Surface Area and Porosity

Transcription

1 Surface Area and Porosity 1 Background Techniques Surface area Outline Total - physical adsorption External Porosity meso micro 2

2 Length 1 Å 1 nm 1 µm mm macro meso micro metal crystallite 1-1 m 1-9 m 1-8 m 1-7 m 1-6 m 1-5 m 1-4 m 1-3 m C-C bond Carbon nanotube cell membrane red blood cell human hair red ant Transistor gate 3 Techniques Mercury intrusion Physical Chemical Temperature Programmed Methods 4

3 Physical 5 Characterization via Material Characterization Physical properties Differentiate Gas Quantity adsorbed on a surface as a function of pressure, volume, and temperature Modeled properties Surface area Pore structure Non-destructuve 6

4 Static V X P X G X 7 Quantity adsorbed - always normalized for mass - cm 3 /g or moles/g Relative pressure - equilibrium pressure divided by saturation pressure - Equilibrium pressure - vapor pressure above the sample - corrected for temperature (thermal transpiration) Saturation pressure - vapor pressure above a liquid Surface energy - solid/fluid interaction, strength, and heterogeneity 8

5 Sample Preparation Clean the surface Remove volatiles Water CO2 Solvents Controlled environment! Inert purge or vacuum Temperature control Avoid Phase Changes 9 Physical Molecules from the gas phase strike the surface. At equilibrium the molecule adsorbs, lose the heat of adsorption, and subsequently desorb from surface. At equilibrium the rate of condensation = the rate of desorption Constant surface coverage at equilibrium. Surface features change the adsorption potential. Surface area models neglect the effects of localized phenomenon. Curve surfaces or roughness provide enhanced adsorption potential. 1

6 Physical Not activated (no barrier) Rapid Weak (< 38 kj/mol) Atomic/Molecular Reversible Non-specific May form multilayers van der Waals/dipole interactions Often measured near the condensation temperature Potential Energy, kj/mol Distance from Surface, Å 11 Chemical Potential Energy, kj/mol May be activated Covalent, metallic, ionic Strong (> 35 kj/mol) May be dissociative Often irreversible Specific - surface symmetry Distance from Surface, Å Limited to a monolayer Wide temperature range 12

7 Isotherm Types I II III n ads IV V VI Constant temperature Quantity adsorbed as a function of pressure Vacuum to atmospheric P Six classifications Quantity is normalized for sample mass 13 Classical View of As the system pressure is increased the formation of a monolayer may be observed. A IV q ads A 14

8 Adsorbed Layer Density The first layer begins to form below 1x1-4 The density continues to increase with pressure/adsorption The monolayer is completed below.1 15 Classical View of As the system pressure is increased (gas concentration also increases) multiple layers sorb to the surface. A B IV q ads A B 16

9 Adsorbed Layer Density The monolayer is completed below.1 The second layer continues to form as pressure is increased The third layer appears at <.5 p/ p o 17 Classical View of As pressure is further increased we may observe capillary condensation in mesopores. A B-C IV q ads A B C 18

10 Adsorbed Layer Density Layer formation continues as increases As approaches 1, the density becomes constant or nearly liquid-like 19 Classical View of As pressure approaches the saturation pressure, the pores are filled and we may estimate total pore volume. A B-C D IV D q ads A B C 2

11 Adsorptives Nitrogen Argon Krypton 21 Nitrogen Broad usage Surface area t-plot Pore size distributions BJH - bulk fluid properties NLDFT - excess density Limitations Strong interactions Slow diffusion <.5 nm pores Reduced precision for materials with < 1m 2 /g (1µmol/g monolayer) 22

12 25 ZSM-5 Faujasite 2 V ads, cm 3 /g e-8 1e-7 1e-6 1e Confinement 23 Argon Pore size distributions H-K calculations Benefits NLDFT - excess density Reduced interaction compared to N2 Molecular size < N 2 and faster diffusion due to size and T (87K) Limitations Ar molecular area not a generally accepted value Statistical t-curves based upon N2 Not used for BJH - bulk fluid methods 24

13 25 2 Nitrogen Argon Faujasite (H + ) V ads, cm 3 /g e-7 1e-6 1e Y zeolite, Ar Nitrogen Argon ZSM-5 (LN 2 ) V ads, cm 3 /g e-8 1e-7 1e-6 1e ZSM-5, Ar 26

14 14 12 Desorption 1 V ads, cm 3 /g e-7 1e-6 1e ZSM-5 Low P Desorption 27 Krypton Surface area estimates - BET Low specific surface area (< 1m 2 /g) Low absolute area - limited sample quantity Benefits High precision, low pressure analysis Limitations Pressure range limited to < 1 torr at 77 K (<.3 ) General agreement with N2 Cost Limited to surface area applications 28

15 Error analysis Gas Law calculations Error Typical values Relative error 29 Error Reduction Probe Temperature, K Reference P ratio Relative Error Ar 77 N2 2/76.26 Kr 77 N2 2.4/76.3 Kr 87 Ar 5/76.7 3

16 Surface Area 31 Surface Area Area from adsorption n m - monolayer N A - Avogadro s number Total area - physical adsorption area of adsorbed molecule - nitrogen or krypton Active area - chemical adsorption area of a surface site - metal atom Stoichiometry 32

17 Type I Isotherm - Langmuir Isotherm Mono-layer adsorption Chemical I Micropore filling n ads Finely divided surface Limiting amount adsorbed as approaches 1 P 33 Langmuir Reduces to the familiar form of the Langmuir equation for associative adsorption At low coverage, the Langmuir equation converges with Henry s Law 34

18 Nitrogen adsorption on Graphitized Carbon n ads, (mmoles/m 2 )/g n ads, (mmoles/m 2 )/g Henry s Law Desorption.1 1e P.1 Henry s Law - Sterling FT Carbopack F - MIC Carbopack F - Kruk Sterling FT - MIC CarboPack F 6 m 2 /g Sterling FT 1 m 2 /g Henry s law constant 19 (mmols/m 2 ) / atm 1e-5 1e-6 1e P 35 Langmuir Estimate of nm Quantity Adsorbed, cm 3 /g X Zeolite,.8nm pores X X Langmuir Transformation, 13x Zeolite 62 m 2 /g p/q, mmhg/(cm 3 /g STP) Pressure, mmhg 36

19 Type II Isotherm Non-porous Macro-porous II Flat Surfaces Uniform surface energy n ads Multilayer adsorption Infinite adsorption as pressure approaches saturation P 37 BET Surface Area Estimate monolayer capacity Multi-layer adsorption Non-porous, Uniform surface Heat of adsorption for the first layer is higher than successive layers. Heat of adsorption for second and successive layers equals the heat of liquefaction Lateral interactions of adsorbed molecules are ignored 38

20 NLDFT estimate for the density of the adsorbed layers ρ p =.99 p =.9 p =.7 p =.5 p =.2 p =.1 p =.1 p =.1 p =.2 p = The density varies with distance from the surface. This is contrast to BET assumptions However, at.5 p/po there are only 3 layers σ 39 BET Equation Similar to Langmuir - a mass balance for each layer is used The first layer is unique and subsequent layers are common E is the heat of liquefaction An infinite series is formed The sum of surface fractions is 1 The total quantity adsorbed is a function of the monolayer and the surface fractions The multilayer may approach infinite thickness as pressure approaches saturation 4

21 BET Equation Linear form of BET 41 BET surface area 42

22 BET estimate of nm Quantity Adsorbed, cm 3 /g Desorption Silica, 1nm pores Linear BET, Lichrosphere 1 1 nm SiO Lic m 2 /g 1/Q(p o /p-1) Relative Pressure, 43 Type IV Isotherm Meso-porous IV Multilayer adsorption n ads Capillary condensation P 44

23 Amorphous Silica-Alumina Quantity adsorbed, cm 3 /g Desorption Amorphous Silica Alumina, 11nm pores BET Surface Area = m 2 /g.6 11 nm pores m 2 /g 1/(q ads (p o /p - 1)) Silica, 4 nm pores MCM-41 Quantity adsorbed, cm 3 /g Desorption BET Surface Area = nm pores m 2 /g 1/(q ads (p o /p - 1))

24 SiO2 SiO2-Al2O3 MCM-41 1 nm pores 25.7 m 2 /g 11 nm pores m 2 /g 4 nm pores m 2 /g Silica, 1nm pores Amorphous Silica Alumina, 11nm pores Silica, 4 nm pores 4 35 Desorption 45 4 Desorption 6 5 Desorption Quantity Adsorbed, cm 3 /g Quantity adsorbed, cm 3 /g Quantity adsorbed, cm 3 /g Fluid Cracking Catalyst,.8nm pores FCC catalyst Quantity Adsorbed, cm 3 /g Desorption BET Surface Area = m 2 /g Y & binder m 2 /g 1/(q ads (p o /p - 1)) BET range reduced to.16 maximum

25 FCC 49 FCC - Rouquerol 5

26 BET surface area summary Nitrogen or Krypton Krypton for low surface area or small sample quantity Isotherm LP to.3 p/p Adjust range used to fit BET parameters for µ-porous materials - Rouquerol transform C must be + Physical constraint Linearity 51 External Surface Area 52

27 t-plot Standard Isotherms Monolayer region is sensitive to isotherm shape Multilayer region is not sensitive to isotherm shape Multilayer region is less dependent on the adsorbent structure IV q ads A B C 53 t-plot Standard Isotherms n ads thickness, Å thickness, Å Slope of a linear region corresponds to area Intercept from a linear region is a pore volume Based on BET surface area 54

28 t-plot Standard Isotherms External Area n ads Flat Surface µ Pore Vol thickness, Å thickness, Å Slope corresponds to external (matrix) area Intercept is the micro pore volume t-curve is critical Statistical curves give comparative results Reference curves are preferred 55 t-plot Standard Isotherms External Area External Area n ads Meso Pore Vol µ Pore Vol Flat Surface Pore Area Flat Surface thickness, Å thickness, Å Low t slope is area Intercept is meso pore volume High t slope is external area 56

29 Halsey BJH Statistical t-curves Harkins-Jura t-plot Jaroniec et. al. Silica Broehkhoff de Boer difficult to use near saturation Thickness, angstroms Halsey Harkins and Jura Jaroniec et. al. Broekhoff de Boer t-plot 58

30 Surface Modifications Quantity Adsorbed, cm 3 /g Desorption Silica, 1nm pores 5 The reference surface may be modified to be similar to the porous material Hydrophilic vs. hydrophobic Thickness, angstroms DFT ODMS t-plot for 13X 25 X Zeolite,.8nm pores 2 Quantity Adsorbed, cm 3 /g Reference curve intercept Quantity Adsorbed, cm 3 /g External area Micropore filling Thickness, angstroms 6

31 Amorphous Silica-Alumina Desorption Amorphous Silica Alumina, 11nm pores Quantity adsorbed, cm 3 /g Negligible micropore volume Capillary condensation at large t values Quantity Adsorbed, cm 3 /g Thickness, angstroms 61 MCM-41 6 Desorption Silica, 4 nm pores 5 Quantity adsorbed, cm 3 /g Ideal t-plot sample 7 6 Area, pore volume, and external area Quantity Adsorbed, cm 3 /g Pore area Thickness, angstroms 62

32 t-plot summary Area Pore area External area (matrix) Pore volume Isotherm LP to.7 p/p Positive or intercept t-curve Reference curve is preferred Statistical curve is convenient 63 Meso-porosity Capillary condensation Fluid has bulk behavior BJH or DH models Adsorbed layer Liquid core 64

33 Meso-porosity BJH models Thickness curve to estimate the adsorbed layer Kelvin equation to estimate the radius of the liquid core 65 Model Isotherms - Kelvin Condensation V = Ad 4 66

34 Amorphous Silica-alumina BJH First V is assumed to be from pore emptying Subsequent V are a combination of pore emptying and thinning of the adsorbed layer Quantity adsorbed, cm 3 /g pore volume, cm 3 /g Desorption Amorphous Silica Alumina, 11nm pores dv/d(log(d)), (cm 3 /g)/å 1 1 width, Å 67 Amorphous Silica-alumina BJH From pore volume and calculated diameter, we can estimate surface area for a cylinder Common to observe the BJH estimate of area is greater than the BET estimate Quantity adsorbed, cm 3 /g Cumulative Pore Area, m 2 /g Desorption Amorphous Silica Alumina, 11nm pores dsa/dd D, angstroms 68

35 Amorphous Silica-alumina Quantity adsorbed, cm 3 /g Desorption Amorphous Silica Alumina, 11nm pores 5 BJH Desorption data has been used - historically Best to use both and Desorption - they should share common features pore volume, cm 3 /g width, Å dv/d(log(d)), (cm 3 /g)/å 69 BJH - PVD Pt/Al2O3 7

36 Pore Area vs BET Area Hg Pore Area Based upon a work function Gas Pore Area Geometric area of a cylinder BET Area Based upon the area occupied by adsorbed nitrogen (krypton) 71 Thank-you 72