ChE 553 Lecture 30 Catalysis By Metals 1

Objective Examine the trends in bonding over the periodic table 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

Trends Over Periodic Table: Cu/Ag/Au H 2 dissociation forbidden (requires d- electrons) O 2 dissociation occurs on W, Ag, nano- gold (o 2 is paramagnetic, can bond to d’s) Leads to selective oxidations 4

Pt/Pd/Ni D’s available Can dissourate H 2, CH, … Wealth of chemistry 5

Ir, Rh, Co, Os, Ru, Fe, Rh, W, Mo, Cr Similar to Pr/Pd/Ni, but stronger bond energies multiple bonding easy. 6

People Try To Explain Trends Based On Differences In Heat Of Adsorption 7 Note: Overall faster than gas phase.

Often No Correlation To Data 8 2NH 3  N H 2

%d character Has Been Used To Correlate Data 9

Often Does Not Work 10 2NH 3  N H 2

Energy Of The Center Of The D-band 11 Te Se Pb 2NH 3  N H 2

Can Understand Variations Via Blowers-Masel 12 2NH 3  N H 2

Processes Catalyzed By Metals 13

Mechanisms Of Reactions On Metals Generally metal catalyzed reactions follow catalytic cycle with adsorbtion, reaction, desorption Form adsorbed radicals Radicals react Molecules desorb 14

Typical Reactions On Metals Simple molecular adsorption reactions Dissociative adsorption reactions Bond scission reactions Addition reactions Recombination reactions Desorption reactions 15

General Rules About Reactions On Metals There must be bare sites on the catalyst to start the reaction. Then at least one of the reactants must adsorb on the bare sites. Then there are a series of bond dissociation reactions, fragmentations, association reactions and single atom recombinations which convert the adsorbed reactants into products Then the products desorb. 16

General Rules Catalytic reactions always go through a catalytic cycle Adsorption Reaction Desorption 17

Example Catalytic Mechanisms: Olefin Hydrogenation 18

Example Catalytic Mechanisms: Paraffin Dehydrogenation 19 H H H H C C H H H H H C C H H H H H H C H C HH H H H H HH H C H C HH H H C H C HH H H H H C C H H H

Example CO Oxidation 20

Example: 5 Center Isomerization 21

Example: 3 Center Isomerization 22

Example Hydroformulation 23

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. 24

Example Exam Question Summarize the adsorption and reaction of gases on metal surfaces. –What are the bonds like? –How do the d-electrons affect the bonding? –What types of reactions would you expect metals to catalyze? –What types of reactions would not be catalyzed by metals? –How do the d-bands affect the reactions? –How and why are reactions on different metals different? 25