Surface Area and Pore Size Distribution

Transcription

1 ABC s of Electrochemistry series Materials Characterization techniques: Surface Area and Pore Size Distribution Ana María Valenzuela-Muñiz February 9, 2012 Department of Chemical and Biomolecular Engineering

2 Outline Introduction Principles Applications Summary Ohio University - Avionics Engineering Center 2

3 Introduction Specific Surface Area: Basic concepts Is a material property of solids which measures the total surface area per unit of mass, solid or bulk volume, or cross-sectional area It is a derived scientific value that can be used to determine the type and properties of a material (e.g. soil). It is defined either by surface area divided by mass (with units of m²/kg), or surface area divided by the volume (units of m²/m³ or m -1 ) It has a particular importance for adsorption, heterogeneous catalysis, and reactions on surfaces 3

4 Introduction Basic concepts Adsorption: Is the adhesion of atoms, ions, biomolecules or molecules of gas, liquid, or dissolved solids to a surface. This process creates a film of the adsorbate (the molecules or atoms being accumulated) on the surface of the adsorbent - Desorption is the reverse process of adsorption - Physisorption: Is a process in which the electronic structure of the atom or molecule is barely perturbed upon adsorption. The weak bonding of physisorption is due to the induced dipole moment of a nonpolar adsorbate interacting with its own image charge in the polarizable solid 4

5 Introduction Basic concepts Adsorption Absorption Physisorption Chemisorption 5

6 Introduction Basic concepts Adsorption Isotherm: Describes the equilibrium of the adsorption of a material at a surface at constant temperature. Is obtained by measuring the amount of gas adsorbed across a wide range of relative pressures at a constant temperature (typically liquid N2, 77K). Conversely desorption Isotherms are achieved by measuring gas removed as pressure is reduced Adsorption isotherms are often used as empirical models, which do not make statements about the underlying mechanisms and measured variables Adsorption hysteresis: Chemical potential of adsorbate during desorption is lower; hence true equilibrium exists. Differences in contact angle during ads/des may also lead to hysteresis. Presence of ink-bottle type pores-narrow neck & wide body. Differences in the shape of the meniscus in the case of cylindrical pores with both ends open. 6

7 Introduction Basic concepts Gas adsorption provides a rapid and quantitative technique for specific surface area and to determine other textural properties of a solid as pore size, total pore volume, and pore volume distribution Pore Volume - Volume of pores accessible to condensed adsorbate Pore size classification Micropores - Less than 2 nm Mesopores - Between 2 and 50 nm Macropores - Greater than 50 nm 7

8 How it works? 1.Adsorbate is introduced in to the manifold 2.The valve to the sample cell is opened allowing the adsorbate to interact with the sample material 3.The pressure is repeatedly measured for the preset equilibration time, if the pressure drops dosing recurs and measurement proceeds until a stable reading is achieved 8

9 How it works? Multilayer formation 9

10 Quantity Adsorbed (cm³/g STP) What will you get? Adsorption isotherm RawCoal ads RawCoal des ElecCoal #42 ads ElecCoal #42 des Relative Pressure (p/p ) 10

11 Isotherm types II III I IV V VI 11

12 Isotherm types IUPAC classification 6 types of isotherms Type I Microporous solids (Langmuir isotherm) Type II Multilayer adsorption on non-porous / macroporous solids Type III Adsorption on non-porous /macroporous solids with weak adsorption Type IV Adsorption on meso porous solids with hysteresis loop Type V Same as IV type with weak adsorbate-adsorbent interaction Type VI Stepped adsorption isotherm, on different faces of solid and/or strong Interaction with surface 12

13 Surface area calculation Most common methods Langmuir (1918) Monolayer adsorption V V a m 1 bp bp BET (1938) Multilayer adsorption V V a m P P P 1 C 1 o CP P o 13

14 Langmuir Equation n p a p = pressure n a m 1 b n a = amount of gas adsorbed, mol/g n a m = mono-layer capacity of sample, mol/g b = Langmuir constant n p a m Assumes adsorption limited to one monolayer The Langmuir equation describes Microporus material exhibiting Type I Isotherms 14

15 BET Equation V a P 1 C 1 P P V C V C o m m P P o V = weight of gas adsorbed P/P 0 =relative pressure Vm = weight of adsorbate as monolayer Multiple-layer adsorption Type II Isotherm Approaches essentially infinite amount adsorbed C = BET constant Stephen Brunauer, Paul Emmett, Edward Teller; Fixed Nitrogen Laboratory (1938). Second most cited chemistry reference over fifty-year period 15

16 BET Equation V a P 1 C 1 P P V C V C o Linear plot m m P P o Intercept slope b 1 C 1, and m V C V C m m V m 1 a o q q R T C e, and C 1 b m m b q a = heat of adsorption, J/mol q o = heat of liquefaction, J/mol R = ideal gas constant, 8.31 J/mol*K T = absolute temperature, K 16

17 BET Equation Vm = weight of adsorbate as monolayer Total Surface area (SA )can then be derived VmN SA M A acs V m 1 b m N = Avagadro s number (6.023x10 23 ) M = Molecular weight of Adsorbate A cs = Adsorbate cross sectional area (16.2Å 2 for Nitrogen) Specific Surface Area (S) is then determined by total Surface area by sample weight SSA SA w 17

18 BET Equation Assumptions Adsorption energy of first layer is greater than that for higher layers Adsorption energies for second and higher layers are equal All adsorption sites on the adsorbent are equivalent Lateral adsorbate attractive forces are ignored 18

19 Porosity Pore Volume Total pore volume is derived from the amount of vapour adsorbed at a relative temperature close to unity (assuming pores are filled with liquid adsorbate). V ads = volume of gas adsorbed V liq = volume of liquid N 2 in pores V liq PaVadsV RT m V m = molar vol. of liquid adsorbate (N 2 =34.7cm 3 /mol) P a = ambient pressure T = ambient temperature 19

20 Porosity Pore Radius The average pore size can be estimated from the pore volume. Assuming cylindrical pore geometry (type A hysteresis) average pore radius (r p ) can be expressed as: r p 2V S Other pore geometry models may require further information on the isotherm hysteresis before applying appropriate model. liq 20

21 Porosity Classification of pores IUPAC a - closed pores b,f - pen only at one end c,d,g - open e - open at two ends (through) Different types and/or shapes of pores will generate different hysteresis types in the adsorption-desorption isotherms 21

22 Data reduction methods to analyze textural properties Langmuir: Provides a means of determining surface area based on a monolayer coverage of the solid surface by the adsorptive BET: The method of Brunauer, Emmet, and Teller is employed to determine surface area on a model of adsorption which incorporates multilayer coverage BJH: The method of Barrett, Joyner, and Halenda is a procedure for calculating pore size distributions from experimental isotherms using the Kelvin model of pore filling. It applies only to the mesopore and small macropore size range deboer t-plot: Commonly used to determine the external surface area and micropore volume of microporous materials. It is based on standard isotherms and thickness curves which describe the statistical thickness of the film of adsorptive on a nonporous reference surface - DFT Plus - MP-Method - Dubinin Plots - Medek - Horvath-Kawazoe technique - Deconvolution by Classical Model Fitting 22

23 Available system General Overview: TriStar II 3020 Located in CEER s Analytical Lab (room 049) at OU Surface Area/Porosity Analyzer Three Sample Positions Saturation Pressure Tube 30 Hour Dewar Two Gas Inlets for Adsorptive Gases Inlet for He for Free Space Monolithic Manifold 23

24 How to do the analysis Steps Degasification Measure Free Space Measure P 0 Dose Equilibrate Backfill Repeat throughout Isotherm 24

25 Sample preparation Sample preparation is an absolute prerequisite for the analysis Make sure the sample is dry and free of any solvent Degasification (degasification time and temperature depends on the sample s characteristics) 25

26 Surface Area and Pore Size Distribution Strengths Accurate Different gasses can be used Limitations Size of the sample Minimum specific area of -N 2, He, CO 2 26

27 Surface Area and Pore Size Distribution Applications Paints and coatings Aerospace Pharmaceutics Electronics Nanotubes Carbon Black Catalysts Ceramics Activated Carbons Fuel Cell Electrodes Adsorbents 27

28 Examples of analysis Specific surface area, pore size, pore volume Sample Surf Area Microp Ext Surf Area Microp Vol Pore diam [m²/g] Area [m²/g] [m²/g] [cm³/g] AVE [nm] Raw RC RC EC Electrolyzed EC EC 44Eth Extracted Eth EC 210Eth EC 44Tol Extracted Tol EC 210Tol

29 m 2 /g Examples of analysis Specific surface area, pore size, pore volume 25 Total Area Micropores RC 44 RC 210 EC 44 EC 210 EC 44Eth EC 210Eth EC 44Tol EC 210Tol 29

30 Pore Volume (cm³/g) Pore Volume (cm³/g) Examples of analysis BJH using a Reference Curve 1.0 MCM-41 BJH Adsorption dv/dlog(w) Pore Volume BJH Adsorption Cumulative Pore Volume Pore Width (Å) 30

31 Examples of analysis DFT Beyond Porosity DFT can also be used to characterize the surface energy 31

32 Summary BET method The method is based on adsorption of gas on a surface The amount of gas adsorbed at a given pressure allows to determine the surface area It is a cheap, fast and reliable method It is very well understood and applicable in many fields Not applicable to all types of isotherms 32

33 Summary Pore structure analysis Adsorption Isotherm BET plot Surface area Pore size distribution Pore radius/pore volume Hysteresis yype Isotherm type t-curve Pore type, Shape, Geometry 33

34 Related Literature At CEER Analytical Methods in Fine Particle Technology; Paul A Webb and Clyde Orr (1997) Porosity and Specific Surface Area Measurements for Solid Materials; Peter Klobes, Klaus Meyer and Ronald G. Munro (2006) [electronic resource] 34

35 Related Literature WebPages Interactive-Software/BET-microporous-sample.aspx Webinars/Physisorption/Physical-Adsorption.aspx 35

36 Acknowledgments Analytical Lab in CEER at Ohio University Micromeritics Instrument Corporation for providing information 36

37 Questions! For more information visit: Contact: