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111TemperatureTemperature--programmed methods programmed methods


Temperature-programmed Methods in Catalysis Research

Procedures and pitfalls of a very commonly used tool


Definitions

• TPD: temperature-programmed desorption• TPR: temperature-programmed reduction or

– temperature programmed reaction• TPO: temperature-programmed oxidation• TPRS: temperature-programmed reaction

spectroscopy


Methods

• TPD: a solid is first exposed to an adsorbate gas under well-defined conditions (wide range of pressure and temperature) and then heated underinert conditions with a temperature program:– parental method: Flash desorption from metal wires in

UHV : G. Ehrlich, Adv. Catal., 1963• All other methods keep solid and reactants in

contact during temperatre-programmed processing• TPD – TPRS most suitable designations


Atmospheres, Reactors and Detectors


Experiment

R.N. Rogers, Anal. Chem., 32, 672, (1960)

High-pressure variantwith inert carrier G1 and reactant G2


Filament

z-transfer rot

Line of sight mass spectrometer nozzle Electric contacts

Metal clips

FHI-AC variable pressure TDS-reactor set-up


Influence of Vacuum TreatmentInfluence of Vacuum Treatment

M. Hävecker, Electronic Structure, Dept. Inorganic Chemistry, Fritz-Haber-Institut (MPG), Berlin, Germany

Strong influence of vacuum treatment both for water and oxygendesorption traces Solid state dynamics!

Strong influence of vacuum treatment both for water and oxygendesorption traces Solid state dynamics!

0 100 200 300 400 500 600Temperature / °C

0

1∗10-9

2∗10-9

3∗10-9

4∗10-9

MS-

Res

pons

e (I.

C.) m/e=18m/e=18

“as is”

0 100 200 300 400 500 600Temperature / °C

0

5∗10-11

1∗10-10

MS-

Res

pons

e (I.

C.) m/e=32m/e=32

after 12h of vacuum

“as is”

Sample K47


AssignmentAssignment ofof relevant MS relevant MS tracestraces


Reaction study of sample “Partie 5010” (hemihydrate) after TDS treatment:

Molecule m/z fragmentsButane 43, 27, 42, 41, 39

Butene 41, 39, 55, 27, 26

Butadiene 39, 54, 27

2,5-dihydro-Furan 41, 70, 39, 42, 27

Furan 68, 39

MSA 26, 54, 98


Aqueous Aqueous vsvs Alcoholic preparationAlcoholic preparation


Hemihydrate:

0 100 200 300 400 500 600

Temperature /°C

0

1∗10-9

2∗10-9

3∗10-9

MS-

Res

pons

e (n

orm

. u.) m/e=18m/e=18

alcoholic

aqueous




Hemihydrate:

0 100 200 300 400 500 600

Temperature /°C

2∗10-11

4∗10-11

6∗10-11

8∗10-11

1∗10-10

MS-

Res

pons

e (n

orm

. u.) m/e=32m/e=32alcoholic

aqueous




Hemihydrate:

0 100 200 300 400 500 600

Temperature /°C

0

5∗10-11

1∗10-10

1.5∗10-10

2∗10-10

2.5∗10-10

MS-

Res

pons

e (n

orm

. u.) m/e=44m/e=44

alcoholic(“Partie 5010”)

aqueous


Reactor StudyReactor Study


Reaction study of sample K 50/16 (fresh VPO) after 2 times TDS treatment:

0 10 20 30 40 50

Time / min

0.8

0.9

1

1.1

1.2

1.3

1.4

MS

-resp

onse

(nor

m. u

.)

MSAMSA

n-butane conversion: 18%n-butane conversion: 18%

m/e=18m/e=18m/e=44m/e=44m/e=26m/e=26

m/e=43m/e=43

m/e=32 (x10)m/e=32 (x10)

m/e=54m/e=54? (MSA?)? (MSA?)

m/e=41, 39m/e=41, 39

•flow of 0.7vol% n-C4H10 / 20vol% O2 / 79.3vol% He•25 ml/min @ 1bar•T=400 °C•30 mg pellet

heating up to 400 °C


Comparison sample F8 “fresh” Comparison sample F8 “fresh” vsvs equilibratedequilibrated


Desorption peak around 300 °C typical for non- conditioned VPO(compare to previous investigations); organics from synthesis

Desorption peak around 300 °C typical for non- conditioned VPO(compare to previous investigations); organics from synthesis

0 100 200 300 400 500Temperature / °C

3∗10-13

4∗10-13

5∗10-13

6∗10-13

MS-

resp

onse

(I. C

.)

0 100 200 300 400 500Temperature / °C

2.5∗10-13

3∗10-13

3.5∗10-13

4∗10-13

4.5∗10-13

MS-

resp

onse

(I. C

.)m/z: 54 m/z: 68fresh

equilibrated


Desorption of organic fragments stemming not from synthesis after reaction (m/e=26: MSA?)

Desorption of organic fragments stemming not from synthesis after reaction (m/e=26: MSA?)

100 200 300 400 500

Temperature / °C

1∗10-12

1.2∗10-12

1.4∗10-12

1.6∗10-12

1.8∗10-12

2∗10-12

MS-

resp

onse

(I. C

.)

m/e=26m/e=26

after reaction

after 1st TDS


m/e=26m/e=26

m/e=39m/e=39

m/e=41m/e=41

m/e=43m/e=43

m/e=55m/e=55

m/e=68m/e=68100 200 300 400 500

Temperature / °C

4∗10-13

6∗10-13

8∗10-13

1∗10-12

1.2∗10-12

1.4∗10-12

1.6∗10-12

1.8∗10-12

2∗10-12

MS-

resp

onse

(I. C

.)

100 200 300 400 500Temperature / °C

5∗10-11

1∗10-10

1.5∗10-10

2∗10-10

2.5∗10-10

3∗10-10

MS-

resp

onse

(I. C

.)

m/e=18

m/e=32

m/e=44

2 types of organic species: product, intermediate(s),no catalyst organics


gaps

• temperature-programmed methods are a prototype area for the existence of gapsbetween surface science and catalysischaracterisation.

• often same methodology and datainterpretation.

• but fundamental differences in boundaryconditions.


TPD: a complex process

• The net rate of TPD is considered as the rationbetween adsorption and desorption

• in static condition: equilibrium• in TPD: disturbance by pumping or gas flow• boundary cases:

– re-adsortion possible (thermodynamic control– re-adsortion suppresed (kinetic control) case of UHV

TDS analysis


TPD and TDS: relation between surfacescience and catalysis charaterisation

• TDS low pressure (probe situation)• TDS: kinetic control, no re-adsorption, usually

(tacitly) first order• no clear distinction between sorptiopn and reaction• TPD high pressure, reaction situation• TPD: transition from kinetic to thermodynamic

regime explicitely studied• changeover in reaction process from sorption to

reaction common


TDS: The method

• A pre-adsorbed species is removed from a well-defined surface by rapid heating (10 K/s) in a well-pumped UHV environment (no equilibration and re-adsorption) (caveat TMP!). (Langmuir unit!)

• Care must be taken to see only desorption from the surface!

Feulner and Menzel, J. Vac. Sci. Technol., 17, (1980), 662


Surface compositionduring reaction:

kinetics


TDS: The observation

• rate of desorption:

r = ν (Θ) Θn exp [-Edes (Θ)/RT]

r = desorption raten = desorption orderΘ= coverage in monolayersν = pre-exponential factor

ν can vary between 1013 and 1018 accordingto transition state


TDS: Complete analysis

Integration to chosenlow desorption

Conventional analysisfor activation energy


TDS: The cheap analysis


TDS: The “elaborate analysis”

Use width and peak desorption temperatures astwo parameters for finding activation energy and pre-factor


TDS: Quality of analysis


The basis of TPD• Simplest case: A solid with adsorption sites S*

gets in contact with a gas G of concentration C and populates in a first order process the vacantadsorption sites.

• Langmuir boundary conditions:– fixed number N of sites S* (F/cm²)– constant adsorption enthalpy dHa (and desorption

enthalpy)– all parmetres are temperature-independent and

coverage-independent


The basis of TPDdN/dT = p k na (N*-N) – knd N

Langmuir model assuming the balance between competitiveadsortion and desorption kinetics

as net effect: the ratio of the two kinetic constants and theadsorbate partial pressure p are the parameters.

k = σ (2πMRT)1/2

The kinetic constant relate to the nature of the adsorbed species bythe molecular mass M (g/mol),

the specific molecular surface area (cm-²), and the gas constant (J/Kmol)


The basis of TPDp = C R T

The partial pressure relates to the gas concentration.

The TPD experiment requires the temperature dependencies of thesorption process: it is assumed that the process is thermally activated(not spontaneously occuring); only a fraction na will be adsorbed:

na = Aa exp (-Ea/RT)

kd = Ad exp (-Ea/RT) desorption process


The basis of TPD

The experiment produces a signal proportional to the change in gas concentration C with temperature

C = -S/F dN/dTS denotes surface area in (cm²/g), F the flow rate (cm³(STP)/sg)

The experiment is time-programmed; one obtains C as function of time t as initial observation data:


The basis of TPD

F + σ (RT/2πM)1/2 na (N*-N)

S N kdC(t) =

It is important to run the experiment strictly linear in temperature:

T = T0 + βT with β being the heating rate in (K/s)

na is the fraction of adsorbing molecules from thestream of species A and Θ is the site occupancy: Θ = N/N*


Reaction rate: first order S + N* Θ Ad exp(-E

d/RT)

F + S N* (1- Θ) σ (RT/2πM)1/2 Aa exp (Ea/RT)

C(T) =

dΘ/dT = -F

S β N*C(T)

One observes the concentration of the adsorbate CT as function of sample temperature

and can relate this to the physically relevant change in surface coveragewith temperature dΘ/dT

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