Adsorption and Catalysis

Transcription

1 Adsorption and Catalysis Dr. King Lun Yeung Department of Chemical Engineering Hong Kong University of Science and Technology CENG 511 Lecture 3 Adsorption versus Absorption H H H H H H H H H Adsorption Surface process Absorption bulk process H 2 adsorption on palladium H H H H H H H H H HH H H H H H H H H H H H H H H H H H 2 absorption palladium hydride

2 Nomenclature Substrate or adsorbent: surface onto which adsorption can occur. example: catalyst surface, activated carbon, alumina Adsorbate: molecules or atoms that adsorb onto the substrate. example: nitrogen, hydrogen, carbon monoxide, water Adsorption: the process by which a molecule or atom adsorb onto a surface of substrate. Coverage: a measure of the extent of adsorption of a specie onto a surface Exposure: a measure of the amount of gas the surface had been exposed to ( 1 Langmuir = 10-6 torr s) adsorbate coverage θ = fraction of surface sites occupied H H H H H H H H H H H H H H adsorbent Types of Adsorption Modes Physical adsorption or physisorption Bonding between molecules and surface is by weak van der Waals forces. Chemical bond is formed between molecules and surface. Chemical adsorption or chemisorption

3 Characteristics of Chemi- and Physisorptions Properties Adsorption temperature Adsorption enthalpy Crystallographic specificity Nature of adsorption Saturation Adsorption kinetic Chemisorption virtually unlimited range wide range ( kjmol -1 ) marked difference for between crystal planes often dissociative and irreversible in many cases limited to a monolayer activated process Physisorption near or below T bp of adsorbate (Xe < 100 K, CO 2 < 200 K) heat of liquifaction (5-40 kjmol -1 ) independent of surface geometry non-dissociative and reversible multilayer occurs often fast, non-activated process Analytical Methods for Establishing Surface Bonds Infrared Spectroscopy Atoms vibrates in the I.R. range chemical analysis (molecular fingerprinting) structural information electronic information (optical conductivity) IR units: wavenumbers (cm-1), 10 micron wavelength = 1000 cm-1 Near-IR: cm-1 Mid-IR: cm-1 Far-IR: cm-1

4 I.R. Measurement I.R. Spectrum of CO 2 O C O Symmetric Stretch Assymmetric Stretch A dipole moment = charge imbalance in the molecule Bending mode

5 I.R. Spectrum of NO on Pt Adsorption decreases Temperature increases Molecular conformation changes I.R. Spectrum of HCN on Pt H- C N 0.15 L HCN, 100 K weak chemisorption H- C N H- C N C N 1.5 L HCN, 100 K physisorption Pt Pt Pt 30 L HCN, 200 K dissociative chemisorption (a) (b) (c) δ(hcn) 2δ(HCN) ν(cn) ν(h-cn)

6 Adsorption Rate R ads = k C x x - kinetic order k - rate constant C - gas phase concentration R ads = k P x x - kinetic order k - rate constant P - partial pressure of molecule R ads = A C x exp (-Ea/RT) Temperature dependency of adsorption processes Frequency factor Activation energy Adsorption Rate Molecular level event R ads = S F = f(θ) P/(2πmkT) 0.5 exp(-ea/rt) (molecules m -2 s -1 ) Sticking coefficient S = f(θ) exp(-ea/rt) where 0 < S < 1 Flux (Hertz-Knudsen) F = P/(2πmkT) 0.5 where P = gas pressure (N m -2 ) m = mass of one molecule (Kg) T = temperature (K) Note: f(θ) is a function of surface coverage special case of Langmuir adsorption f(θ) = 1-θ E(θ), the activation energy is also affected by surface coverage

7 Sticking Coefficient S = f(θ) exp(-ea/rt) where 0 < S < 1 Tungsten S also depends on crystal planes and may be influenced by surface reconstruction. Sticking Coefficient

8 Sticking Coefficient Steering Effects Surface Coverage (θ) Estimation based on gas exposure R ads = dn ads /dt = S F N ads S F t Nearly independent of coverage for most situations Exposure time Molecules adsorbed per unit surface area

9 Adsorption Energetics Potential energy (E) for adsorption is only dependent on distance between molecule and surface d adsorbate surface P.E. is assumed to be independent of: angular orientation of molecule changes in internal bond angles and lengths position of the molecule along the surface Adsorption Energetics Physisorption versus chemisorption repulsive force E(ads) E(ads) < E(ads) Physisorption Chemisorption Chemisorption small minima weak Van der Waal attraction force large minima formation of surface chemical bonds surface attractive forces

10 Physical Adsorption E(d) 0.3 nm Applications: surface area measurement pore size and volume determination pore size distribution Van der Waal forces d nitrogen Note: there is no activation barrier for physisorption fast process metal surface The Brunauer-Emmett-Teller Isotherm BET isotherm where: n is the amount of gas adsorbed at P n m is the amount of gas in a monolayer P 0 is the saturation pressure n at P = P 0 C is a constant defined as: H 1 and H L are the adsorption enthalpy of first and subsequent layers

11 BET Isotherm Assumptions adsorption takes place on the lattice and molecules stay put, first monolayer is adsorbed onto the solid surface and each layers can start before another is finished, except for the first layer, a molecule can be adsorbed on a given site in a layer (n) if the same site also exists in (n-1) layer, at saturation pressure (P 0 ), the number of adsorbed layers is infinite (i.e., condensation), except for the first layer, the adsorption enthalpy (H L ) is identical for each layers. Activated Carbon Surface area ~ 1000 m 2 /g

12 Surface Area Determination BET surface area by N 2 physisorption - adsorption - desorption c = n m = 4.2 x 10-3 mol Area = 511 m 2 /g Plot P/n(P 0 -P) versus P/P 0 calculate c and nm from the slope (c-1/ n m c) and intercept (1/n m c) of the isotherm measurements usually obtained for P/P 0 < 0.2 c = n m = 3.9 x 10-3 mol Area = 480 m 2 /g Chemical Adsorption E(d) r e = equilibrium bond distance Ea(ads) = 0 Applications: active surface area measurements surface site energetics catalytic site determination Ea(des) = - H(ads) d CO = strength of surface bonding = H(ads) Pt surface Note: there is no activation barrier for adsorption fast process, there us an activation barrier for desorption slow process.

13 Chemical Adsorption Processes Physisorption + molecular chemisorption E(d) d CO physisorption chemisorption Chemical Adsorption Processes Physisorption + dissociative chemisorption E(d) H 2 2 H H 2 dissociation chemisorption physisorption atomic chemisorption d Note: this is an energy prohibitive process

14 Chemical Adsorption Processes Physisorption + molecular chemisorption E(d) CO physisorption/ desorption chemisorption physisorption atomic chemisorption d Chemical Adsorption Processes Physisorption + molecular chemisorption E(d) CO direct chemisorption physisorption atomic chemisorption d

15 Chemical Adsorption Processes Energy barrier Ea(ads) ~ 0 Ea(ads) > 0 Chemical Adsorption Processes Energy barrier Chemical Adsorption is usually an energy activated process. -E a des = - E(ads) ~ - H(ads)

16 Formation of Ordered Adlayer Ea(surface diffusion) < kt activated carbon CH 4 Krypton Formation of Ordered Adlayer Chlorine on chromium surface

17 Adsorbate Geometries on Metals Hydrogen and halogens Hydrogen H-H Halogens high electronegativity dissociative chemisorption X-X H-H 2-D atomic gas HH X-X ionic bonding XX compound XX 1-H atom per 1-metal atom Halogen atom tend to occupy high co-ordination sites: (111) (100) Adsorbate Geometries on Metals Oxygen and Nitrogen Oxygen Nitrogen O=O OO N N O=O N N both molecular and dissociative chemisorption occurs. molecular chemisorption σ-donor or π-acceptor interactions. dissociative chemisorption occupy highest co-ordinated surface sites, also causes surface distorsion. molecular chemisorption σ-donor or π-acceptor interactions. (111) (100)

18 Adsorbate Geometries on Metals Carbon monoxide Carbon monoxide C O C O Terminal (Linear) all surface CC metal carbide Bridging (2f site) all surface forms metal carbides with metals located at the left-hand side of the periodic table. molecular chemisorption occurs on d-block metals (e.g., Cu, Ag) and transition metals Bridging (3f hollow) (111) surface Adsorbate Geometries on Metals Ammonia and unsaturated hydrocarbons Ammonia N H 3 NH 2 (ads) + H (ads) NH (ads) + 2 H (ads) N (ads) + 3 H (ads) Ethene 2 HC=CH 2

19 Active Surface Area Measurement Μost common chemisorption gases: hydrogen, oxygen and carbon monoxide furnace catalyst Pulse H 2, O 2 or CO gases thermal conductivity cell (TCD) exhaust carrier gas helium or argon Catalyst Surface Area and Dispersion Calculation furnace 100 µl Pulse H 2 then titrate with O 2 (STP) 1 g 0.10 wt. % Pt/γ-Al 2 O 3 T = 423 K, P = 1 bar 3.75 peaks (H 2 ) 4.50 peaks (O 2 ) thermal conductivity cell (TCD) exhaust Avogrado s number: x Pt lattice constant: a = 3.92 (FCC) carrier gas helium or argon Calculate surface area of Pt and its dispersion.

20 Isotherms Langmuir isotherm S - * + A(g) S-A surface sites Adsorbed molecules H(ads) is independent of θ the process is reversible and is at equilibrium K = [S-M] [S - *] [A] [S-M] is proportional to θ, [S-*] is proportional to 1-θ, [A] is proportional to partial pressure of A Isotherms Langmuir isotherm Molecular chemisorption b = θ (1-θ) P θ = bp 1+ bp Where b depends only on the temperature Dissociative chemisorption θ = (bp) (bp) 0.5 Where b depends only on the temperature

21 Variation of θ as function of T and P θ bp at low pressure θ 1 at high pressure b when T b when H(ads) θ θ b T P P Determination of H(ads) ( InP ) 1/T θ =const = Η (ads) R θ InP θi 0.2 T (P 1, T 1 ) (P 2, T 2 ) T P 1/T

22 Adsorption Isotherms Henry s Adsorption Isotherm Special case of Langmuir isotherm bp << 1 θ = bp V = k P where k = bv

23 The Freundlich Isotherm Adsorption sites are distributed exponentially with H(ads) b i P= θ i (1-θ i ) H(ads) θ = Σθ i N i Σ N i Inθ = θ = RΤ A kp 1/n InP + B Valid for low partial pressure most frequently used for describing pollutant adsorption on activated carbons θ The Temkin Isotherm H(ads) decreases with θ θ = A InBP H(ads) θ Valid at low to medium coverage gas chemisorption on clean metal surfaces

24 Thermal Desorption Spectroscopy L Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases Chemical Adsorption E(d) r e = equilibrium bond distance Ea(ads) = 0 Applications: active surface area measurements surface site energetics catalytic site determination Ea(des) = - H(ads) d CO = strength of surface bonding = H(ads) Pt surface Note: there is no activation barrier for adsorption fast process, there us an activation barrier for desorption slow process.

25 Thermal Desorption Spectroscopy -dna dt d [ -dna ] dt dt L Desorption Rate -dna dt m { } = N dt dt a k exp ( -E d ) RT Linear heating rate T = T 0 + βt dt = β dt Assuming k and E d are independent of coverage and m = 1 (i.e., first order desorption) Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases E d RT p 2 = k β exp( -E d RT ) Thermal Desorption Spectroscopy E d RT p 2 = k β exp( -E d RT ) slope, m Ea TPD provides important information on adsorption/desorption energetics and adsorbate-surface interactions. Determination of E des using different heating rates (β)

26 Thermal Desorption Spectroscopy -dna dt d [ -dna ] Second order desorption dt dt L Assuming k and E d are independent of coverage and m = 2 (i.e., first order desorption) E d 2 = 2(N RT a ) p p k β exp( -E d RT ) Characterized by a shift in the peak maxima toward lower temperature as the coverage increases Thermal desorption spectra of CO on Ni(100) after successive exposure to CO gases Activation Energies for CO Desorption

27 Influence of Surface Overlayer Catalyst poison, strong adsorbates and coke CO desorption Clean catalyst Sulfur-treated catalyst Ordered Adsorbate layer H 2 /Rh(110) O 2 /Rh(110)

28 Thermal Desorption Spectroscopy Ο 2 TPD from Rh(110) Ordered Adsorbate layer benzene/zno(1010)

29 Kelvin Probe Measures the change in work function ( φ) Typical Kelvin probe for adsorption studies Scanning Kelvin probe for surface work function (i.e., elemental and compositional) imaging also known as scanning electrical field microscopy Kelvin Probe Basic principle Vibrating capacitor measures φ φ is the least amount of energy needed for an electron to escape from metal to vacuum. φ is sensitive optical, electrical and mechanical properties of materials

30 The Freundlich Isotherm Adsorption sites are distributed exponentially with H(ads) b i P= θ i (1-θ i ) H(ads) θ = Σθ i N i Σ N i Inθ = θ = RΤ A kp 1/n InP + B Valid for low partial pressure most frequently used for describing pollutant adsorption on activated carbons θ The Temkin Isotherm H(ads) decreases with θ θ = A InBP H(ads) θ Valid at low to medium coverage gas chemisorption on clean metal surfaces

31 Thermal Desorption Spectroscopy L Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases Chemical Adsorption E(d) r e = equilibrium bond distance Ea(ads) = 0 Applications: active surface area measurements surface site energetics catalytic site determination Ea(des) = - H(ads) d CO = strength of surface bonding = H(ads) Pt surface Note: there is no activation barrier for adsorption fast process, there us an activation barrier for desorption slow process.

32 Thermal Desorption Spectroscopy -dna dt d [ -dna ] dt dt L Desorption Rate -dna dt m { } = N dt dt a k exp ( -E d ) RT Linear heating rate T = T 0 + βt dt = β dt Assuming k and E d are independent of coverage and m = 1 (i.e., first order desorption) Thermal desorption spectra of CO on Pd(100) after successive exposure to CO gases E d RT p 2 = k β exp( -E d RT ) Thermal Desorption Spectroscopy E d RT p 2 = k β exp( -E d RT ) slope, m Ea TPD provides important information on adsorption/desorption energetics and adsorbate-surface interactions. Determination of E des using different heating rates (β)

33 Thermal Desorption Spectroscopy -dna dt d [ -dna ] Second order desorption dt dt L Assuming k and E d are independent of coverage and m = 2 (i.e., first order desorption) E d 2 = 2(N RT a ) p p k β exp( -E d RT ) Characterized by a shift in the peak maxima toward lower temperature as the coverage increases Thermal desorption spectra of CO on Ni(100) after successive exposure to CO gases Activation Energies for CO Desorption

34 Influence of Surface Overlayer Catalyst poison, strong adsorbates and coke CO desorption Clean catalyst Sulfur-treated catalyst Ordered Adsorbate layer H 2 /Rh(110) O 2 /Rh(110)

35 Thermal Desorption Spectroscopy Ο 2 TPD from Rh(110) Ordered Adsorbate layer benzene/zno(1010)

36 Kelvin Probe Measures the change in work function ( φ) Typical Kelvin probe for adsorption studies Scanning Kelvin probe for surface work function (i.e., elemental and compositional) imaging also known as scanning electrical field microscopy Kelvin Probe Basic principle Vibrating capacitor measures φ φ is the least amount of energy needed for an electron to escape from metal to vacuum. φ is sensitive optical, electrical and mechanical properties of materials