Coprecipitation

December 16 2008

  

Archy, OK, Jesper and Maria

Outline

 

Precipitation

“The forming of a solid phase within a liquid phase”

Ex. Fe(OH)3, AgCl, BaSO4

http://www.dartmouth.edu/~chemlab/chem3-5/qual_cat/graphics/procedure/proc6.gif

Coprecipitation

Mechanisms• Surface adsorption• Mixed-crystal formation• Occlusion• Mechanical entrapment

Overview of process

1. Atomic mixing 2. Adding of precipitation agent 3. Calcination

Analytical Study of Oxalates Coprecipitation, Marta et al

1. Atomic mixing of precursors

Metaloxides or metalsalts are common precursors. The solubility of the salts or MOs are limiting.An acid or a polar solvent is usually employed.Common acids: HNO3, HAc, HClPolar solvents are traditionally alcohols or water or a mixture of the two.

2. Adding of precipitating agent

Temperature range 0-80°C Precipitating agents are typically oxalic acid, ammonium oxalat or ammonium hydroxidThe adding is often done by titration

Simultaneous precipitation

1. Formation of TiOC2O4 (Ti : C2O4 ratio 1:1, pH ≤ 2)(C4H9O)4Ti + H2C2O4·2H2O → TiOC2O4 + 4C4H9OH2. Conversion of TiOC2O4 to soluble Na2TiO(C2O4)2 (Ti : C2O4 ratio 1:2)TiOC2O4 + Na2C2O4 → Na2TiO(C2O4)2

pH range 2.5-3.5: TiO(C2O4)22-

3. Addition of Ba(CH3COO)2 resulting in simultaneous precipitation of (Ba + Ti) in the form of oxalatesTiO(C2O4)22- + 2H2O → TiOC2O4(H2O)2 ↓ + C2O42-

Ba2+ + C2O42- → BaC2O4↓

Chemical coprecipitation of mixed (Ba + Ti) oxalates precursor leading to BaTiO3 powders, Potdar et al, 1998

Simultaneous precipitation

4. BaTiO3 powders are produced after pyrolysis in air of the mixed oxalates [BaC2O4 + TiOC2O4(H2O)2] precursor

Chemical coprecipitation of mixed (Ba + Ti) oxalates precursor leading to BaTiO3 powders, Potdar et al, 1998

Parameters

• pH• Solubility• Temperature

pH

Constant vs variable pH Layered Double Hydroxides (LDHs) cation pairs: Mg(II)-Al(III) and Zn(II)-Cr(III) anions: terephthalate (TA) and dodecylsulfate (DS)• varying the cation combination, interlamellar anion and pH

control, with or without submission to hydrothermal treatment

Comparative study of the coprecipitation methods for the preparation of Layered Double Hydroxides, Crepaldi et al, 2000

pH - Crystallinity

Comparative study of the coprecipitation methods for the preparation of Layered Double Hydroxides, Crepaldi et al, 2000

pH – Specific Surface Area and average pore diameter

Comparative study of the coprecipitation methods for the preparation of Layered Double Hydroxides, Crepaldi et al, 2000

pH - dependence

Journal of Materials Science 25 (1990) 3634-3640

Competing reactions

In the coprecipitation process, a number of processes compete. In this generalized example metal ions were added to an oxalate-diethylamine media

The competing precipitants are Ox and OH-:

• pH• pOx• Solubility

Studies of theoretical and experimental precipitation conditions of Y+3, Ba+2, and Cu+2 ions in oxalate-diethylamine media in the preparation of YBa2Cu3Oy superconductor, Chen et al, 1992

pH region and Solubility

Studies of theoretical and experimental precipitation conditions of Y+3, Ba+2, and Cu+2 ions in oxalate-diethylamine media in the preparation of YBa2Cu3Oy superconductor, Chen et al, 1992

Studies of theoretical and experimental precipitation conditions of Y+3, Ba+2, and Cu+2 ions in oxalate-diethylamine media in the preparation of YBa2Cu3Oy superconductor, Chen et al, 1992

Aggregation rates

Aggregation rate depends on:• pH • Concentration• Ionic strength

The generally accepted theory for agglomoration of particles comes from the DLVO-theory

Studies of theoretical and experimental precipitation conditions of Y+3, Ba+2, and Cu+2 ions in oxalate-diethylamine media in the preparation of YBa2Cu3Oy superconductor, Chen et al, 1992

Derjaguin Landau Verwey Overbeek

The DLVO-theory was first presented in the 1940's. It describes the force between charged particles interacting through a liquid medium.It combines the van der Waals attractive forces with the counterion double layer repulsive forces.

β − 1 = kBT, the thermal energy scale.κ − 1 = Debye-Hückel screening lengthr = center to center particle distanceZ = constant surface chargeλB= is the Bjerrum length

Aggregation rate, Ionic strength

Ionic strength: measure of the particle concentration of all ions in a solution.Fe2O3 particles were held @ pH 5.7. 1-200 mM NaCl was employed as a variator of ionic strength.

Aggregation rate increases with increased ionic strength

Kinetic stability of hematite nanoparticles: the effect of particle size, He et al, 2007

Aggregation rate, pH

FeO3 particle solution ranged from pH 5-9.

Aggregation rate increases with pH increase, the result is more obvious close to the IEP of the particles.The same is has shown to be true in similar experiments using Y2O3 particles in solution.

Kinetic stability of hematite nanoparticles: the effect of particle size, He et al, 2007

Aggregation rate, particle concentration

13, 44, 132 and 440 mg/L of 65 nm Fe2O3 particles was used.

Aggregation rate is increased with increasing concentrationsSimilar experiments indicating the same results have been done with kaolinite particles as well as colloidal polystyrene particles

Kinetic stability of hematite nanoparticles: the effect of particle size, He et al, 2007

Coprecipitation

Chemical precipitation is widely used in industry. Especially to synhesize complex metaloxides.

Common metal oxides: BaTiO3,Y3Al5O12,YBa2Cu3O7

Advantages of Coprecipitation

• Technical simplicity• Low manufacturing

costs• High reproducability• Fine particle size

Challenges• Difficult to control chemical

composition• Time consuming• Upscaling issue