Atkins & de Paula: Atkins’ Physical Chemistry 9e

1. Atkins & de Paula: Atkins’ Physical Chemistry 9e Chapter 23: Catalysis

2. 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.

3. 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.

4. Chapter 23: Catalysis • 23.2(a) The Michaelis-Menten mechanism of enzyme catalysis • Michaelis–Menten mechanism, a mechanism for enzyme-catalysed reactions.

5. 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.

6. 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

7. Chapter 23: Catalysis 23.2(c) Mechanism of enzyme inhibition

8. 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

9. 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.

10. 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. X-ray fluorescence XPS AES

11. Chapter 23: Catalysis • reconstruction, modification of the substrate surface layers in response to adsorbates. • low-energy electron diffraction (LEED), electron diffraction by surfaces.

12. 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.

13. 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.

14. 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. • No adsorption beyond monolayer • All surface sites are equivalent. • Adsorption does not depend on the coverage (no interaction between adsorbates) Excersize Example 23.4,5

15. 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

16. 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.

17. 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.

18. 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. • Eley–Rideal mechanism, a reaction in which a gas–phase molecule collides with another molecule already adsorbed on the surface.

19. 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.