Adsorption and Surface Characterisation Techniques

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Apr 2004 Dr Mark Osborne

Colloids and Surface Science

Contact Details:

Dr Mark Osborne

Chichester II

Office 2R214 (Second Floor)Lab 2R112 (Ground Floor)

Phone: 8328

Email: [email protected] @ www.sussex.ac.uk/Users/kaf18/courses


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Course Synopsis Adsorption at the gas/solid interface:

Heterogeneous catalysis

Langmuir and BET isotherms Measurement of gas adsorption

Surface characterisation techniques: Light vs TEM and SEM microscopies Dynamic light scattering X-ray photoelectron spectroscopy

Auger and LEED characterisation Texts: Intro to Colloid and Surface Science, D.J.Shaw

Physical Chemistry, P.W. Atkins


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Adsorption at the G/S interfaceTerminologies:

Non-adsorbed gas adsorptive

Adsorbed gas adsorbate

Underlying substrate adsorbant

Adsorptionoccurs at the surface only

Absorptionpenetration into the bulk

Sorption generic term used when adsorption and absorptionprocesses cannot be distinguished experimentally


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Physical vs Chemical AdsorptionTwo types of adsortpion:

Physisorption weak, non-specific Van-der-Waals

(dipole-dipole) interactions

Chemisorption strong, specific formation of chemical bonds(usually covalent)

Traditionally distinguished by adsorption enthalpies Hadsi.e Hads > -25 kJ/mol = physisorption FeN2 -10 kJ/mol

Hads < -40 kJ/mol = chemisorption Fe-N -150 kJ/mol

However, physisorption is always exothermic Hads


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Claussius-Clapeyron EquationDecreasing the temperature increases amount of gas adsorbed

Since for A + S AS Kads

= [AS]/[A][S] = 1/pA

And Gads = - RTlnKads = Hads TSads so lnK= - +

Therefore can measure

Hads directly by isosteric (constantcoverage) calorimetry.

Measure partial gas or vapour pressurepA required to maintain

constant surface coverage at different temperatures T

Then = - or =

HadsRT

SadsR

HadsRT2

lnpAT

V

lnpA(1/T)

HadsR

V


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Determination of EnthalpiesDeriving a linear Claussius Clapeyron relation by recognising

- = then =

So a plot of lnpA vs 1/T is a straight line with gradient

e.g A volume of N2 is absorbed on charcoal at 4.8 atm and

190 K. A pressure of 32 atm is required to maintain the samevolume when the temperature is raised to 250 K

T (K) 190 250

1000/T 5.3 4ln p 1.6 3.5

(1/T)

T

1T2

Hads

R

lnpA

T V

(1/T)

THadsR

Hads 3.5-1.6

R 4 5.3

4 5 61

23

4 = = -1.5

Hads = -1.5 x 8.314= -12 kJ/mol


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Experimental MethodsSolid surfaces pre-treatment - to remove physisorbed gasesand condensates to < 1% monolayer ~ 1013 molecules/cm2

Evacution (outgassing) at 10-4 Torr (0.08 atm)

High temperature

Chemical desorption e.g C on Pt reacted with O2 CO desorbs

Cleavage in high vacuum e.g mica, silicon

Gravimetric high adsorption volumes

Weigh substrate before and after

adsorption using microblance e.g QCM Then m change in quartz oscillating frequency

http://www.q-sense.com/qcmd_tech.html

~m


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Further MethodsVolumetric measurement slow, accurate Evacuate A, B and close tap

Fill volume A with gas to a pressure P1 Open tap, measure P2 and calculate

total volume from P1VA=P2(VA+VB)

Repeat with known weight of solid in B Then drop in P2 corresponds to Vads from P1VA=P2(VA+VB+Vads)

Chromatographic retention fast, not accurate

Column filled with solid Flow carrier gas He + Absorbate A

A is depleted by adsorption until surface is saturated

A @ P1 B ~ 10-4

Solid

A + B @ P2

Detectorresponse

TimeSolidHe + N2


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Heterogeneous CatalysisPhysisorption provides lowenergy pathway to the

dissociation of adsorbategas and reaction takesplace on 2D surface

Note:

Hp


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Comments on the PE curvesP(R) Van der Waals attraction 1/R6 + Born repulsion exp(-R)

C(R) M-X potential but with dissociation of X2 at large R

Heat of physisorption Hp ~ kT


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Catalytic SeriesCatalytic activity depends on strength (Hads) of chemisorption

Weak high mobility but low adsorbate coverage low activity

Strong high coverage and high mobility high activity

Very strong adsorbate immobilised low activity

Hence the volcanocurve fora gas on different surfaces

e.g hydrogenation of ethene

on transition metal surfaces

Adsorbate strengths O2 > C2H2 > C2H4 > CO > H2 > CO2 > N2

Series I

SeriesII+III

Adsorption and Surface Characterisation Techniques

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