CATALYST NETWORK COMPRISING A NOBLE METAL WIRE MADE OF A DISPERSION-STRENGTHENED NOBLE METAL ALLOY

Abstract

A catalyst network comprising at least one noble metal wire that contains at least one dispersion-strengthened noble metal alloy. The invention also relates to a catalyst system containing at least one catalyst network according to the invention, and to a method for the catalytic oxidation of ammonia in which a catalyst network according to the invention is used.

Claims

1. A catalyst network comprising at least one noble metal wire, wherein the at least one noble metal wire contains at least one dispersion-strengthened noble metal alloy which, in addition to at least one noble metal, comprises at least one non-noble metal, and wherein the at least one non-noble metal is selected from the group consisting of zirconium, cerium, scandium and yttrium and the at least one non-noble metal is at least partially present as an oxide.

2. The catalyst network according to claim 1, wherein the catalyst network has a three-dimensional structure.

3. The catalyst network according to claim 1, wherein the proportion of the at least one noble metal is at least 50 wt. % based on the total weight of the dispersion-strengthened noble metal alloy.

4. The catalyst network according to claim 1, wherein the dispersion-strengthened noble metal alloy consists of the at least one noble metal, the at least partially oxidized non-noble metal and impurities.

5. The catalyst network according to claim 1, wherein the at least one noble metal is selected from the group consisting of platinum, palladium, rhodium and mixtures thereof.

6. The catalyst network according to claim 1, wherein the dispersion-strengthened noble metal alloy is a platinum alloy.

7. The catalyst network according to claim 1, wherein the dispersion-strengthened noble metal alloy contains 0.005 wt. % to 1.0 wt. % of the at least one non-noble metal.

8. The catalyst network according to claim 1, wherein the at least one non-noble metal is at least 70% oxidized.

9. The catalyst network according to claim 1, wherein the at least one noble metal wire comprises a plurality of filaments.

10. The catalyst network according to claim 1, wherein the at least one noble metal wire has a two- or three-dimensional structure.

11. The catalyst network according to claim 10, wherein the at least one noble metal wire comprises a helical longitudinal portion.

12. The catalyst network according to claim 1, wherein the catalyst network comprises at least one further noble metal wire.

13. The catalyst network according to claim 12, wherein the at least one further noble metal wire comprises a platinum alloy.

14. A catalyst system for the catalytic oxidation of ammonia, comprising at least one catalyst network according to claim 1.

15. A method for the catalytic oxidation of ammonia in which a fresh gas containing ammonia is conducted over at least one catalyst network according to claim 1.

Description

[0063] FIG. 1 schematically shows a vertically positioned flow reactor 1 for the heterogeneous catalytic oxidation of ammonia. The catalyst system 2 forms the actual reaction zone of the flow reactor 1. The catalyst system 2 comprises a plurality of catalyst networks 4 which are arranged one behind the other in the flow direction 3 of the fresh gas and behind which a plurality of catchment networks 5 can be arranged. The effective catalyst network diameter can be up to 6 m. The networks used are in each case textile fabrics produced by means of machine weaving or knitting of noble metal wires.

[0064] A corresponding flow reactor is used, for example, for the synthesis of hydrocyanic acid according to the Andrussow process. As fresh gas, an ammonia-methane-air mixture under increased pressure is introduced in flow direction 3. Upon entry into the catalyst system 2, the gas mixture is ignited and a subsequent exothermic combustion reaction takes place. The overall reaction is:

##STR00001##

[0065] This exothermic reaction increases the temperature of the gases to up to 1100 C.

Compression Test

[0066] Catalyst networks were woven from wires having a diameter of 76 m. A linear warp wire and a helical weft wire having 17.5 turns/cm were used. FIG. 2 shows the structure of the helical wire. Both wires contained a PtRh10 alloy, a standard industrial alloy. In the case of the catalyst networks according to the invention, the helical wire was dispersion strengthened.

[0067] Stacks of 24 catalyst networks (LW: 50 mm50 mm) were each subjected to a weight of 1 kg over a period of 3 days at an ambient temperature of 1200 C. The thickness of the network stacks was measured at 16 points before and after heat treatment. Both network packages had a thickness of approximately 8 mm at the beginning of the test. The compression was determined according to the formula

[00001]$\mathrm{compression}=\frac{\left(\mathrm{thickness}\right)}{\mathrm{initialthickness}}$

[0068] Table 1 compares the results of the compression test. The network package using the catalyst networks according to the invention (EB) showed a significantly lower compression than the network package in which the networks were made entirely of non-dispersion-strengthened standard wire (VB).

TABLE-US-00001 TABLE 1 Compression [%] VB 40.5% EB 22.5%

[0069] The compression stability of the tested catalyst network package was improved by using a dispersion-strengthened platinum wire. In connection with this, a positive effect on the long-term efficiency of a corresponding catalyst system is expected.

Efficiency in the Flow Reactor

[0070] In a flow reactor as shown in FIG. 1, the efficiency of catalyst systems consisting of 10 catalyst networks, each having a composition analogous to that in the comparative example and in the example according to the invention involving the oxidation of ammonia was compared.

[0071] Fresh gas was introduced from above into reactor 1 under increased pressure. The fresh gas was an ammonia-air mixture having a nominal ammonia content of 10.7 vol. % and a preheating temperature of 175 C. When entering the catalyst system 2, the gas mixture is ignited and a subsequent exothermic combustion reaction occurs. The following main reaction takes place:

##STR00002##

[0072] In this case, ammonia (NH.sub.3) is converted to nitrogen monoxide (NO) and water (H.sub.2O). The nitrogen monoxide (NO) formed reacts in the outflowing reaction gas mixture (symbolized by the directional arrow 7 indicating the flow direction of the outflowing reaction gas mixture) with excess oxygen to form nitrogen dioxide (NO.sub.2), which is reacted with water in a downstream absorption system to form nitric acid (HNO.sub.3).

[0073] The measurement of the catalytic efficiency (i.e., the product yield of NO) has the following sequence: [0074] 1. It is ensured that the catalyst system is suitable for the complete conversion of the ammonia used. That means that NH.sub.3 in the product gas is no longer present in a significant amount, which is checked by means of mass spectrometric analysis of the product gas. [0075] 2. Simultaneous removal of a sample of NH.sub.3/air upstream of the catalyst packing and a sample from the product gas downstream in respectively independent evacuated pistons. The mass of the gas is determined by weighing. [0076] 3. The NH.sub.3/air mixture is absorbed in distilled water and titrated by means of 0.1 N sulfuric acid and methyl red after color change. [0077] 4. The nitrous product gases are absorbed in 3% sodium peroxide solution and titrated by means of 0.1 N sodium hydroxide solution and methyl red after color change. [0078] 5. The catalytic efficiency Eta results from Eta=100CO.sub.n/C.sub.a, where C.sub.a is the average NH.sub.3 concentration of 7 individual measurements in the fresh gas in percent by weight and Co is the mean NOx concentration of 7 individual measurements expressed as percent by weight of the NH.sub.3 which has been oxidized to form NOx. [0079] 6. Separately, the volumetric proportion of N.sub.2O in the product gas is determined by means of gas chromatography.

[0080] Table 2 compares the catalyst efficiency of the catalyst systems (yield of NO in %) for different fresh gas flow rates. The abbreviation tN/m.sup.2d stands for tons of nitrogen (from ammonia) per day and a standardized effective cross-sectional area of the catalyst system of one square meter.

TABLE-US-00002 TABLE 2 Fresh gas flow rate Efficiency [%] 22.5 tN/m.sup.2d VB 94.9 5 bar EB 95.2 30 tN/m.sup.2d VB 92.2 9 bar EB 92.3 55 tN/m.sup.2d VB 89.7 9 bar EB 90.4

[0081] Both test systems showed comparable catalytic efficiency at all tested fresh gas flow rates. The results demonstrate that the use of the dispersion-strengthened platinum alloy does not impair the catalytic activity of the catalyst network.