Atkins & de Paula: Atkins Physical Chemistry 9e

Atkins & de Paula: Atkins Physical Chemistry 9e

Atkins & de Paula: Atkins Physical Chemistry 9e Chapter 23: Catalysis Chapter 23: Catalysis catalyst, a substance that accelerates a reaction but undergoes no net chemical change. enzyme, a biological catalyst. homogeneous catalyst, a catalyst in the same phase as the reaction mixture. heterogeneous catalyst, a catalyst in a different phase from the reaction mixture. HOMOGENEOUS CATALYSIS 23.1 Features of homogeneous catalysis acid catalysis, catalysis by the transfer of a proton from an acid to the substrate. base catalysis, catalysis by the transfer of a proton from the substrate to a base. Chapter 23: Catalysis 23.2 Enzymes active site, the region of an enzyme molecule at which reaction takes place.

substrate, the species that reacts in the presence of an enzyme. lock-and-key model, a model of enzyme action in which the substrate and the active site have complementary shapes. induced-fit model, a variation of the lock-and-key model in which the substrate causes a conformational change in the active site. Chapter 23: Catalysis 23.2(a) The Michaelis-Menten mechanism of enzyme catalysis MichaelisMenten mechanism, a mechanism for enzyme-catalysed reactions. E S ES k a , k a ES P E kb

v kb [ES] ka d [ES] [E][S] k a [E][S] k a[ES] kb [ES] 0 [ES] dt k a kb KM k a kb [E][S] ; Michaelis constant ka [ES] [E]0 [E]0 [E] [ES], [S] [S]0 [ES]

1 K M /[S]0 kb [E]0 v ; Michaelis - Menten equation 1 K M /[S]0 Chapter 23: Catalysis 23.2(a) The Michaelis-Menten mechanism of enzyme catalysis maximum velocity, the greatest reaction rate for a given concentration of substrate. kb [E]0 v 1 K M /[S]0 [S]0 K M : v kb [S]0 [E]0 KM

[S]0 K M : v vmax kb [E]0 kb [E]0 vmax v 1 K M /[S]0 1 K M /[S]0 1 1 KM v vmax vmax 1 ; Lineweaver - Burk plot [S]

0 Chapter 23: Catalysis 23.2(b) The catalytic efficiency of enzymes turnover number (or catalytic constant), the number of catalytic cycles (turnovers) performed by the active site in a given interval divided by the duration of the interval; kcat = kb = vmax/[E]0. catalytic efficiency, = kcat/KM = kakb/(ka + kb). Excersize Example 23.1 Chapter 23: Catalysis 23.2(c) Mechanism of enzyme inhibition E S ES k a , k a ES P E

kb [E][I] KI [EI] [ES][I] ESI ES I K I [ESI] EI E I [E]0 [E] [EI] [ES] [ESI] 1 [I] KI

and 1 [I] K I [E]0 [E] [ES] K [E][S] K [ES] KM , [S] [S]0 [E]0 M [ES] [ES] M [ES] [S]0 [S]0

kb [E]0 vmax v kb [ES] K M /[S]0 K M /[S]0 1 K M 1 v vmax vmax [S]0 Chapter 23: Catalysis 23.2(c) Mechanism of enzyme inhibition competitive inhibition, inhibition in which the inhibitor binds only to the active site of the enzyme; > 1 and = 1. uncompetitive inhibition, inhibition in which the

inhibitor binds to a site of the enzyme that is removed from the active site, but only if the substrate is already present; = 1 and > 1. non-competitive inhibition (or mixed inhibition), inhibition in which the inhibitor binds to a site other than the active site; > 1 and > 1. Excersize Example 23.2 Chapter 23: Catalysis HETEROGENEOUS CATALYSIS 23.3 THE GROWTH AND STRUCTURE OF SOLID SURFACES adsorption, the attachment of particles to a surface . adsorbate, the substance adsorbed. adsorbent (or substrate), the substance on which another substance in adsorbed. desorption, the detachment of an adsorbed substance. 23.3(a) Surface growth step, a discontinuity between two otherwise flat layers. terrace, a flat region of a surface.

Chapter 23: Catalysis 23.3(b) Surface composition and structure ultrahigh vacuum (UHV), pressures lower than about 107 Pa. photoemission spectroscopy, photoelectron spectroscopy applied to surfaces. Auger electron spectroscopy (AES), spectroscopy based on the Auger effect. Auger effect, the emission of a second electron after high energy radiation has expelled another. X-ray fluorescence, the generation of fluorescence by the Auger effect. scanning Auger electron microscopy (SAM), a technique for mapping the spatial variation over a surface. XPS X-ray fluorescence AES

Chapter 23: Catalysis reconstruction, modification of the substrate surface layers in response to adsorbates. low-energy electron diffraction (LEED), electron diffraction by surfaces. Chapter 23: Catalysis electron energy loss spectroscopy (EELS or HREELS), a technique in which the energy loss suffered by a beam of electrons is monitored when they are reflected from a surface. reflectionabsorption infrared spectroscopy (RAIRS), a technique for obtaining the infrared absorption spectrum of the adsorbate. surface-enhanced Raman scattering (SERS), strong enhancement of the Raman spectrum of the adsorbate. surface-extended X-ray absorption fine structure spectroscopy (SEXAFS), spectroscopy that makes use of the oscillations in X-ray absorbance observed on the high-frequency side of an absorption edge. molecular beam scattering (MBS), the scattering of a beam of adsorbate molecules by a surface.

Chapter 23: Catalysis 23.4 THE EXTENT OF ADSORPTION fractional coverage, , the fraction of adsorption sites occupied. rate of adsorption, the rate of change of fractional coverage; d/dt. flash desorption, a technique in which a sample is suddenly heated and the resulting rise of pressure is interpreted in terms of the amount of adsorbate originally on the sample. gravimetry, the determination of fractional coverage by measurement of mass. quartz crystal microbalance (QCM), the determination of mass that makes use of the modification of the crystals vibrational frequency by an adsorbate. 23.4(a) Physisorption and chemisorption physisorption, adsorption by van der Waals interaction between the adsorbate and the substrate. chemisorption, adsorption by the formation of a chemical bond. Chapter 23: Catalysis 23.4(b) Adsorption isotherms adsorption isotherm, the relation between fractional coverage and partial pressure of a

substrate. Langmuir isotherm; based on the 3 assumptions. 1) No adsorption beyond monolayer 2) All surface sites are equivalent. 3) Adsorption does not depend on the coverage (no interaction between adsorbates) A(g) M(surface) AM(surface ) ka , kd d k a pN (1 ) k d N 0 at equilibriu m dt k Kp K a 1 Kp kd

For adsorption with dissociati on, d k a p{N (1 )}2 k d ( N ) 2 0 at equilibriu m dt ( Kp)1/ 2 1 ( Kp)1/ 2 Excersize Example 23.4,5 Chapter 23: Catalysis isosteric enthalpy of adsorption, the standard enthalpy of adsorption at a fixed surface coverage; adH =RT2( ln K/T). BET isotherm, V/Vmon = cz/(1 z){1 (1 c)z}, z = p/p*. Temkin isotherm, = c1 ln(c2p). Freundlich isotherm, = c1p1/c2.

BET isotherm Excersize Example 23.6 Chapter 23: Catalysis 23.5 The rates of surface processes second harmonic generation (SHG), the process of generating radiation of twice the incident frequency (by a surface layer). precursor state, the initial state of an adsorbate molecule on a surface before it forms a chemical bond. 23.5(a) The rate of adsorption sticking probability, s, the proportion of collisions with a surface that lead to adsorption; s = (1 )s0. Chapter 23: Catalysis 23.5(b) The rate of desorption

half-life for adsorption, t1/2 = (ln 2)/kd (kd =Ae-E /RT). temperature-programmed desorption (TPD), the observation of a surge in desorption rate when the temperature is raised linearly. thermal desorption spectroscopy (TDS), anther name for temperature-programmed desorption. 23.5(c) Mobility on surfaces field-ionization microscopy (FIM), a technique that portrays the electrical characteristics of a surface by using the ionization of noble gas atoms. d Chapter 23: Catalysis 23.6 Mechanisms of heterogeneous catalysis co-adsorption, the joint adsorption of two or more adsorbates. LangmuirHinshelwood mechanism, a reaction that takes place by encounters between molecular fragments and atoms adsorbed on the surface. AB P

v k r A B K A pA K B pB k r K A K B pA pB A , B v 1 K A pA K B pB 1 K A pA K B pB (1 K A pA K B pB ) 2 EleyRideal mechanism, a reaction in which a gasphase molecule collides with another molecule already adsorbed on the surface. AB P v v k r pB A

k r KpA pB 1 KpA KpA 1, v k r pB KpA 1, v k r KpA pB Chapter 23: Catalysis 23.7 Catalytic activity at surfaces molecular beam reactive scattering (MBRS), reactive scattering between a molecular beam and adsorbed molecules. pulsed beams, a technique in which a molecular beam is chopped into short slugs.

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