Mesopore volume

If a Type I isotherm exhibits a nearly constant adsorption at high relative pressure, the micropore volume is given by the amount adsorbed (converted to a liquid volume) in the plateau region, since the mesopore volume and the external surface are both relatively small. In the more usual case where the Type I isotherm has a finite slope at high relative pressures, both the external area and the micropore volume can be evaluated by the a,-method provided that a standard isotherm on a suitable non-porous reference solid is available. Alternatively, the nonane pre-adsorption method may be used in appropriate cases to separate the processes of micropore filling and surface coverage. At present, however, there is no reliable procedure for the computation of micropore size distribution from a single isotherm but if the size extends down to micropores of molecular dimensions, adsorptive molecules of selected size can be employed as molecular probes. 

Fig. 16. Mesopore volume as a function of bum-off for PAN derived carbon fiber porous monoliths [28].

The support and the catalysts were characterised by means of nitrogen adsorption, XPS, TPD and SEM. The nitrogen adsorption isotherms were determined at 77 K in a Coulter Omnisorp 1000 CX equipment, and were analysed by the BET equation (SBet), and by the t-plot for mesopore surface area (Smeso) and micropore and mesopore volume (Vmicr0, Vmeso), using the standard isotherm for carbon materials. The catalyst samples were previously outgassed at 120 °C. 

It is important to stress that the capacitive behaviour of the microporous carbons could be further improved by enhancing the mesopore volume. The presence of mesopores plays a crucial role for the ion transportation to the active surface. Hence, a development of mesopores in these materials, and a strict control of the micropore-mesopore volume ratio is necessary. 

The studied solids exhibit great differences in term of porosity (purely microporous, micro-mesoporous and mesoporous). As it is well known, mesoporosity of USY (CBV series) increases as the aluminum content decreases. Large part of mesoporosity is in the range 2-3.6 nm according to the difference between mesoporous volume obtained by N2 or Hg measurements. Pore size estimation gave similar results whatever the technique. 

The difference between the eluted volume found with p-xylene (Veiutedpxylene) and TIPCyC6 (VeiutedTIPCyC6) allows to evaluate the occluded mesopores volume (Vocduded) as ... 

It was found that more than 70 % of the mesopore volume of the highly dealuminated Y zeolite was externally accessible [7]. Interestingly these results are in accordance with... 

Unit cell FWHMb parameter (nm) Specific surface area (m2/g) Mesopore External Specific mesopore volume (cm3/g) Mesopore diameter (nm) Wall thickness0... 

Control over the material s shape at the nanoscale enables further control over reactants access to the dopant, and ultimately affords a potent means of controlling function which is analogous to that parsimoniously employed by Nature to synthesize materials with myriad function with a surprisingly low number of material s building blocks. A nice illustration is offered by the extrusion catalytic polymerization of ethylene within the hexagonal channels of MCM-41 mesoporous silica doped with catalyst titanocene.36 The structure is made of amorphous silica walls spatially arranged into periodic arrays with high surface area (up to 1400 m2g 1) and mesopore volume >0.7 mLg-1. In this case, restricted conformation dictates polymerization the pore diameter... 

A number of models have been developed for the analysis of the adsorption data, including the most common Langmuir [49] and BET (Brunauer, Emmet, and Teller) [50] equations, and others such as t-plot [51], H-K (Horvath-Kawazoe) [52], and BJH (Barrett, Joyner, and Halenda) [53] methods. The BET model is often the method of choice, and is usually used for the measurement of total surface areas. In contrast, t-plots and the BJH method are best employed to calculate total micropore and mesopore volume, respectively [46], A combination of isothermal adsorption measurements can provide a fairly complete picture of the pore size distribution in sohd catalysts. Mary surface area analyzers and software based on this methodology are commercially available nowadays. 

Thus, either type I or type IV isotherms are obtained in sorption experiments on microporous or mesoporous materials. Of course, a material may contain both types of pores. In this case, a convolution of a type I and type IV isotherm is observed. From the amount of gas that is adsorbed in the micropores of a material, the micropore volume is directly accessible (e.g., from t plot of as plot [1]). The low-pressure part of the isotherm also contains information on the pore size distribution of a given material. Several methods have been proposed for this purpose (e.g., Horvath-Kawazoe method) but most of them give only rough estimates of the real pore sizes. Recently, nonlocal density functional theory (NLDFT) was employed to calculate model isotherms for specific materials with defined pore geometries. From such model isotherms, the calculation of more realistic pore size distributions seems to be feasible provided that appropriate model isotherms are available. The mesopore volume of a mesoporous material is also rather easy accessible. Barrett, Joyner, and Halenda (BJH) developed a method based on the Kelvin equation which allows the calculation of the mesopore size distribution and respective pore volume. Unfortunately, the BJH algorithm underestimates pore diameters, especially at... 

Prior to nitrogen adsorption experiment to determine surface properties, ACC sample was degassed at 130°C under vacuum (up to 10 torr) for 12 h. The adsorption data were obtained at the Central Laboratory of Middle East Technical University (METU) with a Quantachrome Autosoib-l-C/MS apparatus over a relative pressure ranging from 10" to 1. The BET specific surface area, total pore volume, micropore volume, mesopore volume, and pore size distribution, PSD, of ACC were yielded by using the software of the apparatus. 

The pore size distribution (PSD) for mesopores and micropores were calculated using DFT (Density Funetional Theory) method and the resulting distribution curves are given in Fig. 21.3. Total mesopore volume (V ) was determined as 0.022 cm g and total mieropore volume (V. ) was determined as 0.709 em g from the eorresponding method. The ACC consists of pores mainly in micro character (<20 A) as seen both from Fig. 21.3. The pH value of ACC used in this study had been previously found to be 7.4 [4]. 

Indeed, both the surfaee area and mesopore volume of the sample containing 32 wt.% of Co-POM were about 70-76 % of the eorresponding eharacteristics of the initial support. 

Mesopore surface area mesopore volume " avarage mesopore diameter broad mesopore... 

The BET specific surface area, mesopore volumes, and pore wall thickness of the calcined and water-treated samples are given in Table 3. BET surface area of the samples prepared with Cm surfactants were found to be less affected by hydrothermal treatment. When the samples synthesized without TPA+ subjected to hydrothermal treatment the sharp inflection in the isotherm became very broad indicating wide distribution of pores. In contrast, the mesopore distribution of the samples prepared with TPA was found to be less affected by hydrothermal treatment. For the samples prepared without TPA, the mesopore volume was found to decrease sharply and the pore diameter was broadened over a large range indicating loss of the mesopore structure. Addition of TPA was found to minimize the structural collapse and thereby helps to preserve the mesoporosity. 

A good-quality CeMCM-41 material with Si/Ce=50 was synthesized by hydrothermal method. For the purpose of comparison a pure siliceous MCM-41 was prepared using the same composition without cerium. Thermogravimetric curves for the synthesized uncalcined samples exhibit shape characteristic for the MCM-41-type materials. The specific surface area of CeMCM-41 evaluated from nitrogen adsorption was equal to 850 m2/g, whereas the pore width and mesopore volume of this material were equal to 3.8 nm and 0.8 cm3/g, respectively. In contrast to the pure silica MCM-41, the CeMCM-41 material exhibits medium and strong acid sites as revealed by thermogravimetric studies of n-butylamine thermodesorption. 

The pore diameter of the resulting materials, their specific surface area and mesopore volume depend on the size of attached ligands (see Table 1). The presence of bonded alkylsilyl groups (appropriate ratio C H) and a successful removal of all surfactant molecules and pyridine (complete absence of nitrogen) was shown by means of the elemental analysis. Table 1 contains also the surface coverage of the bonded ligands. 

Solvent11 Composition Surface area, 2 -1 m g Framework mesopore volume cm3 g 1 Textural pore volume cm3 g1 Pore diameter nm... 

Texture parameters from adsorption isotherms, i. e, total surface area (Stot), external surface area (Sext), mesopore surface area(Sme). mesopore volume (VTO) and mesopore diameter... 

The total surface area S,0i ofll09m2/g, external surface area Sexi of 110m2/g and mesopore volume Vmc of 0.806 cm3/g were evaluated from the nitrogen adsorption isotherm using the comparison plot method [11], The size of the MCM-41 hexagonal channels can be characterized by their hydraulic diameter Dme, whose size of 3.2 nm was calculated according to the formula... 

The porous volumes measured by N2 adsorption are listed in Table 3. After the boronation, the total porous volumes (Vt) of the samples increase, corresponding to the increase of benzene adsorption capacity mentioned above. This should be resulted from the following aspects (1) The average mass of zeolite crystallite decrease and the number of crystal particles in unit weight of sample increases after the boronation owing to a limited introduction of trivalent atoms and Na+cations as counterions, as well as a severe dissolution of silicon. Thus, the total porous volume (mL/g) and the adsorption capacity increase. (2) The transformation of pore size occurs during the boronation. As shown in Table 3, the mesoporous volumes increase and the microporous volumes decrease after the boronation, meaning that some micropores are developed into mesopores due to the removal of silicon from the framework. This is also one of the important reasons why the total porous volumes as well as the adsorption capacities increase after the boronation. 

The pore size distributions of the PCH after deposition of aluminium oxide species onto the surface have been depicted in figure 5 A decrease in intensity is noticed in the micropore-as well as in the mesopore region, with increasing concentration of Al(acac)j. There is no clear evidence for a shift of the maxima in the pore size distribution towards smaller pore size, however only for the largest Al-conceiitration there is an indication in that direction. The Al-grafted PCHs are still characterized by sufficiently large surface areas, micropore and mesopore volumes (table 1). From these values and also from the pore size distributions it can be deduced that the bonding of Al-species onto the PCH surface occurs in the micropores as well as in the mesopores. 

As can be seen from Table 1 all fresh samples exhibited a remarkable porosity the specific surface area, SBET was between 160 and 480 m2/g and the mesopore volume, VpN2 was in the range of 0.7- 1.4 cm3/g and those volumes appeared to be the largest in Pt doped samples. Micropore volumes were virtually insignificant in all samples as can be seen from t-plots... 

The BET specific surface area [28] was calculated in the relative pressure range between 0.04 and 0.2. The total pore volume was determined from the amount adsorbed at a relative pressure of 0.99 [28], The primary mesopore volume and external surface area were evaluated using the as-plot method [24, 28, 29] with the reference adsorption isotherm for macroporous silica [29], The pore size distributions were determined using the Kruk-Jaroniec-Sayari (KJS) equation [30] and the calculation procedure proposed by Barrett, Joyner and Halenda (BJH) [31]. 

Data for calcined samples dioo - XRD (100) interplanar spacing, Sbet - BET specific surface area, V, - total pore volume, Vp - primary mesopore volume, Sex - external surface area, wkjs - primary mesopore diameter. Data for uncalcined samples mreS due - mass percent of residue at 1263 K, mSdir - mass decrease in the temperature range of the surfactant decomposition and desorption of the decomposition products (between about 373 and 623 K). Notes a - no peak on XRD spectrum, d,0o cannot be evaluated, b - no linear region on the Os-plot, which would be suitable for the Vp and Sex evaluation. XRD and adsorption data (except for those for HR-A2 sample) taken from Refs. 24 and 26. Thermogravimetric data for DS-AD taken from Ref. 19. 

---