Nanostructured Catalysts
Abhaya K. Datye
University of New Mexico
Issues
• Control of surface composition
• Facile synthesis via self assembly
• Aggregation of nanoparticles
Control of Surface Composition
and Structure in Nanoparticles
• 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 Pd-Ag Catalysts
During Selective Hydrogenation
of Trace Acetylene in Ethylene
K. Lester, Y. Jin, H. Zea and A. K. Datye
University of New Mexico, Center for Microengineered Materials and
Department of Chemical & Nuclear Engineering, Albuquerque, NM 87131,
USA
E. G. Rightor1, R. J. Gulotty1, J. J. Maj1, J. Blackson1, M. Holbrook2 and C.
Michael Smith3 The Dow Chemical Company, 1Midland, MI, 48674,
2Plaquemine, LA 77565, 3Freeport, TX 77541, USA.
Financial support provided by the U. S. DOE, Office
of Basic Energy Sciences, grant DE-FG0398ER14917 and by the Dow Chemical Company
Restructuring of Pd-Ag Catalysts
During Selective Hydrogenation
of Trace Acetylene in Ethylene
•
•
•
•
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 Æ C2H4 + H2Æ C2H6
Ethylene selectivity is a key objective in this process.
Catalysts are subject to thermal runaway due to the exothermic
reaction
Operating 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
0.35
Pd/Al2O3
No pretreatment
Selectivity to
Oligomers
0.3
0.25
0.2
Pd+Ag/Al2O3
0.15
500 C Pretreatment
0.1
0.05
0
-25
-20
-15
-10
-5
0
5
10
Delta Temperature
15
20
25
High Temperature Reduction Causes a Drop in
Activation Energy for Ethylene Hydrogenation
on Bimetallic Pd-Ag catalysts
Selectivity = moles ethylene produced
moles acetylene reacted
C2H2 Æ C2H4 Æ C2H6
Arrhenius plot for 0.5 Pd - 0.5 Ag/SiO2
catalysts
0.5Pd-0.5Ag / SiO2 Catalysts - Selectivity Vs
Delta Temperature.
-13
Eactivation (kcal/mol) = 20.5 + 0.4
0.5
0
ln(Ethene formed)
Ethylene Selectivity
1
-0.5
-1
-1.5
-2
-15
-17
Eactivation (kcal/mol) = 13.5 + 0.3
-2.5
0
10
20
Delta Temperature (oC)
Reduced at 500 C
Reduced at 100 C
30
40
-19
0.0024
0.0026
0.0028
1/T (1/K)
Reduced at 500 C
Reduced at 100 C
∆T = reaction temperature – clean up temperature
Clean up temperature is the temperature at which 99% of acetylene conversion is
obtained
Ethylene Hydrogenation Is A
Structure-Insensitive
Reaction
Why should the activation energy
for ethylene hydrogenation be
affected by pretreatment?
Effect of CO adsorption on
Activation Energy for Ethylene
Hydrogenation
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 desorption
of CO can change the activation energy for
ethylene hydrogenation
On Pd/SiO2, CO is
adsorbed mainly in a
bridged mode
There is no effect of
pretreatment
Bridge
Bridge
Linear
Reduced at 70 C
Linear
Reduced at 400 C
On Pd-Ag/SiO2, we see more
linearly bound CO than
bridged. High temperature
reduction further affects the
relative concentrations of linear
vs bridged CO
Bridge
Linear
Bridge
Linear
Reduced at 70 C
Reduced at 400 C
Effect of Reduction Temperature
Pd-Ag alloy, with some phase
segregation
Ag redistributes causing a
breakup of the Pd ensembles
Low temperature reduction
High Temperature
reduction
Pd/ SiO2
We see no effect of pretreatment on
ethylene hydrogenation activation energy
Activation Energy as
Activation Energy
(Apparent) kcal/mol
Pretreatment
a function
100 C of Pretreatment
500 C
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
bound than linearly bonded CO
Heats of Adsortion of the Adsorbed CO Species on the Various
Pd-Cointaining Solids at Low (E0) and High (E1) Coverage [1]
Linear CO species
Bridged CO Species
Sample
E0 (kcal/mol) E1(kcal/mol) E0 (kcal/mol) E1(kcal/mol)
Pd (Cl-free)/Al2O3
22
13
40
22
Pd (Cl)/Al2O3
22
13
40
18
Pd (Cl-f)/CeO2/Al2O3
22
13
40
22
Pd (Cl)/CeO2/Al2O3
22
13
40
16
Pd (Cl)/La2O3/CeO2/Al2O3
22
13
40
25
(Cl-f): Chlorine free solid
(Cl): Chlorine containing solid
[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
-13
ln(Ethane formed)
Eactivation (kcal/mol) = 20.5 + 0.4
-15
-17
Eactivation (kcal/mol) = 13.5 + 0.3
-19
0.0024
0.0026
1/T (1/K)
Reduced at 500 C
Reduced at 100 C
0.0028
Schematic of Restructuring
Phenomena in Pd-Ag
High Temperature Reduction
High Temp Oxidation
PdO
Ag
Pd-Ag alloy
Enrichment of Ag on Pd surface
Pd
Low Temp Oxidation
Ag2O
Summary
• High temperature pretreatments cause
restructuring of Pd and Ag
• Reducing the number of Pd nearest
neighbors affects selectivity to oligomer
formation
• By modifying the adsorption of coadsorbed
CO, we can control the activation energy
for ethylene hydrogenation and modify the
selectivity for the reaction
Aerosol Synthesis of Nanostructured Catalysts
Mangesh Bore, Hien Pham, Timothy Ward,
C. J. Brinker, Abhaya Datye
Financial Support provided by NSF – NIRT, Center
for Ceramic and Composite Materials and by the
Materials Corridor Council
Autoclave Route
Reaction Mixture
Autoclave
150 oC
48 hours
Filtration
Calcination
• 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 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 Structure
Y. 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)
Comparison
Aerosol Synthesis
Autoclave Synthesis
• Continuous process
• Batch process
• Reaction time seconds
• Reaction time hours
• Spherical particles
• Irregular shapes
• 3-D interconnected pore
structure (local order is
hexagonal)
• Most common is the 2-D
structure
TEM
Regular shapes
Particle consists of small ordered domains of pores
After Hydrothermal Stability Test at 750°C
SiO2
10% water vapor, 2 hours
Si/Al 20
Aluminum incorporation improves the hydrothermal stability of mesoporous silica
particles.
Hydrothermal Stability Test (batch vs. aerosol route)
10% water vapor, 2 hours
1600
Si/Al molar ratio 20
1400
Surface Area (sq m/gm)
1200
Batch Si-Al
Aerosol Si-Al
1000
800
600
Aerosol Silica
400
Davisil silica gel
200
Batch Silica
0
Initial
500
550
600
Temperature (C)
650
700
750
TEM/STEM images of Au/NH2-MCM-41
The average Au nanoparticle diameter is small (~1 nm), and
the nanoparticles are dispersed inside the pores.
3-aminopropyltrimethoxysilane is used as the amine source
TEM Images of Ordered Nanocrystal/Silica Nanostructures
Before calcination
[100]
A
20 nm
B
C
Courtesy of Hongyou Fan, Jeff Brinker
Sandia
National
Laboratories
Diffusion of Three-Dimensional Metal Particles
on an Oxide Substrate: Implications for the
Sintering of Heterogeneous Catalysts
Lani Miyoshi Sanders
Abhaya K. Datye
Univ. of New Mexico, Albuquerque, NM
Brian Swartzentruber
Sandia National Labs, Albuquerque, NM
Experimental Approach:
Atom-Tracking Scanning
Tunneling Microscopy of
Pd/TiO2(110)
Conventional STM
Si-Ge AdDimer on
Si(001)
Each image takes several
seconds, missing many rotation
events…
Atom Tracking
g
TipSTM
y
time resolution 1000x
t1
t2
x
Developed by Brian Swartzentruber, Sandia Labs
TiO2(110) Surface
3Åx6.5Å
Unit cell
100x100Å2
From: M.J.J.Jak, Ph.D. Dissertation, Nov.
2000.
Depositing
Pd
300x300Å2
Fast deposit @ 4 W, 2 s
300x300Å2
Slow deposit @ 3 W, 4 min
Diffusion
Characteristics
Presence of
small, mobile
particles
rapidly
decays due to
pinning and
growth
Diffusion is
essentially confined
to the [001] direction
Step
decoration is
prevalent only
on steps
perpendicular
to [001]
400x600Å2
Diffusing particles hop discretely with length
of underlying unit cell of substrate
Y (Angstrom)
Atom-Tracking of Pd
Particle Diffusion [001]
165
165
160
160
155
155
150
150
145
145
140
140
135
135
130
145
155
X (Angstrom)
130
100x100Å2
0
10
20
30
Time (s)
40
50
Diffusion Coefficient (cm^2/s)
Scaling Analysis for Pd Particle
Diffusion
1E-14
n=0.86±0.09
1E-15
1E-16
n=1.07±0.10
n=1.06±0.10
1E-17
1
10
Particle Size (# of Atoms)
100
42°C
36°C
25°C
3-D Monte Carlo model gives insights into
decreased motion of larger particles
disordered surfaces
hopping
d-1
high surface free energy
very small particles
shift in
diffusion
mechanism
grow
shift in
scaling law
larger particles
periphery diffusion
d-7
faceting
300x300Å2