CA1105235A - Method of making stoichiometric lead and bismuth pyrochlore compounds using an alkaline medium - Google Patents
Method of making stoichiometric lead and bismuth pyrochlore compounds using an alkaline mediumInfo
- Publication number
- CA1105235A CA1105235A CA324,813A CA324813A CA1105235A CA 1105235 A CA1105235 A CA 1105235A CA 324813 A CA324813 A CA 324813A CA 1105235 A CA1105235 A CA 1105235A
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- Prior art keywords
- lead
- bismuth
- source
- cations
- nitrate
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/86—Inert electrodes with catalytic activity, e.g. for fuel cells
- H01M4/90—Selection of catalytic material
- H01M4/9016—Oxides, hydroxides or oxygenated metallic salts
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01G—COMPOUNDS CONTAINING METALS NOT COVERED BY SUBCLASSES C01D OR C01F
- C01G55/00—Compounds of ruthenium, rhodium, palladium, osmium, iridium, or platinum
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
- C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
- H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
- H01B1/06—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of other non-metallic substances
- H01B1/08—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of other non-metallic substances oxides
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M12/00—Hybrid cells; Manufacture thereof
- H01M12/04—Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type
- H01M12/06—Hybrid cells; Manufacture thereof composed of a half-cell of the fuel-cell type and of a half-cell of the primary-cell type with one metallic and one gaseous electrode
Abstract
(U.S. 938,374) ABSTRACT OF THE DISCLOSURE
A liquid solution method of preparing electrically conductive pyrochlore compounds having the formula:
A2B2O7-y is disclosed wherein A is selected from lead, bismuth and mixtures thereof, B is selected from ruthemium, iridium and mixtures thereof, and 0<y<1.
The method involves reacting A and B cations to yield a pyrochlore oxide by precipitation of A and B cations from an aqueous solution source of these cations in a liquid alkaline medium having a pH of at least about 13.5 in the presence of an oxygen source sufficient to complete the desired stoichiometry, at a temperature below about 200°C for a sufficient time for reaction to occur.
A liquid solution method of preparing electrically conductive pyrochlore compounds having the formula:
A2B2O7-y is disclosed wherein A is selected from lead, bismuth and mixtures thereof, B is selected from ruthemium, iridium and mixtures thereof, and 0<y<1.
The method involves reacting A and B cations to yield a pyrochlore oxide by precipitation of A and B cations from an aqueous solution source of these cations in a liquid alkaline medium having a pH of at least about 13.5 in the presence of an oxygen source sufficient to complete the desired stoichiometry, at a temperature below about 200°C for a sufficient time for reaction to occur.
Description
1~ 35 BA~KGR~UND OF THE INVENTION AND PRIOR ART STATEMENT
2 The present invention is directed to a method of pre~
paration ~f pyrochlore structure compounds. More par~icularly, 4 ~he present ~n~ention is directed to a method of prep~ring stoichiometric lead-containingand bismuth-containing ruthenate 6 and i:idate pyrochlores. These pyrochlores havemanyuses includ-7 ing use as oxygen electrodes in electrochemical devices.
A number of various types o~ electrochemical 9 devices have been developed over ~e past few years for 0 the production of electrical energy by electrochemical reac-11 tion and obversely for the consumption of electrical energy 12 to e~ectuate electrochemical reactions. Many ¢ these 13 devices rely upon a reaction involving oxygen (or air) as part of the mechanism to acco~plish the desired result. ~or example, such devices may contain oxygen electrodes which 6 are oxygen reduc~ng cathodes in which oxygen i9 catalyt~cally elec-;roreduced, Alternatively, such devices may contain 18 oxygen electrodes which catalyze the evolution of oxygen 19 from water~ In general, these electrodes are known in the art as oxygen electrodes. Thus, metal-oxygen batteries, 21 metal-air batteries, ~uel cells, electrolyzers, metal elec-2~ trowinning devices, etc " are among the well-known electro-,~
~
~l~S~3S
1 chemical devices which may contain oxygen electrodes.
2 Typically, such devices contain electrocatalyst materials at
paration ~f pyrochlore structure compounds. More par~icularly, 4 ~he present ~n~ention is directed to a method of prep~ring stoichiometric lead-containingand bismuth-containing ruthenate 6 and i:idate pyrochlores. These pyrochlores havemanyuses includ-7 ing use as oxygen electrodes in electrochemical devices.
A number of various types o~ electrochemical 9 devices have been developed over ~e past few years for 0 the production of electrical energy by electrochemical reac-11 tion and obversely for the consumption of electrical energy 12 to e~ectuate electrochemical reactions. Many ¢ these 13 devices rely upon a reaction involving oxygen (or air) as part of the mechanism to acco~plish the desired result. ~or example, such devices may contain oxygen electrodes which 6 are oxygen reduc~ng cathodes in which oxygen i9 catalyt~cally elec-;roreduced, Alternatively, such devices may contain 18 oxygen electrodes which catalyze the evolution of oxygen 19 from water~ In general, these electrodes are known in the art as oxygen electrodes. Thus, metal-oxygen batteries, 21 metal-air batteries, ~uel cells, electrolyzers, metal elec-2~ trowinning devices, etc " are among the well-known electro-,~
~
~l~S~3S
1 chemical devices which may contain oxygen electrodes.
2 Typically, such devices contain electrocatalyst materials at
3 one or more of their electrodes and precious metals, such as
4 ~latinum (on carbon support) and silver (on carbon ~nd other supports), are frequently employed as electrocatalysts.
6 In addition~ various electr~catalytic alloys, 7 compounds and compound mixtures have been developed for 8 these electrochemic~l devices ~o achieve more desirable 9 systems, For example, U,S. Patent No. 3,536,533 (Ritamura) descrlbes the use of an alloy of gold, silver, palladium 11 and at least one o platinum, rhodium and ruthen~um as a fuel 12 cell electrode electrocatalyst, and U.S. Patent No.3,305,4~2 13 (Jones et a~) describes the use of a combination of platinum 14 and ruthenium oxides &S an elec~rocatalyst. However, both ~eferences describe these catalysts as fuel cell anode (or 16 fuel oxidation) catalys~s. O'Grady et al~ Technical Report 17 No. 37, "Ruthenium Oxide Catalysts For The Oxygen Electrode", 18 Contract No., N0014-67-A-0404-0006 (AD-779-899) Of~ice of 19 Naval Research, May 1974 (National Technical Information Service) describes the use of ru~henium oxide as an electro-21 chemical catalyst for both the generation ~ oxygen and the 22 r~duction of oxygen. U.S. Patent No. 3~405,010 (Kordesch et 23 al) teaches that spinel type electrode catalysts have been 24 found to produce better activation of the electrode and 25 improved electrolyte repellency of the electrode by the 26 inclusion o~ ruthenium. Thus, the prior art describes vari-27 ous types of electrodes including those which ~tilize iridium 28 and/or ruthenium-containing catalysts.
: ' ll~SZ3~
1 Heretofore9 many pyrochlore compounds such as the 2 pyrochlore compounds Pb2RU2o7-y ~lattice parameter of 10~2 3 Pb2Ir207_y (lattice parameter of 10.27L~), Bi2Ir207_y, 4 Bi2Rh207_y~ Pb~Rh207_y~ Pb2Pt20~_y and Cd2~e27-y~ commonly referred to as lead ru~henate, lead iridate~ bismuth iridate, 6 bismuth rhodate, lead rhodate, lead platinate and cadmdum 7 rhenate, respecti~ely, and similar compounds, have been 8 known. For example, Longo, Raccah and Goodenough, MatO
g Res. Bull., ~ol~ 4, pp. 191-202 (1969), haYe described the lo compounds Pb2RU2o7-y and Pb2Ir~07_y and their preparation at elevated temperatures which are in excess ~f 700C. Sleight, 2 Mat. Res. Bull~, Vol. 6, p. 775 (1971) has also described 13 the compounds Pb2Ru207ry and Pb2Ir207_y (including the pyro-4 chlore compound Pb2RU2o6~ having a lattice parameter of 10.27 ~) and their preparation at 700C and3000 atmospheres 16 of pre8sure. UOS. Patent No. 3,682,840 (Van Loan) descr~bes 17 the preparation of lead ruthenate at temperatures of 800C.
18 and higher. However, none of these references teach that 19 lead or bismuth-containing compounds may be made by the pre-sent invention wherein they are prepared in an alkaline medium 21 a~ temperatures below about 200C, as claimed herein.
22 United States Patents Nos. 3,769,382 (Kuo et al) 23 and 3,951,672 (Langley et al) both disclose the preparation 24 o lead ruthenate and lead irida~e us~ng various techniques at temperatures of at least about 600C, and preferably at 26 higher temperatures. Likewise, however, these references 27 fail to recognize that the lead and bismuth pyrochlores made 28 by the method of the present invention are obtained at gen-11~5~S
erally lower temperatures as more specifically recited belo~.
2 Bouchard and Gillscn, Mat. Res~ Bu~l~, Vol~ 6, 3 ppO 669-680 (1971j describe Bi2Ru207 and Bi2Ir207 preparation 4 an~ properties~ including the fact that ~ese compounds have high conductivity and small Seebeck coe~ficients. However~
6 there is no teaching that these compounds may be~made by the 7 method of the present invention. Derwent's Basic 8 Abstract Journal, Section E, Chemdoc, Week No. Y25~ Abstract 9 No. 320 (August 17, 1977), Derwent Accession No. 44866Y/25 o describes electrodes for electrolys~s of alkaline and car-11 bonate 801ution~ which ~mprise nickel-plated steel strips 12 coated with high conductivity layers containing Cd2Re207, 13 Pb2Re27-y or Ni2Re2o7~ These compounds are prepared by 14 impregnating perrhenic acid and a metal nitrate such as Cd n~trate onto a nic~el strip and baking at 350Co 6 However, these compounds are all rhenates rather than ru-7 thenates or ir~dates and are not taught to be prepared 8 by the very method of the present invention.
19 It isseen thatmuch ofthe aboveprior art dealingwith 20 the synthesis of the electrically conductive pyrochlore 21 structure oxides have taught ~ynthesi~ temperatures at 22 least as high as 600C. These highly elevated temperatu~es 23 have been employed because they have been ccnsidered 24 necessary to overcome the diffusional limitations encountered 2s in solid state reactions. These highly elevated tempera 26 tures, however, result in the formation of sintered products 27 wi~h low surace areas. This is a disadvantageous condi-28 tion for materials used in catalytic and electrocatalytic ~ 3 5 1 applications since the concen~ration of available ca~a-2 lytically active sites is limited.
3 It would be desirable from both an energy conserva-4 tion standpoin~ and a maxlmization of surface area s~and-poin~ to carry out these materials syntheses a~ significa~y 6 lower temperatures, eOg. ~elow 300C, bu~ the kinetics o 7 solid state react~ons axe unfavorably sluggish. Solution 8 syntheses offer one possible approach to achieving these g very low temperat~re reactionsO For example Trehoux, 0 Abraham and Thomas, Journal of Solid State Chemlstry, Vol.21, pp. 203-209 (1977) and C.R. AcadO Sc. Paris,t. 281 ppo379~380 2 ~1975) de~cribe the ~lution preparation of ~ pyrochlore 13 compound of the formula K~.14Bio~27[BiO.27Bil.73]
14 [04.90Hl.l]OHoA8. The synthesis is carried out by adding a b~gmuth nitrate solution to a solution of 17~ potassium 6 hydroxide containing an excess of potassium hypochlor~te.
7 The reaction is carried out in this medium for 2 h,ours in 8 a reflux type of apparatus at a temperature slightly higher 19 than 100C~ The method of synthesls and the product pre-pared are diferent in many respects from the synthesis 21 method and products herein. The compound'prepared in the 22 cited reference i8 not an oxide but rather an oxy-23 hydroxide which has a significant amount o protons incor-24 porated into She bulk structure. Proton nucle æ magnetic resonance experiments on the materials of the present 26 invention S[lOW that they are oxides wh~ch do not have 27 significant amounts of protons incorporated into the struc-28 ture. The pyrochlore synthesized by Trehoux et al is not a 1 1~ 5 2 3 5 ; - 6 -1 ruthenium or iridium-contain~ng compound and, in fac~,-is 2 believed not to be an electrically conductive pyrochlore.
3 The potassium hydroxide solution used in the Trehoux 4 reference ser~es nDt only as a reactisn medium bu~ also
6 In addition~ various electr~catalytic alloys, 7 compounds and compound mixtures have been developed for 8 these electrochemic~l devices ~o achieve more desirable 9 systems, For example, U,S. Patent No. 3,536,533 (Ritamura) descrlbes the use of an alloy of gold, silver, palladium 11 and at least one o platinum, rhodium and ruthen~um as a fuel 12 cell electrode electrocatalyst, and U.S. Patent No.3,305,4~2 13 (Jones et a~) describes the use of a combination of platinum 14 and ruthenium oxides &S an elec~rocatalyst. However, both ~eferences describe these catalysts as fuel cell anode (or 16 fuel oxidation) catalys~s. O'Grady et al~ Technical Report 17 No. 37, "Ruthenium Oxide Catalysts For The Oxygen Electrode", 18 Contract No., N0014-67-A-0404-0006 (AD-779-899) Of~ice of 19 Naval Research, May 1974 (National Technical Information Service) describes the use of ru~henium oxide as an electro-21 chemical catalyst for both the generation ~ oxygen and the 22 r~duction of oxygen. U.S. Patent No. 3~405,010 (Kordesch et 23 al) teaches that spinel type electrode catalysts have been 24 found to produce better activation of the electrode and 25 improved electrolyte repellency of the electrode by the 26 inclusion o~ ruthenium. Thus, the prior art describes vari-27 ous types of electrodes including those which ~tilize iridium 28 and/or ruthenium-containing catalysts.
: ' ll~SZ3~
1 Heretofore9 many pyrochlore compounds such as the 2 pyrochlore compounds Pb2RU2o7-y ~lattice parameter of 10~2 3 Pb2Ir207_y (lattice parameter of 10.27L~), Bi2Ir207_y, 4 Bi2Rh207_y~ Pb~Rh207_y~ Pb2Pt20~_y and Cd2~e27-y~ commonly referred to as lead ru~henate, lead iridate~ bismuth iridate, 6 bismuth rhodate, lead rhodate, lead platinate and cadmdum 7 rhenate, respecti~ely, and similar compounds, have been 8 known. For example, Longo, Raccah and Goodenough, MatO
g Res. Bull., ~ol~ 4, pp. 191-202 (1969), haYe described the lo compounds Pb2RU2o7-y and Pb2Ir~07_y and their preparation at elevated temperatures which are in excess ~f 700C. Sleight, 2 Mat. Res. Bull~, Vol. 6, p. 775 (1971) has also described 13 the compounds Pb2Ru207ry and Pb2Ir207_y (including the pyro-4 chlore compound Pb2RU2o6~ having a lattice parameter of 10.27 ~) and their preparation at 700C and3000 atmospheres 16 of pre8sure. UOS. Patent No. 3,682,840 (Van Loan) descr~bes 17 the preparation of lead ruthenate at temperatures of 800C.
18 and higher. However, none of these references teach that 19 lead or bismuth-containing compounds may be made by the pre-sent invention wherein they are prepared in an alkaline medium 21 a~ temperatures below about 200C, as claimed herein.
22 United States Patents Nos. 3,769,382 (Kuo et al) 23 and 3,951,672 (Langley et al) both disclose the preparation 24 o lead ruthenate and lead irida~e us~ng various techniques at temperatures of at least about 600C, and preferably at 26 higher temperatures. Likewise, however, these references 27 fail to recognize that the lead and bismuth pyrochlores made 28 by the method of the present invention are obtained at gen-11~5~S
erally lower temperatures as more specifically recited belo~.
2 Bouchard and Gillscn, Mat. Res~ Bu~l~, Vol~ 6, 3 ppO 669-680 (1971j describe Bi2Ru207 and Bi2Ir207 preparation 4 an~ properties~ including the fact that ~ese compounds have high conductivity and small Seebeck coe~ficients. However~
6 there is no teaching that these compounds may be~made by the 7 method of the present invention. Derwent's Basic 8 Abstract Journal, Section E, Chemdoc, Week No. Y25~ Abstract 9 No. 320 (August 17, 1977), Derwent Accession No. 44866Y/25 o describes electrodes for electrolys~s of alkaline and car-11 bonate 801ution~ which ~mprise nickel-plated steel strips 12 coated with high conductivity layers containing Cd2Re207, 13 Pb2Re27-y or Ni2Re2o7~ These compounds are prepared by 14 impregnating perrhenic acid and a metal nitrate such as Cd n~trate onto a nic~el strip and baking at 350Co 6 However, these compounds are all rhenates rather than ru-7 thenates or ir~dates and are not taught to be prepared 8 by the very method of the present invention.
19 It isseen thatmuch ofthe aboveprior art dealingwith 20 the synthesis of the electrically conductive pyrochlore 21 structure oxides have taught ~ynthesi~ temperatures at 22 least as high as 600C. These highly elevated temperatu~es 23 have been employed because they have been ccnsidered 24 necessary to overcome the diffusional limitations encountered 2s in solid state reactions. These highly elevated tempera 26 tures, however, result in the formation of sintered products 27 wi~h low surace areas. This is a disadvantageous condi-28 tion for materials used in catalytic and electrocatalytic ~ 3 5 1 applications since the concen~ration of available ca~a-2 lytically active sites is limited.
3 It would be desirable from both an energy conserva-4 tion standpoin~ and a maxlmization of surface area s~and-poin~ to carry out these materials syntheses a~ significa~y 6 lower temperatures, eOg. ~elow 300C, bu~ the kinetics o 7 solid state react~ons axe unfavorably sluggish. Solution 8 syntheses offer one possible approach to achieving these g very low temperat~re reactionsO For example Trehoux, 0 Abraham and Thomas, Journal of Solid State Chemlstry, Vol.21, pp. 203-209 (1977) and C.R. AcadO Sc. Paris,t. 281 ppo379~380 2 ~1975) de~cribe the ~lution preparation of ~ pyrochlore 13 compound of the formula K~.14Bio~27[BiO.27Bil.73]
14 [04.90Hl.l]OHoA8. The synthesis is carried out by adding a b~gmuth nitrate solution to a solution of 17~ potassium 6 hydroxide containing an excess of potassium hypochlor~te.
7 The reaction is carried out in this medium for 2 h,ours in 8 a reflux type of apparatus at a temperature slightly higher 19 than 100C~ The method of synthesls and the product pre-pared are diferent in many respects from the synthesis 21 method and products herein. The compound'prepared in the 22 cited reference i8 not an oxide but rather an oxy-23 hydroxide which has a significant amount o protons incor-24 porated into She bulk structure. Proton nucle æ magnetic resonance experiments on the materials of the present 26 invention S[lOW that they are oxides wh~ch do not have 27 significant amounts of protons incorporated into the struc-28 ture. The pyrochlore synthesized by Trehoux et al is not a 1 1~ 5 2 3 5 ; - 6 -1 ruthenium or iridium-contain~ng compound and, in fac~,-is 2 believed not to be an electrically conductive pyrochlore.
3 The potassium hydroxide solution used in the Trehoux 4 reference ser~es nDt only as a reactisn medium bu~ also
5 as a c~nstituent i~ the reaction since potassium is incor-
6 porated into the A site of the pyrochlore. In the method
7 o the present invention the alkali solution ~mployed is
8 801ely a react~on medium with no measurable am4unt of
9 alkali metal cations incorporated in the pyrochlore com-0 pound which results fro~ the synthesis.
MorgensSern-Badar~u and Michel, Ann. Chim.
2 Vo~. 6, pp. lO9 e~ seq. (especially at 109-113) (1971), and 13 C. Ro Acad Sc~ Paris, Vol. 271, Seire C pp. 131~-13~6 14 tl970) report the soluticn preparation of pyrochlore com-pounds having the formula Pb2Sn206 xH20 where Oc x Cl. The 6 conditionsof preparation are strictly defined as follows:
7 equimolar quantities of lead and tin are reacted from solu-8 tlon in the presence of the complexing agent nitrilo-tri-19 acetic acid (NITA) such that the concentration of [NITA~/
~pb2~] o 2. The pH of the reaction medium is fixed at 11 21 and ~he reaction i9 carried out for several hours at 80C.
22 The compound prepared by Morgenstern-Badarau et al ls a 23 hydrated oxide whereas materials made by the method of the 24 present invention are oxides~ The pyrochlore prepared in this reerence, while it does contain lead, is not a lead 26 ruthenate or iridate pyrochlore inanywaysimilar tothe mater-27 ials preparedby the method ofthe present invention. In fact 28 the pyrochlore preparedbyMorgenstern-Badarau and Michel is - . . . - , ~ 2 3~j .~ - 7 -1 believed not to be electrically conductive. While the 2 presence of a complexing agent is required in the synthesis 3 descsibed in the cited reference, no such complexing agent 4 is required in the method of preparation of the present 5 invention. Furthermore, the specified range o~ pH of the 6 synthesis medium in the method of the present invention 7 clearly differs rom the range o~ pH within which the 8 method of the cited reference will operate, In act the 9 Morgenstern-Badarau and Michel, Ann. Chim., Vol. 6j ppolO9~
lo 124 (1971) reference clesrly sta~es that no solid product cômpound can be obtained if conditions wh~ch are coincldent 12 with those specified for the present invention ~pH~ 13.5, 13 temperature ~ 80C, zero concentratio~ of c~mplexing agent) 14 are employed.
In summary, there exists a formidable body o~
16 prior art describing the existence of various pyrochlores, 17 the~r potential uses including uses as dielectric materials, 18 and describing various metals and metal oxides as electro-19 catalyst materi~ls~ Notwithstanding such prior art, there is no suggestion or teaching that the specified lead-21 colltaining or bismuth-containing pyrochlore compounds 22 may be made by the method of the present inven-23 tion.
The present invention is directed to a method o~
26 preparing compounds having the formula:
27 A2B27-y (1) 28 wherein A is selected from the group consisting of lead, 1 bismuth and mixtures thereof, B is selected from ~he 2 group consisting of ruthenium, iridium and mixtures 3 thereo~, and wherein y is a value such that 4 O~y~l.
The compounds made by the method of the present 6 in~ention, as represented by formula (1) above, display 7 the py~ochlore crystal structure. Pyrochlore structl1re 8 oxides are represented by ~he general formula A2B2060' 9 wherein A and B are metal cations~ A detailed descr~ption 0 of their cryrtallographic structure may be found in 11 Structural Inor~anic ChemistrY, Fourth Ed~tion by Ao Fo 2 Wells, ~larendon Press, Oxford, 1975. Briefly, oxides of 13 this type d~splay a face-centered cubic structure having 14 a unit c211 d~mension of about 10A. The B cations are octa-; .. . _ . _ .
hedrally coordinated by oxygen anions (0). The ~tructural framework is formed by a three dimensional array of these corner shared octahedra, each sharing corners w~th six 18 others. This framework has the composition B206. As Wells 1~ describes, this framework of oc~ahedra is "based on the 20 diamond ne~, having large holes which contain the 0' and 21 two A atoms, wh~ch themselve~ form a cupri~e-like net A20' 22 interpen~trating the octahedral framework". The octahedra 23 are actually arranged in tetrahedral clusters. These 24 ~lusters of octahedra are then tetrahedrally arranged 80 25 as to form the large holes in the structuxe described by 2~ ~ells. Each of these large holes may also be defined by 27 our tetrahedrally arranged puckered, hexagonal rings which 28 are formed by the corner sh æ ed octahedra. The A cat~ons - ~ .
523~ 1 , . .
,, 1.
reside in the center of these puckered hexagonal rings and are ~ coordinated by the six 0 anions which define the rings plus two ; more 0' cations at a slightly differen~ distance. These 0' anions : reside at the center of the large holes in the octahedral frame-work. It is the 0' anions which may be partially or totally ab-sent, leading to the general pyrochlore oxide formula A2B207-y where O<y~l. Thus, the compounds made by the method of the pre-sent invention are referred to as pyrochlore compounds, and are stoichiometric pyrochlores of the above formula, containing lead and/or bismuth A cations and containing ruthenium and/or iridium B cations.
- Tn general terms, the method of the present invention involves reacting approximately stoichiometric amounts of A and B cations to yield a pyrochlore oxide by precipitation of A andB
cations from an aqueous solution source of these cations in a liquid alkaline medium in the presence of an oxygen source suffi-cient to complete the desired stoichiometry at a temperature below about 200C for a sufficient time for reaction to occur.
The synthesis occurs entirely in a solution medium where the re-action kinetics are quite favorable and not so restrictive as is found in solid state reactions notwithstanding the low reaction temperature employed in the present method.
The aqueous solution source of reactant (A and B) cat-ions is meant by definition to include any aqueous solution which will dissolve ionic A and B cations. This metal cation contain-ing solution may be prepared using A source materials which in-clude lead nitrate, lead oxide, lead !
_ g _ .. . .. .... . . ... .. . .
~ 3 ~
MorgensSern-Badar~u and Michel, Ann. Chim.
2 Vo~. 6, pp. lO9 e~ seq. (especially at 109-113) (1971), and 13 C. Ro Acad Sc~ Paris, Vol. 271, Seire C pp. 131~-13~6 14 tl970) report the soluticn preparation of pyrochlore com-pounds having the formula Pb2Sn206 xH20 where Oc x Cl. The 6 conditionsof preparation are strictly defined as follows:
7 equimolar quantities of lead and tin are reacted from solu-8 tlon in the presence of the complexing agent nitrilo-tri-19 acetic acid (NITA) such that the concentration of [NITA~/
~pb2~] o 2. The pH of the reaction medium is fixed at 11 21 and ~he reaction i9 carried out for several hours at 80C.
22 The compound prepared by Morgenstern-Badarau et al ls a 23 hydrated oxide whereas materials made by the method of the 24 present invention are oxides~ The pyrochlore prepared in this reerence, while it does contain lead, is not a lead 26 ruthenate or iridate pyrochlore inanywaysimilar tothe mater-27 ials preparedby the method ofthe present invention. In fact 28 the pyrochlore preparedbyMorgenstern-Badarau and Michel is - . . . - , ~ 2 3~j .~ - 7 -1 believed not to be electrically conductive. While the 2 presence of a complexing agent is required in the synthesis 3 descsibed in the cited reference, no such complexing agent 4 is required in the method of preparation of the present 5 invention. Furthermore, the specified range o~ pH of the 6 synthesis medium in the method of the present invention 7 clearly differs rom the range o~ pH within which the 8 method of the cited reference will operate, In act the 9 Morgenstern-Badarau and Michel, Ann. Chim., Vol. 6j ppolO9~
lo 124 (1971) reference clesrly sta~es that no solid product cômpound can be obtained if conditions wh~ch are coincldent 12 with those specified for the present invention ~pH~ 13.5, 13 temperature ~ 80C, zero concentratio~ of c~mplexing agent) 14 are employed.
In summary, there exists a formidable body o~
16 prior art describing the existence of various pyrochlores, 17 the~r potential uses including uses as dielectric materials, 18 and describing various metals and metal oxides as electro-19 catalyst materi~ls~ Notwithstanding such prior art, there is no suggestion or teaching that the specified lead-21 colltaining or bismuth-containing pyrochlore compounds 22 may be made by the method of the present inven-23 tion.
The present invention is directed to a method o~
26 preparing compounds having the formula:
27 A2B27-y (1) 28 wherein A is selected from the group consisting of lead, 1 bismuth and mixtures thereof, B is selected from ~he 2 group consisting of ruthenium, iridium and mixtures 3 thereo~, and wherein y is a value such that 4 O~y~l.
The compounds made by the method of the present 6 in~ention, as represented by formula (1) above, display 7 the py~ochlore crystal structure. Pyrochlore structl1re 8 oxides are represented by ~he general formula A2B2060' 9 wherein A and B are metal cations~ A detailed descr~ption 0 of their cryrtallographic structure may be found in 11 Structural Inor~anic ChemistrY, Fourth Ed~tion by Ao Fo 2 Wells, ~larendon Press, Oxford, 1975. Briefly, oxides of 13 this type d~splay a face-centered cubic structure having 14 a unit c211 d~mension of about 10A. The B cations are octa-; .. . _ . _ .
hedrally coordinated by oxygen anions (0). The ~tructural framework is formed by a three dimensional array of these corner shared octahedra, each sharing corners w~th six 18 others. This framework has the composition B206. As Wells 1~ describes, this framework of oc~ahedra is "based on the 20 diamond ne~, having large holes which contain the 0' and 21 two A atoms, wh~ch themselve~ form a cupri~e-like net A20' 22 interpen~trating the octahedral framework". The octahedra 23 are actually arranged in tetrahedral clusters. These 24 ~lusters of octahedra are then tetrahedrally arranged 80 25 as to form the large holes in the structuxe described by 2~ ~ells. Each of these large holes may also be defined by 27 our tetrahedrally arranged puckered, hexagonal rings which 28 are formed by the corner sh æ ed octahedra. The A cat~ons - ~ .
523~ 1 , . .
,, 1.
reside in the center of these puckered hexagonal rings and are ~ coordinated by the six 0 anions which define the rings plus two ; more 0' cations at a slightly differen~ distance. These 0' anions : reside at the center of the large holes in the octahedral frame-work. It is the 0' anions which may be partially or totally ab-sent, leading to the general pyrochlore oxide formula A2B207-y where O<y~l. Thus, the compounds made by the method of the pre-sent invention are referred to as pyrochlore compounds, and are stoichiometric pyrochlores of the above formula, containing lead and/or bismuth A cations and containing ruthenium and/or iridium B cations.
- Tn general terms, the method of the present invention involves reacting approximately stoichiometric amounts of A and B cations to yield a pyrochlore oxide by precipitation of A andB
cations from an aqueous solution source of these cations in a liquid alkaline medium in the presence of an oxygen source suffi-cient to complete the desired stoichiometry at a temperature below about 200C for a sufficient time for reaction to occur.
The synthesis occurs entirely in a solution medium where the re-action kinetics are quite favorable and not so restrictive as is found in solid state reactions notwithstanding the low reaction temperature employed in the present method.
The aqueous solution source of reactant (A and B) cat-ions is meant by definition to include any aqueous solution which will dissolve ionic A and B cations. This metal cation contain-ing solution may be prepared using A source materials which in-clude lead nitrate, lead oxide, lead !
_ g _ .. . .. .... . . ... .. . .
~ 3 ~
- 10 -1 chloride, lead acetate, lead carbonate, lead citra~e, lead 2 oxalate~ bismuth n~trate, bismuth oxide, bismu~h cl~loride, 3 bismuth oxalate and bismuth oxychloride as well as mixtures 4 thereof~ Desirably, the A source material used in preparing the aqueous solution source of A and B cations is either a 6 lead source material or a bismuth source material, although, 7 as mentioned~ m~xtures of these may be used. .~mong the men-8 tioned A source materials, preferred are lead and bismuth 9 nitrates~ Tb~ E source materials used in preparing the aqueous solution source of A and B cations include ruthenium
11 chloride, ruthenium nitrate, ruthenium nitrosyl nitrite~ ir-
12 idium chloride, iridium hydroxide and iridium oxalic acid as 3 ~ell as m~xtures thereof. Desirably, the B source material 4 is either a ruthenium source or an iridium source, although mixtures thereo~ may be employed. The preferred B ~ource 6 materials include ruthenium nitrate and iridium chloride.
7 The aqueous solution source of A and B cations is 8 prepared by dissolving appropriate amounts of A source mater-19 ial and B source material in aqueous solvent. In some cases 20 water is adequate for this dissolution. When necessary, the 21 A and B source materials may be dissolved in aqueous acid 22 solutions, the acid solutions being just strong enough to 23 cause the A and B source materials to dissolve. Acids such 24 as nitric or hydrochlorlc may be used but nitric acid is pre-25 ~erred.
26 The A source material ~nd B source m terial are dis-27 solved in ~elative amounts so as to achieve, in general, an 28 initial reactant A to B ~on ratio which is approximately ll(~SZ3~
. . .
1 stoichiometric, i.e. about 1.0:1Ø Desirably, this ratio 2 is within the range of about 0.95:1,0 ~o about 1.5:1Ø In 3 the preferred embodiments the A to B ion ratio is in the 4 range o about 1.0:1.0 to about 1.2:1Ø However, when more than 1.0:1.0 ratio of A to B ion is employed, a minor amount 6 of A cation-rich compound (i.e. lead and/or bismuth-rich 7 material) may be obtained with a major amount of the desired 8 stoichiometric material, and the A cation-rich compound may 9 be removed~e.g. by leachings.
o Preparation of the aqueous solution source of A and 11 B cations in the manner just described assures atomic scale l2 mixlng of these cations and thereby provides favorable kinetics
7 The aqueous solution source of A and B cations is 8 prepared by dissolving appropriate amounts of A source mater-19 ial and B source material in aqueous solvent. In some cases 20 water is adequate for this dissolution. When necessary, the 21 A and B source materials may be dissolved in aqueous acid 22 solutions, the acid solutions being just strong enough to 23 cause the A and B source materials to dissolve. Acids such 24 as nitric or hydrochlorlc may be used but nitric acid is pre-25 ~erred.
26 The A source material ~nd B source m terial are dis-27 solved in ~elative amounts so as to achieve, in general, an 28 initial reactant A to B ~on ratio which is approximately ll(~SZ3~
. . .
1 stoichiometric, i.e. about 1.0:1Ø Desirably, this ratio 2 is within the range of about 0.95:1,0 ~o about 1.5:1Ø In 3 the preferred embodiments the A to B ion ratio is in the 4 range o about 1.0:1.0 to about 1.2:1Ø However, when more than 1.0:1.0 ratio of A to B ion is employed, a minor amount 6 of A cation-rich compound (i.e. lead and/or bismuth-rich 7 material) may be obtained with a major amount of the desired 8 stoichiometric material, and the A cation-rich compound may 9 be removed~e.g. by leachings.
o Preparation of the aqueous solution source of A and 11 B cations in the manner just described assures atomic scale l2 mixlng of these cations and thereby provides favorable kinetics
13 for the low temperature, solution medium synthesiQ that follows.
4 The liquid alkaline medium is meant by defi~ition to include any liquid allcaline medium which w~ll promote 16 reaçtion between the A ions and B ions from the mentioned 17 aqueous solution source of A and B cations and will effect 18 the precipitation of the desired pyrochlore structure. The l9 liquid alkaline medium may be any which satisfies this defi-nition and includes aqueous basic solutions of alkali metal 21 hydroxides. Thus, the liquid alkaline medium may desirably 22 be an aquecus basic solution containing a base selected from 23 the group consisting of sodium hydroxide, rubidium hydroxi~e, 24 ce8ium hydroxide~ potassium hydroxide and mlxtures thereo~.
Desirably, su~icient base is included so as to render a 26 liquid alka~ine medium having a pH of at least about 13.5.
.
27 Preferably, suficient base is employed so as to produce a 28 li~uid alkaline medium having a pH of between about 14 and ll~S~
1 15.5. Exact amo~nts of base material need not be specified 2 since p~ determination is within the purview of the artisan.
3 It is also found to be helpfu~, although not neces-4 sary, to saturate the alkaline reaction medium with respect to one or more of the reactan~ cations (ancL especially with 6 respect to the st alkali soluble cation reactant) prior to 7 comb~nation of the aqueous solution source o~ A and B cations 8 with the a~lGaline reaction medium. Th~s may be done so as 9 to avoid large discrepancies between cation ratios in the 10 reacted product and in the in~tial reaetant m1xture due to 11 possible solubility in the alkaline reaction medium o~ one or l2 more of the reactant cations. Thus,differential solubility 13 apparently explains why the initial reactant A to B ion ratio 4 may be set at a level higher than 1.0:1.0, even though a l5 stoichiometric pyrochlore (A:B - 1.0:1.0) is desired. This 16 i~ particularly true in the case of lead-containing wrochlores 17 gince lead is found to have a solubility in the alkaline reac-18 tion media which is several orders of magnitude greater than 19 ruthenium or iridium.
~t should be noted that the alkaline medium acts 21 solely as a reaction medium and not as a constituent in the 22 reaction. This is supported by the fact that the pyrochlores 23 made by the method of this invention show less than 0.02Zo (by 24 weight) alkali metal cation as measured by atomic absorption.
The oxygen source i5 meant to include by definition 26 any source which will provide the oxygen needed to form the 27 pyrochlore compound. The oxygen source may be any of the A
2~ source mater1al, the B source mater~al, the alkaLine l~quid 5~23 1 medium (in the form of dissolved oxygen) or combinations 2 thereof. In any event, an essential aspect of the present 3 invention compound preparation is the inclusion of adequate 4 oxygen to permit the formation of the desired pyrochlore structure. It is important to note that an essential aspect 6 of this invention is that the provision of oxygen be carried 7 out only to the extent necessary for the stabilization of 8 stoichiometric pyrochlore. For example, in the case of lead g ruthenate, the stoichiometric pyrochlore contains lead in the form of Pb2+ only. Providing a significantly more oxidizing 11- environment than that necessary for the stabilization of Pb2+
12 will lead to the formation of Pb4 and consequently the 13 synthesis of lead-rich pyrochlore, rather than the desired
4 The liquid alkaline medium is meant by defi~ition to include any liquid allcaline medium which w~ll promote 16 reaçtion between the A ions and B ions from the mentioned 17 aqueous solution source of A and B cations and will effect 18 the precipitation of the desired pyrochlore structure. The l9 liquid alkaline medium may be any which satisfies this defi-nition and includes aqueous basic solutions of alkali metal 21 hydroxides. Thus, the liquid alkaline medium may desirably 22 be an aquecus basic solution containing a base selected from 23 the group consisting of sodium hydroxide, rubidium hydroxi~e, 24 ce8ium hydroxide~ potassium hydroxide and mlxtures thereo~.
Desirably, su~icient base is included so as to render a 26 liquid alka~ine medium having a pH of at least about 13.5.
.
27 Preferably, suficient base is employed so as to produce a 28 li~uid alkaline medium having a pH of between about 14 and ll~S~
1 15.5. Exact amo~nts of base material need not be specified 2 since p~ determination is within the purview of the artisan.
3 It is also found to be helpfu~, although not neces-4 sary, to saturate the alkaline reaction medium with respect to one or more of the reactan~ cations (ancL especially with 6 respect to the st alkali soluble cation reactant) prior to 7 comb~nation of the aqueous solution source o~ A and B cations 8 with the a~lGaline reaction medium. Th~s may be done so as 9 to avoid large discrepancies between cation ratios in the 10 reacted product and in the in~tial reaetant m1xture due to 11 possible solubility in the alkaline reaction medium o~ one or l2 more of the reactant cations. Thus,differential solubility 13 apparently explains why the initial reactant A to B ion ratio 4 may be set at a level higher than 1.0:1.0, even though a l5 stoichiometric pyrochlore (A:B - 1.0:1.0) is desired. This 16 i~ particularly true in the case of lead-containing wrochlores 17 gince lead is found to have a solubility in the alkaline reac-18 tion media which is several orders of magnitude greater than 19 ruthenium or iridium.
~t should be noted that the alkaline medium acts 21 solely as a reaction medium and not as a constituent in the 22 reaction. This is supported by the fact that the pyrochlores 23 made by the method of this invention show less than 0.02Zo (by 24 weight) alkali metal cation as measured by atomic absorption.
The oxygen source i5 meant to include by definition 26 any source which will provide the oxygen needed to form the 27 pyrochlore compound. The oxygen source may be any of the A
2~ source mater1al, the B source mater~al, the alkaLine l~quid 5~23 1 medium (in the form of dissolved oxygen) or combinations 2 thereof. In any event, an essential aspect of the present 3 invention compound preparation is the inclusion of adequate 4 oxygen to permit the formation of the desired pyrochlore structure. It is important to note that an essential aspect 6 of this invention is that the provision of oxygen be carried 7 out only to the extent necessary for the stabilization of 8 stoichiometric pyrochlore. For example, in the case of lead g ruthenate, the stoichiometric pyrochlore contains lead in the form of Pb2+ only. Providing a significantly more oxidizing 11- environment than that necessary for the stabilization of Pb2+
12 will lead to the formation of Pb4 and consequently the 13 synthesis of lead-rich pyrochlore, rather than the desired
14 stoichiometric pyrochlore. While it is advantageous to bubble air or oxyge~ through the reaction medium when one wishes to 16 synthesize lead-rich or bismuth-rich pyrochlore, such practice 17 may not be desirable in preparing stoichiometric compounds, 18 unless inclusion of A cation-rich compound is acceptable. The 19 preferred practice of the present invention does not involve bubbling air or oxygen through the reaction medium but rather 21 it entails merely carrying out the reaction in the presence of 22 ambient atmosphere or with a blanket of oxygen or oxygen-23 containing gas over the reaction solution.
24 No criticality exists as to whether the aqueous solution source of A and B cations is added to the alkaline 26 medium or whether the alkaline medium is added to the aqueous 27 solution source of reactant cations. However, the former is 28 usuall~ practiced to i~sure that all o the cations see an ~las23s excess of alk21ine medium. In ~eneral~ at least about 1~0 2 liter o liqlid alkaline medium is used per sum to~al mole 3 o me~al c~ion reactan~. As mentloned, the reaction may be 4 carried out at temperatures below about 200~. Desirably~
the reaction tempera~ure is within the ra~ge o~ about 10 to 6 about 100C, Pre~erably, tne reactiQn ls carr~ed ou~ a~
~ temperatures ~ithin the range of about 50 to about 80C.
8 During the reaction period the alkaline medium may 9 be replaced with fresh alkaline medium, and although this is 10 not necessary for successful practice of the invention, it 11 is a preferred practice.
12 It has been found that the nonstoichiometric pyro-13 chlores (lead-rich and bismuth-rich) have a finite solubility 14 in alkaline media, and more specifically that the solubility
24 No criticality exists as to whether the aqueous solution source of A and B cations is added to the alkaline 26 medium or whether the alkaline medium is added to the aqueous 27 solution source of reactant cations. However, the former is 28 usuall~ practiced to i~sure that all o the cations see an ~las23s excess of alk21ine medium. In ~eneral~ at least about 1~0 2 liter o liqlid alkaline medium is used per sum to~al mole 3 o me~al c~ion reactan~. As mentloned, the reaction may be 4 carried out at temperatures below about 200~. Desirably~
the reaction tempera~ure is within the ra~ge o~ about 10 to 6 about 100C, Pre~erably, tne reactiQn ls carr~ed ou~ a~
~ temperatures ~ithin the range of about 50 to about 80C.
8 During the reaction period the alkaline medium may 9 be replaced with fresh alkaline medium, and although this is 10 not necessary for successful practice of the invention, it 11 is a preferred practice.
12 It has been found that the nonstoichiometric pyro-13 chlores (lead-rich and bismuth-rich) have a finite solubility 14 in alkaline media, and more specifically that the solubility
15 increases as the A to B ratio of the pyrochlore increases. As
16 an example, in the case of lead-rich lead ruthenate, it is
17 found that the Pb4+ component of lead-rich pyrochlores pre-
18 ferentially dissolves. This preferential dissolution of Pb4
19 from the pyrochlore can be accelerated by maintaining the
20 lead ion concentration in the alkaline medi~m at as low a
21 level as possible, and this may be achieved by frequently
22 replacing the alkaline medium with fresh alkaline medium
23 during the course of a reaction. Thus, frequent replacements
24 of the alkaline medium tend to destabilize Pb4+ in the pyro-
25 chlore, to retard lead-rich compound formation and to conse-
26 quently favor the formation of stoichiometric pyrochlore.
27 The dissolution of Pb4+ from the pyrochlore may also
28 be accelerated by keeping the concentration of alkaline medium 523~
1 as high as is reasonably possible. It is found, in general, 2 that the reduction of the A to B ratio towards 1.0:1.0 for all 3 of the pyrochlores under discussion is favored by very con-4 centrated alkaline media.
The described reaction is carried out for a time 6 sufficient for reaction to occur. With many reactant combi-7 nations, at least a partial reaction occurs almost instantly.
8 In any event, the length of time over which the reaction should g be allowed to proceed is a matter of choice. Within limits, lo howe~er, the longer tne reaction time, the greater the ex-11 tent of reaction. As a pract~cal matter, a sign~icant am~unt 12 of reacti~n product is obtained by reacting for about one 13 day, and gençrally a reaction time of about 3 to about 7 days 14 ~s advantageous.
After the reaction is completed, t~e reaction produc~
16 may be separated by known separation means, These separation 17 techniques include filtrat~on and centrifugation. Various 18 post treatments m2~ be employed as des~red, These mlght in-19 clude heat ~reatme~ts to improve the crystallinity o~ ~he 20 product and/or washing in various ~edia ln order to leach out 21 any A cation-rich products and/or unreacted metal species.
22 In a preferred embodiment of this invention, the 23 pyrochlore reaction product that has been separated from the 24 reaction medium may be washed in an organic liquid such as 25 methanol or acetone before the pyrochlore is dried. The 26 residual surface species left by such an organic liquid wash 27 will decompose during the drying step, thereby generating a 28 locally reducing atmosphere at the surface of the pyrochlore.
1 In the case of lead ruthenate, for example, this temporary 2 locally reducing atmosphere will tend to destabilize any Pb4~
3 ions which may have been present and thus insure the formation 4 of stoichiometric pyrochlore, as desired. The reaction product ultimately obtained includes one or more of the pyrochlore 6 compounds of ormula (l) above.
7 Ang the stoichiometric pyrochlore compounds ob-8 tained by the method of the present invention are:
9 Pb2RU2o7-y (2) 0 Pb2Ir207_y (3) 11 Pb~iRu207_y (4) 12 P B~Ir207_y (5) 13 Pb~BibRu2o7-y (6) 14 Pb2KucIrdo7-y (1 j and the like, wherein y is as defined, and wherein a~b32 16 and c~ds~. Also, included àre the bismuth counterparts to 17 the foregoing and other variations within the scope of 18 formula (l) which should now be apparent to the ar~isan.
19 As mentioned, the above pyrochlores pro &ced by the method of the present invention exhibit a high electronic conduc-21 tivity, thus making them particularly useful for electrode 22 applications, e.g. as oxygen electrodes.
23 The present invention will be more fully apprecia-24 ted in view of the foll~wing examples. However, these 2s examples are presented for illustrative purposes, and the 26 present invention should not be construed to be limited 27 thereto:
1 A stoichiometric lead ruthenate pyrochlore, e.g.
2 Pb2Ru2O7_y, is prepared as follows:
3 Pb(NO3)~ and Ru(NO3)3 are combined in aqueous 4 solution in an approximately 1.5:1.0 molar ratio of lead to 5 ruthenium, that is about 4.92 grams of Pb(NO3)2 and about 6 2.84 grams of Ru(NO3)3 (in aqueous solution) are added to 7 250 ml of distilled water. This solution, after being stirred, 8 is then added to 500 ml of 12M potassium hydroxide which has 9 been preheated to 75C. Precipitation of a solid occurs 10 immediately. The reaction is carried out, with stirring, 11 for approximately 260 hours with seven interruptions for 12 replacement of the alkaline medium with fresh alkaline medium 13 (12M KOH). The solid is then separated by vacuum filtration, 14 washed in distilled water, and dried at 100C. X-ray diffraction 15 shows that the reacted product is a crystalline material 16 exhibiting the pyrochlore crystal structure. Furthermore, 17 the X-ray diffraction pattern is in agreement with the X-ray 18 data presen;ed by LO~8G, ~accah a.ld ~oodenough, ,~at. ~es. Bu'l., 19 Vol. 4, pp. 191-202 (1969), for Pb2Ru20;_y~ Thus, by using 20 the method of the present invention stoichiometric, or non 21 lead-rich, pyrochlore is synthesized. The surface area of 22 the ~ynthe9iæed product, measured by the BET N2 absorption 23 method, is 142 m2/g ~ .
A stoichiometric lead ruthenate pyrochlore, e.g.
26 Pb2RU2o7-y~ is prepared as follows:
27 Pb(NO3)2 and Ru(NO3)3 are combined in aqueous solu-28 tion in an approximately 1.0:1.0 molar ratio of lead to ruth-ll~S23S
; enium, that is about 3.28 gr~ms of Pb(NO3)2 and about 2.84 grams of Ru(NO3)3 (in aqueous solution) are added to 250 ml of distill-ed water. This solution, after being stirred, is then added to 500 ml of 9M potassium hydroxide which has been preheated to 75 C
Precipitation of a solid occurs immediately. The reaction is carried out, with stirring for 64 hours. The solid is then separated by vacuum filtration, washed in hot distilled water, washed in methanol and dried at 100C. X-ray diffraction shows that the reacted product is a crystalline material exhibiting the pyrochlore crystal structure. Furthermore, the x-ray diffrac-tion pattern is in agreement with the x-ray data presented by Longo, Raccah and Goodenough, Mat. Res. Bu11., Vol. 4, pp. 191-202 (1969), for Pb2Ru207-y. Thus, by using the method of the present invention stoichiometric, or non lead-rich, pyrochlore is synthesized. The ~urface area of the synthesized product, meas-ured by the BET N2 adsorption method, is 77 m2/g.
To illustrate the utility of the compound which is ob-tained by the method of Example 2, electrocatalytic performance data are obtained in 3N KOH at 75C. In these test, the material is fabricated into test electrodes consisting of the catalyst, abinder, a ~etproofing agent and a support. Teflon~ serves as both a binder and wetproofing agent for all the electrodes tested.
Gold expanded metal screen is used as the support.
Electrodes are fabricated by mixing a weighed amount of catalyst with a few drops of water, adding a measured volume * Trade Mark .. ' - ..
523~
1 of Teflon 42 suspension, and mixing vigorously to precipitate 2 the Teflon. The gummy product is then spread on weigh~d gold 3 Exmet screen and is pressed dry between filter paper. The 4 electrode is then cold pressed for 0.5 min. at 200 psi, is allowed to air dry for 30 min. and is then hot pressed in an 6 inert atmosphere at 325C, 500 psi for 0.5 min. After cooling, 7 the electrode is weighed to determine its loading and then 8 placed in the electrochemical cell for testing.
9 The electrochemical half-cell used or testing is lo of the interface maintaining type and consists of a jacketed 11 lqiuid phase cell compartment. The liquid side contains the 12 platinum wire counter electrode, a saturated calomel reference 13 electrode (in contact by Lugin capillary), and magnetic stirrer.
14 The gas side contains the gas (oxygen) inlet and outlet and a stopcock to drain off any condensate. The working electrode 16 is held in place (between the two compartments) between two 17 Teflon discs with a gold current collector pressing against 18 it.
19 The cell is connected to a Princeton Applied Research Model 173 potentiostat with a programmer and logarithmic 21 current converter. Constant rate potential sweep measure-22 ments are ccnducted. Outputs of potential and log o current 23 are recorded on an x-y plotter, and the resulting potential 24 vs. log current density plot, referred to as a performance curve, is used to e~aluate the electrode activity.
26 Table I shows performance data for the electro-27 catalytic reduction of oxygen in 3N KOG at 75C using the 28 stoichiometric pyrochlore of Example 2. Also included in 523~
: 2 Table I are data for a stoichiometric lead ruthenate, of 6 m ~g :
surface area, prepared by conventional solid state reaction tech-niques. The data in Table I show that the stoichiometric pyro-- chlore of Example 2 does have significant electrochemical activity for oxygen reduction. Furthermore, the data in Table I shows that the stoichiometric pyrochlore of Example 2 (which is prepared : at low temperature out of solution and consequently has a rela-tively high surface area) has oxygen electro-reduction capability superior to the lower surface area, stoichiometric pyrochlore . 10 synthesized by conventional solid state techniques. .
TABLE I
. Activity Data For The Electro-Reduction Of Oxygen In 3N KOH at 75C
Potential (mV vs. R~IE) Pb2Ru~o ~y Prepared By Current Density Pb2Ru2O7-y Convent~onal Solid State (mA/cm2) Of Example 2 Reaction Techniques 0.5 1074 938 1.0 1074 920 20 5.0 998 886 The subject matter of this application is related to that of commonly-assigned copending Canadian Applications S.N.
315,786; 315,846 and 316,046.
1 as high as is reasonably possible. It is found, in general, 2 that the reduction of the A to B ratio towards 1.0:1.0 for all 3 of the pyrochlores under discussion is favored by very con-4 centrated alkaline media.
The described reaction is carried out for a time 6 sufficient for reaction to occur. With many reactant combi-7 nations, at least a partial reaction occurs almost instantly.
8 In any event, the length of time over which the reaction should g be allowed to proceed is a matter of choice. Within limits, lo howe~er, the longer tne reaction time, the greater the ex-11 tent of reaction. As a pract~cal matter, a sign~icant am~unt 12 of reacti~n product is obtained by reacting for about one 13 day, and gençrally a reaction time of about 3 to about 7 days 14 ~s advantageous.
After the reaction is completed, t~e reaction produc~
16 may be separated by known separation means, These separation 17 techniques include filtrat~on and centrifugation. Various 18 post treatments m2~ be employed as des~red, These mlght in-19 clude heat ~reatme~ts to improve the crystallinity o~ ~he 20 product and/or washing in various ~edia ln order to leach out 21 any A cation-rich products and/or unreacted metal species.
22 In a preferred embodiment of this invention, the 23 pyrochlore reaction product that has been separated from the 24 reaction medium may be washed in an organic liquid such as 25 methanol or acetone before the pyrochlore is dried. The 26 residual surface species left by such an organic liquid wash 27 will decompose during the drying step, thereby generating a 28 locally reducing atmosphere at the surface of the pyrochlore.
1 In the case of lead ruthenate, for example, this temporary 2 locally reducing atmosphere will tend to destabilize any Pb4~
3 ions which may have been present and thus insure the formation 4 of stoichiometric pyrochlore, as desired. The reaction product ultimately obtained includes one or more of the pyrochlore 6 compounds of ormula (l) above.
7 Ang the stoichiometric pyrochlore compounds ob-8 tained by the method of the present invention are:
9 Pb2RU2o7-y (2) 0 Pb2Ir207_y (3) 11 Pb~iRu207_y (4) 12 P B~Ir207_y (5) 13 Pb~BibRu2o7-y (6) 14 Pb2KucIrdo7-y (1 j and the like, wherein y is as defined, and wherein a~b32 16 and c~ds~. Also, included àre the bismuth counterparts to 17 the foregoing and other variations within the scope of 18 formula (l) which should now be apparent to the ar~isan.
19 As mentioned, the above pyrochlores pro &ced by the method of the present invention exhibit a high electronic conduc-21 tivity, thus making them particularly useful for electrode 22 applications, e.g. as oxygen electrodes.
23 The present invention will be more fully apprecia-24 ted in view of the foll~wing examples. However, these 2s examples are presented for illustrative purposes, and the 26 present invention should not be construed to be limited 27 thereto:
1 A stoichiometric lead ruthenate pyrochlore, e.g.
2 Pb2Ru2O7_y, is prepared as follows:
3 Pb(NO3)~ and Ru(NO3)3 are combined in aqueous 4 solution in an approximately 1.5:1.0 molar ratio of lead to 5 ruthenium, that is about 4.92 grams of Pb(NO3)2 and about 6 2.84 grams of Ru(NO3)3 (in aqueous solution) are added to 7 250 ml of distilled water. This solution, after being stirred, 8 is then added to 500 ml of 12M potassium hydroxide which has 9 been preheated to 75C. Precipitation of a solid occurs 10 immediately. The reaction is carried out, with stirring, 11 for approximately 260 hours with seven interruptions for 12 replacement of the alkaline medium with fresh alkaline medium 13 (12M KOH). The solid is then separated by vacuum filtration, 14 washed in distilled water, and dried at 100C. X-ray diffraction 15 shows that the reacted product is a crystalline material 16 exhibiting the pyrochlore crystal structure. Furthermore, 17 the X-ray diffraction pattern is in agreement with the X-ray 18 data presen;ed by LO~8G, ~accah a.ld ~oodenough, ,~at. ~es. Bu'l., 19 Vol. 4, pp. 191-202 (1969), for Pb2Ru20;_y~ Thus, by using 20 the method of the present invention stoichiometric, or non 21 lead-rich, pyrochlore is synthesized. The surface area of 22 the ~ynthe9iæed product, measured by the BET N2 absorption 23 method, is 142 m2/g ~ .
A stoichiometric lead ruthenate pyrochlore, e.g.
26 Pb2RU2o7-y~ is prepared as follows:
27 Pb(NO3)2 and Ru(NO3)3 are combined in aqueous solu-28 tion in an approximately 1.0:1.0 molar ratio of lead to ruth-ll~S23S
; enium, that is about 3.28 gr~ms of Pb(NO3)2 and about 2.84 grams of Ru(NO3)3 (in aqueous solution) are added to 250 ml of distill-ed water. This solution, after being stirred, is then added to 500 ml of 9M potassium hydroxide which has been preheated to 75 C
Precipitation of a solid occurs immediately. The reaction is carried out, with stirring for 64 hours. The solid is then separated by vacuum filtration, washed in hot distilled water, washed in methanol and dried at 100C. X-ray diffraction shows that the reacted product is a crystalline material exhibiting the pyrochlore crystal structure. Furthermore, the x-ray diffrac-tion pattern is in agreement with the x-ray data presented by Longo, Raccah and Goodenough, Mat. Res. Bu11., Vol. 4, pp. 191-202 (1969), for Pb2Ru207-y. Thus, by using the method of the present invention stoichiometric, or non lead-rich, pyrochlore is synthesized. The ~urface area of the synthesized product, meas-ured by the BET N2 adsorption method, is 77 m2/g.
To illustrate the utility of the compound which is ob-tained by the method of Example 2, electrocatalytic performance data are obtained in 3N KOH at 75C. In these test, the material is fabricated into test electrodes consisting of the catalyst, abinder, a ~etproofing agent and a support. Teflon~ serves as both a binder and wetproofing agent for all the electrodes tested.
Gold expanded metal screen is used as the support.
Electrodes are fabricated by mixing a weighed amount of catalyst with a few drops of water, adding a measured volume * Trade Mark .. ' - ..
523~
1 of Teflon 42 suspension, and mixing vigorously to precipitate 2 the Teflon. The gummy product is then spread on weigh~d gold 3 Exmet screen and is pressed dry between filter paper. The 4 electrode is then cold pressed for 0.5 min. at 200 psi, is allowed to air dry for 30 min. and is then hot pressed in an 6 inert atmosphere at 325C, 500 psi for 0.5 min. After cooling, 7 the electrode is weighed to determine its loading and then 8 placed in the electrochemical cell for testing.
9 The electrochemical half-cell used or testing is lo of the interface maintaining type and consists of a jacketed 11 lqiuid phase cell compartment. The liquid side contains the 12 platinum wire counter electrode, a saturated calomel reference 13 electrode (in contact by Lugin capillary), and magnetic stirrer.
14 The gas side contains the gas (oxygen) inlet and outlet and a stopcock to drain off any condensate. The working electrode 16 is held in place (between the two compartments) between two 17 Teflon discs with a gold current collector pressing against 18 it.
19 The cell is connected to a Princeton Applied Research Model 173 potentiostat with a programmer and logarithmic 21 current converter. Constant rate potential sweep measure-22 ments are ccnducted. Outputs of potential and log o current 23 are recorded on an x-y plotter, and the resulting potential 24 vs. log current density plot, referred to as a performance curve, is used to e~aluate the electrode activity.
26 Table I shows performance data for the electro-27 catalytic reduction of oxygen in 3N KOG at 75C using the 28 stoichiometric pyrochlore of Example 2. Also included in 523~
: 2 Table I are data for a stoichiometric lead ruthenate, of 6 m ~g :
surface area, prepared by conventional solid state reaction tech-niques. The data in Table I show that the stoichiometric pyro-- chlore of Example 2 does have significant electrochemical activity for oxygen reduction. Furthermore, the data in Table I shows that the stoichiometric pyrochlore of Example 2 (which is prepared : at low temperature out of solution and consequently has a rela-tively high surface area) has oxygen electro-reduction capability superior to the lower surface area, stoichiometric pyrochlore . 10 synthesized by conventional solid state techniques. .
TABLE I
. Activity Data For The Electro-Reduction Of Oxygen In 3N KOH at 75C
Potential (mV vs. R~IE) Pb2Ru~o ~y Prepared By Current Density Pb2Ru2O7-y Convent~onal Solid State (mA/cm2) Of Example 2 Reaction Techniques 0.5 1074 938 1.0 1074 920 20 5.0 998 886 The subject matter of this application is related to that of commonly-assigned copending Canadian Applications S.N.
315,786; 315,846 and 316,046.
Claims (14)
PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A method of preparing compounds having the formula:
A2B2O7-y wherein A is selected from the group consisting of lead, bismuth and mixtures thereof, wherein B is selected from the group con-sisting of ruthenium, iridium and mixtures thereof, wherein y is a value such that 0 ? y ? 1, comprising:
reacting A cations and B cations, in approximately stoichiometric amounts, from an aqueous solution source of these cations in a liquid alkaline medium having a pH of at least about 13.5 in the presence of an oxygen source at a temperature below about 200°C for a sufficient time for reaction to occur.
A2B2O7-y wherein A is selected from the group consisting of lead, bismuth and mixtures thereof, wherein B is selected from the group con-sisting of ruthenium, iridium and mixtures thereof, wherein y is a value such that 0 ? y ? 1, comprising:
reacting A cations and B cations, in approximately stoichiometric amounts, from an aqueous solution source of these cations in a liquid alkaline medium having a pH of at least about 13.5 in the presence of an oxygen source at a temperature below about 200°C for a sufficient time for reaction to occur.
2. The method of claim 1 wherein said aqueous solution source contains A source material selected from the group con-sisting of lead nitrate, lead oxide, lead chloride, lead acetate, lead carbonate, lead citrate, lead oxalate; bismuth nitrate, bismuth oxide, bismuth chloride, bismuth oxalate, bismuth oxy-chloride and mixtures thereof and B source material selected from the group consisting of ruthenium chloride, ruthenium nitrate, ruthenium nitrosyl nitrite, iridium chloride, iridium hydroxide and iridium oxalic acid.
3. The method of claim 2 wherein said liquid alkaline medium is an aqueous basic solution of alkali metal hydroxide.
4. The method of claim 3 wherein said aqueous basic solution contains a base selected from the group consisting of sodium hydroxide, rubidium hydroxide, cesium hydroxide, potas-sium hydroxide and mixtures thereof.
5. The method of claim 4 wherein said reacting is performed within the temperature range of about 10°C to about 100°C.
6. The method of claim 5 wherein said pH is within the range of about 14 to about 15.5.
7. The method of claim 6 wherein said reacting is performed within the temperature range of about 50°C to about 80°C.
8. The method of claim 1 wherein, during said react-ing, sufficient fresh alkaline medium is added to the reaction to retard formation of A cation-rich compound and to facilitate the desired reaction.
9. The method of claim 1 wherein, after said react-ing, the product obtained is subjected to leaking to remove undesired impurities.
10. The method of claim 1 wherein A is lead.
11. The method of claim 10 wherein said aqueous solu-tion source contains as A source material lead nitrate and con-tains B source material selected from the group consisting of ruthenium nitrate and iridium chloride.
12. The method of claim 1 wherein A is bismuth.
13. The method of claim 12 wherein said aqueous solu-tion source contains as A source bismuth nitrate and contains B source material selected from the group consisting of ruthen-ium nitrate and iridium chloride.
14. The method of claim 10 or 12 wherein, after said reacting, the product obtained is subjected to leaking to remove undesired impurities.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US938,374 | 1978-08-31 | ||
| US05/938,374 US4176094A (en) | 1977-12-02 | 1978-08-31 | Method of making stoichiometric lead and bismuth pyrochlore compounds using an alkaline medium |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1105235A true CA1105235A (en) | 1981-07-21 |
Family
ID=25471324
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA324,813A Expired CA1105235A (en) | 1978-08-31 | 1979-04-03 | Method of making stoichiometric lead and bismuth pyrochlore compounds using an alkaline medium |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPS5537479A (en) |
| CA (1) | CA1105235A (en) |
| CH (1) | CH639354A5 (en) |
| DE (1) | DE2915634A1 (en) |
| FR (1) | FR2434784A1 (en) |
| GB (1) | GB2029385B (en) |
Families Citing this family (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS63148695U (en) * | 1987-03-20 | 1988-09-30 | ||
| JP4568124B2 (en) * | 2005-01-14 | 2010-10-27 | 学校法人同志社 | Air electrode and air secondary battery using the air electrode |
| JP2010170998A (en) * | 2008-12-24 | 2010-08-05 | Mitsubishi Heavy Ind Ltd | Electrode catalyst for fuel cell and its selection method |
| GB201021352D0 (en) | 2010-12-16 | 2011-01-26 | Johnson Matthey Plc | Catalyst layer |
| GB201213832D0 (en) * | 2012-08-03 | 2012-09-19 | Johnson Matthey Plc | Cathode |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1816105C3 (en) * | 1967-12-20 | 1974-01-24 | E.I. Du Pont De Nemours And Co., Wilmington, Del. (V.St.A.) | Electrically conductive bismuth ruthenium oxide and its use for producing an electrical resistor |
-
1979
- 1979-04-03 CA CA324,813A patent/CA1105235A/en not_active Expired
- 1979-04-18 DE DE19792915634 patent/DE2915634A1/en not_active Withdrawn
- 1979-04-26 CH CH395779A patent/CH639354A5/en not_active IP Right Cessation
- 1979-05-01 JP JP5396679A patent/JPS5537479A/en active Pending
- 1979-05-22 GB GB7917775A patent/GB2029385B/en not_active Expired
- 1979-08-30 FR FR7921775A patent/FR2434784A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| JPS5537479A (en) | 1980-03-15 |
| CH639354A5 (en) | 1983-11-15 |
| FR2434784B1 (en) | 1984-02-24 |
| FR2434784A1 (en) | 1980-03-28 |
| GB2029385A (en) | 1980-03-19 |
| GB2029385B (en) | 1982-10-27 |
| DE2915634A1 (en) | 1980-03-20 |
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