Water-Gas Shift Reaction

Water-Gas Shift Reaction

Dimitris I. Kondarides

Laboratory of Heterogeneous CatalysisDepartment of Chemical Engineering

University of Patras

GR 26504 Patras, GREECE

RESTOENE Workshop8-10 June 2011

Residencia la Cristalera, Miraflores de la Sierra, Madrid, Spain

1. Introduction

2. Industrial WGS catalysts and reactors

• Effect of the nature of the support

• Effect of the nature of the metallic phase

• Effect of addition of alkali promoters

• Effect of metal loading and crystallite size

• Effect of primary crystallite size of the support

4. Conclusions

Outline

• Thermodynamics

• Industrial catalysts and reactors

• HTS catalysts

• LTS catalysts

• Reaction mechanism and kinetics

3. WGS catalysts for fuel cell applications

“Water-Gas” is a synthesis gas containing H2 and CO, originally made by passing steam over red-hot coke or coal.

Introduction

Heat supply for this endothermic reaction was usually provided by alternating the steam with an air stream.

Water gas apparatushttp://chestofbooks.com/crafts/scientific-american/sup4/Apparatus-For-The-

Production-Of-Water-Gas.html

COH HC O+ ↔ ++ ↔ ++ ↔ ++ ↔ +2 2

∆ = 131.2 kJ/molH

2 2C OO C+ ↔+ ↔+ ↔+ ↔

∆ = 393.5 kJ/molH −

2 2C CO2 O+ ↔+ ↔+ ↔+ ↔

∆ = 221 kJ/molH −

Introduction

2 2H OC CO H+ ↔ ++ ↔ ++ ↔ ++ ↔ +

oxygen,

1 131.2

24 2H OCH CO 3H+ ↔ ++ ↔ ++ ↔ ++ ↔ + 3 206.3

24 2

1C

2OH CO 2H+ ↔ ++ ↔ ++ ↔ ++ ↔ + 2 35.6−

24 2CH 2 HCO CO 2+ ↔ ++ ↔ ++ ↔ ++ ↔ + 1 247.4

2H /CO ratio ∆ (kJ/mol)H

With the exception of partial oxidation, reactions are generally endothermic.

The molar ratio of H2 to CO varies depending on the source of carbon/oxygen.

Steam reforming reactions are mostly used when the ultimate objective is generation of pure hydrogen.

or CO2.

Today, water gas (synthesis gas) can be manufactured by the reaction of a carbonaceous material (e.g., coal, coke, natural gas, naphtha, etc.) with steam,

The water-gas shift (WGS) reaction

The reaction has been first reported in 1888 and was then used widely as a source of hydrogen for the Haber process for the manufacture of ammonia.

E. MethanationF. Ammonia synthesisG. NH3 separation

Α. Steam reformingB. HT-WGSC. LT-WGSD. CO2 adsorption

2 2+ ↔ ++ ↔ ++ ↔ ++ ↔ +2CO H O CO H

41 1∆Η= . kJ/mol−

The reaction was catalyzed by oxides of iron and chromium(BASF) at 400-500 οC.

The WGS reaction is used to produce H2, to reduce CO content in a H2-rich stream or to adjust the CO/H2 ratio of water gas.

Ammonia plant

The water gas is used extensively in the industry for the manufacture of

� Ammonia

� Methanol

� Hydrogen

� Hydrocarbons (Fischer-Tropsch process)

� Hydrotreating� Hydrocracking of petroleum fractions� Hydrogenations in the petroleum refining and petrochemical industry

� Metals (reduction of the oxide ore)

E. MethanationF. Ammonia synthesisG. NH3 separation

Α. Steam reformingB. HT-WGSC. LT-WGSD. CO2 adsorption

Industrial applications

H2 production process

The WGS reaction is a reversible, moderately exothermic and equilibrium limited.

100 200 300 400 500 6000

2

4

6

8

10

ln K

p

Temperature (oC)

Variation of the equilibrium constant of

the WGS reaction with temperature

Thermodynamics of the WGS reaction

2 2+ ↔ ++ ↔ ++ ↔ ++ ↔ +2CO H O CO H

41 1∆Η= . kJ/mol−High CO conversions can only be achieved at low temperatures, however with favorable kinetics at higher temperatures.

4 7 2

2

5693.5 49170ln( ) 1.077 ln 5.44 10 1.125 10 13.148

eqK T T T

T T

− −= + + × − × − −

The equilibrium constant decreases by a factor of 80 with increase of temperature from 200 to 400 oC.

2 2

2

H CO

H O CO

P

P PK

P P≈≈≈≈

Thermodynamically, the efficiency of the WGS reaction is maximized at low temperature, high water and low hydrogen concentration.

0 100 200 300 400 5000

10

20

30

40

50

60

70

80

90

100

Convers

ion o

f C

O (

%)

Temparature (oC)

3% CO-10% H2O

3% CO-10% H2O-6% CO

2

3% CO-10% H2O-20% H

2

3% CO-10% H2O-20% H

2-6% CO

2

The industrial realization of WGS takes place in a series of adiabatic converters where the water gas is converted in two stages.


2 2+ ↔ ++ ↔ ++ ↔ ++ ↔ +2CO H O CO H

41 1∆Η= . kJ/mol−

Variation of the equilibrium CO conversion

with feed composition and temperature

CO levels typically achieved at the exit

of the HTS and LTS reactors

200 300 400 5000

1

2

3

4

5

% C

O a

t exit (

dry

basis

)

Temperature (oC)

3%CO + 10%H2O

+ 6%CO2

+ 20%H2

+ 6%CO2 + 20%H2

The industrial realization of WGS takes place in a series of adiabatic converters where the water gas is converted in two stages.


2 2+ ↔ ++ ↔ ++ ↔ ++ ↔ +2CO H O CO H

41 1∆Η= . kJ/mol−

A high temperature shift (HTS) reactor is used for rapid CO conversion

CO levels typically achieved at the exit

of the HTS and LTS reactors

200 300 400 5000

1

2

3

4

5

% C

O a

t exit (

dry

basis

)

Temperature (oC)

HT shift

LT shift

Inter-bed cooling

and a LTS reactor is

used to shift equilibrium toward H2 production.

Water gas

HT CO shift LT CO shift

WGS reactors

2 2+ ↔ ++ ↔ ++ ↔ ++ ↔ +2CO H O CO H

41 1∆Η= . kJ/mol−

Water gas


WGS reactors

Industrial catalysts and operating conditions

The catalyst used in HTS reactors is usually promoted Fe/Cr oxide.

HTS reactor

Typically, the inlet temperature is 300-360 oC and the total pressure between 10 and 60 bar.

T: 350 oC

P: 10-60 bar

�� 450 oC

Fe/Cr oxideUnder normal operating conditions, the temperature rises progressively through the catalyst bed and can increase up to 500 oC.

The CO content can be reduced to 4% or lower. CO: ~ 45%�� 4%

When used in conjunction with LTS, the exit gas of the HTS unit must be cooled.

This is usually done by quenching with water, thus providing additional steam to the process.

2 2+ ↔ ++ ↔ ++ ↔ ++ ↔ +2CO H O CO H

41 1∆Η= . kJ/mol−

Water gas


WGS reactors

Industrial reactors and catalysts

The catalyst used in LTS reactors is usually promoted Cu/Zn/Al.

LTS reactor

The inlet temperature is 190-230 oC and the total pressure does not usually exceed 40 bar.

T: 350 oC

P: 10-60 bar

�� 450 oC

Fe/Cr oxideInlet CO concentration varies between 1 and 5% depending on the performance of the HTS installed upstream.

Exit temperatures can reach 280 oC and the CO content is typically reduced to < 0.5%. CO: ~ 45%�� 4%

T: 200 oC

P: < 40 bar

�� 280 oC

Cu/Zn/Al oxide

CO: 1- 4% �� <0.5%

Commercial LTS catalysts are kinetically limited at <190 oC even at moderate space velocities.

Because of the relatively low melting point of Cu (1084 oC), the catalyst is sensitive to deactivation caused by sintering and the maximum operating temperature should not exceed 280 oC.

Industrial HTS converters exclusively apply Fe-based catalysts because of their excellent thermal stability, poison resistance and good selectivity.

Industrial HTS catalysts

The commercially available catalysts are applied in the form of pellets, containing 8-12% Cr2O3, and small amounts of CuO (~2%) as an activity and selectivity enhancer.

The stable ion phase under reaction conditions is Fe3O4 (magnetite).

This is combined with chromia, which minimizes catalyst sintering by textural promotion.

Specific surface area 30-100 m2/g,

depending of Cr2O3 content and

calcination temperature

Industrial HTS catalysts

The commercially available catalysts are applied in the form of pellets, containing 8-12% Cr2O3, and small amounts of CuO (~2%) as an activity and selectivity enhancer.

The catalyst is usually unsupported and available commercially in tablet or ring form.

(a) Toxicity of the water-soluble Cr6+ ions

Major drawbacks

(b) Low volumetric catalytic activity (GHSV= 10000-15000 h-1), especially at low temperatures where CO conversion is favored thermodynamically.

Effects of promotion of Fe-Cr oxide catalyst on CH4 formation and C2+ hydrocarbon production

Ratnasamy and Wagner, Chem. Rev. 51 (2009) 325-440

CH4 C2+ hydrocarbons

Commercial Fe/Cr/Cu catalysts must be activated before operation using a specific process to control reduction of the oxides to the catalytically active sites.

Industrial HTS catalysts – Activation

Improper deactivation has a detrimental effect on the activity and life of the catalyst.

The catalyst is typically prepared via a precipitation process in the form of Fe2O3

(hematite).

The active Fe3O4 catalyst (magnetite) is formed upon reduction with gas mixtures containing H2, N2, CO, CO2 and H2O.

2 3 3 43Fe O + 2Fe O OH + H→→→→2 2 16 3∆Η= . kJ/mol−

2 3 3 4 23Fe O + 2Fe O CO + CO→→→→ 24 8∆Η= . kJ/mol+

Special care should be taken to avoid further reduction of the active phase (Fe3O4) toward lower oxides, carbides or metallic Fe.

Metallic Fe catalyzes the unwanted methanation and Fischer-Tropsch reactions

2 4 2CO + 3H CH + H O→→→→ 206 2∆Η= . kJ/mol−

These side reactions are unwanted because they lead in:

Industrial HTS catalysts – Activation

(a) Consumption of H2

2

2 2

[CO] [H ]

[CO ] [H O]R

++++====

++++

(b) Development of hot spots in the reactor

(c) Lowering of the mechanical strength of the catalyst pellets and, therefore, increase of the pressure drop in the reactor.

In industrial practice, a reduction factor (R) is used,

which allows prediction of the degree of catalyst reduction as a function of feed composition.

When R < 1.2, there are usually no problems related to reduction of magnetite.

The opposite is true for R > 1.6.

The activated Fe3O4 catalyst is pyrophoric.

Upon exposure to air, the catalyst must be re-reduced and stabilized by surface oxidation using an inert gas with low concentration of oxygen.

Fe2O3-Cr2O3 catalysts have a lifetime of 3-5 years depending, mainly, on the temperature of operation.

The commercial LTS catalyst is composed of copper, zinc oxide, and alumina.

Industrial LTS catalysts

Water gas


WGS reactors

Utilization of this sulfur-sensitive catalyst became possible only after the development of highly efficient hydrodesulfurization technolo-gies using Co(Ni)-MoO3-Al2O3 catalysts (< 1.0 ppm sulfur).

Exit temperatures can reach 380 oC and the CO content is typically reduced to < 0.5%.

Cu/Zn/Al commercial catalysts are applied in the form of tablets, extrusions, or spheres and are usually produced by co-precipitation of metal nitrates.

The active phase is copper, which remains active at temperatures as low as 190-200 oC.

ZnO provides some protection of Cu from sulfur poisoning by reaction with adsorbed sulfur compounds.

ZnO and Al2O3 protect copper against sinteringduring activation.

CuO-ZnO-Al2O3

The principal deactivation mechanism for LTS catalysts is poisoning by sulfur and chlorides contained in the process gas.

Industrial LTS catalysts

The deactivation process begins as soon as the catalyst is placed on stream and CO leakage is usually detected within 6-12 months.

Many plants have installed guard beds of LTS catalysts immediately ahead the main LTS unit (about ¼ the size of the main unit).

The aim is to promote WGS and to sacrificially screen poisons from the main bed.

Trapping of S and Cl “poisons” over LTS catalysts.SYNETIX (Katalco 83-3)

Mechanism of sulfur retention

Mechanism of chloride protection

2 222Cu + Cu HH S + S →→→→

59 4−∆Η= . kJ/mol

Care must be taken to avoid steam condensation and to minimize the re-oxidation of the catalyst upon shutdown.

Industrial LTS catalysts – Activation

Water gas


WGS reactors

Careful startup, inert purging to prevent condensation and sequestration during shutdown of industrial reactors can increase the life of the catalyst from months to years.

Before reduction, the composition of the catalyst is typically:32-33 wt.% CuO34-53 wt.% ZnO, and15-33 wt.% Al2O3

The reduction of CuO is highly exothermic and is carried out at temperatures not exceeding 220-230 oC to avoid sintering.

When formulated properly and operated under standard LTS conditions, Cu-ZnO-Al2O3

catalyst lasts a few years.

T: 200 oC

P: < 40 bar

�� 280 oC

Cu/Zn/Al oxide

CO: 1- 4% �� <0.5%

For the same reason, reaction temperature should not exceed 300 oC.

The kinetics and reaction mechanism of the WGS reaction have been investigated extensively and various mechanisms have been proposed.

WGS mechanisms and kinetics

Cu/Cu+ Fe2+/Fe3+The reactants induce a cyclic change of the oxidation state of the catalytic material.

(A) Redox mechanism

Η2Ο + Red �� H2 + Ox

CO + Ox �� CO2 + Red

Water decomposes toward H2 and O on the reduced catalyst surface

This is followed by reduction of the catalyst by CO, which leads to evolution of CO2.

CO + Η2Ο�� I �� CO2 + Η2

HCOOH

Acidic oxide �� CO +H2O

Metal/metal oxide �� CO2 +H2

Chemisorbed CO and H2O interact on the catalyst surface to form and intermediate, which then decomposes to yield reaction products.

(B) Associative mechanism

Fields of research in WGS catalysis

� Replacement of Cr by non-toxic elements.

� Development of more active catalysts that can operate at gas hourly space velocities above 40000 h-1 (e.g., promotion with noble metals).

� Development of sulfur tolerant WGS catalysts

� Development of catalysts that can operate at lower steam to gas ratios (lower operating costs).

Why is there still interest in the WGS reaction ?

Fuel Cell applications

88--10% CO10% CO

CO+HCO+H22O O COCO22+H+H22

T= 350 T= 350 -- 400400OOCC

<50 ppm CO<50 ppm CO

HH22

Fuel, air, steamFuel, air, steam

Steam reforming of fuele.g. CH4, CH3OH,

C2H5ΟH, gasoline, ect.

Electricity

Heat

PEM

Fuel Cell

33--5% CO5% CO

steamsteam

0.30.3--1% CO1% COHigh Temperature

WGS

Low Temperature

WGS

steamsteam

CO+HCO+H22O O COCO22+H+H22

T= 190 T= 190 -- 240240OOCC

COCO ++ 1/21/2ΟΟ22 COCO22

T=T=120120--151500OOCC

Preferential oxidation of

CO

airair

H2 for fuel cell applications

Commercially available WGS catalysts (Cu-ZnO, Fe-Cr) can not be used in fuel cell application, due to problems related to:

� Volume, weight and cost (30-50% of the fuel processor)

� Transient response to changes in feed composition and temperature

� Pyrophoricity

� Deactivation in the presence of excess steam

� A lengthy precondition step is necessary for catalyst activation

Advantages of noble metal catalysts

� High activity at a wider temperature range

� No need for activation prior to use

� No degradation on exposure to air or temperature cycles

� Availability of wash-coating technologies, which may result in

- reduced size and weight

- improved ruggedness

H2 for fuel cell applications

Identification of the key parameters which determine catalytic activity of supported noble metal catalysts

To develop active, selective and stable LT-WGS catalysts suitable for Fuel Cell applications

• Effect of the nature of the support

• Effect of the nature of the dispersed metallic phase

• Effect of addition of alkali promoters

• Effect of metal loading and crystallite size

• Effect of primary crystallite size of the support

WGS catalysts for fuel cell applications

The WGS reaction may take place over noble metals (e.g., Au, Pt-group metals) dispersed on metal oxide supports.

All catalysts were prepared with the wet impregnation method, followed by reduction with H2 at 300oC.

(NH3)2Pt(NO2)2

Ru(NO)(NO3)3

(NH3)2Pd(NO2)2

Rh(NO3)3

Commercial oxide powders used as supports

““ReducibleReducible”” oxides oxides ““IrreducibleIrreducible”” oxidesoxides

CeO2 (3.3 m2/g)

TiO2 (50 m2/g)

MnO (0.4 m2/g)

YSZ (12 m2/g)

La2O3 (7.0 m2/g)

Al2O3 (83 m2/g)

MgO (22 m2/g)

SiO2(144 m2/g)

x% Me/MOxMe= Pt, Ru, Rh, Pd

x= 0 - 5 wt.%

Metal precursors

Catalysts

0.5%Pt/MOxThe catalytic performance of Pt is improved significantly when supported on “reducible” rather than on “irreducible” metal oxides.

WGS activity of Pt catalysts supported on

commercial oxide supports.

200 300 400 5000

20

40

60

80

100

TiO2

La2O

3

CeO2

YSZ

MnO

Al2O

3

MgO

SiO2

Convers

ion o

f C

O (

%)

Temperature (oC)

Experimental conditions

Temperature range: 150 – 550oC

Mass of catalyst: 100 mg

Particle size: 0.18 < dp < 0.25 mm

Total flow rate: 200 cm3/min

Feed composition: 3%CO + 10% H2O

(balance He)

Effect of the nature of the support – Pt catalysts

A strong effect of the support on

catalytic activity of Pt is observed.


0.5%Pt/MOx

1.2 1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

CeO2

YSZ

Al2O

3

La2O

3 TiO2

MnO

MgOSiO

2

TO

F (

s-1)

1000/T (K-1)

Turnover frequencies (TOF) of Pt catalyst

dispersed on the indicated metal oxides

The TOF of Pt supported on TiO2, CeO2

and La2O3 is 1-2 orders of magnitudehigher than that of Pt supported on irreducible oxides.

For example, at 250oC, TOF of Pt/TiO2 is:

~ 90 times higher than that of Pt/SiO2

~ 22 times higher than that of Pt/Al2O3.

0.5%Pt/MOx

4921.4CeO2

8615.2TiO2

10013.3SiO2

5417.2MgO

2424.6La2O3

1827.4MnO

6224.9Al2O3

6627.9YSZ

Dispersion

%

Ea

(kcal/mol)

Metal

Oxide

Apparent activation energy (Apparent activation energy (EEaa) and ) and

dispersion of platinumdispersion of platinum

1.2 1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

CeO2

YSZ

Al2O

3

La2O

3 TiO2

MnO

MgOSiO

2

TO

F (

s-1)

1000/T (K-1)

Turnover frequencies (TOF) of Pt catalyst

dispersed on the indicated metal oxides


0.5%Ru/MOx

1.6 1.7 1.8 1.9 2.0 2.1

0.01

0.1

1

CeO2

Al2O

3TiO

2

YSZ

TO

F (

s-1)

1000/T (K-1)

200 300 400 5000

20

40

60

80

100

Co

nvers

ion

of C

O (

%)

Temperature (oC)

TiO2

CeO2

YSZ

Al2O

3

Arrhenius plot of TOFs of Ru dispersed on

the indicated oxidesCatalytic performance of Ru supported on

selected commercial oxides.

Effect of the nature of the support – Ru catalysts

The WGS activity of NM catalysts depends strongly on the nature of the support.

200 300 400 5000

20

40

60

80

100

Ru

Rh

Pd

Pt

Convers

ion o

f C

O (

%)

Temperature (oC)

0.5%M/TiO2

Effect of reaction temperature on the

conversion of CO over Pt, Rh, Ru and Ru

catalysts supported on TiO2.

Effect of the nature of the dispersed metal

0.5%M/TiO2

1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

Rh

Pd

Ru

Pt

TO

F (

s-1)

1000/T (K-1)

Arrhenius plot of TOFs obtained over

Pt, Rh, Ru and Pd dispersed on TiO2


The apparent activation energy of the reaction is practically the same for all metals examined:

15.7 – 17.1 kcal/mol

This implies that the dominating

contribution to Ea originates from a

reaction step associated with the support (e.g. water adsorption/ activation, surface reaction, etc.)

Ea does not depend on the nature

of the metal but, mainly, on the nature of the support.

TOF follows the order:

Pt > Rh > Ru > Pd

with Pt being about 20 times more active than Pd.

0.5%M/TiO2

1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

Rh

Pd

Ru

Pt

TO

F (

s-1)

1000/T (K-1)




0.5%M/CeO2

1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

Pd

Ru

Pt

Rh

TO

F (

s-1)

1000/T (K-1)

Ea= 24 kcal/mol


Pt, Rh, Ru and Pd dispersed on CeO2

The dependence of TOF on the nature of the dispersed metal is relatively weak.

0.5%M/TiO2

1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

Rh

Pd

Ru

Pt

TO

F (

s-1)

1000/T (K-1)



x%Pt/TiO2

� Conversion of CO at a given temperature increases significantly with increasing Pt loading in the range of 0.1 – 5.0%.

1.6 1.8 2.0 2.2 2.41E-7

1E-6

1E-5

0.10.5

2.0

5.0

r CO (

mo

l.s

-1.g

ca

t-1)

1000/T (K-1)

200 300 400 5000

20

40

60

80

100

Pt loading

(wt.%)

0.0

0.1

0.5

2.0

5.0Co

nve

rsio

n o

f C

O (

%)

Temperature (oC)

Effect of metal loading / crystallite size - Pt/TiO2

� The activation energy of the reaction does not practically change.

x%Pt/TiO2

Reaction rate does not depend on Pt loading and crystallite size but only on the amount of exposed surface metal atoms.

1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

1.2<dPt

<3.1 (nm)

Pt loading

(wt. %)

0.1

0.5

2.0

5.0

TO

F (

s-1)

1000/T (K-1)

1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1<16.2 (nm)

2.0 (600oC, 2h)

5.0 (600oC, 2h)

5.0 (650oC, 4h)

5.0 (700oC, 4h)

TO

F (

s-1)

1000/T (K-1)

TOFs of CO conversion obtained over Pt/TiO2

catalysts of variable metal loading and crystallite size

The rates of transfer of catalytically active species to or from the support are fast, compared to the overall reaction rate.

Effect of metal loading / crystallite size - Pt/TiO2

Ru/TiO2

TOF does not depend on the structural and morphological characteristics of the dispersed metal, including loading, dispersion and mean crystallite size.

1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

Ru loading

(wt. %)

0.1

0.5

1.0

2.0

5.0

TO

F (

s-1)

1000/T (K-1)

1.0 < dRu < 4.5 (nm)

1.4 1.6 1.8 2.0 2.2 2.4

0.01

0.1

1

Pt loading

(wt. %)

0.1

0.5

1.0

5.0

TO

F (

s-1)

1000/T (K-1)

2.0 < dPt < 9.1 (nm)Pt/CeO2

Effect of metal loading / crystallite size

UV PC P25 AT

Composition 100 100 75 100(% anatase)

Crystallite 9 9 23 30

size (nm)

Surface 238 159 41 8area (m2/g)

Characteristics of the ΤiΟ2 powders

used as supports

X-ray diffractograms of commercial TiO2

powders used as supports

20 30 40 50 60 70 80

(310)(220)

(211)

(111)(101)

*

*

****

(110)

(213) (301)

(215)

(220)(116)

(204)

(211)(105)

(200)

(112)

(004)

(103)

(101)

AT

P25

UV

PC

Diffraction angle (2θ)

Inte

nsity

UV: Hombikat UV100, Sachtleben ChemiePC: PC-500, Millenium ChemicalsP25: P-25, Degussa

AT: AT-1, Millenium Chemicals

Effect of the morphology of the support -TiO2

0.5%Pt/TiO2

The catalytic performance of Pt is improved when supported on TiO2

with small primary crystallite size (high surface area).

200 300 400 5000

20

40

60

80

100

TiO2

support

UV

PC

P25

AT

Convers

ion o

f C

O (

%)

Temperature (oC)

Effect of the type of the TiO2 support on the

catalytic performance of dispersed Pt.


0.5%Pt/TiO2

� Reaction rate per surface Pt atom increases by more than two orders of magnitude (a factor of 120 at 250οC) by decreasing the primary crystallite size of the TiO2

carrier from 35 to 16 nm.

1.6 1.8 2.0 2.2 2.40.01

0.1

1

10

35

25 18

16 nm

AT P25

PC

UV

TO

F (

s-1)

1000/T (K-1)

15 20 25 30 350

2

4

6

8

TO

F a

t 2

50

oC

(s

-1)

Primary crystallite size of TiO2 (nm)

10

12

14

16

18

Activa

tio

n E

ne

rgy,

(kca

l/m

ol)

� This is accompanied by a decrease of the apparent activation energy of the reaction from 16.9 to 11.9 kcal/mol.


The catalytic performance of Pt is significantly improved when supported on TiO2

with smaller primary particle size.

Why ?

Redox mechanism

The strong dependence of TOF on the type of TiO2 support employed may be related to the effect of TiO2 crystallite size on its reducibility.

Associative mechanism

Structural characteristics of TiO2 may influence the type, number, density and reactivity of surface hydroxyl groups, which play a key role in the formation of formate-type intermediates.

R.J. Gorte and coworkers

� J. Phys. Chem. 100 (1996) 18128� J. Phys. Chem. 100 (1996) 785� Appl. Catal. B 17 (1998) 101

It has been reported that reducibility of small oxide clusters depends on its size (at least for CeO2 and La2O3)

Hydroxyl concentration on TiO2 surface decreases with increasing crystallite size.


100 200 300 400 500 600

500 ppm

AT

UV

PC

P25

H2 c

onsum

ptio

n

Temperature (oC)

0.5%Pt/TiO2

H2-TPR over preoxidized Pt/TiO2 catalysts

PtOx �� Pt

TiO2 � TiO2-x

Ti4+ + H(a) + O2- �� Ti3+ + OH-

Surface reduction of TiO2 is enhanced with decrease of

particle size of TiO2

dTiO2

Reducibility of TiO2 – H2 TPR

1000 800 600 400 200

(e)

(d)

(c)

(b)

(a)

SiO2

(*)(*)

AT

Realtiv

e I

nte

nsity,

a.u

.

Raman shift, cm-1

UV

PC

P25

O2, 30°C

SiO2

Pt/TiO2 catalysts

Ram

an

in

ten

sit

y (

a.u

.)

Raman shift (cm-1)

O2, 30oC

In situ Raman spectra obtained over preoxidized Pt/TiO2

catalysts under O2 flow at 30oC

AT

P25

PC

UV

SiO2

485 SiO2 (reactor)

Peak(cm-1)

Assignment

398

516 TiO2 (anatase)

640}

447

612TiO2 (rutile)}

0.5%Pt/TiO2Oxidizing

atmosphere

Reducibility of TiO2 – Raman spectroscopy

1000 800 600 400 200

(f)

(e)

(d)

(c)

(b)

(a)

SiO2

30oC

150oC

250oC

350oC

450oC

4.3% H2/N

2

Pt/TiO2(UV) catalyst

270315355

Rela

tive I

nte

nsity,

a.u

.

Raman Shift, cm-1

4.3%H2/N2

Ram

an

in

ten

sit

y (

a.u

.)

Raman shift (cm-1)

Pt/TiO2(UV)

SiO2

30οC

150οC

250οC

350οC

450οC

Peak

(cm-1)Assignment

355

315 Ti2O3

270}

30oC TiO2

150-250oC Ti2O3

350-450oC TiO, Ti2O(Raman silent)

H2 reduction


In situ Raman spectra obtained upon exposure of the

Pt/TiO2 catalysts to H2/N2 mixture at 30-450 oC.

1000 800 600 400 200

460

(A)

(c)

(b)

(a)

Pt/TiO2 catalysts

270320360

(*)

(*)

Rela

tive I

nte

nsity,

a.u

.

Raman shift, cm-1

UV

PC

P25

H2, 150°C

Ram

an

in

ten

sit

y (

a.u

.)

Raman shift (cm-1)

4.3%H2/N2

P25

PC

UVT= 150oC

dTiO2


In situ Raman spectra obtained upon exposure of the indicated

Pt/TiO2 catalysts to H2/N2 mixture at 150 oC.

1000 800 600 400 200

440

Pt/TiO2 catalysts

(B)

(c)

(b)

(a)

(*)

(*)

315355 270

Rela

tive

Inte

nsity,

a.u

.

UV

PC

P25

H2, 250°C

Raman shift (cm-1)

Ram

an

in

ten

sit

y (

a.u

.)

4.3%H2/N2

1000 800 600 400 200

PC

P25

UV

T= 250oC

P25

PC

UV

The reducibility of TiO2

increases with decrease of particle size.

dTiO2


In situ Raman spectra obtained upon exposure of the indicated

Pt/TiO2 catalysts to H2/N2 mixture at 150 oC.

100 200 300 400 500 600

500 ppm

UV

PC

P25

AT

CO

2 o

r H

2 p

rodu

ction

Temperature (oC)

0.5%Pt/TiO2

PtOx + xCO �� Pt + xCO2

TiO2 + xCO �� TiO2-x + xCO2

CO(M) + 2OH -(S) �� CO2(g) + H2(g) +O2-(S)

The reducibility of Pt/TiO2 catalysts increases with decrease of particle

size of the support.

Reducibility of TiO2 – CO TPR

CO-TPR over preoxidized Pt/TiO2 catalysts

H2-TPR, Raman, CO-TPR

200 250 300 350 400 450 5000

20

40

60

80

100

Pt/Al2O

3

Cu

MnCr

Fe

Ni

Co

Ti

Temperature (oC)

Convers

ion o

f C

O (

%)

The WGS activity of noble metals is improved when dispersed on reducible oxides (MOx) with small crystallite size.

Reducibility and MOx crystallite size

How can we use this result ?

Conversion of CO as a function of reaction temperature

obtained over 0.5%Pt/MOx/Al2O3

0.5%Pt /10%MOx/Al2O3

MOx can be dispersed on high surface areasupports, such as Al2O3 and TiO2




Feed composition:

3%CO + 10% H2O

150 200 250 300 350 400 4500

20

40

60

80

100

Gd

Nd

Ce

Ho

Pt/TiO2

Y

Temperature (oC)

Co

nve

rsio

n o

f C

O (

%)



obtained over 0.5%Pt/MOx/TiO2

0.5%Pt /10%MOx/TiO2




Feed composition:

3%CO + 10% H2O

Dispersion of Pt on combined mixed oxide catalysts results in materials with enhanced WGS activity.



obtained over Pt catalysts supported on mixed oxides

200 250 300 350 400 450 5000

20

40

60

CeOx/TiO

2

NdOx/TiO

2

CeOx/Al

2O

3

TiO2

Al2O

3

Co

nvers

ion

of C

O (

%)

Temperature (oC)




Feed composition:

3%CO + 10% H2O6% CO2 + 20% H2

Grenoble, Estadt, Ollis, J. Catal. 67 (1981) 90.

The relatively weak (factor of 20 - 30) dependence of TOF on the nature of the dispersed metal reflects the narrow range of strengths of CO interaction with the metals investigated.

Volcano-type correlation between TOF of metals

and their respective CO heats of adsorption.

Can we alter the metal-CO bond strength?

� Addition of promoters (e.g. alkalis)

� Use bimetallic catalysts

� Metal-support interactions (doping)

� Electrochemical promotion (NEMCA)

� ….

Methods to improve activity of noble metals

200 250 300 350 4000

20

40

60

80

100

Co

nvers

ion o

f C

O (

%)

Temperature (oC)

alkali (wt. %)

0.0 alkali

0.34 Cs

0.10 K

0.018 Li

0.06 Na

Effects of addition of alkalis on the WGS activity of Pt/TiO2 catalyst

1.8 2.0 2.2

1E-6

1E-5

1E-4

1000/T(K-1)

r CO(m

ol.g

cat-1

s-1)

alkali (wt. %)

0.0 % alkali

0.34% Cs

0.10% K

0.018% Li

0.06% Na

0.5%Pt/(TiO2-alkali) alkali : Pt = 1:1

Effect of addition of alkalis

200 300 400 5000

20

40

60

80

100

Convers

ion o

f C

O (

%)

Temperature (oC)

Cs content

(wt.%)

0.0

0.17

0.34

0.68

1.7 1.8 1.9 2.0 2.1 2.2 2.3

1E-6

1E-5

1E-4

Ea=16 kcal/mol

1000/T (K-1)

r CO(m

ol.g

ca

t-1s

-1)

x% Cs

0.0

0.17

0.34

0.68

Arrhenius plot of reaction rates over Cs-

promoted 0.5%Pt/TiO2 catalystsEffect of addition of Cs on the catalytic

performance of 0.5%/TiO2 catalysts

0.5%Pt/TiO2 - x% Cs


0.0 0.2 0.4 0.60.0

0.4

0.8

1.2

1.6

220oC

250oC

TO

F (

s-1)

Cs content (wt.%)0.00 0.05 0.10 0.15 0.20

0.0

0.4

0.8

1.2

1.6

220oC

250oC

Na content (wt.%)

TO

F (

s-1)

x% Cs x% Na

Effect of alkali-promotion on TOFs of Pt/TiO2 catalysts

In both cases, the maximum is observed for alkali:Pt ratios of 1:1


H2-TPD patterns obtained over alkali-promoted Pt/TiO2

Effect of addition of alkalis – H2 TPD

100 200 300 400

Promoter

none

Li

Na

Cs

K

Temperature (oC)

Alkali, X:Pt=1

� Hydrogen adsorbed on the metal

� Hydrogen adsorbed on the support

� Hydrogen adsorbed at the metal-support interface

The adsorption strength of sites located at the metal-support interface is affected strongly by the presence of alkali promotes

Desorption temperature reflects the chemisorption strength of surface sites

H2-TPD patterns obtained over alkali-promoted Pt/TiO2

x% Csx% Na


100 200 300 400 500

Na content

(wt.%)

100 ppm

0.00

0.017

0.12

0.20

0.06

Temperature (oC)

100 200 300 400 500

Cs content

(wt.%)

100 ppm

0.17

0.68

0.34

0.00

Temperature (oC)

100 200 300 400

Promoter

none

Li

Na

Cs

K

Temperature (oC)

Alkali, X:Pt=1

Effects of alkali promotion on the desorption temperature

(Tmax) of hydrogen adsorbed at the metal-support interface

Desorption temperature (Tmax) reflects the chemisorption strength of surface sites

0 1 2 3200

220

240

260

280 Promoter

none

Li

Na

K

Cs

Alkali:Pt atomic ratio

Tm

ax (

oC

) 100 200 300 400 500

wt.% Na

0.2

0.06

0.0

Temperature (oC)

Ru/Na-TiO2

100 200 300 400 500

wt.% Cs

0.68

0.34

0.0

Temperature (oC)

Pd/Cs-TiO2


Dependence of TOF at 250 oC on the desorption temperature

of hydrogen adsorbed at the metal-support interface

Lo

g (

turn

over

rate

)

-∆Hads (CO), kJ/mol

Lo

g (

turn

over

rate

)

-∆Hads (CO), kJ/mol

M/Al2O3

T= 300oC

Effect of addition of alkalis – H2-TPD

200 220 240 260 280

0.5

1.0

1.5

Promoter

none

Li

Na

K

Cs

Tmax

( oC)

TO

F250

oC (

s-1)

Alkali promotion of TiO2 affects the chemisorption strength of sites located at the metal-support interface

The effect is qualitatively similar to that observed for M/Al2O3 catalysts

CO TPR patterns obtained over alkali-

promoted Pt/TiO2 catalysts

100 200 300 400 500 600

H2

CO2

0.20% Na

0.12% Na

0.68% Cs

Unpromoted

500 ppm

CO

2,

H2 p

rodu

ctio

n

Temperature (oC)

Effect of addition of alkalis – CO TPR

Alkali promotion also affects the reducibility of TiO2

Alkali promotion of TiO2 affects the chemisorption strength of sites located at the metal-support interface

Qualitatively similar results were obtained upon doping of TiO2

with alkaline earth metals.

Performance of optimized catalyst

Effect of space velocity on the catalytic performance of 0.5%Pt/1%CaO-TiO2 catalyst

under realistic feed compositions

0.5%Pt/(1%CaO-TiO2)

200 250 300 350 400 4500

20

40

60

80

100

GHSV (h-1)

4000

7400

10000

Con

vers

ion o

f C

O (

%)

Temperature (oC)

200 250 300 3500

20

40

60

80

100

Convers

ion o

f C

O (

%)

Temperature (oC)

GHSV (h-1)

4000

7400

10000

HTS conditions: 9.7% CO38.7% H2O44.8% H2

6.8% CO2

LTS conditions: 1.6% CO29.9% H2O52.2% H2

16.3% CO2

0 10 20 30 40 50 600

20

40

60

80

100 Pt/TiO

2

Pt/TiO2(1% CaO)

T=300OC

Time-on-stream (h)

Con

vers

ion o

f C

O (

%)

Performance of optimized catalyst

0.5%Pt/(1%CaO-TiO2)

Long-term stability tests of Pt/TiO2 and Pt/TiO2(1%CaO) catalysts. Feed composition: 3%CO, 10% H2O, 20%H2, 6%CO2; T= 300 oC; SV= 29000 h-1.

4000 3500 2250 2000 1750 1500

1384

1690

1560

1625

1525

2120

2175

2068

2062

2060

36673711

3667

15661945

1690

1572

15251622

1837

36653727

1435

1579

2081

21122185

3603

450oC

400oC

350oC

300oC

250oC

200OC

150oC

100oC

75oC

25oC

Ab

so

rba

nce

(a

.u.)

Wavenumber (cm-1)

FTIR spectra obtained upon heating the

preoxidized Pt/TiO2(PC) catalyst under

1%CO flow from 25 to 450 oC.

0.5%Pt/TiO2 (PC) ν(CO) region

ν(OH) region

Formates/carbonates (support)

CO(Pt) + OH(TiO2) � [HCOO](Pt/TiO2)

(formate mechanism)

Mechanistic studies – CO TPR/DRIFTS

2250 2000 1750

2060

1830

1945

350oC

300oC

250oC

200OC

150oC

100oC

Abso

rba

nce

(a.u

.)

Wavenumber (cm-1)

0.5%Pt/TiO2 (PC)

O

C

O

C

O

C

TiOTiO22

Pt0

TiTi3+3+

Ptδ-

O

CCreation of new adsorption sites [Ti3+-Pt] at the metal-support interface

ΤΤiOiOxx

Retated to the SMSI effect ??

e-

Mechanistic studies – CO TPR/DRIFTS

Mechanistic studies – Active sites

2200 2000 1800 1600

1940

Ab

so

rban

ce

(a

.u.)

2043

1975

1745

2025

1820

450

400

350

300

250

200

150

100

25

wavenumber (cm-1)

Pt/TiO2-Cs(0.68%)

2200 2000 1800 1600

2056

1965

1755

2030

1815

450

400

350

300

250

200

150

100

25

Absorb

an

ce

(a.u

.)

wavenumber (cm-1)

Pt/TiO2-Na(0.2%)

2200 2000 1800 1600

2056

1830

1935

450

400

350

300

250

200

150

100

25

Abso

rba

nce

(a

.u.)

wavenumber (cm-1)

Pt/TiO2

FTIR spectra obtained following adsorption of CO (1%) at 25oC

and subsequent heating at 450οC under He flow




2200 2000 1800 1600

2063

1800

1740

17001925

25

450

400

350

300

250

200

150

100

Ab

so

rba

nce

(a

.u.)

wavenumber (cm-1)

2200 2000 1800 1600

1837

2077

450

400

350

300

250

200

150

100

25

Ab

sorb

ance (

a.u

.)

wavenumber (cm-1)

Pt/TiO2-CaO(1%) Pt/TiO2-WO3(4%)

X

2200 2000 1800 1600

400

350

300

250

200

150

100

1628

2050

2085

25

Ab

so

rba

nce (

a.u

.)

wavenumber (cm-1)

Pt/SiO2

X




2200 2000 1800 1600

2045

19752087

1630

1570

1822

400

350

300

250

200

150

100

25

Ab

so

rba

nce (

a.u

.)

wavenumber (cm-1)

2200 2000 1800 1600

2045

1995

1628

2091

1827

400

350

300

250

200

150

100

25

Ab

so

rba

nce

(a

.u.)

wavenumber (cm-1)

Pt/CeO2 Pt/YSZAdsorption sites at the metal-support interfaceare directly related with catalytic activity

The LF band in the ν(CO)

region is the fingerprintof active WGS catalysts

0,00 0,02 0,04 0,061E-8

1E-7

1E-6

1E-5

210OC

230OC

250OC

270OC

r CO(m

olg

ca

t-1s

-1)

PCO

(atm)

x% CO, 10%H2O, 20%H2, 6%CO2

Kinetic studies – Power low expression

0,04 0,08 0,12 0,16 0,20

1E-8

1E-7

1E-6

1E-5

r CO(m

ol.g

cat-1

s-1)

PH

2O (atm)

250OC

230OC

0,0 0,1 0,2 0,3 0,4 0,51E-8

1E-7

1E-6

1E-5

r CO(m

ol.g

ca

t-1s

-1)

PH

2

(atm)

250OC

230OC

0,00 0,05 0,10 0,15 0,201E-8

1E-7

1E-6

1E-5

r CO(m

ol.g

ca

t-1s

-1)

PCO

2

(atm)

250OC

230OC

3% CO, x%H2O, 20%H2, 6%CO2

3% CO, 10%H2O, x%H2, 6%CO23% CO, 10%H2O, 20%H2, x%CO2

T=210 T=210 –– 270 270 ooCC

2 22H O CCO O H====

cb dar k PP P P

(((( ))))0exp /= −= −= −= −

ak k E RT

0.5====a

1====b

CO0 6%= −= −= −= −P

2H O0 18%= −= −= −= −P

0≈≈≈≈c

0.7= −= −= −= −d

2CO0 19%= −= −= −= −P

2H0 50%= −= −= −= −P

1

0

- 10.31 mol g −−−−====k s

-

a

110.8 kcal mol====E

Kinetic studies – Power low expression

T=210 T=210 –– 270 270 ooCC

2 22H O CCO O H====

cb dar k PP P P

(((( ))))0exp /= −= −= −= −

ak k E RT

0.5====a

1====b

CO0 6%= −= −= −= −P

2H O0 18%= −= −= −= −P

0≈≈≈≈c

0.7= −= −= −= −d

2CO0 19%= −= −= −= −P

2H0 50%= −= −= −= −P

1

0

- 10.31 mol g −−−−====k s

-

a

110.8 kcal mol====E

1.5 1.5 -- 3% CO3% CO

10 10 -- 20%H20%H22OO

20 20 -- 40%H40%H22

6%CO6%CO22

1,3 1,4 1,5 1,6 1,7 1,8 1,9 2,0 2,1 2,21E-7

1E-6

1E-5

Ea=10.8kcal/mol

1000/T (K-1)

r CO(m

olg

cat-1

s-1)

3% CO, 10% H2O, 20% H

2, 6% CO

2

3% CO, 20% H2O, 20% H

2, 6% CO

2

3% CO, 10% H2O, 40% H

2, 6% CO

2

1.5% CO, 10% H2O, 20% H

2, 6% CO

2

Fitting of experimental data to the power-low expression

Conventional catalysts and reactors cannot be used in fuel cell applications, mainly due to restrictions in volume and weight.

Noble metal catalysts with proper structural and morphological characteristics may be considered as promising candidates as WGS catalysts for fuel cell applications.

Conclusions

The WGS technology is well established and widely used in large scale steady-state operations, including manufacture of ammonia, methanol, refinery hydrogen Fischer-Tropsch synthesis, etc.

References

• “Effect of morphological characteristics of TiO2-supported noble metal catalysts on their activity for the water-gas shift reaction”, P. Panagiotopoulou, D.I. Kondarides, J. Catal. 225 (2004) 327-336.

• “Effect of the nature of the support on the catalytic performance of noble metal catalysts for the Water-Gas Shift Reaction”, P. Panagiotopoulou, D.I. Kondarides, Catal. Today 112 (2006) 49-52.

• “Particle size effects on the reducibility of titanium dioxide and its relation to the Water-Gas Shift activity of Pt/TiO2 catalysts”, P. Panagiotopoulou, A. Christodoulakis, D.I. Kondarides, S. Boghosian, J. Catal. 240 (2006) 114-125.

• “Water-gas shift activity of doped Pt/CeO2 catalysts”, P. Panagiotopoulou, J. Papavasiliou, G. Avgouropoulos, T. Ioannides, D.I. Kondarides, Chem. Eng. J.134 (2007) 16-22.

• “A comparative study of the water-gas shift activity of Pt catalysts supported on single (MOx) and composite (MOx/Al2O3, MOx/TiO2) metal oxide carriers”, P. Panagiotopoulou, D.I. Kondarides, Catal. Today, 127 (2007) 319-329.

• “Effects of alkali additives on the physicochemical characteristics and chemisorptive properties of Pt/TiO2

catalysts”, P. Panagiotopoulou, D.I. Kondarides, J. Catal. 260 (2008) 141-149.

• “Kinetic and mechanistic studies of the water-gas shift reaction over Pt/TiO2 catalyst”, C.M. Kalamaras, P. Panagiotopoulou, D.I. Kondarides, A.M. Efstathiou J. Catal. 264 (2009) 117-129.

• “Effects of alkali-promotion of TiO2 on the chemisorptive properties and water-gas shift activity of supported noble metal catalysts”, P. Panagiotopoulou, D.I. Kondarides, J. Catal. 267 (2009) 57-66.

• “Chemical reaction engineering and catalysis issues in distributed power generation systems”, P. Panagiotopoulou, D.I. Kondarides, X.E. Verykios, Ind. Eng. Chem. Res. 50 (2011) 523-530.

• “Effects of promotion of TiO2 with alkaline earth metals on the chemisorptive properties and water-gas shift activity of supported platinum catalysts”, P. Panagiotopoulou, D.I. Kondarides, Appl. Catal. B 101 (2011) 738-746.

Thank you very much for your attention!!

Documents

Water-Gas Shift Reaction