1. W. F. SchneiderCBE 40445 CBE 40445 Lecture 15 Introduction to Catalysis William F. Schneider Department of Chemical and Biomolecular Engineering Department of Chemistry and Biochemistry University of Notre Dame wschneider@nd.edu Fall Semester 2005

2. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 What is a “Catalyst” A catalyst (Greek: καταλύτης, catalytēs) is a substance that accelerates the rate of a chemical reaction without itself being transformed or consumed by the reaction. (thank you Wikipedia)Greekratechemical reaction A + B C ΔGΔG EaEa uncatalyzed A + B + catalyst C + catalyst ΔGΔG Ea′Ea′ catalyzed k(T) = k 0 e -Ea/RT E a ′ < E a k 0 ′ > k 0 k′ > k ΔG = ΔG

3. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Catalysts Open Up New Reaction Pathways CH 3 C O CH 2 C CH 3 OH propanone propenol H2CH2C HO C CH 3 ‡ ‡ propanone propenol

4. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Catalysts Open Up New Reaction Pathways CH 3 C O CH 2 C CH 3 OH propanone propenol OH − CH 2 C CH 3 O−O− + H 2 O −OH − Base catalyzed propanone propenol intermediate ‡‡ rate = k[OH − ][acetone]

5. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Catalysts Open Up New Reaction Pathways CH 3 C O CH 2 C CH 3 OH propanone propenol + H 2 O Acid catalyzed H3O+H3O+ CH 3 C OHOH + −H3O+−H3O+ propenol different intermediate ‡‡ propanone rate = k[H 3 O + ][acetone]

6. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Types of Catalysts - Enzymes The “Gold Standard” of catalysts Highly specific Highly selective Highly efficient Catalyze very difficult reactions  N 2  NH 3  CO 2 + H 2 O  C 6 H 12 O 6 Works better in a cell than in a 100000 l reactor Triosephosphateisomerase “TIM” Cytochrome C Oxidase Highly tailored “active sites” Often contain metal atoms

7. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Types of Catalysts – Organometallic Complexes Perhaps closest man has come to mimicking nature’s success 2005 Noble Prize in Chemistry Well-defined, metal-based active sites Selective, efficient manipulation of organic functional groups Various forms, especially for polymerization catalysis Difficult to generalize beyond organic transformations Polymerization: Termination:

8. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Types of Catalysts – Homogeneous vs. Heterogeneous Homogeneous catalysis Single phase (Typically liquid) Low temperature Separations are tricky Heterogeneous catalysis Multiphase (Mostly solid-liquid and solid-gas) High temperature Design and optimization tricky Zeolite catalystCatalyst powders

9. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Types of Catalysts: Crystalline Microporous Catalysts Regular crystalline structure Porous on the scale of molecular dimensions  10 – 100 Å  Up to 1000’s m 2 /g surface area Catalysis through  shape selection  acidity/basicity  incorporation of metal particles 10 Å 100 Å Zeolite (silica-aluminate) Silico-titanate MCM-41 (mesoporous silica)

10. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Types of Catalysts: Amorphous Heterogeneous Catalysts Amorphous, high surface area supports  Alumina, silica, activated carbon, …  Up to 100’s of m 2 /g of surface area Impregnated with catalytic transition metals  Pt, Pd, Ni, Fe, Ru, Cu, Ru, … Typically pelletized or on monoliths Cheap, high stability, catalyze many types of reactions Most used, least well understood of all classes SEM micrographs of alumina and Pt/alumina

11. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Important Heterogeneous Catalytic Processes Haber-Bosch process  N 2 + 3 H 2 → 2 NH 3  Fe/Ru catalysts, high pressure and temperature  Critical for fertilizer and nitric acid production Fischer-Tropsch chemistry  n CO + 2n H 2 → (CH 2 ) n + n H 2 O, syn gas to liquid fuels  Fe/Co catalysts  Source of fuel for Axis in WWII Fluidized catalytic cracking  High MW petroleum → low MW fuels, like gasoline  Zeolite catalysts, high temperature combustor  In your fuel tank! Automotive three-way catalysis  NO x /CO/HC → H 2 O/CO 2 /H 2 O  Pt/Rh/Pd supported on ceria/alumina  Makes exhaust 99% cleaner

12. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Heterogeneous Catalytic Reactors Design goals  rapid and intimate contact between catalyst and reactants  ease of separation of products from catalyst Packed Bed (single or multi-tube) Slurry Reactor Fluidized Bed Catalyst Recycle Reactor

13. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Automotive Emissions Control System “Three-way” Catalyst CO  CO 2 HC  CO 2 + H 2 O NO x  N 2 Pt, Rh, Pd Alumina, ceria, lanthana, … Most widely deployed heterogeneous catalyst in the world – you probably own one! Monolith reactor

14. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Length Scales in Heterogeneous Catalysis Mass transport/diffusion Chemical adsorption and reaction

15. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Characteristics of Heterogeneous Supported Catalysts Surface area:  Amount of internal support surface accessible to a fluid  Measured by gas adsorption isotherms Loading:  Mass of transition metal per mass of support Dispersion:  Percent of metal atoms accessible to a fluid support MMM

16. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Rates of Catalytic Reactions Pseudo-homogeneous reaction rate  r = moles / volume · time Mass-based rate  r′ = moles / mass cat · time  r′ = r / ρ cat Heterogeneous reactions happen at surfaces Area-based rate  r′′ = moles / area cat · time  r′′ = r′ / SA,SA = area / mass Heterogeneous reactions happen at active sites Active site-based rate  Turn-over frequency TOF = moles / site · time  TOF = r′′ / ρ site TOF (s −1 ) Hetero. cats. ~10 1 Enzymes ~10 6 TOF (s −1 ) Hetero. cats. ~10 1 Enzymes ~10 6

17. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Adsorption and Reaction at Solid Surfaces Physisorption: weak van der Waals attraction of a fluid (like N 2 gas) for any surface  E ads ~10 – 40 kJ/mol  Low temperature phenomenon  Exploited in measuring gross surface area Chemisorption: chemical bond formation between a fluid molecule (like CO or ethylene) and a surface site  E ads ~ 100 – 500 kJ/mol  Essential element of catalytic activity  Exploited in measuring catalytically active sites

18. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Comparing Physi- and Chemisorption on MgO(001) 1.77 1.51 2.10 2.60 CO 2 SO 2 Physisorbed CO 2 -2 kcal mol -1 GGA Chemisorbed SO 2 (“sulfite”) -25 kcal mol -1 GGA SO 3 Chemisorbed SO 3 (“sulfate”) -50 kcal mol -1 GGA 1.66 1.48 1.45 2.12 2.58 MgO(001) supercell 1.48 1.25 Mg O :O: surf : : 2- C OO :O: surf : : 2- S OO : :O: surf : : 2- S OO O Schneider, Li, and Hass, J. Phys. Chem. B 2001, 105, 6972 Calculated from first-principles DFT

19. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Measuring Concentrations in Heterogeneous Reactions Kinetics Fluid concentrations  Traditionally reported as pressures (torr, atm, bar)  Ideal gas assumption: P j = C j RT Surface concentrations  “Coverage” per unit area n j = moles j / area  Maximum coverage called monolayer 1 ML: n j,max = ~ 10 15 molecules / cm 2  Fractional coverage θ j = n j / n j,max 0 ≤ θ j ≤ 1 θ j = 1/6 Rate = f(P j,θ j ) Metal particle surface

20. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Adsorption Isotherms Molecules in gas and surface are in dynamic equilibrium A (g) + M (surface) ↔ M-A Isotherm describes pressure dependence of equilibrium Langmuir isotherm proposed by Irving Langmuir, GE, 1915  (1932 Noble Prize)  Adsorption saturates at 1 monolayer  All sites are equivalent  Adsorption is independent of coverage Site conservation θ A + θ* = 1 + Equilibrium rate ads = rate des

21. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Using the Langmuir Isotherm Example: CO adsorption on 10% Ru/Al 2 O 3 @ 100°C P CO (torr) 100150200250300400 CO ads (μmol/g cat ) 1.281.631.771.942.062.21 n CO,∞ = 2.89 μmol/g cat K = 0.0082

22. ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ ● ○ W. F. SchneiderCBE 40445 Brunauer-Emmett-Teller Isotherm (BET) Solid Surface ΔH ads ΔH cond ΔH ads / ΔH cond Relaxes Langmuir restriction to single layer adsorption  Monolayer adsorption; multilayer condensation Useful for total surface area measurement  Adsorption of boiling N 2 (78 K)