Surface Reactivity What determines the surface reactivity? What is physisorption? What is chemisorption? What are the trends in Reactivity? What is the underlying fundament?

Physisorption By Taylor expansion Attractive part Repulsive part (0,0,d) (x,y,z) d + - = (x,y,z), By Taylor expansion Attractive part Repulsive part Result:

Physisorption Often is the less accurate Lennar-Jones potential used with n=12 Molecule Enthalpy kJmol-1 CH4 -21 CO -25 CO2 N2 O2 Remember: Physisorption is only polarizaion, there is no exchange of electrons Where do we utilize physisorption???

The Chemical Bond The simplest chemical bonding: Hydrogen molecule H2+ Saa= Sbb= 1 since and were already normalized. Sab=S the overlap intergral

Simplified Approach Consider a simple two level system: Consider the limit where S is small (small overlap)

Homonuclear system < 0 and S > 0 Strength of bond b is proportional to the overlap

Homonuclear system < 0 and S > 0 Energy in the chemical bond If E < 0 will the molecule be stable and the work required for dissociation will be Ediss= - E. b is proportional to the overlap

The Chemical Bond So why is Cl2 much more reactive than O2 Ebond = -4.55 eV 1s He ``He2´´ Ebond > 0 eV N N2 Ebond = -9.84 eV 5s 1p 6s 2p O O2 Ebond = -5.20 eV 2s So why is Cl2 much more reactive than O2 and why does Ne2 not exist?

The heteronuclear system s* E- s E+ 1sa E1s 1sb E1s Sb b2/d

Molecules in general strong weak no bonding atomic molecular atomic orbital orbital orbital anti bonding Big overlap Small overlap bonding

Electronic structure of the solid .. 4 p 4 s 3 d Energy Atom Metal Density of states (DOS)

Trends in the periodic system To the left the outer atomic orbital tends to be extended and will be more localized as we go to the right where they become filled. Thus the tendency for forming bond will decrease when going to the left. From metallic to covalent to none. When going down the periodic system the outer orbital increases and becomes more extended and delocalized i.e. forms better bonding. Eventually even Xe can form covalent bonding and the highest melting point of metals have to be found in the third transition series where there is an additional bonding effect. gas Filling =localized=less strong bonding Increasing size= delocalization= increasing bonding metals

What is the work function EF EVac Sp-band Core level 1 Core level 2 Metal Adsorbed atom Free atom Work function Vacuum level Fermi level ea Ionization energy

Jellium model of a solid surface metal vacuum + - electrons ionic cores distance density + - dipole Notice exponential decay can be probed by STM

The solid surface The energy distribution of the electrons will follow a Fermi-Dirac Distribution since the electon gas is strongly degenerate, I.e. many electons on few states: where is the chemical potential of the electrons which at T= 0 K is Notice how this expression becomes the usual Boltzman distribution for large temperatues where the electron gas is getting diluted on many states

Free electron metals Typically for metals sp-bands like Ca, Mg, Al, Cu, K, Cs; ect. Evaporation of electrons Increase T or lower F When does metals melt?

The solid surface k will be continous and they will occupy a sphere in k-space with radius kF and volume .

The Tight Binding Model LCAO Where F are the individual atomic wave functions a e0 Core levels Construct Block waves

The Tight Binding Model Anti-bonding states Bonding states

Metals vs. Insulators E Why is a metal a conductor? EF Evac Valence band Conduction Free electron metal Transition metal Insulator sp-band d-band E Why is a metal a conductor? Why are metal reactive? Why may insulatords break down at elevated temperatures? Band gap

Simple model of transition metals eF E DOS sp-band W W/2 d-band There will however always be a repulsive Term proportional to the overlap:

Simple model of transition metals There is an attractive term from the sp electrons roughly 5eV. Why do we get a maximum? Why does the interaction increase with increasing d number? Which metals have the highest melting points?

The Newns-Anderson Model What happens when an atom with an energy level Fa, ea approaches a surface with valence electron at Yk, ek ? Fa, ea EF Yk The affinity level becomes filled when crossing EF Fa, ea na(e)

The Newns-Anderson model Now we should try to explain what happens when an atom approaches a metal surface When a simple atomic level approaches a surface with a constant sp-band will the level broaden: Why is it broadened?

The Newns-Anderson model When a simple atomic level approaches a surface with a simple sp-band will the level broaden and be lowered in bonding energy, I.e. be chemisorbed

The Newns-Anderson model If we again have a narrow atomic level approaching a broad sp-band on which there is superimposed a narrow d-band will we again see a broadening and a lowering, but when the interaction to the d-band is turned on will we have a splitting into bonding and anti-bonding levels. Non-bonding orbitals

Summary of The Newns-Anderson model Simple sp-band results in broadening and lowering: Evac EF Metal Adsorbate levels Distance from surface

Summary of The Newns-Anderson model Narrow d-band on top of sp-band results in splitting: Evac EF Metal Adsorbate levels Distance from surface

Summary of The Newns-Anderson model For molecules this may mean that the anti bonding orbitals get occupied resulting in bonding to the surface, but weakening of the internal bonding the essence of catalysis sp-band Evac EF Metal Adsorbate levels Distance from surface d-band s s*

Free electron metals Cl orbital is occupied and it will be Cl- Li 2s is not populated so it will be positively charged Li+

Free electron metals + - E + - E

Trend in atomic chemisorption energies

Trend in atomic chemisorption energies Attractive part Repulsive part

Trend in atomic chemisorption energies will be fixed for fixed adsorbate and fixed site and will reflect a proportionality for different adsorbates x Vad2 or b2 decreases with filling because d orbital becomes more localized, while increases with n since they become more extended

Trend in atomic chemisorption energies Remember Ebond=Esp+DEd-hyp Since Esp~-5 eV we will start out with strong bonding that decreases with incresing f (filling of d-band) f=1, n=5 Why is Gold so noble?

Trend in molecular chemisorption energies

Trend in molecular chemisorption energies For the late transition metals (from Fe,Ru,Os) Attractive part Repulsive part Not as easily interpreted as the atomic trends, but CO adsorbs stronger when going to the left, at least to the middle part of the TM

Trends in surface reactivity For the late transition metals the 2p is more important than the 5s The bonding increases when going to the left since the d-band moves upwards faster than f decreases If we look at a fixed f then we can evaluate the influence of strain or compression of a metal

Trends in surface reactivity EF Ed p*

The overall Picture Physisorption Molecular Chemisorption Atomic Chemisorption Reaction Desorption?

Density Functional Theory (DFT) z Repeat super cell in xyz Use a plane basis set for describing eletrons Solve the Schødinger equation in an approximate manner Accurate within 0.1-0.3 eV x

N2 approaching Ru(0001) Ex: DFT Calculation It costs 9.8 eV to dissociate N2 in the gas phase while on Ru(0001) it only cost 1.4 eV i.e. Catalyst

Trends in Reactivity The hight in the transition state will basically go as the molecular bonding energy. For the late transition metals: Cu, Ag, and Au cannot dissociate CO Ni can barely, but Pd and Pt cannot. In principle they should be able to do this by the final state is bonded to weakly (I.e. C and O) So this is a final state effect.

Sabatier´s Principle Perfect Too Reactive Too Noble

Separating of electronic and geometric effects Geometrical

Active Sites on Ruthenium Surface Particle

Size Distribution Before and After Annealing B5 sites, (110) geometry B5 sites, (311) geometry 600 particles analyzed TEM PDSP01 Ru/MgAl O 2 4 P: 50 bar ; Temp.: 475 °C gas: H :N = 3:1 2 2 170 hours at 500 °C. 6,0 2½ hours at C. H. Jacobsen, S. Dahl, P. L. Hansen, E. Toernqvist, H. Topsøe, D. V. Prip, P. B Møenshaug, and I. Chorkendorff, J. Mol. Catal. A: Chemical 163 (2000) 19. 5,0 500 °C Activity [mmol/(g*s] 4,0 3,0 50 100 150 200 250 300 Time [h]

Designing catalysis by alloying? Ni, Cu / Fe Cu / Co Cu, Pd, Ag / Ru Cu, Pd, Ag / Rh Ag / Pd Cu, Pd, Ag, Pt, Au / Ir Ag, Au / Pt Ru, Ir / Co Fe, Ru, Rh, Ir, Pt / Ni Ni, Pt / Cu Fe, Co / Rh Fe, Co, Ni, Cu / Pd Co, Ni, Cu, Rh, Pd, Pt / Ag Fe, Co, Ru / Ir Fe, Co, Ni, Cu, Ru, Rh / Pt Fe, Co, Ni, Cu, Rh, Pd, Pt / Au Fe, Co / Cu Ru, Rh, Ir / Pd Fe, Ru, Ir / Ag Ru, Ir / Au Co, Rh, Pd/Fe Ag, Ir, Pt, Au / Fe Rh, Pd, Ag, Pt, Au / Co Cu, Pd, Ag, Au / Ni Ag, Au / Cu Fe, Co, Ni, Pt, Au / Ru Ni, Pt / Rh Au / Pd Curve shape + alloy - phase-separation substrate atoms segregate to the surface adsorbate atoms segregate to the surface Designing reactivity

Thermal Experiment: CH4 Dissociation at 530 K 2 min annealing to 1100 K 3e-7 Initial sticking probability 700 K flash 2e-7 1e-7 1 2 3 4 Ni Coverage [ML] R. C. Egeberg and I. Chorkendorff, Catal.Lett. In press 2001.

Universality in Heterogeneous Catalysis Brøndsted-Evans-Polany (BEP) relationship: There is a linear relation between bonding energy and activation energy barrier. Why is it structure dependent? For a given substrate and site the TS all look alike Why is it adsorbate independent? In TS the molecules lose indentity Why is the relationship between the activation energy and the adsorption energy linear? When the molecule have lost identity in TS the energy is determined by the products

The Industrial Ammonia synthesis The Real Ammonia reactor The schematic reactor inlet outlet A B D C The equilibrium curve The optimum operating line

The Industrial Ammonia synthesis The equilibrium curve The optimum operating line C. H. Jacobsen J. Catal. 205 (2002) 382

The Concept of Optimal Catalyst Curve 2:1 80bar 420oC 3:1 200bar 450oC 5% 5% Defines optimal Catalyst 90% 90% High ammonia conc. 90% requires low bonding energy 2:1 80bar 420oC 3:1 200bar 450oC Low ammonia conc. 5% requires higher bonding energy Claus Jacobsen et al. J. Catal. 205 (2002) 382