Catalytic materials

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Catalytic materials

• Part of module Advanced course, Heterogeneous
  Catalysis
• Literature
• From Elaine McCash, Surface Chemistry Oxford
  Univ. Press 2001, 10 pages.
• From J. T. Richardson, "Principles of Catalyst
  Development," Plenum Press, NewYork NY, 1989,
  Chap. 4 33 pages.

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Heterogeneous catalysts examples

• Metals
• Dispersed on a support
• Porous (e.g.Raney nikkel)
• Bulk metals
• Oxides
• Single metal oxides
• Mixed oxides
• Zeolites
• Sulfides

• Acids
• Bases
• Carbides
• Nitrides
• Molten salts
• Anchored catalysts
• homogeneous complexes
• enzymes

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Ideal structures
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Surfaces are described using Miller-indices

• Find the unit cell and place the surface in an
  x-y-z- coordinate system
• Identify the intercepts with the x, y og z axes
• Identify the reciproke of the intercept
• If the intercept is a fraction multiply up to get
  integer indeces (usually called (hkl))

Animation from http//www.doitpoms.ac.uk/tlplib/m
iller_indices/lattice_index.php
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Typical low-index surfaces (most common)

• Examples of surfaces on a somple cubic crystal
• 100 010 001 (not shown)

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Sphere models

• Illustrates that
• 111 is most dense
• 100 is more open
• 110 is most open, with furrows between the rows
  in the top layer
• Key point The surface is different from the bulk
• Neighbouring atoms in a single direction only
  gives rise to the surface energy

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A metal particle (ideal) exposes different
crystal faces
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Unit-cellSmallest repeating unit that describes
the entire surface
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Distances and atom densities (atoms/surf.area)
can be determined and is tabulated for different
metals
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Real surfaces also expose steps, kinks and
missing atoms

• Surface defects are often important for catalysis
• Atoms in defect sites are even less coordinated
  (have fewer nearest neighbours) than atoms in
  dense faces
• Open to bond reactants

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Bonding (chemisorption) involves interacting
orbitals
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Metals Surface structure and atom distance can
explain bonding Example ethylene on nickel

• Ni-Ni 0.25 nm gives Ni-C-C 105
• Ni-Ni 0.35 nm
• gives Ni-C-C 123
• 100 and 110 is active, form reactive adsorbates
• 111 is less active

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Activity for ethylene hydrogenation
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Example Ammonia synthesis on Fe

• Depending on the crystal plane the activity is
  very different
• The 111-face is much more active than 100 and 110
• The real Fe catalyst can be modelled based on the
  activity of a Fe-111 single crystal and the
  fraction of the surface that exposes the 111 plane

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Real particles exposes many different faces and
edge/corner sites

• Real particles are non-ideal
• Cubo-octahedric particle is often used as a model
  for real particles
• Exposes 100, 110, 111 and a range of edges and
  corners

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Crystallite size is important

• With increasing particle radius
• Fraction low-index planes increases
• Fraction corner and adge sites decreases
• For some reactions this influences the activity
  (structure sensitive reactions)
• Other reactions occur with equal rates on all
  atoms (insensitive or facile reactions)

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Structure sensitve reaction cyclohexane
dehydrogenation/hydrogenolysis

• With increasing metal particle size
• Increasing dehydrogenation
• Favoured by low-index planes (dense planes)
• Hydrogenolysis decreases
• Favoured by corner/edge sites
• Can be used in catalyst design

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Large particlesTransition between dense planes
give rise to more active sites
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Oxides Semiconductors

• (n) og (p)-type semiconductors can be active
  catalysts
• Typical mechanisms involve red-ox processes -
  electron exchange

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Example Ethanol decomposition

• The Fermi-level increases from top to bottom in
  the table, leading to
• Dehydrogenation increases
• Dehydration decreases
• Same type of trend seen in other oxide-catalysed
  reactions, e.g photo-catalysis

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Insulators Solid acids and bases
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Acidity varies with materials
Silica-alumina is a strong acid
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Oxsides Zeolites

• Crystalline materials built by oxygen tetrahedra
  connected by metal atoms (Si and Al)
• Tetrahedra linked by common O-atoms to form
  octahedra (sodalite cages)
• Sodalite-cages are building blocks for a range of
  structures

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Example A-zeolite

• 4-ring openings
• Cavities with 0.7 nm openings
• Adsorbs water and other small molecules, used in
  separation, drying etc.

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Faujasite og mordeniteExamples of 12-ring
structures
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Simulation shows molecular sieve effect
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ZSM-53-dimensional network of pores
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3 different mechanisms for selectivity in zeolites

• Reactants selected due to size, large molecules
  are not transported into the structure
• Products can not diffuse out
• Transition state can not be formed