CHBE 452 Lecture 28 Mechanism of Catalyst Action 1

Last Time We Introduced Catalysis Homogeneous catalysts Acids and bases Metal salts Enzymes Radical initiators Solvents Heterogeneous catalysts Supported metals Metal oxides, nitrides, sulfides Solid acids and bases Polymer bound species 2 Increase rates by or more

Today: Mechanisms Of Catalyst Action Catalysts can be designed to help initiate reactions. Catalysts can be designed to stabilize the intermediates of a reaction. Catalysts can be designed to hold the reactants in close proximity. Catalysts can be designed to hold the reactants in the right configuration to react. 3

Mechanism Of Catalysts, Continued Catalysts can be designed to block side reactions. Catalysts can be designed to sequentially stretch bonds and otherwise make bonds easier to break. Catalysts can be designed to donate and accept electrons. Catalysts can be designed to act as efficient means for energy transfer. 4

Mechanism Of Catalysis Continued Is is also important to realize that: One needs a catalytic cycle to get reactions to happen. Mass transfer limitations are more important when a catalyst is present. 5

Today: The Role Of Catalysts In Initiating Reactions, Stabilizing Intermediates Catalysts initiate reactions by help creating active centers (i.e. a radical or ion). Active center could be catalyst itself Could be a radical R-O. Catalysts stabilize intermediates. Catalyst binds to intermediate, lowering the free energy of the reactive intermediates. Raises intermediate concentration. Intermediates less reactive. 6

How Could Catalysts Change H 2 +Br 2  2HBr Mechanism Br 2 → 2Br Br+H 2 →HBr+H H+Br 2 →HBr+Br 2Br→Br 2 7

Key Principles Of Catalytic Mechanisms Catalysts bind intermediates at distinct sites. Mechanism often the same in gas phase & on catalyst. Initiation much faster – in effect do not need initiation reacion. 8

Example Of Catalysts Initiating Reactions C 2 H 6  C 2 H 4 +H 2 (12.41) C 2 H 6  2CH 3 (12.42) CH 3 +C 2 H 6  C 2 H 5 +CH 4 (12.43) C 2 H 5  C 2 H 4 +H (12.44) H+C 2 H 6  C 2 H 5 +H 2 (12.45) 2CH 3  C 2 H 6 (12.46) 2c 2 H 5  C 4 H 10 (12.47) CH 3  C 2 H 5  C 3 H 8 (12.48)  9 Gas phase mechanism

Catalysis By NO 2 Consider adding NO 2 (12.50) Catalysts can initiate reactions. The mechanisms are similar to the mechanisms without a catalyst, but the initiation process is much faster with the catalyst. Effect 10 9 (small for catalysis) 10

Some Examples Of Reactions Initiated By Catalysts 11

Initiation Often Not Enough. Also Need To Stabilize Intermediates 12

Stabilization Of Ionic Intermediates 13 Note big barrier to first step

Acid Catalyzed Reaction 14

Stabilization Of Intermediates. Can We Have Too Much of a Good Thing? When we stabilize intermediates we increase the intermediate concentration. We also decrease the reactivity of the intermediates. Which wins? 15

Experimental Evidence HCOOH  H 2 +CO 2 16 (12.75)

Sabatier’s Principle The best catalysts are substances which bind the reactants strongly, but not too strongly. 17

Consider H 2 +Br 2  HBr Rideal Eley mechanism Complicated derivation (see text) 18 (12.76) (12.85) (12.86)

For Unlimited Sites 19 Figure The rate of HBr formation as calculated from Equation (12.85), with [S] = 1e14/cm 2 and  p = 0.5, T = 500K,.

For Finite Number Of Sites 20 Figure The rate of HBr formation calculated from Equation (12.85), with [S] from Equation (12.87) and  p = 0.5, T = 500K,. (12.87)

Common Plots In Literature Sachtler-Frahenfort plots: Use heat of oxidation per mole of oxygen as surrogate for heat of formation of product. Tanaka-Tamaru plots: Use heat of oxidation per mole of metal as surrogate for heat of formation of product. 21

Comparison Of Sachtler-Frahrenfort And Tanaka-Tamaru 22 Figure A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.

Summary So Far Catalysts work by initiating reactions, stabilizing intermediates. Leads to increase in rates – need other effects to get to Can stabilize too much. 23

Example: Constructing Sachtler- Frauhenfort And Tanaka-Tamaru Plots Table 12.E.1 some data for the rate of ethylene hydrogenation on a number of metals 0 C. Next lets construct a Sachtler-Fahrenfort plot of the data. 24

Solution 25

Comparison Of Sachtler-Frahrenfort And Tanaka-Tamaru 26 Figure A Sachtler-Frahrenfort and Tanaka-Tamaru plot for the hydrogenation of ethylene.

Stabilizing Intermediates Not Entire Effect Leads to increases in rates – -need other effects to get to Does not lead to selectivity 27

Summary 28 Catalysts work by initiating reactions, stabilizing intermediates. Leads to increase in rates – need other effects to get to Can stabilize too much. Other effects connect selectivity.

Query What did you learn new in this lecture? 29