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Atomic Tailoring of Catalyst Surfaces for High Selectivity
Partial Oxidation of Propane
J. Gleaves, R. Fushimi, G. Yablonaky, M. Rude, D. French, P. Buzzeta, S. Mueller, J. Swisher, J. Searcy Washington University
A. Gaffney The Rohm and Haas Company
Funded by the NSF’s GOALI (Grant Opportunities for Academic Liaison with Industry) Initiative
Scientific Motivation
The selectivity of a metal oxide catalyst is a function of its bulk
structure and surface composition. A variety of metal oxide crystal
structures exhibit catalytic activity, but their selectivity depends strongly
on the preparation procedure, which in turn influences the surface
composition and structure. At present, there is no way to predict how a
change in the preparation procedure will affect the surface composition,
and no way to predict which surface composition will give the highest
selectivity.
Research Objective
The focus of our current research is on the development of highly selective catalysts and the optimum process conditions for the
selective conversion of propane to acrylic acid. The project uses a novel new synthesis approach to create nanoscale gradients
in the surface composition of bulk industrial catalysts. The new approach will be generally applicable to catalytic processes
involving mixed metal oxide and supported metal catalysts.
Pulsed Oxidation of (VO)2P2O7 under Vacuum Conditions
Oxygen pulse response curves (≈1 x 1015 O atoms/pulse)
T = 450 °C
C3H4O2 + 2H2O
1
Oxygen Conversion
0.9
0.8
0.7
n O2
2-
Propane
After oxidizing a catalyst sample it was then exposed to a series of butene-argon pulses and
the transient response of either butene or one of several reaction products was monitored.
Selective oxidation of propane to acrylic acid
C3H8 + 2 O2
Preliminary experiments combining atomic beam deposition and TAP pulse response
experiments were performed using (VO)2P2O7 as a substrate. The base catalyst consisted of
200 m VPO particles that had been equilibrated for several hundred hours in a butane-oxygen
feed at reaction conditions. Surface modified catalysts were prepared by depositing copper
and tellurium atoms on a portion of the base catalyst. During the deposition process the
catalyst was maintained at room temperature. Coverages ranged from ~1015 to 1017 atoms per
sample, but the maximum coverage was always less than 1/100 of a monolayer.
Kinetic tests of unmodified and modified catalysts were performed by pulsing oxygen-argon and
butene-argon mixtures over 140 mg samples hled at 430 C. Catalyst samples were first
exposed to between 200 and 1000 oxygen pulses (pulse size 1015 molecules per pulse), and
then to 50 to 500 butene pulses. Each sample was exposed to a series of oxidation-reduction
cycles.
To develop a more detailed understanding of how the surface
composition of a catalyst influences its activity-selectivity we will focus on
changing the surface and near surface composition of mixed metal oxide
catalysts by changing either the oxygen concentration or the
concentration of one or more metal constituents. In effect, the bulk
crystal structure and its attendant electronic properties will remain intact
while the surface composition is altered.
Catalytic Selective
Oxidation-Reduction Cycle
Preliminary Experiments: Combining surface synthesis and
kinetic characterization
nO
4 2
Reactor equilibrated VPO
0.6
0.5
After several
oxidation-reduction
cycles
0.4
0.3
Propane
activation
site
Oxygen
activation
site
H+
a+
Ma
0.0
0
b+
Mb
Time (s)
0.1
0
Pulse
Number
4.0
0
0
100
200
300
400
500
Butene Reaction over VPO based Catalysts
Surface phase
VPO
Phase B
Acrylic
acid
0.2
500
Butene
conversion
n e-
Furan
production
R. K. Grasselli, Surface properties and catalysis by nonmetals,
1983, 273 -288
Atomic tailoring of technical
catalysts particles
0.0
Key Challenges
1. Uniform, precise coverage change
2. Kinetic analysis changes composition
Time
(s)
1.0
ROH
100
0.0
Time
(s)
1.0
RH
Pulse
Number
Furan
production
Butene
conversion
ROH
Transition Metal Concentration
Physical characterization
Changing the Surface Transition Metal
Composition of Bulk Catalysts
Creating Nanoscale Concentration Gradients of Transition Metal
Species on Bulk Metal Oxide Catalysts
Transition metal source
Nanoscale concentration gradients of
transition metal species on the surface of
metal oxide catalyst particles can be created
by immersing the particles in a dilute beam
Atomic beam
of transition metal atoms. The atoms are Laser beam
Catalyst
produced by focusing the light from a pulsed
particles
excimer laser onto the surface of a metal
target which is contained in a vacuum
Vibrate bed
chamber. The laser pulse ablates the metal
surface producing a pulsed beam of metal
atoms. The particles are contained in a
shallow tubular reactor that is continuously
(Vacuum - 10-8 torr)
agitated so that the surface of the particles Sample holder in
are randomly exposed to the atomic beam. transfer arm
After deposition, the catalyst particles are
transferred,
under
vacuum,
to
a
microreactor where they can be tested.
Changing the surface concentration of a transition metal species inConclusions
a bulk catalyst in a precise controllable manner is a much more difficult
Atom Deposition
problem. The addition of a metal by standard methods (e.g., incipient
wetness, CVD) generally involves a number of reaction steps that are not
well defined. With standard methods the change in the catalytic properties
of a catalyst cannot be directly related to the change in the transition metal
surface concentration.
The key experimental problem is to develop a method to add
different transition metal atoms to the surface of a catalyst in precisely
known amounts so that the change in the concentration of the transition
metal can be directly related to changes in catalyst performance.
Chamber
Cu pulses
Reaction
of
Butene
over
Surface
Modified
VPO
+ O2
Sample vibrator assembly
Transition metal source
Oxygen-enriched
nanolayer
+M
(VO)2P2O7
Metal-enriched
nanolayer
Gatevalve
(separates deposition chamber
and reactor chamber)
(VO)2P2O7
+ O2
Butene
Oxygen, metal-enriched
nanolayer
Furan
Time
(s)
1.0
Pulse
Number
100
Furan Yield versus Pulse Number
Comparison of the normalized furan yield as a
function of pulse number for reactor equilibrated
VPO, VPO modified with copper, and VPO modified
with tellurium. Furan production was determined by
calculating the moments of individual pulse response
curves. The furan yield changes with each butene
pulse as the VPO surface oxygen is depleted. The
rate of change is clearly altered by the addition of
metal atoms, and the maximum in the furan yield
occurs earlier in the reduction cycle on the metal
modified samples.
Two copper samples
Pulse number
Comparison of two different VPOCu samples prepared from the
same base sample and
approximately the same number of
Cu atoms.
Conclusions
Furan
(VO)2P2O7
0.0
Pulse Number
Laser beam path
Butene
1.0
100
.1s
POx, TRx, tRx
(VO)2P2O7
Time
(s)
Pulse
Number
Normalized yield
Submonolayer Change
in Surface Composition
Normalized yield
0.0
Metal Oxide
Particle
10
0
VPO - Cu deposition (Total coverage < .005 monolayers of Cu atoms)
Kinetic characterization
RH
Pulse
Number
This project is concerned with the development of highly selective catalysts for the
selective conversion of short chain hydrocarbons. To date the focus has been on the
development of a novel new synthesis technique that uses atomic beam deposition to
precisely alter the surface composition of bulk industrial catalysts.
It is important to note that the new synthesis technique is very general in nature, and that
any metal, including refractory metals (e.g., tungsten) can be deposited on practically any
substrate particle. Substrate materials include, but are not limited to metal oxide, and
supported metal catalysts, polymeric particles, ceramic particles, and particles with
semiconducting, superconducting, or photocatalytic properties, and particles that are
biologically active.
Heterogeneous Kinetics and Particle Chemistry Laboratory
New Technologies:
Microreactors and Bioreactors
http://crelonweb.wustl.edu
http://www.mikroglas.de
[email protected]