PC IV • G re n zflä c h e n • W S 2011/12

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Heterogene Katalyse
- mikroskopisch -
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“The reaction which takes place at the surface of a
catalyst may occur by interaction between molecules or
atoms adsorbed in adjacent spaces on the surface, … or it
may take place directly as a result of a collision between
a gas molecule and adsorbed molecule or atom on the
surface.”
Langmuir, 1941
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Desorption
Irvin Langmuir
1881 - 1957
Diffusion
Chemisorption
Cyril Hinshelwood
1897 - 1967
Reaktion
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Eric Rideal
1890 -1974
Proc. R. Soc. London, Ser. A 178, 429 (1941)
D.D. Eley, ~1955
100
Langmuir-Hinshelwood
3
RLH [10 ]
80
60
40
20
0
0
2
4
6
8
10
pA (with pB held constant)
1.0
Eley-Rideal
0.8
RER []
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Generic Kinetics
0.6
0.4
0.2
0.0
0
2
4
6
pA (with pB held constant)
8
10
Pd(110)
Pt(210)
4.0
1
transition
point
CO
3.0
PCO2
2.0
LITD
1.0
0.25
T = 500 K
-6
PO2 =17 x 10 mbar
0
0
0.5
1.0
2.0
3.0
4.0
5.0
O CO
-6
0.75
Reaction Rate rco2 [rel. u.]
O
PCO2 [10 mbar]
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CO2 Formation
0
100
300
500
700
Ir(110)
0
100
300
500
0
6.0
700
Pt(110)
-6
PCO [10 mbar]
0
100
300
500
Temperature [ºC]
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CO solo,
Arrhenius Verhalten
O voradsorbiert, CO Strahl
CO sucht
Reaktionspartner
pseudo 1. Ordnung
Ertl & coworkers, J. Chem. Phys. 73, 5862 (1980)
Surf. Sci. 107, 207-219 (1981)
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ohne Katalysator
E=500
CO(g) + 1/2 O2(g)
284
O
247
96
O
C
O
E=-284
134
O
38
mit Katalysator
C
CO2(g)
CO2(a)
0
~ -33
Einheiten kJ/mol
PC IV • Grenzflächen • WS 2011/12
13030 J. Phys. Chem., Vol. 100, No. 31, 1996
suspected, however, that certain highly facile gas-surface
reactions might occur directly at the point of impact between
an incident reagent and an adsorbate, by way of a so-called
Eley-Rideal (ER) mechanism.70 In one limit, an ER mechanism could involve a single gas-surface collision. Alternatively, the incident reagent could “bounce” many times before
reaction, being accommodated to varying degrees in the
process.95 For us, the working definition of an ER reaction is
that it should occur before the reagents have become equilibrated
at the surface. Notice also that ER reactions are far more
exothermic than their LH counterparts, with an excess energy
close to the heat of adsorption of the incident species.
The idea that reactions can occur directly as an incident
reagent strikes an adsorbate is strongly supported by detailed
theoretical calculations.96-103 Over 20 years ago, Elkowitz,
McCreedy, and Wolken96 investigated the reaction of an incident
H atom with an H atom adsorbed on tungsten by running
classical trajectories on a model PES. Tully97 has examined
the reaction of O atoms incident on adsorbed C atoms adsorbed
on platinum using similar methods, but treating the flow of
energy from the reaction zone into the crystal more accurately.
The most recent theoretical studies of ER reactions have
involved quantum mechanical models, beginning with model
2-D calculations,98,99 which clearly established that product
molecules should show a high degree of vibrational excitation.
Very recently, Jackson and Persson101,103 have included rotational motion for the first time by performing 3-D quantum
calculations, using a flat-surface approximation.
Until relatively recently, the experimental evidence for the
ER mechanism came largely from kinetic measurements,
relating the rate of reaction to the incident flux and to the surface
coverage and temperature.70 For example, it has been found104
that the abstraction of halogens from Si(100) by incident H
proceeds with a very small activation barrier, consistent with
an ER mechanism. In order to proVe that a reaction can occur
on essentially a single gas-surface collision, however, dynamical measurements are required. The first such studies concerned
the determination of internal state distributions of product
molecules. Highly vibrationally excited hydrogen molecules
have been observed105,106 issuing from cells in which H atoms
are produced on a hot tungsten filament. Considering the
energetics of the possible reactions, the excited molecules have
been attributed to ER reactions between the incident atoms and
Rettner et al.
Figure 8. Polar plot of the angular distributions of the HCl product
of the reaction of H atoms with Cl/Au(111) for Ts ) 100 K. (Signal
intensities are proportional to the distance from the origin.) Results
are displayed for incidence energies of 0.07 and 0.37 eV, as labeled.
The angle of the incidence beam (60°) is indicated graphically by the
arrow. These results are attributed to an ER mechanism. The broken
line corresponds to cos f, which is expected in the absence of
dynamical factors. The tics are placed at 10° intervals.
H atoms with Cl/Au(111). Clear evidence was again found110
for an ER mechanism. In this case, in addition to a fast, highenergy component in the TOF distributions (corresponding to
the ER product), they found a second, low-energy component
corresponding to an LH product. These observations confirmed
an earlier proposal107 that this reaction occurs by concurrent
ER and LH processes. The solid lines in Figure 8 show the
angular distribution of the ER component. These clearly peak
away from the surface normal and change with Ei, demonstrating
a “memory” of the initial parallel momentum and energy. The
dashed line shows a cosine distribution, which is close to that
of the LH component for this system.
The most recent studies of ER reactions have progressed
beyond the issue of proving whether or not they occur, moving
on to the task of characterizing the reaction dynamics of
carefully chosen systems. In addition to velocity and angular
distributions, quantum-state distributions have now been ob107
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Modellrechnung
10
Model output NH 3 -%)
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NH3 / Ru(0001)
1
Supported catalyst
0.1
0.01
0.001
0.0001
0.0001
Single crystal
0.001
0.01
0.1
1
10
Experimental output (NH3-%)
Stolze & Nørskov, J. Catal. 110, 1 (1988)
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(335)
steps
( 11 13 19 )
kinks
High Miller indexed fcc surface with steps / kinks
BALSAC plot
Au/Ru(001)
Θ= 0.15 ML
Behm & Co, Phys. Rev. Lett. 67, 3279 (1991)
N2
-10
Ru(0001)
Ea = 0.4 ± 0.1eV
0
-11
log S
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Θ= 0.03 ML
-12
-13
1-2% Au on Ru(0001)
Ea = 1.3 ± 0.2eV
-14
0.0015
0.0020
0.0025
0.0030
0.0035
1/T (1/K)
Chorkendorf & Nørskov, Phys. Rev. Lett. 83, 1814 (1999)