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Laboratory of Heterogeneous CatalysisDepartment of Chemical EngineeringUniversity of PatrasGR 26504 Patras, GREECE
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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“Water-Gas” is a synthesis gas containing H2 and CO, originally made by passing steam over red-hot coke or coal.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.
Thermodynamics of the WGS reaction
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.
Thermodynamics of the WGS reaction
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
HT CO shift LT CO shift
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
HT CO shift LT CO shift
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
HT CO shift LT CO shift
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
HT CO shift LT CO shift
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.
Effect of the nature of the support – Pt catalysts
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
Effect of the nature of the support – Pt catalysts
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
Effect of the nature of the dispersed metal
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)
Arrhenius plot of TOFs obtained over
Pt, Rh, Ru and Pd dispersed on TiO2
Effect of the nature of the dispersed metal
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
Arrhenius plot of TOFs obtained over
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)
Arrhenius plot of TOFs obtained over
Pt, Rh, Ru and Pd dispersed on TiO2
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.
Effect of the morphology of the support -TiO2
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.
Effect of the morphology of the support -TiO2
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.
Effect of the morphology of the support -TiO2
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
Reducibility of TiO2 – Raman spectroscopy
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
Reducibility of TiO2 – Raman spectroscopy
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
Reducibility of TiO2 – Raman spectroscopy
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
Experimental conditions
Mass of catalyst: 100 mg
Total flow rate: 200 cm3/min
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 (
%)
Reducibility and MOx crystallite size
Conversion of CO as a function of reaction temperature
obtained over 0.5%Pt/MOx/TiO2
0.5%Pt /10%MOx/TiO2
Experimental conditions
Mass of catalyst: 100 mg
Total flow rate: 200 cm3/min
Feed composition:
3%CO + 10% H2O
Dispersion of Pt on combined mixed oxide catalysts results in materials with enhanced WGS activity.
Reducibility and MOx crystallite size
Conversion of CO as a function of reaction temperature
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)
Experimental conditions
Mass of catalyst: 100 mg
Total flow rate: 200 cm3/min
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
Effect of addition of alkalis
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
Effect of addition of alkalis
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
Effect of addition of alkalis – H2 TPD
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
Effect of addition of alkalis – H2 TPD
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
Mechanistic studies – Active sites
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
Mechanistic studies – Active sites
FTIR spectra obtained following adsorption of CO (1%) at 25oC
and subsequent heating at 450οC under He flow
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!!