WO1985003239A1

WO1985003239A1 - Fine particles with a surface coating of metal or metal compound, particulalrly a catalytically active substance, and a method of producing them

Info

Publication number: WO1985003239A1
Application number: PCT/SE1985/000036
Authority: WO
Inventors: Jan-Erik Anders Otterstedt; Ha^okan Arne CARLÖ; Ann-Kristin Elisabeth Askengren; Per-Arne Dahlqvist
Original assignee: Eka Ab
Priority date: 1984-01-30
Filing date: 1985-01-29
Publication date: 1985-08-01
Also published as: ES539929A0; DK435485A; JPS61501134A; JPS61501133A; DK435485D0; ES8608057A1; NO853818L; EP0169876A1; FI853777A0; SE8400426D0; FI853777L; BR8504995A; SE8400426L

Abstract

Solid sol particles having a surface area of at least 100 m2/g have a surface coating of a metal or a sparingly soluble metal compound, in particular a catalytically active substance, and are optionally coherent as aggregates. The solid surface-coated sol particles are obtained by preparing a liquid-borne sol of carrier particles having a surface area accessible to the liquid of at least 100 m2/g, preferably 100-1000 m2/g, and by dissolving a soluble metal compound in the sol and precipitating it thereform on the surface of the carrier particles. The soluble compound may be one component (anionic or cationic) of the sparingly soluble metal compound that is dissolved in a very low concentration in the sol, the other component of the sparingly soluble metal compound being supplied at a supply rate which allows preferential adsorption or attraction of the precipitating sparingly soluble compound on and against, respectively, the sol particles, instead of formation of crystal nuclei and precipitation of the sparingly soluble metal compound in the liquid phase of the sol. A surface coating of metal on the carrier particles is achieved by first precipitating the metal thereon as a reducible sparingly soluble metal compound which is thereafter reduced to metal. Alternatively, a soluble compound of the desired metal can be dissolved in the sol in order that this should adsorb or attract the metal-containing ion or molecule of the compounds, whereupon these ions or molecules are directly reduced to metal. A liquid-borne sol of solid carrier particles with a surface coating of catalytically active metal or metal compound or a precursor thereof is used for producing a catalyst by spray drying or for producing a catalyst mass by coating the surface of a catalyst support with the surface-coated solid carrier particles.

Description

FINE PARTICLES WITH A SURFACE COATING OF METAL OR METAL COMPOUND, PARTICULARLY A CATALYTICALLY ACTIVE SUBSTANCE, AND A METHOD OF PRODUCING THEM

In connection e.g. with the refining of petroleum, use is made of a large number of catalytically active substances, such as oxides and sulphides of Fe, Co, Mo, Ni, W etc, which are highly insoluble, and also metals, such as Pt, Pd etc. In most cases, these catalytically active substances are deposited on a catalyst support in order to obtain a physical shape suited for the catalytic process and to economize on the catalytic substances which are generally very expensive. In all catalytic processes, one normally aims at having the catalytically active substances present as large a surface as possible to the material to be treated.

One problem in this connection is that the catalytically active substances easily form colloidal solutions and precipitates which are difficult to handle, inter alia because highly diluted solutions must be used in the process. Such colloidal solutions and the preparation thereof were studied in depth by The Svedberg and Zsigmundy about 50 years ago. No actual development of the technique developed at that time seems to have been published in the literature.

Another problem when using catalytically active substances is that the current techniques for their use entails an unnecessary waste of material, i.e. it is difficult to keep the weight quantity of the substances low in relation to the extent of the accessible catalytically active surface. The large weight quantity of valuble substance has induced the experts to make great efforts to recover the substances although the methods of recovery are often very complicated.

Similar difficulties may arise if it is desirable to use other metals or metal compounds for different purposes, e.g. for adsorption purposes, when it is contemplated to rely on the capability of the metal or metal compound of adsorbing other substances. In such cases, too, the metal or metal compounds may be so valuble that substantial efforts have been spent in order to recover said material, after it has been used for the intended adsorption.

Swedish patent specifications SE-B-345,393 and SE-B-408,375 disclose a method for obtaining a large surface area of a catalytically active material on the surface of carrier particles, the catalytically active material being precipitated on carrier particles which may be in the form of a suspension of synthetic or natural minerals. As an example of suspended carrier materials, mention is made of "AEROSIL" which is a highly porous spray-dried silica and which when used is suspended in a suitable liquid. By adjusting the concentration of hydroxyl ions and causing it to increase at a low rate of at most 0.1 pH units during the addition of the two components which are to react with each other and to form a coating on the suspended particles, the precipitation of the catalytically active substance on the surface of the carrier particles will be in the form of fine particles having a diameter of 10-50 Å. A relatively thick coating of the catalytically active material is therefore obtained on the carrier particles which themselves consist of aggregate particles. However, this known method suffers from a number of drawbacks, e.g. limitations as regards the obtainable degree of dispersion for the catalytically active substance and a relatively large consumption of catalytically active material. Similar methods for producing supported catalysts are disclosed in German patent specifications DE-B2-1,963,952 and DE-B2-1,963,827. One object of the present invention is to achieve extremely fine particles with a surface coating of a metal or a sparingly soluble metal compound, in particular a catalytically active substance. Another object of the invention is to achieve a novel method of producing fine catalytically active particles. Yet another object of the invention is to achieve particles coated with a metal or a metal compound and having, in relation to the mass of the particles, a low content of metal or metal compound material located in the very surface of the particles. A further object of the invention is to achieve a method of depositing and/or precipitating thin coatings of insoluble or sparingly soluble substances on surfaces of fine particles. A still further object of the invention is to achieve a catalyst mass having a coating of solid sol particles which in turn have a surface coating of a metal or a sparingly soluble metal compound, particularly a catalytically active substance. Still another object of the invention is to achieve a catalyst mass which has its catalytically active material concentrated in the surface of the catalyst particles in order that only a small portion of the catalyst partides need be dissolved for recovering the catalytically active material from a spent catalyst mass. A further object of the invention is to produce a catalyst mass having such a low content of catalytically active material that the losses of catalytic material are minimal when disposing of spent catalyst mass.

The above-mentioned objects and still further objects of the invention will appear from the following description of the invention and from a number of advantageous embodiments thereof. The present invention is based on the idea of depositing from a solution, preferably but not necessarily an aqueous solution, a metal, a metal compound or a precursor of the metal or metal compound on monodisperse solid sol particles in suspension, such as sols of e.g. silica or aluminium oxide. By this technique it is possible first to prepare monodisperse sols having a very well-defined particle size, use being made of a well-known technique for preparing sols. According to the invention a liquid-borne monodisperse sol of carrier particles is first prepared, having a surface area accessible to the liquid of at least 100 m²/g, and thereafter dissolving in this sol a soluble metal compound which is thereafter precipitated as metal or a sparingly soluble metal compound on the carrier particles. By using a monodisperse sol a maximally large particle surface will be available for receiving the precipitated substance which is not the case if suspended aggregate particles are used as carrier particles. In order to deposit a sparingly soluble compound on the sol particles, the components of the sparingly soluble compound are therefore supplied in such a manner that precipitation occurs on the surface of the sol particles. This may be effected in that the components, under vigorous agitation of the sol, are supplied simultaneously at different pointsor supplied, successively after each other, such that one component is attracted by the particle surface prior to or in connection with the reaction with the other component or such that the newly formed compound is preferentially attracted by the sol particles without any major aggregates of the insoluble compound having time to form. In this context, "major aggregates" refers to so-called oligomeric or more polymeric aggregates. The addition of the components must be effected under conditions promoting colloidal stability. Thus, the electrolyte content in the solution should not attain a level which destabilizes, coagulates, the sol particles, i.e. not exceed a critical content which is dependent on the surface area of the sol particles and the valency of the electrolyte. The pH value is also of great importance for the stability of the sols. Therefore, it is necessary to operate at a pH which counteracts destabilization of the colloid concerned. The same considerations should be taken in respect of temperature.

In one method of carrying out the invention, a soluble compound of one component (anionic or cationic) of the sparingly soluble metal compound is dissolved in a very low concentration in a liquid-borne sol of carrier particles, the other component of the sparingly soluble metal compound being supplied at a supply rate which allows preferential adsorption or attraction of the precipitating sparingly soluble compound on the sol particles instead of an excess of the limit of solubility of the compound in the liquid phase of the sol and, hence, formation of crystal nuclei of the sparingly soluble metal compound therein. The reason for the established effect has however not been quite elucidated. It might depend on adsorption or attraction, as indicated, or on the fact that the low concentrations cause such a slow formation of crystal nuclei that the individual crystal nuclei have a shorter distance up to the surface of the sol particles than to each other, such that they are attracted to the surface of the sol particles and not to each other. A surface coating of metal on sol particles may also be achieved in that the metal is first precipitated on the sol particles as a reducible sparingly soluble metal compound which is thereafter reduced to metal. Alternatively, a soluble compound of the desired metal can be dissolved in the sol in order that this should adsorb or attract the metal-containing ion or molecule of the compound, whereupon these ions or molecules are directly reduced to metal.

The novel features of the invention are recited in the accompanying claims.

Thus, according to one aspect of the invention it is suggested to dissolve a soluble compound of the desired metal or metal compound (i.e. a precursor of the desired final catalytically active substance) in the monodisperse sol, the soluble compound being supplied in such an amount that substantially all metal ions or molecules in the solution can be adsorbed against or be attracted by the surface of the sol particles by surface chemical forces, such as surface charge, van der Waals' forces, dispersion forces etc. A very slow addition is thereafter effected of a substance which precipitates the desired catalytically active substance or a precursor thereof directly on the surface of the sol particles by reacting with the ions or molecules attracted or adsorbed against it.

The production of the coated sol particles may be effected in several stages, i.e. if the particles initially lack a surface chemical nature suitable for the deposition with respect to the final catalytically active substance or any other desired substance, it is possible similarly to build up several layers to change the surface charge of the initial sol particles from negative to positive and vice versa. The same technique can be used if it is desirable to deposit a mixture of catalytically active substances on the particles, e.g. a mixture of cobalt oxide and molybdenum oxide or a mixture of cobalt sulphide and molybdenum sulphide. According to the invention, the sol particles will thus serve as carriers for the catalytically active substance or substances. To make it possible to apply a coating to the surface of the sol particles without a separate precipitation of the catalytically active substance occurring in the liquid phase (aqueous phase) of the sol, it is necessary 1) that the carrier particles (sol particles) are monodisperse and have a large available surface area, such that the carrier surface per unit volume of the sol is large, and 2) that the precipitation of the desired substance (or precursor thereof) occur so slowly that the resulting substance will have time to diffuse up to the carrier surface and be deposited thereon without excess of the limit of solubility of the substance in the liquid phase of the sol and without any spontaneous formation of crystal nuclei therein. The simplest way of carrying out this aspect of the invention is using water as the liquid medium in which the coating of the surface of the sol particles is brought about. However, other liquid media are usable provided the contemplated substances are soluble in the liquid media and can be precipitated therefrom with the technique here described.

Generally, the substances used for coating or depositing must be such substances as precipitate as highly sparingly soluble compounds and do not easily crystallize. if the compounds are liable to crystallization, they should not undergo so-called Ostwald ripening, since such ripening implies a growth of crystal grains at the expense of the formation of new crystal nuclei on adjoining surface portions of the carrier particles. It is believed that the most suitable substances for coating or depositing are those which are least apt to spontaneously form nuclei when supersaturated solutions of the substances are formed. Mostly, this is the case with compounds which are highly insoluble and which crystallize very slowly, e.g. barium sulphate, cobalt sulphide etc. From experiments conducted it seems as if optimum conditions for .obtaining a satisfactory coating of the carrier particles (i.e. such that the surface of the carrier particles is no longer in contact with the solution) exist if the deposited substance is amorphous or exhibits but very faint X-ray diffraction lines and, thus, is microcrystalline at the most. Substances or materials of this type are

1) SiO₂, e.g. from Si(OH)₄ Aluminium oxide, e.g. Al(OH)₃·3H₂O Aluminium silicates ZrO₂, e.g. from ZrOCl₂ Al(PO)₄, e.g. from A1³⁺ and PO₄ ^3-

2) Most insoluble hydroxides and oxide hydrides of polyvalent metals.

3) Insoluble metal sulphides and metal selenides.

4) Silicates and carbonates of polyvalent metals. 5) Metals, e.g. precious metals, such as Pd and Pt.

The carrier in the form of a sol must have a very low solubility in the liquid phase used, generally and preferably water, or at least have a very low dissolution rate in the liquid phase.

In order that a sol should be usable in the invention, the particles of the sol should be monodisperse and have a very large surface area, calculated on the accessible surface, for although particles which are large and have a spongy structure have a large internal surface, this surface is not always accessible to the solution from which the coating is precipitated or deposited. The required size of the accessible surface is determined by a number of factors of which the surface chemical effect of the surface is one of the most important. The surface accessible to the solution must be at least 100 m ²/g in order to obtain practical production rates. A practical range of surface areas is 100-1000 m²/g. As regards silicic acid sols as carriers for the deposited catalytically active substances or other substances, sols having a surface area of 300-700 m²/g, corresponding to a particle size of about 4-9 nm, can be used to advantage.

As indicated above, it is not necessary that the particles of the initial sol have a surface charge of the polarity necessary for depositing the desired final catalytically active substance or other substance. Thus, silicic acid sols (which have a negative surface charge) may first be modified with a polyaluminium compound, e.g. from basic aluminium chloride, in order to have a positive surface charge, if necessary for depositing or precipitating the desired catalytically active substance or its precursor. In some cases, the polarity may also be affected by the pH of the liquid medium. In the case of e.g. a silicic acid sol, the surface of the sol is about neutral at a pH of 2-3 and the negative charge increases with increasing pH. Depending on the system, it is thus possible to use the pH as an aid for adjusting the surface charge. Another means for increasing the negative charge of a silicic acid particle is to react aluminium with the surface of the silicic acid particles with formation of negative SiAl₄ ^-1 sites. Such a modified silicic acid contains a considerable negative charge also at acid pH, as opposed to unmodified silicic acid sols. Whether the adsorption or attraction is obtained by the surface, charge or by any other mechanism, it is necessary that the carrier particles have or are first imparted the ability to adsorb or attract one or more of the components of the desired coating (e.g. by ensuring that the surface of the carrier particles has a suitable surface charge), so as to make it possible by a slow controlled precipitation rate to direct the precipitation to the surface of the sol particles.

Another requirement regarding the carrier, i.e. the sol particles, is that they have a low dissolution rate or are insoluble under the deposition or precipitation conditions since it would otherwise be difficult to deposit or precipitate other substances on the surface of the particles.

The conditions for the deposition must be such that the sol particles (or modified particles) of the carrier material have a sufficiently large total surface for a surface chemical adsorption or attraction of substantially all ions or molecules which are present in the solution and should be included in the subsequently precipitated or deposited substance. The other ion(s) or molecule(s) to be combined with the ions or molecules adsorbed or attracted to the sol particles, must thereafter be supplied at such a low rate that the chemical compound or compounds to be precipitated or deposited will have time to diffuse up to the surface of the particles and be deposited thereon without the limit of solubility being exceeded and without any spontaneous formation of crystal nuclei in the solution.

As an extreme example of an insufficient amount of carrier surface one may consider glass spheres subjected to agitation in water. If a diluted solution of barium acetate and a solution of sodium sulphate are added under agitation, there is formed a cloud of fine barium sulphate crystals, and no coating of barium sulphate is formed on the glass spheres. If, on the other hand, use is made of a corresponding amount of silica or aluminium silicate in the form of sol particles having a surface area of a few hundred square metres per gram and if pH is about 9, the solution remains clear upon a very slow addition of sodium sulphate and barium acetate, and no barium sulphate precipitates as a separate precipitate. A reference test without any silicic acid sol will show that a precipitate of barium sulphate rapidly forms and rapidly settles.

The rate at which a coating can be built up on the sol particles without the formation of new free crystal nuclei in the solution depends on the crystallinity of the coating and varies over a very wide range depending on what substances are combined to form the coating. The invention is also usable in connection with catalysts of the type where the surface of sol particles comprises a catalytically active metal, such as Pt or Pd. One method of producing such catalytically active surfaces is to reduce a metal compound together with the sol, such that the metal is directly precipitated on the surface of the sol particles. Another way is to first precipitate an insoluble compound of the metal on the surface of the sol and thereafter convert this compound to a catalytically active form of the metal. The liquid-borne sol of surface-coated particles can be used for different purposes, e.g. for producing an adsorbent or a catalyst by spray drying. For producing a catalyst, it is highly advantageous to coat the surface of a catalyst support with said surface-coated solid sol particles. This gives an advantageous increase of the activity of the resulting catalyst mass in that the accessible surface area of catalytically active substance will thus be increased 2-3 times as compared with the case where the surface coating of active material is deposited directly on the surface of the catalyst support instead of the surface of the sol particles.

The invention will be described in greater detail hereinbelow by way of a few Examples which should however not be considered limitative of the invention. In these examples, monodisperse sols were used.

EXAMPLE 1 Comparative example without sol By the addition of NaOH to 50 ml water in a beaker, pH is adjusted to 9.7. 150 g of an aqueous solution containing 0.256 g CoCl₂·6H₂O, and 160 g of an aqueous solution containing 0.272 g Na₂S.9H₂O are added dropwise and simultaneously to the beaker at a rate corresponding to 10 g solution every 27 minutes. The temperature of the solutions is maintained at 25°C. A black precipitate of CoS is immediately formed upon the addition of the first drops of the sulphide and chloride solutions.

EXAMPLE 2 CoS on silicic acid sol

To 50 ml silicic acid sol containing 4.85% SiO₂ and having a surface area of 392 m ²/g (corresponding to a particle size of about 7 nm) and a pH of 9.7 are added 160 g of an aqueous solution containing 0.2 g CoCl₂·6H₂O, and 160 g of an aqueous solution containing 0.272 g Na₂S·9H₂O dropwise and simultaneously under vigorous agitation at a rate which corresponds to 10 g solution every 27 minutes. The solutions are maintained at a temperature of 25°C. The sol solution is gradually coloured brownish black during the addition of the two solutions, but no separate precipitation of CoS occurs. When the sol solution is ultracentrifuged, black sol particles settle and a clear supernatant is formed.

The sediment contains CoS in an amount corresponding to the added amount of sulphide and cobalt.

EXAMPLE 3 CoS on aluminium-modified sol

To 50 ml silicic acid sol which contains 4.85% SiO₂ and has a surface area of 392 m²/g (corresponding to a particle size of about 7 nm) and a pH of 9.7 and whose surface has been modified with sodium aluminate, such that 25% of the atoms in the surface are aluminium atoms, are added 160 g of an aqueous solution containing 0.256 g CoCl₂·6H₂O, and 160 g of an aqueous solution containing 0.272 g Na₂S·9H₂O dropwise and simultaneously under vigorous agitation at a rate corresponding to 10 g solution every 27 minutes. The solutions are maintained at a temperature of 25°C. The sol solution is gradually coloured black during the addition of the solutions, but no precipitation of CoS occurs. When the sol solution is ultracentrifuged, black sol particles will settle and a clear supernatant be formed. The sediment contains CoS in an amount corresponding to the added amount of sulphide and cobalt.

EXAMPLE 4 CoS on silicic acid sol Other particle size of sol

To 50 ml silicic acid sol which contains 3.94% SiO₂ and has a surface area of 110 m²/g (corresponding to a particle size of about 25 nm) and a pH of 9.7 and whose surface has been modified with sodium aluminate, such that 25% of the atoms of the surface are aluminium atoms, are added 160 g of an aqueous solution containing 0.256 g CoCl₂·6H₂O, and 160 g of an aqueous solution containing 0.272 g Na₂S·9H₂O dropwise and simultaneously at a rate which corresponds to 10 g solution every 27 minutes. The temperature of the solutions is maintained at 25°C. The sol solution is gradually coloured blackish brown during the addition of the two solutions, but no precipitation of CoS occurs. When the sol solution is ultracentrifuged, blackish brown sol particles will settle and a clear supernatant be formed. The sediment contains CoS in an amount corresponding to the added amount of sulphide and cobalt.

EXAMPLE 5

CoS on silicic acid sol

Effect of content of added solutions

To the same sol as in Example 3 are added 40 g of an aqueous solution containing 0.256 g CoCl₂·6H₂O, and

40 g of an aqueous solution containing 0.272 g Na₂S·9H₂O.

The solutions are added dropwise and simultaneously under vigorous agitation at a rate which corresponds to

10 g solution every 27 minutes. The temperature of the solutions is maintained at 25°C. During the addition of the solutions, the surface of the sol particles is gradually coated with black cobalt sulphide. No separate cobalt sulphide precipitation takes place.

EXAMPLE 6 Nickel sulphide on aluminium-modified silicic acid sol

Example 3 is repeated with the exception that 160 g of an aqueous solution containing 0.256 g NiCl₂·6H₂O is added together with the sodium sulphide solution. The sol solution is gradually coloured black during the addition of the two solutions, but no precipitation of nickel sulphide occurs. When the sol solution is ultracentrifuged, black sol particles settle and a clear supernatant is formed. The sediment contains NiS in an amount corresponding to the added amount of sulphide and nickel.

EXAMPLE 7

Molybdenum sulphide on aluminium-modified silicic acid sol

To 100 g silicic acid sol which contains 11.5% SiO₂ and has a surface area of 476 m²/g (corresponding to a particle size of about 6 nm) and a pH of 10 and whose surface has been modified with sodium aluminate, such that 25% of the atoms in the surface are aluminium atoms, is added dropwise and under vigorous agitation 45 g of a 0.2 molar Na₂S·9H₂O solution. pH is thereafter decreased to 4.3-4.4 by the addition of a 0.5 molar HCl solution under vigorous agitation. 12.8 g of a 5% ammonium molybdate solution, (NH₂)₆Mo₇O₂₄·4H₂O, is thereafter added to the sol solution under vigorous agitation. pH is maintained constant at 4.3-4.4. The test is conducted at room temperature. The sol solution is gradually coloured blackish grey during the addition of the ammonium molybdate solution, but no precipitation of molybdenum sulphide occurs, upon ultracentrifuging, a blackish grey sediment settles in which the added molybdenum and sulphide can be quantitatively found. If the experiment is repeated in the absence of aluminiummodified silicic acid sol, a blackish grey precipitate of molybdenum sulphide will be obtained in the reaction vessel.

EXAMPLE 8

Barium sulphate on aluminium-modified silicic acid sol

To 100 ml silicic acid sol containing 12.2% SiO₂ and having a surface area of 470 m²/g (corresponding to a particle size of about 6 nm) and a pH of 9.7, are added 20 g of an aqueous solution containing 0.0956% by weight of barium nitrate, and 20 g of an aqueous solution containing 0.1% by weight of sodium sulphate dropwise and simultaneously under vigorous agitation at a supply rate corresponding to 10 g solution every 27 minutes. The temperature of the solutions is maintained at 25°C. The sol solution remains clear during the addition of the barium nitrate and sodium sulphate solutions, and no formation of barium sulphate can be observed. When the sol solution is ultracentrifuged, the sol particles settle and a clear supernatant is formed. Barium sulphate is found in the sediment in an amount corresponding to the added amount of sulphate and barium. EXAMPLE 9

Cobalt + molybdenum oxide-hydroxides on modified silicic acid sol

0.2 g Co(NO₃)₂·6H₂O is dissolved in 100 g silicic acid sol in which the surface of the sol particles has been modified with sodium aluminate, such that 25% of the atoms in the surface are aluminium atoms. The sol contains 3.9% SiO₂ and has a surface area of 110 m²/g

(corresponding to a particle size of about 25 nm) . A solution of 0.2 g (NH₄ )₆Mo₇O₂₄·4H₂O in 100 ml water is prepared and added to the sol dropwise and under vigorous agitation at room temperature. pH is adjusted to 10.5 with ammonium hydroxide. The solution is left under agitation for 1 hour at room temperature. After centrifuging, the sample consists of a pale pink sediment and a clear colourless supernatant. The added amount of molybdenum and cobalt is quantitatively found in the sediment. If the experiment is conducted in the absence of silicic acid sol, there is obtained a bluish precipitate when pH is adjusted to 10.5 with ammonium hydroxide.

EXAMPLE 10

Cobalt- and molybdenum sulphides on modified silicic acid sol Example 9 is repeated but the resulting sediment is dried. The dried sediment is thereafter treated with hydrosulphuric acid at a temperature in the range of

300-500°C, the oxides being converted to cobalt sulphide and molybdenum sulphide. EXAMPLE 11

Platinum on silicic acid sol

A first solution is prepared by adding 2 ml hydrazine hydrate under agitation to a silicic acid sol containing 3.94% SiO₂ and having a surface area of 110 m²/g. 4.5 ml of an H₂PtCl₆ solution containing 30 g Pt/1 and being acidified with HCl so as to be 5.8 molar with respect to HCl is diluted with 20.5 ml water to obtain a second solution. At a supply rate of 0.37 ml/min the second solution is added dropwise to the first solution under agitation. During the addition of the second solution to the first solution, an ammonium hydroxide solution is also added dropwise, so that pH never falls below 4.3. The resulting solution is slowly heated to 50°C. Upon reduction to metallic platinum, the solution is coloured black, but no separate precipitation of platinum occurs. If the reduction is effected in the absence of silicic acid sol, a black precipitate is obtained. After completed reduction in the presence of sol, the sample is centrifuged. A black sediment is then obtained and a clear colourless supernatant. The added amount of platinum is quantitatively found in the sediment. EXAMPLE 12

Platinum on aluminium silicate sol Example 11 is repeated but the first solution is instead a solution of 2 ml hydrazine hydrate in 25 g silicic acid sol whose surface has been modified with sodium aluminate, such that 25% of the atoms are aluminium atoms, this silicic acid sol containing 3.87% SiO₂ and having a surface area of 110 m ²/g (corresponding to a particle size of about 22 nm). Upon centrifuging, a blackish brown sediment is formed and a clear colourless supernatant. The supplied amount of platinum is quantitatively found in the sediment.

EXAMPLE 13 Molybdenum sulphide on surface-modified silicic acid sol In a beaker is introduced 50 g silicic acid sol which contains 4.5 g sol particles and 45.5 g water and in which the surface of the silicic acid particles has been modified with sodium aluminate, such that 25% of the atoms in the surface are aluminium atoms. The sol has a surface area of 110 m²/g and contains 3.87% SiO₂ and 0.07% Al₂O₃. A first solution of 0.18 g

(NH₄)₆Mo₇O₂₄·4H₂O in 39.83 g deionized water and a second solution of 1.0 g Na₂S·9H₂O in 39 g deionized water are prepared. These two solutions are added simultaneously and dropwise at a rate of 3.7 g every 10 minutes to the silicic acid sol. Agitation is carried out during the entire addition process and pH is maintained constant at 4.3 by means of a 0.5 molar HCl solution. The weighed amount of Na₂S is 5 times larger than the amount of molybdenum salt for obtaining a 1/10 monolayer of MoS₂ on the surface of the sol particles. The test was conducted at room temperature. The sol solution is gradually coloured blackish grey during the addition of the ammonium molybdate solution, but no separate precipitation of molybdenum sulphide occurs. Upon ultracentrifuging, a blackish grey sediment settles on the bottom, in which the added molybdenum and sulphide can be quantitatively found. If this experiment is repeated in the absence of aluminium-modified silicic acid sol, a blackish grey precipitate of molybdenum sulphide is obtained in the reaction vessel.

Iri all of the above Examples, it is found when studying the coated sol particles in an electron microscope that the insoluble salt has actually been precipitated on the surface of the particles.

Claims

1. Solid particles, c h a r a c t e r i z e d by being of colloidal size and having a surface area of at least 100 m²/g and a surface coating of metal or a sparingly soluble metal compound, in particular a catalytically active substance, or aggregates of such particles.

2. Particles or aggregates of such particles as claimed in claim 1, c h a r a c t e r i z e d in that the solid particles are particles of silicic acid sol, modified silicic acid sol, aluminium oxide sol or aluminium silicate sol.

3. Particles or aggregates of particles as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the surface coating on the particles consists of silver metal, gold metal, ruthenium metal, platinum metal, palladium metal, cobalt sulphide, nickel sulphide, molybdenum sulphide + cobalt sulphide, molybdenum sulphide + nickel sulphide, tungsten sulphide + cobalt sulphide or tungsten sulphide + nickel sulphide.

4. A method of producing fine particles with a surface coating of metal or a sparingly soluble metal compound, c h a r a c t e r i z e d in that a liquidborne monodisperse sol of carrier particles having a surface area accessible to the liquid of at least 100 m²/g is prepared and that a soluble metal compound is dissolved in the sol and precipitated therefrom as metal or a sparingly soluble metal compound on the surface of the carrier particles.

5. Method as claimed in claim 4, c h a r a c t e r i z e d in that the soluble metal compound dissolved in the sol consists of one component (anionic or cationic) of a sparingly soluble metal compound and is added to the sol in a deficient amount in relation to the ability of the carrier particles for surface chemical adsorption or attraction of the dissolved com ponent under prevailing conditions, and that the complementary component of the sparingly soluble metal compound is supplied at such a low supply rate that the sparingly soluble metal compound precipitating upon said supply will have time to diffuse up to the surface of the carrier particles and be deposited thereon without excess of the limit of solubility of the sparingly soluble metal compound in the liquid phase of the sol and without any spontaneous formation of crystal nuclei of the sparingly soluble metal compound therein, and that if it is desirable to obtain a surface coating of the metal on the carrier particles, the metal is first precipitated as a reducible sparingly soluble metal compound which is thereafter reduced to metal, optionally after previous drying of the sol particles.

6. Method as claimed in claim 4, c h a r a c t e r i z e d in that the soluble compound of the metal is dissolved in the liquid-borne sol of solid carrier particles which are capable of surface chemical adsorption or attraction of the metal-containing ion or molecule of the dissolved metal compound, the soluble metal compound being supplied in a deficient amount in relation to the ability of the carrier particles for said surface chemical adsorption or attraction under prevailing conditions, and that the metal-containing ions or molecules are thereafter reduced to metal.

7. Method as claimed in claim 4 or 5, c h a r a c t e r i z e d in that a mixture of sparingly soluble metal compounds is deposited as a surface coating on the fine particles by successively supplying the metals in the form of soluble compounds with metal-containing ions or molecules of an alternatingly positive and negative charge and by thereafter supplying the complementary components of the sparingly soluble metal compounds.

8. The use of a liquid-borne sol of solid particles having a surface coating of a catalytically active metal or a sparingly soluble metal compound or a precursor of this metal or metal compound for producing a catalyst by spray drying or for producing a catalyst mass by coating the surface of a catalyst support with said surface-coated solid sol particles.

9. Use as claimed in claim 8, in which the sol particles are coated with a surface coating of silver metal, gold metal, ruthenium metal, platinum metal, palladium metal, cobalt sulphide, nickel sulphide, molybdenum sulphide + cobalt sulphide, molybdenum sulphide + nickel sulphide, tungsten sulphide + cobalt sulphide or tungsten sulphide + nickel sulphide.

10. Use as claimed in claim 8 or 9, in which the surface-coated solid particles are particles of silicic acid sol, modified silicic acid sol, aluminium oxide sol or aluminium silicate sol.