sd talk 2012 v35 - uweburghaus.us · historic reference: this is a very old idea, see h.s. taylor,...

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North Dakota State UniversityFargo, North Dakota

Active sites in heterogeneous catalysis – the case of copper and gold catalystsUwe BurghausDepartment of Chemistry, North Dakota State Universityhttp://www.ndsu.edu/chemistry/people/faculty/burghaus.html

The concept of active sites in heterogeneous catalysis dates back to Langmuir’s days, and it was explicitly introduced as early on as 1925. Clear identification of the catalytically active sites on a solid catalyst surface, however, remains scientifically challenging. We tackeled this problem by using electron beam lithography, in order to fabricate silica supported Cu cluster samples. It turned out that the reactivity towards CO2 adsorption scales with the rim length of the clusters rather than their surface area. Molecular beam scattering, kinetics, and spectroscopic techniques were combined to characterize the active sites. The second system discussed is nanogold, supported on silica. Adsorption probabilities of CO depend distinctly on the Au cluster size, with a reactivity maximum at ~3 nm. This cluster size is consistent with the legendary reactivity enhancement seen for the CO-oxidation reaction. Again, that effect is related to active sites which here depend on cluster size.

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Active sites in heterogeneous catalysis – the case of copper and gold catalysts.Uwe BurghausDepartment of Chemistry, North Dakota State University

Evgueni Kadossov, Junjun Shan,Mallikharjuna Komarneni, AshishChakradhar, Jordan Schmidt,Stephano Cabrini, Scott Dhuey,Leonidas Ocola, Dan Rosenmann ,Ming Lu

Talk download: www.uweburghaus.us

You know what a surface is. You know what chemistry is.

Surface Chemistry

We are physical chemists / surface chemists

Surface & Chemistry

[email protected]


Last three years

DoE

DoE-EPSCoR

NSF-CAREER

General theme are supported cluster systems

I will highlight one “concept”: active sites

silica

[email protected]


Cu-EBL• Active sites in heterogeneous

catalysis

Au-PVD• How special is nanogold?

• Techniques

• Model catalysts


EBL: Electron beam LithographyPVD: Physical vapor Deposition

These are the measuring techniques we use

Thermal Desorption Spectroscopy

Molecular beam scattering

Modeling of the results

Joint projects

Ambient pressure GC reactor


We use kinetics techniques: thermal desorption spectroscopy

Polanyi Wigner equation-dN/dT = ν/β exp[-Ed/RTs]Binding energies

temperature

pres

sure


tem

pera

ture

time

pumps

mass spec.sample

We study gas-surface dynamics – adsorption probabilitiesNorth Dakota State UniversityFargo, North Dakota

θsat

Ads

orpt

ion

prob

abili

ty

Coverage (surface particle density)

Langmuirian adsorption dynamics

North Dakota State University

These are standard models for gas-surface dynamics

Ads

orpt

ion

prob

abili

ty


precursor assisted adsorption- Kisliuk’s model

Ads

orpt

ion

prob

abili

ty


adsorbate assisted adsorption

Industrial catalysts look like this

Metal nanoparticleson supports

CO

O2

CO2

metal nanoparticle

support

Photos from RUB, Germany

Surface ChemistryClass 6/7 - catalysis


Model catalyst

Metalcluster

Metal oxide support

Inverse model catalyst

Metal support

Metal oxide cluster

Bimetallic systems / alloys

Metal cluster

Metal cluster

Metal support

Metal oxide thin film

Thin films

Metal support

Support

Clusters

Model-Nano-Array Catalysts

Model systems for industrial catalysts look like this

scanning electron microscopy

I focus on one topic in this talk ...

Def.: Specific site on a catalyst where the chemical reaction takes place.Historic reference: This is a very old idea, see H.S. Taylor, Proc. R. Soc. Lond. Ser. A 108 (1925) 105

Key concept

Why is this important?

Molecular level understandingUnderstanding the mechanism of the reactionFormation of intermediatesDesigning better catalyst

Concept: active sites – from magic to a scientific disciplineSurface ChemistryClass 6/7 - catalysis

Common terms in this regardCUS sites: coordinately unsaturated sitesExample: vacancy sites

Lewis acid sites: CUS that behave as electron withdrawing sites (Lewis acid type center)Lewis base sites: electron donor sites (proton-abstracting sites) (Bronsted base center)

Active sites: real and recent examples

Scanning tunneling microscopy: J. Kibsgaard, J.V. Lauritsen, E. Laegsgaard, B.S. Clausen, H. Topsoe, F. Besenbacher, JACS 128 (2006) 13950

STM figures reproduced with permission of F. Besenbacher, Denmark

Hydrodesulphurization catalystre

activ

ity

cluster area rim length

reac

tivity

Active sites:NO BLACK MAGICIT’S REAL !

electrochemical hydrogen evolution reaction2H+ + 2e- H2

T.F. Jaramillo, K.P. Jorgensen, J. Bonde, J.H. Nielsen, S. Horch, Ib Charkendorff, Science 317 (2007) 100

PVD

Oxygen-Plasma-Assisted Molecular Beam Epitaxy

reflection high-energy electron diffraction gun

heated substrate

oxygen plasma source (activated oxygen radicals

and ions)

phosphor screen

effusion cells electron beam

evaporation sources

cryoshroud/baffles

atom specific

flux detection

by atomic absorption

Most common is Physical Vapor Deposition - PVD

Figure and photo fromZ.Q. Yu, S. Thevuthasan, L.V. Saraf - PNNL

support

Vapor

A fancy version of this atPacific Northwest National Laboratories


We also use Electron Beam Lithography - EBL

support

resistdeveloped resist

metal

write patternremove

developed resist

deposit metal remove resist

Electron beam writing!

Electron Beam Lithography

Berkeley Molecular Foundry

Stefano Cabrini

Jordan Schmidt

Student summer research

NDSU /Berkeley Molecular FoundryArgonne

Electron Beam Lithography

We looked atAuCuMoCuOxMoOx


We used these EBL samples to determine the active sites


• Nanocopper clusters on silica• EBL

Let’s start with copper


We can predict adsorption sites using EBL: copper on silica

We know all geometrical parameters:diameter = 11.9 ± 0.3 nmlattice constant = 100 ± 2 nmheight = 5 nm


statistical analysis


EBL allows for a priori knowledge of active sites

Scanning electron microscopy

970 960 950 940 930 920

0

3000

6000

9000

12000

15000

Cu 2p3/2

XPS

inte

nsity

(a.u

.)

binding energy (eV)

Cu 2p1/2

Cu

CuO

We know the chemical composition:Cu vs. Cu2O / CuO

X-ray photoelectron spectroscopy

We know all geometrical parameters:diameter = 11.9 ± 0.3 nmlattice constant = 100 ± 2 nmheight = 5 nm

rim lengthsurface area

= active sitescandidates


Sample cleaning conserves morphology

SEM after the experiments35 nm Copper clusters


Sample cleaning conserves morphology

SEM before and after the experiments77 nm Mo clusters

A

150 nm


That’s the prediction from the known geometrical parameters

63 nm / 12 nm 63 nm / 35 nm 35 nm / 12 nmrims 2.3 0.4 0.6 0.1 3.9 0.6terraces 13 1 1.2 0.3 10 2both 6.0 0.8 1.0 0.2 6 1

R = θsat(large)/θsat(small)

Terrace and rim

R = 6

Terrace

R = 13

Rim

R = 2.3

θsat: saturation coveragelarge vs. small clusters

geometrical prediction

The only problem left: measure θsat

We study gas-surface dynamics – adsorption probabilitiesNorth Dakota State UniversityFargo, North Dakota

θsat


That’s the experimental result

0 5 10 15 20 25 30 35 400.0

0.2

0.4

0.6

0.8

1.0

63 nm CuO

SiO2Si

1-ad

sorp

tion

prob

abili

ty o

f CO

2

time (sec)

Ei = 0.54 eVTs = 95 K

12 nm CuO

Molecular beam adsorption transients CO2 copper/silica

1 2 3 4 5 6

1.5

1.8

2.1

2.4

2.7

ratio

of Θ

63 /

Θ12

number of measurements

Statistical analysis

R = θsat(large)/θsat(small)


Experimental results ...

63 nm / 12 nm 63 nm / 35 nm 35 nm / 12 nmrims 2.3 0.4

2.0 0.30.6 0.10.8 0.2

3.9 0.62.7 0.3

terraces 13 1 1.2 0.3 10 2both 6.0 0.8

4.5 0.71.0 0.21.2 0.3

6 14.0 0.6 Experimental CO/CuO

Experimental CO2/CuOTheory

TheoryTheory

RimCO2/CuO

Terrace and rimCO/CuO


One more hint ... thermal desorption spectroscopy ...

100 150 200 250 300

β

CO

(m/e

= 2

8) T

DS

yiel

d (a

.u.)

temperature (K)

200100

520

1

60

χ (s)

0

αCO

100 120 140

CO

2 (m/e

= 4

4) T

DS

yiel

d (a

.u.)

temperature (K)

χ (s)

10030

41

10

0

CO2

Terrace and rimCO/CuO

RimCO2/CuO

Theory literature

Def.: Specific site on a catalyst where the chemical reaction takes place.

Key concept

Concept: active sites – from magic to a scientific discipline

Perhaps not that new, but ...STM vs. kineticsPVD vs. EBL


Copper is interesting because ...

Why Cu?

Methanol synthesis

Water splitting

Low temperatureCO oxidation

Methanol steam reforming

Water gas shift reaction

WhyCO & CO2 ?

Old concepts______________________________________________

Flue gas10-15% CO2

Air separation0.037%

MeOH

Natural gas(methane CH4)

coalGasification Syngas

(CO, CO2, H2)

CO2

Hydrogen

Water splitting

Surface ChemistryClass 12

New ideas_________________________________________________

Copper is interesting because ...

Methanol Economy

George Andrew Olah (born 1927 in Budapest). Hydrocarbon chemistryAwarded Nobel Prize in Chemistry in 1994

http://en.wikipedia.org/wiki/George_Olah

http://www.usc.edu/dept/chemistry/loker/faculty/Olah.html

From: G.A. Olah, A. Goeppert, G.K. Surya Parkash, Beyond Oil and Gas: The Methanol Economy. Wiley-VCH 2004

G.A. Olah et al.

“History” of energy production• Wood economy• Coal economy (industrial revolution, steam engines)• Oil economyWhat is next?:• Hydrogen economy• Methanol economy• Solar economy

One

of t

he so

urce

s I h

ave

used

to p

repa

re th

is c

lass

.

Other EBL projects ... (no details)

Alkanes and thiophene adsorption on Mo clustersEBL-Mo, EBL-MoOx

Gold clusters

PVD-Au EBL-Au

100 150 200 250 300 350

CO

TD

S yi

eld

(m/e

= 2

8) χCO (L)8421

0.50.20.1

αβ PVD

F

two peaksCO TDS

one peak

100 150 200 250 300 350

χCO (L)8421

0.50.10.05

CO

(m/e

=28

) TD

S yi

eld

(a.u

.)

temperature (K)

EBLα

Literature: Somorjai, Kasemo Jacobs, P. W.; Riberio, F. H.; Somorjai, G. A.; Wind, S. J., Catal. Lett. 1996, 37, 131Wong, K.; Johansson, S.; Kasemo, B., Faraday Discussions - 114 1996, 105, 237.

molecular adsorption of alkanes

EBL: Electron Beam Lithography PVD: Physical Vapor Deposition

adsorption probabilities of hydrogen are very small


Cu-EBL• Active sites in heterogeneous

catalysis

Au-PVD• How special is nanogold?

• Techniques

• Model catalysts



Here we just used physical vapor deposition (PVD)

Quantum size effects

Coordination

Oxygen activation

Surface defects

Quantum size effects

Coordination

Oxygen activation

Surface defects

For example:M. Haruta, Size and support-dependency in the catalysis of gold, Catalysis Today 1997, 36, 153 T.V. Choudhary and D.W. Goodmann, Oxidation catalysis by supported gold nano-clusters., Topics in Catalysis 2002, 21, 25 M.S. Chen and D.W. Goodman, The Structure of Catalytically Active Gold on Titania, Science 2004, 306, 252

Nanogold is interesting because ...

Gold

TiO2

Cluster size

Rat

e2 nm


Growth morphology of nanogold/silica ...

3D2D 2D

Nucleation Cluster growth

nucleating & growth

0 20 40 60 80 100

Au

(69

eV) A

ES in

tens

ity (d

N/d

E)

Au

(69

eV)/O

(503

eV

) AES

ratio

Au deposition time (sec)

Auger


2D


We can distinguish nucleation and growth regimes ...

100 150 200 250 300 350

0 10 20 30 40 50

TDS

peak

are

a (a

.u.)

Au exposure (sec)

CO

(m/e

= 2

8) T

DS

yiel

d (a

.u.)

temperature (K)

2

4

8

20

αβ

χAu(sec)

0 10 20 30 40 50

TD

S pe

ak a

rea

(a.u

.)


TDS


0 10 20 30 400

20

40

60

80

100

Au particle size (nm)

Num

ber o

f clu

ster

s

Cluster size of nanogold ...

scanning electron microscopy (SEM)

3.5 nm

10 sec Au exposure

AESTDSSEM

PVD-Au morphology EBL-Cu morphology

We know what the samples look like.

Most SEM images are from Ming Lu, Brookhaven National labs.


0 10 20 30 40 50

0.25

0.30

0.35

0.40

initi

al a

dsor

ptio

n pr

obab

ility

of C

O


Determining active sites ...


Large gold clusters are non-reactive.


Consistency with theory ...

Catalysis Today 72 (2002) 63–78

Sabine Schimpf, Martin Lucas, Christian Mohr, Uwe Rodemerck, Angelika Brückner, Jörg Radnik, Herbert Hofmeister, Peter ClausResult and image from


400 500 600 700 800 9000.25

0.30

0.35

0.40

0.45

initi

al a

dsor

ptio

n pr

obab

ility

of C

O

surface annealing temperature (K)

Consistency check ... sintering

Large gold clusters are non-reactive.


Nanogold: main result

0 10 20 30 40 50

0.25

0.30

0.35

0.40

initi

al a

dsor

ptio

n pr

obab

ility

of C

O


3.5 nm

Cluster size

Rat

e

2 nm

We measure active sites again.


?


nanogold

0 10 20 30 40 50 60 70 80 90 100 110

0.3

0.4

0.5

0.6

0.7

0.8

initi

al a

dsor

ptio

n pr

obab

ility

of C

O

Cu exposure time (min)

Ei = 0.39 eVTs = 90 K

nanocopper

Is nanogold special?

0 10 20 30 40 50

0.25

0.30

0.35

0.40

initi

al a

dsor

ptio

n pr

obab

ility

of C

O


CZM

First paper was perhaps this one: Y. Kim, M. Boudard, Langmuir 7 (1991) 2999

landing siteX

surface

diffusion zone

Concept: capture zone model and nano copperSurface ChemistryClass 6/7 - catalysis

0 10 20 30 40 50 60 70 80 90 100 110

0.3

0.4

0.5

0.6

0.7

0.8

initi

al a

dsor

ptio

n pr

obab

ility

of C

O

Cu exposure time (min)

Ei = 0.39 eV

Ts = 90 K

surface

clusters

adsorbates

surface

Diffusion rate: k E RTD D D= ν exp( / )

τ D Dk= 1/

Desorption rate:

Surface residence time:

k E RTd d d= ν exp( / )

τ d dk= 1/

See e.g. M. Bowker, P. Stone, R. Bennett, N. Perkins, Surface Science 497 (2002) 155

Size of capture zone Nhop d D= τ τ/

Concept: details of the capture zone modelSurface ChemistryClass 6/7 - catalysis


D.W. Goodman(1945-2012)

Θ


Interested in details?

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

CO

ads

orpt

ion

prob

abili

ty

CO coverage (ML)

0.09

0.39

0.86

χCu = 2.5 min

Ei (eV)


Interested in details?

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

CO

ads

orpt

ion

prob

abili

ty

CO coverage (ML)

0.09

0.39

0.86

χCu = 2.5 min

Ei (eV)

Ads

orpt

ion

prob

abili

ty


First paper was perhaps this one: Y. Kim, M. Boudard, Langmuir 7 (1991) 2999

landing siteX

surface

diffusion zone

Concept: capture zone model and nano copperSurface ChemistryClass 6/7 - catalysis

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

CO

ads

orpt

ion

prob

abili

ty

CO coverage (ML)

0.09

0.39

0.86

χCu = 2.5 min

Ei (eV)

Def.: Specific site on a catalyst where the chemical reaction takes place. Key concept:active sites.

Conclusions and summary

[email protected]

Copper catalyst

0 10 20 30 40 50

reac

tivity

for C

O a

dsor

pttio

n


Gold catalyst

So what?

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BRIM™ - discovering the edge

http://www.topsoe.com/Research/BRIM_story.aspx

Outlook

Images and text from Haldor Topsoe.

The team

[email protected]

Cu and Au projects - NDSUEvgueni KadossovJunjun ShanMallikharjuna KomarneniAshish ChakradharJordan Schmidt

Berkeley - samplesStephano CabriniScott Dhuey

Chicago - samplesLeonidas OcolaDan Rosenmann

Brookhaven - SEMMing Lu

Perhaps useful for student homework assignments:

http://www.uweburghaus.us/publications/publications.html

Identifying rims along nano-sized clusters as catalytically active sites – the case of CuOx/silica model catalysts nanofabricated by electron beam lithography,Chemical Physics Letters, 544 (2012) 70-72,by A. Chakradhar, J. Shan, M.R. Komarneni, M. Lu, U. Burghaus

Rim effects in the adsorption of CO2 on silica supported copper oxide clusters - utilizing electron beam lithography,Journal of Physical Chemistry C, in pressby J. Shan, A. Chakradhar, M. Komarneni, U. Burghaus

Adsorption dynamics of CO on copper and gold clusters supported on silica - how special is nanogold?,Chem. Phys. Lett., 517 (2011) 59-61,by J. Shan, K. Komarneni, U. Burghaus

Adsorption kinetics and dynamics of CO on silica supported Au nanoclusters-utilizing physical vapor-deposition and electron beam lithography Journal of Molecular Catalysis A: Chemical 321 (2010) 101-109by E. Kadossov, S. Cabrini, U. Burghaus

Short summaries

More details

http://www.uweburghaus.us/publications/NSFCAREERPapers.pdf Complete list of our EBL papers

Complete publication list

Electron beam lithographyModel nano-array-catalysts

13.8 nm Au dots 1) A layer of e-beam resist will be deposited on the substrate.2) Pattern will be written with the electron beam. 3) The pattern will be developed. 4) The exposed resist will be removed chemically.5) UV ozone plasma is used to clean the substrate. 6) With a metal evaporator, a thin layer of metal will be deposited on the patterned and 20° tilted substrate.

Tilting the metal evaporator reduces the spot size further. 7) The remaining resist will be removed chemically.8) The metal deposited on the surface initially forms small dots of variable size. 9) Annealing the sample collapses all the small dots in a single drop leading to an extremely

narrow particle size distribution and a patterned surface. 10) Argon ion plasma etching is used to further reduce the dot size.

Finally, dots smaller than 10 nm can be obtained.

7.5 nm Au dots

Samples and SEM images from S. Cabrini, Berkeley Molecular Foundry as part of a joint project

sd talk 2012 v35 - uweburghaus.us · historic reference: this is a very old idea, see h.s. taylor,...

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