K A V I M O N I C A , V .M . T E C H . ( C A T A LY S I S T E C H N O L O G Y )

NANOPLATINUM IN CATALYSIS

NANOMATERIALS

• Materials with particles of nanometer size (1-100 nm)

• Unique properties at the nanoscale are the driving force for the exploitation of Nanomaterials.

• Nanoparticles have a high surface to volume ratio.

• As the crystal domains become smaller, the ratio of the corner to edge atoms to terrace atoms increases.

GENERAL PROPERTIES OF PLATINUMmetal Pt

Atomic number 78

Mass number 195.08

Electronic configuration [Xe]4f14 5d9 6s1

Structure FCC

Lattice constant(nm) 0.392

Metallic radius(nm) 0.1385

Density (g/cm3) 21.41

Melting temperature 1769

Boiling temperature 4170

First ionisation energy KJ/mol 866

Standard reduction potentials, V Pt4+/Pt 1.0Pt2+ /Pt 1.18

Conductivity m ohm-cm -1 94.34

NANOPLATINUM

• Platinum nanoparticles are usually in the form of suspension or colloid.• They can be made with sizes between 2-20 nm depending on

reaction conditions.• In the form of nanoparticles, platinum has a substantially

higher effectiveness because of the increased surface area.• At sizes of 10 nm, 20% of Pt atoms directly interact with

surroundings of nanoparticles.

VARIATION IN PROPERTIESMACRO TO NANO

Particle size of a material has significant influence on their properties.

Nanoparticles vary from their bulk counterpart in the following properties.

Electrical and electronic properties Mechanical properties Magnetic properties Chemical properties Thermal properties Optical properties

1.ELECTRICAL AND ELECTRONIC PROPERTY

• From continuous to discrete energy levels

CONTINUOUS TO DISCRETE

• The spacing between energy states gets larger as the volume gets smaller.• At the bottom, a single atom has just one energy

state per sublevel.• As atoms are group together to form particles,

there are as many splits per sublevel as there are atoms in a particle.• As the volume increases to the size of a solid, the

spacing between splits gets so tight that the sublevel is best characterised as a band.

DISCRETE ENERGY LEVELS

• Consider sodium atom with one valence electron.• At large separations the two electrons have the

same energy.• When two sodium atoms are close to each other,

these outer electron orbitals start to overlap. As a result, outermost energy subshell begins to split• At larger separations ,the two electrons have the

same energy.

SIZE INDUCED METAL INSULATOR TRANSITION

QUANTUM CONFINEMENT

• The charge and energy are quantised and confined(Quantum confinement)

• The spacing between adjacent energy levels in a band is given by approximate relationship

• 𝝈=• The spacing therefore increases as size decreases and when it

becomes greater than the thermal energy kT , the atoms behave as individuals and may lose its metallic properties.

• Conductivity decreases as size decreases.• Electronic structure is altered from continuous band to discrete

electronic levels

2.CHEMICAL PROPERTY

• Reactivity and catalytic property• It is mainly due to electronic and geometric effect• At the nanoscale, there is an increases in surface to volume

ratio• So significant proportion of atoms in nanoscale structures are

actually surface atoms.• Presence of dangling electrons and kinks (as the size is

reduced)exists.• Thus extent of chemical reaction that occurs on surface is

amplified.

REACTIVITY

• The catalytic activity in structure sensitive processes is directly related to the existence of sites with low energy electronic fluctuations.• The fluctuations are dominant in transition metal atoms of

high coordination number.• Different facets shows different reactivity and it increases in

the following order:• FCC(111)<FCC(100)<FCC(110)<FCC(hkl)

REACTIVITY

Facet Coordination number

(100) 8

(110) 7

(111) 9

Number of (hkl) faces for each shape.

Shape Number of (111) faces Number of (100) faces Number of (110) faces

Tetrahedron 4 0 0

Cube 0 6 0

Octahedron 8 0 0

Decahedron 10 0 0

Dodecahedron 12 0 0

Truncated octahedron 8 6 0

Cuboctahedron 8 6 0

Icosahedron 20 0 0

Guisbiers et al. Nanoscale Research Letters 2011 6:396  

CATALYTIC ACTIVITY

• The catalytic activation energy is the energy quantity that must be overcome in order for a chemical reaction to occur in presence of a catalyst.

• The low the catalytic activation energy is, the most active the catalyst is.

• It is thus an important kinetic parameter linked to the chemical activity

• The size-dependent catalytic activation energy, Eca could be obtained by,

• (Eca/Eca,∞)=(Tm/Tm, ∞)

• Therefore, catalytic activation energy decreases with size.

3.THERMAL PROPERTY

•=1- • =• D =size of the structure• A=surface area• V=volume• =bulk melting enthalpy(J/m3)• =surface energy in the solid and liquid

phase(J/m2)

SOLID SURFACE ENERGY OF PLATINUM

Guisbiers et al. Nanoscale Research Letters 2011 6:396

MATERIAL PROPERTIES OF PLATINUM

Materials properties Platinum

Tm,∞ (K) [40] 2,041.5

ΔHm,∞ (kJ/mol) [40] 22

ΔHsub, ∞ (kJ/mol) [41] 565

γl (J/m2)[40] 1.866

γs (J/m2) [13] 2.482

Guisbiers et al. Nanoscale Research Letters 2011 6:396

MELTING POINT

• Property is a consequence of average coordination number of participating atoms.• Surface energy corresponds to the number of broken bonds of

surface atoms relative to bulk situation.• Large surface is related to large surface energy.• The work necessary to increase the surface by adding atoms is

proportional to the increment of the area dA, the proportionality factor is the surface free energy• Dw=γdA

SIZE-DEPENDENT MELTING TEMPERATURE OF PLATINUM VERSUS THE SIZE FOR DIFFERENT SHAPES.

GUISBIERS ET AL. NANOSCALE RESEARCH LETTERS 2011 6:396

SIZE-DEPENDENT MELTING TEMPERATURE OF PLATINUM VERSUS THE SIZE FOR DIFFERENT SHAPES.

GUISBIERS ET AL. NANOSCALE RESEARCH LETTERS 2011 6:396

•  At the nanoscale, the shape which exhibits the highest melting temperature is the one which minimizes the most the Gibbs' free energy

(G = H - TS) and is then the favored one.• The four most-stable shapes among the ones

considered are 1. dodecahedron2. truncated octahedron3. icosahedron and 4. cuboctahedron.

4.MAGNETIC PROPERTY

• Platinum is actually paramagnetic • Superparamagnetism arises from structural

changes associated with size effects• When it is sufficiently small,it acts like single

magnetic spin that is subject to brownian motion.• Its response to a magnetic field is qualitatively

similar to response of a paramagnet but larger.

5.OPTICAL PROPERTY

• Absorption and scattering of light• Change in colour and transparency• Mainly due to two reasonsIncreased level spacing as the system becomes

more confinedSurface plasmon resonance

APPLICATIONS

• Automotive catalytic converters• Petroleum reforming catalysts• Electrocatalysts• Magnetic nanopowders• Polymer membranes• Cancer therapy• Coatings ,plastics, nanofibers and textiles• And in various chemical reactions

REACTIONS

• Pt is catalytically active for both oxidation and reduction reactions.• 2NOx → xO2 + N2

• 2CO + O2 → 2CO2

• CxH2x+2 + [(3x+1)/2]O2 → xCO2 + (x+1)H2O.

PEM FUEL CELLS

• The catalyst is a special material that facilitates the reaction of oxygen and hydrogen.• It is usually made of platinum nanoparticles very

thinly coated onto carbon paper or cloth.• The catalyst is rough and porous so that the

maximum surface area of the platinum can be exposed to the hydrogen or oxygen.• The platinum-coated side of the catalyst faces

the PEM.

PEMFC

• Hydrogen is oxidized at the anode into protons and oxygen is reduced at the cathode to produce water.• Both reactions can be catalyzed by platinum.• While hydrogen oxidation over platinum is

intrinsically very fast, oxygen reduction over platinum is very slow. • Carbon-supported platinum (Pt/C) catalysts have

higher active surface areas and are the materials of choice in today’s fuel cells.

PEMFC

CATALYTIC REFORMING

• Pt is used along with rhenium on alumina or silica support in petroleum reforming process (Platforming).• Catalytic reforming process involves

dehydrogenation, dehydrocyclization and isomerisation.• Hydrocracking and coking are the undesirable

reactions in the catalytic reforming process.

PT AS CATALYST

• Pt is catalytically active for oxidation reduction reactions.• Hydrogenation of cyclohexene to cyclohexane

and styrene to ethyl benzene.• Dehydrogenation of isobutane to isobutene.• Hydrogenation of benzoic acid to cyclohexane

carboxylic acid• Hydrogenation of dimethyl teraphthalate to 1,4-

cyclohexane dimethanol.

REFERENCES

• Greogory Guisbiers ,Size dependent catalytic and melting properties of Pt-Pd nanoparticles ,Nanoscale research letters .

• Nanotechnology: Principles and Practices by Sulabha K.Kulkarni,3rd edition.

• Nanoscience and Technology ,Nanocatalysis ,edited by U.Heiz and U.Landman.

• Nanotechnology -understanding small systems by Ben Rogers,Sumita Pennathur Jesse Adams,2nd edition.

• Nanomaterials by B.Viswanathan.

• Principles of Nanoscience and Technology by M.A.Shah Tokeer Ahmad.

• Rao Huang a, Yu-Hua Wen , Zi-Zhong Zhu and Shi-Gang Sun, Structure and stability of platinum nanocrystals: from low-index to high-index facets,J.Mater.chem., 2011, 21

• • Zhi-You Zhou , Na Tian , Jun-Tao Li , Ian Broadwell and Shi-Gang Sun Nanomaterials of high surface

energy with exceptional properties in catalysis and energy storage• Chem. Soc. Rev., 2011, 40