Gold nanoparticles as catalysts
for oxidation reactions
Institute of Chemical Technology (ITQ, CSIC - UPV)
Angeles Pulido ([email protected])
Valencia (Spain)
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
Contents
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
Reactions
Heterogenous
catalyzed processes
Selective formation
of desired products
Mild reaction
conditions
Use of molecular oxygen as oxidazing agent
Aim
Low temperature CO oxidation with O2 by gold-based catalysts
Synthesis of new catalysts based on
supported gold sub-nanoparticle AIM
Synthesis is still
quite challenging
Activity?
New features?
Bulk NP (2- 4 nm)
Size decreases
Activity increases
Inert Active Controlled
Synthesis
Computational
Chemistry
Reaction Mechanism
Active sites Catalyst Morphology
Support effects
Results: O2 dissociation over Au NP (1 nm)
• Synthesis of Au NP on functionalized Multi-Wall Carbon Nanotubes
• Narrow size distribution (1.1 ± 0.5 nm)
• O2 dissociation: 16O2/18O2 isotopic exchange
• CO oxidation by molecular oxygen
Experimental
16O2/18O2 isotopic exchange on Au NP with 1.1 nm diameter
Results: O2 dissociation over Au NP (1 nm)
At 25 oC the amount of 18O2 and 16O2 decreases with time,
but formation of 16O18O 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
• Periodic DFT (GGA-PAW) with VASP code
• G point, Cutoff = 500 eV • Au38 cluster in a 20x20x20 Å box
Computational details
(100) facet
(111) facet
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
Adsorption of molecular O2
tbt A tbt B
bb
-22
-10
8
22
0 -
-10 -
-20 -
-30 -
-40 -
ΔE (k
cal/
mol)
Dissociation of molecular O2
bb
tbt B
Results: O2 dissociation over Au NP (1 nm)
R
TS
P
O2 dissociation requires only 7.6 kcal mol-1
O2 recombination needs over 30 kcal mol-1
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.
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)
Gold clusters of increasing size (Aun, n < 40)
Au1 Au2 Au3 Au4 Au5
Defects on the graphene and graphene oxide sheets
Au19 Au39
1 nm
Single vacancy
N-doped Vacancy pair
Pyridinic Defect
Oxygen containing
O2 dissociation over Au NP (<1 nm) - Methods
Periodic model
Graphite Graphene
SC (8 x 8)
Space group P1
Unit cell parameters
a = 19.60 Å, c= 20.00 Å and ϒ = 120 °
Unit cell composition C128
1 layer
Space group P63/mmc
Unit cell parameters
a = 2.45 Å, c= 6.64 Å and ϒ = 120 °
Unit cell composition C4
[0001]
O2 dissociation over Au NP (<1 nm) - Methods
Periodic DFT model
C128
Single vacancy
C127
▪ Electronic structure at the DFT level
(GGA- PW91 functional)
▪ Plane wave basis sets (cut-off 400eV)
▪ Projector-augmented wave (PAW) method
▪ Atomic coordinates fully relaxed
▪ Charge population analysis (Bader)
Calculations performed using VASP code
Vacancy pair
C126
O2 dissociation over Au NP (<1 nm) - Methods
Au1(g) + S Au1S (AEint, kJ/mol), where S represents the graphene support model.
2.07 2.09
2.07
Single vacancy
ΔE > 2 eV
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)
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.
Results: O2 dissociation over Au NP (<1 nm)
2.28 2.29
Au1(g) + S Au1S (AEint, kJ/mol), where S represents the graphene support model.
Vacancy pair
~ 2 eV
ΔE
Ea > 0
1.97
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)
Au5
Au5
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
3D NP
Results: O2 dissociation over Au NP (<1 nm)
~ 2.1
Single vacancy graphene sheet
Increasing size of the Au
NP does not weaken the
bonding to the support
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)
ΔE (
kcal m
ol-
1)
-15
-30
0
15
1 2 3
1
2
3
1.43 1.88 4.67
1 2 3
1.45 1.98 4.48
O2 + AunS (O2)AunS (ΔE, kcal mol-1)
Smaller Au clusters lead to
smaller O-O bond activation
Ea ~ 8 – 9 kcal mol-1
Results: O2 dissociation over Au NP (<1 nm)
A. Pulido, et. al New J. Chem. 2011, 35, 2153.
Geometry Optimization (Minimun)
UC Volume: 8000 Å3
UC composition: Au38O2
H ψ = E ψ (t ~ V, Nelec)
Geometry Optimization (TS searching)
UC Volume: 6654.20 Å3
UC composition: C127Au39O2
Frequency Calculations
When dealing with such a complex systems as
heterogeneous catalysts a realistic model of the
catalysts/process has to be used.
4 PROC 4004 s 7747 s
275 s 538 s 64 PROC
~100 times (H ψ = E ψ)
~ 250 times (H ψ = E ψ)
~ 72 times (H ψ = E ψ) Non-stop
4 PROC 4.6 d 9.0 d
7.6 h 14.9 h 64 PROC
4 PROC 11 d 22.4 d
19 h 1.6 d 64 PROC
4 PROC 3.4 d 6.5 d
5.5 h 10.8 h 64 PROC
THESE CALCULATIONS CAN ONLY BE AFFORDED
WITH THE USE OF SUPERCOMPUTATION.
RES
RES
RES
RES
Results: O2 dissociation over Au NP (<1 nm)
Acknowledgements
COMPUTATIONAL RESOURCES
RES (UV Tirant)
AND YOU FOR YOUR KIND ATTENTION
FUNDING
CONSOLIDER Project Juan de la Cierva Program
PEOPLE
Prof. Avelino Corma, Dr. Mercedes Boronat,
Dr. Patricia Concepcion and Dr. Ernest Mendoza
Gold nanoparticles as catalysts
for oxidation reactions
Institute of Chemical Technology (ITQ, CSIC - UPV)
Angeles Pulido ([email protected])
Valencia (Spain)
ΔE (
kJ
mol-
1)
-50
-100
0
50
1
2
3
1 2 3
O2 + AunS (O2)AunS (ΔE, kJ mol-1)
Ea ~ 45 – 55 kJ mol-1
Au supported sub-nanoparticles over graphene-like
materials are expected to preserve gold catalytic
performance for O2 activation
Results: O2 dissociation over Au NP (<1 nm)
Gold: Au(100) vs Au(111) facets
Au(100) Au(111)
Unit
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
Au39
(1:4:9:12:9:4)
~1 nm
Au1
(1) Au6
(1:4:1)
Au5
(1:4)
Au14
(1:4:9) Au18
(1:4:9:4)
Au19
(1:4:9:4)
(100)
(111)
Support optimized structures
2.541
1.950
2.541 1.727
1.455
1.727
1.455
1.458
1.458
1.455
1.454
1.407 1.406
1.407
2.575
2.575
2.575
Results: Gold atom over N doped graphene
Au1(g) + S Au1S (AEint, kJ/mol), where S represents the graphene support model.
2.94 2.24
N-doped
-96
Pyridinic Defect
2.35
2.34
2.35 -129
T. Irawan, I. Barke and H. Hövel, Appl. Phys. A 80, 929–935 (2005)
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
(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.
Gold on single layer graphene
High Angle Annular Dark Field (HAADF)
Bright Field Scanning Transmission Electron Microscopy (BF STEM)
R. Zan, U. Bangert, Q. Ramasse, K. S. Novoselov
Nano Lett. 2011, 11, 1087–1092
Top Related