Catalytic alloys to promote intermediate temperature methane splitting and methods thereof

Abstract

The present disclosure relates to a reactor for the production of hydrogen and carbon from methane splitting, comprising: a gas inlet for methane flow, a reaction chamber for methane splitting, functioning between 750 °C and 850 °C, a gas outlet for hydrogen flow, a bottom outlet for carbon removing by gravity from the reaction chamber; wherein the reaction module comprises a plurality of gas flow channels; wherein an inner surface of each gas flow channel of the plurality of gas flow channels comprises a plurality of nickel and / or iron alloy catalyst domains for contacting with the methane flow for catalyzing the methane splitting. Furthermore, the present disclosure relates to a process for continuous methane splitting using the reactor. Furthermore, the present disclosure relates a process for the segregation of the plurality of nickel and / or iron alloy catalyst domains.

Description

D E S C R I P T I O NCATALYTIC ALLOYS TO PROMOTE INTERMEDIATE TEMPERATURE METHANE SPLITTING AND METHODS THEREOFTechnical field

[0001] The present disclosure relates to the field of catalytic reactors, specifically to intermediatetemperature catalytic methane splitting reactor operating between 750 °C and 850 °C. The reactor is designed for the continuous production of high-purity hydrogen and graphitic carbon, with no CO2emissions, using a combination of a stainless-steel reaction module, with a plurality of nickel (Ni) and / or iron (Fe) catalyst domains.Background

[0002] Currently, most hydrogen produced worldwide comes from the steam reforming of natural gas and other hydrocarbons or coal gasification. These processes are gradually becoming environmentally and economically less appealing due to extensive CO2emissions. Among many alternatives, methane splitting, also known as methane decomposition, cracking, or pyrolysis, has the potential to uproot the current status quo:CH4(g) 2H2(g) + C(s), AH0= 74.8 kJ mol1

[0003] Catalytic methane splitting can seamlessly substitute reforming reactors in hydrogen production facilities, thanks to the similarities with hydrocarbon reforming processes. The methane splitting reaction does not emit CO2, which makes it more environmentally and economically appealing.

[0004] Methane splitting systems can be operated at various conditions using different catalytic materials. To work below high temperatures, <850 °C, Ni and Fe and alloys thereof are widely accepted as the most efficient catalysts.

[0005] Intermediate temperature catalytic methane splitting has shortcomings that have prevented its industrial use to produce hydrogen, namely: i) mechanical obstruction of the catalyst due to carbon growth over active sites, ii) catalyst destruction due to active metal detaching from the catalyst, and iii) reactor clogging from uncontrolled carbon growth.

[0006] At intermediate temperatures, from 750 °C to 850 °C, the deactivation of catalysts from coke formation over catalyst active sites is mostly associated with Ni catalysts. Ni is the most active metal, but it suffers from exceedingly high turnover frequencies of adsorbates in the active sites, compared to the carbon adatom diffusivity away from those sites. This imbalance makes pure Ni catalysts very unstable at intermediate temperatures, as carbon swiftly accumulates around and on top of dehydrogenation sites.Adding Fe to Ni-based materials is widely accepted to decrease the gas turnover frequency on the surface and to increase carbon diffusivity in the catalyst phase.

[0007] There are two carbon-growth mechanisms on the catalyst particle: i) base-growth mechanism; and ii) tip-growth mechanism.

[0008] The base-growth mechanism is characterized by carbon structures nucleating on the surface domains or particles of the catalyst layer, gradually covering the domain surface, and generally forming filamentous structures that grow away from the metal particle. In this case, the end of the carbon structure, opposite to the static metal particle where it grows, is free from metal.

[0009] The tip-growth mechanism is characterized by carbon nucleation at the metal-support interface and the growth of filamentous carbon structures between metal particles and the support material, originating in the detachment of metal particles from the surface of the support material. In this latter case, the carbon structures entrain metal particles during their growth. Therefore, metal particles are effectively lifted from the initial catalyst, causing a permanent physical disruption of the catalyst structure and gradually washing off the active materials. Consequently, the tip-growth mechanism must be suppressed for stable catalysis.

[0010] Reactor clogging due to carbon build-up is another under-addressed technical challenge, despite its eminent significance in ensuring long-term continuous process operation. Document W02013004398A8 [1] explores the movement of carbon away from the reaction zone, considering a mobile amorphous carbon bed over which the methane-derived carbon grows. In this approach, a carbon stream is continuously fed to the reactor, which is then continuously removed from the reactor, together with the carbon product deposited during the reactor pass. Document WO2016154666A1 [2] considers a similar approach in which iron ore is used as cheap catalytic material continuously fed and removed from the system, along with the carbon allotropes forming over it. These approaches address issues associated with clogging of flow paths with carbon. However, this is achieved using complex reactor designs, including moving parts and the continuous discharge and disposal of catalytic materials, which originate low thermal energy efficiencies and provide low to no hydrogen production within an intermediate temperature window, <850 °C.General Description

[0011] Current background art still lacks an effective intermediate temperature methane splitting system, which economically produces hydrogen and graphitic carbon, while avoiding carbon clogging of flow paths by solid carbon products. New and improved reactor designs should be considered to deal with the clogging of flow paths by carbon while reaching high hydrogen productivity, keeping the catalyst in the reactor, and providing means for removing carbon products online.

[0012] The present disclosure relates to a reactor for the production of hydrogen and carbon from methane splitting, comprising: a gas inlet for methane flow, a reaction chamber for methane splitting, a gas outlet for hydrogen flow, a bottom outlet for carbon removal by gravity from the reaction chamber; wherein the reaction chamber comprises a reaction module; wherein the reaction module comprises a plurality of gas flow channels; wherein an inner surface of each gas flow channel of the plurality of gas flow channels comprises a plurality of nickel and / or iron alloy catalyst domains for contacting with the methane flow for catalyzing the methane splitting. Preferably the catalyst domains comprise a plurality of Ni and / or Fe alloy catalyst domains.

[0013] In an embodiment, the reaction chamber has an operation temperature that ranges from about 750 °C to 850 °C; preferably from about 750 °C to 800 °C.

[0014] In an embodiment, the size of the domains ranges from 10 nm to 50 nm, preferably from 16 nm to 20 nm.

[0015] In an embodiment, the reaction chamber has an operation pressure that ranges from about 1 x 105Pa to 5 x 105Pa (1-5 bar).

[0016] In an embodiment, the reaction module is a cylinder or a plurality of plates, preferably the geometry is selected from a tubular shape, or a cubical shape or a parallelepipedal shape.

[0017] In an embodiment, when the reaction module is a plurality of plates, the gas flow channel results from the distance of each plate relative to the adjacent plate. Preferably, the plates are placed parallel to each other.

[0018] In an embodiment, the plurality of gas flow channels has a cross-section selected from: circular or rectangular.

[0019] In an embodiment, each gas flow channel of the plurality of gas flow channels is a tube.

[0020] In an embodiment, the reaction module comprises at least a heating device for controlling the temperature reaction, preferably an electrical heating resistance or a thermal fluid.

[0021] In an embodiment, the reactor comprises a temperature measuring device arranged inside the support, preferably a thermocouple.

[0022] In an embodiment, the reactor, according to any of the previous claims, wherein the reaction module is made of a metal alloy that contains Fe and Ni, preferably a stainless-steel alloy, or any other material comprising a stainless-steel alloy, preferably a stainless steel alloy coating, or any other metal alloy.

[0023] In an embodiment, the reaction module comprises an electrically insulating material, preferably vermiculite.

[0024] In an embodiment, each plate has a distance from a following plate ranging from 1 mm to 10 mm; preferably from 2 mm to 4 mm.

[0025] In an embodiment, each plate has a length ranging from 1 cm to 20 m; preferably from 5 cm to 10 m; more preferably from 20 cm to 5 m; more preferably from 1 m to 3 m.

[0026] In an embodiment, each tube of the plurality of gas flow channels has an equivalent diameter ranging from 0.25 cm to 2 cm; preferably, from 0.5 cm to 1 cm.

[0027] In an embodiment, the inner surface of each gas flow channel is rugose.

[0028] In an embodiment, the plurality of Ni and / or Fe catalyst domains is alloyed with an element of the list consisting of: molybdenum (Mo), chromium (Cr), tungsten (W), manganese (Mn), vanadium (V), silicon (Si), cobalt (Co), or a combination thereof.

[0029] In an embodiment, each Ni and / or Fe catalyst domain of the plurality of Ni or Fe catalyst domains has a size ranging from 5 nm to 50 nm; preferably from 10 nm to 30 nm; more preferably from 15 nm to 25 nm. Measurement of the mesopore size can be carried out by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), among others.

[0030] In an embodiment, each Ni and / or Fe catalyst domain of the plurality of Ni or Fe catalyst domains has a mass amount of Ni ranging from 1 % to 90 % (wt.eiement / wt.domain); preferably from 20 % to 70 % (wt.element / wt. domain; more preferably from 40 % to 60 % (wt.eiement / wt.domain).

[0031] In an embodiment, each Ni and / or Fe catalyst domain of the plurality of Ni or Fe catalyst domains has a mass amount of Fe ranging from 1 % to 90 % (wt.eiement / wt.domain); preferably from 1 to 50 % (wt.eiement / wt.domain); more preferably from 5 % to 20 % (wt.eiement / wt.domain).

[0032] In an embodiment, each Ni and / or Fe alloy catalyst domain of the plurality of Ni and / or Fe alloy catalyst domains may be a crystal, a segregation, or a particle.

[0033] In the present disclosure, domain is defined as a crystalline region containing a specific metallic composition, segregated from the neighboring structures. The terms "segregation" and "exsolution", as used in the context of the present disclosure, are defined as a process in which a solid solution becomes unstable and separates into two or more distinct phases, e.g., metal alloys, at least part of which emerge on the outer surface of the material. These terms will be used hereafter.

[0034] In an embodiment, the reactor further comprises a second gas inlet for introducing a gas shaving- off flow for removing the carbon produced on the catalyst domains, allowing the shaving-off of the plurality of catalyst domains.

[0035] In an embodiment, the gas shaving-off stream is a hydrogen stream, preferably a pure hydrogen stream.

[0036] In an embodiment, the bottom outlet is cone or funnel shaped.

[0037] In an embodiment, the reactor comprises a container arranged below the bottom of the reactor for carbon collection.

[0038] In an embodiment, the reactor comprises a third gas inlet to introduce a gas methane flow to the container, for protecting the produced carbon.

[0039] It is also disclosed the process for continuous methane splitting, comprising feeding the reactor with a methane rich flow, wherein the reactor comprises: a gas inlet for methane flow, a reaction chamber for methane splitting, a gas outlet for hydrogen flow, a bottom outlet for carbon removal by gravity from the reaction chamber; wherein the reaction module comprises a plurality of gas flow channels; wherein an inner surface of each gas flow channel of the plurality of gas flow channels comprises a plurality of Ni and / or Fe alloy catalyst domains; wherein the operation temperature ranges from about 750 °C to 850 °C; preferably from about 750 °C to 800 °C; adding a shaving-off stage for detaching the carbon from the catalyst domains.

[0040] In an embodiment, the support comprises the plurality of Ni and / or Fe alloy catalyst domains is obtained by following steps: providing the reaction module made of a metal alloy that contains Fe and Ni; preferably a stainless- steel alloy, or any other material comprising a stainless-steel alloy; adding a gas flow of hydrogen and methane with a proportion from about 0:100 to 90:10 of hydroge methane; preferably from about 25:75 to 75:25 of hydroge methane; and more preferably from about 40:60 to 60:40 of hydrogemmethane; wherein the temperature of the gas flow is from about 750 °C to 1000 °C; preferably from 800 °C to 900 °C; wherein the pressure of the gas flow is from about 1 x 105Pa to 100 x 105Pa (1 bar to 100 bar); preferably from 1 x 105Pa to 10 x 105Pa (1 bar to 10 bar).

[0041] The present disclosure relates a catalytic methane splitting reactor comprising a stainless-steel reaction module, comprising a plurality of Ni and / or Fe catalyst domains, which are created with a high- temperature gas flow, rich in hydrogen and / or methane.

[0042] In an embodiment, the catalyst domains are a set of Ni and Fe crystallites, which are segregated from a stainless-steel matrix. The domains are created with a high-temperature gas flow with a hydrogemmethane composition from about 10:90 to 100:0, where the temperature ranges from about750 °C to 1000 °C.

[0043] In an embodiment, the reactor hosts a metal alloy catalyst slab comprising active metals dispersed in a complex alloy matrix organized in grains; active Ni:Fe alloy is confined within a complex matrix of Ni, Fe, Mo, Cr, W, Mn, V, Si, Co, and other metals or metalloids in low concentration. The formed carbon particles should be cyclically shaved-off from the catalyst for a long-term stable methane splitting reaction. This is achieved using hydrogen, which causes the selective carbon methanation at the catalyst / carbon interface, making the carbon particles drop-off. The disclosed catalytic reactor allows stable and continuous methane splitting, producing hydrogen and carbon without CO2 emissions.

[0044] In another embodiment, the catalyst domains are shaved-off using hydrogen, which selectively removes the carbon at the catalyst / carbon interface, enabling sustained operation over long periods.

[0045] The reactor of the present disclosure ensures stable and continuous production of hydrogen and graphitic carbon, without CO2emissions.

[0046] The reactor of the present disclosure exploits the 100 % selectivity of the methane splitting reaction under the specified conditions, producing only hydrogen and carbon. Methane is fed into the reactor, purified hydrogen exits the system, and carbon is periodically removed.

[0047] In another embodiment, the use of Ni and / or Fe domains in the support of the reactor enhances catalyst stability.

[0048] In another embodiment, Ni and / or Fe domains can contain Mo, Cr, W, Mn, V, Si, Co, or a combination thereof, to prevent destructive carbon deposition mechanisms.

[0049] To run the methane splitting reaction at intermediate temperatures, it is necessary to have a stable, active, and low-cost catalyst. Although the most active catalysts for the reaction are ceramic- supported Ni nanoparticles, these deactivate very fast, normally remaining active for less than 100 hours on-stream. The present disclosure concerns active reaction modules for running the methane splitting reaction to produce hydrogen and tubular graphitic carbon in a stable, high-power density, and energyefficient fashion. This disclosure also describes an effective way to shave-off the formed carbon structures to a given particle length / size to prevent carbon clogging and mechanical destruction of the catalytic layer. The disclosed catalytic reaction module should be employed in an appropriate reactor design. The reactor comprehends an upper chamber containing the reaction modules, placed vertically such that the released carbon particles freely fall to the bottom of the reactor. The bottom of the reactor should be cone-shaped, designed to collect the produced carbon particles.

[0050] It is considered that the exsolution of metals, comprising Ni, Fe, and combinations thereof, from a pre-existing, complex multimetallic alloy matrix under methane splitting process conditions, results in the stabilization of alloy crystals and / or domains with nanometer sizes, narrow size distributions and uniform spatial distributions on the outer surface of the alloy matrix. This exsolution process, also referred as segregation process, may be driven by temperature, a change in the composition of the gas surroundingthe starting solid solution, or a combination of both factors. This exsolution process may be driven by exposure of a starting complex, multimetal alloy precursor to medium-temperature methane splitting reaction conditions. Under specific exsolution conditions, said alloy crystals and / or domains become exposed on the outer surface, therefore available for gas-solid catalysis, but additionally remain partially embedded within the matrix from which they exsolved, which adds to greater stabilities against metal crystal growth and detachment. The composition and size of such alloy crystals and / or domains may be adjusted by tuning the composition of the starting precursor complex alloy using conventional metallurgical techniques.

[0051] The size of the Ni-based alloy domains may be from 5 nm to 50 nm, preferably from 10 to 30 nm, and more preferably from 15 to 25 nm. Smaller metal domains display minimum or no methane-splitting catalytic activity, while larger particles display lower catalytic activity too.

[0052] According to a particular embodiment of the disclosure, the complex precursor alloy contains additional metals. These additional metals comprise Mo, Cr, W, Mn, V, Si, Co, and any combination thereof. It is considered that these additional metals play a role in the formation and stabilization of surface-exposed, catalytically active alloy domains. High-melting point metals, e.g., those with melting points equal to or greater than 1800 °C, enhance the strength of the catalytic alloy at elevated temperatures and reduce atomic diffusivity, which contributes to retaining the mechanical integrity of the alloy catalyst under thermal stress and reducing, carburizing reaction environments.

[0053] Additionally, these additional metals contribute to diluting, spacing, and stabilizing catalytic domains and / or crystals on the surface of the metal alloy slab. The methane splitting reaction occurs at the surface of these active metal domains. During this process, hydrogen easily evolves, while carbon predominantly forms fibrous structures. These fibrous structures comprise multi-walled carbon nanotubes, carbon nanofibers, and combinations thereof. These catalytic systems are highly stable and operate for thousands of hours without the catalyst deactivating, if there is room for the carbon particles to grow.

[0054] The reaction module can be integrally made of the catalytic alloy or be produced by depositing the catalytic alloy over a supporting structure. The module should comprehend a bundle of channels or gaps to promote contact between the gas phase and the catalyst and allow the carbon particles to be removed. The reaction module can be made of plates organized parallel to each other, or as metal tubes organized in a module. Since the methane splitting reaction is endothermic, inside the plates, or between the plates, of the plate-and-frame module, or in the shell side of the tubes - in the case of metal tubes, either electrical resistances should be placed or a thermal fluid should be circulated. The preferential implementation uses ceramic plate substrates coated with a thin metal alloy layer or just metal alloy plates, with electrical resistances inserted in between the slabs, positioned vertically. When using alloy plates or foils directly, an electrically insulating material, such as vermiculite (folded up to enclose theelectrical resistances) may be used in between the folded metal foils or plates, which should be closed mechanically or welded at the open edge. In the case of the plate-and-frame configuration, the distance between two consecutive plates should be from 1.5 mm to 10 mm with a more preferential distance from 2 mm to 4 mm; the depth of the plate-and-frame macro-support should be from 5 cm to 10 m, and more preferably from 20 cm to 1 m. In the case of the tube bundle, the supporting tubes should display an equivalent diameter from 0.25 cm to 2 cm, and, more preferably, from 0.5 cm to 1 cm. The equivalent diameter is such that it should encompass at least four times the size of the carbon particles to guarantee that no clogging of each channel, gap, or tube occurs during carbon detachment. The equivalent diameter is calculated as four times the cross-section area of the gas flow channel, divided by the respective channel perimeter.

[0055] According to the disclosure, the reaction module operates at a temperature from about 750 °C to 850 °C, and preferably from about 750 °C to 800 °C. These reaction temperatures are required to favor higher equilibrium conversion. Methane splitting is an endothermal reversible reaction, and, as such, the methane conversion per reactor pass is limited by the thermodynamic equilibrium for the reaction at the reaction conditions. For example, at 1 x 105Pa (1 bar) total pressure and 650 °C, the equilibrium, i.e. maximum methane conversion is ca. 72 %, with an equilibrium hydrogen concentration of 84.4 %; at 1 x 105Pa (1 bar) total pressure and 750 °C the maximum methane conversion is 88 %, with an equilibrium hydrogen concentration of 93.6 %; and at 1 x 105Pa (1 bar) total pressure and 800 °C the maximum methane conversion is 92.5 % with an equilibrium hydrogen concentration of 96.1 %. According to the disclosure, the reactor produces hydrogen-rich streams, which may be easily and cheaply purified downstream, and the unreacted methane returned to the reactor. The reactor allows a quasi-continuous operation of the methane splitting process, with periods for the carbon detachment - shaving-off the carbon particles. This shaving-off process is performed to control the length / size of fibrous carbon materials, since there is no catalytic deactivation, but carbon size has to be controlled to avoid clogging.

[0056] The carbon detachment - shaving off - is performed by supplying back hydrogen, which reacts with the attached carbon at the catalyst surface to selectively produce methane and allow the carbon particle to peel off [3], The time between shaving-off steps is directly related to the length / size of produced carbon allotropes, allowing precise control of the solid carbon dimensions.

[0057] To promote the reverse carbon hydrogenation reaction, assuming a constant reaction temperature, the hydrogen concentration should be increased, the reaction pressure should be increased, or both. At reaction temperatures above 700 °C, the hydrogen concentration under thermodynamic equilibrium is greater than 90 %, defined as the volumetric fraction of hydrogen relative to all gases. Therefore, supplying pure hydrogen may no longer be effective in achieving the interface methanation of the adhering carbon layer at the catalyst surface. When operating at temperatures above 700 °C, toproduce the shaving-off of the carbon particles, the total pressure of the reactor should be increased to about 1 x 105Pa to 5 x 105Pa (2 to 5 bar) while supplying pure hydrogen.Brief Description of the Drawings

[0058] The following figures provide preferred embodiments for illustrating the description and should not be seen as limiting the scope of the disclosure.

[0059] Figure 1: Simplified drawing of a metal alloy plate, before and after activation.(1.1) Original alloy grains;(1.2) Segregated active phase grain (N i, Fe, Co) - partially confined;(1.3) Segregated non-active phase grain - confining;

[0060] Figure 2: Drawing of a plate and frame configuration reactor.(2.1) Metal alloy coated plates (ceramic or metallic);(2.2) Metal alloy coating - active catalyst;(2.3) Gap between plates - reaction zone, for producing hydrogen and growing carbon;(2.4) Embedded electrical resistances and / or thermocouples;

[0061] Figure 3: Drawing of a tubular reactor, with ceramic tubes coated in the bore side with a catalytic metal layer, or the tubes are made of the catalytic metal.(3.1) Outer shell;(3.2) Metal alloy coated tubes (ceramic or metallic);(3.3) Interior of the tube - where metal alloy catalyst is coated and reaction happens;(3.4) Gaps between the outer sides of the tubes - usable for utilities;(3.5) Electrical resistance;(3.6) Thermocouple;

[0062] Figure 4: Simplified schematic of the intermediate temperature methane splitting reactor and a carbon collection module, which is periodically emptied using a valve placed at the bottom of the reactor, for the carbon removal.(4.1) Reaction compartment;(4.2) Reaction module;(4.3) Upper chamber;(4.4) Three-way valve, which should allow to connect the top reaction chamber to i) the product outlet to the hydrogen separation process; and ii) the hydrogen inlet for the shaving-off stage;(4.5) Reaction product flow outlet;(4.6) Inlet of the hydrogen during the shaving-off stage;(4.7) Methane inlet to the methane splitting reactor;(4.8) Methane inlet valve;(4.9) The lower part of the reactor is designed for dropping off the produced carbon particles;(4.10) On-off valve for withdrawing the carbon particles;(4.11) Docked container for collecting the carbon particles withdrawn for the reactor;(4.12) On-off valve for evacuating the container and filling it with methane, previous to dock to the reactor;

[0063] Figure 5: H2 production profile, expressed in grams of H2 produced per gram of catalyst per hour, of a Hastelloy C-276 substrate during a methane splitting reaction.

[0064] Figure 6: X-ray diffractograms of a Hastelloy substrate, before and after exposure to activation conditions.

[0065] Figure 7: H2 production profile, expressed in grams of H2 produced per gram of catalyst per hour, of a stainless steel 316 substrate during a methane splitting reaction.

[0066] Figure 8: Simplified schematic of an example intermediate temperature methane splitting reactor, heated with embedded electrical resistances.(8.1) Reaction compartment;(8.2) Reaction module;(8.3) Ceramic insulator;(8.4) Top ceramic plate;(8.5) Three-way valve, which should allow to connect the top reaction chamber to i) the product outlet to the hydrogen separation process; and ii) the hydrogen inlet for the shaving-off stage;(8.6) Reactor outlet (gases);(8.7) Inlet of the hydrogen during the shaving-off stage;(8.8) Methane inlet to the methane splitting reactor;(8.9) Methane inlet valve;(8.10) The lower part of the reactor is designed for dropping off the produced carbon particles;(8.11) Reflective funnel;(8.12) On-off valve for withdrawing the carbon particles;(8.13) Reactor outlet (solids).Detailed Description

[0067] The present disclosure relates to the field of catalytic reactors, specifically to an intermediatetemperature catalytic methane splitting reactor operating between about 750 °C and 850 °C, preferablybetween about 750 °C and 800 °C. The reactor is designed for the continuous production of high-purity hydrogen and graphitic carbon, with no CO2emissions, using a novel catalyst, which is based on segregated Ni and Fe domains in the reaction module. This is either coated with the stainless-steel alloy or comprises of said stainless steel alloy that enables the formation of the Ni and Fe exsolutions.

[0068] The disclosure describes methane splitting, also known as methane decomposition or cracking, systems for transforming methane into hydrogen and solid carbon. Specifically, the present disclosure presents an intermediate-temperature catalytic methane splitting reactor - from 750 °C to 850 °C, for producing high-purity hydrogen and graphitic carbon. The reactor comprises a compartment that hosts the reaction module containing the Ni and Fe exsolutions, wherein the methane splitting reaction occurs.

[0069] In an embodiment, the catalyst comprises catalytic metals or metal alloys - Ni, Fe, Mo, Cr, W, Mn, V, Si, Co, or a combination thereof, dispersed on the stainless-steel support.

[0070] Stable reactor operation is described, by the implementation of periodic shaved-off, using hydrogen as shaving gas. The disclosed catalytic membrane reactor allows the stable and continuous production of hydrogen and carbon, without CO2emissions.

[0071] The disclosure describes the use of solid catalyst systems in the reactor - activated metal alloys. The catalyst comprehends catalytic metals or metal alloys - Ni, Fe, Co, and any combination thereof - segregated from an initial alloy matrix, based on these active materials and also stable non-catalytic materials, such as Mo, Cr, W, Mg, Va, and Si.The following pertains to the reaction module

[0072] It was observed that it is possible to produce stable catalytic metal alloy surface nanopatches based on metal slabs made of Ni and Fe alloys loaded in a more complex alloy containing at least Mo and Cr. Metal plates of stainless steel 310 and 316, or Hastelloy C-276, are metal alloys with very high catalytic activities. However, to make these metal alloys catalytically active, they need to be pre-treated, such as the Ni:Fe nanopatches surface of the metal. This is achieved by treating the metal alloy at high temperatures under a hydroge methane atmosphere. The temperature and the carbon - formed during the methane splitting reaction - promote the diffusion of Ni and Fe alloy to the metal surface, making it far more active. Processes for increasing the surface morphology of the metal alloy, such as a sand treatment, can also increase the catalytic activity. The temperature for pre-treating the metal plate alloys should be from 750 °C to 1000 °C, and more preferably from 800 °C to 900 °C. The pre-treating gas composition should be from 10:90 to 100:0 of hydrogemmethane, preferably from 25:75 to 75:25, and more preferably from 40:60 to 60:40 and the pressure should be from about 1 x 105Pa to 100 x 105Pa (1 bar to 100 bar), and preferably from about 1 x 105Pa to 10 x 105Pa (1 bar to 10 bar). Measurement of the mesopore size can be carried out by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), among others.

[0073] According to the disclosure, the active phase of the reaction modules should be composed of Ni and Fe. The catalytic activity of Ni is too high, and to improve the morphological stability of the catalyst surface, Ni should be alloyed with Fe. The size of the Ni-based alloy domains should be from 5 nm to 50 nm, preferably from 10 nm to 30 nm, and more preferably from 15 nm to 20 nm. Smaller metal domains display minimum or no methane-splitting catalytic activity, while larger particles display lower catalytic activity too. The atomic concentration of Ni in the metal alloy, which is defined as the mass percentage of Ni in the starting complex alloy percursor, varies from 1 % to 80 % (wt.eiement / wt.domain), preferably from 20 % and 70 % (wt.eiement / wt.domain). The concentration of Fe, in the metal alloy, varies from 1 % to 90 % (wt.eiement / wt.domain), preferably from 5 % to 50 % (wt.eiement / wt.domain). Measurement of the mesopore size can be carried out by transmission electron microscopy (TEM), atomic force microscopy (AFM), X-ray diffraction (XRD), among others.

[0074] The methane splitting reaction occurs at the surface of the confined Ni:Fe alloy domains. The formed hydrogen easily evolves, while carbon forms mostly crystal fibrous structures such as multiwall carbon nanotubes and carbon nanofibers. The formed carbon is mechanically prevented from diffusing through the side interfaces of the active metal crystal and reaching its bottom; thereby, the so-called tip growth is hampered. These catalytic systems are highly stable and operate for thousands of hours without the catalyst deactivating.

[0075] The reaction module serves to promote contact between the gas phase and active metals and should allow: i) the free drop of the formed carbon particles; ii) maximize the catalytically active surface per volume of the reactor; iii) minimize the construction costs; iv) minimize the hydrogen volume needed for the shaving-off the formed carbon particles; and v) keep the reaction temperature at the set-point since the methane splitting reaction is endothermic. A preferential module design, which fulfills these requirements, considers using plates to host the active catalyst - Figure 2 - assembled as a plate and frame module. Alternatively, a bundle of active metal tubes - Figure 3 - assembled as a module may be used. These plates or tubes should be composed of the catalytic metal-based alloy, or be used as a supporting structure to host a layer of catalytic active alloy.

[0076] The geometry of the modules should facilitate the unimpeded growth of the carbon particles. In the preferential case of the plate-and-frame macro-support module - Figure 2 - the distance between two consecutive plates should be from 1.5 mm to 10 mm, with a more preferential distance from 2 mm to 4 mm; the depth of the plate-and-frame macro-support should be from 5 cm to 10 m, and more preferably from 20 cm to 1 m. In the preferential case of the tube bundle, the module comprehends tubular channels that go through the entire module, with an inner equivalent diameter varying from 0.25 cm to 2 cm, and, more preferably, from 0.5 cm to 1 cm; larger diameters are required when the set size for the carbon particles is also large, or the length of the tubular channels are lengthy. The preferential length of the tubular channels varies from 1 cm to 20 m, and, more preferably, from 30 cm to 3 m. Longertubular channels are prone to clogging with the produced carbon particles. In the preferential case of ceramic-supported metal plates, the electrical resistances may be inserted in the ceramic core of the plates - Figure 2; in the preferential case of the metal alloy foil be folded over a layer of a thermally stable and electrical insulated material such as vermiculite, the electrical resistance wires may be inserted between the two metal surfaces electrically insulated using the vermiculite layers. In the preferential case of the catalytic module being made of a bundle of tubes, the electrical resistances should be placed on the shell side. The reaction module can be made of the catalytic alloy or made of another material (ceramic or metallic) coated with a layer of the catalytic alloy.

[0077] Since the methane splitting reaction is endothermic, reaction modules should allow local heating using electric resistances. The heating power of these heating systems can be at least equal to the reaction enthalpy and preferably two to three times above for assisting during the start-up of the reactor to heat it more quickly to the steady-state temperature. The reactor should have precise temperature control, with a maximum deviation of ± 50 °C and, more preferably, a maximum deviation of ± 15 °C. In a preferential implementation, the reaction modules may comprehend special holes for thermal controlling - e.g. inserting a thermocouple, or thermal heating - e.g., inserting a tube for circulating a thermal fluid or electrical heating resistances - Figure 2, feature 2.4 and Figure 3, feature 3.4. Figure 2 illustrates a preferential implementation of thermal control and thermal heating.

[0078] A preferential implementation of the methane-splitting reactor is sketched in Figure 4. Since it works in the temperature range from about 750 °C to 850 °C, preferably from about 750 °C to 800 °C, the reactor should be well thermally insulated and electrically or thermally heated. The methane-splitting reactor comprehends a top chamber - cf. Figure 4, feature 4.1, followed by the reaction module - cf. Figure 4, feature 4.2, and the cone for collecting the carbon particles - cf. Figure 4, feature 4.9. The reaction module or modules should fit into the reactor, leaving a chamber between the upper part of the reaction and the top of the reaction module - cf. Figure 4, feature 4.3; this chamber is named the top reactor chamber. Moreover, the reaction module should fit into the reactor, such as if a gas feed enters through the top of the reactor, it can only reach its bottom if it goes through the tubular channels / gaps of the reaction module. This top reactor chamber is connected to a feed tube, as depicted in Figure 4, feature 4.4. This tube is connected to a three-way valve, which allows either to exit the reaction stream (made of hydrogen and unreacted methane) during the production stage - cf. Figure 4, feature 4.5 - or to feed the hydrogen needed for shaving-off the carbon particles - Figure 4, feature 4.6.

[0079] The methane inlet should be fed to the bottom of the reactor - cf. Figure 4, feature 4.7; the inlet methane flowrate should be such that the hydrogen concentration at the product stream is > 60 % and preferably is >70 %. The reaction total pressure should be kept from about 1 x 105Pa to 5 x 105Pa (1 to 5 bar), during the production stage. When working at temperatures higher than 700 °C, the reaction equilibrium conversion is >82 %, and the hydrogen equilibrium concentration is >90 %, at atmosphericpressure. Increasing the operating pressure increases the kinetics of the methane spitting reaction but overloads the hydrogen separation process - normally a Pressure Swing Adsorption (PSA) unit or a carbon molecular sieve membrane module or modules - and the thermal energy recuperation from the product stream.

[0080] During the shaving-off stage, the reactor's methane inlet valve - Figure 4, feature 4.8 - should be closed, and the top 3-way valve should be turned to hydrogen feeding - Figure 4, feature 4.4. The total pressure of the reactor should increase with the hydrogen feeding. A different implementation considers a 3-way valve for the methane inlet - Figure 4, feature 4.8; during the shaving-off stage, the reactor's methane 3-way valve is turned to exit, through a back pressure regulator, for venting partially the reactor - Figure 4, feature 4.8 -and the top 3-way valve turned to hydrogen feeding - Figure 4, feature 4.4; the pressure of the inlet hydrogen can be increased for a faster shaving-off stage. The inlet hydrogen enters through the top of the reaction module, filling up the reaction tubular channels or gaps and promoting the carbon particles peeling off and recovering the carbon product. The bottom of the reactor should be conical to collect the carbon particles that fall, especially during the shaving-off stage - cf. Figure 4, feature 4.9. During this stage, the reactor may be made to vibrate with an external vibration unit, preferably an axial vibration unit, to promote a better detachment of the carbon particles. After filling up the conical bottom of the reactor, the carbon particles must be removed. This can be done by connecting this conical part through a valve - cf. Figure 4, feature 4.10, to a container - cf. Figure 4, feature 4.11. This container should be evacuated to prevent oxygen or inert gas from entering the reactor, or, preferably, it should be evacuated and filled with methane. In a preferential implementation, the carbon receiver container also has an on / off valve for venting the air and filling it with methane or just leaving it under vacuum - cf. Figure 4, feature 4.12. The dead volumes that may exist when coupling the reactor to the container, must be purged as well.

[0081] After recovering and purifying the produced hydrogen from the product stream, the remaining stream is mostly made of unreacted methane but also of contaminants originally present in the feeding stream. These contaminants must be removed before recycling the unreacted methane to the reactor. This can be easily achieved with absorption, adsorption, and membrane processes depending on the present contaminants.The following pertains to shaving-off the carbon particles

[0082] Shaving-off is required to control the length / size of fibrous carbon materials. Catalytic deactivation does not occur in the disclosed alloys, but carbon size has to be controlled to avoid clogging. During the carbon detachment stage - shaving-off, pure hydrogen should be supplied to the top of the reaction channels / gaps, as indicated in Figure 4, feature 4.3. The direction valve - Figure 4, feature 4.4 - should be turned to link the hydrogen input - Figure 4, feature 4.6. to the channels of the reaction models, closing the production flow - Figure 4, feature 4.5. Methane splitting is an equilibrium-limited reactionthat displays higher equilibrium conversions for higher temperatures and lower pressures. During the shaving-off stage, the reactor's pressure should be set such that the methanation reaction kinetics is fast enough. For example, if the reactor is operated at 800 °C and 1 x 105Pa (1 bar), the reaction equilibrium conversion is ca. 92.5 %, and the hydrogen equilibrium concentration is ca. 96.1 %, while at 800 °C and 5 x lO5Pa (5 bar), the reaction equilibrium conversion is co. 73 %, and the hydrogen equilibrium concentration is co. 84.4 %. If working at 800 °C, the shaving-off pressure should be co. 5 x 105Pa (5 bar) for a fast methanation and then for a fast shaving-off stage.The following pertains to general methods

[0083] The metal alloy layer of Ni:Fe:Cr:Mo:W (atomic composition of 58:17:16.5:4:4.5) was deposited in an alumina plate by plasma-enhanced magnetron sputtering, with a thickness of 15-pm-thick. The alloy displayed a face-centered cubic nanocrystalline structure with a grain size of ca. 35 nm and a uniform composition.

[0084] Methane conversion and hydrogen production rates were determined online, during methane splitting reaction tests utilizing a mass spectrometer Pfeiffer Vacuum ThermoStar GSD 301.Example 1

[0085] A Hastelloy C-276 slab, with a 4 cm2exposed area, was loaded on an appropriate single plate and frame reactor configuration. The reactor was produced from Cr-coated steel310. It was introduced in an oven, which provided electrical heating. The reactor was loaded horizontally, with the exposed active area facing down. This slab was heated up under a reducing atmosphere up to 750 °C at a rate of 1 °C min1. The reducing flow was set to 10 mL min1and composed of H2 diluted in Ar at a 1:1 proportion and 1 x 105Pa (1 bar). After reaching the reaction temperature, the reducing atmosphere was replaced by pure CH4 feed (10 mL min1) to finish the activation process and promote methane splitting. Figure 5 displays the hydrogen production activity profile obtained for this material. It was observed that the area normalized hydrogen production activity increases over time, achieving a maximum of 200 gH2 nr2h1, which cannot be much increased due to equilibrium limitations of the reaction, as the methane conversion - co. 70 % - was already close to the equilibrium conversion. This activity was maintained for 4 hours, without any sign of catalyst deactivation.

[0086] X-ray diffractometry was employed before and after the reaction - Figure 6. Hastelloy C-276 presents a single phase of face-centered crystal structure - Ni-based alloy with dissolved Fe, Cr, Mo, and W atoms. After activation treatment and reaction, this phase is split into several new separate crystal structures. This splitting is responsible for generating the active sites.Example 2

[0087] A steel316 slab, with a 3 cm2exposed geometric area, was sanded to increase the effective available area for reaction. This slab was loaded in an appropriate single plate and frame reactorconfiguration, with the exposed catalytic area facing downwards (horizontal position). The reactor was produced from Cr-coated steel316. It was introduced in an oven that provided electrical heating. The system was heated up under an inert atmosphere up to 750 °C at a rate of 1 °C min1. The inert flow of Ar was set to 10 mL-min1. After reaching the reaction temperature, a reducing atmosphere was introduced to activate the catalyst, for 1 hour. The reducing atmosphere was composed of H2 diluted in Ar at a proportion of 1:1 and 1 bar, at a flow of 10 mL min1. After 1 hour, the reducing atmosphere was replaced by 10 mL min1of pure CH4 to finish the activation process and promote methane splitting. Figure 7 displays the hydrogen production activity profile obtained for this material. As described in the prior example, the area normalized hydrogen production activity increases over time, achieving a maximum of 200 gH2 nr2h1. In this example, the period of activity increasing was much longer, taking 10 hours until the maximum activity was reached - activation continued during the methane splitting reaction. The peak activity was maintained for 10 hours, without any sign of catalyst deactivation. Afterwards, the reactor was stopped, being totally full of carbon.Example 3

[0088] A set of steel slabs, with a 400 cm2exposed area each, were loaded in a plate and frame reactor. The plates were loaded, as depicted in Figure 4, in a vertical position, with gaps between the plates to allow gas flow and to introduce resistances for local heating. The slabs were made from Hastelloy C-276, as used in the first example. The resistances were made from nichrome (Ni:Cr at 1:4 ratio) coils, enclosed in alumina sleeves. The reactor was heated up under a reducing atmosphere up to 700 °C at a rate of 1 °C min1. The reducing atmosphere was used to activate the catalyst. The reducing atmosphere was composed of H2 diluted in Ar at a proportion of 1:1 and 1 x 105Pa (1 bar), at a flow of 100 mL min1. After reaching the reaction temperature, the reducing atmosphere flowed for an additional 1 hour; the reducing atmosphere was then replaced by 100 mL min1of pure CH4 to promote methane splitting. Methane flow was maintained for 10 hours - afterward, hydrogen flowed instead for a period of 30 min. Pure methane and pure hydrogen were flowed alternately for these periods, to promote methane conversion and shaving-off of produced carbon particles, respectively. The reactor was shown to produce hydrogen and carbon at a near-constant activity of 100 gH2 nr2h1, with the produced hydrogen and unreacted methane being vented out of the reactor for purification and recycling. The produced carbon was detached from the slabs during shave-off, with a valve in the lower part of the reactor being opened during this period, to allow carbon removal from the reactor. This reactor operated for 10 cycles, 105 hours, without any signs of deactivation or clogging.Example 4

[0089] A preferential implementation of the catalytic methane splitting reactor is depicted in Figure 8. The splitting reaction occurs in the reaction module - Figure 8, feature 8.2, placed in the upper part of the reactor. Surrounding the reaction module is a ceramic block with three roles: a) thermal insulation; b)preventing the gas circulation between the lower and upper parts of the reactor - Figure 8, feature 8.3; and c) hosting electrical resistances to heat up the reaction module. Above the reaction module, there is another ceramic block - Figure 8, feature 8.4 -for thermal insulation and for hosting electrical resistances, which is needed for keeping the reaction module at the reaction temperature. This block comprehends holes for allowing the gas product evolution. The reaction module is located at the cylindrical volume of the reactor - Figure 8, feature 8.1. Beneath is the conical volume, made with this shape for receiving the carbon particles produced at the reaction module. This reactor zone is coated with a ceramic layer of thermal insulation Figure 8, feature 8.10. The volume under the reaction module should be free to allow the carbon particles to drop freely. To mitigate the thermal losses, a funnel should be placed to reflect back the infrared radiation from the reaction module, as pictured in Figure 8, feature 8.11. This funnel should be coated with a low-emissivity material on the face turned downwards to decrease the heat losses. To extend the service time of the cut valve used to exit the carbon particles, this may be cooled using water - Figure 8, feature 8.12. The envelope of the reactor should be of a stainless stain suitable for high temperatures, which should be <600 °C; stainless steel 310 and 316 are suitable options. External to the reactor should be an insulation coating to minimize thermal losses.

[0090] This project has received funding from the European Commission through the European Union's Horizon 2020 programme - FET Proactive research and innovation action - under grant agreement No. 952219 (112CO2).

[0091] The term "comprising" whenever used in this document is intended to indicate the presence of stated features, integers, steps, components, but not to preclude the presence or addition of one or more other features, integers, steps, components or groups thereof.

[0092] As used herein, the term "about" a number refers to that number plus or minus 10% of that number. The term "about" a range refers to that range minus 10% of its lowest value and plus 10% of its greatest value.

[0093] Where ranges are given, endpoints are included. Furthermore, it is to be understood that unless otherwise indicated or otherwise evident from the context and / or the understanding of one of ordinary skill in the art, values that are expressed as ranges can assume any specific value within the stated ranges in different embodiments of the invention, to the tenth of the unit of the lower limit of the range, unless the context clearly dictates otherwise. It is also to be understood that unless otherwise indicated or otherwise evident from the context and / or the understanding of one of ordinary skill in the art, values expressed as ranges can assume any subrange within the given range, wherein the endpoints of the subrange are expressed to the same degree of accuracy as the tenth of the unit of the lower limit of the range.

[0094] The disclosure should not be seen in any way restricted to the embodiments described and a person with ordinary skill in the art will foresee many possibilities to modifications thereof.

[0095] The above described embodiments are combinable.

[0096] The following claims further set out particular embodiments of the disclosure.References:[1] Method for parallel production of hydrogen and carbon-containing products. WO 2013 / 004398 A8, 2013.[2] Pocock G, Cornejo A, Chua HT. A process for producing hydrogen and graphitic carbon from hydrocarbons. WO 2016 / 154666 Al, 2016.[3] Magalhaes Mendes AM, Mateos Pedrero C, Dias Catarino M. Catalytic methane decomposition and catalyst regeneration, methods and uses thereof. WO 2020 / 12287 Al, 2020.

Claims

C L A I M S1. Reactor for production of hydrogen and carbon from methane splitting, comprising: a gas inlet for methane flow, a reaction chamber for methane splitting, a gas outlet for hydrogen flow, a bottom outlet for carbon removing by gravity from the reaction chamber; wherein the reaction chamber comprises a reaction module; wherein the reaction module comprises a plurality of gas flow channels; wherein an inner surface of each gas flow channel of the plurality of gas flow channels comprises a plurality of nickel and / or iron alloy catalyst domains for contacting with the methane flow for catalyzing the methane splitting.

2. Reactor according to the previous claim, wherein the reaction chamber has an operation temperature that ranges from about 750 °C to 850 °C; preferably from about 750 °C to 800 °C.

3. Reactor according to any of previous claims, wherein the domains have a size ranging from 10 nm to 50 nm, preferably from 16 nm to 20 nm.

4. Reactor according to any of previous claims, wherein the reaction chamber has an operation pressure that ranges from about 1 x 105Pa to 5 x 105Pa.

5. Reactor according to any of previous claims, wherein the reaction module is a cylinder or a plurality of plates.

6. Reactor according to any of the previous claims, wherein the reaction module has a geometry selected from a tubular shape, a cubical shape or a parallelepipedal shape.

7. Reactor according to any of the previous claims, wherein the plurality of gas flow channels has a cross-section geometry selected from circular or rectangular.

8. Reactor according to any of the previous claims, wherein each gas flow channel of the plurality of gas flow channels is a tube.

9. Reactor according to the previous claim, wherein the reaction module comprises at least a heating device for controlling the temperature reaction, preferably an electrical heating resistance or a thermal fluid.

10. Reactor according to any of the previous claims, comprising a temperature measuring device arranged inside the support, preferably a thermocouple.

11. Reactor according to any of the previous claims, wherein the reaction module is made of a metal alloy that contains iron and nickel, preferably a stainless-steel alloy, or any other material comprising a stainless-steel alloy, preferably a stainless-steel alloy coating, or any other metal alloy.

12. Reactor according to any of the previous claims, wherein the reaction module comprises an electrically insulating material, preferably vermiculite.

13. Reactor according to the previous claim 4, wherein each plate has a distance from a following plate ranging from 1.5 mm to 10 mm; preferably from 2 mm to 4 mm.

14. Reactor according to any of the previous claims, wherein each plate has a length ranging from 1 cm to 20 m; preferably from 5 cm to 10 m; more preferably from 20 cm to 5 m; more preferably from 1 m to 3 m.

15. Reactor according to the previous claim 6, wherein each tube of the plurality of gas flow channels has an equivalent diameter ranging from 0.25 cm to 2 cm; preferably, from 0.5 cm to 1 cm.

16. Reactor according to any of the previous claims, wherein the inner surface of each gas flow channel is rugose.

17. Reactor according to the previous claims, wherein the inner surface comprises domains with an element of the list consisting of: molybdenum (Mo), chromium (Cr), tungsten (W), manganese (Mn), vanadium (V), silicon (, Si), cobalt (Co), or a combination thereof.

18. Reactor according to any of the previous claims, wherein each nickel and / or iron catalyst domain of the plurality of nickel or iron catalyst domains has a size ranging from 5 nm to 50 nm; preferably from 10 nm to 30 nm; more preferably from 15 nm to 25 nm.

19. Reactor according to any of the previous claims, wherein each nickel and / or iron catalyst domain of the plurality of nickel or iron catalyst domains has a mass amount of nickel ranging from 1 % to 90 % (wt.element / wt. domain); preferably from 20 % tO 70 % (wt. ele ent / wt. omain)).

20. Reactor according to any of the previous claims, further comprising a second gas inlet for introducing a gas shaving-off flow, for removing the carbon deposition in domains.

21. Reactor according to the previous claim, wherein gas shaving-off stream is a hydrogen stream, preferably a pure hydrogen stream.

22. Reactor according to any of the previous claims, wherein the bottom outlet is cone or funnel shaped.

23. Reactor according to any of the previous claims, comprises a container arranged below a bottom of the reactor, for carbon collection.

24. Reactor according to any of the previous claims, comprising a third gas inlet to introduce a gas methane flow to the container, for protecting the produced carbon.

25. Process for continuous methane splitting using the reactor described in any of the previous claims, comprising: feeding the reactor with a methane rich flow, wherein the reactor comprises a reactor for the production of hydrogen and carbon from methane splitting, comprising: a gas inlet for methane flow, a reaction chamber for methane splitting, a gas outlet for hydrogen flow, a bottom outlet for carbon removal by gravity from the reaction chamber; wherein the reaction chamber comprises a reaction module; wherein the reaction module comprises a plurality of gas flow channels; wherein an inner surface of each gas flow channel of the plurality of gas flow channels comprises a plurality of nickel and / or iron alloy catalyst domains for contacting with the methane flow for catalyzing the methane splitting; wherein the operation temperature ranges from about 750 °C to 850 °C; preferably from about 750 °C to 800 °C; adding a shaving-off stage for shaving the carbon from the catalyst domain.

26. Process according to the previous claim, wherein the reaction module comprising the plurality of nickel and / or iron alloy catalyst domains is obtained by following steps: providing the reaction module made of metal alloy that contains iron and nickel, preferably a stainless-steel alloy, or any other material comprising a stainless steel alloy; adding a gas flow of hydrogen and methane with a proportion from about 10:90 to 100:0 of hydroge methane; preferably from about 25:75 to 75:25 of hydroge methane; and more preferably from about 40:60 to 60:40 of hydrogemmethane; wherein the temperature of the gas flow is from about 750 °C to 1000 °C; preferably from 800 °C to 900 °C;wherein the pressure of the gas flow is from about 1 x 105Pa to 100 x 105Pa; preferably from 1 x 105Pa to 10 x 105Pa.

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