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Pore condensation

M. Thommes, G. H. Findenegg. Pore condensation and critical-point shift of a fluid in controlled-pore glass. Langmuir 70 4270-4277, 1994. [Pg.74]

A. de Keizer, T. Michalski, G. H. Findenegg. Fluids in pores experimental and computer simulation studies of multilayer adsorption, pore condensation and critical point shifts. Pure Appl Chem (55 1495-1502, 1991. [Pg.74]

The principle underlying surface area measurements is simple physisorb an inert gas such as argon or nitrogen and determine how many molecules are needed to form a complete monolayer. As, for example, the N2 molecule occupies 0.162 nm at 77 K, the total surface area follows directly. Although this sounds straightforward, in practice molecules may adsorb beyond the monolayer to form multilayers. In addition, the molecules may condense in small pores. In fact, the narrower the pores, the easier N2 will condense in them. This phenomenon of capillary pore condensation, as described by the Kelvin equation, can be used to determine the types of pores and their size distribution inside a system. But first we need to know more about adsorption isotherms of physisorbed species. Thus, we will derive the isotherm of Brunauer Emmett and Teller, usually called BET isotherm. [Pg.183]

Here the phenomenon of capillary pore condensation comes into play. The adsorption on an infinitely extended, microporous material is described by the Type I isotherm of Fig. 5.20. Here the plateau measures the internal volume of the micropores. For mesoporous materials, one will first observe the filling of a monolayer at relatively low pressures, as in a Type II isotherm, followed by build up of multilayers until capillary condensation sets in and puts a limit to the amount of gas that can be accommodated in the material. Removal of the gas from the pores will show a hysteresis effect the gas leaves the pores at lower equilibrium pressures than at which it entered, because capillary forces have to be overcome. This Type IV isotherm. [Pg.188]

Zukal, A. 2006. Adsorption and pore condensation of krypton on mesoporous silicas at 77 K. Microporous Mesoporous Mater 92 220-226. [Pg.308]

The low-temperature physisorption (type I isotherm) of hydrogen in zeolites is in good agreement with the adsorption model mentioned above for nanostructured carbon. The desorption isotherm followed the same path as the adsorption, which indicates that no pore condensation occurred. The hydrogen adsorption in zeolites depends linearly on the specific surface areas of the materials and is in very good agreement with the results on carbon nanostructures [24]. [Pg.126]

The pure siliceous MCM-41 sample (reference) synthesized earlier by the same procedure [4, 5] showed the typical high surface area, well resolved [100], [110], [200] and [210] diffraction peaks in the XRD pattern and an N2 adsorption isotherm (IUPAC type IV) revealing a sharp inflection in the curve at ca. p/po=0.33 due to pore condensation typical for a narrow pore size distribution around a value of 28 A. The siliceous composite samples obtained, using combinations of the C6 and C 4 templates and different synthesis... [Pg.102]

Capillary condensation can be illustrated by the model of a conical pore with a totally wetting surface (Fig. 2.12). Liquid will immediately condense in the tip of the pore. Condensation continues until the bending radius of the liquid has reached the value given by the Kelvin equation. The situation is analogous to that of a bubble and we can write... [Pg.17]

However, the experimental data of figure 6.1 show exactly the opposite the bridged silanols of Kieselgel 40 (smaller pores) condense more readily than the ones of Kieselgel 60. [Pg.129]

Contrary to gas adsorption and pore condensation, in which the pore fluid wets the pore walls, that is, the contact angle is in the range of 0 < 9 < nil nil, mercury is a nonwetting fluid, that is, the contact angle is in the range of nil < 9 < n [151]. Consequently, for a nonwetting liquid, it is necessary to apply a positive excess of hydrostatic pressure, AP, to allow the fluid to penetrate the pores of radius, r. [Pg.212]

In Chapter 2, the structure of these materials and, in Chapter 3, the syntheses methods were described. In Figure 6.14, the adsorption isotherm of N2 at 77 K on the mesoporous molecular sieve MCM-41 is shown [67], The existence of capillary condensation is obvious from the isotherm. This fact implies the existence of pores in the mesopore range, that is, between 2 and 50 nm, which, in modern terms, is the nanoporous region [2], Capillary condensation in mesopores is generally associated with a shift in the vapor-liquid coexistence in pores in comparison with the bulk fluid. That is, a fluid confined in a pore condenses at a pressure lower than the saturation pressure at a given temperature, given that the condensation pressure depends on the pore size and shape, and also on the strength of the interaction between the fluid and pore walls [2,4,5,41],... [Pg.298]

Specifically, pore condensation represents a confinement-induced shifted gas-liquid-phase transition [20], This means that condensation takes place at a pressure, P, less than the saturation pressure, of the fluid [2,4,5], The x = P/P0 value, where pore condensation takes place, depends on the liquid-interfacial tension, the strength of the attractive interactions between the fluid and pore walls, the pore geometry, and the pore size [20],... [Pg.298]

The fact that the isotherms become reversible at temperatures above Tc(h) may seem to indicate that the distinction between the pore condensate and vapour must... [Pg.206]

Adsorption in mesopores is usually characterized by pore condensation and hysteresis. This can be due to either the occurrence of a metastable state or the presence of a pore network. We will first treat the former mechanism and discuss the network model later. [Pg.118]

The pore condensation hysteresis of two fluids (CHF3 and C2F6) in mesoporous silicas with open cylindrical pores of uniform size (MCM-41 and SBA-15), and in a silica with large cellular mesopores which are accessible only via micropores or narrow mesopores, has been studied over a wide temperature range up to the critical point of the fluids. From the sorption isotherms in MCM-41 and SBA-15 the hysteresis onset-temperapore 7h and the corresponding pore condensation pressure plpo)H was determined for several materials of different pore radius R. [Pg.177]

Fig. 2. Temperature dependence of the pore condensation and pore evaporation pressures plpo for CHF3 in MCM-41 (sample M2) and SB A-15 (samples SI and S3). Open symbols pore condensation full symbols pore evaporation. Arrows indicate the hysteresis temperature 7h. Fig. 2. Temperature dependence of the pore condensation and pore evaporation pressures plpo for CHF3 in MCM-41 (sample M2) and SB A-15 (samples SI and S3). Open symbols pore condensation full symbols pore evaporation. Arrows indicate the hysteresis temperature 7h.
The primary aim of this work has been to compare the limits of pore condensation hysteresis of fluids in silica materials of grossly different pore morphologies. For materials with open ended cylindrical pores of uniform pore width (MCM-41 and SBA-I5) we have determined the hysteresis temperatures 7h of the two fluids in samples with pore widths from 4 to 10 nm. Our results yield a linear dependence of the hysteresis temperature increment, AZh = Tc-Th, on the inverse pore radius, MR, in agreement with similar results reported in the literature. [Pg.182]

In addition, we have found (Figure 3) that the pore condensation pressure at the hysteresis temperature, plpt n, in samples of different pore radius is a nearly linear function of the respective reduced hysteresis temperature TIT n- The locus of these points in a plpn vs. TITc diagram marks the border line between pore condensation with and without hysteresis hysteresis phase diagram) for fluids in materials with open-ended cylindrical pores. [Pg.182]

Table 1 presents the textural parameters of the different materials studied using adsorption/ desorption isotherms before and after modifications or catalytic testing, corresponding to BET surface area, the total pore volume and the proportion of the micropore volume. The adsorption isotherm was found to be in agreement with the ones reported for MCM-41 materials with similar pore sizes [5]. Pore condensation of N, signified by a steep increase in the adsorbed volume in the N2 adsorption isotherm, was observed at a relative pressure (P/Po) of 0.26. Using Kelvin s equation, compensating for the multilayer adsorption the pore size was determined to be 2.5 nm. [Pg.390]

Experimental adsorption isotherms obtained with well-characterized materials have been used to correlate the critical pore condensation pressure, p, with effective pore width. This is shown in Figure 1. The pressure vector should be such that no pair of adjacent pore size classes exhibits the values of p that falls between consecutive pressure points. To do this, a smooth least squares interpolating spline routine was used to estimate the value of p for each size class and also at the geometric mean of adjacent classes. In this way, a pressure vector with the desired properties and of twice the length of the pore size vector is generated. Once the pressure vector is established, the model matrix can be calculated. [Pg.75]

The model isotherm for each pore size class was calculated by methods described previously [9], modified to account for cylindrical pore geometry. These calculations model the fluid behavior in the presence of a uniform wall potential. Since the silica surface of real materials is energetically heterogeneous, one must choose an effective wall potential for each pore size that will duplicate the critical pore condensation pressure, p, observed for that size. This relationship is shown in Figure 2. The Lennard-Jones fluid-fluid interaction parameters and Cn/kg were equal to 0.35746 nm and 93.7465 K, respectively. [Pg.75]

In addition to the importance of the M41S materials for size- and shape-selective applications, these materials have been also regarded as a suitable mesoporous model adsorbent for testing theoretical predictions of pore condensation. Pore condensation represents a first order phase transition from a gas-like state to a liquid-like state of a pore fluid in presence of a bulk fluid reservoir, which occurs at a pressure p less than the saturation pressure po at gas-liquid coexistence of the bulk fluid [6,7]. In this sense pore condensation can be regarded as a shifted gas-liquid bulk phase transition due to confinement of a fluid to a pore. Recent work has shown that in fact the complete phase diagram of the confined fluid is shifted to lower temperature and higher mean density as compared with the bulk coexistence curve [e.g., 8,9]. [Pg.260]

A characteristic feature associated with pore condensation is the occurrence of sorption hysteresis, i.e pore evaporation occurs usually at a lower p/po compared to the condensation process. The details of this hysteresis loop depend on the thermodynamic state of the pore fluid and on the texture of adsorbents, i.e. the presence of a pore network. An empirical classification of common types of sorption hysteresis, which reflects a widely accepted correlation between the shape of the hysteresis loop and the geometry and texture of the mesoporous adsorbent was published by lUPAC [10]. However, detailed effects of these various factors on the hysteresis loop are not fully understood. In the literature mainly two models are discussed, which both contribute to the understanding of sorption hysteresis [8] (i) single pore model. hysteresis is considered as an intrinsic property of the phase transition in a single pore, reflecting the existence of metastable gas-states, (ii) neiM ork model hysteresis is explained as a consequence of the interconnectivity of a real porous network with a wide distribution of pore sizes. [Pg.260]

Pore condensation and sorption hysteresis are of great importance for the characterization of porous media by the analysis of appropriate sorption isotherms (e.g., nitrogen, argon and... [Pg.260]

In Figure 1 nitrogen sorption isotherms at 77 K for the pristine B and impregnated B-Fc203 MCM-48 silica materials are shown. Both sorption isotherms exhibit similar shape, i.e reversible pore condensation at p/po < 0.4. The MCM-48 silica phases exhibit no microporosity as revealed by measurements in the low pressure region [13]. Different methods were used to analyze the nitrogen sorption isotherms to obtain surface and pore size... [Pg.262]

Figure 4. Adsorption/desorption isotherms of argon at 77 K on the same MCM-48 silicas as in figure 3. In all cases pore condensation accompanied by sorption hysteresis is observed. Figure 4. Adsorption/desorption isotherms of argon at 77 K on the same MCM-48 silicas as in figure 3. In all cases pore condensation accompanied by sorption hysteresis is observed.
The results for nitrogen, argon and krypton adsorption on pristine MCM-48 materials can be summarized as follows (i) Argon sorption isotherms at 87 K (T/Tc = 0.58, where Tc is the critical temperature of the bulk fluid) reveal for all MCM-48 silica phases used in this study pore condensation but no hysteresis at relative pressures p/po < 0.4. With increasing pore size... [Pg.265]

These results indicate that the shape of sorption isotherms of pure fluids on MCM-48 silicas, i.e. the occurence of pore condensation and sorption hysteresis as well as details of the hysteresis loop depend on the pore size and temperature. The observed hysteresis loops are of type HI, indicating that networking effects are not dominant for sorption hysteresis in the MCM-48 silica materials studied here, despite the fact that MCM-48 consists of a unique three dimensional pore network. [Pg.266]

As indicated before the locus of the phase diagram of a confined fluid compared to the coexistence curve of the bulk fluid is of importance for the occurence of pore condensation and hysteresis, i.e. for the shape of the sorption isotherm. It is expected that the critical temperature and the triple point temperature will be shifted to lower values for a confined fluid compared to a bulk fluid, i.e. the smaller the pore width the lower the critical temperature and triple point temperature of the pore fluid [8,9,20]. Pore condensation occurs whenever the pore... [Pg.266]

In Eq. (31), Pq is the saturation pressure and Pc is the pore condensation pressure. The assumed exponential dependence of the condensation pressure on the adsorption free energy change is similar in basis to the Polanyi potential theory [101] and the Frenkel-Halsey-Hill (FHH) theory [102-104]. In the HK method, the mean free energy change due to adsorption is calculated... [Pg.232]


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See also in sourсe #XX -- [ Pg.215 ]




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Pores, capillary condensation

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