Adsorption of gases on solids (focus on physisorption)

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1 Adsorption of gases on solids (focus on physisorption)

2 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

3 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 Adsorption PHYSISORPTION : CHEMISORPTION physical bond : chemical bond

4 Adsorption and Reaction at Surfaces Adsorption PHYSISORPTION CHEMISORPTION WEAK, LONG RANGE BONDING Van der Waals interactions (e.g. London dispersion, dipole-dipole).. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. DH ads = kj mol -1 SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. DH ads = kj mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. No surface reactions. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. 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..

5 Adsorption and Reaction at Surfaces Adsorption PHYSISORPTION CHEMISORPTION 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. DH ads = kj mol -1 SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. DH ads = kj mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. No surface reactions. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. 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..

6 Adsorption and Reaction at Surfaces Adsorption PHYSISORPTION WEAK, LONG RANGE BONDING Van der Waals interactions (e.g. London dispersion, dipole-dipole).. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. DH ads = kj mol -1 CHEMISORPTION STRONG, SHORT RANGE BONDING Chemical bonding involving orbital overlap and charge transfer. SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. DH ads = kj mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. No surface reactions. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. 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..

7 Adsorption and Reaction at Surfaces Adsorption PHYSISORPTION WEAK, LONG RANGE BONDING Van der Waals interactions (e.g. London dispersion, dipole-dipole).. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. Η ads = kj mol -1 potential energy CHEMISORPTION STRONG, SHORT RANGE BONDING Chemical bonding involving orbital overlap and charge transfer. SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes Nplace on 2 transition 0 metals but not on gold or mercury. z Η ads = kj mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. No surface reactions. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. Surface reactions may take place:- Dissociation, reconstruction, catalysis. MONOLAYER ADSORPTION Langmuir Isotherm used to model adsorption equilibrium..

8 Adsorption and Reaction at Surfaces Adsorption potential energy PHYSISORPTION WEAK, LONG RANGE BONDING 2N Van der Waals interactions (e.g. London dispersion, dipole-dipole).. NOT SURFACE SPECIFIC N Physisorption takes place between all molecules 2 0 on any surface providing the temperature is z low enough. Η ads = kj mol -1 CHEMISORPTION STRONG, SHORT RANGE BONDING Chemical bonding involving orbital overlap and charge transfer. SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. Η ads = kj mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. No surface reactions. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. Surface reactions may take place:- Dissociation, reconstruction, catalysis. MULTILAYER ADSORPTION BET Isotherm used to model adsorption equilibrium.

9 Adsorption and Reaction at Surfaces Adsorption PHYSISORPTION WEAK, LONG RANGE BONDING Van der Waals interactions (e.g. London dispersion, dipole-dipole).. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. Η ads = kj mol -1 CHEMISORPTION STRONG, SHORT RANGE BONDING Chemical bonding involving orbital overlap and charge transfer. SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. Η ads = kj mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. No surface reactions. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. 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..

10 Adsorption and Reaction at Surfaces Adsorption PHYSISORPTION WEAK, LONG RANGE BONDING Van der Waals interactions (e.g. London dispersion, dipole-dipole).. NOT SURFACE SPECIFIC Physisorption takes place between all molecules on any surface providing the temperature is low enough. Η ads = kj mol -1 CHEMISORPTION STRONG, SHORT RANGE BONDING Chemical bonding involving orbital overlap and charge transfer. SURFACE SPECIFIC E.g. Chemisorption of hydrogen takes place on transition metals but not on gold or mercury. Η ads = kj mol -1 Non activated with equilibrium achieved relatively quickly. Increasing temperature always reduces surface coverage. No surface reactions. Can be activated, in which case equilibrium can be slow and increasing temperature can favour adsorption. 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..

11 Adsorption and Reaction at Surfaces

12 potential energy Lennard-Jones Potential physisorption repulsive Coulombic interactions Pauli repulsion Η ads < 50 kjmol -1 attractive van der Waals interactions Physisorption 0 z DH ads N 2 physisorbed z Z physisorbed

13 Chemisorption 3 activated chemisorption Lennard-Jones Potential chemisorption un-activated chemisorption potential energy 2N potential energy 0 N 2 z 0 N 2 z N 2

14 Lennard-Jones Potential un-activated chemisorption 2N Chemisorption 1 potential energy 0 Η ads > 50 kjmol -1 N 2 z DE diss DH ads DH ads N 2 physisorbed z Z chemisorbed 2N chemisorbed Z physisorbed

15 potential energy 0 Lennard-Jones Potential activated chemisorption DE ads z Chemisorption 2 2N DE diss N 2 DH ads DH ads N 2 physisorbed z Z chemisorbed 2N chemisorbed

16 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

17 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)

18 Langmuir adsorption Rate 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. dissociative adsorption).

19 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, = θ 2 second order -adsorbate recombines during desorption.

20 langmuir adsorption Hence if r a = r d 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 multilayer formation 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

Lecture 5. Solid surface: Adsorption and Catalysis

Lecture 5. Solid surface: Adsorption and Catalysis Lecture 5 Solid surface: Adsorption and Catalysis Adsorbtion G = H T S DG ads should be negative (spontaneous process) DS ads is negative (reduced freedom) DH should be negative for adsorption processes