Transcript Dissociative Chemisorption Probed by STM

Connecting Small Molecule
Reaction Dynamics to Catalysis
Ian Harrison
Department of Chemistry, University of Virginia
Charlottesville, VA
Catalysis ↔ Surface Science
Surface Science Model
Real Catalyst
=
Crystalline nanoparticle catalyst
P = 10 atm (~104 Torr)
T = 1000 K

?
Single-crystal surface
P = 10-11 Torr
Ts = 20-1200 K; Tg ~300 K
Pressure, materials, and non-equilibrium gaps
Harnessing Dynamical Information
Surface Temperature, Ts [K]
556
100
CH4/Ni(100)
Ts = 475 K
23, J = 2
10-2
10-3
10-4
13, J = 2
10-5
Schmid et. al.
Juurlink et. al.
E0 = 65 kJ/mol
Thermal Pop.
Tn = 400 K
10-6
417
385
357
Nielsen et al. (3 mbar)
PC-MURT; E0 = 65kJ/mol
10-6
10-7
Ea = 59 kJ/mol
10-8
10-9
Ea = 70 kJ/mol
10-7
10-10
0
(a)
455
CH4/Ni(100)
Initial Sticking Coefficient
Initial Sticking Coefficient
10-1
500
10-5
20
40
60
80
100
Normal Translational Energy [kJ/mol]
1.8
(b)
2.0
2.2
2.4
2.6
2.8
1000/Ts [K-1]

Non-equilibrium dissociative sticking coefficients can be high.

m-TST model can harness this high S/N dynamical information.
Reactivity & Energy Flow at Surfaces
Kinetic Theory
Experiments
Master Equation - MURT
Nonequilibrium Expts
(e.g. Tg  Ts)
←Nonequilibrium
& Equilibrium
PC-MURT
Equilibrium Expts
Transition State
Properties
←Equilibrium
only
canonical-TST
slow
Ab Initio
Improved Design and Engineering
of Catalytic & Nanoscale Processes
at Surfaces
←Energy Flow
fast
DFT
Functional
Development
Electronic Structure Theory
Activation Energies for Surface Reactions
A couple of multibillion-dollar-a-year surface reactions are:
 Steam reforming of natural gas (methane dissociation on Ni catalysts).
 Si homoepitaxy via UHV-CVD (silane dissociation on Si(100)).
CH4 on Ni
100
Activation Energy [kJ/mol]
Experimental
PC - MURT
Ea = 70 kJ/mol
E0 = 65 kJ/mol
110
90
80
70
60
50
Ni(100)
40
30
20
Ni(111)
10
{
{
Expt
Ab Initio
DFT
Activation Energy [kJ/mol]
120
SiH4 on Si(100)
80
Ab Initio
DFT
60
PC-MURT
40
Ea(1000 K) = 31 kJ/mol
20
E0 = 19 kJ/mol
Expt
Ab Initio
DFT
0
0
1985
1990
1995
2000
2005
Publication Year
Abbott et al., J. Chem. Phys. 121, 3792 (2004).
1988
1992
1996
2000
Publication Year
Kavulak et al., J. Phys. Chem. B 109, 685 (2005).
Microcanonical Unimolecular Rate Theory
AB



Molecule first interacts with only a
few local surface atoms (oscillators).
Energy of the transient “physisorbed
complex” (PC) is randomized by the
initial collision and/or rapid
intramolecular vibrational energy
redistribution (IVR).

All states at E* become equally
probable and react with common
ki(E*)s.
PCs are approximately adiabatically
isolated because thermalization is
slow compared to the picosecond
timescale for desorption at E* > E0.
PC System
AB
Reaction
Metal
Reservoir at Ts
Energy
Exchange
AB( p )
R ( E , E ')
R ( E ', E )
F (E)
kR ( E )

 AB( p ) 
AB( g ) 
 A( c )  B( c )

kD ( E )
PC
Energy

Desorption
A(c) + B(c)
AB(g)
E0
E*
ED
Ead
E
ER = E0 + ED
Reaction Coordinate
Physisorbed Complex – Microcanonical
Unimolecular Rate Theory (PC-MURT)
Using RRKM rate constants,
‡( E *  E )
W
0,i ,
ki ( E*)  i
h ( E *)
E0,i = threshold energy for i
the steady state approximation yields,
‡ ( E*  E )
*)
W
k
(
E
R
0
R
S ( E *) 

kR ( E*)  kD ( E*) WR‡( E*  E0 ) WD‡ ( E*)
 A purely statistical result!
PC-MURT

Convolving the individual energy distributions gives
the overall PC energy distribution,
f ( E)  
E
0
f t ( Et )
EEt
0
f v ( Ev )
EEv Et
0
f r ( Er )
 f s ( E  Et  Ev  Er ) dEr dEv dEt

The experimentally observed sticking coefficient is
calculated as,

S   S ( E*) f ( E*) dE*
0
 Predictions possible for any experiment
Dynamical Constraints


Typically, for smooth flat metal surfaces and alkanes

Normal translational energy scaling of S applies.

En = Et cos2J; parallel momentum is approximately conserved.
Local normal energy scaling may apply for corrugated
surfaces, e.g., SiH4/Si(100)-(2x1)


Introduces a new parameter D fixed independently by molecular
beam experiments; <En> = Et {(1-D) cos2J + 3D sin2J}.*
Certain rotational or vibrational modes may also be
spectator degrees of freedom.
*
Xia and Engstrom, J. Chem. Phys. 101, 5329 (1994)
PC-MURT Parameters

Desorption‡ → freely rotating molecule at ∞

Dissociation‡ → 3 Adjustable Parameters {E0, s, D}

E0, the threshold energy for dissociative chemisorption.

s, the number of surface oscillators that freely exchange energy
within the physisorbed complex.

D, a lumped frequency for the molecule-surface normal vibration
and the 3 frustrated rotations at the reactive transition state.

Average Relative Discrepancy
ARD 
Stheory  Sexpt
min  Stheory , Sexpt 


minimum fixes the 3 parameters
Synopsis of PC-MURT for CH4/Ni(100)
Dissociative sticking
probabilities for:
Ts = 475 K
10
23, J = 2
-3
Thermal “bulb” equilibrium
experiments.
661 K
627 K
570 K
10-5
(a)
Juurlink et. al.
E0 = 65 kJ/mol
Thermal Pop.
10-7
20
40
60
80
100
E0 = 65 kJ/mol
D = 170 cm-1  ARD = 43%
s=2
10
10
-1
10-6
0
95 110
(b)
10-5
10-5
Ea = 70 kJ/mol
10-6
Ea = 59 kJ/mol
1000/Ts [K-1]
Ts = 475 K
<Et>
90 kJ/mol
10-3
70 kJ/mol
10-4
50 kJ/mol
10-5
10-6
30 kJ/mol
10-7
10 kJ/mol
0.8 1.1 1.4 1.7 2.0 2.3 2.6
1000/Tn [K-1]
(d)
PC - MURT
Ea = 70 kJ/mol
E0 = 65 kJ/mol
100
10-7
80 100 120 140
10-2
1.2 1.6 2.0 2.4 2.8 3.2 3.6
110
10-4
60
10-9
120
10-3
40
10-8
(ii)
Nielsen et al.
(3 mbar)
20
Normal Translational Energy [kJ/mol]
10-1
10-4
10-8
(iii)
80
10-3
PC-MURT
(E0 = 65 kJ/mol)
10-2
10-10
0.5
65
Et = 53 kJ/mol, Tn = 811 K
Et = 43 kJ/mol, Tn = 570 K
Et = 24 kJ/mol, Tn = 757 K
10-2
90
80
70
60
50
Ni(100)
40
30
20
10-9
Schmid et al., J. Chem. Phys. 117, 8603 (2002)
Expts: Juurlink et al., Phys. Rev. Lett. 83, 868 (1999)
Homblad et al., J. Chem. Phys. 102, 8255 (1995)
Nielsen et al., Catal. Lett. 32, 15 (1995)
50
980 K
895 K
806 K
716 K
625 K
10-5
Normal Translational Energy [kJ/mol]
Activation Energy [kJ/mol]
Extract transition state
parameters for comparison to
electronic structure theory.
Initial Sticking Coefficient

Tn [K]
10-4
3% CH4 in He
35
(c)
0
10-3
10-6
Normal Translational Energy [kJ/mol]
(i)
10-2
10-1
Tn ~ 400 K
0
10-1
3% CD4 in He
20
Schmid et. al.

716 K
10-4
10-6
10-6
836 K
742 K
10-4
13, J = 2
931 K
897 K
805 K
10-3
534 K
10-5
Ts = 475 K
962 K
894 K
Initial Sticking Coefficient
10
10-2
Initial Sticking Coefficient
10-1
-2
100
10-1
Initial Sticking Coefficient
Laser pumped and thermally
populated molecular beam
experiments.
Ts = 475 K
Initial Sticking Coefficient

100
Initial Sticking Coefficient

Ni(111)
10
{
{
Expt
Ab Initio
DFT
Expt
Ab Initio
DFT
0
1.0
1.5
2.0
-1
1000/Ts [K ]
2.5
3.0
1985
(iv)
1990
1995
2000
Publication Year
Abbott et al., J Chem Phys 121, 3792 (2004)
2005
Comparison of Ts= 475 K Experiments
Surface Temperature, Ts [K]
556
100
CH4/Ni(100)
Ts = 475 K
23, J = 2
10-2
10-3
10-4
13, J = 2
10-5
Schmid et. al.
Juurlink et. al.
E0 = 65 kJ/mol
Thermal Pop.
Tn = 400 K
10-6
385
357
Nielsen et al. (3 mbar)
PC-MURT; E0 = 65kJ/mol
10-6
10-7
Ea = 59 kJ/mol
10-8
10-9
10-10
0

417
Ea = 70 kJ/mol
10-7
(a)
455
CH4/Ni(100)
Initial Sticking Coefficient
Initial Sticking Coefficient
10-1
500
10-5
20
40
60
80
100
Normal Translational Energy [kJ/mol]
1.8
(b)
2.0
2.2
2.4
2.6
2.8
1000/Ts [K-1]
7 order of magnitude difference in dissociative sticking coefficient
Application of PC-MURT to CH4/Ni(100):
Comparison of f(E*)s at Ts = 475 K
0.15
Nonequilibrium eigenstate
resolved experiment (23,
J = 2; Et = 93 kJ/mol).

Thermal bulb experiment
at 1010 x higher pressure.
0.20

f rv ( E rv )   E rv  E J  2,2
0.10
3

0.15
S ( E *)
0.10
f ( E* )
ft ( Et )
f s ( Es )
0.05
0.05
0.00
0.00
0
50
100
150
Energy [kJ/mol]
Schmid et al., J. Chem. Phys. 117, 8603 (2002)
Nielsen et al., Catal. Lett. 32, 15 (1995)
Microcanonical Sticking Coefficient
TS  475 K
fT ( E * )
Distribution [mol/kJ]


S   S ( E*) f ( E*) dE*
0

200
E* = 169 kJ/mol; SBeam = 0.15
E* = 17 kJ/mol; ST = 4.8 x 10-8
Fractional Energy Uptakes for
Thermal Sticking of CH4/Ni(100)
<E*>R
<Ev>R
Mean Energy [kJ/mol]
120
50
<Er>R
<Et>R
<Es>R
100
80
E0 = 65 kJ/mol
60
40
20
Fractional Energy Uptake (%)
140
0
fv
fs
fr
ft
30
20
10
200
(a)
40
400
600
Temperature [K]
800
1000
200
(b)
400
600
800
1000
Temperature [K]

Fractional energy uptakes are defined as f j  E j

At T = 500 K, f g  fv  f r  ft ~ 75%; f s  25%
R
E*
R
Mode Selective Chemistry: CD2H2/Ni(100)

Mode selective chemistry is
sometimes observed at surfaces!

Dissociative sticking coefficients for
CD2H2 rovibrational eigenstates and
for a thermally populated molecular
beam are shown at left.

CD2H2 may have insufficient mode
coupling  slow IVR/collisional state
mixing.

Slow IVR requires a full dynamical
theory.
100
Initial Sticking Coefficient, S 0
10-1
10-2
10-3
10-4
10-5
Ts = 473 K
10-6
10-7
CH4
CD2H2
Tn = 423 K
Tn = 423 K
|20> State
23
10-8
|11> State
10-9
40
50
60
70
80
Translational Energy, Et [kJ/mol]
Beck et al. Science 302, 98 (2003)
 The PC-MURT (lines) fails.
Effusive Beam S(Tg,Ts ) for CH4/Pt(111)
Surface Temperature, Ts [K]


PC-MURT can also be used to
predict S(Tg, Ts) sticking for
effusive molecular beams
where Tg = Tt = Tv = Tr; unlike
in supersonic beams.
Note the many opportunities to
measure “effective activation
energies”, “Ea”(Ts)  Ea(T), in
these non-equilibrium
experiments.
Generally,
" Ea "(T j )   kb ln S   E j
 T j1 


2000
R
667
500
400
10-1
Tg = 1000 K
10-2
10-3
10-4
10-5
Tg = 100 K
10-6
10-7
10-8
10-9
 Ej
1000
100
Initial Sticking Coefficient

10-10
0.5
Thermal effusive beam
Thermal ambient gas
Nonequilibrium effusive beam
1.0
1.5
1000/Ts [K-1]
2.0
2.5
CH4 Reactivity Induced by Surface & Gas
Surface Temperature, Ts [K]
1250
1000
833
714
625
556
Surface Temperature, Ts [K]
500
625 500
417
357 313
10-3
10-4
Initial Sticking Coefficient, S
Initial Sticking Coefficient, S
227
CH4/Pt(111)
CH4/Pt(111)
Tg
680 K
600 K
10-5
500 K
10-6
400 K
295 K, angle
integrated
10-7
295 K
10-4
Ts
10-5
1100 K
1000 K
900 K
10-6
800 K
700 K
10-7
10-8
0.8
(a)
278 250
10-3
1.0
1.2
1.4
1.6
1000/Ts [K-1]
1.8
1.6
2.0
2.0
(b)
E0 = 49 kJ/mol
D = 330 cm-1  ARD = 41%
s=2
2.4
2.8
3.2
3.6
4.0
4.4
1000/Tg [K-1]
K. DeWitt et al. J. Phys. Chem. B
110, 6705 (2006)
Alkane Dissociative Chemisorption
PC-MURT Derived Threshold Energies
Fe
Co
Ni

CH4 below, C2H6 above.

Reduction in E0 from CH4 to
C2H6 on Pt(111) is
substantial; 52.5 ± 3 kJ/mol
→ 26.5 ± 3 kJ/mol.

DFT calculations for Pt(110)
yield E0 = 38.5 ± 2 kJ/mol
for both CH4 & C2H6.*

C2H6 E0: Final state effects,
dynamics, energy transfer?
E0 = 65 kJ/mol
Ru
Rh
Pd
E0 = 59 kJ/mol
E0 = 26.5 kJ/mol
Os
Ir
Pt
E0 = 39 kJ/mol
E0 = 52.5 kJ/mol
Abbott et al., J. Chem. Phys. 119, 6407 (2003) [Ni]
DeWitt et al., J. Phys. Chem. B 110, 6705, 6714 (2006) [Pt]
Abbott & Harrison. J. Phys. Chem. B 109, 10371 (2005) [Ir]
Abbott & Harrison. J. Catal. in press (2007) [Ru]
*Anghel
et al., Physical Review B 71, 113410 (2005);
Chem. Phys. Lett. 413, 289-293 (2005)
CH4 Dissociation on Flat Metal Surfaces
versus Nanocatalysts
Temperature, T [K]
CH4 Thermal Dissociative Sticking Coefficient, ST
1000
667
500
400
333

PC-MURT predicts thermal dissociative
sticking coefficients on flat metal surfaces
many orders of magnitude higher than
apparent values measured on
nanocatalysts.

A surprising result since stepped surfaces,
as found on high curvature nanocatalysts,
are typically thought to be more active
than flat surfaces.

Presumably, C build-up quickly limits the
number of active sites available on the
nanocatalysts. Not a bare surface limit.

There seems to be substantial opportunity
for improving CH4 reforming catalysts.
10-1
Ir(111)
Pt(111)
Ru(0001)
Ni(100)
10-2
10-3
10-4
10-5
10-6
10-7
10-8
10-9
Ir (2 nm)
Pt (2 nm)
Ni (7 nm)
Ru (3 & 6 nm)
10-10
10-11
}
Nanocatalyst
(diameter)
10-12
1.0
1.5
2.0
2.5
3.0
-1
1000/T [K ]
Abbott & Harrison, J. Catal 254, 27-38 (2008)
Wei & Iglesia. Angew. Chem. Int. Ed. 43, 3685 (2004) [Ir]
Wei & Iglesia. J. Phys. Chem. B 108, 4094 (2004) [Pt]
Wei & Iglesia. J. Catal. 224, 370 (2004) [Ni]
Wei & Iglesia. J. Phys. Chem. B 108, 7253 (2004) [Ru]
Carstens & Bell . J. Catal. 161, 423 (1996) [Ru]
Dissociative Chemisorption & Associative
Desorption Dynamics of H2/Cu(111)
Can the PC-MURT provide at least a statistical baseline for
the dissociative chemisorption/desorption dynamics?
H2(g) + Cu(111) ↔ 2 H(c)
 Employ detailed balance at thermal equilibrium to predict the
associative desorption fluxes.
Two PC-MURT models:
 2 parameter (E0 = 79 kJ/mol, s = 1) model with active rotations.*
 3 parameter (E0 = 62 kJ/mol, D = 490 cm-1, s = 1) model with rotation
as a spectator to the dissociation dynamics.†
* Abbott
& Harrison, J. Chem. Phys. 125, 024704 (2006); †Abbott & Harrison, J. Phys. Chem. A 111, 9871 (2007)
Detailed Balance at Thermal Equilibrium
Desorption Flux = Dissociative Sticking Flux
D0 = SF0
H 2( p )
R( E , E ')
R( E ', E )
F (E)
kR ( E )

 H 2( p ) 

 H(c)  H(c)
H 2( g ) 


Also applicable to state-resolved flux balances,
kD ( E )
D( Et , Ev , Er ,J,;T )
 S ( Et , Ev , Er ,J,;T ) F0 f MB ( Et , Ev, Er,J,;T )
PC
Energy
H2(g)
H(c) + H(c)
E0
E*
ED
Ead
E
Reaction Coordinate
T = Tg = Ts
ER = E0 + ED
H2/Cu(111) PC-MURT: Rotation as a Spectator
State-Averaged Chemisorption Experiments
100
100
H2/Cu(111)
Ts = 120 K
10-1
10-2
10-3
Tn = 2100 K
10-4
Tn = 2000 K
Tn = 1740 K
10-5
Sticking Coefficient, S
Sticking Coefficient, S
10-1
D2/Cu(111)
Ts = 120 K
E0 = 62 kJ/mol
D = 490 cm-1
s=1
ARD = 570%
10-2
10-3
10-4
Tn = 2100 K
10-5
Tn = 1650 K
Tn = 1465 K
Tn = 1460 K
10-6
Tn = 1235 K
10-6
Tn = 1170 K
Tn = 1135 K
10-7
10-7
0
20
40
0
60
Translational Energy, Et [kJ/mol]
(a)
Experiment
PC-MURT
(b)
20
40
60
80
Translational Energy, Et [kJ/mol]
Rettner et al. J. Chem. Phys. 102, 4625 (1995); Michelsen et al. J. Chem. Phys. 98, 8294 (1993)

Absolute dissociative sticking coefficients for thermally populated molecular
beams of H2 & D2 at specified nozzle temperatures.
H2/Cu(111) PC-MURT: Rotation as a Spectator
State-Averaged Desorption Experiments
1.5
Cos 
Cos10
Cos12
1.0
Angular Distribution, P()
0.5
0.0
Ts = 600 K
Cos12
Cos14
1.0
0.5
0.0
Ts = 370 K
1.0
Cos19
Cos22
0.5
D2/Cu(111)
Ts = 600 K
10
1.0
Angular Distribution, P()
1.5
Ts = 800 K
D2/Cu(111)
Cos14
0.5
E0 = 62 kJ/mol
D = 490 cm-1
s=1
ARD = 24%
0.0
H2/Cu(111)
Cos12
1.0
Cos13
Experiment
PC-MURT
0.5
0.0
0.0
-60
(a)
-40
-20
0
20
40
60
Desorption Angle [degrees]
-60
(b)
-40
-20
0
20
40
60
Desorption Angle [degrees]
Rettner et al. J. Chem. Phys. 94, 7499 (1991)

Angular distributions and cosnJ fits for H2 & D2 recombinative desorption at various
surface temperatures.

5D quantum calculations* predict cos25J for H2 on Cu(111) at Ts = 1000 K.
*
Gross et al. Phys. Rev. Lett. 73, 3121 (1994) with E0 = 70 kJ/mol; E0 = 48.5 kJ/mol is recent DFT expectation.
Mean Translational Energy, <Et> [kJ/mol]
D2/Cu(111) PC-MURT: Rotation as a Spectator
State-Averaged Desorption Experiments
200
D2/Cu(111)
Ts = 1000 K
to infinity

5D quantum calculations,*
like the 1-D van Willigen
model,† predict increasing
<Et> with J.

Catastrophe for 1-D model
as J →90°.

PC-MURT behaves
correctly as J →90°.
5D quantum
Experiment
1D model
PC-MURT
150
100
50
2kBTs
0
0
10 20 30 40 50 60 70 80 90
Desorption Angle, J [degrees]
Comsa & David Surf. Sci. 117, 77 (1982)
*
Gross et al. Phys. Rev. Lett. 73, 3121 (1994) with E0 = 70 kJ/mol.
†
van Willigen, Phys. Lett. 28A, 80 (1968)
H2/Cu(111) PC-MURT: Rotation as a Spectator
Eigenstate-Resolved Desorption Experiments
Product Energy Distribution, P(Et)
0.0030
H2/Cu(111)
( = 0, J = 1)
0.0025
Experiment
PC-MURT
Ts = 370 K
0.0020
Ts = 600 K
0.0015
Ts = 900 K
0.0010
0.0005
0.0000
0
20
40
60
80
100
120
Translational Energy, Et [kJ/mol]
Murphy & Hodgson J. Chem. Phys. 108, 4199 (1998)
By detailed balance, Hodgson’s
desorption experiments yield:
S ( Et ; v, J , Ts )relative 
P( Et ; v, J , Ts )
f MB ( Et ;Ts )
H2/Cu(111) PC-MURT: Rotation as a Spectator
Eigenstate-Resolved Desorption Experiments
Surface Temperature, Ts [K]
1000
667
500
400
333
100
80
58 kJ/mol
Sticking Coefficient, S(Ts)
10-2
10-3
10-4
39 kJ/mol
10-5
10-6
10-7
19 kJ/mol
10-8
10-9
H2/Cu(111)
10-10
( = 0, J = 1)
D2/Cu(111)
10-12
1.0
(a)
Effective Activation Energy, "Ea(Ts)" [kJ/mol]
Et
10-1
10-11
" Ea (Ts )"  kB
286
5 kJ/mol
( = 0, J = 2)
H2/Cu(111)
D2/Cu(111)
60
2.0
2.5
1000/Ts [K-1]
3.0
3.5
R
 Es
( = 0, J = 2)
Thus,
Es
40
R
 " Ea (Ts )" Es
and expts show,
20
Es
0
1.5
 Es
( = 0, J = 1)
 ln S
Ts1
0
(b)
10
20
30
40
50
60
70
Translational Energy, Et [kJ/mol]
for Et  E0  Ev
 50 kJ/mol
at the lowest Ets.
For one surface oscillator, the PC-MURT analytically requires,
 " Ea (Ts )"
 1
Et
R
H2/Cu(111) PC-MURT: Rotation as a Spectator
Eigenstate-Resolved Desorption Experiments
Hodgson’s experiments clearly
demonstrate that the surface is not a
spectator!
Later, we will show that the fractional energy uptakes for surmounting
the thermal activation energy for dissociation at 925 K are roughly:
ft = 42%
fs = 41%
fv = 17%
So, the surface plays an essential role in the dissociation dynamics.
H2/Cu(111) PC-MURT: Rotation as a Spectator
Rotationally-averaged, vibrationally-resolved P(Et )
0.0025
H2/Cu(111)
Ts = 925 K
=1
0.0020
=0
Experiment
PC-MURT
0.0015
0.0010
0.0005
Product Energy Distribution, P(Et)
Product Energy Distribution, P(Et)
0.0025
=1 =0
0.0020
Experiment
PC-MURT
0.0015
0.0010
0.0005
0.0000
0.0000
0
(a)
D2/Cu(111)
Ts = 925 K
=2
20
40
60
80
100
120
Translational Energy, Et [kJ/mol]
0
140
(b)
20
40
60
80
100
120
140
Translational Energy, Et [kJ/mol]
Rettner et al. J. Chem. Phys. 102, 4625 (1995); Michelsen et al. J. Chem. Phys. 98, 8294 (1993)

Qualitative agreement

Averaged over rotational states, J = 0 – 6
E0 = 62 kJ/mol
D = 490 cm-1
s=1
H2/Cu(111) PC-MURT: Rotation as a Spectator
Eigenstate-Resolved Desorption Experiments
Rotational Energy, Er [kJ/mol]
4.4
14.4
29.6
49.5
73.3
100
=0
=1
H2/Cu(111)
Ts = 925 K
80
60
40
20
0
2
4
6
Rotational State, J
(a)
8
0
Mean Translational Energy, <Et> [kJ/mol]
Mean Translational Energy, <Et> [kJ/mol]
0
Rotational Energy, Er [kJ/mol]
10
7.2 15.0 25.4 38.3 53.4 70.3
=0
=1
=2
D2/Cu(111)
Ts = 925 K
80
E0 = 62 kJ/mol
D = 490 cm-1
s=1
ARD = 16 %
60
40
Experiment
PC-MURT
20
0
(b)
2.1
100
2
4
6
8
10
12
14
Rotational State, J
Rettner et al. J. Chem. Phys. 102, 4625 (1995); Michelsen et al. J. Chem. Phys. 98, 8294 (1993)

Mean translational energies for H2 & D2 as a function of rotational state agree
well with PC-MURT for J ≤ 6. These are the key J states at thermal energies.

Divergence for J ≥ 7 shows that rotational energy begins to facilitate sticking at
high J and Er ≥ 40 kJ/mol (n.b., at 925 K, <Er(T)>= kBT = 7.7 kJ/mol).
H2/Cu(111) PC-MURT: Rotation as a Spectator
Eigenstate-Resolved Desorption Experiments
Rotational State, J
Rotational State, J
5
0..2 3 4
6
7
8
0... 4 5 6 7 8
10
9
9 10 11 12 13 14
10-1
10-3
P,J /gn(2J+1)
P,J /gn(2J+1)
10-2
10-3
10-4
10-5
10-5
Experiment
PC-MURT
D2/Cu(111)
Ts = 925 K
10-7
0

E0 = 62 kJ/mol
D = 490 cm-1
s=1
ARD = 221 %
10-4
10-6
H2/Cu(111)
Ts = 925 K
10-6
(a)
=0
=1
=2
10-2
=0
=1
20
40
60
Rotational Energy, Er (kJ/mol)
0
80
(b)
20
40
60
80
Rotational Energy, Er (kJ/mol)
Arrhenius fit lines through PC-MURT rotational energy distributions for
recombinative desorption of H2 & D2 are for Tr = Ts = 925 K.
H2/Cu(111) PC-MURT: Rotation as a Spectator
Vibrational Energy Distribution for Desorption
H2
D2


J
Expt (%)
PC-MURT(%)
0
1
0-10
0-7
96.7
3.3
100
82.4
16.8
0.8
100
90.9
9.0
99.9
75.3
22.7
1.9
99.9
∑P,J
0
0-14
1
0-12
2
0-8
∑P,J
Somewhat less vibrational energy in associatively
desorbed hydrogen than theoretically predicted.
Early rather than Late Barrier for Dissociation!

Experiments show more translational energy and less
vibrational energy release in the associatively desorbing
hydrogen than the PC-MURT predicts.

By detailed balance, dissociative chemisorption favors
translational energy over vibrational energy as compared to
the statistical PC-MURT predictions.

The measured vibrational efficacy for dissociative
chemisorption is only 50% of the translational efficacy.*

Appropriate to an early transition state on the dissociative
potential energy surface according to the Polanyi rules.†
*Rettner
et al. J. Chem. Phys. 102, 4625 (1995)
Acc. Chem Res. 5, 161 (1972)
† J.C.Polanyi,
Early not Late Barrier!
Transition State Characteristics
GGA-DFT
PC-MURT
E0 = 48 kJ/mol
D = 405 cm-1
E0 = 62 kJ/mol
D = 490 cm-1
s=1
GGA-DFT: late barrier
Db‡ > 33% beq
b
Z
• GGA-DFT invariably predicts late
barriers for the dissociation of
diatomic molecules but the
H2/Cu(111) dynamics provide
evidence for an early barrier.
• Impact on Brønsted-Evans-Polanyi
correlations:
 Ea  a DE
LDA-DFT: early barrier
where a < 0.5 for early barriers
Db‡ < 10% beq
and a > 0.5 for late barriers.
H2/Cu(111) PC-MURT: Rotation as a Spectator
Thermal & Effusive Beam Sticking
Surface Temperature, Ts [K]
2000 1000
667
500
400
333
286
100
Tg = 2100 K
Initial Sticking Coefficient, S
10-1
10-2
10-3
Tg = 300 K
Tg = Ts
10-6
10-7
H2/Cu(111)
10-8
zero degree
angle integrated
Expt. for H2/Cu(110)
10-9
10-10
0.5
1.0
1.5
2.0
1000/Ts [K-1]
2.5
3.0
 More than an order of magnitude
variation in ST if rotations are
active. Dynamics matter!
 Rettner employed an erf-model
with over 100 parameters to
predict the 925 K thermal sticking
for D2/Cu(111) based on his
eigenstate-resolved desorption
experiments.*
10-4
10-5
D2/Cu(111)
ST(925)expt = 3.29 x 10-4
ST(925)PC-MURT = 2.16 x 10-4 (no rotations)
ST(925)PC-MURT = 1.4 x 10-5 (with rotations)
3.5
 The 3-parameter PC-MURT is in
good agreement with the ST(925
K) for D2/Cu(111) and the
H2/Cu(110) measurements.†
*Rettner
et al. Faraday Discuss. 96, 17 (1993)
et al., JVST A 9, 1693 (1991)
†Campbell
Energy Uptake in Thermal Sticking of H2/Cu(111)
Temperature, T [K]
1000
667
500
400
333
286
1000
667
50
H2/Cu(111)
60
Fractional Energy Uptakes [%]
H2/Cu(111)
80
Mean Energies [kJ/mol]
Temperature, T [K]
<E*>R
<Et>R
<Es>R
<Ev>R
E0 = 62 kJ/mol
40
20
400
333
286
40
ft
fs
fv
30
20
10
0
0
1.0
(a)
500
1.5
2.0
2.5
1000/T [K-1]
3.0
1.0
3.5
(b)
1.5
2.0
2.5
3.0
3.5
1000/T [K-1]

Mean energies and fractional energy uptakes, fi = EiR/E*R, calculated by
PC-MURT are shown for thermal sticking from an ambient gas.

Molecular normal translational energy contributes most to overcoming E0.

More than 40% of the reactive energy comes from surface phonons!
PC-MURT Predictions with Active Rotation:
D2/Cu(111) Associative Desorption
Rotational State, J
0
9 10 11 12 13 14
Mean Translational Energy, <E t> [kJ/mol]
0... 4 5 6 7 8
Rotational Energy, Er [kJ/mol]
=0
=1
=2
10-2
P,J /gn(2J+1)
10-3
10-4
10-5
10-6
D2/Cu(111)
Ts = 925 K
0
20
40
60
Rotational Energy, Er (kJ/mol)
80
2.1
7.2 15.0 25.4 38.3 53.4 70.3
100
=0
=1
=2
D2/Cu(111)
Ts = 925 K
80
E0 = 79 kJ/mol
s=1
60
Experiment
PC-MURT
40
20
0
2
4
6
8
10
12
14
Abbott & Harrison,
J. Chem. Phys. 125,
024704 (2006)
Rotational State, J

Rotational temperatures predicted by the PC-MURT are Tr ~ 6000 K. Lines
through the experimental solid points are for Tr = Ts = 925 K.

The Boltzmann plots of the experimental data indicate that rotation is a spectator
until Er ~ 40 kJ/mol is exceeded.

The initial rise of the experimental <Et> with J seems to be a modest dynamical
effect, the subsequent fall in <Et> can be rationalized by the statistical PC-MURT
predictions.
H2/Cu(111) Dynamics: Rotation as a Spectator
Rotational Energy, Er [kJ/mol]
4.4
14.4
29.6
49.5
Rotational Energy, Er [kJ/mol]
73.3
0
100
Mean Translational Energy, <Et> [kJ/mol]
Mean Translational Energy, <Et> [kJ/mol]
0
=0
=1
H2/Cu(111)
Ts = 925 K
80
60
40
20
0
2
(a)
4
6
8
5
6
7
8
=0
=1
=2
80
2
4
6
8
10
12
Rotational State, J
10
0... 4 5 6 7 8
9 10 11 12 13 14
10-1
P,J /gn(2J+1)
P,J /gn(2J+1)
10-3
10-4
10-5
10-4
10-5
10-6
H2/Cu(111)
Ts = 925 K
D2/Cu(111)
Ts = 925 K
10-7
10-6
0
(c)
=0
=1
=2
10-2
10-3
20
40
60
Rotational Energy, Er (kJ/mol)
0
80
(d)
Consequently, at thermally
accessible energies,
rotation is effectively a
spectator degree of
freedom and dynamical
steering is of negligible
importance.
14
Rotational State, J
10-2

20
0
=0
=1
The dissociation dynamics
appear to transition from
rotation as a spectator for
Er < 40 kJ/mol to rotation
as statistically participatory
for Er > 40 kJ/mol.
40
(b)
9

60
Rotational State, J
0..2 3 4
7.2 15.0 25.4 38.3 53.4 70.3
D2/Cu(111)
Ts = 925 K
10
Rotational State, J
2.1
100
20
40
60
Rotational Energy, Er (kJ/mol)
Abbott & Harrison, J. Phys. Chem. A 111, 9871 (2007)
80
Summary

The MURT local hot spot model was used to explain and simulate a variety of
activated dissociative chemisorption/associative desorption dynamics.

Benchmark transition state characteristics can be extracted by low parameter
MURT analysis of diverse experiments with high dynamic range.

MURT may be helpful in closing the “nonequilibrium gap” between surface
science and catalysis (e.g., CH4 beam experiments and thermal catalysis).

Most of the energy for thermal activated dissociative chemisorption comes
from the gas but surface phonons cannot be neglected – even for H2 on
Cu(111)!

Dynamical effects can sometimes produce order of magnitude changes in
dissociative sticking coefficients (e.g., if rotation is a spectator) and hence are
vital to know about. The MURT can provide statistical baseline predictions
against which dynamical effects can be identified when they occur (e.g., early
transition states; a < 0.5 in Brønsted-Evans-Polanyi correlations).
Acknowledgements


National Science Foundation
American Chemical Society Petroleum Research Fund
For a brief description of MURT and references: http://faculty.virginia.edu/harrison/murt.htm
Heather Abbott
MURT Kinetics Alumni:
Leticia Valadez and Kristy DeWitt
Dr. Heather Abbott – Humboldt Fellow, FHI, Berlin
Dr. Alex Bukoski – Resident, Veterinary Anesthesiology, U. Florida
Dr. Kristy DeWitt – Optical Air Data Systems
Dan Blumling – Ph.D. student, Penn State
Dave Kavulak – Ph.D. student, UC Berkeley
Prof. Kurt Kolasinski
(West Chester University)
Synopsis of PC-MURT for CO2/Rh(111)
Rotation a spectator
Frequencies from GGA-DFT
E0 = 73 kJ/mol
s=2
Goodman et al., Surf. Sci. 140, L239 (1984)
Sibner et al., J. Chem. Phys. 89, 1163 (1989);
103, 6677 (1995)
Coulston & Haller, J. Chem. Phys. 95, 6932
(1991)
1000
667
500
10-2
CO2/Rh(111)
10-3
-4
10
400
333
286
Tg = 1000 K
Angular Distribution, P(J)
10-5
10-6
10-7
10-8
10-9
10-10
Tg = Ts
Tg = 300 K
10-11
10-12
10-13
zero degree
angle integrated
Goodman et al.
10-14
1.0
1.5
CO2/Rh(111)
Ts = 500 K
1.0
O ~ 0.1 ML
0.8
0.6
0.4
Experiment
PC-MURT
0.73 cos9.4J  0.27 cosJ
0.2
0.0
2.0
2.5
3.0
0
3.5
10
40
CO2/Rh(111)
Ts = 500-1000 K
30
20
1000 K
900 K
800 K
700 K
0
30
40
50
Desorption Angle, J [ ]
1000/Ts [K ]
10
20
o
-1
600 K
500 K
40
Mean Vibrational Energy [kJ/mol]
CO oxidation dynamics
by detailed balance.
Initial Sticking Coefficient, S

CO2 dissociative
sticking in thermal bulb.
Mean Translational Energy [kJ/mol]

Surface Temperature, Ts [K]
Experiment
PC-MURT
65% Thermal
CO2/Rh foil
Ts = 584 K
30
1 = 1330 cm-1
2 = 665 cm-1
3 = 2350 cm-1
20
10
0
0
20
40
60
Desorption Angle, J [ ]
o
80
1
2
3
Vibrational Mode, i
Abbott & Harrison, J. Phys. Chem. C 111, 13137 (2007)
60
Synopsis of PC-MURT for SiH4/Si(100)


Thermally populated
molecular beam experiments
Thermal nonequilibrium
experiments (UHV-CVD)
10-1
10-1
1% SiH4 in H2
10-2
<Et,>
Tn
92 kJ/mol
80 kJ/mol
423 K
599 K
62 kJ/mol
44 kJ/mol
34 kJ/mol
463 K
328 K
423 K
Tn ~ 289 K to 825 K
Initial Sticking Coefficient
Dissociative sticking
probabilities for
Initial Sticking Coefficient

1% SiH4
10-2
Ts = 1173 K
in He
Ts = 1173 K
in H2
Ts = 973 K
in H2
10-3
0.8
E0 = 19 kJ/mol
D = 230 cm-1  ARD = 15%
s=2
Engstrom et al. J. Vac Sci. and Tech. A 13, 2651
(1995)
For other references see:
Kavulak et al. J. Phys. Chem. B 109, 685 (2005)
1.0
1.1
1000/Ts [K-1]
40
50
60
70
80
90
<Et,> Translational Energy [kJ/mol]
1.2
(b)
10-2
80
Experimental
Tg = 300 K
Activation Energy [kJ/mol]
Corrugated Si(100)-(2x1)
surface
Initial Sticking Coefficient

0.9
(a)
10-3
"Ea"(1000 K) = 26 kJ/mol
Mercier et al. (0.005 mbar)
10-4
Liehr et al. (0.0004 mbar)
Liehr et al. (0.004 mbar)
Ab Initio
DFT
60
PC-MURT
40
Ea(1000 K) = 31 kJ/mol
20
E0 = 19 kJ/mol
Behm et al. (0.002 mbar)
Gates et al. (0.006 mbar)
10-5
0
0.8
(c)
1.0
1.2
1000/Ts [K-1]
1.4
1988
1.6
(d)
1992
1996
Publication Year
2000
Synopsis of PC-MURT for CH4/Ni(100)
Dissociative sticking
probabilities for:
Ts = 475 K
10
23, J = 2
-3
Thermal “bulb” equilibrium
experiments.
661 K
627 K
570 K
10-5
(a)
Juurlink et. al.
E0 = 65 kJ/mol
Thermal Pop.
10-7
20
40
60
80
100
E0 = 65 kJ/mol
D = 170 cm-1  ARD = 43%
s=2
10
10
-1
10-6
0
95 110
(b)
10-5
10-5
Ea = 70 kJ/mol
10-6
Ea = 59 kJ/mol
1000/Ts [K-1]
Ts = 475 K
<Et>
90 kJ/mol
10-3
70 kJ/mol
10-4
50 kJ/mol
10-5
10-6
30 kJ/mol
10-7
10 kJ/mol
0.8 1.1 1.4 1.7 2.0 2.3 2.6
1000/Tn [K-1]
(d)
PC - MURT
Ea = 70 kJ/mol
E0 = 65 kJ/mol
100
10-7
80 100 120 140
10-2
1.2 1.6 2.0 2.4 2.8 3.2 3.6
110
10-4
60
10-9
120
10-3
40
10-8
(ii)
Nielsen et al.
(3 mbar)
20
Normal Translational Energy [kJ/mol]
10-1
10-4
10-8
(iii)
80
10-3
PC-MURT
(E0 = 65 kJ/mol)
10-2
10-10
0.5
65
Et = 53 kJ/mol, Tn = 811 K
Et = 43 kJ/mol, Tn = 570 K
Et = 24 kJ/mol, Tn = 757 K
10-2
90
80
70
60
50
Ni(100)
40
30
20
10-9
Schmid et al., J. Chem. Phys. 117, 8603 (2002)
Expts: Juurlink et al., Phys. Rev. Lett. 83, 868 (1999)
Homblad et al., J. Chem. Phys. 102, 8255 (1995)
Nielsen et al., Catal. Lett. 32, 15 (1995)
50
980 K
895 K
806 K
716 K
625 K
10-5
Normal Translational Energy [kJ/mol]
Activation Energy [kJ/mol]
Extract transition state
parameters for comparison to
electronic structure theory.
Initial Sticking Coefficient

Tn [K]
10-4
3% CH4 in He
35
(c)
0
10-3
10-6
Normal Translational Energy [kJ/mol]
(i)
10-2
10-1
Tn ~ 400 K
0
10-1
3% CD4 in He
20
Schmid et. al.

716 K
10-4
10-6
10-6
836 K
742 K
10-4
13, J = 2
931 K
897 K
805 K
10-3
534 K
10-5
Ts = 475 K
962 K
894 K
Initial Sticking Coefficient
10
10-2
Initial Sticking Coefficient
10-1
-2
100
10-1
Initial Sticking Coefficient
Laser pumped and thermally
populated molecular beam
experiments.
Ts = 475 K
Initial Sticking Coefficient

100
Initial Sticking Coefficient

Ni(111)
10
{
{
Expt
Ab Initio
DFT
Expt
Ab Initio
DFT
0
1.0
1.5
2.0
-1
1000/Ts [K ]
2.5
3.0
1985
(iv)
1990
1995
2000
Publication Year
Abbott et al., J Chem Phys 121, 3792 (2004)
2005
Synopsis of PC-MURT for CH4/Pt(111)
100
100
Tn = 680 K
10-1
10-2
CH4 at Tn = 680 K
10-3
CH4 at Tn = 300 K
Initial Sticking Coefficient
Initial Sticking Coefficient
Ts = 800 K
Eb = 1.27 eV
Eb = 0.62 eV
10-1
Eb = 0.48 eV
Eb = 0.42 eV
10-2
10-3
CD4 at Tn = 680 K
10-4
10-4
0.0
(a)
0.2
0.4
0.6
0.8
1.0
1.2
0.5
1.4
Normal Translational Energy [eV]
Luntz & Bethune J. Chem. Phys. 90, 1274 (1989);
1.5
(b)
2.5
3.5
4.5
5.5
1000 / Ts
Harris et al. Phys. Rev. Lett. 67, 652 (1991)

Dissociative sticking probabilities for thermally populated supersonic
molecular beam experiments by Luntz & Bethune.

Extract transition state parameters for comparison to electronic
structure theory. (E0 = 43, 64, 75, and 81 kJ/mol are EST calculations)
For details see: Bukoski et al. J Chem Phys 118, 843 (2003)
E0 = 56 kJ/mol
D = 125 cm-1
s=3

ARD = 34%
Synopsis of PC-MURT for CH4/Ir(111)
100
100
10-1
Initial Sticking Coefficient
Initial Sticking Coefficient
Mullins et al.
PC-MURT
Ts = 1000 K
300  Tn  832 K
10-2
10-3
10-4
10-4
10-1
10-2
4
8
10-3
Thermally populated
molecular beam
experiments.

Thermal equilibrium and
nonequilibrium
experiments.
En = 40 kJ/mol
En = 30 kJ/mol
10-5
0
(a)
20
40
60
80
100
120
0.8
140
1.2
1.6
2.0
2.4
2.8
-1
1000/Ts [K ]
(b)
Translational Energy [kJ/mol]
10-3
10-1
Tg = T s
Tg = 300 K
10-2
10-4
Initial Sticking Coefficient
Initial Sticking Coefficient

10-4
12
10-5
"Ea" = 27 kJ/mol
10-5
"Ea" = 43 kJ/mol
"Ea" = 53 kJ/mol
10-6
PC-MURT
10-7
Mullins et al. (~10-4 mbar)
Ea = 48 kJ/mol
10-3
E0 = 39 kJ/mol
D = 185 cm-1  ARDMB = 88%
s=1
10-4
Ea = 72 kJ/mol
10-5
10-6
PC-MURT (E0 = 39 kJ/mol)
10-7
[c.f., EST calculations of E0 = 15 and 76 kJ/mol]
Weinberg et al. (~1 mbar)
-3
Weinberg et al. (~10 mbar)
10-8
10-8
0.8
(c)
Dissociative sticking
probabilities for:
Tn = 669 K
10-5
0

En = 107 kJ/mol
1.0
1.2
1.4
1.6
-1
1000/Ts [K ]
1.8
2.0
0.8
(d)
1.0
1.2
1.4
1.6
1.8
2.0
-1
1000/T [K ]
Abbott & Harrison, J. Phys. Chem. B 107, 10371 (2005)
Seets et al. J. Chem. Phys. 107, 10229 (1997)
Jachimowski et al. Surf. Sci. 393, 126 (1997)
Synopsis of PC-MURT for CH4/Ru(0001)
Surface Temperature, Ts [K]
1000
500
400
333
286

CH4/Ru(0001)
Ru(0001)
Ts = 600 K
Tn = 700 K
10-1
Initial Sticking Coefficient
Initial Sticking Coefficient
667
100
10-2
10-3
CH4
CD4
10-2
Dissociative sticking
probabilities for:
10-3
10-4

Thermally populated
molecular beam
experiments.

Thermal bulb experiments.

Supported catalysts.
10-5
10-6
En = 83.0 kJ/mol, Tn = 1057 K
En = 51.5 kJ/mol, Tn = 656 K
10-7
En = 44.5 kJ/mol, Tn = 656 K
En = 41.5 kJ/mol, Tn = 535 K
10-8
10-4
40
(a)
50
60
70
1.0
80
Normal Translational Energy [kJ/mol]
1.5
2.0
2.5
3.0
3.5
1000/Ts [K-1]
(b)
Surface Temperature, Ts [K]
1000
CH4/Ru(0001)
Ts = 600 K
Tn = 450-1250 K
10-3
10-2
10-3
10% CH4 in Ar
10-4
100% CH4
25% CH4 in He
10-5
Initial Sticking Coefficient
Initial Sticking Coefficient
10-1
3% CH4 in He
500
400
333
286
10-6
0
20
40
60
80
100
Normal Translational Energy [kJ/mol]
CH4/Ru(0001)
Tg = Ts
10-4
Egeberg et al.
Wu & Goodman
Ru/Al2O3
Ru/SiO2
10-5
E0 = 59 kJ/mol
D = 155 cm-1  ARD = 316%
s=2
10-6
10-7
Tg = 300 K
10-8
10-9
10-10
3% CH4 in H2
(c)
667
10-2
100
10-11
1.0
120
(d)
1.5
2.0
2.5
1000/Ts [K-1]
Abbott & Harrison, J. Catal 254, 27-38 (2008)
3.0
3.5
Luntz et al., J. Chem. Phys. 116, 5781 (2002)
Chorkendorff et al., J. Chem. Phys. 110, 2637 (1999)
Egeberg et al., Surf. Sci. 497, 183 (2002)
Wu & Goodman, J. Chem. Phys. 110, 2637 (1999)
C2H6/Pt(111): Effusive Beam Experiments
Surface Temperature, Ts [K]
Surface Temperature, Ts [K]
1250 833 625 500 417 357 313 278
250
1250 833 625 500 417 357 313 278
10-2
10-2
Tg = Ts
10-3
Tg = Ts
Tg
10-4
680 K
600 K
10-5
500 K
400 K
10-6
zero degree
angle integrated
Initial Sticking Coefficient
Initial Sticking Coefficient
C2H6/Pt(111)
500 K
10-4
400 K
10-5
295 K
10-6
zero degree
angle integrated
10-7
0.8
(a)
Tg
680 K
600 K
10-3
295 K
10-7
250
1.2
1.6
2.0
2.4
2.8
3.2
1000/Ts [K-1]
3.6
4.0
0.8
(b)
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
1000/Ts [K-1]
DeWitt et al., J. Phys. Chem. B 110, 6714 (2006)
E0 = 24 kJ/mol
D = 215 cm-1
s = 10
ARD = 53 %
E0 = 29 kJ/mol
D = 90 cm-1
s=2
ARD = 556 %
AngleIntegrated
ARD = 13 %
C2H6/Pt(111): Supersonic Beam Expts
100
100
C2H6/Pt(111)
Initial Sticking Coefficient
Initial Sticking Coefficient
C2H6/Pt(111)
10-1
10-2
10-3
Tn = 770-900 K
10-1
10-2
Tn = 770-900 K
Tn = 472-823 K
Tn = 472-823 K
10-4
10-3
50
(a)
75
100
125
150
175
200
225
Normal Translational Energy [kJ/mol]
50
(b)
75
100
125
150
175
200
Normal Translational Energy [kJ/mol]
Schoofs et al., Surf. Sci. 215, 1 (1989); Newell et al., Faraday Discuss. 105, 193 (1996)
E0 = 24 kJ/mol
D = 215 cm-1
s = 10
ARD = 3032 %
225
Increasing Tnozzle
increases S.
E0 = 29 kJ/mol
D = 90 cm-1
s=2
ARD = 48 %
Recommended Transition State Parameters
for C2H6 on Pt(111)
Temperature, T [K]
1000
667
500
400
333
286
10-1
C2H6/Pt(111)
Initial Sticking Coefficient, S
10-2
Ea = 31 kJ/mol
10-3
"Ea " = 26 kJ/mol
E0 = 26.5 ± 3 kJ mol-1
D = 153 ± 63 cm-1
s = 2 (or 10)
10-4
10-5
Translational, vibrational, and
surface energy certainly help
facilitate dissociation.
10-6
Ea = 37 kJ/mol
10-7
10-8
Tg = Ts
10-9
Tg = 300 K
Rodriguez & Goodman (1 Torr)
10-10
1.0
1.5
2.0
2.5
3.0
3.5
1000/T [K-1]
Rodriguez & Goodman, J. Phys. Chem. 94, 5342 (1990)
The role of rotational energy is
less clear – rotation might even
inhibit dissociation.