Catalysis by Gold for PROX reaction
by
Dr. P. SANGEETHA
National Center for Catalysis Research
(NCCR)
Indian Institute of Technology-Madras
Catalysis by Gold
• Bulk gold is catalytically inactive!
• When gold is dispersed on supports with a high surface
area, its catalytic properties change substantially
• Supported gold nanoparticles are highly active in the oxidation of
CO at low temperatures [Haruta et al., Chem. Lett. (1987)].
2 CO + O2→2 CO2
[from Haruta et al., Gold Bull. (2004)].
The “Bond & Thompson” model
• The active site for catalysis is
theinterface between particle and
support.
• The interface is formed by gold
atoms in a more or less oxidised
state.
Ref:
Bond & Thompson, Catal. Rev.-Sci. Eng.
(1999).
•Bond & Thompson, Gold Bulletin (2000)
Introduction: Fuel cell
• Converts chemical energy of
reactants directly into
electrical energy
• High-efficiency
• Low-emission
• Poisoning by CO
Picture from: www.news.cornell.edu/photos/pem300.gif
Introduction: fuel processors
Sulfurfree
Steam
CH4
H2O
Water-gas shift rxns.
5-10
10%
3%
0.5%
PROX ppm
HTS CO LTS CO
Reformer CO
PEM
CO + H2O  CO2 + H2
HTS: High temperature shift; LTS: Low temperature shift;
PROX preferential oxidation; PEM: Fuel
• 350-550 C
• Fe-oxides,
Chromia
• 200-300 C
• CuO/ZnO/Al2O3
Choudhary and Goodman, Catal. Today 77 (2002) 65.
• Ambient-200 C
• Noble metal on
oxide carrier
Why gold catalyst?
PROX
CO + 0.5 O2 = CO2
H = -280 kJ/mol
H2 + 0.5 O2 = H2O
H = -240 kJ/mol
Prior art
CO conversion (temp. 50-350 ℃ ):
Ru/Al2O3 > Rh/Al2O3> Pt/Al2O3> Pd/Al2O3 (0.5 wt.%)
But CO selectivity decrease with increased temperature (Sco<
40%)
New art
Supported nano-gold catalyst showed high CO activity even at
sub-ambient temperature. (Cco> 90%)
Lower hydrogen consumption.
Gold Nanoparticles
 Au has long been known as being catalytically far less active than
other transition metals.
 Because of its inertness, Au was formerly considered as an
ineffective catalyst.
 This assumption was based on studies where Au was present as
relatively large particles (diameter > 10 nm) or in bulk form such as
single crystal.
 Haruta et al. have shown exceptionally high CO oxidation activity
on supported nano-Au catalysts even at sub-ambient temperatures
(200 K).
Supported Nano-Au catalysts exhibit:
• an extraordinary high activity for low-temperature catalytic combustion
• Partial oxidation of hydrocarbon
• Hydrogenation of unsaturated hydrocarbons
• Reduction of nitrogen oxides
• Propylene epoxidation
• Methanol synthesis
• Environmental catalysis
Activity of gold catalysts
Preparation
method
Reducible or
irreducible
oxides
Depositionprecipitation method,
Photo-deposition
method,
Impregnation
method etc.
Stabilized the
small gold
particles
Small gold
particles
Supports
Promoter
Support and its effect
• Transition metal oxide series
– Only they can form hydroxides or hydrated oxides in the presence of
alkali.
• Activity: TiO2, Fe2O3, Co3O4> Al2O3, SiO2 (Reducible > Irreducible)
• Suitable support can stabilize the gold nanoparticle and maintain high
dispersion.
• Addition of MgO, MnOx and CuOx improve the CO oxidation activity and
selectivity.
Rate per surface metal atom (s-1)
Gold particle size and activity
0.15
0.1
CO oxidation
Au/TiO2 (273K)
0.05
0
Au/TiO2
• TOF decrease in the
diameter from 4 nm
Pt/SiO2
• TOF steady from 4nm
Pt/SiO2 (437 K)
5
10
15
20
Mean diameter of metal particles (nm)
M. Haruta, J. New. Mat. Electrochem.
Systems 7 (2004) 163
• CO is absorbed only on
steps, edges and corner
sites. Thus, smaller Au
particles are preferable.
The structure of Catalytically Active Gold on
Titania
• Cluster size and morphology, particle thickness and
shape
• Support effects:
Nature of the support material, Surface defects, Metal-Support
charge transfer, Au- support interface.
• Metal oxidation state
• Au-oxide contact area
Particle size effect
Au/TiO2
The TOF decreases with a decrease
in the diameter from 4 nm.
Pt/SiO2
The TOF is steady independent
of particle diameter from 4 nm.
The hemispherical particles with flat
planes strongly attached to the TiO2.
 CO is absorbed only on steps, edges
and corner sites.
Preparation of gold catalyst
Preparation
Advantage
Disadvantage
Impregnation
•Easy
•No support limit
•Wide Au size distribution
•Cl- can not remove
Co-precipitation
•High dispersion
•Easy
•Narrow Au size distribution
•Support limit
•Support may cover Au
surface
Deposition-precipitation
• Narrow Au size distribution
• Easy
• Au size <6 nm
•Support limit
Photo-deposition
•Au size <4 nm
•Easy
•Only semiconductor metal
oxide
Gas phase grafting
•Au size <6nm
•No support limit
•Complex
•Wide Au size distribution
Liquid phase grafting
•Au size <6nm
•No support limit
•Long prepare period
Colloid mixing
•Easy
•Au size <6nm
•Support limit
•Low activity
Deposition-precipitation method
HAuCl4  H 2 O  AuCl3 (H 2 O)   Cl 
Low pH
AuCl3 (H 2 O)  AuCl3 (OH)   H 
AuCl3 (OH)   H 2O  AuCl2 (H 2 O)(OH)  H   Cl 
AuCl2 (H 2 O)(OH)  AuCl2 (OH) 2  H 
AuCl2 (OH) 2  H 2 O  AuCl(OH)3  H   Cl 
AuCl(OH)3  H 2  Au(OH) 4  H   Cl 
High pH
T iOH AuCl(OH)3  T i- O - Au(OH)3  H   Cl 
1. Residual Cl- increase with pH value
decrease
2. Actual gold loading increase with
decrease in pH value
Moreau et al., 2005
Photo-deposition method
• Photochemical reactions:
TiO2 + hν e- + p+ (hν> E bg)
H2O  H+ + OHOH- + p+  OH0  1/2H2O + 1/4O2(g)
M++ e-  M0
mM0  Mm (M= Ag, Pt, Au)
• Key influential factors: irradiation time, light source, pH,
concentration of metal ion, surfactant
• pH effect:
Ag+
OH-
Ag+
＋＋＋＋＋＋
TiO2
Extremely low pH
AgOH
TiO2
100% deposition
At suitable pH
OH-
AgOH
Ag2O
OH-
pH too high
Zhang et al., Langmuir 19 (2003) 8230.
Possible Mechanism
Eley-Rideal type:
C
O
(a)
H
(b)
(C)
a. Adsorption of CO and H2 and dissociation of H2 on a gold particle,
b. Reaction of gas phase O2 with adsorbed H atom and
c. Reaction of the resulting oxidizing species with adsorbed CO to give
CO2.
Rossignol, et al., J. Catal. 230 (2005) 476.
I. Preferential oxidation of CO in H2 stream on
Au/CeO2-TiO2 catalysts
Introduction
• A series of Au catalyst supported on CeO2-TiO2 with various CeO2 contents
were prepared.
• CeO2-TiO2 was prepared by incipient-wetness impregnation with aqueous
solution of Ce(NO3)3 on TiO2.
• Gold catalysts were prepared by deposition-precipitation method at pH 7 and
65°C.
• The catalysts were characterized by XRD, TEM and XPS.
• The preferential oxidation of CO in hydrogen stream was carried out in a
fixed bed reactor.
• Adding suitable amount of CeO2 on Au/TiO2 catalyst could enhance CO
oxidation and suppress H2 oxidation at high reaction temperature (>50 deg.C)
• Additives such as La2O3, Co3O4 and CuO were added to Au/CeO2-TiO2
catalyst and tested for the preferential oxidation of CO in hydrogen stream.
Supported catalysts
Preparation by Incipient wetness method:
• Au/CeO2-TiO2
(1:9)
Preparation by Deposition-precipitation method:
• Au/CoOx-CeO2-TiO2 (0.5:1:9)
• Au/La2O3-CeO2-TiO2 (0.5:1:9)
• Au/CuOx-CeO2-TiO2 (0.5:1:9)
XRD
 XRD peaks indexed to (111), (200),
(220) and (311) at 2θ values of 28.5, 33.1,
47.5, and 76.7, respectively, which are
well consistent with face centered cubic
fluorite structured CeO2 (JCPDS 431002).
 XRD peaks also showed the presence
of anatase phase of TiO2.
 No peaks related to Au phase were
discernible in the diffraction patterns of
Au/CeO2-TiO2 catalyst.
XRD patterns of Au/CeO2-TiO2 (a)Au/CeO2TiO2(1: 1); (b) Au/CeO2-TiO2(1: 9);
(c) Au/CeO2-TiO2(2: 8); (d) Au/CeO2-TiO2(3: 7)
TEM
 The average gold particle sizes were
2.3–2.5 nm; they were nearly the same
at various Ce/Ti ratios.
 The nanosize support was beneficial
for gold species to diffuse into the pore
of the support during preparation.
 From TEM pictures, gold particles
were dispersed uniformly on the
support. CeO2 was the black shadow
with 20–30 nm diameters, and the slight
gray background was TiO2.
TEM image of Au/CeO2-TiO2(1:9)
XPS
o XPS spectrum of 1% Au/CeO2TiO2 (1:9) catalyst surface with two
strong peaks at 84.7 eV and 88.3 eV
corresponding to Au 4f7/2 and Au
4f5/2, respectively.
o The metallic gold species was
about 93% on the CeO2-TiO2
catalyst surface and only 7 % of Au+
dispersed on the support.
o Although the accurate mechanism
of CO oxidation by gold was
unclear, the presence of metallic
gold improved CO oxidation was a
fact without doubt.
XPS Au 4f spectra of Au/CeO2-TiO2 (1:9)
catalyst.
PROX Reaction
Au/CeO2-TiO2(1:9) showed the maximum CO
conversion at room temperature and increased with
temperature to attain 100 % conversion.
With an increase in the Ce/Ti ratio from 1:9 to 3:7,
there was a decrease in the CO conversion at room
temperature.
Increasing Ce/Ti ratio accurately affected the
oxidizing ability of the catalyst; Ce/Ti ratio of 1:9
could reach optimum activity for PROX reaction.
Au/CeO2-TiO2(1:9) showed the highest CO
conversion of 94 % and selectivity of 91 % at
25°C. It had better catalytic performance than other
Ce/Ti catalysts, which may be due to its fine gold
particle size and the nature of Ce4+ at low levels
of loading.
Influence of Ce/Ti ratio on Au/CeO2-TiO2
catalyst :CO conversion and selectivity for
PROX reaction.
(□) CeO2-TiO2(1:1); (○) CeO2-TiO2(10:90); (▲)
CeO2-TiO2(20:80);
(◇) CeO2-TiO2(30:70);(■) TiO2(Degussa) ; (★)
Effect of different additives
 Au/CeO2-TiO2 and Au/CoOx-CeO2TiO2 had higher activities at room
temperature, and decreased with
temperature due to the competition of H2
oxidation.
 The selectivity of Au/CuOx-CeO2TiO2 was much better than Au/CeO2TiO2 and other additives such as La and
Co.
 The CO conversion for Au/CuOxCeO2-TiO2 approached to 100% when
the temperature was above 65°C.
CO conversion and selectivity for the
PROX over Au/CeO2-TiO2
Effect of Promoter
• CeO2 plays the role of promoter, thus the promoter not only stabilizes the
gold particle size, but the gold supported on promoter interface may form
having perimeter of increased activity.
• The morphology of promoting oxide comprises the
(i) crystalline or amorphous character
(ii) the type of crystalline structure that may affect the oxygen binding energy,
abundance of the oxygen vacancies and
(iii) size and shape of the oxide phase affecting the length of the perimeter of
active Au/promoter oxide interfaces
• The small size of gold nanoparticles, amorphous active oxide CeO2 , TiO2
and the synergistic effect of CeO2-TiO2 support favours higher activity in
preferential oxidation of CO in hydrogen stream.
Conclusion
• A series of Au/CeO2-TiO2 catalysts were prepared by depositionprecipitation method and various additives were added and tested for PROX
reaction.
•
From XRD and TEM analysis, it indicated that gold particle dispersed highly
on the supports and formed particle size less than 3 nm.
• Au/CeO2-TiO2 (1: 9) catalyst was very active for PROX reaction, and
showed CO conversion of 94 % and CO selectivity of 91% at 25°C.
• When the temperature increased, the CO conversion reached 100% at 50°C
and then decreased.
• Au/CuO-CeO2-TiO2 (0.5: 1: 9) demonstrated >95% CO conversion when the
temperature was higher than 65 °C and the CO selectivity also improved
substantially.