centro de física de materiales

centro de física de materiales

Non-adiabatic effects in the reactivity of molecules at metal surfaces

CFM

Ricardo Díez MuiñoCentro de Física de MaterialesCentro Mixto CSIC-UPV/EHU

San Sebastián

1st nanoICT SymposiumDonostia – San Sebastian, February 26 2008

http://www.csic.es/


contributors to this work

CFM

H. Fabio BusnengoUniversidad de Rosario

Argentina

Maite AlducinCFM San Sebastián

Spain

Antoine SalinUniversité de Bordeaux

France

J. Iñaki JuaristiUPV San Sebastián

Spain

Gisela BocanDIPC San Sebastián

Spain

Fernando MingoCFM San Sebastián

Spain

http://www.csic.es/


outline

- introduction

- surface face and reactivity

- non-adiabatic effects: electronic excitations

- conclusions

CFM

centro de física de materialesCFM

What is catalysis? The effect produced in facilitating a chemical reaction, by the presence of a substance, which itself undergoes no permanent change.

A+B P direct reactionA+B+C P+C catalyzed reaction

A and B are reactantsC is the catalyst

P is the reaction product

A+B

P

gas/solid interfaces and heterogeneous catalysis



A+B P direct reactionA+B+C P+C catalyzed reaction

A and B are reactantsC is the catalyst

P is the reaction product

A+B

P


Heterogeneous catalysis: The catalyst is in a different phase solid surfaces.



A+B

P


Heterogeneous catalysis: The catalyst is in a different phase solid surfaces.

The chinese symbol for catalyst

is the same as the one for marriage broker

(matchmaker)


ammonia synthesis: 3H2(g) + N2 (g) ↔ 2NH3(g) (catalyzed by Fe surface)

global context: chemical industry

world production: 130 million tons (year 2000),

~200US$/ton


Catalysis in car industry:

In car engines, CO, NO, and NO2 are formed.

Catalytic converters reduce such emissions by adsorbing CO and NO onto a catalytic surface, where the gases undergo a redox reaction.

CO2 and N2 are desorbed from the surface and emitted as relatively harmless gases:

2CO + 2NO → 2CO2 + N2

global context: car industry

http://www.csic.es/


catalysis and nanoscale

CFM

- in the nanoscale, chemical properties can be changed - tunability of electronic properties (optimization of reactivity)


catalysis and nanoscale

CFM

In general, Au is a noble metal, the most inert bulk metal.

The chemical properties of Au dramatically change in the nanoscale. For instance, Au can act as a catalyst and transform CO into CO2 when it comes in the form of nanoparticles.

- in the nanoscale, chemical properties can be changed - tunability of electronic properties (optimization of reactivity)

Au as a catalyst


outline

- introduction

- surface face and reactivity- non-adiabatic effects: electronic excitations

- conclusions

CFM


Fe (111)

Fe (100)Fe (110)

rates of ammonia synthesis over five iron single-crystal surfaces

surface face and reactivityN2 + 3H2 2NH3

http://www.csic.es/


stic

king

coe

ffici

ent S

0

0.25 0.50 0.75 1.000.00

0.25

0.50

0.75

1.00

W(100)

T=800K

T=300K

T=100K

impact energy Ei (eV)

no threshold threshold

normal incidence

Rettner et al. (1988)Beutl et al. (1997)

surface face and reactivity: measurements of N2 dissociation on W surfaces

0.25 0.50 0.75 1.000.00

0.25

0.50

0.75

1.00

W(110)

T=800K

Pfnür et al. (1986)Rettner et al. (1990)

http://www.csic.es/

unidad de física de materialesUFM

theoretical method

In our particular case:

- DFT - GGA (PW91) calculation with VASP

- Plane-wave basis set and US pseudopotentials

- periodic supercell: 5-layer slab and 2x2 surface cell

- 30 configurations = 5610 ab-initio values

- interpolation through the corrugation reducing procedure [Busnengo et al., JCP 112, 7641 (2000)]

- adiabatic calculation of the molecule / surface interaction through a multidimensional potential energy surface (PES)

- classical dynamics in the adiabatic PES


0.0 0.5 1.0 1.5 2.0 2.50.0

0.1

0.2

0.3

0.4

0.5 N2/W(110)i=0

0.5 1.0 1.5 2.0 2.5

N2/W(110)i=60


stic

king

coe

ffici

ent S

0


classical dynamics in the 6D-PES

http://www.csic.es/


0.0 0.5 1.0 1.5 2.0 2.50.0

0.1

0.2

0.3

0.4

0.5 N2/W(110)i=0

0.5 1.0 1.5 2.0 2.5

N2/W(110)i=60


stic

king

coe

ffici

ent S

0

Alducin et al., PRL 97, 056102 (2006); JCP 125, 144705 (2006)

classical dynamics in the 6D-PES


theory

http://www.csic.es/

unidad de física de materiales

for thermal energies, long distances matter

UFM

0 25 50 75 1000.00

0.25

0.50

0.75

1.00

final

Z=2.0

Z=3.25

W(110)

final

Z=2.0

Z=3.25

W(100)

impact energy Ei (meV)

prob

abili

ity o

f rea

chin

g Z

Z=2Å

Z=3.5Å

1 1 .2 1 .4 1 .6 1 .8 2 2 .2

1 .5

2

2 .5

3

3 .5

4

=0o

r(Å)

Z(Å

)

potential wellW(110)

potential well

Alducin et al., PRL 97, 056102 (2006)

http://www.csic.es/


Difficult access to the precursor well

Once in the precursor well, molecules easily dissociate

in summary, dynamics matters

http://www.csic.es/


outline

- introduction

- surface face and reactivity

- non-adiabatic effects: electronic excitations- conclusions

CFM


non-adiabatic effects: electron-hole pair excitationschemicurrents

Exposure (ML)

Gergen et al., Science 294, 2521 (2001).

vibrational promotion of electron transfer

Huang et al., Science 290, 111 (2000) White et al., Nature 433, 503 (2005)

NO on Cs/Au(111)electron emission asa function of initial

vibrational state

http://www.csic.es/


non-adiabatic effects: electron-hole pair excitations

friction of a single chemisorbed CO molecule

Takaoka et al., PRL 100, 046104 (2008).

CO on Pt(997)migration to step edges

electron excitationduring H2 dissociation

Nieto et al., Science 312, 86 (2006).

H2 on Pt(111)sticking coefficient


description of electronic excitations by a friction coefficient

classical equations of motion

mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – (ri)(dri/dt)

for each atom “i” in the molecule

friction coefficient

adiabaticforce:

6D DFT PES

- damping of adsorbate vibrations: Persson and Hellsing, PRL49, 662 (1982)

- dynamics of atomic adsorption Trail, Bird, et al., JCP119, 4539 (2003)

previously used for:friction coefficient:

effective medium approximation

n(z)

z

n0

bulk metal

n0

=n0kFtr(kF)

effective medium: FEG with electronic density n0

http://www.csic.es/


probability of dissociative adsorption: N2 on W(110)

non-adiabatic

adiabatic

stic

king

coe

ffici

ent

initial kinetic energy (eV)

polar angle of incidence

i=0

i=45

i=60

0.2

0.4

0.2

0.4

0.5 1.0 1.5 2.0 2.50.0

0.1

0.2

0.3

but for this system, dissociation is

roughly decided atZ=2.5A

Juaristi et al., PRL 2008 (in press)

http://www.csic.es/


probability of dissociative adsorption: H2 on Cu(110)

[Salin, JCP 124, 104704 (2006)]

non-adiabatic

adiabatic

0.25

0.50

0.75

1.00

0.25

0.50

0.75

stic

king

coe

ffici

ent

initial kinetic energy (eV)0.5 1.0 1.5 2.0

0.00

0.02

0.04

0.06


i=0

i=45

i=60


http://www.csic.es/


classical equations of motion

mi(d2ri/dt2)=-dV(ri,rj)/d(ri) – (ri)(dri/dt)

for each atom “i” in the molecule

friction coefficient velocity

n(z)

z

n0

bulk metal



energy loss of reflected molecules: N2 on W(110)

i=0i=45i=60

energy losses in the reflected molecules due to electronic excitations are < 100 meV

Ei=1.5 eV


some conclusions

CFM

the reactivity of diatomic molecules on surfaces

can be very dependent on the particular surface face

a local description of the friction coefficient shows that

electronic excitations play a minor role

in the dissociation of diatomic molecules on metal surfaces

in the particular case of N2/W, we have shown

that the differences in reactivity arise from the

dynamics at long distances (>3 Å) from the surface

http://www.csic.es/


thank you for your attention1st nanoICT Symposium

Donostia – San Sebastian, February 26 2008

http://www.csic.es/


gas/surface dynamics

CFM

dissociative adsorption

molecularadsorptiondesorption

some elementary reactive processes at surfaces

from the fundamental point of view, the goal is to understand how solid surfaces and nanostructures can be used to promote gas-phase chemical reactions


surface face and reactivity

- surface roughness (work function)- unique active sites at the surface

two possible reasons for the difference in reactivityover different faces:

the rate-limiting step in ammonia formation is the dissociative adsorption of N2 on the surface

http://www.csic.es/


surface face and reactivity

- surface roughness (work functions)- unique active sites at the surface

two possible reasons for the difference in reactivityover different faces:

the rate-limiting step in ammonia formation is the dissociative adsorption of N2 on the surface

most reactive surfaces have C7 sites

(seven nearest neighbors)

http://www.csic.es/



UFM

0 25 50 75 1000.00

0.25

0.50

0.75

1.00

final

Z=2.0

Z=3.25

W(110)

final

Z=2.0

Z=3.25

W(100)

The amount of N2 molecules thatare able to reach

Z=3.25 Å is much smaller

in the W(110) face

Z=2Å

Z=3.25Å


prob

abili

ity o

f rea

chin

g Z

http://www.csic.es/


W(100) W(110)

adsorption energyDFT = 7.4 eV

Exp. = 6.6-7 eVadsorption distance

DFT = 0.63 Å

adsorption energyDFT = 6.8 eVExp. = 6.6 eV

adsorption distanceDFT = 1.15 Å

final state features: N adsorption on W

http://www.csic.es/


approach to surface: vertical over a surface atom

2.62Z(Å): 2.5

1091 meV

223 meV

868 meV

2.651.9Z(Å):

705 meV

395 meV

310 meV

W(100) W(110)

dynamic trapping in a well in both faces

http://www.csic.es/



UFM

0 25 50 75 1000.00

0.25

0.50

0.75

1.00

final

Z=2.0

Z=3.25

W(100)

Z=2Å

Z=3.25Å


prob

abili

ity o

f rea

chin

g Z

http://www.csic.es/


1 1 .2 1 .4 1 .6 1 .8 2 2 .2

1 .5

2

2 .5

3

3 .5

4

1 1 .2 1 .4 1 .6 1 .8 2 2 .2

1 .5

2

2 .5

3

3 .5

4

1 1 .2 1 .4 1 .6 1 .8 2 2 .2

1 .5

2

2 .5

3

3 .5

4

1 1 .2 1 .4 1 .6 1 .8 2 2 .2

1 .5

2

2 .5

3

3 .5

4

=45o

=270o

=90o

125o=90o

54o

=0o

r(Å) r(Å)

r(Å)r(Å)

Z(Å

)Z(

Å)

Z(Å)

Z(Å

)

some elbow plots

for the N2/W(110)

system

distance between contour lines = 0.2eV

E<0

E>0E=0

http://www.csic.es/

Today's key factors in the development of chemical processes are the concepts of "zero-waste" and "100

% selectivity".

In the field of catalysis, the chemistry at interphases, referred to as heterogeneous catalysis,

is promising to achieve both goals.


accuracy of adiabatic DFT calculations

Kohn-Sham equations solved self-consistently

exchange-correlation term

is not exact

Hammer et al., PRB 59, 7413 (1999)

RPBE functionalprovides better

atomic chemisorptionenergies

http://www.csic.es/


influence of the exchange correlation functional in the sticking coefficient: N2/W(110)

Pfnür et al., JCP 85 7452 (1986)

0,5 1,0 1,5 2,0 2,50,0

0,1

0,2

0,3

0,4

0,5

0,0 0,5 1,0 1,5 2,0 2,50,0

0,1

0,2

0,3

0,4

0,5


i=0


i=60

stic

king

coe

ffici

ent

initial kinetic energy (eV)

PW91

the RPBE XC functional:

- fits better the experimental chemisorption energies- describes better the interaction very near the metallic surface

- but better dynamics??

not in this case!0,5 1,0 1,5 2,0 2,5

0,0

0,1

0,2

0,3

0,4

0,5

0,0 0,5 1,0 1,5 2,0 2,50,0

0,1

0,2

0,3

0,4

0,5

RPBE

Bocan et al., JCP 2008 (in press)

http://www.csic.es/

centro de física de materiales

Documents

chemical reaction

chemical properties

permanent change

catalytic surface

redox reaction

reactivity of molecules

fe surfaceglobal context

heterogeneous catalysiscentro