Adsorption of gases on solids(focus on physisorption)

Adsorption

• Solid surfaces show strong affinity towards gas molecules that it comes in contact with and some amt of them are trapped on the surface

• the process of trapping or binding of molecules to the surface is called adsorption

• Desorption is removal of these gas molecules from the surface

Adsorption• Two types of adsorption

– Physical adsorption : generally a molecule is physisorbed if the bond energy is less than 10 kcal/mole

– Chemical adsorption : chemisorbed if bond energy is more than 10 kcal /mole

• In chemisorption there is a direct chemical bond involved while Vander waals forces are responsible for physical bonding

PHYSISORPTION : CHEMISORPTION

“physical” bond : chemical bond

MONOLAYER ADSORPTIONLangmuir Isotherm used to model adsorption

equilibrium..

MULTILAYER ADSORPTIONBET Isotherm used to model adsorption

equilibrium.

Surface reactions may take place:- Dissociation, reconstruction, catalysis.

No surface reactions.

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

adsorption.

Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces

surface coverage.

∆Ηads = 50 … .. 500 kJ mol-1∆Ηads = 5 … .. 50 kJ mol-1

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

NOT SURFACE SPECIFICPhysisorption takes place between all molecules on

any surface providing the temperature is low enough.

WEAK, LONG RANGE BONDINGVan der Waals interactions (e.g. London

dispersion, dipole-dipole)..

CHEMISORPTIONPHYSISORPTION

Adsorption and Reaction at SurfacesAdsorption

MONOLAYER ADSORPTIONLangmuir Isotherm used to model adsorption

equilibrium..

MULTILAYER ADSORPTIONBET Isotherm used to model adsorption

equilibrium.

Surface reactions may take place:- Dissociation, reconstruction, catalysis.

No surface reactions.

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

adsorption.

Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces

surface coverage.

∆Ηads = 50 … .. 500 kJ mol-1∆Ηads = 5 … .. 50 kJ mol-1

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

NOT SURFACE SPECIFICPhysisorption takes place between all molecules on

any surface providing the temperature is low enough.

STRONG, SHORT RANGE BONDINGChemical bonding involving orbital overlap and

charge transfer.

CHEMISORPTIONPHYSISORPTION

Adsorption and Reaction at SurfacesAdsorption

MONOLAYER ADSORPTIONLangmuir Isotherm used to model adsorption

equilibrium..

MULTILAYER ADSORPTIONBET Isotherm used to model adsorption

equilibrium.

Surface reactions may take place:- Dissociation, reconstruction, catalysis.

No surface reactions.

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

adsorption.

Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces

surface coverage.

∆Ηads = 50 … .. 500 kJ mol-1∆Ηads = 5 … .. 50 kJ mol-1

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

NOT SURFACE SPECIFICPhysisorption takes place between all molecules on any surface providing the temperature is low

enough.

STRONG, SHORT RANGE BONDINGChemical bonding involving orbital overlap and

charge transfer.

WEAK, LONG RANGE BONDINGVan der Waals interactions (e.g. London

dispersion, dipole-dipole)..

CHEMISORPTIONPHYSISORPTION

Adsorption and Reaction at SurfacesAdsorption

MONOLAYER ADSORPTIONLangmuir Isotherm used to model adsorption

equilibrium..

Surface reactions may take place:- Dissociation, reconstruction, catalysis.

No surface reactions.

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

adsorption.

Non activated with equilibrium achieved relatively quickly. Increasing temperature

always reduces surface coverage.

∆Ηads = 50 … .. 500 kJ mol-1∆Ηads = 5 … .. 50 kJ mol-1

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

NOT SURFACE SPECIFICPhysisorption takes place between all molecules on

any surface providing the temperature is low enough.

STRONG, SHORT RANGE BONDINGChemical bonding involving orbital overlap and

charge transfer.

WEAK, LONG RANGE BONDINGVan der Waals interactions (e.g. London

dispersion, dipole-dipole)..

CHEMISORPTIONPHYSISORPTION

Adsorption and Reaction at SurfacesAdsorption

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N20

MULTILAYER ADSORPTIONBET Isotherm used to model adsorption

equilibrium.

Surface reactions may take place:- Dissociation, reconstruction, catalysis.

No surface reactions.

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

adsorption.

Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces

surface coverage.

∆Ηads = 50 … .. 500 kJ mol-1∆Ηads = 5 … .. 50 kJ mol-1

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

NOT SURFACE SPECIFICPhysisorption takes place between all molecules on

any surface providing the temperature is low enough.

STRONG, SHORT RANGE BONDINGChemical bonding involving orbital overlap and

charge transfer.

WEAK, LONG RANGE BONDINGVan der Waals interactions (e.g. London

dispersion, dipole-dipole)..

CHEMISORPTIONPHYSISORPTION

Adsorption and Reaction at SurfacesAdsorption

z

pote

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l ene

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2N

N20

MONOLAYER ADSORPTIONLangmuir Isotherm used to model adsorption

equilibrium..

MULTILAYER ADSORPTIONBET Isotherm used to model adsorption

equilibrium.

Surface reactions may take place:- Dissociation, reconstruction, catalysis.

No surface reactions.

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

adsorption.

Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces

surface coverage.

∆Ηads = 50 … .. 500 kJ mol-1∆Ηads = 5 … .. 50 kJ mol-1

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

NOT SURFACE SPECIFICPhysisorption takes place between all molecules on

any surface providing the temperature is low enough.

STRONG, SHORT RANGE BONDINGChemical bonding involving orbital overlap and

charge transfer.

WEAK, LONG RANGE BONDINGVan der Waals interactions (e.g. London

dispersion, dipole-dipole)..

CHEMISORPTIONPHYSISORPTION

Adsorption and Reaction at SurfacesAdsorption

MONOLAYER ADSORPTIONLangmuir Isotherm used to model adsorption

equilibrium..

MULTILAYER ADSORPTIONBET Isotherm used to model adsorption

equilibrium.

Surface reactions may take place:- Dissociation, reconstruction, catalysis.

No surface reactions.

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

adsorption.

Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces

surface coverage.

∆Ηads = 50 … .. 500 kJ mol-1∆Ηads = 5 … .. 50 kJ mol-1

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

NOT SURFACE SPECIFICPhysisorption takes place between all molecules on

any surface providing the temperature is low enough.

STRONG, SHORT RANGE BONDINGChemical bonding involving orbital overlap and

charge transfer.

WEAK, LONG RANGE BONDINGVan der Waals interactions (e.g. London

dispersion, dipole-dipole)..

CHEMISORPTIONPHYSISORPTION

Adsorption and Reaction at SurfacesAdsorption

Adsorption and Reaction at Surfaces

Lennard-Jones Potentialphysisorption

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∆Ηads < 50 kJmol-1

Physisorption

z

attractive van der Waals interactions

repulsive Coulombic interactionsPauli repulsion

∆Ηads

N2physisorbed

Z physisorbed

0

Lennard-Jones Potentialchemisorption

Chemisorption 3

z

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l ene

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2N

N20

zpo

tent

ial e

nerg

y

N2

N20

activated chemisorption un-activated chemisorption

Z physisorbed

N2physisorbed

Lennard-Jones Potentialun-activated chemisorption

z

pote

ntia

l ene

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∆Ηads > 50 kJmol-1

Chemisorption 1

z

∆Ηads

∆Ηads

Z chemisorbed

2Nchemisorbed

∆Εdiss

2N

N20

Lennard-Jones Potentialactivated chemisorption

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Chemisorption 2

zN2

physisorbed

∆Ηads

∆Ηads

Z chemisorbed

2Nchemisorbed

∆Εdiss

2N

N2∆Εads0

Isotherm

• Isotherm is an equation which relates the amt of substance attached to the surface to its concentration in the gas phase at a fixed temperature.

• Langmuir: for monolayer adsorption• BET : for multilayer

Langmuir

• This simplest physically plausible isotherm is based on three assumptions:

• 1. Adsorption cannot proceed beyond monolayer coverage.• 2.All surface sites are equivalent and can accommodate, at

most, one adsorbed atom.• 3.The ability of a molecule to adsorb at a given site is

independent of the occupation of neighboring sites.• The dynamic equilibrium is• A(g)+M(surface) ↔AM(surface)

Langmuir adsorptionRate of adsorption = collision rate { P/(2pmkT)0.5 } X probability of finding a suitable site f(Θ) X prob of being in correct orientation κ X probability of overcoming the adsorption barrier exp(-Ea /RT) (this may be 1 if there is no barrier, e.g.physisorption)

f(Θ) is dependant on the type of adsorption:

(1- Θ) if the adsorbate requires a vacant site

(1- Θ)2 if the adsorbate requires two vacant sites (e.g. dissociativeadsorption).

Langmuir adsorption

Rate of desorption = rate constant k = A exp(-Ed /RT) X coverage term g(θ)

g(θ) tells us the kinetic order of desorption:

= 1 zero order -constant coverage, e.g. a solid or multilayer film,

= θ first order -adsorbate remains intact ,

= θ2 second order -adsorbate recombines during desorption.

langmuir adsorption

Hence if ra = rd at eqm ,

P = (2pmkT)0.5 (A/k) {g(θ)/f(θ)} exp(Ea -Ed )/RT

Ea -Ed is just the heat of adsorption, q

For non-dissociative (Langmuir) adsorption with no multilayerformation g(θ)=θ and f(θ)=(1-θ),

giving P =θ / b (1-θ)

where 1/b = (2pmkT)0.5 (A/k) exp(-q)/RT