Surface Properties of HCl Modified Ag-ACFs
Won-Chun Oh♠ and Young-Shin Ko1
Department of Advanced Materials & Science Engineering, Hanseo University, Chungnam 356-706, Korea
1Department of Science Education, Seoul National University of Education, Seoul 137-700, Korea
♠e-mail: wc_oh@hanseo.ac.kr
(Received October 1, 2005; Accepted December 12, 2005)
Abstract
Silver impregnated activated carbon fibers were post-modified using hydrochloric acid. Adsorption behaviors, SEM morphologies, and functional groups for the silver impregnated ACFs were compared with those of post-modified ACFs.
Adsorption isotherms were used to characterize SBET, the pore structure and volume of silver-activated carbon fibers (ACFs) before and after acid post-treatment. In order to the reveal the causes of the differences surface states after the samples were washed with hydrochloric acid, outer surface and pore structure were investigated by SEM. And the type and quality of various functional groups were studied from FT-IR spectra and Boehm titration method. Finally, the quantitative properties in silver contents were also examined by EDX spectra.
Keywords: Adsorption, Functional group, SEM/EDX, FT-IR, Boehm titration, Silver-ACF
1. Introduction
Activated carbon fibers as an adsorbent are a compara- tively modern form of porous carbon material with a number of significant advantages over the more traditional powder or granular forms. These include a developed surface area, microporosity, thermal stability, high adsorption capacity as well as very high rates of adsorption from the gas and liquid phase. The micropore size is in general uniform and can often be controlled by adjusting the conditions of preparation of the fibers or by post-preparation modification like metal treatment. For activated carbon fiber (ACF), the characteri- stics of their inner and outer surfaces play also a significant role in their primary application. Considerable effect has been devoted to characterize, and to comprehend the chemical properties of carbon surfaces, as well as their microstructure [1, 2]. Carbon surface chemistry presumes traditionally the existence of variety of surface complexes formed by com- bining carbon atoms with other elements such as oxygen, nitrogen, hydrogen, phosphor and sulfur. The presence of heteroatoms such as oxygen, which can form ketones, carboxyls, phenols, ethers, and lactones; nitrogen in the form amines; and phosphor as phosphates determine the acidity and basity of the activated carbon fiber. The aims of oxida- tion like acid treatments of carbon surface are obtaining a hydrophilic surface state with a large number of oxygen- containing surface functional groups. Oxygen-containing surface functional groups have the acid-base characters or redox properties. Their properties can be used in the prepa- ration of carbon-supported metal catalysts with transition metal complexes. The oxidation of carbon surfaces is a
frequently used method in the preparation of carbon-based materials. Various chemicals have been used as oxidizers such as nitric, hydrochloric and sulfuric acid, perchlorate, permanganate and noble metals [3, 4]. Activated carbon and their fiber treated with metal have been known as tradition- ally outstanding porous materials for the removal organics and toxic species [5-7], and antibacterial properties [8].
The objective of this paper is to study in detail the perfor- mance of surface transition and to investigate the effects of physical and chemical properties for the two kinds of post- modified ACFs. The difference in the method of acid treat- ment results in dramatically different structural and chemical properties of sorbents. For characterization of post-treatment effects for metal-ACFs, the changes in physicochemical pro- perties of these samples were presented by nitrogen adsorp- tion isotherm and surface properties. Surface morphologies were investigated by SEM-EDX analysis, FTIR results and properties of surface functional groups by Boehm titration.
2. Experimental
2.1. Raw materials
The ACFs used as a raw material were prepared from commercial carbon fibers (T-300 Amoco, USA). The carbonized carbon fiber was heated first at 823 K, and the carbon fibers were activated by steam diluted with nitrogen in a cylinder quartz glass tube in the temperature range of 1053~1073 K for 30 min. These ACFs were washed with deionized water and dried for 24 h at ambient temperature.
Science
The 0.01 M sulfuric acid at boiling temperature was used in the oxidation treatment to increase the functional groups without damage of the ACF surface. The oxidized ACFs were washed and dried at 323 K for 24 h.
2.2. Silver impregnation
For silver impregnation, 2 g of activated carbon fiber was dipped into 100 ml of 0.05 and 0.1 M silver nitrate solution and stirred for 24 h at room temperature. These samples were dried at 383 K for 48 h in an N2 atmosphere, and named as Ag-ACF [9]. For the acid post-treatment, 2 g of Ag-ACFs was dipped into 100 ml of 0.01, 0.05 and 0.1 M hydrochloric acid solution and stirred for 12 h at room temperature. After removal of the liquid, the post-treated ACFs were dried completely in an oven at vacuum state.
Table 1 shows nomenclatures of HCl modified ACFs.
2.3. Measurement
Nitrogen isotherms were measured using BET surface area measuring apparatus (ASAP 2010, Micromeritics) at 77 K.
Scanning electron microscopy (SEM, JSM-5200 JOEL, Japan) was used to observe the surface morphology and pore struc- ture of HCl post-modified Ag-activated carbon fiber and the modified silver state. For the elemental analysis of metal contents in acid post-modified Ag-ACFs, EDX was used.
The acid post-modified Ag-ACFs were examined by a KBr method using Fourier transform infrared (FT-IR) spectro- scopy. Discs were prepared by mixing 1 mg of powdered Ag-ACFs with 600 mg of KBr (for FT-IR spectroscopy) in mortar, and then pressed the resulting mixture under the pressure of 450 Pa for 3 minutes. The spectra of the samples were measured between 4000 and 500 cm–1 using a spectrophotometer (FTS 3000MX, Biored Co.).
2.4. Boehm titration
A Boehm titration method [10] was used for the identi- fication of oxygenated surface groups on the carbon surfaces.
1 gram of Ag-ACFs was placed in 50 ml of the following 0.05 M solutions: sodium hydroxide, sodium carbonate,
sodium bicarbonate, and hydrochloric acid. The elenmeyer flasks were sealed and shaken for 24 h and then 5 ml of each filtrate was pipetted and excess of base and acid was titrated with HCl and NaOH, respectively. The numbers of acidic sites of various types were calculated under the assumption that NaOH neutralizes carboxylic, phenolic, and lactonic groups; Na2CO3, carboxylic and lactonic groups; and NaHCO3, only carboxylic groups. The number of surface basic sites was calculated from the amount of hydrochloric acid, which reacted with the Ag-ACFs.
3. Results and Discussion
Fig. 1 shows the adsorption isotherms of N2 at 77 K on the post-modified Ag-ACFs. All of the samples tested gave Type I isotherms characterized by plateau that is nearly horizontal to the p/p0 axis. The knees of the isotherms at about P/P0=0.1 were very sharp. It is well known that the adsorption on microporous materials at lower P/P0 due to micropore filling leads to Type I isotherm. Thus, it is easy to figure out that the pores in tested samples are mainly micropores. When the relative pressure is over 0.95, nitrogen uptake can be observed, indicating that mesopore and/or macropore develop in tested samples. It is consider that the increase of total surface area and micropore volume was related to the removal of metallic silver and its compounds with the fixation of oxygen groups at the entrance and on the walls of micropore. This trend would be highly dependent on the treated concentration of silver salts on the starting fibers as well as the HCl treatment condition such as acid/
fiber ratio, initial acid strength, and temperature and duration process. The estimated texture parameters are summarized in Table 2 for both HCl post-modified and the non-post modi- fied ACFs. The data in Table 2 demonstrate that HCl treat- ment of the Ag-ACFs leads to an appreciable gain of the total surface area and small increases in the pore volume with some pore opening of the average pore dimensions.
Accordingly, an apparent de-blocking of the average pore radii is observed, indicating effective attack by HCl, particu- larly on micropores, by causing partial destruction of their structure, which would result in the gain of total surface area
Table 1. Nomenclatures of HCl Post-modified Ag-ACFs
Sample Nomenclature
0.05 M AgNO3 + ACF
0.05 M AgNO3 + ACF + 0.01 HCl (post treatment) 0.05 M AgNO3 + ACF + 0.05 HCl (post treatment) 0.05 M AgNO3 + ACF + 0.1 HCl (post treatment)
Ag0.05-ACF Ag0.05-ACF-0.01HC Ag0.05-ACF-0.05HC Ag0.05-ACF-0.1HC 0.1 M AgNO3 + ACF
0.1 M AgNO3 + ACF + 0.01 HCl (post treatment) 0.1 M AgNO3 + ACF + 0.05 HCl (post treatment) 0.1 M AgNO3 + ACF + 0.1 HCl (post treatment)
Ag0.1-ACF Ag0.1-ACF-0.01HC Ag0.1-ACF-0.05HC Ag0.1-ACF-0.1HC
along with slight changes in the pore volume of the fibers.
The erosive action of HCl to the Ag-ACFs was also confirmed by scanning electron microscopy (SEM). In order to the reveal the causes of the differences surface states after the samples were washed with hydrochloric acid, outer
surfaces were investigated by SEM. Fig. 2 and Fig. 3 are shown the surface morphologies of the Ag-ACFs before and after post-treatment with hydrochloric acid. In these figures, it can clearly observe the highly homogeneous surface struc- ture and distribution and growth state of silver and silver compounds on the surface of Ag-ACFs before treated with acid. It is also noted that a much part of surfaces are blocked and dotted by silver and silver agglomerates after the treat- ment. Differences in the degree of blocking effects depend on the type and concentration of treated-silver. It is believed that silver distribution and their crystal particles affected to pore structures like a SBET, micropore volume and pore radius. The growth state of silvers and particle size on the surface of Ag0.05-ACF-HC series after treated with acid increase with increasing mole ratio of hydrochloric acid. In the case of silver, silver adsorbed on the activated carbon fiber surface is known to be initially reduced to form silver metal nuclei which are aggregating to form silver particles and silver compounds, and then greater size of their com- pounds and particles is obtained as the reduction is continued.
Fig. 4 shows the FT-IR spectra of the carbon fiber materials tested. The weak band of O-H stretching vibrations at 3400 cm−1 was due to surface hydroxyl groups and chemisorbed water. And the presence of absorption bands at 1600-1850 cm−1 range indicated that some of the acidic surface groups are cyclic anhydrides and lactone structure. The band at near 1550 cm−1 and mutually overlapping absorption bands within the 1150-1450 cm−1 range have been discussed previously [11]. According to Fanning et al. [12], it should be empha- sized that the presence of bands at 1730 cm−1, 1620 cm−1, 1550 cm−1 can be respectively attributed to the stretching vibrations of C=O moieties in carboxylic, ester, lactonic or anhydride groups (1730 cm−1), quinine, enol, cyclic β-ketones and/or ion-radical structures (1620 cm−1), and conjugated systems like diketones, keto-esters and keto-enol structures.
From FT-IR result, it is indicate that acid post-treatment gave rise to a greater increase in carboxylic acid and lactone groups including C=O absorption bonds. These changes demonstrate the specific interaction between CH3OH mole- cules and surface structures containing C=O groups. The
Fig. 1. Adsorption isotherms obtained from HCl post-modified Ag-ACFs; (a) Ag0.05-ACF-HC and (b) Ag0.1-ACF-HC series.
Table 2. Comparison of Physical Parameters of HCl Post-modified Ag-ACFs
Sample Parameter
SBET (m2/g) Micropore volume
(mL/g) External surface area
(m2/g) Average pore diameter (Å)
Ag0.05-ACF Ag0.05-ACF-0.01HC Ag0.05-ACF-0.05HC Ag0.05-ACF-0.1HC
12211232 13341367
0.652 0.625 0.637 0.669
643.2 639.6 641.5 643.3
16.51 16.51 16.54 16.55 Ag0.1-ACF
Ag0.1-ACF-0.01HC Ag0.1-ACF-0.05HC Ag0.1-ACF-0.1HC
12231134 12561313
0.655 0.653 0.657 0.659
632.5 631.5 632.8 639.3
15.45 16.56 16.58 16.51
absorption band of CH3 bending vibrations occurs in the region of 1443 cm−1. A strong band at 1025 cm−1 is assigned to C-O stretch from derived from bonding of grapheme layer and hydroxyl groups. In this work, the band at the 1031 cm−1 is assigned to C-O stretch of doubly bridging methoxy species. Assignment of this band is analogous to that discussed previously [2]. The sites for dissociation of such a weak Bronsted acid may be described as involving Lewis centers. These sites are considered to be formed by a very Lewis acid site placed near a basic site. The υ (C-O) mode of the methoxy groups at near 1025 cm−1 depends on the chemical structure of the adsorption sites. The absorption frequency of υ (C-O) of adsorbed carbon monoxide is often presented as an indicator characterizing the local coordi- nation. Because water molecules adsorbed on the surface of activated carbon fibers is participate in specific interaction like hydrogen bonds, the absorption bands in the 1600-1550 cm−1 region can also be described by OH bending vibrations.
Some complicated absorption bands in the 1650-1500 cm−1 region suggests that aromatic ring bands and C=C vibrations
overlap the above mentioned C=O stretching vibration bands and deformed OH bands. The major goal of surface oxidation by acid treatment is to control of metal contents onto the carbon surfaces and to obtain a more hydrophilic surface with a relatively large number of functional groups contain- ing oxygen.
The results obtained from method proposed by Boehm are collected Table 3. The chemical investigations and analysis of functional group following carbon fiber surface modifi- cation are set out in this table. As mentioned above, the transformations of FT-IR spectra are due to an alternation of the carbon surface via introduction of oxygen groups and removal of some carbon atoms from matrix by post-modifi- cation with hydrochloric acid. The oxidized surface chemical structure of the samples tested was shown highly diverse performance. Oxidation with hydrochloric acid as oxidants results in the increase of the number of acidic groups for all samples. Moreover, the reduction in the number base sites is noticed. Oxidation with hydrochloric acid introduces a large number of carboxylic groups. Strongly oxidized carbon samples
Fig. 2. SEM images obtained from Ag0.05-ACF-HC series; (a) Ag0.05-ACF, (b) Ag0.05-ACF-0.01HC, (c) Ag0.05-ACF-0.05HC and (d) Ag0.05-ACF-0.1HC
have acidic groups at different acidic strength and surface
oxides. The surface acidity increases with increasing of the amount of acid post-treated, while basity decrease with increasing of the amount of acid post-modified. The most
Fig. 3. SEM images obtained from Ag0.1-ACF-HC series; (a) Ag0.1-ACF, (b) Ag0.1-ACF-0.01HC, (c) Ag0.1-ACF-0.05HC and (d) Ag0.1- ACF-0.1HC.
Fig. 4. Infrared spectra recorded from HCl post-modified Ag-ACFs; (a) Ag0.05-ACF, (b) Ag0.05-ACF-0.01HC, (c) Ag0.05-ACF-0.05HC, (d) Ag0.05-ACF-0.1HC, (e) Ag0.1-ACF, (f) Ag0.1-ACF-0.01HC, (g) Ag0.1-ACF-0.05HC and (h) Ag0.1-ACF-0.1HC.
Table 3. Number of Surface Species (meq/g) Obtained from Boehm Titration
Sample Functional group (meg/g)
Carboxylic Lactonic Phenolic Acidic Basic
Ag0.05-ACF Ag0.05-ACF-0.01HC Ag0.05-ACF-0.05HC Ag0.05-ACF-0.1HC
0.420.44 0.781.53
0.210.23 0.230.27
0.220.37 0.340.68
0.851.04 1.352.48
0.470.44 0.440.25 Ag0.1-ACF
Ag0.1-ACF-0.01HC Ag0.1-ACF-0.05HC Ag0.1-ACF-0.1HC
0.300.30 0.320.42
0.210.24 0.250.27
0.410.45 0.490.53
0.920.99 1.061.22
0.650.12 0.120.11
Fig. 5. EDX elemental micro-analysis spectra obtained from HCl post-modified Ag-ACFs; (a) Ag0.05-ACF, (b) Ag0.05-ACF-0.01HC, (c) Ag0.05-ACF-0.05HC, (d) Ag0.05-ACF-0.1HC, (e) Ag0.1-ACF, (f) Ag0.1-ACF-0.01HC, (g) Ag0.1-ACF-0.05HC and (h) Ag0.1-ACF- 0.1HC.
severe effect of oxidation is observed for all samples. These results obtained may contribute to the lowest local pH of this carbon surface with removal of metallic silver and silver compounds due to acid treatment. An influence of the acidic groups controlled on the carbon fiber surface by acid treat- ment is also demonstrated to the proper distribution and selective introduction of the amounts of metallic silver with increasing of acidic groups calculated from Boehm titration.
The titration method discussed above provides information about species that behave as acids or bases in aqueous solutions. This excludes a significant number of species, for example, aldehydes, esters, ethers, ketones, etc from being detected on the surface. Those functional groups play impor- tant role in the adsorption performance when the adsorbate has polar properties has availability to hydrogen bonding.
In other to the chemical elemental microanalysis of HCl post-modified Ag-ACFs, prepared samples were analyzed by EDX. The EDX spectra for post-modified Ag-ACFs were shown in Fig. 5. From this figure, it shows the presence of C, O, Ag and Cl. In case of most of Ag-ACF series, C and Ag are present as major elements in the Ag-ACF series post- modified in HCl solutions. And, these results presented for each Ag-ACF series were shown the spectra corresponding to almost all tested samples increase in oxygen with decreas- ing of silver contents with increasing of the concentration of HCl solutions. It is well correlated with the change of contents of treated silver and concentration of post-treated HCl. Notes that a increasing of the amount of C and O contents with decreasing treated silver contents is observed for the all tested samples, which becomes more homogene- ous as treated silver is removed and as the carbon fiber surface is oxidized. The results of EDX elemental micro- analysis of Ag-ACFs transformed as a function of concen- tration of HCl solution are listed Table 4. The oxygen surface groups are expected to be located at the edges of the basal planes, which are relatively weak sites of carbon struc- ture and oxidation progresses slowly into the basal planes.
4. Conclusion
In this study, silver impregnated activated carbon fibers
were modified using hydrochloric acid. Adsorption behaviors, SEM morphologies, and functional groups for the silver impregnated ACFs were compared with those of post-modi- fied Ag-ACFs. Adsorption isotherms were used to charac- terize SBET, the pore structure and volume of Ag-ACFs before and after acid post-treatment. In order to the reveal the causes of the differences surface states after the samples were washed with hydrochloric acid, outer surface and pore structure were investigated by SEM. And the type and quality of various functional groups were studied from FT- IR spectra and Boehm titration method. Finally, the quanti- tative properties by EDX spectra were examined, which observed homogeneous surface state as treated silver was removed and as the carbon fiber surface was oxidized.
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Table 4. EDX Elemental Microanalysis of HCl Post-modified Ag-ACFs
Sample C (%) O (%) Ag (%) Cl (%) Others (%)
Ag0.05-ACF Ag0.05-ACF-0.01HC Ag0.05-ACF-0.05HC Ag0.05-ACF-0.1HC
89.689.6 91.690.9
6.388.55 8.909.20
1.920.62 0.570.33
1.730.71 1.030.47
0.32 (Cu) 0.17 (Cu) 0.21 (Cu) 0.22 (Cu) Ag0.1-ACF
Ag0.1-ACF-0.01HC Ag0.1-ACF-0.05HC Ag0.1-ACF-0.1HC
84.384.4 89.589.4
6.226.65 7.117.97
4.974.99 1.551.18
4.473.97
1.741.34 0.51 (Cu)
