Mesoporous solids adsorption onto

Figure 7.42 Types of gas sorption isotherm - microporous solids are characterised by a type I isotherm. Type II corresponds to macroporous materials with point B being the point at which monolayer coverage is complete. Type III is similar to type II but with adsorbate-adsorbate interactions playing an important role. Type IV corresponds to mesoporous industrial materials with the hysteresis arising from capillary condensation. The limiting adsorption at high P/P0 is a characteristic feature. Type V is uncommon. It is related to type III with weak adsorbent-adsorbate interactions. Type VI represents multilayer adsorption onto a uniform, non-porous surface with each step size representing the layer capacity (reproduced by permission of IUPAC).

The BET isotherm plots for the Cr(VI)/Sn02 catalyst (Figure 4) show that tlie isotherm for uncalcined catalyst is typical for adsorption onto a microporous solid (type 1 isotherm). This form of isotherm is retained after calcination at 300°C, but after calcination at higher temperatures the isotherm changed drastically in form. Calcination at 1000°C resulted in an isothenn characteristic of a type III non-porous solid. Whilst after calcination at 600°C the material exhibited intermediate behaviour and was mesoporous. 

Adsorption onto mesoporous soHds, i.e. those with pores larger than ca. 20 A, continues to increase at partial pressures well above those at which uptake on microporous solids is complete. Multilayer uptake is possible and the BET treatment is applicable, particularly until any sharp increase of uptake due to... 

A detailed discussion of adsorption onto mesoporous solids is beyond the scope of this text, but certain features relevant to microporous solids should be described. Firstly, microporous solids can themselves contain mesoporosity. The most important example of this is observed in zeolites such as Y or mordenite that have been treated after synthesis to remove aluminium from the framework (Section 6.2.3). The migration of silica leaves mesopores that are evident from nitrogen adsorption isotherms and directly visible by electron microscopy. The presence of secondary mesopores enhances diffusion and catalytic properties. Conversely, mesoporous solids that are well ordered on the mesoscale can contain disordered micropores in their walls. The mesoporous channels of calcined SBA-15, for example, are connected by micropores that result from removal of block copolymer chains that run between the large channels in the as-synthesised material. This is observed from nitrogen... 

All the empirical equations dealt with in Section 3.2 are for adsorption with monolayer coverage, with the exception of the Freundlich isotherm, which does not have a finite saturation capacity and the DR equation, which is applicable for micropore volume filling. In the adsorption of sub-critical adsorbates, molecules first adsorb onto the solid surface as a layering process, and when the pressure is sufficiently high (about 0.1 of the relative pressure) multiple layers are formed. Brunauer, Emmett and Teller are the first to develop a theory to account for this multilayer adsorption, and the range of validity of this theory is approximately between 0.05 and 0.35 times the vapor pressure. In this section we will discuss this important theory and its various versions modified by a number of workers since the publication of the BET theory in 1938. Despite the many versions, the BET equation still remains the most important equation for the characterization of mesoporous solids, mainly due to its simplicity. 

A typical N2 adsorption measurement versus relative pressure over a solid that has both micropores and mesopores first involves essentially a mono-layer coverage of the surface up to point B shown in isotherm IV (lUPAC classification) in Figure 13.1. Up to and near point B the isotherm is similar to a Langmuir isotherm for which equilibrium is established between molecules adsorbing from the gas phase onto the bare surface and molecules desorbing from the adsorbed layer. The volume of adsorbed N2 that covers a monolayer volume, hence the surface area of N2 can then be determined from the slope of the linearized Langmuir plot when P/V is plotted against P ... 

A related system involves thin (one or few monolayers) of surface species either covalently bonded or adsorbed onto the solid substrate. Since the effective surface concentration (Cs) is quite smaller than for thicker films, the PBD signal can be too weak to measure. One way to overcome this problem involves increasing the active area of the electrode while maintaining the geometrical area, by using porous electrodes. In that way, the adsorption of redox intermediates (e.g., CO) of methanol fuel cells on mesoporous metal electrodes have been studied successfully... 

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