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II. Synthetic Aspects

Hand-Outs: 21. II. Synthetic Aspects Chemical (Vapor) Transport H. Sch äfer, Chemical Transport Reactions , 1964. Nonvolatile reactants/products moved along an activity/temperature gradient at temperatures low compared to direct volatilization of the solid.

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II. Synthetic Aspects

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  1. Hand-Outs: 21 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Nonvolatile reactants/products moved along an activity/temperature gradient at temperatures low compared to direct volatilization of the solid. Gaseous Species Solid to be Transported Transport Agent

  2. Hand-Outs: 21 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Nonvolatile reactants/products moved along an activity/temperature gradient at temperatures low compared to direct volatilization of the solid. Gaseous Species Solid to be Transported Transport Agent Standard Set-Up: crude ZnS(s) and I2(s) placed into closed container; enough I2(s) to give 0.1-1.0 atm at ca. 900C; ends of container are heated to 800C and 900C, creating a temperature (and pressure) gradient in the tube. I2: transport agent (very common; also HCl(g) and O2(g)) H > 0 (endothermic): ZnS(s) transported from high T to low T. ZnS(s, crude) + I2 ZnS(s, pure) I2(g) + ZnI2(g) + S2(g) 900C 800C

  3. Hand-Outs: 21 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 ZnS(s, crude) + I2 ZnS(s, pure) I2(g) + ZnI2(g) + S2(g) 900C 800C • “Local” equilibrium pertains: heterogeneous reaction faster than diffusion of ZnI2 + S2; • Diffusion of I2 is also not significant with respect to “background” gas = I2(g); • Rate-determining transport of ZnI2 and S2 depends on • (i) difference in equilibrium pressures at 800C and 900C; • (ii) diffusion coefficients of gases; • (iii) cross-sectional area and length of the reaction tube. Transport Rate: (mg/hr or mg/day)

  4. Hand-Outs: 21 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Principles: (1) Heterogeneous reaction of the gas B on the solid A; (2) Gas (B + C) motion in the container; (3) Heterogeneous reaction to reform solid A. Usually rate-determining step

  5. Hand-Outs: 21 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Principles: (1) Heterogeneous reaction of the gas B on the solid A; (2) Gas (B + C) motion in the container; (3) Heterogeneous reaction to reform solid A. Usually rate-determining step Nature of the Gas Motion depends on Total Gas Pressure in the Container (Closed): (a) Low total pressure (< 103 atm) Mean free path of gas molecules  container dimensions – Molecular Flow (tendency to equalize pressure throughout the container) (b) High total pressure (> 103 atm) Uniform gas density (constant total pressure throughout system) but a nonuniform composition (gradient) – Diffusion (tendency to equalize concentration throughout the container) (NOTE: rate of diffusion decreases as total pressure increases) (c) Very high total pressure – Convection (tendency to equalize temperature throughout the container)

  6. Hand-Outs: 22 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Kinetics: What is the rate of chemical transport? (# moles A(s) transported, nA, in time t) A(s, crude) + B A(s, pure) B(g) + C(g) T1 T2 From Stoichiometry: From Diffusion Theory:

  7. Hand-Outs: 22 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Kinetics: What is the rate of chemical transport? (# moles A(s) transported, nA, in time t) A(s, crude) + B A(s, pure) B(g) + C(g) T1 T2 From Stoichiometry: From Diffusion Theory: Gas “DIFFUSION” Gas “FLOW” H. Schäfer et al., Z. anorg. Allg. Chem. 286, 27-55 (1956) cB, cC = concentrations of gases (moles/cm3) A = cross sect. area (cm2) D = diffusion coeff. for B(g) + C(g) (cm2/sec) s = length (cm) t = time of experiment (sec)

  8. Hand-Outs: 22 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Kinetics: What is the rate of chemical transport? (# moles A(s) transported, nA, in time t) A(s, crude) + B A(s, pure) B(g) + C(g) T1 T2 AND

  9. Hand-Outs: 22 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Kinetics: What is the rate of chemical transport? (# moles A(s) transported, nA, in time t) A(s, crude) + B A(s, pure) B(g) + C(g) T1 T2 AND Estimates: (P.W. Atkins, Physical Chemistry)

  10. Hand-Outs: 22 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Kinetics: What is the rate of chemical transport? (# moles A(s) transported, nA, in time t) A(s, crude) + B A(s, pure) B(g) + C(g) T1 T2

  11. Hand-Outs: 22 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Kinetics: What is the rate of chemical transport? (# moles A(s) transported, nA, in time t) A(s, crude) + B A(s, pure) B(g) + C(g) T1 T2 Chemical Controls Maximize pB: Reaction Thermodynamics Physical Controls Wide, short tubes; Higher temperatures

  12. Hand-Outs: 23 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Thermodynamics: What is the direction of chemical transport? A(s, crude) + B A(s, pure) B(g) + C(g) T1 T2 High T to Low T (Hot-to-Cold)? -OR- A(s, pure) A(s, crude) + B B(g) + C(g) T2 T1 Low T to High T (Cold-to-Hot)?

  13. Hand-Outs: 23 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Thermodynamics: What is the direction of chemical transport? Conditions: pTOT = pB + pC = 1 atm; T1 = 1073 K, T2 = 1273 K CalculatepB(T2) and pB(T1) and then pB = pB(T2)  pB(T1)

  14. : constant S0, vary H0 Hand-Outs: 23 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 pTOT = pB + pC = 1 atm; T1 = 1073 K, T2 = 1273 K pB = pB(T2)  pB(T1) = pB(H0): plot… Exothermic Endothermic pB(T2) > pB(T1) pB(T2) < pB(T1)

  15. : constant S0, vary H0 Hand-Outs: 23 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 pTOT = pB + pC = 1 atm; T1 = 1073 K, T2 = 1273 K pB = pB(T2)  pB(T1) = pB(H0): plot… Exothermic Endothermic pB sizable; xB(T2) < xB(T1) A(s) forms at T1 Hot-to-Cold pB sizable; xB(T2) > xB(T1) A(s) forms at T2 Cold-to-Hot pB(T2) > pB(T1) pB(T2) < pB(T1)

  16. : constant S0, vary H0 Hand-Outs: 23 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 pTOT = pB + pC = 1 atm; T1 = 1073 K, T2 = 1273 K pB = pB(T2)  pB(T1) = pB(H0): plot… Exothermic Endothermic pB sizable; xB(T2) < xB(T1) A(s) forms at T1 Hot-to-Cold With S0 > 0, only endothermic equilibrium creates transport conditions. pB(T2) < pB(T1)

  17. : constant S0, vary H0 Hand-Outs: 23 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 pTOT = pB + pC = 1 atm; T1 = 1073 K, T2 = 1273 K pB = pB(T2)  pB(T1) = pB(H0): plot… Exothermic Endothermic With S0 < 0, only exothermic equilibrium creates transport conditions. pB sizable; xB(T2) > xB(T1) A(s) forms at T2 Cold-to-Hot pB(T2) > pB(T1)

  18. Hand-Outs: 24 II. Synthetic Aspects Chemical Transport “Rules” H. Schäfer, Chemical Transport Reactions, 1964 • A reaction supports transport only when no solid is present on one side of the chemical • equation; • A reaction with an extreme equilibrium position (large |H0|) gives no measurable transport; • Sign of H0 determines the transport direction: • Exothermic reactions transport from low T to high T; • Endothermic reactions transport from high T to low T; • When H0 = 0, p = 0 and no transport takes place; • For any value of S0 0, there is a value of H0 that gives maximum transport; • For large |S0|, transport is only possible when H0 and S0 have the same sign: • Transport becomes significant when ln Kp is ca. 0; • If S0 is small, then, depending on the sign of H0, transport can take place in either • direction; • Reactions with large, positive S0, transport can only occur from high T to low T (H0 > 0); • Maximum transport value increases with an increasing magnitude of S0, when H0 changes • correspondingly and p becomes larger. Justifies I2 as useful transport agent: M-I bonds are weaker than other M-X bonds, so H0 typically small.

  19. Hand-Outs: 25 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 7 Nb(s) + 8 NbCl5(s)  5 Nb3Cl8(s) Transport Equilibrium: H0 = + 457.3 kJ/mol Nb3Cl8; S0 = + 487.9 J/Kmol Nb3Cl8; Therefore, G0~ 0 at 937 K = 664 C Therefore, (a) Optimum controlled transport conditions will happen around 900 K; (b) Use Kp(T) and pTOT~ 1 atm to determine partial pressures of gases; (c) Endothermic equilibrium: transport to low T, place reactants in hot end.

  20. Hand-Outs: 25 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Transport Equilibrium:

  21. Hand-Outs: 25 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Transport Equilibrium: 5 Nb3Cl8(s)  7 Nb(s) + 8 NbCl5(g) NbCl4(g) NbCl5(g) Product grows here Transport Temperatures pi= 0.2-0.3 atm

  22. Hand-Outs: 25 II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964 Transport Equilibrium: Estimate Transport Rate: Tube: 20 cm long, 1 cm diameter T1 = 700 K, T2 = 800 K p(NbCl5) = 0.25 atm. (Need ca. 0.065 g NbCl5 for ca. 1 atm pressure) NOTE: (1) Generally favorable to keep temperature gradient small; (2) At high temperatures, 5 Nb3Cl8(s)  7 Nb(s) + 8 NbCl5(g); (3) Another competing phase is “Nb3Cl7(s)” = Nb6Cl14(s), so even cooler transport temperatures chosen in the experiment (ca. 650-700 K); increases time by factor of 7-10.

  23. Hand-Outs: 26 II. Synthetic Aspects Examples of Chemical Transport H. Schäfer, Chemical Transport Reactions, 1964 Low conversion due to protective skin that grows on the metal surface and prevents further reaction; 2 mg I2 / cm3 gives nearly complete conversion. H0 = 300 kJ/mol: transport from low T to high T; efficient to purify noble metals. W filaments in an atmosphere with a small partial pressure of WCl6(g0 can sustain themselves by transport. H0 < 0, so W is cold (thicker) parts of filament transport to hot (thinner) parts of the filament.

  24. Hand-Outs: 26 II. Synthetic Aspects Examples of Chemical Transport H. Schäfer, Chemical Transport Reactions, 1964 A small amount of I2 transports red P by forming P2I4(g), which then froms metal phosphides and regenerates the transport agent. Disproportionation Reactions: often endothermic processes and typically S0 > 0, so they are good candidates for transport equilibria:

  25. Hand-Outs: 26 II. Synthetic Aspects Examples of Chemical Transport H. Schäfer, Chemical Transport Reactions, 1964 Separation / Purification Reactions: mixture of M(s) and M(s) (1) M(s) transports; M(s) volatilizes (2) M(s) transports; M(s) does not: Nb(s) and NbC(s) (3) M(s) and M(s) transport in same direction – 2 different temperature ranges; impure (4) M(s) transports by exothermic equilibrium; M(s) by endothermic equilibrium: Cu(s) and Cu2O(s) mixture using HCl(g) as transport agent… Exothermic: cold-to-hot, so load mixture in cold end of the tube.

  26. Hand-Outs: 26 II. Synthetic Aspects Examples of Chemical Transport H. Schäfer, Chemical Transport Reactions, 1964 Separation / Purification Reactions: mixture of M(s) and M(s) (1) M(s) transports; M(s) volatilizes (2) M(s) transports; M(s) does not: Nb(s) and NbC(s) (3) M(s) and M(s) transport in same direction – 2 different temperature ranges; impure (4) M(s) transports by exothermic equilibrium; M(s) by endothermic equilibrium: Cu(s) and Cu2O(s) mixture using HCl(g) as transport agent… Exothermic: cold-to-hot, so load mixture in cold end of the tube. NOTE: 600 C Cu2O 1100 C

  27. Hand-Outs: 26 II. Synthetic Aspects References Reginald Gruehn and Robert Glaum, “New results of chemical transport as a method for the preparation and thermochemical investigation of solids,” Angewandte Chemie, International Edition (2000), 39(4), 692-716. M. Lenz and Reginald Gruehn, “Developments in Measuring and Calculating Chemical Vapor Transport Phenomena demonstrated on Cr, Mo, W, and Their Compounds,” Chemical Reviews (Washington, D. C.) (1997), 97(8), 2967-2994. Mercouri Kanatzidis, Rainer Pottgen, Wolfgang Jeitschko, “The metal flux: A preparative tool for the exploration of intermetallic compounds,” Angewandte Chemie, International Edition (2005), 44(43), 6996-7023.

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