Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 www.elsevier.com/locate/colsurfa Fumed oxides modified due to pyrolysis of cyclohexene V.M Gun’ko a,*, R Leboda b, V.I Zarko a, J Skubiszewska-Zieba b, W Grzegorczyk b, E.M Pakhlov a, E.F Voronin a, O Seledets b, E Chibowski b a b Institute of Surface Chemistry, 31 Prospect Nauki, Kiev 03680, Ukraine Faculty of Chemistry, Maria Curie-Sklodowska University, Lublin 20031, Poland Received 25 May 2002; accepted December 2002 Abstract Mixed oxides such as fumed X/silica (X 0/Al2O3 (AS), TiO2 (TS)) and CVD-TiO2/fumed silica, initial and covered by carbon deposit formed on cyclohexene pyrolysis, were characterized by means of carbonization kinetics, nitrogen adsorption Á/desorption, water adsorption, and electrophoresis methods Catalytic capability of AS (per gram of the adsorbent) in cyclohexene pyrolysis (973 K for h) is greater at a low concentration of alumina, but for fumed TS, it is maximal at large CTiO2 Concentration of pyrocarbon formed on AS or TS is Á/4 times larger than that on silica under the same pyrolysis conditions, and fumed TS can stronger catalyze pyrolysis than CVD-titania/silica does A marked impact of the nature of X/SiO2 on cyclohexene pyrolysis results in variations of the structural, adsorptive, and electrokinetic properties of C/X/SiO2 Alterations in the pore size distributions (main maxima at Rp between 0.5 and nm) can be due to the availability of various channels between primary particles (5 Á/50 nm) packed in relatively dense aggregates and between aggregates in more loose agglomerates of aggregates differently filled by grafted pyrocarbon The dependencies of z potential of C/X/SiO2 particles on pH are closer to that of C/SiO2 than X/SiO2, i.e carbon deposit covers mainly the X/SiO2 interface and X phase possessing catalytic activity in cyclohexene pyrolysis # 2003 Elsevier Science B.V All rights reserved Keywords: Fumed mixed oxides; Pyrolysis kinetics; Carbon/mixed oxides; Nitrogen adsorption; Pore size distribution; z Potential Introduction * Corresponding author Present address: Laboratory of Surface Electro-Physics, Institute of Surface Chemistry, National Academy of Sciences of Ukraine, 17 General Naumov Street, Kiev 03164, Ukraine Tel.: '/380-44-4449627; fax: '/380-44-424-3567 E-mail address: vlad-gun@carrier.kiev.ua (V.M Gun’ko) Individual and mixed X/SiO2 oxides (e.g silica, alumina/silica, titania/silica, etc.) of different origin covered by carbon deposit of the pre-graphite structure prepared by means of pyrolysis of organic precursors can be used as adsorbents, fillers, etc [1 Á/10] Features of C/X/SiO2 materials depend not only on the carbon concentration (CC), 0927-7757/03/$ - see front matter # 2003 Elsevier Science B.V All rights reserved doi:10.1016/S0927-7757(02)00606-4 104 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 pyrolysis temperature, organic precursor structure, and silica substrate type, but also on the concentration (CX) and distribution of the second oxide in X/SiO2 (e.g X 0/Al2O3, TiO2) possessing catalytic activity in pyrolysis and related reactions These properties can also alter due to variations in the substrate topology (porous, nonporous but highly dispersed with large or small polydispersity, different types of particle packing in aggregates and agglomerates, broad or narrow particle size distributions, etc.) and technique of X/SiO2 synthesis [1 Á/15] Many of the important characteristics of oxide surfaces covered by carbon deposit (porosity, adsorptive ability, acidity of surface sites, etc.) can be studied upon adsorption of polar and nonpolar probe molecules [2 Á/11] Various pyrocarbon/silicas (CS) synthesized by pyrolysis of organic and organometallic precursors on fumed silica, silica gel, X/silica (X 0/TiO2, ZnO, etc.) as substrates and interaction of CS with different adsorbates, as well as coking of mixed oxide catalysts due to side processes were investigated previously [2 Á/6,11 Á/15] It should be noted that the size of basal planes in pyrocarbon prepared using organic or organometallic precursors with a relatively small molecular size (e.g methylene chloride, cyclohexene, acetylacetonates of Ti, Co, Ni, Cr, Zn, Zr, etc.) at 750 Á/950 K can reach only several nanometers [16,17], and pyrocarbon possesses a relatively low specific surface area, e.g SBET of carbon deposit per se is approximately 100 Á/120 m2 g(1 at CC !/20 wt.% in C/fumed silica [3] Additionally, pyrocarbon can form mainly on the outer surface of porous silica particles at CC !/ 10 wt.%, but at low CC, carbon deposit can cover mainly the pore walls [7 Á/10,18]; nevertheless, a significant portion of the oxide surface can remain practically pure and accessible for adsorbate molecules over a broad CC range But in the case of mixed oxide substrates (e.g TiO2/SiO2) or pyrolysis of organometallic precursors on silica gel (resulting in the formation of new oxide, silicate or even pure metal clusters), the pyrocarbon distribution depends on the allocation of active sites (Brønsted and Lewis acid sites) catalyzing carbonization [2 Á/6] Previously, we studied such properties or characteristics of X/SiO2 (X 0/Al2O3, TiO2, Al2O3/ TiO2 (AST)) as the X phase structure, particle morphology, concentration and distribution of Brønsted and Lewis acid sites, as well as molecular and dissociative adsorption of water, surface modification by organic and organosilicon compounds, etc [2 Á/6,11,19Á/26] Clearly, the appearance of an X phase (which can catalyze pyrolysis and related reactions of organics) in X/SiO2 can be responsible for significant changes in the characteristics not only of oxide surfaces itself but also of carbon deposit in C/X/SiO2 Elucidation of this impact is useful in order to control the physicochemical properties of hybrid C/X/SiO2 materials, and controlled variations in CC and CX, and synthetic conditions allow one to prepare C/X/SiO2 adsorbents with desirable porosity, polarity of surface, active surface site distribution, etc Despite numerous investigations of highly dispersed or porous oxides covered by carbon deposit, many questions related to their structures (especially for mixed fumed oxides) after carbonization of organics and their interactions with such adsorbates as water (e.g its associative desorption and dissociative adsorption from air or alterations in the z potential and particle aggregation in aqueous suspensions, as adsorbents are frequently utilized in aqueous media) still remain unstudied or unclear Therefore, the aim of this work was to study carbonization kinetics, structural and adsorptive characteristics, and z potential of carbon/ X/fumed silica (X 0/Al2O3, TiO2) materials synthesized by using cyclohexene pyrolysis depending on the concentration of X oxides in fumed X/SiO2 or silica covered by titania using a chemical vapor deposition (CVD) technique A choice of C6H10 as a precursor was conditioned by a relatively small size of its molecules (to provide their effective interaction with active sites of the X/SiO2 surfaces in channels of aggregates of primary particles) and the absence of Cl (e.g CH2Cl2 was used previously to prepare carbon deposit on fumed silica [3]), which can effectively interact with Ti or Al atoms in mixed oxides at high temperatures that can result in uncontrolled and undesirable changes in the oxide matrix structure V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 Experimental 2.1 Materials Highly dispersed fumed silica (Aerosil A-175 and A-300 having different specific surface areas) and fumed X/SiO2 (X 0/Al2O3 (AS), TiO2 (TS)) at different concentrations (CX) of X oxide phase (Tables and 2) (pilot plant at the Institute of Surface Chemistry, Kalush, Ukraine) were used as the initial materials to prepare C/X/SiO2 adsorbents Additionally, TS and AST samples at CTiO2 ]/65 wt.% (SBET between 30 and 74 m2 g(1) were used for the comparison of kinetics of carbonization reaction CVD-titania/fumed silica (Institute of Surface Chemistry, Kiev) synthesized by means of CVD technique using chemisorption of TiCl4 on fumed silica A-300 as a substrate, and then hydrolyzed at 423 K was described in detail elsewhere [21] The titania phase in fumed TS is crystalline and consists of a blend of anatase (major portion) and rutile, but in the case of CVD-TS, titania particles represent only anatase, whose phase transition to rutile does not occur even at 1100 K for h due to the silica impact [21] Alumina in AS is totally amorphous as well as pure fumed silica or silica in fumed AS and TS [19 Á/25] Carbosil (CS) samples (Maria Curie-Sklodowska University, Lublin) were synthesized using cyclohexene pyrolysis on oxide substrates at 973 K for h The technique of CS sample preparation was described in detail elsewhere [1,26 Á/28] 2.2 Kinetics of carbonization Pyrocarbon deposition on oxide samples was carried out using controlled cyclohexene pyrolysis under isothermal conditions (973 K for h) in a flow reactor (coupled with the scales) in the He'/ 5% C6H10 stream To reduce the impact of external diffusion phenomena on dynamics of carbon deposit formation, the low-weighted amounts of samples ( :/100 mg) and flow velocity of the reaction mixture of 63 dm3 h (1 were used Kinetics of carbon grafting was controlled by means of the gravimetric method 105 2.3 Nitrogen adsorption Nitrogen (spectra analyzed grade) adsorption Á/ desorption isotherms were recorded at 77.35 K using a Micromeritics ASAP 2010 (V-2.00) adsorption analyzer The structural characteristics of studied materials calculated on the basis of nitrogen adsorptionÁ/desorption are summarized in Tables Á/3 2.4 Electrophoresis Electrophoretic investigations were performed using a ZetaPlus (Brookhaven Instruments) z potential apparatus Deionized distilled water (pH 6.95), and 0.2, 1, and 2.5 g of solids per liter of water were utilized to prepare the suspensions, which were then treated with an ultrasonic bath for Á/3.5 h The pH values measured by an OP208/1 precision digital pH-meter were adjusted by the addition of 0.1 M HCl or NaOH solutions The suspension salinity was changed using NaCl solution 2.5 Computation The specific surface area (SBET) was calculated according to the standard BET method [29,30] using adsorption data at relative pressures (p/p0) between 0.06 and 0.2, where p and p0 denote the equilibrium and saturation pressures of nitrogen, respectively The pore volume VBJH was evaluated from adsorption data using the BarrettÁ/Joyner Á/ Halenda (BJH) method [31]; then, VBJH and SBET were used to estimate the average pore diameter (DBJH) The specific surface area of mesopores (SK) was calculated using the Kiselev equation [32] Also, the mesopore volume distributions dVmes/dRp were calculated with desorption data according to the improved theory of capillary condensation Á/evaporation (with Broekhoff Á/ deBoer Á/Dollimore Á/Heal Á/Dubinin Á/Ulin corrections to the Kelvin equation) [33 Á/35], which links the adsorbed layer thickness (t) and the radius (Rp !/1 nm) of filled pores (emptied on desorption), using local isotherm approximation (LIA) with the program package described in detail elsewhere [36 Á/38] 106 Sample A-300 C/A300 A-175 C/A175 AS1 C/AS1 AS3 C/AS3 AS8 C/AS8 AS23 C/AS23 CAl2O3 (wt.%) CC (wt.%) SBET (m2 g (1) SK (m2 g (1) SDS (m2 g (1) VBJH (cm3 g (1) DBJH (nm) DAJ1, p /p0 B/0.1 DAJ2, p /p0 B/0.85 DFRDA, p /p0 B/0.1 1.3 23 Á/ 8.5 308 378 244 320 43 44 0.65 0.96 7.5 8.6 2.367 2.300 2.598 2.574 2.196 2.171 Á/ 8.3 185 203 168 177 25 25 0.50 0.56 9.2 9.4 2.328 2.313 2.569 2.572 2.186 2.174 Á/ 31.8 Á/ 27.5 Á/ 31.8 Á/ 25.6 203 169 185 199 303 195 347 321 158 140 146 156 241 160 283 266 22 25 29 29 42 30 45 43 0.42 0.38 0.39 0.43 0.66 0.45 0.80 0.77 7.3 7.9 7.5 7.5 7.6 8.1 8.0 8.2 2.406 2.235 2.332 2.311 2.355 2.323 2.348 2.319 2.611 2.564 2.588 2.586 2.592 2.581 2.588 2.579 2.183 2.144 2.186 2.173 2.195 2.180 2.194 2.176 SK is the specific surface area of mesopores determined by Kiselev method, SDS is the specific surface area of micropores determined by Dubinin Á/Stoeckli method, VBJH and DBJH are the total pore volume and the pore diameter, respectively, estimated according to the BJH method, DAJ is the fractal dimension estimated using Avnir Á/ Jaroniec method, DFRDA is the fractal dimension estimated using the fractal analogue of the Dubinin Á/Astakhov equation V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 Table Structural parameters of initial fumed silica, AS, C/SiO2, and C/AS Sample CTiO2 (wt.%) CC (wt.%) SBET (m2 g (1) SK (m2 g (1) SDS (m2 g (1) VBJH (cm3 g (1) DBJH (nm) DAJ1, p /p0 B/0.1 DAJ2, p /p0 B/0.85 DFRDA, p /p0 B/0.1 TS2 C/TS2 TS5 C/TS5 TS17 C/TS17 TS22 C/TS22 TS33 C/TS33 TS9 C/TS9 TS20 C/TS20 TS36 C/TS36 1.7a 5a 17a 22a 33a 9b 20b 36b Á/ 20.1 Á/ 20.0 Á/ 17.4 Á/ 18.7 Á/ 23.1 Á/ 26.3 Á/ 17.3 Á/ 29.7 313 281 306 268 286 274 247 215 215 166 235 184 84 76 114 83 264 247 252 235 242 233 214 189 188 149 198 152 65 60 87 71 42 34 42 33 41 35 32 30 29 24 32 26 17 15 20 15 0.71 0.77 0.71 0.74 0.69 0.72 0.63 0.60 0.55 0.47 0.57 0.43 0.17 0.16 0.25 0.22 8.1 9.3 8.2 9.4 8.4 9.0 8.7 9.6 9.2 9.8 8.6 8.2 7.3 7.4 8.1 9.2 2.354 2.317 2.361 2.340 2.350 2.320 2.375 2.329 2.358 2.336 2.332 2.304 2.353 2.313 2.413 2.324 2.584 2.570 2.591 2.577 2.582 2.577 2.585 2.573 2.581 2.574 2.583 2.578 2.598 2.589 2.615 2.580 2.191 2.178 2.195 2.187 2.188 2.180 2.197 2.181 2.191 2.185 2.194 2.174 2.192 2.177 2.212 2.178 SK is the specific surface area of mesopores determined by Kiselev method; SDS is the specific surface area of micropores determined by Dubinin Á/Stoeckli method; VBJH and DBJH are the total pore volume and the pore diameter, respectively, estimated according to the BJH method; DAJ is the fractal dimension estimated using Avnir Á/ Jaroniec method, DFRDA is the fractal dimension estimated using the fractal analogue of the Dubinin Á/Astakhov equation a CVD-TS b Fumed TS V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 Table Structural parameters of initial fumed and CVD-titania/silica and C/TiO2/SiO2 107 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 108 Table Mesopore specific surface areas and fractal dimensions of oxide and carbon/oxide adsorbents Adsorbent SdB (m2 g (1) Svt (m2 g (1) DFRDA, xmax 0/ 3.5 nm A-300 C/A-300 AS23 C/AS23 TS33 C/TS33 TS36 C/TS36 255 328 294 271 195 152 89 71 249 314 284 265 166 123 89 66 2.476 2.512 2.358 2.424 2.494 2.460 2.493 2.477 SdB and Svt are the specific surface area of mesopores calculated using the t or v ,t methods, respectively DFRDA is the fractal dimension calculated using the fractal analogue of the Dubinin Á/Astakhov equation at xmax 0/3.5 nm Calculation of fractal dimension (D ) [39] was performed using adsorption data with the linear relationship between adsorption (ln U ) and nitrogen pressure (ln ln(p0/p )) [40] at p/p0 5/0.1 with no capillary condensation in mesopores, and at p/ p0 B/0.85 with a marked capillary condensation It is pertinent to note that deviations of the t-curves from the standard t-curve for nitrogen [29] are observed for all the studied samples only at p/p0 !/ 0.75, for which ln U (U is the reduced adsorption) is a nearly linear function of ln ln(p0/p ), as determination coefficient:/0.98 Á/0.99 at 0.001 B/ p /p0 B/0.1 and :/0.80 Á/0.84 at p /p0,max 0/0.85, i.e the larger the maximal p/p0, the worse is the fit on linearization, i.e DAJ value is less uniquely determined The fractal analogue of the DubininÁ/ Astakhov equation (FRDA) [41] was utilized to estimate fractal dimension DFRDA at p/p0 5/0.1 and the pore half-width range between xmin 0/0.2 nm and xmax 0/2.5 nm corresponding to main maxima in the pore size distributions of fumed oxides linked to micropores (Rp B/1 nm) and the narrowest mesopores at Rp between and 2.5 nm Contribution of micropores was estimated using the Dubinin Á/Stoeckli (DS) equation [42] with consideration for adsorption in mesopores using SK for corresponding corrections It should be recognized that the SK values can be overestimated, and the greater the capillary effect (or larger hysteresis loop in adsorption Á/desorption isotherms), the larger is the SK overestimation [4 Á/ 6,43] Consequently, microporosity can be underestimated when SK is used for corrections of isotherms to calculate the micropore parameters On the other hand, calculations of the micropore parameters without such a correction result in an enhancement of SDS by a factor 2Á/3; however, VDS increases to a lesser extent An increase in the upper limit of Rp on integration for SDS up to nm results in an enhancement of SDS by half (with consideration of adsorption in mesopores using SK) However, calculations without correction on adsorption in mesopores give the SDS values of 190Á/200 m2 g(1 (Rp between 0.2 and nm) and 100Á/130 m2 g(1 (Rp between 0.2 and nm) for A300 or C/A-300, respectively Consequently, fumed oxide microporosity expanded to Rp B/2 nm can correspond to 60Á/65% of a total surface area, i.e the pore character of studied adsorbents can be close to that of microporous adsorbents This circumstance allows us to utilize the FRDA equation [41] at xmax 0/2.5 (Tables and 2) or 3.5 nm (Table 3) To compare the results summarized in Tables and 2, the structural parameters of some adsorbents were calculated using t or v,t methods [29,44,45], and obtained values of the mesopore specific surface area (SdB and Svt, Table 3) are close to SK (Tables and 2) One can conclude that calculations of the micropore and mesopore parameters (Tables Á/3) using series of adsorption equations give results, which differ relatively slightly and typically are within the accuracy of the methods used It should be pointed out that some equation parameters depend on the nature of adsorbents, but some others are independent [29]; nevertheless, the calculated structural characteristics are closely related (Tables Á/3) that confirms the reliability of the obtained results and the possibility to use the mentioned equations to calculate the structural parameters of hybrid adsorbents To compute the pore size distribution f(Rp), the modified DubininÁ/Astakhov equation [46,47] ln a0   ni  L   X W Ax ln f i ( kb W i (1) (where L 0/3 or [44]; Wi is the current and W0 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 the limiting adsorption of vapor; A 0/RgT ln(p0/p) is the differential molar work equal (with inverse sign) to the variation in the Gibbs free energy, where Rg denotes the gas constant; ni is the equation parameter (typically varied between 0.5 and 4); bN2 0/0.33; x is the micropore half-width or pore radius of mesopores) was used as a local isotherm Ul in the overall adsorption isotherm in the form of Fredholm integral equation of the first kind: U(T; p)0 g xmax Ul (T; p; x)f (x) dx; (2) xmin where f(x) is the unknown distribution function of a given parameter x To calculate the f (x ) function, the regularization method can be used, as solution of Eq (2) is well-known ill-posed problem due to a strong influence of noise components on the experimental data, which not allow one to utilize exact inversion formulas or iterative algorithms [48,49] For this purpose, the CONTIN program package [49] was modified to utilize adsorption equations to estimate the pore size and adsorption energy distributions [44] The weights of the Wi /W0 coefficients in Eq (1) were estimated within the scope of LIA using adsorption data One of the reasons of an increase in L up to in Eq (1) with the varied ni parameters was a large non-uniformity of hybrid C/X/SiO2 materials, as it is known [16,29,46,47] that ni in Eq (1) depends not only on the pore size distribution but also on the nature of adsorbent surface, and f(Rp) is stable at L ]/3 [44] Additionally, a subsequent increase in L up to results in only small changes in the f(Rp) distributions which become smoother [38] The k value (:/10 nm kJ mol (1 for hybrid adsorbents) was estimated comparing parameter magnitudes obtained using adsorption equations dependent and independent on the nature of adsorbents with LIA Additionally, the pore size distribution was calculated using the overall isotherm equation [50] a0 g rk (p) f (Rp ) dr' rmin g rmax rk (p) w Rp t(p; Rp )f (Rp ) dr; (3) where rmin and rmax are the minimal and maximal half-width or pore radius, respectively; w 0/1 for 109 slit-like pores and for cylindrical pores; 2gnm cos u ; (4) Rg T ln(p0 =p) a cz t(p; Rp )0 m SBET ( z 1 ' 1(nb ( n)zn(1 ( (nb ' 1)zn ' 1(nb ' n)zn'1  2 ; ' (c ( 1)z ' 12(cb ( c)zn ( 12(cb ' c)zn'1 rk (p)0t(p; Rp )' (5) where b 0/exp(Do /RgT ), Do is the excess of the evaporation heat due to the interference of the layering on the opposite wall of pores (determined as a varied parameter using LIA), t(p ,Rp) the statistical thickness of adsorbed layer, am the monolayer capacity, c 0/cs exp((Qp(/Qs)/RgT ), cs the BET coefficient for adsorption on flat surface (calculated using LIA), Qs and Qp are the adsorption heat on flat surface and in pores, respectively, z0/p/p0, and n 0/d /tm is the number (non-integer) of statistical monolayers Qs and Qp values were estimated on the basis of ab initio (B3LYP/631G(d,p) or larger basis sets) calculations of nitrogen molecule interaction with different adsorbents, and the nitrogen adsorption energy distributions f(E ) determined using adsorption data for similar oxides and pyrocarbon/oxides (Qp :/2Qs for narrow micropores at Rp B/0.4 nm) Desorption data were utilized to compute the f(Rp) distribution with Eq (3) and regularization procedure According to Nguen and Do [50], Eqs (3) Á/(5) are valid not only for mesopores but also for micropores (in Ref [50], xmin 0/0, but micropores at the half-width xmin B/0.2 nm are inaccessible for nitrogen molecules, and therefore, one can assume that xmin 0/0.2 nm in Eq (3) has no influence on the positions of f(Rp) peaks even for micropores) Results and discussion 3.1 Kinetics of cyclohexene pyrolysis Differences in the characteristics of Brønsted (Å/ SiO(H)M Å/, M 0/Al, Ti), Lewis (incompletely Ocoordinated Ti or Al atoms), and other sites on TS 110 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 and AS surfaces depending on the synthetic technique (pyrogenic, CVD, etc.) and X concentration (as changes in CX lead to alterations in the size and shape of X particles and X/SiO2 interface structure) reflect in pyrolysis of organics on these oxides [3 Á/6] Alumina/silica catalyses pyrolysis at CX 5/8 wt.% stronger (per gram of the adsorbent) than AS23 and CVD-TiO2/SiO2 (Fig 1, CX 0/0 corresponds to A-300) This result is due to several reasons Firstly, the lower the CAl2O3, the greater is the Brønsted acidity of Å/SiO(H)Al Å/ [51,52]; secondly, formation of individual alumina phase in AS at great CAl2O3 causes the appearance of large number of sixfold O-coordinated Al atoms [20], which are not Lewis acid sites and can also be responsible for a reduction of the Brønsted acidity of neighboring bridging hydroxyls, i.e the number of strong Brønsted and Lewis sites can be reduced in AS with increasing CAl2O3, which is responsible for diminution of the catalytic activity of AS23 in acid Á/base reactions Thirdly, SBET of AS (Table 1) is typically larger than that of fumed TS (Table 2); therefore, normalized CC (divided by SBET, Fig 1(c) and (d)) is larger for TS36 than those for AS1 or AS8, and similar result is observed for the normalized carbonization rate constant (Fig 2), i.e catalytic activity of TS36 per m2 is the largest among studied X/SiO2 oxides Fumed TS and AST at CTiO2 ]/65 wt.% (Fig 1(e)) are more active in cyclohexene pyrolysis than fumed TS at low CTiO2 or CVD-TS (Fig 1(b)) due to the differences in the titania/silica interface structure, and distribution and morphology of titania phase per se [19,21,22,25] In the case of CVD-TS, titania is distributed in the form of relatively large particles, whose size strongly grows with CTiO2 Therefore, the specific catalytic activity (per m2) of CVD-TS depends slightly on CTiO2 and maximal for TS33 at CTiO2 0/33 wt.% (Figs 1(b) and 2) The reaction rate (Fig 2) depends slightly on the carbonization time (maximal for TS36 at t :/0), i.e entire poisoning of active surface sites of X/SiO2 is not observed on cyclohexene pyrolysis at 973 K for h, since pyrocarbon does not cover totally the oxide surfaces (additionally, pyrocarbon can be a catalyst of this reaction) Comparing kinetic data for AS and TS (Figs and 2), one can conclude that in the case of AS, the Al2O3/SiO2 interfaces with Brønsted and Lewis sites catalyze pyrolysis of organics, but for TS, both titania phase and TiO2/SiO2 interfaces possess the catalytic capability, which impacts the pyrocarbon distribution on the oxide matrix However, titania per se in TS and AST, may be, plays the main role, as the normalized reaction rate is larger for fumed TS at CTiO2 0/36 wt.% (Fig 2); however, the largest number of Å/Si Ã/O Ã/TiÅ/ (or Å/SiÃ/O(H) Ã/TiÅ/) bridges is observed in fumed TS at CTiO2 :/20 wt.% [19,21] For CVD-TS, the reaction rate can be lower due to distribution features of CVD-TiO2 in the form of relatively large particles, which cannot provide a great catalytic activity of CVD-TS due to a small contribution of the surface area of titania particles to the overall specific surface area of CVD-TS In the case of AS, the reaction rate is maximal at low CAl2O3 (Figs and 2), as Brønsted acidity decreases with CAl2O3 These catalytic features of X/SiO2 can reflect in the structural adsorptive characteristics of pyrocarbon and C/X/SiO2 as a whole 3.2 Morphology of X/SiO2 and C/X/SiO2 Isotherms of nitrogen adsorption Á/desorption on fumed silica and pyrocarbon/fumed silica [3] demonstrate the absence of the saturation effect, as the adsorption plateau does not appear even at p/p0 0/1 Such a plateau is typically observed for silica gels when pores are filled in a significant portion and nitrogen adsorption does not practically rise with increasing p /p0 !/0.9, as a contribution of the outer surface of silica gel particles to the specific surface area is very small and capillary condensation occurs mainly in pores In the case of fumed X/SiO2 and C/X/SiO2, the isotherm shape is close to that for fumed silica and C/SiO2, but their position drops down with decreasing VBJH and SBET (Tables and 2) Fumed oxides possess a complicated structural hierarchy with nonporous spherical primary particles (diameter: Á/50 nm) packed in aggregates (100 Á/500 nm) and agglomerates of aggregates (!/ mm) possessing strongly different apparent densities due to changes in the type of packing of particles in aggregates (apparent density:/0.7 g V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 111 Fig (a, b) Relative and (c, d) normalized (divided by SBET) carbon contents as a function of the carbonization time for silica and mixed oxides: (a, c) AS; (b, d) CVD-TS (open symbols) and fumed TS (solid symbols); and (e) fumed TS and AST at large CTiO2 ]/65 wt.% 112 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 Fig Carbonization reaction rate as a function of carbonization time for (a) fumed silica and AS; and (b) CVD-TS (open symbols) and fumed TS (solid symbols) cm (3) and agglomerates (:/0.04 Á/0.10 g cm (3) [53 Á/57] Firstly, adsorption of nitrogen can occur near contacts between adjacent primary particles in aggregates corresponding to ‘micropores’ and narrow ‘mesopores’, then in channels (mesopores) formed by clusters of neighboring particles in aggregates and also on the external surfaces of aggregates, which are not closely packed in agglomerates, and between aggregates, i.e in large mesopores or even macropores Clearly, complete filling of all these free inter-particle volumes (especially in agglomerates) cannot be reached upon nitrogen adsorption on fumed oxides, possessing the apparent density of about Á/3% of the specific density, even at p/p0 0/1 This circumstance causes the corresponding shape of isotherms at p /p0 0/1 with the plateau independent on the X phase type and its concentrations in X/ SiO2 However, the appearance of X phase is responsible for a reduction of the adsorption due to a decline in the pore volume and specific surface area with increasing CX especially for fumed TS (Tables and 2) Additionally, the hysteresis loop in isotherms for fumed silicas is relatively small (i.e capillary effect is weak) in contrast to mesoporous silica gels [2 Á/5], but it is slightly larger for mixed X/SiO2 and C/X/SiO2 due to known changes in the particle (primary and swarm) morphology in comparison with the initial fumed silica [53 Á/57] The absence of the strong capillary effects for fumed oxides causes more exact estimation of the specific surface area of mesopores using the Kiselev equation in comparison with silica gels (whose SK can be nearly twice as large as SBET [2 Á/6]); as a result, the sum SK'/ SDS is close to SBET (Tables and 2) and the SK values are akin to SdB and Svt (Table 3) For all TS samples, carbonization leads to reduction of the specific surface area (Table 2), and the isotherms for C/TiO2/SiO2 lie below those of TiO2/SiO2 The pore volume decreases for all C/ fumed TS samples (large CC per m2 of the adsorbent), but for C/CVD-TS (smaller CC per m2), it is seen only for TS22 and TS33 (Table 2) For C/SiO2 and C/AS3 (AS3 has a minimal specific surface area and porosity among AS samples), SBET and VBJH are enlarged, but for other C/AS samples, they tend to diminish (especially for C/ AS8) in comparison with those for initial AS (Table 1) This effect for silica (A-300, A-175) can be linked with relatively low CC values in C/ SiO2 (the lower the CC value, the larger is the specific surface area of carbon deposit per se) [2 Á/ 6,26], as silica does not catalyze pyrolysis in contrast to AS and TS (Figs and 2) These structural effects can be elucidated in detail on the basis of analysis of the pore volume (Fig 3) and size (Figs 4Á/6) distributions and changes in adsorbent fractality (Tables 1Á/3) The dVmes/dRp functions (Fig 3) obtained using LIA have two main maxima corresponding to the mesopore radius of Â/1.5 and Â/2.5 nm, i.e these V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 113 Fig Mesopore volume distribution for (a) initial fumed silica, C/SiO2, AS1, and C/AS1; (b) initial CVD-TS and C/CVD-TS; (c) AS and C/AS; and (d) fumed TS and C/TS Rp values are smaller than the size of primary particles of fumed oxides, and the main dVmes/dRp peak shifts towards smaller Rp in comparison with that of the particle size distribution of fumed silica (or X/SiO2) with increasing average particle size and decreasing SBET (A-300 0/A-175 and lower) [55] These small mesopores can be formed by primary particles having relatively tight contacts in aggregates (apparent density of aggregates is about 30% of the specific density [53,54]), i.e these pores represent an accessible surface of primary particles in aggregates giving a significant contribution to the specific surface area increasing with the aggregate size The spatial structure of these mesopores can be complicated due to random packing of primary particles of different sizes in aggregates Clearly, formation of carbon deposit (especially at CC !/10 wt.% [18]) can occur mainly on the outer surface of aggregates or in broad channels in aggregates (if oxide is relatively inert, such as silica, in pyrolysis of organics) and, with consideration of a possible pore structure (particle packing is random and not too dense), formation of the pore plugs is less probable for 114 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 Fig Pore size distributions calculated with the regularization procedure using (a, b) Eq (1) at L0/3 for adsorption data (regularization parameter a0/0.01), and (c, d) Eq (3) for desorption data (a0/0.001); (a, c) silica, alumina (SBET :/160 m2), and AS; and (b, d) silica, titania (SBET :/60 m2), and TS fumed oxides than that for silica gels having mainly cylindrical pores between more closely packed primary particles with smoother pore walls (i.e a micropore contribution in SBET for meso- porous silica gels can be smaller than that for fumed oxides) [2 Á/6] For C/AS, the intensity of the first and second peaks in dVmes/dRp (Fig 3(a) and (c)) is minimal V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 115 Fig Pore size distributions for pyrocarbon/oxides calculated with the regularization procedure using (a, b) Eq (1) and L0/3 for adsorption data (regularization parameter a 0/0.01), and (c, d) Eq (3) for desorption data (a0/0.001); (a, c) C/silica and C/AS; and (b, d) C/silica and C/TS for C/AS1 having maximal CC and minimal SBET and VBJH values among C/AS samples (Table 1) However, a maximal reduction of the last parameters due to pyrocarbon grafting is observed for C/AS8, but AS8 possesses larger VBJH and SBET than those for AS1 Minimal values of CC, DSBET 0/SBET(C /AS)(/SBET(AS), and DVBJH 0/ VBJH(C/AS)(/VBJH(AS) among C/AS samples are characterized for C/AS23 due to the availability of individual amorphous alumina phase, which possesses a lower catalytic capability in pyrolysis in comparison with the alumina/silica interface (Brønsted acidic sites) Therefore, the intensity of dVmes/dRp (Fig 3) at Rp between and nm for C/AS23 is maximal among C/AS samples The first and second peaks of dVmes/dRp (Fig 3) are larger for CVD-TS (and C/CVD-TS) than those for fumed TS due to the smaller sizes of a major portion of CVD-TS particles (namely silica particles in TS, as formation of CVD-titania was performed on the A-300 substrate, and CVDtitania rather represents individual relatively large particles than a continuous layer covering the silica substrate [21]), and SBET of CVD-TS is significantly larger than that of fumed TS (Table 2) Additionally, the DBJH and VBJH values (Table 2) and dVmes/dRp functions (Fig 3) show that channels in aggregates of fumed TS can be narrower than those of CVD-TS that corresponds to a marked diminution of the porosity (SBET, 116 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 Fig Pore size distribution of oxides and carbon/oxides calculated with the regularization procedure using Eq (1) and L 0/4, and automatically chosen regularization parameter a (determined between 0.005 and 0.0005) V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 VBJH) of fumed TS at CTiO2 ]/20 wt.% in comparison with CVD-TS It should be noted that in the case of TS (and C/TS), the third peak (shoulder) of dV /dRp is observed at Rp :/20Á/25 nm and it slightly shifts towards larger Rp with increasing CTiO2 (Fig 3) These mesopores can be linked with the inter-aggregate volumes in agglomerates and broad channels in aggregates between relatively large TiO2 particles (!/20 nm), which can be filled by nitrogen only in part at p /p0 !/0.9 A similar shoulder of dV /dRp is also observed for silica and AS (Fig 3(a) and (c)), but it is less marked than that of TS; however, its shift is seen, e.g for A-300 and A-175 The pore size distribution calculated using Eq (1) as a kernel of Eq (2) at L 0/3 (ni and Wi /W0 optimized with LIA) and fixed regularization parameter a 0/0.01 (parts (a) and (b) of Figs and 5), or L 0/4 and unfixed a (determined on the basis of F -test and confidence regions for nonnegative solutions of Eq (2) [56]) (Fig 6) for adsorption data or utilizing Eq (3) for desorption data (parts (c) and (d) of Figs and 5) with the regularization procedure (a 0/0.001) demonstrate marked changes in f(Rp) depending on the nature of X, and concentrations CX in X/SiO2 and CC in C/X/SiO2 It is pertinent to note that the f(Rp) distributions at Rp between and 10 nm (Figs 4Á/ 6) are akin to dVmes/dRp (Fig 3) but differ in halfwidth of peaks; however, f(Rp) gives more informative picture of micro- and mesopore changes over a large Rp range up to 100 nm Thus, utilization of different isotherm equations for adsorption and desorption data with LIA or overall adsorption (Eq (2)) results in relatively close pore size distributions (Figs Á/5) that lend support to the validity of the used methods Contact zones between adjacent spherical primary particles can be considered as slit-like micropores (narrow channels in aggregates are ‘cylindrical’ micropores) with a different accessibility dependent on the size of adsorbate molecules, which impact, e.g SBET for fumed silica determined with different adsorbates [53] Micropores give large one or even two f(Rp) peaks at Rp between 0.4 and 0.9 nm (Figs 4Á/6), whose halfwidth depends on the regularization parameter value (a 0/0.01 Á/0.0005) and used local isotherm 117 with Eq (1) at L 0/3 or and Eq (3) Thus, all the studied samples possess marked microporosity, whose contribution to f (Rp) seems large due to the log-scale for Rp (in the linear Rp scale, micropores give a very narrow peak as half-width equals to 0.3 Á/0.7 nm) Fumed TS at CTiO2 ]/20 wt.%, titania and alumina (SBET B/160 m2 g(1), which consist of larger primary particles (forming larger channels in aggregates) than AS or silica, have more marked peaks in f(Rp) at Rp !/2 nm (Fig 4) However, after carbonization, new f(Rp) peaks appear at Rp !/5 nm and their intensity is larger for C/AS1 (Fig 5(a)) and C/fumed TS (Fig 5(b)) containing greater amounts of pyrocarbon, i.e these f (Rp) peaks can be linked to the porosity of grafted carbon Some details of the pore size distributions can be elucidated using the regularization procedure with unfixed a (Fig 6) Relative intensity of the second f(Rp) peak grows with the size of primary particles (Fig 6(a) for A-300 and Fig 6(b) for A-175), but micropore contribution (first f(Rp) peak) of C/A300 is higher than that of C/A-175; therefore, changes in SBET of C/A-300 (in comparison with A-300) is larger than that of C/A-175 (Table 1) as well as changes in f(Rp) for C/SiO2 relative silica (notice that CC for C/A-300 and C/A-175 is very close) However, the first f(Rp) peak for A-175 shifts toward smaller Rp in comparison with that for A-300 (due to changes in slit-like micropores between neighboring primary particles with increasing their diameters) and relative contribution of the second peak is greater Changes in f (Rp) for AS1 and AS23 due to grafted carbon are different, and in the case of C/AS23, pyrocarbon is more uniform (Fig 6(c) and (d)), as the alumina phase distribution (i.e distributions of active sites, which catalyze pyrolysis) depends strongly on CAl2O3 in fumed alumina/silica [20], and CC in C/AS1 is larger (Table 1) The titania phase distributions in CVD-TS and fumed TS are different, as CVDtitania consists of relatively large particles having weak contacts with the silica substrate [21]; therefore, pore size distribution in CVD-TS depends mainly on the silica phase and f (Rp) for A-300 and CVD-TS33 or C/A-300 and C/CVD-TS33 are similar (Fig 6(a) and (e)) and relatively large 118 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 amounts of grafted carbon (formed mainly on the titania surface) have a weak impact on the pore size distribution of the silica phase At the same time, the distribution function f (Rp) of carbon/ fumed TS36 is similar to that of C/AS23 (Fig 6(d) and (f)); however, the catalytic capability of TS36 per m2 is the greatest (Figs and 2) Thus, changes in the pore (channels in aggregates) structure of fumed oxides due to carbon grafting (Tables and and Figs 3Á/6) strongly alter from those observed for C/silica gels or C/ CVD-titania/silica gel [2 Á/6] because of the differences in (a) structural hierarchies, (b) packing of primary globules of silicas, and (c) distribution of the second oxide phase [20 Á/25] Typically, the specific surface area of C/fumed X/SiO2 decreases smaller due to carbon grafting (Tables and 2) than that for C/silica gels or C/CVD-titania/silica gel at the same concentration of pyrocarbon deposit [2 Á/6], as in the case of fumed oxides, the porosity is linked with more open pores (channels) formed by primary particles in aggregates, which are less dense than large silica gel particles having mesopores with a relatively narrow size distribution, which influences the carbon phase structure Also, contribution of the external surfaces of aggregates of primary particles of fumed oxides is significantly larger than that for silica gel particles This conclusion is also supported by the shape of f (Rp) (Figs Á/6) and dV /dRp(Rp) (Fig 3) showing marked inter-particle volumes at Rp up to 50 nm in C/X/SiO2 in contrast to silica gels [2 Á/6] Pyrocarbon grafting on the fumed silica surfaces leads even to a gain of the specific surface area (Table 1); however, in the case of X/SiO2 (X 0/ fumed titania, alumina, CVD-titania), the SBET values typically go down (Tables and 2) It is worth noting that apparent density of C/X/SiO2 can rise in comparison with that of initial X/SiO2 However, a diminution of the amount of oxide per cm3 of C/X/SiO2 can be found, and as a result, SBET is lowered, as the specific surface area of the carbon deposit (:/100 m2 g(1 at large CC) [3] can be lower than that of fumed silica Fractal dimension of C/X/SiO2 materials is lower than that of X/ SiO2 (Tables and 2) This can be caused by an enhancement of the density of adsorbents due to reduction of the free volume between primary particles in aggregates (fumed oxides are mass fractal [56]), whose walls become smoother, that gives a diminution of the specific surface area (in m2 g(1), i.e surface fractality changes due to the alterations in the surface topology (roughness alters per cm3) [2 Á/6] Additionally, the specific surface area of the carbon deposit at large CC values [3,7 Á/10,18] is typically lower than that of fumed silica Notice that at CTiO2 5/17 wt.%, the VBJH value is magnified due to carbon grafting onto both fumed and CVD-TS; however, the SBET value decreases (Table 2) In the case of AS, VBJH typically reduces (Table 1) This effect can be linked with the differences in CC, as the pyrocarbon concentration is typically lower for C/TiO2/ SiO2 than that for C/Al2O3/SiO2 However, fractal dimension of C/fumed TS is smaller than that of C/Al2O3/SiO2 (except C/AS1) or C/CVD-TS (Tables and 2) Comparison of DAJ at different p /p0 (or U ) and fractal dimensions (Df) estimated on the basis of the density of aggregates (as fumed oxides are mass fractal) (Df,1 :/2.5 from equation rcluster 0/rparticle(dcluster/dparticle)Df(3) and agglomerates of aggregates (Df,2 :/2.1 Á/2.2) [53,54] shows that DAJ2 (p/p0 B/0.85) is close to Df,1 Therefore, one can assume that filling of inter-particle volume by nitrogen occurs only in aggregates at large p /p0 values, but filling of the inter-aggregates volumes in agglomerates (adsorption on the outer surface of aggregates of primary particles) takes place in an insignificant portion The bulk silica (specific density :/2.2 g cm (3) volume corresponds to :/ 0.32 cm3 g(1 of aggregates (as the apparent density of aggregates is :/0.7 g cm (3) and the free volume in aggregates is about 0.68 cm3 g(1, which is in agreement with VBJH of fumed silica (Table 1) Consequently, this pore volume VBJH filled at p/p0 :/0.98 (as VBJH was estimated at such a pressure) corresponds the free volume in aggregates practically without contribution of a large free volume in agglomerates This conclusion is in agreement with the shape of isotherms, which not have the plateau at p/p0 0/1 Changes in DFRDA and DAJ with increasing CC are close (Tables and 2) and their minima are linked / V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 with C/A-300, C/TS2, and C/AS1 It should be mentioned that the observed differences between DFRDA and DAJ values are in agreement with the results obtained previously [41] However, an increase in xmax to 3.5 nm gives the DFRDA values (Table 3) between DAJ1 and DAJ2 (Tables and 2) The observed dependencies of the D values on the utilized p/p0 range (DAJ1, DAJ2) or xmax value for DFRDA can be caused by nonuniformity of X/SiO2 and C/X/SiO2 particles (primary particles, aggregates, and agglomerates), i.e these materials can be characterized by combination of mass (primary particles of initial fumed oxides), surface (portion of tiny CVD-titania and pyrocarbon particles), and pore (oxide aggregates and large pyrocarbon particles) fractal [56] components The nature of the mixed oxide surfaces influences the water adsorption energy distributions f(E ) (calculated with modified BET equation akin to Eq (5) as a kernel in Eq (2) assuming c 0/ exp((E(/QL)/RT ), where QL is the liquefaction heat of water) (Fig 7) due to the differences in amounts of Brønsted and Lewis acid sites (adsorption of water on studied AS and TS samples was described in detail elsewhere [20,21]) The first f (E ) peak corresponds to water molecules weakly bound to the surface (in large adsorbed water clusters or molecules adsorbed on weak surface sites) and having one or two hydrogen bonds per molecule The second f (E ) peak is linked to strongly bound water molecules interacting directly with Brønsted and Lewis acid sites (first adsorbed monolayer) The last peak energy is greater for AS than that for silica and TS (AS possesses stronger Brønsted acidity than TS has [20 Á/23]); however, it shifts toward larger energy for titania, but its intensity is very low Notice that the activation energy (E") of desorption of intact water molecules from these oxides (e.g E" 0/67 kJ mol (1 at peak temperature Tmax 0/458 K for AS23, 63 kJ mol(1 and Tmax 0/438 K for TS22, and 71 kJ mol (1 and Tmax 0/480 K for TiO2 [24]) is in agreement with f(E ) shown in Fig Additionally, the second f(E ) peak for AS1 lies at larger energy than that for AS3 or AS23, which can be connected with greater acidity of AS at low CAl2O3 [51,52] 119 Fig Water adsorption energy distributions calculated using modified BET equation and regularization procedure at a 0/ 0.01 for adsorption data 3.3 X/SiO2 and C/X/SiO2 particles in aqueous suspension Properties of C/X/SiO2 particles in the aqueous suspensions can be linked to the carbon deposit morphology, which are of importance for applications of these materials in liquid media For C/ SiO2, an increase in CC makes the surface more basic, and a minimum of electrokinetic potential z (pH) observed for pure silica suspensions at pH:/2.5 Á/3 disappears even at low CC (a similar effect has been seen for pure silica with increasing salinity of the suspension) (Fig 8) [57] An enhancement of the basic properties of C/SiO2 surface is linked with basic sites of carbon phase 120 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 Fig Electrokinetic potential as a function of pH at (a) different salinity of pure silica suspension (z (pH) is also shown for C/A-300 at CC 0/9.3 wt.%); and (b) different CC (pyrolysis of CH2Cl2 at 673 Á/823 K for 40 Á/120 min) in C/A-300 at 0.01 M NaCl (possessing pre-graphite structure and partially oxidized graphene clusters), for which a pH value of the isoelectric point (IEP) can be about [3,57Á/ 59] In case of AS, the carbon deposit forms first of all on the strongest acidic or basic sites (possessing a maximal catalytic capability in pyrolysis), as the shape of the z (pH) curves for C/AS changes not only at pH B/5 (negative charge of particles is provided by acidic sites) but also at pH !/6 (Fig 9) For C/AS, pH(IEP) is close to 4, but for pure Fig z Potential as a function of pH for (a) AS and C/AS; (b) fumed TS and C/TS; and (c) CVD-TS and C/CVD-TS at different concentrations of CX in C/X/SiO2 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 AS, it is below Therefore, one can assume that pyrocarbon in C/AS is distributed nonuniformly, mainly on the alumina/silica interfaces and alumina phase, as they possess the catalytic ability in pyrolysis of organics Additionally, the z (pH) curves for C/AS are relatively close to that for C/ SiO2; consequently, silica surface and carbon deposit in C/X/SiO2 are mainly accessible for liquid water A similar effect is observed for C/ TS (Fig 9), but it is not too great as that for C/AS may be due to closer electrokinetic properties of titania and silica than those of alumina and silica [57] There is tendency of an enhancement of pH(IEP) with increasing CC practically independent on the nature of X/SiO2 substrate or salinity of the suspension (Fig 10); additionally, the range of pH(IEP) for C/X/SiO2 materials is more narrow (2.5 Á/4.5) than that of X/SiO2 [57] (Fig 10) These effects can be connected with the formation of pyrocarbon on more active (in pyrolysis) patches of the X/SiO2 surfaces such as the X/silica interface and X phase possessing Brønsted and Lewis acid sites or sites active in redox reactions (anatase), i.e silica phase in C/X/SiO2 is covered by pyrocarbon to a lesser extent and pH(IEP) of C/X/SiO2 corresponds to average value of silica (pH(IEP) :/2.2) and carbon deposit Thus, in the case of such mixed oxides as AS and TS, formation of carbon deposit due to pyrolysis of cyclohexene occurs mainly on surface patches of the X/SiO2 interface and X phase possessing the catalytic capability in pyrolysis, which results in the similarity of the electrokinetic behavior of C/X/SiO2 and C/SiO2 particles in the aqueous suspensions, as X phase is shielded by pyrocarbon to a greater extent than silica Conclusions For fumed alumina/silica, mainly the interfaces with Brønsted and Lewis sites catalyze cyclohexene pyrolysis, but in the case of TS, both the titania phase and TiO2/SiO2 interfaces possess the catalytic capability, which impacts the carbon structure and distribution However, titania phase per se in TS plays an important role, as the normalized reaction rate is larger for fumed 121 TS36, but for AS, the reaction rate is maximal at low CAl2O3 For CVD-TS, the pyrolysis rate can be lower also due to distribution features of CVDTiO2 in the form of relatively large particles, which cannot provide a great specific catalytic activity of CVD-TS due to a small contribution of the surface area of titania particles to the specific surface area of CVD-TS as a whole These effects result in the differences in the pyrocarbon distribution on oxide support particles The SBET value of C/fumed silica (low CC) is larger than that for the initial silica in contrast to pyrocarbon/silica gels The absence of the plateau and small hysteresis loops are observed in isotherms of nitrogen adsorptionÁ/desorption on fumed silica, alumina/silica, titania/silica, CVDTS, and C/X/SiO2, as capillary effects are weak due to features of channels (free volume) in aggregates formed with near-spherical primary particles The difference in dVmes/dRp functions for C/X/SiO2 is due to the formation of various channels in aggregates of primary particles, whose accessible surface area determines the SBET and Smes values to a greater extent than the pore volume, which also depends on the packing types of primary particles not only in aggregates but also in agglomerates The SBET values typically go down for C/X/SiO2 in comparison with those of X/SiO2, and fractal dimension drops due to grafting of tiny carbon particles in the free volume of aggregates and on their outer surfaces (surfaces become smoother); however, in the case of pure silica, carbon particles can be mainly formed on the outer surface of aggregates, which reflects in smaller changes in fractal dimension of C/A-175 (relative A-175) in comparison with other C/X/ SiO2 adsorbents Thus, changes in the distribution of X oxide in X/SiO2 result in the alterations in pyrocarbon distribution and its concentration in C/X/SiO2, which are maximal for AS1 (per g) and TS36 (per m2) Changes in electrokinetic behavior of hybrid particles in the aqueous suspensions due to pyrocarbon grafting show that C deposit can be nonuniform, as it covers mainly surface patches of the X/silica interface and X phase possessing great catalytic activity in pyrolysis of organics than silica, and in the case of pure silica, pyrocarbon 122 V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 Fig 10 pH of the isoelectric point for aqueous suspensions of C/X/SiO2 as a function of (a) CC in C/A-300 at different salinity; (b) CAl2O3 in C/AS; (c) CC in C/A; (d) CTiO2 and CC in C/fumed TS; and (e) CTiO2 in CVD-TS and C/CVD-TS, and CC in C/CVD-TS V.M Gun’ko et al / Colloids and Surfaces A: Physicochem Eng Aspects 218 (2003) 103 Á/124 represents particles formed mainly on the outer surface of aggregates of primary silica particles, and SBET of C/SiO2 is larger than that of SiO2 Consequently, changes in the structure of silica and X/SiO2 (X distribution, X particle morphology, X concentration, etc.) allow one to control the distribution and amount of pyrocarbon (grafted first of all on more active sites), nonuniformity of hybrid surfaces, and as a result the overall properties of C/X/SiO2 adsorbents [14] [15] [16] [17] [18] [19] [20] Acknowledgements Financial support from the State Committee for Scientific Research (KBN, Warsaw, Project No T09A 03611) is gratefully acknowledged This research was partially supported by NATO (Grant EST.CLG.976890) [21] [22] [23] References [24] [1] (a) R Leboda, Mater Chem Phys 31 (1992) 243; (b) R 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