Mesopore analysis

As explained in Chapter 7, since the multilayer isotherm path is rather insensitive to differences in surface chemistry, for routine mesopore analysis it is possible to make use of a universal form of nitrogen isotherm. However, most activated carbons are highly microporous and the determination of the micropore size distribution remains a more difficult problem. Indeed, as discussed in Chapter 8, even the assessment of the total micropore volume presents conceptual difficulties. We should therefore regard the measurement of a nitrogen adsorption isotherm as only the first stage in the characterization of a microporous carbon. 

Table 3 Mesopore analysis of sample B - application of the BJH method...

One of the biggest problems in the area of mesopore analysis is the hysteresis effect, that is the adsorption isotherm is different from the desorption isotherm. The hysteresis loop formed in the isotherm covers only a portion of the isotherm. The desorption branch of the isotherm always has a higher amount of material adsorbed than does the adsorption branch, which from any semi-thermodynamic point of view makes sense. Several complications have been postulated for the phenomenon. Complicating the subject is a specific case that leads to what appears to be hysteresis, for example partial chemisorption and plastic deformation of the adsorbent. The solution to non-specific hysteresis may be found in density function theory (DFT) calculations of which several have been quite insightful. 

The above analysis, which includes the last term of Eq. (217), will be referred to as the mesopore analysis . An analysis without this last term, which is identical to the analysis for microporous materials described previously will be referred to as micropore an ysis . Essentially, the non-inter-preted micropore analysis uses Eq. (217) without the last term and sets the pore radius, in place of Eq. (196) equal to t obtained from Eq. (194). (Simply doing this does not yield the same value for t as obtained from the mesopore analysis due to the interactions between the parameters in the fitting routine.)... 

Does it Matter Whether to Use a Micropore or a Mesopore Analysis ... 

Table 36 gives the summary of the mesopore analysis using the above method. From the mesopore analysis and the X-ray data, the wall thickness is calculated. With the exception of the desorption data for the last data set, which is designated C-22, the wall thickness is calculated to be between 0.60 and 0.87 nm, which is fairly reasonable according to the criterion of Eq. (190). 

As with the mesopores analysis, a major point in having reasonable results is the use of an adequate -plot for the given adsorbate. In this case, values of t can be evaluated from an empirical relation given by Harkins and Jura [114] ... 

In this kind of determination, as in die ease of die mesopores analysis through adsorption—desorption measurements, it must be taken into account that the size available to flux is not the real size of the pore. So it is necessary to add the thickness of the adsorbate layer, t, which is a function of relative pressure, as shown above. A good approximation for determining t is again the Halsey correlation. 

Jessop et al have developed a novel procedure for computing the pore size distribution from nitrogen isotherm data. The method is based on the application of mean-field theory for the calculation of a set of isotherms corresponding to pores of given width and it is claimed that the molecular model provides a realistic representation of the adsorbed fluid in pores of all sizes and that the method can therefore be used for both micropore and mesopore analysis. A limited number of comparisons have been made with more conventional methods of pore size analysis, but it is probably too early to judge the success of this interesting approach. 

The mesoporous character of MCM-41 overcomes the size limitations imposed by the use of zeolites and it is possible to prepare the complex by refluxing the chiral ligand in the presence of Mn +-exchanged Al-MCM-41 [34-36]. However, this method only gives 10% of Mn in the form of the complex, as shown by elemental analysis, and good results are only possible due to the very low catalytic activity of the uncomplexed Mn sites. The immobihzed catalyst was used in the epoxidation of (Z)-stilbene with iodosylbenzene and this led to a mixture of cis (meso) and trans (chiral) epoxides. Enantioselectivity in the trans epoxides was up to 70%, which is close to the value obtained in solution (78% ee). However, this value was much lower when (E)-stilbene was used (25% ee). As occurred with other immobilized catalysts, reuse of the catalyst led to a significant loss in activity and, to a greater extent, in enantioselectivity. 

Similarly, monometallic Rh, Pd, and Au and bimetallic Pt-Rh and Pt-Pd nanowires were prepared in FSM-16 or HMM-1 by the photoreduction method [30,33,34]. The bimetallic wires gave lattice fringes in the HRTEM images, and the EDX analysis indicated the homogeneous composition of the two metals. These results show that the wires are alloys of Pt-Rh and Pt-Pd. Mesoporous silica films were also used as a template for the synthesis of uniform metal particles and wires in the channels [35,36]. Recently, highly ordered Pt nanodot arrays were synthesized in a mesoporous silica thin film with cubic symmetry by the photoreduction method [37]. The... 

Detailed analysis of the DDIF spectra showed a constant microporosity for almost all samples. This lends strong support to the hypothesis that the original rocks after burial, compaction and the initial diagenesis had a common pore geometry. Such a base rock appears to be dominated by the micropores with approximately 2/3 micropores, 1 /3 mesopores and very few macropores. The effect... 

Capek, L., Kreibich, V., Dedecek, J. et al. (2005) Analysis of Fe species in zeolites by UV-VIS-NIR, IR spectra and voltammetry. Effect of preparation, Fe loading and zeolite type, Microporous Mesoporous Mater., 80, 279. 

Structural and textural characterisation of pure SBA-15 and hybrid GFP/SBA-15 Pure SBA-15 and GFP/SBA-15 hybrid were characterised by X-ray powder diffraction, HRTEM and volumetric analysis. Calcined SBA-15 (Fig. 1, curve A) show the typical XRD pattern of an ordered hexagonal network of mesopores with (10), (11) and (20) reflections. The presence of well resolved (11) and (20) peaks indicate that the calcined material used for the preparation of the hybrid materials have a long-range order. The hexagonal XRD pattern was still clearly observed in the hybrid material (GFP/SBA-15), as all the three main reflections were found (Fig. 1, curve B), indicating that the sonication and the GFP physical adsorption does not affect the framework integrity of the material. 

The parent siliceous materials of the SBA-15 and MLV types were synthesised after typical procedures [2], The carbon replicas were prepared via polymerisation, catalysed by ferric chloride, of pyrrole introduced into the mesopores of matrices [3]. The products were characterised by the nitrogen adsorption, TEM, and thermal analysis (DTG, DTA). The nitrogen content in carbons was determined using elemental analysis, XPS, and EDX, while the Si and Fe contents, with XPS. The replicas of SBA-15 and MLV-0.75 are denoted as CMK-3Nx and OCM-.Nx, respectively, were x refers to the number of g of FeCl3 per 1 g of silica used for preparations. 

Solid-state NMR spectroscopy was used for studying the formation of cubic mesoporous aluminophosphate thin films and powders. The analysis of the initial gel, the as-deposited materials and the thermally-treated materials elucidated the changes in the coordination of phosphorus and aluminium atoms and thus revealed how the framework formation and condensation proceeds. The consolidation process in thin films was different than the process in powders. Most probably this could be attributed to the effect of glass substrate. 

Mesoporous materials (SBA-15 and Al-SBA-15 with various Si/Al ratios) were synthesized and investigated in relation to their capacity to be used as adsorbents for depollution of the contaminated air or wastewater. The compositional and the structural properties were determined by XRD, N2 isotherms, NMR, chemical analysis and XPS. The acidity and adsorption properties of the solids were checked by adsorption microcalorimetry using various basic or polluting molecules in gas phase. 

Multiwall carbon nanotubes (MWCNTs) have been synthesized by catalytic chemical vapor deposition (CCVD) of ethylene on several mesoporous aluminosilicates impregnated with iron. The aluminosilicates were synthesized by sol-gel method optimizing the Si/Al ratios from 6 to 80. The catalysts are characterized by nitrogen adsorption, X-ray diffraction, 27A1 NMR, thermogravimetric analysis (TGA) and infrared. The MWCNTs are characterized by TGA and transmission and scanning electron microscope. 

Liquid-phase breakthrough experiments were also developed in order to characterize mesopores. The principle of the methodology relied on the analysis of the diffusion and adsorption of molecular probes with various molecular dimensions and adsorption strength. The relative proportion of occluded and accessible mesopores in the studied dealuminated Y zeolite could then be estimated. To allow this estimation, it is necessary to use molecular probes that can or cannot penetrate into the microporosity of the Y zeolite (see Figure 2). 

The mesopores are with regular and well-defined shapes and have a broad pore size distribution. N2 adsorption analysis revealed also a broad pore size distribution centered around 28 nm. The... 

The mesoporous molecular sieve SBA-15 has been functionalized with aminopropyl moieties via grafting. Further treatment of the 3-aminopropyl-modified material with glutardialdehyde (GA) results in GA-ATS-SBA-15. The modified silica materials were characterized by NMR and IR spectroscopy as well elemental analysis confirming the successful modification. Furthermore, the elemental analysis suggests that two of three amino moieties of the 3-aminopropyl modified material react further with... 

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