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Atom Deposition Chamber. P Ox , T Rx , t Rx. VPO. (VO) 2 P 2 O 7. Butene conversion. Furan production. Oxygen pulse response curves (≈1 x 10 15 O atoms/pulse). + O 2. Kinetic characterization. n 2. n 4. T = 450 °C. Normalized yield. Oxygen-enriched nanolayer. RH. RH. ROH.

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Heterogeneous Kinetics and Particle Chemistry Laboratory

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Heterogeneous kinetics and particle chemistry laboratory

Atom Deposition Chamber

POx, TRx, tRx

VPO

(VO)2P2O7

Butene

conversion

Furan

production

Oxygen pulse response curves (≈1 x 1015 O atoms/pulse)

+ O2

Kinetic characterization

n

2

n

4

T = 450 °C

Normalized yield

Oxygen-enriched

nanolayer

RH

RH

ROH

ROH

Butene

Butene

Furan

Furan

100

100

Pulse Number

Pulse Number

0.0

0.0

Time (s)

1.0

1.0

Time (s)

Submonolayer Change

in Surface Composition

(VO)2P2O7

(VO)2P2O7

+ M

Pulse Number

Metal-enriched

nanolayer

Vibrate bed

Physical characterization

Preliminary Experiments: Combining surface synthesis and kinetic characterization

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.

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.

Normalized yield

(VO)2P2O7

+ O2

Scientific Motivation

100

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.

Pulse Number

Two copper samples

100

0.0

1.0

Time (s)

Pulse Number

Oxygen, metal-enriched

nanolayer

C3H8 + 2 O2 C3H4O2 + 2H2O

0.0

1.0

Time (s)

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.

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.

Pulse number

500

Propane

O2-

O2

Oxygen Conversion

Pulsed Oxidation of (VO)2P2O7under Vacuum Conditions

Catalytic Selective Oxidation-Reduction Cycle

Pulse

Number

Reactor equilibrated VPO

0.00

4.00

Propane

activation

site

Time (s)

Oxygen

activation

site

After several oxidation-reduction cycles

H+

Ma

Mb

a+

b+

Phase B

Surface phase

Selective oxidation of propane to acrylic acid

Butene Reaction over VPO based Catalysts

n e-

Acrylic

acid

R. K. Grasselli, Surface properties and catalysis by nonmetals, 1983, 273 -288

Atomic tailoring of technical catalysts particles

Key Challenges

1. Uniform, precise coverage change

2. Kinetic analysis changes composition

VPO - Cu deposition (Total coverage < .005 monolayers of Cu atoms)

Metal Oxide

Particle

Furan

production

Butene

conversion

Transition Metal Concentration

Changing the surface concentration of a transition metal species in a bulk catalyst in a precise controllable manner is a much more difficult 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.

Conclusions

Changing the Surface Transition Metal Composition of Bulk Catalysts

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.

Comparison of two different VPO-Cu samples prepared from the same base sample and approximately the same number of Cu atoms.

Furan Yield versus Pulse Number

Cu pulses

Creating Nanoscale Concentration Gradients of Transition Metal Species on Bulk Metal Oxide Catalysts

Transition metal source

.1s

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 of transition metal atoms. The atoms are produced by focusing the light from a pulsed excimer laser onto the surface of a metal target which is contained in a vacuum 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 agitated so that the surface of the particles are randomly exposed to the atomic beam. After deposition, the catalyst particles are transferred, under vacuum, to a microreactor where they can be tested.

Sample vibrator assembly

Atomic beam

Laser beam

Reaction of Butene over Surface Modified VPO

Catalyst particles

Transition metal source

Conclusions

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.

Laser beam path

(Vacuum - 10-8 torr)

Sample holder in transfer arm

Gatevalve

(separates deposition chamber

and reactor chamber)

Atomic Tailoring of Catalyst Surfaces for High SelectivityPartial Oxidation of PropaneJ. 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 CompanyFunded by the NSF’s GOALI (Grant Opportunities for Academic Liaison with Industry) Initiative

Heterogeneous Kinetics and Particle Chemistry Laboratory


Heterogeneous kinetics and particle chemistry laboratory

New Technologies:

Microreactors and Bioreactors

http://crelonweb.wustl.edu


Heterogeneous kinetics and particle chemistry laboratory

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