Upload
vinh-vat-va
View
33
Download
0
Embed Size (px)
Citation preview
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
z
pote
ntia
l ene
rgy
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
ntia
l ene
rgy
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
z
pote
ntia
l ene
rgy
∆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
pote
ntia
l ene
rgy
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
rgy
∆Ηads > 50 kJmol-1
Chemisorption 1
z
∆Ηads
∆Ηads
Z chemisorbed
2Nchemisorbed
∆Εdiss
2N
N20
Lennard-Jones Potentialactivated chemisorption
z
pote
ntia
l ene
rgy
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