Dr Qiao Chen. Interface Chemistry: Surface and Heterogeneous Catalysis

Transcription

1 Interface Chemistry: Surface and Heterogeneous Catalysis Dr Qiao Chen Chichester III, Office 3R506 NanoLab 2R114, 2R108 Phone:

2 Outline 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

3 Challenges in surface science Q: what do we want to know about a surface? 1. What s on the surface? Surface composition, elements, functional groups 2. What s the surface structure? Periodicity and orientation of the adsorbate. 3. What s the surface energy? Bonding, interaction, charging, work function and electronic states. 4. What s the physical and chemical properties? Catalyst, electrodes, corrosion resist, bio-compatible, non-sticking, low friction,..the limit is your imagination. Can we control the surface physical and chemical properties of materials? 2.5 dimension material science.

4 1. Challenges in surface science difficulty Q: How do we know about a surface? 1. Surface is normally very reactive, need high vacuum to protect. P<10-9 mbar, UHV----- Ultra High Vacuum (expensive) 2. Surface concentration is low 1 monolayer of CO in c(2x2) structure on Cu (100) surface = 1 molecule per 13Å 2 =7.7x10 15 molecule per cm 2 =1.3x10-8 mol per cm 2 With sample 1mm 2 area contains 1.3x10-10 mol. 3. Short lived transition state. Broaden spectrum, poor resolution Lifetime <10-14 sec, need fast spectroscopy 4. Too small to be seen. A flat-lying benzene molecule occupies 16Å 2 =0.16nm 2 =1.6x10-15 mm 2

5 1. Challenges in surface scienc The positive side Q: How do we know about a surface? a. Surface is normally very reactive, need high vacuum to protect. Hugely advanced in technology towards nanotechnology and space technology Diamond Leiber s research group, Harvard b. Surface concentration is low Ultra sensitive spectroscopy c. Short lived transition state. Broaden spectrum, poor resolution Ultra fast spectroscopy, laser instrument, terahertz electronics d. Too small to be seen. Ultra high resolution, Scanning probe microscopy Silicon nanowire device. Science 302, 1377 (2003).

6 Single crystal surfaces vs amorphous Simple structure High purity Well defined Measurable crystal dimension Clear bonding geometry A cubic crystal

7 Fundamental aspects of surface science Surface Miller index and surface symmetry z Miller index of a surface is the reciprocals of the fractional intercepts Fractional intercepts: h*, k*, l* h=1/h* k=1/k* l=1/l* So, no or fractions, but all integers z x a A cubic crystal y (110) a (100) (111) y x

8 Surface density and surface energy z (110) a y (100) (111) x a 2 a a Density=1/(a 2 2) Surface density: (111) > (100) > (110) Surface energy: (111) < (100) < (110) Density=1/(a 2 ) a 2 Density=1/(a 2 3/2) Higher density=stronger inter-atomic interaction Higher surface energy=higher reactivity

9 Adsorption at the G/S interface Terminologies: Non-adsorbed gas adsorptive Adsorbed gas adsorbate Underlying substrate absorbant Adsorption occurs at the surface only Absorption penetration into the bulk Sorption generic term used when adsorption and absorption processes cannot be distinguished experimentally

10 Physical vs Chemical Adsorption Two types of adsorption: Physisorption weak, non-specific Van-der-Waals (dipole-dipole) interactions Chemisorption strong, specific formation of chemical bonds (usually covalent) Traditionally distinguished by differences in adsorption enthalpies H ads i.e H ads > -25 kj/mol = physisorption Fe N 2-10 kj/mol H ads < -40 kj/mol = chemisorption Fe-N -150 kj/mol However, physisorption is always exothermic H ads <0, Why? Chemisorption generally exothermic too?

11 Physisorption vs Chemisorption Single atom C(R) P(R) P(R) = Van der Waals attraction 1/R 6 + Born repulsion exp(-ar) C(R) = M-X potential Heat of physisorption Hp ~ kt << Hc Heat of chemisorption Activation energy for chemisorption Ea is where P(R) = C(R) Chemisorbed bond length Rc (1-2Å) < Rp (5-10Å) physisorbed By Dongbo Li, et al, Nature Materials 7, (2008)

12 Heterogeneous Catalysis Diatomic Activated Chemisorption E a R c H c H dis H p Physisorption provides low energy pathway to the dissociation of adsorbate gas and reaction takes place on 2D surface (reduced degrees of freedom) Non-Activated Chemisorption R p

13 Comments on the PE curves P(R) = Van der Waals attraction 1/R 6 + Born repulsion exp(- R) C(R) = M-X potential but with dissociation of X 2 at large R Heat of physisorption H p ~ kt << H c Heat of chemisorption Activation energy for chemisorption E a is where P(R) = C(R) Chemisorbed bond length R c (1-2Å) < R p (5-10Å) physisorbed Importantly activation energy E a << H dis for X+X formation Physisorption lowers E a for X+X formation to ~ kt hence catalysis e.g 2HI H 2 + I 2 E a = 184 kj/mol no catalyst E a = 59 kj/mol on Pt D HI = 294 kj/mol

14 Adsorption coordinates Atop site Bridge site

15 Potential energy surfaces of adsorption Adsorption of H 2 on W(100) Non-dissociative adsorption dissociative adsorption By Prof. Steve Holloway, Liverpool University

16 Summary So far we have learned: 1) Challenge in Surface Science 2) Miller index of single crystal surfaces 3) Physisorption vs Chemisorption 4) Potential energy surface of reaction on surface

17 Thermodynamics of Adsorption Adsorptive gas has 3D degrees of freedom z Adsorbate 2D restricted motion or 1D pinned vibrations Therefore physisorption always results in a decrease in entropy S ads < 0 From G ads = H ads T S ads y x x y For spontaneous adsorption G ads < 0 So H ads < 0 and physisorption is enthalpically driven What will a decrease in temperature do at equilibrium?

18 Example of Chemisorption CO feedback bonding Electron configuration Fe: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 6 Fe: Frontier orbitals: 3d, 4p C: 1s 2 2s 2 2p 2 O: 1s 2 2s 2 2p 4 Chemisorption through charge transfer, electron sharing and bonding. MO of CO molecule

19 Clausius-Clapeyron Equation Decreasing the temperature increases amount of gas adsorbed Since for A(g) + M(s) AM(s) K ads = [AM]/[A][M] = 1/p A when [AM]=[M] H ads And G ads = - RTlnK ads = H ads T S ads so lnk = - + RT S ads R Therefore can measure H ads directly by isosteric (constant coverage) calorimetry. Measure partial gas or vapour pressure p A required to maintain constant surface coverage (volume) V at different temperatures T lnp Then A H = - ads or lnp A = H ads T RT 2 V (1/T) V R

20 Determination of Enthalpies Deriving a linear Claussius Clapeyron relation by recognising 1 (1/T) lnp - = then A H = ads (1/T) T 2 T T R T V So a plot of lnp A vs 1/T is a straight line with gradient H ads R Example:A volume of N 2 is absorbed on charcoal at 4.8 atm and 190 K. A pressure of 32 atm is required to maintain the same volume when the temperature is raised to 250 K T (K) /T ln p H ads = = -1.5 R H ads = -1.5 x = -12 kj/mol

21 Experimental Methods Surface pre-treatment - to remove physisorbed gases and condensates to < 1% monolayer ~ molecules/cm 2 Evacuation (outgassing) at 10-4 Torr (0.08 atm) High temperature Chemical desorption e.g C on Pt reacted with O 2 CO desorbs Cleavage in high vacuum e.g mica, silicon Gravimetric high adsorption volumes Weigh substrate before and after adsorption using microbalance e.g QCM Then m c f change in quartz oscillating frequency c = 17.7 ng Hz -1 cm -2 for a 5 MHz quartz crystal ~ m ng 10-9 g

22 Further Methods Volumetric measurement slow, accurate Evacuate A, B and close tap P 1 B ~ 10-4 Fill A with gas of volume V A at pressure P 1 Open tap, measure P 2 and calculate total volume from P 1 V A =P 2 (V A +V B ) Repeat with known weight of solid in B Then drop in P 2 corresponds to V ads from P 1 V A =P 2 (V A +V B +V ads ) A + P 2 Solid Chromatographic retention fast, not accurate Detector response Column filled with solid Flow carrier gas He + Absorbate A He + N 2 Solid Time A is depleted by adsorption until surface is saturated

23 Catalytic Series Catalytic activity depends on strength ( H ads ) of chemisorption Weak high mobility but low coverage Low activity Strong high coverage and high mobility High activity V. strong adsorbate immobilised Low activity Hence the volcano curve for a gas on different surfaces e.g hydrogenation of ethene on transition metal surfaces 4d 3d Adsorbate strengths O 2 > C 2 H 2 > C 2 H 4 > CO > H 2 > CO 2 > N 2

24 Properties of catalysts Catalysts Function Examples Metals Hydrogenation Dehydrogenation Ni, Fe, Pt, Pd, Ag, W Semiconductor Oxides Oxidation NiO, ZnO, Bi 3 O 2, MoO 3, V 2 O 5, TiO 2 Insulating Oxides Dehydration Al 2 O 3, MgO, SiO 2 Acids Polymerisation Isomerisation Cracking Alkylation H3PO4, SiO3/Al2O3, Zeolites

25 Adsorption isotherms Surface coverage of adsorbate is a function of adsorptive gas or vapour pressure = adsorption isotherm (constant temp) 5 types classified by Brunauer in 3 categories V ads =Adsorbed gas volume NH 3 + charcoal N 2 + silica Monolayer I III Multilayer II Br 2 + silica Benzene H 2 O + + Fe 2 O 3 charcoal V IV Condensation in pores/capillaries

26 Interpretation of isotherms Pre 1916 adsorbed layers described qualititatively as either: Condensed liquid film (2D liquid) Compressed gaseous layer (2D gas) Langmuir (1916) recognised the rapid decrease in attractive force as 1/R 6 with increasing R: Our present conception of the structure of atoms and molecules makes it impossible for us to conceive of any appreciable force which one atom or molecule can exert directly on others at distances greater than two or three Angstrom units I Langmuir, Trans. Farad. Soc. 1921, 17, 4 hence Molecules striking a surface already covered also condense but usually evaporate much more rapidly than from the first layer. Hence except when the vapour is nearly saturated, the amount of material adsorbed on a plane surface rarely exceeds that contained in a layer one atom (or molecule) deep I Langmuir, Phys. Rev. 1915, 6, 79

27 Summary 1.Bonding in Chemisorption 2.Therodynamics of adsorption 3.Clausius Clapeyron equation measuring enthalpy 4. Methods to measure surface coverage and surface area. 5. Adsorption strength vs catalytice activity. 6. Adsorption isotherms

28 Langmuir isotherm Langmuir postulate of monolayer correct for chemisorption and physisorption at high T and low P (multilayers are reduced) Langmuir model assumes: 1) Only one adsorbed molecular layer 2) All adsorption sites are equivalent (lattice model) 3) H ads is independent of surface coverage Monolayer adsorption V m =20 V=10 =0.5 V ads = KpV m 1 + K p p p 1 = + V ads V m KV m Define coverage =V ads /V m p 1 Kp 1 Kp p K V ads p V ads 1 V m 1 { KV m p NH3 on Charcoal p

29 Langmuir derivation For the dynamic equilibrium k a A(g) + M(s) AM(surf) V ads = volume of gas adsorbed k a = rate of adsorption exp(-e a /kt) V m = volume of monolayer k d = rate of desorption exp(-e d /kt) k d V ads V m Then rate of change of surface coverage = due to adsorption/desorption d = k a pn(1- ) dt d = k d N dt Adsorption desorption where N=total number of adsorption sites N(1- ) = number of vacant sites p = partial pressure of adsorbate A Adsorption probability proportional to: pressure p, and number of vacancy N(1- ) Desorption probability proportional to: number of adsorbates N Each proportional constant k a or k d is determined by temperature T and the activation energy k=exp(-e a /kt)

30 Langmuir derivation At equilibrium the rate of change is 0 so Define the equilibrium constant K K=k a /k d Rearranging gives Kp(1 ) Rate of adsorption=rate of desorption k a pn(1- )= k d N V Kp V Kp ads Kp (1 Kp) Hence 1 1. Langmuir isotherm is independent to total adsorption sites, or the size of sample 2. Coverage is not linear proportional to p. 3. Temperature dependent behaviour is determined by K through difference between adsorption energy barrier and desorption energy barrier. 4. K has the unit of atm At low pressure, Kp<<1, is linearly proportional to the p, at high pressure, Kp>>1, approach to1: called as saturated coverage (Monolayer) m

31 Adsorption with dissociation Langmuir isotherm strictly valid for adsorption without dissociation (physisorption) But chemisorption is generally associated with dissociation of adsorbate Rate of change of coverage due to adsorption now depends on probability of each atom finding a vacant site = no. of vacancies for each atom = N(1- ) d Probabilities are independent so for diatomic = k a pn 2 (1- ) 2 dt Rate of change of coverage due to desorption depends on frequency of atomic encounters to form diatomic = surface coverage of each atom = N d Again independent probabilities so for diatomic = -k d N 2 2 dt K = 10 atm Equating adsorption and desorption rates leads to (k a p/k d ) 1/2 - (k a p/k d ) 1/2 = (1+(Kp) 1/2 )=(Kp) 1/2 and finally = (Kp)1/2 1+(Kp) 1/2 Surface coverage now depends more weakly on pressure (p 1/2 ) p atm K = 1 atm -1 K = 0.1 atm -1

32 BET model of adsorption Langmuir fails to account for multilayers hence Brunauer, Emmett and Teller BET model (1938) Considered how monolayer acts as substrate for further adsorption Assumes adsorption enthalpy of multi-layers independent of # of layers } H L adsorption enthalpy for subsequent layers H 1 adsorption enthalpy for monolayer

33 BET Without derivation Balancing evaporation and condensation rates between layers gives V ads V m = cp 0 p (p 0 -p)[p 0 +p(c-1)] where p0 = vapour pressure of bulk liquid of adsorbate Vm = monolayer capacity (as in Langmuir) c = exp[( H L H 1 )/RT] (equivalent to K in Langmuir)

34 Properties of BET curves V ads V m = cp 0 p (p 0 -p)[p 0 +p(c-1)] and the linearized equation Rearranging gives p V ads (p 0 -p) [p 0 +p(c-1)] cp 0 V m = = 1 + (c-1) p cv m cv m p 0 p V ads (p 0 -p) When c > 1 ( H L > H 1 ) BET describes type II isotherm Then at low p: (p 0 p) ~ p 0 BET reduces to Langmuir When c <= 1 ( H L <= H 1 ) BET describes type III

35 Monolayer capacity in BET BET with c > 1 most general isotherm but how do we get V m without adsorption curve reaching the asymptotic limit of the Langmuir isotherm? From the linearized equation Plotting ~ linear 0.05 <= p/p 0 <= 0.3 with Gradient = and Intercept = So V m = p p vs V ads (p 0 -p) p 0 (c-1) V m c 1 V m c 1 (gradient + intercept) Intercept often small << gradient and so leads to only 5-10% error in calculated monolayer capacity V m if omitted

36 BET at low pressure P<<p 0 V ads V m = cp 0 p (p 0 -p)[p 0 +p(c-1)] cp0 p p ( p cp) 0 0 cp ( p cp) 0 Langmuir Isotherm Kp 1 Kp When K=c/p 0

37 Determining BET monolayer volume How to measure? Usually use N 77 K (b.p) to determine surface capacities since cheap, inert and produces well defined knee (saturation) on many surfaces 1. Isolate valve, pump down B without solid, measure P 1. P 1 B ~ Open valve, measure P 2 This is to measure the dead volume in B, V B A + P 2 P 1 V A =P 2 (V A +V B ) With solid: 3. Isolate valve, pump down B, setup P 1. Solid 4. Open valve, measure P 2. P 1 V A =P 2 (V A +V B )+P 2 V ads P 2 (V A +V B )=P 2 (V A +V B )+P 2 V ads

38 Determining BET monolayer volume How to measure? P 2 (V A +V B )=P 2 (V A +V B )+P 2 V ads Using volumetric analysis determine V ads = (P 2 /P 2-1)x(V A +V B ) vs P 2 where P 2 = P 1 V A /(V A +V B ) measured with and without the solid surface. get isotherm from V ads vs p 2 /p 0 and V m from p 2 /V ads (p 0 -p 2 ) vs p 2 /p 0 p V ads (p 0 -p) = 1 + (c-1) p cv m cv m p 0 get mass adsorbed M m = x V m where is density of liquid N 2 From mass how might we determine BET surface area?

39 BET surface area Determine number of molecules adsorbed = N Av M m /M N2 from ratio of monolayer mass to molecular mass of absorbate From real gas Van der Waals constant b (N 2 =3.9x10-5 m 3 /mol) can calculate area per molecule ~ (b/n Av ) 2/3 = 16.2x10-20 m 2 /N 2 So for a non-porous (no absorption), planar solid substrate BET surface area = number of molecules adsorbed x area per molecule expressed as area per unit mass (m 2 /g) of solid substrate For a particulate substrate (powder) area related to grain size Area per unit mass = 3/ R where = mass/4 R 3 /3 is the solid density and assumes monodisperse spherical particles

40 Low pressure isotherms But what about dependence of V ads on H ads? Langmuir assumes H ads independent V ads H ads increases (less ve) with increasing V ads Enthalpy modified by increasing adsorbateadsorbate interactions as coverage increases most energetically favourable sites occupied first Temkin Isotherm - assumes H ads changes linearly with p V ads = k ln (n p) or V ads = k ln n + k ln p Freundlich Isotherm - assumes H ads logarithmic change with p V ads = k p 1/n or log V ads = log k + n log p

41 Anomalous isotherms Stepwise isotherms associated with the formation of complete monolayers before each subsequent layer commences 90 K on C 3000 K Isotherm shows two distinct steps corresponding to the completion of the first and second monolayers Favoured if H ads remains more exothermic near completion than H L for commencing the next layer

42 Desorption d dt = k d N N: Total number of adsorption sites : Coverage Desorption rate constant Arrhenius relationship First order desorption k Aexp( E / RT) Integration ln( 0) ln( ) d knt d d Half-life t1 ln 2 / kd 2

43 What is a catalyst?? Alters the rate of reaction Highly selective Does it participate in the reaction?? How does it change the rate?? Offers an alternate path with low E. Does it affect H R, G R, and Eq. constant?? Does it affect yield & selectivity?? Does it initiate a reaction??

44 Catalytic process Every catalytic reaction is a sequence of elementary steps, in which reactant molecules bind to the catalyst, where they react, after which the product detaches from the catalyst, liberating the latter for the next cycle

45 Heterogeneous catalysis Potential energy diagram of a heterogeneous catalytic reaction, with gaseous reactants and products and a solid catalyst. Note that the uncatalyzed reaction has to overcome a substantial energy barrier, whereas the barriers in the catalytic route are much lower.

46 Characteristic of a catalyst? Accelerate the reaction Small amounts of catalyst Not consumed in the reaction Not chemically degraded Change the operating temperature level Influence the product distribution Efficiency depends on activity, properties & life of the catalyst Examples: Ammonia synthesis Promoted iron SO 2 oxidation Venadium Pentaoxide Cracking Sylica, alumina Dehydrogenation Platinum, Molybdenum

47 Classification and key components of catalysts Classification: Homogenous catalysis Heterogeneous catalysis key components: Promoter, carrier, accelerator, inhibitor

48 Promoter: is an additive which has no catalytic properties of its own but enhances the activity of a catalyst Promoter results in: Increase of available surface area Stabilization against crystal growth and sintering Improvement of mechanical strength Examples: Alumina, SiO 2 Carrier: principally serve as a framework on which catalyst is deposited - no catalytic properties of its own Carrier results in: Highly porous nature - increase of available surface area Improve stability Improves the heat transfer characteristics Examples: Alumina, Asbestos, Carborundum, Iron oxide, Manganese, Activated carbon, Zinc oxide

49 Accelerator: are substances which can be added to a reacting system to maintain the activity of a catalyst by nullifying the effects of poisons Poisons: substances which reduce the activity of a catalyst. They are not deliberately added but are unavoidably deposited during the reaction. Examples: Sulfur, Lead, Metal ions such as Hg, Pd, Bi, Sn, Cu, Fe etc. Inhibitor: substances added to the catalyst during its manufacture to reduce its activity. Coking/Fouling: deposition of carbonaceous material on the surface of the catalyst - Common to reactions involving hydrocarbons

50 Catalytic rates on surfaces Eley-Rideal mechanism Heterogeneous catalysis depends on at least one reactant being adsorbed (usually chemisorbed) onto a surface Eley-Rideal mechanism For reaction between an adsorptive gas phase molecule B and adsorbate A the rate of formation of the product P depends on p B = partial pressure of adsorptive gas 1 st order in p B V A /V m = = surface coverage of adsorbate 1 st order in d[p] dt = kp B

51 Catalytic rates on surfaces Eley-Rideal mechanism d[p] dt = kp B from the Langmuir isotherm KpA A 1 Kp A Hence d[p] dt = k AB p B Kp A (1+Kp A ) What are k AB and K? k AB is the rate constant for., K is the adsorption equilibrium constant for A, k a /k d What is the order of reaction at high and low pressures? High p Kp A /(1+Kp A ) ~ 1 = 1 st order in p B Low p (1+Kp A ) ~ 1 = 2 nd order

52 Catalytic rates on surfaces Eley-Rideal mechanism Temperature dependent production rate: dp [ ] dt d[p] dt = k AB p B Kp A (1+Kp A ) At low T, increase T will increase k AB T At High T, increase T will decrease the surface coverage of A.

53 Catalytic rates on surfaces Langmuir-Hinshelwood Most reactions require both molecules A and B to be adsorbed to the surface before encounter lasts sufficiently long for formation of product P So reaction rate expected to be 2 nd order in surface coverage d[p] In this case = k AB A B = dt For A and B following Langmuir isotherms k AB V A V B V m 2 V A V m K A p A (1+K A p A +K B p B ) V B V m = = K B p B (1+K A p A +K B p B ) d[p] dt So = k AB K A K B p A p B (1+K A p A +K B p B ) 2

54 Catalytic rates on surfaces Langmuir-Hinshelwood derivation A A For A: k p (1 ) k a A A B d A For B: k p B (1 ) k B a B A B d B Use equilibrium constant K=k a /k d, instead of rate constant K p (1 ) A A A B A Target: separate A and B K p (1 ) A A A B A K p (1 ) (1) (2) B B A B B Divide Langmuir isotherm A against B A KApA KApA A B B KBpB K p K p K p K p B B A A B B B B B VB KBpB B V 1 K p K p m A A B B B B K p Into equation (2) (1 KApA ) K p B B B B B B B

55 Deriving other catalytic rates Deuteration of NH 3 on Pt surface with dissociated D 2 at low coverage So the rate is 1 st order in NH 3 and D coverage NH3 D But in this case the Langmuir isotherm with dissociation applies Low surface coverage of D 2 occurs at low gas pressure Therefore 1+(Kp D2 ) 1/2 ~ 1 so Langmuir reduces to V/V m = D =(Kp D2 ) 1/2 Reaction rate depends additionally on undissociated NH 3 coverage d[nh So 2 D] = dt k NH2D (K D2 p D2 ) 1/2 K NH3 p NH3 1+K NH3 p NH3

56 Hydrogenation of Alkenes Raney Ni by Murray Raney in 1926 H H + Chemisorption + H 2 dissociation Dynamic equilibrium of bond breaking and reformation with surface atoms + H H Finely divided Ni H H For the preservation of edible fats of structure CH2(OOCR)CH(OOCR )CH2(OOCR ) where R, R, R are hydrocarbons with many double bonds H or H 1 st hydrogenation step 2 nd hydrogen encounter leads to alkane formation and desorption Reformation of double bond leads to isomers

57 Summary So far we have learned: 1) Challenge in Surface Science 2) Miller index of single crystal surfaces 3) Physisorption vs Chemisorption 4) Activated chemisorption vs non-activated chemisorption 5) Potential energy surface of reaction on surface 6) Properties of catalysts 7) Langmuir isotherm 8) Langmuir isotherms for dissociation adsorption 9) BET isotherm 10) Temkin isotherm 11) Freundlich Isotherm 12) Desorption kinetics 13) Eley-Rideal mechanism 14) Langmuir-Hinshelwood mechanism 15) Langmuir-Hinshelwood mechanism for dissociation adsorption

58 Structure of Solid Surfaces Surface analysis techniques categorised into spectroscopy analyses surface composition microscopy interrogates surface structure Essential starting point of surface analysis is a clean surface Reduce contaminating gases by high vacuum 10-6 Torr ~ 10-8 atm Still many collisions with surface since for gas of mass m at press p and temp T Collision rate Z = p = 3.5x10 22 p (Torr) / (Tm) 1/2 cm -2 s -1 1 atm = 760 Torr (2 mkt) 1/2 ~ collisions cm -2 s -1 for air (RMM m = 29 gmol K A typical metal surface contains ~ atoms cm -2 So 10% of surface atoms are struck by a gas molecule every second Or every 10 seconds every surface atom will have undergone a collision

59 Ultra-high vacuum techniques With Ultra-high vacuum (UHV) techniques at Torr collision rate is reduced to collisions s -1 Or each surface atom undergoes collision with gas every 10 5 s or once a day! Good for experiment UHV achieved in differential pumping in stages Rotary vane pump removes gas by positive displacement from 760 Torr to 25 Torr Sorption pump removes gases by absorption in porous materials or molecular sieves 25 to 10-4 Torr Cryopump promotes adsorption at pump surface rather than surface under study 10-4 to 10-8 Torr Ion pumps gas ionised by e - bombardment and removed by trapping ions on a cathode Torr UHV chamber - stainless steel frame + alumino-silicate glass ports (low leeching properties) + malleable metals valves such as gold to vacuum grease contamination Ion bombardment (Ar + ) or pulsed lasers ablate remaining gas but can induce surface reconstruction New surface generated by annealing (high temp, slow cooling cylces), in situ vapour deposition, the cleaving of a fresh plane (single crystal) or mechanical crushing (metals) Problem the pressure gap upto atm difference between experiment and industrial catalysis

60 Photoelectron spectroscopy Essentially 3 types of photoelectron spectroscopy (PS) UV (UPS) Uses discharge to induced He 1s2p to 1s nm UV used to ionize electrons from valence molecular orbitals Kinetic energy of e - = photon energy valence binding energy X-ray (XPS) Uses e- bombardment to induce X-ray transitions in Mg ev or Al ev by L to K shell e - relaxation X-rays used to ionize core electrons from surface atoms. Binding energy of core electron characteristic of element. Augar electron spectroscopy (AES) Analyses energy of a secondary electron produced by L to K relaxation in XPS or following electron impact ionization

61 Excitation process Incident X-ray h or electron beam Primary electron Secondary Auger electron with KE = K 1 -L 1 -L 23 Vacuum BE of secondary e- hv 2p = L 23 2s = L 1 Transition energy of relaxing e- 1s = K 1 Occurs when relaxation energy K 1 -L 1 > L 23 binding energy of valence electrons

62 A basic experimental setup h = ½mv 2 + BE + - V Electrostatic analyser X-ray Source Or Electron beam 5 nm Photoelectrons + Detector UHV Torr Sample To vacuum pump Extent of deflection by electric field E = V/x across analyser dependent on ½mv 2 of ionised core electron Therefore measure electron flux (intensity) as a function of applied voltage since K.E V

63 The XPS process Ejected photoelectron with KE = h BE - Incident X- ray h Vacuum continuum 2p = L 23 2s = L 1 Binding energy BE of core s electron 1s = K 1

64 Surface sensitivity Most spectroscopic techniques cannot differentiate between the surface and bulk of a substrate due to weak signal to noise ratio (SNR) A 10 nm layer of adsorbate on a 1 mm substrate = 10-8 /10-3 = 10 ppm Too weak for NMR but Surface sensitive techniques interrogate only a few microns of substrate so I Ads /I Bulk > ppm In XPS the ionization process is not surface sensitive at the atomic scale since X-rays can penetrate > 1 m into sample But detection of photoelectrons is surface sensitive since only ionized electrons that are produced ~ 1 nm from the surface can escape substrate Probability of a photoelectron leaving the surface decreases with increasing depth due to inelastic scattering in the bulk that stops electrons from the bulk escaping

65 Inelastic mean free path photoelectron travels distance through a solid without being scattered is called inelastic mean free path (IMFP) Low energy electrons: lattice interaction. High energy electron: Less portion energy loss in the scattering process.

66 IMFP vs Photoelectron energy Variation of IMFP vs photoelectron energy described by universal curve for most metals, inorganic solids and polymers though absolute IMFPs are different IMFP vs KE on a log-log scale shows < 10 Å for 15 < KE < 350 ev < 20 Å for 10 < KE < 1400 ev Using X-rays from Mg ev or Al ev leads to electron emission with KEs in the range ev depending on the binding energy So XPS is inherently surface specific to < 5 nm Energy (ev) Some materials require tunable synchrotron X-rays to match the photoelectron KE to the minimum in the IMFP curve IMFP (nm)

67 The survey scan Each element at the surface of a substrate gives rise to peaks in the photoelectron spectrum at kinetic energies determined by h = ½mv 2 + BE + h = incident X-ray energy (~ eV) BE = the core electron binding energy BE? = spectrometer work function ~ 5 ev So rearranging gives the binding energy BE = h -½mv 2 - Example Palladium analysed with Mg ev 3x10 5 Signal Kinetic Energy (ev) x Auger 3d p 3s 4s 4p Signal

68 Chemical shifts Exact binding energy of electron depends additionally on: 1. the formal oxidation state of the atom 2. the local chemical and physical environment Gives rise to chemical shifts in peak positions that are interpretable because: 1. XPS probes core levels that have discrete energies 2. XPS is a one electron process Atoms of higher +ve oxidation states exhibit higher BE due to extra coulombic interaction between photoelectron + ion core.

69 Chemical shifts in Polyacrylic acid Survey scan ev of thin film poly (acrylic-acid) shows O 1s and C 1s photoelectrons Core-line scans of C 1s and O 1s show chemical shifts of due to the 3 different carbon and 2 different oxygen environments

70 XPS of a nanocomposite O 1s Si 2sSi 2p Silica Sol SiO 2 N 1s C 1s Polypyrrole Bulk Powder H N + n O 1s Silica rich surface 100nm N 1s C 1s Si 2s Si 2p Nanocomposite

71 Depth profiling with XPS Chemical composition can be probed at different depths by either: 1. Tuning X-ray energy (synchrotron) and hence e - KE across universal curve 2. Changing path length of electron through solid to detector Path length altered by changing angle of detection w.r.t surface normal Probability photoelectron is detected depends on distance travelled by the electron through solid and not the depth of the atom from which it originates Then depth probed given by d = x cos d x Analyser For a detection angle = 75 o, d=x/4

72 Profiling semiconductors oxide elemental 10º corresponds to electrons departing with trajectories at a grazing angle to the surface 90º corresponds to emission perpendicular to the surface Core line Si 2p spectra 10 o -90º to the surface plane = (90- ) Si 2p peak of the oxide (BE ~ 103 ev) increases at grazing incidence elemental Si of substrate (BE ~ 99 ev) dominates at normal incidence

73 Pros and Cons of XPS Advantages 1. Quantitative elemental analysis ( area under peak = [A] ) 2. Information of oxidation state and molecular environment 3. Highly surface specific 2-5 nm and depth profiling ability 4. Non-destructive Disadvantages 1. UHV conditions (10-10 Torr) can modify sample (volatiles) 2. Sample charging (insulators) can introduce errors in BE 3. Cannot detect H atoms (unlike NMR) since no core electrons 4. Often excess of C, O and Si from pump oils and oxidised surfaces 5. Peak fitting not quantitative for unresolved peaks

74 Low energy electron diffraction LEED favoured for studying 2D surface structure or the arrangement of atoms on a surface as per X-ray diffraction for determining 3D crystal structure Electron energies in the range KE = ev (compare to X-rays > 1200 ev): Electron energies must be high enough for De Broglie wavelength = h/(2me) 1/2 ~ ev to be less than interatomic spacing of surface atoms Low enough to ensure electrons are scattered from surface atoms only Phosphor screen Sample 0 V Electron gun KE = e - x Voltage -ve charged grids filter inelastic secondary e -

75 Bragg diffraction Low energy electron waves are elastically scattered from the high core electron density surrounding surface atom centres For constructive interference between reflected waves the path difference d must be a whole number of wavelengths d = a sin = n a d sin 1/V 1/2 since = h/(2me) 1/2 sin 1/a also so Diffraction pattern contracts with increasing electron gun voltage (E=eV) and lattice spacing a First order angular separation of diffraction peaks sin = /a so LEED pattern is a display of the real space 2D arrangement of atoms in reciprocal space (1/a)

76 Optical vs Electron Microscopy Condenser lens Objective lens Ocular lens Light source = nm K.E = 100 kev = 4 pm Object plane Intermediate image plane Electron gun Condenser coil Objective coil Projector coil Retina or Camera Image plane Phosphor screen

77 Electron microscope components Invented by Ernst Ruska 1933 commercialised by Siemens 1939 Electron gun - thermionic emission by heating 2400K e- acceleration by ~100 kv = h/(2mev) 1/2 ~ 4 pm Housing - HV 10-6 Torr to maximize mean free path of electron Lenses - Magnetic field lens made of two electromagnetic coils separated by small gap to concentrate lines of force Condenser - condenses divergent electron beam from gun to sample dimensions (grid support a few mm) SEM objective - focuses incident e - to point source on specimen TEM objective - focuses transmitted e - through points in sample back to a points on the image plane Projector lens - provides magnification 800,000 x 0.4 nm = 320 m image = limit of eye Electron trajectory Wehnelt cap -ve Anode +ve Focusing force Same sense 3 Tesla

78 Sample preparation TEM Resolution ultimately limited by contrast = signal (transmitted e-) to background (scattered e-) ratio SBR Sample thickness = mean free path dependent on density ~ 100 nm to maximize electron throughput Ultrathin specimen supported on 3 mm metal grid and e - imaged through mesh Particulates smaller than grid opening suspended on a carbon film 5-10 nm Biological samples (transparent) apply negative stain 1%-3% uranyl acetate or water phosphotungstic acid in water SEM Resolution ultimately limited by size of interaction volume (beam depth) Depends on incident electron beam energy and surface composition Metal decoration by sputter coating important for reducing: 1. interaction volume 2. surface charging of insulators 3. beam damage to soft matter Then resolution limited by metallic grain size

79 Structure of SEM

80 Detection mechanism for SEM Secondary electron detector sei Back scattering detector ebsd Energy-dispersive X-ray spectroscopy edx

81 SEI Back scatted electron

82 EBSD Back scatted electron is sensitive to the atomic weight.

83 Effect of detection mode Secondary electron mode Electrons detected from first 5-50 nm of surface with KE < 50 ev << incident beam (no reflected electrons) So lower contrast (low flux electron flux) But smaller interaction volume so higher resolution ~ 5 nm Backscattered emission mode Electrons detected with KE > 50 ev ~ incident beam KE Electrons backscatter from laterally extended interaction volume ~ m so high contrast (high electron flux) But low resolution ~ 10 nm

84 EDX EDX for element analysis

85 Effect of metal decoration No Coating Lines due to charging effects Gold Gold Palladium Iridium

86 Scanning Electron Microscope Electron beam focused by objective coil to ~ 40 pm and raster scanned across sample by scanning coil Geometry only allows detection of electrons reflected from one side of scanned sample Position of cathode ray beam on monitor synchronised with SEM beam on sample by scan generator

87 Resolution and Interaction Volume Resolution limited by 1. Focus size of electron 5-30 kv = h/(2me) 1/2 ~ 40 pm but with angular aperture ~ 0.01 so radius = / = 4 nm ~ 10 x TEM 2. Contrast beam current (electron flux) beam diameter 3. Interaction volume dependent on beam energy and surface composition Carbon 6 40 kv kv Gold kv Secondary e 5 nm - mode Backscattered mode 10 nm Multiple elastic and inelastic scattering results in re-emission from laterally extended volume surrounding beam

88 Effect of interaction volume Diatom (unicellular silica secreting algae) imaged with different beam accelerating voltages resulting in different penetration depths and interaction volumes Fine detail revealed at 5kV Decreased resolution at 20kV Bar is 1µm in object space, Magnification = x 4000 = 8 mm in image space

89 Summary 1) Challenge in Surface Science 2) Miller index of single crystal surfaces 3) Physisorption vs Chemisorption 4) Activated chemisorption vs non-activated chemisorption 5) Potential energy surface of reaction on surface 6) Properties of catalysts 7) Langmuir isotherm 8) Langmuir isotherms for dissociation adsorption 9) BET isotherm 10) Temkin isotherm 11) Freundlich Isotherm 12) Desorption kinetics 13) Eley-Rideal mechanism 14) Langmuir-Hinshelwood mechanism 15) Langmuir-Hinshelwood mechanism for dissociation adsorption 16) XPS 17) SEM

90 Fundamental aspects of surface science Surface Miller index and surface symmetry z Miller index of a surface is the reciprocals of the fractional intercepts Fractional intercepts: h*, k*, l* h=1/h* k=1/k* l=1/l* So, no or fractions, but all integers z x a A cubic crystal y (110) a (100) (111) y x

91 Heterogeneous Catalysis Diatomic Activated Chemisorption E a R c H c H dis H p Physisorption provides low energy pathway to the dissociation of adsorbate gas and reaction takes place on 2D surface (reduced degrees of freedom) Non-Activated Chemisorption R p

92 Clausius-Clapeyron Equation Decreasing the temperature increases amount of gas adsorbed Since for A(g) + M(s) AM(s) K ads = [AM]/[A][M] = 1/p A when [AM]=[M] H ads And G ads = - RTlnK ads = H ads T S ads so lnk = - + RT S ads R Therefore can measure H ads directly by isosteric (constant coverage) calorimetry. Measure partial gas or vapour pressure p A required to maintain constant surface coverage (volume) V at different temperatures T lnp Then A H = - ads or lnp A = H ads T RT 2 V (1/T) V R

93 Langmuir isotherm Langmuir postulate of monolayer correct for chemisorption and physisorption at high T and low P (multilayers are reduced) Langmuir model assumes: 1) Only one adsorbed molecular layer 2) All adsorption sites are equivalent (lattice model) 3) H ads is independent of surface coverage Monolayer adsorption V m =20 V=10 =0.5 V ads = KpV m 1 + K p p p 1 = + V ads V m KV m Define coverage =V ads /V m p 1 Kp 1 Kp p K V ads p V ads 1 V m 1 { KV m p NH3 on Charcoal p

94 Adsorption with dissociation Langmuir isotherm strictly valid for adsorption without dissociation (physisorption) But chemisorption is generally associated with dissociation of adsorbate Rate of change of coverage due to adsorption now depends on probability of each atom finding a vacant site = no. of vacancies for each atom = N(1- ) d Probabilities are independent so for diatomic = k a pn 2 (1- ) 2 dt Rate of change of coverage due to desorption depends on frequency of atomic encounters to form diatomic = surface coverage of each atom = N d Again independent probabilities so for diatomic = -k d N 2 2 dt K = 10 atm Equating adsorption and desorption rates leads to (k a p/k d ) 1/2 - (k a p/k d ) 1/2 = (1+(Kp) 1/2 )=(Kp) 1/2 and finally = (Kp)1/2 1+(Kp) 1/2 Surface coverage now depends more weakly on pressure (p 1/2 ) p atm K = 1 atm -1 K = 0.1 atm -1

95 Low pressure isotherms But what about dependence of V ads on H ads? Langmuir assumes H ads independent V ads H ads increases (less ve) with increasing V ads Enthalpy modified by increasing adsorbateadsorbate interactions as coverage increases most energetically favourable sites occupied first Temkin Isotherm - assumes H ads changes linearly with p V ads = k ln (n p) or V ads = k ln n + k ln p Freundlich Isotherm - assumes H ads exponential with p V ads = k p 1/n or log V ads = log k + n log p

96 BET Without derivation Balancing evaporation and condensation rates between layers gives V ads V m = cp 0 p (p 0 -p)[p 0 +p(c-1)] where p0 = vapour pressure of bulk liquid of adsorbate Vm = monolayer capacity (as in Langmuir) c = exp[( H L H 1 )/RT] (equivalent to K in Langmuir)

97 BET at low pressure P<<p 0 V ads V m = cp 0 p (p 0 -p)[p 0 +p(c-1)] cp0 p p ( p cp) 0 0 cp ( p cp) 0 Langmuir Isotherm Kp 1 Kp When K=c/p 0

98 Catalytic rates on surfaces Eley-Rideal mechanism d[p] dt = kp B from the Langmuir isotherm KpA A 1 Kp A Hence d[p] dt = k AB p B Kp A (1+Kp A ) What are k AB and K? k AB is the rate constant for., K is the adsorption equilibrium constant for A, k a /k d What is the order of reaction at high and low pressures? High p Kp A /(1+Kp A ) ~ 1 = 1 st order in p B Low p (1+Kp A ) ~ 1 = 2 nd order

99 Catalytic rates on surfaces Eley-Rideal mechanism Temperature dependent production rate: dp [ ] dt d[p] dt = k AB p B Kp A (1+Kp A ) At low T, increase T will increase k AB T At High T, increase T will decrease the surface coverage of A.

100 Catalytic rates on surfaces Langmuir-Hinshelwood Most reactions require both molecules A and B to be adsorbed to the surface before encounter lasts sufficiently long for formation of product P So reaction rate expected to be 2 nd order in surface coverage d[p] In this case = k AB A B = dt For A and B following Langmuir isotherms k AB V A V B V m 2 V A V m K A p A (1+K A p A +K B p B ) V B V m = = K B p B (1+K A p A +K B p B ) d[p] dt So = k AB K A K B p A p B (1+K A p A +K B p B ) 2

101 The XPS process Ejected photoelectron with KE = h BE - Incident X- ray h Vacuum continuum 2p = L 23 2s = L 1 Binding energy BE of core s electron 1s = K 1

102 Inelastic mean free path photoelectron travels distance through a solid without being scattered is called inelastic mean free path (IMFP) Low energy electrons: lattice interaction. High energy electron: Less portion energy loss in the scattering process.

103 Chemical shifts in Polyacrylic acid Survey scan ev of thin film poly (acrylic-acid) shows O 1s and C 1s photoelectrons Core-line scans of C 1s and O 1s show chemical shifts of due to the 3 different carbon and 2 different oxygen environments

104 Depth profiling with XPS Chemical composition can be probed at different depths by either: 1. Tuning X-ray energy (synchrotron) and hence e - KE across universal curve 2. Changing path length of electron through solid to detector Path length altered by changing angle of detection w.r.t surface normal Probability photoelectron is detected depends on distance travelled by the electron through solid and not the depth of the atom from which it originates Then depth probed given by d = x cos d x Analyser For a detection angle = 75 o, d=x/4

105 Detection mechanism for SEM Secondary electron detector sei Back scattering detector ebsd Energy-dispersive X-ray spectroscopy edx