BET Report

Indian Institute of Technology Delhi Department of Chemical Engineering

CHL727: Heterogeneous Catalysis and Catalytic Reactors

Lab Report BET Surface Area Analysis Supervisor: Professor K.K. Pant Date:

Submitted by: Hemlata Malav Priyanka Gupta Sanjna Mandrai Shubam Gupta Vinita Kumari

2011CH10084 2011CH70177 2011CH10113 2011CH70182 2011CH70188

Aim To determine the BET surface area of the given catalyst (MCM-41) using Micromeritics ASAP 2010.

Theory: The BET theory is aimed to explain the physical adsorption of gas molecules on a solid surface and is taken to be the basis for an important analysis technique used for the measurement of the specific surface area of a material. This theory is an extended version of the Langmuir theory, which is a theory developed for monolayer molecular adsorption, for multilayer adsorption with the following assumptions: 1. 2. 3. 4. 5.

Homogeneous surface. No lateral interactions between molecules of each adsorption layer. Uppermost layer is in equilibrium with vapor phase. For the first layer- heat of adsorption and for higher layers- heat of condensation At saturation pressure, the number of layers becomes infinite i.e. the gas molecules physically adsorb on a solid in layers infinitely.

The resulting equation using these assumptions, known as BET equation is given as:

1 𝑃 𝑣 [( 𝑜 ) −1] 𝑃

=

𝑐−1 𝑃 ( ) 𝑣𝑚 𝑐 𝑃𝑜

+

1 𝑣𝑚 𝑐

where     

V = the volume (at STP) of gas adsorbed at pressure P PO = the saturation pressure which is the vapour pressure of liquefied gas at the adsorbing temperature P = the equilibrium pressure of the adsorbate at the adsorbing temperature. VM = the volume of gas (STP) required to form an adsorbed monomolecular layer. C = a constant related to the energy of adsorption given by-

𝑐=

exp(𝐸1 − 𝐸𝐿 ) 𝑅𝑇

where E1 is the heat of adsorption for the first layer and EL is the heat of liquefaction/condensation. The BET equation is an adsorption isotherm and can be plotted as a straight line with 1/v[(PO/P) – 1] on the y-axis and P/PO on the x-axis according to the experimental results. This plot is called a BET plot. The value of the slope s and the y-intercept l of the line are

used to calculate the monolayer adsorbed gas quantity vm and the BET constant ‘C’ varies from solid to solid- low values represent weak gas adsorption typical of low surface area solids, organics and metals in particular. The following relations are used-

1 𝑣𝑚 = 𝑠+𝑙 𝑠 c=1+ 𝑙 The BET equation is widely used in surface science for the calculation of the surface areas of various types of catalysts by physical adsorption of gas molecules.

Principle of operationA sample contained in an evacuated sample tube is cooled to very low (cryogenic) temperature and then exposed to analysis gas, mixture of N2 and He gas, at a series of precisely controlled pressures. The number of adsorbed gas molecules increases with each incremental increase in pressure. The pressure at which the adsorption equilibrium occurs is measured and universal gas law is applied to find the quantity of gas adsorbed. The adsorbed film thickness increases over time due to adsorption. The micropores in the surface are instantaneously filled, then the free surface and after it the large pores are filled. The process may continue to the point of bulk condensation of the analysis gas. Then the desorption process begins in which pressure is reduced systematically resulting in release of adsorbed molecules. As with adsorption the changing quantity of the gas on the solid surface is quantified. The two data sets represent the adsorption and the desorption isotherms.

LimitationsSometimes, the calculated single–point value may not go through a maximum during the gradual decrease in slope in the BET plot. Also that the graph may not be linear in nature in region of lower relative pressures. If there is no truly linear region, then it can be said that the BET equation is invalid for that particular sample.

Apparatus-

Micromeritics ASAP 2010

The ASAP 2010- (Accelerated Surface Area and Porosimetry System) has enough felxibility in the gas selection and high vacuum for high-resolution low surface area measurements. It is based on the theory of physisorption to obtain adsorption and desorption isotherms and more inference about the surface area and porosity of a solid material. It is capable of performing various measurements such as surface area analysis plus pore size and pore volume distribution, typically using gases like: Nitrogen, Carbon dioxide, Carbon monoxide, Oxygen, Argon and Butane, as the standard gas. It calculates the BET and Langmuir surface areas, average and total pore volumes, BJH pore size distribution and perform micro-pore analysis using various techniques such as HorvathKawazoe, Dubinin (both Astokhov and Radushkevich models), MP-Method, t-plot method, and Saito-Foley method.

Techniques: t-plot It is used to mathematically model multi-layer formation to calculate a layer “thickness, t” as a function of increasing relative pressure (P/Po). The resulting t-curve is compared with the experimental isotherm in the form of a t-plot, i.e. experimental volume adsorbed is plotted versus statistical thickness for each experimental P/Po value. The linear range of this graph lies between monolayer and capillary condensation. The slope of this t-plot (V v/s t) is equal to the “external area”, i.e. the

area of those pores other than micropores. Mesopores, macropores and the outside surface is able to form a multiplayer, whereas micropores which have already been filled cannot contribute further to the adsorption process. It is recommended to initially select P/Po range 0.2 – 0.5, and subsequently adjust it to find the best linear plot.

Barrett-Joyner-Halenda (BJH) Method It uses modified Kelvin equation. We know that Kelvin equation predicts pressure at which adsorptive will spontaneously condense (and evaporate) in a cylindrical pore of a given size. Condensation occurs in pores that already have some multilayers on the walls. Therefore, the pore size is calculated from combining the modified Kelvin equation and the selected statistical thickness (t-curve) equation.

Dollimore Heal (DH) Method Extremely similar calculation to BJH and hence, gives very similar results. Differs only in minor mathematical details.

OBSERVATIONS1. 2. 3. 4. 5.

Saturation pressure : 763.62518 mm Hg Correlation coefficient : 25.9965 Weight of (tube + catalyst) = 36.3055 gm Weight of tube = 35.7875 gm Weight of catalyst = 0.5177 gm

Data table: P/P0

P/ VA (PO – P)

43.79810 60.48724 75.24563

Volume of adsorbed gas (cm3/gm of catalyst) 219.9858 236.3629 249.1444

0.057320403 0.079142991 0.098431427

0.000276 0.000364 0.000438

87.79890 103.22732 119.32848 134.79208 148.14983

259.5286 272.9192 289.6722 311.5749 336.6670

0.114834635 0.134981392 0.155998316 0.176159170 0.193554217

0.000500 0.000572 0.000638 0.000686 0.000713

P(mm Hg)

Graph-

P/ VA (PO – P) vs P/P0 0.0008

P/ VA (PO – P)

0.0007 0.0006 0.0005 0.0004 0.0003 0.0002 0.0001 0

0

0.05

0.1

0.15

0.2

0.25

P/P0

Calculation: 1. 2. 3. 4. 5.

Intercept l = 1/vmc = 0.000110 ± 0.000021 Slope s = (c-1)/ vmc = 0.003274 ± 0.000157 Volume of one molecular layer of gas, vm = 1/ (l + s) = 295.553791 cm3/ gm Molecular cross- section area = 0.1620 nm2 BET surface area, S = 1286.6044 ± 60.3414 m2/g

Conclusion: 1. The BET surface area of the catalyst MCM-41 was found to be 1286.6044 m2/g. Because of the inaccuracy in measurement, according to the rule of significant digits, the BET surface area will be 1290 m2/g. 2. In accordance with IUPAC classification, this isotherm can be classified as a Type IV isotherm. Isotherm shows hysteresis nature because of capillary condensation at higher partial pressures which matches with a Type IV category isotherm and is associated with meso-porosity. Hysteresis falls into H3 category of IUPAC classification, hence it can be concluded that the given sample contains disordered lamellar pore structures, slit and wedge, shape pores. In this case, pore blocking, percolation, and cavitation effects play an important role.