Seminar core slides malaysia dec 2013

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Seminar materials for 3 day seminar conducted at Sciencegates, MPOB, UNITEN.

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Seminar core slides malaysia dec 2013

  1. 1. Characterization of Powders & Porous Solids A sharing session . . . Mr Mohd Zulkiffli A Bakar
  2. 2. Itinerary (11.12.13) TIME TOPIC 0900 ~ 1300 Gas Sorption 1300 ~ 1400 Lunch 1400~1700 Mercury porosimetry Chemisorption 1700~1730 Q&A REMARKS
  3. 3. Itinerary (12.12.13) TIME TOPIC 0900 ~ 1300 Gas Sorption 1300 ~ 1400 Lunch 1400~1700 Microporosity 1700~1730 Q&A REMARKS
  4. 4. Itinerary (13.12.13) TIME TOPIC 0900 ~ 1300 Gas Sorption , Microporosimetry 1300 ~ 1400 Lunch 1400~1700 MPOB 1700~1730 Q&A REMARKS
  5. 5. History of sorption science
  6. 6. History sorption science
  7. 7. Pioneers of sorption science
  8. 8. Main Characteristics of Powders and Porous Solids Particle size Surface area Porosity
  9. 9. Why We Care About Particle Size and Surface Area These characteristics control many properties of materials: Flowability; “Filter-ability” Viscosity-Reology; Agglomeration; Dusting tendency; Settling rate; Activity/Reactivity rate (e.g. of catalyst); Dissolution rate (of pharmaceutical); Gas absorption; Hydration rate (of cement); Moisture absorption; Entry into lungs (shape dependency too); Combustion rate (of fuel) Etc…
  10. 10. What is Particle Size? SEM of real ibuprofen particles
  11. 11. A Concept of Equivalent Sphere Due to symmetry, size of sphere is completely determined by only one parameter – it’s diameter (radius) Other properties of sphere are easily computed from its size: 1 V = πd 3 6 S = πd 2 m= ρ 6 πd 3 Sphere is just a convenient model! This is why it is found throughout the particle size analysis
  12. 12. Different Equivalent Spheres
  13. 13. Particle Size Measurement Techniques Direct observation (image analysis) Sieving; Sedimentation – settling rate; Coulter counter – electrozone sensing; Gas adsorption – BET (SSA back extrapolation to size); Permeability (gas or liquid) e.g. Blaine, FSSS Light scattering – laser diffraction and Photon Correlation Spectroscopy / Dynamic Light Scattering
  14. 14. And What Do They Measure Direct observation (image analysis) – usually some 2-D representation of a particle. Which dimension is viable?; Sieving – combination of particle size and shape; Sedimentation – settling rate. Stokes Law (spheres, straight line settling); Coulter counter – electrozone sensing; Gas absorption / Permeability – surface area. Extrapolate to average particle size only. – BET (SSA back extrapolation to size); Light scattering – equivalent scatterers;
  15. 15. Particle Size by Direct Observation Google for ImageJ
  16. 16. Dynamic Light Scattering (DLS) DLS measures Brownian motion and relates this to the size of the particles. The larger the particle the slower the Brownian motion will be. Smaller particles are “kicked” further by the solvent molecules and move more rapidly. The velocity of Brownian motion is defined by a property known as the translational diffusion coefficient (D). The size of a particle is calculated from the translational diffusion coefficient by using the Stokes-Einstein equation: d (H ) = kT 3πη D d(H) – hydrodynamic diameter, D – translational diffusion coefficient, k – Boltzmann’s constant, T – temperature, η - viscosity
  17. 17. What We Measure in DLS? The diameter that is measured in DLS is a value that refers to how a particle diffuses within a fluid so it is referred to as a hydrodynamic diameter The diameter that is obtained by this technique is the diameter of a sphere that has the same translational diffusion coefficient as the particle The translational diffusion coefficient will depend not only on the size of the particle “core”, but also on any surface structure, as well as the concentration and type of ions in the medium Particle core Shell formed by solvent particles, ions etc. Low conductivity medium will produce an extended double layer of ions around the particle, reducing the diffusion speed and resulting in a larger, apparent hydrodynamic diameter. Thus, the measurements are usually done in 10mM NaCl (ISO13321 Part 8 1996)
  18. 18. How DLS Works The dark spaces in the speckle pattern produced by light scattering are where the phase additions of the scattered light are mutually destructive. The bright spots of light in the speckle pattern are where the light scattered from the particles arrives with the same phase and interfere constructively. The observed signal depends on the phase addition of the scattered light falling on the detector. In example A, two beams interfere and “cancel each other out” resulting in a decreased intensity detected. In example B, two beams interfere and “enhance each other” resulting in an increased intensity detected.
  19. 19. How DLS Works For a system of particles undergoing Brownian motion, a speckle pattern is observed where the position of each speckle is seen to be in constant motion. This is because the phase addition from the moving particles is constantly evolving and forming new patterns. The rate at which these intensity fluctuations occur will depend on the size of the particles. Figure above schematically illustrates typical intensity fluctuations arising from a dispersion of large particles and a dispersion of small particles. The small particles cause the intensity to fluctuate more rapidly than the large ones. It is possible to directly measure the spectrum of frequencies contained in the intensity fluctuations arising from the Brownian motion of particles, but it is inefficient to do so. The best way is to use a device called a digital auto correlator.
  20. 20. How an Auto Correlator Works If the intensity of a signal is compared with itself at a particular point in time and a time much later, then for a randomly fluctuating signal it is obvious that the intensities are not going to be related in any way, i.e. there will be no correlation between the two signals. However, if the intensity of signal at time t is compared to the intensity a very small time later (t+δt), there will be a strong relationship or correlation between the intensities of two signals. Perfect correlation is indicated by unity (1.00) and no correlation is indicated by zero (0.00). If the signals at t+2δt, t+3δt, t+4δt etc. are compared with the signal at t, the correlation of a signal arriving from a random source will decrease with time until at some time, effectively t = ∞, there will be no correlation. If the particles are large the signal will be changing slowly and the correlation will persist for a long time. If the particles are small and moving rapidly then correlation will reduce more quickly.
  21. 21. Different Forms of Particle Size Distribution Consider 2 populations of spherical particles of diameter 5nm and 50nm present in equal numbers. If a number distribution of these 2 particle populations is plotted, a plot consisting of 2 peaks (positioned at 5 and 50nm) of a 1 to 1 ratio would be obtained. If this number distribution was converted into volume, then the 2 peaks would change to a 1:1000 ratio (because the volume of a sphere is proportional to d3). If this was further converted into an intensity distribution, a 1:1000000 ratio between the 2 peaks would be obtained (because the intensity of scattering is proportional to d6 from Rayleigh’s approximation). In DLS, the distribution obtained from a measurement is based on intensity.
  22. 22. Schematics of Zetasizer Nano
  23. 23. Measurement of Porosity and Specific Surface Area by Gas Adsorption
  24. 24. ? ? ? Quiz ? ? ? ? Name 2 methods to measure particle size A - Laser scattering - Optical ( microscopy)
  25. 25. What are Porous Materials? Non-porous solid Low specific surface area Low specific pore volume Porous solid High specific surface area High specific pore volume Porous materials have highly developed internal surface area that can be used to perform specific function. Almost all solids are porous except for ceramics fired at extremely high temperatures F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999
  26. 26. ? ? ? Quiz ? ? ? ? Looking at the diagram, how to tell if a particle is porous? A Porous if and only if value of pore depth is larger than value of pore width
  27. 27. Measure of Porosity There are three parameters used as a measure of porosity; specific surface area, specific pore volume or porosity, and pore size and its distribution. Specific Surface Area, m2/g = Total surface area, m2 Mass of the solid, g Porosity, % = Volume of pores X 100 Volume of solid (including pores) Porosity Pore size and its distribution Specific Pore volume, cm3/g = Total pore volume, cm3 Mass of the solid, g
  28. 28. Concept of Porosity: Open vs. Closed Pores Inter-connected (open) Closed Open pores are accessible whereas closed pores are inaccessible pores. Open pores can be inter-connected, passing or dead end. Passing (open) Dead end (open) F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999
  29. 29. Size of Pores (IUPAC Standard) Micropores Zeolite, Activated carbon, Metal organic framework 2 nm Mesopores Macropores Mesoporous silica, Activated carbon Sintered metals and ceramics 50 nm Porous material are classified according to the size of pores: material with pores less than 2 nm are called micropores, materials with pores between 2 and 50 nm are called mesopores, and material with pores greater than 50 nm are macrospores Sing, K. S. W. et al. Reporting Physisorption Data for Gas/Solid Systems. Pure & Appl. Chem. 57, 603-619 (1985).
  30. 30. Shapes of Pores Cylindrical Conical Spherical or Ink Bottle Slits Pore Shapes Interstices F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 1-25, 1999
  31. 31. ? ? ? Quiz ? ? ? ? Will pore size be the same as particle size ? A Particle size measures external cross-sectional diameter, while pore size measures measures mean internal pore diameter
  32. 32. Experimental Techniques
  33. 33. Techniques for Porosity Analysis Gas adsorption Small Angle Neutron scattering Mercury porosimetry Techniques Small angle X-ray scattering TEM SEM
  34. 34. Techniques for Porosity Analysis Can measure only open pores Pore size : 0.4 nm – 50 nm Easy Established technique Gas adsorption Small Angle Neutron scattering Mercury porosimetry Techniques Small angle X-ray scattering TEM SEM
  35. 35. Techniques for Porosity Analysis Gas adsorption Small Angle Neutron scattering Mercury porosimetry Techniques Small angle X-ray scattering TEM SEM Similar to gas adsorption Can measure only open pores Pore size >1.5 nm Easy Established technique
  36. 36. Techniques for Porosity Analysis Gas adsorption Small Angle Neutron scattering Mercury porosimetry Techniques Small angle X-ray scattering TEM SEM Provide information regarding pore connectivity Pore size can be measured if the materials contains ordered pores Rarely used for pore analysis
  37. 37. Techniques for Porosity Analysis Gas adsorption Small Angle Neutron scattering Mercury porosimetry Techniques Small angle X-ray scattering TEM SEM Pore size > 5nm Rarely used for pore analysis
  38. 38. Techniques for Porosity Analysis Gas adsorption Small Angle Neutron scattering Mercury porosimetry Techniques Small angle X-ray scattering TEM SEM Any pore size Open + Close porosity
  39. 39. Techniques for Porosity Analysis Gas adsorption Small Angle Neutron scattering Mercury porosimetry Techniques Small angle X-ray scattering TEM SEM Any pore size Open & Close porosity Costly
  40. 40. Theory of Adsorption
  41. 41. Adsorption Process Adsorptive - adsorbate before being adsorbed on the surface Adsorbate - the gas adsorbed on the surface of solids Adsorbent - the solid where adsorption takes place Adsorption is brought by the forces acting between the solid and the molecules of the gas. These forces are of two kinds: physical (physiosorption) and chemical (chemisorption)
  42. 42. Physisorption vs Chemisorption PHYSISORPTION CHEMISORPTION WEAK, LONG RANGE BONDING Van der Waals interactions STRONG, SHORT RANGE BONDING Chemical bonding involved. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. Hads = 5 ….. 50 kJ mol-1 Hads = 50 ….. 500 kJ mol-1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. No surface reactions. Surface reactions may take place:- Dissociation, reconstruction, catalysis. MULTILAYER ADSORPTION BET Isotherm used to model adsorption equilibrium. MONOLAYER ADSORPTION Langmuir Isotherm is used to model adsorption equilibrium. http://www.soton.ac.uk
  43. 43. Adsorption Process Gas molecules admitted under increasing pressure to a clean, cold surface. 1 1 2 2 3 3 1. Diffusion to adsorbent surface 2. Migration into pores of adsorbent 3. Monolayer builds up of adsorbate Data treatment techniques find the quantity of gas that forms the first layer. S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
  44. 44. Adsorption Process V a = f (W , T , I , P ) where V a = volume of gas adsorbed; W = weight of adsorbent; P = pressure of the adsorbate; Adsorbate T = temperature; I = interaction between adsorbate and adsorbent. If W, T, and I are made constant, the above equation can be written as :  p Equation of adsorption = f o Va  p  isotherm   where Adsorbent p pressure of adsorbate o = & J. E. p saturated pressure of adsorptive S. Lowell 3rd Ed.Shields, Powder Surface Area and Porosity, Chapman & Hall, New York, 1991
  45. 45. Gas Sorption: Isotherm Desorption isotherm Va Adsorption isotherm p po  p V a = f  po      where p pressure of adsorbate o = p saturated pressure of adsorptive Isotherm is a measure of the volume of gas adsorbed at a constant temperature as a function of gas pressure. Isotherms can be grouped into six classes.
  46. 46. Va Gas Sorption: Isotherm Type I or Langmuir P/Po Va Concave to the P/Po axis Exhibited by microporous solids ( < 2nm ) Type II B P/Po 1 Exhibited by nonporous or macroporous solids ( > 50nm ) Unrestricted monolayer-multilayer adsorption Point B indicates the relative pressure at which monolayer coverage is complete 1 S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
  47. 47. Va Gas Sorption: Isotherm Convex to the P/Po axis Exhibited by nonporous solids Type III P/Po Va Type IV P/Po 1 Exhibited by mesoporous solids Initial part of the type IV follows the same path as the type II S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. 1 Chapman & Hall, New York, 1991
  48. 48. Gas Sorption: Isotherm Type V Va Highly uncommon Exhibited by mesoporous solids P/Po Va Type VI P/Po 1 Exhibited by nonporous solids with an almost completely uniform surface 1 S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
  49. 49. Gas Sorption: Hysteresis Va Hysteresis P/Po 1 Hysteresis indicates the presence of mesopores. Hysteresis gives information regarding pore shapes . Types I, II and III isotherms are generally reversible but type I can have a hysteresis. Types IV and V exhibit hysteresis. S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991
  50. 50. Gas Sorption: Hysteresis Type B Type C Type D Type E Va Type A P/Po 1 Cylindrical P/Po Slits 1 P/Po 1 P/Po Conical 1 P/Po 1 Bottle neck
  51. 51. Adsorption Theories: Langmuir P 1 P = + Va Vm b Vm where Adsorbate Va = volume of gas adsorbed at pressure P ; Vm = volume of gas required to form monolayer; b = empirical constant; and P = pressure of adsorbate. Assumptions: homogeneous surface (all adsorption sites energetically identical) Adsorbent monolayer adsorption (no multilayer adsorption) no interaction between adsorbed I. Langmuir The Constitution and Fundamental Properties of Solids and Liquids. Part I. Solids. molecules J. Am. Chem. Soc., 1916, 38 (11), 2221-2295
  52. 52. ) The Langmuir adsorption isotherm Basic assumptions surface uniform (∆Hads does not vary with coverage) monolayer adsorption, and no interaction between adsorbed molecules and adsorbed molecules immobile Case I - single molecule adsorption when adsorption is in a dynamic equilibrium A(g) + M(surface site) AM the rate of adsorption rads = kads (1-θ) P the rate of desorption rdes = kdes θ at equilibrium rads = rdes ⇒ kads (1-θ) P = kdes θ rearrange it for θ let θ = (kads / kdes)P ⇒ 1+(kads / kdes)B0P B0 = k ads k des θ= A case I B0 is adsorption coefficient Cs BP = 0 C∞ 1+ B0 P 56
  53. 53. ) The Langmuir adsorption isotherm (cont’d) Case II - single molecule adsorbed dissociatively on one site A-B(g) + M(surface site) A-M-B θ=θΑ=θ the rate of A-B adsorption rads=kads (1−θΑ )(1Β Β)PAB=kads (1−θ )2PAB −θ the rate of A-B desorption rdes=kdesθΑθΒ =kdesθ2 at equilibrium rads = rdes ⇒ rearrange it for θ Let. θ= kads (1−θ )2PAB= kdes θ2 A A B case II (kads/ kdes)P AB 1+ (kads/ kdes)P AB k B 0 = ads k des ⇒ Cs (B0PAB)1/2 θ= = C∞ 1+ (B0PAB)1/2 57 B
  54. 54. The Langmuir adsorption isotherm (cont’d) Case III - two molecules adsorbed on two sites A(g) + B(g) + 2M(surface site) A-M + B-M the rate of A adsorption rads,A = kads,A (1− θΑ− θΒ) PA the rate of B adsorption rads,B = kads,B (1− θΑ− θΒ) PB the rate of A desorption rdes,A = kdes,A θΑ the rate of B desorption rdes,B = kdes,B θΒ at equilibrium ⇒ rads ,A = rdes ,A and ⇒ rads ,B = rdes ,B case III kads,A(1−θΑ−θΒ)PA=kdes,AθΑ and kads,B(1−θΑ−θΒ)PB=kdes,BθΒ rearrange it for θ θ A = where A B Cs ,A B0,APA = C∞ 1+ B0,APA + B0,B PB B0 ,A = kads ,A k and B0 ,B = ads ,B kdes ,A kdes ,B θB = Cs ,B B0,B PB = C∞ 1+ B0,APA + B0,B PB are adsorption coefficients of A & B. 58
  55. 55. The Langmuir adsorption isotherm (cont’d) A B case II case I C BP θ= s = 0 C∞ 1+ B0 P B0 = B A A B0 = k ads k des Adsorption Adsorption Strong kads>> kdes C θ = s →1 B0>>1 C∞ kads>> kdes θ= Weak kads<< kdes θ= Cs = B0P C∞ B0<<1 Cs ,A = C∞ 1 + B0,A PA + B0,B PB C B0,B PB θB = s ,B = C∞ 1 + B0,A PA + B0,B PB k k B0 ,A = ads ,A and B0 ,B = ads ,B kdes ,A kdes ,B θA = Cs (B0PAB)1/2 θ= = C∞ 1+ (B0PAB)1/2 k ads k des Cs →1 B0>>1 C∞ kads<< kdes θ= Cs = (B0P)1/2 B0<<1 C∞ case III B0,A PA Cs ,A B0,A PA = C∞ B0,A PA + B0,B PB A, B both strong C B0,B PB θ B = s ,B = C∞ B0,A PA + B0,B PB A strong, B weak A weak, B weak θA = θA = Cs,A / C∞ →1 θB = Cs,B / C∞ = (B0,B / B0,A) P B PA θ A = Cs ,A / C∞ = B0 ,A PA θ B = Cs ,B / C∞ = B0 ,B PB 59
  56. 56. Langmuir adsorption isotherm Cs (B0PAB)1/2 case II θ = = C∞ 1+ (B0PAB)1/2 C B0,A PA θ A = s ,A = Case III C∞ 1 + B0,A PA + B0,B PB C B0,B PB θ B = s ,B = C∞ 1 + B0,A PA + B0,B PB Strong adsorption kads>> kdes Weak adsorption kads<< kdes Amount adsorbed case I C BP θ= s = 0 C∞ 1+ B0 P θ= θ= Cs →1 C∞ Cs = B0 P C∞ mono-layer large B0 (strong adsorp.) moderate B0 small B0 (weak adsorp.) Pressure Langmuir adsorption isotherm established a logic picture of adsorption process It fits many adsorption systems but not at all The assumptions made by Langmuir do not hold in all situation (error?) Solid surface is heterogeneous , heat of adsorption is not a constant at different θ 60 Physisorption of gas molecules on a solid surface can be more than one layer
  57. 57. Adsorption Theories: BET P 1 ( C − 1)  P  = +   Va ( P − P o ) Vm C Vm C  P o  S.Brunauer, P.Emmett, E.Teller Adsorption of Gases in Multimolecular Layers, J. Am. Chem. Soc., 1938, 60 (2), pp 309–319 where V a = volume of gas adsorbed at pressure P ; V m = volume of gas required to form monolayer; C = BET constant (related to energy of adsorption of 1st layer); and P Modification of Langmuir = relative pressure of adsorbate. o P isotherm Adsorbate (a) Adsorbent (b) (c) Both monolayer and multilayer adsorption Assumptions: gas molecules physically adsorb on a solid in layers infinitely; there is no interaction between each adsorption layer; the Langmuir theory can be applied to each layer.
  58. 58. Specific Surface Area Calculation P (C − 1)  P  1 =  o + o Va ( P − P ) VmC  P  VmC At least three data points in the relative pressure range 0.05 to 0.30 Y = mX + i 1 Vm = m+i Total surface area = 1 V[(Po/P)-1] V m N av Acs Weight of adsorbate 0-1 0-2 0-3 P/Po Total surface area SSA (Specific surface area) = Weight of sample
  59. 59. Single Point BET Single-point method offers the advantage of simplicity and speed, often with little loss in accuracy. ( Vm = Va 1 − P P o ) i.e. Vm = 1/slope A relative pressure of 0.3 gives good general agreement with the multi-point method. Correction of single point “error” at P/P0 = 0.3 by multiplying the single point BET value by C/C-2 decreases the difference. Sample No. Multi-point BET (m2/g) Uncorrected single-point (m2/g) Uncorrected difference (%) Corrected single – point Corrected difference (%) (m2/g) 1 4.923 4.241 -13.9 4.948 0.51 2 4.286 3.664 -14.5 4.275 -0.26 3 8.056 6.867 -14.8 8.011 -0.56 4 5.957 5.194 -12.8 6.060 +1.73
  60. 60. Pore Size Distribution Va The distribution of pore Broad pore size volume with respect to distribution pore size is called a pore Narrow pore size size distribution. distribution Pore volume = ∑ Va d Unimodal pore size distribution Pore diameter, d Va Multimodal pore size distribution Pore diameter, d
  61. 61. Pore Size Distribution Va Adsorption or desorption isotherm. The desorption isotherm is preferred over adsorption isotherm. (P/Po)des (P/Po)ads 1 P/Po ∆Gdes = RT(lnPdes - lnP0) ∆Gads = RT(lnPads - lnP0) ∆Gdes < ∆Gads
  62. 62. Pore Size: Kelvin Equation p    2γV ln o  = cos θ  p  r k RT   where        pressure of adsorbate   = o  saturated pressure of adsorbate ;      γ = liquid surface tension; V = molar volume of condensed adsorbate; p p rk θ Multilayer formation occurs in parallel to = mean radius of curvature of the liquid meniscus; capillary condensation. k Capillary condensation is described by the R = real gas constant; Kelvin equation. r T = temperature; θ = contact angle between the solid and condensed phase.
  63. 63. Pore Size: Kelvin Equation Prior to condensation, some adsorption has taken place on the walls of the pore, rk does not represent the actual pore radius. rk rp = rk + t t θ Actual radius of the pore Adsorbed layer Kelvin radius of the pore Thickness of the adsorbed layer
  64. 64. Methods for Calculation of Pore Size Distribution BJH (Barrett, Halenda) method Joyner and Mesoporous solids DH (Dollimore Heal) method Dubinin-Astakhov method HK (Horvath-Kawazoe) method Microporous solids Saito-Foley method NLDFT (Non Local Density Functional Theory) and Monte Carlo simulation method Microporous and Mesoporous solids
  65. 65. Questions . . . anyone ?
  66. 66. Porosity Analyzer Outgassing station Analysis station Liquid nitrogen bath
  67. 67. Steps for Measurement 1. Sample Preparation 2. Adsorption Analysis 3. Interpretation
  68. 68. Sample Preparation (Outgassing) Vacuum Helium Adsorbate Po Surface contamination is removed by application of: Temperature Flowing gas (helium or nitrogen) or vacuum Backfill can be done using helium or adsorbate gas. Sample Cell Outgassing station Analysis station According to IUPAC standards, materials should be outgassed for at least 16 hours.
  69. 69. Adsorption Analysis Vacuum Helium Adsorbate Po Sample Cell Outgassing station Analysis station Adsorbate (nitrogen, argon, carbon dioxide, krypton) Analysis temperature (liquid nitrogen, liquid argon, 0 oC) Quantity of sample (1 mg sample is sufficient) Number of points (single point, five points, seven points, eleven points, full analysis)
  70. 70. Interpretation Pore size & distribution Points P/Po Volume adsorbed 1 2 3 Pore shape Weight of sample Results Specific surface area Pore volume
  71. 71. Common Adsorbates Gas Temperature Cross sectional area (nm2) N2 -195.8 oC (liquid nitrogen) -183 oC (liquid argon). 0.162 Ar -183 oC (liquid argon). -195.8 oC (liquid nitrogen) 0.142 CO2 -78 oC, -25 oC, 0 oC 0.195 CO -183 oC (liquid argon) 0.163 Kr -195.8 oC (liquid nitrogen) 0.205 O2 -183 oC (liquid argon) 0.141 0 oC, 25 oC 0.469 C4H10
  72. 72. Choice of Adsorptive 0.55 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 n-b uta ne Kr yp ton Nit rog Ca en rbo nm on oo xid e Ca rbo nd iox ide Ar go n 0.00 Ox yg en Cross-sectional area, nm 2 0.50 N2(g) in N2(l) is the most commonly used adsorbate. Not completely inert. Dipole movement and thus can have localized adsorption. Cross-sectional area of 0.162 nm2 is questionable. S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991 Quantachrome Autosorb-I Operational Manual
  73. 73. Choice of Adsorptive 0.55 Ar(g) in Ar(l) is preferable but because of unavailability of Ar(l) (87K), N2(l) (77 K) is used. Ar can reach to somewhat smaller pores than N2. Accurate measurement of micropores is possible using Ar. 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 n-b uta ne Kr yp ton Nit rog Ca en rbo nm on oo xid e Ca rbo nd iox ide Ar go n 0.00 Ox yg en Cross-sectional area, nm 2 0.50 S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991 Quantachrome Autosorb-I Operational Manual
  74. 74. Choice of Adsorptive 0.55 In case of activated carbon, CO2 is often the most preferred adsorbate. Adsorption analysis of CO2 takes less time. Limited to micropore analysis. 0.45 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05 n-b uta ne Kr yp ton Nit rog Ca en rbo nm on oo xid e Ca rbo nd iox ide Ar go n 0.00 Ox yg en Cross-sectional area, nm 2 0.50 S. Lowell & J. E. Shields, Powder Surface Area and Porosity, 3rd Ed. Chapman & Hall, New York, 1991 Quantachrome Autosorb-I Operational Manual
  75. 75. Validity of BET - Method The BET method depends on the crosssectional area of adsorbate. Monolayer structure is same on all the surface. Localized monolayer coverage. P 1 (C − 1)  P  = +   V ( P − P o ) Vm C Vm C  P o  SSA = V L av A M Adsorbate Adsorbent K. S. W. Sing, The Use of Nitrogen Adsorption for the Characterisation of Porous Materials, Colloids and Surfaces, 187 – 188, 2001, 3 - 9
  76. 76. Validity of Kelvin Equation   ln    p p  2γ V  = cos θ o   r k RT  rk θ Is relation between the meniscus curvature and the pore size and shape valid? Is it applicable for micropores and narrow mesopores? Does surface tension varies with pore width? F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 203, 1999
  77. 77. Shape of Microporous Materials Type I isotherms don’t have hysteresis. Va Pore shape cannot be determined by isotherm. Type I or Langmuir P/Po 1 As various methods for pore size calculation are based on shape of pores, reliability of pore size calculation is questionable. F. Rouquerol, J. Rouquerol, K. S. W. Sing, Adsorption by Powders and Porous Solids, Academic Press, 439-446, 1999
  78. 78. Choice of Method Mesopores Micropores Macropores 2 nm Methods Assumption 50 nm Pore Shape Brunauer MP method Dubinin-Astakhov method HK (Horvath-Kawazoe) method Saito-Foley method Based on .. Cylindrical or Slit shaped de Boer’s t-method - Polanyi potential theory Independent of Kelvin equation Slit Everett and Powl method Independent of Kelvin equation Cylindrical HK method P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 – 152, 1997 Quantachrome Autosorb-I Operational Manual
  79. 79. Choice of Method Mesopores Micropores Macropores 2 nm Methods 50 nm Assumption Pore Shape BJH (Barrett, Joyner and Halenda) method DH (Dollimore Heal) method Based on .. Cylindrical, Slit-shaped Kelvin equation Cylindrical t-method P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 – 152, 1997 Quantachrome Autosorb-I Operational Manual
  80. 80. Choice of Method Mesopores Micropores Macropores 2 nm Methods 50 nm Assumption Pore Shape NLDFT (Non Local Density Functional Theory) and Monte Carlo simulation method Based on .. Cylindrical and slit Statistical thermodynamics P. A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 53 – 152, 1997 Quantachrome Autosorb-I Operational Manual
  81. 81. Quantachrome INSTRUMENTS Physisorption Methods and Techniques
  82. 82. Micro and Mesopore Size Determination by Gas Sorption First: Quantitative estimation of micropore volume and area… T-plot and DR methods.
  83. 83. Volume adsorbed Multilayer adsorption Type II, IV After the knee, micropores cease to contribute to the adsorption process. Low slope region in middle of isotherm indicates first few multilayers, on external surface including meso and macropores… before the onset of capillary condensation Relative Pressure (P/Po)
  84. 84. Estimation of Micropores... the t-plot method This method uses a mathematical representation of multi-layer adsorption. The thickness, t, of an adsorbate layer increases with increasing pressure. The t-curve so produced is very similar in appearance to a type II isotherm. For every value of P/Po, the volume adsorbed is plotted against the corresponding value of “t”. If the model describes the experimental data a straight line is produced on the t-plot...
  85. 85. The t-plot Statistical thickness Resembles a type II A statistical multilayer A statistical monolayer Relative Pressure (P/Po)
  86. 86. t-plot Method (mesoporous only) Slope = V/t = A 1 2 3 4 t( ) 5 6 7
  87. 87. t-plot Method showing a “knee” C C X X X B B X X A A X 1 2 3 X 4 5 6 7 t( ) Slope A - slope B = area contribution by micropores size C
  88. 88. ? ? ? Quiz ? ? ? ? What is an αs plot? αs (for Ken Sing) is a comparison A plot like the t-plot but its slope does not give area directly.
  89. 89. Estimation of Micropores Dubinin-Radushkevich (DR) Theory   T 2  P0   2 W = W0 exp  − B  log    β  P       W = volume of the liquid adsorbate W0 = total volume of the micropores B = adsorbent constant β = adsorbate constant A linear relationship should be found between log(W) and log2(Po/P)...
  90. 90. Estimation of Micropores Log (W) Dubinin-Radushkevich (DR) Plot Extrapolation yields Wo 0 Log2(Po/P)
  91. 91. Pore Size Determination Requires a recognition and understanding of different basic isotherm types.
  92. 92. t-plot Method (in the presence of micropores) Intercept = micropore volume 1 2 3 4 t( ) 5 6 7
  93. 93. Types of Isotherms Type II Volume adsorbed Type I Type V Type IV Type III Relative Pressure (P/Po)
  94. 94. Volume adsorbed Types of Isotherms Limiting value (plateau) due to filled pores and essentially zero external area. Type I or pseudo-“Langmuir” Steep initial region due to very strong adsorption, for example in micropores. Relative Pressure (P/Po)
  95. 95. ? ? ? Quiz ? ? ? ? Why pseudo Langmuir? A Langmuir applies to monolayer limit, not volume filling limit.
  96. 96. Types of Isotherms Volume adsorbed Absence of hysteresis indicates adsorption on and desorption from a non-porous surface.. Type II Low slope region in middle of isotherm indicates first few multilayers Rounded knee indicates approximate location of monolayer formation. Relative Pressure (P/Po)
  97. 97. Types of Isotherms Volume adsorbed Example: krypton on polymethylmethacrylate Type III Lack of knee represents extremely weak adsorbate-adsorbent interaction BET is not applicable Relative Pressure (P/Po)
  98. 98. Types of Isotherms Volume adsorbed Type IV Closure at P/Po~0.4 indicates presence of small mesopores (hysteresis would stay open longer but for the tensilestrength-failure of the nitrogen meniscus. Rounded knee indicates approximate location of monolayer formation. Hysteresis indicates capillary condensation in meso and macropores. Low slope region in middle of isotherm indicates first few multilayers Relative Pressure (P/Po)
  99. 99. Types of Isotherms Volume adsorbed Example: water on carbon black Type V Lack of knee represents extremely weak adsorbate-adsorbent interaction BET is not applicable Relative Pressure (P/Po)
  100. 100. Types of Hysteresis Volume adsorbed Large pores/voids Gel Mesopores MCM Relative Pressure (P/Po)
  101. 101. MesoPore Size by Gas Sorption (BJH)
  102. 102. ? ? ? Quiz ? ? ? ? Analyzer measures volume of pores: Yes or No? A NO! It measures what leaves supernatent gas phase
  103. 103. Pore Size Distribution Hysteresis is indicative of the presence of mesopores and the pore size distribution can be calculated from the sorption isotherm. Whilst it is possible to do so from the adsorption branch, it is more normal to do so from the desorption branch... Micropore (Greek micro = small): 0 nm - 2 nm diameter Mesopore (Greek meso = middle): 2nm - 50 nm diameter Macropore (Greek macro = large): >50 nm diameter
  104. 104. Adsorption / Desorption Adsorption = Desorption = multilayer formation meniscus development
  105. 105. Kelvin* Equation P 2 γV ln = cos θ P0 rRT 4.15 rk ( A ) = log( P / P0 ) * Lord Kelvin a.k.a. W.T. Thomson
  106. 106. Pore Size rp = rk + t rp = actual radius of the pore rk = Kelvin radius of the pore t = thickness of the adsorbed film
  107. 107. Statistical Thickness, t • Halsey equation • Generalized Halsey • deBoer equation • Carbon Black STSA
  108. 108. BJH Method (Barrett-Joyner-Halenda) rpore = rKelvin + t Pore volume requires assumption of liquid density!
  109. 109. Pore Size Distribution dV/dlogD Artifact 40 Pore Diameter (angstrom)
  110. 110. Amount adsorbed ~ 0.42 Relative Pressure (P/Po)
  111. 111. Pore Size Data • Volume and size of pores can be expressed from either adsorption and/or desorption data. • The total pore volume, V, is taken from the maximum amount of gas adsorbed at the “top” of the isotherm and conversion of gas volume into liquid volume. • The mean pore diameter is calculated from simple cylindrical geometry: 4V d= A where A is the BET surface area.
  112. 112. Pore size analysis of MCM 41 (Templated silica) by N2 sorption at 77 K 600 E xp . N itrogen sorp tion at 77 K in M C M 41 D F T - Isoth erm Volume [cc/g] 500 400 300 200 100 0 0.2 0.4 P /P 0 0 .6 0 .8 1
  113. 113. Pore size analysis of MCM 41: Calculations compared 0.3 Dv(d) [cc/Å/g] 0.25 BJH-Pore size distribution DFT-Pore size distribution 0.2 0.15 0.1 0.05 0 15 23 31 39 Pore Diameter [Å] 47 55
  114. 114. Calculation Models
  115. 115. Comparisons • P/Po range 1x10-7 to 0.02 0.01 to 0.1 0.05 to 0.3 > 0.1 > 0.35 0.1 to 0.5 Gas Sorption Calculation Methods Mechanism micropore filling sub-monolayer formation monolayer complete multilayer formation capillary condensation capillary filling in M41S-type materials Calculation model DFT, GCMC, HK, SF, DA, DR DR BET, Langmuir t-plot (de-Boer,FHH), BJH, DH DFT, BJH
  116. 116. Different Theories of Physisorption Surface area Pore volume Pore size BET Total pore vol DR ave Langmuir BJH t-plot (µpore vol) DR DH DR (µpore vol) MP and t-plot DA DFT BJH HK αs plot (BJH) (DFT) SF (DH) (DH) (DFT)
  117. 117. HK & SF Horvath-Kawazoe & Saito-Foley • HK • Direct mathematical relationship between relative pressure (P/Po) and pore size. Relationship calculated from modified Young-Laplace equation, and takes into account parameters such as magnetic susceptibility. Based on slit-shape pore geometry (e.g. activated carbons). Calculation restricted to micropore region (≤ 2nm width). • SF • Similar mathematics to HK method, but based on cylindrical pore geometry (e.g. zeolites). Calculation restricted to micropore region (≤ 2 nm diameter).
  118. 118. DA & DR Dubinin-Astakov and Dubinin-Radushkevic • DA • Closely related to DR calculation based on pore filling mechanism. Equation fits calculated data to experimental isotherm by varying two parameters, E and n. E is average adsorption energy that is directly related to average pore diameter, and n is an exponent that controls the width of the resulting pore size distribution. The calculated pore size distribution always has a skewed, monomodal appearance (Weibull distribution). • DR • Simple log(V) vs log2(Po/P) relationship which linearizes the isotherm based on micropore filling principles. “Best fit” is extrapolated to log2(Po/P) (i.e. where P/Po = 1) to find micropore volume.
  119. 119. BET • The most famous gas sorption model. Extends Langmuir model of gas sorption to multi-layer. BET equation linearizes that part of the isotherm that contains the “knee” , i.e. that which brackets the monolayer value. Normally solved by graphical means, by plotting 1/(V[(Po/P)]-1) versus P/Po. Monolayer volume (Vm) is equal to 1/(s+i) where s is the slope and i is the y-intercept. Usually BET theory is also applied to obtain the specific surface area of microporous materials, although from a scientific point of view the assumptions made in the BET theory do not take into account micropore filling. Please note, that for such samples the linear “BET” range is found usually at relative pressures< 0.1, in contrast to the classical BET range, which extends over relative pressures between 0.05 – 0.3.
  120. 120. Langmuir • Adsorption model limited to the formation of a monolayer that does not describe most real cases. Sometimes can be successfully applied to type I isotherms (pure micropore material) but the reason for limiting value (plateau) is not monolayer limit, but due to micropore filling. Therefore type I physisorption isotherm would be better called “pseudo-Langmuir” isotherm.
  121. 121. t-plot Statistical Thickness • Multi-layer formation is modeled mathematically to calculate a layer “thickness, t” as a function of increasing relative pressure (P/Po). The resulting t-curve is compared with the experimental isotherm in the form of a t-plot. That is, experimental volume adsorbed is plotted versus statistical thickness for each experimental P/Po value. The linear range lies between monolayer and capillary condensation. The slope of the t-plot (V/t) is equal to the “external area”, i.e. the area of those pores which are NOT micropores. Mesopores, macropores and the outside surface is able to form a multiplayer, whereas micropores which have already been filled cannot contribute further to the adsorption process. • It is recommended to initially select P/Po range 0.2 – 0.5, and subsequently adjust it to find the best linear plot.
  122. 122. BJH & DH Barrett, Joyner, Halenda and Dollimore-Heal • BJH • Modified Kelvin equation. Kelvin equation predicts pressure at which adsorptive will spontaneously condense (and evaporate) in a cylindrical pore of a given size. Condensation occurs in pores that already have some multilayers on the walls. Therefore, the pore size is calculated from the Kelvin equation and the selected statistical thickness (t-curve) equation. • DH • Extremely similar calculation to BJH, which gives very similar results. Essentially differs only in minor mathematical details.
  123. 123. Other Methods • FRACTAL DIMENSION • The geometric topography of the surface structure of many solids can be characterized by the fractal dimension D, which is a kind of roughness exponent. A “flat” surface is considered D is 2, however for an irregular (real) surface D may vary between 2 and 3 and expresses so the degree of roughness of the surface and/or porous structure. The determination of the surface roughness can be investigated by means of the modified FrenkelHalsey Hill method, which is applied in the range of multilayer adsorption.
  124. 124. Example Data : Microporous Carbon
  125. 125. BET : Not strictly applicable
  126. 126. Example Data : Microporous Carbon • Tag all adsorption points • Analyze behavior • Note knee – transition from micropore filling to limited multilayering (plateau).
  127. 127. Example Data : Microporous Carbon • Use Langmuir (Monolayer model) / DR for Surface Area, Micropore Volume • Usue Langmuir in range of 0.05 -> 0.2 (monolayer)
  128. 128. Example Data : Microporous Carbon • Langmuir Surface Area
  129. 129. Example Data : Microporous Carbon • DR Method for surface area, micropore volume • Choose low relative pressure points (up to P/P0 = 0.2)
  130. 130. Example Data : Microporous Carbon • Reports micropore surface area, and micropore volume. • Note Langmuir, DR surface areas very close (1430 m2/g vs. 1424 m2/g)
  131. 131. Example Data : Macroporous Sample Little or no “knee”, isotherm closes at 0.95
  132. 132. Example Data : Macroporous Sample • BET Plot = OK • Surface area ca. 8m2/g (low) • Note hysteresis above P/P0 = 0.95 ∴Pores > 35 nm
  133. 133. Example Data : Macroporous Sample Intercept = (-), no micropore volume.
  134. 134. Example Data : Macroporous Sample BJH Shows pores > 20nm, to over 200 nm
  135. 135. Example Data : Mesoporous Silica Hysteresis => mesopores Also micropores ?? Test using tmethod
  136. 136. Example Data : Mesoporous Silica BET Surface area = 112m2/g Classic mesoporous silica !
  137. 137. Example Data : Mesoporous Silica Intercept ~ 0 Look at tabular data MP SA = 8m2/g (total SA = 112) Statistical Thickness => Use de Boer for oxidic surfaces = silicas
  138. 138. Example Data : Mesoporous Silica Use BJH – shows narrow pore size distribution in 14-17nm range (mesopores)
  139. 139. Questions from audience?
  140. 140. MicroPore Size by Gas Sorption
  141. 141. Available Calculation Models
  142. 142. Pore filling pressures for nitrogen in cylindrical pores at 77 K, (Gubbins et al. 1997)
  143. 143. Pore filling pressures for nitrogen in cylindrical silica pores at 77 K (Neimark et al., 1998)
  144. 144. Pore size analysis of MCM 41 by silica by N2 sorption at 77 K 0.3 600 0.25 Dv(d) [cc/Å/g] Volume [cc/g] 500 Exp. Nitrogen sorption at 77 Kin M 41 CM DFT- Isotherm 400 300 200 100 0 BJH -Pore size distribution DFT-Pore size distribution 0.2 0.15 0.1 0.05 0.2 0.4 P/P0 0.6 0.8 1 0 15 23 31 39 Pore Diam [Å] eter 47 55
  145. 145. Gas- and liquid density profiles in a slit pore by GCMC (Walton and Quirke,1989)
  146. 146. NLDFT / GCMC (Monte Carlo) Kernel File Applicable Pore Diameter Range Examples NLDFT– N2 - carbon kernel at 77 K based on a slit-pore model 0.35nm-30 nm Carbons with slit-like pores, such as activated carbons and others. NLDFT– N2 – silica equilibrium transition kernel at 77 K, based on a cylindrical pore model 0.35nm- 100nm Siliceous materials such as some silica gels, porous glasses, MCM-41, SBA15, MCM-48 and other adsorbents which show type H1 sorption hysteresis. NLDFT– N2 - silica adsorption branch kernel at 77 K, based on a cylindrical pore model 0.35nm-100nm Siliceous materials such as some controlled pore glasses, MCM-41, SBA-15, MCM-48, and others. Allows to obtain an accurate pore size distribution even in case of type H2 sorption hysteresis NLDFT– Ar zeolite/silica equilibrium transition kernel at 87 K based on a cylindrical pore model 0.35nm -100nm Zeolites with cylindrical pore channels such as ZSM5, Mordenite, and mesoporous siliceous materials (e.g., MCM-41, SBA-15, MCM-48, some porous glasses and silica gels which show type H1 sorption hysteresis).
  147. 147. NLDFT / GCMC (Monte Carlo) Kernel File Applicable Pore Diameter Range Examples NLDFT – Ar-zeolite/silica adsorption branch kernel at 87 K based on a cylindrical pore model 0.35nm-100nm Zeolites with cylindrical pore channels such as ZSM5, Mordenite etc., and mesoporous siliceous materials such as MCM-41, SBA-15, MCM-48, porous glasses some silica gels etc). Allows to obtain an accurate pore size distribution even in case of H2 sorption hysteresis. NLDFT – Ar-zeolite / silica equilibrium transition kernel based on a spherical pore model (pore diameter < 2 nm) and cylindrical pore model (pore diameter > 2 nm) 0.35nm-100nm Zeolites with cage-like structures such as Faujasite, 13X etc. , and mesoporous silica materials (e.g., MCM-41, SBA15, porous glasses, some silica gels which show H1 sorption hysteresis). NLDFT – Ar-zeolite / silica adsorption branch kernel at 87 K based on a spherical pore model (pore diameter < 2 nm) and cylindrical pore model (pore diameter > 2 nm) 0.35nm-100nm Zeolites with cage-like structures such as Faujasite, 13X, and mesoporous silica materials (e.g., MCM-41, SBA15, controlled-pore glasses and others). Allows to obtain an accurate pore size distribution even in case of H2 sorption hysteresis.
  148. 148. NLDFT / GCMC (Monte Applicable Pore Carlo) Kernel File Diameter Range Examples NLDFT – Ar - carbon kernel at 77 K based on a slit-pore model 0.35 nm - 7 nm Carbons with slit-like pores, such as activated carbons etc. NLDFT - CO2 - carbon kernel at 273 K based on a slit-pore model 0.35nm-1.5 nm Carbons with slit-like pores, such as activated carbons etc. GCMC – CO2 - carbon kernel at 273 K based on a slit-pore model 0.35nm-1.5 nm Carbons with slit-like pores, such as activated carbons etc.
  149. 149. RECENT ADVANCES IN THE PORE SIZE ANALYSIS OF MICRO- AND MESOPOROUS MOLECULAR SIEVES BY ARGON GAS ADSORPTION
  150. 150. Micropore Size Characterization • Physical adsorption in micropores, e.g. zeolites occurs at relative pressures substantially lower than in case of adsorption in mesopores. • Adsorption measurements using nitrogen at 77.4 K is difficult, because the filling of 0.5 - 1 nm pores occurs at P/Po of 10-7 to 10-5, where the rate of diffusion and adsorption equilibration is very slow.
  151. 151. Advantages of Using Argon • Advantage to analyze such narrow micropores by using argon at liquid argon temperature (87.3 K). • Argon fills these micropores (0.5 – 1nm) at much higher relative pressures (i.e., at relative pressures 10-5 to 10-3) compared to nitrogen.
  152. 152. Advantages of Higher Temperature & Pressure • Accelerated diffusion. • Accelerated equilibration processes. • Reduction in analysis time.
  153. 153. Argon Adsorption at 87.3 K versus Nitrogen Adsorption at 77.4 K 350 N2/77K Ar/87 K Volume [cm3] 280 210 140 70 0 10-6 ZEOLITE | 10.5.2001 5 10-5 5 10-4 5 10-3 5 10-2 5 10-1 5 100 P/P0 The different pore filling ranges for argon adsorption at 87.3K and nitrogen adsorption at 77.4K in faujasite-type zeolite are illustrated above.
  154. 154. Micropore Size Calculation • Difficulties are associated with regard to the analysis of micropore adsorption data. • Classical, macroscopic, theories [1] like DR and semiempirical treatments such those of HK and SF do not give a realistic description of micropore filling • This leads to an underestimation of pore sizes for micropores and even smaller mesopores [2]. [1] F. Rouquerol, J. Rouquerol & K. Sing, Adsorption by Powders & Porous Solids, Academic Press, 1999 [ 2 ] P. I Ravikovitch, G.L. Haller, A.V. Neimark, Advcances in Colloid and Interface Science 76-77 , 203 (1998)
  155. 155. New Calculation • To overcome the above mentioned problems we introduce a new method for micropore analysis based on a Non-local Density Functional Theory (NLDFT) model by Neimark and Co-workers [3-5]. • The new DFT-method is designed for micromesopore size characterization of zeolitic materials ranging in size from 0.44 to 20 nm using high-resolution low-pressure argon adsorption isotherms at 87.3 K. [3] P.I. Ravikovitch, G.L. Haller, A.V. Neimark, Advances in Colloid and Interface Science, 76 – 77 (1998), 203 -207 [4] A.V. Neimark, P.I Ravikovitch, M. Gruen, F. Schueth, and K.K. Unger, J. Coll. Interface Sci., 207, (1998) 159 [5] A.V. Neimark, P.I. Ravikovitch, Microporous and Mesoporous Materials (2001) 44-45, 697
  156. 156. Systematic, Experimental Study • To evaluate the application of argon sorption for micro- and mesopore size analysis of zeolites and mesoporous silica materials including novel mesoporous molecular sieves of type MCM-41 and MCM-48. • The sorption isotherms were determined using a static volumetric technique • Samples were outgassed for 12 h under vacuum (turbomolecular pump) at elevated temperatures (573 K for the zeolites and 393 K for MCM41/MCM-48).
  157. 157. Results 25 Argon adsorption isotherms at 87 K on MCM-41, ZSM-5 and their 50-50 mixture. Adsorption, [mmol/g] 20 15 10 MCM-41 5 ZSM-5 50-50 0 0 0.2 0.4 0.6 P/Po 0.8 1
  158. 158. Results 25 Adsorption, [mmol/g] 20 15 MCM-41 10 ZSM-5 50-50 5 0 0.000001 0.00001 0.0001 0.001 P/Po 0.01 0.1 1
  159. 159. ZSM 0.12 0.4 0.35 0.1 0.3 0.25 0.2 0.06 0.15 histogram 0.04 0.1 integral 0.05 0.02 0 0 -0.05 1 10 D, [Å] 100 1000 Vcum, [cm3/g] dV/dD [cm3/g 0.08
  160. 160. MCM 0.2 0.7 0.18 0.6 0.16 dV/dD [cm3/g] /g] 0.12 0.4 0.1 histogram 0.06 0.3 integral 0.08 0.2 0.04 0.1 0.02 0 0 1 10 100 D, [Å] 1000 Vcum, [cm3/g] 0.5 0.14
  161. 161. Evaluation of DFT Algorithm 20 18 16 Adsorption, [mmol/g] 14 12 experimental 10 NLDFT fit 8 6 4 2 0 0.000001 0.00001 0.0001 0.001 P/Po 0.01 0.1 1
  162. 162. Pore Size Distribution 0.1 0.6 0.09 0.5 0.08 0.07 dV/dD [cm3/g] 0.06 0.05 0.3 histogram 0.04 0.2 0.03 integral 0.02 0.1 0.01 0 0 1 10 100 D, [Å] 1000 Vcum [cm3/g] 0.4
  163. 163. Discussion • Argon sorption at 77 K is limited to pore diameters smaller than 12 nm. i.e. no pore filling/pore condensation can be observed at this temperature for silica materials containing larger pores. • This lack of argon condensation for pores larger than ca. 12 nm is associated with the fact, that 77 K is ca. 6.8 K below the bulk triple point [4,5] . [4] M. Thommes, R. Koehn and M. Froeba, J. Phys. Chem. B (2000), 104, 7932 [5] M. Thommes, R. Koehn and M. Froeba, Stud. Surf. Sci. Catal., (2001), 135 17
  164. 164. Discussion • These limitation do not exist for argon sorption at its’ boiling temperature, i.e. ca. 87 K. • Pore filling and pore condensation can be observed over the complete micro- and mesopore size range .
  165. 165. Discussion • Results of classical, and semi-empirical methods (e.g., BJH, SF etc) indicate that these methods underestimate the pore size considerably. • Deviations from the DFT-results are often in a range of ca. 20 % for pore diameters < 10 nm.
  166. 166. Summary • Our results indicate that argon sorption data at 87 K combined with the new NLDFT-methods provides a convenient way to achieve an accurate and comprehensive pore size analysis over the complete micro-and mesopore size range for zeolites, catalysts, and mesoporous silica materials.
  167. 167. Acknowledgements • Special thanks go to Alex Neimark and Peter Ravikovitch at TRI Princeton, New Jersey, USA.
  168. 168. References to research work of nitrogen, argon and krypton in MCM-48/MCM-41 materials (1) M. Thommes, R. Koehn and M. Froeba, “ Systematic Sorption studies on surface and pore size characteristics of different MCM-48 silica materials”, Studies in Surface Science and Catalysis 128, 259 (2000) (2) M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of nitrogen, argon and krypton in mesoporous MCM-48 silica materials” J. Phys. Chem. B 104, 7932 (2000) (3)M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of pure fluids in mesoporous MCM-48 silica, MCM-41 silica and controlled pore glass, Studies in Surface Science and Catalysis, 135, 17 (2001) (4)M. Thommes, R. Koehn and M. Froeba, “Characterization of porous solids: Sorption and pore condensation behavior of nitrogen, argon and krypton in ordered and disordered mesoporous silica materials (MCM-41, MCM-48, SBA-15, controlled pore glass, silica gel) at temperatures above and below the bulk triple point”, Proceedings of the first topical conference on nanometer scale science and engineering” (G.U. Lee, Ed) AIChE Annual Meeting, Reno, Nevada, November 4-9, 2001 (5)M. Thommes, R. Koehn and M. Froeba, “Sorption and pore condensation behavior of pure fluids in mesoporous MCM-48 silica, MCM-41 silica and controlled pore glass at temperatures above and below the bulk triple point”, submitted to Applied Surface Science, (2001)
  169. 169. Rapid Micropore Size Analysis by CO2 Adsorption
  170. 170. o CO2 Adsorption at 0 C on Carbon
  171. 171. RAPID MICROPORE ANALYSIS • The advantages of micropore analysis with Quantachrome’s Density Functional Theory (DFT) and CO2 include: • Speed of analysis; with the higher diffusion rate at 273.15K, analysis times are reduced as much as 90%. • Carbon dioxide at 273.15K permits probing pores from about 2 angstroms (0.2 nm).
  172. 172. DFT ADVANTAGE DFT has recently been applied to describe the behavior of fluids that are confined in small pores. The current popular gas sorption models, e.g. BJH, HK, SF, DA, etc., assume that the density of the adsorbed phase remains constant, regardless of the size of the pores that are being filled. Packing considerations suggest that these models are less than satisfactory for analyses of pores less than 2 nm.
  173. 173. DFT “Fitting” • For a given adsorbate-adsorbent system, DFT calculates the most likely summation of "ideal isotherms“ calculated from "ideal pores" of fixed sizes needed to match the experimental results.
  174. 174. CO2 for Speed! • Typically, micropore analyses with nitrogen as adsorbate will require 24 hours or more to run. • Using carbon dioxide as adsorbate provides several advantages. – Carbon dioxide molecules are slightly thinner than nitrogen molecules (2.8 angstroms radius vs. 3.0 angstroms) and will fill smaller pores than nitrogen. – The use of carbon dioxide allows the measurements to be made at 273.15K, typically with an ice/water bath. – There is no longer any need to provide and maintain or replenish a level of liquid nitrogen during the analysis.
  175. 175. CO2 Benefits • At this temperature, the diffusion rate of molecules moving through small and tortuous micropores is much higher than at 77.35K. This so-called "activated adsorption" effect led to the popularization of the use of carbon dioxide to characterize carbonaceous material since the early 1960s.
  176. 176. CO2 Benefits • This higher diffusion rate is responsible for reducing the analysis time to a few hours for a complete adsorption experiment. The faster rate also provides for the possibility of using larger samples than with nitrogen adsorption, thus reducing sample weighing errors. • Pore size distributions thus obtained are comparable to those from a 24-hour nitrogen/77.35K analysis.
  177. 177. N2 Adsorption @ 77K: 40 hours
  178. 178. CO2 adsorption at 273K: 2.75 hours
  179. 179. o CO2 Adsorption at 0 C Density Functional Theory Micropore Distribution
  180. 180. o CO2 Adsorption at 0 C Monte Carlo Simulation Micropore Distribution
  181. 181. How to do it? • Hardware requirements for this new method are minimal: – a wide- mouth dewar and – a water-level sensor. • The proprietary Quantachrome Autosorb® software provides the DFT data reduction capabilities to do the rest. Pore size distributions from about 2 angstroms can be determined from the data taken at 273.15K. • Currently, calculation parameters are optimized for studies on carbon surfaces.
  182. 182. BIBLIOGRAPHY for Rapid Micropore Size Analysis by CO2 Adsorption 1. J. Garrido, A. Linares-Solano, J.M. Martin-Martinez, M. Molina-Sabio, F. Rodriguez-Reinoso, R. Torregosa Langmuir, 3, 76, (1987) 2. F. Carrasco-Martin, M.V. López-Ramón, C. Moreno-Castilla. Langmuir, 9, 2758 (1993) 3. P. Tarazona. Phys.Rev.A 31, 2672 (1985) 4. N.A. Seaton, J.P.R.B. Walton, N. Quirke. Carbon, 27, 853 (1989) 5. C. Lastoskie, K.E. Gubbins, N. Quirke. J.Phys.Chem., 97, 4786 (1993) 6. J.J. Olivier. Porous Materials 2, 9 (1995) 7. P.I. Ravikovitch, S.C. Ó Domhnaill, A.V. Neimark, F. Schüth, K.K. Unger. Langmuir, 11, 4765 (1995) 8. A.V. Neimark, P.I. Ravikovitch, M. Grün, F. Schüth, K.K. Unger. COPS-IV, 1997 (in press) 9. P.I. Ravikovitch P.I., D. Wei, W.T. Chuen, G.L. Haller,A.V. Neimark. J.Phys.Chem., May 1997 10. E.J. Bottani, V. Bakaev, W.A. Steele. Chem.Eng.Sci. 49, 293 (1994) 11. M.M. Dubinin. Carbon 27, 457 (1989)
  183. 183. Questions from the floor ?
  184. 184. CHEMISORPTION & CATALYSIS
  185. 185. Catalysis & Catalysts Facts and Figures about Catalysts Life cycle on the earth Catalysts (enzyme) participates most part of life cycle e.g. forming, growing, decaying Catalysis contributes great part in the processes of converting sun energy to various other forms of energies e.g. photosynthesis by plant CO2 + H2O=HC + O2 Catalysis plays a key role in maintaining our environment Chemical Industry ca. $2 bn annual sale of catalysts ca. $200 bn annual sale of the chemicals that are related products 90% of chemical industry has catalysis-related processes Catalysts contributes ca. 2% of total investment in a chemical process 189
  186. 186. What is Catalysis Catalysis Catalysis is an action by catalyst which takes part in a chemical reaction process and can alter the rate of reactions, and yet itself will return to its original form without being consumed or destroyed at the end of the reactions (This is one of many definitions) Three key aspects of catalyst action taking part in the reaction • it will change itself during the process by interacting with other reactant/product molecules altering the rates of reactions • in most cases the rates of reactions are increased by the action of catalysts; however, in some situations the rates of undesired reactions are selectively suppressed Returning to its original form • • After reaction cycles a catalyst with exactly the same nature is ‘reborn’ In practice a catalyst has its lifespan - it deactivates gradually during use 190
  187. 187. Action of Catalysts Catalysis action - Reaction kinetics and mechanism Catalyst action leads to the rate of a reaction to change. This is realised by changing the course of reaction (compared to non-catalytic reaction) Forming complex with reactants/products, controlling the rate of elementary steps in the process. This is evidenced by the facts that uncatalytic The reaction activation energy is altered those formed in non-catalytic reaction The rates of reactions are altered (both catalytic energy The intermediates formed are different from desired and undesired ones) reactant produc reaction process t Reactions proceed under less demanding conditions Allow reactions occur under a milder conditions, e.g. at lower temperatures for those heat sensitive materials 191
  188. 188. Action of Catalysts It is important to remember that the use of catalyst DOES NOT vary ∆G & Keq values of the reaction concerned, it merely change the PACE of the process Whether a reaction can proceed or not and to what extent a reaction can proceed is solely determined by the reaction thermodynamics, which is governed by the values of ∆G & Keq, NOT by the presence of catalysts. In another word, the reaction thermodynamics provide the driving force for a rxn; the presence of catalysts changes the way how driving force acts on that process. e.g CH4(g) + CO2(g) = 2CO(g) + 2H2(g) ∆G°373=151 kJ/mol (100 °C) ∆G°973 =-16 kJ/mol (700 °C) At 100° ∆G° =151 kJ/mol > 0. There is no thermodynamic driving force, the C, 373 reaction won’t proceed with or without a catalyst At 700° ∆G° = -16 kJ/mol < 0. The thermodynamic driving force is there. However, C, 373 simply putting CH4 and CO2 together in a reactor does not mean they will react. Without a proper catalyst heating the mixture in reactor results no conversion of CH4 and CO2 at all. When Pt/ZrO2 or Ni/Al2O3 is present in the reactor at the same temperature, equilibrium conversion can be achieved (<100%). 192
  189. 189. Types of Catalysts & Catalytic Reactions The types of catalysts Classification based on the its physical state, a catalyst can be gas liquid solid Classification based on the substances from which a catalyst is made Inorganic (gases, metals, metal oxides, inorganic acids, bases etc.) Organic (organic acids, enzymes etc.) Classification based on the ways catalysts work Homogeneous - both catalyst and all reactants/products are in the same phase (gas or liq) Heterogeneous - reaction system involves multi-phase (catalysts + reactants/products) Classification based on the catalysts’ action Acid-base catalysts Enzymatic Photocatalysis Electrocatalysis, etc. 193
  190. 190. Applications of Catalysis Industrial applications Almost all chemical industries have one or more steps employing catalysts Petroleum, energy sector, fertiliser, pharmaceutical, fine chemicals … Advantages of catalytic processes Achieving better process economics and productivity Increase reaction rates - fast Simplify the reaction steps - low investment cost Carry out reaction under mild conditions (e.g. low T, P) - low energy consumption Reducing wastes Improving selectivity toward desired products - less raw materials required, less unwanted wastes Replacing harmful/toxic materials with readily available ones Producing certain products that may not be possible without catalysts Having better control of process (safety, flexible etc.) Encouraging application and advancement of new technologies and materials And many more … 194
  191. 191. Applications of Catalysis Environmental applications Pollution controls in combination with industrial processes Pre-treatment - reduce the amount waste/change the composition of emissions Post-treatments - once formed, reduce and convert emissions Using alternative materials … Pollution reduction gas - converting harmful gases to non-harmful ones liquid - de-pollution, de-odder, de-colour etc solid - landfill, factory wastes … And many more … Other applications Catalysis and catalysts play one of the key roles in new technology development. 195
  192. 192. Research in Catalysis Research in catalysis involve a multi-discipline approach Reaction kinetics and mechanism Reaction paths, intermediate formation & action, interpretation of results obtained under various conditions, generalising reaction types & schemes, predict catalyst performance… Catalyst development Material synthesis, structure properties, catalyst stability, compatibility… Analysis techniques Detection limits in terms of dimension of time & size and under extreme conditions (T, P) and accuracy of measurements, microscopic techniques, sample preparation techniques… Reaction modelling Elementary reactions and rates, quantum mechanics/chemistry, physical chemistry … Reactor modelling Mathematical interpretation and representation, the numerical method, micro-kinetics, structure and efficiency of heat and mass transfer in relation to reactor design … Catalytic process Heat and mass transfers, energy balance and efficiency of process … 196
  193. 193. Catalytic Reaction Processes Understanding catalytic reaction processes A catalytic reaction can be operated in a batch manner Reactants and catalysts are loaded together in reactor and catalytic reactions (homo- or heterogeneous) take place in pre-determined temperature and pressure for a desired time / desired conversion Type of reactor is usually simple, basic requirements Withstand required temperature & pressure Some stirring to encourage mass and heat transfers Provide sufficient heating or cooling Catalytic reactions are commonly operated in a continuous manner Reactants, which are usually in gas or liquid phase, are fed to reactor in steady rate (e.g. mol/h, kg/h, m3/h) Usually a target conversion is set for the reaction, based on this target required quantities of catalyst is added required heating or cooling is provided required reactor dimension and characteristics are designed accordingly. 197
  194. 194. Catalytic Reaction Processes Catalytic reactions in a continuous operation (cont’d) Reactants in continuous operation are mostly in gas phase or liquid phase easy transportation The heat & mass transfer rates in gas phase is much faster than those in liquid Catalysts are pre-loaded, when using a solid catalyst, or fed together with reactants when catalyst & reactants are in the same phase and premixed It is common to use solid catalyst because of its easiness to separate catalyst from unreacted reactants and products Note: In a chemical process separation usually accounts for ~80% of cost. That is why engineers always try to put a liquid catalyst on to a solid carrier. With pre-loaded solid catalyst, there is no need to transport catalyst which is then more economic and less attrition of solid catalyst (Catalysts do not change before and after a reaction and can be used for number cycles, months or years), In some cases catalysts has to be transported because of need of regeneration In most cases, catalytic reactions are carried out with catalyst in a fixed-bed reactor (fluidised-bed in case of regeneration being needed), with the reactant being gases or liquids 198
  195. 195. Catalytic Reaction Processes General requirements for a good catalyst Activity - being able to promote the rate of desired reactions Selective - being to promote only the rate of desired reaction and also retard the undesired reactions Note: The selectivity is sometime considered to be more important than the activity and sometime it is more difficult to achieve (e.g. selective oxidation of NO to NO2 in the presence of SO2) 199
  196. 196. Catalytic reaction processes Stability - a good catalyst should resist to deactivation, caused by the presence of impurities in feed (e.g. lead in petrol poison TWC. thermal deterioration, volatility and hydrolysis of active components attrition due to mechanical movement or pressure shock A solid catalyst should have reasonably large surface area needed for reaction (active sites). This is usually achieved by making the solid into a porous structure.
  197. 197. Example Heterogeneous Catalytic Reaction Process The long journey for reactant molecules to 1. travel within gas phase 2. cross gas-liquid phase boundary 3. travel within liquid phase/stagnant layer 4. cross liquid-solid phase boundary 5. reach outer surface of solid 6. diffuse within pore 7. arrive at reaction site 8. be adsorbed on the site and activated 9. react with other reactant molecules, either being adsorbed on the same/neighbour sites or approaching from surface above Product molecules must follow the same track in the reverse direction to return to gas phase Heat transfer follows similar track gas phase reactant molecule 1 2 3 gas phase liquid phase / stagnant layer 45 6 porous solid pore 78 9 201
  198. 198. Solid Catalysts Catalyst composition Catalyst Active phase Support Where the reaction occurs (mostly metal/metal oxide) Promoter Textual promoter (e.g. Al - Fe for NH3 production) Electric or Structural modifier Poison resistant promoters Support / carrier Increase mechanical strength Increase surface area (98% surface area is supplied within the porous structure) may or may not be catalytically active 202
  199. 199. Solid Catalysts Some common solid support / carrier materials Alumina Inexpensive Surface area: 1 ~ 700 m2/g Acidic Silica Inexpensive Surface area: 100 ~ 800 m2/g Acidic Other supports Active carbon (S.A. up to 1000 m2/g) Titania (S.A. 10 ~ 50 m2/g) Zirconia (S.A. 10 ~ 100 m2/g) Magnesia (S.A. 10 m2/g) Lanthana (S.A. 10 m2/g) Zeolite mixture of alumina and silica, often exchanged metal ion present shape selective acidic Active site porous solid pore 203
  200. 200. Solid Catalysts Support Preparation of catalysts Precipitation To form non-soluble precipitate by desired reactions at certain pH and temperature Adsorption & ion-exchange Drying & firing precursor add acid/base precipitate filter & wash solution with pH control or deposit the resulting precipitation precipitate Support I-exch. Na+ S-Na+ + Ni 2+ S-Ni 2+ Amount adsorbed Cationic: S-OH+ + C+ → SOC+ + H+ Anionic: S-OH- + A- → SA- + OH+ Drying & firing Concentration Support Impregnation Fill the pores of support with a metal salt solution of sufficient concentration to give the correct loading. Dry mixing Physically mixed, grind, and fired Drying & firing Soln. of metal Pore saturated pellets precursor 204
  201. 201. Solid Catalysts Preparation of catalysts Reduction if elemental metal is the active phase 40 100 Sulphidation BET S.A. Commonly used Pre-treatments BET S.A. m2/g Catalysts need to be calcined (fired) in order to decompose the precursor and to received desired thermal stability. The effects of calcination temperature and time are shown in the figures on the right. 75 50 25 0 500 600 700 800 900 Temperature ° C 0 0 Time / hours if a metal sulphide is the active phase Activation Typical catalyst life span Can be many years or a few mins. Activity Some catalysts require certain activation steps in order to receive the best performance. Even when the oxide itself is the active phase it may be necessary to pre-treat the catalyst prior to the reaction Induction period Normal use Time dead 205 10
  202. 202. Adsorption On Solid Surface Adsorption Adsorption is a process in which molecules from gas (or liquid) phase land on, interact with and attach to solid surfaces. The reverse process of adsorption, i.e. the process n which adsorbed molecules escape from solid surfaces, is called Desorption. Molecules can attach to surfaces in two different ways because of the different forces involved. These are Physisorption (Physical adsorption) & Chemisorption (Chemical adsorption) Physisorption Chemisorption force van de Waal chemcal bond number of adsorbed layers multi only one layer adsorption heat low (10-40 kJ/mol) high ( > 40 kJ/mol) selectivity low high temperature to occur low high 206
  203. 203. Adsorption On Solid Surface Adsorption process Adsorbent and adsorbate Adsorbent (also called substrate) - The solid that provides surface for adsorption high surface area with proper pore structure and size distribution is essential good mechanical strength and thermal stability are necessary Adsorbate - The gas or liquid substances which are to be adsorbed on solid Surface coverage, θ The solid surface may be completely or partially covered by adsorbed molecules define Adsorption heat θ= number of adsorption sites occupied number of adsorption sites available θ = 0~1 Adsorption is usually exothermic (in special cases dissociated adsorption can be endothermic) The heat of chemisorption is in the same order of magnitude of reaction heat; the heat of physisorption is in the same order of magnitude of condensation heat. 207
  204. 204. Adsorption On Solid Surface Applications of adsorption process Adsorption is a very important step in solid catalysed reaction processes Adsorption in itself is a common process used in industry for various purposes Purification (removing impurities from a gas / liquid stream) De-pollution, de-colour, de-odour Solvent recovery, trace compound enrichment etc… Usually adsorption is only applied for a process dealing with small capacity The operation is usually batch type and required regeneration of saturated adsorbent Common adsorbents: molecular sieve, active carbon, silica gel, activated alumina. Physisorption is an useful technique for determining the surface area, the pore shape, pore sizes and size distribution of porous solid materials (BET surface area) 208
  205. 205. Adsorption On Solid Surface Characterisation of adsorption system Adsorption isotherm - most commonly used, especially to catalytic reaction system, T=const. The amount of adsorption as a function of pressure at set temperature Adsorption isobar - (usage related to industrial applications) The amount of adsorption as a function of temperature at set pressure Adsorption Isostere - (usage related to industrial applications) Adsorption pressure as a function of temperature at set volume V3>V2 V2>V1 T2 >T1 T3 >T2 T4 >T3 P3>P2 P2>P1 P1 V4>V3 V1 Pressure T1 Vol. adsorbed Vol. adsorbed P4>P3 T5 >T4 Pressure Adsorption Isotherm Temperature Adsorption Isobar Temperature Adsorption Isostere 209
  206. 206. Adsorption On Solid Surface Five types of physisorption isotherms are found over all solids I Type I is found for porous materials with small pores e.g. charcoal. It is clearly Langmuir monolayer type, but the other 4 are not amount adsorbed II Type II for non-porous materials III Type III porous materials with cohesive force between adsorbate molecules greater than the adhesive force between adsorbate molecules and adsorbent IV Type IV staged adsorption (first monolayer then build up of additional layers) Type V porous materials with cohesive force between adsorbate molecules and adsorbent being greater than that between V 1.0 relative pres. P/P0 adsorbate molecules 210
  207. 207. Adsorption On Solid Surface Other adsorption isotherms Many other isotherms are proposed in order to explain the observations The Temkin (or Slygin-Frumkin) isotherm rads=kads(1-θ)P ≡ rdes=kdesθ B0P b1eQs / RTP θ= ⇒ θs = 1+ B0P 1+b1eQs / RTP ∆H of ads Assuming the adsorption enthalpy ∆H decreases linearly with surface coverage From ads-des equilibrium, ads. rate ≡ des. rate Langmuir Temkin θ where Qs is the heat of adsorption. When Qs is a linear function of θi. Qs=Q0-iS (Q0 is a constant, i is the number and S represents the surface site), the overall coverage  [b1eQs / RTP RT  1+ b1P  θ = ∫ θsdS = ∫ dS = ln 0 0 (1+ b eQs / RTP ( i  i 1+ b1Pexp− RT )  1  1 1 When b1P >>1 and b1Pexp(-i/RT) <<1, we have θ =c1ln(c2P), where c1 & c2 are constants Valid for some adsorption systems. 211
  208. 208. Adsorption On Solid Surface The Freundlich isotherm rads=kads(1-θ)P ≡ rdes=kdesθ B0P b1eQi / RTP θ= ⇒ θi = 1+ B0P 1+b1eQi / RTP ∆H of ads assuming logarithmic change of adsorption enthalpy ∆H with surface coverage From ads-des equilibrium, ads. rate ≡ des. rate Langmuir Freundlic h θ where Qi is the heat of adsorption which is a function of θi. If there are Ni types of surface sites, each can be expressed as Ni=aexp(-Q/Q0) (a and Q0 are constants), corresponding to a fractional coverage θi, θi Ni ∞[b eQ/ RTP / (1+b eQ/ RTP)]⋅ aeQ/Q0 dQ the overall coverage 1 1 0 i ∑ ∫ θ= = ∑N i i ∞ ∫ ae 0 Q/Q 0 dQ the solution for this integration expression at small θ is: lnθ=(RT/Q0)lnP+constant, or as is the Freundlich equation normally written, θ = c1 p1/ C2 1/c2=RT/Q0 where c1=constant, Freundlich isotherm fits, not all, but many adsorption systems. 212
  209. 209. Adsorption On Solid Surface BET (Brunauer-Emmett-Teller) isotherm Many physical adsorption isotherms were found, such as the types II and III, that the adsorption does not complete the first layer (monolayer) before it continues to stack on the subsequent layer (thus the S-shape of types II and III isotherms) Basic assumptions the same assumptions as that of Langmuir but allow multi-layer adsorption the heat of ads. of additional layer equals to the latent heat of condensation based on the rate of adsorption=the rate of desorption for each layer of ads. the following BET equation was derived P/ P 1 c −1 0 = + (P / P ) 0 V(1− P / P ) cV cV 0 m m Where P - equilibrium pressure P0 - saturate vapour pressure of the adsorbed gas at the temperature P/P0 is called relative pressure V - volume of adsorbed gas per kg adsorbent Vm -volume of monolayer adsorbed gas per kg adsorbent c - constant associated with adsorption heat and condensation heat Note: for many adsorption systems c=exp[(H1-HL)/RT], where H1 is adsorption heat of 1st layer & HL is liquefaction heat, so that the adsorption heat can be determined from constant c. 213
  210. 210. Adsorption On Solid Surface Comment on the BET isotherm BET equation fits reasonably well all known adsorption isotherms observed so far (types I to V) for various types of solid, although there is fundamental defect in the theory because of the assumptions made (no interaction between adsorbed molecules, surface homogeneity and liquefaction heat for all subsequent layers being equal). BET isotherm, as well as all other isotherms, gives accurate account of adsorption isotherm only within restricted pressure range. At very low (P/P0<0.05) and high relative pressure (P/P0>0.35) it becomes less applicable. The most significant contribution of BET isotherm to the surface science is that the theory provided the first applicable means of accurate determination of the surface area of a solid (since in 1945). Many new development in relation to the theory of adsorption isotherm, most of them are accurate for a specific system under specific conditions. 214
  211. 211. Adsorption On Solid Surface Use of BET isotherm to determine the surface area of a solid At low relative pressure P/P0 = 0.05~0.35 it is found that P/ P 1 c −1 0 = + (P / P ) ∝(P / P ) 0 0 V(1− P / P ) cV cV 0 m m Y = a +b X P / P0 V (1− P / P0 ) The principle of surface area determination by BET method: A plot of P / P0 V (1− P / P0 ) P/P0 against P/P0 will yield a straight line with slope of equal to (c- 1)/(cVm) and intersect 1/(cVm). For a given adsorption system, c and Vm are constant values, the surface area of a solid material can be determined by measuring the amount of a particular gas adsorbed on the surface with known molecular cross-section area Am, V Vm - volume of monolayer adsorbed gas molecules calculated from the As = AmNm = Am m × 6022 ×1023 . plot, L VT,P VT,P - molar volume of the adsorbed gas, L/mol Am - cross-section area of a single gas molecule, m2 * In practice, measurement of BET surface area of a solid is carried out by N2 physisorption at liquid N2 temperature; for N2, Am = 16.2 x 10-20 m2 215
  212. 212. Adsorption On Solid Surface Summary of adsorption isotherms Name Note Langmuir Temkin Freundlich BET Isotherm equation θ= Cs BP = 0 C∞ 1+ B0P θ =c1ln(c2P) θ =c1p1/ C 2 Application Chemisorption and physisorption Useful in analysis of reaction mechanism Chemisorption Chemisorption Chemisorption and physisorption Easy to fit adsorption data P/ P 1 c −1 0 = + (P / P ) Multilayer physisorption 0 V(1− P / P ) cV cV 0 m m Useful in surface area determination 216
  213. 213. Mechanism of Surface Catalysed Reaction Langmuir-Hinshelwood mechanism A + B This mechanism deals with the surface-catalysed reaction in which that 2 or more reactants adsorb on surface without dissociation A(g) + B(g) The rate of reaction A(ads) + B(ads) P (the desorption of P is not r.d.s.) ri=k[A][B]=kθAθB B0,A PA  θA =   From Langmuir adsorption isotherm (the case III) we know 1+ B0,A PA + B0,B PB  B0,B PB θ B = We then have 1 + B0,A PA + B0,B PB      B0,A PA B0,B PB kB0,A B0,B PA PB  = ri = k  1 + B P + B P  1 + B P + B P  1 + B P + B P 0 ,A A 0 ,B B  0 ,A A 0 ,B B  0 ,A A 0 ,B B  When both A & B are weakly adsorbed (B0,APA<<1, B0,BPB<<1), ri = kB0,A B0,B PA PB = k' PA PB 2nd order reaction When A is strongly adsorbed (B0,APA>>1) & B weakly adsorbed (B0,BPB<<1 <<B0,APA) ri = kB0,A B0,B PA PB = kB0,B PB = k' ' PB B0,A PA 1st order w.r.t. B 217 P
  214. 214. Mechanism of Surface Catalysed Reaction Eley-Rideal mechanism B A P This mechanism deals with the surface-catalysed reaction in which that one reactant, A, adsorb on surface without dissociation & other reactant, B, approaching from gas to react with A + B(g) A(g) A(ads) P (the desorption of P is not r.d.s.) The rate of reaction ri=k[A][B]=kθAPB From Langmuir adsorption isotherm (the case I) we know θ A = B0,A PA 1+ B0,A PA  B P  kB P P ri = k  0,A A PB = 0,A A B  1+ B P  1 + B0,A PA 0,A A   When both A is weakly adsorbed or the partial pressure of A is very low (B0,APA<<1), ri = kB0,A PA PB = k' PA PB 2nd order reaction We then have When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1) ri = kB0,A PA PB = kP B B0,A PA 1st order w.r.t. B 218
  215. 215. Mechanism of Surface Catalysed Reaction Mechanism of surface-catalysed reaction with dissociative adsorption The mechanism of the surface-catalysed reaction in which one reactant, AD, dissociatively adsorbed on one surface site AD(g) A(ads) + D(ads)+ B(g) P B A B P (the des. of P is not r.d.s.) The rate of reaction ri=k[A][B]=kθADPB From Langmuir adsorption isotherm (the case I) we know We then have (B0,ADPAD )1/ 2 P = k (B0,ADPAD )1/ 2 PB ri = k 1/ 2 B 1/ 2 1 + (B0,AD PAD ) 1 + (B0,ADPAD ) (B0,ADPAD )1/ 2 θ AD = 1/ 2 1 + (B0,ADPAD ) When both AD is weakly adsorbed or the partial pressure of AD is very low (B0,ADPAD<<1), 1/ ri = k (B0,ADPAD ) PB = k' PAD2 PB 1/ 2 w.r.t. B The reaction orders, 0.5 w.r.t. AD and 1 When A is strongly adsorbed or the partial pressure of A is very high (B0,APA>>1) 1/ 2 ri = k (B0,AD PAD ) PB (B0,ADPAD ) 1/ 2 = kP B 1st order w.r.t. B 219
  216. 216. Mechanism of Surface Catalysed Reaction Mechanisms of surface-catalysed rxns involving dissociative adsorption In a similar way one can derive mechanisms of other surface-catalysed reactions, in which dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another associatively adsorbed reactant B on a separate surface site dissociatively adsorbed one reactant, AD, (on one surface site) reacts with another dissociatively adsorbed reactant BC on a separate site … The use of these mechanism equations Determining which mechanism applies by fitting experimental data to each. Helping in analysing complex reaction network Providing a guideline for catalyst development (formulation, structure,…). Designing / running experiments under extreme conditions for a better control … 220
  217. 217. Need to ask ?
  218. 218. Quantachrome INSTRUMENTS Chemisorption 3 © 2004 Quantachrome Instruments
  219. 219. 3. Chemisorption Techniques 3.1 Introduction: Physisorption/Chemisorption 3.2 Classical Models 3.3 Active Metal Area Measurement 3.4 Adsorption Thermodynamics 3.5 Pulse vs. Static 3.6 Temperature Programmed Analyses © 2004 Quantachrome Instruments
  220. 220. The Nature of Gas Sorption at a Surface • When the interaction between a surface and an adsorbate is relatively weak only physisorption takes place. • However, surface atoms often possess electrons or electron pairs which are available for chemical bond formation. • This irreversible adsorption, or chemisorption, is characterized by large interaction potentials which lead to high heats of adsorption. © 2004 Quantachrome Instruments
  221. 221. Physisorption vs Chemisorption Property Physisorption Chemisorption Forces van der Waals Chemical bonding < 40 50-200 Rare 60–100 Isothermal Reversibility Complete Slow or none Extent Multilayers Monolayer ∆Hads (kJ mol-1) Ea (kJ mol-1) © 2004 Quantachrome Instruments
  222. 222. On The Nature of Chemisorption • Chemisorption is often found to occur at temperatures far above the critical temperature of the adsorbate. • As is true for most chemical reactions, chemisorption is usually associated with an activation energy, which means that adsorbate molecules attracted to a surface must go through an energy barrier before they become strongly bonded to the surface. © 2004 Quantachrome Instruments
  223. 223. Adsorption Potentials P otential E nergy P Hp Hc A C Potential energy curves for molecular (non-dissociative) adsorption © 2004 Quantachrome Instruments
  224. 224. Adsorption Potentials Potential Energy P A X+X Hdissoc. Hact. X2 C Potential energy curves for activated adsorption © 2004 Quantachrome Instruments
  225. 225. Adsorption Potentials Potential Energy P X+X Hdissoc. X2 C A Potential energy curves for non-activated adsorption © 2004 Quantachrome Instruments
  226. 226. Isobars (b) Quantity adsorbed (c) (a) Temperature Isobaric variation in quantity adsorbed with temperature. Physisorption isobar (a) represents lower heat of adsorption than chemisorption isobar (b). © 2004 Quantachrome Instruments
  227. 227. On The Nature of Chemisorption • Because chemisorption involves a chemical bond between adsorbate and adsorbent, unlike physisorption, only a single layer of chemisorbed species can be realized on localized active sites such as those found in heterogeneous catalysts. • However, further physical adsorption on top of the chemisorbed layer and diffusion of the chemisorbed species into the bulk solid can obscure the fact that chemisorbed material can be only one layer in depth © 2004 Quantachrome Instruments
  228. 228. Quantachrome INSTRUMENTS Classical Models 3.2 © 2004 Quantachrome Instruments
  229. 229. 3.2 Classical Models 3.2.1 Langmuir 3.2.2 Freundlich 3.2.3 Temkin © 2004 Quantachrome Instruments
  230. 230. Adsorption Process Adsorbate Adsorptive Active Sites (Adsorbent) © 2004 Quantachrome Instruments
  231. 231. Irving Langmuir (1881-1957) Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903 1903-1906 M.A. and Ph.D. in 1906 from Göttingen. 1906-1909 Instructor in Chemistry at Stevens Institute of Technology, Hoboken, New Jersey. 1909 –1950 General Electric Company at Schenectady where he eventually became Associate Director 1913 -Invented the gas filled, coiled tungsten filament incandescent lamp. 1919 to 1921, his interest turned to an examination of atomic theory, and he published his "concentric theory of atomic structure" . In it he proposed that all atoms try to complete an outer electron shell of eight electrons © 2004 Quantachrome Instruments
  232. 232. Irving Langmuir (1881-1957) 1927 Coined the use of the term "plasma" for an ionized gas. 1932 The Nobel Prize in Chemistry "for his discoveries and investigations in surface chemistry" 1935-1937 With Katherine Blodgett studied thin films. 1948-1953 With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation. © 2004 Quantachrome Instruments
  233. 233. 3.2.1 Langmuir’s “Kinetic” Approach rate of adsorption = ka P(1-θ) where θ is the fraction of the surface already covered with adsorbate, i.e.,θ = V/Vm rate of desorption = kd θ Suggests a dynamic equilibrium. Is it? © 2004 Quantachrome Instruments
  234. 234. Langmuir (continued…) At equilibrium (any pressure) ka P(1-θ) = kd θ from which θ = V/Vm = KP/(1+KP) where K = ka / kd. In its linear form, the above equation can be expressed as: 1/V = 1/Vm + 1/(VmKP) © 2004 Quantachrome Instruments
  235. 235. Or, if you prefer… Confining adsorption to a monolayer, the Langmuir equation can be written V KP = Vm 1 + KP where V is the volume of gas adsorbed at pressure P, Vm is the monolayer capacity (i.e. θ=1) expressed as the volume of gas at STP and K is a constant for any given gas-solid pair. Rearranging in the form of a straight line (y=ab+x) gives P 1 P = + V KVm Vm © 2004 Quantachrome Instruments
  236. 236. Langmuir Plot 1/V Slope = 1/(VmK) Intercept = 1/Vm 1/P 1/V = 1/Vm + 1/(VmKcP1/s) © 2004 Quantachrome Instruments
  237. 237. Temperature Dependent Models generally K = Ko exp(q/RT) where Ko is a constant, R is the universal gas constant, T is the adsorption temperature and q is the heat of adsorption • Langmuir:K is constant;q is constant at all θ • Temkin: assumed that q decreases linearly with increasing coverage • Freundlich: assumed that q decreases exponentially with increasing coverage © 2004 Quantachrome Instruments
  238. 238. Temkin Temkin assumed that q decreases linearly with increasing coverage, that is, Q=qo(1- λ θ) Where qo is a constant equal to the heat of adsorption at zero coverage (θ = 0) and λ is a proportionality constant. © 2004 Quantachrome Instruments
  239. 239. Temkin θ = A ln P + B or, since θ = V/Vm V = Vm A lnP + VmB Where A = RT/qo λ θ and B = A ln Ko + 1/ λ θ © 2004 Quantachrome Instruments
  240. 240. Temkin Plot V Slope = VmA Intercept = VmB Ln(P) V = Vm A lnP + VmB © 2004 Quantachrome Instruments
  241. 241. * m Multiple Temkin Plots to find V extrapolated V experimental * denotes “temperature invariant” or “thermally irreversible” quantity Ln(P) Temp H Temp M © 2004 Quantachrome Instruments Temp L
  242. 242. Freundlich Temkin assumed that q decreases exponentially with increasing coverage, that is, Q = -qm lnθ Where qm is a constant equal to the heat of adsorption at θ = 0.3679 © 2004 Quantachrome Instruments
  243. 243. Freundlich lnθ = C lnP + D or, since θ = V/Vm ln(V/Vm) = C lnP + D Where C=RT/ qm and D = C lnKo © 2004 Quantachrome Instruments
  244. 244. Freundlich (continued…) Ln(V) Slope = C Intercept = D + ln(Vm) Ln(P) Ln(V/Vm) = C lnP + D © 2004 Quantachrome Instruments
  245. 245. * m Multiple Temkin Plots to find V extrapolated Ln(V) experimental * denotes “temperature invariant” or “thermally irreversible” quantity Ln(P) Temp H Temp M © 2004 Quantachrome Instruments Temp L
  246. 246. Quantachrome INSTRUMENTS Active Metal Area 3.3 © 2004 Quantachrome Instruments
  247. 247. 3.3 Active Metal Area 3.3.1 Principles of Calculation 3.3.2 Choice of Adsorbate 3.3.3 Active Site Size Calculation 3.3.4 Metal Dispersion 3.3.5 Accessible vs non-accessible sites © 2004 Quantachrome Instruments
  248. 248. Active Site Quantification • Because the formation of a chemical bond takes place between an adsorbate molecule and a localized, or specific, site on the surface of the adsorbent, the number of active sites on catalysts can be determined simply by measuring the quantity of chemisorbed gas © 2004 Quantachrome Instruments
  249. 249. Active Site on a Catalyst • Metal on support. • Island-like crystallites • Not all metal atoms exposed. • Adsorption technique perfectly suited. (cf Chemical analysis of entire metal content ) © 2004 Quantachrome Instruments
  250. 250. 3.3.1 Principles of Calculation Monolayer Volume, Vm = volume of gas chemisorbed in a monomolecular layer © 2004 Quantachrome Instruments
  251. 251. Methods to Determine Vm = volume of gas chemisorbed in a monomolecular layer •Extrapolation • Bracketing • Langmuir • Temkin • Freundlich © 2004 Quantachrome Instruments
  252. 252. Extrapolation method First (only?)isotherm Volume Adsorbed Vm Pressure (mm Hg) © 2004 Quantachrome Instruments
  253. 253. The second isotherm Volume Adsorbed combined Weak only Pressure (mm Hg) © 2004 Quantachrome Instruments
  254. 254. The difference isotherm Volume Adsorbed combined Weak only Strong Pressure (mm Hg) © 2004 Quantachrome Instruments
  255. 255. Vm from Pulse Titration … will be covered in 3.5.2 © 2004 Quantachrome Instruments
  256. 256. Metal Area Calculation To Calculate Metal Surface Area: A = (Vm) x (MXSA) x (S) x 6.03 x 10-3 (units m2/g) where MXSA = metal cross sectional area (Å2) and S = stoichiometry = metal atoms per gas molecule To calculate metal area per gram of metal, Am: Am = A x l00/L where L = metal loading (%) = known value from chemical analysis © 2004 Quantachrome Instruments
  257. 257. Stoichiometry The gas-sorption stoichiometry is defined as the number of metal atoms with which each gas molecule reacts. Since, in the gas adsorption experiment to determine the quantity of active sites in a catalyst sample, it is the quantity of adsorbed gas which is actually measured, the knowledge of (or at least a reasonably sound assumption of) the stoichiometry involved is essential in meaningful active site determinations (area, size, dispersion). © 2004 Quantachrome Instruments
  258. 258. 3.3.2 Choice of Adsorbate Chemisorption Physisorption • CO or H2 on Pt, Pd • N2 at 77K at 40 oC • Ar at 87K • CO or H2 on Ni • Kr at 77K • CO2 at 273K For metal-only area For total surface area (& dispersion etc) and pore size © 2004 Quantachrome Instruments
  259. 259. 3.3.3 Active Site Size Calculation To calculate average crystallite size: d = (L x 100 x f )/AD (units Å) where f = shape factor = 6 ρ = density of metal (g/ml) © 2004 Quantachrome Instruments
  260. 260. Shape Factor & Crystallite Size The default shape factor of 6 is for assumed cubic geometry. Consider a cube of six sides (faces) each of length l. then the total surface area, ΣA = 6l2. The volume of the cube is given by l3 or, in terms of total area, substitute ΣA /6 for l2 to give V= lΣA/6 For a cube whose mass is unit mass, its volume is given by 1/ ρ (where ρ is the density of the material). V=1/ρ © 2004 Quantachrome Instruments
  261. 261. Shape Factor & Crystallite Size For the same cube of unit mass, the area is then the area per unit mass A and l is rewritten d (crystallite size), the length required to give a cube whose mass is unity. Equating both terms for volume: dA/6=1/ ρ or d=6/A ρ For a supported metal, the loading, L, must be taken into consideration. d=L6/A ρ Other geometries can be treated in a similar fashion. For example, a rectangular particle whose length is three times its width has a shape factor of 14/3. © 2004 Quantachrome Instruments
  262. 262. 3.3 Metal Dispersion Supported metals It is most likely that the catalyst exists as a collection of metal atoms distributed over an inert, often refractory, support material such as alumina. At the atomic level it is normal that these atoms are assembled into island-like crystallites on the surface of the support. © 2004 Quantachrome Instruments
  263. 263. 3.3 Metal Dispersion • In the case of supported metal catalysts, it is important to know what fraction of the active metal atoms is exposed and available to catalyze a surface reaction. • Those atoms that are located inside metal particles do not participate in surface reactions, and are therefore wasted. © 2004 Quantachrome Instruments

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