chemisorption and physisorption - max planck society · isotherm isotherm...
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Surface area determination Surface area determination -- physisorptionphysisorptionand chemisorptionand chemisorptionand chemisorptionand chemisorption
• Motivation• Physisorption• Chemisorption• Chemisorption• Outlook
Literature:Literature:
1. DIN ISO 9277: BET method2. DIN 66136: Dispersion measurement of metals2. DIN 66136: Dispersion measurement of metals3. DIN 66134: BJH method4. Rouquerol F, Rouquerol J, Sing K. Adsorption by Powders and Porous Solids. London:
Academic Press; 1999.5 N i k AV Si KSW Th M S f A d P i H db k f5. Neimark AV, Sing KSW, Thommes M. Surface Area and Porosity. Handbook of
Heterogeneous Catalysis. Wiley-VCH Verlag GmbH & Co. KGaA; 2008.6. Bergeret G, Gallezot P. Particle Size and Dispersion Measurements. Handbook of
Heterogeneous Catalysis. Wiley-VCH Verlag GmbH & Co. KGaA; 2008.
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Heterogeneous Catalysis. Wiley VCH Verlag GmbH & Co. KGaA; 2008.7. Quantachrome manuals
Surface area determination Surface area determination -- physisorptionphysisorptionand chemisorptionand chemisorptionand chemisorptionand chemisorption
What do we obtain?What do we obtain?
• specific surface area of a substance• specific surface area of a part of a substance (i e of the metal particles on• specific surface area of a part of a substance (i.e. of the metal particles on a support)
Why do we need such quantities?
• comparability of different charges of the same substance• comparability of catalyst activity (... not given per gram)
Idea: surface area is a measure for the number of catalytic centers (but for sure not the number of active sites – no one knows how an active site looks like)
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Surface area determination Surface area determination -- physisorptionphysisorption
Weak adsorbate – adsorbens interactions – first level approximation:
Adsorption energy is constant in the first layer and different butp gy ynearly constant in the following multilayers
We measure the (in principle) whole surface area!We measure the (in principle) whole surface area!
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
PhysisorptionPhysisorptionWhatWhat dodo wewe measuremeasure?? -- isothermsisothermsWhatWhat do do wewe measuremeasure? ? isothermsisotherms
IUPAC isotherm classification
• Saturation behaviour for high p/p0• concave – convex• „sharp knee point“ B
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
HowHow do do wewe measuremeasure??VolumetricVolumetric measurementmeasurementVolumetricVolumetric measurementmeasurement
ideal gas law:
nRTpV =
ideal gas law:
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
1. Dead volume measurement with He1. Dead volume measurement with He
V1T1 Problems:Problems:
V2 is unknown, T2 is not well defined:
Quantachrome :
V2T2
1. V2 at RT with He2. Pressure change with and without
bath yields volume of „cold“ andy „„warm“ zone volume in V2
• Cold and warm zone changeoverCold and warm zone changeovervolume should be minimized!• Some adsorbens absorb also He!
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
2. Measurement with N2. Measurement with N22
V1T11. Certain amount of N2 is dosed
in V12 Equilibration with V yields2. Equilibration with V2 yields
new (lower) p1= p23. Step one is repeated until the
d i ddesired p4. the lost amount is now in V2,
i.e. in the „cold“ and „warm“ zone volume and the residual in the calculation is the adsorbedamount of N22
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Analysis Analysis ofof thethe isothermsisothermsthethe BETBET methodmethodthethe BET BET methodmethod
11/ pCpp −
Brunauer, Emmett and Teller in the 1930s
( )( ) 00
0 11/1
/pp
CnC
Cnppnpp
mma
•+=−
with na : adsorbed amount
bxay += bxay +
banm +
=1
1+=bCBET constant C:Monolayer capacity
ba + a
Lana mms = nmam 162.0= for N2Specific surface area
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
L: Avogadro number
Analysis Analysis ofof thethe isothermsisothermsthethe BETBET methodmethodthethe BET BET methodmethod
How to do?
Fit with the BET equation inFit with the BET equation in the range ofp/p0 = 0.05 ... 0.3
Sometimes even at lowerupper limit
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
BET BET equationequation, BET , BET constantconstant CC,,LangmuirLangmuir equationequation asas limitlimit forfor C=1C=1Langmuir Langmuir equationequation asas limitlimit forfor C 1C 1
( )( )0 11/ pCpp
•−
+=( )( ) 00/1 pCnCnppn mma
+−
EE
e RTEE L
C−
=1
From the original theory:
E i th d ti f th fi t l dE1 is the adsorption energy of the first layer and EL the liquefaction energy,
with E E this transforms into the Langmuir equationwith E1 = EL this transforms into the Langmuir equation
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Isotherm Isotherm classificationclassification
IUPAC isotherm U C so eclassification
• Saturation behaviour for high p/p0( filli )(pore filling)
• concave – convex• „sharp knee point“ B
Type I: Langmuir, „saturation“
Type II: C > 2, sigmoid (S-shaped)yp g ( p )non- or macroporous
Type III: C < 2, weak interactions
Type IV: C > 2, mesoporous, „saturation“
Type V: C < 2, like III with „saturation“
Type VI: stepwise (layer by layer) adsorption
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Type VI: stepwise (layer by layer) adsorption
Adsorption and desorption hysteresisAdsorption and desorption hysteresis
H1: narrow distribution of relatively uniformH1: narrow distribution of relatively uniform (cylindrical-like) pores-delayed condensation and no network effects, analysis of the desorption branchanalysis of the desorption branch
H2: complex pore structure, network effects are i t t l i f th d ti b himportant, analysis of the adsorption branch
H3: non-rigid aggregates of plate-like particlesor assemblages of slit-shaped pores- no saturation
H4: complex materials containing both micropores and mesopores
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Neimark AV, Sing KSW, Thommes M. Surface Area and Porosity. Handbook of Heterogeneous Catalysis, Wiley; 2008.
Adsorption and desorption hysteresis,Adsorption and desorption hysteresis,some effectssome effectssome effectssome effects
Often at p/p0=0.42 for N2independent of the adsorbent
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Neimark AV, Sing KSW, Thommes M. Surface Area and Porosity. Handbook of Heterogeneous Catalysis, Wiley; 2008.
BET BET methodmethod –– exampleexample 11# 10904# 10904# 10904# 10904
• only typeII and typeIV isotherms• only typeII and typeIV isotherms• the p/p0 range should give a
positive linear slope i h li i i• in the linearity region
1/ ≈ma nnmust be fulfilled
• for this is fulfilled200100 << C
For negative slope or non-linearity the BET method is not applicable!
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
BET BET methodmethod –– exampleexample 11# 10904# 10904# 10904# 10904
C=40 ... 55
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
BET BET methodmethod –– exampleexample 22# 9828# 9828# 9828# 9828
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
BET BET methodmethod –– exampleexample 22# 9828# 9828# 9828# 9828
C = -370C 370
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
BET BET methodmethod –– exampleexample 22# 9828# 9828 -- LangmuirLangmuir# 9828 # 9828 LangmuirLangmuir
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
BET BET methodmethod –– exampleexample 11# 10904# 10904 -- LangmuirLangmuir# 10904 # 10904 LangmuirLangmuir
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Reference Reference materialsmaterials forfor thethe BET BET methodmethod,,measurementmeasurement errorserrorsmeasurementmeasurement errorserrors
DIN ISO 9277:
measurement errors:
• I t t ( V T )• Instrument (p, V, T, vacuum)• Scatter of different measurement
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Surface area determination Surface area determination -- chemisorptionchemisorption
Strong adsorbate – adsorbens interactions – first level approximation for metalon a support:
Adsorption energy on the metal is constant in the first layer and stronglyexceeds the adsorption energies on the support or in multilayers
We measure (in principle) the metal surface area!
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
HowHow do do wewe measuremeasure??PretreatmentPretreatment,, choicechoice ofof adsorbateadsorbatePretreatmentPretreatment, , choicechoice ofof adsorbateadsorbate
Pretreatment is different from physisorption
• removal of water• reduction of the metal (only!)
Quantachrome: Flow cell, reduction with H2/Ar or CO, evacuationQ , 2 ,
Choice of adsorbate is crucial
• only adsorption on the metal • no irreversibel subsurface absorptionp• no volatile reaction products (carbonyles)• no bulk reaction
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
HowHow do do wewe measuremeasure??VolumetricVolumetric measurementmeasurementVolumetricVolumetric measurementmeasurement
ideal gas law:
nRTpV =
ideal gas law:
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Chemisorption:Chemisorption:measurementmeasurementmeasurementmeasurement
Two isotherms:
1. Total amount2. Reversible amount
( ti ft t ti(evacuation after saturationand a second isotherm)
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Chemisorption:Chemisorption:analysisanalysisanalysisanalysis
Linear extrapolation to p = 0 yields the volume of an adsorbed monolayer vm
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Chemisorption:Chemisorption:analysisanalysisanalysisanalysis
1001v )(100122414
12 metalgmwt
am
LnvA mm
M−=Specific metal surface area:
wtM
mnvD m 100
22414•=metal dispersion:
Vm volume of an adsorbed monolayerL Avogadro numbern chemisorption stoichiometrym sample massam surface area occupied by a metal atomwt metal loadingM atomic mass of the metal
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
ComparisonComparison ofof different adsorptives different adsorptives forfor PtPt, , PdPdandand RhRhandand RhRh
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Kunimori K, Uchijima T, Yamada M, Matsumoto H, Hattori T, Murakami Y. Appl Catal 1982;4(1):67-81.
Chemisorption Chemisorption –– thethe heatheat ofof adsorptionadsorption
Triangles:Heat of adsorption for CO/Pt(110) is far from being constant!
Dirk Rosenthal, Electronic Structure, Department Inorganic Chemistry, Fritz-Haber-Institut der MPG, Berlin, Germany
Stuck A, Wartnaby CE, Yeo YY, Stuckless JT, AlSarraf N, King DA. An improved single crystal adsorption calorimeter. Surf Sci 1996;349(2):229-40.