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Langmuir 1995,11, 708-710
Evaluation of Low Concentrated Hydrophilic Sites on Microporous Carbon Surfaces with an X-ray Photoelectron Spectroscopy Ratio Method Y. Kaneko,**tK. Ohbu,? N. Uekawa,$ K. Fujie,* and K. Kaneko* Surface Science Research Center, Lion Corporation, 7-13-12,Hirai, Edogawa-ku, Tokyo 132, Japan, and Department of Chemistry, Faculty of Science, Chiba University, 1-33 Yayoi, Inage, Chiba 263, Japan Received September 26, 1994. I n Final Form: January 3, 1 9 9 P The surface of activated carbon fiber (ACF)was oxidized by a H202 solution or was reduced by Hz gas and the change in the water adsorption isotherm as a result of such surface treatments was examined at 303 K. The surface oxidation by H2Oz increased the amount of adsobed water in the low-pressureregion, while the reduction by H2 decreased the low-pressure adsorption of water. The surface oxidation also increased slightly the intensity of the XPS C1, tail in the high-energy region. The XPS ratio spectrum, using the spectrum of the reduced ACF as the standard, gave an explicitpeak due to the presence of surface functional grops. The ratio peak area had a linear relationship with the amount of adsobed water in the low-pressure region. Table 1. Micropore Structure Introduction Many microporous solid materials with extremely high sample Wa (mbg-9 a, (m2*g-l) w (nm) surface areas and of a specific chemical nature have 0.636 1402 0.94 CEL attracted much attention from both a fundamental and CEL-OX 0.647 1396 0.93 practical point of view. Activated carbon fibers (ACFs) 0.636 1402 0.94 CEL-red PAN 0.286 759 0.75 have uniform micropores and a high surface area and 0.243 601 0.81 PAN-ox their surface properties have been studied intensively. PAN-red 0.209 535 0.82 ACFs show excellent adsorption characteristics for various g a ~ e s land - ~ for organic compounds dissolved in ~ a t e r , ~ - ~ reflectance FTIR,10J2in addition to the traditional chemibut they cannot easily adsorb water molecules in the cal titration method,13it is still quite difficult to evaluate micropore region because of the weak interaction of water the surface-functional groups of high burned-off carbons molecules with the graphitic micropore walL7 Obviously, such as ACFs. In this paper a new approach is presented functional groups on the ACF surface influence sensitively for analyzing functional groups on the surface of ACFs, the adsorption characteristics of water. Modification of present in very low concentrations, by applying a surfacethe ACF surface with hydrophilic substances changes the sensitive XPS method. extent and strength of water adsorption dramatically.* Furthermore, we have reported that removal of the polar Experimental Section functional groups from the ACF surface is effective from improvement of the adsorption of trace organic compounds Cellulose-(CEL)and polyacrylonitrile(PAN)-basedACFs were used. The samples were chemically treated as follows: (a) from aqueous s ~ l u t i o n . Accordingly, ~,~ a reliable charoxidation by HzOz, ACF was oxidized in 31% H202 aqueous acterization and quantization of the surface functional solution at 298 Kfor 24 h and then dried in vacuo after washing; groups is indispensable to arrive a t a n understanding of (b) reduction by Hz, ACF was heated in hydrogen at 1273 K for the adsorption properties of hydrophobic microporous 15 min. The ACF samples thus treated with H2Oz or with Hz solids such as ACFs. Although many attempts have been are denoted CEL-ox (or PAN-ox) and CEL-red (or PAN-red), made to elucidate the nature of the surface of carbonous respectively. The micropore structures were determined by N2 solids, for instance by temperature-programmed desorpadsorption at 77 K. The H2O adsorption isotherms in the lowt i ~ n , ~ JX-ray O photoelectron spectroscopy (XPS),ll diffuse pressure region were measured gravimetricallyat 303 K. XPS spectral changes as a result of the surface treatments were Pa at 298 K, using Mg Ka measured under a vacuum of < + Lion Corporation. radiation (6 kVl30 mA), with the aid of a Shimazu ESCA 850 * Chiba University. apparatus. The binding energy was calibrated with respect to @Abstractpublished in Advance ACS Abstracts, February 15, the binding energy of Au 4f712 (83.8 eV). 1995.
(1)Kaneko, K.; Ishii, C.; Ruike, M.; Kuwabara, H. Carbon 1992,30, 1075. (2)Katori, T.; Shimizu, K.; Sindo, N.; Maeda, T. J. Chem. SOC., Faraday Trans. 1992, 88, 1075. (3) Sato, M.; Sukegawa, T.; Suzuki, T.; Hagiwara, S.; Kaneko, K. Chem. Phys. Lett. 1991, 181, 526. (4) Kaneko, Y.; Abe, M.; Ogino, K. Colloid Su$. 1988, 37,21. (5) Kaneko, Y.; Minoura, Y.; Agui, W.; Abe, M.; Ogino, K. J.Colloid Interface Sci. 1988, 121, 161. (6) Asakawa, T.; Ogino, K.; Yamabe, K. Bull. Chem. Soc. Jpn. 1985, 58, 2009. (7) Segarra, E. I.; Grandt, E. D. Chem. Eng. Sci., in press. (8)Matsumoto, A.; Ruike, M.; Suzuki, T.; Kaneko, K. Colloid Surf. 1993. 15. - _ _ _ , .74. -, -. (9) Rodriquez-Reinoso,F.; Molina-Sabio, M.; Monecas, M. A. J.Phys. Chem. 1992,96, 2707. (10)Singredjo, L.; Kapteijin, F.; Moulijn, J. A.; Martin-Martinez, J.-M.; Boehm, H. P. Carbon 1993,31,213. (11)Takahagi, T.; Ishitani, A. Carbon 1984,22, 43.
Results and Discussion Porosity and Water Adsorption. The NZadsorption isotherms of all ACF samples are of type I according to the ITJPAC classification, indicating that ACFs are representative microporous systems. The N2 adsorption isotherms were analyzed by the a,plot1* using the SPE method.15 The standard data of nonporous carbon black were used for construction of the high-resolution a,plot. The high-resolution a,plot has a strong upward deviation below a, = 0.3, suggesting the presence of narrow (12) Fanning, P. E.; Vannice, M. A. Carbon 1993,31,721. (13)Boehm, H. P. Angew. Chem. 1966,5,533. (14)Sing, K. S. W. Carbon, 1989, 27, 5 . (15)Kaneko, K.; Ishii, I. Colloid Surf. 1992, 67, 203.
0743-7463/95/2411-0708$09.00/0 0 1995 American Chemical Society
Letters
Langmuir, Vol. 11,No. 3, 1995 709
Binding energy /eV
Binding energy /eV
PIP0
Figure 1. Total-range water adsorption isotherm of CEL (a) and the low-pressure range water isotherms of CEL samples (b): A, CEL; W, CEL-ox; 0, CEL-red. -'""I
PIP,
fr
0.1
0
PIP0
Figure 2. Total-range water adsorption isotherm of PAN (a) and the low-pressure range water isotherms of PAN samples (b): A, PAN; W, PAN-ox; 0 , PAN-red. micropores whose widths are less than 0.7 nm. Consequently, it is difficult to evaluate the surface area correctly, even by the SPE method. The micropore structure, the pore volume W,, the surface area &, and the average pore width w ,are listed in Table 1. In the case of CEL the pore volume, the surface area, and the pore width are almost invariant before and after the oxidation or reduction treatments have been carried out. In the case of PAN
Figure 3. XPS spectra of surface-treated ACF samples (a) CEL and (b) PAN: -, ACF; - - -,ACF-ox; - - -, ACF-red. both the pore volume and the surface area decrease slightly with the surface treatment. The average pore width increases slightly, and the decrease in the pore volume and the surface area should be caused by a decrease of fraction of smaller micropores. Figure 1 shows water adsorption isotherms of CEL, CEL-ox, and CEL-red. The total-range isotherm of CEL is shown in Figure l a . The low-pressure region isotherms are shown in Figure lb. All adsorption isotherms of these CEL samples are essentially of type 111, which means that these surfaces are hydrophobic regardless of the surface treatment.16 However, the effect of the surface treatment is clearly observed in the low-pressure region of the adsorption isotherm. The surface oxidation by HzOz increases the initial uptake, while the surface reduction by Hz decreases it. The total-range water adsorption isotherm of a n as-received PAN and the low-pressure region isotherms of all PAN samples are shown in parts a and b of Figure 2, respectively. Although the water adsorption isotherm of PAN is basically of type 111, the uptake in the low-pressure region is much greater than that of the CEL samples. The surface of PAN itself has a polar nature due to presence of the C-N bonds, and the water adsorption isotherm of PAN has a much larger uptake in the low-pressure region than that of CEL. The surface reduction by Hz lowers markedly the low-pressure uptake. As the low-pressure uptake depends sensitively on the chemical state of the surface, it is valuable to determine the low-pressure uptake. We evaluated the low-pressure uptake of water by plotting the data according to the Langmuir equation. WL(H,O) should be proportional to the number of water adsorption sites per unit of surface area, that is, to the concentration of surface functional groups. The correlation of w~(H20) with the XF'S result will be presented below. Ratio XPS Spectra and Surface Chemical Structure. The surface chemical changes ofACF samples were examined by XF'S. Figure 3 shows the XPS spectral (16)Dubinin, M. M.; Serpinsky, V. V. Carbon 1981,19,402.
Letters
710 Langmuir, Vol. 11, No. 3, 1995
"2i2
2i4
'
2;(6 2iU Binding energy /eV
2hO
'
2621
Binding energy ieV
Figure 4. Ratio spectra of the XPS C 1s for CEL and CEL-ox (a) and for PAN and PAN-ox (b): -, as-received ACF; - - -, oxidized ACF.
relatted peak area
Figure 5. Linear relationshipbetween the low-pressurewater adsorption and the area of the XPS C Is ratio peak: 0, CEL;
,. PAN.
changes of the CI, peak of CEL and PAN samples. The C1, peak coincides with that of graphite (284.6 eV). The peaks of CEL samples are not symmetrical and they show a weak shoulder in the tail a t the high-energy side, which is associated with the presence and concentration of the
functional groups. A slight intensity differencein the highenergy tails is observed, which indicates the concentration changes of the surface functional groups. The XPS literature on carbon fiber" mentions that, the =COH, =CO, and -C(O)OH groups show peaks a t 286,287, and 288.6 eV, respectively. A quantitative comparison of the high-energy tails is desirable, but the differences are too small to evaluate the concentration of the surface functional groups. Although we observe more marked differences in the high-energy tails corresponding to the PAN samples treated in a different way, direct relative comparison between the PAN and CEL results is preferable. As both the CEL-red and PAN-red spectra have the lowest intensity a t the high-energy side and the contribution by the surface functional groups can be neglected in these samples, the intensity ratios in relation to each reduced sample were determined. Figure 4 shows the ratio spectra for CEL and PAN. A distinct difference between asreceived and oxidized samples is shown even for the CEL system. Both ratio spectra have the peak at the same position a t about 290 eV, which may be assigned to -C(O)OH species. The ratio spectrum of PAN-ox has a strong peak a t 288.6 eV, while the ratio spectrum of PAN has a peak a t 288.3 eV and a shoulder at 286.7 eV. Probably, PAN-ox has mainly -C(O)OH surface functional groups, whereas PAN has =CO functional groups in addition to -(CO)OH. A careful deconvolution of the ratio-spectra will provide a more detailed picture of the nature of the functional groups in ACFs a t low concentration, although for this purpose we have to examine more microporous carbon samples before arriving a t a conclusive interpretation. Determinationat SurfaceFunctional Groups. The peak areas of XPS ratio spectra should be proportional to the concentration of the surface functional groups, and hence the peak area corresponding to the spectrum in Figure 4 can be associated with WL(H~O), as shown in Figure 5. A good linear relationship between the number of hydrophilic sites on the carbon surface and the XPS ratio peak area for CEL and PAN samples is found. Therefore, this method can be used to determine very low concentrations on coverages of hydrophilic sites on hydrophobic carbon surfaces, even when dealing with high burned-off activated carbons such as ACFs. In principle this method can be widely applied to determine hydrophilic site coverages on various hydrophobic solids.
Acknowledgment. The authors are grateful to Dr. M. Abe and Dr. K. Nishiyama of Science University of Tokyo for their cordial support. This work was partially supported by the Grant-in-Aid of Ministry of Education, Japanese Government. LA9407673
