Presentation on theme: "Speeding up the approach to equilibrium"— Presentation transcript:
1 Speeding up the approach to equilibrium CatalysisSpeeding up the approach to equilibrium
2 HistoryKirchoff in 1814 noted that acids aid hydrolysis of starch to glucoseFaraday (and Davy) studied oxidation catalysts in the 1820’sCatalyst defined by Berzelius in 1836A compound, which increases the rate of a chemical reaction, but which is not consumed by the reactionDeacon, Messel, Mond, Ostwald, Sebatier processes (HCl, SO2 oxidation, water gas shift, ammonia oxidation, ethene hydrogenation)20th C: ammonia production, cracking reactions, hydrocarbon production, catalytic converters etc.Catalysis science developed by Langmuir, Emmett, Rideal and others.
3 CatalysisWhen 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.
4 Heterogeneous catalysis Chemisorption and catalysisDiffusion of reactantsAdsorptionSurface diffusionReactionDesorptionDiffusion of products
5 2 main mechanisms Langmuir-Hinshelwood Reaction between adsorbates Eley-RidealReaction between adsorbate and incoming molecule
8 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.pAQAfastRDSkhetABRate = k qAFor Langmuir adsorption
9 LH model for unimolecular reaction Two limiting casesLow pressures/Weak bindingKp<<1Rate ≈ kKpRate linearly dependent on gas pressureFirst order reactionSurface coverage very lowHigh pressures/Strong bindingKp>>1Rate ≈ kRate independent of gas pressureZero order reactionSurface coverage almost unity
10 LH model for bimolecular reaction Langmuir-Hinshelwood reaction with surface reaction as rdspAQAfastRDSkhetAABBpBQBRate = k qA qB
14 LH model for bimolecular reaction Rate = k qA qB
15 LH model for bimolecular reaction pArateFor constant PBQB >> QARate limited bysurface concentration of AQB << QARate limited bysurface concentration of B
16 Eley-Rideal bimolecular surface reactions pAQAfastRDSkhetAABBpBAn adsorbed molecule may react directly with an impinging gas molecule by a collisional mechanism
17 Eley-Rideal bimolecular surface reactions rate = k QA pB= k KApA pB / (1+KApA)QA = 1pArateFor constant PBHigh pressureStrong bindingKApA >> 1rate = k pB …….. zero order in AkexpNote: For constant pA, the rate is always first order wrt pBLow pressureWeak bindingKApA << 1rate = khet KA pA pB …….. first order in Akexp
18 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-Scannot proceed when A blocks all sites.
19 Transition State Model of Catalyst Activity #homadsorbed reactantsadsorbed products#hetLangmuir-Hinshelwood KineticsAdsorption of reactants and desorption of products are very fast. DEads and DEdes very small.Surface Reaction is RDS: DEhetDEhomDEadsDEdesDEhetpotential energyreactantsproductsreaction co-ordinate
20 Principle of Sabatier A “volcano” curve 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
21 Catalyst Preparation For a catalyst the desired properties are high and stable activityhigh and stable selectivitycontrolled surface area and porositygood resistance to poisonsgood resistance to high temperatures and temperature fluctuations.high mechanical strengthno uncontrollable hazardsOnce a catalyst system has been identified, the parameters in the manufacture of the catalyst areIf 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 H2 in the reactor.
22 Case studies Ammonia synthesis (Haber-Bosch) Hydrogenation of CO (Fischer-Tropsch)
23 Ammonia synthesisA: Steam reforming B: High temperature water-gas shift C: Low temperature water-gas shift D: CO2 absorption E: Methanation F: Ammonia synthesis G: NH3 separation.
24 Ammonia reactants Steam reforming CH4(g) + H2O(g) CO(g) + 3 H2(g) 15-40% NiO/low SiO2/Al2O3 catalyst ( C)products often called synthesis gas or syngasWater gas shiftCO(g) + H2O(g) CO2(g) + H2(g)Cr2O3 and Fe2O3 as catalystcarbon dioxide removed by passing through sodium hydroxide.CO2(g) + 2 OH-(aq) CO32-(aq) + H2O(l)
26 Mechanism 1 N2(g) + * N2* 2 N2* + * 2N* 3 N* + H* NH* + * 4 NH* + H* N2*2N2* + *2N*3N* + H*NH* + *4NH* + H*NH2* + *5NH2* + H*NH3* + *6NH3*NH3(g) + *7H2(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.
27 CO+3H2CH4+H2O ( DG298, -140 kJ/mol) Hydrogenation of COHydrogenation of CO is thermodynamically favourable; the first example, methanation catalysed by nickel was reported by Sabatier and Senderens in 1902CO+3H2CH4+H2O ( DG298, -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 catalystsnCO+(2n+1)H2CnH2n+2+nH2O2nCO+(n+1)H2=CnH2n+2+nCO2Since syngas (CO + H2) 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
28 Fischer-TropschIron 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.
29 FT mechanismadsorption and cleavage of CO and the stepwise hydrogenation of surface carbide giving methylene and other speciesMaitlis, 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
30 Other mechanisms?Boudouard reaction. Important in methanation (over Nickel).2CO C + CO2Some evidence that hydrogenation of adsorbed carbon leads to formation of hydrocarbons.Also an important side (undesired) reaction in some hydrocarbon conversion reactions (coking)