ChE 551 Lecture 29 Catalysis By Metals

Metals Work By Same Mechanisms As Other Catalysts Metal catalysts can help initiate reactions Metal catalysts can stabilize the intermediates of a reaction Metal catalysts can hold the reactants in close proximity and in the right configuration to react Metal catalysts can be designed to block side reactions Metal catalysts can stretch bonds and otherwise make bonds easier to break Metal catalysts can donate and accept electrons Metal catalysts can act as efficient means for energy transfer

Mechanisms Of Reactions On Metals Generally metal catalyzed reactions follow catalytic cycle with adsorbtion, reaction, desorption Form adsorbed radicals Radicals react Molecules desorb

Typical Reactions On Metals Simple molecular adsorption reactions Dissociative adsorption reactions Bond scission reactions Addition reactions Recombination reactions Desorption reactions

Dissociative adsorption H2 + 2S  2Had Adsorption On Metals Molecular Adsorption CO + S  COad Dissociative adsorption oxidative addition H2 + 2S  2Had

Molecular vs Dissociative Adsorption Figure 5.12 The metals which dissociate CO, NO, H2, O2 and CO at various temperatures.

Bond Fragmentation Reactions

Association Reactions

Reactions Continued

Recombinative Desorption (Reductive Elimination)

Displacement Reaction

-scission R2CDCH2(ad)  R2C=CH2 + D(ad) (14.15)

Example Catalytic Mechanisms: Olefin Hydrogenation

Principles Of Catalytic Reaction Metals can help initiate reactions Metals can stabilize the intermediates of a reaction Metals can hold the reactants in close proximity and in the right configuration to react Metals can stretch bonds and otherwise make bonds easier to break Metals can donate and accept electrons

Mechanism On Surface Similar To Radical Reactions In Gas Phase – But Radicals Bound To Surface

Electrons In Metals Solvate Radicals Metals are solvents for radicals. They lower the energy of radical species which allows initiation-propagation reactions to occur. Adsorbed radicals have lower energies than gas phase radicals which leads to higher concentrations. Intrinsic barriers of species adsorbed on metals similar to gas phase radicals.

Electrons In Metals Solvate Radicals Figure 14.18 The electron density extending out from a metal surface. (Note 1 bohr =0.52Å)

Metals Stabilize Intermediates

Multiple Radicals One key difference between gas phase and surface is that di & tri radicals are stable on metals N Gives possibilities for interesting chemistry

Metals Initiate Reactions Figure 5.12 The metals which dissociate CO, NO, H2, O2 and CO at various temperatures.

Activation Energy For H2 Dissociation Metal Heat of dissociative adsorption of H2 Intrinsic barrier for dissociative adsorption of H2 Pt -13 kcal/mole <6 kcal/mole Si -54 kcal/mole ~80 kcal/mole Al ~-70 kcal/mole >90 kcal/mole K

D-bands Help Tear Bonds Apart Figure 12.20 A diagram of the key interactions during the dissociation of hydrogen on platinum.

Redox Chemistry (14.53) (14.54) (14.55) (14.56) (14.57)

Metals Hold Reactants In Correct Configuration To React Figure 14.20 Balandin's suggested multiplet for the decomposition of ethanol a) to form ethylene b)to form acetyladehyde. The asterisks in the figures represent places on the surface where reaction can occur.

Metals Hold The Reactants In The Correct Configuration To React (12.91) Figure 12.15 The active site for reaction (12.91) on a palladium catalyst.

Structure Sensitive Reactants Figure 14.22 The rate of nitric oxide dissociation on several of the faces of platinum along the principle zone axes of the stereographic triangle. Adapted from Masel[1983].

Structure Sensitive Reactions   Figure 14.21 The rate of the reaction N2 + 3H2  2NH3 over an iron catalyst as a function of size of the iron particles in the catalyst. Data of Boudart et al [1975]

Different Reactions Show Different Structure Sensitivity

A picture of a supported metal Practical Catalysts Are Supported Structures With Multiple Exposed Faces Figure 12.4 A picture of a supported metal catalyst.

Summary Metals can help initiate reactions - in particular they facilitate bond scission processes Metals can stabilize the intermediates of a reaction particularly radical intermediates Metals can lower the intrinsic barriers to bond scission. The d-electrons promote bond scission and bond formation. The s-electrons promote redox chemistry. D’s convert forbidden reactions to allowed reactions. Produces tremendous rate enhancements.

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