GB2188251A - Methanation and steam reforming catalyst - Google Patents

Abstract

A catalyst for a methanation or steam reforming reaction is made by plasma spraying a powder onto a substrate in order to coat the substrate with catalytic material for the reaction. The catalytic material preferably comprises NiO and NiAl2O4 in association and the substrate is preferably metallic, for example an aluminium bearing ferritic alloy.

Description

SPECIFICATION Methanation and Steam Reforming Catalyst This invention relates to the manufacture of catalysts suitable for the production of methane from one or more oxides of carbon, i.e.

methanation, and for steam reforming reactions.

Methanation is a known chemical reaction for converting CO or CO2 to methane by a catalytic reaction with hydrogen under pressure according to the reaction schemes (1) and (2) below: CO+3H2eCH4+H20 (1) CO2+4H2 < CH4+2H2O (2) Methanation has been or is industrially important, for example for removing CO from the gas stream in ammonia synthesis, for purifying hydrogen by removing CO and for converting CO and hydrogen to methane.

Nickel-alumina is described as a catalytic material for methanation reactions in for example US Patent No.4 124629 and US Patent No. 4 451 580 (equivalent to UK Patent No. 2 106 415 B) where the latter describes providing the catalytic material from an aqueous dispersion onto a substrate to give a supported catalyst. Supported catalysts made in this may, however, suffer from loss of adhesion of the catalytic material to the substrate during use.

Nickel-alumina is also a known catalytic material for the steam reforming of methane, i.e. the reaction according to the scheme (3) below: CH4+H2O < CO+3H2 (3) Reaction (3) is thus the reverse of methanation reaction (1) above. Also, hydrocarbons other than methane may be steam reformed to give mixtures of carbon monoxide and hydrogen.

The present invention is concerned with production of supported catalysts suitable for methanation andlorsteam reforming reactions where the catalytic material is tenaciously adhered to the substrate and in one aspect comprises a method of making a catalyst suitable for the production of methane by the reaction of one or more oxides of carbon with hydrogen and/or for the steam reforming of hydrocarbons comprising plasma spraying a powder onto a substrate in order to coat the substrate with a catalytic material for the reaction.

Plasma spraying is a known technique belonging to a group of techniques known generically as 'thermal spraying' which involves spraying a solid material by melting it and projecting the molten material with high kinetic energy onto a target which, in the present invention, is the substrate.

Material suitable for thermal spraying is described in Thin Solid Films, 95(1982) 219--225, a paper by K. T. Scott and J. L. Woodhead entitled "Gel Processed Powders for Plasma Spraying".

In plasma spraying, the material in powdered form is rapidly heated in a DC arc-plasma and then projected at high velocity in the molten or plastic state onto the substrate surface where it quenches and adheres to produce the coating. For example, the heat source in the plasma process may be an electric arc struck between two electrodes, the anode of which serves as a nozzle. By suitable design, the arc is constricted and stabilized within the latter, generating temperatures up to 20,000"K with the ionised gases being ejected at exit velocities of several hundred metres per second.

This arc plasma is a source of high thermal and kinetic energy to entrained material. Most guns use nitrogen, argon or helium as the main plasma gas and operate at power levels between 20 and 80 KW.

The art describes preparing catalysts by plasma spraying in, for example UK Patents Nos. 1 346 943 and 1 413 166; T. J. Koska, J. E. Rutter, R. H. Kirchoff, US Nat. 750718; and W. J. Thomson, J. M. Arndt, K. L. Wright, Prep. Pap.-Am.Chem.Soc., Div. Fuel Chem.25(2), (1979). However, plasma spraying has not been extensively used for making catalysts, probably because coatings produced thereby are dense and of low surface area; moreover, it has not hitherto been used to make methanation or steam reforming catalysts.

Coatings of catalytic material produced by the invention have been found to be tenaciously adherent such that the resulting catalysts may be worked and shaped without the coatings spalling.

Moreover, although the coatings have a very low surface area (e.g. less than 1 m2g~1), the catalysts may surprisingly have sufficient activity for methanation and steam reforming reactions.

The catalytic material may comprise a catalytically active constituent in combination with a ceramic oxide constituent such as known in the art for methanation reactions. Examples of the catalytically active constituent are compounds of Mo, W, Cr, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag and Au.

Ni is preferred because of its known value in the catalysis of methanation reactions and its low current cost. Examples of the ceramic oxide constitutent are HfO2, PbO, ZrO2, CeO2, Ce2O3, TiO2, Nub205, Ta205, SnO2, In205 SiO2, A1203, La203, ThO2, MgO, SrO, P205 and BaO. Awl202 is preferred because of its known value in the catalysis of methanation reactions. It should be noted that Al203, where used, may be in the form of an aluminate in the coating, e.g. as NiAI2O4.

The powder for plasma spraying may be prepared by, for example co-precipitating it from an aqueous solution containing ions corresponding to the catalytically active constituent and to the ceramic oxide constituent using a precipitating agent such as aqueous sodium carbonate solution, followed by calcining the resulting co-precipitate.

The substrate may be made of a ceramic material or of a metal. Examples of suitable ceramic materials are mullite, cordierite, silicon carbide, silicon nitride, zirconia and barium titanate.

Examples of suitable metals are aluminium bearing iron base alloys, aluminium, stainless steels and high Ni content steels. An example of an aluminium bearing iron base alloy has a composition by weight of 10to30%Cr,1to10%Al,OtoO.5%Candthe balance iron. Other examples of such alloys may contain Y, for example 0.1 to 3.0% by weight. Such alloys are available in the UK under the UK Registered Trade Mark "FECRALLOY".

Not all catalytic materials that can be plasma sprayed are necessarily suitable for use with all substrates. For example, a specific catalytic material may not adhere satisfactorily to a specific substrate.

The suitablility of any combination of catalytic material and substrate in the practice of this invention may therefore have to be determined experimentally.

The substrate is preferably in a form such that the pressure drop is low when reactants are passed through the final supported catalyst. Examples of such forms include tubes and honeycomb structures. Thus, the substrate may be a body fabricated, at least in part, of corrugated metal defining channels through the body, for example comprising spirally wound alternate plain and corrugated sheets wound in "Swiss-roll" type fashion. Such a body may be held together by welding or by any suitable externally applied fastening means. A plurality of such bodies may, after treatment by the method of this invention, by arrranged randomly in a container to constitute a catalyst device, for example as described in UK Patent Specification No. 2 077 136A.

Several ways of carrying out the invention will now be particularly described, by way of example only, as follows. Reference will be made to the accompanying drawing wherein.

Figure 1 shows the relationship between methane production and temperature for a catalyst of the invention and for a comparison catalyst.

EXAMPLE 1 Preparation of Catalyst An aqueous solution of Ni(NO3)2 - 6H2O and Al(NO3)3 - 9H2O was prepared and added to an aqueous solution of Na2CO3 - 1OHIO to produce a co-precipitate comprising Ni and Al hydroxides. The co-precipitate was washed repeatedly in distilled water by slurrying and decanting, then dried and calcined at 450 C for 2 hours in air to give a powder of NiO/NiAl2O4 suitable for plasma spraying. The powder was found to have a surface area of 171.9 m2g-'.

The above-prepared powder was plasma sprayed onto 0.002 in FECRALLOY steel sheet that had been pretreated by grit blasting. This gave a coating of catalyst material of about 20 pm thickness and a surface area of 0.3 m2g-1. The phase composition and crystallite sizes in the coating were NiO (66%, 103 ) NIAIPO, (23%, 131 A) and Ni (11%). The plasma spraying was carried out using a Metco 3MBnMB plasma system wherein the plasma gas was nitrogen/hydrogen.

Measurement of Catalytic Activity Samples of the above-prepared supported catalyst were supported in silica tubular reactors (8 mm internal diameter) in a catalyst test rig. The samples were evacuated, flushed with helium and pre-reduced in the flowing hydrogen (80 cm3 min-') at 550"C for 0.5 h. where the rate of temperature increase was 50"C min1. The samples were then cooled in flowing hydrogen to ca 100"C and then allowed to cool naturally to room temperature in static hydrogen.

The reactors and catalyst samples were then flushed with a mixed feed of hydrogen (64 cm3 min-') and carbon monoxide (16 cm3 min-')for O.5 to 1.0 h which was equivalent to a gas space velocity of 2.4x 105 h-1.The following reaction occurred: CO+3H2=CH4+ H20 The effluent gas from the reactor was passed through a trap (room temperature) and was analysed by gas chromatography.

By way of comparison, the powder prepared as above was also tested for its catalytic activity in the same way as described above. The powder (0.0200 g; < 250 pm) was supported on silica frits in the reactors and covered with a silica wool plug.

The results obtained at different temperatures are summarised in Figure 1 wherein triangular data indicators show results obtained with the catalyst of the invention and circular data indicators show results obtained with the powder. The figure shows, not surprisingly, that the activity of the powder is greater at lower temperatures in view of its much greater surface area. However, the respective activities are seen to converge at higher temperatures ( > 450 C). The "bite" temperature of the catalyst, i.e. the temperature at which significant amounts of methane begin to be formed, is seen to be 300340 C.

EXAMPLE 2 Preparation of Catalyst A powder was prepared as described in Example 1 and plasma sprayed onto 0.0012 in FECRALLOY steel sheet that had been pre-treated by grit blasting. This gave a coating of catalyst material of about 50 pm thickness and a surface area of < 0.1 m2g-1.The phase composition and crystallite sizes in the coating were NiO (54%, 93A), NiAl2O4 (23%,105 ) and Ni(23%, 109A).The plasma spraying was carried out using a Plasma Technik F4 burner powered by the Metco system used in Example 1 wherein the plasma gas was argon/ hydrogen.

The supported catalyst so-produced was fabricated into cylindrical catalyst bodies having longitudinal channels therethrough by spirally winding alternate plain and corrugated sheets of the supported catalyst in 'Swiss-roll' type fashion and holding the sheets together by welding. The coating of the catalyst material remained tenaciously bonded to the steel substrate throughout this fabrication treatment. Thirty-five such catalyst bodies (each 17 mm long, 12 mm diameter) were stacked into a q in tubular reactor to form a catalyst bed of 24 in length as described in UK Patent Application Publication No. 2 103 953 A. The resulting bed was found to have satisfactory properties for the catalysis of methanation reactions under the following operating conditions: pressure (25 bar), bed inlet temperature (320"C) and gas hourly space velocity (ca. 10 000 h-1).

Claims (10)

1. A method of making a catalyst suitable for the production of methane by the reaction of one or more oxides of carbon with hydrogen andlor for the steam reforming of hydrocarbons comprising plasma spraying a powder onto a substrate in order to coatthe substrate with a catalytic materialforthe reaction.

2. A method according to claim 1 wherein the catalytic material comprises a catalytically active constituent in combination with a ceramic oxide constituent.

3. A method according to claim 2 wherein the catalytically active constituent is nickel and the ceramic oxide constituent is alumina.

4. A method according to claim 2 or claim 3 wherein the powder is made by coprecipitating it from an aqueous solution containing ions corresponding to the catalytically active constituent and to the ceramic oxide constituent, followed by calcining.

5. A method according to any of the preceding claims wherein the substrate is made of a ceramic material.

6. A method according to any of claims 1 to 4 wherein the substrate is made of metal.

7. A method according to claim 6 wherein the metal is an aluminium bearing iron base alloy.

8. A method according to claim 6 or claim 7 wherein the substrate comprises a body fabricated at least in part of corrugated metal defining channels through the body.

9. A method of making a catalyst substantially as described herein with reference to either of the examples.

10. A catalyst made by a method according to any ofthe preceding claims.