1. Nanochemistry NAN 601 Dr. Marinella Sandros
Instructor:
Dr. Marinella Sandros
Reaction mechanisms and Catalysis

2. Reaction Mechanism
“A sequential series of simple reactions which combine to form a larger, balanced chemical equation.”

3. Reaction Mechanism

4. Elementary Reactions

5. Elementary Reactions For the overall reaction
NO2 + CO  CO2 + NO
The elementary reactions of the mechanism are
2NO2  NO + NO3 R1 = k1[NO2]2
molecularity = 2
NO3 + CO  NO2 + CO2 R2 = k2[NO3][CO]
For any elementary reaction
the order of the reaction wrt a reactant is its stoichiometric coefficient in that step
the molecularity of the reaction is the sum of the stoichiometric coefficients for that step. The molecularity corresponds to the number of molecules that actually collide in that step!
NO3 is an intermediate.

6. Reaction Mechanisms Rule #1
For any step, the order of that step w.r.t. a reactant is its stoichiometric coefficient in that step.
For step 2 in the previous reaction
NO3 + CO  NO2 + CO2 R2 = k2[NO3][CO]

7. Rate-Limiting Step Controls Overall Rate
NO2 + CO  CO2 + NO
Two elementary reactions:
2NO2  NO + NO3 (slow) R1 = k1[NO2]2
NO3 + CO  NO2 + CO2 (fast) R2 = k2[NO3][CO]
Overall reaction:
NO2 + CO  CO2 + NO Rate = R1 =k1[NO2]2
The rate law for the overall reaction is the rate law for the rate-limiting step!

8. overall rate = rate of slowest step
Reaction Mechanisms
Rule #2
For a multistep reaction in which one step is much slower than the others,
overall rate = rate of slowest step

9. Rate Constants and Equilibrium

10. Reaction Mechanisms Rule #3
Write the rate equation for the overall reaction in terms of the reactants in the net reaction.
Express intermediates in terms of reactants and equilibrium constants.

11. Reaction Mechanisms and Rate Laws Summary
For any step, the order of that step w.r.t. a reactant is its stoichiometric coefficient in that step.
For a multi-step reaction in which one step is much slower than the others, the overall rate is equal to the rate of the slowest step
Write the rate equation for the overall reaction in terms of the reactants in the net reaction.

12. Experimentally determined rate law:
Examples
2 NO (g) + Br2(g)  2NOBr (g)
Experimentally determined rate law:
rate= k [NO]2[Br]
Show the following mechanism also produces a rate law consistent with experimentally observed one? K1
Step 1: NO (g) + NO (g) N2O2 (g) (fast) Step 2: N2O2 (g) + Br2(g)  2 NOBr (g) (slow)
K-1 k2

13.  Rate= k2 K1[NO]2 [Br2] = k [NO]2[Br2]
Step 1: NO (g) + NO (g) N2O2 (g) (fast) Step 2: N2O2 (g) + Br2(g)  2 NOBr (g) (slow)
K-1 k2
Step 2 is rate limiting!!!
 Rate= k2 [N2O2][Br2]
K1[NO]2 = k-1 [N2O2 ]
K1[NO]2 = [N2O2 ]
k-1
 Rate= k2 K1[NO]2 [Br2] = k [NO]2[Br2]
k-1

14. What is a “Catalyst”
A catalyst (Greek: καταλύτης, catalytēs) is a substance that accelerates the rate of a chemical reaction without itself being transformed or consumed by the reaction. (wikepedia)
k(T) = k0e-Ea/RT
Ea′ < Ea
k0′ > k0
k′ > k
ΔG = ΔG Ea
Ea′
A + B
A + B +
catalyst ΔG ΔG C
C + catalyst
uncatalyzed
catalyzed

15. Catalyst ??
Efficiency depends on activity, properties & life of the catalyst
Examples:
Ammonia synthesis – Promoted iron
SO2 oxidation – Venadium Pentaoxide
Cracking – Sylica, alumina
Dehydrogenation – Platinum, Molybdenum 7

16. Kinetic Vs. Thermodynamic
A reaction may have a large, negative ΔGrxn, but the rate may be so slow that there is no evidence of it occurring.
Conversion of graphite to diamonds is a thermodynamic favor process (ΔG -ve ).
C (graphite) --> C (diamond)
Kinetics makes this reaction nearly impossible (Requires a very high pressure and temperature over long time)

17. Activation Energy

18. Activation Energy
Catalyst lowers the activation energy for both forward and reverse reactions. 7

19. Activation Energy
This means , the catalyst changes the reaction path by lowering its activation energy and consequently the catalyst increases the rate of reaction.

20. Homogeneous Catalysis
A catalyst that is present in the same phase as the reacting molecules.
Example:
The Decomposition of aqueous hydrogen peroxide
2 H2O2 (aq)  2H2O (l) + O2 (g)
Very very very slow!!!!!!

21. Homogeneous Catalysis
Catalyzed by Bromine:
Br2 (aq) + H2O2 (aq)  2 Br- (aq) + 2 H+ (aq) + O2 (g)
2 Br- (aq) + H2O2 (aq) + 2 H+ (aq)  Br2 (l) + 2H2O (l)

22. Homogeneous Catalysis
The two reactions together serve as a catalytic pathway for hydrogen peroxide decompostion.
Both of them must have significantly lower activation energies than the uncatalyzed decomposition.

23. Heterogeneous Catalysis
The catalyst exists in a different phase from the reactant molecules, usually as a solid in contact with either gaseous reactants or with reactants in a liquid solution.
Many industrially important reactions are catalyzed by the surfaces of the solids.

24. Industrial Examples Sulfuric acid synthesis (Contact process)
SO2 + O2, SO3
vanadium oxides
hydration of SO3 gives H2SO4
Ammonia synthesis (Haber-Bosch process)
N2 + H2, NH3
iron oxides on alumina
consumes 1% of world's industrial energy budget
Nitric acid synthesis (Ostwald process)
NH3 + O2, HNO3
unsupported Pt-Rh gauze
direct routes from N2 are uneconomical
Hydrogen production by Steam reforming
CH4 + H2O, H2 + CO2
Nickel or K2O
Greener routes to H2 by water splitting actively sought
Ethylene oxide synthesis
C2H4 + O2, C2H4O
silver on alumina, with many promotors
poorly applicable to other alkenes
Hydrogen cyanide synthesis (Andrussov oxidation)
NH3 + O2 + CH4, HCN
Pt-Rh
Related ammoxidation process converts hydrocarbons to nitriles
Olefin polymerization Ziegler-Natta polymerization
propylene, polypropylene
TiCl3 on MgCl2
many variations exist, including some homogeneous examples
Desulfurization of petroleum (hydrodesulfurization)
H2 + R2S (idealized organosulfur impurity), RH + H2S
Mo-Co on alumina
produces low-sulfur hydrocarbons, sulfur recovered via the Claus

25. Heterogeneous Catalysis
Heterogeneous catalysts are often composed of metals or metal oxides.
Initial step is usually Adsorption of reactants.
Adsorption binding of molecules to a surface, whereas absorption refers to the uptake of molecules into the interior of another substance.

26. Absorption and AdsorptionH H H H H H H H H H H H H H H H H H H H H H H H H H H H
H2 adsorption on
palladium
H2 absorption on
palladium hydride
Adsorption occurs because the atoms or ions at the surface of a solid are extremely reactive.

27. Heterogeneous Catalysis
Example of heterogeneous catalysis is the reaction of hydrogen with ethylene:
C2H4 + H2  C2H6
VERY SLOW!
However in the presence of finely powdered metal such as nickel or palladium at room temperature and under <200 atm of hydrogen pressure.

29. http://chemwiki. ucdavis

30. Adsorption In physisorption The bond is a van der Waals interaction
Adsorption energy is typically kJ/mol. ( much weaker than a typical chemical bond )
Many layers of adsorbed molecules may be formed.

31. Adsorption For Chemisorption
The adsorption energy is comparable to the energy of a chemical bond.
The molecule may chemisorp intact (left) or it may dissociate (right).
The chemisorption energy is kJ/mol for molecules and kJ/mol for atoms.

32. Adsorption and Catalysis
Adsorbent: surface onto which adsorption can occur.
example: catalyst surface, activated carbon, alumina
Adsorbate: molecules or atoms that adsorb onto the substrate.
example: nitrogen, hydrogen, carbon monoxide, water
Adsorption: the process by which a molecule or atom adsorb onto a surface of substrate.
Coverage: a measure of the extent of adsorption of a species onto a surface H
adsorbate
adsorbent H
coverage θ = fraction of surface sites occupied

33. Criteria for a Good Catalyst

34. http://www. biology. hawaii

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36. http://www. biology. hawaii

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38. http://www. biology. hawaii

39. http://www. biology. hawaii

40. http://www. biology. hawaii

41. Quiz 4 2NO (g) + 2N2O(g)  2N2(g) + 2NO2(g) 2 NO2(g)  2NO(g) + O2 (g)
What is the chemical equation for the overall equation?
Why is NO considered a catalyst and not an intermediate?
Is this an example of homogeneous or heterogeneous catalysis?

42. Answer (a) 2 N2O (g)  2N2(g) + O2(g)
(b) An intermediate is produced and then consumed. A catalyst is consumed but then reproduced. NO2 is the intermediate.
(c) Since NO is in the same state as reactant, it is homogeneous.