Process for the preparation of a supported catalyst

Details Process for the preparation of a supported catalyst Process for the preparation of a supported catalyst

1998-12-22

2001-05-29

Dunn, Tom (Department: 1754)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Metal, metal oxide or metal hydroxide

C502S325000, C502S339000, C502S344000, C502S347000

Reexamination Certificate

active

06239065

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a process for the preparation of a supported catalyst having a nanocrystalline structure and a high specific surface, which process makes use of an intensive mechanical grinding step combined with a leaching step.
The invention also relates to the so-prepared, supported catalyst.
The invention further relates to the use of such a supported catalyst as an electrocatalyst in a fuel cell or in an electrolyser.
By the expression “nanocrystalline structure”, there is meant a crystalline structure having a grain size lower than 100 nm.
BRIEF DESCRIPTION OF THE PRIOR ART
“Raney” catalysts have been known for a very long time. As a matter of fact, U.S. Pat. No. 1,628,190 issued on May 10, 1927 in the name of Murray RANEY discloses a process for preparing a finely divided powder of Ni, which consists in melting together a mixture of Ni, Al and Si. After completion of the mixture, Al and Si are leached by treatment of the alloy with a caustic solution, thereby leaving only Ni in the form of a porous or finely divided powder.
Some inventors have also suggested to combine the Raney process with an intensive mechanical grinding in order to obtain porous Ni. Thus, by way of example, the article published in
J. Alloys and Compounds
, vol. 185, (1992), pp. 25-34, disclosed a process wherein a Ni
35
Al
65
is prepared by a mechanical grinding and this alloy is then leached with a solution of 20% KOH in order to dissolve Al.
In European laid-open patent application EP-0,734,765-A1 (1996), reference is made to the use of the above process for the preparation of alloys other than Ni-based alloys. As a matter of fact, in this laid-open application, the alloy manufactured by mechanical grinding is of the formula Al—Me—X where Me is a metal selected from the group consisting of Ni, Co, Fe, Cu, Pd and Ag, and X is different from Me and consists of at least one metal selected from the group consisting of Ag, Co, Pt, Ti, V, Fe, Cu, Zn, Ge, W, Re, Os, Ir, Au, Zr, Nb, Mo, Ru, Rh, Pd, Cd, In, Sn and Sb. After the alloy has been prepared, Al is leached.
All these processes of preparation make use of a combination of elements that are exclusively metallic, thereby generating some problems during the mechanical grinding step. Indeed, metals are subject to cold welding during the process, because of the creation of fresh metal surfaces during the grinding step. Therefore, it becomes difficult to operate the grinding machine without having sticking problems, and to reduce the aggregation of crystallites in order to facilitate their separation and increase the specific surface of the final product. Therefore, it is advantageous to use, as a starting material, at least one non-metallic compound. This compound can be, for example, a metal oxide and can be part of either the catalytic component or of the leachable component.
In international patent application WO 98/32885 published in the name of the Applicant, there is disclosed a process for the preparation of nanocrystalline materials having a high specific surface. The process comprises a first step consisting in mixing the elements of the alloy with other elements acting as dispersing agents. All these elements are subjected together to an intensive mechanical grinding, a fast quenching from the melt or any alternative step, in order to generate a nanocrystalline material. In a second step, the nanocrystalline material is leached in order to totally or partially dissolve the dispersing agents in a leaching solution. The resulting product still has a nanocrystalline structure in addition of having a high porosity thanks to the dissolution of the dispersing agent. Its specific surface is considerably increased and such in turn increases the catalytic properties of the material.
It is worth mentioning that the morphology of the resulting material is similar to the one of a sponge that would have nanometric pores. In the field of catalysis, the species (gas or solvated ions) that must chemically react with each other must stay for a given time on the surface of the catalyst in order to obtain the requested catalytic effect. Therefore, the amount of species that will react as a function of time which is the reaction speed, will be directly related to the available surface of the catalyst. Of course, this available surface is substantially increased if use is made of a nanometric sponge. However, it is also important that the species reach or leave (depending on the chemical reaction) quickly the catalytic surface. This may be a problem if the species have to travel for long time through nanometric channels in the sponge in order to reach internal surfaces. In fact, it is highly probable that the internal surface will be of very small use if the speed of transportation of the species is lower than the reaction speed. Thus, most of the materials forming the heart of the sponge, could be used very little during the catalytic reaction. In the case where the material comprises noble metals such as Pt or Pd, it becomes advantageous to fragment the sponge and disperse the fragments that are so-obtained onto a support in order to maximize their use and therefore reduce the costs and amount of noble metals that are needed.
SUMMARY OF THE INVENTION
The present invention proposes a solution to the problem mentioned hereinabove.
More particularly, the invention proposes a process in which, during the preparation (synthesis) of the nanocrystalline material, chemical elements or compounds that are not soluble by leaching and which do not mainly contribute to catalysis are incorporated into the material in order to provide a support for the catalytic elements or compounds. By way of example, an intensive mechanical grinding of Pt+Ru+MgO can be carried out. Once obtained, the ground mixture can be grounded for a second time with a substantial amount of carbon black. Leaching is then carried out and a catalyst of Pt—Ru supported onto carbon black is obtained. The support which, in the above example, is carbon black, can also be incorporated to the material at a different step of the process, such as during the leaching step. In such a case, the mixture of Pt+Ru+MgO will be grounded and then leached in a solution containing a sufficient amount of carbon black, on which the remaining Pt—Ru will be deposited while MgO is dissolved.
Such a use of a chemical element or compound that is not leachable permits to increase the separation of the particles and nanocristallities and therefore to increase the active surface of the catalyst. Thus, the process according to the invention permits to reduce the amount of species that act as catalysts, species which are often very expensive. On this basis, a first object of the present invention as claimed hereinafter is to provide a process for the preparation of a supported catalyst having a nanocrystalline structure and a high specific surface area, comprising:
in a first step, preparing a nanocrystalline material consisting of a metastable composite or alloy of at least two different chemical elements or compounds containing at least one catalytic species, the so-prepared material having a nanocrystalline structure with crystallites of a size lower than 100 nm; and
in a second step, subjecting the nanocrystalline material to leaching with a leaching solution in order to eliminate totally or partially at least one of the elements or compounds of the material other than the at least one catalytic species,
wherein at least one further chemical element or compound that is non-leachable and acts as a support for the at least one catalytic species, is added to the material during the first step or to the leaching solution during the second step.
The process according to the invention permits to obtain a supported catalyst in the form of a nanocomposite material, a solid solution, an intermetallic compound or a combination thereof, having a nanocrystalline structure and a high specific surface area that is usually higher than 2 m
2
/g.
A second object of the present invention

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