department of chemical & biomolecular engineering university of maryland college park, md, 20742...

Department of Chemical & Biomolecular EngineeringUniversity of MarylandCollege Park, MD, 20742 04/04/2013

Characterizing Porous Materials and Powders

N2 Adsorption Isotherms

Laleh EmdadiAdvisor: Dr. Dongxia Liu


Objective:

To give you a basic understanding of adsorption chemistry and how it is used to characterize powders and porous materials. We focus on physisorption.

Good References to Study:

K. S.W. Sing, R. T. Williams. Physisorption hysteresis loops and the characterization of nonporous materials. Adsorption Science and Technology 22(10) (2004) 773-782.

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K.Y. Foo, B. H. Hameed. Insights into the modeling of adsorption isotherms systems. Chemical Engineering Journal 156 (2010) 2-10.

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Characterizing Porous Materials and Powders - Surface Area - Multipoint BET Method - Single Point BET Method - Multipoint/ Single Point Comparison - Porosity by Gas Adsorption

- Isotherms - Total Pore Volume and Average Pore Radius - Pore Size Distributions (Mesopore): - BJH Method - DH Method

- Surface Area of Microporous Samples by Langmuir Method- Micropore Analysis - V-t Method - Alpha-s Method - MP Method - DR Method - DA Method - HK Method - SF Method

- Density Functional Theory (DFT) and Monte Carlo Simulation Methods

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Surface Area

The Brunauer-Emmett-Teller (BET) method is the most widely used procedure for the determination of the surface area of solid materials and involves the use of BET equation:

Slope (s)Intercept (i)

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Usually N2 is used as the adsorbate and C for nitrogen is (50-250)


Multiple BET Method

The BET equation requires a linear plot of versus which for most solids using nitrogen as adsorbate is restricted to a limited region in the P/P0 range of 0.05 to 0.35. This linear region is shifted to lower relative pressures for microporous materials.

The standard multipoint BET procedure requires a minimum of 3 points in this range. Wm can be obtained from slope and intercept of the BET plot.

A Typical BET Plot

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Multiple BET Method

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The second step in the application of the BET method is the calculation of the surface area. The total surface area of the sample can be expressed as:

N : Avogadro's number (6.0221415 × 1023 molecules/mol),Wm: weight of adsorbate constituting a monolayer of surface coverage

Acs: molecular cross section of the adsorbate molecule, (16.2 A°2 for N2 at 77 K)

M: molar mass (molecular weight) of the adsorbate gas

The second parameter can be calculated is specific surface area of the solid can be calculated as:

St: total surface area,

W: sample weight

http://en.wikipedia.org/wiki/Avogadro's_number


Single Point BET Method

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For routine measurements of surface areas, a simplified procedure may be applied. In this method, only a single point on the adsorption isotherm in the linear region of the BET plot is used ( = 0.3).

The assumption is that the intercept in the BET equation is zero:

Ideal gas equation

For microporous materials P/P0 as high as possible, yet still in the linear region of the BET plot should be chosen!!!


Multipoint/Single Point Comparison

The relative error in single point versus multipoint for determining of surface area of the sample is a function of BET C constant and the relative pressure used.For multipoint BET we have:

For single point BET we have:

As a result the relative error of single point method is:

Usually if you chose the P/P0 properly, this error is very low and so it is acceptable.

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Single Point BET Error

This table shows the relative error for various C calculated using P/P0 of 0.3.

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Isotherms

Non-porous or macroporous

Mesoporous

Not common; N2 adsorption on polyethylene Mesoporous

Microporous (chemisorption)

Uniform non-porous surface

porous materialsconsisting of well-defined cylindrical-like pore channels or agglomerates of compacts ofapproximately uniform spheres

disordered materials and the distribution of pore size and shape is not well defined

non-rigid aggregatesof plate-like particles giving rise to slit-shaped pores

narrow slit pores, but now includingpores in the microporeregion

(IUPAC classifications)


Porosity by Gas Adsorption

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Pores are characterized according to their sizes:a) Pores > 50 nm in diameter are called macropores.b) Pores 2-50 nm in diameter are called mesopores.c) Pores < 2 nm in diameter are called micropores.

Porosity of powders and other porous solids can be characterized by gas adsorption studies. There are 2 common techniques for describing porosity:1) Determination of total pore volume 2) Pore size distribution

For most solid materials, nitrogen at 77 K is the most suitable adsorptive.


Total Pore Volume and Average Pore Radius

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The total pore volume is derived from the amount of vapor adsorbed at a relative pressure close to unity (P/P0 ≈1), by assuming that the pores are then filled with liquid adsorbate. The volume of nitrogen adsorbed (Vads) can be converted to the volume of liquid nitrogen (Vliq) contained in the pores using this equation:

Since pores which would not be filled below P/P0 ≈1 have a negligible contribution to the total pore volume, the average pore size can be estimated from the pore volume.

For example for cylindrical pore:

Pa: ambient pressure, T: ambient temperature

Vm: molar volume of the liquid adsorbate (34.7 cm3/mol for nitrogen)

R: universal gas constant

S: BET surface area


Pore Size Distributions (Mesopore)

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It means the distribution of pore volume versus pore size. The desorption isotherm generally is more appropriate than the adsorption isotherm for this purpose. For some special cases like when a sample exhibits Type H2 hysteresis, the adsorption isotherm is recommended!Mesopore size calculations based on cylindrical pore geometry assumption are done using Kelvin equation:

For nitrogen as adsorbate:rp: actual pore radius

t: thickness of the adsorbed layer; t=3.54 (Vads/Vm) or from de Boer equation:

Vads: volume of nitrogen adsorbed at a given relative pressure

Vm: volume of nitrogen adsorbed at the completion of a monolayer for a nonporous solid of the same composition as the porous sample


BJH Method (for pore size distribution)

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Ac can be calculated from Ap which is the area of each pore (see the AsiQwin software manual for more details).

This method proposed by Barrett, Joyner, and Halenda (BJH). This method is based on the assumption that the initial relative pressure is close to unity, all pores are filled with liquid.

rp: pore radius

Vp: pore volume

rK: inner capillary radius

∆t: thickness of adsorbed layer of nitrogen

Ac: area exposed by the pore from which the physically adsorbed gas is desorbed.


DH Method (for pore size distribution)

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This method proposed by Dollimore and Heal (DH).

∑Ap: areas of all the pores emptied of condensate in previous desorption steps.

∑Lp: lengths of all the pores emptied of condensate in previous desorption steps.

Assuming cylindrical pore geometry, the cumulative pore areas and length can be estimated for each desorption step by:


Surface Area of Microporous Samples by Langmuir Method

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In the absence of meso and/or macropores, a sample containing micropores will exhibit a Type I (see slide 10) or Langmuir isotherm. The Langmuir equation is a limiting case of BET equation for the adsorption of a single molecular layer of adsorbate:

It can be rewritten in the form of a straight line: (Vm can be found from slope)

W: weight of adsorbate at P/P0,

Wm: weight of adsorbate in a monolayer

This method is not applicable to composite materials containing micropores and meso-and/or macropores.


Micropore Analysis (V-t Method)

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The procedure is same as BET surface area measurement, but it extends the pressure range to higher pressures to permit calculation of the external surface area, that is, non-microporous part of the material. A t-plot which is a plot of volume of gas adsorbed versus t (the statistical thickness of an adsorbed film) is used.The t values are calculated as a function of the relative pressure using the de Boer equation:

the Carbon Black equation:

Or the Halsey equation:For nitrogen adsorption

at 77 K


Micropore Analysis (V-t Method)

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Surface area of all the pores can be calculated as:

Vads STP: volume of gas adsorbed, corrected to standard conditions of temperature and pressure

15.47: the conversion of the gas volume to liquid volume

SMP: micropore surface area

St: external surface area


Micropore Analysis (Alpha-s Method)

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An empirical analogue to the t-method was proposed by Sing. It does not need statistical thickness, t which depends on BET surface area of nonporous reference material. Instead the BET area can be replaced by the amount adsorbed at some arbitrarily chosen P/P0 (usually = 0.4) .

It is used for the determination of micropore volumes by extrapolation of Alpha-s curves to αs=0, and for the determination of nonporous surface contributions (from the slopes of linear portions of Alpha-s curves).

In practice, complications arise using this method because it is found to depend on the exact nature of the material chosen as nonporous reference!


Micropore Analysis (MP Method)

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An extension of de Boer’s t-method for micropore analysis was proposed by Mikhail, Brunauer, and Bodor. This method calculate the t independently of the solid by:

t(AP) = 104Vliq/SBET

A V versus t plot is constructed from t versus P/P0 a shown in figure below, using linear slopes constructed for t-value intervals from the origin to 4 AP, 4 to 4.5 AP, 4.5 to 5 AP, etc. The calculations are continued until no further decrease in the slope.

The area of pores in the thickness range from 4 to 4.5 A, for example is the difference between the values calculated from first and second slopes, and so on.


Micropore Analysis (MP Method)

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Pore volumes can similarly be calculated, using the relation:


Micropore Analysis (Dubinin-Radushkevich (DR) Method)

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Dubinin and Radushkevich considered that the fraction of the adsorption volume V occupied by liquid adsorbate at various adsorption potentials ε can be expressed as a Gaussian function:

Micropore volume (V0) and E0 parameter can be calculated from the linear fit of the isotherm data plotted as log (V) versus [log(P/P0)]2.The linear range for these plots is usually found at relative pressures of less than 10-2.

A: free energy of adsorption, A= ε = RT ln (P/P0)

V0: microspore volume

E0: characteristic energy of adsorption

β: affinity coefficient v: liquid molar volume of a given adsorbate

vC6H6: molar volume of benzene as the reference liquid


Micropore Analysis (Dubinin-Astakhov (DA) Method)

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For the microporous materials with heterogeneous distributions or strongly activated carbons, the Dubinin-Radushkevich equation fails to linearize the adsorption data, the Dubinin-Astakhov equation is used:

n and E are calculated re-iteratively by non-linear curve fitting to the adsorption isotherm in the low relative pressure, micropore region. The they are used in this equation where r is pore radius and K is a constant.

W: weight adsorbed at P/P0 and T

W0: total weight adsorbed

E: characteristic energy

n: non-integer value (typically between 1 and 3


Micropore Analysis (Horvath-Kawazoe (HK) Method)

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Many pore size distribution methods are based on capillary condensation phenomenon which is questionable for small confines of micropores. Horvath and Kawazoe proposed a method which is independent of this assumption!The HK method expresses the adsorption potential function within slit-like micropores as a function of the effective pore width:


Micropore Analysis (Horvath-Kawazoe (HK) Method)

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By selecting effective pore widths in the micropore range, the HK equation can be used to calculate the corresponding relative pressures.

From the adsorption isotherms, the amount of adsorption at each of these relative pressures is determined.

Differentiation of weight (or volume) of gas adsorbed relative to the total uptake, W/W0, with respect to the effective pore width yields a pore size distribution in the micropore range.


Micropore Analysis (Saito-Foley (SF) Method)

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HK method is good for materials with a predominance of slit-like pores (like activated carbons), but for some solids like zeolites that usually have cylindrical pore geometry it is better to use the method that Saito and Floey presented:

Everything (even numerical solution of this equation above) and the other parameter definitions except the pore shape are analogous to that of HK method and yields size distributions in the micropore range.


Density Functional Theory (DFT) and Monte Carlo Simulation Methods

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Classical macroscopic theories that we have introduced until now such as DR method and BJH method and also semi-empirical treatments such as HK method and SF method do not give a realistic description of the filling of micropores and even narrow mesopores which leads to an underestimation of pore sizes.

Treatments such as the Density Functional Theory (DFT) or methods of molecular simulation (Monte Carlosimulation (MC) and Molecular Dynamics (MD)) provide a much more accurate approach for pore size analysis and bridge the gap between the molecular level and macroscopic approaches.

DFT is the best suggested method for micro-mesoporous zeolite samples!


Density Functional Theory and Monte Carlo Simulation Methods

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The Non-Local Density Functional Theory (NLDFT) and the Grand Canonical Monte Carlo simulation (GCMC) methods correctly describe the local fluid structure near curved solid walls; adsorption isotherms in model pores are determined based on the intermolecular potentials of the fluid-fluid and solid-fluid interactions. The relation between isotherms determined by these microscopic approaches and the experimental isotherm on a porous solid can be interpreted in terms of a Generalized Adsorption Isotherm (GAI) equation :

N(P/P0): experimental adsorption isotherm data

W: microspore volume

N(P/P0,w): isotherm on a single pore of width W

f(W): pore size distribution functionThis equation can be solved numerically via a fast non-negative least square algorithm.


Library of DFT and GCMC methods in Quantachrome’s Data Reduction Software

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This table is just part of the large library we have, for a complete table please read the manual page 382

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Thank you for your attention

department of chemical & biomolecular engineering university of maryland college park, md, 20742...

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