WO2003059507A1

WO2003059507A1 - Gold catalysts and methods for their preparation

Info

Publication number: WO2003059507A1
Application number: PCT/ZA2003/000008
Authority: WO
Inventors: Elma Van Der Lingen; Mike Bernard Cortie; Lizelle Glaner
Original assignee: Mintek
Priority date: 2002-01-16
Filing date: 2003-01-15
Publication date: 2003-07-24
Also published as: AU2003215039A1

Abstract

A method of producing a catalyst which includes the steps of alloying gold with at least one less noble element to produce a precursor material and removing at least part of the at least one less noble element from the precursor material.

Description

GOLD CATALYSTS AND METHODS FOR THEIR PREPARATION

BACKGROUND OF THE INVENTION

This invention relates to the production of catalytically active gold.

Gold is considered the most inert of all metallic elements, but the metal can show catalytic activity when prepared with specific techniques. For gold to be catalytically active, the gold particle size should be in the nanometer range. Good interaction between the gold particles and a support of at least one transition metal oxide results in a heterogeneous catalyst active even at room temperature and below.

Preparation techniques generally used to prepare gold catalyst systems include deposition-precipitation, co-precipitation and the inverse techniques of these two by which the order of adding the reactants is altered. Deposition-precipitation includes the precipitation of gold hydroxides or gold(0) from chloro auric acid or gold phosphine complexes onto a metal oxide support, whereas co-precipitation involves the simultaneous precipitation of gold hydroxide and metal hydroxide. Precipitation techniques can be varied by adjusting parameters such as reactant concentrations, stirring, ageing, temperature, and calcination conditions.

Prior art methods for the production of catalytically active gold include co- precipitation (Japanese patent No 60 (1985)-238148), deposition-precipitation (US patent No 4,839,327), and immersing a carrier in an aqueous solution containing water soluble salts of gold and transition metals (US patent No 4,698,324). UMMA Y Ul- I HE INVENTION

The present invention is concerned with a different method of preparing gold-based heterogeneous catalysts for oxidation and reduction reactions and for use as electrocatalysts. The method involves alloying of gold with a less noble metal and the dissolution of a less noble metal from a gold-containing intermetallic precursor to form meso-porous gold. The meso-porous gold can be incorporated with a metal oxide system, and a promoter element can be added to improve catalytic activity.

The invention provides a method of producing a catalyst which includes the steps of alloying gold with at least one less noble element to produce a precursor material and removing at least part of the at least one less noble element from the precursor material.

The less noble element (also referred to hereinafter as a base element) may be selected from elements in groups MB and I HA.

The alloy may be prepared in any appropriate way and for example the gold and the at least one less noble element, in suitable ratios, may be melted in a furnace under a protective or inert atmosphere. Use may for example be made of a vacuum arc furnace which is filled with an inert gas such as argon.

A promoter metal may be added to the melt.

The promoter metal may be selected from elements in groups IVB, VB, VIB, VIII, IB, IIB, IIIA, IVA, magnesium and cerium. For example the less noble element may be aluminium and the precursor material may include Au_xAl_y. The promoter metal which may for example be selected from iron, titanium and the PGM (platinum group metals) can be added as metal or, in each case, as Fe_xAl_y, Ti_xAl_y or a PGM/aluminium compound to the melt in various masses, or in any other suitable form.

The base element may be removed from the precursor material using any appropriate technique which may be dependent, at least partly, on the intended use of the resulting catalyst.

For example use may be made of chemical or electrochemical techniques which do not attack nor react with the gold in the precursor material nor the promoter element.

In one form of the invention the removal step is preceded by a step of comminuting the precursor material. An objective in this regard is to reduce the precursor material to a fine powder. This step may be effected by pulverising or milling the precursor material. The duration of the step may be varied according to requirement. Also the comminuting step may be carried out in an appropriate medium such as an alcohol medium.

If the precursor material is reduced to a fine powder then it may be appropriate to remove the base element using a chemical technique e.g. by leaching the powder with a reagent which may be an alkaline or acidic medium. For example use may be made of sodium hydroxide to leach the precursor powder to obtain gold powder with a skeletal or meso-porous structure and with a high surface area.

If the catalyst is to be employed ultimately as an electrocatalytic electrode then the comminuting step will normally not be required. In this instance the precursor material may be directly leached, or electrochemically attacked, to obtain a catalytically active surface.

If an electrochemical technique is employed then the base element can be removed by controlling the electrical potential or current density of the electrical process. This approach provides a porous metal surface on the precursor material.

The milled precursor material and the leaching medium may be agitated or stirred and the leaching may be carried out under controlled temperature conditions e.g. below 10°C.

The strength of the leaching reagent may be varied to control the rate at which leaching is effected.

A dispersant such as (NaP0₃)₆ may be added to the leaching medium to counteract forces between the resulting fine gold particles.

The leached precursor material, i.e. gold powder, may be washed with an alcohol/water solution to remove the leaching medium and thereafter may be dried at room temperature under atmospheric conditions.

The method of the invention may include the step of providing a promoter element for the gold by the removal of the base element from the precursor material.

The support material helps to prevent agglomeration and sintering of the gold particles, which are of a size in a nanometer range, to larger particles, a process which could result in lower catalytic activity. The support material may include a metal oxide which may be selected from the elements in groups IVB, VB, VIB, VIII, IB, MB, IIIA, IVA and cerium, or any appropriate combination thereof.

The catalytically active gold particles, which are produced by the leaching process, may be combined with the support material directly, for example by milling, or the precursor material can be combined with the support material by using a technique selected from the following:

(a) spraying of molten particles of the precursor material onto a metallic support and then selectively dissolving the more reactive metal, i.e. the base element. Thus, in the case in which the precursor material consists mainly of AuAI₂, the aluminium is dissolved;

(b) filling a fibrous support system with alloyed powder or catalyst powder by means of vibratory filling. The powder may be mixed with a suitable fluid such as water or alcohol; (c) milling the precursor material with the support material and then leaching. The milled material may be compacted, sintered and then selectively dissolved n order to produce a geometry with a catalytically active surface;

(d) applying the precursor material, once milled after admixture of a suitable binder, in the form of a paste to the support material and then removing the binder chemically or by means of heat treatment;

(e) electroplating techniques may be employed to produce catalytic gold on a support material. For example the less noble metal may be electroplated onto a gold support which is then heat treated to form a surface alloy. The surface alloy is thereafter chemically or electrochemically treated to remove the less noble metal and form a catalytically active surface; (f) electrodepositing the less noble metal and gold onto an appropriate support material to form gold and less noble metal layers on the support material which thereafter can be processed to obtain a catalytically active surface; and

(g) the leached precursor powder may be impregnated with the support material, or metal salts followed by calcination.

Electrodes which are suitable for use in fuel cells (for example) may be produced from gold particles, made in accordance with the method of the invention, and PTFE, using conventional wet and dry methods.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of examples with reference to the accompanying drawings in which:

Figure 1 shows a skeletal structure which is obtained after leaching of AuAI₂;

Figure 2 is an image in a backscattered mode of small gold particles;

Figure 3 shows a porous AuAI₂ surface, produced by leaching, which is suitable for use as an electrode;

Figure 4 is a secondary electron image of leached AuAI₂ surrounded by Co₃0 ;

Figure 5 is a backscattered electron image of the material of Figure 4, revealing a gold-rich area;

Figure 6 is an image of an Au-Ni skeletal structure; Figure 7 shows the catalytic activity of leach AuAI₂ in a mixed iron/cobalt oxide, support in dry and humid atmospheres;

Figure 8 shows the catalytic activity of leached AuAI₂ on a CuMnOx support for CO oxidation;

Figure 9 illustrates the effect of alloying AuAI₂ with titanium, on catalytic activity; Figure 10 is a secondary electron image of the AuAI₂/Ti material used to produce the curve shown in Figure 9;

Figure 11 shows a number of curves of NOx conversion versus temperature for PGM doped gold catalysts; Figure 12 illustrates propene oxidation curves versus temperature for leach AuAI₂ supported on Al₂0₃;

Figure 13 gives curves which compare the electro-oxidation of formic acid on polycrystalline gold and leached gold alloy, on an absolute current basis; Figure 14 gives curves which are similar to those in Figure 13 but on a current density basis; and

Figure 15 shows curves which are similar to those in Figure 13 but for the electro- oxidation of formaldehyde.

DESCRIPTION OF PREFERRED EMBODIMENTS

As has been indicated in the preamble to this specification the manner in which a catalyst is prepared has a significant influence on its activity, the materials with which it reacts and what is produced, and its operating lifetime. The present invention relates to the preparation of a catalyst using a gold-based alloy as a starting material.

This approach lends itself to the possibility of producing catalysts with promoter metals which have physical properties of a different nature to the properties of conventionally prepared catalysts. As is apparent from the description which follows the catalyst can, if desired, be supported on a metallic precursor alloy and can act therefore as a structural member of a device or appliance.

In one example of the invention gold and aluminium, in predetermined ratios, were melted together in a vacuum arc furnace under a protective argon atmosphere to produce a precursor material consisting mainly of AuAI₂. Iron was added as a promoter metal in the amount of 5-weight per cent iron to AuAI₂. It is to be noted that the use of a promoter metal is not limited to iron for other elements selected from the groups IVB, VB, VIB, VIII, IB, MB, IIIA, IVA, magnesium and cerium, for example, can be added to the melt.

Milling was conducted on the melt in a Zieb mill for a few seconds followed by milling in a microniser for several minutes. An alcohol medium was found to be appropriate for obtaining good milling. The milling process produces fine powder and increases subsequent leaching of the less noble metal.

The milled precursor material was then subjected to a leaching process to remove the less noble component (also referred to herein as the base element) from the precursor material. Leaching may be effected using an alkaline or acidic medium.

The leaching was carried out with varying molarities of sodium hydroxide solution for various time intervals and under various conditions e.g. stirring and temperature. The alcohol medium used during the milling process also results in some leaching of the aluminium from the AuAI₂ compound as it is slightly alkaline.

The leaching temperature was kept below 10°C and an overhead stirrerwas used for agitation. The use of a dispersing agent such as (NaP0_3.6 can be beneficial for, when added to the leaching medium, it counteracts the forces which prevail between the fine gold particles.

The leached milled precursor material, i.e. the gold powder, was washed with an alcohol/water solution to remove all of the leaching agent and the gold powder was dried at room temperature under atmospheric conditions. Figure 1 is a HRSEM (high resolution scanning electron microscope) image of the skeletal structure which is obtained after leaching an AuAI₂ powder particle. Figure 2 is an HRSEM image in the backscattered mode and reveals that small particles with a size distribution of between 2 nm and 6 nm can be obtained by using the method of the invention. A typical BET surface area of 21m²/g can be obtained for gold by this method.

It is to be noted that, depending on the application, it may not be required to mill the precursor material prior to leaching. For example in order to obtain a catalytically active surface which is to be used as an electrode the melted precursor material (without milling) can be leached directly or it can be electrochemically attacked. In the latter case the leaching process is controlled by varying the electrical potential or current density of the electrical apparatus. When the precursor material is leached directly, i.e. without milling, a porous metal surface is produced on the precursor material as is shown in Figure 3, which illustrates leached bulk AuAI₂ suitable for use as an electrode.

Gold with a particle size in the nanometer range appears to require a support to be catalytically active and to prevent agglomeration and sintering of the gold to larger particles, which will result in lower catalytic activity. The support material may be selected from oxidised elements in groups IVB, VB, VIB, VIII, IB, MB, IIIA, IVA and cerium, or any appropriate combination thereof. To produce catalytic gold on a support material the AuAI₂ precursor material, with or without a promoter element, was milled, in the microniser, together with the chosen support material. Leaching of the aluminium can be conducted before or after the milling process.

Figure 4 is an HRSEM secondary electron image of leached AuAI₂ surrounded by Co₃0₄ revealing particles of the support on the leached gold area. Figure 5 is a backscattered electron image of the material of Figure 4, revealing the gold-rich area. Figure 6 is an HRSEM image of a fine Au - 0.33% Ni skeletal structure surrounded by support particles.

Figures 4, 5 and 6 reveal the good contact which was achieved between the catalytically active gold and the support material. Heat treatment of the gold/support can be conducted at temperatures of between 50°C and 500°C for various durations and in different atmospheres before or after the leaching step. A typical heat treatment was undertaken at a temperature of between 100°C and 300°C for several hours in atmosphere.

EXAMPLE 1

Activity tests of the catalysts produced by the method of the invention^' were performed in a continuous micro fixed bed reactor. CO oxidation tests were conducted with a gas mixture comprising 1 vol% CO in air at a total flow rate of 76ml/min. The products in the effluent stream were analysed by a gas chromatograph.

Catalytic gold alone, prepared by the leaching route, showed some activity, approximately 5% for the oxidation of carbon monoxide. Catalytic gold with a support material showed significantly higher activity for the conversion of CO. Figure 7 illustrates that for 4 wt% gold on a mixed iron/cobalt oxide support, a conversion of more than 90% can be obtained in dry and humid atmospheres. In this test 0,5g of the sample was tested at 60°C. Figure 8 shows that activities of more than 80% can be obtained on CuMnOx supports with 1 wt% gold loading. These tests were conducted on 0,25g samples at 60°C. EXAMPLE 2

The effect of alloying AuAI₂ with a promoter, such as titanium, can be seen in Figure 9. Tests were only conducted on the leached gold-titanium powder without the addition of a support material. The tests are conducted on 0,25g samples at 60°C. Alloying in of a promoter enables the elements to be brought into contact with one another at an atomic level. Figure 10 reveals the microstructure of AuAI₂ +5wt% Ti. Energy dispersive X-ray analysis (EDS) indicated that the three alloying elements are present in all the phases in various ratios. Leaching of this material results in gold and titanium in contact with each other at the atomic level, which renders the promoter more effective and increases the catalytic activity.

EXAMPLE 3

Tests for lean-burn NOx reduction activity were conducted on 0.5 g or 1 g of the catalyst. Unsupported leached AuAI₂ catalysts were mixed 1 :4 m/m (-1 :10 v/v) with ground quartz (0.2 - 0.8 mm) and the supported catalysts 1 :3.4 m/m (~1 :2.5 v/v) with quartz. Gas composition was 1000 ppm NO, 1000 ppm propene (C₃H₆), and 10% 0₂ in N₂. Total gas flows of 1000 ml/min or 500 ml/min (space velocity = 60000 or 30000 ml/g_cat/hr) were used and the temperature was ramped at 5°C/min from 25°C to 500°C.

NOx reduction activity over the unsupported PGM promoted leached AuAI₂ catalysts is shown in Figure 11. Low Rh addition has the greatest effect of lowering NOx reduction temperature and increasing the percentage conversion, but significantly narrows the temperature window of activity. Some of the promoted leached AuAI₂ catalysts were supported as 1 wt% on γ-AI₂0₃ and tested for lean NOx reduction by propene. The propene oxidation results are shown in Figure 12. Supporting leached AuAI₂ catalysts on Al₂0₃ significantly affected the oxidation of propene over Al₂0₃ in all cases. Very large shifts of ~200°C to lower T50 are found for NanCo/AI₂0₃ and NanMn/AI₂0₃ versus Al₂0₃ alone.

EXAMPLE 4

The comparison of the electro-oxidation of formic acid on polycrystalline gold and leached gold alloy is shown in Figures 13 and 14. Figure 13 shows the absolute current value, which is much higher on the leached gold alloy, which has a 50χ greater surface area (1.58 cm² vs. 0.03 cm²).

However, when the current is plotted as current density (Figure 14), both cyclic voltammograms are similar, although it seems that the electro-oxidation of formic acid on leached gold alloy is less inhibited by the formation of the oxide monolayer.

The situation is different though in the case of formaldehyde (Figure 15). A much higher current density is obtained on the leached gold alloy, which appears thus to have a catalytic effect on the electro-oxidation of formaldehyde.

Claims

1. A method of producing a catalyst which includes the steps of alloying gold with at least one less noble element to produce a precursor material and removing at least part of the at least one less noble element from the precursor material.

2. A method according to claim 1 wherein the less noble element is selected from elements in groups MB and IIIA.

3. A method according to claim 1 or 2 wherein the gold and the at least one less noble element, in suitable ratios, are melted in a furnace under a protective or inert atmosphere.

4. A method according to claim 3 wherein a promoter metal is added to the melt.

5. A method according to claim 4 wherein the promoter metal is selected from elements in groups IVB, VB, VIB, VIII, IB, IIB, IIIA, IVA, magnesium and cerium.

6. A method according to claim 5 wherein the less noble element is aluminium and the precursor material includes Au_xAl_y, and the promoter metal is selected iron, titanium and platinum group metals.

7. A method according to claim 6 wherein the promoter metal is selected from Fe_xAl_y, Ti_xAly, and a PGM/aluminium compound.

8. A method according to any one of claims 1 to 7 wherein the less noble element is removed using a chemical or electrochemical technique which does not attack nor react with the gold in the precursor material. y. A metnoα according to claim o wnerein an eiectrocnemicai technique is used and the less noble element is removed by controlling the electrical potential or current density used in the technique.

10. A method according to any one of claims 1 to 9 wherein the removal step is preceded by a step of comminuting the precursor material.

11. A method according to claim 10 wherein the comminuting step is carried out in an alcohol medium.

12. A method according to claim 10 or 11 wherein the less noble element is removed from the comminuted precursor material by means of a leaching medium.

13. A method according to claim 12 wherein the comminuted precursor material and leaching medium are agitated and kept at a temperature below 10°C.

14. A method according to claim 12 or 13 which includes the step of adding a dispersant to the leaching medium to counteract forces between the resulting fine gold particles.

15. A method according to any one of claims 1 to 14 wherein the removal of the less noble element results in the provision of a promoter element for the gold.

16. A method according to claim 15 wherein the support material is a metal oxide selected from the elements in groups IVB, VB, VIB, VIII, IB, IIB, IIIA, IVA and cerium, or any appropriate combination thereof. A metnoα according to claim I or ib wnerein tne precursor material is combined with the support material by using a technique selected from the following:

(a) spraying of molten particles of the precursor material onto a metallic support and then selectively dissolving the more reactive metal;

(b) filling a fibrous support system with alloyed powder or catalyst powder by means of vibratory filling;

(c) milling the precursor material with the support material and then leaching; (d) applying the precursor material, once milled after admixture of a suitable binder, in the form of a paste to the support material and then removing the binder;

(e) electroplating;

(f) electrodepositing the less noble metal and gold onto an appropriate support material to form gold and less noble metal layers on the support material which thereafter are processed to obtain a catalytically active surface; and

(g) impregnating the precursor material with the support material.