The Impact of Nanoscience in Heterogeneous Catalysis

1 11Nanoscience and Catalysis 111

The Impact of Nanoscience inHeterogeneous Catalysis

R. Schlögl

Fritz Haber Institut der MPG

www.fhi-berlin.mpg.de

Sharifah Bee Abd Hamid,

Combicat Centre, University of Malaya

[email protected]

A EuropeanPerspective?


Self-organisation: Nanomagnets: Co/Au(111)

Size too small: no remanence at 300 K!

Increase active volume by height increase

Result: nanostructured data storage system

Magnetfeld (T)

J. Kirschner et al.

MPI Mikrostrukturphysik, Halle

300x300nm

Au(111) Co-Nanomagneta

7nm13nm


Nano-CatalysisCatalysis is a multi-

functional phenomenon:It is multi-scale and hence

ever since“nanocatalysis”

A pleonasmus?Or:

What`s new in thenanoera?


Polymer-Metal Interface: Multiscale Approach

Simulation of coarse-grainedBPA-PC liquids (T = 570K)next to metal surface

Specific surface interactionsinvestigated via ab initio calculations (CPMD,…)

Molecular structure coarse-grainedonto bead-spring chain

Delle Site, Abrams, Kremer, MPI Polymer Science


Literature

10

100

1000

10000

1989 1994 1999 2004

[a]

[n]

At every hour every daya paper on nanoscience

is published.

Two reviews per dayoccur on the subject


The EU view

• ERA in nanotechnology

• “nanotechnology is an all-embracing term forvarious aspects of science and technologyinvolved in the study, manipulation and control ofindividual atoms and molecules”…”nanotechnology is predicted to underpin thenext industrial revolution”.

• Integration and integrated teaching as long-term structural targets.


The ERA Nanotechnology

• Nanoelectronics, molecular electronics quantumcomputing

• Nanobiotechnology, drug delivery systems,biocompatible implants, single cell analysis andmanipulation

• Nanomaterials: nanomaterials for structuraltasks (polymers, ceramics, metals)

“The potential for nanotechnology applied tocatalysis and to local reactions (lab on a chip)

offers further fields of developments.


Dispersion

Why Nano

5 nm 10 nm

Autoreduction

After Catal. Ar 673 K

Ru clusters in zeolite Y:

Synthesis ex ion-exchanged Ru-red.

Thermal activation up to673 K.

Use in ammonia synthesisfor 400 h.


Why Nano ?

Catalytic sites operatebest (selective) whenthey are isolated fromeach other to limitexchange of electronsand adsorbates:

by defaultnanostructuredfunctional materials

2 nm

Site Isolation


Size Effects in Catalysis

0

2

4

6

8

10 100 1000 10000

Number of Atoms

Siz

e [n

m]

0

0,05

0,1

0,15

0,2

10 100 1000

Number of valence electronsE

nerg

y ga

p (e

V)

0102030405060708090

100

3 5 7 9

Size [nm]

Con

vers

ion

[%]

C Au/OxideC Au/C

Selective oxidation of glycol to glyoxalwith air (Haruta et al.)

It is not the ground state electronic structurethat matters

Sizeparameter:

37,5


Nanocatalysis10 nm

2 nmWorking hypothesis:

The “nanoeffect” is the kinetic stabilisation ofmetastable materials containing or representingthe active sites. The extent of stabilisation issize-dependent (surface free energy vs.cohesion energy).

It is not size that matters but a local metastablestructure the existence of which under reactionconditions is linked to “size”.


Cs - HPAActive state - HPA

Hydrated - HPA

Dehydrated - HPA

HPA: A metastable phase


0.01 0.02 0.03

0 1 2 3 4 5

FT

(c(k

)*k3 )

R, (Å)

T, (K)

773 773

700

600

500

400

Cs3[PMo12O40]*xH2O

0.01 0.02 0.03 0.04

0 1 2 3 4 5 F

T(c

(k)*

k3 ) R, (Å)

T, (K)

773 773

700

600

500

400

Cs2H[PMo12O40] *xH2O

„Migration“ of Mo from the Keggin-Anion – in situ XAS

10% propene /10% oxygen


In-situ functional analysis: Transformation essential

O + H2O + O2

MOx Cs2H[PMo12O40]

1.98

2.00

3.42

3.44

3.46

3.74

3.76

0.0

373 473 573 673 773

Dis

tanc

e R

, (Å

)

Temperature, (K)

MS

signal Acrolein (m/e = 56)

Mo – O

Mo – Moc

Mo – Moe

1.0


The Combicat M Approach• Replace post-synthetic

defectation by planfulsynthesis of structurallycomplex but chemicallysimple materials.

• Compromise betweenstability underapplication andreactivity for controlledfunctionalisation.

Deactivation tobulk ortho-MoO3

MoO4-2

polymolybdates

Homo-polymer Hetero-polymer

Stabilisation by condensation or supporting into nanostructures

Ball - milling Hydrothermal regeneration


MoO3 for selective oxidation?

A syntone


Control of Solid Formation[Mo]

pH

T

Hx[MopOq]n-x

[Mo7O24]6- [Mo8O26]4-

H2[MoO4]

[Mo2O10]8-[Mo3O14]10-

supramolecular hex

A

B

C

D

E

[Mo36O112]8-[Mo12O40]8-

0 1 2 3 4 5 6-0,02

0,00

0,02

0,04

0,06

0,08

0,10

0,12

0,14

0,16

dpH

/dV

pH


5 nm 5 nm

Control of Nanostructure


Nano and HTHEReaction Control

Ratio [Mo] : [H2O]Ratio [Mo] : [H3O+]

[] structure-directing cations[] complexing agents for cations

Redox potential of solution

Rate of additionMode of addition

Reactor sizeStirring (speed)

Mode of reaction: decreasing, constant, overflow

Temperature

thermodynamic

Intrinsic parameters

coupling

kinetic

Extrinsic parameters


Nano and HTHE

Ageingwashing

DryingCalzination

Cationcomposition

Anioncomposition

Extrinsicvariables

PresentHTHE


Nanostructuring by PVDPt(111) substrate

Evapoartion of iron

Pt (111) substrate

43

Repeated evaporation and oxidation

of iron

Fe O (111)Pt(111) substrate

32a

Oxidation in 1 - 10-4 mbar O2 at 970-1100 K

-Fe O (0001)

Pt(111) substrate

Oxidation in 10 -10 mbar O2 at 870-1000 K

FeO(111)Pt(111) substrate

-6 -7

Iron

Oxygen

A

B

C

D


Models for UnderstandingTurn over frequencyCatalyst

1.0 x 10-3 molecules/site.s.(2)

- Technical catalyst

5.0 x 10-4 molecules/site.s.(1)

-Polycrystaline iron oxide

6.6 x 10-4 molecules/site.s-Single crystal modelcatalyst

(1) K. Coulter, D. W. Goodman, and R. G. Moore, Catal. Lett. 1995, 31, 1.(2) T. Hirano, Appl. Catal. 1986, 26, 119.

200 400 600 800

O1209201Fe2O3 b rxn

dN/d

E

Energy (eV)

Before

after

Fe

OC

O

Fe

200 400 600 800

dN

/dE

Energy (eV)

0 50 100 150 200

0.0

0.2

0.4

0.6

0.8

1.0

1.2

% C

onve

rsio

n

Time (min)

StyreneFe2O3


From Understanding to Mastering

500x450A

0

20

40

60

80

100

0 200 400 600 800 1000Time on stream, min

Eth

ylb

enze

ne

con

vers

ion

s, %

KFe2O3 MWNTs arc d.


What about nanocatalysis

• A.T. Bell: …Together with novelapproaches to nanoparticle synthesis thisknowledge (about catalyst function fromin-situ analysis) is contributing to thedesign and development of new catalysts.(Science 2003, 299, 1688)

• G. A. Somorjai: New synthetic methods ofcatalyst preparation are required forprecise control of size, structure,location…… (Appl. Catal. A, 2001, 222, 3)


Nanocatalysis: A New Paradigm

Understanding catalysis as a functional systemwith higher complexity than one elementaryreaction (the rds).

Respecting its dynamics given by theinteractions catalyst-reactants and catalyst-reactor (models?).Science of synthesizing (not preparing) and

functionally characterizing a dynamicalsupramolecular material.


Nanocatalysis: A Knowledge-basedApproach

Not a “size effect” (nothing new)

But a transition from

Finding a catalyst (by trial and error)

To

Mastering a catalyst (designing)

With (theoretically) pre-determined properties


complexity

system descriptor (p,T,[])

„practical“ catalyst

-reactive-

single crystal

-well defined-

additional model systems

choice? pragmatic

In-situ analysis

Functional definition

Structural definition

The Impact of Nanoscience in Heterogeneous Catalysis

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