Bonding to surfaces

Two classifications distinguished by the magnitude of their enthalpies of adsorption

Physisorption: long-range but weak van der Waals-type interactions with negligible exchange of electrons and enthalpies~Hcond (-HAD<35 kJ/mol)

Chemisorption: formation of a chemical bond (covalent, ionic, metallic) with exchange of electrons and –HAD>35 kJ/mol

Enthalpy of chemisorption depends strongly on surface coverage (interactions)

Spectroscopic techniques (see later) are used to confirm chemisorption

(IR or XPS for example)

Adsorption kinetics

Probability of molecule being associatively adsorbed may defined in terms of a sticking probability, s.

(Z) surface with themolecules ofcollison of rate

surface by the molecules of adsorption of rates

We can write: )1(0 ss

Thus the greater the number of vacant sites, the greater is s.

However, s is often not a linear function of θ. Invoke the precursor state

Precursor stateIf adsorbate collides with the surface and doesn’t stick, it may not simply rebound, but rather form a weak bond (physisorption) and diffuse for a period (losing energy) until a vacant site is located for chemisorption to occur. If the weak bond is initially at a vacant site one refers to intrinsic precursor states, whereas if it is at an occupied site this corresponds to extrinsic precursor states.

For adsorption to proceed, the gas needs to “dump” energy into the solid, if not it will desorb. The longer the gas molecule resides on the surface, the more likely is energy exchange with the surface. Can write an Arrhenius-type relationship between residence time and the enthalpy of adsorption at the precursor adsorption site. See p.11 Attard & Barnes

Lennard-Jones Potentialphysisorption

z

pote

ntia

l en e

rgy

ads < 35 kJmol-1

attractive van der Waals interactions

repulsive Coulombic interactionsPauli repulsion

ads

0

Potential energy profiles

Not activated Activated

Potential energy profilesFor an introduction to PE curves for adsorption see section 2.4 of:

http://www.chem.qmw.ac.uk/surfaces/scc/

MO diagram for chemisorption

1s 1s

σ*

σ

σ* orbitalsσ orbitals

Metal Adsorbed molecule Free molecule

PHYSISORPTION CHEMISORPTION

WEAK, LONG RANGE BONDING

Van der Waals interactions (e.g. London dispersion, dipole-dipole)..

STRONG, SHORT RANGE BONDING

Chemical bonding involving orbital overlap and charge transfer.

NOT SURFACE SPECIFIC

Physisorption takes place between all molecules on any surface providing the

temperature is low enough.

SURFACE SPECIFIC

E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury.

ads = 5 ….. 35 kJ mol-1 ads = 35 ….. 500 kJ mol-1

Non activated with equilibrium achieved relatively quickly. Increasing temperature

always reduces surface coverage.

Can be activated, in which case equilibrium can be slow and increasing temperature can

favour adsorption.

No surface reactions. Surface reactions may take place:- Dissociation, reconstruction, catalysis.

MULTILAYER ADSORPTION

BET Isotherm used to model adsorption equilibrium.

MONOLAYER ADSORPTION

Langmuir Isotherm used to model adsorption equilibrium..

Adsorption and Reaction at Surfaces

Surface structure

Surface defects

Surface science

In order to ensure reproducible results between experiments precise definition of the chemical and structural state of the surface: well-defined surfaces required.

Review 2nd and 3rd year lectures from D. Cunningham on naming crystal and solid structures

Overview

bcc body-centred cubic

fcc face-centred cubic

hcphexagonal close

packed

Close-packed solids

ABA ABC

Close-packed solids

hcp ccp (fcc)

bcc

http://www.chem.qmw.ac.uk/surfaces/scc/

Miller indices

Miller indicesIdentify the intercepts on the x- , y- and z- axes

x = a (at the point (a,0,0) )parallel to the y- and z-axes

Intercepts :    a , ,

Specify the intercepts in fractional co-ordinates In the case of a cubic unit cell each co-ordinate will

simply be divided by the cubic cell constant a a/a , /a, /a    i.e.    1 , ,

Take the reciprocals of the fractional intercepts yielding

Miller Indices :   (100)

Try these

And this

Miller indicesAtkins & dePaula p. 700

fcc (100)

Rotate 45o

CN=8 for surface layer (12 for bulk)

http://www.chem.qmw.ac.uk/surfaces/scc/

fcc(110)

Rotate 45o

CN=7 for surface layer (12 for bulk)

http://www.chem.qmw.ac.uk/surfaces/scc/

fcc(111)

Rotate 45o

CN=9 for surface layer (12 for bulk)

http://www.chem.qmw.ac.uk/surfaces/scc/

Surface energetics

• The most stable solid surfaces are those with :

a high surface atom density

surface atoms of high coordination number

expect

fcc (111) > fcc (100) > fcc (110)

Intersection of surfaces

Surface relaxation

Surface relaxation

Surface reconstruction