Nanoreactor s

8/12/2019 Nanoreactor s

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NANOCATALYSTS

ADEM YILDIRIM

UNAM-Institute of Materials Science and Nanotechnology

MSN 532: Selected Topics in Materials Science and Nanotechnology


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Catalysis

The catalyst accelerate the rate of a chemical reaction(A B) without itself being consumed in the process.

Catalysts generally react with one or more reactants to formintermediates that subsequently give the final reactionproduct, in the process regenerating the catalyst.

Y + X XY

(1) X + C XC(2) Y + XC XYC(3) XYC CZ

(4) CZ C + Z


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Types of Catalysts

Homogeneous catalysts

Homogeneous catalysts function in the same phase as thereactants.

Heterogeneous catalysts Heterogeneous catalysts are those which act in a different

phases than the reactants. Heterogeneous catalysts are generally solids that act on

substrates in a liquid or gaseous reaction mixture. Most nanocatalysts are heterogeneous catalysts for example

metal nanoparticles.


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Hydrogenation

On solids, the accepted mechanism today is called the Horiuti-Polanyi mechanism.

1. Binding of the unsaturated bond, and hydrogen dissociationinto atomic hydrogen onto the catalyst2. Addition of one atom of hydrogen; this step is reversible3. Addition of the second atom; effectively irreversible underhydrogenating conditions.

Examples of Heterogeneous Catalysis


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Examples of Heterogeneous Catalysis

Catalytic convertor

A catalytic converter is a deviceused to reduce the toxicity ofemissions from an internalcombustion engine.

2CO + O2 2CO 22NOx xO 2 + N2

CxH2x+2 + [(3x+1)/2]O 2 xCO

2 + (x+1)H2O


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Nanocatalysis

Nanocatalysis research can be explained as thepreparation of heterogeneous catalysts in thenanometer length scale.

They are very promising and it can be expectedthat use of nanocatalysts can decrease theenergy usage in the chemical processes resultsin a greener chemical industry.

Also they can be used for water and air cleaningprocesses and new generation fuel cells. However, these new features come with new

problems like, thermal stability and separationafter reaction completed.

Parameters like surface area, activity,selectivity, longevity, and durability must bewell characterized.


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Nanocatalysis

Size Effects Technical catalysis has been concerned with small particles for a

long time.

The initial incentive to reduce the size of the particles of activecomponents was to maximize the surface area exposed to thereactants, and thus minimize the specific cost per function .


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Shape Effect The shape of the nanoparticle determines surface atomic arrangement

and coordination. For example, studies with single-crystal surfaces of bulk Pt have shown

that high-index planes generally exhibit much higher catalytic activitythan that of the most common stable planes, such as {111}, {100}, andeven {110}.

Because the high-index planes like; {210}, {410} and {557} have a highdensity of atomic steps, ledges, and kinks, which usually serve asactive sites for breaking chemical bonds.

Nanocatalysis


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Nanocatalysis: Preparation

1. Solution Method: Metal nanoparticles (Pd, Au, Co, Pt so on) are generally prepared by

reduction of organometallic compound solutions in the presence ofsurfactant molecules.


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Nanocatalysis: Preparation

2. Vapor Deposition and Lithographic Methods:

Titania-supported AuGold atoms on the (100) surface of Ni

These methods are expensive and production of large amounts ofcatalyst is impossible and they are only used for kinetic studies and toidentify the size and shape effects for metal nanoparticle catalysts.


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Nanocatalysis: Supports

Mesoporous Materials

MesoporousSilicas In designing andsynthesizing new solid

inorganic catalyststhe aims are to maximizesurface area, activity,selectivity, longevity, anddurability.

A mesoporous material is amaterial containing pores withdiameters between 2 and 50 nm.


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Nanocatalysis: Supports

Examples


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Some Recent Advances inNanocatalysis


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They describe a method for the synthesis of tetrahexahedral (THH) Pt NCsat high purity. The THH shape is bounded by 24 facets of high-index planes~{730}.

Size of particles can be tuned between 20-200 nm by simply changing thereaction time.

Preparation of new NCs

Size control of THH Pt NCs and their thermal stability.SEM images of THH Pt NCs grown at (A) 10, (B) 30, (C) 40, and (D) 50 min.


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Characterization of Tubes Structure and stability.


They found tetrahexahedral PT particles nearly 2 fold active than the sphericalones Particles are stable up 850 0C

f


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For metals important in catalysis and other nanotechnological fields,synthesis by using nanoporous polymer and anodic aluminum films astemplates led to gold, nickel and palladium nanotubes, but with innerdiameters as large as 10 100 nm.

This fact seems to suggest that thin-walled metal nanotubes with

diameters below 10 nm might be unobtainable because of their extremelyhigh surface energies.

They demonstrate the first synthesis of platinum, palladium, and silvernanotubes, with inner diameters of 3 4 nm and outer diameters of 6 7 nm,

by the reduction of metal salts confined to lyotropic mixed LCs of twodifferent sized surfactants.

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In the typical fabrication process, the liquid crystalline phase of hexachloroplatinic acid (H 2PtCl6), nonaethylene glycol monododecyl ether (C 12EO9),polyoxyethylene sorbitan monostearate (Tween 60) and water at a molar ratio of1:1:1:60 was treated with hydrazine.

Preparation of Particles


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TEM images of A) platinum, B) palladium nanotubes

Characterization of Tubes Structure.


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The role of each component in these catalysts, metal particles, oxidesupports, and their interface must be understood, requiring not only catalystsynthesis but characterization and performance in designated catalyticreactions. (reactivity studies)

Parameters to control in transition metal heterogeneous catalysts include;-Particle composition, Size and shape, Support composition and pore size

distribution, Organizational structure of the porous network.

They introduce a synthetic procedure to generate hexagonal structures(SBA15) in neutral pH conditions in the presence of Pt nanoparticles.

Silica Supports

Sili S t


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Characterization of Particles Structure.

TEM images of Pt /SBA-15 catalysts. (a) 1.7 nm, (b)2.9 nm, (c) 3.6 nm, and (d) 7.1 nm. The scale barsrepresent 40 nm.

Silica Supports

Sili S t


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Catalytic Activity.

Ethylene Hydrogenation Turnover Rates and Kinetic Parameters on Pt Catalysts

Silica Supports

Sili S t


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Silica Supports

Colloidal nanoparticles are usually prepared in the presence of organic cappingagents, such as polymers or surfactants, that prevent the aggregation of

nanoparticles in solution.

At high temperatures, typically above 300 0C, however, the organic cappinglayers can decompose and the metal nanoparticles can deform and aggregate.

Many industrially important catalytic processes, including CO oxidation, partialoxidation and cracking of hydrocarbons and combustion reactions, are carried outat temperatures above 300 C.

Silica S pports


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The Pt@mSiO2 coreshell nanoparticles were prepared by polymerizing the silicalayer around the surface of Pt nanoparticles using a sol gel process.

To a pH 10-11 solution of TATB caped Pt Nanoparticles, a controlled amount of 10vol% TEOS diluted with methanol was added to initiate the silica polymerization.

The as-synthesized Pt@SiO2 was calcined at 350 C or higher for 2 h in static airto remove TTAB surfactants to generate Pt@mSiO2 particles.

Silica Supports


Silica Supports


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TEM image of Pt nano crystals Thermal stability of Pt@mSiO2 nanoparticles. TEMimages of Pt@mSiO2 nanoparticles after calcinationat 350 0C (a,b), 550 0C (c) and 750 0C (d).

Silica Supports

Silica Supports


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CO oxidation as a model reaction.

The design of Pt@mSiO2 coreshellnanoparticles enables the direct

access of reactive molecules to thecatalytically active core metals.

The activity of the Pt@mSiO2catalyst was as high as that of TTAB-

capped Pt nanoparticles.

Catalytic Activity of Particles

Silica Supports

Carbon Supports


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Carbon Supports

Mesoporous carbons (OMC) are obtained by nanocasting from ordered

mesoporous silica as a mould.

Porous carbon materials combine chemical inertness, biocompatibility, andthermal stability, and are thus suitable for many different applications.

Carbons are notoriously difficult to separate from solutions. Magnetic silicagel can be synthesized by entrapment of magnetite particles in the forming gel.

In the case of carbon, the resulting material has typically surface areas ofaround 600 m 2 g-1 and a pore volume below 0.2 cm 3 g-1.

Magneticseparation

Carbon Supports


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(1) Selective deposition of magnetic nanoparticles,(2) Protection of the nanoparticles by a nanometer thick carbon layer.(3) Subsequent introduction of the catalytically active component.

Illustration of the synthesis procedureA) ordered mesoporoussilica SBA-15; B) carbon/SBA-15composite; C) B with surfacedepositedcobalt nanoparticles; D) protectedcobalt nanoparticles on C; E) magnetic-ordered mesoporous carbon; F) Pd on E.


Carbon Supports

Carbon Supports


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Carbon Supports


TEM images of Co OMC at different magnification: a) low magnification; b) high magnification.Arrows indicate hollow carbon shells left after the leaching procedure.

Carbon Supports


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After the addition of Co OMC to the Rh6G-containing solution, therewas a change from orange-red to colorless within minutes.

Carbon Supports

Characterization of Particles Magnetic Separation.

Rh6G aqueous solution (left) and after adsorption of the Rh6G on Co OMC and separation of the dye loaded Co OMC by a magnet (right).

Carbon Supports


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The catalyst was used as a slurry in a small, well-stirred reactor thathad a continuous supply of hydrogen, and the hydrogen consumptionwas monitored.

Catalytic Activity.

Carbon Supports

Hydrogenation of octene over 1% Pd-loaded Co OMC. Fist run, second run afterseparation of catalyst and new addition of octene, run to test how separable the catalystis after magnetic removal of catalyst and readmission of hydrogen.

Carbon Supports


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Carbon Supports

Layer-by-layer (LbL) self-assembly approach, which enables the fabrication

of many different core-shell materials.- Silicas, titanias, polymers, silica polymer nano composites, magnetic

materials.- Solid core spheres or hollow core spheres may be produced, depending on

core template removal.

They report on the fabrication of new silica templates with a solid silicacore/mesoporous shell, containing an Au nanoparticle within the silica core.

Carbon Supports


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Carbon Supports

Schematic Illustration for the Synthesis of Au@HCMS Polymer and Carbon Capsules

Carbon Supports


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Carbon Supports

Characterization of Au@SCMS silica templates with core diameter of 80 nm and shell thicknessof 25 nm: (a) SEM image, (b) TEM image, and (c) N2 adsorption/desorption isotherms and thecorresponding pore size distribution (inset).

Carbon Supports


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Carbon Supports

TEM image of (a) Au@HCMS polymer capsules with core diameters of 50 nm andshell thicknesses of 15 nm. (b) Au@HCMS carbon capsules with core diameters of80 nm and shell thicknesses of 25 nm.

Conclusion


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Conclusion

Size and shape controlled preparation of metal

nanoparticles are very promising for greenerheterogeneous catalytic reactions.

Size and shape effects of the particles, the kinetic

pathways and selectivity of the particles must becompletely understood.

Support materials for these nanocatalysts must be wellstudied and role of them and their interfaces in thecatalysis must be understood.

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