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.