TPX Temperature Programmed Techniques - ETH Z · 2016. 10. 7. · Temperature programmed reduction...

Dr. Davide FerriPaul Scherrer Institut� 056 310 27 81� [email protected]

TPX

Temperature Programmed Techniques

Definitions

� Temperature programmed desorption (TPD), orthermal desorption spectroscopy (TDS)

� Temperature programmed reduction (TPR)

� Temperature programmed reaction (TPR)

� Temperature programmed oxidation (TPO)

� Temperature programmed reaction spectroscopy (TPRS)

time

upta

ke, d

esor

ptio

n ra

te…

temeprature

Definitions

� TPD

A solid is first exposed to an adsorbate gas under well-defined conditions (temperature and pressure) and then heated under inert conditions with a temperature program

� TPX but TPD

Solid and reactants in contact during temperature programmed experiment

Information

� Tred, Tdes, Tox, Treact

� Adsorption site strength, quality and quantity� Bond strength, solid-adsorbate� Surface coverage� Quantification of desorption� Adsorption enthalpy + pre-exponential factor for desorption

� Advantages� Experimentally simple� Inexpensive� Access to powders and single crystals

� Disadvantages� Complex determination of activation energies and pre-exponential

factors

temperature

upta

ke,

deso

rptio

n ra

te…

Temperature programmed techniques

� Equipment

gas

airNH3ArCO2COHeH2CH4N2O2H2O

λ�10-3 [W/(cmK)]

0.2770.2700.1900.1830.2671.5741.9720.3740.2750.2850.195

Thermal conductivity

Reactant and carrier gasesTPR: 5 vol.% H2/Ar (or He)TPO: 5 vol.% O2/He temperature temperature

consumptionup

take

(A)

(B)

J.W. Niemantsverdriet, Spectroscopy in catalysis, VCH, Weinheim, 2007

Temperature programmed reduction

MOn + nH2 → M + nH2O

From thermodynamics:

∆G = ∆G0 + nRT ln(pH2O/pH2) < 0

If H2 is the reducing agent,

∆G = nRT ln[(pH2O/pH2)/(pH2O/pH2)eq]

then ∆G <0, if

pH2O/pH2 < (pH2O/pH2)eq

� Reduction of a bulk metal oxide

� Reduction of a bulk metal oxide

BUT!reduction of a supported MO x can produce completely different TPR patterns


thermodynamic data for reduction (400°C)

metal

Ti

V

CrMn

Fe

CoNiCu

Mo

RuRhPdAgIr

(pH2O/pH2)eq

4�10-16

2�10-9

6�10-4

2�10-11

3�10-9

102�10-10

0.70.150

5002�108

2�106

400.021012

1013

1014

3�1017

1013

oxide

TiO2TiOV2O5VOCr2O3MnO2MnOFe2O3FeOCoONiOCuOCu2OMoO3MoO2RuO2RhOPdOAg2OIrO2

10

0

-10

-20

-30

105

100

10-5

10-10

T (K)

1/T�103 (K-1)0 1 2 3 4 5

lnp H

2O/p

H2

p H2O

/pH

2

Fe

Fe3O4FeO

Fe/FeO

Fe/Fe3O4

FeO/Fe3O4

1000 500 200

Fe2O3

� Reduction mechanisms

Rate of reduction of MOn + nH2 → M + nH2O

- d[MOn]/dt = kred[H2]p f([MOn])

kred, rate constant of reduction reactionp, reaction order in H2

t, time

If α is the degree of reduction, if p=0 (excess H2), if linear T-ramp (dT = β dt) and using Arrhenius equation

dα/dT = ν/β e-Ered/RT f(1-α)

ν, pre-exponential factorβ, heating rateEred, activation energy of reduction reaction


� Reduction mechanisms

shrinking core f(α) = 3 (1-α)1/3

nucleation and growth f(α) = (1-α)[-ln(1-α)]2/3


degr

ee o

f red

uctio

n, α

1.0

0.5

0.0

time

metal oxide

f(α

)

1.0

0.5

0.0

degree of reduction, α0.0 0.5 1.0

f(α)

� Activation energy of reduction

ln(β/T2max) = -Ered/RTmax + ln(νR/Ered) + K

if f(1-α) and α(Tmax) independent of β.

temperature (K)

H2

upta

ke

Tmax


600 650 700 750550500

Fe2O3

wet H2

dry H2

temperature (K)

1000/T1.4 1.6 1.81.2

lnβ

/T2 m

ax

-20

-18

-16

-14

wet H2172 kJ/mol

dry H 2111 kJ/mol


� Reduction of a bulk metal oxide…a somewhat different phase diagram

J. Zielinski et al., Appl. Catal. A: General 381 (2010) 191

two-steps mechanism

three-steps mechanism

one-step mechanism?Fe2O3 → Fe


� Effect of exp. conditions


� TPR profile dependent on exp. Conditions� sample amount, H2 conc., H2O, T ramp…

� Effect of fed water on pH2O/pH2?



� Effect of fed water on pH2O/pH2� High ratio, three step mechanism� Low ratio, two step mechanism

α

β1

β2

� Evidence of mechanism from XRD


� Evidence of mechanism from XRD


one-step mech.two-steps mech.

1 h-Fe2O32 c-Fe3O44 c-Fe

very low pH2O/pH2low pH2O/pH2

100 200 300 4000 100 200 300 4000 100 200 300 4000

Rh/SiO2 Fe/SiO2 FeRh/SiO2Fe:Rh 1:1

temperature (°C) temperature (°C)temperature (°C)

H2

upta

ke (

mol

H2/

mol

met

al)

� Supported oxides and bimetallic catalysts

as prepared

after oxidationRh3+→Rh

Rh–O

Rh–ClFe2O3→Fe3O4Fe3O4→Fe Rh3+→Rh

H2→2H*Fe3+(Rh)→Fe(Rh)



� Reduction mechanisms, PrCoO 3

� Co3++1e-�Co2+

� Co2++2e-�Co

Fierro et al., J. Mater. Sci. 23 (1988) 1018

2. nucleation

1. shrinking core Co3++1e-�Co2+

Co2++2e-�Co

fast

slow

700°C

550°C

500°C

150°C

++

+ +

+

200°CLa3Co3O8

300°CLa2Co2O5

La2O3+Co

25 30 35 40 45 50 55 60 65

2θ

25°C♦LaCoO3

Co3O4

x

CoO

*

*

*

CoLaCoO3

0 100 200 300 400 500 600 700

-10

-8

-6

-4

-2

0

TGADTG

temperature (°C)

wei

ght l

oss

/ %

Pd/LaCoO3

No information on Pd

� Reduction of 0.5 wt.% Pd/LaCoO 3: H2-TPR XRD


Chiarello et al., J. Catal. 252 (2007) 127 + 252 (2007) 137

24.32 24.36 24.40 24.44 24.480.0

0.2

0.4

0.6

0.8

1.0

1.2

r.t.50°C100°C140°C210°C320°C600°C

abso

rptio

n / a

.u.

energy (keV)

Pd K-edge

� Reduction of 0.5 wt.% Pd/LaCoO 3: H2-TPR XANES

Pd2+→Pd

TPRPd/LaCoO3

weight loss / %

0 50 100 150 200 250 300 350 400

0.0

0.2

0.4

0.6

0.8

1.0

frac

tion

of P

d2+

T (°C)

LCA-10

-8

-6

-4

-2

0


Chiarello et al., J. Catal. 252 (2007) 127 + 252 (2007) 137

Eyssler et al., J. Phys. Chem. C 114 (2010) 4584

24.3 24.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

24.35 24.45

0.2

0.4

0.6

0.8

1.0

1.2

24.3 24.4

0.0

24.35 24.45energy (keV) energy (keV)

abso

rptio

n (a

.u.)

2 wt.% Pd/LaFeO 3 LaFe0.95Pd0.05O3

� Reduction of Pd-containing perovskites

29°C(He)28°C29°C49°C76°C107°C550°C

29°C (He)29°C28°C35°C66°C95°C

123°C150°C196°C427°C479°C565°C

10 vol.% H2/He, 10°C/min, 50 ml/min


Eyssler et al., J. Phys. Chem. C 114 (2010) 4584

24.3 24.4

0.0

0.2

0.4

0.6

0.8

1.0

1.2

24.35 24.45

0.2

0.4

0.6

0.8

1.0

1.2

24.3 24.4

0.0

24.35 24.45energy (keV) energy (keV)

abso

rptio

n (a

.u.)

2 wt.% Pd/LaFeO 3

� Reduction of Pd-containing perovskites

29°C(He)28°C29°C49°C76°C107°C550°C

29°C (He)29°C28°C35°C66°C95°C

123°C150°C196°C427°C479°C565°C

10 vol.% H2/He, 10°C/min, 50 ml/min


LaFe0.95Pd0.05O3


10008006004002000

temperature (°C)

CeO2

Rh/CeO2

H2

cons

umpt

ion

� Reduction of CeO 2-based catalystsRh/CexZr1-xO2

x, molar content C

eO2

CexZr1-xO2 + δH2 → CexZr1-xO2-δ + δH2O + V0

δ

x

Red. conditions: CeO2 → Ce2O3Ox. conditions: Ce2O3 → CeO2

Boaro et al., Catal. Today 77 (2003) 407

Temperature programmed desorption

� Oxygen mobility in perovskite-type oxides – O 2-TPD� surface oxygen (α): suprafacial catalysis� lattice oxygen (β): intrafacial catalysis

LaMnO3±δ La0.9Ce0.1MnO3±δ

α

β

Temperature programmed desorption

� Distribution of acid sites – NH 3-TPD� which sites are present?� how strong are the sites?� which sites are catalytic relevant?� site structure?

temperature (K)

deso

rptio

nra

te (

a.u.

) NO

Xconversion

(%)

300 400 500 600 700 8000

25

50

75

100

Greenhalg et al., J. Mol. Catal. A 333 (2010) 121

4NO + 4NH3 + O2� 4N2 + 6H2O

� NH3 ads. at given T� NH3 desorption upon heating

TPX Temperature Programmed Techniques - ETH Z · 2016. 10. 7. · Temperature programmed reduction...

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