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A scientific investigation of a useful catalyst .

ByJakob Engbæk

2/19

Acid Rain

• SO2 + … → → H2SO4

NOx + … → → HNO3

The big question: What happened to the Acid Rain?

3/19

A catalyst for removing Sulphur from crude oil was invented and used

SO2 emissions from fuels

502005

10(?)2010

3502000

ppm S in dieselYear

New Sulphur regulations world wide EU :

S-polluted crude oil + H2(g) Desulfurized oil + H2S(g)10-50 bar

300-400 ºC

Natural Sulphur contens

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Different levels of investigation

Traditional studies New studiesAtomic level

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An early example: Mössbauer• The Co-Mo-S phase

Co at Mo sites.CoxMo1-xS2

• Activity of Co-Mo/Al2O3

proportional with Co concentration.

• Geometry on the surface is still unknown

• Reactive part or sites still unknown, has something to do with Co

S S

Mo Co

S S

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Traditional studies

Problems:• Low signals • Complex system• Average over “ large”

area Advantages:• Practical knowledge

(Closer to plant conditions)

A lot has been found.From this it is believed:•Co most likely at corners or edges.

•Active sites are vacancies connected to Co sites.

Result from average over “ large” area → likely local structure

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New Studies with an STM

But first:• What is an STM?• And how does an

STM work?

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New Studies with an STM Properties:• Conducting

substrate• Atomic scale• Real space • UHV conditions

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New Studies with an STM Properties:• Conducting

substrate• Atomic scale• Real space • UHV conditions

(2 kÅ)2

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Model system for MoS2 catalyst

MolecularAdsorption

Active Sites

Morphology?

Synthesize a model system under UHV conditions and use STM to explore the atomic details.

Promoters

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The Herringbone reconstructed Au(111) surface

Nucleation site at elbows for Ni, Co, Fe, Ru, Pd and Mo

bulged

pinched

[1 0]1

[11 ]2

[110]

[112]

(23x 3) unit cell

FCC

HCP

(23x 3) unit cell

Nucleation Site

[11 ]2

[1 0]1

(a) (b)

802 x 795 Å2, 141 x 135 Å2,

3000 x 3000 Å2,

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Mo Deposition on Au(111)

500 Å

• Mo coverage ~10 %

• High degree of dispersion.

• Self-assembled Mo nanoclusters, size 20-30Å.

• Well-defined nanometer spacing.

4035 x 4090 Å2,, insert 890 x 920 Å2

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Sulphating Mo on Au(111)• High degree of dispersion • MoS2 nanoclusters,size 20-30Å • Triangular morphology

807 x 818 Å2

Good model system for HDS catalysts

Mo deposited in H2S (10-6 mbar),Postannealing to 673K

41Å × 42Å

41Å × 42Å

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MoS2 Nanocrystal :

Edge Structure of MoS2 Triangles

Protrusions at edges imaged out of registry

with basal plane S atoms.

One-dimensionalElectronic

Edge States

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What is seen on the surface

Protrusions reflect interstitial region

MoMo

SSSSSS

SS SS

Nanosized, metallic, closed ”wire”

Simulation (DFT)

Mo edge with S dimers

Mo edge with S dimers in MoS2 reproduced by STM simulation

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Thiophene on MoS2 Nanoclusters

Low Temperature

40 x 43 Å2

Thiophene on Au substrate towards perimeter of cluster

Thiophene adsorbed on bright brim (edge state II)

Thiophene (C4H4S)

Size ~ 4 × 5 Å2

Substrate Temperature < 200K

The desorbtion temperature for both species is below room temperature

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S Vacancies at the Mo-edge- Catalytically Active Sites

Energy barrier for vacancy formation surmounted with pre-dissociated hydrogen

atoms at 673 K.

The S covered edges are stable, vacancies are active.

Side view Top view

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MoS2 nanoparticles promoted by Co

MoS2

Mo Edge

CoMoSS Edge

Mo edge

Cobalt S edge

Co inclusion

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References: • Hydrotreating catalysis, Science

and technology. H. Topsøe, B. S. Clausen, F. E. Massoth

• Infrared Spectroscopic Investigations of enviromental DeNOx and Hydrotreating catalysts. Nan-Yu Topsøe.(Doctor Technices thesis)

• Atomic-scale study of a Hydrodesulfurizations Model catalyst. J. V. Lauritzen (PhD Thesis)

• Hydrodesulfurization catalysts.Line S. Byskov (PhD Thesis)

• Structure and electronic properties of MoS2 and metallic nano-scale systems.Mikkel V.Bollinger (PhD Thesis)• S. Mørup et al. Journal of

catalysis 1981