entitled METHOD FOR PREPARING A CATALYST FOR ENVIRONMENTAL DECONTAMINATION BY MEANS OF NON-SELECTIVE REDUCTIVE HETEROGENEOUS ELECTROCATALYSIS

Abstract

A method consisting in depositing coating of a semiconductor such as TiO.sub.2 on the surface of a substrate of activated carbon in the form of grain or powder that acts by an advanced oxidation-reduction mechanism in environmental decontamination processes, by way of a heterogeneous electrocatalysis system applying an electrical potential having a magnitude equal to or greater than that of the bandgap energy of the semiconductor, which is 3.2 eV in the case of anatase TiO.sub.2, such that an electron rises from the valence band to the conduction band, leaving in its place holes, h+, with enough oxidative capacity to be able to oxidise H.sub.2O and form OH radicals.

Claims

1. A method for the preparation of a catalyst for an environmental decontamination by anon-selective reductive heterogeneous electrocatalysis involving a semiconductor of a transition metals oxide,TiO.sub.2, the method comprising the step of and substrate of metal, method comprising the step of, sts consists of partially deposition of a nanometric coating of the TiO.sub.2 on one face of the nanometric coating including active carbon surface of a substrate, in the form of grains or powder, with a roughness established between Ra 0.02 and 50 microns and a surface ranging from 300 to 3000 M.sup.2/gr,through a process of physical deposition in the vapor state by low pressure electric arc; wherein the applying an electric potential of equal or greater magnitude to the breach energy of the forbidden band or to TiO.sub.2 layer together vvith the BandGap of the semiconductor to TiO.sub.2 layer together with the substrate; forminga rectifying potential barrier that favors the transport of electrons from the TiO.sub.2 to the active carbon and originates a reducing mechanism at the same time that inhibits the opposite flow of charges; reinforcing the metal in its reducing function and preventing the recombination of h+ holes and e- electrons at the interface, when the electric potential is established.

2. The method according to claim 1, wherein the coating used is TiO.sub.2 anatase.

3. The method according to claim 1, wherein the coating is TiO.sub.2 in its amorphous or rutile form or a combination of both.

4. The method according to claim 1, wherein the substrate used is a material with a high surface and porous, the such as cellular concrete, porous silica, zeolites or another analogous, for which the deposition of a conductive layer of an intermediate metal between the TiO.sub.2 layer and substrate.

5. The method according to claim 1, wherein the carbon is present in the form of nanotubes on a metallic substrate that has been processed to the form of grains or powder on which TiO.sub.2 is similarly deposited.

6. The method according to claim 1, wherein the TiO.sub.2 coating is mixed with a photocatalytic semiconductor oxide, preferably CdS, SrTiOs, ZnO, Nb.sub.2O.sub.5, MoS, Fe.sub.2O.sub.3, WO.sub.3 o SnOs or a mixture of them.

7. The method according to claim 1,wherein the TiO.sub.2 has been worked with increased O.sub.2 vacancies.

8. The method according to claim 1, wherein the TiO.sub.2 has been doped with any transition metal such as Cu, Mo, V, Ni, W, Fe, Al or with noble metals, such as Ag, Au, Pt, Ru, Rh, Pd.

9. The method according to claim 1, wherein the TiO.sub.2 has been doped with N.sub.2 or some other non-metal, such as B or S.

10. The method according to claim 1, wherein the TiO.sub.2 has been doped with rare earths Ce, La, Pr, Nd, Sm, Gd, Dy or Eu.

Description

DESCRIPTION OF THE INVENTION

[0048] The new technology consists of a decontaminating, disinfecting and biocidal nanometric TiO.sub.2 coating, which is deposited on the surface of an active carbon substrate, with the high surface density and porosity that characterizes it, in the form of grains or powder. The substrate that can be metallic, rough or with a smooth finish, preferably rough, of any roughness that falls within the roughnesses established between Ra 0.02 and 50 microns, but preferably roughnesses between Ra 1.6 to 50 microns, in the form of particles or powder or even in the form of larger bodies, although preferably in the two previous forms; a coating of a photoactive semiconductor of TiO.sub.2 is provided or deposited.

[0049] Preferably, the substrate used will be granulated active carbon (hereinafter AC), with the large surface that characterizes it, which can be between 300 m.sup.2, even less, up to 3000 m.sup.2 per gr., even more.

[0050] TiO.sub.2 is a broadband photosensitive semiconductor. To raise an electron from the valence band to the conduction band, this semiconductor needs to resort to the photoelectric effect through the incidence of an energetic photon, with wavelengths from 400 nm and less. This type of radiation is thus in the ultraviolet range of the light spectrum.

[0051] As the photon strikes, an electron is promoted to the conduction shell, as we have noted, and once here it can move freely through the band. In the place that this electron occupied, in the valence band, there is then a hole with a positive charge (h+) with a strong oxidative character, capable of stealing an electron from any element that affects it and oxidizing it. This is the classic mechanism of heterogeneous photocatalysis that is widely used in advanced oxidation reactions, which are very widespread today, as they are very effective, cheap and stable over time. Its greatest utility is given in liquid and gaseous decontamination processes, as well as in disinfectant, bactericidal and viricidal processes. Solar radiation is widely used for activation.

[0052] However, together with the fundamental requirement of having a large surface area, a very strict requirement reserved for the photocatalyst, which in some cases is impossible to fulfill, is the direct exposure of that large surface to light. Without this exposure, the phenomenon does not take place.

[0053] In our novel device we have solved, very effectively, at least three of the most acute problems that are required of the catalyst and which we will now describe. These are the following:

[0054] 1.sup.st. With AC and, preferably, but not only, the form of TIO.sub.2 in its anatase state (which is the spongy form with the highest surface magnitude of this excellent catalyst), which is ensured by a technology of special disposition and a specific subsequent heat treatment, we have secured a huge active surface for the catalysis and advanced oxidation-reduction processes that we intend to develop;

[0055] 2.sup.nd. Through a process of physical deposition in the state of vapor by low pressure electric arc, we have ensured the requirement of a strong anchoring of the semiconductor on an atomically free surface of the AC, being of crucial importance this last signaling of surface cleanliness to ensure, at the limit of phases, the correct mechanism of transfer and exchange of charges. The junction of the semiconductor TiO.sub.2 with the Active Carbon forms a rectifying potential barrier that favors the transport of electrons from the semiconductor to the element with metallic character, a phenomenon that is automatically established just by putting these elements in contact. Then, the flow of electrons from TiO.sub.2 into the conductor occurs intrinsically, during the equalization of the Fermi energy levels at the interface, resulting from the difference in the values of the output work between the two elements. This fact allows a predisposition to the reduction mechanism if the semiconductor coating is partially established on the surface of the granulated substrate, a question that we have achieved in the system that we have invented. The fact that our technology has a reductive character is very advantageous if we want to reconvert and return the components of the pollutants that would be expelled to the external environment to their initial states, before their formation, preventing the formation of new compounds, which, in turn, could affect the environment. At the same time, the contrary flow mechanism is inhibited, which reinforces the metal in its reducing function and prevents the recombination of the h+ holes and electrons at the interface, when the electric potential is established.

[0056] 3.sup.rd. We have replaced light radiation and the photoelectric effect, as the most widespread classical method in photocatalysis, by the application of an electric potential, which is what it promotes the passage of the electron from the valence band to the conduction band. The magnitude of potential used, as a minimum, has been the one corresponding in energy to that of the photon of light of the ultraviolet range indicated. With this innovative solution, we have extended the activation of the entire immense surface referred to in the first requirement to a space of reduced volume, thanks to the fact that we have literally converted an entire volume into a surface. Now, with this solution, it is not necessary to irradiate the photocatalyst, nor to make light reach its entire surface, an issue that, in some cases, is practically impossible. However, the electrons and the electric potential can reach every corner and surface of the catalyst, no matter how convoluted, microrugal or profuse. The noxious gases or liquids that can be treated with this invention will practically come into contact with the catalyst in all its magnitude, 100%, and there will be no molecules that can escape without having been treated and transformed into harmless and compatible elements with the environment.

[0057] This TiO.sub.2 layer, together with the substrate, which in this case has a metallic character, will act by means of a mechanism of oxide of advanced reduction in environmental decontamination processes through a novel heterogeneous electrocatalysis system, which avoids the demands of light radiation from high energy (Ultraviolet A, B or C light or others even more energetic) that are imposed on TiO.sub.2 and other broadband semiconductors, so that they can act efficiently during various decontamination processes, through the known oxidation mechanisms by heterogeneous photocatalysis. But, above all, this technology avoids the need for the entire surface of the semiconductor to be obligatorily exposed to light, an issue that on countless occasions substantially limits and even makes it impossible, in many cases, the use of the effective treatment photocatalytic in operating volumes tight and convoluted structures.

[0058] In this invention, the application of an electric potential of equal magnitude, or greater, to that of the forbidden band gap energy, or BandGap, of the semiconductor, which, in the case of TiO.sub.2 anatase is 3.2 eV, is sufficient to raise an electron from the valence band to the conduction band, leaving in its place h+ holes with sufficient oxidative capacity to oxidize H.sub.2O and form OH radicals, extremely oxidizing, which will destroy the pollutants, or act, directly the h+, against them, in different types of oxidation-reduction reactions.

[0059] In other words, the novelty of the new technology is based on replacing the photoelectric mechanism, universally used in photocatalysis, which is based on the delivery of energy from quanta of light to a material, in this case a semiconductor, so that it performs the catalytic function; for an electric one, more efficient, versatile and universal in its use. The photocatalytic mechanism, therefore, has serious limitations and demands that are difficult to meet. However, the electricity proposed, through the replacement of radiation by the application of an electric potential to the semiconductor, is of increased quantitative efficiency, easy to implement, simpler, cheaper, effective and does not require the mandatory exposure of the photo catalyst to light.

[0060] The union of the TiO.sub.2 semiconductor with the Active Carbon forms a rectifying potential barrier that favors the transport of electrons from the semiconductor to the element with a metallic character, a phenomenon that is automatically established by simply putting these elements in contact. Then, the flow of electrons from TiO.sub.2 to the conductor occurs intrinsically, during the equalization of the Fermi levels at the interface, product of the difference in the values of the output work between the two elements. This fact allows a predisposition to the reducing mechanism in the system that we have invented, which is very advantageous if you want to reconvert and return the pollutants that would be expelled to the external environment to their initial states, before their formation, preventing the formation of new compounds, which, in turn, could affect the environment. At the same time, the contrary flow mechanism is inhibited, which reinforces the metal in its reducing function and prevents the recombination of the h+ holes and electrons at the interphase, when the electric potential is established.

[0061] An important aspect to add is that the barrier mechanism can even reduce the intrinsic bandgap gap of the semiconductor below 3.2 eV, which reinforces the described mechanism and reduces the energy necessary for the phenomenon to be established, without detriment of its functionality. Whenever we treat different semiconductors to be used, other than the reference par excellence, the TiO.sub.2, the potential to be established will be adjusted to the specific value of the BandGap for the specific semiconductor (s), or above it, which is not difficult to achieve modifying the voltage value of the source.

[0062] There is another very important aspect in the invention and it is the following: a catalytic converter requires a large surface area for efficient performance. It can be said that the more surface a catalyst can have, the more active centers there will be and the greater number of molecules can be catalyzed at the same time. Well, when coating on activated carbon, the TiO.sub.2 will have a large surface to act and this surface will be located in a relatively small volume, like the substrate in question, which can reach 3000 m.sub.2/gr. which is equivalent to a colossal specific surface area. In other words, it could be said, literally, that we have extended a whole possible surface to a volume, with the method used. There is even more: by sintering TiO.sub.2 in its anatase form, which is what we choose for our technology, the surface of TiO.sub.2 also grows, in turn, to a spongy state that can increase up to 1000 times its initial value. Hence, the increase in surface area has a multiplying effect on our technology, which is evidenced in the effective results it demonstrates during its performance.

[0063] Thus, we must emphasize a change in technology that goes from heterogeneous photocatalysis to heterogeneous electrocatalysis.

[0064] The electrocatalytic TiO.sub.2 semiconductor can be deposited by some type of chemical or physical deposition known in the art, such as galvanic, CVD (Chemical Vapor Deposition), Sol Gel, Spin Coating, Molecular Beam Epitaxy, Atomic Beam Epitaxy or PVD (Physical Vapor Deposition). The latter, in turn, by any of the methods used, such as Thermal, Sputtering, Magnetron-Sputtering or Multi Arc; preferably by this last indicated method: the Multi Arch.

[0065] Alternatively, TiO.sub.2 is used in its amorphous or rutile form, instead of TiO.sub.2 anatase or a combination of both, obtained through specific sintering processes and heat treatments.

[0066] On the substrate, a coating of a layer of some other photocatalytic semiconductor can be added, preferably among those indicated below: CdS, SrTiOs, XnO, Nb.sub.2O.sub.5, MoS, FeO.sub.3, WO.sub.3 O SnO.sub.3. We have chosen, preferentially, although not limited to a TiO.sub.2 coating. The anchorage to the substrate of this photocatalytic layer must be strong, as we have indicated, and must ensure a certain transmission of charges, without the possibility of polarization or parasitic resistances that hinder or modify the conditions of potential barrier rectifier or Shottkii, required and accompanied by values strictly determined, which will ensure the correct operation of the decontaminating device. For this purpose, an atomic cleaning is strictly necessary, which ensures the total freedom of the valences exposed in the surface layer of the substrate and their availability to receive the host layer, that of the semiconductor. This requirement can only be guaranteed by massive ionic bombardment, which is exclusive to our technology, with parallel heating through an overpotential applied to the substrate that ensures sputtering acceleration in the Debaev layer or by means of a neutral atom accelerator, which we also use in the process. A variant of atomic cleaning can also be by means of Argon heavy ion plasma, produced from an ion source, also present in our know-how.

[0067] The AC substrate used can have a surface ranging from 300 to 3000 m.sup.2 / gr. Preferably a AC of 800 to 3000 m.sup.2/g will be used, depending on its porosity. Porosity can be microporous (less than 2 nm), mesoporous (2 to 50 nm), and macroporous (more than 50 nm). Preferably mesoporous and macroporous, without ruling out microporous, would be used in specific designations, such as for biocidal clinical treatments. On these particles, in granulometric or powder form, with particle dimensions from 1 to 10 mm for the particulate carbon and less than 1 mm for the powder, the partial deposition of a catalytic TiO.sub.2 coating is carried out, which will be the one that will act, together with the substrate, in advanced oxidation-reduction reactions.

[0068] AC is conductive and is the substratum used par excellence in our new technology. However, we also plan to use, as a substitute for this and when circumstances recommend it, other bases for the coating, such as porous and spongy materials, such as grains or powder of a material with a high surface and porous, as it may well be. be aerated concrete, porous silica, zeolites or other similar.

[0069] In the case that the base material is not conductive, it is envisaged to carry out a deposition of a metal, prior to the coating of the catalytic semiconductor. In such an embodiment the metallic layer would span the entire surface of the substrate.

[0070] In this case, the deposition of a conductive layer of a metal will be carried out before coating, in turn, on this, the electrocatalytic semiconductor.

[0071] The deposition of the catalytic semiconductor can also be carried out on some metal, specifically, a decontaminating coating, where the carbon is present in the form of nanotubes that have been made to grow on a metallic substrate that, to increase the catalyzing surface, has been elaborated to the form of grains or powder. On top of this element, already carbonated, the TiO.sub.2 will be deposited similarly.

[0072] The decontaminating coating can be technologically worked so that the TiO.sub.2 semiconductor presents increased O.sub.2 vacancies to improve its character as an n-type semiconductor, highly advisable in this technology. This improves the transfer of electrons from the 2p orbital of O.sub.2 to the 3d orbitals of TiO.sub.2, with removal of O.sub.2.

[0073] With a similar objective of accentuating the electron donor character of TiO.sub.2 and increasing the efficiency of the n-type semiconductor mechanism, our technology provides for its doping with any transition metal, among which we can point out, by way of example and without limitation, Cu, Mo, V, Ni, W, Fe, Al or with noble metals, such as Ag, Au, Pt, Ru, Rh, Pd. We even envisage a decontamination coating, where the semiconductor has been doped with N.sub.2 or some other non-metal, such as B or S, to improve its electrocatalytic response.

[0074] In this same direction, a decontaminating coating is envisaged where the semiconductor has been doped with rare earths Ce, La, Pr, Nd, Sm, Gd, Dy or Eu for the enhancement of its electrocatalytic response.