"COMPOSITE GRAPHENO-METAL OXIDE PLATELET METHOD OF

PREPARATION AND APPLICATIONS"

Field of the invention

The invention relates to composite graphene-metal oxide materials, the method of preparation and their applications. Taking advantage of the semi-conductor properties of metal oxides, these materials have great potential to be used in organic synthesis, solar cells, solar hydrogen production and synthesis of methanol. In particular graphene-Ti02 has excellent photocatalyt ic activity and can be used to degrade inorganic and organic pollutants in water and air, synthesis of organic compounds, production of hydrogen using solar cells and synthesis of methanol

State-of-the-art

When photo active semiconductor like Ti0 ₂ is illuminated with photons with energy equal or larger than the band gap, electrons (e ^~ ) are excited from the valence band into the conduction band leaving a positive hole (h ⁺ ) ; the electrons migrate to the surface of graphene preventing the direct recombination with the holes and providing an adsorption site for the species to be oxidized. The holes became then available for conducting oxidation reactions on the photocatalyst surface. The photocatalytic activity can be enhanced by: a) Increasing the surface area; b) Decreasing the recombination rate of the photo generated electron-hole ; c) Extend the light absorption to longer wavelengths. In order to improve the photocatalytic activity, it has been proposed the addition of metals to the metal oxides. Thus, Pt was deposited on the surface of Ti0 ₂ . After excitation, the electron migrates to the metal where it is trapped and the electron-hole recombination reduced [ 1 ] ; the concentration of metal should be very small, because large concentrations are detrimental for the photoactivity [1] .

Addition of activated carbon increases the photocalytic activity of Ti0 ₂ due to the adsorption on the substrate of the reactants or the intermediate species on the activated carbon [2,3] . Carbon can be also used to reduce the band gap of Ti02 allowing absorption of photons in the visible region increasing the adsorption efficiency [4-7]

Patent EP0997191 [4] describes the preparation of TiC supported at least partially on the surface of nanoparticles of Ti02. This material was produced by CVD treatment of Ti02 with gaseous hydrocarbons and a reductant agent, the material thus produced was able to photo-oxidize formaldehyde using visible light.

Khan et al . [5] prepared Ti02 modified with carbon by flame pyrolysis using metallic titanium as precursor. The pyrolysis was carried out in the presence of products of combustion (C02 and water vapor) of a flame of natural gas with controlled addition of oxygen. In this material, some atoms of oxygen of the crystalline network were replaced with carbon allowing the absorption of light at wavelengths below 535 nm. This photocatalyst was efficient for water splitting .

In another paper [6], carbon was introduced in the structure of Ti02 by hydrolysis of titanium tetrachloride with tetrabutylammonium followed by the calcinations of one hour at 400 °C. The resultant dark brown material was 5 times more photoactive in the decomposition of 4- chlorophenol than Ti02 doped with nitrogen. In this case, the used of tetrabutylammoniumn in the precipitation process, produced relatively homogenous doped Ti02 particles .

Patent US7524793B2 describes the preparation of photocatalytic Ti02 having atoms of carbon. This photocatalyst was manufactured by mixing a fine grained titanium compound with a specific area of at least 50 m2/g with an organic compound and subsequent thermal treatment at temperatures up to 350 °C. Carbon is present only on the surface of Ti02 particles, which is different from the carbon doped Ti02 described in [6] where carbon was claimed to be inserted inside Ti02 crystals.

On the other hand, it was recently observed that the photocatalytic activity of Ti0 ₂ can be improved by adding carbon nanotubes [8] . Carbon nanotubes have a large electron storage capacity; they can accept and store photo- excited electrons from Ti0 ₂ retarding the recombination of the electron-hole pair. In addition, carbon nanotubes provide a surface area similar to activated carbon and may enhance the photocatalytic activity acting as photosensitizer .

Graphene have very recently attracted considerable attention as a viable and inexpensive alternative to carbon nanotubes in nanocomposite materials. Graphene is essentially a flattened carbon nanotube cut along its axes made of a two-dimensional crystalline sheet of carbon atoms arranged in a honeycomb lattice. It has two faces with no bulk in between, therefore reagents can attach to both graphene faces. The great interest of graphene is because of its ultrathin geometry (is the thinnest known material) and properties such as high charge carrier mobility, excellent thermal conductivity and high mechanical strength (the strongest ever measured) [9].

It was described the use of self-organized hybrid nanostructures of graphene-Ti02 to increase the charge- discharge ratio of lithium batteries [10] . The increase in efficiency was attributed to the increase of the electric conductivity of the graphene-Ti02 electrodes. A determinant step in the preparation of the material was the development of an anionic surfactant mediated growth of self-assembled metal oxide-graphene hybrid nanostructures.

It is been discussed if single nanoparticles of Ti02 are harmful for the human beings as they can penetrate even into the brain blood circulation. The present invention discloses a composite material containing nanoparticles of Ti02 chemically bonded to a graphene platelet surfaces. This composite particle is in the range of micrometers in diameter and possesses no danger for the human beings like TiO nanoparticles may do.

Description

The invention relates the process of synthesis and use of a new composite catalyst of graphene-metal oxide. It is based on a metal oxide that is presented in amorphous, semicrystalline or crystalline and/or oxohydrate and/or hydrated form and graphene and/or reduced graphene oxide.

The composite material is prepared by mixing an aqueous solution of graphene oxide and a solution of a metal source material dissolved in water or a water miscible solvent. After hydrolysis, the metal oxide is attached to graphene oxide by physical and/or chemical interaction. The graphene oxide can be converted into graphene by chemical reduction and/or thermal treatment under hydrogen. A change of color takes place after the reducing process. The composite material has improved photoactivity due to: a) the high surface area; the nanosize metal oxide particles are dispersed on both surface sides of graphene, b) reduced rate of electron hole recombination; the high mobility of electrons and high electron storage of graphene makes the exchange of electrons with the titania easier, and c) adsorption of chemical species to be photodegraded and intermediate products on the surface.

Summary of the invention

The present invention refers to the process of preparation and application of a composite of graphene-metal oxide. The platelets of graphene have been proved to be effective supports for metal-oxide catalysts. In particular, the composite catalyst of platelets of graphene and metal oxides could be used in organic synthesis, solar cells, solar generation of hydrogen, synthesis of methanol, taking advantage of the semi-conductor properties of metal oxides or just by their catalytic properties.

The present invention refers to a composite catalyst, the method of preparation and their applications.

The catalyst of the present invention is composed of nanoparticles of metal oxides attached to platelets of graphene or reduced graphene-oxide . The platelets are composed of one layer or multiple layers.

In a preferred embodiment, the thickness of the platelets of graphene is less than 1000 nm preferably between 1 to 100 nm.

In a more preferential embodiment the size of the nanoparticles of metal-oxide should be between 1 to 100 nm, and the metal-oxide is amorphous, semicrystalline or crystalline and/or in the oxohydrate and/or hydrate form.

In a preferential embodiment, the metal-oxide nanoparticles is selected from the group of Ti02, ZnO, Zr02, Fe203, W03, SrTi03, BaTi03, Nb205, KTa03, Sn02, Ta205, A1203, Ce02, Y203 or a mixture of them.

In other preferential embodiment, the metal oxide is doped or decorated doped material is selected from the group consisting of Pt, Pd, Ni, Cu, Fe, Rh, Ru, N, C or a mixture of them; and the concentration relative to the metal-oxide is between 0.5 to 20 weight percent. The composite material has a surface area between 40 to 500 m2/g, preferably between 60 to 250 m2/g.

The method of preparation of the composite material comprises : a) Preparation of a solution of graphene oxide in water ; b) Preparation of a solution of the metal-oxide precursor in a solvent miscible with water; c) Mixture of both solutions prepared previously in the desired proportion; d) Precipitation of the solution obtained in c) by addition of a basic solution preferentially ammonia; e) Reduction of the graphene oxide by adding a reductant agent, preferentially N2H4 and heating the suspension at a convenient temperature and enough time to obtain a constant change of color of graphene. The heating conditions can change according to the time and temperature of the process; for example, we verified that the constant change of color at temperatures higher than 30 °C for more than 2h, preferentially at 90 °C for about 12 h. ; . f) Filtration and washing of the precipitate.

The material thus obtained can be calcinated in a non- oxidant environment which can be an inert gas, NH3, hydrocarbons, at higher than 400 °C preferentially 450 °C for 2 h.

In a preferential embodiment, the reduction of the graphene oxide to graphene is total or partial; and the composition of graphene is from 0.01 wt . % to 2 wt . % and preferably between 0.1 to 1 wt . % .

The graphene metal-oxide catalyst according to the invention, displays improved photocatalytic activity than nanoparticles of Ti02. As the nanoparticles of Ti02 are attached strongly to both phases of graphene platelets; minimizes the risk of the particles to reach vital organ of living objects.

In an even more preferential embodiment, the composite catalyst is a photocatalyst .

Examples

The present invention will now be described in greater detail with reference to the following examples. The examples are for illustrative purposes only and are not intended to limit the scope of the invention.

Example of preparation of graphene oxide

50 ml of H ₂ S0 ₄ is added to 2 g of graphite at room temperature; the solution is cooled at 0 °C using an ice bath and then 7 g of KMn0 ₄ is added gradually. The mixture is heated at 35 °C and stirred for 2 h. After that, 300 ml of water is added into the mixture at 0 °C (ice bath) . Then H ₂ 0 ₂ (30%) is added until no gas is produced. The solid is filtered, washed with 250 ml of HC1 (0.1 M) and water (500 ml) . The graphene oxide is dried under vacuum at room temperature for 24 h and then triturated using a mortar.

Example of the preparation of the solution of graphene oxide .

75 mg of graphene oxide and 100 ml of water is sonicated using an ultrasonic bath for 7 h. The insoluble graphene is separated by centrifugation at 12000 rpm for 10 minutes.

Examples of preparation of the composite material . Example 1. Preparation of Ti02-graphene composite titanium tetrachloride (6 g) is added dropwise under strong stirring into a 4 % solution of HC1. The stirring is continued until the solution becomes clear. 7 g of the graphene oxide solution is added and the solution stirred for another 30 minutes. Then NH ₃ (28-30%) is added dropwise until the pH becomes 7. In order to reduce the graphene oxide 3 g of N2H4 is added and leave to react overnight at 90 °C. The Ti02-graphene is filtered and washed with water until no chloride was detected (formation of AgCl) and dried at 90 °C overnight. Then the composite material was calcinated at 450 °C for 2h under nitrogen with heating rate of 2°C/min. Fig 1 shows a SEM image of the composite material, it can be seen that 10-15 nm Ti02 particles are present on the graphene platelets.

Example 2. Preparation of Ti02-graphene composite Potassium titanium oxide oxalate (3 g) is dissolved in 100 ml of water and stirred until the solution becomes clear. 3 g of the graphene oxide solution is added and stirred for another 30 minutes. Then NH3 (2 M) is added dropwise until pH 7. The graphene oxide is reduced by addition of 3 g of N2H4, the reduction is carried out overnight at 90 °C. The Ti02-graphene is filtered, washed with water and dried at 90 °C overnight. Then the composite material is calcinated at 450 °C for 2 h under nitrogen with heating rate of 2°C/min.

Example 3. Preparation of Ti02-graphene composite beads. - 1.3 g of hexadecylamine is dissolved in 150 ml of ethanol and 1 ml of KC1 (0.1 M in water) . To this solution 2 g of a solution of graphene oxide is added. Then titanium isopropoxide (4.5 g) is added dropwise under strong stirring; the solution was kept static for 24h. The precipitate is filtered and transferred into a flask. Then 3 g of N2H4 and 20 ml of water are added, the flask was closed and heated at 90 °C overnight. The composite material is calcinated at 450 °C for 2h under nitrogen with heating rate of 2°C/min. SEM images of the beads are shown in Fig 2.

Example 4. Preparation of Zr02-graphene composite. -

6.3 g of zirconylnitrate is dissolved in 100 ml of water and stirred until the solution becomes clear. 4.2 g of graphene oxide solution is added and the solution stirred for another 30 minutes. Then NH ₃ (2M) is added dropwise until the pH becomes 7. In order to reduce the graphene oxide 3 g of N2H4 is added and leave to reacted overnight at 90 °C. The Zr02-graphene is filtered and dried at 90 °C overnight. Then the composite material is calcinated at 450 °C for 2h under nitrogen with heating rate of 2°C/min.

Photocatalytic activity

The photocatalytic activity of the composite material was compared with the commercially available P-25 Ti02 by the photodegradat ion of NO. Figure 3a clearly shows that the percentage of conversion of the Ti02-graphene material is much higher than P25. The conversion obtained with the Ti02-graphene photocatalyst is almost constant more than 90 %. However, P25 conversion has a long unsteady state period of time and reaches a much smaller steady state conversion, 63 % for the same operating conditions. Similarly, the selectivity (percentage of NO converted to N0 ₂ ^~ and N0 ₃ ^~ ) in the composite material is much higher than in P25 (Fig 3 b) .

Description of the Figures

Fig 1 - SEM image of Ti02-graphene composite.

Fig 2.- SEM images of the Ti02-graphene beads. Fig 3.- Photocatalytic degradation of NO by graphene-Ti02 and P 25 Ti02. a) selectivity of conversion, b)percentage of conversion.

References

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