GB2079787A - Alloy with good catalytic activity and method of production thereof - Google Patents

Abstract

A metal alloy having good catalytic activity comprises a metallic refractory base containing carbon and at least one platinum type metal, and its surface has been subjected to an acid intergranular corrosion action causing microfissuration thereof and the appearance of microcrystals of platinoids. It is suitable for use as an exhaust catalyst. A preferred alloy has a composition of: wt% Fe 40-80 Pt group 0.02-2 C 0.02-01 Cr 0-40 Mi 0-40 <IMAGE>

Description

SPECIFICATION Alloy with good catalytic activity and method of production thereof This invention relates to an alloy with a good degree of catalytic activity which can be used in particular but no exclusively for depollution of motor vehicle exhaust gases. and to a method for producing the alloy.

Particularly for facilitating the post-combustion of carbon monoxide and hydrocarbons, the positive action of the noble or platinum type metals of group 8 of the periodic table of the elements and the positive action of mixed oxides based on copper, nickel, cobalt, manganese, and the like which are deposited on refractory carriers such as porous alumina, are known.

These substances generally have satisfactory catalytic activity when they are new but nevertheless suffer from the following disadvantages: 1. A well-known mechanical fragility. This means that if the catalyst is in the form of balls, it suffers from considerable wear in the course of use thereof. Alternatively if it is in the form of a monolithic member, relatively complex mounting means must be attached thereto, in order to avoid rupture thereof by mechanical or thermal shock effects.

2. The activity reduces in a frequently irreversible manner after over-heating or poisoning.

This can occur by accidental over-heating to a temperature exceeding 1 000 C which often happens, due both to the large amount of heat which is given off by the combustion of the unburnt hydrocarbons and the poor thermal conductivity of the refractory carriers, preventing rapid dissipation of the heat produced.

3. The recovery of the platinoids is relatively troublesome and difficult, by virtue of their substantial dilution in a large mass of refractory materials.

The alloy of the invention is in the form of a refractory metal of which it has all the properties: maleability, electrical and thermal conductivity, ductility, elasticity, weldability, etc. It has a high melting temperature, close to 1 450=C. It can be put into the form of thin, expanded and perforated sheets, chips or cuttings, balls, wires, etc.

The alloy can operate continuously up to temperatures of the order of 11 00,C in combustion gases containing 14% of carbon dioxide and water vapour, diluted in nitrogen and containing a carbon monoxide content of from 0 to 4% varying alternatively on respective sides of unitary richness or strength.

Recycling thereof is easy after prolonged use and is effected by simple re-melting under vacuum, re-shaping and re-activation. Recovery of the platinoids that it contains is effected by dissolving the substance in a diluted strong acid and separating the platinoids by settling, centrifuging or filteration.

According to the present invention there is provided an alloy with a good catalytic activity, including a metallic refractory base containing carbon and at least one platinum type metal.

Preferably the refractory metal alloy has a base comprising 40 to 80% of iron, and containing, in the alloyed state, in the mass thereof, small amounts of platinum and/or palladium, ruthenium or rhodium, in amounts of up to 2%, preferably from 0 to 2% in respect of platinum, palladium and ruthenium, and from 0 to 1% in respect of rhodium.

Thus, a composition which is particularly suitable for the production of a tri-functional catalyst contains as follows: Platinum from 0.1 to 0.2% Ruthenium from 0.1 to 0.2% Rhodium from 0.02 to 0.1% Another preferred composition for making a reduction catalyst contains as follows: Platinum from 0.05 to 0.1% Ruthenium from 0.1 to 0.2% Another suitable composition for producing an oxidation catalyst contains as follows: Platinum from 0.1 to 0.2% Palladium from 0.05 to 0.2% A feature of a catalyst alloy according to the invention lies in its good resistance to oxidation at high temperature (above 1000"C) in combustion gases of an internal combustion engine operating at carburation richness values of from 0.80 to 1.10 and preferably from 0.99 to 1.01.This derives from the nature of the base alloy used with the above-mentioned platinum type metal. This is the case for example with an austenitic refractory steel containing 20% of chromium and 20% of nickel, or a ferritic refractory steel containing at least 20% of chromium and 0.05 to 4% of aluminium.

Austenitic refractory steels which can be used may contain different proportions of nickel and chromium, provided that they are between 15% and 50%. Thus, good results are achieved with the following steels, containing: 25% of nickel and 20% of chromium 35% of nickel and 15% of chromium Moreover, the alloy must contain carbon, preferably in a range of from 0.02 to 0.1 % by weight, this permitting it to retain its malleability while imparting thereto the capacity for intergranular corrosion, inducing the production at its surface or in its mass of microfissures at the boundaries of the grains. These provide surface porosity and a characteristic specific surface.

The latter depends both on the size of the grains of the structure, the depth of intergranular corrosion and subsequent chemical treatments (oxalation and oxidation) causing the appearance of micro-crystals of platinoids in the microfissures.

The above-mentioned intergranular corrosion action is produced by first subjecting the alloy to temperatures of from 400"C, preferably from 450"C, to 800"C in tempering treatments. The alloy after treatment in this way is then subjected to intergranular corrosion preferably in an acid aqueous medium. These treatments result in the formation of chromium carbide at the grain boundaries and in the grains, with impoverishment in respect of dhromium in the vicinity.

A preferred process for the preparation of the alloy catalyst according to the invention is as follows: An alloy of chomium, nickel and iron, with a carbon content of from 0.02%, preferably from 0.04%, to 0.1% by weight and a platinum type metal content of from 0.05 to 2%, preferably to 1%, is prepared by fusion under vacuum, in a controlled atmosphere or in air.

The ingot produced is treated by hyper-hardening from a temperature in the region of 1150"C and then laminated or rolled in several successive passes, possibly with hyper-hardening treatments between the rolling passes, to produce a thin metal sheet which is from 0.05 mm to 0.15 mm in thickness, or wires which are from 0.05 to 0.3 mm in diameter. It is also possible after rolling or laminating to form chips, cuttings or turnings of from 0.05 to 0.1 mm in thickness or 'saddles' which are from 0.05 to 0.20 mm in thickness.

The above-indicated substances are again subjected to a hyper-hardening process from a temperature which is in the range of from 1050"C to 1150 C in order to homogenise the alloy and suppress cold hardening, and then tempered at a temperature in the range of from 400 to 800on for a period in the range of from 30 minutes to 10 hours, which constitutes a treatment for making it sensitive to intergranular corrosion.

The surfaces of the product are then cleaned by sanding, anodic dissolution in an acid bath or chemical cleaning. Intergranular corrosion is then effected by soaking in an acid bath permitting rapid microfissuration without substantial loss of material or thickness. The alloy dissolves only at the grain boundaries, so this supposes surface passivation of the remainder of the surface.

This operation is carried out in a chloronitric solution resulting from mixing of concentrated or dilute hydrochloric and nitric acid. Other mixtures are also suitable, for example fluoronitric, sulphonitric, sulphocupric, chlorochromic, sulphochromic or oxalic mixtures. Such interangular corrosion can also be performed by electrolysis, anodic corrosion in a dilute acid medium (hydrochloric, hydrofluoric, sulphuric, oxalic, nitric) or in slaine solution. A controlled acid attack operation is then performed with an acid aqueous bath (preferably 20% HC1 or H2S04 for approximately 2 minutes) to enlarge the fissures and to expose the microcrystals of platinoids in the metal matrix.

The intergranular corrosion can be effected by anodic corrosion in an acid medium which is diluted to 1 % approximately, wherein the alloy forms the anode. The voltage between the electrodes in this case can be of the order of 3 volts and the duration of the treatment for up to 30 minutes.

This is followed by treatment in an aqueous solution of oxalic acid at a concentration in the range of from 5 to 30% and at a temperature in the range of from 60 to 90"C for a period in the range of from 2 to 8 hours.

This results in partial dissolution of the chromium and the iron at the alloy surface and in the microfissures, with the simultaneous deposit of nickel and iron oxalate. After calcination, the resulting catalytic product is a very highly divided phase of oxides which are initimately linked to the platinum type metal microcrystals. The oxalation operation can also be effected by electrolytic deposit of nickel and iron oxalate upon electrolysis of an aqueous solution of oxalic acid, with electrodes of the catalytic alloy. The resulting product is then oxidised either directly when it is used by the gases from the engine, in a furnace which is supplied by the combustion gases from a burner which operates with a mixture which is alternatively weak and rich, between 600 and 700"C, or a temperature in the region of 350"C.

The characteristics of the alloy of the invention can be further improved by containing also from 0 to 3% of at least one activating or stabilising agent such as Ce, Cu, Mo, Ti, La, Ca, Y, Al, W, Mn.

The following examples and tests were carried out using a combustion gas containing: -1.5% of carbon monoxide 0.87% of oxygen -400 ppm of propylene or propane -2000 ppm of nitrogen oxides in a laboratory catalytic reactor formed by a strip of the alloy of the invention which was 0. 1 mm in thickness wound in a spiral configuration with an identical strip which was corrugated, so as to have 400 cells or cavities per cm2. The alloy samples of the invention used in the Examples were made by melting under vacuum, in a controlled atmosphere, or in air, cast, heat hardened at a temperature in the region of 11 50 C.

The hourly spatial speed of gass throughflow was 100 000H-'.

Example 1: A sample of an alloy of the invention containing by weight, 25% of chromium, 20% of nickel, 0.2% of platinum, 0.15% of ruthenium, 0.05% of rhodium, 0.03% of carbon, traces of sulphur and/or phosphorus, balance, including impurities being iron, was laminated or rolled to a thickness of 0.05 mm and hyper-hardened at 1 050 C and then brought to a temperature of 600"C for a period of 8 hours and slowly cooled. It was made up into the form of a catalytic block or a cell-forming monolithic member and immersed for a period of 30 minutes in a mixture of concentrated nitric acid containing 10% of hydrochloric acid to cause intergranular corrosion and microfissuration.

The loss of materials was then in the range of from 0.5 to 3% by weight. The sample was then immersed for two minutes in a 20% hydrochloric acid aqueous solution. The reaction was lively and the sample lost 0.5% of its weight. It was then immersed in a 20% oxalic acid solution which was heated at a temperature of 80"C, for 2 hours. A deposit of powdery nickel and iron oxalate was formed which had good adhesion and which returned the sample to its original weight.

The resulting catalyst was oxidised at a temperature of 350"C to decompose the oxalate formed into powder nickel and iron oxide. This product was tested over the temperature ranges between 200 and 500"C and between 300 and 600"C for a linear rise in temperature in dependence on time. The efficiency R of conversion of each polluting substance was calculated by integration against time and calculated in percent with the results shown in the following Table I, when the catalyst was operated in relation to a gaseous mixture which was alternatively rich and weak, that is to say, which, with respect to stoichiometric equilibrium, had respectively an excess and a deficiency, for example of 2%, of fuel with respect to the oxygen required for burning it.

The same sample of catalyst was tested after being aged successively for 5 hours at 700"C, 800"C, 900"C and 1000"C, and which, in each case, underwent in series tests in the range 200 to 500"C and 300 to 600"C. It was found that the first test after aging gave less good results than the following tests which enjoyed the benefit of activation of the catalyst, which was carried out in the course of the preceding test.

Table I State of the RCO RC3H6 RC3H8 RNO Catalyst Test temperature % % % New 200"-500"C 61 62 67 300"-600"C 90 95 95 New 200"-500"C 64 61 63 300 -600 > C 96 34 92 Aged 5h at 700 C 200"-500"C 24 30 34 300"-600"C 53 63 68 Aged 5h at 700 C 200"-500"C 49 27 52 300"-600"C 82 53 85 Aged 5h at 800 C 200 -500 C 43 21 51 300"-600"C 77 46 83 Aged 5h at 900 C 200 -500 C 8 10 13 300 -600 C 34 40 45 Aged 5h at 900 C 200 -500 C 41 16 45 300 -600 C 72 38 80 Aged 5h at 900 C 200 -500 C 26 32 38 300 -600 C 53 63 71 Aged 5h at 1000 C 200 -500 C 4 5 9 300 -600 C 19 27 30 Product having 200 -500 C 36 14 43 undergone the previous tests 300 -600 C 66 35 77 Product having 200 -500 C 27 33 38 undergone the previous tests 300 -600 C 55 65 71 After being 200 -500 C 31 33 30 maintained for 4 hours at 600 C, weak mixture 300 -600 C 63 66 50 It will be noted that, even after the last operation, the catalyst of the invention still enjoyed a good level of activity.

Example 2 The alloy of the previous Example 1 was subjected to a heat treatment for 4 hours at 650 C and then subjected to intergranular corrosion in a 1 % hydrochloric acid solution, in which it formed the anode.

The voltage between the electrodes was 3V and the treatment period was 10 minutes (preferably two cathodes were used, disposed on respective sides of the flat sample which was to be provided with microfissures).

Under these conditions, the sample lost 1.5% in weight and became fragile (it broke by bending).

After winding of a corrugated or wavy sample and a flat sample to form a cylindrical cellforming block, the product was subjected to an oxalation treatment in a 20% oxalic acid aqueous solution at a temperature of 80 C for a period of 2 hours.

The resulting product had catalytic activity characteristics which were very close to those measured in Example 1 and given in Table 1.

Example 3 The alloy of the previous Examples 1 and 2 was heated for 8 hours at 600 C and then subjected to intergranular corrosion in a 10% oxalic acid solution which also contained 1 % of hydrochloric acid, with cathodic attack, at a voltage of 3V, for 30 minutes, the cathode comprising a metal of the same nature as the anode and being disposed symmetrically on respective sides of the sample in which microfissures were to be formed. After this treatment, it was found that the solution contained, in suspension (it was agitated uniformly and maintained at ambient temperature), iron and nickel oxalate and, in solution, chromium oxalate.

The current was then reversed so that the microfissured sample become the cathode. After electrolytic treatment for 1 hour, the formation of a uniform and homogeneous deposit of nickel and iron oxalate on the sample which then serves as a cathode was observed. The resulting deposit was highly adherent and was olive green in colour.

After being put into the form of a cylindrical cell-forming block, it was found that the product had catalytic activity greater than that produced in Example 1.

Example 4.

The operation was as in Example 2, but with the hydrochloric acid being replaced by ammonium chloride in the course of the anodic intergranular corrosion process. The resulting product had a level of activity which was identical to that measured in the course of Example 1.

There was formation of iron chloride, nickel chloride and chromium chloride in the solution, the iron chloride having a tendency to precipitate in the form of hydroxide in a neutral medium.

After oxalation, the resulting product had a level of activity which was identical to that measured in Example 1.

Claims (16)

1. An alloy with good catalytic activity, including a metallic refractory base containing carbon at at least one platinum type metal.

2. An alloy according to claim 1, in which the metallic refractory base is an alloy including by weight, from 40 to 80% iron, from 0 to 40% chromium and from 0 to 40% nickel.

3. An alloy according to claim 1 or claim 2, in which at least one platinum type metal is present in the range, by weight, of from 0.02 to 2%.

4. An alloy according to claim 3, in which the at least one platinum type metal is one or more of platinum, rhodium ruthenium and palladium.

5. An alloy according to claim 4, in which platinum when present is in the range of from 0.05 to 0.2%, ruthenium when present is in the range of from 0.1 to 0.2%, rhodium when present is in the range of from 0.02 to 0.1%, and palladium when present is in the range of from 0.05 to 0.2%.

6. An alloy according to any one of the preceding claims, including, by weight, from 0.02 to 0.1% of carbon.

7. An alloy according to any one of the preceding claims, including, by weight, from 0 to 3%, of at least one activating or stabilising elemental agent.

8. An alloy according to claim 7, wherein the at least one activating or stabilising agent is one or more of cerium, copper, molybdenum, titanium, lanthanum, calcium, yttrium, aluminium, tungsten or manganese.

9. An alloy according to any one of claims 1 to 7, having a microfissured surface containing microcrystals of platinum type metal resulting from intergranular corrosion.

1 0. An alloy with good catalytic activity substantially as hereinbefore described.

11. An alloy with good catalytic activity substantially as hereinbefore described with reference to any one of Examples 1 to 4.

1 2. A process for the production of an alloy according to any one of the preceding claims, in which a mixture of a metallic refractory base, carbon and at least one platinum type metal, is melted under vacuum in a controlled atmosphere, or in air to an alloy, the resulting alloy is treated by hyper-hardening from a temperature in the region of 1150"C and then converted into thin elements of sheet, wire or chip type, which in turn are successively subjected to hyperhardening from a temperature in the range of from 1050"C to 1150"C and reheating at a temperature in the range of from 400 to 800"C for a period in the range of from 30 minutes to 10 hours, permitting sensitisation to intergranular corrosion; the surface of the alloy is then cleaned and subjected to intergranular corrosion resulting in rapid microfissuration thereof, and then immersed respectively in an acid aqueous bath for a period of about 2 minutes and an oxalic acid solution with a strength in the range of from 5% to 30%, which is heated at a temperature in the range of from 60 to 90"C, for 2 hours approximately, the resulting product then being oxidised at a temperature in the region of 350"C.

1 3. process according to claim 12, in which the alloy contains a metallic refractory base of chromium, nickel and iron, from 0.02 to 0.1% carbon and from 0.05 to 2% platinum type metal, in which intergranular corrosion is carried out in an acid bath, and in which immersion is carried out in an acid aqueous bath of 20% hydrochloric acid.

14. A process according to claim 13, in which intergranular corrosion is carried out in a chloronitric acid bath.

1 5. A process for the production of the alloy according to any one of claims 1 to 11, in which intergranular corrosion is effected by anodic corrosion in an acid medium which is diluted to 1 % approximately, wherein the alloy forms the anode, the voltage between the electrodes being of the order of 3V and the duration of treatment being less than 30 minutes.

16. A process for the production of an alloy according to claim 1 substantially as hereinbefore described.

1 7. An alloy produced by the process according to any one of claims 12 to 1 5.