Physisorption pore size distribution

Figure 6.4 Features of beta zeolite after Fenton treatment, (a) Saito-Foley adsorption pore-size distribution from Ar-physisorption for (O) parent zeolite containing the template (no porosity) ( ) Fenton-detemplated and (V) commercial NH4-form BEA.

The nitrogen physisorption isotherm and pore size distributions for the synthesized catalysts are shown in Figs. 3 and 4. The Type IV isotherm, typical of mesoporous materials, for each sample exhibits a sharp inflection, characteristic of capillary condensation within the regular mesopores [5, 6], These features indicate that both TS-1/MCM-41-A and TS-l/MCM-41-B possess mesopores and a narrow pore size distribution. 

Fast adsorption/desorption kinetics and relatively small (<10 kj/mol) adsorption enthalpies are observed for hydrogen adsorption on many porous materials, which indicates that physisorption on porous materials is suitable for fast recharging with hydrogen [81,82], The narrowest pores make the biggest contribution to hydrogen-adsorption capacity, whereas mesopores contribute to total pore volume, but little to hydrogen capacity, and are detrimental for the overall volumetric capacity. Hence, porous materials with very narrow pores or pore-size distributions are required for enhanced hydrogen capacity at low pressures. 

The pore size distribution based on BJH (Barrett-Joyner-Halenda) calculations, the micropore fraction (t-plot analysis), and the BET (Brunauer-Enunett-Teller) surface area of the catalysts were acquired by physisorption measurements of nitrogen at 77 K (Micrometries Gemini 2360). Prior to BET analysis the samples were evacuated at 373 K for at least 12 h. 

Gas adsorption (physisorption) is one of the most frequently used characterization methods for micro- and mesoporous materials. It provides information on the pore volume, the specific surface area, the pore size distribution, and heat of adsorption of a given material. The basic principle of the methods is simple interaction of molecules in a gas phase (adsorptive) with the surface of a sohd phase (adsorbent). Owing to van der Waals (London) forces, a film of adsorbed molecules (adsorbate) forms on the surface of the solid upon incremental increase of the partial pressure of the gas. The amount of gas molecules that are adsorbed by the solid is detected. This allows the analysis of surface and pore properties. Knowing the space occupied by one adsorbed molecule, Ag, and the number of gas molecules in the adsorbed layer next to the surface of the solid, (monolayer capacity of a given mass of adsorbent) allows for the calculation of the specific surface area, As, of the solid by simply multiplying the number of the adsorbed molecules per weight unit of solid with the space required by one gas molecule ... 

Figure 16. The pore-size distribution for sol—gel-derived birnessite Na(5Mn02 20 as processed into three pore-solid nanoarchitectures xerogel, ambigel, and aerogel. Distributions are derived from N2 physisorption measurements and calculated on the basis of a cylindrical pore model. (Reprinted with permission from ref 175. Copyright 2001 American Chemical Society.)...

Besides specific surface area, silicas are also characterised by their porosity. Most of the silica s are made out of dense spherical amorphous particles linked together in a three dimensional network, this crosslinked network building up the porosity of the silica. Where the reactivity of diborane towards the silica surface has been profoundly investigated, little attention has been paid to the effect of those reactions on the pore structure. However different methods are developed to define the porosity and physisorption measurements to characterise the porosity parameters are well established. Adsorption isotherms give the specific surface area using the BET model, while the analysis desorption hysteresis yields the pore size distribution. 

In calculations of the mesopore size distribution from physisorption isotherms it is generally assumed (often tacitly) (a) that the pores are rigid and of a regular shape (e.g. cylindrical capillaries or parallel-sided slits), (b) that micropores are absent, and (c) that the size distribution does not extend continuously from the mesopore into the macropore range. Furthermore, to obtain the pore size distribution, which is usually expressed in the graphical form AV /Arp vs. rp, allowance must be made for the effect of multilayer adsorption in progressively reducing the dimensions of the free pore space available for capillary condensation. 

It is evident from the above considerations that the use of the physisorption method for the determination of mesopore size distribution is subject to a number of uncertainties arising from the assumptions made and the complexities of most real pore structures. It should be recognized that derived pore size distribution curves may often give a misleading picture of the pore structure. On the other hand, there are certain features of physisorption isotherms (and hence of the derived pore distribution curves) which are highly characteristic of particular types of pore structures and are therefore especially useful in the study of industrial adsorbents and catalysts. Physisorption is one of the few nondestructive methods available for investigating meso-porosity, and it is to be hoped that future work will lead to refinements in the application of the method -especially through the study of model pore systems and the application of modem computer techniques. 

Over the past few years a revised form of density functional theory DFT, has become a powerful tool for the interpretation of physisorption data (Balbuena and Gubbins, 1992, 1993 Lastoskie et al., 1993 Olivier 1995 CrackneO et al., 1995 Maddox and Gubbins, 1995). In particular, the approach must now be regarded as a valuable alternative procedure for evaluating the pore size distribution (Lastoskie et al., 1994 Olivier et al., 1994). 

Optimization of the pore size distribution is important for the control of both the equilibria and the dynamics of physisorption (see Ruthven, 1984 Do et al., 1993). Most activated carbons are highly microporous, but for some purposes it is desirable to extend the range of pore size into the mesopore or macropore range - or even eliminate the microporosity. Progress in this direction has been made by the use of special pre-treatment procedures and the careful control of the conditions of carbonization and activation. In this connection, physisorption measurements have an important role to play in characterizing the material at various stages of manufacture. 

Nitrogen physisorption methods for total surface area (BET), and more recently macropore surface area determination (t-plot) are used to quantify relationships of the amount and type (zeolite, matrix) of surface present. Nitrogen and mercury pore size distribution (NPSD HGPSD) are used to determine sizes of pores within the catalyst. Bulk, particle, and skeletal densities can be measured with standard volumetric apparatus or more recently with sophisticated pychnometers using helium as a fill gas. 

Figure 1.7 shows the physisorption isotherm, obtained using the non continuous volumetric technique, of a mesoporous alumina (type IV isotherm) and the results of analysis procedures (BET transform, /-curve, BJH porous distribution). This solid presents a specific surface area of approximately 200 m /g with the narrow pore size distribution at around 10 nm. The shape of the /-curve shows that it does not contain any micropores. 

This technique is one of the most important and extensively used methods in the characterisation (porous volume, specific surface area and pore size distribution) of porous inorganic materials [40,41]. Nevertheless, real solid/gas interfaces are complex, leading to uncertainties in the assumptions made, and different mechanisms may contribute to physisorption (e.g. monolayer-multilayer ad-... 

K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniewska, Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl. Chem., 57 (1985) 603. K. Kaneko, Determination of pore size and pore size distribution 1. Adsorbents and catalysts, /. Membr. Sci., 96 (1994) 59. 

Mercury porosimetry (for macropores) and BET N2 physisorption (for micro- and mesopores) can determine both the total pore volume and the pore size distribution. 

CW-EPR X-band measurements were performed on a Bruker ESP 300E Spectrometer at a temperature of 120 K. The CuHis complexes are paramagnetic due to the S=l/2 spin of the Cu " ion. Nitrogen physisorption was performed with a Micromeritics ASAP 2400 apparatus. Measurements were done at 77 K. Prior to the measurements the zeolite samples were degassed for 24 hours at 373 K in vacuum. Micropore volumes and pore size distributions were determined with standard BET and BJH theory. Diffuse Reflectance Spectroscopy of the CuHis complex encapsulated zeolite samples were taken on a Varian Cary 5 UV-Vis-NIR spectrophotometer at room temperature. The DRS spectra were recorded against a halon white reflectance standard in... 

Nitrogen physisorption was performed with a Micromeritics ASAP 2400 apparatus. Measurements were done at -196°C. Prior to the measurements the powdered samples were degassed for 24 hours at 300°C in vacuum. Surface areas, pore volumes and pore size distributions were determined with standard BET and BJH theory. 

The SBA-15 mesoporous silica was prepared according to the work of Zhao et al. [7] by mixing tetraethoxysilane, Si(OEt)4, and the Pluronic 123 [(EO)2o(PO)7o(EO)2o] non ionic surfactant in a strongly acidic solution before hydrothermal treatment at 95°C for 72h and calcination in air at 550°C for 6h. The hexagonal P6mm symetry of the mesoporous material was confirmed with XRD [8]. Na physisorption yields a specific surface area of 720 m. g a narrow pore size distribution (15 A) with an average pore size of 64 A (BJH analysis). 

Figure 1 isothenns (left) and pore size distributions (right, as determined by N2-physisorption) of catalysts prepared with fructose using different protocols. 

Figure 3. (Left) Saito-Foley adsorption pore-size distribution based on Ar-physisorption ( ) parent BEA zeolite, ( ) one-pot catalyst, ( ) calcined zeolite at 923 K (H-form), (O) BEA zeolite NH4-form. The pore size distribution of BEA is schematically illustrated. (Right) N2O decomposition performance. Conditions 4.5 mbar N2O in He (pressure, 3 bara) and W/F N20= 900 kgxsxmoF. Two reference catalysts are included for comparison, based on NH4-form commercial samples prepared by conventional Fe ion-exchange.

Nitrogen physisorption measurements were performed on a Micromeritics Tristar 3000 apparatus at -196 °C. Prior to analysis the samples were dried in a helium flow for 14 horns at 120 °C. Surface areas (St), and micropore (Vmicro) and mesopore (Vmeso) volumes were determined using the t-method [13] with the Harkins-Jura thickness equation. There is no standard method for the determination of blocked mesopore volume (Vmeso,bi)- For this we used the pore size distribution from the desorption branch of the isotherm calculated using BJH theory [14]. The total amoimt of Vmeso,bi was determined considering that the volume in pores with a diameter of 2 - 5 run is (partially) blocked. 

In the nitrogen physisorption experiments, all the xSn02-MSM samples synthesized with and without HT treatment showed type IV isotherms with the Hi hysteresis loops appeared at P/Pq around 0.5 0.7 and 0.6 0.8, respectively, which were the characteristic of large mesoporous materials [7,15]. The textural properties of the calcined HxSn02-MSM materials are listed in Tab. 1. The samples possessed high surface area (Sbet = 735 896 m /g), enormous pore volume (Vrotai = 0.81 1.00 cm /g), large pore diameter (Objh 8.9 nm) and narrow pore size distribution (PSD <1.5 nm). Since the pore size and pore... 

The catalyst and support surface areas, porous volumes and pore size distributions were measured by physisorption of N2 at -196°C on a Micromeritics ASAP 2000 instrument. Prior to measurement, samples were outgassed at 150°C under 0,13 Pa. Surface area, porous volume and pore size distribution values were computed, using BET and BJH equations, from the amount of N2 physisorbed at different relative pressure. 

EX AFS and X ANES are generally performed at a synchotron facihty and provide information about neighboring atoms of the atom under investigation. Nitrogen physisorption Used to determine surface area, pore size, pore volume, pore size distribution and further textural properties through analysis of adsorption/desorption isotherms. 

Physisorption of probe molecules is used to determine surface area and characterize the pore size distribution of solid catalysts and materials. Basically, the process of adsorption that takes place on a solid surface involves adsorption processes between a solid (adsorbent) and a gas (adsorptive). Note that the word adsorbate is distinct from adsorptive and corresponds more specifically to the matter in the adsorbed... 

Figure 20.16. physisorption isotherm for sono-aerogel (triangles) and classic gel (squares). Solid symbols correspond to the adsorption branch and open symbols to the desorption one. The inset shows the pore size distribution of the same samples using the BJH model. The continuous line is for the sono and dotted line for the classic gel. 

Figure 1. Differential pore size distributions derived from the desorption branch of nitrogen physisorption at 77 K (STP 273.15 K, 1 atm), (a) Pt5PC, (b) Pt2NP both calcined in air at 573 K.

In addition to the evaluation of the combined micro and mesoporous carbons, also siliceous materials were analyzed. More recently is there also a growing interest in the characterization of siliceous materials with CO2 adsorption and the application of the DR-equation [13]. The application of the DR-equation and the proposed correction on this material will provide fh er insight on the universal applicability of this approach. To this end the N2 and CO2 physisorption characteristics were measured on two purely mesoporous MCM-41 materials. The BJH pore size distribution (N2) of these mesoporous materials shows a narrow pore size distribution around 23 A and 32 A respectively. The corresponding specific surface area (5bet), total pore volume (FloiaO. and results fiom the DR-equation obtained by CO2 adsorption are given in Table 3. 

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