Type of Surface

8/12/2019 Type of Surface

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T ypes of Surface Homogeneous surface Heterogeneous surface

Considering from >> Energy uniformity of the surface Dependence ofheat of adsorption on the surface coverage


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T echniques of Surface Study Atomic Force Microscopy)(AFM)

X-ray Photoelectron

Spectroscopy (XPS)

Scanning Tunnelling Microscope (STM)

SEMetc.
http://en.wikipedia.org/wiki/Image:ScanningTunnelingMicroscope_schematic.pnghttp://en.wikipedia.org/wiki/Image:System2.gifhttp://en.wikipedia.org/wiki/Image:Atomic_force_microscope_block_diagram.png


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Adsorption Absorption - Adsorption a combination of physical

and chemical processes in which asubstance(adsorbate) accumulates on asolid(adsorbent) surface

Attraction of a molecule to asurface can be caused by physical andchemical forces


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Specific Surface Areasof Some Adsorbents .

Alumina Chips 0.50 m2/g (N2)0.46 m2/g (Kr)

Glass Powder. 5.3 m2/gCarbon Black 24.1 m2/g

Carbon Powder 111 m2/gSilica-Alumina Pellets

s.s.a. = 215 m2/gp.v. = 0.60 cm3/ga.p.d. = 115

s.s.a. = specific surface area, p.v. = pore volume, a.p.d. = average pore diameter


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Adsorption of Gas on Solid Surface :Adsorption Isotherm

The free gas and the adsorbed gas are inequilibrium, The fraction coverage ( ) ofthe surface depends on pressure (P) ofthe overlying gas. Variation of with P at a chosen

temperature is calledadsorption isotherm


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T hermodynamic viewpoint, Molecules always prefer to be in a lowerenergy state. A moving gas molecule is normally at ahigher energy state. When gas molecule is adsorbed on the solidsurface, its energy becomes lower.

The difference in the two energy states iscalled theheat of adsorption (H) andthis is the quantity of energy liberatedwhen the molecule is adsorbed


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AdsorptionEnthalpy Adsorption is anexothermic process because

energy is liberated, thereforeenthalpy isalways negative. Adsorption constants areequilibrium constants, therefore they obeyvan 't Hoff's equation:

K is a surface-coveraged constant
http://en.wikipedia.org/wiki/Exothermichttp://en.wikipedia.org/wiki/Enthalpyhttp://en.wikipedia.org/wiki/Enthalpyhttp://en.wikipedia.org/wiki/Jacobus_Henricus_van_%27t_Hoffhttp://en.wikipedia.org/wiki/Jacobus_Henricus_van_%27t_Hoffhttp://en.wikipedia.org/wiki/Jacobus_Henricus_van_%27t_Hoffhttp://en.wikipedia.org/wiki/Enthalpyhttp://en.wikipedia.org/wiki/Exothermic


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Physisorption Basic physical forces :(electrostatic forces)

- dipole-dipole interaction- dispersion interactions or- London-van der Waals force- hydrogen bonding

involves relatively smallheats ofadsorption about -25 kJ per moleor less (exothermic) multilayer adsorption


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Chemisorption involves the formation of strongchemical bonds betweenadsorbate molecules and specificsurface locations known aschemically active site

forces and energies about -40 kJ

per mole (

exothermic)

monolayer adsorption


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SorptionIsotherms Asorption isotherm (also adsorption isotherm) describes the

equilibrium of thesorption of a material at a surface(moregeneral at a surface boundary) at constant temperature.

I t represents the amount of material bound at the surface(the sorbate) as a function of the material present in the gasphase and/or in the solution.

Sorption isotherms are often used as empirical models,[1] which do not make statements about the underlying mechanismand measured variables. They are obtained from measured data

by means ofregression analysis.

T he most frequently used isotherms are the, Freundlichisotherm, the Langmuir isotherm, and the BET model.
http://en.wikipedia.org/wiki/Sorptionhttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Regression_analysishttp://en.wikipedia.org/wiki/Freundlich_isothermhttp://en.wikipedia.org/wiki/Freundlich_isothermhttp://en.wikipedia.org/wiki/Langmuir_isothermhttp://en.wikipedia.org/wiki/Langmuir_isothermhttp://en.wikipedia.org/wiki/Freundlich_isothermhttp://en.wikipedia.org/wiki/Freundlich_isothermhttp://en.wikipedia.org/wiki/Regression_analysishttp://en.wikipedia.org/wiki/Temperaturehttp://en.wikipedia.org/wiki/Sorption


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SorptionIsotherms

Langmuir isotherm Freundlich isotherm Brunauer-Emmett-Teller (BET)


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Langmuir Isotherm

Three assumptions : 1. Adsorption cannot proceed beyondmonolayer coverage

2. All surface site are equivalent andthe surface is uniform

3. The ability of a molecule to adsorbat the given site is independent of

the occupation of neighboring sites


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Langmuirequation Equilibrium between empty surface sitesS* and particles P and filled surfacesites S-P

Fraction coverage, = # active sites occupied

# active sites availableFraction of non-occupied sites = (1- )


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Rate ofadsorption :d = kads. P.N. (1-) (1) dt

Rate ofdesorptiond = - kdes. N. .(2)

dtAt equilibrium, (1) = (2)

Kads

Kdes


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Langmuirequation :

K is a Langmuir adsorption constant (Surface-coveraged constant)

= K P1 + K P

K = KadsKdes
http://en.wikipedia.org/wiki/Image:Langmuir_isotherm2.jpg


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Langmuirequation Let = the fraction of filled surface site= V = volume of sites occupied

Vm volume of max. sites coverage

= K P = V1 + K P Vm

P = P + 1V Vm KVm

P

P/V Slope = 1/V

Intercept = 1/KVm


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Freundlich equation

The Frendlich Adsorption Isotherm is mathematicallyexpressed as

x/m = K p1/n

or

x/m = K c1/n where

x = mass of adsorbate m = mass ofadsorbent p = Equilibrium pressure of adsorbatec = Equilibriumconcentration of adsorbate insolution.

K and 1/n are constants for a given adsorbate and

adsorbent at a particular temperature.
http://en.wikipedia.org/wiki/Masshttp://en.wikipedia.org/wiki/Adsorbatehttp://en.wikipedia.org/wiki/Adsorbenthttp://en.wikipedia.org/wiki/Equilibriumhttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Concentrationhttp://en.wikipedia.org/wiki/Pressurehttp://en.wikipedia.org/wiki/Equilibriumhttp://en.wikipedia.org/wiki/Adsorbenthttp://en.wikipedia.org/wiki/Adsorbatehttp://en.wikipedia.org/wiki/Mass


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T he BET Isotherm (Brunauer, Emmett, Teller)

1. Dealing with multilayer adsorption(physical adsorption)

2. Rads on empty surface = Rdes on 1st

layerRads on 1st layer = Rdes on 2nd layerand so on

3. heat of adsorption of the upper layers areequal to heat of liquefaction of adsorbate,but not for the monolayer


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T he BET equation

= V = cxV

m (1-x)[1+(c-1)x]

X = P/Po; V , x 1 (P=Po)C = exp(Hdes- Hvap)/RT = const.


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T he BET Isotherm

00

11

P

P

C V

C

C V P P V

P

mm

Po = vapor pressure of adsorbate (liquid)P = vapor pressure of gas on surfaceV = volume of surface sites occupiedVm = max. volume forming monolayerC = constant = exp( Hdes- Hvap)/RT


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Langmuir isotherm(red) and BET isotherm(green)
http://en.wikipedia.org/wiki/Image:Adsorption_Isotherms_%28Langmuir_red_%26_BET_green.JPG


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Surface Area Determination Vm= (n mole) (22,414 cm3 /mole)

w(g)

T R

V P n m

Asp

= specific surface areaNA = Avogadro #

o = area occupied per molecule

Asp = Vm NA o (22,414)


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Exercise :

From the BET isotherm of N2 on 1.00 g Fe/Al2O3 surface at 77K, the maximum volumeof N2 to form a monolayer on the surface wasfound to be 2.86 cm3 at STP. Determine thesurface area of Fe/Al2O3 (Given N-N bond = 110 pm)

T he End

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Type of Surface