II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964...

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.

0800 900

2 2 2ZnS(s) I (g) ZnI (g) 1/ 2 S (g)C

Solid to be Transported Transport AgentGaseous Species

Hand-Outs: 21


Nonvolatile reactants/products moved along an activity/temperature gradient at temperatures low compared to direct volatilization of the solid.

0800 900

2 2 2ZnS(s) I (g) ZnI (g) 1/ 2 S (g)C

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. 900C;ends of container are heated to 800C and 900C, 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.

Solid to be Transported Transport AgentGaseous Species

ZnS(s, crude)+ I2

ZnS(s, pure)I2(g) + ZnI2(g) + S2(g)

900C 800C

Hand-Outs: 21


0800 900

2 2 2ZnS(s) I (g) ZnI (g) 1/ 2 S (g)C

ZnS(s, crude)+ I2

ZnS(s, pure)I2(g) + ZnI2(g) + S2(g)

900C 800C

• “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 800C and 900C;(ii) diffusion coefficients of gases;(iii) cross-sectional area and length of the reaction tube.

Transport Rate: (mg/hr or mg/day)310 is desirable.i

i

p

p

Hand-Outs: 21


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.

A(s) B(g) C(g)i j k

Usually rate-determining step

Hand-Outs: 21


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.

A(s) B(g) C(g)i j k

Usually rate-determining step

Nature of the Gas Motion depends on Total Gas Pressure in the Container (Closed):

(a) Low total pressure (< 103 atm)Mean free path of gas molecules container dimensions – Molecular Flow(tendency to equalize pressure throughout the container)

(b) High total pressure (> 103 atm)Uniform gas density (constant total pressure throughout system) but a nonuniform composition (gradient) – Diffusion (tendency to equalizeconcentration throughout the container) (NOTE: rate of diffusion decreases as total pressure increases)

(c) Very high total pressure – Convection (tendency to equalize temperaturethroughout the container)

Hand-Outs: 21


A(s) B(g) C(g)i j k

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)

T2 T1

From Stoichiometry:

B A C A

A B C

and j k

n n n ni i

i in n n

j k

From Diffusion Theory:

Hand-Outs: 22


A(s) B(g) C(g)i j k


A(s, crude)+ B


T2 T1

From Stoichiometry:

B A C A

A B C

and j k

n n n ni i

i in n n

j k

From Diffusion Theory:

B CB BB B

B C 2 2

C B C CC C

B C 2 2

( )

( )

j c cdc DAt dp DAtn DAt p

ds kc jc RT ds sRT

dc k c c dpDAt DAtn DAt p

ds kc jc RT ds sRT

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)

H. Schäfer et al., Z. anorg. Allg. Chem.286, 27-55 (1956)

Hand-Outs: 22

Gas “FLOW”Gas “DIFFUSION”


A(s) B(g) C(g)i j k


A(s, crude)+ B


T2 T1

A BB

2

n i n i DAp

t j t j sRT CA

C2

nn i i DAp

t k t k sRT AND

Hand-Outs: 22


A(s) B(g) C(g)i j k


A(s, crude)+ B


T2 T1

A BB

2

n i n i DAp

t j t j sRT CA

C2

nn i i DAp

t k t k sRT AND

Estimates:1.5

TOT

1.8

,0

0 0

,

1.86 2

T

002

0

OT

0.1cm /se

Mechanical Arguments: ;

Empirical Relation: ;

For 1atm, 273 K c

4.1 10 (cm /sec)

,

TOT

TOT

TOT

TD

p

pD T

D p T

p T

Dp

D

T

(P.W. Atkins, Physical Chemistry)

Hand-Outs: 22


A(s) B(g) C(g)i j k


A(s, crude)+ B


T2 T1

0.84A B 2

TOT

1.8 10 (moles/hr)n i p T A

t j p s

Hand-Outs: 22


A(s) B(g) C(g)i j k


A(s, crude)+ B


T2 T1

0.84A B 2

TOT

1.8 10 (moles/hr)n i p T A

t j p s

Physical Controls

Wide, short tubes;Higher temperatures

Chemical Controls

Maximize pB:Reaction Thermodynamics

Hand-Outs: 22


A(s) B(g) C(g)i j k

Thermodynamics: What is the direction of chemical transport?

A(s, crude)+ B


T2 T1

High T to Low T(Hot-to-Cold)?

A(s, pure)A(s, crude)

+ BB(g) + C(g)

T2 T1

Low T to High T(Cold-to-Hot)?

-OR-

Hand-Outs: 23


A(s) B(g) C(g)

Thermodynamics: What is the direction of chemical transport?

0 0 0( ) ln ( )pG T H T S RT K T

0 01( ) exp expC B

pB B

p p H SK T

p p RT R

Conditions: pTOT = pB + pC = 1 atm;T1 = 1073 K, T2 = 1273 K

1Therefore, ( )

1 ( )Bp

p TK T

Calculate pB(T2) and pB(T1)

and then

pB = pB(T2) pB(T1)

Hand-Outs: 23


pTOT = pB + pC = 1 atm; T1 = 1073 K, T2 = 1273 K

pB = pB(T2) pB(T1) = pB(H0): plot…1

( )1 ( )B

p

p TK T

: constant S0, vary H0

H0 (kJ)

-100 -50 0 50 100

1000

p B

(at

m)

-40

-20

0

20

40 S0 = 0 J/K mol

EndothermicExothermic

pB(T2) > pB(T1) pB(T2) < pB(T1)

A(s) B(g) C(g)

Hand-Outs: 23




( )1 ( )B

p

p TK T


H0 (kJ)

-100 -50 0 50 100

1000

p B

(at

m)

-40

-20

0

20

40 S0 = 0 J/K mol


pB(T2) > pB(T1) pB(T2) < pB(T1)

pB sizable;

xB(T2) > xB(T1)A(s) forms at T2

Cold-to-Hot

pB sizable;

xB(T2) < xB(T1)A(s) forms at T1

Hot-to-Cold

A(s) B(g) C(g)

Hand-Outs: 23

H0 (kJ)

-100 -50 0 50 100

1000

p B

(at

m)

-300

-250

-200

-150

-100

-50

0

50

S0 = +50 J/K mol




( )1 ( )B

p

p TK T



pB(T2) < pB(T1)

pB sizable;

xB(T2) < xB(T1)A(s) forms at T1

Hot-to-Cold

With S0 > 0,only endothermicequilibrium createstransport conditions.

A(s) B(g) C(g)

Hand-Outs: 23

H0 (kJ)

-100 -50 0 50 100

1000

p B

(at

m)

-20

0

20

40

60

80

S0 = 10 J/K mol




( )1 ( )B

p

p TK T



pB(T2) > pB(T1)

pB sizable;

xB(T2) > xB(T1)A(s) forms at T2

Cold-to-Hot

With S0 < 0,only exothermicequilibrium createstransport conditions.

A(s) B(g) C(g)

Hand-Outs: 23

• 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.

II. Synthetic Aspects Chemical Transport “Rules” H. Schäfer, Chemical Transport Reactions, 1964

Justifies I2 as useful transport agent:M-I bonds are weaker than other

M-X bonds, so H0 typically small.

Hand-Outs: 24


7 Nb(s) + 8 NbCl5(s) 5 Nb3Cl8(s)

Transport Equilibrium:3 8 5 4Nb Cl (s) + 4 NbCl (g) 7 NbCl (g)

H0 = + 457.3 kJ/mol Nb3Cl8;S0 = + 487.9 J/Kmol Nb3Cl8;

Therefore, G0 ~ 0 at 937 K = 664 C

4

4 5 4 5

5

7NbCl 47

NbCl NbCl TOT NbCl NbCl4NbCl

; ( ) andp p

pK p K T p p p p

p

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.

Hand-Outs: 25

T (K) Kp p(NbCl5) (atm) p(NbCl4) (atm) PTOT

700 2.3 109 0.9 0.055 0.955

0.95 0.057 1.01

800 4.2 105 0.9 0.22 1.12

0.8 0.21 1.01

900 8.8 102 0.7 0.58 1.28

0.5 0.48 0.98

0.55 0.50 1.05

1000 4.0 101 0.3 0.85 1.15

0.2 0.68 0.88

0.25 0.77 1.02

1100 5.9 103 0.1 0.93 1.03

0.05 0.62 0.67



Hand-Outs: 25

T (K)

700 800 900 1000 1100

p i (

atm

)

0.0

0.2

0.4

0.6

0.8

1.0

NbCl5(g)

NbCl4(g)

5 Nb3Cl8(s) 7 Nb(s) + 8 NbCl5(g)

TransportTemperatures

pi = 0.2-0.3 atm



Product growshere

Hand-Outs: 25

Estimate Transport Rate:Tube: 20 cm long, 1 cm diameterT1 = 700 K, T2 = 800 Kp(NbCl5) = 0.25 atm.

3 8

0.8 2Nb Cl 4 51 0.25 atm (800 K) (0.785 cm )

(1.8 10 ) 9.28 10 mole/hr4 1.00 atm 20 cm

= 0.052 g/hr

n

t

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.



(Need ca. 0.065 g NbCl5 forca. 1 atm pressure)

Hand-Outs: 25

II. Synthetic Aspects Examples of Chemical Transport H. Schäfer, Chemical Transport Reactions, 1964

800 CZn(s) S(g) ZnS(s) Low conversion due to protective skin that grows on

the metal surface and prevents further reaction;

800 C 900 C

2 2 2ZnS(s) I (g) ZnI (g) (1/ 2) S (g)

2 mg I2 / cm3 gives nearly completeconversion.

80 C 200 C

4Ni(s) 4 CO(g) Ni(CO) (g)

H0 = 300 kJ/mol: transport from low Tto high T; efficient to purify noble metals.

2 6W(s) 3 Cl (g) WCl (g) W filaments in an atmosphere with a small partialpressure of WCl6(g) can sustain themselves by transport.H0 < 0, so W is cold (thicker) parts of filament transport to hot (thinner) parts of the filament.

Hand-Outs: 26


very slowM(s) P(red,s) M P (s)a ba b

0

2 2 4

2 4 2

2 P(red,s) 2 I (g) P I (g)

M(s) P I (g) M P (s) 2 I (g)2

H

a b

ba b

A small amount of I2 transports red P by formingP2I4(g), which then froms metal phosphides andregenerates the transport agent.

Disproportionation Reactions: often endothermic processes and typically S0 > 0, sothey are good candidates for transport equilibria:

1000 600

3 (Al) 660Al(s) (1/ 2) AlBr (g) (3/ 2) AlBr(g)

fT

1100 900

4 2Si(s) SiCl (g) 2 SiCl (g)

Hand-Outs: 26


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…

0

2 4Nb(s) 2 I (g) NbI (g)H

Exothermic: cold-to-hot, so loadmixture in cold end of the tube.

19 kJ

3 3 21100 900

92 kJ

2 3 3 2600 900

3 Cu(s) 3 HCl(g) Cu Cl (g) (3/ 2) H (g)

(3/2) Cu O(s) 3 HCl(g) Cu Cl (g) (3/ 2) H O(g)

H

H

Hand-Outs: 26


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…

0

2 4Nb(s) 2 I (g) NbI (g)H

Exothermic: cold-to-hot, so loadmixture in cold end of the tube.

19 kJ

3 3 21100 900

92 kJ

2 3 3 2600 900

3 Cu(s) 3 HCl(g) Cu Cl (g) (3/ 2) H (g)

(3/2) Cu O(s) 3 HCl(g) Cu Cl (g) (3/ 2) H O(g)

H

H

284 kJ

2 21100 900Cu O(s) 2 HCl(g) 2 CuCl(g) H O(g)

H

Cu2O600 C 1100 C

NOTE:

Hand-Outs: 26

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.

II. Synthetic Aspects References

Hand-Outs: 26

Date post:	26-Dec-2015
Category:	Documents
Upload:	aubrie-booth
View:	217 times
Download:	0 times

II. Synthetic Aspects Chemical (Vapor) Transport H. Schäfer, Chemical Transport Reactions, 1964...

Documents