Chapter 23: Catalysis

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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
 Michaelis–Menten 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 
k a  kb [E][S]
KM 

; Michaelisconstant
ka
[ES]
[E]0  [E]  [ES], [S]  [S] 0  [ES] 
v
[E]0
1  K M /[S] 0
kb [E]0
; Michaelis- Mentenequation
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.
v
kb [E]0
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
v
kb [E]0
vmax

1  K M /[S] 0
1  K M /[S] 0
1
1  KM  1


 
; Lineweaver- Burk plot
v vmax  vmax  [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, η = 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]
[EI]
[ES][I]
ESI  ES  I K I 
[ESI ]
EI  E  I
KI 
[E]0  [E]  [EI] [ES]  [ESI ]
  1
[I]
KI
and
   1
[I]
K I
[E]0  [E]  [ES] 
KM 
 K

[E][S]
K [ES]
, [S]  [S] 0  [E]0  M
  [ES]   [ES] M    
[ES]
[S] 0
 [S] 0

v  kb [ES] 
kb [E]0
vmax

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;
α > 1 and α = 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; α = 1 and α > 1.
 non-competitive inhibition (or mixed inhibition),
inhibition in which the inhibitor binds to a site other
than the active site; α > 1 and α > 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 10–7 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.
 reflection–absorption 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, θ, the fraction of adsorption sites occupied.
 rate of adsorption, the rate of change of fractional coverage; dθ/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 crystal’s 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 equilibrium
dt
k
Kp

K a
1  Kp
kd
For adsorptionwith dissociation,
d
 k a p{N (1   )}2  k d ( N ) 2  0 at equilibrium
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; ΔadHθ =RT2( ln K/T)θ.
 BET isotherm, V/Vmon = cz/(1 – z){1 – (1 – c)z}, z = p/p*.
 Temkin isotherm, θ = c1 ln(c2p).
 Freundlich isotherm, θ = 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 – θ)s0.
Chapter 23: Catalysis
23.5(b) The rate of desorption
 half-life for adsorption, t1/2 = (ln 2)/kd (kd =Ae-Ed/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.
Chapter 23: Catalysis
23.6 Mechanisms of heterogeneous catalysis
 co-adsorption, the joint adsorption of two or more adsorbates.
 Langmuir–Hinshelwood mechanism, a reaction that takes place by encounters between
molecular fragments and atoms adsorbed on the surface.
ABP
A 
v  kr A B
K A pA
K B pB
kr K A K B pA pB
, B 
v 
1  K A pA  K B pB
1  K A pA  K B pB
(1  K A pA  K B pB ) 2
 Eley–Rideal mechanism, a reaction in which a gas–phase molecule collides with another
molecule already adsorbed on the surface.
ABP
v
v  kr pB A
kr KpA pB
1  KpA
KpA  1, v  kr pB
KpA  1, v  kr 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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