Principles of heterogeneous catalysis

About 90% of all industrial processes involve heterogeneous catalysis, the

catalyst being typically a solid while the reactants are in gas or in the liquid

phase. The advantage compared to homogeneous catalysts is that they are

easier to prepare, handle and separate from the reaction mixture.

The catalytic process is run inside a reactor typically operated with

continuous flow under steady-state conditions. The rate is determined, continuous flow under steady-state conditions. The rate is determined,

apart from the nature of the catalytically active surface, by external

parameters like temperature, partial pressures and flow rate.

Usually heterogeneous catalysts consist of fine, often nanosized, powders

supported on nominally inert oxide substrates (Alumina, MgO,...) which

exhibit all possible crystallographic faces.

The catalyst is often a metal to which chemical and structural promoters or

poisons are added to enhance the efficiency and /or the selectivity.

A catalytic reaction is said to be homogeneous if the catalyst is in the

same phase as the reactants.

Examples:

1) the molecule chlorine-tris(triphenylphosphine)-rodium(I)[RhCl(PPh3)3]

is added to alkene solutions for hydrogenation reactionsis added to alkene solutions for hydrogenation reactions(Wilkinson catalyst)

2) Nickel(IV) acetylacetonate (benzene synthesis) 3) Dicarbonyl diiodine-iridium(III) (acetic acid synthesis, Cativa process) 4) di-cobalt(II) Octacarbonyl (hydroformylation of akenes to aldehydes)

A classical example are the iron powders used in the Haberprocess to enable the reaction of H and N to NH . The

A catalytic reaction is said to be heterogeneous if thecatalyst is in a different phase than the reactants, e.g.when the reactants chemisorb on a solid surface.The weakening of the internal bonds makes the formationof new bonds with other molecules easier. The productmust have a lower affinity with the catalyst in order to bereleased into the gas phase (desorption).

process to enable the reaction of H2 and N2 to NH3. Thetriple bond between the nitrogen atoms in the N2 moleculesis then broken by dissociative chemisorption.

Other Examples: 1) Pt/Rh (Ostwald process for the production of nitric acid)2) Titanium tetrachloride and an organometallic composite of Al (Ziegler- Natta process for polymerization)3) Chromium oxide for the Phillips process for the polymerization of ethylene to polyethylene.4) Zeolite ZSM-5 for hydrocarbon conversion.

2 CO + 2 NO 2 CO2 + N2

Pt/Pd/Rh

An example of heterogeneous catalysis from everyday lifeis the automobile’s catalytic converter transformingpoisonous gases like carbon monoxide and nitrogen oxides,generated from the combustion of gasoline, into much lessdangerous molecules like carbon dioxide and nitrogen.

:

The catalyst is composed by a thin metal film of Pt, Pd and Rh deposited on a ceramic surface. In this way the amount of expensive metal is minimized and the active surface maximized allowing to keep down costs at an affordable level.

The catalyst causes the dissociative adsorption of NO.The N atoms recombine to N2 which desorbs. The Oatoms react with adsorbed CO yielding CO2 whichdesorbs, too.

Industrial catalysis is still governed by empirical experience.

The single crystal surfaces studied under ultra high vacuum conditions,

necessarily differ from the real catalysis conditions. One speaks in

particular of material and pressure gaps.

The material gap can be bridged by studying the reaction for different

single crystal surfaces including surfaces with well defined defects such

as monoatomic steps or using particles with uniform particle size.

Analysis of the catalyst composition enables the deliberate modification

of the chemical composition studying the way in which the reaction

“digs its own bed”.

The reaction rate is determined by the surface concentration of the

reactants and by barriers to the adsorption which determine the

pressure gap. It can be bridged using lower temperatures to increase the

surface coverage and supersonic molecular beams to study the high

energy tail of the Boltzmann distribution of the reactants

Structure sensitivity arises for H2

dissociation on Fe

Concept of active site

Dissociation barrier for NO = 1,28 eV for the flat terrace

Dissociation barrier at the step = 0.15 eV

NO/Ru(0001)

Concept of active site

Dissociative adsorption at steps

Ag(410): O2ad + Oad

Ag(210): Oad only No O2 !!!!(L. Vattuone et al. PRL 90, 228302 (2003))

13

Pristine Ag(100): O2 only

Ion sputtered Ag(100):O2+Oad(L. Vattuone et al. J. Chem. Phys. 115, 3346 (2001))

Ag(410): O2ad + Oad (L. Savio et al PRL 87, 276101 (2001))

Poisoning of active sites by Sulphur segregation

CLASSIFICATION OF REACTION MECHANISMS

A(gas)+B(gas)→→→→C(gas)+(D(ad) or D(gas))

LangmuirLangmuir--Hinshelwood:Hinshelwood: both reactants are adsorbed at the surface and the reaction occurs between surface species

A(gas)+B(gas)→A(ad)+B(ad)A(ad)+B(ad)→C ↑ + D(ad) (or D ↑)A(ad)+B(ad)→C ↑ + D(ad) (or D ↑)

Four processes are involved: adsorption of A and B at the surfacediffusion of A(ad) and B(ad)reaction between A(ad) and B(ad)desorption of C

Example: Haber-Bosch synthesis of NH3;

EleyEley--RidealRideal: : only one reactant has to be adsorbed at the

surface

A(gas)→A(ad)

B(gas)+A(ad)→C ↑ +D (ad) (or D ↑)

Example: H(g) +D/Cu(111)→ HD ↑

H+Dad�HD energy balance: E(HD)=Ediss(HD)+Ekin(H) + Eads(D)=2.3 eV on Cu(111)

This energy is carried away by the products of the reaction which keep memory of

the velocity of the impinging H atoms

On Ni(110), however, also D2 is formed indicating that part of the impinging H

ends up in a translationally hot precursorwhich may transfer its energy to the D

atoms which then react by collision with other D adatoms.

Eley Rideal Mechanism

Eley Rideal Mechanism

Coadsorption

In the limit in which one can assume that the mutual interaction of the reactants

acts pairwise and independently with interaction energies: εAA , εAB , εBB one has:

If εAA + εBB - 2εAB <0 the two species will repel each other and one has

competitive adsorption

If εAA + εBB - 2εAB >0 the two species will attact each other and one has

cooperative adsorption

Competitive adsorption Competitive adsorption

may inhibit the sticking

probability of one of

the reactants, which

may be enhanced

adding a promoter

(electropositive atoms

like alkali atoms)

Coadsorption: Ethylidyne plus carbon monoxide on Rh(111)

Coadsorption O-Ag(110)

O-Ag(110)

Reaction with water to OH

Poisons and Promoters

Typically electronegative species act as poisons,

electropositive species as promoters

CO methanation on Ni (powders as well as supported clusters). Sulphur

attenuates the catalytic activity of Ni, 10 sites deactivated per S atom

Two possibilities:

1) S blocks 10 sites and the reaction sequence needs for a critical 1) S blocks 10 sites and the reaction sequence needs for a critical

step of the reaction

2) S has an electronic effect which extends on nearest neighbor

sites

If 1) holds changing the poison has little effect,

if 2) holds a less electronegative adsorbate has a smaller effect

phosphorus is less poisoning than S → electronic effect

Electropositive impurities:

Potassium may accelerate certain steps in a reaction.

CO methanation over Ni occurs by CO dissociation and subsequent

reaction of the carbidic carbon produced with H. The carbidic C coverage

saturates at 50% of a ML. Large H2 pressure needed to find sites where to

dissociate. dissociate.

The presence of K does not modify the reaction barrier, but

0.1% of K increase the carbon coverage in working conditions from 0.1 ML

to 0.3 ML on Ni(100).

The reaction is poisoned above T=650 K since carbidic carbon coalesces

into unreactive graphite.

Structural Promoter:

adsorbate which modifies the surface

structure to make it more or less reactive

Modification of the surface structure following adsorption

Adsorbate induce surface restructuring

Determination of the Reaction Rate

a) power laws

Partial pressure of reactants pi and products pj

r= k pAapB

b

useful approach but contains still no information about the progress of the

reaction

b) Attempt to describe the elementary steps: Langmuir approach

The various steps of the reaction are described in terms of rate equations for

adsorption , desorption and surface reaction.

If adsorption and desorption processes are fast then the partial coverages of the If adsorption and desorption processes are fast then the partial coverages of the

reactants are related to the relative partial pressures through adsorption

isotherms.

This approach may look hopeless given the various structural elements of a real

catalyst. However, it works, and the reason why it works is the same of why

catalysts are robust and can stand a wide range of operative conditions

CBA adad →+ k

BBAA

AAA

pbpb

pb

++=Θ

1 BBAA

BBB

pbpb

pb

++=Θ

1

CBA adad →+ k

BBAA

AAA

pbpb

pb

++=Θ

1BBAA

BBB

pbpb

pb

++=Θ

1

2)1( BBAA

BABABA

pbpb

ppbbkr

++=ΘΘ=

For constant T the reaction passes through a maximum since A blocks

the sites for the adsorption of B

c) Microkinetics parameters obtained by DFT calculations of the

elementary steps involved

d) Kinetic Monte Carlo: takes into account the actual neighborhood of the

adsorbed particle detailed insight but heavy computational effort.

the sites for the adsorption of B

Selectivity

A catalyst may lead to different products which then need to be

separated. The selectivity towards one particular reaction channel is thus

often more valuable than the overall reactivity.

Consecutive reactions A�B�C the reaction passes through a maximum

of B and, if this is the desired product it must then be interrupted at such

pointpoint

The reaction has two branches caracterised by reaction rates r1 and r2.

The selectivity is then s1= r1 / (r1 + r2 ) and can be affected by poisoning

one of the pathways

Ostwald process: Ammonia oxidation for the production

of Nitric acid

In industry the catalyst is Pt based and used at T>1000 K

Surface science demostrated that the reaction may

work at T=500 K on RuO2

Mechanism of heterogeneous catalysis

Catalyst consists of Fe3O4 with small concentrations of K, Al, Ca oxides

Surface composition is characterized by the segregation of the additives, which cahnges

further upon reduction.

The catalytically active area consists particles of about 30 nm diameter and a specific

surface area of 20 m2/g exposing metallic Fe covered with a submonolayer of chemisorbed

K and O. Al2O3 and CaO act as structural promoters against thermal sintering

L-H reactions HABER-BOSCH process for NH3 synthesis

N2+3H2 ↔NH3 ; ∆H=-46.1 kJ/mol <0 exothermic process

N2 gas ↔ N2 ad

H2 gas ↔ 2H ad

N2 ad ↔ 2N ad too high energy barrier, rate limiting step

Nad+H ↔ NH adNad+H ↔ NH ad

NH ad + H ad ↔ NH2 ad

NH2 ad +H ad ↔ NH3 ad

NH3 ad ↔ NH3 ↑

The best catalyst exhibits:

a) the highest dissociative adsorption probability for N2

b) The lowest adsorption energy for atomic N

N2 breaks more easily on Mo but the bonding of N with the metal is then too strong thus poisoning the catalyst.On Fe and Ru the dissociation reaction is less efficient but N is less strongly bound and reacts readily with H.

Optimal choice of the catalyst: Volcano plot

Active surfaces have

C7 sites !

Specific reactivity for ammonia synthesis

of Fe surfaces 20 atm and 800 K

N2 Sticking coefficient:

Fe(110) 7 x 10-8

Fe(100) 2 x 10-7

Fe(111) 4 x 10-6

Close to the industrial catalyst

Adsorption states of N2/Fe(111)

T<80K physisorption with molecular axis normal to

the surface γ state adsorption energy of 24 kJ/mol

and νNN=2100 cm-1 (for N15 )

T>80K chemisorption with molecules lying side on (π

bonded species) α state adsorption energy

31kJ/mole νNN=1490 cm-1

T> K dissociation νNFe=450 cm-1 β state

surface nitridesurface nitride

sticking probability of N2/Fe(111)

Full line activated indirect adsorption Eact=3kJ/mol

Dashes direct adsorption channel

Effect of K promoter

N2 Sticking increases to 4 x 10-5

Adsorption energy of α state increases from 31 to 44 kJ/mol

K and O are present in 1:1 ratio on the Fe surface.

They stabilize each other against desorption and reduction.

Since O is electronegative the promotion is effect is reduced but still relevant.

K moreover lowers the adsorption energy of NH3 favouring its desorption

H2 dissociation is easy and not rate limiting.

Since NH3 is the most stable hydrite there is no problem with selectivity

The proposed mechanism allows to calculate the ammonia

production rate up to 300 atm!

Alternative: Mittasch catalyst - alkali promoted Ru

UHV experiment Adsorption on MgO supported Ru at

atmospheric pressure

CO/Pt(111)

CO forms a c(2x4) layer

occupying both on top and

bridge sites which shift to on

top site in presence of O .

The reaction takes place at

the perimeter of the islands !

Microscopic mechanism of CO-O reaction

CO oxidation on Pt(110)

Adsorbate induced removal of the (1x2) reconstruction

CO oxidation proceeds as well on Pt, Pd and Rh but not on Ru at least in UHV.

The reason is due to the fact that at atmospheric pressure and high T Ru

transforms into RuO2, which is the true catalyst.

Theory: reaction

barrier 0.89 eV

Oxidation of Hydrogen on Platinum

coeff.diffusion D

production OHfor constant rate kwith

k

Dllength diffusion

3

3

D =

lD in the μm range

Numerical simulation of the H2 +O2 reaction on Pt(111)