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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


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...
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