Molecules at surfaces and mechanism of catalysis

Gerhard Ertl

Fritz Haber Institut der Max Planck GesellschaftBerlin, Germany

Transformation of energy and matter

The Sun:

nuclear fusion

Photosynthesis

Fossil fuels “Renewable energies”(wind, water, solar, etc.)

Fuel processing(catalysis)

Chemicals(catalysis)

Fuel cell(electrocatalysis)

Nuclearreactor

Electrical energy

Mechanical energy

+ O2Heat Environment

(catalysis)

Catalytic synthesis of ammoniaN2 + 3 H2 Æ 2 NH3(Haber-Bosch process)

Steady-state reaction rate:

= v = f (pi, pj, T, catalyst)dnjdt

Reactor

dnidt

dnidt

dnjdt

¢

i: reactantsj: products

Heterogeneous catalysis

Progress of a chemical reaction

withoutcatalyst

with catalyst

Energy

E

D E

*

E*

E

initial

final state

|=

reaction coordinate

Transition state theory

kr = e · ekBTh

DS /R|= –E*/RT

DG‡

DG‡ – azFei

zFeiazFei

G

x x

+

r+ ∞ exp – r+ ∞ j+ ∞ exp – DG‡ – azFei

RT RT

DG‡

Heterogeneous catalysis Electrocatalysis

2 H2 + O2 Æ 2 H2O / PtHeterogeneous catalysis Electrocatalysis

O2 + 4 Haq + 4 e- 2 H2O

+

Æ̈ Æ̈H2 2 Haq + 2 e-+

U0 = 1.23 V U0 = 0 V DU = U1 – U2 DG0 = –nFDU

1 2

E*

E

initial

final state

|=

reaction coordinate

Transition state theory

kr = e · ekBTh

relaxation times for energy exchangebetween adsorbate and heat bath of solid(= phonons + electrons)

DS /R|= –E*/RT

time scale >>

Associative desorption of hydrogen from Ru(0001)D2

D

H2HDD2

/2kBH2:(2110±450) K

D2:(1720±290) K

200 250 300 350 400 450 500 0 20 40 60 80

TDS-

sign

al [a

.u.]

flux

[a.u

.]

temperature [K] flight time [ms]

A BThermal desorption spectroscopy (TDS) fs-laser induced ToF spectra

3000

2500

2000

1500

1000

500

0

0 1 2 3

time [10-12 s]

surfa

ce te

mpe

ratu

re [ K

]rate [a.u.]

TelTph

Tads : D layer Tads : H layer

D2 yield H2 yield

DtDDtH

D. Denzler, C. Frischkorn, C. Hess, M. Wolf, G. Ertl,Phys.Rev.Lett. 91 (2003), 226102; J.Phys.Chem. B 108 (2004), 14503

O / Pt (111)

J. Wintterlin, R. Schuster, and G. Ertl, Phys.Rev.Lett. 77 (1996), 123.

z

x

z xE*

EE

Ead

tsurf = t0·eEad/RT tsite = t0 ·e

E*/RT¢

O/Ru (0001)

T = 300 K

OC

260

~ 21

DH = 283 kJ/mol

ELH = 100*

CO + 2 O21

CO2ad

COad + Oad CO2

2CO + O2 2CO2

Oad + COad CO2 + 2* O2 + 2* O2,ad 2Oad

CO + * COad

I––

Pt

Catalytic oxidation of CO

Pt at low coverages( )

Oad + COad CO2 Pt 111( )/

Density Functional Theory Study

GGA

1.0 2.0 3.0 4.0

1.0

0.5

0.0

C – O (a) separation / Å

E –

E 0 / e

V

A. Alavi, P. Hu, T. Deutsch, P.L. Silvestrelli, J. Hutter,Phys. Rev. Lett. 80 (1998), 3650

2 H2 + O2 Æ 2 H2O Pt/

Mechanism (?) :

(1) O2 + * Æ 2 Oad(2) H2 + * Æ 2 Had(3) Oad + Had Æ OHad(4) OHad + Had Æ H2Oad(5) H2Oad Æ H2Og + * (T ≥ 170 K)

But :

Important :

T < 170 KInduction period

~ 12 kJ/mol

T > 230 KNo induction period

> 50 kJ/molE* :

(6) H2Oad + Oad Æ 2 OHad

S. Völkening, K. Bedürftig, K. Jacobi, J. Wintterlin and G. Ertl,Phys. Rev. Lett. 83 (1999), 2672.

Oad + Had T = 131 K; p(H2) = 8 ·10-9 mbar

625 s 1000 s

1175 s 1250 s170 ¥ 170 Å2

Oad + Had T = 131 K; p(H2) = 8 ·10-9 mbar

1300 s 1350 s

1400 s 1450 s170 ¥ 170 Å2

OH-frontsT = 111 K; p(H2) = 8 ·10-9 mbar; 2100¥1760 Å2

q

x

fi

H2O O

H2O + O Æ 2 OH

OH

2 OH + 2 H Æ 2 H2O

500 s

t = 0

625 s

250 s

0.3

0.2

0.10

0 20 40 60 80 100

0.3

0.2

0.10

0 20 40 60 80 100

0.3

0.2

0.10

0 20 40 60 80 100

0.3

0.2

0.10

0 20 40 60 80 100

0.6

0.4

0.20

0 20 40 60 80 100

0.3

0.2

0.10

0 20 40 60 80 100

H2OO

OHconc

entr

atio

nxnum LD/

C. Sachs, M. Hildebrand, S. Völkening,J. Wintterlin, and G. Ertl, Science 293 (2001), 1635;

J. Chem. Phys. 116 (2002), 5759

Reaction-diffusion systems

— Diffusion— Heat conductance— Variation of partial pressures— Electric field

Coupling between different parts of the surface via

Spatio-temporal self-organization

Temporal and spatial variation of state variables (surface concentrations) xi

Heartbeats of ultra thin catalyst

Ultra thin (200 nm thick) Pt(110) catalyst during CO oxidation, 5 mmsample diameter, T"="528 K, pO2 = 1 x 10-2 mbar, pCO = 1.85 x 10-3 mbar

F. Cirak, J.E. Cisternas, A.M. Cuitino,G. Ertl, P.Holmes, I. Kevrekidis, M.Ortiz,H.H. Rotermund, M.Schunack, J. Wolff,

Science 300 (2003), 1932

1845 1855 18751865 1885 1895 1905 1915 1925 1935

160

140

120

100

80

60

20

40

Year

Num

ber

( thou

sand

s)

HaresLynxes

PEEM images with 500 µm diameter, steady-state conditions: pO2!=!4!x!10

-4!mbar, pCO!=!4.3!x!10-5!mbar, T!=!448!K

Spiral waves during CO-oxidation on Pt(110)

S. Nettesheim, A. von Oertzen, H.H. Rotermund, G. Ertl, J.Chem.Phys. 98 (1993), 9977

Hurricane Bret over the coast of Texas,August 1999 (photo: NASA, GOES)

Chemical turbulence

Photoemission electron microscope(PEEM) imaging. Dark regions arepredominantly oxygen covered, brightregions are mainly CO covered.

Real time, image size 360 x 360 mm

Temperature T = 548 K, oxygen partialpressure po2 = 4 x 10

-4 mbar, COpartial pressure pco = 1.2 x10-4 mbar.

Global delayed feedback

O2 CO

UHVChamber

PEEM

Delay

Amplifier

Integrator

sample

M. Kim, M. Bertram, M. Pollmann, A. von Oertzen;A.S. Mikhailov, H.H. Rotermund, and G. Ertl,

Science 292 2001 , 1357( )

CO oxidation reaction on Pt(110)

• Suppression of spiral-wave turbulence anddevelopment ofintermittent turbulencewith cascades ofreproducing bubbles

CO oxidation on Pt(110)with delayed global feedback

uniform oscillations

phase turbulence

cellular structures

turbulence

intermittent turbulence

clusters

Retina

10mm

Quantumlevel

Atomiclevel

Heterogeneous catalysis :Dynamics of surface reactions

Spatio- temporalself- organization

Micro-kinetics

Macroscopickinetics

1 Å 100 Å 10 mm 1 cm10-15 sec

10-12 sec

10-6 sec

1 sec

102 sec

Microscopic Mesoscopic Macroscopic