NanostructuredNanostructured CatalystsCatalysts

Abhaya K. DatyeAbhaya K. DatyeUniversity of New MexicoUniversity of New Mexico

IssuesIssues

• Control of surface composition• Facile synthesis via self assembly• Aggregation of nanoparticles

Control of Surface Composition Control of Surface Composition and Structure in and Structure in NanoparticlesNanoparticles

• Selective catalysts often involve more than one element

• Thermodynamics, preparation variables, often dictate the surface composition and structure

• How do we generate tailored surface structures

Restructuring of PdRestructuring of Pd--Ag Catalysts Ag Catalysts During Selective Hydrogenation During Selective Hydrogenation of Trace Acetylene in Ethyleneof Trace Acetylene in Ethylene

K. Lester, Y. Jin, H. K. Lester, Y. Jin, H. ZeaZea and A. K. Datye and A. K. Datye University of New Mexico, Center for Microengineered Materials aUniversity of New Mexico, Center for Microengineered Materials and nd

Department of Chemical & Nuclear Engineering, Albuquerque, NM 87Department of Chemical & Nuclear Engineering, Albuquerque, NM 87131, 131, USAUSA

E. G. RightorE. G. Rightor11, R. J. Gulotty, R. J. Gulotty11,, J. J. MajJ. J. Maj11, J. Blackson, J. Blackson11, , M. HolbrookM. Holbrook22 and and C. C. Michael SmithMichael Smith33 The Dow Chemical Company, The Dow Chemical Company, 11Midland, MI, 48674,Midland, MI, 48674,

22Plaquemine, LA 77565Plaquemine, LA 77565, , 33Freeport, TX 77541Freeport, TX 77541, USA., USA.

Financial support provided by the U. S. DOE, Office of Basic Energy Sciences, grant DE-FG03-

98ER14917 and by the Dow Chemical Company

• Industrial feedstock for the production of ethylene polymers must contain no more than 5-10 ppm of acetylene.

• Selective Hydrogenation of acetylene is used to remove trace acetylene

C2H2 + H2 C2H4 + H2 C2H6

• Ethylene selectivity is a key objective in this process.• Catalysts are subject to thermal runaway due to the exothermic

reaction

Restructuring of PdRestructuring of Pd--Ag Catalysts Ag Catalysts During Selective Hydrogenation During Selective Hydrogenation of Trace Acetylene in Ethyleneof Trace Acetylene in Ethylene

Operating ConditionsOperating Conditions

• Our reaction conditions correspond to the ‘front end’ hydrogenation, where acetylene is present with a large excess of ethylene and also an excess of hydrogen and some CO.

• Feed:30% C2H4, 0.4% C2H2, 0.1% CO, 16% H2 and balance N2.

Hydrocarbon byproduct formation is suppressed on Pd-Ag after HTR

Selectivity to Oligomers vs Delta Temperature

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

-25 -20 -15 -10 -5 0 5 10 15 20 25

Delta Temperature

Sele

ctiv

ity to

O

ligom

ers

Pd/Al2O3

Pd+Ag/Al2O3

No pretreatment

500 C Pretreatment

High Temperature Reduction Causes a Drop in High Temperature Reduction Causes a Drop in Activation Energy for Ethylene Hydrogenation Activation Energy for Ethylene Hydrogenation

on Bimetallic Pdon Bimetallic Pd--Ag catalystsAg catalysts

Arrhenius plot for 0.5 Pd - 0.5 Ag/SiO2 catalysts

Eactivation (kcal/mol) = 20.5 + 0.4

Eactivation (kcal/mol) = 13.5 + 0.3 -19

-17

-15

-13

0.0024 0.0026 0.0028

1/T (1/K)

ln(E

then

e fo

rmed

)

Reduced at 500 C Reduced at 100 C

0.5Pd-0.5Ag / SiO2 Catalysts - Selectivity Vs Delta Temperature.

-2.5-2

-1.5-1

-0.50

0.51

0 10 20 30 40

Delta Temperature (oC)

Eth

ylen

e S

elec

tivity

Reduced at 500 CReduced at 100 C

C2H2 C2H4 C2H6Selectivity = moles ethylene produced

moles acetylene reacted

∆T = reaction temperature – clean up temperatureClean up temperature is the temperature at which 99% of acetylene conversion is obtained

Ethylene Hydrogenation Is A Ethylene Hydrogenation Is A StructureStructure--Insensitive Insensitive

ReactionReactionWhy should the activation energy Why should the activation energy

for ethylene hydrogenation be for ethylene hydrogenation be affected by pretreatment?affected by pretreatment?

Effect of CO adsorption on Effect of CO adsorption on Activation Energy for Ethylene Activation Energy for Ethylene

HydrogenationHydrogenation

If the surface is covered by CO, the activation energy for ethylene reaction is simply the heat of desorption of CO

Therefore, changes in the heat of desorptionof CO can change the activation energy for ethylene hydrogenation

On Pd/SiOOn Pd/SiO22, CO is , CO is adsorbed mainly in a adsorbed mainly in a bridged mode bridged mode There is no effect of There is no effect of pretreatmentpretreatment

Linear

Bridge

Reduced at 70 C Reduced at 400 C

Bridge

Linear

On PdOn Pd--Ag/SiOAg/SiO22, we see more , we see more linearly bound CO than linearly bound CO than bridged. High temperature bridged. High temperature reduction further affects the reduction further affects the relative concentrations of linear relative concentrations of linear vsvs bridged CObridged CO

LinearLinear Bridge

Bridge

Reduced at 70 C Reduced at 400 C

Effect of Reduction Temperature

Low temperature reduction High Temperature reduction

Pd-Ag alloy, with some phase segregation

Ag redistributes causing a breakup of the Pd ensembles

Pd/ SiOPd/ SiO22We see no effect of pretreatment on We see no effect of pretreatment on

ethylene hydrogenation activation energyethylene hydrogenation activation energy

Activation Energy as a function of PretreatmentPretreatment

100 C 500 C

Activation Energy (Apparent) kcal/mol 28 27

The apparent activation energy for ethylene hydrogenation on Pd is consistent with the heat of adsorption of CO. From the literature, the heat of adsorption for bridged CO ranges from 22-40 kcal/mol depending on coverage.

Bridged CO is more strongly Bridged CO is more strongly bound than linearly bonded CObound than linearly bonded CO

Sample E0 (kcal/mol) E1(kcal/mol) E0 (kcal/mol) E1(kcal/mol)Pd (Cl-free)/Al2O3 22 13 40 22Pd (Cl)/Al2O3 22 13 40 18Pd (Cl-f)/CeO2/Al2O3 22 13 40 22Pd (Cl)/CeO2/Al2O3 22 13 40 16Pd (Cl)/La2O3/CeO2/Al2O3 22 13 40 25

(Cl-f): Chlorine free solid (Cl): Chlorine containing solid

Linear CO species Bridged CO Species

Heats of Adsortion of the Adsorbed CO Species on the Various Pd-Cointaining Solids at Low (E0) and High (E1) Coverage [1]

[1] Dulaurent O, Chandes K, Bouly C and Bianchi D, Journal of Catalysis, v 192(#2), 2000

Arrhenius plot for Ethylene Hydrogenation on 0.5 Pd - 0.5 Ag/SiO2 catalysts

Eactivation (kcal/mol) = 20.5 + 0.4

Eactivation (kcal/mol) = 13.5 + 0.3 -19

-17

-15

-13

0.0024 0.0026 0.0028

1/T (1/K)

ln(E

than

e fo

rmed

)

Reduced at 500 C Reduced at 100 C

Schematic of Restructuring Schematic of Restructuring Phenomena in PdPhenomena in Pd--AgAg

Pd-Ag alloy

AgAgHigh Temperature Reduction

High Temp Oxidation

Low Temp Oxidation

Pd

Ag2O

PdO

Enrichment of Ag on Pd surface

SummarySummary• High temperature pretreatments cause

restructuring of Pd and Ag• Reducing the number of Pd nearest

neighbors affects selectivity to oligomerformation

• By modifying the adsorption of coadsorbedCO, we can control the activation energy for ethylene hydrogenation and modify the selectivity for the reaction

Mangesh Bore, Hien Pham, Timothy Ward,C. J. Brinker, Abhaya Datye

Aerosol Synthesis of Nanostructured Catalysts

Financial Support provided by NSF – NIRT, Center for Ceramic and Composite Materials and by the

Materials Corridor Council

Autoclave RouteAutoclave RouteAutoclave

150 oC48 hours

CalcinationReaction Mixture Filtration

• Liquid-Crystal Template Mechanism– Proposed by C. T. Kresge et al., Nature (1992)

J. S. Beck et al., J. A. C. S. (1992)

MCM-41

Irregular shapes

Aerosol RouteAerosol Route

Calcination

• Evaporation Induced Self Assembly (EISA)– Proposed by Jeffrey Brinker et. al., Nature (1999)– Evaporation of solvent leads to ordering of surfactant

structures– Condensation of silica follows the formation of templated

structures to lock in the structure

Control of Particle StructureControl of Particle StructureY. Lu, H. Fan, A. Stump, T.L. Ward, T. Rieker, C.J. Brinker,Nature 398 (1999) 223

Hexagonal nanostructure: interconnected hexagonally packed spherical pores, 1200 m2/g, d=3.2 nm (5 wt% CTAB)

cubic nanostructure: interconnected pores arranged in simple cubic lattice (4.2 wt% B56)

lamellar “onion-skin” structure: concentric shells of silica separated by pore volume, 478 m2/g, d=9.2 nm (5wt% P123)

ComparisonComparison

• Continuous process• Reaction time seconds• Spherical particles• 3-D interconnected pore

structure (local order is hexagonal)

• Batch process• Reaction time hours• Irregular shapes• Most common is the 2-D

structure

Aerosol Synthesis Autoclave Synthesis

Regular shapesParticle consists of small ordered domains of pores

TEM

Si/Al 20

After Hydrothermal Stability Test at 750°C 10% water vapor, 2 hours

Aluminum incorporation improves the hydrothermal stability of mesoporous silica particles.

SiO2

Hydrothermal Stability Test (batch vs. aerosol route) 10% water vapor, 2 hours

0

200

400

600

800

1000

1200

1400

1600

Initial 500 550 600 650 700 750

Temperature (C)

Surf

ace

Are

a (s

q m

/gm

)

Aerosol SilicaDavisil silica gel

Batch Silica

Aerosol Si-Al

Si/Al molar ratio 20

Batch Si-Al

The average Au nanoparticle diameter is small (~1 nm), and the nanoparticles are dispersed inside the pores.

TEM/STEM images of Au/NH2-MCM-41

3-aminopropyltrimethoxysilane is used as the amine source

TEM Images of Ordered Nanocrystal/Silica Nanostructures

B C

Sandia National Laboratories

20 nmA

[100]

Before calcination

Courtesy of Hongyou Fan, Jeff Brinker

Diffusion of ThreeDiffusion of Three--Dimensional Metal Particles Dimensional Metal Particles on an Oxide Substrate: Implications for the on an Oxide Substrate: Implications for the

Sintering of Heterogeneous Catalysts Sintering of Heterogeneous Catalysts

Lani Miyoshi SandersLani Miyoshi SandersAbhaya K. DatyeAbhaya K. Datye

Univ. of New Mexico, Albuquerque, NMUniv. of New Mexico, Albuquerque, NMBrian Brian SwartzentruberSwartzentruber

SandiaSandia National Labs, Albuquerque, NMNational Labs, Albuquerque, NM

Experimental Approach: Experimental Approach: AtomAtom--Tracking Scanning Tracking Scanning Tunneling Microscopy of Tunneling Microscopy of

Pd/TiOPd/TiO22(110)(110)

Atom Tracking Atom Tracking STMSTM g

Tip

x

y t1t2

Conventional STMConventional STM

Developed by Brian Swartzentruber, Sandia Labs

Each image takes several seconds, missing many rotation

events…

time resolution 1000x

Si-Ge Ad-Dimer on Si(001)

TiOTiO22(110) Surface(110) Surface

From: M.J.J.Jak, Ph.D. Dissertation, Nov. 2000.

3Åx6.5ÅUnit cell

100x100Å2

Depositing Depositing PdPd

300x300Å2

300x300Å2Fast deposit @ 4 W, 2 s

Slow deposit @ 3 W, 4 min

Diffusion Diffusion CharacteristicsCharacteristics

400x600Å2

Presence of small, mobile particles rapidly decays due to pinning and growth

Step decoration is prevalent only on steps perpendicular to [001]

Diffusion is essentially confined to the [001] direction

Diffusing particles hop discretely with length of underlying unit cell of substrate

AtomAtom--Tracking of Pd Tracking of Pd Particle Diffusion Particle Diffusion

130

135

140

145

150

155

160

165

145 155X (Angstrom)

Y (A

ngst

rom

)

130

135

140

145

150

155

160

165

0 10 20 30 40 50Time (s)

100x100Å2

[001]

1E-17

1E-16

1E-15

1E-14

1 10 100

Particle Size (# of Atoms)

Diff

usio

n C

oeffi

cien

t (cm

^2/s

)

42°C36°C25°C

Scaling Analysis for Pd Particle Scaling Analysis for Pd Particle DiffusionDiffusion

n=0.86±0.09

n=1.06±0.10n=1.07±0.10

33--D Monte Carlo model gives insights into D Monte Carlo model gives insights into decreased motion of larger particlesdecreased motion of larger particles

shift in diffusion

mechanismshift in

scaling law

faceting

very small particleshigh surface free energydisordered surfaces

hoppingd-1

grow

larger particles

300x300Å2

periphery diffusiond-7