and pore size distribution are derived from the in situ NMR data, and they are shown to be in good agreement with results derived from N2 adsorption. The change of 1H NMR spin−lattice relaxation time of adsorbed H2O with vapor pressure is consistent with the mechanism of water cluster formation at surface groups preceding the occurrence of pore filling. NMR spectra of high pressure H2 gas in nanoporous carbon with preadsorbed D2O proves unambiguously that water preferentially fills the smaller nanopores. These results suggest that water adsorption can potentially be used for the characterization of pore structures of nanoporous carbon, and that in situ NMR is a convenient method for water isotherm measurement with accompanying microscopic information.
1. INTRODUCTION Carbon-based porous materials are critical for applications such as gas separation and storage,1−4 water filtration and decontamination,5−8 and electrochemical supercapacitors.9−11 The performance of porous carbon is significantly improved by large surface area and small pore size, which are routinely characterized by the adsorption of nonpolar gas at cryogenic temperatures, such as N2 adsorption isotherms at 77 K.12,13 The N2 adsorption starts with a monolayer adsorption on the surface, followed by multilayers buildup of N2 molecules as adsorption increases. Such physical adsorption was described in the Brunauer−Emmett−Teller (BET) theory.14−16 For water adsorption in nanoporous carbons (with pore sizes of a few nanometers down to the subnanometer scale), the surface chemistry of the pores can dominate the characteristics of water isotherms and water dynamics.5−11 Such surface chemistry cannot be revealed by N2 adsorption due to its insensitivity to surface groups. Water vapor thus might be a better choice than N2 gas for evaluating surface properties as well as for pore structure analysis. It is well-accepted that the adsorption of water in nanoporous carbon is determined by both surface groups and the pore size (Figure 1).17−23 The primary adsorption sites (PAS) of water, mostly oxygenated functional groups on the pore surface, dominate water adsorption in the low vapor pressure range (P/P0 < 0.3), where P0 is the saturated vapor pressure (P0 = 19.8 mmHg at 22 °C).19,23−27 If the surface density of PAS is high, then water adsorbed on PAS will gradually fill the pores, without apparent steep increases in the isotherms. Here we are concerned with nanoporous carbon, © 2014 American Chemical Society
where the PAS density is relatively low and the water adsorption exhibits an S-shaped isotherm, classified as the type V isotherm,28 with a steep increase in adsorption at the medium to high relative pressure range (0.3 < P/P0 < 0.8).23 The initial growth of small water clusters occurs at PAS, where the cluster size is very limited in the low vapor pressure range (P/P0 < 0.3). Rapid increase of adsorption could occur when cooperative adsorption of several water molecules takes place and bridges adjacent water clusters across pore walls that are preadsorbed at PAS.23 The partial pressure where the sharp rise takes place is therefore closely related to the pore size. This cooperative adsorption could not take place when the PAS density is very low and the pore size is large, making bridging across the pore walls impossible by a few water molecules.23 Under such circumstances, the only viable pore filling mechanism is capillary condensation, which might be a possibility in nanoporous carbon, as hinted at by the numerical results.23 Here, we report an in situ 1H NMR study of water adsorption in three nanoporous carbon samples with average pore sizes of 1.2, 1.4, and 2.5 nm determined in previous studies by various techniques.12,29 These samples possess large surface areas and have rather uniform pore size distribution, as determined by N2 gas adsorption.12 The average pore size is controlled by the activation time. This series of samples is excellent for studying the effect of pore size on water isotherms Received: February 12, 2014 Revised: April 3, 2014 Published: April 3, 2014 8474
dx.doi.org/10.1021/jp501518f | J. Phys. Chem. C 2014, 118, 8474−8480
The Journal of Physical Chemistry C
Article
Figure 1. Illustrations of water adsorption process in nanoporous carbon as a function of relative water vapor pressure: (A) no water adsorption at P/P0 = 0; (B) water is primarily adsorbed on PAS at low relative pressure (P/P0 < 0.3); (C) water clusters start to grow and cooperative adsorption takes place bridging the pore walls at medium to high relative pressure (0.3< P/P0 < 0.8); and (D) nanopores are filled with water (P/P0 = 1).
Figure 2. (A) Water adsorption isotherms in three microporous activated carbons derived from PEEK with different amounts of burnoff at room temperature. Isotherms are fitted to the Mahle model (solid lines, fitting parameters shown in Table 1) and the Talu-Meunier model (dashed lines, Supporting Information) for comparison purpose. (B) The spin−lattice relaxation time of water as a function of relative vapor pressure (scattered data) is fitted to eq 4 (solid lines), with fitting parameters shown in Table 1.
volume of nanoporous carbons. Previous studies showed that the pore sizes of the 20, 35, 91 wt % BO samples are 1.2, 1.4, and 2.5 nm, respectively.12,29 (The pore size of the 91 wt % BO sample is estimated from that of the 95 wt % BO sample derived with the same procedure, except for a slight difference in burn-offs. This should give a good estimate because the H2 adsorption capacity of 91 and 95 wt % are almost identical. The pore size of the 95 wt % BO sample is estimated to be ∼2.4 nm by NMR shift and H2 adsorption.29 A more accurate estimation is given by N2 adsorption, in which a series of nanoporous carbon with high burnoffs (>70 wt %) contain a large pore volume for the pores of 2.5 nm diameter.12 This suggests that 91 and 95 wt % BO samples should contain a large pore volume of 2.5 nm in diameter. Therefore, we use 2.5 nm as our best estimate of the pore size for the 91 wt % BO sample.) The Xray photoelectron spectrometry (XPS) data indicate that the surface chemistry of these nanoporous carbons is largely graphitic, suggesting a locally slit-shaped pore morphology.12 Water adsorption isotherms in these samples were measured by 1H NMR at ∼0.8 T (1H NMR frequency of 34 MHz) at room temperature (∼291 K). The nanoporous carbons are loaded into a quartz sample tube inside the NMR detection coil. The sample tube is connected to a quartz manifold that can be either evacuated by a mechanical pump or filled with water vapor for adsorption. This in situ water vapor loading system has been discussed in detail elsewhere.32−34 A solid-
since they possess a similar PAS. The isotherms are measured by 1H NMR equipped with an in situ vapor/gas loading system. Using a previously proposed water isotherm model,22 we derived the pore size distribution from the measured water isotherm, which is in good agreement with that determined from N2 adsorption. Along with the isotherm, the NMR measurement also provides information on water dynamics and the local environment of adsorbed water at the same time.29−33 This shows that in situ 1H NMR of water, a very easy measurement, can be very useful in determining the pore size distribution of nanoporous carbon and providing information on water dynamics at the same time.
2. EXPERIMENTAL SECTIONS Three nanoporous carbon samples are activated carbon derived from poly(etheretherketone) (PEEK) precursor, first by carbonization at 900 °C in Ar, followed by activation in hot water steam at 900 °C for a given activation time which controls the amount of burnoff (BO), the percentage loss of weight after activation compared to that of before activation.12 The three samples used here have BOs of 20, 35, 91 wt %, respectively. Water steam is used during the activation process to limit pore diameter increase because it is thought to provide a gentler oxidation environment than CO2. Water vapor can help conserve small pore diameters while increasing the pore 8475
dx.doi.org/10.1021/jp501518f | J. Phys. Chem. C 2014, 118, 8474−8480
The Journal of Physical Chemistry C
Article
Table 1. Parameters Characterizing Three Activated Carbons Derived from PEEK with Different BO Using the Mahle Modela BO (wt %)
A (P/P0)
B (P/P0)
ns/D (mmol/g)
ns·VL (cm3/g)
A′ (P/P0)
B′ (P/P0)
τ′/D′ (s)
20 35 91
0.543(6) 0.592(5) 0.775(3)
0.112(8) 0.080(6) 0.067(4)
11.4(2) 9.3(2) 15.3(3)
0.55 0.47 0.79
0.43(2) 0.49(1) 0.64(1)
0.17(5) 0.07(1) 0.01(6)
0.30(3) 0.28(2) 0.17(1)
a A, B, and ns/D are used to fit the water adsorption isotherms to eq 1. A′, B′, and τ′/D′ are used to fit the T1 data to eq 4. The pore size (d, measured through the centers of the first layer of carbon atoms on opposing walls) and BET surface area (SA) are determined in refs 12 and 29.
Figure 3. (A) The correlation of pore size d with the pore filling pressure characterized by A in eq 1. The error bars represent the distribution of the pore filling pressure represented by B. The data obtained from three samples give a straight line. (B) Comparison of pore size distribution determined by water adsorption isotherms and by the differential pore volume determined from N2 adsorption at 77 K in ref 12. The data are converted from the isotherms shown in Figure 2 based on eq 3.
echo sequence with a π/2 pulse of ∼4 μs was employed in the measurement. The spin−lattice relaxation time (T1) was measured by the standard saturation recovery method. The intensity of the 1H NMR signal is calibrated with a test tube of bulk water of known volume (0.11 cm3) corrected with an extrapolation following a Gaussian decay.35 The isotherms of concurrent adsorption of H2 and D2O (99.9%) in the 35 wt % burnoff sample at room temperature were measured by 1H NMR at ∼4.7 T (1H NMR frequency of 200 MHz), equipped with an high-pressure in situ H2 gas loading system up to 10 MPa.29 This gas loading system consists of a sapphire NMR sample tube connected to a H2 gas cylinder via stainless steel tubing, as described previously in detail.29,30,36 Water vapor is introduced into this loading system after being evacuated but prior to exposing it to high pressure gas. The sample is equilibrated with a certain vapor pressure of D2O before measuring the adsorption isotherms of H2 up to ∼10 MPa at room temperature. A single π/2 pulse of ∼10 μs was used for excitation in this high magnetic field measurement.
small water clusters bridging existing clusters at PAS across pore walls takes place, leading to rapid increase of adsorption. Here, we use the pressure at the center of the S-shape isotherm as the pore filling pressure (Pf) such that the amount of adsorbed water at Pf is half of the total amount at saturated vapor pressure P/P0 = 1. The measured isotherms show low water adsorption at low relative pressure range, suggesting that the density of PAS is low for all three samples. This ensures that the pore size plays a critical role in determining Pf. If the surface contains high density of PAS, however, then water could be continuously adsorbed on PAS, covering the entire surface area and eventually filling the pore volume as the relative pressure increases. Continuous adsorption at PAS will obscure the steep increase in water uptake and the effect of pore size.18 The uniqueness of these nanoporous carbon samples derived from PEEK is that the pore size increases as the activation time increases, but the PAS density remains low. Fitting the measured isotherms with analytical models could provide quantitative parameters that characterize the properties of nanoporous carbon. A number of analytical models have been proposed to fit the type-V isotherms.20 It was suggested by Mahle that the symmetric S-shaped isotherm is the consequence of pore size distribution.22 Mahle’s model was originally proposed for the capillary condensation of water in nanopores. Here we simply use the concept of pore size distribution and the analytical form, both are independent of the capillary condensation mechanism. This model provides an accurate and convenient description of the isotherm shape, given by the following:22
3. RESULTS AND DISCUSSION 3.1. Water Adsorption Isotherms. Figure 2A shows water adsorption isotherms of the three samples, plotted as the amount of water uptake (mmol of water per gram of nanoporous carbon) versus the relative water vapor pressure (P/P0) at room temperature. All three isotherms exhibit an Sshaped type V isotherm.16,18 low water adsorption at low relative pressure (0 < P/P0 < 0.3), rapidly increasing water adsorption at medium relative pressure (0.3 < P/P0 < 0.8), and an adsorption plateau approaching the saturated vapor pressure (P/P0 = 1). Such an adsorption process is known to be dominated by two factors: the surface density of PAS and pore size (Figure 1).17−22 At low relative pressure, water is primarily adsorbed on PAS and forms small water clusters (Figure 1B). In the medium range vapor pressure, cooperative adsorption of
n=
ns ⎡ −1⎛ P /P0 − A ⎞ ⎛ − A ⎞⎤ ⎟⎥ ⎟ − tan−1⎜ ⎢tan ⎜ ⎝ B ⎠⎦ ⎝ ⎠ D⎣ B
(1)
where n and ns are the amount of adsorbed water at P and P0, respectively, A and B, in unit of P/P0, are the center and the 8476
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The Journal of Physical Chemistry C
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distribution of pore filling pressure, respectively, and the normalization coefficient D is given by D = tan−1[(1 − A)/B] − tan−1(−A/B). The fitted isotherms (solid lines) are shown in Figure 2 with fitting parameters listed in Table 1. For the purpose of comparison, the isotherm data are also fitted to a semiempirical model based on the association theory of water molecules in micropores (pore size
