Porosity characteristic adsorption isotherms

The BET surface area values are also reported with the distribution of porosity between microporosity (pore diameter <1.8 nm) deduced from N2 adsorption isotherms (t-curves) and mesoporosity (pore diameter > 1.8 nm). The following trend is observed for high atomic M/HPA ratio used for the precipitation, the precipitates exhibited high surface area mainly due to microporosity. However, depending on the nature of the coxmter cation and also of the previous ratio values, the textural characteristics were not similar. In particular, it is interesting to note the presence of mesopores for (NH4)2.4P, CS2.9P, CS2.7P and Cs2.4Si samples. 

Each type of pore is associated with a characteristic type of adsorption isotherm. The appropriate method of characterizing the porosity of an iron oxide is, therefore, to obtain the complete adsorption/desorption isotherm. There are six standard adsorption isotherms for gases (Fig. 5.3). Type I, with enhanced adsorption at low relative... 

In this study, activated carbon fibers (ACFs) deposited by copper metal were prepared by electroplating technique to remove nitric oxide (NO). The surface properties of ACFs were determined by FT-IR and XPS analyses. N2/77K adsorption isotherm characteristics, including the specific surface area, micropore volume were investigated by BET and t-plot methods respectively. And, NO removal efficiency was confirmed by gas chromatographic technique. From the experimental results, the copper metal supported on ACFs appeared to be an increase of the NO removal and a decrease of the NO adsorption efficiency reduction rate, in spite of decreasing the BET S specific surface area, micropore volume, and micro-porosity of the ACFs. Consequently, the Cu content in ACFs played an important role in improving the NO removal, which was probably due to the catalytic reactions of C-NO-Cu. 

Adsorption isotherms at 77 K were simulated for each model material. These data are shown in Figure 2. These isotherms all show standard Type IV behavior characteristic of mesoporous materials. They are described well by BET-type models at low pressures, with capillary rises at high pressures and pore filling at pre.ssures near saturation. The (a) and (b) models have considerably higher maximum adsorption than the (c) and (d) models due to their higher porosity. The (a) and (c) models both have capillary upswings at relative pressures around 0.6, while the (b) and (d) models, which have larger pores, show sharper capillary rises between relative pressures of 0.7 and 0.8. 

The experimental permeation results could be consistently described using Eqs. (9.43b) and (9.47) for Langmuir and Henry sorption respectively as shown by de Lange in a full analysis of sorption, permeation and separation results of five different gases [63]. This description requires knowledge of adsorption isotherms which could be measured only on unsupported membranes. To use these data for calculation of the permeation of supported membranes requires the assumption of equal pore characteristics in both cases. As discussed by de Lange et al. this is probably not correct in the case of silica layers. Based on sorption data a microporosity of about 30% and a pore size distribution with a peak at 0.5 nm is found. Analysis of permeation data point to a pore diameter of = 0.4 nm and a considerably smaller porosity. Table 9.7 summarises the sorption data. H2 and CH4 have relatively low (isosteric) adsorption heats (cf ) while CO2 and isobutane strongly adsorb. 

Even if it is not possible to attain the desired ideal conditions of operation, and less desirable conditions (nonlinear adsorption isotherm, fluctuating numbers of adsorbing centers, induction time, or delay not related to adsorption) occur, the time At is in every case characteristic of the particular catalyst and is sensitive to the smallest alteration in its condition. Thus, one may learn much about differences in particle size, porosity, and bulk factor, as well as about the surface area of a catalyst. The method also is... 

The adsorption isotherm (a plot of the concentration distribution of the solute in the mobile gas and stationary solid phases at a given temperature) is the foundation upon which the surface characteristics of adsorbents are defined. From this one can determine the specific surface area, capillary distribution (from the desorption curve), porosity, and other properties of a solid. Using adsorbates with diverse physical and chemical characteristics, it is possible to define the type of adsorbate-adsorbent interactions involved and the nature of adsorption in the system being examined. 

CFG pore volume distribution for characteristic dimensions were calculated fium the desorption branch of N2 adsorption isotherms at 77 K (Fig. 1). All the CFC samples are seen to possess mesoporous structures with rather narrow pore size distributions but different total porosities V and specific surface areas A. 

The nitrogen adsorption/desorption isotherms allow the specific surface area, pore size distribution as well as the micro/meso ratio to be estimated. The total surface area is quite similar for the investigated samples and it ranges from 329 to 403 m /g being the most developed for the Nt+3M+F composite as shown in Fig. 9.6. The nitrogen adsorption isotherms showed that the carbon materials are typically mesoporous (apart from the material M+F, that is, without nanotubes), and the amount of micropores is very moderate. The micropore volume values for all the samples are comparable var3dng from 0.152 to 0.174 cm /g. The porosity characteristics of all the composites are illustrated in Table 9.1. 

The nitrogen adsorption/desorption isotherms and the specific surface areas or the pore volumes derived from the N2 desorption data show that during compression in argon the mesopore structure is not destroyed. Nitrogen adsorption for the calcined sample after pressure treatment in argon (not shown in Fig.6) and for the initial sample is identical and exhibits porosity characteristic of well ordered MCM-41 silica. 

Figure 1 and 2 include the adsorption isotherms of N, for series A and B respectively. The isotherm shapes are characteristic of microporous solids being mostly type I, but with activation they show some tendency to become type II isotherms, depending on the starting porosity of the carbon, the activating agent and the use of catalyst. tJncatalyzed activation... 

The nitrogen adsorption / desorption isotherms (Fig. 2) are typical of well-defined porous frameworks that are characteristic of either supermicroporosity (MSU-1) or a small mesoporosity (MSU-4) without any textural porosity [14]. In these two compounds, the silica walls (deduced from x-ray diffraction and nitrogen isotherms) are quite thick (< 20 A) [5],... 

Both types of molecular sieves, MCM-36 and MCM-41, demonstrate large BET surface area and high static sorption capacity (see Table 2). Considerable qualitative differences are observed in N2 isotherms, which are shown in Figure 3. The nitrogen isotherm for MCM-41, prepared with cetyltrimethylammonium cation, is type IV [9] and shows the characteristic reversible steep capillary condensation at p/p0 = -0.4 corresponding to the pore opening -40 A [1]. MCM-36 also shows the type IV isotherm with almost linear and reversible uptake increase up to - p/p0 = 0.5, followed by a hysteresis loop. This profile of adsorption/desorption is typical for layered materials with slit-like porosity generated between layers [9],... 

The type II isotherm is associated with solids with no apparent porosity or macropores (pore size > 50 nm). The adsorption phenomenon involved is interpreted in terms of single-layer adsorption up to an inversion point B, followed by a multi-layer type adsorption. The type IV isotherm is characteristic of solids with mesopores (2 nm < pore size < 50 nm). It has a hysteresis loop reflecting a capillary condensation type phenomenon. A phase transition occurs during which, under the eflcct of interactions with the surface of the solid, the gas phase abruptly condenses in the pore, accompanied by the formation of a meniscus at the liquid-gas interface. Modelling of this phenomenon, in the form of semi-empirical equations (BJH, Kelvin), can be used to ascertain the pore size distribution (cf. Paragr. 1.1.3.2). 

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