AFL Aston Catalysis Surface Chemistry Slides
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Prof Adam Lee leeaf@cardiff.ac.uk
8 lectures
Air
CH3010: Catalysis
1
During this course you should become familiar with:
• Experimental reaction kinetics – liquid and gas phase reactions • The importance of surface processes. • How surface structure affects the reactivity of materials. • Adsorption processes, physisorption and chemisorption • Adsorption isotherms - Langmuir and BET models. • Kinetics of surface reactions - Eley Rideal and Langmuir Hinshelwood mechanisms. • Surface analytical techniques, including XPS, AES.
Recommended Reading
• G.A. Somorjai Introduction to Surface Chemistry (Wiley)
• G.Attard, C.Barnes Surfaces (Oxford Chemistry Primer)
• E.McCash Surface Chemistry (OUP)
• P.W Atkins & J de Paula Physical Chemistry, 9th Edition Chapter 21 • “ “ Elements of Phys. Chem., 5th Edition Chapter 1
• Tutorial notes on surface chemistry, useful www.chem.qmw.ac.uk/surfaces
2
Learning objectives
3
Why do we care about kinetics?How fast a process occurs? - can predict practical/economic feasibility - can predict risks (heat release, pressure changes) - alter reactants - improve reactor engineering
What route is followed? - what steps are involved (e.g. dissociation, coupling) - what is the mechanism (how do atoms/molecules interact)
Typical experiments involve: 1. mixing reactants 2. initiating reaction (heat , light )
3. measuring T or P 4. measuring reactant/product concentrations
Crucial that Steps 1-3 >> Step 4
4
Experimental kineticsKinetic theory – reaction rates:
nAkdtAd ][][
−=
Experimental kinetics – concentrations of reactants
Aim of experiment to determine:
• Reaction order n - steps involved • Rate constant(s) k - intrinsic ‘speed’ of reaction, bigger = faster • Activation energy Ea - mechanism
Theory and experiment linked via the integrated rate equations
For reactions that occur over minutes or hours, k < 1x10-2 s-1
This measurement timescale is fine for standard undergraduate techniques:
e.g. pH meter , conductivity , photometry
Key considerations in choosing a method are the half-life t1/2 and mixing time.
5
Case study: 1
6
Case study: 1Consider thermal decomposition of cyclopentene:
Since pV = nRT (i.e. p ∝ n), and 1 mol of any gas occupies same V at same T
Measuring p versus time t allows us to track reaction progress
T = 500oC – 540 oC t1/4 ~1000 sec
Reaction vessel in furnace
Manometer
Cyclopentene
Vacuum pump
Transducer
7
T = 500oC – 540 oC t1/4 ~1000 sec
8
Case study: 1Consider thermal decomposition of cyclopentene:
Since pV = nRT (i.e. p ∝ n), and 1 mol of any gas occupies same V at same T
Measuring p versus time t allows us to track reaction progress
If all cyclopentene decomposes, pressure ↑ x2.
Partial pressure of CP at any time PCP = 2 PCP(t=0) – Pt.
Analysis
• Determination of order, n
1. Integrated plots
Plot ln(PCP) vs t and 1/PCP vs t and compare trend lines via R2 values.
Best fit (R2 closest to 1) predicts order.
BUT
Works best for synthetic data; “real” data difficult to distinguish.
9
Ln(CP)
ln (C
P)
3.9
4.125
4.35
4.575
4.8
Time / min
0 5 10 15 20
R² = 0.9752
1/CP
1 / CP
0
0.0045
0.009
0.0135
0.018
Time /min
0 5 10 15 20
R² = 0.9536
10
2. Quarter lives method
1st order reactions t1/4 independent of PCP(t=0)
2nd order reactions t1/4 ∝ 1/PCP(t=0)
∴examine t1/4 at 3 different values of PCP(t=0) (at fixed T) → PCP(t=0) dependence
Can generalise this method to any value of n.
3. Initial rates method
ln(-Rt=0) = ln(k) + n ln(PCP(t=0))
∴plotting ln(-Ro) vs ln(PCP(t=0)) should be linear with slope = n.
Initial rates Ro can be obtained from the slope of the tangent at t=0 or more accurately by fitting a cubic polynomial to the data.
Need to repeat reax. at different T
• Determination of rate constant
Once we have determined n use appropriate integrated plot to determine k
• Determination of EActivation
Arrhenius plot of ln(k) vs 1/T slope = -EA/R
Ln(CP)
ln (C
P)
4
4.2
4.4
4.6
4.8
Time / min
0 5 10 15 20
Need to repeat reax. at different T11
For reactions with t1/2 ~ 1 msec – 1 sec we usually use a flow method
• Continuous (discharge) flow
Very wasteful of reactants and needs moveable detector
12
A
B Reaction regionMixing region
Product (waste)
12
Case study: 2
Moveable detector
• Stopped flow method
Reactant syringes
Mixing chamber
Light sourcePhotomultiplier
detector
Stopping syringe
13
14
Uses much smaller amounts of reactants and employs a fixed detector
e.g. reduction of 2,2 dichlorophenolindophenol (DCIP) by ascorbic acid
DCIP → products
Can use optical absorption detection method: [DCIP ] ∝ absorbance
Faster reactions (t1/2 < 1 msec) – relaxation methods
For example systems at equilibrium
Monitor time to re-equilibrium system (e.g. [B] or [A]) after perturbation
t* = 1/(k1+k-1) and K = k1/k-1
The perturbation should occur in 10-6 – 10-7 seconds and may b e:
- pressure jump - temperature jump - high electric field pulse - ultrasonic vibrations - light pulse (flash photolysis) ultrafast spectroscopy (fs-ps!)
k1
A ⇔ B k-1
15
• Flash Photolysis – very fast reactions (k~50 000 – 500 000 s-1)
Pre-mixed volume of reactants in a photolysis cell subjected to light pulse producing atoms, radicals & excited states whose concentration is followed with time.
Advantages: - no mixing time
- timescales of reax. limited only by
pulse duration (<10-12 s with lasers)
- species formed in the centre of the cell
so can ignore wall reactions.
16
17
Reaction dynamicsAll previous methods consider systems where reactants have a thermal distribution of energy.
To study elementary reactions ideally want molecules possessing a specific energy, and only undergoing a single collision.
Solution: Molecular beams
Collimation+rapid expansion make beam: 1. intense 2. linear 3. minimal vibrational excitation
Energy can be tuned: 1. varying T 2. seeding beam with inert elements (e.g. He)
NH3 synthesis CH3OH synthesis
C2H4 oxidation
Surface Chemistry& Analysis
Heterogeneous Catalysis
Heterogeneous Catalysis
Chemical Surface
Modification
Chemical Surface
Modification
Corrosion Science
Medical Implants
Polymer ScienceTribology
OpticsCorrosion Science
Medical Implants
Polymer ScienceTribology
Optics
35% of global GNP
Electronic Devices
Gas sensors (CO)
18
Surface chemical processes$10 trillion!!
Gas phase: Controlling parameters: [A], collision rate zAA, collision cross-section σA ..
e.g. O2 P = 1 atm; T = 298 K → zAA = 2.8 x 109 molecule-1 sec-1
Surfaces: Rate of gas molecules colliding with surface (flux) = n x c
;
∴ Rate = P/(2πmkT)1/2 Hertz-Knudsen equation
e.g. O2 P = 1 atm; T = 298K → rate = 5 x 1023 cm-2 sec-1
Surface chemistry phenomenally fast due to higher collision probability → vast collisional cross-section 19
Surface Kinetics
n = N / V = P / (kT)
P,V,T
20
• Substrate (adsorbent) - the solid surface where adsorption occurs
• Adsorbate - the atomic/molecular species adsorbed on the substrate
Surface Terminology
What happens when a molecule collides with a surface?
Nothing - elastic collision, no energy transfer
Atoms/molecules feel attractive potential and “stick” to surface - a process termed ADSORPTION
The sticking probability s is defined as:
s = no.of molecules that stick 0 ≤ s ≤ 1 no. of molecular impacts
21
22
Distance from surface
adsorbate
δ- δ +δ+
Arises from van der Waals forces between a molecule/atom and surface
Consider changes in thermodynamic parameters upon adsorption:
S2 < S1 , ∴ ΔS < 0
Recall, ΔG = ΔH - TΔS Gibbs-Helmholtz equation
Since ΔG <0 for a process to occur spontaneously,
ΔH <0 , i.e adsorption must always be exothermic
3 degrees of freedom S1
2 degrees of freedom S2
AdsorptionSurface
23
Thermodynamics of adsorption
+ve-ve
24
For dissociative adsorption of molecules, e.g. X2, we need to consider energy: 1. breaking the X-X bond - dissociation energy DXX
2. making 2 new X-M (M=metal atom) bonds - bond strength EX-M
For the adsorption to be exothermic:
EX-M > DXX/2.
For N2, EN-M >420 kJ mol-1, H2, EH-M >200 kJ mol-1
∴ N2 adsorption is more specific.
EX-M high enough
Molecular adsorption Dissociation
Metal Metal Metal
Chemisorption • ΔH > 50 kJ mol-1 • chemical adsorption → chemical bond formation • Confined to single monolayer • Adsorption may be activated • Dissociation possible
Physisorption • ΔH < 50 kJ mol-1 • physical adsorption → van der Waals (perm./induced dipoles) • Multilayer adsorption • Unactivated adsorption • Enhanced at low T (Le Chatelier) • Non-dissociative
diffusion “2-D gas”
desorption
A2(g) ⇔ A2(a)
simple equilibrium between gas & adsorbed
Surface lifetime, τ = τo e ΔH/RT Frenkel eqn.
EX-M high enough
A2(g) ⇒ 2 A(a)
adsorbed atoms can still diffuse across surface 25
Adsorption types
Chemisorption
• A true chemical bond forms between the adsorbate and the surface - involves electron transfer.
• Bonds within the adsorbate molecule are weakened. ⇒ fundamental to catalysis!
• Chemisorption process may be associative or dissociative.
e- transfer
26
ΔHads(kJ/mol) Dissociative ads: O2/W -600 N2/W -340 H2/W -160 H2/Ni - 80
Non-Dissociative CO/Ni -130 N2/Ni - 50
Bond dissociation energy of N2 = -950 kJmol-1 (Gas Phase) H2 = -430 kJmol-1
• ΔHads for dissociative adsorption > non dissociative
27
Consider a linear combination of 1s atomic orbitals when going from a diatomic molecule to an infinite solid
+
• Infinite Solid
[ ]n
[ ]n
[ ]n
Electronic Properties of Solids
28
In the solid state bands exist rather than discreet molecular orbitals. → Each band is a continuum of MO’s
Consider band structure for Sodium
1s2
2s2
2p6
3s1
1s Band
2s Band
2p Band
3s BandFermi Energy (EF)
Filled
Filled
Filled
½ full
Atom → extended structure
Band formation
29
Fermi energy
Recall from MO theory
• N atomic orbitals ⇒ N molecular orbitals levels in each band
• Each level has space for two electrons.
• The Fermi level is the highest occupied energy level in band structure (equivalent to the HOMO in a molecule).
• N.B. For a material to be a conductor, the Fermi level must lie in a region with unoccupied states above it.
EF
Insulator Conductor30
Density of States
1. N atomic orbitals ⇒ N levels in each band
2. Band width increases with orbital overlap
Recall:
• Distribn of energy levels within a band depends on band width
Strong overlap → Broad band → Low Density of States
• Density of States = No. energy levels per unit energy in a band
Weak overlap → Narrow band → High Density of States
31
d-orbitals ⇒ poor overlap ⇒ narrow band ⇒ high Density of states
s-orbitals ⇒ good overlap ⇒ wide band ⇒ low Density of states
Filled d states
sp band
sp band
Filled d states
Empty d states
EF
Partially filled d-band metalN(E)
N(E)
EF
Den
sity
of
Sta
tes
Den
sity
of
Sta
tes
Filled d-band metal
EF
32
What is the significance of the density of states?
• Chemisorption involves electron transfer to/from the surface
• Electrons can only be transferred to/from states near the fermi level (Ef).
• Direction of e- transfer can be:
Filled orbital on adsorbate → empty state on metal Filled states on metal → empty orbital on adsorbate
• High DOS at Ef → metal has lots of states available for electron transfer
• Transition metals (d block) are good catalysts
How do we determine the direction of e- transfer?
Recap:
• Combination of atomic orbitals in a extended solid result ⇒ ‘band’ formation
FERMI ENERGY (Ef) = Highest occupied level
DENSITY OF STATES (DoS) = Number of available energy levels at a particular energy.
• DoS at Ef tells us how good/bad solid will be at electron transfer (e.g.important for electronic interaction with adsorbates -see later)
Definition The difference in energy between an electron at rest just outside the metal surface and an electron at the Fermi Energy.
• Electrons are held in a potential well
• φ depends on the depth (W) of the potential well in which the electrons are held, and the number of electrons.
E > 0
E < 0W
EF
φEmpty levels Fermi Level
Bottom of valence band
E(φ)=W-EF
Zero Energy Level
Occupied States
The Workfunction (φ)
Inte
nsity
0.5
1/λ
Threshold = φ
• The work function (φ) is measured as the threshold photon energy required for photoemission
• φ is a measure of surface electronic properties and changes in the presence of adsorbates.
• φ is typically 1-5 eV
36
Work function changes induced by Chemisorption
• Recall chemisorption involves electron transfer
a. Electronegative adsorbates
e-EF
φ
If the LUMO of the adsorbate < EF then charge is transferred to the adsorbate.
Metal Adsorbate
LUMO
1. Ef ↓ hence φ ↑ i.e Δφ is +ve
ΔφEF
37
e-
EF
φ
b. Electropositive adsorbates
ΔφEF
• If the HOMO of the adsorbate >EF then charge is transferred to the metal.
HOMO
1. Ef ↑ hence φ ↓ i.e Δφ is -ve
38
Examples of work function changes
O/Cu(111) - Positive H/Mo(111) - Negative
Cs/W(100) - Negative
Cl/W(100) - Positive
39
Recap
WORK FUNCTION (Φ) = minimum energy for an electron to be emitted from the Fermi Level (Ef)
• Φ changes in the presence of adsorbates
- Chemisorption
- electronegative adsorbate → ΔΦ ALWAYS +ve (e.g. Cl)
- electropositive adsorbate → ΔΦ ALWAYS –ve (e.g. K)
40
Case Study CO/Pt(111)
Δφ varies with fractional coverage (θ)
Need to consider bonding of CO to surface
41
• Molecular Orbital Diagram for CO (with sp mixing):
• Electron configuration: (1σ)2 (2σ)2 (3σ)2 (4σ)2 (1π)4 (5σ)2 (2π)0
3σ
4σ
5σ
6σ
42
The 4σ orbital is localised on O
The 5σ orbital on the C atom
The 2π is anti-bonding
5σ
1πO
C 2π
4σ
So what happens at 0.33 ML?
43
Low coverages
Initial adsorption occurs into preferred on-top site
σ donation to metal dominates, hence φ decreases.
OC
• But CO-CO repulsion prevents occupation of every on-top site.
Vacant Metal d orbital
5σ orbital of CO
O C
Strong σ donor
Full metal d orbital
2π orbital of CO
π acceptor
44
Higher coverages
Occupy less favourable bridge sites.
Charge transfer now dominated by π* back-bonding
⇒ φ increases and returns to that of the clean surface
OCOC
O C
Weak σ donor π acceptor
45
ΔH
ads(
kJ/
Mol
)
- 145
- 96.4
- 48.2
Coverage (monolayers)
Infra-red measurements show:
< 0.33 ML → single band at ~2090 cm-1 → linear Pt-C=O species
> 0.33 ML → second band ~1850 cm-1 → bridging species
• The enthalpy of adsorption (ΔHads) is also coverage dependent
>0.33<0.33
• Is there evidence for such adsorption site changes?
0.33
46
Strong
Weak
Beginning of 20th century, use of nitrogenous fertilisers well established
Principal source NaNO3 (Chile)
Massive demand due to: • population growth • use as explosive
BUT
Sources predicted not to last more than 50 years
Other sources of ‘fixed’ nitrogen required! → ammonia
Process feeds roughly 33 % of world
Dissociative adsorption – NH3 synthesis
47
Haber-Bosch process (1909) commercialised by BASF
Alvin Mittasch tested >4000 catalytic materials in 1000 experiments to achieve direct combination of H2 and N2
1. Adsorption and dissociation of the reactants
N2(g) ⇒ N(a) + N(a) H2(g) ⇒ H(a) + H(a)
2. Diffusion and reaction of N and H atoms, and desorption of ammonia.
N(a) ⇒ NH(a) ⇒ NH2(a) ⇒ NH3(a) ⇒ NH3(g)
The catalyst must ‘get the balance right’:
• Adsorb N2 and H2 dissociatively • Allow N and H diffusion and reaction • Allow ammonia desorption
48
Fritz Haber Carl Bosch (Nobel Prize, Chemistry
1918/1932)
Potential energy diagram for dissociative adsorption
-ΔHchem
Transition State
H
-ΔHphysisorption
H + H
H2
Molecular state
Echem
Distance from Surface (nm)Ener
gy /
eV
Dissociated stateFe
Fe
HH
Gas phase enthalpy of dissociation
49
• Non-activated dissociative chemisorption
• Activated dissociative chemisorption
Crossover > E = 0
Crossover E < 0
- spontaneous dissociation
- need to put energy in to system (heat) to dissociate
e.g. H2 on Fe
e.g. CO on Ta
50
Molecular vs dissociative adsorption of CO
The catalytic activity of different metal surfaces reflects their interaction with
adsorbates.
Of particular importance is the role of the Fermi level.
Dissociated CO Molecular CO
51
The position of EF relative to LUMO of the adsorbate affects electron
transfer process (i.e. degree of back-bonding and hence dissociation)
Carbon Monoxide
Ti V Cr Mn Fe Co Ni
• Same principle applies to dissociation of other diatomics e.g. O2, N2, H2
• Back-bonding (and hence dissociation) can be tuned by adding dopants
e- transfer
• If Ef > LUMO → strong back-bonding → weakens C-O bond → Dissociation
52
P1 V1 = P2 (V1 + V2) - Kinetic theory of gases
P2’,V1,T
P1,V1,T
0,V2,T
P2,V1,TP2,V2,T
P2’,V2,T
P1,V1,T
0,V2,T
X
X
Open valve
Open valve
53
How do we measure adsorption?
P2’ < P2 , due to adsorption of gas molecules onto the surface
By measuring P2 and P2’ can calculate number of molecules adsorbed ∝ [P2 - P2
’] - Volumetric method
Plot of number of molecules adsorbed against equilibrium pressure at constant temperature
A useful quantity is the monolayer uptake - the number of molecules which just completely cover the surface.
This can be used to calculate the surface area of a solid
x
x
x
x
x
xx
Pressure (=P2’)
Num
ber
m
olec
ules
ad
sor
bed,
n
A2(g) ⇔ A2(a)
[A2(a)] = f (p)
Often use V (volume equivalent at stp) or θ (surface coverage) instead of n
54
Adsorption isotherms
Surface
Monolayer
55
By making certain assumptions Langmuir derived a simple equation to describe the adsorption isotherm of any atom/molecule.
Enables us to predict adsorbate coverage (θ) ➔ calculate reaction rates ➔ optimise reaction conditions (T, pressure)
Based on idea chemical equilibria exist during all reactions:
- stabilities of adsorbate vs. gas/liquid - temperature (surface and reaction media) - pressure (liquid conc.) above catalyst0
GAS/LIQUID reactants, products
solvents
CATALYST absorbate
Langmuir adsorption isotherm
56
Equilibrium between gas molecules M, empty surface sites S & adsorbates
e.g. for non-dissociative adsorption
S* + M S----M
Assumption 1: Fixed number of identical, localised surface sites
[S----M] ∝ θ adsorbate coverage
[S*] ∝ vacancies ∝ (1- θ) [M] ∝ gas pressure
∝ PReactants Products
Assumption 2: Adsorption is immobile – no surface diffusion
Assumption 3: Each adsorption site is occupied by only 1 adsorbate (only monolayer adsorption)
57
Equilibrium constant, b is
P)1(]tstanac[Re]oducts[Prb
θ−θ
==
Rearrange in terms of θ,
)bP1(bP+
=θ Langmuir Adsorption Isotherm
b is Langmuir equilibrium constant (∝ sticking-probability s ) - depends on ΔHads
Assumption 4: ΔHads and thus b is temperature & pressure independent
b
58
Rate of adsorption ↑ as b ↑
b=0.1
b=1
b=10
Surface saturated/ poisoned
Tune chemistry
Self-Assembled Monolayers
MoS2Au/(111) hydrodesulfurisation
catalyst
θ is the fractional surface coverage = n/nmonolayer
b is a constant for specific gas-solid equilibrium.
Rerrange Langmuir isotherm to relate experimental measureables, P and n
P/n = 1/bnm + (1/nmonolayer) P
Plot of P/n vs P should be linear with slope 1/nmonolayer
From nmonolayer, and the cross-sectional area of the adsorbate → calculate surface area of solid.
(often use volume of gas adsorbed at STP instead of number molecules)59
Applying Langmuir adsorption isotherm
)bP1(bP+
=θ
[C2H5Cl] Mass [C2H5Cl]/mass(mol/dm13) adsorbed
(g)0 0
1.17E103 3 3.90E1042.94E103 3.8 7.74E1045.87E103 4.3 1.37E1031.17E102 4.7 2.49E1031.76E102 4.8 3.67E103
Langmuir analysis of chloroethane adsorption on charcoal
[C2H
5Cl]
0
1.3
2.5
3.8
5
Mass adsorbed m (g)0 0.0045 0.009 0.0135 0.018
[C2H
5Cl/]m
0.00E+00
1.00E-‐03
2.00E-‐03
3.00E-‐03
4.00E-‐03
[C2H5Cl]0.00E+00 4.50E-‐03 9.00E-‐03 1.35E-‐02 1.80E-‐02
y = 0.1982x + 0.0002
≡P for gases≡V for gases
60
Mass of monolayer = 1/slope = 1/0.198 = 5.05 g
Convert → moles → molecules
Surface area = molecules x area molecule
Langmuir analysis involving two measurementsA carbon sample adsorbs 25 cm3 of nitrogen at a pressure of 10 mbar and 41 cm3 at a pressure of 18 mbar. Making use of the Langmuir adsorption isotherm, determine the monolayer capacity of the sample. [R=8.314 J K-1 mol-1]
θ = 25/Vm at P=10 mbar , where Vm is saturation (monolayer) uptake. θ = 41/Vm at P=18 mbar
and θ = KP/(1+KP)
Thus 25/Vm = 10K/(1+10K) and 41/Vm = 18K/(1+18K)
Rearranging:
1/K = (10Vm - 250)/25 1/K = (18Vm - 738)/41.
Solve simultaneous equations for 1/K → Vm = 205 cm3 (make sure you can do this)
61
62
Langmuir Isotherm – adsorption stops at the monolayer
During physisorption multilayers form – need a better model to account for this
Monolayer volume
Mono- layer
Multilayer
Complex adsorption isotherms
initial irreversible maturation I maturation II dispersion attachment attachment
Medical devices -‐hip implants -‐ prosthetics -‐ scalpels
Fouling -‐Bioreactors -‐ boats -‐ gum disease
SLIME (matrix protein)
Brunauer, Emmett and Teller developed more realistic model that: 1. Allows multilayer adsorption
2. Different enthalpy of adsorption of multilayers and monolayers
V = volume of N2 adsorbed Vm = monolayer volume P0 = saturation N2 vapour pressure at 77K P = applied N2 pressure
c accounts for enthalpies of adsorption
c = exp(ΔHDO - ΔHVAP
O)/RT
ΔHDO = enthalpy of desorption (strength of adsorbate-surface interaction)
ΔHVAPO = enthalpy of vaporisation (adsorbate-adsorbate interaction in multilayer)64
BET adsorption isotherm
ommo PP
cVc
cVPPVP .)1(1
)(−
+=−
BET Adsorption Isotherm
P = 1 + (c-1) Pn(P - Po) nmc nmc Po
A plot of P/V(P-Po) vs P/Po should be a straight line
Gradient = [(c-1)/Vmc], Intercept = 1/Vmc solve for c = Gradient / Intercept (substitute above to find Vm)
Vm → number of N2 molecules adsorbed,
Area occupied by single N2 molecule is 16.5 Å2
Total surface area = (no. N2 molecules) x (16.5 x10-20) m2
Isotherm is only valid for P/P0 = 0.05-0.3, outside it is not linear.65
x
x
xx
xP/V(P-Po)
P/Po
66
Example BET calculation 1. Calculate monolayer volume Vm
Intercept = 1/Vmc = 2x10-4
Gradient = (c-1)/ Vmc = 0.014 Gradient = (c-1) x 1/Vmc
= (c-1) x Intercept
Gradient = (c-1) x (2x10-4) = 0.014 ∴ (2x10-4)c - (2x10-4) = 0.014
c = (0.014 + (2x10-4))/(2x10-4) → c = 71
Intercept = 1/Vmc = 2x10-4 ∴ 1/(2x10-4)c = Vm
1/((2x10-4)x 71) = Vm → Vm = 70.4 cm3
2. Convert Vm to number of N2 molecules
PV = nRT P = 1.01x105 Pa (Nm-2); V = Vm in m3 (1cm3 = 1x10-6 m3)
R = 8.314 Jmol-1K-1; T = 298 K (SATP)
1.10x105 x 70.4 x10-6 = n x 8.314 x 298 ➔ n = 0.0029 moles
Number N2 molecules = n x NA = 0.0029 x 6.022x1023 = 1.75x1021 N2 molecules
3. Multiply by area of a single N2 molecule
Area of 1 N2 molecule = 16.2 Å2 = 16.2x10-20 m2
Surface area = 1.75x1021 x 16.2x10-20 = 283 m2
Determining the heat of adsorption For a gas at pressure P in equilibrium with the condensed phase at temperature T.
Clausius-Clapeyron equation
rearranges to:
where ΔHads = isosteric heat of adsorption (measured at constant θ)
Measure several adsorption isotherms at different temperatures:
Pressure (=P2
’)
Fractional surface
coverage θ
T3
T2
T1
T3 < T2 <T1 (θ increases with decreasing T)
For particular θ read off values for P1, P2, P3
Plot ln P vs 1/T and slope = -ΔHads /R
P3 P2 P1
θ = constant
67
2
lnRTH
dTPd adsΔ
="#
$%&
'
θ
!"
#$%
&−
Δ−=))
*
+,,-
.
122
1 11lnTTR
HPP ads
θ
• All gases Physisorb on any surface when T < condensation temp.
• Reactive gas Chemisorb on reactive surface when T>condensation temp.
• Langmuir isotherm - assumes all adsorption sites are identical - only useful for describing monolayers
• BET isotherm - better model for physisorption - takes account of varying ΔHads of mono- and multilayers - widely applied to surface area analysis
Adsorption summary
Highly organized macro-mesoporous Al2O3
68
Face Centred Cubic (fcc)
Body Centred Cubic (bcc)
Crystal structures
Most common structures for transition metals
69
Miller Indices Quick way to describe surface: 1. Find the intercepts of the plane with the 3 crystal directions or axes in
terms of primitive vectors (a, b, c)
= (2, 1, 3)
2. Take reciprocals
= (1/2, 1, 1/3)
3. Multiply resulting numbers by the smallest number that yields 3 integers ⇒
(h, k, l) notation
i.e. multiply by 6 ⇒ (h, k, l) = (3, 6, 2)
b
a
c
3
21
70
Miller Indices for a simple cubic lattice
z
yx
(100) (010)(100)(100) (010)(010)
(111)(111)
(110)(110)
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(100) (110)
a
a√2
(111)
a√3/2
a√2
fcc
bcc
(100) (110) (111)
Atom packing in exposed crystal faces
72
In the bulk each atom is surrounded by 12 nearest neighbours → STABLE
Consider cleaving a metal crystal to generate a (111) plane
Atoms in the surface have lower coordination number
Bulk fcc(111)9 nearest neighbours12 nearest neighbours
Surface energy
73
fcc(100)
fcc(110)
Lower coordination number → more unstable/reactive → more bonds broken → higher surface energy
8 nearest neighbours
6 nearest neighbours
74
More open faces have higher surface energy i.e. for fcc γ111< γ100< γ110
6 nearest neighbours
9 nearest neighbours
8 nearest neighbours
75
76
Consider the surface decomposition of a molecule A , i.e.
A (g) ↔ A (ads) → Products
Let us assume that : • decomposition occurs uniformly across surface sites (not restricted to a few special sites)
• products are weakly bound to surface and, once formed, rapidly desorb
• the rate determining step (rds) is the surface decomposition step
Under these circumstances, the molecules of A on the surface are in equilibrium with those in the gas phase
➔ predict surface conc. of A from Langmuir isotherm
θ = b.P / ( 1 + b.P )
Assumption 4: ΔHads is coverage independent
Assumption 3: Only 1 adsorbate per site
Unimolecular decomposition
77
Rate of surface decomposition (∴reaction) is given by an equation:
Rate = k θ (assuming that the decomposition of Aads occurs in unimolecular elementary reaction step and that kinetics are 1st order in surface concentration of intermediate Aads)
Substituting for θ gives us eqn. for rate in terms of gas pressure above surface
Two extreme cases: • Limit 1 : b.P << 1 ; i.e. 1st order reaction (with respect to A) with an 1st order rate constant , k' = k.b
This is low pressure (weak binding) limit:
Rate = k b P / ( 1 + b P )
then ( 1 + b.P ) ~ 1 and Rate ~ k.b.P
➔ steady state θ of reactant v. small
78
• Limit 2 : b.P >> 1 ; then ( 1 + b.P ) ~ b.P and Rate ~ k
• i.e. zero order reaction (with respect to A)
This is the high pressure (strong binding) limit : steady state surface θ of reactant ~100%
Rate shows the same pressure variation as θ (not surprising since rate ∝ θ!)
Rate = k b P / ( 1 + b P )
79
Langmuir-Hinshelwood type reaction:
Assume that surface reaction between two adsorbed species is the rds. If both molecules are mobile on the surface and intermix then reaction rate given by following equation for bimolecular surface combination step:
Rate = k θΑ θΒ
Since θ = b.P / ( 1 + b.P ), when A& B are competing for same adsorption sites the relevant equations are:
A (g) ↔ A (ads) B (g) ↔ B (ads)
A (ads) + B (ads) AB (ads) AB (g)rds fast
Bimolecular reactions: 1
80
Look at several extreme limits: •Limit 1 : bA PA << 1 & bB PB << 1
In this limit θA & θB are both very low , and
Rate → k . APA . bBPB = k' . PA. PB 1st order in both reactants
•Limit 2 : bA PA << 1 << bB PB
In this limit θA → 0 , θB → 1 , and
Rate → k . bA PA / (bB PB ) = k' . PA / PB
Substituting these into the rate expression gives :
1st order in A negative 1st order in B
θ = b.P / ( 1 + b.P )
Rat
e
Pure A Pure B[A]/[B]
Competitive Adsorption
81
Eley-Rideal type reaction : Consider same chemistry
A (g) ↔ A (ads) A (ads) + B (gas) AB (ads) AB (gas)
last step is direct reax between adsorbed A* and gas-phase B.
82
A + B ➔ AB
rds fast
Rate = k θΑ [Β]
where [B] is pressure/conc in gas or liquid phase
[A ]/ [B]
Rat
e
A varied
Bimolecular reactions: 2
83
However Without extra evidence cannot conclude above reaction is Eley-Rideal mechanism… last step may be composite and consist of the following stages
B (g) ↔ B (ads) A (ads) + B (ads) AB (ads) AB (g)
with extremely small steady-state coverage of adsorbed B ➔
Test by monitoring rate • vary θΑ
• vary ratio of or over wide range
fast fast
slow
Langmuir-Hinshelwood not Eley-Rideal
B
A
pp
]B[]A[ need free sites
84
Calculated energy diagram
Langmuir-Hinshelwood: CO oxidation over Pt
Highest rate of CO2 production under slightly oxidising conditions: - a high concentration (~0.75 monolayer) of surface O - significant no. of Oa vacancies (empty sites) - CO adsorbs in vacancy with only small energy barrier
Reaction pathway
COO
CO(g)+O(a)
Example 1
CO(a)+O(a)
CO2 (g)
CO(g)+½O2(g)
85
Ru catalyst
O atoms
Eley-Rideal: CO oxidation over Ru
Highest rate of CO2 production under oxidizing conditions: - a high concentration (1 monolayer) of surface O - no surface CO detectable
Calculated energy diagram
Transition state
GAS
SURFACE
CO(g)+O(a)
Example 2
CO2 (g)
86
Oscillating reactions of carbon monoxide oxidation on platinum.
Good for oxididation
‘Inert’ towards O2
Can adsorb CO
Ultra High Vacuum Equipment
• Measurements require Ultra High Vacuum (< 10-7 torr)
- trajectories of electrons/ions used in analysis remain unperturbed
- surface kept free of contamination
Apparatus for Surface Analysis
87
Retarding Field Analyser
Electron analysers
88
Concentric hemispherical analyser (CHA)
+ ve
- ve
- ve
89
X-ray Photoelectron Spectroscopy (XPS)
Typical photon sources: Mg Kα = 1254 eV Al Kα = 1487 eV
Elemental analysis of surfaces
Binding Energy
Kinetic Energy
hνKE = hν - BE
90
KE of emitted electron varies depending on photon energy
Binding Energy eV
XPS spectrum of zirconium
91
Limiting resolution depends on X-ray line width.
- higher resolution achieved using monochromator
92
Monochromated X-rays
Can also use synchrotron source ⇒ continually variable source of monochromated X-Rays
Bremsstrahlung background
93
Auger Electron Spectroscopy (AES)
Can use X-ray or electron excitation source
Typical electron excited Auger spectrum
94
The Auger process
Initial excited state
Auger process
Final state
EAUGER
Ew
Ex
Ey
Ep
e-
Ground state
Photoelectron
95
Energy of emitted Auger electron involving levels w, x and y:
Ewxy ≈ Ew(Z) - Ex(Z) - Ey(Z)
Internal atomic rearrangement – independent of energy source used.
Assign elements in surface using tables of electron binding energy.
e.g. calculate the kinetic energy of the OKLL Auger transition: 493-512 eV (depending on oxide environment)
96
Why are low energy electron spectroscopies surface sensitive? - high KE strong scattering by neighbouring atoms - low KE excitation of
Surface sensitivity
Kinetic Energy eV
Esc
ape
Dep
th /n
m
Universal escape depth curve
97
Electrons excited by backscattering and emitted
Auger/photo electrons excited by incident beam and directly emitted
Excitation source
Auger/photoelectrons excited but not lose too much energy to be emitted
98
Can use XPS/AES to study thin films/coatings
Frank-Van der Merwe
Stranski-Krastanov
Applications
99
Simultaneous Multilayers
Volmer-Weber
100
Id/Io = exp (- d/lcosθ)
θ e-
d
Io = intensity of substrate peak for clean surface
Id = intensity of substrate peak after growth of film
l= escape depth of emitted electron θ = angle between surface normal and detector
d = film thickness
101
Angular Resolved XPS
I1/I2
Angle
I1I2
I1 I2
I1/I2
Angle
I2
I1
I2I1
I2
Fe Cr
102
Electron binding energy shifts depending on atom neighbours - increases if surrounded by electron withdrawing groups (e.g. O, F) - decreases “ “ “ “ donating groups (e.g. K, Ca, H)
Chemical environment
103
Accurate measurement of BE and ‘chemical shift’ tells us - surface functionalisation (e.g. polymer coating) - oxidation state of elements (e.g. rusting)
N 1s XP spectrum of NH3 oxidation catalyst
104
105
106
0.25
0.20
0.15
0.10
0.05
0
θ C2H
4 / M
L
0 1 2 3 4Exposure / L
Monolayer
285 284 283 282
2.81.230.160.06
Expo
sure
/ L
Binding Energy / eV
C 1s Fast XP spectra of C2H4 adsorption on Pt(111)
Multilayer
C2H4
C2H4
Pt(111)
C2H4
C2H4
Ethene adsorption
• Precursor-mediated adsorption
di σ/π-bound
• Single adsorbate
107
Ethylidyne
285 284 283 282500
400
300
200
100
Binding Energy / eV
285 284 283 282
126183
238293
353467
621
Tem
perat
ure / K
Binding Energy / eV
C2H4C 1s Fast XP spectra of C2H4 reaction on Pt(111)
Tem
pera
ture
/ K
CHx
Carbon• Surface coverage = 0.25 ML
H2
H2
Time-resolved XPS
108
• Stable ethylidyne intermediate ΔEact = 75 kJmol-1
3.5 3.7 3.9 4.1 4.3 4.5 T-1 / 10-3 K-1
ln (R
ate)
-7
-8
-9
-10
-11
-12
-13
Eact = 57 ± 3 kJmol-1
ν = 1x1010±.0.5 s-1
Temperature / K 100 200 300 400 500 650
H2 D
esor
ptio
n 3 L C2H4
1st order kinetics
0.1
0.2
0.3
0
0.4
0.5
C :
Pt ra
tio
Carbon C2H3
di-σ C2H4 Total C
C2H4
CHx Cgraphite H2 H2
C2H4,H2
Surface reaction kinetics
Permit quantitative analysis of surfaces • Sensitive to 0.1-1% monolayer (1012 - 1013 atoms/cm2) • Oxidation state information? - if oxidation state of element changes → electron binding energy changes ➔ chemical shift - complex for AES as 3 electrons involved, hard to interpret shifts
• Surface sensitivity depends on kinetic energy of emitted electron - 50-500 eV Auger electrons are highly surface sensitive
• Auger electron energy independent of energy of excitation source
XPS/AES summary
109
RAIRS
Reflection Absorption Infra Red Spectroscopy
Surface vibrational spectroscopy
110
2884
1339
1118
C-H symmetric stretch C-H deformation C-C stretch
RAIRS of ethylidyne Pt(111)
Heat
111
Selection Rules:
Angle of incidence (degrees)
Res
pons
eEp E’p
Es
E’s
Surface
IR
112
Metal Surface Selection Rule
Vibrational mode of surface molecule needs dipole moment perpendicular to the surface to be detected
113
HREELS
High Resolution Electron Energy Loss Spectroscopy
Sample Slits
Electron Gun
Monochromator
Detector
114
Surface
Incident electron beam interacts with vibrating molecule - loses/gains energy
- energy change → vibrational frequency
‘Specular’ detection: θreflection = θincidence
115
Dipole Scattering: Electric field of e- interacts with dipole ┴ to surface due to molecular vibration. Strongest in SPECULAR direction
Impact Scattering: Impact of e- with molecule excites vibration. Not dependent on orientation of dipole. Best viewed OFF SPECULAR
116
CO/Ni(111)
CO/Pt(111)
117
HREELS of C2H4
Why do we see the C=C stretch?
118
HREELS vs RAIRS Resolution > 2meV 0.1meV Pressure Range < 10-8 torr UHV to >1 bar Spectral Range > 10 meV >100 meV
• RAIRS low frequency range is limited by detector, while for HREELS it is limited by elastic peak width.
• Vibrating molecule must have a dipole perpendicular to surface
• In HREELS can observe dipoles parallel to surface using ‘impact mode’ and working off specular
Summary
119
Group frequencies
120
121
Low Energy Electron Diffraction (LEED)
Structural characterisation of surfaces
122
dsinθ = nλ
d
Incident electron beam Diffracted electrons
Surface diffraction occurs due to regularly spaced unit cells
123
dsinθ = nλ
and
sin θ = opposite/hypotenuse = y/L
∴y/L = nλ/d or separation of spots ∝ 1/d (atom/molecule spacing)
θ
LL
y
Sample
124
Possible structures of p(2x2) oxygen unit cell on Cu(100)
LEED pattern for O2/Cu(100)
a
b
+ 0.25 ML Oadsorbed
Clean Cu(001)
125
126
Temperature-programmed desorption
127
Pt(111)
128
Quadrupole Mass Spectrometer
H2
Temperature / K 100 200 300 400 500 650
H2 D
esor
ptio
n 3 L C2H4
Stepwise decomposition
C2H3
CH3
CH2
For a simple 1st order desorption the activation energy (Eact) for desorption
can be calculated from the desorption peak temperature using the
Redhead equation:
β = heating rate (typically 1-1000 K sec-1)
Tp = peak temperature
ν = constant (frequency factor) = 1015 sec-1
Eact = RTp [ln(νTp/β) - 3.46] Redhead equation
129
CO/Pt(112)- stepped surface - get info about preferential adsorption site
Weakly-bound
Strongly-bound
130
TiO2
Can also follow surface reactions
Temperature / K
O
Acetophenone decomposition on TiO2(001)
131
Scanning tunnelling microscopy (STM)
Gerd Binnig & Heinrich Rohrer (Nobel Prize, Physics 1986)
132
TipSample
Electronic Wavefunction
Fermi Level
Vacuum Level
φS φT
At a certain applied potential electrons tunnel to or from tip.
Tunnelling current sensitive to tip sample separation
Maintain a constant tunnelling current while scanning tip across surface.
Motion of tip towards or away from surface ⇒ topography of surface.
133
134
Xenon on nickel (110) CO on platinum (111)
Fe on Cu(111)
Adsorbate-induced reconstruction
N2/Cr(110)
135
Chemical Contrast
Pt/Rh(100)
136
Physical interaction with the surface and tip
Vertical displacement is registered by deflection of laser
Compile a topographic image of surface
Can record images of - insulating samples
- solid liquid interface
- biological samples
Atomic force microscopy (ATM)
137
138
AFM image of contact lens recorded in saline solution
Hydrophobic lens Lens with hydrophilic coating
139
140
• Supported metal particle can expose different crystal faces.
• In addition there are steps & defects within each particle. - these are low coordination sites - region of high potential energy ➔ facilitate bond dissociation
Structure sensitivity
141
Structure Sensitivity occurs when reaction requires specific active sites: (any mix of step, terrace, kink atoms)
Density of steps and dominant crystal face reflects the metal particle size
∴changing particle size modifies rate
Stepped surfaces Stepped + kinked surface
(100)
square
(111)
hex
142
100xNN(%)Dispersion
T
S=
Consider total fraction of available surface sites:
Spherical particles
if Ns = total no. of surface atoms NT = total atoms in particle
For small particles (< 20Å) Dispersion → 1
∴if Activity ∝ SA, then ⇑ particle size will ⇓ rate (per mass of catalyst)
provided exposed surface atom arrangement unchanged
143
Structure sensitive test:
Consider CO + 3H2 → CH4 + H2O
Compare specific TON (per surface site)
Ni (100)
9% Ni/Al2O3
5% Ni/Al2O3
If reaction requires specific (4-coord) active site expect
• constant ΔEact observed
• higher rate over surfaces with most (100) sites larger particles
144
Structure sensitive vs insensitive reaction:
Cyclohexane hydrogenolysis • High step/kink densities → high rates • Reaction requires defect sites
contrast with (de)hydrogenation which proceeds over diverse surface arrangements
Reaction kinetics tell us about the active site
-H2
-CHx
• The importance of surface processes
• How surface structure affects the reactivity of materials
• Adsorption processes, physisorption and chemisorption: - activated vs non-activated adsorption
• Adsorption isotherms: Langmuir and BET models
• Kinetics of surface reactions: - Eley Rideal and Langmuir Hinshelwood mechanisms
• Surface analytical techniques: specifically XPS/AES
145
Summary
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