§10.5 Catalytic reaction

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.

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. C 12 H 22 O 11 + H 2 O  C 6 H 12 O 6 + C 6 H 12 O 6 Substance that appears in the rate equation to a power that is higher than that to which it appears in the stoichiometric equation.

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

5.3 General characteristics of catalyzed reactions 1)Catalyst takes part in the reaction. (CH 3 ) 3 COH  (CH 3 ) 2 C=CH 2 + H 2 O without catalyst: k = 4.8  exp(-32700/T) s -1 with HBr as catalyst: k c = 9.2  exp(-15200/T) dm 3  mol -1  s -1

with HBr as catalyst: 2) t-Bu-Br  (CH 3 ) 2 C=CH 2 + HBr 1) t- Bu-OH + HBr t-Bu-Br + H 2 O By altering reaction path, catalyst lower activation energy of the overall reaction significantly and change the reaction rate dramatically.

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.

3) Selectivity of catalysts (1)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.

(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. 4) Other characteristics:

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 (K a ) as pointed out by Brønsted et al. in the 1920s: Where G a and  is experimental constants.  ranges between 0 ~ 1.

In aqueous solution, the acid may be H + or H 3 O + 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:

5.5 Some phenomena of heterogeneous catalysis The potential curve of adsorption (1) basic principal of heterogeneous catalysis Interaction between molecule and catalyst on catalytic activity 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.

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

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

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

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

For unimolecular reaction According to Langmuir isotherm （ 3 ） kinetics for heterogeneous catalysis Under low pressure, when b A P A << 1 At high pressure, when b A P A >> 1 pApA r r max

When b A p A << 1 + b B p B The adsorption of competing species inhibits the reaction. For example: Decomposition of N 2 O over Ag, CuO or CdO. When b B p B >> 1 For example Decomposition of ammonia over Pt when competing adsorption exists:

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 of L-H mechanism pApA p B = constant r

Ununiformity of solid surface and catalysis PH 3, 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 （ 4 ） Active sites 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.

Fe(100) Fe(111) Fe(211) Fe(110) Fe(210) Active sites in iron catalyst for ammonia synthesis C 7 : active sites

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

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

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.

（ 1 ） Kinds of enzymes: pepsinHydrolysis of proteins diastaseHydrolysis of starch ureasehydrolysis of urea invertasehydrolysis of sucrose zymasehydrolysis of glucose maltaseHydrolysis of maltose Important hydrolytic enzymes oxidation-reduction enzymes SOD(Superoxide Dismutase)Decomposition of superoxide (O 2 - ) NitrogenaseDinitrogen fixation 1) hydrolytic enzymes 2) oxidation-reduction enzymes

(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 ?

Using stationary-state approximation Michaelis constant Discussion: 1) When [S] >> k M : 2) When [S] << k M : When [S] = k M :

Lineweaver-Burk plot Slope: S = k M /r m intercept: I = 1/r m Both r m and k M can be obtained by solving the equations.

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.

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

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

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 A superoxide Dismutase can catalytically decompose 10 5 molecules of hydrogen peroxide in at ambient temperature in 1 s, while Al 2 (SiO 3 ) 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 o C for single cycle is only 10~15%.