Should Undergraduate Specialize

MSN 532: Selected Topics in Materials Science and Nanotechnology

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UNAM-Institute of Materials Science and Nanotechnology

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ADEM YILDIRIM

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NANOCATALYSTS

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Outline

 Catalysis
• Types of Catalysts • Examples of Heterogeneous Catalysis  Nanocatalysis • Preparation • Size Effects • Shape Effect • Support Materials

 Some Recent Advances • Nanocatalyst Preparation • Silica Supports • Carbon Supports

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Catalysis
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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 form intermediates that subsequently give the final reaction product, in the process regenerating the catalyst.

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(1) (2) (3) (4)

X + C → XC Y + XC → XYC XYC → CZ CZ → C + Z

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Y + X → XY

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Types of Catalysts
 Homogeneous catalysts • Homogeneous catalysts function in the same phase as the reactants.

 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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Examples of Heterogeneous Catalysis
 Hydrogenation • On solids, the accepted mechanism today is called the Horiuti1. Binding of the unsaturated bond, and hydrogen dissociation into atomic hydrogen onto the catalyst 2. Addition of one atom of hydrogen; this step is reversible 3. Addition of the second atom; effectively irreversible under hydrogenating conditions.

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Polanyi mechanism.

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Examples of Heterogeneous Catalysis
 Catalytic convertor • A catalytic converter is a device used to reduce the toxicity of emissions from an internal combustion engine.

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2CO + O2 → 2CO2 2NOx → xO2 + N2 CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O

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Nanocatalysis
• Nanocatalysis research can be explained as the preparation of heterogeneous catalysts in the nanometer length scale. • They are very promising and it can be expected that use of nanocatalysts can decrease the energy usage in the chemical processes results in a greener chemical industry. • Also they can be used for water and air cleaning processes and new generation fuel cells. • However, these new features come with new problems like, thermal stability and separation after reaction completed. • Parameters like surface area, activity, selectivity, longevity, and durability must be well 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 active components was to maximize the surface area exposed to the reactants, and thus minimize the specific cost per function.

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Nanocatalysis
 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 activity than that of the most common stable planes, such as {111}, {100}, and even {110}. • Because the high-index planes like; {210}, {410} and {557} have a high density of atomic steps, ledges, and kinks, which usually serve as active sites for breaking chemical bonds.

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

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reduction of organometallic compound solutions in the presence of surfactant molecules.

Nanocatalysis: Preparation 2. Vapor Deposition and Lithographic Methods:
• These methods are expensive and production of large amounts of catalyst is impossible and they are only used for kinetic studies and to identify the size and shape effects for metal nanoparticle catalysts.

Gold atoms on the (100) surface of Ni

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Titania-supported Au

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Nanocatalysis: Supports  Mesoporous Materials • A mesoporous material is a material containing pores with diameters between 2 and 50 nm.

• In designing and synthesizing new solid inorganic catalysts the aims are to maximize surface area, activity, selectivity, longevity, and durability.

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Mesoporous Silicas

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Nanocatalysis: Supports  Examples

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Some Recent Advances in Nanocatalysis ou rs e M

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Preparation of new NCs
• They describe a method for the synthesis of tetrahexahedral (THH) Pt NCs at 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 the reaction time.

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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Preparation of new NCs
 Characterization of Tubes  Structure and stability.
• They found tetrahexahedral PT particles nearly 2 fold active than the spherical

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ones • Particles are stable up 850 0C

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Preparation of new NCs

• They demonstrate the first synthesis of platinum, palladium, and silver nanotubes, 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 two different sized surfactants.

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• This fact seems to suggest that thin-walled metal nanotubes with diameters below 10 nm might be unobtainable because of their extremely high surface energies.

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

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Preparation of new NCs
 Preparation of Particles
In the typical fabrication process, the liquid crystalline phase of hexachloro platinic acid (H2PtCl6), nonaethylene glycol monododecyl ether (C12EO9), polyoxyethylene sorbitan monostearate (Tween 60) and water at a molar ratio of 1:1:1:60 was treated with hydrazine.

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Preparation of new NCs
 Characterization of Tubes  Structure.

TEM images of A) platinum, B) palladium nanotubes

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Silica Supports
• The role of each component in these catalysts, metal particles, oxide supports, and their interface must be understood, requiring not only catalyst synthesis but characterization and performance in designated catalytic reactions. (reactivity studies) • Parameters to control in transition metal heterogeneous catalysts include; -Particle composition, Size and shape, Support composition and pore size • They introduce a synthetic procedure to generate hexagonal structures (SBA15) in neutral pH conditions in the presence of Pt nanoparticles.

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distribution, Organizational structure of the porous network.

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Silica Supports
 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 bars represent 40 nm.

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Silica Supports
 Catalytic Activity.
Ethylene Hydrogenation Turnover Rates and Kinetic Parameters on Pt Catalysts

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Silica Supports
• Colloidal nanoparticles are usually prepared in the presence of organic capping agents, such as polymers or surfactants, that prevent the aggregation of nanoparticles in solution.

• Many industrially important catalytic processes, including CO oxidation, partial oxidation and cracking of hydrocarbons and combustion reactions, are carried out at temperatures above 300 C.

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

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Silica Supports
 Preparation of Particles
• The Pt@mSiO2 coreshell nanoparticles were prepared by polymerizing the silica layer 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 10 vol% TEOS diluted with methanol was added to initiate the silica polymerization.

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

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Silica Supports
 Characterization of Particles • Structure.

TEM image of Pt nano crystals

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Thermal stability of Pt@mSiO2 nanoparticles. TEM images of Pt@mSiO2 nanoparticles after calcination at 350 0C (a,b), 550 0C (c) and 750 0C (d).

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Silica Supports
 Catalytic Activity of Particles

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• The activity of the Pt@mSiO2 catalyst was as high as that of TTABcapped Pt nanoparticles.

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• The design of Pt@mSiO2 coreshell nanoparticles enables the direct access of reactive molecules to the catalytically active core metals.

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• CO oxidation as a model reaction.

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Carbon Supports

• Mesoporous carbons (OMC) are obtained by nanocasting from ordered mesoporous silica as a mould.

• In the case of carbon, the resulting material has typically surface areas of around 600 m2 g-1 and a pore volume below 0.2 cm3 g-1.

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Magnetic separation

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

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

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Carbon Supports
 Preparation of Particles
(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.

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Illustration of the synthesis procedure A) ordered mesoporous silica SBA-15; B) carbon/SBA-15 composite; C) B with surfacedeposited cobalt nanoparticles; D) protected cobalt nanoparticles on C; E) magneticordered mesoporous carbon; F) Pd on E.

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Carbon Supports
 Characterization of Particles • Structure.

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

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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).

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

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Carbon Supports
 Catalytic Activity.
• The catalyst was used as a slurry in a small, well-stirred reactor that had a continuous supply of hydrogen, and the hydrogen consumption was monitored.

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

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Carbon Supports

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

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

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• 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

Carbon Supports
 Preparation of Particles

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

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Carbon Supports
 Characterization of Particles • Structure.

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

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Carbon Supports
 Characterization of Particles • Structure.

TEM image of (a) Au@HCMS polymer capsules with core diameters of 50 nm and shell thicknesses of 15 nm. (b) Au@HCMS carbon capsules with core diameters of 80 nm and shell thicknesses of 25 nm.

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Conclusion
 Size and shape controlled preparation of metal nanoparticles are very promising for greener heterogeneous catalytic reactions.  Size and shape effects of the particles, the kinetic pathways and selectivity of the particles must be completely understood. Support materials for these nanocatalysts must be well studied and role of them and their interfaces in the catalysis must be understood.

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