Porous texture hysteresis loops

The shape of adsorption-desorption isotherms gives the first pieces of information on a porous solid texture. Existence of an inflexion point in the low-pressure region of the high-resolution isotherm is a sign of the microporous character of a solid [6]. NaY zeolite appears microporous (type I isotherm) regardless of probe molecule and temperature. Similarly, USY zeolite is mainly microporous,but a small hysteresis loop at high P/Po values reveals the presence of some mesopores created during dealumination of Y zeolite [8,9]. 

Therefore, these results indicate that Cr-K10 has, at least in part, a pillared structure. The results for Cr-PB indicate (Fig. 1 and Table 1) that this material has a micro-porous structure with some contribution of mesopores (shape of Nj adsorption-desorption isotherm) and a narrow pore volume distribution with a maximum at a pore radius of 2.1 nm. All Cr-PILC studied exhibit hysteresis loop of type H4 [11] which can be attributed to solids with a slit-shaped porous structure. Heat treatment results only in a small decrease of the BET surface area for both Cr-KIO and Cr-PB (Table 1). Sulfidation does not influence significantly the porous texture of both Cr-PILC as well [12]. 

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. 

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