Computational methods in catalysis

Horia Metiu

Department of Chemistry and Biochemistry

University of California at Santa Barbara

Santa Barbara, California

University of California at Santa Barbara

There are two important things about a chemical reaction

Thermodynamic equilibrium (what is the maximum conversion?)

Kinetics (how fast the reaction takes place?)

A catalyst changes the rate of reaction

It is not consumed or created during reaction

It does not change the maximum yield.

Enzyme catalysis

Homogeneous catalysis

Electrocatalysis

Photocatalysis

Heterogeneous catalysis

I’ll focus on ordinary heterogeneous catalysis

It is “easier” than the other areas of catalysis

It is the most important for economy

There are many opportunities for scientific discovery

One can contribute to economy and make some serious money

How important is ordinary heterogeneous catalysis ?

►80 % of chemical industry, 20% of GDPfuels (gasoline, diesel, aviation, heating)commodity chemicals (sulfuric acid, methanol, ethylene, propene, vinyl chloride, ammonia)monomers for all plastics including rubber polymers (polyethylene)cosmeticsdetergentsfuel cells (polymeric and oxide)

Prior to the discovery of oil most chemical industry was based on coal

Germany

Catalyst development: a case study

How catalytic research is done

What one person working in this field can do

The initial use of oil: distillation. heavy fraction: lubricants and lighting. lighter fraction: useless

Cars were invented. gasoline became useful the low quality of gasoline was the limiting factor in engine development (natural oil does not have the right kind of hydrocarbons)

Enter Eugene Houdry

rich, French mechanical engineer, interested in car racing.

Lignite to gasoline: 1922-1929. Pilot plant produced gasoline but not economical (not cheaper, not better). French government withdraws funding.

1930-1933: Goes to US. Switches from lignite to oil. Sponsored by Vaccum Oil Company. After testing hudreds of compounds he finds an aluminosilicate catalyst that produced gasoline of good quality (need branched alkanes and aromatics)

1933 Depression: the company told Houdry to find another sponsor

1933-1937: Sun Oil Company funds his research. Houdry finds a method to regenerate the catalyst. In 1937 a commercial unit started working and producing high octane gasoline (winning WW2)

Houdry also discovered a catalyst for making butadiene from oil, whichis copolymerized to make rubber. Also credited with winning the World War II.

Catalytic converter. Not used because it was poisoned by tetraethyl lead. After tetraethyl lead was banned converters came into use.

Better life through chemistry for us and for Houdry

Great opportunities: Most of the products chemical industry makes now are derived from oil by catalysis:

We need to convert almost all chemical industry from

oil to natural gas, or coal or CO2. We need new catalysts

and new processes for doing this

Is this kind of work possible today?

YES

The methodology is still empirical but computation will play a major role

Catalysis community

The three communities in catalysis

Surface science research

Academic “practical” catalysis research

Industrial catalysis research

The “sociology” of catalysis research community

Computational work:

binding sites and binding energy

spectroscopy

activation energies for reactions

rate constant

kinetic simulation

The main goal is working with systems which provide detailed, firm knowledge.

Surface science:

well defined surface (faces of single crystal)

perfect surface cleanliness (pressure 10-10-10-11 torr )

large number of tools (electron spectroscopy can be used)

Practical catalysis research in academia and industry

Use powder catalysts often amorphous

Use atmospheric pressure or higher

don’t know surface structure (electron microscopy)

surface purity is an issue

Main work:

Making catalysts (pay attention: MgO)

Testing chemical performance

Physical characterization (IR, Raman, EXAFS, NEXAFS, XRD, X-ray fluorescence, atomic analysis, NMR, ESR, Mossbauer, UV-Vis)

Flow gases or liquids through a bed of solid catalyst

Molecules stick to the solid, react, desorb and exit the bed

What are the goals of academic “practical” catalysis research?

Find new and better catalysts

Improve existing catalysts (the 1 % crowd) (refineries)

Explain how existing catalysts work (it is sometimes useful: coking; Houdry)

Find a general organizing principle (explain)

Industrial research: money

conversion

rate

selectivity

long term stability

safety and environmental effects (Cd, Be)

cost (Ru for NH3)

heat management (the thresholds (methane,CO2), ethylene epoxidation)

Performance criteria for academic research in practical catalysis

Selectivity:

make only the compound you want

Partial oxidation of CH4:

methanol, formaldehyde, dimethyl ether, CO, CO2, H2O

CH2= CH2 and CH CH

Hydrogenate acetylene but not ethylene

H2 and CO

oxidize CO to CO2 but do not oxidize hydrogen to water

An exception from these rules:

catalysts that involve new principles or new classes of compounds

If you want to do computations to explain how a catalyst works look for:

important reaction for a product for which demand is growing

inexpensive and non-toxic catalyst

conversion over 40%

excellent selectivity

long lasting

How to model practical catalysts?

Ru catalyst supported on an oxide particle of oxide catalyst

Customary model

Highest area in a crystallite

Must be the most important face

Lowest surface energy

LaOCl(101)&

LaOCl(011)

LaOCl(100)&

LaOCl(010)

LaOCl(111)

LaOCl(110)

Es = 0.519 Es = 0.599 Es = 0.622 Es = 0.820

LaOCl(001)

Es = 0.106

LaOClLa (BSL)O (BSL)Cl (BSL)

CH4 + HCl + 1/2O2 = CH3Cl + H2O oxichlorination

MoS2 important catalyst for oil industry

Platelets

The face shown in the picture is the largest and it is inactive

Catalysis takes place at the edges.

Lesson 1: If the catalyst is crystalline you must consider all faces of the crystallite.

Lesson 2: Look at the role of defects: steps, missing atoms, small clusters on top of the surface (Taylor and active centers).

Oxide catalysts, oxidation reactions and Mars van Krevelen

Lesson 3: The catalyst is what becomes of the initial material in contact with the reactants and the products when it reaches the steady state.

CH4 + HCl + 1/2O2 = CH3Cl + H2O oxichlorination

People have used as catalyst

La2O3

or LaCl3

or LaOCl

Lercher & Dow Chemicals

Which of these is the catalyst?

None of the above

Do three experiments of methane oxichlorination simultaneously:

one with La2O3

one with LaOCl

one with LaCl3

After 10 hours of running the reaction you reach steady state: all three experiments have the same performance; the initial “catalysts” evolve to the same state which is the real catalyst

Lesson 3: The catalyst is what becomes of the initial material in contact with the reactants and the products when it reaches the steady state.

We expose the initial compound simultaneously to HCl and O2

Fact #1: LaCl3 and LaOCl react with O2 to make La2O3

and

Fact #2: La2O3 reacts with HCl to make LaOCl and then LaCl3

HCl puts Cl atoms on the surface and removes O atoms

O2 puts O atoms on the surface and removes Cl atoms

The surface is a mixture of O and Cl

The O to Cl ratio on the surface depends on the ratio of HCl and O2 in the gas and on temperature!!!!

This surface is the real catalyst; this is the surface to be modeled!!!

What to keep in mind when doing computational work:

we are working with imperfect models

we are working with an imperfect theory (DFT)

There are two extreme responses. Response 1:

work on well defined models for systems for which DFT is accurate (a small field; not many practical problems)

binding energies

activation energies and rate constants

spectroscopic quantities

Response 2: Learn how to do useful work in spite of inaccuracies.

Don’t try to get the exact activation energies: order the catalysts according to their activity and pass this information to you experimentalist collaborator.

Chemistry depends on differences in total energy ! binding energies, activation energies, vibrational frequencies

Find simple descriptor of the catalytic processmethane activation: activation energy of breaking the C-H bond

Trends in chemistry depend on differences of energy differencesCompare same reaction (e.g. H2 dissociation) on different catalysts (e.g. Pt, Pd, Ni, Cu). Compare activation energies forH2 dissociation on each metal.

The goal of such calculations is to increase the probability that the experimentalists find a good catalyst

It is very useful to be imbedded in an experimental group

A history lesson: ammonia synthesis

N2+ 3 H2 = 2NH3

Haber’s experiments in Karlsruhe: Osmium

Mittasch (BASF) 200 compounds; iron oxide with alkali; promoter

After Mittasch: 25000 catalysts have been tried; lower limit; We still use the Mittasch catalyst

Can we use theory to help us do better than this?

Professor: “remember now you got the brains”

The discovery of a new catalyst for ammonia synthesis

N2 + 3H2 = 2 NH3

(Norskov group + Topsoe)

N2 + surface = 2 N-surface

H2 + surface = 2 H surface

N-surface + H-surface = NH-surface

NH-surface + H-surface = NH2-surface

NH2-surface + H-surface = NH3-surface

NH3-surface = surface + NH3(gas)

Fact #1: Dissociation of N2 is the rate limiting step

Approximation #1: Bronsted-Evans-Polanyi

The activation energy for N2 dissociation is a linear function of the binding energy of N to the surface (importance; historical note)

Approximation #2: The stronger N binds to the surface the less reactive it is.

Consequences:

if N binds very strongly Evans Polanyi says that it is easy to dissociate N2 but N won’t react (poor catalyst)

if N does not bind strongly enough then you cannot dissociate N2 effectively (poor catalyst)

The best catalyst will bind N with moderate strength.

Note that because of these approximation all you need to do is calculate the binding energy of N !!!!

Calculate binding energy of N on all metals and alloys

Volcano curve: rate of ammonia production versus the binding energy of N to the metal catalyst.

Os, Ru best but Fe is used

Mo dissociates N2 easily but binds N too strongly: poissoning

Co binds N weakly but does not dissociate it efficiently

How about a CoMo alloy?

Topsoe: did experiments and found that

Co3Mo3N is as good as the best activated

Ru catalyst (which is too expensive to use)

Maybe this was a lucky accident!!

The same principle was used to find a new catalyst for the methanationreaction (conversion of CO and H2 to methane)

Best catalysts are Co and Ru but they are too expensive and are not used. The recommended catalyst was Ni

Ni is on one side of the volcano (below Co and Ru) and Fe is on the other side. A NiFe alloy is more efficient than Ni and cheaper.

The art of using DFT to contribute to catalysis science and practice

Catalysis offers computational chemists tremendous opportunitiesfor the next fifty years.

Make a reasonable model for the system

Find a simplifying descriptor of catalytic activity

Compare many catalysts for the same reaction

Give experimentalists systems to try and also systems not to try

Tell them that you are using intelligent, DFT-assisted guessing

This is better than trying 25,000 catalysts at random.

Problems and systems

Methane activation

stranded methane (flared or vented) turn it to liquids

methane as a raw material

now: methane to syn gas (CO + H2)

ammonia

methanol

gasoline

Can we convert methane directly (without syngas)?

methanol, formaldehyde, ethylene

yields are low, temperatures high, selectivity

CO2 utilization:

CO2 as oxidant (power plants):

CH4 +CO2 methanol, formaldehyde, CO + H2

Systems

cation doping

anion doping

oxide clusters on oxides

complex “oxides”: carbonates, phosphates, vanadates, molibdates, perovskites, etc

heterogenize homogeneous catalysts

make artificial enzymes

carbonic anhydrase with Zn cofactor

CO2 bicarbonate + protons

Protein

Active center

Pocket (channel)

Great selectivity

Do difficult chemistry (NH3)

Rate or structure simulations:

Difficulties:

flexibility, water matters, charge transfer is frequent; even remote proteins could matter

DFT for the active center

Classical treatment of protein and water (Monte Carlo or Molecular Dynamics)

Classical mechanics for protein and a continuum electrostatics for water

Blocking the pocket (Viagra)

Polymeric membrane electrolyte

Nafion

Carbon cloth anode Carbon cloth cathode

Carbon black particles

Pt catalyst at the cathode

Pt/Ru catalyst at the anodeH+

H+

H+

H+

H+

H2 or

CH3OH or

HCOOH

O2

What a fuel cell looks like: Electrocatalysis

Problems:

Pt and Pt/Ru too expensive and too rare

CO poisoning

Oxygen reaction too slow

Homogeneous catalysis: one phase

Wacker process:

H2C=CH2 + ½ O2 CH3CHO (acetaldehyde)

H2C=CH2 + ½ O2 CH3CHO (acetaldehyde)

Pd[Cl4]2- is considered to be the catalyst, but CuCl2 and H2O could also be considered catalysts.

Pd and goes back and forth between Pd2+ and Pd0: Cu is shifts from Cu2+ to Cu+; protons and Cl- are produced. Solvation is essential !!!

The reaction takes place efficiently at sites where the water molecules are in the right position. The water is part of the reaction coordinate

The ions interact with long range forces and these cannot be neglected

Photocatalysis:

TiO2 + organics use light and oxidize the organicSelf cleaning glassPaint on buildings