ChE 553 Lecture 29 Catalysis By Metals 1

Objective Apply what we have learned to reactions on metal surfaces 2

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 3

Mechanisms Of Reactions On Metals Generally metal catalyzed reactions follow catalytic cycle with adsorbtion, reaction, desorption Form adsorbed radicals Radicals react Molecules desorb 4

Typical Reactions On Metals Simple molecular adsorption reactions Dissociative adsorption reactions Bond scission reactions Addition reactions Recombination reactions Desorption reactions 5

Adsorption On Metals Molecular Adsorption CO + S  CO ad Dissociative adsorption –oxidative addition H 2 + 2S  2H ad 6

Molecular vs Dissociative Adsorption 7 Figure 5.12 The metals which dissociate CO, NO, H 2, O 2 and CO at various temperatures.

Bond Fragmentation Reactions 8

Association Reactions 9

Reactions Continued 10

Recombinative Desorption (Reductive Elimination) 11

Displacement Reaction 12

 -scission R 2 CDCH 2(ad)  R 2 C=CH 2 + D (ad) (14.15) 13

Example Catalytic Mechanisms: Olefin Hydrogenation 14

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 15

Mechanism On Surface Similar To Radical Reactions In Gas Phase – But Radicals Bound To Surface 16

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. 17

Electrons In Metals Solvate Radicals 18 Figure The electron density extending out from a metal surface. (Note 1 bohr =0.52Å)

Metals Stabilize Intermediates 19

Multiple Radicals One key difference between gas phase and surface is that di & tri radicals are stable on metals NGives possibilities for interesting chemistry 20

Metals Initiate Reactions 21 Figure 5.12 The metals which dissociate CO, NO, H 2, O 2 and CO at various temperatures.

D-bands Help Tear Bonds Apart 22 Figure A diagram of the key interactions during the dissociation of hydrogen on platinum.

Activation Energy For H 2 Dissociation MetalHeat of dissociative adsorption of H 2 Intrinsic barrier for dissociative adsorption of H 2 Pt-13 kcal/mole<6 kcal/mole Si-54 kcal/mole~80 kcal/mole Al~-70 kcal/mole>90 kcal/mole K~-70 kcal/mole>90 kcal/mole 23

Redox Chemistry 24 (14.53) (14.54) (14.55) (14.56) (14.57)

Metals Hold Reactants In Correct Configuration To React 25 Figure 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 26 Figure The active site for reaction (12.91) on a palladium catalyst. (12.91)

Structure Sensitive Reactants 27 Figure 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 28 Figure The rate of the reaction N 2 + 3H 2  2NH 3 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 29

Practical Catalysts Are Supported Structures With Multiple Exposed Faces 30 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. 31