Gas Sorption 7-

Adsorption is universally understood to mean the enrichment of one or more of the components in the region between two bulk phases (the interfacial layer).When a gas or vapour is brought into contact with a solid, part of it is taken up by the solid. The solid

41 that takes up the gas or vapour is called the adsorbent and the gas or vapour attached to the surface of the solid is called the adsorbate.

The interactions between the solid and the molecules in the gas or vapour phase can occur via physisorption or chemisorption. Physisorption is adsorption in which the forces involved are intermolecular forces (van der Waals forces, dispersion forces) of the same kind as those responsible for the imperfection of real gases and the condensation of vapours. Whereas chemisorption, the adsorbate sticks to the solid by the formation of a chemical bond with the surface. The most important features in differentiating between the two are:

 Chemisorption is dependent on the reactivity on the adsorbent and adsorptive and so is specific, whereas physisorpton is non-specific.

 Physisorption is always exothermic, and the energy released upon accommodation to the surface is of the same order of magnitude as an enthalpy of condensation (on the order of 20 kJ/mol). For chemisorptions, the energy is of the same order of magnitude as the energy change in a chemical reaction between a solid and a gas or vapour and so may be exothermic or endothermic and the magnitudes of the energy changes can range from very small to very large.

 The rate of adsorption in chemisorption is dependent on the energy of activation of the chemical reaction but for physisorption the rate of adsorption is very fast.  Chemisorbed molecules are linked to reactive parts of the surface and the

adsorption is necessarily confined to a monolayer. At high relative pressures, physisorption generally occurs as a multilayer.

Gas adsorption measurements are widely used for determining the properties of porous solids such as surface area, pore volume and pore size distribution. The amount of gas adsorbed can be measured either volumetrically or gravimetrically. In a gravimetric measurement, the amount of gas adsorbed is calculated by the increase in sample mass. In a volumetric measurement the amount of adsorption is determined by measuring the volume that the gas occupies, and calculating the difference in pressure before and after.

42 The degree of adsorption, n, is given as a function of the temperature, T, pressure, P and nature of the adsorbent and adsorbate:

n = f(P, T, adsorbate, adsorbent) (2.5)

For isothermal adsorption of a particular system this is simplified to:

n = f(P)T, adsorbent, adsorbate (2.6)

The pressure can be expressed in terms of relative vapour pressures, p/po when working within the limits of vacuum and the saturation vapour pressure:

n = f(p/p0)T, adsorbent, adsorbate (2.6)

43

Figure 2.3 IUPAC classifications of adsorption isotherms

Adsorption isotherms can then be identified as one of the six recognised types of isotherms classified by the IUPAC classification of adsorption isotherms10, illustrated in Figure 2.3. Type I isotherms are observed for the adsorption of gases on microporous solids. The adsorption is limited to the completion of a single monolayer of adsorbate at the adsorbent surface. Type II isotherms are observed for the adsorption of gases on non- porous or macroporous solids. They represent unrestricted monolayer-multilayer adsorption. Type III isotherms are found for adsorbents with a wide distribution of pore sizes. This type of isotherm is obtained when the amount of gas adsorbed increases without limit as its relative saturation approaches unity. Type IV isotherms are seen for mesoporous adsorbents. They have a characteristic hysteresis loop, which is associated with capillary condensation taking place in the mesopores. Type V isotherms is related to the Type III isotherm in that the adsorbent—adsorbate interaction is weak, but is

44 obtained with certain porous adsorbents. Type VI isotherms are obtained with argon or krypton on graphitised carbon blacks at liquid nitrogen temperature, also for some metal-organic frameworks where the adsorbent can flexibly respond to guest molecules. These isotherms represent stepwise multilayer adsorption on a uniform non-porous surface

To determine the surface area and porosity of materials from N2 sorption experiments, Langmuir and BET theories are generally used. Langmuir theory is based on the theory that whenever a gas comes into contact with a solid, equilibrium will be established between the gaseous phase and the adsorbed gases bound on the surface of the solid. The Langmuir isotherm model describes the dependence of the surface coverage of an adsorbed gas on the pressure of the gas above the surface at a fixed temperature and assumes monolayer adsorption on a homogeneous surface. It is based on four assumptions:

1. All adsorption sites are equivalent.

2. There is no interaction between neighbouring adsorbed molecules 3. All adsorption occurs through the same mechanism.

4. At the maximum adsorption, only a monolayer is formed

Surface coverage can be defined as the fraction of the adsorbed sites occupied:

K =

(2.8)

Where K is the equilibrium constant, and -1 are the direct and inverse rate constants,

is the surface coverage and P is the equilibrium pressure.

BET theory is an extension of the Langmuir theory from monolayer adsorption to multilayer adsorption with the following assumptions:

1. Langmuir theory can be applied to each layer

2. Gaseous molecules can physically adsorb on a solid in layers infinitely 3. There are no interactions between adsorption layers

45 This is summarised in the following equation:

(2.9)

Where P and P0 are the equilibrium and saturation pressure, v is the STP volume of adsorbate, vm is STP volume of the amount of adsorbate required to form a monolayer and c is the BET constant.

Polymer surface areas and pore size distributions were measured by nitrogen adsorption and desorption at 77 K using either a Nova surface area analyzer version 10.0 (by myself), Micromeritics ASAP 2420 or ASAP 2020 volumetric adsorption analyzer (by Rob Clowes at the University of Liverpool Chemistry department) and analysed by myself.

Approximately 0.1 g of sample was degassed at 363 or 393 K under vacuum for 12 hours prior to N2 adsorption. Carbon dioxide measurements were measured at 273 and 298 K using a Micromeritics ASAP 2050 extended sorption analyser fitted with a chiller circulator dewar.