surface area and porosity part ii · thommes et. al, (2015), physisorption of gases, with special...

Name of Institute, Faculty, Department1 18.10.17KIT – The Research University in the Helmholtz Association

Dr. Peter G. Weidler Institute for Functional Interfaces IFG KIT

www.kit.edu

Surface Area and PorosityPart II

IFG, KIT Campus North - 2 -

November 2017Dr. Peter G. Weidler Surface Area & Porosity

Overview I

Introduction:

What are surfaces ?

● Importance of surfaces ? Are they ?

● Basic concept of surface area measurement

BET equation

● derivation, concept

● criticism (physical reality)

Basics of gas sorption

● physisorption <--> chemisorption

● specific <--> unspecific



Overview II

Why nitrogen ?

● What if argon, krypton, H2O,.... ?

● --> what is the "real" surface area ?

Other adsorptives

● H2O

● organic gases



Overview III

Self-similarity --> fractal surfaces

● basic concept of fractal surfaces

● How to determine fractal dimensions

● Does it really matter ?

How precise is a specific surface area ?

● linear regressions and the R²....

Talking about errors..... sample preparation

First measurements in lab



Overview IV

Porosity

● What are pores ?

● Definition of pore size

Hysteresis

● IUPAC definitions and pore shape

● Which pore range detectable ?

Determination of porosity

● old method BJH

● advanced method DFT/MC

● some words about Hg-porosimetry

Imaging of porous samples



Overview V

Evaluation of data

● reporting gas sorption data:

● total pore volume, what is it ?

counter-check the SSA-value

● by TEM or REM

● by XRD

● PCS (light scattering)

● ...

Concluding remarks



Basics of gas sorption I

physisorption <--> chemisorption

unspecific <--> specific

--> cross section area of molecule on substrate



Basics of gas sorption II

physisorption

Lennard-Jones potential ε(r) = -A/r6 + B/r-12

more general Mie-potential: w(r) = -A/rn + B/rm (1903)



Basics of gas sorption III

Interaction: roughly 3 categories:

(1) pure electrostatic (Coulomb force)

(2) polarization (dipole force)

(3) quantum mechanics

ad (1): Interac. between charges, permanent dipoles, quadrupols,...

ad (2): induced dipole moments in atoms/molecules by

E-fields of adjacent charges/perm. dipoles

ad (3): covalent/chemical binding forces, charge transfer, repulsion (Pauli exclusion principle)



Basics of gas sorption IV

.... almost most of it is assigned to Van-der-Waals-Force(s):

which is more a zoo of forces with different range:

e.g., Coulomb

charge/charge potential by 1/r

charge/dipole potential by 1/r²

or 1/r4 fixed or freely rotating dipole

dipole/dipole potential by 1/r6

... VdW is either short ranged nor long ranged,

it depends on the origin of the potential / forces !!!



Basics of gas sorption V

Sorption is a dynamic process:

molecules/atoms detach after a certain time

either going „back“ to gas phase

or move on to next stable site



Use of different gases I

nitrogen not only possible gas:

unpolar:

argon (supposed as the new standard gas)

krypton (low surface area ≈ 0.05 m²/g po ≈ 2.6 torr

but, Acs 0.152 nm² ; 0.236 nm²;

commonly adopted value 0.202 nm²

polar molecules:

CO2

H2O !!!

--> intermediate physi/chemi-sorption

--> chem. reaction with surface (formation of Me-OOH)



Use of different gases II

What is the molecular cross section

of Ar, Kr and the other gases ??

--> tabled in books:

But: Ar shows different values on differentsurfaces

--> same SSA obtained ???

if not, what is the reason.....



Use of different gases III

Argon:

main reason for discrepancy:

used at 77.3 K ( liq. nitrogen)

--> use liq. Ar (87 K)

--> at 77.3 K is argon liquid like or solid stateor something between ??

influence of substrate !!

Weidler; IFG, KIT-CN15 12.12.2016

„Reactive surfaces“ I

N2


Clay minerals with respect to swelling non-swelling swelling

1:1 layer silicates kaolinite dickite nacrite chrysotile antigorite…

halloysite

2:1 layer silicates pyrophyllite talc illites micas brittle micas chlorites

smectites e.g. montmorillonites vermiculites

channel and spherical structures

palygorskite, sepiolite, imogolite, allophane

rigid structure with high water contents

„Reactive surfaces“ II


You might think that the subject of water interacting with clay mineral surfaces, water in interlayer space, would be pretty straightforward. Ha! (Moore & Reynolds, 1997)

H2O

non-swellable clay minerals

N2 or H2O

swellable clay minerals

Surface area of non-swellable and swellable clay minerals

„Reactive surfaces“ III


Specific surface area of smectites

Maximum of Specific Surface Area (Smax)

a0b0 (001)-face of unit cellNA Avogadro numberM Mass of unit cell

ASAP

S VP------------------=

AS specific surface area [m²/g]AP surface of a particle

S specific density (e.g. 2.75 g/cm³)VP volume of particle

approximation for particles > 200 nm:

ASmax2a0b0 NA

M---------------------------=

„Reactive surfaces“ IV


N2 adsorption isotherms

As, out = 33 m²/g As, out = 105 m²/g As, out = 72 m²/g

„Reactive surfaces“ V


watervapor adsorptions isotherms

As= 378 m²/g As = 417 m²/g As = 289 m²/g

„Reactive surfaces“ VI


„Reactive surfaces“ VII


Monolayer-capaity ??

Monolayer-capacity ??

watervapor sorption isotherms I


falsk with sample

saturated salt solution

ventilator

exsiccator or box Salt Relative Humidity at RT [%]

LiCl 11

MgCl2 33

Mg(NO3)2 53

NH4NO3 62

NaCl 75

NH4H2PO4 93

KCl 84

amount adsorbed determined by weighing the mass

watervapor sorption isotherms II


measurements over wide rangesof humidity

measurements at different temperatures

→ sorption enthalpy

fully automatic

watervapor sorption isotherms III


watervapor sorption isotherms IV

by Dr. Friedrich, RUB


watervapor sorption isotherms V

Heat of adsorption (latent heat)

→ Clausius-Clapeyron equation

qst = - R T1 T2 /(T1 - T2 ) ln C2/C1

with

Ci equilibrium concentration at Ti temperatureR ideal gas constant


watervapor sorption isotherms VI

by Dr. Friedrich, RUB



Dynamic Vapour Sorption System

any vapor between 20°C and 70°C

– water– alcohol– other organic liquids

Determination of adsorbed mass bybalance

gas vapor sorption isotherms I



gas vapor sorption isotherms II



powders: 10-80 mg

gas vapor sorption isotherms III



Example: FeAlPO4-5 Zeolite; ≈ 40 mg @ 25°c and 40°C



SSABET = 151 m²/g




Pore Volume = 0.63 cm³/g




Influence of temperature T2 > T1



overlapping of the potentials in pores

potentials different for other gases/molecules



kinetic data I



kinetic data II



Langmuir-Equation

V = Vm * K * c / (1 + K * c)

V adsorbed VolumeVm maximum of VK reaction constantc concentration

kinetic data III



Langmuir-Equation for kinetics

V = Vm * K * t / (1 + K * t)

V adsorbed VolumeVm maximum of VK reaction constantt concentration time !

kinetic data IV



Evaluation of


V = Vm * K * t / (1 + K * t)

plot time/mass vs. time

→ linear regression

kinetic data V



kinetic data VI



Evaluation of


from linear regression obtain

slope and intercept

intercept

slope: angle α

kinetic data VII



slope intercept R²25°C 0.0242 0.491 0.999840°C 0.0247 0.289 0.9997

slope = 1/Vmax and intcept = 1/(K * Vmax)

leading to

Vmax (mg) K (1/min)25°C 41.3 0.0540°C 40.5 0.09

kinetic data VIII



Influence of temperature T2 > T1

density ρ

kinetic data IX



summary

– different surfaces with different reactivity

– different surfaces with different accessibilty

– different gases for the detection of this behavior

– more insight in surface properties



Literature

Gregg and Sing, (1982), Adsorption, Surface Area, & Porosity, 2nd ed. Academic Press;pp. 303

Thommes, Lowell, Shields, Thomas, Thommes, (2006), Characterization of PorousSolids And Powders: Surface Area, Pore Size and Density, Springer, The Netherlands

Thommes, (2004), Physical adsorption characterization of ordered and amorphousmesoporous materials,in: G.Q. Lu, X.S. Zhao (Eds.), Nanoporous Materials;Science and Engineering, Imperial College Press, London, UK, pp. 317– 364(Chapter 11)

Thommes et. al, (2015), Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)Pure & Appl. Chem, 87(9-10) pp 1051-1069

Rouquerol, Rouquerol, Sing, (1998) , Adsorption by Powders and Porous Solids: Principles, Methodology and Applications Publisher: Academic Press; 1 ed. , pp. 467



Literature

Peitgen, Jürgens, Saupe, (1992), Bausteine des Chaos. Fraktale Klett-Cotta, pp. 514

Benoit B. Mandelbrot, (1990),The Fractal Geometry of Nature Spektrum Akademischer Verlag, pp. 480

Kindle Edition; W. H. Freeman; file size: 10.1MB

surface area and porosity part ii · thommes et. al, (2015), physisorption of gases, with special...

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