Metal-foam body and method for the production thereof and the use thereof as a catalyst

Abstract

The invention relates to a method for producing a metal-foam body, comprising the steps of (a) providing a metal-foam body A, which consists of nickel, cobalt, copper, or alloys or combinations thereof, (b) applying an aluminum-containing material MP to metal-foam body A so as to obtain metal-foam body AX, (c) thermally treating of metal-foam body AX, with the exclusion of oxygen, to achieve the formation of an alloy between the metallic components of metal-foam body A and the aluminum-containing material MP so as to obtain metal-foam body B, wherein the duration of the thermal treatment is chosen in dependence on the temperature of the thermal treatment and the temperature of the thermal treatment is chosen in dependence on the thickness of the metal-foam body AX. The invention also relates to the metal-foam bodies obtainable by the methods according to the invention and to the use thereof as catalysts for chemical transformations.

Claims

1. A process for producing a metal foam body, comprising the following steps: (a) providing a metal foam body A made of nickel, cobalt, copper, alloys thereof or combinations thereof; (b) applying an aluminum-containing material MP to metal foam body A so as to obtain metal foam body AX; (c) treating metal foam body AX thermally, with exclusion of oxygen, in order to form an alloy between the metallic components of metal foam body A and the aluminum-containing material MP so as to obtain metal foam body B; wherein the duration H of the thermal treatment (in minutes), depending on the temperature T of the thermal treatment (in C.), is chosen as follows: H.sub.min<H<H.sub.max, wherein: maximum duration H.sub.max=d1+(a1d1)/(1+(T/c1){circumflex over ()}b1); and minimum duration H.sub.min=d2+(a2d2)/(1+(T/c2){circumflex over ()}b2); wherein: a1=366.1; b1=129.0; c1=650.9; d1=8.7; a2=33.5; b2=235.5; c2=665.8; d2=1.8; and wherein the metal foam body AX has a thickness D of 10-30 mm and the temperature T of the thermal treatment, depending on the thickness D of the metal foam body AX, is chosen as follows: when D=10 mm, 600 C.<T680 C.; when 10 mm<D20 mm, 600 C.<T675 C.; when 20 mm<D30 mm, 600 C.<T665 C.

2. The process of claim 1, wherein the aluminum-containing material MP is an aluminum-containing powder, and an organic binder is applied to metal foam body A together with, or before, the aluminum-containing powder.

3. The process of claim 1 wherein metal foam body A consists of nickel.

4. The process of claim 1, wherein metal foam body A has an apparent density in the range of from 100 to 1500 kg/m.sup.3.

5. The process of claim 1, wherein metal foam body A has a specific BET surface area of 100 to 20 000 m.sup.2/m.sup.3.

6. The process of claim 1, wherein metal foam body A has a porosity of 0.50 to 0.95.

7. The process of claim 1, wherein the aluminum-containing material MP in step (b) contains metallic aluminum in an amount of 80% to 100% by weight.

8. The process of claim 1, wherein the aluminum-containing material MP is a powder composed of particles, 95% of which have a diameter in the range from 5 to 75 m.

9. The process of claim 1, further comprising the following step: (d) activating the metal foam body B by treatment with a leaching agent.

10. The process of claim 9, wherein the treatment of the metal foam body B with leaching agent is performed for a period of 5 minutes to 8 hours at a temperature of 20 to 120 C., and wherein the leaching agent is an aqueous NaOH solution having an NaOH concentration of 2% to 30% by weight.

11. The process of claim 9, further comprising the following step: (e) post-doping the activated metal foam body B with a promoter element selected from the group consisting of: Mo, Pt, Pd, Rh, Ru, Cu and mixtures thereof.

12. The process of claim 1, wherein metal foam body A has an apparent density of 300 to 600 kg/m.sup.3.

13. The process of claim 12, wherein metal foam body A has a specific BET surface area of 1000 to 6000 m.sup.2/m.sup.3.

14. The process of claim 13, wherein metal foam body A has a porosity of 0.50 to 0.95.

15. The process of claim 14, wherein the aluminum-containing material MP in step (b) contains metallic aluminum in an amount of 90% to 99.5% by weight.

16. The process of claim 15, wherein the aluminum-containing material MP is a powder composed of particles, 95% of which have a diameter in the range from 5 to 75 m.

17. The process of claim 16, further comprising the following step: (d) activating the metal foam body B by treatment with a leaching agent.

18. The process of claim 17, wherein treatment of the metal foam body B with leaching agent is performed for a period of 5 minutes to 8 hours at a temperature of 20 to 120 C., and wherein the leaching agent is an aqueous NaOH solution having an NaOH concentration of 2% to 30% by weight.

19. The process of claim 1, wherein the thermal treatment of step c) takes place in a belt furnace and wherein during thermal treatment, the temperature is first increased from room temperature to about 300 to 350 C. and this temperature is maintained for a period of about 2 to 30 minutes to remove moisture and organic constituents from the coating.

20. The process of claim 19, wherein, after the alloy formation, the metal foam body is cooled to a temperature below 200 C., wherein cooling is achieved in a cooling zone of the furnace.

Description

EXAMPLES

(1) 1. Providing of Metal Foam Bodies

(2) Three metal foam mats (a, b, c) made of nickel were provided (manufacturer: AATM, thickness: 1.9 mm, basis weight: 1000 g/m.sup.2, average pore diameter: 580 m).

(3) 2. Applying of Aluminum Powder

(4) Subsequently, binder solution (polyethyleneimine (2.5% by weight) in water) was first sprayed onto all metal foam mats, followed by application of pulverulent aluminum (manufacturer: Mepura, average particle size: <63 m, containing 3% by weight of added ethylenebis(stearamide)) in the form of a dry powder (about 400 g/m.sup.2).

(5) After the foam mats have been coated, 6 cuboidal foam bodies of different thickness (a1, a2, a3, b1, b2, b3) were produced by stacking individual layers of thickness 1.9 mm (length and width each 25 mm) one on top of another. In order to increase the number of contact points and the contact area, the foam bodies were then compressed by about 30%.

(6) Metal foam bodies a1, a2 and a3: thickness 9 mm (7 layers each of thickness 1.9 mm=thickness 13.3 mm; compression to 9 mm)

(7) Metal foam bodies b1, b2 and b3: thickness 12 mm (9 layers each of thickness 1.9 mm=thickness 17.1 mm; compression to 12 mm)

(8) 3. Thermal Treatment

(9) Thereafter, all metal foam bodies were subjected to a thermal treatment under nitrogen atmosphere in a furnace. This involves first removing the binder thermally at 350 C. for 30 min and then heating up to the maximum temperature within 10 min; this was maintained for a defined period of time (duration of treatment), followed by quenching to below 200 C.

(10) TABLE-US-00001 Treatment Duration of Metal foam body temperature ( C.) treatment (min) a1 (thickness: 679 3 a2 (thickness: 660 15 9 mm a3 (thickness: 679 15 9 mm b1 (thickness: 674 9 12 mm b2 (thickness: 660 15 12 mm b3 (thickness: 679 9 12 mm
4. Determination of Extent of Alloying

(11) At the end, the extent of alloy formation in the metal foam bodies was determined. This was done by examining cross sections of the metal foam bodies under a microscope and scanning electron microscope.

(12) This gave the following result:

(13) While superficial alloy formation had taken place in metal foam bodies a1 and b1, but unalloyed regions remained within the metal foam, no alloy formation took place in the case of metal foam bodies a2 and b2, and alloy formation in metal foam bodies a3 and b3 is so far advanced that no unalloyed regions remained within the metal foam.

(14) Among the findings from prior experiments, moreover, is that: If the temperature for alloy formation is chosen above 680 C., for example 700 C., the aluminum reacts with the nickel in an uncontrolled manner and the shaped body burns off, leaving just powder residues.

(15) This result clearly shows that departure from the thermal treatment conditions according to the invention has the effect that superficial alloy formation leaving unalloyed regions within the metal foam is difficult to achieve.

(16) 5. Determination of the Position of the Limiting Curves of the Heating Time

(17) On the basis of the abovementioned results, the position of the limiting curves for the heating time that, for a given heating temperature, leads to superficial alloy formation leaving unalloyed regions within the metal foam was ascertained by a sigmoidal model (heating time=d+(ad)/(1+(heating temperature/c){circumflex over ()}b)).

(18) The limiting values used for the position of the upper curve (maximum heating time) were the following values:

(19) TABLE-US-00002 Temp ( C.) .fwdarw. Duration (min) 680 .fwdarw. 10 675 .fwdarw. 12 665 .fwdarw. 30 660 .fwdarw. 60

(20) The limiting values used for the position of the lower curve (minimum heating time) were the following values:

(21) TABLE-US-00003 Temp ( C.) .fwdarw. Duration (min) 680 .fwdarw. 2 675 .fwdarw. 3 665 .fwdarw. 20 660 .fwdarw. 30

(22) The following result was found for the position of the limiting curves (reporting of H in minutes and reporting of T in C.): maximum duration H.sub.max=d1+(a1d1)/(1+(T/c1){circumflex over ()}b1), with a1=366.1; b1=129.0; c1=650.9; d1=8.7; and minimum duration H.sub.min=d2+(a2d2)/(1+(T/c2){circumflex over ()}b2), with a2=33.5; b2=235.5; c2=665.8; d2=1.8.
6. Determination of the Interval Limits for the Temperature of the Thermal Treatment Depending on the Thickness of the Metal Foam Bodies Treated

(23) The position of the interval limits for the temperature of the thermal treatment depending on the thickness of the metal foam bodies treated was found from the results presented above and further experience values.

(24) The temperature T for the thermal treatment (in C.), depending on the thickness D of the metal foam body AX (in millimeters), should be selected as follows: when 0 mm