This invention relates to a catalyst structure for performing a combustion reaction, and to a catalytic process which uses that catalyst structure. The catalyst may be used in a reactor, the reactor comprising combustion channels in thermal contact with heat-removal channels or a heat-removal chamber, so heat is provided by a combustion reaction in the combustion channels, and this heat may be used for performing an endothermic chemical reaction such as steam reforming. A plant and process are described in WO 2005/10251 1 (GTL Microsystems

AG) in which methane is reacted with steam, to generate carbon monoxide and hydrogen in a first catalytic reactor; the resulting gas mixture is then used to perform Fischer-Tropsch synthesis in a second catalytic reactor. The reforming reaction is typically carried out at a temperature of about 800 °C, and the heat required may be provided by catalytic combustion in channels adjacent to those in which reforming is carried out, the combustion channels containing a catalyst which may comprise palladium or palladium/platinum on an alumina support in the form of a thin coating on a metallic substrate. An inflammable gas mixture such as a mixture of methane and air is supplied to the combustion channels. Combustion occurs at the surface of the catalyst without a flame. However, it has been found that the combustion reaction tends to occur most vigorously near the start of the combustion channel, which can lead to an unsuitable temperature distribution along the channel; there may also be problems due to instability of the active catalytic material. According to the present invention there is provided a catalyst structure for a combustion reaction, wherein the catalyst structure comprises a substrate with a first end and a second end, a first portion of the substrate adjacent to the first end being devoid of any added catalytic material; a first coating comprising palladium and/or palladium oxide, covering at least a part of length of the substrate that is contiguous with the first portion; and a second coating comprising platinum, the second coating being spaced away from the first portion, wherein either the first coating or the second coating covers at least part of the other coating, wherein the covering coating is porous and permeable to reactant gases to undergo combustion. The substrate may be a metal substrate. The first coating and the second coating each comprise a ceramic as a catalyst support. The metal substrate may be a structural part of a reactor, such as the wall of a flow channel, or it may be a non- structural substrate, such as a foil, that may also be removable. Alternatively, the metal substrate may form the basis of a catalyst pellet.

Where such a catalyst structure is to be used in a flow channel along which the combustible gas mixture is caused to flow, the first portion of the substrate is at the upstream end of the catalyst structure.

In another aspect the invention provides a catalytic combustion process, for achieving combustion of a fuel gas combined with air, so as to achieve a temperature in excess of 750 °C, wherein combustion takes place over a catalyst structure, wherein the fuel gas combined with air is caused to flow past the catalyst structure, and the catalyst structure comprises a substrate with a first end and a second end, the first end being at the upstream end of the catalyst structure, a first portion of the substrate adjacent to the first end being devoid of any added catalytic material; a first coating comprising palladium and/or palladium oxide, covering at least a part of length of the substrate that is contiguous with the first portion; and a second coating comprising platinum, the second coating being spaced away downstream from the first portion, wherein either the first coating or the second coating covers at least part of the other coating, wherein the covering coating is porous and permeable to reactant gases to undergo combustion.

This catalyst may be used to achieve high temperatures such as 800 °C. It has been found to be surprisingly stable, despite the high temperatures. It will be appreciated that the catalyst of the invention has an upstream portion that comprises palladium, and a downstream portion that comprises platinum. At least part of the downstream catalyst portion includes two different catalyst layers on top of each other, the outer layer being porous and gas permeable. The use of palladium as an initial catalyst is desirable because it has a lower light-off temperature, for example it can initiate combustion of methane at a temperature of about 400 °C; platinum, on the other hand, has a light-off temperature for methane at about 700 °C, so it is suitable for use downstream, where the gases have already been heated by combustion.

During operation, the catalyst typically will become poisoned, or otherwise become deactivated, so its activity will deteriorate. This deactivation will typically affect the upstream portions of the catalyst first. With a catalyst structure provided with palladium at the upstream end and platinum at the downstream end, once the deactivation has reached the end of the palladium catalyst the combustion will cease, because the gases will not reach the light off temperature required by the downstream platinum catalyst. This problem is solved by the present invention.

In another aspect the invention provides a catalytic reactor which incorporates such a catalyst structure in a chamber or channel for a combustion reaction. A suitable catalytic reactor defines a multiplicity of first and second flow channels arranged alternately within a block to ensure thermal contact between the first and second flow channels.

The combustible gas mixture comprises a fuel (such as methane) and a source of oxygen (such as air). As the combustible gas mixture follows along the channel in contact with the catalyst, combustion occurs, and the concentrations of the fuel and oxygen decrease.

The first flow channels and the second flow channels may extend in parallel directions, within a reactor block, and the combustible gas mixture and the endothermic reaction mixture flow in the same direction (co-flow). The flow channels may be of length at least 300 mm, more preferably at least 500 mm, but usually no longer than about 1000 mm. A length of between 500 mm and 900 mm, for example 600 mm or 800 mm may be advantageous. It has been found that co-flow operation gives better temperature control, and less risk of hot-spots.

In one embodiment each first flow channel (the channels for the combustion reaction) and each second flow channel (the channels for the endothermic reaction) contains a removable catalyst structure to catalyse the respective reaction, and each catalyst structure may comprise a metal substrate, and incorporate an appropriate catalytic material. Each such catalyst structure may be shaped so as to subdivide the flow channel into a multiplicity of parallel flow sub-channels. Each catalyst structure may include a ceramic support material on the metal substrate, which provides a support for the catalyst.

The metal substrate provides strength to the catalyst structure and enhances thermal transfer by conduction. The metal substrate may be of a steel alloy that forms an adherent surface coating of aluminium oxide when heated, for example a ferritic steel alloy that incorporates aluminium (eg Fecralloy (TM)). The substrate may be a foil, a wire mesh or a felt sheet, which may be corrugated, dimpled or pleated; the preferred substrate is a thin metal foil for example of thickness less than 150 μηι, which is corrugated to define the longitudinal sub-channels. Depending on the size of the flow channels, the substrate may comprise either a single corrugated foil or an assembly of foils, which may be bonded together.

Where the foils are corrugated, the corrugations may be square, rectangular, trapezoidal or hexagonal in cross-section; or arcuate or sinusoidal; or they may be of zigzag shape, defining triangular corrugations, or a sawtooth shape, for example with sloping portions connected by flat peaks. The corrugations typically run parallel to the length of the foils. In some alternative configurations, the corrugations may be non- parallel or even perpendicular to the length of the foil.

In the case of a substrate which comprises an assembly of foils, if the corrugated foils have corrugations that would enable adjacent foils to intermesh then the corrugated foils may be spaced apart by foils that are flat or substantially flat, to ensure they do not intermesh. Such flat foils are not necessary if the adjacent foils have corrugations that are not parallel, or are otherwise shaped to ensure the foils they do not intermesh. For example if the peaks and troughs are defined by flat portions of the profile, and the flat portions in adjacent foils are aligned with each other, then the foils will not intermesh. Where flat foils are used, they may also be corrugated at a very small amplitude, for example to provide a total height of less than about 0.2 mm, for example 0.1 mm, as this makes them slightly less flexible and so easier to work with during assembly. The direction of the corrugation of the substantially flat foil may be lengthwise along the foil or, alternatively, may be transverse. The shape of the corrugations of the flat foils may be sawtooth or rippled. The foils may be of thickness in the range 20-150 μηι, for example 50 μηι. A thicker foil, for example 100 μηι thick, may provide benefits in enhanced heat transfer. By preventing intermeshing of the corrugated foils, either by the provision of flat foils or by the provision of adjacent foils with non-parallel corrugations, the overall height of the insert is more repeatable and controllable than a stack in which identical corrugated foils are deployed.

To form the catalyst structure the metal substrate would be provided with a catalyst on at least some of the surfaces. For example the substrate may be coated with ceramic support material, for example based on alumina, and this would be impregnated with active catalytic material appropriate for the reaction that is to take place in the corresponding channel. The ceramic coating may be applied by techniques such as dip coating, or spraying, to achieve a ceramic thickness between 10 μηι and 100 μηι, depending on the reaction. Where the channel contains a plurality of foils, the coating may be applied to separate foils before they are stacked together, or even after such foils have been bonded together.

The reactor block may comprise a stack of plates. For example, the first and second flow channels may be defined by grooves in respective plates, the plates being stacked and then bonded together. Alternatively the flow channels may be defined by thin metal sheets that are castellated and stacked alternately with flat sheets; the edges of the flow channels may be defined by sealing strips. To ensure the required good thermal contact both the first and the second gas flow channels may be between 10 mm and 2 mm high (in cross-section); and each channel may be of width between about 3 mm and 25 mm. The stack of plates forming the reactor block is bonded together for example by diffusion bonding, brazing, or hot isostatic pressing. The stack of plates provides the requisite structure to ensure that the reactor can resist the differential pressures and thermal stresses that are applied during operation; the catalyst insert does not have to provide structural support. Consequently the catalyst inserts can be non-structural, as they do not have to hold the walls of the channels apart during operation.

The channels may be square in cross-section, or may be of height either greater than or less than the width. The height refers to the dimension in the direction of the stack, in the direction for heat transfer. For example the plates might be 0.5 m wide and 1 .0 m long, or 0.6 m wide and 0.8 m long; and they may define channels 7 mm high and 6 mm wide, or 3 mm high and 10 mm wide, or 10 mm high and 5 mm wide. These dimensions are merely exemplary, and the skilled person will recognise that many different shapes and sizes are equally suitable. Arranging the first and second flow channels to alternate in the stack helps ensure good heat transfer between fluids in those channels. The catalyst structures are inserted into the channels, and can be removed for replacement. A flame arrestor may be provided at the inlet to each flow channel for combustion to ensure a flame cannot propagate back into the combustible gas mixture being fed to the combustion channel. This may be within an inlet part of each combustion channel, for example in the form of a non-catalytic insert that subdivides a portion of the combustion channel adjacent to the inlet into a multiplicity of narrow flow paths which are no wider than the maximum gap size for preventing flame propagation. For example such a non-catalytic insert may be a longitudinally- corrugated foil or a plurality of longitudinally-corrugated foils in a stack. Alternatively or additionally, where the combustible gas is supplied through a header, then such a flame arrestor may be provided within the header.

The present invention also provides a method of performing an endothermic reaction, such as steam reforming, using such a reactor.

The combustion reaction may be carried out at a space velocity between 20 000 and 70 000 /nr. The space velocity, in this document, means the volume of gas supplied to a reactor per hour, measured at standard temperature and pressure (0°C and 1 atmosphere), as a multiple of the free volume of the corresponding reactor channels.

Although the catalyst structure was described above as comprising a first coating and a second coating, in a modification it may also comprise a third coating comprising platinum, the third coating covering at least a downstream part of the second coating, and the third coating being permeable to reactant gases to undergo combustion. This has the effect of increasing the activity of the platinum catalyst towards the second end of the substrate. The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings, in which:

Figure 1 shows a perspective, partly sectional, view of a catalytic reactor of the invention, the section being on the line 1 -1 of figure 2;

Figure 2 shows a flow diagram for the gas flows in the catalytic reactor of figure 1 ; Figure 3 shows a catalyst structure for use in the catalytic reactor of figure 1 ; Figure 4 shows graphically the variation of catalytic activity with temperature; and

Figure 5 shows a diagrammatic side view of a catalyst structure of figure 4, for use in combustion. The steam reforming reaction of methane is brought about by mixing steam and methane, and contacting the mixture with a suitable catalyst at an elevated temperature so the steam and methane react to form carbon monoxide and hydrogen (which may be referred to as synthesis gas or syngas). The steam reforming reaction is endothermic, and the heat may be provided by catalytic combustion, for example of methane mixed with air. In the present invention the combustion takes place over a combustion catalyst within adjacent flow channels within a reforming reactor. The steam/methane mixture may be preheated, for example to over 600 °C, before being introduced into the reactor. The temperature in the reformer reactor therefore typically increases from about 600 °C at the inlet to about 750-800 °C at the outlet. It will be appreciated that the temperature in the combustion channels is higher than the temperature in the reforming channels; the temperature in the combustion channels may reach as high as 820 ° or 850 °C. At such high temperatures conventional combustion catalysts may deteriorate during use.

Referring now to figure 1 there is shown a reactor block 10 suitable for use as a steam reforming reactor, or for use in a steam reforming reactor. The reactor block 10 defines channels for a catalytic combustion process and channels for

steam/methane reforming. The reactor 10 consists of a stack of plates that are rectangular in plan view, each plate being of corrosion resistant high-temperature alloy such as Inconel 625, Incoloy 800HT or Haynes HR-120. Flat plates 12, typically of thickness in the range 0.5 to 4 mm, in this case 2.0 mm thick, are arranged alternately with castellated plates 14 or 15, so the castellations define channels 16 or 17. The castellated plates 14 and 15 are arranged in the stack alternately. The thickness of the castellated plates 14 and 15, typically in the range between 0.2 and 3.5 mm, is in each case 0.9 mm. The height of the castellations, typically in the range 2-10 mm, is 6 mm in each case, and solid bars 18 of the same thickness are provided along the sides. The wavelengths of the castellations in the castellated plates 14 and 15 may be different from each other, but in the embodiment shown in the figure the wavelengths are the same, so that in each case successive fins or ligaments are 7 mm apart. The castellated plates 14 and 15 may be referred to as fin structures. At each end of the stack is a flat end plate 19, which in this case is also of thickness 2.0 mm.

Although only five channels are shown as being defined by each castellated sheet 14 or 15 in figure 1 , there might be many more, for example over seventy channels in a reactor block 10 of overall width about 500 mm.

The stack of plates would be assembled and bonded together typically by diffusion bonding, brazing, or hot isostatic pressing. Into each of the channels 16 and 17 is then inserted a respective catalyst insert 22 or 24 (only one of each are shown in Figure 1 ), carrying a catalyst for the respective reaction. These inserts 22 and 24 comprise a metal substrate and a ceramic coating acting as a support for the active catalytic material. The metal substrate of each insert 22, 24 comprises a stack of corrugated foils and flat foils occupying the respective flow channel 16 or 17, each foil being of thickness less than 0.2 mm, for example 100 μηι; the stacks shown in figure 1 consist of three corrugated foils separated by two flat foils, bonded together. The channels 16 and 17 in this example are 6 mm high and 7 mm wide, while the catalyst inserts 22 and 24 in this case are 5.4 mm high and 6.6 mm wide, so providing a degree of clearance from the walls of the channels 16 and 17. This is necessary to allow for tolerances in manufacture of the reactor block 10.

Referring now to figure 2 there is shown a side view of the assembled reactor block 10. The gas mixture undergoing combustion enters a header 30 at one end of the reactor block 10 (top, as shown) and after passing through a baffle plate flame arrestor 31 follows the flow channels 17 that extend straight along most of the length of the reactor 10. Towards the other end of the reactor block 10 the flow channels 17 change direction through 90° to connect to a header 32 at the side of the other end of the reactor 10 (bottom right as shown), this flow path being shown as a broken line C. The gas mixture that is to undergo the steam methane reforming reaction enters a header 34 at the side of the one end of the reactor block 10 (top left, as shown), passes through a baffle plate 35 and then changes direction through 90° to flow through the flow channels 16 that extend straight along most of the length of the reactor block 10, to emerge through a header 36 at the other end (bottom, as shown), this flow path being shown as a chain dotted line S. The arrangement is therefore such that the flows are co-current; and is such that each of the flow channels 16 and 17 is straight along most of its length, and communicates with a header 30 or 36 at an end of the reactor block 10, so that the catalyst inserts 22 and 24 can be readily inserted into the straight portions of the flow channels 16 and 17 before the headers 30 or 36 are attached.

In the reactor block 10 the combustion channels 17 are 6 mm high and 7 mm wide; and with this size of channels the corresponding catalyst insert 24 comprises a stack of corrugated foils. As an alternative, it may be satisfactory to provide a catalyst insert whose substrate is a single corrugated foil; this is particularly appropriate in channels that are less than 3 mm high. Referring now to figure 3 there is shown a perspective view of one such catalyst structure 40 which consists of a single corrugated foil 41 , which may form part of one of the catalyst inserts 22 or 24, or which may be used on its own as a catalyst structure. Typically such a catalyst structure 40, or the insert 22 or 24, extends over substantially the entire length of the straight portion of the corresponding flow channel 16 or 17. In an alternative, a number of such catalyst structures 40 or inserts 22 or 24 may be inserted end to end along the flow channel 16 or 17, although this may make it harder subsequently to remove them for replacement. The foil 41 is provided with a ceramic coating 42 on both surfaces which may be of alumina, containing active catalytic material.

Palladium and platinum have been previously suggested as suitable active catalytic materials for a combustion catalyst. Platinum is catalytically active in the metal form, rather than the oxide form, and is stable as the metal; but it has a higher light-off temperature than palladium.

Referring now to figure 4, this shows a schematic graphical representation of the variation with temperature (T) of the catalytic activity (A) of palladium (shown as a continuous line) and of platinum (shown as a broken line) for the combustion of methane. Platinum is active at high temperatures, and not particularly active at temperatures below about 600 °C. However, it tends to undergo deactivation at temperatures above about 795 °C. Palladium is catalytically active at lower temperatures in the oxide form, and significantly less active in the metal form until the temperature rises to above about 750 °C. In a low oxygen partial pressure the transformation from the oxide to the metal may occur, with a consequential reduction in the catalytic activity. In addition, palladium oxide is active at lower temperatures than platinum, for example between 400 °C and 700 °C. As the temperature rises from about 500 °C to about 700 °C the palladium oxide gradually decomposes to palladium metal and oxygen, and the activity A therefore decreases. As the temperature rises to still higher values, the catalytic activity A of the palladium metal then increases.

Surprisingly it has been found that by providing the catalyst structure with a first coating which contains palladium oxide, and then providing a downstream portion of the first coating with a second coating which contains platinum, each coating being porous and permeable to the reactant gases, the stability and activity of the catalyst structure is enhanced. This may be because palladium oxide prevents overheating in the start of the flow channel. Further along the channel the oxygen partial pressure will tend to decrease, so the palladium oxide becomes less active, but the platinum comes into effect.

Where the catalyst substrate is a foil, such catalytic coatings are preferably provided on both surfaces of the foil, so as to maximise the surface area of the catalyst structure that contains active catalytic material. Referring now to figure 5 there is shown a diagrammatic side view of a catalyst structure 40 for use in a flow channel 17 in which combustion will occur, showing the details of the ceramic coating 42 on the two surfaces of the corrugated foil 41 . The foil 41 is of a steel alloy that forms an adherent surface oxide of aluminium oxide when heated, for example a ferritic steel alloy that incorporates aluminium (eg Fecralloy (TM)). The foil 41 has been heat treated, so it has such an oxide surface. Extending from the end of the foil 41 which will be adjacent to the inlet end of the flow channel 17 there is a first portion 45 which is not coated with a ceramic coating, and is not provided with any added active catalytic material. The first portion 45 may extend up to 25% of the length of the foil 41 , but is more typically less than 20%, for example about 10% or 15%.

The remainder of the length of the foil 41 is coated on both surfaces with first coatings 46 of alumina which contain palladium, and are devoid of platinum. By way of example the palladium may be at a concentration of 10% (by weight of the alumina), and the coatings 46 are of thickness between 30 and 50 μηι.

Second coatings 47 of alumina which contain platinum, and are devoid of palladium, are then deposited on a downstream section of the first coatings 46, the second coatings 47 extending to the opposite end of the foil 41 from the first portion 45. By way of example the platinum may be at a concentration of 10% (by weight of the alumina), and the second coatings 47 are of thickness between 30 and 50 μηι. The second coatings 47 may extend over as much as 75% of the first coatings 46, typically at least 50% of the length of the first coatings 46. In some cases the catalyst structure 40 may have only the first coatings 46 and the second coatings 47. However, in this example the catalyst structure 40 also includes third coatings 48, which cover a downstream portion of the second coatings 47. The third coatings 48 may be of the same composition and the same thickness as the second coatings 47, and extend to the opposite end of the foil 41 from the first portion 45. They may extend over as much as 75% of the second coatings 47, more typically between 30% and 60% of the length of the second coatings 47. The third coatings 48 have the effect of increasing the overall loading of platinum towards the downstream end of the catalyst structure 40.

The first coatings 46, the second coatings 47, and the third coatings 48 are all porous and gas permeable, to a sufficient extent that reactive gases can diffuse through the thickness of each coating; and where coatings cover each other the reactant gases can diffuse through to the underlying coating. The alumina support, in each case, is a conglomerate of alumina particles, typically of sizes in the range 1 μηι to 12 μηι, with an overall thickness no more than 80 μηι, along with a binder which may be also of alumina but of much smaller particles sizes, between 0.1 and 0.5 μηι, which bond the larger particles together. The coatings are porous by virtue of gaps between adjacent particles, which form a three-dimensional interlinked network through which the reactive gases can diffuse.

The catalyst structure 40 as shown in figure 5 has been found particularly effective, and surprisingly resistant to deterioration in performance. When the catalyst is new, the combustion of methane is initiated by the palladium in the first coatings 46, so that by the time the gas mixture reaches the second coatings 47 it is sufficiently hot for the platinum in the second coatings to be catalytically active. In experimental tests, with the entire catalyst structure 40 at a temperature below 600 °C, it has been found that the palladium in the first coatings 46, along their whole length, performs as if the outer coatings 47 and 48 were not there. The reactant gases diffuse through the outer coatings 47 and 48, but do not undergo combustion in the outer coatings 47 and 48 because the temperature is too low. If portions of the catalyst structure 40 are at a temperature above 600 °C, then the platinum in the outer coatings 47 and 48 starts to have an effect on the overall performance. Some of the methane undergoes combustion within the outer coatings 47 and 48, but methane continues to diffuse through to the underlying first coatings 46 to undergo combustion in the presence of the palladium. Hence, during operation, the outer coatings 47 and 48 are sufficiently gas-permeable that they do not inhibit the reactant gases from reaching the catalytically-active first coating 46. Depending on the temperature, the partial pressure of the components of the gas mixture will change through the thickness of the outer coatings 47 and 48 as components such as methane are removed by reaction; the oxygen is generally provided in excess.

It is presumed that during use, the catalyst structure 40 is gradually deactivated or poisoned, but this deactivation starts at the upstream end of the catalyst structure 40 and gradually works its way downstream.

Even when the deactivation has affected all the exposed parts of the first coatings, combustion can still occur, because the gas mixture can diffuse through the second coatings 47, and combustion can therefore be initiated at the palladium in the parts of the first coatings 46 that are covered by the second coatings 47.

Consequently the catalyst structure 40 only becomes inactive when the deactivation has suppressed the catalytic activity of the palladium along the entire length of the first coatings 46.

In a modification, the second coatings 47 are deposited first, so they are deposited directly on the metal foil 41 , and the first coatings 46 are then deposited onto the same parts of the metal foil 41 as shown in figure 5. Consequently, apart from the first portion 45, palladium (in the first coatings 46) is provided along the entire remaining length of the metal foil 41 ; and a downstream part of the metal foil 41 is provided with platinum (in the second coatings 47). This differs from the catalyst structure 40 only in that the platinum of the second coatings 47 is covered by the first coatings 46, which are gas-permeable. Optionally, this modified structure may also be provided with third coatings 48 exactly as described above.