adsorption calorimetry: fundamentals and

ADSORPTION CALORIMETRY:FUNDAMENTALS AND APPLICATIONS IN

CHARACTERIZATION OF CATALYST

Juan Carlos Moreno-Piraján, PhD.Grupo de Investigación en Sólidos Porosos y CalorimetríaProfesor TitularUniversidad de Los AndesBogotá, Colombia

Sao Pedro, Brazil 18th April 2010

Prof. Juan Carlos Moreno-Piraján GGrupo de Investigación en Sólidos Porosos y Calorimetría

OUTLINE1. INTRODUCTION TO CALORIMETRY

1.1. Definition

1.2. Objectives

1.3. Goals and Basic interest

2. ADSORPTION CALORIMETRY: HISTORY

2.1. Mercury calorimeter

2.2. Dewar Calorimeter

2.3. Tian-Calvet calorimeter

2.4. Local construction calorimeter (Colombia)

3. GENERAL DESCRIPTION OF MICROCALORIMETRIC TECHNIQUE AND FUNDAMENTALS

4. ADSORPTION CALORIMETRY: APLICATIONS

5. CONCLUSIONS.


1. INTRODUCTION TO 1. INTRODUCTION TO CALORIMETRYCALORIMETRY


CALORIMETRYCALORIMETRY1.1 CALOR METRY

HEAT MEASURE

“Science that measures the amount of heat”

Latín Greek

Endothermic Process Exothermic Process


1.2. Objectives and basic interests of theadsorption calorimetry

Calorimetry aims to measure heat.

Heat exchanges occur with most physical or chemical phenomena.

They especially occur during any physical adsorption or immersionphenomen.

The adsorbate-adsorbent energy of interaction (derived from theheat measurements) is a basic data for any modelling andunderstanding.


1.3. Goals and basic interest of adsorptioncalorimetry

Calorimetry should therefore allow to:

Provide basic data for adsorption theory and modelling.

Detect specific interations between adsorbent and adsorbate andto evaluate homogeneity of adsorbent.

Detect phase changes occurring in the adsorbed layer.

Detect any change in the extent of the solid—fluid interface.


1.3. Interesting aspects of AdsorptionCalorimetry

Heat involved in physisorption is small and requieres highsensitivity and stability.

Introduction or removal of adsorbate requieres open calorimetricsystem.

Well-defined starting and final states require careful outgassingprocedure and careful control adsorption experiment.

Assessment of a state function independent from calorimeter andprocedure (enthalpy, energy, not simply a heat) requires carefullydevising the experiment and determining all heat terms.


2. ADSORPTION CALORIMETRY:HISTORY


2.1 Pierre Antoine Favre (1813-1880)

First Professor of Chemistry of the Faculty ofSciences of Marseilles (1854)

Inventor, with Silbermann, on the mercurycalorimeter or “thermometer for calories”, : a very large mercury thermometer (several kg) with a finger hole to accomodate the sample.

First calorimeter for measure heats of adsorption of gases on solids ( from 1854 to 1871).


2.2. James Dewar liquid air adsorption calorimeter

DEWAR 1904

Dewar built a diathermal, phase-change, calorimeter.

Adsorptive came from burette on right.

Heat of adsorption producedvaporization of liquid air.

Air evolved was measured in burette on

left.


2.3. Albert Tian ( 1880 -1972)

Professor of Chemistry at the Faculty of Sciences ofMarseille (1923-1950)

Invented the heat-flux microcalorimeter: isothermal, with a thermopile of 42 junctions+ 7 thermocouples and use Joule or Peltier powercompensation for heat measure.

He invented the multi-shieldedthermostat (Stable within 10-6 K)

Launches a school of calorimetry.


2.4. Edouard Calvet (Marsella,1895 - Marsella,1966)

A. Tian´s student, and then successor.

Introduces, in 1947 the differentialmounting.

Makes the heat-flow microcalorimeteran extremely versatile piece of equipment.

Has a special Institute built by theCNRS, in 1959, for development ofmicrocalorimetry.



The Tian-Calvet microcalorimeter, specially suited for studies ofadsorption or immersion: isothermal, sensitive, easy introduction or removal of gas or liquid. (Jean Rouquerol-Laboartorie Chimie Provence, Universites-Aix-Marseilles, France)


2.5. Adsorption microcalorimeter in Sólidos Porosos y Calorimetría’s Group

3. GENERAL DESCRIPTION OF MICROCALORIMETRIC

TECHNIQUE


Examples for surface sites

Metal sites

Brønsted-sites Lewis-sites



Probing sites by Chemisorption

+ NH3

Desorption TTDS/TPD

Adsorption Δ HCalorimetry

IR, XPS, NMR, UV/Vis...shift of bands (probe/surface)

Diferent methods deliver different information

•Isotherms: number of sites

•Spectroscopies: type, strength (but not number unless extinctioncoefficient known)

•Calorimetry, TDS (thermal desorption spectroscopy)=: number andstrength (but not type)

Specific Adsorption (Chemisorption)

• The physisorption (e.g. N2) gives information on surface area.A ≈ B ≈ C

• Specific adsorption / chemisorption gives information about a particular type of site which depends on the type of probe usedA, B can be distinguished, B and C can maybe distinguished

A B C


E

t

Integral Heats of Adsorption

A probe may chemisorb on different sites under production of differentheats of adsorption

If all sites are covered at once, the evolved heat will be an integral heat

If the number of adsorbed molecules is known, an average/mean heat ofadsorption can be calculated


t

E

Differential Heats of Adsorption

ATdiff n

Qq,

int ⎟⎠⎞

⎜⎝⎛=δδ

• Differential heats of adsorption as a function of coverage can be determined


ADSORPTION CALORIMETRY

The sorptive must be introduced stepwise, i.e. at constanttemperature, the pressure is increased slowly

For each adsorption step, the adsorbed amount must be determined(isotherm)

For each adsorption step, the evolved heat must be determined

The differential heat can then be determined by division of evolvedheat through number of molecules adsorbed in a particular step


Measurement of the Adsorbed Amount

• Via the pressure decrease through the adsorption (no change in number of molecules in system during adsorption)

• Via increase in sample weight

• Via the evolved heat (if heat of adsorption known and constant)

• Spectroscopically (if extinction coefficient of adsorbed species known)


Pressure Decrease Method

• A known number of molecules of the sorptive is introduced into the sample cell

• The sorptive is distributed into three partitions:gas phase, wall adsorption, sample adsorption

• the equilibrium pressure with sample is compared to the equilibrium pressure without sample at equal number of sorptivemolecules in the cell

• from the pressure difference the number of adsorbed molecules can be calculated


Dosing a Known Amount of Gas

• A known number of molecules of the sorptive is introduced into the sample cell

• If we know the volume, temperature and pressure, we can calculate the number of gas molecules

• Need V, T, p


The Dosing Volume

Pressure transducerDosing volume

Vdos

Sample cellGas in

Vacuum

• p, T can be easily measured.• V needs to be determined

Calibration VolumeVcal


Volume Calibration

• A volume can be measured by determining the amount of liquid that it can take up

a) gravimetrically: weight / density of liquidb) volumetrically: add liquid from a burette

• An unknown volume of any shape can then be determined through expansion from gas (an ideal gas that does not stick much to thewalls) from one volume to the other and pressure measurement before and after the equilibration


Calibrating the Dosing Volume

• fill VCal and VDos, same pressure• close valve between VCal and VDos

• set pressure in Vdos to pDos

• open valve, equilibrate


Vdos

Sample cellGas in

Vacuum



• Initial situation:

• After opening valve:

• n, T are constant

DosDosCalCal VpVpnRT +=( )DosCalfin VVpnRT +=

CalDosfin

finCalDos V

pppp

V−−

=

Calibrating the Dosing Volume


Vdos

Sample cell


Gas in

Vaccum


Determination of the amount dosed

RTVpp

n DosafterDosbeforeDosi

)( ,,int,

−=

Vacuum


Vdos

Sample cell


Gas in



4. ADSORPTION CALORIMETRY:APPLICATIONS

A Tian-Calvet heat-flux microcalorimeter for measurement of differential heats ofadsorption

Authors: Brent E Handy, Sanjay B Sharma, Brian E Spiewak and J A Dumesic

Meas Sci. Technol. 4 (1993) 1350-1356. Printed in the UK

0

20

40

60

80

100

120

140

160

0 10 20 30 40 50

CO coverage (μmol/g)

- ΔH a

ds (k

J/m

ol)

0

20

40

60

80

100

120

140

160

0 20 40 60 80 100 120 140

CO coverage (μmol/g)

- ΔH a

ds (k

J/m

ol)

Differential heat of adsorption of carbonmonoxide versus coverage on supported Pt catalysts at 353K.

a

b

a) 4% Pt/SiO2 (o) clean catalyst, (•) catalyst following n-hexanedehydrocyclization reaction.

b) 1% Pt/K(Ba)L-zeolite (o)cleancatalyst, (•)catalyst following n-hexane dehydrocyclization reaction.



0102030405060708090

0 50 100 150

V μmol/g

Q d

if, k

J/m

ol

CuZ-128

CuZ-79

CuZ-150

020406080

100120140160

0 15 30 45 60 75 90 105 120 135 150

V μmol/g

Q d

if, k

J/m

ol

CuZ-128

CuZ-79

CuZ-150

2 29 104 241 329

020406080

100120140160180

0 15 30 45 60 75 90

V μmol/g

Q d

if, k

J/m

ol

CuZ-79CO CuZ-79N2O-CO CuZ-79N2O

2 14 43 85 124 148

Calorimetric study of room temperature adsorption ofN2O and CO on Cu(II)-exchanged ZSM5 zeolites

Vesna Rakic , Vera Dondur b, Spasenka Gajinov, Aline Auroux Thermochimica Acta 420 (2004) 51–57

Differential heats of adsoprtion on CuZ-x (a)adsorption of N2O,(b) adsorption on CO, ( c ) adsorption CO on CuZ-79 previously contactedwith N2O. In all cases, the samples wereactivated at 673 K in vacuum. Its evident thatdifferentials heats of adsorption of N2O are lower than those of CO, indicating a weakerinteraction.


0102030405060708090

0 50 100 150

V μmol/g

Q d

if, k

J/m

ol

CuZ-128

CuZ-79

CuZ-150

020406080

100120140160

0 15 30 45 60 75 90 105 120 135 150

V μmol/g

Q d

if, k

J/m

ol

CuZ-128

CuZ-79

CuZ-150

2 29 104 241 329

020406080

100120140160180

0 15 30 45 60 75 90

V μmol/g

Q d

if, k

J/m

ol

CuZ-79CO CuZ-79N2O-CO CuZ-79N2O

2 14 43 85 124 148



•It is evident from Fig. a that insignificant number of such strong sites exists on CuZ-150 sample.

•On the contrary, the highest amount of the strongest sites for N2O adsorption is found on CuZ-128 sample that exhibits a different distribution of active sites in comparison with under-exchanged sample CuZ-79




• The results presented so far are a clear indication that Cu+ ions, produced by reduction of Cu2+ ions in ion-exchanged zeolite samples, are stronger active sites in CO adsorption that in N2O adsorption. Therefore, it could be concluded that CO would be adsorbed primarily in the case of possible competitive co-adsorption of these two gases

• The profile obtained for CO adsorption on the surface previously contacted with N2O is almost identical to that one obtained for the adsorption of CO on one empty surface, but only in the first part of adsorption procedure.


0,00

0,01

0,02

0,03

0,04

0,05

0,06

0,07

0 1 2 3 4 5 6 7

Isobutane partial pressure /hPa

adso

rbed

am

ount

/mm

olg-1

Act 473 K

Act 573 KAct 723 K

Act 723 K

0

10

20

30

40

50

60

70

0,00 0,01 0,02 0,03 0,04 0,05

adsorbed amount / mmolg-1

Diff.

hea

t of a

dsor

ptio

n /k

Jmol

-1

Act 473 K

Act 573 KAct 723 K

Act 723 K

* Adsorption of isobutane at 313 K on sulfated zirconia activated at various temperatures.

Characterization of catalysts in their active state by adsorption microcalorimetry: Experimental design and application to sulfated zirconiaSabine Wrabetz, Xiaobo Yang, Genka Tzolova‐Müller, Robert Schlögl, Friederike C. Jentoft *

Fritz Haber Institute of the Max Planck Society, Department of Inorganic Chemistry, Faradayweg 4‐6, D‐14195 Berlin, Germany

Journal of Catalysis 269 (2010) 351–358

Isotherm

Differential heats as a function of coverage

These are some results obtained by adsorption microcalorimetry, that is an excellent tool for characterization.

5. CONCLUSIONS

• Calorimetric data are basic: they are a prerequisite for anyunderstanding of adsorption

• To be fully meaningful and independent from the calorimetricprocedure, these data should be expressed in terms of energy orenthalpy of adsorption or inmersion, not in “heats”

• For this purpose, the total calorimetric set-up and procedure must be carefully studied

• Calorimetry is sensitive and quantitative, but not specific, therefore wellcomplemented by spectroscopic thechniques


REFERENCES

• 1. Lecture Prof. Dr.Jean Rouquerol presented in I Jornadas de Sólidos Porosos y Calorimetría, Adsorption and immersion calorimetry: Goals, issues, solutions andapplications, September 21-25 2009.

• 2. Characterization of catalysts in their active state by adsorption microcalorimetry: Experimental design and application to sulfated zirconia, Sabine Wrabetz, Xiaobo Yang, Genka Tzolova-Müller, Robert Schlögl, Friederike C. Jentoft, Journal of Catalysis 269 (2010) 351–358.

• 3. Calorimetric study of room temperature adsorption of N2O and CO on Cu(II)-exchanged ZSM5 zeolites, Vesna Rakic , Vera Dondur b, Spasenka Gajinov, AlineAuroux Thermochimica Acta 420 (2004) 51–57

• 4. A Tian-Calvet heat-flux microcalorimeter for measurement of differential heatsof adsorption Authors: Brent E Handy, Sanjay B Sharma, Brian E Spiewak and J A Dumesic, Meas Sci. Technol. 4 (1993) 1350-1356. Printed in the UK


STUDENTS OF GROUP

THE PROFESSORS!!!!!


THANKS FOR YOUR ATTENTION

adsorption calorimetry: fundamentals and

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