Heterogeneous Catalysis Steps
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Questions and Answers

What is the first step in the process of heterogeneous catalysis?

  • Reactants diffuse to the surface of the catalyst. (correct)
  • The intermediate compound gets desorbed from the surface.
  • The final products diffuse out of the internal pores.
  • Reactants form an intermediate compound.

    • What happens to the reactants in the third step of heterogeneous catalysis?

  • They form an intermediate compound. (correct)
  • They diffuse out of the internal pores.
  • They get desorbed from the surface.
  • They diffuse to the surface of the catalyst.

    • What is the molecularity of a reaction that involves one molecule?

  • Monomolecular (correct)
  • Poly molecular
  • Bimolecular
  • Termolecular

    • What is the rate of catalytic reaction proportional to?

    <p>The surface coverage.</p> Signup and view all the answers

    What is the kinetic order of the reaction when the pressure of reacting gas is very low?

    <p>First order</p> Signup and view all the answers

    What happens to the rate of reaction when the pressure of reacting molecule is sufficiently high?

    <p>It becomes zero order</p> Signup and view all the answers

    What is the last step in the process of heterogeneous catalysis?

    <p>The final products diffuse out of the internal pores and the external surface of the catalyst.</p> Signup and view all the answers

    What is a characteristic of poisoning of catalysts?

    <p>Minute quantities are able to inhibit catalytic reactions</p> Signup and view all the answers

    What is the rate of the catalytic reaction given as?

    <p>R = kb1/2P1/2</p> Signup and view all the answers

    What is the effect of a poison on a catalyst?

    <p>It reduces the effectiveness of the catalyst</p> Signup and view all the answers

    What is an example of a volatile compound that can poison a catalyst?

    <p>Hydrogen sulfide (H2S)</p> Signup and view all the answers

    What is the typical diameter of the microporous crystalline solids known as zeolites?

    <p>Smaller than 2 nm</p> Signup and view all the answers

    What is the classification of pores in a catalyst based on?

    <p>Their size</p> Signup and view all the answers

    What is the purpose of exchanging sodium ions with ammonium ions in zeolite synthesis?

    <p>To create acidic forms of zeolites</p> Signup and view all the answers

    What type of catalyst has a porous structure?

    <p>Porous, bulk-solid catalyst particle</p> Signup and view all the answers

    What is the most common use of synthetic zeolites?

    <p>As catalysts in the petrochemical industry</p> Signup and view all the answers

    What is an example of a supported catalyst?

    <p>Pt on ceramic</p> Signup and view all the answers

    What is the reaction that is inhibited by water vapor as a poison?

    <p>CH3CHO → CH4 + CO</p> Signup and view all the answers

    What is the chemical formula for wood, in terms of its cellulose and lignin components?

    <p>C21H32O14</p> Signup and view all the answers

    What is the product of heating ammonium-zeolite to 700 K?

    <p>Acidic forms of zeolites</p> Signup and view all the answers

    What is the chemical formula for charcoal?

    <p>C7H4O</p> Signup and view all the answers

    What is the purpose of slow pyrolysis in charcoal production?

    <p>To remove volatile constituents from wood</p> Signup and view all the answers

    What is the type of reaction involved in the decomposition of C15H32 into ethene, propene, and octane?

    <p>Cracking reaction</p> Signup and view all the answers

    What is the primary mechanism by which activated carbon removes contaminants from wastewater?

    <p>Physical adsorption</p> Signup and view all the answers

    What is the typical range of pore sizes in activated charcoal or activated carbon?

    <p>1 to 100 nm</p> Signup and view all the answers

    What type of activated carbon has a larger particle size?

    <p>Granular activated carbon</p> Signup and view all the answers

    What is the purpose of impregnated carbon containing I2, Ag, Al, Mn, Zn, Fe, Li, Ca?

    <p>Air pollution control in museums and galleries</p> Signup and view all the answers

    What is the catalyst used in the process of ammonia synthesis?

    <p>Iron</p> Signup and view all the answers

    What is the role of the active site in enzyme catalysis?

    <p>To increase the rate of a chemical reaction</p> Signup and view all the answers

    What is the result of the reaction completing in enzyme catalysis?

    <p>The product(s) leaves the active site, so the enzyme is free to catalyze more reactions</p> Signup and view all the answers

    What is the composition of enzymes?

    <p>Complex protein molecules</p> Signup and view all the answers

    What is the maximum energy required for a reaction to occur in the absence of a catalyst?

    <p>18 Kcal/mole</p> Signup and view all the answers

    Approximately how many different enzymes are capable of catalyzing different kinds of reactions?

    <p>2000</p> Signup and view all the answers

    What is the main component of an enzyme molecule?

    <p>Amino acid units</p> Signup and view all the answers

    What is the function of the active site on an enzyme?

    <p>To bind to the substrate</p> Signup and view all the answers

    What is the optimal pH for enzyme catalysis?

    <p>7.4</p> Signup and view all the answers

    Why do enzymes have high substrate specificity?

    <p>Due to the shape of the active site</p> Signup and view all the answers

    What is the theory that explains the mechanism of enzyme catalysis?

    <p>Lock-Key theory</p> Signup and view all the answers

    What happens to enzymes when they are stored at 5°C or less?

    <p>They are deactivated over time</p> Signup and view all the answers

    Study Notes

    Heterogeneous Catalysis Steps

    • The process of heterogeneous catalysis occurs in five steps:
      • Reactants diffuse to the surface of the catalyst
      • Molecules adhere to suitable sites available for adsorption
      • Reactants bound to the surface form an intermediate compound
      • The intermediate compound gets desorbed from the surface, making it available for adsorption of other molecules
      • The intermediate compound disintegrates to form the final products, which then diffuse out of the internal pores and the external surface of the catalyst

    Kinetic Treatment of Heterogeneous Catalytic Reaction

    • The rate of catalytic reaction is proportional to the surface coverage (θ)
    • The Langmuir isotherm equation is used to describe the adsorption process: θ = b * p / (1 + b * p)
    • There are three cases:
      • Partially covered catalyst surface: R = k * b * p (first-order reaction)
      • Fully covered catalyst surface: R = k (zero-order reaction)
      • Adsorption with dissociation: R = k * b^1/2 * p^1/2

    Poisoning of Catalysts

    • A poison reduces the effectiveness of a catalyst by preferentially adsorbing onto the catalyst surface
    • Characteristics of poisoning:
      • Very minute quantities can inhibit catalytic reactions
      • Poisoning can be reversible or irreversible
      • Examples of poisons include volatile compounds of sulfur, HCN, lead, and arsenic compounds

    Types of Catalysts

    • Non-porous, bulk-solid catalyst particles (e.g. Pt metal sheet)
    • Supported catalysts (e.g. Pt on ceramic, TiCl3 on MgCl2, Pt crystallites on Al2O3)
    • Porous, bulk-solid catalyst particles (e.g. zeolites, silica, Raney nickel, activated charcoals)

    Zeolites

    • Microporous crystalline solids with diameters smaller than 2 nm
    • Consist of silicon, aluminum, and oxygen in a three-dimensional polymeric framework
    • Can have cations such as Na+, K+, Ca2+, Mg2+, water, and/or other molecules within their pores
    • Three common types of zeolites: A, X, and Y
    • Starting materials include sodium aluminate and sodium silicate
    • Synthesis occurs at temperatures between 300-450 K
    • Sodium ions can be exchanged for ammonium ions, and acidic forms of zeolites can be produced by heating the ammonium-zeolite to 700 K

    Charcoal

    • Dark grey residue consisting of carbon and any remaining ash
    • Produced by slow pyrolysis of wood or other substances in the absence of oxygen
    • Activated charcoal has pores ranging from 1-100 nm
    • Physical adsorption is the primary means of removing contaminants from wastewater
    • Types of charcoal include granular activated carbon, powdered activated carbon, and impregnated carbon

    Catalytic Processes and Their Catalysts

    • Ammonia synthesis: iron
    • Sulfuric acid manufacture: nitrogen(II) oxide, platinum
    • Cracking of petroleum: zeolites
    • Hydrogenation of unsaturated hydrocarbons: nickel, platinum, or palladium
    • Oxidation of hydrocarbons in automobile exhausts: copper(II) oxide, vanadium(V) oxide, platinum, palladium
    • Isomerization of n-butane to isobutane: aluminum chloride, hydrogen chloride

    Enzyme Catalysis

    • Enzyme catalysis is the increase in the rate of a chemical reaction by the active site within the enzyme
    • Enzymes form a transition state with reactants, lowering the free energy of the reaction
    • Once the reaction completes, the product leaves the active site, allowing the enzyme to catalyze more reactions
    • Criteria for enzyme catalysis:
      • Enzymes are highly selective and active
      • There are over 2000 different enzymes, each catalyzing different reactions
      • Enzymes are large molecules with cross-sectional dimensions up to 100 nm
      • The active site is a small part of the external surface
      • Enzymes can be deactivated over time at moderate temperatures
      • The rate of catalytic enzyme reaction has a maximum value at pH = 7.4
      • The lock-key theory explains the mechanism of enzyme catalysis

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    Learn the 5 steps involved in the process of heterogeneous catalysis, from reactant diffusion to intermediate compound formation and desorption.

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