1 Lecture 7. Molecular kinetic. Characterization of Catalysts. Catalytic Reactions AY C A TL S I S Prepared by PhD Halina Falfushynska

2 What is a Catalyst ? Catalyst is a substance that increases the rate of the reaction at which a chemical system approaches equilibrium, without being substantially consumed in the process. Catalyst affects only the rate of the reaction,i.e.Kinetics. It changes neither the thermodynamics of the reaction nor the equilibrium composition.

3 Reaction path for conversion of A + B into AB Kinetic Vs. Thermodynamic

4 Activation Energy : The energy required to overcome the reaction barrier. Usually given a symbol E a or ∆G≠ The Activation Energy (Ea) determines how fast a reaction occurs, the higher Activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction Activated Complex Theory

5 Catalyst lowers the activation energy for both forward and reverse reactions. Activation Energy

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Catalysis

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Effect of a catalyst

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Heterogeneous Catalysis Consider the hydrogenation of ethylene: C 2 H 4 (g) + H 2 (g)  C 2 H 6 (g),  H = -136 kJ/mol. –The reaction is slow in the absence of a catalyst. –In the presence of a metal catalyst (Ni, Pt or Pd) the reaction occurs quickly at room temperature. –First the ethylene and hydrogen molecules are adsorbed onto active sites on the metal surface. –The H-H bond breaks and the H atoms migrate about the metal surface. Catalysis

Catalysis

Hydrogenation of ethylene

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20 Catalyst Preparation (1) Unsupported Catalyst Usually very active catalyst that do not require high surface area e.g., Iron catalyst for ammonia production (Haber process) (2) Supported Catalyst requires a high surface area support to disperse the primary catalyst the support may also act as a co-catalyst (bi- functional) or secondary catalyst for the reaction (promoter)

21 Supported Catalyst Nickel clusters SiO 2 Highly dispersed metal on metal oxide

22 Molecules in Zeolite Cages and Frameworks + p-xylene ZSM-5 Y-zeolite Paraffins

Enzymes Enzymes are biological catalysts. Most enzymes are protein molecules with large molecular masses (10,000 to 10 6 amu). Enzymes have very specific shapes. Most enzymes catalyze very specific reactions. Substrates undergo reaction at the active site of an enzyme. A substrate locks into an enzyme and a fast reaction occurs. The products then move away from the enzyme. Catalysis

Enzymes Only substrates that fit into the enzyme lock can be involved in the reaction. If a molecule binds tightly to an enzyme so that another substrate cannot displace it, then the active site is blocked and the catalyst is inhibited (enzyme inhibitors). The number of events (turnover number) catalyzed is large for enzymes ( per second). Catalysis

EnzymesCatalysis

Enzyme activity Temperature and pH affect the activity of an enzyme

Enzymes function best or are most active in specific conditions known as optimum conditions. Optimum Condition

Enzymes application Enzymes for detergents and personal care Enzyme applications in the food industry 28

Medical and Clinical Application Amylase and proteases and lypases used in drugs to cure digestive disorders Papain (papaya proteinase I) ingredient in some toothpastes or mints as teeth-whitener; enzymatic debriding preparations, notably Accuzyme Papacarie, a gel used for chemomechanical dental caries removal 29

Medical and Clinical Application Thrombin obtained from beef plasma used to stop bleeding during operation Enzymes are used as surface disinfectants – Trypsin used in cleaning Glucose oxidase, Peroxidases used in determining the Glucose in blood 30