Surface area and the pore system - N 2 adsorption

The exposure of the catalytic surface on the incoming reactant molecules is an influen-tial factor in determining the activity of the catalyst. Therefore, the higher surface area can minimize the amount of catalyst used in a chemical reaction. However, the high sur-face area does not necessarily correspond to high catalytic activity, if the pore structure is not uniformly distributed over the catalytic surface. Hence, catalytic activity is interde-pendent with the pore size and pore volume together with the surface area, which affects the molecular transportation and reaction pathway. The surface area of a solid catalyst is found by the physisorption of an inert gas such as nitrogen and argon. The number of inert gas molecules adsorbed over the solid material depends on the equilibrium pressure p, the temperature and the nature of gas-solid interaction. An adsorption isotherm of a solid material at constant temperature can be plotted using the equation (2.1), where n is

quantity of gas adsorbed, p is saturation pressure of the gas,p^ois equilibrium pressure and T temperature, respectively [174].

n=f(p

p^o)_T (2.1)

The IUPAC had defined the adsorption together with hysteresis loops, as shown in fig-ure 2.22. The hysteresis loop arises due to adsorption-desorption behaviour from the pore and its connectivity with the pore network in the solid system. The Type I isotherm are commonly found in zeolites and activated carbons which is microporous in nature. Herein, the quantity of gas physisorbed is exclusively dependent on the accessible micropore vol-ume rather on the internal surface area. The high uptake at low _p^po is due to the strong gas-solid interaction, which leads to micropore filling and multilayer adsorption is limited to exposed external surface area. This is generally associated with hysteresis loop 4 (H4) due to narrow silt-like nanopores.

Figure 2.22: Physisorption isotherms (left side and the hysteresis loops (right side) proposed by IUPAC [171]

The macroporous or non-porous adsorbents exhibit Type II isotherm. This is a conse-quence of unrestricted monolayer-multilayer adsorption even at high relative partial pres-sure. Point B shares information on the complexity in the monolayer coverage. The dis-tinct B shows the formation of the monolayer, whereas a gradual B shows no insights on the establishment of a monolayer, as observed from Type III isotherm. The Type III isotherms is a result of weak gas-solid interaction, and cluster formation of adsorbed molecules is favoured.

The Type IV isotherm majorly differs from previous isotherms on non-reversible na-ture of the adsorption and desorption pathways. The mesoporosity of the oxide gel

gener-ates Type IV isotherm. Herein, the capillary condensation is followed after the monolayer-multilayer adsorption. The hysteresis loop 1 and 2 are commonly associated with these type of materials. The former is related to a narrow range of uniform pores, whereas the latter is due to the complexity of pore networks.

The Type V isotherm follows the same weak interaction observed in Type III isotherm, whereas the molecules form clusters, and nanopore filling is observed at high relative pressure. Examples include activated carbon. The Type VI isotherms are observed when the solids exhibit surface uniformity, thereby layer-by-layer adsorptions is preferred. The quantity of the adsorbed layer is represented by the step height. However, the nature of the gas temperature decides the sharpness of the step.

The Brunauer-Emmett-Teller method (BET) is a commonly used method to estimate the surface area of a different catalyst either supported or unsupported. However, materials with low surface area (<2 m²/g) are not recommended to rely on BET estimated surface area. The BET surface area is found by the following two steps: Firstly, the BET plot is obtained from the physisorption isotherm. Secondly, deduction of the monolayer capacity of the adsorbate and calculating the BET surface area using the cross-sectional area of the adsorbed nitrogen molecule (nitrogen occupies 0.162 nm²at 77 K). Equation (2.2) shows the BET equation, where P is the applied pressure, Pois saturation pressure of inert gas, Va is total volume adsorbed at various P values, Vois the first monolayer volume,χis a constant related to the heat of adsorption. It is assumed that, at _P^P

o = 1, the multiplayer corresponds to infinite thickness. The linear dependency of _{V a(P}^P

o−P) and _P^P

o could be seen from figure 2.23. The preferred range for building BET plot is between 0.05-3 relative pressure for its decent linearity. The final value denotes the complete monolayer formation on the adsorbate.

Figure 2.23:Linear BET plot [174]

P

The gas molecules condense at high pressure due to the multilayer formation inside the pore. This phenomenon is known as capillary condensation, which is reported to occur at lower pressure compared to the saturation pressure of the adsorbate gas molecules. The pore volume and pore size distribution can be deduced by utilizing the Barrett-Joyner-Halenda (BJH) method, which is based on the modified Kelvin equation. One has to remember that this method assumes that multilayer is unwinded slowly at a step-wise reduction of ^p_p^o. The modified equation is represented in equation (2.3), whererp,γ,v1

and R is the radius of the cylindrical pore, the surface tension of the condensate, molar volume of the condensate and gas constant, respectively. The summation of the Kelvin radius and thickness of the multilayer film is used to calculate the radius of the pores present in the catalyst.

r_p=−2γv1

RT lnP

P_o (2.3)