Gold nanoparticles as catalysts

for oxidation reactions

Institute of Chemical Technology (ITQ, CSIC - UPV)

Angeles Pulido (apulido@itq.upv.es)

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.

TOWARDS SUB-NANOMETER

SUPPORTED GOLD

NANOPARTICLE CATALYSTS

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 (apulido@itq.upv.es)

Valencia (Spain)

Additional Slides

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

H. O. BAND L. U. BAND

Band decomposition charge

G1V

G2V

G

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

T. Irawan, I Barke and H. Hövel

Appl. Phys. A 80, 929–935 (2005)

STM gold growth on nano-pits

(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

ΔE

int

4 3

1

2

3

4

O2 adsorption on Au(100) facets

2

1

-50

0

rOO ~ 1.43 – 1.45 Å

|q(O2)| ~ 0.84 – 0.87 e-

rOO ~ 1.43 – 1.45 Å

rAuO ~ 2.29 – 2.30 Å

|q(O2)| ~ 0.84 – 0.87 e-

Ea ~ 45 – 55 kJ mol-1