Instructor: Dr. Marinella Sandros

Nanochemistry NAN 601 • Instructor: • Dr. MarinellaSandros Reaction mechanisms and Catalysis

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

Reaction Mechanism

Elementary Reactions

Elementary Reactions • For the overall reaction NO2 + CO  CO2 + NO The elementary reactions of the mechanism are 2NO2 NO + NO3R1 = k1[NO2]2 molecularity = 2 NO3 + CO  NO2 + CO2R2 = k2[NO3][CO] molecularity = 2 • 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.

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 + CO2R2 = k2[NO3][CO]

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!

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

Reaction Mechanisms and Rate LawsSummary • 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.

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

K1 K-1 k2 Step 1: NO (g) + NO (g) N2O2 (g) (fast) Step 2: N2O2 (g) + Br2(g)  2 NOBr (g) (slow) Step 2 is rate limiting!!!  Rate= k2 [N2O2][Br2] K1[NO]2 = k-1 [N2O2 ] K1[NO]2 = [N2O2 ] k-1  Rate= k2K1[NO]2 [Br2] = k [NO]2[Br2] k-1

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

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

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)

Activation Energy

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

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

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!!!!!!

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)

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.

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.

Industrial Examples http://en.wikipedia.org/wiki/Heterogeneous_catalysis

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.

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

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.

http://chemwiki.ucdavis.edu/@api/deki/files/1559/=Catalytic_Hydrogenation_Mechanism.jpghttp://chemwiki.ucdavis.edu/@api/deki/files/1559/=Catalytic_Hydrogenation_Mechanism.jpg

Adsorption • In physisorption • The bond is a van der Waals interaction • Adsorption energy is typically 5-10 kJ/mol. ( much weaker than a typical chemical bond ) • Many layers of adsorbed molecules may be formed.

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 30-70 kJ/mol for molecules and 100-400 kJ/mol for atoms.

coverage θ = fraction of surface sites occupied H H H H adsorbate H H H H H H H H H H adsorbent 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

Criteria for a Good Catalyst

http://www.biology.hawaii.edu/171L/SummerSession1/Lab%204%20Enzyme%20Catalysis.pdfhttp://www.biology.hawaii.edu/171L/SummerSession1/Lab%204%20Enzyme%20Catalysis.pdf

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?

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.

Instructor: Dr. Marinella Sandros