catalysis - Catalysis Lectures W.H Green 5.68J/10.652J...

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Catalysis Lectures W.H. Green 5.68J/10.652J Spring 2003 Handouts: Norskov et al., J. Catalysis Imbihl and Ertl, Chem. Rev. (partial) Homework Major points: 1) Why reactions have barriers, and how catalysts avoid/reduce these barriers. 2) Why thermochemistry of the catalyst-substrate complex is so important. 3) Homogeneous Catalysts and Heterogeneous Catalysts are NOT hugely different. 4) How Homogeneous and Heterogeneous Catalysts are different 5) Chain-Reaction Kinetics in General 6) Some important types of catalysis a. Dissociative Chemisorption (see Norskov et al. J. Catalysis) b. Protons c. Lewis Acids (vacant orbitals) d. Free Radicals 7) Langmuir-Hinshelwood vs. Eley-Rideal 8) Catalyst oscillations / microscopic dynamics WHY REACTIONS HAVE BARRIERS Usually, reactions have barriers because: a) closed shell orbitals repel b) steric strains to reach TS c) most reactions except ion-molecule and radical recombination have some barrier since molecule has to rearrange to reach TS. Many catalysts have a vacant orbital that can accept an electron or electron pair, dramatically lowering barriers, completely changing reaction path: Transition Metals, Lewis acids, Free Radicals. Many catalysts are charged: acids, bases, metal ions, oxides. Some catalysts provide a path that avoids steric strain, e.g. H2O can pass protons around.
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A few catalysts are less dramatic: they don’t change the reaction path much, but instead have some weak interactions which stabilize the TS more than the reactants. Each H-bond is worth 5 kcal/mol; enough van der Waals contacts can add up. THERMOCHEMISTRY IS KEY We are going from catalyst + reactants to catalyst + products, with a fixed free energy difference. We want to avoid any steps with free energy differences much larger than this – they will introduce effective activation energies (even if there is no kinetic barrier beyond the intermediate’s thermochemsitry. Best if the intermediates are all intermediate in energy between reactants+catalyst and products+catalyst. Examples: H 3 O + + enol = intermediate = H 3 O + + ketone Intermediate higher in energy than reactant so there must be a barrier just from thermo. H + enol = intermediate = H + ketone I think intermediate lower in energy than reactant, no barrier caused by thermo. (Will be small kinetic barrier to H addition to double bond).
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