ES2540701A1 - Photo-catalysts zirconium oxide-carbon compounds, preparation procedure and degradation procedure of organic compounds (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

Photo-catalysts made of zirconium-carbon oxide, preparation procedure and process of degradation of organic compounds. The present invention describes new xerogel carbon-zirconium oxide composite materials useful as photocatalysts, as well as its synthesis process and its application in the degradation of organic compounds. These composite materials are active photocatalysts in the degradation of organic molecules under both ultraviolet radiation and visible light. (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

ZIRCONIO-CARBON OXIDE COMPOUND CATALYZERS, PREPARATION PROCEDURE AND ORGANIC COMPOUND DEGRADATION PROCEDURE 5

SECTOR OF THE TECHNIQUE

The present invention is framed in the field of catalysts, in particular catalysts containing zirconium. 10

The main sector of application of the materials object of the invention is the elimination of contaminants, in particular the use of the catalysts described as photo-catalysts for the degradation of organic molecules in water and / or air decontamination processes using ultraviolet (UV) light and 15 visible light.

STATE OF THE TECHNIQUE

Photocatalysis 20

Photocatalysis is an advanced oxidation process that has been used successfully in air and water decontamination. For various reasons, the process of treatment and / or purification of water by heterogeneous photocatalysis with titanium dioxide as a catalyst is, today, one of the photochemical applications that has aroused most interest among the international scientific community. The titanium dioxide is considered the photocatalyst par excellence but the band gap (energy separation between the valence and conduction band) of this is 3.2 eV, therefore, radiation or UV light is required (λ ≤ 387 nm ) to produce the migration of an electron from the valence band to the conduction band necessary for the photocatalysis process. This makes these degradation processes expensive and tedious because they require irradiation with an energetic and relatively dangerous radiation such as ultraviolet light.

Titanium oxide photocatalysts

 35

Titanium oxide, pure or doped, is the most widely used and studied as a photo-catalyst. New strategies are constantly being developed

to improve said photo-catalytic properties of titanium oxide (titania or TiO2), the vast majority of them focused on improving their photo-catalytic activity by preparing hybrid or doped materials. In this way the suitable combination of carbon materials and photosensitive metal oxides such as TiO2 can give rise to high performance materials in photocatalysis, and especially replacing ultraviolet light with visible light making the process more viable.

Alternatives using Zirconium Oxide

In contrast, zirconium oxide has been much less studied and used as a photocatalyst. The band-gap of zirconium oxide is 5 eV, but depending on the method of synthesis, values from 2.3 ev [B. Králik, E.K. Chang, and S.G. Louie Structural properties and quasiparticle band structure of zirconia, Phys. Rev. B 57 (12), 7027, 1998], reaching values similar to titanium oxide, which has a band gap of 3.2 eV in its anatase phase (the most active), in some synthesis [S. Polisetti, P.A. Deshpande, and G. Madras. Photocatalytic Activity of 15 Combustion Synthesized ZrO2 and ZrO2-TiO2 Mixed Oxides, Ind. Eng. Chem. Res. 50 (23), 12915, 2011] being optimally suitable for use as a photo-catalyst. However, no references have been found that demonstrate the photo-activity of pure or doped zirconium oxide in the degradation of contaminants.

 twenty

Importance of the textural properties of photocatalysts.

Finally, it is worth highlighting the importance of the textural properties of the materials developed as photocatalysts, since the elimination of the contaminant also occurs due to its adsorption in the porosity of the same, therefore, in liquid phase processes, the presence of an adequate volume of mesoporosity where the contaminant can be adsorbed is decisive.

Technical problem statement

Therefore, new photo-catalytic materials that are active in the visible spectrum of electromagnetic radiation and capable of suppressing the rapid combination of electrons and photo-generated holes are required for use in a variety of light-based environmental applications solar, ranging from dye-sensitized solar cells (DSC) to removal of contaminants in the aqueous phase or in the gas phase. Therefore, there is therefore a need to provide a simple and economical method of synthesis of photo-active materials that allows obtaining materials with adequate textural properties and that

they are active in the visible spectrum, preferably, or with enhanced photo-catalytic properties using ultraviolet radiation.

OBJECT OF THE INVENTION

 5

The new synthesized materials have been developed in order to be able to decompose polluting organic molecules of water or air, through a simpler, more efficient and less expensive process than those currently available or implanted, which is based on the use of visible radiation instead of other more energetic radiations such as ultraviolet light which makes the advanced processes of water decontamination more expensive and tedious.

The object of the present invention is to obtain carbon-zirconium oxide composite materials active under visible radiation and with improved photo-catalytic properties. The proposed synthesis process makes it possible to obtain 15 materials with high performance as active photocatalysts under visible light, which makes this advanced oxidation process easier for water and air decontamination, either by the use of visible radiation, or for the shortest period of use of ultraviolet radiation for the degradation of the contaminant, and being undoubtedly more photo-active than other materials commonly used and implemented, for example, commercial titanium oxide, P25

New synthesized materials have been developed in order to completely oxidize organic molecules that commonly pollute water or air. The photo-catalytic properties of the material will allow a more simple and less expensive oxidation process than those currently available or implanted, which is based on the possibility of using visible radiation (sunlight) instead of other more energetic radiation such as ultraviolet light. The use of solar radiation not only reduces the energy consumption of decontamination processes but will also provide, for example, a simple method of decontamination and water purification in developing countries.

Photocatalysis is an advanced oxidation process that is already used successfully in air and water decontamination. However, both the improvement of the process of treatment and / or purification of water by photocatalysis, and of the materials used for this purpose, is, today, one of the lines of research that has aroused most interest among the International scientific community Titanium dioxide is

considered the photocatalyst par excellence, but the band gap (energy separation between the valence and conduction band) of this is 3.2 eV, therefore, radiation or UV light (λ ≤ 387 nm) is required to produce the migration significant electrons from the valence band to the conduction band, a necessary step for the photocatalysis process. This makes these degradation processes expensive and tedious as they require the use of energetic and relatively dangerous radiation such as ultraviolet light.

The first object of the present invention is an active zirconium oxide photocatalyst under visible light. Surprisingly, without the zirconium oxide or carbon xerogel 10 used as support being photocatalysts, no references have been found that demonstrate the photo-activity of said pure phases and separately, surprisingly, the combination of both elements results in a photo-catalyst with activity under visible light. In particular, they are photocatalysts, hereinafter "photocatalysts of the invention", which comprise a 15 zirconium oxide phase forming a stable coating on a carbon xerogel support and allowing the use of visible light in the Photo-removal processes of pollutants, also improving their photo-catalytic properties under longer wavelength radiation.

 twenty

Another object of the present invention is a process, hereinafter "process of the invention", for obtaining highly doped carbon xerogels with zirconium oxide exalting its photo-catalytic activity, achieving a synergistic effect between the carbon and the oxide of zirconium that allows the use of visible light in the processes of photo-elimination of contaminants, and also improving its 25 photo-catalytic properties under radiations of greater wavelength.

Another object of the invention is a process for obtaining improved photo-catalysts comprising an additional treatment of the materials of the invention at high temperature in an inert atmosphere. 30

Another object of the invention are the materials obtainable by said process, hereinafter "improved photocatalysts of the invention".

Finally, a final object of the invention is a process for removing contaminants that comprises the use of the materials of the invention. This procedure has advantages over existing ones because, together with the activity

improved, it allows the recovery of these materials, since they precipitate spontaneously when the agitation of the suspension ceases.

DESCRIPTION OF THE FIGURES

 5

Figure 1.- Graphical representation of the adsorption kinetics of orange G in photocatalytic materials, P25 (◆) and CZr40 (●).

Figure 2.- Adsorption isotherm of orange G in the different photo-catalytic materials, P25 (◆) and CZr40 (●). 10

Figure 3.- Graphical representation of the degradation kinetics of orange G in the different photo-catalytic materials under visible radiation, ZrO2 (▲), carbon xerogel (○), P25 (◆) and CZr40 (●), and photolysis ( ■).

 fifteen

Figure 4.- Graphical representation of the evolution of total organic carbon (TOC) during the degradation experiences of orange G under visible radiation using P25 (◆) and CZr40 (●).

 twenty

DESCRIPTION OF THE INVENTION

Zirconium photocatalysts

The first object of the invention relates to photocatalysts comprising a carbonaceous phase and a zirconium oxide phase both homogeneously distributed in the solid, hereinafter "photocatalysts of the invention".

In a particular embodiment, the zirconium oxide phase is highly dispersed in the carbonaceous phase, specifically carbon gel, and has crystal sizes less than 5 nm.

Contrary to what was anticipated in the state of the art, photo-catalysts have been generated from two materials that, by themselves, do not exhibit photo-catalytic activity. In addition, these photocatalysts are active under visible light and have better photocatalytic properties under radiations of greater wavelengths when compared with reference photocatalysts such as P25. In

In particular, the photocatalysts of the invention are active for photo-degradation of contaminants.

Procedure for obtaining zirconium photocatalysts

 5

Another object of the invention is a process, hereinafter "process of the invention", which allows obtaining the photocatalysts of the invention.

The process of the invention allows to obtain highly doped carbon xerogels with zirconium oxide exalting its photo-catalytic activity, by achieving a synergistic effect between the carbon and the zirconium oxide.

The process of the invention comprises the following steps:

1.- Preparation of xerogel carbon-zirconium oxide composite materials

(i) Obtaining a compound hydrogel, organic gel doped with 15 zirconium oxide from a phenolic compound (R), an aldehyde, (F) and a zirconium alkoxide (A)

(ii) Curing of the compound hydrogel resulting from step (i)

(iii) Drying of the hydrogel compound resulting from step (ii) by microwave in an inert atmosphere, resulting in a dry gel. twenty

(iv) Carbonization of the dry gel resulting from step (iii).

Unlike other known procedures, the process of the invention does not employ surfactant to obtain the hydrogel.

 25

In a particular embodiment, obtaining the hydrogel compound of step (i) is carried out in turn through the following steps:

(i) - Obtaining the organic gel compound hydrogel - zirconium oxide

a) Prepare a solution of zirconium alkoxide in an organic solvent. 30

b) Heat the solution (a) below the boiling point of the organic solvent, with stirring.

c) Prepare an aqueous solution containing a phenolic compound and an aldehyde.

d) Add the aqueous solution of section (c) dropwise onto the alkoxide solution 35 obtained in section (b).

e) Gelification of the solution obtained in section (d) at constant temperature until the hydrogel is obtained.

In a particular embodiment, the phenolic compound (R) employed is resorcinol.

 5

In another particular embodiment, the aldehyde (F) employed is formaldehyde.

In another particular embodiment, the zirconium alkoxide is zirconium proproxide (IV).

The other particular embodiment, the microwave drying step, iii), is performed under an inert atmosphere of argon, helium or nitrogen, preferably argon.

Preferably, the organic polymer coated with zirconium oxide and dried in microwave, is also carbonized (iv) in inert gas flow at temperatures between 500-1200 ° C, preferably 900 ° C. fifteen

In another preferred embodiment, the reaction temperature that is reached in sub-stage b) of step i) is between 5 and 98 ° C, preferably 70 ° C

In another preferred embodiment, the carbonization speed ramp of step (iv) 20 is very slow, reaching a temperature between 500 ° C and 1200 ° C, more preferably at 900 ° C, with an increase of between 0.1 and 25 ° C per minute, preferably with an increase of 1 ° C / min.

 Preferably, after reaching the maximum carbonization temperature, it is maintained for 2 hours.

The carbonization is carried out in an inert atmosphere, in particular nitrogen, helium, neon, argon, krypton or xenon, preferably nitrogen.

 30

In a particular embodiment, heptane is used as the reaction medium

Pollutant removal procedure

Another object of the invention is a process for the elimination by photo-degradation of contaminants in solution, in gas or liquid phase, preferably organic pollutants, comprising the contacting of the materials of

the invention or the improved materials of the invention with the solution containing said contaminants under radiation in the ultraviolet and visible spectrum.

In particular, a method of removing organic pollutants in waters characterized by using visible radiation in the presence of the photocatalysts of the invention is the subject of the invention.

In a particular embodiment, the material used is in a proportion from 0.1 mg / mL of suspension, preferably 1mg / mL.

 10

Together with the greater degradation activity achieved with the use of the photocatalysts of the invention, when the stirring of the suspension ceases, the photocatalysts of the invention precipitate spontaneously, while the known photocatalysts remain in suspension. , and, consequently, they need an additional centrifugation process to be separated removed. fifteen

REALIZATION MODES

Next, by way of illustrative but not limiting examples, practical embodiments of the materials of the invention, the process object of the present patent and the use of the photo-catalyst obtained are explained.

Example 1: Photo-catalysts carbon gel - zirconium oxide.

The preparation of the gels was carried out using resorcinol (R), formaldehyde (F), water (W), and zirconium proproxide (IV) as metal alkoxide. The amount of zirconium propoxide to be used is calculated based on the different percentages of metal oxide, by weight, that is desired to be achieved in the final carbonized assuming that all R and F react and a 50% weight loss after carbonization .

  30

Zirconium Propoxide is dissolved in 500 ml of n-heptane, under stirring and at a temperature of 70 ° C. On this solution, and maintaining the stirring and temperature, an aqueous solution of resorcinol and formaldehyde in the form of a 37% aqueous solution is slowly added. The polymer mixture is kept under stirring and at 70 ° C for 24 hours for polymerization and sample curing. Then, the suspension obtained is filtered and the collected solid is exchanged with abundant acetone for 5 days (2 exchanges per day) to exchange the gel water for

acetone and reduce the collapse of the porosity in the drying phase. Subsequently, the solid is dried using microwave under an argon atmosphere at 300 W until constant weighing in periods of 3 min. Finally, the dry gel is carbonized at 900 ° C under argon for 2 hours.

 5

Following the synthesis procedure just described, and specifically, when the amounts of reagents used were 24.8 grams of R, 36.4 grams of F, 33.4 grams of water and 18.2, 31.1 or 48.5 grams of zirconium propylene, a Carbon-zirconium oxide xerogel composites with a real charge of zirconium oxide in the carbonized of 24.1, 34.3 and 45.1% by weight, 10 respectively, hereinafter CZr20, CZr30, CZr40 presenting the characteristics listed in Table 1.

 Coal

 SBET m2 · g-1 W0 (N2) cm3 · g-1 W0 (CO2) cm3 · g-1 L0 (N2) nm L0 (CO2) nm VDFT cm3 · g-1 Smic (CO2) m2 · g-1 V0 , 95 cm3 · g-1

 ZrO2

 4 0.002 0.040 2.46 2.10 0.072 38 0.050

 CZr20

 353 0.138 0.213 0.85 0.56 0.090 759 0.204

 CZr30

 331 0.127 0.183 0.97 0.55 0.321 658 0.430

 CZr40

 409 0.162 0.156 1.17 0.58 0.802 582 0.978

 TiO2 (P25)

 57 0.006 - - - 0.080 - 0.118

Table 1. Textural characteristics of the photo-catalysts of carbon-zirconium oxide and inorganic oxides. fifteen

Example 2: Obtaining improved photo-catalysts.

The photocatalysts obtained in Example 1 are subjected to a post-treatment in the atmosphere of He, N2, Ar or H2 for 12 h at 400 ° C with a heating ramp of 5 ° C / min before being used in the photo-catalytic system. In addition, Degussa® titanium oxide (P25) is used as a reference material, which is subjected to the same post-treatment as composite materials.

Example 3: Photo-catalytic degradation of the Orange G dye under visible radiation. 25

The photocatalysts prepared according to examples 1 and 2 are useful for the degradation of organic pollutants from water using visible radiation. To illustrate its application, the Orange G compound was used as a test molecule.

 30

Orange g

The photo-degradation was performed using a visible light lamp of 10-14 W of power, an initial concentration of orange G in water of 10 mg / l (after saturation 5 of the material), 250 ml of solution and a concentration of 1 mg / ml photocatalyst. The evolution of the photo-degradation was performed by taking aliquots of the sample from time to time being analyzed by UV spectrophotometry at λ = 487 nm. The methodology followed is detailed below:

1.- The adsorption kinetics of the dye are performed on the photo-catalytic material of 10 where the time necessary to reach the adsorption equilibrium is obtained. To do this, 250 mg of material is introduced into 250 ml of a solution of 20 mg / l of dye in the dark and stirring. Aliquots of solution are taken every so often until constant dye concentration (measured by UV spectroscopy).

2.- The adsorption isotherm is performed to calculate the adsorption capacity of the material, for which 25 mg of sample is introduced into 25 ml of solutions with a concentration between 5 and 50 mg / ml of dye, under darkness and agitation . After the time necessary to reach equilibrium (calculated by step 1) the solutions are filtered and the concentration is measured by UV spectroscopy. The adsorption capacity of the sample is obtained by applying the Langmuir isotherm.

3. Saturation of the material is carried out under darkness and in agitation. For this, 250 mg of treated sample are added to 250 ml of a solution of suitable concentration so that after saturation the residual concentration is 10 mg / ml.

4. Photo-degradation is performed under visible light, following the evolution of degradation by taking aliquots from time to time by measuring its concentration by UV spectroscopy.

5. The evolution of total organic carbon (TOC) present in the solution during the photo-degradation experience is studied to analyze the degree of mineralization of the contaminant. Said degree of mineralization was measured by an analyzer 30

Shimadzu V-CSH, calculating the TOC present in water samples by eliminating the value of inorganic carbon to the value of total coal.

Example 4: Photo-catalytic degradation of the Orange G dye under ultraviolet radiation. 5

The photocatalysts prepared according to examples 1 and 2 are useful for the degradation of organic pollutants from water using ultraviolet radiation. To illustrate its application, the Orange G compound was used as a test molecule. 10

The photo-degradation was performed using a low pressure mercury vapor light lamp of 15 W of power radiating at λ = 250 nm, an initial concentration of orange G in water of 10 mg / l (after saturation of the material), 250 ml of solution and a photocatalyst concentration of 1 mg / ml. The evolution of the photo-15 degradation was performed by taking sample aliquots from time to time being analyzed by UV spectrophotometry at λ = 487 nm. The methodology followed is detailed below:

1.- The adsorption kinetics of the dye are carried out on the photo-catalytic material from which the time necessary to reach the adsorption equilibrium is obtained. To do this, 250 mg of material is introduced into 250 ml of a solution of 20 mg / l of dye in darkness and stirring. Aliquots of solution are taken every so often until constant dye concentration (measured by UV spectroscopy).

2.- The adsorption isotherm is performed to calculate the adsorption capacity of the material, for which 25 mg of sample is introduced into 25ml of solutions of a concentration between 5 and 50 mg / ml of dye, under darkness and agitation. After the time necessary to reach equilibrium (calculated by step 1) the solutions are filtered and the concentration is measured by UV spectroscopy. The adsorption capacity of the sample is obtained by applying the Langmuir isotherm. 30

3. Saturation of the material is carried out under darkness and in agitation. For this, 250 mg of treated sample are added to 250 ml of a solution of suitable concentration so that after saturation the residual concentration is 10 mg / ml.

4. Photo-degradation is performed under ultraviolet light, following the evolution of degradation by taking aliquots from time to time by measuring its concentration by UV spectroscopy.

5. The evolution of total organic carbon (TOC) present in the solution during the photo-degradation experience is studied to analyze the degree of mineralization of the contaminant. Said degree of mineralization was measured by a Shimadzu V-CSH analyzer, calculating the TOC present in the water samples by eliminating the value of inorganic carbon to the value of total coal. 5

PREFERRED EMBODIMENT

Example 5.- Photo-catalytic degradation of Orange G using xerogel carbon-zirconium oxide composites as photo-catalysts under irration with visible or ultraviolet light.

The experimental photo-catalytic system and the methodology described in example 3 and 4 have been used in discontinuous mode. The materials described according to examples 1 and 2 have been used as catalyst.

For comparative purposes, the most industrially used photocatalyst, titanium oxide, P25, from Degussa® was tested.

Previously, the kinetics and adsorption isotherm of orange G are performed in the materials treated data collected in Figures 1 and 2, respectively. It is observed how the adsorption capacity of the composite material is much greater than that of P25 due to the greater pore volume of this (Table 1). Once the equilibrium time of the kinetics and the adsorption capacity of the isotherm is obtained (Figures 1 and 2), the photo-catalytic degradation is carried out (Figure 3). In addition, the photolysis of the dye is performed in the absence of material to ensure that the degradation produced is due to the presence of the catalyst.

It is noted that orange G does not degrade under visible light in the absence of catalyst. With the use of the titanium oxide pretreated in H2 as a photocatalyst, a degradation of the orange G of about 35% is obtained at 170 min, while for the xerogel carbon-zirconium oxide composite material it is 77%, while that zirconium oxide such as carbon xerogel does not show photo-catalytic activity under visible radiation (Figure 3). Obtaining a clear synergistic effect between C and metal oxide.

 35

The TOC analysis shows that the degradation of the sample by visible radiation is carried out until the complete mineralization of the contaminant because the TOC continues

the same trend as the removal of the contaminant, achieved a reduction of 70% at 170 min for the xerogel carbon-zirconium oxide composite material and only 25% for P25 (Figure 4). This is an important aspect, because it means that no oxidation intermediates are generated, which in general are more polluting than the starting contaminant itself. 5

Under ultraviolet light it is observed that in this case due to the use of more energetic radiation the time necessary for total degradation is less than 30 min. It looks like the P25 improves but slightly the degradation of the contaminant. As for the carbon-zirconium oxide composite, the improvement is significantly greater, reaching 80% degradation after 9 min. Therefore, the performance of the composite material is significantly higher both under ultraviolet and visible light, and can be used in a wider range of wavelengths.

As mentioned in the introduction for the use of semiconductor oxides, such as titanium oxide, in the photo-degradation with visible light a reduction of the band-gap or the use of an efficient sensitizer is required. These results show that the carbon gel acts as an efficient photosensitizer.

 In addition, the adsorption of the reagents is facilitated due to the greater surface area of the 20 composite materials compared to the P25, facilitating their interaction with the active centers and therefore, their degradation.

On the other hand, the use of these composite materials facilitates the dispersion of zirconium oxide so that it reduces the possibility of hollow-electron recombination, which is larger at larger particle sizes, since if the particle size is Small the distance that the electron or the photo-generated hole must travel to the surface for the redox reaction is less and therefore less is the possibility of recombination, increasing the photo-catalytic activity of the composite material, in our case: the sizes of Zirconium oxide crystals are less than 5 nm versus 30 sizes of 30 nm crystal containing the P25 or 40 nm of zirconium oxide.

The better dispersion of zirconium oxide (observed by X-ray diffraction) due to the high surface area of the carbonaceous material allows a greater generation of oxygen vacancies or partially reducible oxides that reduce the band-gap allowing the degradation through the use of light visible. In addition, the zirconium oxide phase

observed by means of ray diffraction is the cubic one, which can be responsible for the reduction of the band-gap and therefore, of its excellent performance under visible light.

In addition with the use of zirconium oxide, a new possibility of such efficient materials has been opened, and even with improved performance, such as those of 5 titanium oxide in photo-catalytic applications.

In the global contaminant elimination balance, the xerogel composites of synthesized inorganic carbon-oxide show better performance because the removal of the contaminant by adsorption must also be considered, and in this sense due to the presence of the carbon phase the properties are improved textural of the inorganic phase favoring its adsorption. Therefore, the xerogel carbon-zirconium oxide composite material has the best degradation and adsorption performance. In addition, the volume of mesopores increases with the increase in% of zirconium oxide present in the carbon phase, increasing its adsorption capacity

Claims (16)

1. Photo-catalyst comprising a carbonaceous phase and a zirconium oxide phase, both homogeneously distributed in the solid.  5 2. Photocatalyst according to the preceding claim characterized by being active under visible light. 3. Photocatalyst according to claims 1 or 2, characterized in that the zirconium oxide phase dispersed on the carbon gel phase has crystal sizes of less than 5 nm. 4.- Procedure for obtaining carbon gel composite materials - zirconium oxide comprising the following steps:  fifteen (i) Obtaining a compound hydrogel, organic gel doped with zirconium oxide from a phenolic compound (R), an aldehyde, (F) and a zirconium alkoxide (A). (ii) Curing of the compound hydrogel resulting from step (i) (iii) Drying of the hydrogel compound resulting from step (ii) by microwave in an argon atmosphere giving rise to a dry gel. (iv) Carbonization of the dry gel resulting from step (iii). 5. Method according to previous claim characterized in that step i) comprises, in turn, the following sub-stages: a) Preparation of a solution of alkoxide was zirconium in an organic solvent b) Heating the solution (a) below the boiling point of the organic solvent, with stirring. c) Preparation of an aqueous solution containing a phenolic compound and an aldehyde. d) Add dropwise the aqueous solution of section (c) on the alkoxide solution obtained in section (b). e) Gelification of the solution obtained in section (d) at constant temperature until hydrogel is obtained 35 6. Method according to any of claims 4 to 5, characterized in that the solvent used is heptane. 7. Method according to any of claims 4 to 6, characterized in that the phenolic compound (R) used is resorcinol. 5 8. Method according to any of claims 4 to 7, characterized in that the aldehyde (F) used is formaldehyde. 9. Method according to any of claims 4 to 8, characterized in that the zirconium alkoxide is zirconium proproxide (IV). 10. Method according to any of claims 4 to 9, characterized by maintaining throughout the process a constant temperature between 5 and 98 ° C, preferably 70 ° C 15 11. Method according to any of claims 4 to 10, characterized in that the organic polymer coated with zirconium oxide and dried in microwave, is charred (iv) in inert gas flow at temperatures between 500-1200 ° C, preferably at 900 ° C. twenty 12. Method according to any of claims 4 to 11, characterized in that the carbonization speed ramp reaches a temperature between 500 ° C and 1200 ° C, more preferably at 900 ° C, with an increase of between 0.1 and 25 ° C per minute, preferably with an increase of 1ºC / min. 25 13. Process for obtaining improved photocatalysts comprising subjecting a photocatalyst according to claims 1 to 3 to a treatment at temperatures between 100 and 600 ° C in an atmosphere of He, N2, Ar or H2, or with any mixture formed between these gases. 30 14.- Photo-catalysts obtainable by the method according to the preceding claim. 15.- Photo-degradation process of organic compounds in solution, in gas or liquid phase, comprising bringing the photocatalysts 35 according to claims 1 to 3 or 14 into contact with the suspension containing said compounds under radiation in the visible and ultraviolet spectrum. 16.- Method according to previous claim applied to the elimination of organic pollutants in waters characterized by using visible radiation in the presence of photocatalysts.