PC4259 Chapter 4 Adsorption on Solid Surfaces & Catalysis When atom or molecule is trapped by an attractive interaction on a solid surface, it becomes an adsorbate with adsorption energy Eads Physisorption: Eads  100 meV, attracted by van der Waals force, little change in electronic configurations Chemisorption: Eads  0.5 eV, chemical bond is formed between adsorbate and substrate, significant changes in electronic configurations

- - r Van der Waals (London) Interaction p1 p2 + + Neutral atoms can induce (fluctuating) dipole moments in each other p1 p2 + - + - r ( = polarizability) p1 =0 but p12 ≠ 0 Interaction between mutual induced dipoles: Full potential energy: Repulsion between atoms at small distance ~ Lennard-Jones potential

Physisorption Potential Modeled as the interaction of an induced adsorbate dipole with its image dipole Physisorption potentials of He atoms on some metals calculated with jellium model

Chemisorption: electronic structures of adsorbate & surface go through significant reconfiguration, form chemical bond (metallic, covalent or ionic) DFT calculation results of charge densities of some chemisorbed atoms on a jellium substrate E-donation from Li E-capture by Cl In chemisorption, Eads ~ 1 eV/atom = 96.5 kJ/mol = 23.1 kcal/mol

Dissociative chemisorption: a molecule dissociates, and the breaking species form chemical bonds with surface (e.g., O2  O + O on Fe) Dissociation energy of molecule AB: Ediss Ediss = 4.5 eV, 5.2 eV and 9.8 eV for H2, O2 and N2 Dissociative adsorption energy: For O2 on Fe, since O + Fe bond strength is ~ 4.2 eV, the dissociative Eads is ~ 3.2 eV

Transition Between Physisorption & Chemisorption states Z’ Molecular physisorption & dissociative chemisorption potential curves intersect at transition point z’ Activation energy for chemisorption Eact Precursor state for chemisorption Barrier from precursor to chemisorption state: a = Eact + d

Evolution of molecular bond in chemisorption bridge site a = 0.5 eV Transition point on-top site a = 0.7 eV H2 on Pd(100), bridge site on-top site H2 on Cu(100)

Desorption from Surface Desorption: Adsorbed species gain sufficient energy to leave the surface Thermal desorption: desorption process activated by thermal energy (e.g., by raising temperature) Stimulated desorption: desorption activated by energy transfer from photons, electrons, ions,… Reaction before desorption: adsorbed atoms form molecules, then the molecules leave surface

Activation Energy for Desorption Physisorbed & non-dissociative chemisorbed species: Edes = Eads Desorption of recombined dissociative chemisorbed species: Edes = Eads + Eact

Arrangement of Adsorbates on Surface Depends on coverage , adsorbate-substrate & adsorbate-adsorbate interactions, and T  , in unit of ML (monolayer), can be measured using XPS, AES or EELS Low  & high T,  2-D gas phase High  & low T,  2-D order phase High  & high T,  2-D liquid phase Phase diagram & transition

Types of Adsorbate-adsorbate Interactions Van der Waals attraction between mutually induced dipoles, important only for physisorbed inert gas at low T Dipole force between permanent dipoles of adsorbed molecules (e.g. H2O, CO, NH3), or due to charge transfer in bond formation, often repulsive due to parallel dipoles Orbital overlap between adsorbates at neighboring sites, often repulsive due to Pauli exclusion Substrate-mediated interactions: Adsorbate disturbs electronic or mechanic structures (e.g. charge transfer or elastic distortion) at nearby sites, make them more favorable or unfavorable for others to occupy, corresponding effective attraction or repulsion Mainly consider nearest neighbor (nn) and next (or 2nd) nearest neighbor (nnn) interactions

If nn-interaction repulsive but nnn-interaction is attractive  H2 on graphite at low T Quite Common

Adsorption sites on hexagonal surfaces of metals CO take on-top sites on Rh(111), but bridge sites on Ni(111)

Si(111) -Ga Each Ga atom bonds with three Si atoms on surface, so all Si dangling bonds are saturated, while the dangling bond on top of a Ga atom is completely empty, satisfying electron counting rule

Si(111) -Pb STM image More than one adsorbate may be accommodated in each supercell Need both STM (or LEED) and XPS (or AES) data

Si(111) -Sb trimer

Superstructures formed by both adsorbed & substrate atoms Simple two-layer case fl + fu = 1 fu fl Si(111) -Ag

Dynamic Adsorption & Desorption Measurements To find out binding energy, activation barrier for adsorption, etc. A flux F can come from a gas-phase ambient of pressure p: A flux can also be generated by a gas doser, a molecule beam or an evaporator in vacuum At constant F or p for a period t, Ft or pt is the total exposure Unit of Ft: monolayer (ML) pt is often in unit of Langmuir (L), 1 L = 10-6 torr-s

Adsorption Kinetics Under a flux F, surface coverage  increases at a rate: Probability of sticking or sticking coefficient:  = condensation coefficient, reflecting effects of orientation (steric factor), energy dissipation of adsorbed particles f() = coverage factor, represents the probability of finding available adsorption sites. Sticking may be hindered by adsorbates already on surface exp(-Eact/kT) = Boltzmann factor, comes in if there is a barrier for adsorption

Langmuir adsorption model: each adsorption site only accommodate 1 particle,   1 ML Non-dissociative adsorption (n = 1)  Dissociative adsorption of diatomic molecule (n = 2) Dissociative adsorption of n-atom molecules n = order of adsorption (non-activated)

In physisorption or atomic chemisorption with Edes >> kT, initial sticking coefficient s0  1 & independent of T In dissociative chemisorption with a physisorption precursor state of binding energy d and a barrier to chemisorption a, s0 depends on T Molecule precursors of coverage p Rate to desorb: Rate to chemisorption Initial sticking coefficient:

Eact = a - d from Arrhenius plot: ln(1/s0 -1) vs 1/T Initial sticking coefficient in dissociative chemisorption Eact = a - d from Arrhenius plot: ln(1/s0 -1) vs 1/T

Coverage factor in nth-order activated chemisorption If precursor physisorption can occur at all sites, conversion to chemisorption requires n empty sites, introducing ka(1 - )n factor Overall coverage factor: (K = ka/kd)

Sb4 chemisorption on Si surfaces (n = 4) T-dependence of K Case of decreasing K at higher T, indicating εa > εd,

Mass Spectrometer for desorption measurement Sample TemperatureControl Isothermal desorption: T fixed Programmed desorption: T varies with time

(Polanyi-Wigner equation) Desorption rate: If adsorbates occupy identical sites, for nth-order desorption (e.g. n adsorbed atoms recombine first and then desorb as a molecule) (Polanyi-Wigner equation) n = 0: desorption of 2-D dilute gas in equilibrium with a 2-D solid, e.g. adatoms on a multilayer film In isothermal desorption (T fixed):

Edes from Arrhenius plot Isothermal desorption of 2-D gas of Ag in equilibrium with 3 different 2-D solid phases Edes from Arrhenius plot

1st-order (n = 1) Isothermal Desorption : attempt frequency ~ 1013 s-1 (0 = 1 ML, Eads = 3 eV) 2nd-order (n = 2, e.g. O + O  O2) kinetics for associative diatomic molecular desorption: (in Homework 8)

Temperature Programmed Desorption (TPD) Analyze bonding and reaction properties of adsorbed species Sample Programmed heating Mass spectrometer Linear T ramping: T(t) = T0 + t When T is low, desorption rate is low due to Boltzmann factor At a very large t (or T), surface is run out of adsorbates, desorption rate is also low. At Tm, desorption flux reaches a peak

0th-order TPD First-order TPD TPD n = 0 Peak is reached right be before all adsorbates have desorbed First-order TPD TPD n = 1 Peak at: In 1st-order TPD, Tm is independent of 0

Edes from 1st-order TPD 2nd-order TPD ~ 1013 s-1 m Tm decreases as 0 increases Spectra are more symmetric

TPD spectra show a combination of a few kinetic models Inhomogeneous substrate Multilayer desorption 0th-order followed by 1st-order

Adsorption Isotherm The coverage  on a surface in equilibrium with a gas ambient of pressure p satisfies , or: with In first-order Langmuir adsorption system & Langmuir adsorption isotherm

HREELS: for adsorbate bond configurations of atoms and molecules Also can be detected with optical scattering method Bond orientation from polarization dependence Large shift

Electron Stimulated Desorption (ESD) Through excitation of electronic system of adsorbates Desorption of ionic or neutral species

Electron Stimulated Desorption Ion Angular Distribution (ESDIAD) Flying away direction At low  0.5<<1 e H H O 0.2 <  <1 H+ ESDIAD from Ru(0001)

Adsorption Induced Work Function Variation Dipole moment p = qd : intrinsic & induced In-plane dipole has no effect

Cs-Induced Work Function Variation Cs: large ion size, e-donor Dipole-dipole interaction introduces a depolarization factor:  = polarizability

Cs adsorption on Semiconductor With: On p-type GaAs Bands bend downward Evac  EC negative electron affinity high-flux photo-cathode

Adsorption Induced Change in LDOS near EF Ni(111)-O Depletion of LDOS at EF 6 L 100 L 1000 L Surfactants: adsorbates to purposely modify surface property

Find a reaction path with lower barriers Kinetic Barrier in Chemical Reaction CO oxidation: CO + ½O2  CO2 Energy gain: Hr = 283 kJ/mol O2 dissociation barrier: ~ 5 eV Haber-Bosch synthesis of NH3 ½N2 + 3/2H2  NH3 Energy gain: Hr = 46 kJ/mol Find a reaction path with lower barriers N2 dissociation barrier: ~ 9.8 eV!

O2, H2 and N2 may easily dissociate when adsorbed on some surfaces Basis of Heterogeneous Catalysis: Chemical reaction via adsorption-dissociation-reaction-desorption path often only encounters moderate barriers Catalyst: accelerates certain chemical reaction, but is not consumed in reaction

Gerhard Ertl: 2007 Nobel Prize in Chemistry for his pioneering studies of chemical processes on solid surfaces. He developed quantitative description of how H organizes on surfaces of catalytic metals such as Pt, Pd, and Ni. He also produced key insights into mechanism of Haber-Bosch process of NH3 synthesis Haber-Bosch synthesis of NH3 on Fe N2 dissociation not a major obstacle, but H addition to N is up-hill

CO oxidation on Pt(111): main barrier now is only 105 kJ/mol, while in gas phase O2 dissociation requires ~ 490 kJ/mol Catalyst to convert CO to CO2, NO to N2 and HC to H2O in a car exhaust contains Pt, Pd, Rh and Ir

LDOS(EF), d-band center & Reactivity LDOS at EF and surface reactivity are closely correlated E Noble metal EF Transition metal EF d-band sp-band Downward shift of d-band center & increase of N2 dissociation barrier on Ru(0001) induced by adsorption of N, O or H, DOS at EF in noble or transition metals

Raise nitrogen sticking probability by 102 K as electronic promoter in NH3 synthesis Enhance LDOS at EF Lower physisorption potential curve of N2 Raise nitrogen sticking probability by 102

Poisoning of catalyst Poisoning often occurs due to coverage of S or graphitic C On clean Pd(100), H2 dissociation is barrier-less On p(22)S/Pd(100), H2 dissociation barrier = 0.1 eV On c(22)S/Pd(100), H2 dissociation barrier = 2 eV, blocked S adsorption shifts Pd d-band downward, surface becomes more repulsive for H2 adsorption & dissociation

General suitability of material as catalyst: should be just moderately reactive Methanation of CO CO + 3H2  CH4 + H2O Fischer-Tropsch reaction facilitated by Fe-Co catalysts doped with K & Cu Volcano curve