Method for the preparation of metal oxide hollow nanoparticles

Abstract

The present invention concerns a method for the preparation of metal oxide hollow nanoparticles comprising the steps of: providing an aqueous suspension of nanoparticles of an oxide of a first element having at least two oxidation states coupled with a substrate; placing in contact the aqueous suspension with an aqueous solution of a salt of a second element having at least two oxidation states having a standard reduction potential lower than the standard reduction potential of said first transition metal to obtain hollow nanoparticles.

Claims

1. A method for the preparation of metal oxide hollow nanoparticles comprising the steps of: providing an aqueous suspension of nanoparticles of an oxide of a first element having at least two oxidation states coupled with a substrate; placing in contact said aqueous suspension with an aqueous solution of a salt of a second element having at least two oxidation states having a standard reduction potential lower than the standard reduction potential of said first element to obtain said metal oxide hollow nanoparticles.

2. The method according to claim 1, characterized in that said oxide of said first element having at least two oxidation states is selected from the group consisting of Mn.sub.3O.sub.4, MnO.sub.2, Co.sub.3O.sub.4, Fe.sub.3O.sub.4, PbO.sub.2, and CeO.sub.2.

3. The method according to claim 1, characterized in that said salt of said second element having at least two oxidation states is a salt of a metal selected from the group consisting of Fe(II), Sn(II), V(III), Ti(III), Cr(II), and Ce(III).

4. The method according to claim 1, characterized in that the difference between the standard reduction potential of said first element having at least two oxidation states and the standard reduction potential of said second element having at least two oxidation states is at least 0.15 V.

5. The method according to claim 1, characterized in that the aqueous suspension of nanoparticles is heated to a temperature ranging from 20 to 100 C.

6. The method according to claim 1, characterized in that said substrate is selected from the group consisting of silica, carbon, alumina, zinc oxide, zirconium oxide, titanium dioxide, alkaline oxides, and alkaline earth oxides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described in detail with reference to the figures of the accompanying drawings, wherein:

(2) FIG. 1 illustrates a TEM image of nanocrystals of AuMn.sub.3O.sub.4 deposited on SiO.sub.2 after a calcination phase as described in example 1;

(3) FIG. 2 illustrates a TEM image of hollow nanocrystals of AuMn.sub.xFe.sub.yO.sub.z (where [Fe]/[Fe]+[Mn]>99%) deposited on SiO.sub.2 obtained as described in example 1;

(4) FIG. 3 illustrates the catalytic oxidation performance of the carbon monoxide of the materials obtained with the method according to example 1;

(5) FIG. 4 illustrates a TEM image of hollow nanoparticles of Mn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2 obtained by carrying out the replacement reaction at 90 C. and with a replacement level M.sub.Fe/(M.sub.Fe+M.sub.Mn)=99.4%;

(6) FIG. 5 illustrates a TEM image of hollow nanoparticles of Mn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2 obtained by carrying out the replacement reaction at 20 C. and with a replacement level M.sub.Fe/(M.sub.Fe+M.sub.Mn) 35.7%;

(7) FIG. 6 illustrates a TEM image of nanoparticles of Mn.sub.3O.sub.4 deposited on a carbon support as described in example 3;

(8) FIG. 7 illustrates a TEM image of hollow nanoparticles of Mn.sub.3-xFe.sub.xO.sub.4/carbon obtained at 90 C. as illustrated in example 3 and with a replacement level M.sub.Fe/(M.sub.Fe+M.sub.Mn)=99.0%;

(9) FIG. 8 illustrates a TEM image of nanoparticles of Mn.sub.3O.sub.4 deposited on a support of SiO.sub.2 as described in example 4;

(10) FIG. 9 illustrates a TEM image of hollow nanoparticles of Mn.sub.3-xCe.sub.xO.sub.4/SiO.sub.2 obtained at 90 C. as illustrated in example 4 and with a replacement level M.sub.Ce/(M.sub.Ce+M.sub.Mn)=23.5%;

(11) FIG. 10 illustrates a TEM image of hollow nanoparticles of Mn.sub.3-xSn.sub.xO.sub.4/SiO.sub.2 obtained as illustrated in example 4;

(12) FIG. 11 illustrates the trend of the replacement level as a function of the quantity of Fe.sup.2+ in the formation of nanoparticles of Mn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2.

BEST MODE FOR CARRYING OUT THE INVENTION

(13) Further characteristics of the present invention will be evident from the following description of some merely illustrative non-limiting examples.

Example 1

(14) Preparation of Hollow Nanocatalysts of AuMn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2

(15) The heterodimer nanoparticles of AuMnO were synthesized as illustrated in J. Am. Chem. Soc. 2014, 136, 2473-2483.

(16) 2200 mg of SiO.sub.2 are dispersed in 600 ml of hexane and subsequently a solution of heterodimer nanoparticles of AuMnO in hexane (which contains 24 mg of Au) is added dropwise under stirring.

(17) The mixture is sonicated for 30 minutes, then the products are collected by centrifugation and dried at 60 C. for one night and calcined at 450 C. for 3 hours in static air with a heating speed of 5 C./min. Nanoparticles of AuMn.sub.3O.sub.4 are obtained supported on SiO.sub.2 (FIG. 1).

(18) 300 mg of the calcined product AuMn.sub.3O.sub.4/SiO.sub.2 are then dispersed in 150 ml of water by means of sonication. The mixture is heated to 90 C. for 15 minutes under stirring.

(19) An aqueous solution of Fe(ClO.sub.4).sub.2 is added rapidly (successive experiments were repeated increasing the quantity of Fe.sup.2+: 0.025, 0.05, 0.1, 0.2 or 5 mmoles of Fe.sup.2+). After stirring for 90 min at 90 C., the product is collected by centrifugation and washed twice with water and once with ethanol. Hollow nanoparticles of AuMn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2 are obtained (FIG. 2). The sample is dried at 60 C. for one night and used as a heterogeneous catalyst for the oxidation of carbon monoxide. The galvanic replacement level can be controlled by acting on the quantity of Fe.sup.2+ precursor. The replacement level of the end product is calculated as:

(20) $\frac{M_{Fe}}{\left(M_{Fe}+M_{Mn}\right)}$
where M.sub.Fe is the number of moles of Fe and M.sub.Mn is the number of moles of manganese in the end product in the suspension. When the replacement level is lower than 70%, it increases linearly with the increase in the quantity of the Fe(II). To reach a replacement level greater than 99%, an excess of Fe.sup.2+ is required with respect to the theoretical stoichiometric value. Table 1 shows the data in terms of replacement level of the initial sample of AuMn.sub.3O.sub.4 supported on SiO.sub.2 not subject to galvanic replacement (AuMn.sub.3O.sub.4/SiO.sub.2) compared with those of the sample after galvanic replacement with increasing quantities of Fe.sup.2+ (AuMn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2-GR1, AuMn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2-GR2, AuMn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2-GR3, AuMn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2-GR4, AuMn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2-GR5).

(21) TABLE-US-00001 TABLE 1 Fe2+ added Au wt % Mn wt % Fe wt % $\frac{M_{\mathrm{Fe}}}{\left(M_{\mathrm{Fe}}+M_{Mn}\right)}$ AuMn.sub.3O.sub.4/ 0 mmol 0.92 4.42 0 0% SiO.sub.2 AuMn.sub.3x 5 mmol 0.82 0.05 5.92 99.1% Fe.sub.xO.sub.4/SiO.sub.2- GR1 AuMn.sub.3x 0.2 mmol 1.04 1.54 3.32 67.9% Fe.sub.xO.sub.4/SiO.sub.2- GR2 AuMn.sub.3x 0.1 mmol 1.05 3.50 1.59 30.9% Fe.sub.xO.sub.4/SiO.sub.2- GR3 AuMn.sub.3x 0.05 mmol 0.90 3.52 0.78 17.9% Fe.sub.xO.sub.4/SiO.sub.2- GR4 AuMn.sub.3x 0.025 mmol 0.95 4.71 0.35 6.8% Fe.sub.xO.sub.4/SiO.sub.2- GR5

(22) The catalytic performance of these materials in the oxidation of carbon monoxide varies with the replacement level. The T50 and T90 (temperatures for which 50% and 90% carbon monoxide conversion is achieved respectively) during the heating and cooling transients of the reaction process are shown in FIG. 3, as a function of the replacement level. The sample AuMn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2-GR4 with a replacement level of 18% has proved to be the most active catalyst with the lowest T50 and T90 of the materials tested.

(23) The best performance of the catalyst is therefore obtained with nanoparticles having a replacement level of 18%.

Example 2

(24) Preparation of Mn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2 Nanoparticles

(25) The nanoparticles of Mn.sub.3O.sub.4 were synthesized as illustrated in Chem. Mater. 2009, 21, 2272-2279.

(26) 700 mg of SiO.sub.2 are dispersed in 150 ml of hexane. A solution in hexane of nanoparticles of Mn.sub.3O.sub.4 (containing overall 80 mg of Mn.sub.3O.sub.4) is then added dropwise under stirring.

(27) The mixture is sonicated for 30 minutes, then the products are collected by centrifugation, dried at 60 C. for one night and calcined at 350 C. for 2 hours under static air with a heating speed of 5 C./min. Nanoparticles of Mn.sub.3O.sub.4 supported on SiO.sub.2 are obtained.

(28) 20 mg of the product obtained, Mn.sub.3O.sub.4/SiO.sub.2 are dispersed in 10 ml of H.sub.2O with sonication. The mixture is heated to different temperatures (Tamb, 40, 60, 90 C.) under stirring. 1 ml of an aqueous solution of Fe(ClO.sub.4).sub.2 (0.0366 mmol) is then injected and left to react at the temperature for 90 min.

(29) The product is collected and washed with water 3 times and then once with ethanol. The sample is dried at 60 C. for one night.

(30) The results in terms of replacement level are reported in table 2 and illustrated in FIGS. 4 and 5.

(31) TABLE-US-00002 TABLE 2 Mn Fe M.sub.Fe/(M.sub.Fe + M.sub.Mn) GR-90 C. 0.02 wt % 3.91 wt % 99.5% GR-60 C. 0.19 wt % 3.67 wt % 95.0% GR-40 C. 1.81 wt % 1.56 wt % 45.9% GR-20 C. 1.70 wt % 0.96 wt % 35.7% GR-90 C.: sample Mn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2 obtained after heating to 90 C.; GR-60 C.: sample Mn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2 obtained after heating to 60 C.; GR-40 C.: sample Mn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2 obtained after heating to 40 C.; GR-20 C.: sample Mn.sub.3-xFe.sub.xO.sub.4/SiO.sub.2 obtained after heating to 20 C.

(32) It should be noted that the replacement level can be modulated also according to the reaction temperature.

Example 3

(33) Preparation of Nanoparticles of Mn.sub.3-xFe.sub.xO.sub.4/Carbon

(34) The synthesis of the nanoparticles of Mn.sub.3O.sub.4 and the procedures for deposition on the support are the same as those illustrated in example 2; for this experiment the silica support was replaced with a carbon support (FIG. 6).

(35) After the deposition, no calcination was performed since calcination in air to remove the ligands would cause a significant loss of material (during heating not only the ligands are lost but partially also the carbon).

(36) The galvanic replacement procedures carried out are the same as those of example 2 (FIG. 7).

(37) It can be noted that the use of a different support does not affect the method subject of the invention.

Example 4

(38) Preparation of Mn.sub.3-xCe.sub.xO.sub.4/SiO.sub.2 nanoparticles

(39) 20 mg of nanoparticles of MnO were synthesized according to the procedure reported in Chem. Mater. 2009, 21, 3183-3190.

(40) The deposition procedures are the same as those used in example 1.

(41) The powders obtained are dried at 60 C., then calcined at 450 C. for 3 hours in air to remove the ligands and to oxidize the MnO to Mn.sub.3O.sub.4.

(42) 20 mg of the product obtained, Mn.sub.3O.sub.4/SiO.sub.2 are dispersed in 10 ml of H.sub.2O by means of sonication and the suspension is heated to 90 C. under stirring. 1 ml of an aqueous solution of CeCl.sub.3 (0.5 mmol) is then injected and left to react at constant temperature for 90 min. The product obtained is collected and washed with water three times and with ethanol once. Subsequently the product is dried at 60 C. for one night.

Example 5

(43) Preparation of Mn.sub.3-xSn.sub.xO.sub.4/SiO.sub.2 Nanoparticles

(44) 20 mg of nanoparticles of MnO were synthesized according to the procedure reported in Chem. Mater. 2009, 21, 3183-3190.

(45) The deposition procedures are the same as those used in example 1.

(46) The powders obtained are dried at 60 C., then calcined at 450 C. for 3 hours in air to remove the ligands and to oxidize the MnO to Mn.sub.3O.sub.4.

(47) 20 mg of the product obtained, Mn.sub.3O.sub.4/SiO.sub.2 are dispersed in 10 ml of H.sub.2O by means of sonication and the suspension is heated to 90 C. under stirring. 1 ml of an aqueous solution of SnCl.sub.2 (0.005 mmol SnClo.sub.2 and 0.1 ml HCl) is then injected, and left to react at constant temperature for 90 min. The product obtained is collected and washed with water three times and with ethanol once. Subsequently the product is dried at 60 C. for one night (FIG. 10).

(48) The replacement level data are reported in table 3.

(49) TABLE-US-00003 TABLE 3 Mn (% by weight) Sn (% by weight) Starting material 2.41 0 (Mn.sub.3O.sub.4/SiO.sub.2) Material after 0 0.76 galvanic replacement (Mn.sub.3-xSn.sub.xO.sub.4/SiO.sub.2)

Example 6

(50) Comparison Between the Nanoparticles Obtained with the Method of the Invention and the Nanoparticles Obtained According to the Known Art.

(51) The nanoparticles obtained with the method of the invention were produced as described in example 2.

(52) In particular, the nanoparticles of Mn.sub.3O.sub.4 were synthesized as illustrated in Chem. Mater. 2009, 21, 2272-2279.

(53) The SiO.sub.2 support is dispersed in hexane. A solution in hexane of nanoparticles of Mn.sub.3O.sub.4 is then added dropwise under stirring.

(54) The mixture is sonicated for 30 minutes, then the products are collected by centrifugation, dried at 60 C. for one night and calcined at 350 C. for 2 hours under static air with a heating speed of 5 C./min. Nanoparticles of Mn.sub.3O.sub.4 supported on SiO.sub.2 are obtained.

(55) 200 mg of the product obtained, Mn.sub.3O.sub.4/SiO.sub.2 (containing approximately 3.08% by weight of Mn) are dispersed in 100 ml of H.sub.2O with sonication. The mixture is heated to 90 C. under stirring. An aqueous solution of Fe(ClO.sub.4).sub.2 is then injected (successive experiments were repeated increasing the quantity of Fe.sup.2+: 0.0118, 0.0235, 0.047, 0.086, 0.133 mmol Fe.sup.2+) and left to react at the temperature for 90 min.

(56) The product is collected and washed with water 3 times and then once with ethanol. The sample is dried at 60 C. for one night.

(57) The results in terms of replacement level are reported in FIG. 11.

(58) Comparing the data obtained with the data reported in Oh et al., Galvanic Replacement Reactions in Metal Oxide Nanocrystals, Science (2013) 340, 964-968, FIG. 1E, it can be noted that the method according to the invention allows a replacement level to be obtained comparable to much lower concentrations with respect to those used in this known art document.

(59) In particular, the concentrations compared are the following:

(60) TABLE-US-00004 Concentration of Fe.sup.2+ (for a theoretical Quantity replacement Quantity of Mn Solvent of Fe.sup.2+ of 100%) Ho et al. 1 mmol Mn 15 ml 1 ml Fe.sup.2+ 1.0 M xylene Sample 200 mg 100 ml 10 ml Fe.sup.2+ 0.0112 M according MnOx/SiO.sub.2 H.sub.2O to the with ~3.08% invention by weight Mn (i.e. 0.112 mmol Mn)

(61) It follows that the method according to the invention allows optimization of the galvanic replacement reaction kinetics with respect to the methods known in the art.

REFERENCES

(62) [1] Oh et al., Galvanic Replacement Reactions in Metal Oxide Nanocrystals, Science (2013) 340, 964-968; [2] Zhang et al., Self-templated synthesis of hollow nanostructures Nano Today (2009) 4, 494-507; [3] Wang et al., Surfactant-free synthesis of Cu2O hollow spheres and their wavelength-dependent visible photocatalytic activities using LED lamps as cold light sources, Nanoscale Research Letters (2014), 9:624.