Adsorption of Atoms and Molecules. Physisorption Chemisorption Surface Bonding Kinetics of Adsorption/Diffusion/Desorption (Scattering Dynamics)

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Brandon Osborne
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1 Adsortion of Atoms and Molecules Physisortion Chemisortion Surface Bonding Kinetics of Adsortion/Diffusion/Desortion (Scattering Dynamics)

2 Outcomes of Collision Process Rebound (elastically or inelastically) V(r) E Elastic Scattering Inelastic Scattering Accomodation (thermalizing) Adsortion Adsortion Physisortion Chemisortion Surface Reaction Desortion r True elastic scattering is a urely quantum mechanical henomenon H, HD, He Physisortion of Atoms and Molecules Weak bonding, Van der Waals Interaction Binding Energy ~ Bulk (adsorbate) vaorization energy Physisortion will occur for any gas solid system for a suitable range of T, P. Multilayers can form Tyical E ~ 1 to 40 kj mol 1

3 Chemisortion of Atoms and Molecules Chemisortion associated with charge transfer (see examle below) Binding Energy E ~ 50 to 400 kj mol 1 Chemisorbed layer ~ 1 atom (molecule) thick, or less M +X (g) EA M +X (g) Chemisortion Physisortion Reactive Chemisortion of Molecules Chemisortion ossibly associated with molecular decomosition E.g. Dissociative adsortion (charge transfer will occur also!) M +A (g) D o Dissociation energy M +A (g) M +A (a) Chemisortion M +A (a) Physisortion

4 Oxygen on Pt(111) Chemisorbed molecular secies too M +A (g) M +A (g) E A Electron affinity M +A (g) M +A (a) Physisortion M +A (a) M +A (a) Chemisortion Other Reactive Processes Catalysis (A + B (ads) AB) Substrate reaction (Oxidation etc.) Desortion (+ Chemistry with a sledge hammer!) Xe M + B (a) M+ B (g) Xe M + B (a) M+ B (a)

5 Physisortion arises from disersion forces Consider non reacting secies Instantaneous fluctuations in charge distribution interact with instantaneous diole moments (also multiole moments) in neighboring secies. r Consequence: attractive interaction of the form E = C / r 6 where C = f( I i, i, i ) I = ionization otential, olarizability, = diole moment. Disersion forces above a surface The 6 1 Lennard Jones otential is commonly used to describe both Van der Waals and overla reulsive interaction otentials 1 6 dv j r j E 4 r r 1 6 E Ej E Dr Cr Ndv j Vsubstrate Substrate atoms/unit volume E R C R C R For airwise interactions with all surface secies: Works best for insulating substrates

6 Electrostatic interaction(s) give rise to attractive forces above metallic faces also E.g. H above a erfect conductor substrate a R V( R) 1 8 er 3 R Comare E LJ 1/R 3 Physisortion energy (of Xe) on a metal face Factors affecting binding energy Roughness Local adsorbate density Proximity of substrate Bulk vaorization energy Geometry in binding sites Adsorbate adsorbate interactions Order/disorder henomena, hase transitions Distinction between adsorbate and substrate Long + short range interactions 1eV/atom(molecule) = 3.05 kcal/mol = kj/mol.

Chemisortion: the chemical bond Chemisortion is site secific Ato site Hollow Bridge Sequential filling of binding sites Binding energies deend on crystal face Stes, defects affect adsortion energies

7 Chemisortion: the chemical bond Chemisortion is site secific Ato site Hollow Bridge Sequential filling of binding sites Binding energies deend on crystal face Stes, defects affect adsortion energies D alloyed layers, comound layers can exist when no such bulk hase is known Adsortion chemistry on extended surface is often very analogous to inorganic/cluster chemistry Heats of Chemisortion E.g. again dissociative adsortion M +A (g) D o Dissociation energy M +A (a) Chemisorbed State M +A (a) M +A (g) H ads

8 Predictions from Heats of Adsortion Given dissociative adsortion, is molecular or atomic desortion referred? Is H ads < E A or H ads > E A? Ni H or W O M +A (g) M +A (g) D o E A (M +A (a) ) +A (g) M +A (a) Molecular desortion H ads E A Atomic desortion M +A (a) Trends in Heats of Adsortion

9 Activated dissociative adsortion V(r) E aades A ads E aaads A gas r H ads = E aaads E aades A ads Routes for Activated dissociative adsortion?

10 Charge transfer on adsortion: Electroositive adsorbate Electron transfer to surface e.g. alkali metals Charge transfer on adsortion: Electronegative adsorbate Neutral atom Electron transfer to adsorbate e.g. adsorbed O, Cl, F

11 Isolated CO 6 5 n 1 4 n 3 b C O CO Surface bonding back donation to 5 donation Ato or bridge bonding CO Blyholder model

12 Isolated O molecules O Surface interaction leads to dissociative adsortion! Strong ionic bonding of O with surface Adsorbate Induced Work function changes on adsortion/charge transfer Work function change given by Helmholtz equation d x10 N in V in Debye N in atoms cm 3.3 x10 Cm/ Debye.,, /. 30 Electronegative adsorbates (O,Cl,F,H,..) cause work functions to increase. Electroositive adsorbates (Li,Na,K,Ba ) cause work functions to decrease Physisorbates also decrease work functions (usually) E.g. NH 3

13 Diole deolarization E.g. Adsorbed alkali, Cs initially ve. Dioles of like olarization cause deolarization. Toing s Model N 9 1 N 4 = olarizability 3 Sticking Kinetics: Probabilities for adsortion, robability sticking coefficients. rate of adsortion dn Adsorbate density/m S rate of collision de Exosure/m Ra S R 1 a f s* ka / mkt mkt S f s* k a No adsortion if incident molecule/atom hits occuied site(s). Collision rate, atoms/m at P,T Comare non dissociative S( )/ S( 0) S( )/ S f o Fraction of available sites Condensation coefficient on an emty site Rate constant for adsortion and dissociative adsortion f() = 1 S o ~ 1 for O /W(100) f() = (1 S o ~ 10 6 for N /Fe(111)

14 Precursors in sticking coefficients Kinetics: Adsortion Isotherms. Adsortion isotherms are measures of equilibrium (steady state) between gas ressure P and coverage Useful for determining H ads, surface coverage, mechanistic information, and hase equilibria in D systems. ln P H H ads ads ex o T RT RT Assumed, at equilibria R d = R a ; desortion and adsortion rates are balanced. Constant Isosteric heat of adsortion

15 Kinetics of Desortion f ' Used R k f ' k More generally d d d n dn n E Equally Rd() t nn ex dt RT for n th order desortion kinetics [1] n = 1 anticiated for desortion of atoms, undissociated molecules n = n = 0 recombinative desortion, e.g. Cl, C N (NCCN) free sublimation with no change in surface area, D hase equilibria Info from isosteric heat of adsortion, isothermal desortion, or TPD (temerature rogrammed desortion) TPD (Temerature Programmed Desortion) Conventionally a linear heating rate T T t Substitution in [1] and setting o d dn 0 dt dt P* at T [] E RT E ex n 1 RT 1 [3] E RT N E ex n o RT t

16 Assuming is fixed (known) in TPD [] rearranged Can often ignore the last term Given T, and guess of, can get E directly

17 E RT 1 Better method; vary. E ex 0 n 1 1 RT 1 Dwrt.... T E 1 E E ex 0 RT R RT T E ex RT T E 1 ln C RT Gradient = E/R.

, n E d T, T E d const., n E d, T, T, E d const., E d, T, o T, I, const.

18 Why is often taken as 10 13? Transition State Theory. Summary of quick TPD analysis methods Vary measure use assume find T T E d const., n E d T, T E d const., n E d, T, T, E d const., E d, T, o T, I, const., E do, E d () n = recombinative desortion, C N (NCCN)/Cu(001) E RT E RT Redhead 1 E ex RT 1T E ln ln RT T E 1 ln C RT Redhead Ciftlikli, Hinch. Langmuir (010).

Adsorption of gases on solids (focus on physisorption)

Adsorption of gases on solids (focus on physisorption) Adsorption Solid surfaces show strong affinity towards gas molecules that it comes in contact with and some amt of them are trapped on the surface