Slide 1: Introduction to Gas Adsorption
HORIBA Scientific Particle Characterization Introduction to Gas Adsorption Or everything you ever wanted to know about physisorption but were afraid to ask! John A. Guerin Key Account Business Development Manager 12/15/2022 © 2022 HORIBA, Ltd. All rights reserved.
Slide 2: What is gas adsorption
What is gas adsorption • Adsorption – the accumulation of a substance at a surface of a material • Gas adsorption – the build up of gas molecules on the surface of a solid, which is reversable • Not to be confused with aBsorption – the accumulation and distribution of a substance throughout a materi
Slide 3: Why is it important
Why is it important • Adsorption occurs in nature • Materials which separate gases (gas purification, gas separation, poison gas protection – gas masks) • The surface of pharmaceutical APIs and excipients affects performance • Activated carbons adsorb impurities from water for purification • Paints,
Slide 4: What causes a gas to adsorb
What causes a gas to adsorb • Van der Waals forces – weak intermolecular forces which increase as molecules become closer together • Within a material, Van der Waals forces are satisfied by the neighboring molecules of the substance • However, at the edge of a solid or the interface with surrounding
Slide 5: Some definitions
Some definitions • Adsorptive – gas available to be adsorbed • Adsorbate – gas which has been adsorbed • Adsorbent – the material adsorbing the gas Adsorptive Adsorbate Adsorbent © 2022 HORIBA, Ltd. All rights reserved.
Slide 6: What causes a gas to adsorb continued
What causes a gas to adsorb continued • In nature, at this interface or edge of a solid, the Van der Waals forces attract water molecules or other vapors from the surrounding atmosphere. • This satisfies the unmet potential energy • Flat surfaces are lowest in adsorption energy • Adsorption energy i
Slide 7: Classification of pores
Classification of pores • Macropores – > 50 nm diameter • Mesopores – 2 to 50 nm diameter • Micropores – < 2 nm diameter • Generally, materials with larger pore sizes tend to have lower total surface area. • Materials with smaller pore sizes, alternatively, tend to have higher surface areas. • And o
Slide 8: What else causes adsorption to occur
What else causes adsorption to occur • In a gas adsorption experiment, the sample, which has been placed in a sample holder and appropriately prepared (we’ll get to that), is immersed in a cryogen (typically LN2) which cools the surface of the sample – in the case of LN2, to approximately 77.3K. • G
Slide 9: The Adsorption Process
The Adsorption Process Increasing gas pressure causes the beginning of multi-layer At low pressure, isolated sites coverage. Smaller pores in the on the sample surface begin sample will fill first. The BET to adsorb gas molecules equation is used to calculate the surface area. As gas pressure increa
Slide 10: What effect does particle size have on surface area?
What effect does particle size have on surface area? A 1 cm3 cube would Trisecting the same cube measure 6 cm2 in area into 8, ½ x ½ cm cubes 1 x 1 x 6 faces creates 12 cm2 of area ½ x ½ x 6 faces = 1.5 cm2 1.5 x 8 = 12 cm2 © 2022 HORIBA, Ltd. All rights reserved. 10
Slide 11: A few additional terms
A few additional terms • Isotherm – the raw data from a test at constant temperature (typically 77.3K) plotted as volume of gas adsorbed (y axis) vs pressure (x). • Saturation pressure – the pressure of a gas which is in equilibrium with its liquid – expressed as Po • Relative pressure – a means to
Slide 12: Types of Isotherms as defined by IUPAC
Types of Isotherms as defined by IUPAC Microporous solids with limited external Microporous solids having pores over a surfaces e.g., activated carbon, zeolites, wider range – possibly as large as 2.5 nm. porous oxides. Nonporous or macroporous solids. Point Rarely seen. Here, interaction B if sharp
Slide 13: What data can I obtain?
What data can I obtain? • The Isotherm is the raw data – again, volume of gas adsorbed (y) vs pressure (normally expressed as relative pressure – P/Po) (x) • Using different ranges of the pressure axis, different characteristics of a material may be calculated. • Surface area – Langmuir, BET (multi-
Slide 14: Isotherms – most common types
Isotherms – most common types Filling of macropores Vads Vads Vads P/Po P/Po P/Po Micropore filling at low Build up of gas on Development of Filling of mesopores pressures surface / higher multi-layer (adsorption) and emptying energy sites (filling adsorption (desorption) may of the monolayer) demon
Slide 15: Data collection
Data collection • In this presentation we will focus upon surface area calculations • Early work by Langmuir (1919) on charcoal indicated that gas molecules adsorb in only a single layer after which negligible adsorption occurred • Brauneur, Emmitt and Teller (BET) in 1938 theorized that multi-layer
Slide 16: Adsorption layers
Adsorption layers Ideal ith layer 3rd layer Actual 2nd layer 1st layer © 2022 HORIBA, Ltd. All rights reserved. 16
Slide 17: The BET equation (in linear form)
The BET equation (in linear form) 𝑷𝑷 𝑪𝑪−𝟏𝟏 𝑷𝑷 𝟏𝟏 = + 𝑽𝑽𝒂𝒂 𝑷𝑷𝒐𝒐−𝑷𝑷 𝑽𝑽𝒎𝒎𝑪𝑪 𝑷𝑷𝒐𝒐 𝑽𝑽𝒎𝒎𝑪𝑪 y = m(x) + b Where: Va = Volume of adsorbed gas P = Absolute pressure Po = Saturation pressure Vm = Mono-layer gas volume – what we’re seeking! 𝐸𝐸1 −𝐸𝐸𝐿𝐿 C = BET C constant…
Slide 18: The BET equation (linear) for multi-point analysis
The BET equation (linear) for multi-point analysis 𝑷𝑷 𝑪𝑪−𝟏𝟏 𝑷𝑷 𝟏𝟏 = + 𝑽𝑽𝒂𝒂 𝑷𝑷𝒐𝒐−𝑷𝑷 𝑽𝑽𝒎𝒎𝑪𝑪 𝑷𝑷𝒐𝒐 𝑽𝑽𝒎𝒎𝑪𝑪 y = m (x) + b 𝑃𝑃 𝑉𝑉𝑎𝑎[(𝑃𝑃0 −P) 𝐶𝐶 − 1 𝑉𝑉𝑚𝑚 𝐶𝐶 1 𝑣𝑣𝑚𝑚 𝑐𝑐 𝑃𝑃 © 2022 HORIBA, Ltd. All rights reserved. 𝑃𝑃0 18
Slide 19: And from this linear regression
And from this linear regression C −1 slope = Vm C 1 intercept = Vm C 1 Vm = slope + intercept slope C=1+ intercept © 2022 HORIBA, Ltd. All rights reserved. 19
Slide 20: Calculating surface area from Vm
Calculating surface area from Vm 𝑉𝑉𝑚𝑚 ∗ 𝐿𝐿 ∗ α𝑚𝑚 𝑆𝑆𝐵𝐵𝐵𝐵𝐵𝐵 = 𝑀𝑀 Where: SBET – Total surface area under test Vm – Monolayer volume L – Avagadro’s number - 6.02 x 1023 molecules/mol αm – Cross sectional area of gas molecule - 0.162 nm2 for N2 M – molar volume – 22414 L / mol Dividing…
Slide 21: Single point calculation from the BET equation
Single point calculation from the BET equation • When C is very large – 1/VmC approaches 0 • Meaning y-intercept is very close to 0 • Regression can be forced through the origin without substantial error (change to slope) • Likewise, C-1≈ C. Thus, C-1/VmC becomes 1/Vm , simplifying our equation to:
Slide 22: So how do we use all of this?
So how do we use all of this? • We take advantage of these Van der Waals forces • We prepare a sample by removing these adsorbed molecules thus “reenergizing” the surface of the material we wish to measure. • Next, we perform a controlled adsorption of gas on the surface to measure it Horiba SA-9603
Slide 23: Preparation options
Preparation options • Preparation of a sample requires − Time − Elevated temperature * − Clean environment (in a sample tube) Note: Rule of thumb – apply temperature which will not change the structure of the material but will drive off adsorbed water / vapor. Note 2: If prep temp is unknown, a gene
Slide 24: How does preparation work? AKA Degassing
How does preparation work? AKA Degassing • Typically, a sample is placed in a clean sample tube. • The sample tube is connected to an instrument and a source of heat • Heat is then applied, and while being heated…. • A source of dry gas flows through the sample tube Sample tube with sample Sample wi
Slide 25: That's interesting but what really happens?
That’s interesting but what really happens? • As the temperature rises, the molecules adsorbed on the surface are heated • They begin to move more rapidly and overcome the weak Van der Waals forces • Then as they break free from the surface of the sample material, they are swept away by the flowing
Slide 26: Types of gas adsorption instruments
Types of gas adsorption instruments • Volumetric – manifold of known volume, volume of sample tube known, PV=nRT Gas dosed according to pre-defined table of pressures or volumes • Dynamic – flowing gas, TCD on gas inlet and on gas outlet. Conductivity difference of gas on inlet vs outlet measured as
Slide 27: Comparison of gas adsorption instruments
Comparison of gas adsorption instruments Volumetric Dynamic Gravimetric • Requires volume • Typically, very fast (for • Uses a microbalance in a calibration data single point SA) “chamber” • Typically, slow, step • Good tool for in process • Very small amount of wise data collection control of produ
Slide 28: Why does Horiba promote flowing gas / dynamic?
Why does Horiba promote flowing gas / dynamic? • Dynamic instruments are of straightforward design • Lower cost to purchase • Very robust and reliable • Very low maintenance costs • Extremely fast data collection • Excellent choice for production environments • Self sealing bulkhead fittings avoid c
Slide 29: Now to the measurement
Now to the measurement We must know the precise mass of the sample we are testing – but it must be the mass after preparation! • This requires a tare weight of the empty sample cell • A known mass of sample is added (Many times people record this mass.) • Preparation (degassing) of the sample takes
Slide 30: The Horiba approach – Dynamic gas alternative
The Horiba approach – Dynamic gas alternative The SA-9600 Series of Surface Area Instruments Degas stations Analysis stations 9601 Single Point 9603 Single Point 9601 Multi Point 9603 Multi Point © 2022 HORIBA, Ltd. All rights reserved. 30
Slide 31: Automatic dynamic sorption process
Automatic dynamic sorption process • Analysis initiated from PC software or integrated computer • Gas flows across sample • 1cm3 N2 is injected for calibration • Dewar is raised / sample cools • Gas begins adsorbing • When adsorption ends, Dewar is lowered • Gas desorbs rapidly providing integratabl
Slide 32: Small total surface area vs large
Small total surface area vs large Small Surface Area Large Surface Area © 2022 HORIBA, Ltd. All rights reserved. 32
Slide 33: BET Multi-point example
BET Multi-point example 1st point 2nd point 3rd point © 2022 HORIBA, Ltd. All rights reserved. 33
Slide 34: Plot at least 3 points to obtain multi-point plot
Plot at least 3 points to obtain multi-point plot BET C constant = 145.84 Surface area = 11.36 m2/g © 2022 HORIBA, Ltd. All rights reserved. 34
Slide 35: Example data – Calcium Carbonate
Example data – Calcium Carbonate Grand Grand Column1 Sample 1 Sample 2 Sample 3 Column1 Sample 1 Sample 2 Sample 3 Totals Totals Test 1 2.28 2.47 2.13 Test 1 2.32 2.4 2.26 Test 2 2.31 2.36 2.2 Test 2 2.32 2.4 2.26 Test 3 2.21 2.3 2.12 Test 3 2.27 2.28 2.25 Average 2.27 2.38 2.15 2.26 Average 2.30 2.
Slide 36: Example data - Kaolin
Example data - Kaolin Grand Grand Column1 Sample 1 Sample 2 Sample 3 Column1 Sample 1 Sample 2 Sample 3 Totals Totals Test 1 21.29 21.89 21.72 Test 1 21.47 21.11 21.8 Test 2 21.27 21.83 21.66 Test 2 21.26 21.77 21.48 Test 3 20.95 21.55 21.4 Test 3 20.66 21.37 21.06 Average 21.17 21.76 21.59 21.51 Av
Slide 37: Other materials
Other materials Material Single point Multi-point Alumina 173.74 177.76 Bentonite 12.31 12.29 Calcium Hydroxide 13 13.35 Graphite 5.75 5.97 Hydroxyapatite 56.28 56.88 Kaolinite 7.9 8.1 © 2022 HORIBA, Ltd. All rights reserved. 37
Slide 38: © 2022 HORIBA, Ltd. All rights reserved.
© 2022 HORIBA, Ltd. All rights reserved. 38