WO2023138806A1 - Method for producing noble metal meshes on flatbed knitting machines - Google Patents

Abstract

The present invention relates to a method for producing a two-layer noble metal mesh on a flatbed knitting machine which has a first and a second needle bed. The method comprises: providing at least one wire containing noble metal; and knitting the noble metal mesh. The first and second layers of the noble metal mesh are knitted simultaneously on the first and second needle bed, and a supporting mesh is knitted on the first and second needle bed using a supporting thread. An abutting edge of the supporting mesh is connected to the two layers of the noble metal mesh via connecting stitches and knitted via both needle beds in the knitting rows containing the connection.

Description

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

The present invention relates to a method for producing a two-layer precious metal mesh on a flat knitting machine which has a first and a second needle bed. The method includes providing at least one wire containing precious metal and knitting the precious metal mesh. Simultaneously, the first and second layers of precious metal net are knitted on the first and second needle beds and a support net is knitted using a supporting thread on the first and second needle beds. An abutting edge of the support mesh is connected to the two layers of the noble metal mesh via connecting knit stitches and the knit rows containing the connection are knitted over both needle beds.

The heterogeneous noble metal-catalyzed oxidation of ammonia (NH3) to nitric acid (HNO3, Ostwald process) or the production of hydrogen cyanide (HCN, Andrussow process) is of great importance due to the central relevance of the products for the chemical industry. The catalyst systems used for this purpose are usually installed in the form of gas-permeable flat structures in the reaction zone of a flow reactor in a plane perpendicular to the direction of flow of the fresh gas. Collection or catchment systems for recovering evaporated catalytically active components are also often based on such net-like structures. Expediently, a plurality of meshes are usually arranged one behind the other and combined to form a stack of catalyst meshes. The individual nets consist of fine noble metal wires, which mainly contain platinum (Pt), palladium (Pd), rhodium (Rh) or alloys of these metals. Catchment nets in particular can also contain other components such as nickel.

A number of processes are known for producing such knitted goods, for example weaving, warp-knitting and knitting. Weaving and knitting are particularly suitable for the production of rectangular webs of netting with a homogeneous material distribution and structure. They offer little flexibility in terms of shape and material variability of the products to be manufactured. In an additional process step, the nets must be cut from the fabricated webs to the size and shape suitable for the reactors, with waste containing precious metal components being produced. In addition, the machines used require long set-up times and a high use of materials. In comparison, knitting offers greater flexibility: the knitting pattern, wires (both in terms of thickness and material), needles and tension of the wire used can be used to vary both the basis weight and the structure, stretchability and strength of the available knitted fabrics. Another advantage is the significantly shorter set-up times for the machines. In addition, the use of different materials in a knitted fabric is possible. In so-called intarsia knitting, for example, different areas are made of different yarns or wires. In principle, the length of a knitted fabric is not limited, but in the case of flat knitting machines, the maximum width is predetermined by the width of the needle beds.

Flat knitting machines or circular knitting machines can be used for knitting. In circular knitting machines, the needles are arranged in a circular needle bed and the thread or wire is fed accordingly in a circular movement. Such machines primarily allow the production of tubular knitwear. On flat knitting machines, on the other hand, the shape and size of the knitted fabric can be varied. The latter can also have more than one needle bed, between which the yarn or wire is guided back and forth by means of thread guides during production.

The use of two needle beds allows the production of a single-layer or two-layer knitted fabric. In principle, a single-layer knit can be created in two ways: On the one hand, if you only work on one needle bed, i.e. only stitches are formed on one of the needle beds and knitted together. On the other hand, stitches created on the first needle bed can be knitted with stitches created on the second needle bed, i.e. the thread is guided back and forth within a back and/or back course between the two needle beds. Accordingly, a two-layer knitted fabric is created when knitting is carried out in parallel on both needle beds and the stitches produced on the first row of needles are not knitted or only selectively knitted over the edge stitches with the stitches produced on the second needle bed. With this production method, two single-layer knitted fabrics or layers can also be produced in parallel on the first and second needle beds.

The knitting of precious metal nets is described in EP 0544710 A1. Flat knitting machines are used for this purpose in EP 3795728 A1. It is described that separate layers produced on different needle beds can be connected to each other by means of a connecting row of knitting on one side of the two knitting surfaces. This allows the production of knitted fabrics with greater widths than previously possible. EP 0364153 A1 describes the use of an additional noble metal-free thread when knitting a noble metal mesh, which increases process stability. The additional thread is knitted in parallel with the precious metal and can be removed from the homogeneous knitted fabric produced, consisting of precious metal material and thread material, after the knitting process if required. In EP 3779005 A1, this method was further developed in such a way that the additional thread and the precious metal are not knitted in parallel, but simultaneously. Precious metal-free areas can thus be retained in the finished knitted fabric, which has led to a reduction in the use of precious metals.

However, it has been shown that the simultaneous knitting of wires or threads containing precious metals and non-precious metals poses particular challenges to the knitting process and, presumably due to the different material properties, warping of the knitted fabric and thus instabilities in the production process can occur. Such instabilities can manifest themselves, on the one hand, in wire breaks during the knitting process and, on the other hand, in irregularities in terms of structure and mechanical properties, both of which can result in a product of poor quality.

The object of the present invention was therefore to provide a method with high process stability for the production of noble metal nets on flat knitting machines using wires containing noble metal and threads free of noble metal.

The object is achieved by a method for producing a two-layer precious metal mesh on a flat knitting machine, the flat knitting machine having a first and a second needle bed, comprising the steps

- providing at least one wire containing precious metal,

- Provision of at least one precious metal-free supporting thread,

- knitting a first layer of the precious metal mesh on the first needle bed and a second layer of the precious metal mesh on the second needle bed, the first layer having the butt edges S11 and S12 and the second layer having the butt edges S21 and S22, the butt edge S11 at least partially abutting the butt edge S21 and the butt edge S12 at least partially butt the butt edge S22,

- Knitting a supporting net on the first and second needle bed using the at least one precious metal-free supporting thread, the supporting net having a hem SH and an edge KH opposite the hem, the hem SH butts against an abutting edge S11 and S21 of the two layers of the precious metal mesh, with all knitting processes taking place simultaneously, characterized in that the abutting edge SH of the supporting mesh is connected to the respective abutting edges S11 and S21 of the two layers of the noble metal mesh by at least one knitting stitch and that the supporting mesh in the rows of knitting on which the abutting edge SH of the supporting mesh is connected to the respective abutting edges of the two layers of the noble metal mesh S11 and S21 is knitted with connecting stitches from both needle beds.

Within the scope of the invention, it was surprisingly recognized that knitting a support mesh over both needle beds in the area in which it is connected to the layers of the precious metal mesh has a stabilizing effect on the knitting process. In other words, the at least partial embedding of the precious metal mesh in a single-layer support structure reduces process disruptions in the form of tears in the precious metal-containing wire or irregularities in the precious metal mesh.

The method according to the invention relates to knitting on a flat knitting machine with a first and a second needle bed.

Knitting is characterized by the production of the resulting knitted fabric in rows, in which interlocking stitches are formed. During the knitting process on a flat knitting machine with two needle beds, a row of knitting is first formed on at least one of the needle beds. The next knitting row is then formed in the knitting direction, the part of the knitted fabric containing the first knitting row being referred to as “below” in the following. The thread or wire is guided from one side of the needle bed or beds to the other side and back. If reference is made below to a "thread", the respective version should also apply to a corresponding "wire". A knitting row therefore includes a forward row and a back row, which results from the thread guide. The thread can be guided in a knitting row over only one or both needle beds. Typically, for a full row of knitting, the thread may be passed first on the first needle bed (row row), then the thread is passed in the opposite direction on the same needle bed or the second needle bed (row row). However, the thread can also be guided alternately on the two needle beds in a forward and/or reverse row, with connections being produced between stitches on the two needle beds within a row of knitting. The thread guide can on the take place over the entire width of the needle bed or needle beds, but it is also possible to knit over parts of the width of the needle beds. Depending on the width of the needle bed you are knitting, you can determine the width and shape of the knitted fabric.

In the method according to the invention, all knitting processes take place simultaneously. This means that several partial areas of the knitted fabric to be produced are knitted simultaneously on the two needle beds, so that the partial areas are not produced one after the other in several knitting steps.

The method includes providing at least one wire containing noble metal.

A wire containing noble metal is understood to mean a wire which consists of at least one noble metal or contains a significant proportion (>50% by weight) of noble metal. In the context of the present invention, noble metals are understood as meaning a metal selected from the group consisting of platinum metals, gold and silver. Platinum metals are the metals of the so-called platinum group, i.e. platinum (Pt), palladium (Pd), iridium (Ir), rhodium (Rh), osmium (Os) and ruthenium (Ru).

The at least one wire containing noble metal preferably consists of platinum, a platinum alloy, palladium or a palladium alloy. A platinum-based alloy contains more than 50 wt. A palladium-based alloy contains more than 50% by weight of palladium.

Preferably, the at least one noble metal-containing wire consists of an alloy selected from the group consisting of platinum with 1-15% by weight rhodium, platinum with 1-15% by weight rhodium and 0.1-20% by weight palladium, platinum with 1-15% by weight rhodium, 0.1-20% by weight

Palladium and 0.1 - 5% by weight ruthenium, platinum with 1 - 15% by weight rhodium, 0.1 - 40% by weight

Palladium and 0.001 - 5% by weight iridium, platinum with 1 - 15% by weight rhodium, 0.1 - 20% by weight

Palladium and 0.001 - 5% by weight tantalum, platinum with 1 - 15% by weight rhodium, 0.001 - 5% by weight

Iridium and 0.001 - 5% by weight tantalum, palladium with 1 - 25% by weight platinum, palladium with 1 - 20% by weight platinum and 1 - 15% by weight rhodium, palladium with 1 - 25% by weight tungsten, palladium with 1 - 15% by weight nickel, palladium with 0.001 - 5% by weight rhodium, palladium with 1 - 15% by weight copper, palladium with 1-15% by weight copper and 1-15% by weight nickel and palladium with 1-30% by weight cobalt. Wires containing noble metals are preferably used which have a diameter of 40-150 μm, preferably 50-130 μm.

The at least one wire containing noble metal can be in the form of a round wire, that is to say 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.

The at least one noble metal-containing wire can include a plurality of wires, in this case also referred to as filaments, which can preferably be twisted together. The filaments can all consist of the same material, i.e. all contain precious metals, or consist of different materials, which in turn do not all have to contain precious metals.

In many cases it can be advantageous to knit together two or more wires containing precious metals. In other words, several noble-metal-containing wires can be guided together when forming a mesh. When knitting with several wires, in one embodiment the wires containing noble metal consist of the same material; in further embodiments wires containing noble metal consisting of at least two different materials can be used. The multiple wires may have the same or different diameters.

In the method according to the invention, the first layer of the two-layer precious metal mesh is knitted simultaneously on the first needle bed and the second layer of the precious metal mesh is knitted on the second needle bed. In other words, during the knitting process, two parts of the precious metal mesh to be produced are knitted simultaneously on one needle bed each, so the two layers are not produced one after the other.

In the context of the present invention, “two-layer noble metal nets” are understood to mean noble metal nets that have two layers, whereby the layers can be connected to one another via one or more of their respective abutting edges or can also not be connected at their abutting edges. The two layers lie on top of one another, ie they overlap at least partially in the area of their surface area. Precious metal meshes which, as in the case of the present invention, are connected to one another on one side via the abutting edges of the two layers are also referred to as two-layer noble metal meshes. A single-layer precious metal net is available when unfolded along the common abutting edge.

The first and second layers of the two-layer precious metal mesh each have two abutting edges, i.e. each layer has an abutting edge on one side and another abutting edge on the other side. The first layer has the butt edges S11 and S12 and the second layer has the butt edges S21 and S22. The sides of the precious metal layers are to be understood here in relation to the position of the abutting edges on the two needle beds perpendicular to the knitting direction. Depending on the shape of the particular ply, the abutting edges of a ply may not intersect, as in the case of a rectangular ply, or may intersect, such as in the case of a semi-circular ply.

Butt edges can be connected to each other, preferably by at least one connecting knit stitch. However, abutting edges can also abut without being connected. The butt edges are formed during the knitting process in the knitting direction from bottom to top. The two precious metal layers can also have an upper and/or lower edge or abutting edge, the presence of these edges depends on the shape of the respective layer. In addition to the two lateral abutting edges, a rectangular layer has, for example, a lower and an upper edge or abutting edge, while a semi-circular layer has no other edges or abutting edges.

In this context, the lower edges are to be understood as meaning those edges which are formed first in the knitting direction, ie are located at the bottom of the knitted fabric. Accordingly, the upper edges are to be understood as those which are formed later in the direction of knitting.

An abutting edge of the first layer of the noble metal mesh S11 at least partially abuts an abutting edge of the second layer of the noble metal mesh S21. In addition, the respective other abutting edges of the first and second layer of the precious metal mesh, ie abutting edge S12 and abutting edge S22, at least partially abut one another. “At least partially abutting” is to be understood here as meaning that the abutting edges abut over parts of their length in the knitting direction, ie over parts of the knit rows that form them. In other words, the abutting edges can also be at least partially offset from one another.

The first and the second layer of the two-layer precious metal mesh can be connected at least partially at their abutting edges by connecting knit stitches. Below is closed understand that the two layers are connected to one another at their edge meshes via meshes that are formed from the at least one noble metal-containing wire. This connection of the first and second layer of the two-layer precious metal mesh can be done on only one side or on both sides. The connection or connections can be made over the entire length or over parts of the length of the butt edges. It can be advantageous that the connection is made on one side over the entire length of the butt edges.

In further embodiments, the connection can be made on both sides of the first and second layer of the two-layer noble metal mesh at least over parts of their respective abutting edges, whereby these partial connections can be made at the same height, i.e. in the same knitting row or the same knitting rows or at different heights. The partial connections can be made over the same length of the butt edges on both sides, i.e. over the same number of knit rows. However, the partial connections can also be made via a different length of the abutting edges on both sides. In a preferred embodiment, the partial connections are made at least partially over the same courses of knitting. In such an embodiment, knitting or circular knitting is carried out at least partially, in other words a ring-like structure is created via these knitting rows. It has been shown that knitting such an at least partially ring-like structure can have a further positive effect on the stability of the knitting process.

The reference for the height of the knitted fabric is the bottom row of the knitted fabric. Even if the knitting of a layer is started in different knitting rows, the height of the total knitted fabric remains the same, in other words the layers of the knitted fabric at different heights can comprise a different number of knitting rows.

Preferably, the first and the second layer of the two-layer precious metal mesh can be knitted from a wire containing precious metal or wires containing precious metal of the same composition. However, the two layers can also be knitted from wire containing precious metal or wires of different composition containing precious metal.

Wire containing precious metals of the same or different diameters can be used to knit the first and second layers of the two-layer precious metal mesh. It has proven to be favorable that the first and the second layer of the precious metal mesh are knitted from wire containing precious metal or wires containing precious metal of the same diameter. The first and the second layer of the two-layer precious metal net can be knitted in the same or different knitting patterns. Different knitting patterns can arise, for example, from different stitch lengths, floats or catches. In preferred embodiments, the first and second layers of precious metal mesh are knitted in the same knit pattern.

The first and the second layer of the two-layer noble metal net can have the same or different lengths in the knitting direction and/or the same or different widths perpendicular to the knitting direction. The first and the second layer of the precious metal mesh preferably have the same length and the same width.

The first and second layers of the two-layer noble metal mesh can have the same shape or different shapes. Particularly preferably, the first and second layers of the two-layer noble metal mesh have the same shape. It can be advantageous for the first and the second layer of the two-layer noble metal mesh to have the shape of a semicircle. It can be particularly advantageous that these two semicircles have the same width and the same length.

In preferred embodiments, the first and second layers of the two-layer noble metal mesh are congruent, in other words the two layers can have the same length, the same width and the same shape.

The method includes providing at least one non-precious metal supporting thread.

Suitable non-precious metal support threads can be selected through routine experimentation and taking into account the ultimate use of the two-layer precious metal mesh and any additional steps in the manufacturing process. Preferred non-precious metal support threads can be removed after the production of the precious metal mesh, for example by dissolving in acidic or basic media, cutting off, melting or flaming. Such support threads can be natural or synthetic, organic in nature or inorganic in nature. Examples of suitable materials are polyamides, polyesters, cellulosic fibers, cotton, acrylic-styrene polymers, nylon, PVA and other vinyl polymers, alginate, copper, silver, aluminum, or even low melting point metals such as tin and lead alloys. The at least one non-precious metal supporting thread can consist of only one thread-like element; such individual thread-like elements are also referred to as filaments in the case of threads that are free of noble metal. The precious metal-free supporting 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.

In the method according to the invention, a supporting mesh is knitted on the first and second needle bed simultaneously with the first and second layer of the two-layer noble metal mesh using the at least one noble metal-free supporting thread. In other words, the support mesh is knitted at the same time as the two layers of the precious metal mesh to be manufactured, so the two layers and the support mesh are not manufactured one after the other.

“Support mesh” means the areas of the knitted fabric that are at least partially knitted using the non-precious metal support thread. The support mesh can also include other threads or wires.

The knitted fabric includes all the knit courses formed during the process. The knitted fabric comprises at least the first and second layer of the two-layer precious metal mesh and the support mesh. However, the knitted fabric can also include other parts or areas. Preferably, all rows of the knitted fabric are knitted over the same width of the needle beds, in other words, the knitted fabric is preferably rectangular.

The support net has an abutting edge SH and an opposite edge KH. Depending on the shape of the support net, the abutting edge SH and the opposite edge KH may not intersect, as in the case of a rectangular support net, or intersect, such as in the case of a semi-circular support net. The hem SH and the edge KH are formed during the knitting process in the knitting direction from bottom to top.

The abutting edge SH of the supporting mesh abuts an abutting edge S11 and S21 of the first and second layer of the two-layer precious metal mesh. In other words, the edge of the support mesh that abuts the two layers of the two-layer precious metal mesh is referred to as the abutting edge SH. Accordingly, the abutting edge SH encompasses the rows of knitting in which both the layers of the two-layer precious metal mesh and the support mesh are knitted. With the opposite edge KH is accordingly that of the abutting edge SH opposite edge of the supporting net in the same knitting rows, ie the abutting edge SH and the edge KH have the same length in the knitting direction. The edge KH does not touch one of the two layers of the two-layer precious metal mesh.

The shape of the edge and abutting edge of the support net is not further restricted. However, it has proven to be advantageous that the shape of the support network is designed in such a way that the overall shape of the knitted fabric is rectangular. In other words, the support mesh supplements the layers of the noble metal mesh in such a way that the shape of the knitted fabric, comprising the noble metal layer or noble metal layers and support mesh, is rectangular overall. Accordingly, the shape of the support mesh preferably correlates with the shape of the first and second layers of the precious metal mesh. For example, the support mesh preferably has a concave shape if the first and second layers of the precious metal mesh have a convex shape.

In preferred embodiments, the abutting edge SH has the same length as the abutting edges S11 and S12. In other words, the abutting edge SH comprises the same rows of knitting as the abutting edges S11 and S12, so the support mesh abuts the abutting edges of the layers of the two-layer noble metal mesh over the entire length of the layers of the precious metal mesh.

The abutting edge SH of the support mesh and the respective abutting edges S11 and S21 of the first and second layer of the two-layer noble metal mesh are connected to one another via at least one knitting stitch. In other words, the support mesh is at least partially connected to the two layers of the two-layer precious metal mesh during the knitting process. This achieves a stabilization of the knitting process. It can be preferred that the abutting edge SH of the support mesh is connected to the respective abutting edges S11 and S21 of the first and second layer of the two-layer precious metal mesh over its entire length.

The connecting knitted stitch or the connecting knitted stitches can be formed from the at least one noble metal-containing wire or the noble metal-free supporting thread. The connecting knitted stitch or the connecting knitted stitches are preferably formed from the non-precious metal supporting thread.

The support net is knitted with connecting stitches from both needle beds in the row or rows in which the butt edge SH of the support net is connected to the respective butt edges of the two layers of the noble metal net S11 and S21. Under "connecting stitches" are such knitting stitches to understand that between the first and the second needle bed are formed. In other words, the support net is at least partially knitted in one layer. It has been shown that the single-layer design of the support mesh in the area or areas that are connected to the layers of the precious metal mesh leads to an increase in process stability.

The precious metal-free supporting thread can be knitted in parallel with the at least one precious metal-containing wire, i.e. stitches can be formed during knitting that comprise precious metal-containing wire and supporting thread. In these cases, the relevant part of the resulting knitted fabric contains both precious metal and the material of the supporting thread. In other embodiments, the supporting thread can also be used for parts or areas of the knitted fabric that do not contain any wire containing precious metal, i.e. in these cases the resulting knitted fabric contains areas with and areas without wire containing precious metal.

The support network can span multiple areas. For example, the support mesh can also include rows of knitting in which no wire containing precious metal is used, ie in which only the support thread free of precious metal is knitted. In other words, the knitted fabric can include areas that do not contain any wire containing precious metal. In this case, the support network can also include other edges in addition to the abutting edge SH and the edge KH. Such edges can be formed in rows of knitting in which no loops of the layers of precious metal mesh are knitted. In other words, the knitted fabric can also have knit rows that are only formed by areas of the support mesh.

It can be preferred that the support net is knitted in two layers in the areas with knit rows that do not contain any wire containing precious metal. In other words, the support net cannot be knitted with connecting stitches between the first and second needle beds in these knitting courses. These two layers can be connected at the edge stitches via connecting knit stitches, so it can be advantageous for the support net to have a tubular structure in these areas. It can also be preferred that the support mesh is knitted in a single layer in the areas that do not include any wire containing precious metal.

In preferred embodiments, the support mesh comprises areas knitted in one layer and areas knitted in two layers which do not comprise any wire containing precious metals. In preferred embodiments, the support mesh abuts the two layers of the two-layer precious metal mesh only at the abutting edges S11 and S21. It can also be preferred that the support mesh also abuts the two layers of the two-layer precious metal mesh at the respective other edges or abutting edges of the two layers of the two-layer noble metal mesh S12 and S22. The abutting edge of the supporting net SH and the two abutting edges of the two layers of the two-layer precious metal net S12 and S22 can be connected to one another via at least one knitting stitch of the supporting thread. However, such a connection does not have to be made; in such embodiments, these abutting edges are not connected to one another via knit stitches.

It can be preferred that the support mesh also abuts the respective lower and/or upper edges of the two layers of the two-layer precious metal mesh. The support mesh can only abut the lower or upper edges of the two layers of precious metal mesh or the lower and upper edges. In such embodiments, the support mesh surrounds the two layers of the two-layer precious metal mesh on at least two sides.

It can be advantageous for the support mesh to surround the two layers of the two-layer precious metal mesh on more than two sides. In preferred embodiments, the support mesh surrounds the sides of the two layers of the two-layer precious metal mesh by at least 50%. This means that at least 50% of the circumference of the two layers of the two-layer noble metal mesh is surrounded by the supporting mesh, more preferably at least 60%, even more preferably at least 80%. It can be particularly preferred that the support mesh completely surrounds the two layers of the two-layer noble metal mesh.

Preferably, all areas of the support network can be knitted from non-precious metal support threads of the same composition. However, different areas can also be knitted from non-precious metal support threads of different compositions. Different areas of the support mesh can be knitted in the same or different knit patterns.

The method according to the invention can include further steps.

The method may include providing at least one additional wire or filament. Suitable additional wires or threads can be used depending on the intended use and/or function in the manufacturing process or for the subsequent use of the precious metal mesh to be chosen. For example, the additional wire can be a wire made from a non-noble metal that is suitable for stabilizing the noble metal network when used in the reactor, for example it can be a steel or stainless steel wire. In these cases, the method comprises the simultaneous knitting of areas of the knitted fabric using the at least one further wire or thread. In this case, the knitted fabric can include parts or areas that only contain the additional wire or thread or parts or areas that contain both the additional wire or thread and the precious metal-containing wire or parts or areas that contain both the additional wire or thread and the precious metal-free supporting thread or parts or areas that contain both the additional wire or thread and the precious metal-containing wire as well as the precious metal-free supporting thread.

The support mesh can preferably be removed in a further step. Suitable methods are known in principle to those skilled in the art and depend on the type of non-precious metal supporting thread used and the precious metal-containing wire. For example, the support mesh can be decomposed, dissolved, melted, flamed, or cut off.

In a further step, a connecting stitch or several connecting stitches between the first and second layer of the two-layer precious metal mesh can be removed on at least one side, in other words a connection of the two layers made via the edge stitches can be separated on at least one side. As a result, a single-layer precious metal mesh can be produced which is at least partially connected to the two layers on only one side, i.e. via only one abutting edge. In other words, a single-layer noble metal mesh can be obtained in this way.

The invention is explained in more detail below with reference to figures and an example. However, it is not limited to these embodiments.

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

Figure 2 schematically shows the thread routing of an exemplary right row between the two needle beds of a flat knitting machine.

FIG. 3 shows embodiments of two-layer noble metal nets that can be produced using the method according to the invention on flat-bed knitting machines with two needle beds. Figure 1 shows a schematic of 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 a plurality of catalyst nets 6 arranged one behind the other in the direction of flow 5 of the fresh gas. Typically, the catalyst nets 6 are knitted nets which are produced, for example, by knitting together wire with a diameter of 76 μm from various platinum-rhodium alloys. In addition, catchment nets 4 can be provided.

Figure 2 shows schematically the thread routing in an exemplary front row of a knitting row between two needle beds 10 and 11 of a flat knitting machine. Needles that are used during the knitting process are highlighted, the direction of thread guidance is indicated by arrows. Each circle symbolizes a needle. In Figure 2A the thread guide for a thread or wire 12 for a right side row is illustrated in which only stitches are knitted on the first needle bed. In the event that only stitches are knitted on the first needle bed 10 in the back row and all further complete rows of knitting, a single-layer knitted fabric is obtained. If only stitches are knitted on the second needle bed 11 in the back rows, a two-layer fabric is obtained. The knitting pattern used in figure 2A also contains a float: stitches are only formed on every second needle of the needle bed. Figure 2B shows the right side row for a single layer knit where the thread or wire 12 stitches are knitted on the first and second needle beds 10 and 11.

FIG. 3 shows embodiments of two-layer noble metal nets that can be produced using the method according to the invention on flat-bed knitting machines with two needle beds.

Figure 3 A and B show a two-layer precious metal mesh 100 made of two rectangular layers 101 and 102, only the first layer 101 being visible in the front view in Figure 3 A. Figure 3 B shows an open view of the precious metal mesh 100 along the abutting edges 111 of the first layer 101 or the abutting edge 112 of the second layer 102. The abutting edge 112 of the first layer is congruent with the abutting edge 113 of the second layer 102. The "opening" is to be understood virtually at this point and serves to better understand the Invention. The two layers 101 and 102 are connected along their abutting edges 112 and 113 to the abutting edge 120 of a support net 130 (the connection to the abutting edge 113 is no longer shown in the representation in FIG. 3 B for the sake of clarity). The support net 130 is knitted in a single layer from a support thread, preferably a cotton thread, from connecting stitches from both needle beds. The edge 121 of the support mesh 130 opposite the abutting edge 120 does not abut any other sides or edges of the precious metal mesh 20'.

Figure 3C shows a two-layer precious metal mesh 200 surrounded on two sides by a support mesh. Only the frontal view of a first rectangular layer 201, which lies congruently over a further layer, is shown. Both layers are connected to two supporting mesh areas 230 and 231 via their respective two abutting edges (only the abutting edges of the first layer 201 (210 and 211) can be seen in the figure). The first area of the support mesh 230 is made in one layer. The second area 231 can be manufactured in one or two layers.

Figure 3D shows a view analogous to Figure 3C of a two-layer noble metal mesh 300 which is surrounded by a support mesh on two adjacent sides. In the case of a noble metal mesh of this embodiment, for example, the lower area 331 of the support mesh can be started with the knitting, after which the two layers of the noble metal mesh 300 and a further area 330 of the support mesh are produced simultaneously. In the corresponding rows of knitting, stitches are thus formed from the support material and the wire containing precious metal. The two areas of the support network 330 and 331 can be conveniently connected to one another.

Figure 3E shows a view analogous to Figure 3C of a two-layer noble metal mesh 400 surrounded on all four sides by a support mesh. In the illustration, the support network is appropriately divided into areas 430, 431, 432 and 433; these areas are preferably connected to one another. In the embodiment shown in Figure F, a two-layer precious metal mesh 500 is also surrounded on four sides with support mesh areas (530, 531, 532, 533). One of the support mesh areas 532 does not abut the noble metal mesh 500 but is spaced from the noble metal mesh by a gap 534 . In this embodiment too, all areas are produced simultaneously, ie during a single knitting process, in the knitting direction from bottom to top.

Figure 3G shows a view analogous to Figure 3C of a two-layer precious metal mesh 600 made up of two semi-circular layers (only the first layer 601 is visible). The two layers are connected on one side along the respective abutting edge (610 in the case of the first layer 601) to the support net 630 via its abutting edge 620. FIG. 3H also shows a two-layer noble metal mesh 700 formed from two semicircular layers, in which the layers are surrounded by two supporting mesh areas 730 and 731 at both abutting edges.

example and comparative example

In the example according to the invention and in the comparative example, a rectangular layer (100 cm wide, 200 cm long) was knitted on a flat knitting machine with two needle beds on each needle bed with a PtRh5 wire (76 μm diameter), the two layers being connected to one another on one side. A cotton yarn was used as the support thread.

In the comparative example, a single-layer support mesh area was knitted with the cotton yarn on both layers on the non-joined side. A single-layer knitted fabric with two areas (precious metal layer and support mesh) was knitted on each of the two needle beds.

In the example according to the invention, a single-layer support net knitted over both needle beds was knitted on the non-joined side of the two layers. The two precious metal layers were therefore only knitted with stitches on one needle bed each, while the supporting net was knitted with stitches on both needle beds.

The knitted fabric of the example according to the invention had a more uniform structure compared to the knitted fabric of the comparative example. Such irregularities in the knit represent potential mechanical weak points that have a negative effect on the use of the net in the reactor.

Definition of the reference symbols used

1 flow reactor

2 catalyst system

3 Catalyst Pack

4 catchment nets

5 flow direction

6 single layer catalyst mesh

10 First bed of needles

11 Second needle bed

12 thread/wire

100, 200, 300, 400, 500, 600, 700 precious metal mesh , 201, 301, 401, 501, 601, 701 First layer of precious metal mesh

Second layer of precious metal mesh, 210, 310, 410, 510, 610, 710 abutting edges of the first layer of precious metal mesh, 211, 311, 411, 511, 611, 711, 113 abutting edges of the second layer of precious metal mesh, 230, 330, 430, 530, 630 V ated support mesh area

Gap between precious metal mesh and support mesh

Claims

Expectations: 1. A method for producing a two-layer precious metal mesh on a flat knitting machine, the flat knitting machine having a first and a second needle bed, comprising the steps - providing at least one wire containing precious metal, - Provision of at least one precious metal-free supporting thread, - knitting a first layer of the precious metal mesh on the first needle bed and a second layer of the precious metal mesh on the second needle bed, the first layer having the butt edges S11 and S12 and the second layer having the butt edges S21 and S22, the butt edge S11 at least partially abutting the butt edge S21 and the butt edge S12 at least partially butt the butt edge S22, - Knitting of a supporting net on the first and second needle bed using the at least one non-precious metal supporting thread, the supporting net having an abutting edge SH and an edge KH opposite the abutting edge, the abutting edge SH abutting an abutting edge S11 and S21 of the two layers of the precious metal netting, with all knitting processes taking place simultaneously, characterized in that the abutting edge SH of the supporting netting has the respective abutting edges S11 and S21 of the two layers of the precious metal netting s is connected via at least one knitting stitch and that the support net is knitted with connecting stitches from both needle beds in the rows on which the butt edge SH of the support net is connected to the respective butt edges of the two layers of the precious metal net S11 and S21. 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. The method according to any one of the preceding claims, wherein the first and the second layer of the two-layer precious metal mesh are knitted from a precious metal-containing wire or precious metal-containing wires of the same composition. 4. The method according to any one of the preceding claims, wherein the first and the second layer of the noble metal mesh are knitted from a noble metal-containing wire or noble metal-containing wires of the same diameter. 5. The method according to any one of the preceding claims, wherein the first and the second layer of the noble metal net are knitted in the same knitting pattern. 6. The method according to any one of the preceding claims, wherein the first and second layers of the noble metal network are congruent. 7. The method according to any one of the preceding claims, wherein the first and the second layer of the two-layer precious metal mesh are connected at least partially at their abutting edges by at least one connecting knitting stitch. 8. The method according to any one of the preceding claims, wherein the abutting edge SH of the support mesh has the same length as the abutting edges S11 and S12 of the first and second layer of the two-layer precious metal mesh. 9. The method according to any one of the preceding claims, wherein the shape of the support mesh correlates with the shape of the first and second layers of the two-layer precious metal mesh. 10. The method according to any one of the preceding claims, wherein the abutting edge SH of the support mesh is connected over its entire length to the respective abutting edges S11 and S21 of the first and second layer of the two-layer precious metal mesh. 11. The method according to any one of the preceding claims, wherein the support mesh surrounds the sides of the first and second layers of the two-layer precious metal mesh by at least 50%. 12. The method according to the preceding claim, wherein the support network comprises a plurality of regions. 13. The method according to the preceding claim, wherein the method comprises a further step in which the support mesh is removed. 14. The method according to any one of the preceding claims, wherein the method comprises a further step in which a connecting stitch or stitches removed between the first and second layer of precious metal mesh on one side.