Method for producing noble metal meshes on flatbed knitting machines

Abstract

The present invention relates to a method for producing a noble metal mesh on a flatbed knitting machine which comprises at least one needle bed and at least one knitted fabric take-up device which is spaced apart at a distance (D) from the at least one needle bed. Here, first of all at least part of a supporting mesh is knitted over at least the length of the distance between the needle bed and a take-up device of the flatbed knitting machine. Within the context of the invention, it has been recognized to be surprising that the knitting of a supporting mesh with the use of a noble metal-free supporting thread over a minimum length before the beginning of the knitting of the noble metal mesh has a stabilizing effect on the knitting process.

Description

[0001] Process for the production of precious metal nets on flat knitting machines

[0002] The present invention relates to a method for producing a precious metal mesh on a flat knitting machine. Initially, at least a portion of a support mesh is knitted over at least the length of the distance between the needle bed and a take-off device of the flat knitting machine.

[0003] The heterogeneous noble metal-catalyzed oxidation of ammonia (NH3) to nitric acid (HNO3, Ostwald process) or the production of hydrogen cyanide (HCN, Andrussow process) are of great importance due to the central relevance of the products to the chemical industry. The catalyst systems used for this purpose are usually installed in the reaction zone of a flow reactor in the form of gas-permeable sheet materials in a plane perpendicular to the flow direction of the fresh gas. Collection or catchment systems for the recovery of vaporized catalytically active components are also frequently based on such mesh-like structures. It is usually advisable to arrange several meshes one behind the other and combine them to form a catalyst mesh stack. The individual meshes consist of fine precious metal wires, predominantly containing platinum (Pt), palladium (Pd), rhodium (Rh), or alloys of these metals.Catchment nets in particular can also contain other components such as nickel.

[0004] A number of processes are known for producing such nets as knitted fabrics, including weaving, knitting, and crocheting. Weaving and crocheting are particularly suitable for producing rectangular net panels with a homogeneous material distribution and structure. They offer little flexibility in terms of the shape and material variability of the manufactured products.

[0005] Knitting offers greater flexibility in comparison: the knitting patterns, wires (both in terms of thickness and material), needles, and wire tension used allow for easy variation of the fabric's weight, structure, stretchability, and strength. A further advantage is the significantly shorter machine set-up times. Furthermore, the use of different materials in one knitted fabric is possible; in so-called intarsia knitting, for example, different sections are made from different yarns or wires. The length of a knitted fabric is, in principle, unlimited; however, in the case of flat knitting machines, the maximum width is determined by the width of the needle beds.

[0006] Flat knitting machines or circular knitting machines can be used for knitting. On circular knitting machines, the needles are arranged in a circular needle bed, and the thread or wire is fed in a circular motion. These machines are primarily used to produce tubular knitwear. On flat knitting machines, however, the shape and size of the knitted fabric can be varied. Flat knitting machines can also have more than one needle bed, between which the yarn or wire is guided back and forth during production using thread guides.

[0007] The use of two needle beds allows the production of a single-layer or double-layer knitted fabric. A single-layer knitted fabric can essentially be created in two ways: Firstly, when only one needle bed is used, i.e. only stitches on one of the needle beds are formed and knitted together. Secondly, stitches created on the first needle bed can be knitted with stitches created on the second needle bed, i.e. the thread is passed back and forth between the two needle beds within a forward and / or backward row. A two-layer knitted fabric is created when knitting takes place on both needle beds simultaneously and the stitches created on the first row of needles are not connected, or are only connected sporadically, to the stitches created on the second needle bed via the edge stitches. With this production method, two single-layer knitted fabrics or layers can also be produced simultaneously on the first and second needle bed.

[0008] To keep the knitted fabric taut during production, knitting machines typically have one or more take-up devices located below the stitch-forming area of ​​the needle beds. These take-up devices grip the fabric being knitted and exert a pulling force on it.

[0009] The knitting of precious metal nets is known and described, for example, in EP 0364153 A1. In EP 3779005 A1, flat knitting machines are used for this purpose. Precious metal nets with more complex 3-dimensional structures can also be produced in this way, as described, for example, in EP 1358010 A2. EP 0364153 A1 describes the use of an additional precious metal-free thread when knitting a precious metal net, which increases process stability. The additional thread is knitted parallel to the precious metal and can be removed from the produced homogeneous knitted fabric, consisting of precious metal material and thread material, after the knitting process, if necessary. In EP 3779005 A1, this process was further developed so that the additional thread and the precious metal are not knitted in parallel, but simultaneously. In this way, precious metal-free areas can be preserved in the finished knitted fabric, which leads to a reduction in the use of precious metals.

[0010] However, it has been shown that knitting with wires containing precious metals poses particular challenges to the knitting process. In particular, it has been observed that the mechanical stress on the resulting knitted fabric caused by the direct knitting of the precious metal-containing wire on the needle beds of the flat knitting machine can have a detrimental impact on the production process and quality. The stiffness of the wire can prevent the formed stitches from shedding, leading to blockage of the precious metal knit on the needles or between the needle beds. This can cause the knitted fabric to warp, resulting in instabilities in the production process. Such instabilities can manifest themselves, on the one hand, in wire breakage during the knitting process and, on the other hand, in irregularities in terms of structure and mechanical properties, both of which result in a product of poor quality.Such instabilities and the accumulation of stitches and knitted fabric are also detrimental to the load on the needle bed; this can lead to increased damage to the needle bed and the carriage unit for thread guidance.

[0011] The object of the present invention was therefore to provide a method with high process stability for the production of precious metal nets on flat knitting machines using wires containing precious metals.

[0012] The object is achieved by a method for producing a precious metal net on a flat knitting machine, wherein the flat knitting machine has at least one needle bed and at least one knitted fabric take-off device spaced at a distance D from the at least one needle bed, comprising the steps

[0013] - Providing at least one precious metal-free support thread, - Providing at least one precious metal-containing wire,

[0014] - knitting a support net using at least one precious metal-free support thread,

[0015] - Knitting the precious metal net using the at least one precious metal-containing

[0016] Wire, characterized in that initially at least part of the support net is knitted over at least the length of the distance D.

[0017] Within the scope of the invention, it was surprisingly discovered that knitting a support net using a precious metal-free support thread over a minimum length before starting to knit the precious metal net has a stabilizing effect on the knitting process. In other words, pre-knitting a support net at least over the length of the distance between the needle bed and the take-off device reduces process disruptions in the form of blockages of the resulting precious metal-containing knit on the needles or between the needle beds, breaks in the precious metal-containing wire, or irregularities and defects in the precious metal knit in the form of faulty rows and / or connections.

[0018] The method according to the invention relates to knitting on a flat knitting machine with at least one needle bed.

[0019] Knitting is characterized by the row-by-row production of the resulting knitted fabric, in which interlocking connections, typically in the form of loops, are formed. In addition to loops, depending on the knitting pattern, knitted fabrics can also comprise other connections known to those skilled in the art in the form of other thread guides, such as floats or catches. In the context of this application, loops are always understood to mean other possible thread guide forms for connection. If reference is made below to a "thread", the respective embodiment should also apply to a corresponding "wire". In the knitting direction, the knitted fabric is formed row by row, whereby in the following reference is made to the part of the knitted fabric containing the first knitting row as "bottom". The thread is guided from one side of the needle bed or needle beds to the other side and back.A knitting row therefore comprises a forward and a reverse row, which is determined by the thread guidance. The thread guidance can be carried out over just one needle bed in a knitting row or, in the case of a knitting machine with two needle beds, over both needle beds. The thread guidance can be carried out across the entire width of the needle bed(s), but knitting can also be carried out over only parts of the width of the needle bed(s). The width and shape of the knitted fabric can be determined by the width of the needle bed(s).

[0020] When reference is made in this application to the length of the knitted fabric, this always means the length from the first knitting row to the last knitting row.

[0021] During the knitting process on a flat knitting machine with a needle bed, the first part of a knitting row is first formed in one direction (forward row), then the thread is guided in the opposite direction (back row).

[0022] During the knitting process on a flat-bed knitting machine with two needle beds, the first part of a knitting row is formed on at least one of the needle beds. Typically, the yarn for a complete knitting row is first guided on the first needle bed (forward row), then the yarn is guided in the opposite direction on the same needle bed or the second needle bed (wrong row). However, the yarn can also be guided alternately on both needle beds in a forward and / or wrong row, creating connections between stitches on the two needle beds within a knitting row. This allows the formation of a single-layer or a double-layer knitted fabric.For the purposes of the present invention, "two-layer knitted fabrics" are understood to mean knits that have two layers, where the layers may or may not be connected to one another at one or more of their respective butting edges. The two layers lie on top of one another, meaning they overlap at least partially across their surface area. Knits that are connected to one another on one side via the butting edges of the two layers are also referred to as two-layer knitted fabrics. By folding the fabric open along the common butting edge, a single-layer knit is obtained. A combination of single-layer and two-layer regions within one knit is also possible.

[0023] The lower edges are those edges that are formed first in the knitting direction, i.e., those located at the bottom of the knitted fabric. Accordingly, the upper edges are those that are formed later in the knitting direction. A flat knitting machine suitable for the method according to the invention has at least one fabric take-off device, which is arranged at a distance D from the at least one needle bed.

[0024] Knitted fabric take-off devices are known in principle to those skilled in the art; they can, for example, be one or more rollers, rolls, belts, hooks, plates, needles, or combs. Particularly preferably, the knitted fabric take-off devices of the present invention comprise at least one roller or roll. A knitted fabric take-off device can optionally consist of one roller or roll, or alternatively of two or more rollers or rolls. If a knitted fabric take-off device according to the invention comprises two rollers or rolls, these are preferably arranged such that the knitted fabric can be pressed between the rollers or rolls. The at least two rollers or rolls can be operated in opposite directions. In this case, the contact pressure between two rollers or rolls can preferably be adjusted such that the precious metal mesh is sufficiently fixed to exert a take-off force, but is not irreversibly deformed.Knit take-off devices can be arranged statically or movably beneath the at least one needle bed. A static take-off device means that the distance between the knit take-off device and the needle bed remains unchanged during the knitting process. A static take-off device can also have moving parts, such as rollers or rolls.

[0025] The flat knitting machine can optionally also be equipped with several fabric take-off devices.

[0026] For example, the flat knitting machine can have a first knitted fabric take-off device arranged at a distance D from the at least one needle bed and a second knitted fabric take-off device arranged at a distance D2 from the at least one needle bed, where D2 is greater than D. Within the scope of the invention, the value for D is preferably at least 3 cm, in particular at least 2 cm and particularly preferably at least 1 cm, measured from the contact point of the knitted fabric on the needle bed to the point at which the knitted fabric is in contact with the knitted fabric take-off device.

[0027] Within the scope of the invention, the value for D is preferably at most 20 cm, in particular at most 15 cm, and most preferably at most 10 cm, measured from the point of contact of the knitted fabric with the needle bed to the point where the knitted fabric is in contact with the take-off device. Within the specified range for D, it can be ensured that the knitted fabric can be sufficiently pressed.

[0028] The second fabric take-off device is thus arranged farther from the needle bed than the first fabric take-off device. The distance D2 can preferably be at least 25 cm, in particular at least 30 cm, and preferably at least 35 cm, measured from the contact point of the knitted fabric on the needle bed to the point where the knitted fabric is in contact with the fabric take-off device. The distance D2 can preferably be at most 100 cm, in particular at most 80 cm, and preferably at most 60 cm, measured from the contact point of the knitted fabric on the needle bed to the point where the knitted fabric is in contact with the fabric take-off device.

[0029] The first and second fabric take-off devices can each be static or movable. For example, both fabric take-off devices can be static. In preferred embodiments, the flat knitting machine has a fabric take-off device that is statically arranged at a distance D2 from the at least one needle bed, and a fabric take-off device that is movably arranged between the at least one needle bed and the statically arranged fabric take-off device. The movable fabric take-off device, in such cases also referred to as an auxiliary take-off device, has a minimum distance D from the at least one needle bed.

[0030] The method comprises providing at least one precious metal-free support thread.

[0031] Suitable non-precious metal support threads can be selected through routine testing, taking into account the final intended use of the precious metal mesh and any additional steps in the manufacturing process. Preferred non-precious metal support threads can be removed after the precious metal mesh has been manufactured, for example, by dissolving them in acidic or basic media, cutting them off, melting them, or flaming them. Such support threads can be natural or synthetic, organic, or inorganic. Examples of suitable materials include polyamides, polyesters, cellulose fibers, cotton, acrylic-styrene polymers, nylon, PVA and other vinyl polymers, alginate, copper, aluminum, or even low-melting-point metals such as tin and lead alloys.

[0032] The at least one precious-metal-free support thread can consist of only one thread-like element; such individual thread-like elements are also referred to as filaments in the case of precious-metal-free threads. The precious-metal-free support thread can also consist of more than one filament, which can advantageously be twisted together. These multiple filaments can be made of the same or different materials.

[0033] The method according to the invention comprises knitting a support net using the at least one precious metal-free support thread.

[0034] The "support mesh" refers to the areas of the knitted fabric that are knitted without the use of precious metal-containing wire. The support mesh is therefore precious metal-free, but can also include additional threads or wires.

[0035] The knitted fabric comprises all knitting rows formed during the process. The knitted fabric comprises at least the support mesh and the precious metal mesh. However, the knitted fabric can also comprise other parts or regions. The shape of the knitted fabric is not further restricted; for example, it can be round, rectangular, or trapezoidal; the shape is determined by the outer edges of the entire knitted fabric. It may be preferable for all knitting rows of the knitted fabric to be knitted over the same width of the needle beds; in other words, the knitted fabric can preferably be rectangular.

[0036] In the method according to the invention, at least a portion of the support mesh is first knitted over at least the length of the distance D. In other words, before knitting begins, the precious metal mesh is knitted over at least the length D using the precious metal-free support thread. Preferably, at least 5 knitting rows are knitted using the precious metal-free support thread. Knitting to a minimum length ensures that the knitted fabric is tensioned by the knit take-up device, in particular that it is tensioned evenly, before the more difficult-to-process precious metal-containing wire is knitted in. This presumably ensures that stitches or knitted parts containing the stiff precious metal-containing wire are pulled off the needles during the knitting process. This increases process stability and reduces defects in the precious metal mesh caused by the knit take-up device.

[0037] In the event that the flat knitting machine has more than one fabric take-off device at different distances from the at least one needle bed, it may be advantageous for at least part of the support net to be knitted initially over at least the length of the distance up to at least one fabric take-off device that is further away than distance D. The more distant fabric take-off device can, for example, be arranged at a distance D2 from the at least one needle bed, where D2 is greater than D. In other words, it may be advantageous for at least part of the support net to be knitted over at least the length D2.In the case of two fabric take-off devices, the first fabric take-off device, located at a distance D, can optionally be deactivated when the support net has reached the second fabric take-off device, located at a distance D2, and the second fabric take-off device can exert a take-off force on the support net. This can be particularly advantageous if the first fabric take-off device comprises two rollers or rolls, in particular counter-rotating rollers or rolls, with which the fabric can be pressed.

[0038] In one possible embodiment, the more distant fabric take-off device, located at a distance D2, consists of only one roller or roll. Using a single roller or roll prevents the precious metal mesh from being irreversibly deformed between two rollers during the knitting process.

[0039] It may also be advantageous for the support net to be knitted only up to the nearest knitting take-off device; in other words, even in the case of several knitting take-off devices, the support net can initially only be knitted over the minimum length D.

[0040] On flat-knitting machines equipped with a static take-up device at a distance D2 from the at least one needle bed and a movable auxiliary take-up device, a portion of the fabric of length D is typically first knitted until the fabric can be tensioned by the auxiliary take-up device. As the knitting process continues, the auxiliary take-up device moves toward the static take-up device as the length of the fabric increases, up to a distance D2. This carries the resulting fabric along.

[0041] Within the scope of the invention, a knitted fabric take-off device, in particular at a distance D and / or at a distance D2, exerts a take-off force on the knitted fabric, in particular on the support net, the precious metal net or both.

[0042] For knitting precious metal nets, a minimum pull-off force is preferably required. The pull-off force exerted by the first, second, or both fabric pull-off devices, in particular by at least one roller or at least one roll, on the knitted fabric, in particular containing the precious metal net, can preferably be at least 5 N / m, in particular at least 10 N / m, and most preferably at least 15 N / m. This minimum pull-off force preferably prevents the knitted fabric from jamming.

[0043] The pull-off force exerted on the knitted fabric by the first, second, or both pull-off devices, in particular by at least one roller or at least one roll, can preferably be at most 350 N / m, in particular at most 250 N / m, and most preferably at most 150 N / m. This allows particularly good precious metal nets to be produced. For example, the aforementioned maximum pull-off force can prevent the precious metal net from tearing during knitting.

[0044] The pull-off force can be adjusted by the specialist on the knitted fabric take-off device.

[0045] The support net can be knitted in a single or multi-layered manner. It can be knitted on a single needle bed or, if the flat knitting machine has more than one needle bed, on multiple needle beds. The support net can have different sections that are single-layered and / or double-layered.

[0046] If the flat knitting machine has two needle beds, the support net can be knitted with connecting stitches from both needle beds. "Connecting stitches" here refer to the knitting stitches formed between the first and second needle beds. In other words, the support net is at least partially knitted in a single layer.

[0047] The support net can be knitted across the entire width or only across part of the width of at least one needle bed. The shape of the support net, in particular the first part of the support net with the minimum length D, is not further restricted; it can, for example, be rectangular, round, or triangular. Combinations of these shapes are also possible, especially if the support net comprises multiple sections.

[0048] The method according to the invention comprises providing at least one wire containing precious metal.

[0049] A precious metal-containing wire is understood to be a wire that consists of at least one precious metal or that contains a significant proportion (> 50 wt%) of precious metal. For the purposes of the present invention, precious metals are understood to be a metal selected from the group consisting of platinum group metals, gold, and silver. Platinum group metals are understood to be the metals of the so-called platinum group, i.e., platinum (Pt), palladium (Pd), iridium (Ir), rhodium (Rh), osmium (Os), and ruthenium (Ru).

[0050] Preferably, the at least one precious metal-containing wire consists of platinum, a platinum alloy, palladium, or a palladium alloy. A platinum alloy contains more than 50 wt.% platinum; other alloying constituents include, in particular, palladium, rhodium, and ruthenium. A palladium alloy contains more than 50 wt.% palladium; other alloying constituents include, in particular, platinum, rhodium, and ruthenium.

[0051] Preferably, the at least one precious metal-containing wire consists of an alloy selected from the group consisting of platinum with 1 - 15 wt.% rhodium, platinum with 1 - 15 wt.% rhodium and 0.1 - 20 wt.% palladium, platinum with 1 - 15 wt.% rhodium, 0.1 - 20 wt.%

[0052] Palladium and 0.1 - 5 wt.% ruthenium, platinum with 1 - 15 wt.% rhodium, 0.1 - 40 wt.%

[0053] Palladium and 0.001 - 5 wt.% iridium, platinum with 1 - 15 wt.% rhodium, 0.1 - 20 wt.%

[0054] Palladium and 0.001 - 5 wt% tantalum, platinum with 1 - 15 wt% rhodium, 0.001 - 5 wt%

[0055] Iridium and 0.001 - 5 wt% tantalum, palladium with 1 - 25 wt% platinum, palladium with 1 - 25 wt% platinum and 1 - 15 wt% rhodium, palladium with 1 - 25 wt% tungsten, palladium with 1 - 15 wt% nickel, palladium with 0.001 - 5 wt% rhodium, palladium with 1 - 15 wt% copper, palladium with 1 - 15 wt% copper and 1 - 15 wt% nickel and palladium with 1 - 30 wt% cobalt.

[0056] Preferably, precious metal-containing wires are used which have a diameter of 40 - 150 pm, preferably 50 - 130 pm.

[0057] The at least one precious metal-containing wire can be designed as a round wire, i.e., with a round cross-section. In another embodiment, the wire can be designed as a flattened round wire or as a wire with a different cross-section.

[0058] The at least one precious metal-containing wire can comprise several wires, in this case also referred to as filaments, which can preferably be twisted together. The filaments can all be made of the same material, i.e., all contain precious metals, or they can be made of different materials, which in turn do not all have to contain precious metals.

[0059] The method according to the invention comprises knitting the precious metal mesh using the at least one precious metal-containing wire. According to the invention, the precious metal mesh is knitted after at least a first portion of the support mesh has been knitted; in other words, the knitted fabric is knitted at least to a length of the distance D before the precious metal-containing wire is knitted. The precious metal mesh can be knitted over the same width of the at least one needle bed as the first portion of the support mesh, but the precious metal mesh can also be knitted narrower or wider than the first portion of the support mesh.

[0060] In many cases, it can be advantageous to knit two or more precious metal-containing wires together. In other words, when forming a stitch, several precious metal-containing wires can be guided together, and the several precious metal-containing wires are knitted in parallel. When knitting with several precious metal-containing wires, in one embodiment, the precious metal-containing wires consist of the same material; in other embodiments, precious metal-containing wires consisting of at least two different materials can be used. The several wires can be the same or different; in particular, they can have the same or different diameters and / or the same or different compositions.

[0061] The precious metal mesh can be knitted with one or two layers; in other words, the precious metal mesh can be knitted as a single- or double-bedded fabric. Preferably, the first and second layers of a two-layer precious metal mesh can be knitted from a precious metal-containing wire or precious metal-containing wires of the same composition. However, the two layers can also be knitted from precious metal-containing wire or precious metal-containing wires of different compositions. Precious metal-containing wire of the same or different diameters can be used to knit the first and second layers of a two-layer precious metal mesh.

[0062] The first and second layers of a two-layer precious metal mesh can be knitted in the same or different knitting patterns. Different knitting patterns can be created, for example, by different stitch lengths, stitch connections, floats, or catches. In preferred embodiments, the first and second layers of a two-layer precious metal mesh are knitted in the same knitting pattern.

[0063] The first and second layers of a two-layer precious metal mesh can have the same length or different lengths in the knitting direction and / or the same width or different widths perpendicular to the knitting direction. Preferably, the first and second layers of a two-layer precious metal mesh have the same length and the same width; in other words, the first and second layers of a two-layer precious metal mesh are preferably congruent.

[0064] The first and second layers of a two-layer precious metal mesh can have the same shape or different shapes. Particularly preferably, the first and second layers of a two-layer precious metal mesh have the same shape. It may be advantageous for the first and second layers of a two-layer precious metal mesh to have the shape of a semi-ellipse, in particular the shape of a semicircle. It may be particularly advantageous for these two semi-ellipses or semicircles to have the same width and the same length.

[0065] When knitting on several needle beds, it can also be advantageous to knit several precious metal-containing wires that are not knitted in parallel and are, for example, guided between the needle beds, inserted into the resulting knitted fabric or form connections between the layers.

[0066] The precious-metal-free support thread can be knitted parallel to the at least one precious-metal-containing wire, i.e., loops, catches, or floats can be formed during knitting that comprise the precious-metal-containing wire and the precious-metal-free support thread. In these cases, the relevant part of the resulting knitted fabric contains both the precious-metal-containing wire and the material of the support thread.

[0067] The support net can comprise only a part that is knitted with a minimum length D before the precious metal net is knitted. The support net can also comprise further areas or parts. For example, it can comprise the first area of ​​minimum length D, which is knitted before the precious metal net is knitted, and at least one further area that is knitted simultaneously with the precious metal net, i.e., which is knitted in the same knitting rows as the precious metal net. The support net can also comprise an area that is knitted during and / or after the precious metal net is knitted. The support net can therefore completely or partially surround the precious metal net. The different areas of the support net can be connected to the precious metal net, but it can be advantageous if the different areas are not connected or are only partially connected to one another.In preferred embodiments, a first region of minimum length D can be knitted first, followed by a first precious metal mesh, and then a further support mesh region and a further precious metal mesh. In other words, several precious metal meshes can be knitted, separated from each other by at least one support mesh region.

[0068] In preferred embodiments, the support mesh surrounds the precious metal mesh by at least 50%. This means that at least 50% of the circumference of the precious metal mesh is surrounded by the support mesh, more preferably at least 60%, even more preferably at least 80%. It may be particularly preferred for the support mesh to completely surround the precious metal mesh.

[0069] The knitted fabric can therefore have knitting rows comprising a precious metal-containing wire and a precious metal-free supporting thread, whereby the precious metal-containing wire and the precious metal-free supporting thread can be knitted completely or partially in parallel, or even completely or partially simultaneously. Such knitting rows can have regions that do not comprise a precious metal-containing wire. Regions that do not comprise a precious metal-containing wire can be connected to regions that comprise a precious metal-containing wire by loops or catches of the precious metal-containing wire and / or the precious metal-free supporting thread, but the regions can also be unconnected. In other words, the regions of the precious metal mesh can have connections with the regions of the supporting mesh within a knitting row.Consecutive knitting rows can have such a connection throughout or can have no connection at all; any combination is also possible. For example, consecutive knitting rows can alternate between such connections, but several consecutive knitting rows can also have no connection, followed by one or more knitting rows with a connection. The precious metal mesh can therefore be fully or partially connected to the corresponding areas of the support mesh on at least one side. In the case of partial connection, this can be achieved via one or more consecutive knitting rows.

[0070] It may be preferred for the support net to be knitted in two layers in the areas with knitting rows that do not contain a precious metal-containing wire if the flat knitting machine has two needle beds. In other words, the support net can be knitted in these knitting rows without connecting stitches between the first and second needle beds. These two layers can be connected at the edge stitches via connecting knitting stitches; it may therefore be advantageous for the support net to have a tubular structure in these areas. It may also be preferred for the support net to be knitted in one layer in the areas that do not contain a precious metal-containing wire. In preferred embodiments, the support net comprises single-layer knitted areas and double-layer knitted areas that do not contain a precious metal-containing wire.

[0071] Preferably, all sections of the support mesh can be knitted from precious-metal-free support threads of the same composition. However, different sections can also be knitted from precious-metal-free support threads of different compositions. Different sections of the support mesh can be knitted in the same or different knitting patterns.

[0072] The method according to the invention may comprise further steps.

[0073] The method may comprise providing at least one additional wire or thread. Suitable additional wires or threads may be selected depending on the intended use and / or function in the manufacturing process or for the subsequent application of the precious metal mesh. For example, the additional wire may be a wire made of a non-precious metal suitable for stabilizing the precious metal mesh during use in the reactor; for example, it may be a steel or stainless steel wire. In these cases, the method comprises the simultaneous knitting of regions of the knitted fabric using the at least one additional wire or thread.In this case, the knitted fabric may comprise parts or regions that contain only the additional wire or thread, or parts or regions that contain both the additional wire or thread and the precious metal-containing wire, or parts or regions that contain both the additional wire or thread and the precious metal-free supporting thread, or parts or regions that contain both the additional wire or thread and the precious metal-containing wire and the precious metal-free supporting thread.

[0074] Preferably, the support mesh can be removed in a further step. Suitable methods are known in principle to those skilled in the art and depend on the type of precious metal-free support thread and the precious metal-containing wire used. The support mesh can, for example, be decomposed, dissolved, melted, flamed, or cut off. The invention is explained in more detail below with reference to illustrations and exemplary embodiments. However, it is not limited to these embodiments.

[0075] To determine the pull-off force within the scope of the invention, the following procedure was used:

[0076] First, support nets, such as a cotton net, were knitted on a flatbed knitting machine, with a pocket for a rod being knitted into each support net. Rods were inserted into the knitted pockets. Free-hanging weights of varying weights were attached to the rods.

[0077] Then, precious metal nets (wire: PtRh5, d=76 pm) with a mesh width of one meter were knitted, each of which was subjected to a continuous pull-off force by an attached weight.

[0078] The knitting results of the precious metal mesh were assessed visually. Good results were considered to be achieved if there was no material jamming (the knitted fabric jamming at the needle bed) and no tears in the precious metal mesh.

[0079] The value for the take-off force (N / m) specified in the present application is the applied take-off force divided by the fabric width at the needle bed.

[0080] The pull-off force always takes into account the weight of the push-in rod.

[0081] Figure 1 shows a schematic representation of a flow reactor for the heterogeneous catalytic combustion of ammonia.

[0082] Figure 2 shows partial sections perpendicular to the machine's longitudinal axis of a flat knitting machine with at least one take-off device.

[0083] Figure 3 shows examples of knitted fabrics with differently shaped support nets that can be produced according to the invention.

[0084] Figure 4 shows examples of knitted fabrics that can be produced according to the invention, with differently shaped support nets that also surround the precious metal net laterally. Figure 1 schematically shows a vertically positioned flow reactor 1 for the heterogeneous catalytic combustion of ammonia. The catalyst system 2 forms the actual reaction zone of the flow reactor 1. It comprises a catalyst packing 3 and downstream catchment nets 4. The catalyst packing 3 comprises several catalyst nets 6 arranged one behind the other in the flow direction 5 of the fresh gas. Typically, the catalyst nets 6 are knitted nets, which are produced, for example, by knitting wire with a diameter of 76 pm made of various platinum-rhodium alloys. Catchment nets 4 can also be provided.

[0085] Figure 2 shows partial sections perpendicular to the machine's longitudinal axis of flat knitting machines 200, each with two needle beds 201 and 202. The flat knitting machine 200' in Fig. 2 A has a take-off device 203' at a distance D from the needle beds 201 and 202'. The device 200" shown in Fig. 2 B has two take-off devices 203" and 204" at distances D and D2 from the needle beds 201" and 202". The take-off device 203" is shown here as comprising two rollers, between which the resulting knitted fabric can be passed, but it can also be any other take-off device known to the person skilled in the art. The same applies to the take-off device 204", which is shown as a single roller.

[0086] Figure 3 shows examples of knitted fabrics 300 that can be produced according to the invention, comprising rectangular-shaped precious metal nets 302 with differently shaped support nets 301. In all Figures 3 A to F, the support nets border, across the entire width, on regions of the knitted fabric 302 that were knitted from wire containing precious metal. The support nets of Figures 3 A to D each have only a first region (30T-301 iv ) of length D, which is shaped differently. In Figures E and F, the support nets also have a second area (303 v , 303 vi ) which is located in the knitted fabric above the precious metal-containing area (302 v ,302 vi ) is arranged.

[0087] Figure 4 shows examples of knitted fabrics 400 that can be produced according to the invention with differently shaped support nets that also surround the precious metal net laterally. The first area of ​​length D (40T-401 vi) each has an at least partially rectangular shape, but all other shapes are also possible. In Fig. 4 A, the support net comprises a second region 404', which laterally surrounds the knitted fabric region 402' containing precious metal. In the embodiment shown, there is no connection between the precious metal-containing region 402' and the support net region 404'; the knitted fabric has a gap 405'. In the embodiment shown in Fig. 4 B, the lateral support net region 404" is partially connected to the precious metal-containing region 402" (406"). The embodiment of the support net shown in Fig. 4 C has a further region 403"' above the precious metal-containing region 402"'. Fig. 4 D shows a support net that surrounds the precious metal-containing region 402 iv surrounded on three sides (401 iv , 403 iv , 404 iv ) and is completely connected to it. Fig. 4 E also shows such a design, but the knitted area containing precious metals is 402v semicircular in shape. In Fig. 4 F, the support net surrounds the precious metal-containing knitted fabric 402 vi complete, with no connection between the adjacent edges on one side (405 vi ).

[0088] Combinations of the different designs, shown as examples in Figures 3 and 4, are also conceivable. The different sections of the support nets can be single-layered or double-layered, and any combination is also possible.

[0089] Definition of the reference symbols used

[0090] 1 flow reactor

[0091] 2 Catalyst system

[0092] 3 catalyst packs

[0093] 4 catchment nets

[0094] 5 Flow direction

[0095] 6 catalyst networks

[0096] 200 flat knitting machine

[0097] 201 First needle bed

[0098] 202 Second needle bed

[0099] 203 trigger device

[0100] 204 trigger device

[0101] 300. 400 knitted fabrics

[0102] 301. 401 First support net area

[0103] 302. 402 Precious Metals Network

[0104] 303, 304 Additional support net area

[0105] 404 Lateral support net area

[0106] 405 Gap between precious metal net and support net

[0107] 406 connections between precious metal net and support net

Claims

CLAIMS:

1. A method for producing a precious metal net on a flat knitting machine, wherein the flat knitting machine has at least one needle bed and at least one knitted fabric take-off device spaced at a distance D from the at least one needle bed, comprising the steps - Provision of at least one precious metal-free support thread, - Providing at least one wire containing precious metals, - knitting a support net using at least one precious metal-free support thread, - Knitting the precious metal net using the at least one precious metal-containing wire, characterized in that first at least a part of the support net is knitted over at least the length of the distance D.

2. The method according to claim 1, wherein the at least one noble metal-containing wire consists of platinum, a platinum alloy, palladium or a palladium alloy.

3. Method according to one of the preceding claims, wherein the at least one noble metal-containing wire has a diameter of 40 - 150 pm.

4. A method according to any one of the preceding claims, wherein two or more precious metal-containing wires are knitted in parallel.

5. The method according to any one of the preceding claims, wherein the noble metal-free support thread comprises a material selected from the group consisting of polyamides, polyesters, cellulose fibers, cotton, acrylic-styrene polymers, nylon, vinyl polymers, alginate, copper, aluminum, tin alloys and lead alloys.

6. Method according to one of the preceding claims, wherein the flat knitting machine has several fabric take-off devices.

7. The method according to claim 6, wherein firstly at least a part of the support net is knitted over at least the length of the distance up to a knitted fabric take-off device further away than distance D.

8. The method according to claim 7, wherein the knitted fabric take-off device further away than distance D is arranged at a distance D2 and at least a part of the support net is knitted over at least the length D2.

9. Method according to one of the preceding claims, wherein the support network comprises several regions.

10. A method according to any one of the preceding claims, wherein the support mesh surrounds at least 50% of the precious metal mesh.

11. A method according to any one of the preceding claims, wherein the support mesh comprises a region which is knitted after the knitting of the precious metal mesh.

12. A method according to any one of the preceding claims, wherein the method comprises a further step of removing the support net.

13. Method according to one of the preceding claims, wherein the knitted fabric take-off device at the distance D and / or the knitted fabric take-off device at the distance D2 exerts a take-off force on the support net and / or the precious metal net, wherein the exerted take-off force is at least 5 N / m, in particular at least 10 N / m.

14. Method according to one of the preceding claims, wherein the knitted fabric take-off device comprises at least one roller or roll.

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