§10.5 Catalytic reaction

1. §10.5Catalytic reaction

2. 5.1 Catalysts and catalysis catalyst Substance that changes the rate of a chemical reaction without themselves undergoing any chemical change. catalysis The phenomenon of acceleration or retardation of the speed of a chemical reaction by addition of small amount of foreign substances to the reactants.

3. 5.2 type of catalysis Homogeneous catalysis Heterogeneous catalysis Biological catalysis / enzyme catalysis 1) Homogeneous catalysis the catalyst is present in the same phase as the reactant. Example: Hydrolysis of sucrose with inorganic acid. C12H22O11 + H2O  C6H12O6 + C6H12O6 Substance that appears in the rate equation to a power that is higher than that to which it appears in the stoichiometric equation.

4. 2) Heterogeneous catalysis: the catalyst constitutes a separate phase from the reaction. Examples: Haber’s process for ammonia synthesis; contact oxidation of sulphur dioxide; Hydrogenation of alkene, aldehyde, etc.

5.  5.3 General characteristics of catalyzed reactions • Catalyst takes part in the reaction. (CH3)3COH  (CH3)2C=CH2 + H2O without catalyst: k = 4.8  1014 exp(-32700/T) s-1 with HBr as catalyst: kc = 9.2  1012 exp(-15200/T) dm3mol-1s-1

6. with HBr as catalyst: 1) t- Bu-OH + HBr t-Bu-Br + H2O 2) t-Bu-Br  (CH3)2C=CH2 + HBr By altering reaction path, catalyst lower activation energy of the overall reaction significantly and change the reaction rate dramatically.

7. 2) No impact on the thermodynamic features of the reaction (1) Cannot start or initiate a thermodynamically non-spontaneous reaction; (2) Can change the rate constant of forward reaction and backward reaction with the same amplitude and does not alter the final equilibrium position. Catalyst can shorten the time for reaching equilibrium. (3) Is effective both for forward reaction and backward reaction. Study on the catalyst for ammonia synthesis can be done with easy by making use of the decomposition of ammonia.

8. 3) Selectivity of catalysts • The action of catalyst is specific. Different reaction calls for different catalyst. • Hydrogenation? Isomerization? (2) The same reactants can produce different products over different catalysts.

9. 4) Other characteristics: (1) The chemical composition of catalyst remains unchanged at the end of the reaction; (2) Only a small amount of catalyst is required; (3) Catalyst has optimum temperature; (4) Catalyst can be poisoned by the presence of small amount of poisons;anti-poisoning. (5) The activity of a catalyst can be enhanced by promoter; (6) catalyst usually loaded on support with high specific area , such as activated carbon, silica.

10. 5.4 kinetics of homogeneous catalysis For homogeneous reaction, the reactant is usually named as substrate. When C is some acid, rate constant is proportional to dissociation constant (Ka) as pointed out by Brønsted et al. in the 1920s: Where Ga and  is experimental constants.  ranges between 0 ~ 1.

11. In aqueous solution, the acid may be H+ or H3O+ but in general it may be any species HA capable of being a proton donor (Brønsted acid) or a electron acceptor (Lewis acid). Dehydration of acet-aldehyde catalyzed by different acids. For base-catalyzed reaction there also exists:

12. 5.5 Some phenomena of heterogeneous catalysis (1) basic principal of heterogeneous catalysis Interaction between molecule and catalyst on catalytic activity The potential curve of adsorption When the interaction between molecules and catalyst is weak, the activation is insufficient. When the interaction between molecules and catalyst is very strong, it is difficult for the succeeding reaction to occur.

13. diffusion adsorption reaction diffusion desorption （2） Mechanism of heterogeneous catalysis A surface reaction can usually be divided into five elementary steps: 1) diffusionof reactants to surface; 2) adsorption of reactants at surface; 3) reactionon the surface; 4) desorption of product from surface; 5) diffusion of product from surface. Which is r.d.s.?

14. A B A (g) + B + Many surface reactions can be treated successfully on the basis of the following assumptions: 1) the r.d.s. is a reaction of adsorbed molecules; 2) the reaction rate per unit surface area is proportional to . For unimolecular reaction over catalyst Catalyzed isomerization or decomposition

15. A-B A B A (g) B (g) + Transition state A A B A (g) + + B (g) A-B + For bimolecular reaction over catalyst Langmuir-Hinshelwood mechanism (L-H mechanism) Langmuir-Rideal mechanism (L-R mechanism)

16. Langmuir-Rideal mechanism (L-R mechanism) Hydrogenation of ethylene Langmuir-Hinshelwood mechanism (L-H mechanism) Synthesis of ammonia

17. r rmax pA （3） kinetics for heterogeneous catalysis For unimolecular reaction According to Langmuir isotherm Under low pressure, when bAPA << 1 At high pressure, when bAPA >> 1

18. When bApA << 1 + bBpB when competing adsorption exists: The adsorption of competing species inhibits the reaction. For example: Decomposition of N2O over Ag, CuO or CdO. When bBpB >> 1 For example Decomposition of ammonia over Pt

19. r pB = constant pA The situation of the L-R mechanism is the same as that of unimolecular reaction over catalyst. For L-H mechanism, small modification should be made. Rate~ partial pressure relation ofL-H mechanism

20. （4） Active sites Ununiformity of solid surface and catalysis 10-9 PH3, which is insufficient for formation of monolayer,can destroy completely the activity of Pt catalyst toward oxidation of ammonia. In 1926, Talyor proposed the active site model 1) Only the molecules adsorbed on the active sites can lead to reaction. 2) The fraction of active sites on the catalyst surface is very low.

21. Fe(111) Fe(211) Fe(100) C7: active sites Fe(110) Active sites in iron catalyst for ammonia synthesis Fe(210)

22. Where are the active sites? The active site is in fact atom cluster comprising of several metal atoms. Atom cluster Increase of the degree of subdivision will increase the ununiformity of catalyst surface and increase the number of active sites. Adsorption of species on the edges of a calcites crystal

23. (5) Poison of catalyst If bB is very large, even at low pB, A will be very small. The reaction of A will be greatly retarded. The impurities with high b is catalyst poison.

24. 5.6 Enzyme catalysis Enzymes are biologically developed catalysts, each usually having some one specific function in a living organism. Enzymes are proteins, ranging in molecular weight from about 6000 to several million. Some 150 kinds have been isolated in crystalline form. The diameter of enzyme usually ranges between 10 ~ 100 nm. Therefore, the enzyme catalysis borders the homogeneous catalysis and the heterogeneous catalysis.

25. 1) hydrolytic enzymes 2) oxidation-reduction enzymes （1）Kinds of enzymes: Important hydrolytic enzymes oxidation-reduction enzymes

26. ? (2) Kinetics of enzyme catalysis A rather widely applicable kinetic framework for enzymatic action is that known as the Michaelis-Menten Mechanism (1913). Enzyme-substrate complex

27. Using stationary-state approximation Michaelis constant Discussion: 1) When [S] >> kM: 2) When [S] << kM: When [S] = kM:

28. Lineweaver-Burk plot Slope: S = kM/rm intercept: I = 1/rm Both rm and kM can be obtained by solving the equations.

29. Many enzyme systems are more complicated kinetically than the foregoing treatment suggests. There may be more than one kind of enzyme-substrate binding site; sites within the same enzyme may interact cooperatively. Often, a cofactor is involved. Luciferase (荧光素酶) is a generic name for enzymes commonly used in nature for bioluminescence. http://en.wikipedia.org/wiki/Image:Luciferase-1BA3.png

30. substrate enzyme (2)Outstanding characteristics of enzyme catalysis 1) High selectivity: Lock and key Even 10-7 mol dm-3 urease can catalyze the hydrolysis of urea (NH2CONH2) effectively. However, it has no effect on CH3CONH2.

31. Chirality of enzyme catalysis John Warcup Cornforth 1975 Noble Prize Great Britain 1917/09/07 for his work on the stereochemistry of enzyme-catalyzed reactions

32. 2) High efficiency Activation energy of hydrolysis of sucrose is 107 kJ mol-1 in presence of H+, while that is 36 kJ mol-1 in presence of a little amount of saccharase, corresponding to a rate change of 1022. A superoxide Dismutase can catalytically decompose 105 molecules of hydrogen peroxide in at ambient temperature in 1 s, while Al2(SiO3)3, an industrial catalyst for cracking of petroleum, can only crack one alkane molecules at 773K in 4 s. 3) Moderate conditions Nitrogenase in root-node can fix dinitrogen from dinitrogen and water at ambient pressure and atmospheric pressure with 100 % conversion. While in industry, the conversion of dinitrogen and dihydrogen to ammonia over promoted iron catalyst at 500 atm and 450 ~ 480 oC for single cycle is only 10~15%.