Surface Area and Porosity Part II

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1 Surface Area and Porosity Part II Dr. Peter G. Weidler Institute for Functional Interfaces IFG 1 KIT The Research University in the Helmholtz Association KIT Name of Institute, Faculty, Department

2 Overview Introduction: What are surfaces? -2- I 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

3 Overview -3- II Why nitrogen? What if argon, krypton, H2O,...? --> what is the "real" surface area? Other adsorptives H2O organic gases

4 Overview 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? -4- III linear regressions and the R²... Talking about errors... sample preparation First measurements in lab

5 Overview -5- 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

6 Overview Evaluation of data reporting gas sorption data: -6- total pore volume, what is it? counter-check the SSA-value V by TEM or REM by XRD PCS (light scattering)... Concluding remarks

7 Basics of gas sorption I physisorption <--> chemisorption unspecific <--> specific --> cross section area of molecule on substrate -7-

8 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) -8-

9 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) -9-

10 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!!!

11 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

12 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 nm² ; nm²; commonly adopted value nm² polar molecules: CO2 H2O!!! --> intermediate physi/chemi-sorption --> chem. reaction with surface (formation of Me-OOH)

13 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 different surfaces --> same SSA obtained??? if not, what is the reason

14 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 state or something between?? influence of substrate!!

15 Reactive surfaces I N Weidler; IFG, KIT-CN

16 Reactive surfaces II Clay minerals with respect to swelling 16 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 Weidler; IFG, KIT-CN

the subject of water interacting with clay mineral surfaces, water in interlayer space,

17 Reactive surfaces III Surface area of non-swellable and swellable clay minerals non-swellable clay minerals N2 or H2O swellable clay minerals H2O 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) Weidler; IFG, KIT-CN

Reactive surfaces IV Specific surface area of smectites Maximum of Specific Surface Area (Smax) AP AS = ----------------- S VP S AS AP VP specific surface area [m²/g] surface of a particle specific

18 Reactive surfaces IV Specific surface area of smectites Maximum of Specific Surface Area (Smax) AP AS = S VP S AS AP VP specific surface area [m²/g] surface of a particle specific density (e.g g/cm³) volume of particle approximation for particles > 200 nm: 2a0 b 0 NA ASmax = M a0b0 NA M (001)-face of unit cell Avogadro number Mass of unit cell Weidler; IFG, KIT-CN

19 Reactive surfaces V N2 adsorption isotherms As, out = 33 m²/g As, out = 72 m²/g As, out = 105 m²/g Weidler; IFG, KIT-CN

20 Reactive surfaces VI watervapor adsorptions isotherms As= 378 m²/g As = 289 m²/g As = 417 m²/g Weidler; IFG, KIT-CN

21 Reactive surfaces VII Weidler; IFG, KIT-CN

22 watervapor sorption isotherms I Monolayercapacity?? Monolayercapaity?? Weidler; IFG, KIT-CN

23 watervapor sorption isotherms exsiccator or box ventilator falsk with sample saturated salt solution II Salt Relative Humidity at RT [%] LiCl 11 MgCl2 33 Mg(NO3)2 53 NH4NO3 62 NaCl 75 KCl 84 NH4H2PO4 93 amount adsorbed determined by weighing the mass Weidler; IFG, KIT-CN

24 watervapor sorption isotherms III measurements over wide rangesof humidity measurements at different temperatures sorption enthalpy fully automatic Weidler; IFG, KIT-CN

25 watervapor sorption isotherms IV by Dr. Friedrich, RUB Weidler; IFG, KIT-CN

26 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 temperature R ideal gas constant Weidler; IFG, KIT-CN

27 watervapor sorption isotherms VI by Dr. Friedrich, RUB Weidler; IFG, KIT-CN

28 gas vapor sorption isotherms I Dynamic Vapour Sorption System any vapor between 20 C and 70 C water alcohol other organic liquids Determination of adsorbed mass by balance

29 gas vapor sorption isotherms II

30 gas vapor sorption isotherms III powders: mg

31 Example: FeAlPO4-5 Zeolite; c and 40 C

32 Example: FeAlPO4-5 Zeolite; c and 40 C

33 Example: FeAlPO4-5 Zeolite; c and 40 C SSABET = 151 m²/g

34 Example: FeAlPO4-5 Zeolite; c and 40 C

35 Example: FeAlPO4-5 Zeolite; c and 40 C Pore Volume = 0.63 cm³/g

36 Influence of temperature T 2 > T1

37 overlapping of the potentials in pores potentials different for other gases/molecules

38 kinetic data I

39 kinetic data II

40 kinetic data III Langmuir-Equation V = Vm * K * c / (1 + K * c) V Vm K c adsorbed Volume maximum of V reaction constant concentration

41 kinetic data IV Langmuir-Equation for kinetics V = Vm * K * t / (1 + K * t) V Vm K t adsorbed Volume maximum of V reaction constant concentration time!

42 kinetic data V Evaluation of Langmuir-Equation for kinetics V = Vm * K * t / (1 + K * t) plot time/mass vs. time linear regression

43 kinetic data VI

44 kinetic data VII Evaluation of Langmuir-Equation for kinetics from linear regression obtain slope and intercept slope: angle α intercept

45 kinetic data 25 C 40 C VIII slope slope = 1/Vmax intercept R² and intcept = 1/(K * Vmax) leading to 25 C 40 C Vmax (mg) K (1/min)

46 kinetic data IX Influence of temperature T 2 > T1 density ρ

47 summary different surfaces with different reactivity different surfaces with different accessibilty different gases for the detection of this behavior more insight in surface properties

48 Literature Gregg and Sing, (1982), Adsorption, Surface Area, & Porosity, 2nd ed. Academic Press;pp. 303 Thommes, Lowell, Shields, Thomas, Thommes, (2006), Characterization of Porous Solids And Powders: Surface Area, Pore Size and Density, Springer, The Netherlands Thommes, (2004), Physical adsorption characterization of ordered and amorphous mesoporous materials, in: G.Q. Lu, X.S. Zhao (Eds.), Nanoporous Materials; Science and Engineering, Imperial College Press, London, UK, pp (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 Rouquerol, Rouquerol, Sing, (1998), Adsorption by Powders and Porous Solids: Principles, Methodology and Applications Publisher: Academic Press; 1 ed., pp

49 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

Specific Surface Area and Porosity Measurements of Aluminosilicate Adsorbents

Specific Surface Area and Porosity Measurements of Aluminosilicate Adsorbents ORIENTAL JOURNAL OF CHEMISTRY An International Open Free Access, Peer Reviewed Research Journal www.orientjchem.org ISSN: 0970-020 X CODEN: OJCHEG 2016, Vol. 32, No. (5): Pg. 2401-2406 Specific Surface