IE904093A1

IE904093A1 - Supported catalysts and a method of preparing them

Info

Publication number: IE904093A1
Application number: IE409390A
Authority: IE
Original assignee: Rhone Poulenc Chimie
Priority date: 1989-11-27
Filing date: 1990-11-13
Publication date: 1991-06-05
Also published as: FI905824A; GR3020638T3; ES2090113T3; FI905824A0; EP0430744A1; PT95999A; KR910009335A; BR9005980A; CA2030682A1; ATE138591T1; DE69027184T2; FR2654953B1; MX171972B; DE69027184D1; JPH0747288A; JPH03181335A; NO905099D0; KR970005545B1; PT95999B; RU2027505C1

Abstract

Supported catalyst and process for its manufacture. The supported catalyst, useful especially in the treatment of exhaust gases of internal combustion engines, comprises a porous support on which catalytically active elements are deposited, and more particularly cerium in combination with other elements. The cerium is deposited according to the invention solely on a peripheral crown ring of specified width of the support by impregnation with a colloidal solution of a cerium compound.

Description

The invention concerns a supported catalyst and a method of preparing it.

A supported catalyst is understood as being a catalyst of porous material, with active elements deposited on the surface, particularly by an impregnation process.

It relates more particularly to a catalyst in which at least one active element has been deposited on part of the carrier, namely a peripheral ring.

Supported catalysts have been known for a very long tine and used in many processes, such as treating petroleum cuts or treating effluents, e.g. treating exhaust fumes from internal combusion engines.

The catalysts generally comprise a carrier obtained by shaping a porous material such as alumina. Thus the carrier may have various shapes such as spheres, cylindrical extrusions or extrusions with a polylobe section or a section of various shapes such as a cartwheel.

The porous materials generally used are alumina, silica, resin, zeolites or the like.

These carriers generally have a large specific surface area, e.g. over 20 m /g, as a neans of obtaining a large area which is made catalytically active by depositing active elements.

The active elements may be divided into several categories: - catalytically active elements proper: these are generally elements of the platinum family, such as palladium, rhodium and the like - promoting elements, which encourage the catalytic activity of the above elements.

Nowadays various elements are generally inpregnated on to the whole area of the carrier, except for elements of the platinum family and more specifically the element platinum. It is not possible to control the depth to which the element penetrates into the carrier or the concentration of that element in each part of the area.

In view of the speed at which the reagents are diffused in the catalyst, it is kncwn that only a peripheral part of the catalyst is effective. Consequently only part of the elements deposited will really be effective.

The active elements are generally very expensive iraterials and significantly affect the cost of carrying out a process.

Attenpts have therefore been made for a very long time to reduce the quantity of active elements in catalysts, while maintaining a level of catalytic activity which is high and at least equivalent.

A solution to this problem has been proposed by Hoesktra and described in US Patent No. 3674680.

The solution oonsists briefly of giving the carrier or catalyst special shapes, so cis to obtain a catalyst of a thickness which is no mare than twice the thickness of the diffusion layer of the reagents. Thus the greater part of the catalyst is in contact with the reagents.

However, this limitation on thickness makes it necessary to have conplex shapes, resulting in catalysts which are brittle and expensive to produce.

A method has also been proposed for inpregnating metals of the platinum family, enabling the impregnation to be carried out solely within a ring round the periphery of the carrier. The process uses a solution of platinum salt. However, it cannot be adapted to deposit other elements, e.g. transition metals or rare earths.

A particular purpose of the present invention is to avoid these disadvantages by proposing a method of impregnating elements other than elements of the platinum family, over a ring of well defined thickness at the periphery of a carrier, and by proposing catalysts which have a homogeneous content of active elements over a ring at the periphery of the carrier.

To this end the invention proposes a catalyst comprising a carrier for mad by shaping a porous material, over the surface of which active elements have been deposited, characterised in that at least one of said active elements, other than an element of the platinum family, is deposited in a homogeneous concentration over a ring at the periphery of the carrier, the ring having a well defined width.

It is advantageous for the width of the peripheral ring to be substantially equal to the depth to which the reagents are diffused in the catalyst in question.

Thus all or nearly all the active elements are in contact with the reagents. This makes it possible to obtain catalytic activity at least equivalent to that of a conventional catalyst, with a far lower mass of active elements.

In a preferred embodiment of the invention, the width of the peripheral ring is constant.

In addition, the porous material forming the carrier has active surface locations. Most porous materials used to form catalyst carriers are mineral oxides, which tend to polarise and become electrically charged when suspended in an aqueous solution. Ibis is explained in an article by J. P. Brunelle Preparation of catalysts by adsorption of netal conplexes on mineral oxides published in Fine.Appl.Chem 50, 9-10 (1978, p 211 ff.

Thus most mineral oxides are anphoteric. That is to say, the reactive surface locations can react equally well with electronegative compounds (anions) and with electropositive conpounds (cations).

The pH for which electrical neutrality of the surface is obtained corresponds to the isoelectric point of the material.

The anphoteric, acid or basic character will be more or less strong, according to how high the concentration of reactive locations is as one moves away from the isoelectric point of the material.

Some exanples of mineral oxides of an anphoteric character are alumina, titanium oxide, zirconium oxide and cerium oxide. Zinc and magnesium oxides have a basic character. Silica also has an anphoteric character but not a very strong one.

Carriers suitable for the invention are thus those made of a porous material of an anphoteric, acid or basic character.

Hcwever, materials suitable for the invention must also be capable of being shaped by any kncwn process, such as extrusion, moulding, granulation or the like.

Mineral oxides may of course be used, alone or in mixtures, and may contain additives such as stabilising agencs.

The catalyst of the invention is obtained by impregnating the carrier with a colloidal solution of a oonpound of the element to be deposited, also described as a sol. A sol is in fact a metastable equilibrium which can be destroyed by changing the pH. Thus when the sol enters the pores of the carrier, the reactive surface locations therein change the pH of the sol, causing the element to be deposited, while the liquid phase of the sol continues to be diffused towards the centre of the carrier.

As the diffusion speed is constant throughout the carrier and the concentration of reactive locations in the carrier is homogeneous, the sol will be neutralised at a surface with a profile parallel and identical with that of the outer surface of the carrier.

However, if the elements are to enter the pores of the carrier, the dimensions of the colloids forming the sol must be smaller then the diameter of the pores of the carrier, and particularly smaller than the micropores, which are those forming the greater part of the surface.

The width of the peripheral ring in which the element is deposited is determined and controlled by the pH of the sol with which it is inpregnated.

As for the content of the element deposited, this is controlled by the concentration of that element in the sol.

By way of exanple, for a carrier made of alumina, the micropores of which are generally smaller than 1000 A in diameter and preferably smaller than 100 A, the suitable sols are those comprising colloids with a dimension smaller than 10G0 A, and preferably smaller than 100 A, e.g. of the order of 80 A or less. Sols of cerium oxide which have colloids of the order of 60 A are perfectly suitable. They make it possible to obtain alumina based catalysts containing cerium, which is deposited solely over a ring at the periphery of the carrier.

The concentration of elements in the peripheral ring is not critical and is determined according to the element deposited and the catalyst required.

The concentration of the above element in the rest of the carrier is advantageously close or equal to zero.

The carrier may be impregnated by any known methods. However, the dry inpregnation method is preferred, since it gives better control of the width of the inpregnated ring.

Pore inpregnation is understood as being inpregnation of a porous carrier with a volume of solution of sol substantially equal to the pore volume of the carrier.

The carrier thus inpregnated is dried then given heat treatment to activate the catalyst.

Some exanples of colloidal solutions of a cerium conpound are colloidal solutions of cerium hydroxide, cerium hydroxynitrate or any other conpound which can be deconposed to ceric oxide by heat treatment.

The concentration of cerium (expressed in cerium oxide) in the colloidal impregnating solution is not critical; it depends on the quantity of cerium required in the catalyst. In a preferred embodiment of the invention, however, the concentration may be from 20 to 400 g/1 (expressed in Ce02).

The pH of the solution is not critical either. It is selected according to the stability of the sols used and the acid or basic character of the carrier to be inpregnated. Thus for an alumina carrier the pH of the colloidal cerium solution will advantageously be from 0.2 to 2.

It is possible to deposit other elements on the carrier by conventional inpregnation with solutions of salts which can be decomposed to oxides by heat treatment.

The catalysts of the invention are used in many applications.

For example, the catalysts of the invention are perfectly suitable for treating exhaust fumes from internal combustion engines. Thus the catalysts described in European Patent No. 27069 can be made by the method of the invention. These contain iron and cerium as active elements, in addition to the precious metals of the platinum family normally used for this application. Cerium, which is an expensive element, is deposited solely within a ring approximately 400 microns wide at the periphery of the carrier, corresponding to the diffusion layer of gases in the catalyst.

Thus the cost of producing the catalyst is significantly reduced, while the catalytic properties are maintained at least at an equivalent level.

Other advantages, details and purposes of the invention will emerge more clearly from the following exanples, which are given purely to illustrate the invention.

Exanple 1 100 g of alumina spheres of gamma structure are prepared by the methods described in French Patents No, 1449904 and 1386364.

The spheres have a specific surface area of 100 m2^g ancj a tota^ pore volume of 0.90 cm /g. The volume of micropores with a diameter greater than 10C0 A is 0.20 enr/gr and the mean diameter of the micropores is 200 A.

A colloidal suspension of cerium hydroxide is also prepared, by the method described in European Patent No. 239 477.

This horaodisperse suspension has the following properties: - concetnration of Ce02 : 300 g/1 - mean dimensions of colloids : 5 nm Ihe colloidal suspension is diluted to obtain a solution containing 52 g/1 of Ce02 at pH 1.

The ICO g of alumina is kept rotating, for exanple in a q rotating bowl, and 90 cm of the solution is sprayed onto it.

After 30 minutes’ contact the spheres are dried at 150°C then calcined in air at 400°C for 3 hours.

The carrier contains 4.7% by weight of cerium, expressed as Ce, relative to the total mass of the carrier.

All the cerium is contained in a peripheral ring 400 microns wide. The concentration of cerium in the ring is 10% by weight.

Exanple 2 Exanple 1 is repeated except that the cerium solution is at pH 0.5 and its concantration is 78.3 g/1.

The alumina spheres have a specific surface area of 100 m /g and a total pore volume of 1.20 cm/g, of which 0.45 cm/g corresponds to the volume of micropores with a dimension greater than 1000 A. The mean diameter of the micropores □ corresponding to a volume of 0.75 cm/g is about 200 A.

The volume of cerium solution sprayed on is 120 cm .

The spheres are dried and calcined as in Example 1.

The carrier contains 9.4% of cerium relative to the total carrier, and all the cerium is deposited on a surface ring 300 microns wide.

Example 3 (comparative) 100 g of alumina spheres with the properties of the spheres in Example 1 is inpregnated with a 52 g/1 aqueous solution (90 cm ) of cerous nitrate, under the operating conditions described in Example 1.

The carrier thus obtained contains 4.7% by weight of Ce relative to the total carrier. Hcwever, as indicated in Table I below, the cerium is spread over the whole surface of the carrier.

The various carriers are analysed by X-ray fluorescence to determine the profile of the concentration of cerium throughout the thickness.

Alumina spheres 3.2 nm in diameter are recovered for this purpose. Spheres forming a statistically representative sample are divided into two equal parts, and the concentration of cerium from the perhiphery of the sphere to the centre is determined by X-ray fluorescence, using an EDAX apparatus.

The results obtained are expressed by the mean weight percentage of cerium for each slice or ring of carrier. They are given in the table below.

Ring Surface -------> centre Total Exanple 0-100 pm 100-200 200-300 300-400 400-500 500-1600 1 6.6% 9.6 9.8 9.0 0.1 0 4.7 2 28.1% 28.3 28.2 0.1 0 0 9.4 3 4.5% 4.6 4.7 4.8 4.8 4.8 4.7 Ihe results clearly shew the advantage of the carriers according to the invention and of the impregnating method which enables the cerium to be deposited solely on a peripheral layer of the carrier.

Other active elements such as other promoters, like iron or metals of the platinum family, nuy of course be inpregnated before or after the depositing of the element in question, either onto the whole area of the carrier or solely onto a peripheral ring. The method of the invention is particularly suitable for this purpose.

Claims

1. Supported catalysts of the type comprising a carrier obtained by shaping a porous material with catalytically active elements deposited on it, characterised in that at least one of said active elements not belonging to the platinum family is deposited in homogenous concentration on a peripheral ring of said carrier, of well defined width.

2. The catalysts of Claim 1, characterised in that the width of the peripheral ring of the carrier is constant.

3. The catalysts of Claim 1 or 2, characterised in that the porous material has active surface locations.

4. The catalysts of Claims 1, 2 or 3, characterised in that the porous material is a mineral oxide with an acid, basic or anphoteric character.

5. The catalysts of any of the preceding Claims, characterised in that the porous material is selected from the group conprising alumina, silica, titanium oxide, zirconium oxide, cerium oxide or a mixture thereof.

6. The catalysts of any of the preceding Claims, characterised in that the active element impregnated into the peripheral ring of the carrier is cerium.

7. The catalysts of any of the preceding Claims, characterised in that they conprise catalytically active elements inpregnated over the whole area of the carrier.

8. The catalysts of any of the preceding Claims, characterised in that they conprise catalytically active elements of the platinum family, inpregnated onto a peripheral ring of the carrier, of a width equal to or different from that corresponding to the active element cerium.

9. The catalysts of Claim 7 or 8, characterised in that they conprise promoting elements.

10. The catalysts of any of the preceding Claims, characterised in that the concentration of cerium element in the carrier, outside the peripheral ring, is zero.

11. A method of preparing the catalysts of any of the preceding Claims, of the type conprising inpregnating a carrier of porous material with a solution of a conpound of an element to be deposited, drying the carrier thus inpregnated and subjecting it to heat treatment to reactivate it, characterised in that the inpregnation of the active element to be deposited on a peripheral ring of said carrier is effected by inpregnation with a colloidal solution of a conpound of said element.

12. The method of Claim 11, characterised in that the conpound is a hydroxide, a nitrate hydroxide.

13. The method of Claim 10 or 11, characterised in that the particles of colloidal solution have a dimension no greater than that of micropores in the porous material forming the carrier.

14. The method of Claim 13, characterised in that the dimension of the particles of colloidal solution is no greater than 1C00 A.

15. The method of Claim 14, characterised in that the dimension of the particles of colloidal solution is less than 100 A.

16. The method of any of Claims 10 to 15, characterised in that the colloidal solution is a colloidal solution of cerium hydroxide or cerium hydroxynitrate.

17. The method of Claim 16, characterised in that, for a porous material with an am photeric character such as alumina, the colloidal solution of cerium oxide has a pH from 0.2 to 2.

18. The method of Claim 16 or 17, characterised in that the concentration of cerium oxide in the colloidal inpregnating solution is from 20 to 400 g/1.

19. A catalyst for treating exhaust fumes from an internal combustion engine, of the type conprising an alumina based carrier and catalytically active elements such as iron, cerium or precious metals, characterised in that the cerium is deposited on a peripheral ring of the carrier by the method described in any of Claims 10 to 19.

20. A catalyst according to Claim 1 or 19, substantially as hereinbefore described and exemplified.

21. A method of preparing a catalyst according to Claim 1 or 19, substantially as hereinbefore described and exemplified.

22. A catalyst according to Claim 1 or 19, whenever prepared by a method claimed in a preceding claim.