Heterogeneous Catalysis by Supported Metals

1
Heterogeneous Catalysis by Supported Metals

• By far the most widely used industrial catalysts
  are heterogeneous formulations of inorganic
  solids. Zeolites and clays are widely used in
  cracking processes, and similar solids are
  employed as supports for finely dispersed metals.
• Catalytic converters use supported metals of the
  platinum group to promote both oxidation and
  reduction reactions of exhaust components.
• Catalytic chemistry? Design of materials?
  Optimization?

2
General Classification of Bulk Solids as Catalysts
3
General Classification of Bulk Solids as Catalysts
4
Industrial Processes Catalyzed by Inorganic Solids
5
Industrial Processes Catalyzed by Inorganic Solids
6
Heterogeneous Catalysis Topics of Study

• Characterization of the Surface of Catalytic
  Inorganic Solids
• Surface area, pore size
• Structural analysis techniques
• Kinetics of Heterogeneous Catalysis
• Simple kinetic analysis of unimolecular and
  bimolecular reactions on dispersed metal
  catalysts
• Control of Automobile Emissions
• Structure and composition of catalytic converters
• Legislated reactions and developments in catalyst
  technology
• Cat. converter design considerations

7
Physical Forms of Catalytic Surfaces

• Three broad forms of catalytic surface can be
  distinguished on the basis of general physicals
  structure.
• Non-porous, bulk-solid catalyst particle
• Pt metal foil
• Porous, bulk-solid catalyst particle
• Zeolites, silica, Raney nickel
• Supported catalyst with discontinuous coverage
  on the external surface of a support
  material.
• Pt on ceramic (cat. Converters), TiCl3 on MgCl2
  (Ziegler-Natta polymerization)
• The majority of heterogeneous reactions occur at
  the interface of the solid and fluid, with
  limited activity from the bulk solid phase.
  Exposed surface area per unit mass is therefore a
  key design parameter.

8
Surface Area Langmuir Isotherm

• The Langmuir isotherm is the simplest
  representation of adsorption equilibrium. Key
  assumptions include (Gates pp195-197)
• all adsorption sites are equivalent
• interactions between adsorbed molecules are
  negligible
• only one adsorbing molecule can be bonded to each
  site
• The rate of adsorption of A is
• qv fraction of vacant sites
• The rate of desorption of A is
• qA fraction of occupied sites
• At equilibrium, these rates are equal, and qv
  qA 1, giving us
• or
• KA kads/kdes (adsorption
• equilm constant)

9
Surface Area N2 Adsorption

• Knowledge of the volume of N2 required to
  establish an adsorbed monolayer on the solid (Vm)
  is sufficient to estimate the surface area of the
  solid.
• cross-sectional area of an adsorbed N2 molecule
  is taken to be 0.162 nm2
• Vm is usually estimated using the BET isotherm
• Areas derived from this procedure are useful
  indicators of available surface, but the activity
  of the surface for a particular reaction depends
  on the reactant which will have different
  adsorption characteristics.

10
Modeling of the Adsorption Behaviour of Solids
11
Typical Surface Areas of Catalysts and Supports
12
Pore Size Distribution

• The effectiveness of a catalyst particle is
  governed by the balance of reaction rates and
  transport processes. While surface area can be a
  useful indicator of available reaction sites,
  pore size and pore size distribution are key
  parameters in transport equations.
• Mercury penetration and N2 desorption
  measurements provide information on pore sizes,
  as shown below for alumina.
• Figure 6-4 Pore size distribution
  of g-AI203. Curve 1, distribution of
  macropore sizes as determined by
  mercury penetration curve 2,
  complete size distribution
  determined by a combination of
  Hg porisometry and N2 adsorption.

13
Zeolites Fuel Production by Catalytic Cracking

• Zeolites are crystalline aluminosilicates
• which, depending on the Si/Al ratio,
• template agents and preparation,
• have unique micropore structures.
• Virtually all acid catalyzed
  reactions can be conducted with
  acidic form of zeolites, provided the
  reactant is small enough to enter the
  pores.
• The acidic sites in HZSM-5 are
  strong enough to protonate paraffins,
  leading to widespread use as an
  industrial cat. cracking catalyst.

14
HZSM-5 Cracking Reactivity Si/Al72, 538C, 1 atm
a Zeolite Crystallite size
15
High Surface Area Forms of Metals

• Platinum blacks are finely divided powdered forms
  of the metal prepared by reduction of aqueous
  solutions of H2PtCl6.
• Raney metals are skeleton forms prepared by
  leaching out of Al from the binary alloy.
• Supported metals, as shown at
• right, are highly dispersed on an
• inorganic support. Once the
• support is impregnated with a
• suitable solution (often aqueous)
• of the precursor and dried,
• reduction (often with H2) of the
• metal on the surface generates
• metal crystallites.

Figure 6-38 Electron micrograph of a supported
metal catalyst, Rh/SiO2. The metal crystallites
are present on the surfaces of primary particles
of SiO2
16
High Surface Area Forms of Supported Metals

• Plots generated by data obtained for the
• chemisorption of hydrogen on a series of
• rhodium metal catalysts supported on
• silica at ambient temperature.
• The horizontal axis indicates the
• percentage of the solid material which
• is rhodium (dispersed on the surface).
• The left-hand vertical axis is the volume
• (V) of hydrogen adsorbed at _at_ STP by
• unit mass of solid.
• The right-hand vertical axis (H2 / Metal)
• is the number of hydrogen molecules
• adsorbed per rhodium atom present
• (surface and bulk), evaluated from the
  corresponding value of V.

17
Kinetic Treatment of Surface Reaction Data

• Relatively few industrial processes (ammonia and
  methanol syntheses aside) are operated under
  conditions that make chemical reaction rates of
  practical importance. In most cases rates are
  large enough to reach a mass transfer limitation
  or essentially complete conversion.
• Kinetics experiments have
• yielded important insight
• into plausible catalytic
• mechanisms, under the
• assumption that stage 3
• in the overall reaction is
• rate determining.

18
Langmuir-Hinshelwood Kinetics

• The langmuir-Hinshelwood treatment of
  heterogeneous reaction kinetics assumes that the
  rate is proportional to product of the fractional
  coverages (q) of the reactants, since these are
  proportional to the corresponding surface
  concentrations.
• In a bimolecular reaction, the reactants must
  compete for equivalent surface sites if this
  treatment is to be applied. The reaction rate
  will be
• which, after applying the Langmuir isotherm as a
  model of surface adsorption, yields
• In some cases, reactants are adsorbed onto
  different sites. Although strictly outside the
  bounds of this simple treatment, it is often
  applied in the absence of alternate
  techniques/information.

19
Langmuir-Hinshelwood Kinetics

• Idealized plot of the initial rate of
  isomerization of cis-2-butene to trans-2-butene
  on a silica-alumina catalyst at 358K as a
  function of the pressure of cis-2-butene. The
  heavy line corresponds to the pressure range
  accessible in the experimental study. The dashed
  extensions to extrapolation on the basis of the
  Langmuir adsorption
• isotherm.
• Apparent reaction orders
• (x) at various points
• are indicated.

20
Langmuir-Hinshelwood Kinetics Examples

• Tabulated below are several examples of rate
  expressions for surface catalyzed reactions and
  their apparent interpretations according to the
  Langmuir-Hinshelwood scheme.