ChE 553 Lecture 23 Catalysis By Surfaces 1

Objective For Today Ask How Surfaces Can Catalyze Reactions 2

3 Catalysis Definition Ostwald defined a catalyst as a substance which changed the rate of reaction without itself being consumed in the process Not being consumed  catalyst does change

Freshman Chemistry View Of Catalysts “Catalysts lower barriers to chemical reactions” Catalysts often raise barriers to elementary steps in a reaction mechanism! 4

Key Mechanism Of Catalyst Action Stabilizing intermediates by bonding to them The Increase in intermediate concentration leads to a substantial increase in rate. 5

6 Catalytic Reaction Occurs Via A Catalytic Cycle: reactants + catalyst  complex complex  products + catalyst

7 Example: Rhodium Catalyzed CH 3 OH+CO  CH 3 COOH Methyl radicals are not stable in the gas phase or solution but they are stable when bound to rhodium. The higher concentration leads to a higher rate

Example To Illustrate How Increasing Barriers Can Increase Rates consider A  A ad  B with the formation of B rate determining The rate of reaction, r, is given by r= K ad k B P A /(1+K ad P A ) Where K ad is the equilibrium constant for adsorption, k B is the rate constant for the production of B and P A is the pressure of A 8

Note the correct rate equation is r= K ad k B P A /(1+K ad P A ) I am assuming that the last term is small 9

Model That I will derive In Lecture 22 K ad = K 0 * exp((ΔG ad +X)/RT) Where ΔG ad is free energy of adsorption on some reference surface, and X is the change in free energy in moving to some other surface k B =k o *exp(-(Ea+0.5 X)/RT So as you increase X, you increase the activation energy for the formation of B. 10

Now Combine The Equations r = K 0 * exp((ΔG ad )/RT)* k o *exp(-(Ea)/RT)* exp((0.5*X)/RT) When X=0, r=r 0 the rate on the reference surface, Combining r = r 0 *exp((0.5*X)/RT) 11

Now Consider How Increases In X Changes The Rate From before k B =k o *exp(-(Ea+0.5 X)/RT r = r 0 *exp((0.5*X)/RT) Notice that when we increase X, we increase the activation barrier for B formation, yet the rate of B formation goes up! 12

Physical Interpretation The rate is given by r= k B θ B When we increase X, we increase θ B and decrease k B. As long as the increases in θ B are larger than the decrease in k B the net rate will increase Most catalysts work by increasing θ B. The rate increases even though the activation barrier for the rate determining step goes up. 13

Limit To The Analysis θ B cannot be bigger than 1 r= k B θ B = K ad k B P A /(1+K ad P A ) At low coverages increases in θ B dominate so the net rate will increase Once θ B gets close to 1, θ B cannot increase any more. The rate decreases with increasing X. 14

Plot of actual case from lect 24 15

Net Result Is Volcano Behavior 16 (12.75)

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

What Types Of Reactions Will Be Increased By Metal Surfaces? Bulk metals have many free electrons Free electrons rapidly exchange with radicals –radical intermediates stabilized Free electrons neutralize carbocations –Carbocation reactions slowed down 18

19 A Selection Of The Reactions Catalyzed By Supported Metals Key intermediates, hydrogen atoms, oxygen atoms, methyls ethyls …

Insulating Oxides Stabilize Ionic Intermediates Insulating oxides have no free electrons –radical intermediates not stabilized Insulating oxides are ionic –Ionic intermediates, i.e. carbocations can be stabilized 20

Table 12.2-Some Reactions Commonly Catalyzed By Solid Acids And Bases 21 ReactionExampleTypical Application Isomerization (Rearranging the structure of a molecule) CH 2 =CHCH 2 CH 3  CH 3 CH=CHCH 3 Octane Enhancement Monomer Production Paraxylene Production Alkylation (Making too little molecules into a bigger one) CH 3 CH=CHCH 3 + CH 3 CH 2 CH 2 CH 3  (CH 3 CH 2 )CH(CH 3 )(C 4 H 9 ) Pharmaceutical Production Monomer Production Fine Chemicals Butane + olefin  octane Cracking (Taking a big molecule and making it into two littler ones). C 12 H 24  C 7 H 14 + C 5 H 10 Crude Oil Conversion Digestion

Semiconductors Are In Between Semiconductors have fewer free electrons, and also be ionic Usually use semiconductors when you want to stabilize some radicals e.g. oxygen atoms or sulfur atoms but not other radicals, such as hydrogen atoms. 22

23 A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides (Semiconductors)

24 A Selection Of The Reactions Catalyzed By Transition Metal Oxides, Nitrides, And Sulfides

Summary Most catalysts work by stabilizing intermediates Intermediate concentration goes up Activation energy for product formation goes up (not down)! –Rate constant for product formation goes down Net effect is an increase in rate Metals, semiconductors, insulators stabilize different intermediates – gives different chemistry 25