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