1. Quantachrome
I N S T R U M E N T S
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2. Quantachrome
I N S T R U M E N T S
Chemisorption 3

3. 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

4. Quantachrome
I N S T R U M E N T S
Introduction
3.1

5. 3.1 Introduction 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

6. 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.

7. Physisorption vs Chemisorption

8. 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.

9. Adsorption Potentials
Potential energy curves for molecular (non-dissociative) adsorption

10. Adsorption Potentials
Potential energy curves for activated adsorption

11. Adsorption Potentials
Potential energy curves for non-activated adsorption

12. 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).

13. 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

14. Quantachrome
I N S T R U M E N T S
Classical Models
3.2

15. 3.2 Classical Models
Langmuir
Freundlich
Temkin

16. Adsorption Process
Adsorptive
Adsorbate
Active Sites (Adsorbent)

17. Irving Langmuir ( )
Graduated as a metallurgical engineer from the School of Mines at Columbia University in 1903
M.A. and Ph.D. in 1906 from Göttingen.
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

18. Irving Langmuir ( )
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"
With Katherine Blodgett studied thin films.
With Vincent Schaefer discovered that the introduction of dry ice and iodide into a sufficiently moist cloud of low temperature could induce precipitation.

19. 3.2.1 Langmuir’s “Kinetic” Approach
rate of adsorption = ka P(1-)
where  is the fraction of the surface already covered with adsorbate, i.e., = V/Vm
rate of desorption = kd 
Suggests a dynamic equilibrium. Is it?

20. Langmuir (continued…)
At equilibrium (any pressure)
ka P(1-) = kd 
from which
 = 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)

21. Or, if you prefer…
Confining adsorption to a monolayer, the Langmuir equation can be written
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

22. Langmuir Plot 1/V = 1/Vm + 1/(VmKcP1/s) Slope = 1/(VmK) 1/V
Intercept = 1/Vm
1/V = 1/Vm + 1/(VmKcP1/s)

23. 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 
Temkin: assumed that q decreases linearly with increasing coverage
Freundlich: assumed that q decreases exponentially with increasing coverage

24. Temkin
Temkin assumed that q decreases linearly with increasing coverage, that is,
Q=qo(1-  )
Where qo is a constant equal to the heat of adsorption at zero coverage ( = 0) and  is a proportionality constant.

25. Where A = RT/qo   and B = A ln Ko + 1/  
Temkin
 = A ln P + B
 or, since  = V/Vm
V = Vm A lnP + VmB
Where A = RT/qo    and  B = A ln Ko + 1/  

26. Temkin Plot
Ln(P) V
Slope = VmA
Intercept = VmB
V = Vm A lnP + VmB

27. Multiple Temkin Plots to find
experimental
extrapolated V
* denotes “temperature invariant” or “thermally irreversible” quantity
Ln(P)
Temp H
Temp M
Temp L

28. Freundlich
Temkin assumed that q decreases exponentially with increasing coverage, that is,
Q = -qm ln
Where qm is a constant equal to the heat of adsorption at  =

29. Where C=RT/ qm and D = C lnKo
Freundlich
ln = C lnP + D
 or, since  = V/Vm
ln(V/Vm) = C lnP + D
Where C=RT/ qm and D = C lnKo

30. Freundlich (continued…)
Ln(P)
Ln(V)
Slope = C
Intercept = D + ln(Vm)
Ln(V/Vm) = C lnP + D

31. Multiple Temkin Plots to find
experimental
extrapolated
Ln(V)
* denotes “temperature invariant” or “thermally irreversible” quantity
Ln(P)
Temp H
Temp M
Temp L

32. Quantachrome
I N S T R U M E N T S
Active Metal Area
3.3

33. 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

34. 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

35. 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 )

36. 3.3.1 Principles of Calculation
Monolayer Volume, Vm = volume of gas chemisorbed in a monomolecular layer

37. Methods to Determine Vm
= volume of gas chemisorbed in a monomolecular layer
Extrapolation
Bracketing
Langmuir
Temkin
Freundlich

38. Extrapolation method Vm First (only?)isotherm Volume Adsorbed
Pressure (mm Hg)
First (only?)isotherm

39. The second isotherm combined Weak only Volume Adsorbed
Pressure (mm Hg)

40. The difference isotherm
combined
Weak only
Volume Adsorbed
Strong
Pressure (mm Hg)

41. Vm from Pulse Titration
… will be covered in 3.5.2

42. 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

43. 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).

44. 3.3.2 Choice of Adsorbate Chemisorption CO or H2 on Pt, Pd at 40 oC
CO or H2 on Ni
For metal-only area (& dispersion etc)
Physisorption
N2 at 77K
Ar at 87K
Kr at 77K
CO2 at 273K
For total surface area and pore size

45. 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)

46. 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, A = 6l2.
The volume of the cube is given by l3 or, in terms of total area, substitute A /6 for l2 to give
V= lA/6
For a cube whose mass is unit mass, its volume is given by 1/  (where  is the density of the material).
 V=1/

47. 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/  or
 d=6/A 
For a supported metal, the loading, L, must be taken into consideration.
 d=L6/A 
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.

48. 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.

49. 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.

50. Exposed metal atoms
Since these islands vary in size due to both the intrinsic nature of the metal and the support beneath, plus the method of manufacture more or less of the metal atoms in the whole sample are actually exposed at the surface. It is evident therefore that the method of gas adsorption is perfectly suited to the determination of exposed active sites.
support
Exposed active sites
Adsorbed gas

51. Metal Dispersion
Dispersion is defined as the percentage of all metal atoms in the sample that are exposed.
The total amount of metal in the sample is termed the loading, χ , as a percentage of the total sample mass, and is known from chemical analysis of the sample.

52. Metal Dispersion The dispersion, δ, is calculated from:
Where M is the molecular weight of the metal, Na is the number of exposed metal atoms found by adsorption and WS is the mass of the sample.

53. 3.3.5 Accessible vs. Non-accessible Sites
Adventitious moisture
Reducing gas accessibility
Diffusion
Purge
Physisorption blocks
Bulk hydride
Spillover
Stoichiometry
Characterization gas vs. Process gas

54. Spatial Ordering
There may exist a number of different adsorption sites that involve different numbers of metal atoms per adsorbate molecule.

55. Adsorption Thermodynamics
Quantachrome
I N S T R U M E N T S
Adsorption Thermodynamics
3.4

56. 3.4 Adsorption Thermodynamics
Isosteric Heats from Isotherms
See also activation energy under 3.6.1

57. 3.4.1 Heats of Adsorption
Whenever a gas molecule adsorbs on a surface, heat is (generally) released, i.e. the process of adsorption is exothermic.
This heat comes mostly from the loss of molecular motion associated with the change from a 3-dimensional gas phase to a 2-dimensional adsorbed phase.
Heats of adsorption provide information about the chemical affinity and the heterogeneity of a surface, with larger amounts of heat denoting stronger adsorbate-adsorbent bonds.
There are at least two ways to quantify the amount of heat released upon adsorption: in terms of (i) differential heats, q, and (ii) integral heat, Q.

58. Differential Heats of Adsorption
q, is defined as the heat released upon adding a small increment of adsorbate to the surface.
Its value depends on (i) the strength of the bonds formed and (ii) the degree to which surface is already covered.
i.e a plot of q vs. θ provides a curve illustrating the energetic heterogeneity of the surface.
Use it to fingerprint surface energetics and to test of the validity of any Vm evaluation method used (see earlier) since each method assumes a different relationship between q and θ.

59. Differential Heats of Adsorption
Since q can, and most often does, vary with θ, it is convenient to express it as an isosteric heat of adsorption, that is, at equal surface coverage for different temperatures.
Thus, obtain two or more isotherms at different temperatures.
Determine pressures corresponding to equal coverage at different temperatures.
Construct an Arrhenius plot of (lnP) versus (1/T). Values for q at any given coverage, θ, can be calculated from the Arrhenius slopes, m.

60. and R is the universal gas constant.
Slopes of (lnP) vs. (1/T).
where
m = d lnP/d(1/T)
and R is the universal gas constant.

61. Integral Heat of Adsorption
This is simply defined as the total amount of heat released, Q, when one gram of adsorbent takes up X grams of adsorbate. It is equivalent to the sum, or integral, of q over the adsorption range considered, that is:
where Vm is expressed in mL at STP, and θ ideally ranges from θmin = 0 to θmax = maximum coverage attained experimentally.

62. Experimental Approaches
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I N S T R U M E N T S
Experimental Approaches
3.5

63. 3.5 Experimental Approaches
Pulse
Static

64. Preparation Techniques
• Sample is heated under inert flow to remove adsorbed moisture. Whilst reduction step creates moisture, we don’t ant the reducing gas to compete for diffusion to surface.
• Reduce with H2: can be pure hydrogen or diluted with nitrogen or argon. Higher concentrations give higher space velocities for the same volumetric flow rate.

65. Preparation Techniques (continued…)
Purging with inert gas (normally helium) strips excess reducing gas quickly. Can shorten prep time and/or give more reproducible data since hydrogen is difficult to pump.
Cooling is done under vacuum/flow to ensure continued removal of residual reducing gas… though it is the hot removal step (above) which is critical. That is, don’t cool before removing as much reducing gas as possible.

66. Chemisorption Techniques
Vacuum method
Flow methods

67. Vacuum Technique • Sample is heated under inert flow • Reduced with H2
• Purged with inert, cooled under vacuum/flow
• Adsorbate dosed to obtain isotherm
• Calculate the amount adsorbed

68. Static (volumetric) Setup
adsorptives
manifold
Turbo- molecular (drag) pump
vent
Flow “U” cell
diaphragm pump
furnace

69. Setup
Filler rod goes here
Quartz wool
sample
capillary

70. 3.5.2 Flow (Pulse) Chemisorption

71. Flow Types of Analysis TPR TPO TPD Monolayer by Titration BET
A flow system permits multi-functional catalyst characterization :
active sites
support

72. Overview
Analysis is done by detecting changes in gas composition downstream of sample.
Detector senses
abstraction of reactive species during adsorption
evolution of previously adsorbed species during desorption
decomposition products
Signal detection
Standard: thermal conductivity detector
Optional: mass spectrometer

73. ChemBET™ 3000 TPR

74. Flow Diagram A 1 B A 2 3 4 OUT IN CLICK FOR BYPASS & LONGPATH

75. Flow/Static (FloStat™) Flow Diagram
to vent
heated zone (vapor option) A
heater
vapor source (optional) B 1 2
to mass spec (optional) 3
oil-free high vacuum 4 5
Schematic representation only. Some vacuum volumetric components omitted for clarity.

76. TPRWin™ Software Data Acquisition

77. Overview Quartz flow-through cell allows
T/C #1
T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
Quartz flow-through cell allows
high-temperature (up to 1100 degC)
in-cell temperature monitoring
Two t/c’s if necessary, one to DAQ, one to MassSpec.
mass spectrometer sampling port.

78. Pulse Titration
Metal area, dispersion and crystallite size are calculated from the amount of analysis (reactive) gas adsorbed.
Variable volumes of analysis gas are injected into the inert carrier gas stream, which continuously flows over the sample.
Detector measures the volume of gas that remains unadsorbed by the sample. Subtraction from the total amount injected gives the total amount adsorbed to within 1uL accuracy.

79. Titration Pulse Titration of Active Sites H2 or CO titration
N2 and He carrier respectively
Constant temperature (room temp?)
Multiple injections until saturation M H H2 CO He N2

80. Titration Data Acquisition

81. Titration
LOAD
INJECT
signal
injections

82. Titration Calculations
1. Calculate total nominal volume of reactive gas adsorbed by comparison with calibration injection or average of last n (three) peaks
(note: peak area represents gas not adsorbed!)
Total vol adsorbed =
(Peak Avg - Peak1) + (Peak Avg - Peak2) + (Peak Avg - Peak3) etc
x nominal injection volume = Vnom (units ml)

83. Titration Calculations
2. Convert to STP:
 (Vnom) x (273/rt) x (Pamb/760) = Vstp (units ml)
 3. Convert to specific volume adsorbed:
 Vstp /sample wt = Vsv (units ml/g)
 4. Convert to micromoles per gram (weight as supplied ):
 Vsv / 22.4 = Vm (units mmole/g)

84. Requirements for Different Analysis Types
Long cell
Short cell
Std. cell
5% H2
100% H2
5% O2
100% N2
100% He
30% N2
Inj.
Furnace
Mantle
Dewar
Long path
TPR 
()
TPO
TPD
Metal Area* *
BET
Mixed Gases: the gases to be mixed should have significantly different thermal conductivities. Hydrogen (for TPR for example) should be blended with nitrogen or argon (inert carrier gases) and not be blended with helium (inert but too similar to hydrogen with respect to thermal conductivity). Similarly, when performing pulse titration, hydrogen should be injected into nitrogen or argon carrier, not into helium. Carbon monoxide should be injected into helium, not nitrogen! Choose one gas from family “1” and the other gas from family “2”…
1) Helium, hydrogen
2) Nitrogen, argon, carbon monoxide, carbon dioxide L
* Using H2 active gas. If using CO, substitute 100% CO for 100% H2 & 100% He for 100% N2.

85. Temperature Programmed (TP) Experiments
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I N S T R U M E N T S
Temperature Programmed (TP) Experiments
3.5

86. 3.6 Temperature Programmed (TP) Experiments
TP-Reduction
TP-Oxidation
TP-Desorption
TP-Reaction

87. 3.6.1 TP-Reduction
Metal oxides are readily characterized by their ease of reduction.
CeO2  CeO2-x + x/2O2
TPR profiles represent that ease of reduction as reduction rate as a function of increasing temperature.
2CeO2 + H2  Ce2O3 + H2O

88. Temperature Programmed Reduction
A low concentration of pre-mixed hydrogen (e.g.5%) in nitrogen or argon (or other reducing gas for custom research applications) flows over the sample as it is heated during a linear increase (ramp) in temperature.
Peak reduction temperature is also a function of heating rate and may be used to calculate activation energy for the reduction process.

89. TPR Temperature Programmed Reduction Metal oxide to metal
5% hydrogen reactive gas
Balance N2 or Ar (not He ! ...unless MS)
Ramp rate
Activation Energy
H2O M H2
It is usual to react the unreduced catalyst, typically a metal oxide which may be supported or not, with a reducing gas, typically hydrogen diluted in an inert carrier gas – typically nitrogen. The change in hydrogen concentration is monitored as a function of increasing sample temperature. MO

90. TPR tmax signal temperature
The resulting temperature programmed reduction (also known as TPR) profile represents the relative ease with which the sample reacts with the hydrogen, or reduces. The peak in the profile represents the maximum reaction rate, and its temperature is related to the activation energy of the reduction process. Two or more well separated peaks are an indication that two or more distinct unreduced phases may be present in the sample.
temperature

91. Linearly ramped furnace is essential for standard TP profiles
TPR
Linearly ramped furnace is essential for standard TP profiles
The resulting temperature programmed reduction (also known as TPR) profile represents the relative ease with which the sample reacts with the hydrogen, or reduces. The peak in the profile represents the maximum reaction rate, and its temperature is related to the activation energy of the reduction process. Two or more well separated peaks are an indication that two or more distinct unreduced phases may be present in the sample.

92. TPR Profiles for Different Heating Rates3
tmax 2
signal
temperature
The resulting temperature programmed reduction (also known as TPR) profile represents the relative ease with which the sample reacts with the hydrogen, or reduces. The peak in the profile represents the maximum reaction rate, and its temperature is related to the activation energy of the reduction process. Two or more well separated peaks are an indication that two or more distinct unreduced phases may be present in the sample. 1
time

93. TPR Profiles for Different Heating Rates

94. Heating Rate & Peak Temperature
TPR Profile
Heating Rate  (K-1)
Peak Temperature (Tmax) 1 10
874 2 15
902 3 20
928

95. Kissinger (Redhead) Equation

96. 3.6.2 TP-Oxidation
Temperature programmed oxidation (using 2%-5% O2 in He for example) is performed in a manner analogous to TPR.
TPO can be particularly useful for looking at carbons:
Carbon supports (graphite vs. amorphous)
Carbon deposits from coking
Carbides

97. TPO carbon Temperature Programmed Oxidation
Metals and carbon to oxides
2-5% oxygen reactive gas
balance He (not N2 !)
Ramp rate
Activation Energy
CO + CO2 O2
Temperature Programmed Oxidation is used to evaluate a sample’s ease of oxidation. This analysis results in a fingerprint profile of oxidation rate (reduction in concentration of oxidizing species) as a function of increasing time and temperature. It is usual to react carbon samples or a reduced catalyst, ie a metal (which may be supported or not), with an oxidizing gas, typically oxygen diluted in an inert carrier gas – typically helium. The change in oxygen concentration is monitored as a function of increasing sample temperature. Carbon dioxide is mildly oxidizing and is occasionally used according to the following scheme: C + CO2 = 2CO. M C
carbon

98. TPO: Signal vs. Temperature

99. TPO: Signal & Temp. vs. Time

100. Temperature Programmed Oxidation
Zhang and Verykios reported that three types of carbonaceous species designated as C, C, and C were found over Ni/Al2O3 and Ni/CaO±Al2O3 catalysts in the TPO experiments.
Zhang ZL and Verykios XE,. Catal. Today (1994).
Goula et al identified two kinds of carbon species on Ni/CaO Al2O3 catalysts from TPO experiments. The high-temperature peak was assigned to amorphous and/or graphite forms of carbon. The lower temperature peak suggested a filamentous form.
Goula MA, Lemonidou AA and Efstathiou AM, J Catal (1996).

101. 3.6.3 Temperature Programmed Desorption
The monitoring of desorption processes is equally easy.
A pure unreactive carrier gas carries evolved species from the sample to the detector as the user-programmable furnace heats the sample.
This technique is commonly employed to determine the relative-strength distribution of acidic sites by means of ammonia desorption.

102. TPD Temperature Programmed Desorption
Remove previously adsorbed species
Helium/Nitrogen purge
Ramp rate
Activation Energy
NH3
NH3 MO

103. Ammonia TPD

104. Pyridine TPD Multiple acid sites revealed by peak deconvolution
Physisorbed pyridine is clearly evident in the first sample (low temp.), but absent in the second.

105. TPD
Increasing mass
tmax
signal
temperature

106. Overview Quartz flow-through cell allows
T/C #1
T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
Quartz flow-through cell allows
high-temperature (up to 1100 degC)
in-cell temperature monitoring
Two t/c’s if necessary, one to DAQ, one to MassSpec.
mass spectrometer sampling port.

107. With Mass Spectrometer
T/C #1
T/C #2
Modified cell holder
Capillary to mass spec.
Gas flow
Capillary or capillary connector to
mass spectrometer
Tube ends
just below port connection
In-situ thermocouple
¼” swagelok® compression fitting

108. 3.6.4 TP-Reaction
Essentially everything that is not standard TPR or TPO!!
Can be a single reactive gas, or a mixture of reactants… akin to microreactor work.
Need not be done over a bare metal surface… might have one reactive species preadsorbed on the surface
e.g.