Concept 04 / 06Surface Chemistry
Catalysis: Homogeneous, Heterogeneous, Shape-Selective and Enzyme Catalysis
Catalysts, promoters and poisons, homogeneous and heterogeneous catalysis with examples, adsorption theory of heterogeneous catalysis, activity and selectivity, shape-selective catalysis by zeolites, and enzyme catalysis: characteristics, mechanism, optimum conditions and examples.
A catalyst alters the rate of a reaction without being consumed. Promoters enhance a catalyst's activity (molybdenum for iron in the Haber process); poisons reduce it (CO or for iron; arsenic compounds for platinum in the contact process).
Homogeneous catalysis: reactants and catalyst in the same phase.
(lead chamber process).
(acid-catalysed ester hydrolysis).
(inversion of cane sugar).
Heterogeneous catalysis: catalyst in a different phase, usually a solid with gaseous or liquid reactants.
(contact process); (Haber); (Ostwald); hydrogenation of vegetable oils with Ni.
Adsorption theory of heterogeneous catalysis. Steps: diffusion of reactants to the surface; adsorption on active sites (free valencies, which is why rough surfaces, cracks and finely divided catalysts are more active); reaction on the surface through an activated complex; desorption of products, freeing the sites; diffusion of products away. Transition metals are good catalysts because of variable oxidation states and ability to form bonds with adsorbates.
Activity is the ability to accelerate a reaction (chemisorption strength must be moderate). Selectivity is the ability to direct a reaction to a particular product: from CO and , Ni gives , gives , and Cu gives HCHO.
Shape-selective catalysis. The catalyst's pore structure and the size of reactant and product molecules control the reaction. Zeolites are microporous aluminosilicates with honeycomb-like cavities; ZSM-5 converts alcohols directly into gasoline (petrol) by dehydration to a mixture of hydrocarbons.
Enzyme catalysis. Enzymes are complex nitrogenous organic compounds (proteins) produced by living cells; they are biochemical catalysts.
Examples: invertase (sucrose to glucose and fructose), zymase (glucose to ethanol and ), diastase (starch to maltose), maltase (maltose to glucose), urease (urea to ammonia and ), pepsin (proteins to peptides in the stomach), trypsin (proteins to amino acids in the intestine).
Characteristics: extremely efficient (one enzyme molecule may transform a million substrate molecules per minute); highly specific; most active at an optimum temperature ( to ) and optimum pH (usually to ) (see figure); activity enhanced by activators (such as , , , ) and coenzymes (often vitamins); reduced by inhibitors and poisons.
Mechanism (lock and key): the substrate fits into the active site of the enzyme to form an enzyme-substrate complex, which decomposes to products and regenerates the enzyme: .
Key Takeaways:
- Promoters boost activity; poisons reduce it.
- Heterogeneous catalysis: adsorption, surface reaction, desorption.
- Selectivity: same reactants, different products with different catalysts (CO + ).
- ZSM-5 zeolite: shape-selective alcohol to gasoline.
- Enzymes: specific, efficient, optimum () and pH ( to ); lock and key.
- Pitfall: an enzyme is not "used up", but heating or extreme pH denatures it permanently.
Brain-Teaser 1: What products form from CO and over (a) Ni, (b) , (c) Cu? What property of catalysts does this show?
Solution: (a) ; (b) ; (c) HCHO. It shows selectivity: a catalyst directs the same reactants along a particular pathway.
Brain-Teaser 2: Why are finely divided metals better catalysts than lumps?
Solution: Heterogeneous catalysis occurs on the surface through adsorption at active sites (corners, edges, defects). Finely divided metal has vastly more surface area and more such sites per unit mass, so more reactant molecules are adsorbed and react at the same time.
Brain-Teaser 3: Why does an enzyme-catalysed reaction slow down above about , while most uncatalysed reactions keep speeding up?
Solution: Like any reaction, the enzymatic rate first increases with temperature. But above the optimum, the protein's three-dimensional structure unfolds (denaturation), destroying the active site, so activity drops sharply. Uncatalysed reactions have no such structure to lose.
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