Adsortion of Atoms and Molecules Physisortion Chemisortion Surface Bonding Kinetics of Adsortion/Diffusion/Desortion (Scattering Dynamics)
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
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
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)
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 1 9 3 For airwise interactions with all surface secies: Works best for insulating substrates
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 = 96.47 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 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
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
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?
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
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
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 15 3.76 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
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)
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
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
Assuming is fixed (known) in TPD [] rearranged Can often ignore the last term Given T, and guess of, can get E directly
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.
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).