Chapter 2 Experimental
2.1 Catalyst preparation
34
Chapter 2
35 Co-precipitation is most commonly utilised when a homogenous distribution of catalyst components or precursors with a definite stoichiometry are desired. A nucleus has to form in order for a solid to precipitate out of a homogeneous solution. The formation of a precipitated particle is governed by the free energy of agglomerates of the components of the solution. The total free energy, G, is determined by equation 2:
G = Gbulk + Ginterface + Gothers (2)
where Gbulk is the difference of the free energy between solution and solid species, Ginterface
is the free energy change related to the formation of the interface and Gothers summarises all other contributions.
Several possible methods may be used to carry out the precipitation process. In the batch process, the pH and all other parameters, excluding temperature, change continuously during the precipitation due to the consumption of the metal species. The advantage of this method is the simple way in which the product is obtained. A more rigorous process involves the simultaneous addition of the precursors, controlling the pH and the reagent ratios at all times.
The control of these parameters prevents phase transitions of the precipitates formed first.
This may altogether be avoided by using a continuous process. In the continuous process all the parameters, including temperature, concentration, pH and residence times of the precipitate are kept constant or altered as needed [1].
Hydroxyapatite (HAp) is best prepared by co-precipitation. The characteristics of HAp prepared by this method include high surface area, fine particle size, non-stoichiometry and low crystallinity. These characteristics depend on several factors such as pH, aging time, temperature, and type and concentration of the starting materials. In the precipitation, calcium is supplied as a chloride, nitrate, acetate, hydroxide, carbonate or sulphate salt and phosphorus in the form of ammonium phosphate, phosphoric acid or sometimes sodium and potassium phosphate salts. Nitrates are the preferred metal salts, as NO3- ions are not as easily incorporated into the lattice of HAp. Ammonium salts are preferred as a source of phosphorus as opposed to sodium and potassium salts for similar reasons. Maintaining the pH
36 of the reaction mixture between 9 and 11 is very important as nucleation of HAp occurs in a basic pH range. Adjusting the pH of the solution is best done with ammonia gas or aqueous ammonia as opposed to metal hydroxide salts (such as KOH or NaOH) as this avoids trace contamination of metals that could possibly play a role in the catalysis. The advantage of using an ammonia solution is that NH4+ can be removed from the precipitate by decomposition into the gas phase upon heating. It is generally accepted that using a higher pH tends to eliminate the formation of carbonate species in the HAp powders.
Catalyst materials may consist of one or several active components deposited on a high surface area support. The primary advantage of using a support is the optimal dispersion of the catalytically active phase and to stabilise this against sintering. The support used must also be inert. A commonly employed technique used to deposit the active phase on a support is wet impregnation. The impregnation technique involves the inclusion of a precursor into the pore space of the support. The precursor, at this point of the preparation method, is still dissolved in solution and cannot become fixed on the surface. The precursor is generally fixed on the surface by the thermal decomposition of the precursor thus forming the catalytically active phase.
The catalysts used in this study were prepared by two different wet chemical techniques. The techniques chosen were wet impregnation and co-precipitation, because of their simplicity and ease of scale up for industrial applications.
2.1.1 Synthesis of hydroxyapatite
Hydroxyapatite (HAp) was synthesised using a co-precipitation method [2-4]. A solution of diammonium hydrogen orthophosphate, (NH4)2HPO4 (5.6 g), (1.00 mol) (Merck, 98.5 %) was adjusted to pH 11.0 using a dilute ammonia solution (Rochelle Chemicals, 25 %) and then diluted to 200 mL with double distilled water. The pH was measured using a pH 211 Microprocessor pH meter (HANNA Instruments). Similarly, a calcium nitrate tetrahydrate, Ca(NO3)2.4H2O (23.7 g), (0.167 mol) (Merck, 98 %) solution was prepared and the pH adjusted to 11.0 and this solution was then diluted to 200 mL using double distilled water.
37 The (NH4)2HPO4 solution was added drop-wise to the Ca(NO3)2.4H2O solution with constant and vigorous stirring at room temperature over a period of 1 hour. The pH of the mixture was maintained between 9.50 and 11.0 throughout, using diluted ammonia. The resultant white gelatinous precipitate was heated to and maintained at 85 °C for 1.5 hours and then allowed to cool to room temperature. Thereafter, the precipitate was filtered under vacuum and washed repeatedly until the filtrate was neutral, thus having ensured the removal of excess diluted ammonia. The HAp was then dried in an oven overnight at 120 °C and thereafter, calcined at 550 °C (reached from room temperature at a ramp rate of 1.5 °C min-1) for 6 hours.
2.1.2 Synthesis of the iron supported on hydroxyapatite catalyst
Iron in the form of iron oxide, -Fe2O3, was supported on the surface of HAp using a wet impregnation technique [2]. A 1 wt% loading of iron supported on Hap catalyst was synthesised by a single impregnation of the support (HAp) with a ferric nitrate, Fe(NO3)3.9H2O (Merck, 97 %), solution. The Fe(NO3)3.9H2O (0.19 g) was dissolved in 20 mL of double distilled water and added drop-wise to the HAp (2.0 g) with continuous stirring.
The mixture was left to stir for 6 hours after which excess water was removed by gentle heating and the resultant compound was dried overnight at 120 °C. The powdered form of the catalyst material was calcined at 550 °C at a ramp rate of 1.5 °C min-1 for 6 hours. Similarly 3 and 9 wt% iron supported on HAp catalysts were prepared. The catalysts were denoted 1- WET, 3-WET and 9-WET where the number denoted the loadings of 1, 3, and 9 wt% iron respectively.
2.1.3 Synthesis of iron substituted hydroxyapatite catalyst
Weight percentage loadings of 1, 3 and 9 wt% iron substituted HAps were prepared using a co-precipitation method [4-6]. The metal nitrate precursor solution was prepared by adding calculated masses of Ca(NO3)2.4H2O (46.7 g) and Fe(NO3)3.9H2O (1.2 g) to 60 mL of double distilled water. This solution was then diluted to 200 mL with double distilled water. A 100 mL solution of (NH4)2HPO4 (11.3 g) was then prepared and adjusted to pH 11.0. This phosphorus containing solution was then diluted to 160 mL using double distilled water. The
38 metal nitrate solution was added drop-wise to the phosphate solution over a period of 1 hour with constant and vigorous stirring at room temperature. The pH was maintained at 11.0. A brown gelatinous precipitate that formed was heated to and maintained at 80 °C for 1.5 hours to ensure removal of NH4+ in solution. The precipitate was then filtered under vacuum and washed thoroughly to ensure the removal of excess unreacted precursor ions. The precipitate was then dried overnight in an oven at 120 °C and then powdered. The powdered catalyst material was calcined at a temperature of 550 °C at a ramp rate of 1.5 °C min-1 for 6 hours.
The 3 and 9 wt% iron substituted Hap catalysts were prepared in a similar manner.