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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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Apr 2004 Dr Mark Osborne
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