Surface Area, Pore Size and More :

1. Surface Area, Pore Size and More:  Theory and Application of Porous Materials Characterization Methods Gas Adsorption Measurements with particular focus on Microporous Materials Liquid Intrusion Porosimetry with particular focus on Meso- and Macroporous Materials Other Methods of Pore Size measurement - Capillary Flow Porometry and Electroacoustics Catalyst Characterization using Chemisorption and Temperature Programmed Analyses Dynamic Water Vapor Sorption - Adsorption, Absorption, Hydrophobicity, Hydrophilicity

2. Inert Gas Adsorption • What can be measured using this technique? • Who would be interested in such results? • A brief overview of measurement fundamentals. • Microporous materials • Carbons • Zeolites • Metal organic frameworks • Instrument selection for these materials • Specific features of benefit to analyzing microporous materials • Mesoporous/nonporous materials • Carbon black • Ceramics • Pigments • Alumina • Silica • Metal powders • Pharmaceuticals • Instrument selection for these materials • Specific features of benefit to analyzing meso-/nonporous materials

3. Inert Gas Adsorption • What can be measured using this technique? • Specific Surface Area • How low? • Depends on instrument sensitivity and amount of sample (more later!) • How high? • No limit • Pore Size Distribution • Min, max? • As small as the smallest gas molecule that can be adsorbed • Pore Volume • No limit • Heats of Adsorption • More later

4. Inert Gas Adsorption • Who would be interested in such results? • Everyone who needs to understand how pore structure affects material performance. • Surface Area • affects dissolution rates. • affects electron/ion current density at electrode interface with electrolyte. • affects adsorption capacity. • represents surface free energy available for bonding in tabletting and sintering.

5. Inert Gas Adsorption • Who would be interested in such results? • Everyone who needs to understand how pore structure affects material performance. • Pore Size Distribution • affects diffusion rates. • affects molecular sieving properties. • affects surface area per unit volume.

6. Measurement Overview • Two techniques available • Dynamic flow (uses different concentrations of the adsorbing gas, i.e. gas mixtures)… this will only be covered in discussion session • Vacuum-volumetric, better to say “Manometric” (uses different pressures of the adsorbing gas)… our main focus

7. What is a Gas Sorption Analyzer? • Does it actually measure surface area and pore size? • NO!! It simply records various pressures of gas in the sample cell due to adsorption and desorption. The instrument then calculates the amount (as STP volume) of gas adsorbed/desorbed.Surface area, pore size are calculated by PC software (iQWin, NovaWin, Quadrawin). • Pressure measurements are critical!

8. Adsorption/Desorption • Adsorption is the sticking of gas molecules onto the surface of a solid… all available surfaces including that surface inside open pores. • Increasing the pressure of gas over a solid causes increasing adsorption. • Temperature dependent

9. Adsorption/Desorption • Desorption is the removal of gas molecules from the surface of a solid… all available surfaces including that surface inside open pores. • Decreasing the pressure of gas over a solid causes increasing desorption. • Done at same temperature as adsorption.

10. Movie time!

11. So, How Does It Work? • Basic Construction • Removable sample cell • Dosing manifold • Pressure transducers • Vacuum system • Analysis gas • Valves to move gas in and out of manifold and sample cell • Sample thermostat (dewar, furnace, cryostat)

12. So, How Does It Work? • Basic Construction • Removable sample cell • A long stemmed piece of glassware that holds the sample during degassing (preparation) and analysis. • Available in different stem diameters and bulb sizes.

13. So, How Does It Work? • Basic Construction • Dosing manifold • A chamber of known (i.e. calibrated) physical volume from which gas is added to and removed from the sample cell during adsorption and desorption respectively (think burette).

14. So, How Does It Work? • Basic Construction • Pressure transducers • Used to both quantitatively determine the amount of gas adsorbed/desorbed and the pressures at which the sorption is measured.

15. So, How Does It Work? • Basic Construction • Vacuum system • Vacuum pump(s) generate sub-atmospheric pressure conditions. • Rotary oil pumps for low vacuum applications. • Turbo pump backed by oil-free diaphragm pump for high vacuum applications.

16. So, How Does It Work? • Basic Construction • Analysis gas • Nitrogen is used most often. • Argon is recommended for micropore size measurements. • Krypton is used for very low surface area and thin film applications. • Multiple gases can be connected at one time, though only one is actively used.

17. So, How Does It Work? • Basic Construction • Valves to move gas in and out of manifold and of sample cell • Automatically operated to fill the dosing manifold to a pressure sufficient to yield a datum point at a specified target pressure (or at target sorbed amount) • Magnetic latching valves… no heat generated during pressure equilibration

18. So, How Does It Work? • Basic Construction • Sample thermostat (dewar, furnace, cryostat) • Dewar holds cryogenic liquids (liquefied gases) like liquid nitrogen (LN2) and liquid argon (LAr) • Furnace: used for chemisorption measurements at temperatures above ambient • Cryostat: for advanced research applications, overcomes limitations of restricted choice of temperatures available with liquefied gases in a dewar.

19. Basic Construction Pressure transducer(s) Analysis gas to vacuum Manifold Sample cell

20. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Manifold, transducer and sample cell are evacuated. Sample cell

21. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Manifold, transducer and sample cell are evacuated… and cell is cooled. Sample cell

22. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Intermediate valve status. Sample cell

23. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Analysis gas is admitted to build some pressure in the manifold. Sample cell

24. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold A steady pressure in the manifold is recorded, P1. Sample cell

25. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Gas expands from manifold into sample cell; pressure drops in the manifold, rises in sample cell. Sample cell

26. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Gas is adsorbed by the sample, pressure drops further in both volumes. Sample cell

27. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Eventually the pressure equilbrates. Final pressure, P2, is recorded. Sample cell

28. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Process is repeated at higher and higher pressures. Sample cell

29. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Adsorption measurements are complete... Getting ready to desorb! Sample cell

30. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold In desorption, some gas is removed from the manifold while the sample cell remains isolated. Sample cell

31. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Manifold is isolated and desorption P1 is measured. Sample cell

32. Basic Operation Pressure transducer(s) Analysis gas to vacuum Manifold Gas is expanded from sample cell into manifold, pressure drops in the sample cell, rises in the manifold… P2 (desorption) Sample cell

33. A More Realistic Representation

34. Sample Temperature Control • As the coolant evaporates, the level sensor signals the dewar drive to compensate for the change in level, thereby maintaining a constant and small cold zone. cabinet level sensor sample cell 90 hr dewar drive shaft dewar support arm

35. What’s Really Measured • The pressure of gas notcurrently adsorbed by the sample, just filling thevoid volume. • To know quantitatively what is adsorbed, the instrument calculates: • The dose amounts, i.e. amount of gas moved into (adsorption) or out of (desorption) the cell by the end of an equilibration period. • The amount of gas remaining unadsorbed (in the void volume) at that time. • The difference is what is adsorbed.

36. What’s Measured • To calculate the gas amounts dosed (in/out) the instrument must know: • P1 • P2 • Volume of the manifold • Temperature of the manifold

37. What’s Measured • To know the volume of the manifold: • it is calibrated using a special cell and glass cyclinder (rod). • All instrument manifolds are factory calibrated. • To know the temperature of the manifold: • it is constantly monitored by a solid state sensor.

38. What’s Measured • To calculate the gas amounts not adsorbed the instrument must know: • Volume of the void volume (sample cell) • Temperature of the void volume (sample cell)

39. What’s Measured • To know the volume of the sample cell the instrument can: • Measure it by expanding helium from the manifold (as part of initializing the analysis) • Use a previously measured value • Use a stored value based on expanding nitrogen into an empty cell, correcting for sample volume (the so-called NOVA method)

40. What’s Measured • To know the temperature of the sample cell (in coolant) the instrument: • is told it as an analysis parameter. • To ensure that the volume of cell in coolant remains constant: • a coolant level sensor and dewar elevator mechanism combine to maintain level of coolant around the sample cell.

41. Small Cold Zone = Sensitivity Level sensor Coolant level controlled here creates a small cold zone. Quantachrome’s instruments

42. Working Equation PV = nRT nads = ndosed - nvoid nads = (PV/RT)man. - (PV/RT)cell

43. Refinements • Corrections for “non-ideality” of gas, especially at cryogenic temperatures. • Compensation for the slight change in temperature of that part of the sample cell not in coolant (“TempComp”). • Determination of “saturation vapor pressure” of the coolant, known as Po.

44. What Is The Result? It’s called an “isotherm” Amount adsorbed Equilibrium pressure

45. What Is The Result? The values on the y-axis are calculated frompressure measurements (and temperature values) Amount adsorbed The values on the x-axis arepressure measurements. Equilibrium pressure

46. What Is The Result? Desorption curve may overlay on, or appear to left of, the adsorption curve Amount adsorbed The values on the x-axis are in fact expressed as relative pressure, P/Po Relative pressure

47. Very Low Pressure Behavior (micropore filling) Amount Adsorbed Relative Pressure (P/Po)

48. Low Pressure Behavior (monolayer) The “knee” Amount Adsorbed Relative Pressure (P/Po)

49. Medium Pressure Behavior (multilayer) Amount Adsorbed Relative Pressure (P/Po)

50. High Pressure Behavior (capillary condensation) Amount Adsorbed Relative Pressure (P/Po)