GB2112414A - Palladium-based alloy - Google Patents

Abstract

The alloy contains: ruthenium, 4 to 8 wt.%; at least one rare-earth element 0.3 to 8 wt.% (total); palladium, the balance. The alloy can be useful for purification of hydrogen by diffusion and as a catalyst for chemical processes, as well as processes associated with transfer of hydrogen.

Description

SPECIFICATION Palladium-based alloy The present invention relates to palladium-based alloys. In this specification, all percentages are by weight.

Palladium-based alloys are used as membranes for purification of hydrogen by diffusion, as catalysts in the chemical industry, for processes associated with hydrogen transfer, as well as in medicine, electrical engineering, and instrument manufacture as electrodes, thermo-couples, electrical contacts, potentiometric materials, and soldering compositions, for example.

To obtain super-pure hydrogen by way of purification by diffusion of commercial hydrogen or isolation thereof from hydrogen-containing mixtures, use is made of hydrogen-permeable membranes made of palladium and palladium-based alloys. At the present time use if made of alloys containing up to 40% of silver (cf. German Patent No. 2,305,595; U.S. Patent No. 3,247,648: A. A. Rodina et al., Journal of Physical Chemistry, 1979, No. 5, p. 1350 in Russian). A maximum hydrogen-permeability is inherent in alloys containing 7.6-9.6% of silver. Also employed are alloys containing 1 8 to 25% of silver. To obtain a higher stability of the latter alloys, other elements such as indium are incorporated (cf.

A. A. Rodina et al.,JournalofphysicalChemistry, 1980, No. 6, p. 1551). However, silver-containing alloys are less active in chemical processes associated with hydrogenation and dehydrogenation.

Other palladium-based alloys employed for hydrogen purification and containing gold, copper, boron, nickel, rhodium, cerium, yttrium, and platinum either possess insufficient mechanical strength (e.g alloys with gold and copper additions), or get broken under the effect of hydrogen and other aggressive medium (cf. A. G. Knapton, Platinum Metals Review, 1 977, V21 (2), p. 44).

Incorporation into palladium or additions of gold, copper, and likewise silver lowers catalytic activity of palladium in dehydrogenation reactions. Incorporation of platinum additions make palladium more durable, increases its catalytic activity, but considerably reduces the values of hydrogenpermeability of the alloys. Furthermore, in the majority of these alloys two hydride phases a an /3 are formed which when present together lower the selectivity of catalysts made from palladium alloys and serve as the cause of destruction under the effect of hydrogen.

U.S. Patent No. 3,238,700 discloses a membrane for purification of hydrogen which is made of a palladium alloy containing 4.5% of ruthenium. Hydrogen-permeability of this alloy exceeds that of pure palladium; however, this alloy has but a short service life when operated in the atmosphere of hydrogen and hydrocarbon upon multiple cycles of heating and cooling and has a low selectivity in carrying out catalytic processes, which is likely to be associated with the presence of two hydride phases ( and ).

During operation of the membrane there occurs the transition a > ,B which results in breaking of the membrane. Furthermore, the presence of two hydride phases substantially lowers the selectivity of catalytic processes of hydrogenation and dehydrogenation, owing to different mechanisms and kinetics of reactions occurring at active centres of these phases.

What is desired is an alloy based on palladium which would not break in an atmosphere of hydrogen and would make it possible to increase selectivity of catalytic processes.

The present invention provides a palladium-based alloy which contains ruthenium and a rare-eartY element and has the following composition: ruthenium 4 to 8% by mass rare-earth element 0.3 to 8% by mass palladium the balance.

The rare-earth elements are scandium, yttrium, and the lanthanides.

An alloy of the above-specified composition is stable in operation; it withstands 4 times as many heating-cooling cycles as the prior art alloy containing 5% of ruthenium, balance palladium.

The range of content of ruthenium is limited by values of hydrogen-permeability and durability of a particular alloy. Hydrogen-permeabiiity of alloys at a minimum (4%) and maximum (8%) content of ruthenium does not substantially differ from that of an alloy having an optimal composition with 6% of ruthenium (cf. V. M. Gryaznov et al., DAN SSSR, 1973, vol. 211, No. 3, p. 624). If the content of ruthenium were more than 8%, hydrogen-permeability of the alloys would become substantially reduced. Alloys containing more than 10% of ruthenium would already be binary ones and would be non-processable.

Palladium-based alloys containing less than 4% of ruthenium, though possessing sufficient hydrogen-permeability, would have an insufficient durability in operation in an atmosphere of hydrogen.

The lower limit of the (total) content of the rare-earth element(s) (REE) is defined by the presence of hydride phases. If the REE content were below 0.3% a considerable amount of the undesirable hydride ,B-phase would be present. If the REE content were above 8% the majority of the alloys would be binary and non-processable.

Consequently, violation of the above-specified range of contents of ruthenium and REE would not provide the expected results.

Various modifications of palladium alloys according to the present invention are possible.

For alloys containing lanthanum and yttrium the following compositions are recommended as the most efficient for the purpose of high hydrogen-permeability, heat-resistance, and selectivity in catalytic processes: ruthenium 4 to 8% lanthanum 0.3 to 2% palladium the balance; and ruthenium 4 to 8% yttrium 0.3 to 8% palladium the balance.

Alloys of palladium with ruthenium and REE may be melted in an electric-arc vacuum furnace with a non-consumable tungsten electrode on a copper water-cooled hearth in an atmosphere of purified helium under a superatmospheric pressure of from 600 to 800 mm Hg. The preliminary vacuum in the furnace chamber is not less than 3.10-4 mm Hg. The alloying additions of rare-earth metals are introduced into the alloys through an intermediate ligature. The composition of the alloys is controlled by chemical analysis.

All the smelted alloys have a fine-grain structure and are within the range of a solid solution.

Membrane-foils with a thickness of 100 jum can be produced from the alloys by the method of cold deformation with intermediate vacuum annealing.

For measurement of hydrogen-permeability, the thus-made foils are fixed along the periphery in a reactor cell into which hydrogen is introduced from one side. The amount of hydrogen that has passed through the foil is determined chromatographically. During operation the foils are subjected to cyclic heating at a temperature within the range of from 50 to 4000C.

The accompanying drawing is a graph of the relationship between hydrogen-permeability (QH (ml/s.cm2 x 102) and temperature (OC) of some palladium alloys, namely: a well-known alloy of palladium and 6% ruthenium (curve 1), and palladium-ruthenium alloys with additions of lanthanum: Pd -- 6% Ru - 0.3% (curve 2); Pd - 6% Ru - 0.6% La (curve 3); and Pd - 6% Ru - 1% La (curve 4).

The arrows on the curves show the direction of temperature variation.

It is seen from the graph that, within the investigated temperature range, a very insignificant hysteresis is observed in curve 2, whereas it is absent in curves 3 and 4. The absence of hysteresis in the hydrogen-permeability curves for the alloys Pd -6% Ru - 0.6% La and Pd t 6% Ru - 1% La points to the absence of the a ss p transition in these alloys in operation of these alloys in an atmosphere of hydrogen or hydrogen-containing media.

The presence, in the alloys, of only one hydride a-phase substantially increases heat-resistance and extends service life of membranes made of these alloys.

Similar values of hydrogen-permeability and the absence of hysteresis in the curves of relationship between hydrogen-permeability and temperature are exhibited by alloys of palladium-ruthenium with additions of yttrium, cerium, neodymium, and samarium.

In carrying-out catalytic processes (hydrogenation of pentadiene and nitrobenzene) the starting feed is supplied into the reactor from one side of the foil membrane under a vapour pressure of 10 mm Hf, and from the other side hydrogen is supplied under a pressure of 1 atm. The rate of supply of vapours of the starting compounds is 75 ml/min.

EXAMPLE 1 Alloys of palladium with ruthenium and additions of lanthanum have been produced; their compositions and strength characteristics are shown in Table 1.

TABLE 1 Hardness Tensile Alloy Hv kg/mm2 strength kg/mm2 Pa-6%Ru 116 60 Pa-6%Ru-0.3%La 126 75 Pa-6%Ru-0.6%La 140 90 Pa-6%Ru-1%La 146 95 It is seen from Table 1 that additions of lanthanum increase the mechanical strength of the known Pa~6% Ru alloy.

Higher selectivity of the catalytic process on the membrane made from an alloy with a lanthanum addition is shown in pentadiene hydrogenation. The process parameters are shown in Table 2.

TABLE 2 Reaction products, % Temperature Alloy oc pentane pentene-1 pentene-2 pentadiene Pa-6%Ru 50 53 2 11 balance 120 23 2.5 74.5 none Pa-6%Ru-1%La 50 0.8 11.5 38 balance 120 none 28 72 none In the reaction of hydrogenation of pentadiene at the temperature of 120 C the membrane of a palladium alloy with 6% of ruthenium withstood 150 thermal cycles of heating and cooling; a membrane of a palladium alloy with 6% ruthenium and 0.3% of lanthanum withstood 500 cycles, while membranes made of alloys of palladium with 6% of ruthenium and 0.6% of lanthanum and palladium with 6% of ruthenium and 1% of lanthanum remained unbroken during the entire operation period (800 cycles).

EXAMPLE 2 Alloys of palladium with ruthenium and additions of yttrium have been produced; their compositions and strength characteristics are shown in Table 3.

TABLE 3 Hardness Tensile Alloy Hv kg/mm2 strength kg/mm2 Pa-6%Ru 116 60 Pa-6%#u-0.3%Y 131 78 Pa-6%Ru-1%Y 148 98 Pa-6%Ru-2%Y 154 118 Pa-4%Ru-8%Y 134 102 It is seen that the addition of yttrium increases the mechanical strength of the known Pa-Ru alloy.

Improvement of the selectivity of a catalystic process on membranes made from alloys with additions of yttrium is shown in hydrogenation of nitrobenzene. The process characteristics are shown in Table 4.

TABLE 4 Reaction products % Temperature nitroso- cyclo- nitro Alloy 0C aniline benzene hexylamine benzene P'a -6% Ru 170 66 2 12 20 250 72 traces 28 none Pa-6%Ru-0.3%Y 170 69 traces 8 23 250 78 none 22 none Pa-6%Ru-1%Y 170 85 none 4 11 250 100 - - - Pa-6%Ru-2%Y 170 93 none none 7 250 100 - - - The alloy containing palladium, 6% of ruthenium, and 2% of yttrium in this reaction at the temperature of 2500C withstands 4 times as many cycles as the alloy comprising palladium and 6% of ruthenium.

The addition of yttrium increase the mechanical strength, heat-resistance, and selectivity of the prior art alloy.

EXAMPLE 3 In much the same manner there have been prepared alloys of palladium with ruthenium and additions of cerium, neodymium, and samarium; their compositions and mechanical strength characteristics are shown in Table 5.

TABLE 5 Hardness Tensile Alloy kg/mm2 strength kg/mm2 Pa -6% Ru 116 60 Pa-6%Ru-1%Ce 138 86 Pa-6%Ru-5%Ce 162 104 Pa-6%Ru-2%Nd 146 92 Pa-6%Ru-2%Sm 140 89 Pa-6%Ru-7%Sm 170 132 Additions of cerium, neodymium, and samarium increase the mechanical strength of palladium and, like lanthanum and yttrium, enhance the heat-resistance and extend the service life of membranes made of these alloys, as well as improving the selectivity of catalytical processes carried out on them.

Claims (2)

1. A palladium-based alloy containing: ruthenium 4to8wt.% at least one rare-earth element 0.3 to 8 wt.% (total) palladium the balance.

2. A palladium-based alloy according to claim 1, substantially as described in any of the Examples given.