Gold nanoparticles as catalysts
for oxidation reactions
Institute of Chemical Technology (ITQ, CSIC - UPV)
Angeles Pulido ([email protected])
Valencia (Spain)
Contents
 Context
 Aim
 Results
 O2 dissociation over supported gold nanoparticles
(Au NP ~ 1 nm )
 O2 dissociation over supported gold sub-nanoparticles
(Au NP < 1 nm)
 Computational Cost
Context
GREEN CHEMISTRY
▪ Towards Sustainable development
▪ Design of new and more efficient chemical processes
▪ Optimize the use of raw materials and energy
▪ Minimize the generation of byproducts
Stoichiometic
Heterogenous
Reactions
catalyzed processes
Selective formation
of desired products
Mild reaction
conditions
Aim
Use of molecular oxygen as oxidazing agent
Low temperature CO oxidation with O2 by gold-based catalysts
Bulk
NP
(2- 4 nm)
Size decreases
New features?
Activity?
Activity increases
Inert
AIM
Controlled
Synthesis
Synthesis is still
Active
quite challenging
Synthesis of new catalysts based on
supported gold sub-nanoparticle
Computational
Reaction Mechanism
Support effects
Chemistry
Catalyst Morphology
Active sites
Results: O2 dissociation over Au NP (1 nm)
Experimental
• Synthesis of Au NP on functionalized Multi-Wall Carbon Nanotubes
• Narrow size distribution (1.1 ± 0.5 nm)
• O2 dissociation:
16O /18O
2
2
isotopic exchange
• CO oxidation by molecular oxygen
Results: O2 dissociation over Au NP (1 nm)
16O /18O
2
2
isotopic exchange on Au NP with 1.1 nm diameter
At 25 oC the amount of
but formation of
16O18O
18O
2
and 16O2 decreases with time,
is only observed at 80 oC. Therefore,
more energy is required for recombination than dissociation.
Results: O2 dissociation over Au NP (1 nm)
Theoretical investigation
• Find the mechanism for O2 dissociation on small isolated Au NP
• Study the possibility of generating an oxide overlayer
(111) facet
(100) facet
Computational details
• Periodic DFT (GGA-PAW) with VASP code
• G point, Cutoff = 500 eV
• Au38 cluster in a 20x20x20 Å box
Results: O2 dissociation over Au NP (1 nm)
Adsorption of molecular O2
Dissociation of molecular O2
tbt B only 7.6 kcal mol -1
O2 dissociation requires
tbt A
tbt B
O2 recombination needs over 30 kcal mol -1
TS
bb
Eads O2
(kcal/mol)
r(OO)
(Å)
qO2
tbt A
-23
1.37
-0.61
tbt B
-24
1.36
-0.64
bb
-23
1.46
-0.87
ΔE (kcal/mol)
0-
22
-10 -
8
-20 -30 -40 -
R
-10
-22
P
bb
Results: O2 dissociation over Au NP (1 nm)
Surface gold oxidation is energetically favourable
L. Alves et. al, J. Am. Chem. Soc. 2011, 133, 10251.
TOWARDS SUB-NANOMETER
SUPPORTED GOLD
NANOPARTICLE CATALYSTS
O2 dissociation over Au NP ( < 1 nm)
Graphene (a 2D network
of sp2 C atoms) is a zero
band gap semiconductor
Electronic properties of graphene could provide
new features in heterogeneous and/or photo-catalysis
Properties and catalytic performance of
gold clusters (Aun, n < 40) supported over
defective graphene sheets were investigated
O2 dissociation over Au NP (<1 nm) - Methods
Gold clusters of increasing size (Aun, n < 40)
1 nm
Au1
Au2
Au3
Au4
Au5
Au19
Au39
Defects on the graphene and graphene oxide sheets
Single
vacancy
Vacancy
pair
Oxygen
containing
N-doped
Pyridinic
Defect
O2 dissociation over Au NP (<1 nm) - Methods
Periodic model
Graphene
Graphite
SC (8 x 8)
1 layer
[0001]
Space group P63/mmc
Space group P1
Unit cell parameters
Unit cell parameters
a = 2.45 Å, c= 6.64 Å and ϒ = 120 ° a = 19.60 Å, c= 20.00 Å and ϒ = 120 °
Unit cell composition C4
Unit cell composition C128
O2 dissociation over Au NP (<1 nm) - Methods
Periodic DFT model
▪ Electronic structure at the DFT level
(GGA- PW91 functional)
▪ Plane wave basis sets (cut-off 400eV)
C128
Single
vacancy
▪ Projector-augmented wave (PAW) method
Vacancy
pair
▪ Atomic coordinates fully relaxed
▪ Charge population analysis (Bader)
Calculations performed using VASP code
C127
C126
Results: O2 dissociation over Au NP (<1 nm)
Single
vacancy
ΔE > 2 eV
2.09
2.07 2.07
Interaction between the gold atom
and the single vacancy graphene
sheet is similar to the reported for
metal oxides (TiO2 or MgO) used
as supports in gold based catalysts.
A gold atom strongly binds to the three under-coordinated C
atoms around the vacancy site.
Deposited Au atoms are positively charged (ρe Au  Graphene)
Au1(g) + S  Au1S (AEint, kJ/mol), where S represents the graphene support model.
Results: O2 dissociation over Au NP (<1 nm)
Vacancy
pair
ΔE
Ea > 0
~ 2 eV
2.28 2.29
1.97
Au1(g) + S  Au1S (AEint, kJ/mol), where S represents the graphene support model.
Results: O2 dissociation over Au NP (<1 nm)
Single vacancy
Vacancy pair
Pyridinic Defect
Graphene sheets are chemically activated by the presence
of C vacancies leading to “trapped” gold atoms
Is it Au NP growth favorable on defective graphene?
Results: O2 dissociation over Au NP (<1 nm)
Au - Au interaction is stronger
than Au – C sp2 network and
metal clustering is favored
over deposition of isolated
atoms on the graphene surface
2D and 3D structures
of Au5 clusters can be
formed
Au5
Au5
3D
NP
Results: O2 dissociation over Au NP (<1 nm)
Single vacancy graphene sheet
Increasing size of the Au
NP does not weaken the
bonding to the support
~ 2.1
Gold
particle
shape
is
not
modified by multiple interaction
with the support as happens
with metal oxide supports.
Results: O2 dissociation over Au NP (<1 nm)
1.43
1.88
ΔE (kcal mol -1)
15
1
2
4.67
3
0
2
-15
1
-30
Ea ~ 8 – 9 kcal mol-1
1.45
3
1.98
4.48
Smaller Au clusters lead to
3
smaller
O-O2 bond activation
1
O2 + AunS  (O2)AunS (ΔE, kcal mol-1)
A. Pulido, et. al New J. Chem. 2011, 35, 2153.
Results: O2 dissociation over Au NP (<1 nm)
When dealing with such a complex systems as
THESE CALCULATIONS CAN ONLY BE AFFORDED
heterogeneous catalysts a realistic model of the
WITH THE USE OF SUPERCOMPUTATION.
catalysts/process has to be
used.
UC Volume: 6654.20 Å3
UC Volume: 8000 Å3
UC composition: Au38O2
4 PROC
64 PROC
4 PROC
64 PROC
4 PROC
64 PROC
4 PROC
64 PROC
4004 s
275 s
UC composition: C 127Au39O2
H ψ = E ψ (t ~ V, Nelec)
7747 s
538 s
Geometry Optimization (Minimun)
9.0 d
4.6 d
~100 times (H ψ = E ψ)
14.9 h
7.6 h
Geometry Optimization (TS searching)
22.4 d
11 d
~ 250 times (H ψ = E ψ)
1.6 d
19 h
Frequency Calculations
6.5 d
3.4 d
~ 72 times (H ψ = E ψ) Non-stop
10.8 h
5.5 h
RES
RES
RES
RES
Acknowledgements
COMPUTATIONAL RESOURCES
RES (UV Tirant)
PEOPLE
Prof. Avelino Corma, Dr. Mercedes Boronat,
Dr. Patricia Concepcion and Dr. Ernest Mendoza
FUNDING
CONSOLIDER Project
Juan de la Cierva Program
AND YOU FOR YOUR KIND ATTENTION
Gold nanoparticles as catalysts
for oxidation reactions
Institute of Chemical Technology (ITQ, CSIC - UPV)
Angeles Pulido ([email protected])
Valencia (Spain)
Additional
Slides
Results: O2 dissociation over Au NP (<1 nm)
ΔE (kJ mol -1)
50
1
2
3
0
-50
-100
2
1
Ea ~ 45 – 55 kJ mol-1
3
Au supported sub-nanoparticles over graphene-like
materials are expected to preserve gold catalytic
performance for O2 activation
O2 + AunS  (O2)AunS (ΔE, kJ mol-1)
Gold: Au(100) vs Au(111) facets
Unit
Au(100)
Au(111)
Unit
Au(111) hexagonal packing is more compact
Molecule adsorption is preferred over Au(100) surface
O2 activation has lower barrier over Au(100) facets
Au(100) gold clusters
Au1
(1)
Au5
(1:4)
Au6
(1:4:1)
(100)
Au14
(1:4:9)
Au18
(1:4:9:4)
Au19
(1:4:9:4)
(111)
Au39
(1:4:9:12:9:4)
~1 nm
Support optimized structures
1.458
1.454
2.541
2.541
1.455
1.727
1.727
1.455
1.950
1.455
1.458
1.407
2.575
1.407
1.406
2.575
2.575
Band decomposition charge
G
G1V
G2V
H. O. BAND
L. U. BAND
Results: Gold atom over N doped graphene
Pyridinic
Defect
N-doped
2.94
-96
2.24
-129
2.34
2.35
2.35
Au1(g) + S  Au1S (AEint, kJ/mol), where S represents the graphene support model.
STM gold growth on nano-pits
FIGURE 3. STM image of sample A after evaporation of 0.10ML gold.
18±1 clusters on (100×100)nm2, height distribution (1.5± 0.4)nm.
Calculated coverage (with (1)) 0.032 ML
T. Irawan, I. Barke and H. Hövel, Appl. Phys. A 80, 929–935 (2005)
STM gold growth on nano-pits
T. Irawan, I Barke and H. Hövel
Appl. Phys. A 80, 929–935 (2005)
Gold on single layer graphene
R. Zan, U. Bangert, Q. Ramasse,
K. S. Novoselov
Nano Lett. 2011, 11, 1087–1092
(a) BF and (b) HAADF image of monolayer graphene regions with 0.2 Å of Au evaporated on top. Au
nanocrystals are clearly visible in both images; the HAADF image furthermore reveals single Au atoms.
Hydrocarbon contamination is manifest as wormlike background in the BF and as dark-gray cloudlike
contrast in the HAADF image. (c) HAADF image of Fe atoms on monolayer graphene. Note again the
hydrocarbon deposit, which hosts the atoms. (d) HAADF image of a monolayer graphene region with 0.2 Å
Cr evaporated on top. Cr atoms are spread over wide areas in noncrystalline agglomerates predominantly
amidst hydrocarbon deposits. The frame width in all images is 10 nm.
High Angle Annular Dark Field (HAADF)
Bright Field Scanning Transmission Electron Microscopy (BF STEM)
O2 adsorption on Au(100) facets
1
rOO ~ 1.43 – 1.45 Å
0
r
~ 2.29 – 2.30 Å
ΔEint
4 AuO
3 |q(O2)| ~ 0.84 – 0.87 e2
-50
Ea ~ 45 – 55 kJ mol-1
2
1
rOO ~ 1.43 – 1.45 Å
|q(O2)| ~ 0.84 – 0.87 e-
4
3