WO2018154292A1 - Multicomponent metallic alloys - Google Patents

Multicomponent metallic alloys Download PDF

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Publication number
WO2018154292A1
WO2018154292A1 PCT/GB2018/050448 GB2018050448W WO2018154292A1 WO 2018154292 A1 WO2018154292 A1 WO 2018154292A1 GB 2018050448 W GB2018050448 W GB 2018050448W WO 2018154292 A1 WO2018154292 A1 WO 2018154292A1
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hea
metals
heas
metallic
hep
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PCT/GB2018/050448
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French (fr)
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Kirill YUSENKO
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Swansea University
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    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C30/00Alloys containing less than 50% by weight of each constituent
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C5/00Alloys based on noble metals
    • C22C5/04Alloys based on a platinum group metal

Definitions

  • the invention concerns multicomponent metallic alloys and processes for preparing the 5 same.
  • the invention is directed to new high entropy alloys ('HEAs'); to catalyst compositions comprising said HEAs; and to the use of said HEAs as electrochemical catalysts.
  • the invention also provides a method for preparing multicomponent metallic alloys which include, but are not limited to, said HEAs.
  • HEAs multicomponent alloys comprising at least five principal metallic elements, each in an amount between 35 and 5 atomic percent (at %) are referred to as HEAs.
  • HEA 20 emphasizes the role of entropy in favouring the formation of single-phase disordered alloys without precipitation of multiple ordered and partially disordered phases' 31 .
  • HEAs can have a range of novel properties, such as their excellent specific strength, superior mechanical performance at high temperatures, exceptional ductility and fracture toughness at cryogenic temperatures,
  • HEAs can be utilised in many applications including (i) structural applications, such as in aerospace engineering and civil transportations; (ii) superconducting electromagnets, such as magnetic resonance imaging (MRI) scanners, nuclear magnetic resonance (NMR) machines, and particle accelerators, like several other type II superconductors; (iii) high-temperature applications, such as gas
  • Hexagonal close packed (hep) metals show high hardness, as well as excellent mechanical and chemical stability.
  • the design of hep- structured HEAs based on these elements can open tantalizing possibilities for materials development.
  • the improvement of mechanical properties and oxidation stability in rhenium alloys is known as the "rhenium effect" and plays an important role in refractory alloys design and development 151 . Therefore, platinum group metals ('PGMs'), i.e. Ru, Rh, Pd, Os, Ir and Pt, and/or refractory metals, i.e.
  • rhenium can be therefore considered ideal candidates for a new class of /7cp-structured HEAs.
  • Ir, Pd, Pt and Rh each have fee crystal structures, while Os, Ru and Re each have hep crystal structures at ambient conditions. All of these metals have high importance as construction materials for extreme conditions and show high activity as heterogeneous catalysts.
  • PGM-based and/or Re-based alloys are not easily accessible for development and have barely been considered as principle elements for multicomponent alloys and HEAs.
  • Pd, Rh and Ru were used as principal elements in 7 multicomponent alloys, mainly as a family of 4-component whereas Ir, Os, Pt and Re do not appear to have been considered as principle elements in any HEA [4] .
  • These latter elements are particularly promising candidates for single-phase HEA development, because they form continuous solid solutions in a broad range of compositions with each other and display high catalytic activity and exceptional mechanical, chemical and thermal stability under extreme conditions.
  • PGM-based and/or Re-based HEAs preferably hep- structured HEAs, and in particular for such HEAs that show exceptional mechanical, chemical and/or thermal stability, and/or that show pronounced electrocatalytic activity.
  • a HEA comprising at least five principle elements, each principle element being present in an amount from 5 to 35 atomic percent (at.%), wherein said principle elements comprise: one or more hexagonal close packed (hep) metals selected from osmium (Os), ruthenium (Ru) and rhenium (Re), and/or one or more face centered cubic (fee) metals selected from iridium (Ir), palladium (Pd), platinum (Pt) and rhodium (Rh).
  • hep hexagonal close packed
  • fee face centered cubic metals selected from iridium (Ir), palladium (Pd), platinum (Pt) and rhodium (Rh).
  • references herein to a hep metal is to any metallic element that has a hexagonal close packed crystalline structure at standard temperature and pressure
  • reference herein to a fee metal is to any metallic element that has a face centered cubic crystalline structure at standard temperature and pressure.
  • said principle elements comprise one or more metals selected from osmium, rhenium, iridium and platinum. More preferably, said principle elements comprise five or six metals selected from osmium, ruthenium, rhenium, iridium, palladium, platinum and rhodium.
  • This disclosure not only represents the first successful creation of PGM- and/or rhenium- based HEAs and/or the first successful creation of single-phase /7cp-structured HEAs, but also shows that such HEAs display high stability under extreme temperatures and pressures, and possess potent electrocatalytic activity.
  • the HEAs of the invention may comprise a mixture of hep metals and fee metals. Therefore, said HEAs can be simply classified by the ratio between fee metal (e.g. Rh, Ir, Pt and Pd) principle elements and hep metal (e.g. Ru, Re and Os) principle elements. Further, data suggests that the maximum solubility of fee metals in hep metals to be less than 40 at.%, and the solubility of hep metals in fee metals to be up to, and preferably less than, 20 at.%. These values can be used to predict the formation of two-phase mixtures or single-phase, either or fcc-structured, HEAs. Therefore, in some preferred embodiments, the HEA is a single-phase /7cp-structured alloy. In these embodiments, the principle elements preferably comprise zero or less than 40 at.% of fee metals.
  • fee metal e.g. Rh, Ir, Pt and Pd
  • hep metal e.g
  • the HEA is a single-phase fcc-structured alloy.
  • the principle elements preferably comprise zero or up to 20 at.%, more preferably zero or less than 20 at.% of hep metals.
  • the HEA has a composition as indicated in Table 1 below
  • HEAs which contain 5 or 6 metals selected from PGMs and rhenium, were prepared by a novel process of thermal decomposition of a single-source precursor, the first successful example of HEA preparation that does not require direct melting at high temperature or mechanical alloying, and this process can be further extended to the synthesis of other, multicomponent metallic systems.
  • a process represents a new approach in the design and optimization of multicomponent alloys in general, and in particular for refractory high-entropy alloys for a broad range of applications.
  • a process for preparing a multicomponent metallic alloy preferably a process for preparing a HEA comprising multiple (preferably at least 5) principle elements, each of which being present in an amount from 5 to 35 at.%, and most preferably a process for preparing the HEA according to the first aspect of the invention.
  • the process comprises: a. providing a solution comprising a combination of two or more iso formula solid crystalline salts, wherein each of said salt comprises a metallic coordination cation or a metallic coordination anion, and wherein each metal to be alloyed is present within at least one of said coordination cation(s) and/or said coordination anion(s); b.
  • said thermal decomposition is carried out in a static or flowing (preferably flowing) reducing atmosphere.
  • a reducing atmosphere is to an atmospheric condition in which oxidation is prevented by removal of oxidizing gases such as oxygen, and/or in which one or more reducing gases such as hydrogen are present.
  • the reducing atmosphere may comprise one or more inert gases, preferably as a mixture with said one or more reducing gases.
  • said thermal decomposition is carried out in hydrogen flow.
  • This thermal decomposition single-source precursor strategy is based on the fact that isoformular solid crystalline salts containing coordination cations and coordination anions are usually isostructural and can be co-crystallized with formation of multicomponent salt's solid- solutions.
  • the thermal decomposition of said solid solutions in a reducing atmosphere results in the formation of multicomponent metallic alloys under relatively mild conditions (preferably at or below 800 °C, more preferably between 700-800 °C).
  • relatively mild conditions preferably at or below 800 °C, more preferably between 700-800 °C.
  • the reaction of water solutions of results in precipitation of salt has low solubility in water and can be easily filtered.
  • Further thermal decomposition at or below 800 °C in hydrogen stream results in the formation of a metastable nanostructured alloy.
  • said isoformular solid crystalline salts are selected from salts in which the coordination cation is of formula (I) and/or the coordination anion is of formula (II):
  • M' is a metal, preferably selected from cobalt, chromium, iridium, osmium, rhodium and ruthenium, more preferably selected from iridium, osmium, rhodium and ruthenium; M" is a metal, preferably selected from iridium, osmium, palladium, platinum, rhenium, titanium, vanadium and zirconium, more preferably selected from iridium, osmium, palladium, platinum and rhenium; and X is selected from fluorine, chlorine, bromine and iodine.
  • said single source precursor is prepared according to the following scheme:
  • HEAs of the first aspect of the invention show pronounced electrocatalytic activity, as evidenced using methanol electro-oxidation as a model reaction. Therefore, according to a third aspect of the invention, there is provided a catalyst composition comprising the HEA of the first aspect. Further, according to fourth aspect, the invention provides for the use of the aforementioned HEA as an electrochemical catalyst, preferably for the oxidation of methanol and/or preferably in a methanol oxidation fuel cell.
  • any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.
  • Figure 2 shows a Room temperature compressibility curve for hcp- lro.24Oso.21 Reo.i6Rho.i8Ruo.20 HEA in comparison with PGMs (orange - Os, blue - Ir, green - Re, red - Ru, dashed lines - Rh); insert shows c/a pressure dependence.
  • Figure 3 shows TEM images and elemental distribution in the single-phase hcp-
  • Figure 4 shows TEM images and elemental distribution in the two-phase HEA with nominal composition and Figure 5 shows cyclic voltammograms (scan rate recorded in 1 M MeOH / 1 M
  • a mixture of the chloropentaammine salts was dissolved in a hot water.
  • a mixture of the hexachlorometallates was also dissolved in a hot water. 2-3 drops of concentrated HCI solution were added to suppress hydrolysis. Hot solutions were mixed with intensive stirring and cooled to room temperature. The brownish crystalline precipitates were filtered, washed with cold water, acetone and dried in air. These single source precursors were air stable and did not need any protective atmosphere.
  • Metallic alloys were then prepared in powder form by thermal decomposition of said single-source precursors in 5-vol.%-H 2 /95-vol.%-N 2 stream (25-30 minutes at 1073 K followed by cooling to room temperature for 10-12 h) according to the following general equation, wherein a, b, c, d and e are as indicated above:
  • the Morphology and elemental compositions of the HEAs synthesized in Example 1 were analysed using a Hitachi S-4800 Field Emission scanning-electron microscope equipped for energy dispersive X-ray spectroscopy (EDS). The average elemental composition was obtained from 20 individual points.
  • High-resolution transmission electron microscopy (TEM) was performed with a probe-corrected FEI Titan G2 60-300 transmission electron microscope operated at 300 eV.
  • EDS was performed with a SuperX Bruker spectrometer equipped with 4 silicon drift detectors.
  • a wide-angle position sensitive detector based on Mythen-2 Si strip modules was applied to collect the diffraction data.
  • the detector was moved at constant angular speed with 10 s scan time at each temperature and 60 s waiting time in order to let the temperature stabilize.
  • the powdered alloys were sealed in 0.5 mm quartz capillaries in vacuum, and heated in the capillary furnace from 25 to 1200 °C with axial rotation 1121 .
  • the diffraction images were recorded under continuous ⁇ -rotation of the DAC from -3 to +3° with the acquisition time of 1 second.
  • the sample was loaded in a diamond anvil cell equipped with conically supported Boehler Almax type diamond anvils (300 ⁇ culet sizes) [12al .
  • Pressure was determined using ruby placed in the pressure chamber and solid Ne as internal standard. Neon serving as pressure-transmitting medium was loaded at about 1.5 kbar using the gas-loading system installed at the ID-15B beam-line. Radial diffraction high- temperature and high-pressure data were calibrated, corrected and integrated using of the FIT2D [13] and refined using TOPAS software 1141 .
  • Si (NIST SRM 640c) powder was used as external standard for calibration.
  • the single-phase H EA adopts a single phase hep crystal
  • the HEA shows regular thermal expansion as single phase, without distortions or phase transformations up to 1500 K.
  • hep- has relatively high thermal expansion and small ⁇ parameter, which is typical for Os and Re (see Table 2).
  • Corresponding c/a ratio increases with temperature, as characteristic for Os and Ru.
  • c/a ratio for pure Re decreases with temperature, which is quite rare for /7cp-structured metals.
  • Table 2 Thermal expansion coefficients and bulk moduli for
  • Phase stabilities of pure PGMs and Re have been intensively investigated under high- pressure up to 304 Gpa for Pt, 50 Gpa for Rh, 56 Gpa for Ru, 77 Gpa for Pd, 640 for Re and 750 Gpa for Os [16al . All metals seem to be quite incompressible (see Table 2 above). Only elemental Ir, investigated up to 65 Gpa, shows a 14-layered hep-based superstructure above 59 Gpa [16bl . The corresponding equations of state for all PGM and Re are known in detail and the pure metals (except Ir) show no HP— HT transformations.
  • Pure Os investigated up to 750 Gpa shows anomalies in the cell parameters ratio at 150 Gpa and 440 Gpa, which has been attributed to the change of the Fermi surface for valence electrons 11601 .
  • This electronic topological transition is mainly characteristic for /7cp-structured metals and can be detected as a peculiarity in the c/a behaviour under pressure. Despite this, the same transition has been predicted to occur in pure Ir, and at the much lower pressure of 80 Gpa. Since their development, HEAs have barely been investigated under high-pressure.
  • Vo is the specific volume at ambient pressure, Bo the bulk modulus, and the pressure derivative of bulk modulus (see Table 2 and Figure 2).
  • the hep- HEA has low bulk modulus in comparison with pure PGMs. This can be a result of the large differences in compressibility for Os and other metals.
  • the c/a ratio for hcp-hEA increases with pressure without any anomaly, which suggests no electronic topological transition above 45 Gpa.
  • High-resolution transmission electron microscopy (HR-TEM) images show clear pseudomorphism of the metallic conglomerates with shape similar to the starting single- source precursor ( Figure 3). Large concentration of structural defects such as stacking faults and inter-growths could be expected due to the low preparation temperature. Nevertheless, according to HR-TEM the h HEA displays a relatively low number of twins and no planar defects. The elaboration of gases and hydrogen creates small pores of 3-5 nm in diameter, which suggests structural porosity of the material. According to EDX data, all elements have homogeneous distribution along the phase.
  • Each particle contains narrow regions of fee- and /7cp-intergrowths and high concentration of planar defects.
  • Such structure is also typical for nano-twinned fee- and /7cp-structured alloys and plays an important role in their mechanical properties, especially mechanical deformation 1171 .
  • this two-phase sample with nominal composition only the
  • Ir and Pt metals show detectable segregation: Ir concentrates inside grains and Pt forms agglomerates on the interfaces between individual grains. This result is unexpected and can be associated with a mechanism of thermal decomposition where metals are individually reduced at various temperatures.
  • ternary 1181 and quaternary 1191 alloys based on PGMs have been proposed as heterogeneous substrates for electrocatalysis. Particular attention has been given to active PGM-based multicomponent catalysts for methanol electro-oxidation, including the CoPtRu, OsPtRu, IrOsPtRu, NiPtRuZr, and NiPtRhRu systems 1201 .
  • methanol electro- oxidation has been chosen as a model reaction.
  • the electrooxidation of methanol in acidic media can be briefly described as a two-step process including the absorption and dehydrogenation of the molecule on the metallic surface and the oxidation of the adsorbate to CO2. So far, the best activity in this process has been shown by Pt— Ru alloys; but investigations have also been performed for Pd— Pt— Rh, Ir— Pt and Os [23] .
  • Cyclic voltammograms were recorded following the adsorption of methanol at 0.1 V versus standard hydrogen electrode (SHE) on pure metals and HEAs prepared via single-phase precursor route. All platinum-group metals were shown to catalyse methanol oxidation.
  • Thermal degradation of single source precursors allows for the preparation of PGM- and/or rhenium-based HEAs, in particular the first single phase /7cp-structured HEA.
  • These novel HEAs display high stability under extreme conditions: heating up to 1500 K and compressing up to 45 Gpa do not result in any phase change of the alloy. Cyclic voltammetry suggests electro-catalytic activity of the alloys, particularly for methanol oxidation.

Abstract

The invention concerns multicomponent metallic alloys and processes for preparing the same. In particular, the invention is directed to new high entropy alloys ('HEAs'); to catalyst compositions comprising said HEAs; and to the use of said HEAs as electrochemical catalysts. The invention also provides a method for preparing multicomponent metallic alloys which include, but are not limited to, said HEAs.

Description

Multicomponent Metallic Alloys
Field of the Invention
The invention concerns multicomponent metallic alloys and processes for preparing the 5 same. In particular, the invention is directed to new high entropy alloys ('HEAs'); to catalyst compositions comprising said HEAs; and to the use of said HEAs as electrochemical catalysts. The invention also provides a method for preparing multicomponent metallic alloys which include, but are not limited to, said HEAs.
Background of the Invention
10 Over many thousands of years, traditional metallic alloys have been developed according to a 'base element' paradigm. This strategy begins with one, or rarely two, principal elements, such as iron in steels or nickel in superalloys, and a minor alloying approach is used to obtain alloys with enhanced properties. In sharp contrast, a novel paradigm for alloy design was proposed about a decade which involves the mixing of multiple elements in an
Figure imgf000002_0001
15 equimolar or near-equimolar composition to form multicomponent alloys, thus eschewing the long established 'base element1 concept of alloy development.
Unlike conventional alloys, which contain one and rarely two principle elements, multicomponent alloys comprising at least five principal metallic elements, each in an amount between 35 and 5 atomic percent (at %) are referred to as HEAs. Their name
20 emphasizes the role of entropy in favouring the formation of single-phase disordered alloys without precipitation of multiple ordered and partially disordered phases'31. There has been growing interest in HEAs as they can have a range of novel properties, such as their excellent specific strength, superior mechanical performance at high temperatures, exceptional ductility and fracture toughness at cryogenic temperatures,
25 superparamagnetism, and superconductivity. HEAs can be utilised in many applications including (i) structural applications, such as in aerospace engineering and civil transportations; (ii) superconducting electromagnets, such as magnetic resonance imaging (MRI) scanners, nuclear magnetic resonance (NMR) machines, and particle accelerators, like several other type II superconductors; (iii) high-temperature applications, such as gas
30 turbines, rocket nozzles, and nuclear construction; (iv) aerospace applications, where lightweight materials tolerant to high temperatures are in demand; (v) cryogenic applications such as rocket casings, pipework, and liquid O2 or N2 equipment; and (vi) catalytic applications, such as methanol oxidation for, e.g. fuel cell applications.
1 The single-phase disordered structures typical for HEAs are considered responsible for the abovementioned outstanding properties. However, the formation of inter-grain precipitates also plays an important role in their mechanical and chemical features. Truly single-phase HEAs are rare and the search for systems with high thermal stability is challenging due to the large number of experimental parameters to be taken into consideration141.
To date, most HEAs have been produced using liquid-phase methods, including high temperature arc melting, induction melting or Bridgeman solidification. However, many single-phase HEAs prepared from the melt display metastable nature and undergo phase separation during or after heating. The formation enthalpy of intermetallic compounds is often high and cannot be compensated by configuration entropy.
The most widely studied single-phase HEAs consist of low-density first-raw transition metals or of refractory elements (Mo, Nb, Ta, W, Zr). Up to now, mainly body centred cubic (bed) and face centred cubic (fee) HEAs have been intensively investigated. However, several experimental and theoretical works were recently performed to develop hexagonal close packed (hep) -structured HEAs. In the 4f transition metal alloy family, GdHoLaTbY, DyGdLuTbTm and DyGdLuTbY were found to contain a mixture of /7cp-structured alloy and unknown phases'61; whereas was reported to be mainly hep phase after heat
Figure imgf000003_0001
treatment of mechanically alloyed powders'71. The melting of hexagonal metals leads to hep- structured alloys containing only to three or four-components, which are normally in equilibrium with ordered phases or quasicrystals, as in These results mark
Figure imgf000003_0002
the failure of the traditional melting routes for the synthesis of single-phase /7cp-structured HEAs and thermodynamic limitations in relative stability of /7cp-structured alloys in comparison with ordered intermetallic phases. Therefore, there remains a need for improved methods of producing stable multicomponent metallic alloys, more particularly methods for producing stable HEAs, and most particularly methods for preparing stable hep-structured HEAs.
Hexagonal close packed (hep) metals (in particular Os, Re, Ru, Ti and Zr) show high hardness, as well as excellent mechanical and chemical stability. Thus, the design of hep- structured HEAs based on these elements can open tantalizing possibilities for materials development. As an example, the improvement of mechanical properties and oxidation stability in rhenium alloys is known as the "rhenium effect" and plays an important role in refractory alloys design and development151. Therefore, platinum group metals ('PGMs'), i.e. Ru, Rh, Pd, Os, Ir and Pt, and/or refractory metals, i.e. rhenium, can be therefore considered ideal candidates for a new class of /7cp-structured HEAs. Ir, Pd, Pt and Rh each have fee crystal structures, while Os, Ru and Re each have hep crystal structures at ambient conditions. All of these metals have high importance as construction materials for extreme conditions and show high activity as heterogeneous catalysts. However, due to their high melting points, PGM-based and/or Re-based alloys are not easily accessible for development and have barely been considered as principle elements for multicomponent alloys and HEAs. Pd, Rh and Ru were used as principal elements in 7 multicomponent alloys, mainly as a family of 4-component
Figure imgf000004_0001
whereas Ir, Os, Pt and Re do not appear to have been considered as principle elements in any HEA[4]. These latter elements are particularly promising candidates for single-phase HEA development, because they form continuous solid solutions in a broad range of compositions with each other and display high catalytic activity and exceptional mechanical, chemical and thermal stability under extreme conditions.
Therefore, there is a clear need for PGM-based and/or Re-based HEAs, preferably hep- structured HEAs, and in particular for such HEAs that show exceptional mechanical, chemical and/or thermal stability, and/or that show pronounced electrocatalytic activity.
Statements of the Invention
According to a first aspect, there is provided a HEA comprising at least five principle elements, each principle element being present in an amount from 5 to 35 atomic percent (at.%), wherein said principle elements comprise: one or more hexagonal close packed (hep) metals selected from osmium (Os), ruthenium (Ru) and rhenium (Re), and/or one or more face centered cubic (fee) metals selected from iridium (Ir), palladium (Pd), platinum (Pt) and rhodium (Rh).
Reference herein to a hep metal is to any metallic element that has a hexagonal close packed crystalline structure at standard temperature and pressure, Similarly, reference herein to a fee metal is to any metallic element that has a face centered cubic crystalline structure at standard temperature and pressure.
In preferred embodiments, said principle elements comprise one or more metals selected from osmium, rhenium, iridium and platinum. More preferably, said principle elements comprise five or six metals selected from osmium, ruthenium, rhenium, iridium, palladium, platinum and rhodium.
This disclosure not only represents the first successful creation of PGM- and/or rhenium- based HEAs and/or the first successful creation of single-phase /7cp-structured HEAs, but also shows that such HEAs display high stability under extreme temperatures and pressures, and possess potent electrocatalytic activity.
The HEAs of the invention may comprise a mixture of hep metals and fee metals. Therefore, said HEAs can be simply classified by the ratio between fee metal (e.g. Rh, Ir, Pt and Pd) principle elements and hep metal (e.g. Ru, Re and Os) principle elements. Further, data suggests that the maximum solubility of fee metals in hep metals to be less than 40 at.%, and the solubility of hep metals in fee metals to be up to, and preferably less than, 20 at.%. These values can be used to predict the formation of two-phase mixtures or single-phase, either
Figure imgf000005_0001
or fcc-structured, HEAs. Therefore, in some preferred embodiments, the HEA is a single-phase /7cp-structured alloy. In these embodiments, the principle elements preferably comprise zero or less than 40 at.% of fee metals.
In alternative embodiments, the HEA is a single-phase fcc-structured alloy. In these embodiments, the principle elements preferably comprise zero or up to 20 at.%, more preferably zero or less than 20 at.% of hep metals.
In particularly preferred embodiments, the HEA has a composition as indicated in Table 1 below
Figure imgf000005_0002
The above HEAs, which contain 5 or 6 metals selected from PGMs and rhenium, were prepared by a novel process of thermal decomposition of a single-source precursor, the first successful example of HEA preparation that does not require direct melting at high temperature or mechanical alloying, and this process can be further extended to the synthesis of other, multicomponent metallic systems. As a consequence, such a process represents a new approach in the design and optimization of multicomponent alloys in general, and in particular for refractory high-entropy alloys for a broad range of applications.
Therefore, according to a second aspect of the invention there is provided a process for preparing a multicomponent metallic alloy, preferably a process for preparing a HEA comprising multiple (preferably at least 5) principle elements, each of which being present in an amount from 5 to 35 at.%, and most preferably a process for preparing the HEA according to the first aspect of the invention. The process comprises: a. providing a solution comprising a combination of two or more isoformular solid crystalline salts, wherein each of said salt comprises a metallic coordination cation or a metallic coordination anion, and wherein each metal to be alloyed is present within at least one of said coordination cation(s) and/or said coordination anion(s); b. co-crystallizing said combination of isoformular solid crystalline salts to form a single source precursor, wherein said precursor is a multicomponent solid solution comprising said metallic coordination cation(s) and said metallic coordination anion(s); c. placing said single source precursor in a reducing or inert environment and applying sufficient heat to affect thermal decomposition and formation of a multicomponent metallic alloy.
Preferably, said thermal decomposition is carried out in a static or flowing (preferably flowing) reducing atmosphere. Reference herein to a reducing atmosphere is to an atmospheric condition in which oxidation is prevented by removal of oxidizing gases such as oxygen, and/or in which one or more reducing gases such as hydrogen are present. The reducing atmosphere may comprise one or more inert gases, preferably as a mixture with said one or more reducing gases. Most preferably, said thermal decomposition is carried out in hydrogen flow.
This thermal decomposition single-source precursor strategy is based on the fact that isoformular solid crystalline salts containing coordination cations and coordination anions are usually isostructural and can be co-crystallized with formation of multicomponent salt's solid- solutions. The thermal decomposition of said solid solutions in a reducing atmosphere results in the formation of multicomponent metallic alloys under relatively mild conditions (preferably at or below 800 °C, more preferably between 700-800 °C). For example, the reaction of water solutions of
Figure imgf000007_0004
results in precipitation of
Figure imgf000007_0003
salt has low solubility in water and can be easily filtered. Further thermal decomposition at or below 800 °C in hydrogen stream results in the formation of a metastable nanostructured
Figure imgf000007_0005
alloy. As the skilled reader would readily understand, similar reactions can be performed with other metallic coordination cation- and/or metallic coordination anion-containing isoformular salts to co-crystallize multiple (e.g. 5 or 6) metallic component salts to prepare a single source precursor for thermal decomposition into multicomponent metallic alloys, preferably HEAs. As these single source precursors are stable salts that can be obtained from, e.g., water solutions, this synthesis strategy can be easily scaled up and used as an environmentally friendly route for the preparation of multicomponent metallic alloys, in particular HEAs. Further, the thermal decomposition process occurs at a relatively low temperature in non- equilibrium conditions and, in many cases, it provides metastable metallic alloys that are unavailable by conventional synthetic processes.
In preferred embodiments, said isoformular solid crystalline salts are selected from salts in which the coordination cation is of formula (I) and/or the coordination anion is of formula (II):
Figure imgf000007_0001
wherein M' is a metal, preferably selected from cobalt, chromium, iridium, osmium, rhodium and ruthenium, more preferably selected from iridium, osmium, rhodium and ruthenium; M" is a metal, preferably selected from iridium, osmium, palladium, platinum, rhenium, titanium, vanadium and zirconium, more preferably selected from iridium, osmium, palladium, platinum and rhenium; and X is selected from fluorine, chlorine, bromine and iodine. In particularly preferred embodiments, said single source precursor is prepared according to the following scheme:
Figure imgf000007_0002
And formation of said HEA by thermal decomposition occurs in hydrogen according to the following scheme:
Figure imgf000008_0001
wherein or less.
Figure imgf000008_0002
The HEAs of the first aspect of the invention show pronounced electrocatalytic activity, as evidenced using methanol electro-oxidation as a model reaction. Therefore, according to a third aspect of the invention, there is provided a catalyst composition comprising the HEA of the first aspect. Further, according to fourth aspect, the invention provides for the use of the aforementioned HEA as an electrochemical catalyst, preferably for the oxidation of methanol and/or preferably in a methanol oxidation fuel cell.
In the claims which follow and in the preceding description of the invention, except where the context requires otherwise due to express language or necessary implication, the word "comprises", or variations such as "comprises" or "comprising" is used in an inclusive sense i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.
All references, including any patent or patent application, cited in this specification are hereby incorporated by reference. No admission is made that any reference constitutes prior art. Further, no admission is made that any of the prior art constitutes part of the common general knowledge in the art. Preferred features of each aspect of the invention may be as described in connection with any of the other aspects.
Other features of the present invention will become apparent from the following detailed description of certain embodiments. Generally speaking, the invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including the accompanying claims and drawings). Thus, features, integers, characteristics, compounds or chemical moieties described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein, unless incompatible therewith.
Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.
Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.
Embodiments of the present invention will now be described by way of example only with reference to the following Experimental Section wherein: Figure 1 shows thermal expansion curves for
Figure imgf000009_0006
pure PGMs[9] and rhenium1101. Above: temperature dependence of atomic volumes; below: temperature dependence of c/a ratio.
Figure 2 shows a Room temperature compressibility curve for hcp- lro.24Oso.21 Reo.i6Rho.i8Ruo.20 HEA in comparison with PGMs (orange - Os, blue - Ir, green - Re, red - Ru, dashed lines - Rh); insert shows c/a pressure dependence.
Figure 3 shows TEM images and elemental distribution in the single-phase hcp-
Figure imgf000009_0008
Figure 4 shows TEM images and elemental distribution in the two-phase HEA with nominal composition
Figure imgf000009_0007
and Figure 5 shows cyclic voltammograms (scan rate recorded in 1 M MeOH / 1 M
Figure imgf000009_0009
H2SO4 after 5 min of methanol absorption at 0.1 V on (a) single-phase hcp- lro.24Oso.21 Reo.i6Rho.i8Ruo.20 HEAs. ; (b) two-phase lro.i8Oso.25Pto.14Reo.15Rho.14Ruo.14. Blue line: first cycle after absorption; grey line: second cycle after absorption (background).
Experimental Section
Example 1 : HEA Synthesis
Two single-phase HEAs and fcc-
Figure imgf000009_0002
Figure imgf000009_0001
source precursors.
Starting
Figure imgf000009_0003
were prepared according to published protocols from soluble chlorides
Figure imgf000009_0004
obtained from Sigma Aldrich. were obtained
Figure imgf000009_0005
from Sigma Aldrich11 11. Single-source precursors were synthetized from water solutions according to the following procedure: Amounts of for each HEA were calculated according to the
Figure imgf000010_0003
following general equation, wherein a + b is 1 or less, and c + d + e is 1 or less:
Figure imgf000010_0001
A mixture of the chloropentaammine salts was dissolved in a hot water. A mixture of the hexachlorometallates was also dissolved in a hot water. 2-3 drops of concentrated HCI solution were added to suppress hydrolysis. Hot solutions were mixed with intensive stirring and cooled to room temperature. The brownish crystalline precipitates were filtered, washed with cold water, acetone and dried in air. These single source precursors were air stable and did not need any protective atmosphere. Metallic alloys were then prepared in powder form by thermal decomposition of said single-source precursors in 5-vol.%-H2/95-vol.%-N2 stream (25-30 minutes at 1073 K followed by cooling to room temperature for 10-12 h) according to the following general equation, wherein a, b, c, d and e are as indicated above:
Figure imgf000010_0002
Example 2: HEA Morphology and Elemental Analysis
The Morphology and elemental compositions of the HEAs synthesized in Example 1 were analysed using a Hitachi S-4800 Field Emission scanning-electron microscope equipped for energy dispersive X-ray spectroscopy (EDS). The average elemental composition was obtained from 20 individual points. High-resolution transmission electron microscopy (TEM) was performed with a probe-corrected FEI Titan G2 60-300 transmission electron microscope operated at 300 eV. EDS was performed with a SuperX Bruker spectrometer equipped with 4 silicon drift detectors. In situ high-temperature powder X-ray diffraction data for the powdered samples were collected at the 11 1 beam-line at the DIAMOND light source λ = 0.494984 A. A wide-angle position sensitive detector based on Mythen-2 Si strip modules was applied to collect the diffraction data. The detector was moved at constant angular speed with 10 s scan time at each temperature and 60 s waiting time in order to let the temperature stabilize. The powdered alloys were sealed in 0.5 mm quartz capillaries in vacuum, and heated in the capillary furnace from 25 to 1200 °C with axial rotation1121. Room temperature compressibility curves were collected up to 45 Gpa at the ID-15B beam-line, ESRF, (λ = 0.410962 A, MAR 555 flat panel detector, beam size
Figure imgf000010_0004
The diffraction images were recorded under continuous ω-rotation of the DAC from -3 to +3° with the acquisition time of 1 second. The sample was loaded in a diamond anvil cell equipped with conically supported Boehler Almax type diamond anvils (300 μηι culet sizes)[12al. Pressure was determined using ruby placed in the pressure chamber and solid Ne as internal standard. Neon serving as pressure-transmitting medium was loaded at about 1.5 kbar using the gas-loading system installed at the ID-15B beam-line. Radial diffraction high- temperature and high-pressure data were calibrated, corrected and integrated using of the FIT2D[13] and refined using TOPAS software1141. Si (NIST SRM 640c) powder was used as external standard for calibration.
The single-phase H EA adopts a single phase hep crystal
Figure imgf000011_0002
structure with
Figure imgf000011_0003
g/cm3 (room temperature cell parameters). Atomic volumes taken additively according to alloys composition suggest V/Z = 13,877 A3, which corresponds to a 1 % negative deviation from the linear dependence of alloy's atomic volumes (Zen's law) [15].
It is well known that single-phase HEAs are quite rare and usually stable only in a limited temperature interval. For example, well-documented reversible phase separation in the bec- AlxCoCrFeNi HEA family occurs above ca. 600 0Ci16i. In contrast, the single-phase hep- HEA was heated in vacuum up to 1500 K to prove its thermal
Figure imgf000011_0006
stability by high-temperature powder X-ray diffraction (see Figure 1). The volumetric thermal expansion coefficient for
Figure imgf000011_0009
was estimated as: a(T)=
Figure imgf000011_0004
by fitting the corresponding dataset to
Figure imgf000011_0001
where
Figure imgf000011_0005
is the atomic volume at reference temperature (293 K), with
Figure imgf000011_0008
for the hep alloy. The HEA shows regular thermal expansion as single phase, without distortions or phase transformations up to 1500 K. hep-
Figure imgf000011_0007
has relatively high thermal expansion and small αι parameter, which is typical for Os and Re (see Table 2). Corresponding c/a ratio increases with temperature, as characteristic for Os and Ru. In contrast, c/a ratio for pure Re decreases with temperature, which is quite rare for /7cp-structured metals. Table 2: Thermal expansion coefficients and bulk moduli for
Figure imgf000012_0002
Figure imgf000012_0001
Figure imgf000012_0009
Higher concentration of fcc-structured metals in the alloy were found to result in the formation of two-phase mixtures. Nevertheless,
Figure imgf000012_0003
which is
Figure imgf000012_0008
alloy has been prepared as a single-phase fcc-structured alloy. Single-phase
Figure imgf000012_0004
has fee crystal structure with
Figure imgf000012_0007
(room temperature cell parameters). Atomic volumes
Figure imgf000012_0005
taken additively according to alloy's composition suggest which
Figure imgf000012_0006
corresponds to a 1 % positive deviation from Zen's law.
Phase stabilities of pure PGMs and Re have been intensively investigated under high- pressure up to 304 Gpa for Pt, 50 Gpa for Rh, 56 Gpa for Ru, 77 Gpa for Pd, 640 for Re and 750 Gpa for Os[16al. All metals seem to be quite incompressible (see Table 2 above). Only elemental Ir, investigated up to 65 Gpa, shows a 14-layered hep-based superstructure above 59 Gpa[16bl. The corresponding equations of state for all PGM and Re are known in detail and the pure metals (except Ir) show no HP— HT transformations. Pure Os investigated up to 750 Gpa shows anomalies in the cell parameters ratio at 150 Gpa and 440 Gpa, which has been attributed to the change of the Fermi surface for valence electrons11601. This electronic topological transition is mainly characteristic for /7cp-structured metals and can be detected as a peculiarity in the c/a behaviour under pressure. Despite this, the same transition has been predicted to occur in pure Ir, and at the much lower pressure of 80 Gpa. Since their development, HEAs have barely been investigated under high-pressure. A hydrostatic compression of fcc-structured CrMnFeCoNi HEA reveals its martensitic transformation to hep phase at 14 Gpa, which suggests the structural instability of HEAs under high-pressure even at room
Figure imgf000013_0007
However, to demonstrate the high stability of the H EAs of the invention, the stability of
Figure imgf000013_0006
at room temperature under compression up to 45 Gpa was evaluated, using Ne as pressure transmitting media. The alloy did not undergo any phase transitions, and Its compressibility curve can be fitted with the third-order Birch-Murnaghan equation of state:
Figure imgf000013_0001
where Vo is the specific volume at ambient pressure, Bo the bulk modulus, and the
Figure imgf000013_0008
pressure derivative of bulk modulus (see Table 2 and Figure 2). The hep-
Figure imgf000013_0002
HEA has low bulk modulus in comparison with pure PGMs. This can be a result of the large differences in compressibility for Os and other metals. The c/a ratio for hcp-hEA increases with pressure without any anomaly, which suggests no electronic topological transition above 45 Gpa.
High-resolution transmission electron microscopy (HR-TEM) images show clear pseudomorphism of the metallic conglomerates with shape similar to the starting single- source precursor (Figure 3). Large concentration of structural defects such as stacking faults and inter-growths could be expected due to the low preparation temperature. Nevertheless, according to HR-TEM the h
Figure imgf000013_0003
HEA displays a relatively low number of twins and no planar defects. The elaboration of gases and hydrogen creates small pores of 3-5 nm in diameter, which suggests structural porosity of the material. According to EDX data, all elements have homogeneous distribution along the phase.
HR-TEM images for the two-phase sample do not show
Figure imgf000013_0004
clear phase separation, which could be expected as per PXRD data (Figure 4). Each particle contains narrow regions of fee- and /7cp-intergrowths and high concentration of planar defects. Such structure is also typical for nano-twinned fee- and /7cp-structured alloys and plays an important role in their mechanical properties, especially mechanical deformation1171. In this two-phase sample with nominal composition only the
Figure imgf000013_0005
fcc-structured Ir and Pt metals show detectable segregation: Ir concentrates inside grains and Pt forms agglomerates on the interfaces between individual grains. This result is unexpected and can be associated with a mechanism of thermal decomposition where metals are individually reduced at various temperatures.
Example 3: HEA Electrocatalytic Activity
Recently, several ternary1181 and quaternary1191 alloys based on PGMs have been proposed as heterogeneous substrates for electrocatalysis. Particular attention has been given to active PGM-based multicomponent catalysts for methanol electro-oxidation, including the CoPtRu, OsPtRu, IrOsPtRu, NiPtRuZr, and NiPtRhRu systems1201.
However, the investigation of the catalytic activity of HEAs is at a very early stage. In fact, only
Figure imgf000014_0002
HEAs were probed for methanol electro-oxidation1211. The AICoCrTiZn alloy was successfully tested for the catalytic degrading of azo-dye solutions'221. All studies concluded reporting the high electrocatalytic activity of HEAs in comparison with ordinary binary and ternary alloys.
To evaluate the electrocatalytic activity of the HEAs of the invention, methanol electro- oxidation has been chosen as a model reaction. The electrooxidation of methanol in acidic media can be briefly described as a two-step process including the absorption and dehydrogenation of the molecule on the metallic surface and the oxidation of the adsorbate to CO2. So far, the best activity in this process has been shown by Pt— Ru alloys; but investigations have also been performed for Pd— Pt— Rh, Ir— Pt and Os[23].
Electrocatalytic activity was assessed by cyclic voltammetry, which was performed using an Autolab PGSTAT potentiostat (Eco-Chemie) controlled by a PC with GPES software from the same manufacturer. A few miligrams of powder were dispersed on screen printed glassy carbon electrodes (GC-SPEs, DropSens C110, S = 0.12 cm2). The tests were performed in a drop of 1 M MeOH / 1 M H2S04 after 5 minutes conditioning at 0.1 V (vs. SHE), and background measurements were performed in a drop of 1 M H2SO4. Cyclic voltammograms were recorded following the adsorption of methanol at 0.1 V versus standard hydrogen electrode (SHE) on pure metals and HEAs prepared via single-phase precursor route. All platinum-group metals were shown to catalyse methanol oxidation. The
HEA and the two-phase sample with nominal composition
Figure imgf000014_0001
were also found to show significant catalytic effect in the anodic oxidation of methanol in sulfuric acid solution (Figure 5).
The enhancement of the kinetics of methanol electrooxidation is a well-documented effect for PGM alloys with transition metals, but the reason for this is still unclear. Either the alloying metal modifies the electronic structure of platinum valence bonds, therefore weakening COads binding and allowing the adsorbate to be removed more easily; or platinum atoms are responsible for the adsorption of methanol molecules and the transition metal provides surfaces oxides which can oxidize the adsorbate at lower potentials. The latter might entail that nanostructured alloys favour electro-oxidation, as shown by the oxidation potential of the two-phase
Figure imgf000015_0002
the single-phase hcp-
Figure imgf000015_0001
SHE).
Conclusion Thermal degradation of single source precursors allows for the preparation of PGM- and/or rhenium-based HEAs, in particular the first single phase /7cp-structured HEA. These novel HEAs display high stability under extreme conditions: heating up to 1500 K and compressing up to 45 Gpa do not result in any phase change of the alloy. Cyclic voltammetry suggests electro-catalytic activity of the alloys, particularly for methanol oxidation.
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Claims

Claims
1. A high entropy alloy (HEA) comprising at least five principle elements, each principle element being present in an amount from 5 to 35 atomic percent (at.%), wherein said principle elements comprise: one or more hexagonal close packed (hep) metals selected from osmium, ruthenium and rhenium, and/or one or more face centered cubic (fee) metals selected from iridium, palladium, platinum and rhodium.
2. The HEA according to claim 1 , wherein said principle elements comprise one or more metals selected from osmium, rhenium, iridium and platinum.
3. The HEA according to claim 2, wherein said principle elements comprise five or six metals selected from osmium, ruthenium, rhenium, iridium, palladium, platinum and rhodium.
4. The HEA according to any of claims 1 to 3, wherein said principle elements comprise a mixture of hep metals and fee metals.
5. The HEA according to any of claims 1 to 4, wherein said HEA is a single-phase hep structured alloy.
6. The HEA according to claim 5, comprising zero or less than 40 at.% of fee metals.
7. The HEA according to any of claims 1 to 4, wherein said HEA is single phase fee structured alloy.
8. The HEA according to claim 7, comprising zero or up to 20 at.% of hep metals.
9. The HEA according to claim 4, having one of the following compositions:
Figure imgf000018_0001
10. A process for preparing a multicomponent metallic alloy, the process comprising: a. providing a solution comprising a combination of two or more isoformular solid crystalline salts, wherein each of said salt comprises a metallic coordination cation or a metallic coordination anion, and wherein each metal to be alloyed is present within at least one of said coordination cation(s) and/or said coordination anion(s);
b. co-crystallizing said combination of isoformular solid crystalline salts to form a single source precursor, wherein said precursor is a multicomponent solid solution comprising said metallic coordination cation(s) and said metallic coordination anion(s);
c. placing said single source precursor in a reducing or inert environment and applying sufficient heat to affect thermal decomposition and formation of a multicomponent metallic alloy.
1 1. The process according to claim 10, wherein said thermal decomposition is carried out in a static or flowing reducing atmosphere.
12. The process according to claim 1 1 , wherein said thermal decomposition is carried out in hydrogen flow.
13. The process according to any of claims 10 to 12, wherein thermal decomposition occurs at a temperature at or below 800 °C.
14. The process according to any of claims 10 to 13, wherein said isoformular solid crystalline salts are selected from salts in which the coordination cation is of formula (I) and/or the coordination anion is of formula (II):
Figure imgf000019_0001
wherein M' is a metal, preferably selected from cobalt, chromium, iridium, osmium, rhodium and ruthenium; M" is a metal, preferably selected from iridium, osmium, palladium, platinum, rhenium, titanium, vanadium and zirconium; and X is selected from fluorine, chlorine, bromine and iodine).
15. The process according to any of claims 10 to 14, wherein the multicomponent metallic alloy is a HEA comprising multiple principle elements, each of which being present in an amount from 5 to 35 at.%.
16. The process according to claim 15, for preparing the HEA according to any of claims 1 to 9.
17. The process according to claim 16, wherein said single source precursor is prepared according to the following scheme:
Figure imgf000020_0001
and formation of said HEA by thermal decomposition occurs in hydrogen according to the following scheme:
Figure imgf000020_0002
wherein a + b is 1 or less, and c + d + e is 1 or less.
18. A catalyst composition comprising the HEA according to any of claim 1 to 9.
19. A use of the HEA according to any of claims 1 to 9 as an electrochemical catalyst.
20. The use according to claim 19, for the oxidation of methanol.
21. The use according to claim 19 or claim 20, in a methanol oxidation fuel cell.
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