Presentation on theme: "Catalysis Speeding up the approach to equilibrium."— Presentation transcript:
Catalysis Speeding up the approach to equilibrium
History Kirchoff in 1814 noted that acids aid hydrolysis of starch to glucose Faraday (and Davy) studied oxidation catalysts in the 1820’s Catalyst defined by Berzelius in 1836 A compound, which increases the rate of a chemical reaction, but which is not consumed by the reaction Deacon, Messel, Mond, Ostwald, Sebatier processes (HCl, SO 2 oxidation, water gas shift, ammonia oxidation, ethene hydrogenation) 20 th C: ammonia production, cracking reactions, hydrocarbon production, catalytic converters etc. Catalysis science developed by Langmuir, Emmett, Rideal and others.
Catalysis When we consider a catalytic reaction, we may imagine that the reaction mechanism consists of many different steps. Catalyst must be a reactant in one of the first steps in the mechanism and a product in one of the last steps.
Heterogeneous catalysis Chemisorption and catalysis Diffusion of reactants Adsorption Surface diffusion Reaction Desorption Diffusion of products
2 main mechanisms Langmuir-Hinshelwood Reaction between adsorbates Eley-Rideal Reaction between adsorbate and incoming molecule
LH model for unimolecular reaction Decomposition occurs uniformly across the surface. Products are weakly bound and rapidly desorbed. The rate determining step (rds) is the surface decomposition step. Rate = k A For Langmuir adsorption pApA fast RDS k het A B
LH model for unimolecular reaction Two limiting cases High pressures/ Strong binding Kp>>1 Rate ≈ k Rate independent of gas pressure Zero order reaction Surface coverage almost unity Low pressures/ Weak binding Kp<<1 Rate ≈ kKp Rate linearly dependent on gas pressure First order reaction Surface coverage very low
LH model for bimolecular reaction Langmuir-Hinshelwood reaction with surface reaction as rds Rate = k A B pApA fast RDS k het A AB B pBpB
Langmuir adsorption of mixed components
LH model for bimolecular reaction Rate = k A B
LH model for bimolecular reaction pApA rate For constant P B Rate limited by surface concentration of A Rate limited by surface concentration of B
Eley-Rideal bimolecular surface reactions pApA fast RDS k het A AB B pBpB An adsorbed molecule may react directly with an impinging gas molecule by a collisional mechanism
Eley-Rideal bimolecular surface reactions rate = k p B k K A p A p B / (1+K A p A ) = 1 pApA rate For constant P B Low pressure Weak binding K A p A << 1 rate = k het K A p A p B …….. first order in A k exp High pressure Strong binding K A p A >> 1 rate = k p B …….. zero order in A k exp Note: For constant p A, the rate is always first order wrt p B
Diagnosis of mechanism If we measure the reaction rate as a function of the coverage by A, the rate will initially increase for both mechanisms. Eley-Rideal: rate increases until surface is covered by A. Langmuir-Hinshelwood: rate passes a maximum and ends up at zero, when surface covered by A. B + S B-S cannot proceed when A blocks all sites.
Transition State Model of Catalyst Activity reactants products E hom potential energy E ads E des E het reaction co-ordinate Langmuir-Hinshelwood Kinetics Adsorption of reactants and desorption of products are very fast. E ads and E des very small. Surface Reaction is RDS: E het transition state # hom adsorbed reactants adsorbed products # het
Principle of Sabatier When different metals are used to catalyse the same reaction, it is generally observed that the reaction rate can be correlated with the position of the metal in the periodic table: A “volcano” curve
Catalyst Preparation For a catalyst the desired properties are high and stable activity high and stable selectivity controlled surface area and porosity good resistance to poisons good resistance to high temperatures and temperature fluctuations. high mechanical strength no uncontrollable hazards Once a catalyst system has been identified, the parameters in the manufacture of the catalyst are If the catalyst should be supported or unsupported. The shape of the catalyst pellets. The shape (cylinders, rings, spheres, monoliths) influence the void fraction, the flow and diffusion phenomena and the mechanical strength. The size of the catalyst pellets. For a given shape the size influences only the flow and diffusion phenomena, but small pellets are often much easier to prepare. Catalyst based on oxides are usually activated by reduction in H 2 in the reactor.
Case studies Ammonia synthesis (Haber-Bosch) Hydrogenation of CO (Fischer-Tropsch)
Ammonia synthesis A: Steam reforming B: High temperature water-gas shift C: Low temperature water-gas shift D: CO 2 absorption E: Methanation F: Ammonia synthesis G: NH 3 separation.
Ammonia reactants Steam reforming CH 4 (g) + H 2 O(g) CO(g) + 3 H 2 (g) 15-40% NiO/low SiO 2 /Al 2 O 3 catalyst ( C) products often called synthesis gas or syngas Water gas shift CO(g) + H 2 O(g) CO 2 (g) + H 2 (g) Cr 2 O 3 and Fe 2 O 3 as catalyst carbon dioxide removed by passing through sodium hydroxide. CO 2 (g) + 2 OH - (aq) CO 3 2- (aq) + H 2 O(l)
Ammonia Synthesis Fe/K catalyst exothermic
Mechanism 1N 2 (g) + * N2*N2* 2N 2 * + * 2N* 3N* + H* NH* + * 4NH* + H* NH 2 * + * 5NH 2 * + H* NH 3 * + * 6NH 3 * NH 3 (g) + * 7H 2 (g) + 2* 2H* Step 2 is generally rate-limiting. Volcano curve is therefore apparent with d-block metals as catalysts. Ru and Os are more active catalysts, but iron is used.
Hydrogenation of CO Hydrogenation of CO is thermodynamically favourable; the first example, methanation catalysed by nickel was reported by Sabatier and Senderens in 1902 CO+3H 2 CH 4 +H 2 O ( G 298, -140 kJ/mol) In their classic 1926 papers Fischer and Tropsch showed that linear alkenes and alkanes (as well as some oxygenates) are formed at 200–300°C and atmospheric pressure over Co or Fe catalysts nCO+(2n+1)H 2 C n H 2n+2 +nH 2 O 2nCO+(n+1)H 2 =C n H 2n+2 +nCO 2 Since syngas (CO + H 2 ) is readily available from a variety of fossil fuels, including coal, the Fischer–Tropsch process became industrially important for economies which had good supplies of cheap coal but which lacked oil
Fischer-Tropsch Iron catalysts give mainly linear alkenes and oxygenates, while cobalt gives mostly linear alkanes. Ruthenium, one of the most active catalysts but one which, owing to its expense is little used industrially, can give high molecular weight hydrocarbons; rhodium catalysts make significant amounts of oxygenates in addition to hydrocarbons, while nickel gives mainly methane. Catalyst can be immobilised on Kieselguhr (diatomaceous silicate earth), alumina, active carbon, clays and zeolites.
FT mechanism adsorption and cleavage of CO and the stepwise hydrogenation of surface carbide giving methylene and other species Maitlis, P. M.; Quyoum, R.; Long, H. C.; Turner, M. L. Appl. Catal. A: General 1999, 186, Towards a Chemical Understanding of the Fischer-Tropsch Reaction: Alkene Formation
Other mechanisms? Boudouard reaction. Important in methanation (over Nickel). 2CO C + CO 2 Some evidence that hydrogenation of adsorbed carbon leads to formation of hydrocarbons. Also an important side (undesired) reaction in some hydrocarbon conversion reactions (coking)