surface area and porosity part iv - centro federal de ... · (recommendations 1984) pure &...
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Name of Institute, Faculty, Department1 10/15/17KIT – The Research University in the Helmholtz Association
Dr. Peter G. Weidler Institute for Functional Interfaces IFG KIT
www.kit.edu
Surface Area and PorosityPart IV
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
IFG, KIT Campus North - 3 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Overview II
Why nitrogen ?
● What if argon, krypton, H2O,.... ?
● --> what is the "real" surface area ?
Other adsorptives
● H2O
● organic gases
IFG, KIT Campus North - 4 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
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
IFG, KIT Campus North - 5 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
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
IFG, KIT Campus North - 6 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
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
IFG, KIT Campus North - 7 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity I
What is porosity ?
What are pores ?
--> one definition:
anything deeper than wide is a pore.
Why are pores/porosity of interest ?
sorption kinetics (process kinetics)
surface area (inner and outer surface area)
density of material
distribution of surface are in a volume
--> fractality
IFG, KIT Campus North - 8 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity II
Definition of pore sizes (IUPAC) :
micro pores : < 2 nm
meso pores : > 2 nm to < 50 nm
macro pores : > 50 nm
Now what about nanopores ?
no definition of "nanopores"
if you depend on nano, then maybe you are
1.78 *109 nm high and 75 *1012 ng heavy....
nano-pores or for the hard-cores femto-, atto- , chico-, harpo-, zeppo-,groucho-, gummo-pores.......
IFG, KIT Campus North - 9 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity III
Geo-Water-Soil-Construction-people:
also (macro- und micro-) pores, but on a different scale--> 0.2 µm to cm
Detection of porosity
indication by sorption isotherm
(1) at certain p/po steep rise in adsorption branch
(2) divergence of desorption- and adsorption branch
-> hysteresis
IFG, KIT Campus North - 10 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity IV
Classification after IUPAC
I Langmuir
II typical multi-layer
IV hysteresis
III, V, VI rare species
IFG, KIT Campus North - 11 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity V
Classification after IUPAC
H1 narrow pore size distribution
H2 broader psd, smallest size
not clear from desorption
H3 aggregates of plate-like
particles
H4 plate-like system with
micro-porosity
IFG, KIT Campus North - 12 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Basic concept XIV
--> adsorption at low partial pressure p/po....
IFG, KIT Campus North - 13 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Basic concept XVI
... increasing number of molecules, i.e. partial pressure p/po....
IFG, KIT Campus North - 14 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Basic concept XVI
... finally reaching the filling of "all" pores...
IFG, KIT Campus North - 15 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity VI
Obtaining pores sizes and distributions
Kelvin-equation:
γ surface tension
ρl density of liquid
t: statistical film thickness
de-Boer plot
IFG, KIT Campus North - 16 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity VII
Kelvin-equation : instability during desorption
emptying of pores depends on pore width and
corresponding pressure p
p1/po << p2/po
IFG, KIT Campus North - 17 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity VIII
Kelvin-equation
IFG, KIT Campus North - 18 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity IX
Kelvin-equation -->
for larger pores >> 100 nm
gas sorption --> resolution low
small step in p/po results in high step in pore size !!
--> Mercury Hg-porosimetry
Washburn-equation Dp = 1470 kPa µm/ PHg
smaller pores demand higher pressure
1µm --> 1470 kPa (14.7 bar) 1nm --> 1.47 MPa (14.7 kbar)
(mbar = hPa)
IFG, KIT Campus North - 19 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity X
Kelvin-equation -->
... problem with large pores..
... what about the small pores ?
IFG, KIT Campus North - 20 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity XI
Comparison of different models and measurements
IFG, KIT Campus North - 21 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity XII
Deviation for micro-pores and the smaller meso-pores !!!
IFG, KIT Campus North - 22 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Porosity XIII
Solution:
better models -->
DFT / MC calculations
Calculation of the interaction of single gas-molecules with
surfaces (atoms) with different conditions:
oxygen-surfaces
carbon-surfaces
with defined pore geometry (more to that later)
IFG, KIT Campus North - 23 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
DFT/MC I
Density Functional Theory / Monte Carlo Simulation
IFG, KIT Campus North - 24 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
DFT/MC II
Density Functional Theory / Monte Carlo Simulation
Calculation ofmodel isothermsfor defined
- pore geometry
- pore surface chemistry
- adsorbate
IFG, KIT Campus North - 25 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
DFT/MC III
Generalized Adsorption Isotherm (GAI) equation
IFG, KIT Campus North - 26 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
DFT/MC IV
Generalized Adsorption Isotherm (GAI) equation
IFG, KIT Campus North - 27 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
DFT/MC V
Criticism :
limited kernels available
N2, Ar, CO2
missing Kr, H2O
pore geometry: cylinder, spheres, slits
and combination with adsorbates
computational effort immense
IFG, KIT Campus North - 28 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
pore geometry I
✔ pore volume determined
✔ pore size determined
--> pore shape ???
where is the information about pore shape ?
IFG, KIT Campus North - 29 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
pore geometry II
type of hysteresis
correspondence of isotherm
with DFT/MC kernel
--> fit quality parameter
imaging ! (TEM, REM, ...)
IFG, KIT Campus North - 30 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
pore geometry III
type of pore geometries:
these plus spheres
IFG, KIT Campus North - 31 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
pore geometry IV
independent of pore geometry...
... but you still do not know the pore geometry 8o(
IFG, KIT Campus North - 32 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
pore geometry V
IFG, KIT Campus North - 33 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Counter-checking I
SSA obtained by gas sorption
determine particle diameter by TEM
(if particles exhibit regular shape)
--> find specific density
--> find shape model
--> calculate SSA
IFG, KIT Campus North - 34 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Counter-checking II
SSA obtained by gas sorption
determine particle diameter by XRD
(if particles are single crystals)
--> find specific density
--> find shape model
--> calculate SSA
IFG, KIT Campus North - 35 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Counter-checking III
What if calculated SSA deviates from measured SSA ?
if calculated < measured :
-- porosity
-- smaller particles present
-- TEM particles statistics not complete
if calculated > measured :
-- TEM particles statistics not complete
-- XRD no single crystals
IFG, KIT Campus North - 36 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Counter-checking IV
example: magnetite ρ = 5.2 g/cm³ ©by Frederik
IFG, KIT Campus North - 37 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Counter-checking V
example: magnetite ρ = 5.2 g/cm³ ©by Frederik
IFG, KIT Campus North - 38 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Counter-checking VI
example: magnetite ρ = 5.2 g/cm³
crystallite size determined by XRD: 33 nm
assuming cubes : SSA = 35 m²/g
particle size determined by TEM : 35 nm
assuming cubes : SSA = 33 m²/g
BET: m²/g (practical)
IFG, KIT Campus North - 39 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Last word on sizes
simple model: cubes edge a or sphere diameter d
cube split, every step n edge an+1 = an / 2
what is number of particles and SSA ?
cube edge a1 = 10000nm (10µm)
specific density = 2.5 g/cm³
IFG, KIT Campus North - 40 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Last word on sizes
after 14 step:
edge = 1.22 nm
one particle split into 5.50 1011 particles
SSA one particle: 0.2 m²/g
after 14 steps : 1200 m²/g
IFG, KIT Campus North - 41 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Last word on sizes
IFG, KIT Campus North - 42 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Last word on sizes
IFG, KIT Campus North - 43 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Last word on sizes
Last remark:
sorption data based on mass, e.g., mg stuff /kg
or
based on surface e.g., mg stuff /m² ???
last can lead to that material, different on mass bases
adsorbed same amount on surface base !!!
IFG, KIT Campus North - 44 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Last word on sizes
sample: Mat A Mat B
sorption: 1 mg/kg 10 mg/kg
SSA: 10 m²/g 100 m²/g
--> 1 µg/m² for both
which material is better ??
If interested on surface phenomena: use per m²
IFG, KIT Campus North - 46 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Example: MOF DAY 2 26.6 Å
IFG, KIT Campus North - 47 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Example: MOF DAY 2
IFG, KIT Campus North - 49 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Summary
surfaces are tough business
somewhat imprecise
got to know the limitation resp.
parameters applied
same applies to porosity
pore shape limited
... even not possibly for most material
use specific surface area in relation to
surface processes
IFG, KIT Campus North - 50 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
Thanks to following person for their support:
Prf. Virginia Ciminelli and Angela de Mello Ferreira
Paulo and Frederik
Pof. Wander Vasconcelos and Eduardo
Sebastian
Matthias Thommes (Quantachrome)
IFG, KIT Campus North - 51 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
muito obrigado !!
IFG, KIT Campus North - 52 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
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)
Sing et. al, (1985), REPORTING PHYSISORPTION DATA FOR GAS/SOLID SYSTEMSwith Special Reference to the Determination of Surface Area and Porosity(Recommendations 1984)Pure & Appl. Chem, 57(4) pp 603-619
Rouquerol, Rouquerol, Sing, (1998) , Adsorption by Powders and Porous Solids: Principles, Methodology and Applications Publisher: Academic Press; 1 ed. , pp. 467
IFG, KIT Campus North - 53 -November 2017Dr. Peter G. Weidler Surface Area & Porosity
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
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