Core-shell particles with catalytic activity

Abstract

The present invention pertains to novel core-shell particles comprising a core of iron oxide and a shell of cobalt oxide, characterized in that they are spherical with a number average diameter, as measured by TEM, of between 1 and 20 nm. This invention is also directed to their uses in the manufacture of a catalyst, and to the method for preparing these particles, by precipitating cobalt oxide onto magnetite or hematite particles which are themselves precipitated from Fe(III) and optionally Fe(II) salts.

Claims

1. A method for the preparation of spherical core-shell particles, comprising the successive steps of: (a) preparing an aqueous solution comprising a ferric salt, at a temperature of less than 50 C.; (b) adding at least one base to said aqueous solution, so as to obtain a suspension of iron oxide particles having a pH value of from 10 to 14; (c) washing the suspension; (d) adding a strong acid to the washed suspension to peptize the washed suspension; (e) reacting at least one base with said peptized suspension, until the pH reaches a value from 10 to 14, at a temperature of from 50 to 95 C.; (f) adding a cobalt salt to the heated suspension in order to obtain spherical particles having a core of iron oxide and a shell comprising cobalt oxide.

2. The method according to claim 1, characterized in that the aqueous solution comprising the ferric salt further includes a ferrous salt in a molar ratio of Fe(III) to Fe(II) of 2:1, whereby the iron oxide particles are magnetite particles.

3. The method according to claim 1, characterized in that the ferric salt is ferric nitrate, ferric chloride or ferric hydroxide.

4. The method according to claim 2, characterized in that the ferrous salt is ferrous nitrate, ferrous chloride or ferrous hydroxide.

5. The method according to claim 1 characterized in that the cobalt salt is cobalt nitrate, cobalt chloride or cobalt sulphate.

6. The method according to claim 1, characterized in that the strong acid is nitric acid or hydrochloric acid.

7. The method according to claim 3, characterized in that the ferric salt is ferric chloride.

8. The method according to claim 4, characterized in that the ferrous salt is ferrous chloride.

9. The method according to claim 5, characterized in that the cobalt salt is cobalt nitrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) This invention will be further understood in light of the following non-limiting examples which are given for illustration purposes only, and also in connection with the attached drawings in which:

(2) FIG. 1 is a TEM image showing, on the left side, core-shell particles of this invention and, on the right side, platelets of cobalt oxide;

(3) FIG. 2 is a HR-TEM image of a core-shell particle of this invention;

(4) FIG. 3 is a TPR plot of core-shell particles of this invention compared to magnetite;

(5) FIG. 4 is a TPR plot of core-shell particles of this invention adsorbed on a porous carrier.

EXAMPLES

Example 1

Synthesis and Characterization of Core-Shell Particles

(6) Magnetite particles were first synthetized via a slightly modified Massart method. To this end, 9.02 g of FeCl.sub.3.xH.sub.2O and 3.26 g of FeCl.sub.2.xH.sub.2O were mixed together in 380 ml of water. To this solution were added from 10 to 40 ml of ammonia: magnetite formation was visible as a black precipitate. The particles were washed with 300 ml water, until the pH of the supernatant was constant. The magnetite particles were then peptized with 40 ml of a 2M HNO.sub.3 solution. The precipitate was recovered with a magnet and redispersed in water.

(7) The following Table 1 summarizes the size of the magnetite particles as a function of the ammonia amount:

(8) TABLE-US-00001 TABLE 1 NH.sub.4OH (ml) Particle size (nm) 10 12.2 20 7.6 40 3.5

(9) 50 ml of a 20 g/l magnetite suspension were then mixed with 10 ml of NaOH solution. The mixture was heated up to 70 C. Then 10 ml of a Co(NO.sub.3).sub.2.xH.sub.2O solution was slowly added to the magnetite suspension at the speed of 0.2 ml/min. The Co concentration in the solution was chosen so as to achieve a final Co/Fe wt % varying in the 3-60% range.

(10) The precipitate was then washed with water, the supernatant was removed and the so-obtained slurry was freeze-dried. The particle suspensions show an extended stability in a wide range of pH values. The isoelectric point is at pH=7-8. At pH below 2, the particles dissolve and at pH above 12 the ionic strength is high and the particles settle quite fast.

(11) Mean iron particle size, calculated on the basis of the TEM image analysis, is comprised in the 3-12 nm range and inversely proportional to the amount of ammonia used in the synthesis. The polydispersity of the systems (span=0.6) is in agreement with similar aqueous procedures reported in the literature. The image analysis of the Co coated particles showed no homogeneous nucleated Co oxide particles (see FIG. 1a which illustrates core-shell particles of about 7.5 nm). A control experiment further showed how, with no magnetite particles in the hot basic solution, Co oxide precipitates as hexagonal plates of about 20 nm (see FIG. 1b).

(12) Moreover, there appeared to be no statistically relevant size difference between the pure magnetite particles and the Co-coated ones. Rather, as evident from the EDX analysis, Co-enriched regions are formed on the top of the magnetite particles. High resolution TEM (HR-TEM) characterization also provided visual evidence of this new layer, as shown on FIG. 2 which illustrates core-shell particles with a Co/Fe ratio of 60 wt %. The Co-rich phase is visible as a lighter region on the surface of the particles. Furthermore, lattice fringes analysis showed that core and shell crystal structure are aligned on the same direction.

(13) TPR characterizations were also performed on freeze-dried powders by flowing a 5% H.sub.2 in N.sub.2 mixture at 40 ml/min, heating the samples at 5 C./min. The TPR characterization as well supports the presence of a core-shell structure. The behaviour in a reducing environment of these iron oxide particles perfectly matched magnetite TPR profile reported in the literature. From the TPR plots illustrated on FIG. 3, one could derive that the part of the Co interacted with the magnetite structure, influencing Fe reducibility, and that part of it also contributed to the formation of the cobalt-rich shell. The reduction rate was so fast in any way that the presence of pure Co.sub.3O4 and Fe.sub.3O.sub.4 phases could not be detected.

(14) Similar TPR plots of these core-shell particles supported on mesoporous silica (EMS 385 supplied by Eurosupport as extrudates) having a mean pore size of 41 nm (span=0.7), a specific pore volume of 0.47 cm.sup.3/g and a specific surface area of 125 m.sup.2/g showed, on the contrary, that the Co-rich and the iron-rich phases of the core-shell particles behaved more closely like pure CO.sub.3O.sub.4 and Fe.sub.3O.sub.4 when increasing the Co/Fe ratio. It could then be derived that the support had a beneficial affect in the retention of the core-shell structure under reducing conditions.