Silica Gel Carbonized Due to Pyrolysis ... - ACS Publications

Langmuir 2000, 16, 3227-3243

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CVD-Titania/Silica Gel Carbonized Due to Pyrolysis of Cyclohexene V. M. Gun’ko,† R. Leboda,*,‡ M. Marciniak,‡ W. Grzegorczyk,‡ J. Skubiszewska-Zie¸ ba,‡ A. A. Malygin,§ and A. A. Malkov§ Institute of Surface Chemistry, 31 Prospect Nauki, 252022 Kiev, Ukraine; Faculty of Chemistry, Maria Curie-Skłodowska University, 20031 Lublin, Poland; and St.-Petersburg State Institute of Technology (Technical University), 9803 St.-Petersburg, Russia Received May 6, 1999. In Final Form: December 13, 1999 Titania/silica gels covered by a carbon layer using pyrolysis of cyclohexene at 973 K and containing different amounts of titania (CTiO2) and carbon deposit (CC) have been studied by means of pyrolysis kinetics, nitrogen adsorption/desorption, TEM, IR spectroscopy, differential thermogravimetry, and theoretical methods. The pyrolysis rate depends on the concentration and the characteristics of a titania phase, as anatase, which forms at a lower synthetic temperature in comparison with rutile, catalyzes this process more strongly than rutile does. The adsorbent mesoporosity decreases with increasing concentrations of titania and carbon covering the oxide surface mainly in mesopores, but in the case of C/SiO2, carbon can be also grafted onto the outer surface of silica. The microporosity (maximal for binary systems C/SiO2 or TiO2/SiO2) of carbon/titania/silica gels is relatively low and changes slightly with increasing deposit concentration. The influence of carbon on the specific surface area of the adsorbents is weaker than that of titania due to not only the difference in the morphology of these deposits per se but also the types of their distributions and contacts between grafted matters and substrate surfaces. Carbon deposit reduces the amount of adsorbed water to a greater extent than titania does. Theoretical modeling of C/TiO2/SiO2 and pyrolysis of cyclohexene has been performed using different quantum chemical methods and molecular mechanics.

Introduction Porous oxides covered by carbon due to carbonization of organic precursors can be of interest as effective adsorbents for both polar and nonpolar compounds.1,2 The physicochemical properties of carbon deposit/mixed oxides can depend nonlinearly on the amounts of carbon (CC) and different oxide components and alter due to the changes in the substrate origin and techniques of oxides and carbon preparation.1-4 Features of the properties of carbonized oxide surfaces can be elucidated using adsorption/desorption of probe compounds, as these phenomena depend not only on the adsorbent texture but also on variations in dispersion and polar (electrostatic) compo* To whom correspondence should be addressed. Fax: 48 81 533 33 48. † Institute of Surface Chemistry. ‡ Maria Curie-Skłodowska University. § St.-Petersburg State Institute of Technology (Technical University). (1) (a) Leboda, R. Mater. Chem. Phys. 1992, 31, 243. (b) Leboda, R. Mater. Chem. Phys. 1993, 34, 123. (c) Leboda, R.; Gierak, A.; Charmas, B.; Łodyga, A. React. Kinet. Catal. Lett. 1993, 50, 63. (d) Leboda, R.; Łodyga, A.; Gierak, A. Mater. Chem. Phys. 1997, 51, 216. (e) Leboda, R.; Łodyga, A.; Charmas, B. Mater. Chem. Phys. 1997, 55, 1. (f) Gun’ko, V. M.; Skubiszewska-Zie¸ ba, J.; Leboda, R.; Zarko, V. I. Langmuir, in press; (g) Villieras, F.; Leboda, R.; Charmas, B.; Bardot, G.; Gerard, G.; Rudzinski, W. Carbon 1998, 36, 1501. (h) Leboda, R.; SkubiszewskaZie¸ ba, J.; Grzegorczyk, W. Carbon 1998, 36, 417. (2) (a) Kamegawa, K.; Yoshida, H. J. Colloid Interface Sci. 1993, 159, 324; 1995, 172, 94. (b) Bandosz, T. J.; Jagiełło, J.; Putyera, K.; Schwarz, J. A. Langmuir 1995, 11, 3964. (c) Bandosz, T. J.; Jagiełło, J.; Putyera, K.; Schwarz, J. A. Carbon 1994, 32, 659. (d) Bandosz, T. J.; Jagiełło, J.; Andersen, B.; Schwarz, J. A. Clays Clay Miner. 1992, 40, 306. (e) Jankowska, H.; Swiıtkowski, A.; Choma, J. Active Carbon; Harwood: New York, 1991. (3) Leboda, R.; Gun’ko, V. M.; Marciniak, M.; Malygin, A. A.; Malkin, A. A.; Grzegorczyk, W.; Trznadel, B. J.; Pakhlov, E. M.; Voronin, E. F. J. Colloid Interface Sci. 1999, 218, 23. (b) Leboda, R.; Marciniak, M.; Gun’ko, V. M.; Grzegorczyk, W.; Malygin, A. A.; Malkov, A. A. Colloid. Surf. A., submitted for publication; (c) Leboda, R.; Turov, V. V.; Marciniak, M.; Malygin, A. A.; Malkov, A. A. Langmuir, in press.

nents of the energy of interaction between adsorbate molecules and adsorbent patches of the different origin.1,3 Carbonized silicas (carbosils, CS) synthesized by means of pyrolysis of organic precursors on fumed silica or silica gel as substrates and their interactions with polar and nonpolar gaseous or liquid compounds were investigated using several methods.1-5 These studies showed that adsorption of polar molecules (e.g., water) occurred predominantly on the oxide phase (bonding to hydroxyls) or due to interaction with the oxidized fragments of the carbon deposit, the amount of which depended on a synthetic technique and the availability of oxygen in a carrier gas or precursors. Nonpolar molecules mainly adsorbed onto nonpolar carbon fragments (tiny globules with a pregraphite structure having a relatively low porosity) covering an oxide surface [typically, the outer surface of particles with large amounts (>10 wt %) of grafted carbon] by a nonuniform layer. Clearly, in the case of carbonized mixed oxides, the surface structure can be more complex than that of individual oxides, especially when one of them [or their interface having, e.g., Brønsted acid sites tM1O(H)M2t] can catalyze pyrolysis (and related reactions) that causes an additional (4) (a) Turov, V. V.; Leboda, R.; Bogillo, V. I.; Skubiszewska-Zie¸ ba, J. Langmuir 1995, 11, 931. (b) Leboda, R.; Skubiszewska-Zie¸ ba, J.; Sidorchuk, V. V.; Tertykh, V. A.; Zarko, V. I. J. Non-Cryst. Solids 1995, 189, 265. (c) Turov, V. V.; Leboda, R.; Bogillo, V. I.; SkubiszewskaZie¸ ba, J. Adsorp. Sci. Technol. 1996, 14, 319. (d) Turov, V. V.; Leboda, R. Adv. Colloid Interface Sci. 1999, 79, 173. (e) Turov, V. V.; Leboda, R.; Bogillo, V. I.; Skubiszewska-Zie¸ ba, J. Langmuir 1997, 13, 1237. (f) Turov, V. V.; Leboda, R.; Bogillo, V. I.; Skubiszewska-Zie¸ ba, J. J. Chem. Soc., Faraday Trans. 1997, 93, 4047. (g) Turov, V. V.; Leboda, R.; Skubiszewska-Zie¸ ba, J. J. Colloid Interface Sci. 1998, 206, 58. (5) (a) Leboda, R.; Dıbrowski, A.; Skubiszewska-Zie¸ ba, J. International Conference on Silica Science and Technology, “Silica 98”; Mulhouse, France, 1-4 Sept. 1998, p 501. (b) Leboda, R.; Dıbrowski, A. In Adsorption on New and Modified Inorganic Sorbents; Dıbrowski, A., Tertykh, V. A., Eds.; Studies in Surface Science and Catalysis 99; Elsevier: Amsterdam, 1996; p 115.

10.1021/la990555b CCC: $19.00 © 2000 American Chemical Society Published on Web 02/19/2000

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Table 1. Surface Characteristics of the Silica and Titania/Silica Adsorbents sample

Ts °C

CTiO2, wt %

SBET, m2/g

Smes, m2/g

SK, m2/g

SDA, m2/g

SDS, m2/g

V, cm3/g

VDA, cm3/g

VDS, cm3/g

rp, nm

DTW

nTW

x e 0.1

KSK K21 K24 K28 K61 K62 K64 K68

200 200 200 600 600 600 600

5.2 15.2 24.4 3.0 5.1 9.8 17.9

377 349 281 228 355 334 318 281

342 273 213 200 328 287 282 216

535 463 351 295 528 469 458 419

55 60 50 41 52 52 48 34

60 85 61 85 63 64 56 43

0.98 0.88 0.69 0.54 0.94 0.89 0.85 0.76

0.154 0.169 0.132 0.014 0.152 0.149 0.138 0.133

0.091 0.069 0.072 0.054 0.072 0.063 0.075 0.058

5.2 5.0 4.9 4.8 5.3 5.3 5.4 5.4

2.2030 2.2129 2.2009 2.1877 2.1942 2.2011 2.2020 2.1895

1.852 1.842 1.861 1.968 1.868 1.855 1.855 1.874

2.3669 2.3832 2.3774 2.5044 2.3532 2.3485 2.3521 2.3407

nonuniformity of the carbon phase due to the dependence of its distribution (in pores or on the outer surface of substrate particles) depending on the distribution of different active sites on the oxide matrices. These effects are responsible for complicated dependencies of the specific surface area, pore structure, volume and size distributions, etc. on the amounts of the deposits and synthetic conditions.1-5 Also, the problem of coking of heterogeneous catalysts is important, as this process results in deactivation of their surfaces.6-8 Coke, possessing a polyaromatic pregraphitic character, forms on the external and internal surfaces of the acid-base catalysts and its distribution depends on its concentration, as initially (CC < 8%) it forms mainly on the internal surfaces.8 Additionally, the carbon deposition depends on acidity of the active sites distributed inside and outside of the catalyst particles. The activation energy of coking was found to be dependent on the heat of sorption, giving an indication of the strength of active sites in this process. Coke formation involves thermal elimination of hydrogen or paraffins from carbenium (carbonium) ions or disproportionation of two adsorbed adjacent ions, their cyclization (“snake swallowing its own tail” mechanism), and many other reactions over the 250-450 °C range. Notice that there is a marked relationship between the basicity of adsorbed organics and the weight percentage of coke, which forms on acidic catalysts, and coking of the alumina/silica surface is significantly greater than that of the silica surface which is “inert”. Also, formation of radicals can contribute reactions causing growth of the carbon deposit even on non-acid-base “inert” surfaces such as silica gel and different processes in the gas phase (near the surface), especially at T > 450 °C, when deactivation of acidic catalysts occurs also due to decay of active sites.6-8 Despite the numerous studies of carbonized porous oxides and deactivation of oxide catalysts due to coking and related processes, many questions concerning the structures of the carbon deposit per se and the carbon/ mixed oxide interfaces, as components of mixed oxides can possess different catalytic abilities in carbon phase formation, and interaction with various adsorbates still remain unclear. Therefore, the aim of this work was to study structural features and the properties of the surfaces of carbon/silica, titania/silica gel, and carbon/titania/silica (6) (a) Cumming, K. A.; Wojciechowski, B. W. Catal. Rev.sSci. Eng. 1996, 38, 101. (b) Corma, A., Fornes, V., Imelik, B., Naccache, C., Coudurier, G., Ben Tarrit, Y.,Vedrine, J. C., Eds. Catalysis by Acids and Bases; Studies in Surface Science and Catalysis 20; Elsevier: Amsterdam, 1985. (c) Wojciechowski, B. W.; Corma, A. Catalytic Cracking; Dekker: New York, 1986. (d) Butt, J. B.; Petersen, E. E. Activation, Deactivation and Poisoning of Catalysts; Academic Press: New York, 1988. (7) (a) Lisovskii, A. E.; Aharoni, Ch. Catal. Rev.sSci. Eng. 1994, 36, 25. (b) Hughes, R. Deactivation of Catalysts; Academic Press: London, 1984. (8) (a) Bhatia, S.; Beltramini, J.; Do, D. D. Catal. Rev.sSci. Eng. 1989-90, 31, 431. (b) Buyanov, R. A. Coking of Catalysts; Nauka: Novosibirsk, Russia, 1983. (c) Fenelonov, V. B. J. Porous Mater. 1996, 2, 263. (d) Fenelonov, V. B.; Procudina, N. A.; Okkel, L. G. J. Porous Mater. 1996, 3, 23.

DAJ x e 0.85 2.4186 2.4301 2.4266 2.4145 2.4066 2.4152 2.4156 2.4012

gel adsorbents depending on the amounts of carbon and titania by using experimental and theoretical methods. Experimental Section Materials. Commercial mesoporous silica gel KSK-2 (Russia) with average pore radius rp ≈ 5 nm and the specific surface area SBET ) 377 m2/g (Table 1) was utilized as an initial material to prepare titania/silica gel (CVD-TSG) adsorbents by means of a chemical vapor deposition (CVD) technique. Synthesis of these mixed oxides (using chemisorption of TiCl4 and hydrolysis of residual Ti-Cl bonds at different temperatures) was described in detail elsewhere.3 The CVD-TSG samples synthesized at Ts ) 200 °C (labeled K2-i where i is the number of the CVDhydrolysis cycles) contain different amounts of anatase. The titania phase in the K6-i samples (Ts ) 600 °C) consists of mainly rutile with a minor portion of anatase.3 Grafted titania represents a nonuniform layer with particles of different sizes having relatively weak contacts with the silica substrate, as a characteristic band at 950 cm-1 linked to the tSi-O-Tit bridges is absent in the IR spectra of all the CVD-TSG samples. Carbon/ oxide samples [K2-i-j and K6-i-j where j is the number (1 or 2) of samples having different concentrations of pyrocarbon (≈10 wt % at j ) 1; ≈20 wt % at j ) 2)] were synthesized using pyrolysis of cyclohexene at the silica gel and CVD-TSG surfaces at 973 K and a constant partial pressure of 51.3 mmHg. Before pyrolysis, samples were heated at the reaction temperature to remove adsorbed water and other compounds up to a constant weight. The cyclohexene pyrolysis was performed in a flow reactor under isothermal conditions and gravimetric control for 5 h. The technique of oxide carbonization was described in detail elsewhere.1 Methods. (a) IR. The infrared spectra were recorded using a Specord M80 (Karl Zeiss, Jena, Germany) spectrophotometer. For observation over the 1600-400 cm-1 range, a sample (1-5 mg) was stirred with a drop of Nujol mull and placed between KBr glasses. The IR spectrum of pure Nujol was subtracted from the obtained one, and then the resulting spectrum was smoothed using the FFT filter. To record the spectra over the 4000-3500 cm-1 range (with mean errors (1 cm-1), the pressed samples (28 × 2 mm) of oxides were utilized. (b) Pyrolysis Kinetics. Kinetics of carbonization of CVDtitania/silica gel due to pyrolysis of cyclohexene was studied under isothermal conditions (973 K) and at a constant partial pressure of cyclohexene for 5 h with consideration for the changes in the specific surface area assuming a linear dependence of SBET on the carbonization time. (c) Nitrogen Adsorption. The nitrogen adsorption-desorption isotherms at 77.4 K were obtained using a Micromeritrics ASAP 240 5 N (V-1.01) adsorption analyzer. The specific surface area (SBET) was calculated according to the BET method9 utilizing the nitrogen adsorption data at P/P0 (P and P0 were the equilibrium and saturation pressures) between 0.06 and 0.2. Average pore radius (rp) values (estimated using the values of (9) (a) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surface; 6th ed.; Wiley: New York, 1997. (b) Gregg, S. J., Sing, K. S. W., Stoeckli, H. F., Eds. Characterization of Porous Solids; Soc. Chem. Industry: London, 1979. (c) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity, 2nd ed.; Academic Press: London, 1982. (d) Toth, J. Adv. Colloid Interface Sci. 1995, 55, 1. (e) Barret, E. B.; Yoyner, L. G.; Halenda, P. P. J. Am. Chem. Soc. 1951, 73, 373. (f) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moskou, L. Pure Appl. Chem. 1985, 57, 603. (g) Fenelonov, V. B. Porous Carbon; IC: Novosibirsk, Russia, 1995.

CVD-Titania/Silica Gel

Langmuir, Vol. 16, No. 7, 2000 3229

SBET and pore volume V calculated at a high P/P0 value) were slightly larger than those obtained using the Barrett-JoynerHalenda method9e for the adsorption data. The surface area and the volume (VDA) of micropores (at rp e 1 nm) were estimated, based on the Dubinin-Astakhov (DA) equation10 L

ln(a) )

[ ( ) ( )] Wi

∑ ln W i

A

-

ni

(1)

βEi

0

Θ)

where L ) 1 or 2, Wi is the current and W0 the limiting adsorptions of vapor, A ) RT ln(P0/P) is the differential molar work equal (with inverse sign) to the variation in the Gibbs free energy, β is the similarity coefficient expressing the ratio between the characteristic free energies of adsorption of the test (nitrogen) and reference vapor (benzene) (βN2 ) 0.33), and Ei is the characteristic adsorption energy, which is inversely proportional to the pore half-width r0; P/P0 > 1.5 × 10-4. Additionally, the Dubinin-Stoeckli (DS) equation10

a)

)[

(

( )]

W0 -mr02A2 r0 exp 1 + erf 2 2V* D δDx2

(2)

(where r0 is the pore half-width at a maximum of the distribution, δ is the distribution dispersion

D ) (1 + 2mδ2A2)0.5;

m ) (β k)-2;

erf(z) )

2 xπ

∫e

z -t2

0

dt

∫

as

a0

A da

(3)

where σ is the surface tension of nitrogen at 77.4 K, a0 is the amount of adsorbate at an initial point of the hysteresis loop, and as is the limiting value of adsorption. Calculation of the fractal dimension (D)13 of the adsorbents was performed on the basis of the adsorption data using the Frenkel-Halsey-Hill equation14 in the form14a

[ ( ( ))]

ln(Θ) ) const + (DAJ - 3) ln ln

P0 P

[(

(4)

where Θ denotes the relative adsorption a/am (am is the volume of adsorbed gas for the monolayer coverage calculated with the BET equation), at a low coverage at P/P0 e 0.1 with no capillary (10) (a) Dubinin, M. M. Carbon 1985, 23, 373. (b) Dubinin, M. M. In Progress in Surface and Membrane Science; Cadenhead, D. A., Ed.; Academic Press: New York, 1975; Vol. 9, p 1. (c) Dubinin, M. M.; Stoeckli, F. J. Colloid Interface Sci. 1980, 75, 34. (d) Dubinin, M. M. Adv. Colloid Interface Sci. 1968, 2, 217. (e) Dubinin, M. M. Izv. AN USSR, Ser. Khim. 1980, No 1, 22. (f) Dubinin, M. M.; Kataeva, L. I. Izv. AN USSR, Ser. Khim. 1977, No 3, 516. (g) Dubinin, M. M.; Kataeva, L. I.; Ulin, B. I. Izv. AN USSR, Ser. Khim. 1977, No 3, 510. (11) Kiselev, A. V.; Dreving, V. P.; Runov, A. D. Dokl. AN USSR 1945, 46, 310. (12) (a) Gun’ko, V. M. Zh. Fiz. Khim. 1989, 63, 506. (b) Gun’ko, V. M.; Lukyanchuk, V. M.; Chuiko, A. A. Catal. Catal. 1992, N 28, 82. (c) Lukyanchuk, V. M.; Gun’ko, V. M.; Tarkovskaya, I. A. Catal. Catal. 1992, N28, 73. (13) (a) Kaneko, K.; Sato, N.; Suzuki, T.; Fujiwara, Y.; Nisikawa, K.; Jaroniec, M. J. Chem. Soc., Faraday Trans. 1991, 87, 179. (b) Avnir, D. Ed. The Fractal Approach to Heterogeneous Chemistry; Wiley: Chichester, England, 1989. (14) (a) Avnir, D.; Jaroniec, M. Langmuir 1989, 5, 1431. (b) Xu, W.; Zerda, T. W.; Yang, H.; Gerspacher, M. Carbon 1996, 34, 165. (c) Zerda, T. W.; Yang, H.; Gerspacher, M. Rubber Chem. Technol. 1992, 65, 130. (d) Jarze¸ bski, A. B.; Lorenc, J.; Pajık, L. Langmuir 1997, 13, 1280. (e) Jaroniec, M.; Kruk, M.; Olivier, J. Langmuir 1997, 13, 1031.

)

3 - DTW F , xmaxnµAn γ n n 3 - DTW , xminnµAn µDTW-3/nADTW-3 (5) γ n

(

)]

[where F ) 3 - D/xmax3-D - xmin3-D; µ ) (kβ)-n; n (nTW in Tables 1 and 2) is the equation parameter (eq 5 at n ) 2 is close to a fractal analogue of the Dubinin-Radushkevich equation);15 xmax and xmin are the maximal and minimal half-widths of pores; k is the constant; γ denotes the incomplete gamma function] for an estimation of the fractal dimension DTW, nTW, and distribution function f(DTW) at P/P0 e 0.1. The specific surface area of mesopores (Smes) and their distribution (as dSmes/drp) were calculated according to the improved theory of capillary condensation/evaporation including some corrections of the equations, which link the adsorbed layer thickness (t) and the radius of emptied pores,10e-g using the integral form12

[∫

Vmes ) V*

and k is the constant of 10-12 kJ nm/mol9,10) was used for calculation of the surface area (SDS) and the volume (VDS) of micropores utilizing a portion of the isotherms at 1.0 × 10-5 e P/P0 e 0.2. The SDA, VDA, SDS, and VDS values were calculated with and without correction of the isotherm due to adsorption in mesopores. The mesopore surface area (SK) was determined using the Kiselev equation11

SK ) σ-1

condensation in mesopores and at P/P0 < 0.85 (or 0.9) with a significant capillary condensation.14d For all the studied samples, ln Θ is a near linear function of ln(ln(P0/P)) up to P/P0 ≈ 0.9. However, according to Terzyk et al.,15 the use of eq 4 for micromesoporous solids, such as the studied C/TiO2/SiO2 samples, can be doubted. Therefore, we also utilize another isotherm equation15

(

)

2t(y) 2t(y) 1+ a(y) dy rp(y) - t(y) rp(y) - t(y) P0 P0 2t(y) dy t(y) P P r (y) - t(y) p

P0

P

∫

∫

Smes ) 2

1V′mes(x)

x0

rp(x)

]

∫

dx

(6)

(7)

(where V* is the volume of gram of adsorbed nitrogen, t(P) is the thickness of the adsorption layer, and rp(P) is the radius of mesopores from which desorption occurs at the pressure P; x ) P/P0; x0 ) 0.05; V′, a′, and t′ are the derivatives) using the developed program package.12 To obtain continuous dependencies a(x), rp(x), t(x), etc. on the basis of the discrete experimental data aex(x), the corresponding spline-functions as(x) were used. Additionally, the minimization of the function

Θ(λ1) )

∫

x2

x1

[as(x) - at(x,λi)]2

dx x

(8)

(instead of the corresponding sums) was utilized in order to calculate unknown parameters λi of different adsorption equations at(x,λi) at relative pressures x1 e x e x2. Notice that the calculations of the SDA and VDA values were performed with eq 1 at L ) 1 using some corrections (and with no them) in the adsorption isotherms with regard to the nitrogen adsorption into mesopores (utilizing Smes as more appropriate than SK).12 To compute the adsorption energy distribution, the modified BET equation9

Θ)

cx[1 - (n + 1)xn + nxn+1] (1 - x)[1 + (c - 1)x - cxn+1]

(9)

where n is the number of adsorbed layers, c ) γeE-QL/RT, QL is the liquefaction heat, E is the adsorption energy, and γ is the constant (≈1), was used as a local isotherm Θl in the overall adsorption isotherm written in the form of Fredholm integral equation of the first kind (15) (a) Terzyk, A. P.; Gauden, P. A.; Rychlicki, G.; Wojsz, R. Colloid. Surf. A 1999, 152, 293. (b) Terzyk, A. P.; Gauden, P. A.; Rychlicki, G.; Wojsz, R. Colloid. Surf. A 1998, 136, 245. (c) Terzyk, A. P.; Gauden, P. A.; Rychlicki, G.; Wojsz, R. Colloid. Surf. A 1997, 126, 67. (d) Terzyk, A. P.; Gauden, P. A.; Rychlicki, G.; Wojsz, R. Colloid. Surf. A 1996, 119, 175. (e) Wojsz, R.; Terzyk, A. P. Comput. Chem. 1996, 20, 427. (f) Wojsz, R.; Terzyk, A. P. Comput. Chem. 1997, 21, 83. (g) Terzyk, A. P.; Gauden, P. A.; Rychlicki, G.; Wojsz, R. Langmuir 1999, 15, 285.

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Table 2. Surface Characteristics of the Carbon Coated Adsorbents sample

CC, wt %

K0-1 K0-2 K0-3 K21-1 K21-2 K24-1 K24-2 K28-1 K28-2 K61-1 K62-1 K62-2 K64-1 K64-2 K68-1 K68-2

10.13 9.80 20.51 10.18 20.44 10.65 20.69 10.31 20.50 10.70 10.11 20.33 9.78 20.11 9.71 19.94

CTiO2, wt %

SBET, m2/g

Smes, m2/g

S K, m2/g

SDA, m2/g

SDS, m2/g

V, cm3/g

VDA, cm3/g

VDS, cm3/g

rp, nm

DTW

nTW, eq 5

x e 0.1

5.2 5.2 15.2 15.2 24.4 24.4 3.0 5.1 5.1 9.8 9.8 17.9 17.9

338 374 289 293 252 236 208 188 152 306 295 255 291 237 252 204

273 292 209 250 201 215 159 150 95 283 269 262 273 200 223 175

477 541 388 401 330 320 267 253 190 438 415 361 408 329 357 279

57 64 53 47 42 35 36 32 30 47 45 39 57 38 36 32

77 88 60 58 47 40 42 38 41 49 46 46 77 43 43 37

0.84 0.92 0.66 0.71 0.55 0.55 0.44 0.42 0.31 0.76 0.74 0.59 0.74 0.56 0.65 0.49

0.157 0.177 0.140 0.130 0.117 0.103 0.099 0.087 0.079 0.136 0.128 0.108 0.157 0.107 0.102 0.091

0.060 0.071 0.080 0.047 0.058 0.069 0.056 0.054 0.037 0.061 0.059 0.038 0.060 0.051 0.032 0.047

5.0 4.9 4.8 4.8 4.4 4.6 4.2 4.5 4.0 5.0 5.0 4.7 5.1 4.7 5.2 4.8

2.1954 2.2012 2.2011 2.1969 2.1904 2.1995 2.1961 2.2051 2.2019 2.1926 2.2026 2.1951 2.1921 2.1915 2.1881 2.1864

1.866 1.856 1.856 1.862 1.873 1.858 1.864 1.848 1.854 1.870 1.852 1.865 1.871 1.872 1.878 1.879

2.3576 2.3525 2.3584 2.3548 2.3512 2.3694 2.3665 2.3640 2.3667 2.3517 2.3554 2.3449 2.3511 2.3553 2.3345 2.3406

Θ(T, P) )

∫

xmax

o

Θl(T, P, x) f(x) dx

DAJ x e 0.85 2.4283 2.4087 2.4257 2.4253 2.4232 2.4233 2.4327 2.4211 2.4367 2.4048 2.4158 2.4307 2.4183 2.4100 2.4154 2.4169

(10)

where f(x) is the unknown distribution function of a given parameter x (adsorption energy, pore size, fractal dimension). To calculate the f(x) function, the constrained regularization method16,17 can be used, as solution of eq 10 is a well-known ill-posed problem due to a strong influence of noise components in experimental data, which do not allow us to utilize exact inversion formulas or iterative algorithms.17 For this purpose, the CONTIN program package17 was modified by one of the authors of this work to apply different adsorption equations [e.g., eq 1 at L ) 1-8 or the Dubinin-Radushkevich equation for f(rp); eq 9, deBoer-Hill9 and deBoer-Hill-Toth9d equations for f(E); eq 5 for f(DTW)] to estimate the distributions of the nitrogen adsorption energy, pore size, and fractal dimension. For calculations of the f(rp) distributions of the carbon/titania/silica adsorbents, eq 1 at L ) 4 was used as a kernel of eq 10. The weights of the terms in eq 1 were estimated using the local isotherm approximation method. One of the reasons of the increase in L up to 4 for f(rp) was a high nonuniformity of complex C/TiO2/SiO2 materials (it is known9,10 that ni in eq 1 depends not only on the pore size distribution but also on the nature of adsorbent surface) and a maximal stability of the f(rp) shape at L ) 3-5. Notice that the CONTIN program package was tested using different model isotherms with the given parameter magnitudes (xt) and the obtained results of all the tests were positive; i.e., the distribution functions f(x) obtained with the CONTIN had peaks only around the given xt values. (d) DTG. Water desorption from the samples was explored over the 25-700 °C range using a Q 1500 D (Paulik & Paulik) apparatus under quasi-isothermal and quasi-isobaric conditions.18 The apparatus increased the temperature at β0 ) 3-5 K/min until the beginning of an intensive decomposition (desorption); then the heating rate was small up to the end of this intensive desorption. After that, the rate was again increased to β0 until the beginning of the next intensive desorption. (e) Transmission Electron Microscopy. TEM micrographs of CS samples were obtained using a BS 540 (Tesla) apparatus (accelerating voltage 80 kV, resolution 0.8 nm, magnification ×7000-×12000). Microscope samples were prepared using the method of direct platinum-carbon replication. This method consists of evaporation of carbon and platinum onto the surface of adsorbent, followed by treatment of the samples in hydrofluoric acid to dissolve the oxide phases.

Results and Discussion According to the IR spectra (Figure 1), a marked concentration of tSiOH groups is observed at the silica (16) Szombathely, M.v.; Brauer, P.; Jaroniec, M. J. Comput. Chem. 1992, 13, 17. (17) (a) Provencher, S. W. Comput. Phys. Comm. 1982, 27, 213. (b) Provencher, S. W. Comput. Phys. Comm. 1982, 27, 229. (18) Paulink, F. Special Trends in Thermal Analysis; Wiley: Chichester, England, 1995.

Figure 1. IR spectra of initial silica gel (KSK) and CVD-TSG (K24, K28, K61).

Figure 2. IR spectra of initial silica gel (KSK), CVD-TSG (K28, K68), fumed titania/silica (TS), and CVD-titania/fumed silica A-300 (CVD-TS).

gel surface even after several CVD cycles in the titania synthesis and rehydration of the samples. Also, the absorbance intensity at 950 cm-1 (Figure 2) does not practically change on deposition of titania, independent of the number of the CVD cycles. This band corresponds to Si-O vibrations in the tSi-O-Tit bridges (plus some contribution of adsorbed water).19 For example, in the case of fumed titania/silica (TS at 20.5 wt % TiO2), the intensity of this band is significantly higher than that of CVDtitania/fumed silica (CVD-TS at 22.2 wt % TiO2) or CVDTSG (Figure 2) (synthesis and properties of CVD-titania/


Langmuir, Vol. 16, No. 7, 2000 3231

Figure 3. Amount of the carbon deposit as a function of the pyrolysis time.

fumed silica and fumed TS were described in detail elsewhere3,19). Initial silica gel or CVD-TSG can adsorb water more intensively than CVD-TS or fumed silica (A300) do; therefore, the absorbance intensity at 950 cm-1 is higher for KSK (due to a contribution of water to this band) than that of A-300, or higher for K2-i and K6-i than that of CVD-titania/fumed silica (Figure 2). It should be noted that an increase in absorbance at 700-500 cm-1 and > 3750 cm-1 is due to an increase in the titania content in CVD-TSG. Thus, the IR data attest that CVD-titania covers the silica gel surface by a nonuniform layer possessing supposedly cluster structure with weak contacts (mainly hydrogen and electrostatic bonding) between the coverage with titania and the silica substrate. The structure of the titania phase (as well as conditions of CVD synthesis) can affect reactions (coking, cracking, isomerization, disproportionation, etc.) of organics occurring at the mixed oxide surface due to the catalytic ability of titania (anatase) and titania/silica in both acid-base and redox reactions.20 To elucidate this effect, the kinetics of the carbon deposition onto the surface of silica gel and CVD-titania/silica gels was studied using pyrolysis of cyclohexene. Typically, the catalytic reaction rate strongly decreases during an induction period (τ*, relaxation time) (Figures 3-5) as follows21

v ) v∞ + (v0 - v∞) × exp(-t/τ*)

(11)

where v0 is the initial reaction rate and v∞ is the steadystate reaction rate. The effects akin to that observed in Figure 4a are linked to the carbon deposit formation on (19) (a) Gun’ko, V. M.; Zarko, V. I.; Turov, V.V.; Voronin, E. F.; Tischenko, V. A.; Chuiko, A. A. Langmuir 1995, 11, 2115. (b) Gun’ko, V. M.; Zarko, V. I.; Chibowski, E.; Dudnik, V. V.; Leboda, R.; Zaets, V. A. J. Colloid Interface Sci. 1997, 188, 39. (c) Gun’ko, V. M.; Zarko, V. I.; Turov, V. V.; Leboda, R.; Chibowski, E.; Holysz, L.; Pakhlov, E. M.; Voronin, E. F.; Dudnik, V. V.; Gornikov, Yu. I. J. Colloid Interface Sci. 1998, 198, 141. (d) Gun’ko, V. M.; Voronin, E. F.; Zarko, V. I.; Pakhlov, E. M.; Chuiko, A. A. J. Adhesion Sci. Thechnol. 1997, 11, 627. (e) Gun’ko, V. M.; Zarko, V. I.; Chuikov, B. A.; Dudnik, V. V.; Ptushinskii, Yu. G.; Voronin, E. F.; Pakhlov, E. M.; Chuiko, A. A. Int. J. Mass Spectrom. Ion Process 1998, 172, 161. (f) Gun’ko, V. M.; Zarko, V. I.; Turov, V. V.; Leboda, R.; Chibowski, E. Langmuir 1999, 15, 5694. (g) Gun’ko, V. M. Teoret. Eksperim. Khim., submitted for publication. (20) (a) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. Rev. 1996, 25, 61. (b) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (c) Brown, G. E., Jr.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gratzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Chem. Rev. 1999, 99, 77. (21) Kiperman, S. L. Fundamentals of Chemical Kinetics in Heterogeneous Catalysis; Khimiya: Moscow, 1979.

Figure 4. (a) Relative reaction rate R ) mC/(t × SBET(t)) and (b) derivatives dR/dt as functions of the reaction time.

the most active surface sites. Typically, fast coking of the catalysts results in a strong reduction in their catalytic abilities6-8 as coke forms on the most active sites. In the case of a heterogeneous surface involving several oxide phases, the total reaction rate v∞ of the carbonization of a precursor at the surface can be represented as the sum of several terms

v∞ )

∑i ki0fi(Θi) + kCf(ΘC)

(12)

where ki0 and kC are the rate constants of the carbonization of the ith oxide and carbon phases, respectively, and fi(Θi) is the function of the coverage of ith oxide phase (e.g., f(Θ) ) (Θi0 - Θi)m where Θi0 is an initial portion of the surface corresponding to the ith oxide phase).21 It is known that anatase or anatase/silica can catalyze both acid-base and redox reactions; therefore, coking of the surface upon dehydrogenation of organics is due to their interaction with Brønsted (tMO(H)Mt at M ) Ti, Si) and Lewis acid (incompletely O-coordinated Ti atoms) sites or radical sites (such as MO•) with electron transferring (titania is a semiconductor).6-8,20 The effectiveness of CVD-titania as a catalyst of the carbonization of organics can depend on the crystalline nature of titania (anatase or rutile, as the catalytic capability of anatase is greater than that of rutile20), the particle distribution in pores or at the outer surface of the support (which affects the accessible surface area of the titania and silica phases), and the particle size distribution (typically, the smaller particles the higher catalytic effect). Therefore, the reaction rate v of the CVDTSG carbonization is higher than that of the pristine silica gel (Figures 3-5), as silica is practically “inert” in similar reactions.6-8 Also, the reactions on the samples including

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Gun’ko et al.

Figure 5. (a) Rate of the carbon deposit growth CC/t during the relaxation time (τ*) corresponding to a minimal R value; (b) changes in the carbon content during τ*.

anatase (K2i) have greater v values (depending also on the titania content) than those for samples K6i, which have a large portion of rutile. Rutile/silica provides the reaction rate close to that of pure silica depending slightly on the CTiO2 value. A linear increase in the v value with increasing reaction time at t > τ* can be linked with a linear growth in the content of the carbon phase (increase in the ΘC value in eq 12), possessing a large number of active sites (e.g., nonclosed cycles) able to interact with different gaseous reactants to form closed aromatic cycles through dehydrogenation. One can assume that the reaction mechanism does not change with increasing CTiO2 and CC, as the derivatives dv/dt for all the studied samples are close related with consideration for a decrease in the SBET values at t > τ* (Figure 4b). According to the TEM micrograph (Figure 6a), pyrocarbon particles formed on silica gel (sample K0-2) is relatively large (i.e., they were formed mainly at the outer surface of the substrate) and their distribution is nonuniform. In the case of samples K21-2 (Figure 6b) and K24-1 (Figure 6c,d) with the anatase/silica substrate, the pyrocarbon distribution is more uniform and its particles are smaller than in the case of the silica gel matrix. This effect can be caused by the distribution of CVD-titania (anatase) on the silica gel surface in the form of very tiny particles (clusters) mainly in pores, that results in formation of the carbon particles in pores in a larger portion in comparison with K0-i samples. In the case of titania synthesized at higher temperature (rutile/ anatase blend) with formation of dense and segregated particles, the pyrocarbon phase distribution (Figure 6e,f) is nonuniform and similar to that of K0-2 (Figure 6a). The changes in the distribution, morphology and properties of the titania and carbon phases in the K2-i-j and K6-i-j samples can reflect in the dependencies of adsorption of different adsorbates (e.g., nitrogen and water) on the amounts and accessibility of different phases and synthetic conditions. Adsorption Data and Structure of Carbonized Oxides. The adsorption strongly decreases with increasing concentrations of CVD-titania and carbon deposit (Figure 7 and Tables 1 and 2); however, the isotherm type does not change, which suggests unchanging porosity origin of the samples. The observed changes in the adsorption are linked to a marked decrease in the specific surface area (SBET) (mainly of mesopores (Tables 1 and 2, Smes) and the pore volume (V) due to the grafting of titania and carbon, which fill and block pores. Notice that an increase in the amount of grafted carbon less decreases

the SBET value (relative to SBET of CVD-TSG) than titania (i.e., CTiO2) impacts SBET independent of the temperature of the CVD-TiO2 synthesis (Figure 8). However, the higher temperature (Ts ) 600 °C) corresponds to the lower changes in the SBET values for carbonized CVD-TSG relative to the corresponding titania/silicas (Figure 8 and Table 2). Some segregation of the TiO2 phase on heating19,22 with increasing CTiO2 and Ts can result in denser tiny particles embedded in micropores and narrow mesopores and larger particles formed in broader mesopores or at the outer surface of the silica particles [notice that rp for K6-i larger than that of K2-i samples (Table 1)], which can influence the formation of pyrocarbon particles of different sizes depending on the CTiO2 value (e.g., dense rutile particles performed at 600 °C cannot affect the pyrolysis more strongly than less dense anatase particles synthesized at 200 °C do). These result in an increase in the microporosity of CVD-TSG samples (K2i) (Figure 8e and Table 1). Notice that a low concentration of the carbon can also impact the microporosity (a small-scale nonuniformity of the surface) (Table 2 and Figure 8e). Calculations of the surface area per cm3 of the adsorbents (Figure 8f) show smaller changes (maximal ∆max ≈ 65 m2/g) than that of SBET at ∆SBET max ≈ 225 m2/g (Figure 8c) or Smes (Figure 8b) due to formation of the guest phases (TiO2, pyrocarbon) in the pore volume, as the V values decrease strongly (Figure 8d). Clearly, this effect can be responsible for the changes in microporosity, dependent on the deposit concentration and its origin (Figure 8e), and the rearrangement of the pore size distributions (Figures 9-11). However, the changes in the microporosity are complicated (Figure 8e), there is tendency of reduction of the surface area (SDS, SDA) and volume (VDS, VDA) of micropores with increasing concentrations of the deposits (Tables 1 and 2) and these values are lower for K2-i-2 (or K6-i-2) than those of K2-i-1 (K6-i-1). The pore size distributions (Figure 9) were computed using eq 1 at L ) 4, ni ) i and the precalculated weights of the terms with the regularization procedure CONTIN.17 According to these data, deposition of carbon (Figure 9b) shifts the distribution of pores at rp near 1-2 nm toward the larger sizes and a portion of micropores vanishes due to grafted carbon; however, the surface area of micropores (Tables 1 and 2, SDA, SDS) does not change. Additionally, the number of narrow mesopores at rp between 1 and 2 nm decreases for K0-3. (22) Crocker, M.; Herold, R. H. M.; Wilson, A. E.; Mackay, M.; Emeis, C. A.; Hoogendoorn, A. M. J. Chem. Soc., Faraday Trans. 1996, 92, 2791.


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Figure 6. TEM micrographs of (a) carbon/silica gel K0-2 (magnification ×12000) and carbon/titania/silica gels (b) K21-2 (×12000), (c) K24-1 (×12000), (d) K24-1 (×7000), (e) K64-1 (×12000), and K64-1 (×9000).

The CVD-titania impact on the microporosity of silica gel is similar to that of carbon (Figure 9c,e). Notice that the micropore size distribution is broader for carbon/ titania/silica than that of carbon/silica or titania/silica (Figure 9). A similar dependence was obtained for the δ value (distribution dispersion) in eq 2, which is minimal for K0-3 (δ ) 0.13 nm) and K61 (δ ) 0.17 nm) and significantly larger for C/CVD-TSG samples (up to 1 nm or larger). The number of mesopores markedly reduces at rP between 1 and 2 nm and at rp between 5 and 8 nm (Figures 9 and 10), which corresponds to a significant reduction in mesoporosity (Tables 1 and 2, Smes, V) of the adsorbents due to occupation of mesopores by tiny carbon or titania particles. Also, the temperature of the CVD-

titania synthesis influences the mesopore size distributions (Figures 9-11) due to an increase in the titania phase segregation and changes in its particle morphology with elevating temperature giving a higher density of titania (with a major portion of rutile3) particles filling mesopores of silica gel. This effect can also reduce the catalytic capability of segregated titania particles (K6-i samples) in pyrolysis of cylcohexene. Notice that in the case of CVDTiO2/SiO2, the phase transition of anatase to rutile can be inhibited by the silica matrix, for example, heating of the CVD-TiO2/fumed silica samples at 1100 K for 2 h does not change the anatase structure.19c Similar phenomena are possible for CVD-TSG; therefore, sample pretreatment and cyclohexene pyrolysis at 973 K do not transform

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Figure 7. Adsorption/desorption isotherms of nitrogen (77.4 K) onto (a) silica gel (KSK), carbon/silica gel and CVD-TSG; and (b) carbon/CVD-TSG.

anatase to rutile entirely, as a marked difference in their catalytic efficiencies is observed (Figures 3-5). The larger catalytic effect of anatase (K2i samples) in comparison with rutile (K6-i samples) increases nonuniformity of the surface, which causes the larger changes in the pore size distributions (Figures 9d,f, 10, and 11). These structural effects can also reflect in the changes in the nitrogen adsorption energy distribution (notice that the CONTIN program package was successfully utilized recently to calculate the nitrogen adsorption energy distribution for carbon blacks23 and carbon/titania/silica gel3b). To calculate the adsorption energy distribution (Figure 12), eq 9 was utilized with the regularization method at n ) 2 and xmax ) P/P0 corresponding to Θ ≈ 2. The f(E) distribution for carbon/silica gels changes due to grafted carbon per se (interaction between nonpolar nitrogen molecules with polyaromatic fragments of the carbon deposit can be larger than that for silica surface and the energy of the second peak (corresponding to nitrogen adsorption onto the basal plane of pregraphite structures of carbon deposit) increases by ∼1 kJ/mol), but the dependence of f(E) on CC for K0-2 and K0-3 is insignificant (Figure 12a). Perhaps, a larger catalytic effect of anatase (K2-i samples) on the cyclohexene pyrolysis is responsible for the changes in the carbon phase structure (spatial, energetic) in comparison with that of the K6-i-j samples, i.e., “catalytic” and “noncatalytic” carbon deposits possess different structural and energetic nonuniformities. Notice that the first maximum of the f(E) distributions (∼5.4 kJ/mol) related to nitrogen interacting with other nitrogen molecules (e.g., from the second or subsequent adsorbed (23) Puziy, A. M.; Matynia, T.; Gawadzik, B.; Poddubnaya, O. I. Langmuir 1999, 15, 6016.

Gun’ko et al.

layers), as it is close to QL at 77.35 K. The energy of the second peak (∼8.4 kJ/mol) is in agreement with the corresponding values of the “pure” isosteric adsorption enthalpy (qst - QL) calculated using the DTW values (eq 10 from ref 15c), as the qst - QL changes are in the 3.03-0.96 kJ/mol range at P/P0 between 0.0095 and 0.1 for all the studied samples. The E values of the second peak for carbon/silica gel and titania/silica gel are close, as the energy of nitrogen interaction with titania is large than that of silica, and it is lower for carbon/titania/silica gel than that of carbon/silica or titania/silica (Figure 12). This effect can be connected with changes in the pore structure of “catalytic” carbon deposit of carbon/titania/silica gels (especially for K2-i-j samples) in comparison with that of “noncatalytic” one of carbon/silica gels (K0-i samples). The highest peak of f(E) can be linked with nitrogen molecules adsorbed in micropores of oxides, where the adsorption potential is large due to relatively strong electrostatic fields.9 Notice that in the case of K28-2 sample with the maximal concentration of deposits (TiO2 + C), the energy of the last peak is minimal as well as the E value of the second peak (Figure 12). Complicated changes in the topology of micropores and mesopores due to grafted titania and pyrocarbon are responsible for the nonlinear relationships between fractal dimensions and the deposit concentrations (Figure 13a,b), micropore surface area (Figure 13c) or Smes (Figure 13d). The DTW and DAJ values were calculated at low pressures P/P0 < 0.1 (Figure 13a-c) in order to elucidate features of micropores of C/CVD-TSG samples or at higher pressures P/P0 < 0.85 on mesopore filling (Figure 13d). Fractal dimension DTW of titania/silica reduces more substantially than that of carbon/silica/titania (Figure 13a). Additionally, peaks of SDS(DTW) correspond to binary systems (Figure 13c), i.e., carbon/silica or titania/silica, as its values for ternary matters below 50 m2/g (except K21-1 and K64-1). A similar picture is observed for Smes(DAJ) (Figure 13d) characterizing the relationship between the mesopore surface area and DAJ computed at P/P0 < 0.85. These effects are linked with a reduction of the free volume of mesopores due to grafted titania and pyrocarbon; however, the surface area per cubic centimeter decreases in a small way in comparison with that per gram of the adsorbent (Figure 8). Thus, on the basis of the calculated parameters, one can assume that the carbon deposit is more porous per se and looser than CVD-titania is. Calculations of the fractal dimension distribution f(DTW) with eq 5 as a kernel of eq 10 show that all the deposits increase the nonuniformity of the adsorbents as the f(DTW) functions are broadened (Figure 14). However, this effect is larger titania synthesized at lower temperature (Figure 14, K28) that that of titania prepared at 600 °C (K68). The impact of pyrocarbon in K0-3 and K68-2 relatively small in respect to broadening of f(DTW), but in the case of K28-2, the distribution is narrower than that for K28, i.e., the adsorbent nonuniformity decreases due to the carbon deposit. Consequently, the half-width (or total width) of the f(DTW) distribution can be used as a measure of the topological nonuniformity of the adsorbents. An increase in the CTiO2 and CC values (Tables 1 and 2) results in a decrease in the surface area of mainly mesopores (Smes), as micropores give smaller contributions to the surface area (SDA, SDS) and the pore volume (VDA, VDS), which depend more weakly than the parameters of mesopores on the CTiO2 and CC values especially at larger deposit concentrations (Figures 8-12). Rejection of correction of the isotherms due to adsorption in mesopores results in an increase of SDA, SDS and VDS approximately


Langmuir, Vol. 16, No. 7, 2000 3235

Figure 8. Surface area (a) SBET, (b) Smes, (e) SDS, (c) changes in SBET, and (d) total pore volume and surface in square meters per cubic centimeters ((f) calculated with consideration of the specific densities of the components and the pore volume) as functions of the deposit content (titania or titania plus carbon).

by a factor of 2; i.e., microporosity does not exceed 30% of the total porosity of the samples. The results of the calculations of the micropore surface area using eq 2 at P/P0 < 0.2 are close to the SDA values (SDS is slightly larger than SDA due to features of equations used to calculate these values as SDA ∼ W0E0, but SDS is the integral of some function of rp 10) as well as to Smi obtained using the deBoer method9 incorporated in the software of the Micromeritrics adsorption analyzer. However, this method gives the micropore volume Vmi ( 5 nm) formed at the outer surface of the matrix (carbon deposit can be formed at the outer surface of titania particles; however, the outer surface of mesoporous silica gel particles is very small) can represent the plugs blocking of the mesopores that


Langmuir, Vol. 16, No. 7, 2000 3237

Figure 10. Mesopore size distribution for oxides (a) and carbon/ oxides (b) (calculations on the basis of the theory of capillary evaporation10 using the program package12).

Figure 12. Nitrogen adsorption energy distributions for KSK, K0-2, and K0-3 (a), and K24, K24-2, K28, K28-2, K68, and K68-2 (b) (CONTIN with eq 9).

adsorbents), x is the relative content of carbon, and S0 (SBET) is the specific surface area of an initial adsorbent. A relative change in the surface of the volume unit

Ψ)

SCS S0(1 - x)

(14)

is linked to the Θ value2a as follows

Θ)

1-Ψ 1-λ

(15)

where

λ) Figure 11. Mesopore size distribution for carbon/oxide samples K6-i-j (calculations on the basis of the theory of capillary evaporation10 using the program package12).

reflects in the reduction of the nitrogen adsorption with increasing CTiO2 and CC. To estimate the deposit distribution on the substrate surface, the models developed to describe the coke grafting8c,d can be used. The coverage of the adsorbent surface by carbon deposit is given by

Θ)1-

SCS - SCx S0(1 - x)

(13)

where SCS ) SBET is the specific surface area of Carbosil, SC is the accessible surface area of the carbon phase (we do not know exact values of SC, typically estimated from adsorption of probe molecules, which do not practically adsorb onto oxide surfaces; therefore, SC was roughly estimated from other structural characteristics of the

SCx S0Θ(1 - x)

(16)

Clearly, λ > 1 corresponds to formation of individual particles at the outer surface of adsorbent, λ < 1 on pore filling, and λ ≈ 1 on formation of a film covering the whole surface. The Ψ values decrease with decreasing relative surface area per gram of adsorbent.8c,8d The calculations of the Θ, λ, and Ψ values as functions of the amounts of carbon, titania, and titania + carbon deposits using the SBET(x) functions (Tables 1 and 2) show (Figure 15) that the titania phase possesses tighter contacts with the substrate than carbon has on silica or titania/silica (λC > λTiO2) due to the differences in the texture of these phases and in the particle distributions in pores and on outer surfaces and deposit morphology. Thus, carbon particles can be formed in mesopores [or even on the outer surface of the silica gel particles, especially at CC ≈ 20 wt %, as λ > 1 for K0-3 and the Ψ values are close to 1 for all the C/SiO2 samples (Figure 15,b)] more loosely in comparison with titania. However, in the case of C/TiO2/SiO2 samples, a marked decrease in the Ψ values are observed with

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Figure 13. Relationship between the fractal dimensions DTW (a, c) or DAJ (b, d) and the deposit concentrations (a, b) or the specific surface area of micropore (SDS) (c) or mesopore (Smes) (d) at P/P0 e 0.1 (a-c) and P/P0 e 0.85 (d).

Figure 14. Fractal dimension distributions f(DTW) for pristine silica gel (KSK), carbon/silica gel (K0-3), CVD-titania/silica gel (K28, K68), and carbon/titania/silica gel (K28-2, K68-2).

increasing total amounts of the C + TiO2 deposits. This effect can be explained by formation of a major portion of carbon deposit (as well as titania) in mesopores, as titania, grafted in mesopores, catalyzes pyrolysis. Water Desorption. The changes in the nature of the silica gel surface due to deposition of titania and carbon cause the strong changes in water adsorption/desorption (Figures 16-19 and Table 3). The relative amounts of water adsorbed (or desorbed) per gram or square meter of the adsorbents are lower for the first series of the samples (K2-i-j) than those of K6-i-j at the close amounts of titania and carbon deposits. This effect is related to the weakly bound water (water molecularly adsorbed and desorbed mainly at T < 376 K) to a greater

extent than to the strongly bound water desorbed at T > 376 K (Table 3). Notice that water associatively desorbed from silica or silica/titania is observed up to 1000 K, but its major portion desorbs at T < 700 K.19e,25 The activation energy of desorption of intact water from fumed silica or CVD-titania/fumed silica is 60-70 and 100-250 kJ/mol for associative desorption with the participation of different hydroxyls in the reactions (e.g., for adjacent OH groups, especially in dense islands with 5-10 hydroxyls, E* is lower than that for single groups). Typically, the DTG curves of water desorption from silicas in air include two-three maxima (360-375, 395-430, 600-680 K).19e In the case of water desorption in quasi-isothermal and quasi-isobaric conditions, the main peak for all of the samples lies at 375 K (Figures 16-18), which can correspond to the activation energy of 70-80 kJ/mol. The second low maximum is observed for some samples at 510-570 K. Notice that temperature and E* of desorption of a major portion of water from titania or titania/silica are typically lower than those for pure silica at a greater amounts of water adsorbed onto the binary oxide.19e The carbon deposit on the surface of silica gel without titania (samples K0-1, K0-2, K0-3) causes a more significant reduction in water adsorption than the same amounts of the summary deposit (TiO2 + C) (Figure 19a) due to the hydrophobic properties of carbon deposit. The (25) (a) Zhuravlev, L. T. Colloid Surf. A 1993, 74, 71. (b) Dunken, H.; Flammersheim, H. J.; Franke, S.; Wittkoff, H. Z. Chem. 1985, 25, 93. (c) Zhuravlev, L. T. Pure Appl. Chem. 1989, 61, 1969. (d) Krylova, I. V.; Filonenko, A. P.; Sitonite, Yu. P. Zh. Fiz. Khim. 1967, 41, 2839. (e) Hoffman, P.; Kno¨zinger, E. Surf. Sci. 1987, 188, 181.


Langmuir, Vol. 16, No. 7, 2000 3239

Figure 16. (a) TG data of water desorption and (b) relative desorption values per square meter for the first series of samples K2-i.

Figure 15. (a) Coverage of a substrate and (b) changes in the surface area in the volume unit as functions of the deposit content. (c) λ characterizes the relationship between an accessible surface area of a deposit and an inaccessible surface area of a covered adsorbent.

surface fractality has a large effect on the weakly bound water (Figure 19b) than that for the strongly bound water, which represents both molecularly and dissociatively adsorbed water, as an increase in the porosity (or reduction of fractal dimension D) leads to an increase in the adsorption of intact water in mesopores (weakly bound water), but it does not practically change the amount of hydroxyls per square nanometer (strongly bound water). Notice that the differences in the properties of the strongly and weakly bound water adsorbed by modified silica gels

Figure 17. DTG data of water desorption for the first series of the carbonized CVD-TSG samples K2-i-j.

are well reflected in the dependence of the free energy of these waters on the amounts of grafted carbon and titania.3c,4 To elucidate features of carbonized CVD-titania/silica gel samples, theoretical modeling of the complex particles and corresponding reactions can be performed. Quantum Chemical Modeling. The calculations of the cluster models of CVD-TSG and carbonized oxides were performed using quantum chemical semiempirical NDDO method.19b Also several larger models (one to two thousand atoms) were designed using molecular mechanics (MM) with the MM+ force field from the HyperChem Lite program package.26 Dehydrogenation and formation of aromatic compounds was studied using ab initio and

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Gun’ko et al. Table 3. TG Data over 25-300 °C Range for Water Desorption (Where Temperature of TG Deflection Point Is 102 °C for All the Samples)

Figure 18. Relative desorption values per square meter for the second series of the samples K6-i.

Figure 19. (a) Amount of desorbed water as a function of the total deposit content. (b) Relationships between fractal dimension DAJ (P/P0 e 0.9) and amounts of desorbed water weakly and strongly bonded to the surface.

semiempirical PM327 methods with the Gaussian 9428 and GAMESS29 program suits. Also, several relatively small titania and titania/silica clusters (Table 4) were calculated using ab initio method (SBK and MINI basis sets29). Formation of titania clusters in micropores or mesopores between primary nanoscaled silica globules (the coordination number of these globules can be 4-8) can cause a small decrease in the volume and the surface area of (26) (a) Allinger, N. L. J. Am. Chem. Soc. 1997, 99, 88127. (b) Berkert, U.; Allinger, N. L. Molecular Mechanics; American Chemical Society: Washington, 1982. (c) HyperChem Lite; Hypercube, Inc., HC49000100: Gainesville, FL, 1996. (27) (a) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 209. (b) Stewart, J. J. P. J. Comput. Chem. 1989, 10, 221.

sample

TGw, gH2O/ gads

TGw, mgH2O/ m2ads

TGmax300, gH2O/ gads

TGmax300, mgH2O/ m2ads

TGs, gH2O/ gads

TGs, mgH2O/ m2ads

KSK K01 K02 K03 K21-1 K21-2 K24-1 K24-2 K28-1 K28-2 K61-1 K62-1 K62-2 K64-1 K64-2 K68-1 K68-2

0.800 0.458 0.426 0.325 0.450 0.350 0.320 0.250 0.250 0.175 0.500 0.540 0.380 0.490 0.380 0.400 0.275

2.12 1.36 1.14 1.12 1.54 1.39 1.36 1.20 1.33 1.15 1.63 1.83 1.49 1.68 1.60 1.59 1.35

0.985 0.750 0.724 0.578 0.620 0.484 0.508 0.390 0.378 0.262 0.718 0.742 0.515 0.677 0.519 0.570 0.403

2.61 2.22 1.94 2.00 2.12 1.92 2.15 1.88 2.01 1.72 2.35 2.52 2.02 2.32 2.19 2.26 1.98

0.185 0.292 0.298 0.253 0.170 0.134 0.188 0.140 0.128 0.087 0.218 0.202 0.135 0.187 0.139 0.170 0.128

0.39 0.86 0.80 0.88 0.58 0.53 0.79 0.68 0.68 0.57 0.72 0.69 0.53 0.64 0.59 0.67 0.63

micropores and a greater reduction of the volume and the surface area of mesopores (Table 1). The segregation of the titania phase on heating3,19,22 with increasing CTiO2 causes formation of a portion of relatively large particles in mesopores and the plugs blocking some part of pores, which cause a significant reduction in the total porosity (Figure 8). Notice that in the case of fumed silica substrate, the size of the CVD-titania particles can reach to 100150 nm at CTiO2 > 20 wt %.19 However, the pore structure of silica gel (Table 1, rp) restricts the growth of the titania particles by the pore size but the largest titania (or carbon1g) particles (> 10 nm) can be formed at the outer surface of silica gel particles. Also, decreases in the V and S values are larger than it should be expected on formation of an uniform layer of titania due to segregation of the grafted phases into the larger particles, which can block mesopores of KSK (Figures 9-12). We modeled the titania particles embedded between silica globules then covered by the carbon phase (Figure 20). The calculations of the fragments of these particles (initially relaxed using the MM optimization) by the NDDO (as well as the PM3) method show that the highest occupied molecular orbital (HOMO) is localized at the carbon phase (polyaromatics with a portion of OH, COC, and CdO groups) for both carbon/silica and carbon/titania/silica clusters (Figure 21). Also, the band gap between the HOMO and the lowest unoccupied MO (LUMO) is a small value ( 1.5] for MOH, t M1O(H)M2t groups (Table 4) (e.g., for one OH group bonded to the smallest fullerene C20, qO ) -0.24 and qH ) 0.21 (PM3)) causing a strong interaction with such polar adsorbates as water. Thus, to control the components of the interaction energy between the surface of carbonized CVD-titania/silica gel and polar or nonpolar adsorbates, one can vary the amounts of the carbon or titania deposits and the temperatures of the CVD process and pyrolysis. The structural differences between silica and titania (anatase) cause marked distortions in several layers of oxides at the CVD-TSG interface. These distortions are (30) Zarko, V. I.; Gulko, O. V.; Voronin, E. F.; Pakhlov, E. M.; Chuiko, A. A. Dokl. AN Ukr. 1993, N3, 127.

characterized by marked bond elongations (∆r/rmax ≈ 0.09) and significant changes in valence angles (∆φ/φmax ≈ 0.2) relative to the bulk of solids. Clearly, the strained bonds such as asymmetrical tSi-O-Tit bridges are unstable and can be easily hydrolyzed on synthesis of the titania layers including the stage of hydrolysis of Ti-Cl bonds.3,19 The heat of hydrolysis of the tSi-O-Tit linkages can reach a great magnitude (>100 kJ/mol per tSi-O-Tit bond according to NDDO and ab initio calculations) and this value grows with increasing number of neighboring tSi-O-Tit linkages up to 500 kJ/mol (NDDO) when all the Si and Ti atoms at the interface are bonded with another phase (Figure 20). The ab initio calculations of hydrolysis of the tTi-O-Sit bonds in the cluster with four polyhedrons (Figure 22) as follows

tTi-O-Sit + H2O f tTi(OH)-O(H)Sit (17) tTi-O-Sit + H2O f tTiOH + HOSit (18) using the SBK basis set suggest a high exothermicity of these reactions: ∆Et ) -106 kJ/mol for eq 17 (Figure 22c) and -148 kJ/mol for eq 18 (Figure 22b). Also, the activation energy of hydrolysis of the interface bonds decreases after reactions similar to eq 17 in comparison with eq 18.19 Therefore, the number of the linkages between CVDtitania and silica gel substrate can be relatively small due

3242


Gun’ko et al.

Figure 22. Hydrolysis of (a) Si-O-Ti bond with (b) its breaking (formation of two terminal hydroxyls) and (c) formation of bridging and terminal OH groups.

kJ/mol (eq 21). The PM3 calculation of the activation energy (E*) of reaction 19 at n ) 1 gives E* ) 296 kJ/mol. The ab initio (6-31G(d,p)) calculations of the E* value using the PM3 geometry give 672 and 585 kJ/mol for the geometry of the transition state calculated at the 6-31G(d,p) level (with the QST3 method from Gaussian 94). The activation energy calculated with consideration for electron correlation and exchange effects with the combined B3LYP exchange and correlation functional in DFT (B3LYP/6-31G(d,p)) decreases E* to 501 kJ/mol. However, this value is too large for the studied carbonization reactions, as the corresponding factor exp(-E*/kT) equals only 1.27 × 10-27 at 973 K; i.e., the reaction rate is too small. Consequently, unimolecular reactions such as eq 19 are unlikely for cyclohexene pyrolysis at CVD-TSG surface, and the process occurs at the oxide surface with participation of the surface active sites. The stage, which can limit the formation of aromatics from cyclohexene molecules at the surface, is formation of active particles with breaking bonds, for example, carbenium ions, H+, H•, or other radicals, which can be formed due to interaction of organic molecules with active sites of titania (semiconductor, which can be active in redox reactions) or the titania/silica interface (Brønsted acid sites tTiO(H)Sit catalyze formation of carbenium ions) with great ease than upon interaction with silica (insulator, which is relatively inert as a catalyst). The energy of formation of electron-hole (e--h+) pairs at the titania surface (as the beginning of redox reactions) close to the band gap (Eg) between the valence zone and zone of conductivity (≈3 eV for titania and ≈9 eV for silica, but Eg < 0.5 eV for carbon).20 Localization of such pairs on the tTiOH bonds gives {tTi4+OH•}+ + {tTi3+OH},20b promoting elimination of OH•, which can attach H• from organic molecules with formation of adsorbed radicals CnHm•. Notice that the probability of processes

TiO2 + hν f TiO2* + e- + h+ Aads + e- f A•Figure 23. Schemes of reactions 20 (a) and 21 (b).

A•-ads + Cgas f B•-ads

to their intensive hydrolysis during the titania phase synthesis. This is in agreement with the IR data (Figures 1 and 2) attesting a great amount of tSiOH groups on the surface of CVD-TSG and the absence of the Si-OTi band at 950 cm-1, independent of the number of the CVD cycles.3,19 The carbon/oxide samples were performed using pyrolysis of cyclohexene, which can undergo dehydrogenation up to benzene, forming then high polyaromatic compounds.6-8 According to the PM3 calculations (starting from cyclohexane), the reactions

C6H12 f C6H12-2n + nH2

(19)

are endothermic for all n values except for n ) 3 (∆Et ) -43 kJ/mol relative to the product at n ) 2) when benzene forms. The endothermicity strongly increases at n > 3 (389-426 kJ/mol relative to the energy of the products of a preceding reaction), but at n < 3, ∆Et is 53 kJ/mol (n ) 1) and 50 kJ/mol (n ) 2). Subsequent reactions related to formation of molecules of aromatic series (Figure 23)

2C6H6 f C12H10 + H2

(20)

C12H10 + C6H6 f C18H12 + 2H2

(21)

are exothermic, as ∆Et ) -53 kJ/mol (eq 20) and -124

A•-ads + R• f D-ads is higher than that of processes

Aads + h+ f A•+ A•+ads + Cgas f B•+ads A•+ads + R• f D+ads due to the differences between the electronic structures of studied organics and titania (titania as a semiconductor has a lower band gap Eg) influencing the mentioned reactions.19,20 The energy of the reaction

C6H12 f C6H11• + H•

(22)

with transition from the singlet state to the triplet one is ≈330 kJ/mol (PM3), which slightly greater than the Eg value of titania. Fast tunneling of electrons from titania to adsorbed molecules (the lifetime of adsorption complex9a τ ) τ0exp(Q/kT) is 10-15 ns at 970 K, if the heat of adsorption Q ≈ 40 kJ/mol for studied compounds; i.e., τ is quite long for this electron transferring) promotes elimination of radicals H• and RC• with subsequent formation of new C-C bonds, as the first stage of carbon


deposit formation. The band gap for the carbon phase (polyaromatics) is smaller than that of titania; therefore, the formation of the individual carbon phase leads to an increase in the probability of the electron transferring to the adsorbed molecules, giving an increase in the reaction rate with increasing CC (Figures 3 and 4). The activation energy of ion formation due to proton transferring from B sites to organic molecules can be relatively small, as E* ) 100-130 kJ/mol according to ab initio calculations of interaction between short olefins and B sites of zeolite models.31 Such a small E* value causes the carbon phase formation primarily on the most active B sites of oxides (even at relatively low temperatures6-8) that corresponds to the greatest reaction rate decreasing to v∞ during τ*. After poisoning of these sites (e.g., tTiO(H)Sit at the titania/silica interface) by carbon deposit, the reaction can continue at lower v on weaker B sites (tTiO(H)Tit, tSiOH) or radical sites at the oxide and carbon deposit surfaces. These effects cause a larger nonuniformity of the carbon/titania/silica gel surfaces in comparison with that of carbon/silica, which reflects in the changes in the adsorption energy distribution due to interaction of adsorbates with carbon or titania phases (Figure 12) or in the nonlinear dependence of fractal dimension on CC and CTiO2 (Figure 13). Conclusion The analysis of the obtained results attests that the CVD-titania and carbon deposit cover the silica gel surface by the nonuniform layers (dependent on the substrate origin), which represent tiny particles with relatively broad size distributions. The porosity (SBET and V) of modified silica gel decreases near linearly with increasing amounts of grafted matters filling the free volume of mesopores (anatase + carbon) and coating the outer surface of the matrix (carbon). The relative changes in the surface area per cubic centimeter of the adsorbent (31) Evleth, E. M.; Kassab, E.; Jessri, H.; Allavena, M.; Montero, L.; Sierra, L. R. J. Phys. Chem. 1996, 100, 11368.

Langmuir, Vol. 16, No. 7, 2000 3243

is smaller due to formation of tiny deposit particles in mesopores of silica gel; i.e., the deposit is responsible for an additional porosity in the volume of the initial mesopores of silica gel. Grafting of titania particles causes a more significant decrease in the accessible surface area and the pore volume than that of the carbon deposit due to the differences in the morphology and topology of the titania and carbon phases and their distributions and contacts with the substrate surfaces. The titania phase can be distributed mainly in the pores and, in the minor portion, at the outer surface of the silica gel particles, but the carbon deposit forms globules at both outer (especially at great CC values for carbon/silica gel samples) and inner surfaces of the adsorbents. However, for CVD-TSG samples, the carbon deposit quickly forms on the active sites of the titania/silica interface or titania phase (especially in the case of anatase), therefore, a marked portion of carbon is packed in pores. The titania phase (anatase) prepared at lower temperature (200 °C) can more strongly catalyze the cyclohexene pyrolysis than titania obtained at 600 °C (represents mainly rutile). “Catalytic” carbon in C/anatase/silica (K2-i-j) samples differs from “noncatalytic” carbon in C/silica (K0-i) samples in respect to their distribution on the matrix, spatial, and energetic structures of globules. Theoretical modeling and the utilization of the powerful regularization procedure in order to analyze the adsorption data allow us to elucidate some features of formation of the carbon and titania deposits at the silica gel surface. Acknowledgment. The authors thank Dr. E. M. Pakhlov for the IR spectra of titania/silica samples. G.V.M. is grateful to Dr. S. W. Provencher for the CONTIN program package with the corresponding papers and Dr. T. L. Petrenko for the use of the Gaussian 94 program package. Financial support from the State Committee for Scientific Research (KBN, Warsaw), Project No. 3 T09A 03611, is gratefully acknowledged. LA990555B

Silica Gel Carbonized Due to Pyrolysis ... - ACS Publications

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