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Langmuir 2003, 19, 10426-10433
Carbon Dispersion and Morphology in Carbon-Coated Nanocrystalline MgO Maxim S. Mel’gunov,* Elena A. Mel’gunova, Vladimir I. Zaikovskii, and Vladimir B. Fenelonov Boreskov Institute of Catalysis, Prospekt Akad. Lavrentieva 5, 630090 Novosibirsk, Russian Federation
Alexander F. Bedilo and Kenneth J. Klabunde Department of Chemistry, Kansas State University, 111 Willard Hall, Manhattan, Kansas 66506 Received August 5, 2002. In Final Form: July 29, 2003 Nanoscale carbon/MgO composite adsorbents have been prepared by chemical vapor deposition (CVD) of carbon over aerogel-prepared nanocrystalline MgO with propylene as the carbon precursor, and by pyrolysis of Mg(OH)2 aerogels modified with resorcinol. A new textural approach including adsorption methods and HREM has been applied for comprehensive characterization of their textural properties. Carbon has been found to form twisted isolated graphite-like fragments designated as nanoislands (1-2 graphite-like monolayers) coating the MgO surface in the case of carbon CVD and a thicker (2-3 graphitelike monolayers) coating in the case of pyrolysis of resorcinol-modified Mg(OH)2 aerogels. The average number of carbon nanoislands grows with an increase of carbon loading whereas their mean size remains approximately the same. The nanoislands have very high specific surface areas close to the maximum surface area possible for carbon. The discovered peculiarities of carbon deposition allow one to achieve controlled carbon/mineral (hydrophobic/hydrophilic) composition of the surface in the surface coverage range of 20-70%.
Introduction Carbon-mineral materials (CMMs) find various applications as catalysts, biocatalysts, and adsorbents.1-9 These materials have the so-called mosaic-like surface structure with the simultaneous presence of both hydrophilic (mineral) and hydrophobic (carbon) phases on their surface.1,10-12 Particular hydrophilic/hydrophobic compositions of the surface depends on the preparation conditions as well as on the type of the mineral compound. For example, it was shown recently that restricted mobility of carbon precursors over the surface of SiO2 gels and γ-Al2O3 in the kinetic regime of deposition results in homogeneous distribution of carbon clusters.10-12 Unrestricted mobility over a smooth surface of a SiO2 aerogel without narrow mesopores leads to a similar result. However, unrestricted mobility of carbon precursors in * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Fenelonov, V. B. Porous Carbon; BIC: Novosibirsk, 1995. (2) Kyotani, T.; Sonobe, N.; Tomita, A. Nature 1988, 331, 331. (3) Radovic, L. R.; Rodriguez-Reinoso, F. In Chemistry and Physics of Carbon, A series of Advances; Thrower, P. A., Ed.; Marcel Dekker Inc.: New York, 1997; Vol. 25, p 243. (4) Carrot, J. P.; Sing, K. S. W. Colloids Surf. 1986, 21, 21. (5) Rao, K. S. R.; Rao, P. K. Appl. Catal. 1990, 62, L19. (6) Rudzinski, W.; Gierak, A.; Leboda, R.; Dabrowski, A. Fresenius’ J. Anal. Chem. 1995, 7-8, 667. (7) Leboda, R. Chromatographia 1981, 14, 524. (8) Fenelonov, V. B.; Chernov, Yu. L.; Rachkovskaya, L. N.; Kryukova, G. N.; Mashkov, O. A.; Gavrilov, V. Yu.; Plaksin, G. V. React. Kinet. Catal. Lett. 1984, 35, 255. (9) Bandosz, T.; Jagiello, J.; Putyera, K.; Schwarz, J. A. Langmuir 1995, 11, 3964. (10) Mel’gunov, M. S.; Fenelonov, V. B.; Gorodetskaya, T. A.; Leboda, R.; Charmas, B. J. Colloid Interface Sci. 2000, 229, 431. (11) Mel’gunov, M. S.; Fenelonov, V. B.; Leboda, R.; Charmas, B. Carbon 2001, 39, 357. (12) Fenelonov, V. B.; Procudina, N. A.; Okkel’, L. G. J. Porous Matter. 1996, 3, 23.
dense-packed porous SiO2 gels with a considerable amount of narrow mesopores seems to result in carbon deposition inside these narrow mesopores. Carbon trapping inside narrow mesopores results in a decrease of carbon fraction on the CMM surface. The present paper reports on the carbon deposition over nanocrystalline aerogel-prepared (AP) MgO. AP alkaline earth metal (AEM) oxides are well-known as efficient destructive sorbents in reactions with hazardous compounds and air pollutants containing halogenated hydrocarbons, phosphorus, sulfur, etc.13-19 Nanocrystalline MgO transformed into MgCl2 in a reaction with 1-chlorobutane is an effective catalyst for 1-chlorobutane dehydrohalogenation.19 However, due to their enhanced hydrophility nanocrystalline AP AEM oxides rapidly deactivate in the presence of water vapor and CO2 during storage with access to the atmosphere. One of the possible ways to improve the hydrophobic properties of these materials is the formation of an “intelligent” carbon coating.20 Carbon chemical vapor deposition (CVD)21-24 from the gas phase (13) Li, Y.-X.; Klabunde, K. J. Langmuir 1991, 7, 1388. (14) Li, Y.-X.; Koper, O. B.; Atteya, M.; Klabunde, K. J. Chem. Mater. 1992, 4, 323. (15) Koper, O. B.; Li, Y.-X.; Klabunde, K. J. Chem. Mater. 1993, 5, 500. (16) Hooker, P. D.; Klabunde, K. J. Environ. Sci. Technol. 1994, 28, 1243. (17) Li, Y.-X.; Li, H.; Klabunde, K. J. Environ. Sci. Technol. 1994, 28, 1248. (18) Mishakov, I. V.; Bedilo, A. F.; Chesnokov, V. V.; Filimonova, S. V.; Martyanov, I. N.; Klabunde, K. J.; Volodin, A. M.; Parmon, V. N. Chem. Mater., submitted for publication. (19) Fenelonov, V. B.; Mel’gunov, M. S.; Mishakov, I. V.; Richards, R. M.; Chesnokov, V. V.; Volodin, A. M.; Klabunde, K. J. J. Phys. Chem. B 2001, 105, 3937. (20) Bedilo, A. F.; Sigel, M. J.; Koper, O. B.; Mel’gunov, M. S.; Klabunde, K. J. J. Mater. Chem. 2002, 12, 3599.
10.1021/la0206965 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/05/2003
Carbon-Coated Nanocrystalline MgO
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Table 1. Textural Characteristics of As-Prepared AP-MgO and CVD-Prepared C-AP-MgO Composites sample
as-prep AP-MgO
4h
8h
12 h
16 h
24 h
carbon content, X, g of carbon/g of C/MgO composite true density, FCMgO, g of C-AP-MgO/cc of solid phase true density of carbon, FC, g of carbon/cc of carbon total surface area, ACMgO, m2/g of C-AP-MgO external surface area, Aext, m2/g of C-AP-MgO volume of pores inside aggregates, Vmes, cc/g of C-AP-MgO partial AS area of MgO, AMgO, m2 of MgO/g of C-AP-MgO partial AS area of carbon, AC, m2 of carbon/g of carbon total surface area of carbon, ACT, m2 of carbon/g of carbon no. of carbon nanoislands per 100 nm2, [N]
0 3.10
0.0203 3.02 1.39 424 184 0.45 279 (7059) (12970) 3
0.0522 3.01 2.02 437 154 0.42 265 3299 5652 7
0.0759 2.97 1.97 448 139 0.42 194 3341 5760 10
0.0828 2.92 1.81 451 133 0.42 179 3290 5663 11
0.1105 2.88 1.85 467 123 0.39 100 3318 5701 15
allows comprehensive control of the carbon localization, morphology, and dispersion. The texture of the initial AEM support should not change considerably during CVD. Thus, the carbon morphology, localization, and dispersion in C/MgO composites can be analyzed using a new textural method based on adsorption studies of CMMs.10-12,25 By measuring the specific surface area26,27 of the CMM, ACMM, that of the initial support, A0, the specific accessible surface (AS) area, AC, of carbon,28,29 and the amount of carbon deposited on the surface, one can obtain a set of empirical parameters carrying important information about the properties of deposited carbon. In this paper this method is combined with high-resolution electron microscopy (HREM) to study the carbon localization, morphology and dispersion in C/AP-MgO composites. Experimental Section Preparation of starting AP-MgO is described elsewhere.13-19 Briefly, the procedure involves reaction of Mg ribbon with methanol under an inert atmosphere to obtain magnesium methoxide, mixing the latter with toluene, dropwise addition of deionized water, followed by vigorous stirring for several hours. This results in the formation of a hydroxide gel, which is dried in an autoclave under supercritical conditions. The obtained hydroxide is heated under dynamic vacuum ( 7 nm appear due to the carbon deposition. Estimation of the Individual Surface Areas of Carbon and MgO in C/MgO Composites Using CO2 Adsorption at 273 K. According to Gregg and Sing,26 sensitivity of polar or chemically active gases, especially CO2 at 273 K, to the nature of the surface can be used to measure the polarity of surfaces. The latter characteristic allows estimation of individual surface areas of components in a composite. For example, Parekh and Weller32 proposed a technique to measure the MoO3 surface area in MoO3/Al2O3 catalysts by O2 chemisorption. Because O2 both chemisorbs and physically adsorbs over MoO3, the following procedure has been applied to measure the chemisorption impact. O2 adsorption isotherms were measured twice at 77 K, separated with evacuation at 195 K to desorb O2 physically adsorbed during the first adsorption run. The difference between the first and the second isotherms was considered as O2 chemisorption and was used to evaluate individual surface areas of components. Recently, it has been shown that there is no need in using chemisorption for the purpose.28,29 Physical adsorption can be applied if one takes into account individual character of adsorption on different surfaces.28,29 In this study the latter technique has been used for evaluation of partial surface areas of MgO and carbon separately using CO2 adsorption. CO2 adsorption isotherms are shown in Figure 5. The first run isotherms are characterized by a sharp increase at low CO2 pressure ( 1.0 corresponds to carbon deposition as separate clusters on the surface of a mineral support. Adsorption data allow estimation of the total surface area of carbon per weight of carbon,11 including areas of AS measured by CO2 adsorption and IS, which is in contact with the MgO surface as
ACT ) AC(1 + 1/λ)
(8)
The corresponding values of ACT are reported in Table 1. So high values of the carbon surface area are actually quite plausible, except, maybe, for the value obtained for the 4 h sample. It was estimated recently that small carbon fragments consisting of several dozens of atoms could have the specific surface area up to 6000 m2/g.34 So high surface areas result from a considerable contribution of “edges” of carbon fragments. Modeling of Carbon Clusters. Figure 7 presents a model of a hydrocarbon molecule that can be considered as a precursor of a small carbon cluster. The “surface area“ of such a molecule depends on the total number of carbon atoms in it, n, and the number of carbon atoms that form the “edge“, ned. Another assumption has to be made to calculate the surface area. Let us consider all carbon atoms to be hexagonaly packed like in a graphite monolayer with no heteroatoms attached, and each carbon atom to be modeled as a triangle prism with edge, a, and height, h. The value of a can be estimated from the length of the carbon-carbon bond in aromatic molecules, l ) 0.142 nm, using a simple geometrical relation a ) lx3 ≈ 0.25 nm. The value of h can be assumed to be ∼0.35 nm, which represents the “thickness” of a benzene molecule. Then, the surface areas of the top and bottom sides of such prism can be calculated as (34) Kaneko, K.; Ishi, C.; Ruike, M.; Kuwabara, H. Carbon 1992, 30, 1075.
(10)
The corresponding edge sides of the prisms form the edge side of the carbon cluster. Likewise, the top and bottom sides of the prisms form the top and bottom sides of the cluster. In this model the accessible surface area of a carbon cluster, which lies flat on a flat surface of a support, can be expressed as a sum of the top and edge sides of the cluster:
(
)
N0 nedAed N0 ) Abas 1 + (11) AC ) (nAbas + nedAed) nM nAbas M where M ) 12 g/mol is the molecular mass of carbon. The total surface area is expressed as a sum of top, bottom and edge sides of a cluster:
(
ACT ) Abas 2 +
)
nedAed N0 nAbas M
(12)
The values of AC and ACT calculated using this model are shown in Figure 8 along with experimentally measured values. One can see that the experimental values of AC correspond to model clusters that have about 40-50% of carbon atoms at the edge. The model cluster shown in Figure 7 has exactly 40% of carbon atoms at the edges. Its planar size is about 1.0 × 2.0 nm. Thus, it can serve as a model of actual carbon nanostructures in our samples. The data in Figure 8 suggest that, if one assumes AC to be measured correctly, the values of ACT should be in a range of 4650-4850 m2/g. The actual measured values of ACT are in the range 5650-5700 (Table 1). The deviation from the model is 18-20%, which is very close to the experimental error of about 10%. This deviation can be explained by various reasons, e.g., rough (not ideally flat) surface of MgO nanocrystalls, limitations of the considered model, possible experimental errors of measuring the values of λ used for calculation of ACT (eqs 7 and 8), etc. However, none of these reasons should alter the main conclusions that the measured values of AC and ACT are plausible, and carbon is localized on the MgO surface as separate thin (monolayerlike) highly dispersed clusters. Moreover, the values of AC and ACT do not depend on the amount of deposited carbon. Hence, the mean size of the carbon clusters does not vary much with the carbon loading.
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Langmuir, Vol. 19, No. 24, 2003
Mel’gunov et al. Table 2. Textural Characteristics of As-Prepared CCOP-AP-MgO Composites total surface area, ACMgO, m2/g of composite surface area after carbon burning off, A0, m2/g of MgO carbon content, X, g/g of composite MgO AS area, AMgO, m2/g of composite carbon AS area, AC, m2/g of carbon relative increase of total surface area, ψ degree of MgO surface coverage, θ partial composition of composite surface, χ ratio of AS to IS areas of carbon fragments, λ total surface area of carbon, ACT, m2/g of carbon
Figure 9. Kinetic dependence of carbon CVD over AP-MgO.
Kinetics of Carbon Deposition. This idealized model of carbon localization gives a good correlation between the data obtained by the adsorption and HREM techniques, and also the kinetics of carbon deposition. The amount of deposited carbon (in mg per m2 of the surface area of the initial AP-MgO) is plotted in Figure 9. The best fit of this dependence is the first-order kinetics equation a (1 exp(-kt)) with the mean deviation of 1.3 × 10-5 g. According to known literature data, the first-order kinetics is typical for carbon CVD processes.35,36 Thus, on the basis of these two factors we assume the first-order kinetics to be the best fit in our case. The value of a is 0.53 ( 0.15 mg of carbon per m2 of starting AP-MgO. It can be considered as a limiting amount of carbon that can be deposited under chosen starting conditions. Then, the value of k equal to 3.54 × 10-2 h-1 presents the first-order kinetics rate constant. The “theoretical” amount of carbon necessary to cover 1 m2 of a flat surface by a layer of graphite-like carbon of a monolayer thickness has been estimated10 as 0.78 mg/ m2. One can consider that such a coincidence between the “theoretical” and measured values of a is an additional evidence for the formation of a very thin carbon film over the MgO surface, close in thickness to that of a graphitelike monolayer. One can estimate the half-time period as
τ)
ln 2 ) 19 ( 5 h k
(13)
If one assumes that carbon covers the surface of AP-MgO as a graphite-like monolayer, one-half of the MgO surface will be covered in 14-24 h. The actual values of θ for 12 and 16 h samples are 0.48 and 0.52, correspondingly (Figure 3). Thus, one-half of the MgO surface is covered in about 14 h, which is within the range obtained from the kinetic data. Taking into account the constant mean size of carbon clusters, one can also estimate the mean surface concentration of carbon nanoislands. Assuming that their average size does not depend on the amount of deposited carbon (as follows from the stable AS of carbon) and is equal to 7 × 7 carbon hexagons (about 100 carbon atoms), the mass of such averaged nanoisland, x, will be about 100 × 12/ 6.02 × 1023 ∼ 2.0 × 10-21 g. The number of nanoislands, N, can be considered as a ratio of the total mass of carbon to the weight of a single nanoisland as X/x. Finally, the concentration of nanoislands over MgO surface can be considered as (35) Tesner, P. A. In Chemistry and Physics of Carbon; Thrower. P. A., Ed.; Marcel Dekker: New York, 1984; Vol. 19, p 65 (see also references within). (36) Bru¨ggert, M.; Hu, Z.; Hu¨ttinger, K. J. Carbon 1999, 37, 2021 (and references within).
[N] )
X xAMgO(1 - X)
593 429 0.1024 338 2490 1.54 0.12 0.43 5.4 2951
(14)
The values of [N] presented in Table 1 show that the surface concentration of nanoislands steadily grows with carbon loading from 3 islands/100 nm2 for the 4 h sample to 15 islands/100 nm2 for the 24 h sample. C-AP-MgO Composite Prepared from AP-Mg(OH)2 Modified with Resorcinol. In this section we shall consider the structure of CCOP-AP-MgO composite prepared from aerogel-prepared Mg(OH)2 modified with resorcinol20 and compare it with the texture of CVDprepared C-AP-MgO composites. Textural characteristics of CCOP-AP-MgO sample are reported in Table 2. It was mentioned in our previous publication20 that after burning of carbon from CCOP-AP-MgO composite the texture of the resulting MgO sample becomes very similar to that of AP-MgO synthesized without any modifying agents. Thus, the CCOP-AP-MgO composite can be considered as interlinked MgO nanocrystalls covered with carbon, which does not significantly influence the texture of the MgO carcass. Thus, we can consider the texture of CCOP-AP-MgO composite similarly to the “host”-“guest” texture of CVDprepared C-AP-MgO composites. The total surface area of the MgO carcass (m2/g of MgO), including the AS area of the opened MgO and the IS area of the “carbon”-“MgO” interface, can be assumed to be equal to the surface area of the resulting MgO formed after burning carbon off, A0. In this assumption, all textural parameters introduced by eqs 3 and 7-10 have similar physical meanings as for the CVD-prepared composites. AS areas of carbon and MgO were calculated in the same manner as in the case of CVD-prepared samples and shown in Table 2 together with the values of θ, χ, ψ, and λ. In general, these values indicate that carbon deposition results in an increase of the absolute surface area value at the expense of the AS of carbon clusters. In comparison with CVD-prepared C/MgO composites, the surface of carbon clusters in CCOP-AP-MgO is much more accessible than the surface of nanoislands (λ ) 5.4). The surface of MgO is covered with carbon to a significantly lower degree (θ ) 0.12 versus θ ) 0.72 for 24 h sample). However, the composition of the composite surface is not very different from that of CVD-prepared 24 h sample (χ ) 0.43 vs 0.79). The measured value of λ ) 5.4 indicates that the model of carbon morphology elaborated for CVD nanoislands can hardly be applied for description of carbon morphology in CCOP-AP-MgO. Such a high value of λ is evidence of an increased thickness of carbon fragments compared to the carbon nanoislands in CVD-prepared samples, which is supported by the HREM data. Conclusion Carbon deposited by CVD over nanocrystalline MgO has a distinctive morphology of small isolated graphitelike one-carbon-atom thick fragments, which are desig-
Carbon-Coated Nanocrystalline MgO
Langmuir, Vol. 19, No. 24, 2003 10433
nated as carbon nanoislands. The number of nanoislands over the MgO surface linearly increases with the carbon loading whereas the average size does not change. According to the CO2 and N2 adsorption data and HREM, the surface composition of a carbon/MgO composite can be varied in a controlled way between 20 and 70% of the MgO surface covered with carbon using a small amount of deposited carbon. The localization of carbon deposited over the AP-MgO surface is very similar to that observed for mesoporous silica gel when carbon precursors are chemically immobilized on the SiO2 surface.10 It appears that the carbon precursors interact with the AP-MgO surface in such way that this interaction prevents them from migrating over the surface to form large clusters or nanoparticles. Carbon-coated MgO composites prepared by heat treatment of resorcinol-modified Mg(OH)2 aerogel show a similar type of carbon localization. The main difference between these two types of materials is the size of carbon clusters.
V ) volume, cc/g, (depending on subscript index could be pore, solid phase, etc. volume) x ) mean mass of a carbon nanoisland, g X ) mass of carbon per gram of C/MgO composite
Notations
ψ ) ratio of a textural parameter (surface area, volume, etc.) in a composite to that of the initial support taking into account the mass change θ ) ratio of the MgO surface area covered with carbon to the total MgO surface area χ ) part of a composite surface area attributed to carbon λ ) accessibility of carbon surface F ) true density, g/cm3; reciprocal to the volume of solid phase
Alphabetical Entities A ) specific surface area, m2/g a ) preexponential factor in first-order kinetic reaction, g/m2 a(P/P0) ) adsorption isotherm, cc/m2 STP (standard temperature and pressure) h ) thickness of carbon graphite-like monolayer, nm k ) rate constant in first-order kinetic reaction, h-1 M ) molecular mass, g/mol N0 ) Avogadro number n ) number of carbon atoms in a cluster [N] ) surface concentration of carbon nanoislands, clusters/nm2
Subscript Indexes bas ) attributed to carbon atoms forming a basal plane of a carbon cluster CMgO ) attributed to C/MgO composite C ) attributed to carbon CT ) carbon total. Used with ACT to express the total surface area of carbon ed ) attributed to carbon atoms forming an edge of a carbon cluster MgO ) attributed to initial AP-MgO support m ) attributed to mesopores s ) attributed to solid phase a ) attributed to mesoporous aggregates of nanoparticles Greeks
Acknowledgment. The financial support of the U.S. Army Research office and CRDF (Project RC1-2340-NO02) is acknowledged with gratitude. LA0206965
