Abstract:
A hydrogen oxidation catalyst which is formed of a Dawson-type polyoxometalate compound represented by general formula (I). X a [P 2 M b O 61 Ru c (L) d ] (I) (In the formula, X represents a monovalent cation independently selected from among an alkali metal cation, a tetraalkyl ammonium cation and a tetraalkyl phosphonium cation; M represents a transition metal independently selected from among V, Nb, Mo and W; L represents a ligand independently selected from among H 2 O and an organic ligand, provided that at least one L is H 2 O; a represents the number of cations (X) necessary for neutralizing the electrical charge of the compound as a whole; b represents an integer of 12-17 and c represents an integer of 1-6, provided that the total of b and c is equal to 18; and d represents an integer that is equal to c.)

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a hydrogen oxidation catalyst capable of oxidizing a hydrogen molecule (H 2 ) to hydrogen ions (H + ), and more particularly, it relates to a hydrogen oxidation catalyst formed of a specific ruthenium-containing polyoxometalate compound. 
       BACKGROUND ART 
       [0002]    Fuel cells are devices for extracting electric energy from fuels such as hydrogen and ethanol by an electrochemical reaction, and have a low environmental load due to their low carbon dioxide emission, and therefore have attracted attention in recent years. In fuel cells, platinum fine particles and platinum compounds are generally known as a catalyst or mediator for promoting a hydrogen oxidation reaction (i.e., a reaction represented by formula (1) below, in which hydrogen molecule (H 2 ) is oxidized to hydrogen ions (H + )) and oxygen reducing reaction (i.e., a reaction represented by formula (2) below, in which an oxygen molecule reacts with hydrogen ions and electrons to produce water). 
         [0000]      H 2 →2H + +2 e   −   (1)
 
         [0000]      O 2 +4H + +4 e   − →2H 2 O   (2)
 
         [0003]    However, platinum is very expensive. Also, in fuel cells, the precipitation of platinum as platinum particles causes the degradation of the components of the fuel cell, including the platinum catalyst, electrolyte membrane, and others (Patent Literature 1). Accordingly, as a material capable of oxidizing hydrogen a material which is alternative to platinum or is free of platinum is required. Although Patent Literature 2 describes a redox fuel cell in which a catholyte solution containing a polyoxometalate redox couple is at least partially reduced at the cathode in operation of the cell, and is at least partially re-generated by reaction with an oxidant after the reduction at the cathode, it does not describe that the redox couple has hydrogen-oxidizing ability. 
       CITATION LIST 
     Patent Literature 
       [0004]    Patent Literature 1: Japanese Unexamined Patent Publication No. 2009-289681 
         [0005]    Patent Literature 2: Japanese Unexamined Patent Publication No. 2010-541127 
       SUMMARY OF INVENTION 
     Problems to be Solved by the Invention 
       [0006]    The present inventors have made intensive research with a view to solve the above problems, and as a result have found that a specific ruthenium-containing polyoxometalate compound has hydrogen-oxidizing ability and is useful as hydrogen oxidation catalyst. 
       Solution to Problem 
       [0007]    The present invention provides a hydrogen oxidation catalyst which is formed of a Dawson-type polyoxometalate compound represented by general formula (I): 
         [0000]      X a [P 2 M b O 61 Ru c (L) d ]  (I)
 
         [0000]    wherein X represents a monovalent cation independently selected from an alkali metal cation, a tetraalkyl ammonium cation and a tetraalkyl phosphonium cation;
   M represents a transition metal independently selected from vanadium (V), niobium (Nb), molybdenum (Mo) and tungsten (W);   L represents a ligand independently selected from H 2 O and an organic ligand, provided that at least one L is H 2 O;   a represents the number of cations X needed for the electrical charge of the overall compound to be neutral;   b is an integer of from 12 to 17 and c is an integer of from 1 to 6, provided that the total of b and c is equal to 18; and   d is an integer which is equal to c.   
 
         [0013]    In the above general formula (I), X is a monovalent cation independently selected from alkali metal cations, preferably lithium, ion, sodium ion, potassium ion, and rubidium ion; quaternary ammonium cations, preferably tetraalkylammonium cations, such as tetramethylammonium cation, tetraethylammonium cation, and tetrabutylammonium cation; and quaternary phosphonium cations, preferably tetraalkylphosphonium cations, such as tetramethylphosphonium cation, tetraethylphosphonium cation, and tetrabutylphosphonium cation. 
         [0014]    M is a transition metal independently selected from vanadium (V), niobium (Nb), molybdenum (Mo) and tungsten (W), and M is preferably tungsten (W). 
         [0015]    L is a ligand independently selected from H 2 O and organic ligands such as pyridine and sulfoxide derivatives (such as dimethyl sulfoxide), provided that at least one L is H 2 O. L is preferably H 2 O. While not wishing to be bound by any theory, it is believed that the part on the ruthenium atom to which H 2 O coordinates as a ligand is an active site. Accordingly, the polyoxometalate compound of the present invention has at least one H 2 O as ligand L. 
         [0016]    “a” is the number of cations X needed for the electrical charge of the overall polyoxometalate compound to be neutral. The number b of metal M is an integer of 12 to 17, and the number c of ruthenium is an integer of 1 to 6, provided that the total of the number b of metal M and the number c of ruthenium is equal to 18. The number d of the ligand is an integer which is equal to the number c of ruthenium. 
         [0017]    In one preferred embodiment of the present invention, examples of the Dawson-type polyoxometalate compound represented by the above general formula (I) are K 7 [α 1 -P 2 W 17 O 61 Ru(H 2 O)] and K 7 [α 2 -P 2 W 17 O 61 Ru(H 2 O)]. In a more preferred embodiment of the present invention, examples of the polyoxometalate compound represented by the above general formula (I) is K 7 [α 1 -P 2 W 17 O 61 Ru(H 2 O)]. 
       Effects of the Invention 
       [0018]    According to the present invention, a hydrogen oxidation catalyst having hydrogen oxidizing ability is provided. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1( a )  represents the fundamental structure of a Dawson-type polyoxometalate anion [α-P 2 W 18 O 62 ] 6− ,  FIG. 1( b )  represents the structure of [α 1 -P 2 W 17 O 61 Ru] 7−  which is one example of α 1 -isomers of [α-P 2 W 18 O 62 ] 6− , and  FIG. 1( c )  represents the structure of [α 2 -P 2 W 17 O 61 Ru] 7−  which is one example of α 2 -isomers of [α-P 2 W 18 O 62 ] 6− . 
           [0020]      FIG. 2  represents the cyclic voltammogram of K 7 [α 1 -P 2 W 17 O 61 Ru(H 2 O)] as determined in 0.5M H 2 SO 4 . 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0021]    The embodiments of the present invention will be described with reference to the drawings. 
         [0022]    As described above, the present invention provides a catalyst formed of a specific polyoxometalate compound containing ruthenium. 
         [0023]    The structure of Dawson-type polyoxometalates is briefly described as follows. Dawson-type polyoxometalates have a plurality of octahedral fundamental units in which six oxide ions (O 2− ) coordinate to metal M, and there exist isomers of α-form, β-form, γ-form, δ-form, ε-form, etc., depending whether adjacent octahedrons share edges or not or whether adjacent octahedrons share apexes or not. The Dawson-type polyoxometalate compounds have a structure in which 18 octahedrons are condensed so as to share apexes, and have M and two atoms such as boron, silicon, sulfur, and phosphorus, incorporated in the structure. Generally, both ends in the molecular long axis direction of this compound are referred to as cap regions and the site located between the two cap regions is referred to as a belt region. 
         [0024]    Next,  FIG. 1( a )  illustrates the fundamental structure of Dawson-type polyoxometalates, taking [α-P 2 W 18 O 62 ] 6−  as an example. As illustrated in  FIG. 1( a ) , [α-P 2 W 18 O 62 ] 6−  has two regions referred to as caps, and a region located between them, called as a belt. An isomer in which one tungsten present in the belt region is substituted with another atom (in  FIG. 1( b ) , ruthenium (Ru(III))) as shown in  FIG. 1( b )  is α 1 -isomer (i.e., [α 1 -P 2 W 17 O 61 Ru] 7− ), and an isomer in which the tungsten present in a cap region is substituted with another atom (in  FIG. 1( c ) , ruthenium (Ru(III))) as shown in  FIG. 1( c )  is α 2 -isomer (i.e., [α 2 -P 2 W 17 O 61 Ru] 7− ). The α 1 -isomer is racemic. In  FIGS. 1( b ) and ( c ) , octahedrons represent a fundamental unit in which six oxide ions (O 2− ) coordinate to tungsten (W 6+ ), and tetrahedrons (black colored regions) represent another fundamental unit in which phosphorus (P) shares oxygen with tungsten which constitutes the octahedrons. In addition, in  FIGS. 1( b ) and ( c ) , ruthenium is represented as a spherical body so that the position of the ruthenium is easily recognized. Although not shown in  FIGS. 1( a ), ( b ), and ( c ) , in the polyoxometalate compound of the present invention, ligands are coordinated to the ruthenium atom. 
         [0025]    Although the ruthenium-containing polyoxometalate compound of the present invention has been explained with reference to  FIGS. 1( a ), ( c ), and ( c ) , the present invention is not limited to the compounds shown in  FIGS. 1( b ) and ( c ) . Ruthenium may be present in both cap regions and belt region. 
         [0026]    The method for preparing the ruthenium-containing polyoxometalate compound of the present invention is not particularly limited as long as the tungsten in the corresponding precursor can be substituted with ruthenium, and ligands independently selected from H 2 O and an organic ligand can coordinate to the ruthenium. For example, K 10 [α 1 -P 2 W 17 O 61 Ru(H 2 O)], which is a preferred ruthenium-containing polyoxometalate compound of the present invention, can be obtained by, for example, preparing K 10 [α 2 -P 2 W 17 O 61 ].15H 2 O via a well-known method (R. Contant, W. G. Klemperer, O. Yaghi, Inorg. Synth. 1990, 27, 104-118.), reacting the resulting K 10 [α 2 -P 2 W 17 O 61 ].15H 2 O with Ru 2  (benzene) 2 Cl 4  to synthesize K 7 [P 2 W 17 O 61 Ru III (H 2 O)] (a mixture of α 1 -form and α 2 -form), and subsequently, reacting the resulting K 7 [P 2 W 17 O 61 Ru III (H 2 O)] (a mixture of α 1 -form and α 2 -form) and dimethyl sulfoxide (dmso) to form K 8 [P 2 W 17 O 61 Ru II (dmso)] (a mixture of α 1 -form and α 2 -form), purifying it using the solubility difference to obtain K 8 [α 1 -P 2 W 17 O 61 Ru II (dmso)] (α 1 -form only), and heating K 8 [α 1 -P 2 W 17 O 61 Ru II (dmso)] (α 1 -form only) in water to exchange dmso with water. 
         [0027]    The hydrogen oxidation catalyst formed of the polyoxometalate compound of the present invention can be used after being supported on a carrier, for example, oxides such as silica, alumina, magnesia, and titania; carbon materials such as activated carbon and carbon nanotube; etc. The type, shape, and dimensions of the carrier can be appropriately selected according to the usage of the catalyst. 
       EXAMPLES 
       [0028]    The present invention will be further explained with reference to the following examples, and it should be understood that the scope of the present invention is not limited by these examples. 
       Synthesis for K 7 α 1 -P 2 W 17 O 61 Ru(H 2 O) (α 1 -Form) 
       [0029]    K 7 α 1 -P 2 W 17 O 61 Ru(H 2 O) (α 1 -form) was synthesized via synthesis steps 1 to 4 as follows. 
       Synthesis Step 1 
     Synthesis for K 10 [α 2 -P 2 W 17 O 61 ].15H 2 O 
       [0030]    K 10 [α 2 -P 2 W 17 O 61 ].15H 2 O was prepared via a well-known method (R. Contant, W. G, Klemperer, O. Yaghi, Inorg. Synth. 1990, 27, 104-118.) 
       Synthesis Step 2 
     Synthesis for K 7 [P 2 W 17 O 61 Ru III (H 2 O) ] (a Mixture of α 1 -Form and α 2 -Form) 
       [0031]    Ru 2 (benzene) 2 Cl 4  (0.085 g, 0.17 mmol), K 10 [α 2 -P 2 W 17 O 61 ].15H 2 O (1.592 g, 0.33 mmol), and water (20 mL) were charged into a 100 mL Teflon® inner tube-type autoclave, and were allowed to react for 5 hours at 170° C. After cooling the autoclave, the resulting solution was filtered, a 15 ml of acetone was added to the resulting filtrate, and the resulting mixture was agitated for one hour at room temperature. The precipitates generated were separated out by centrifugal separation, and a 100 mL of acetone was added to the resulting liquid. The mixture thus obtained was agitated for 30 minutes at room temperature to obtain a dark-brown liquid. The dark-brown liquid was allowed to stand for overnight in a refrigerator. The solid matters precipitated during standing were filtered out, and the solid matters were washed with a 100 mL of acetone and dried at 70° C. to obtain K 7 [P 2 W 17 O 61 Ru III (H 2 O)] (a mixture of α 1 -form and α 2 -form) (yield: 0.67 g, yield percentage: 42% (weight basis)). 
       Analysis Results 
       [0032]    IR (KBr): ν=1090 (s), 1079 (s), 1055 (sh), 1014 (m), 950 (s), 915 (s), 817 (sh), 777 (vs) cm − 1. Cyclic voltammogram (in 0.5M KH 2 PO 4  aqueous solution (pH 4.3)): E 1/2 (Ru V/IV)= 1068 mV, E 1/2 (Ru IV/III )=719 mV, and E 1/2 (Ru III/II )=180 mV. Anionic MS (CH 3 CN—H 2 O): calculated m/z value for [P 2 W 17 O 61 RuOH 3 ] 4   − =1070.9331, measured m/z value=1070.9338. 
       Synthesis Step 3 
     Synthesis for K 8 [α 1 -P 2 W 17 O 61 Ru II (dmso)]-4KCl-22H 2 O 
       [0033]    K 7 [P 2 W 17 O 61 Ru(H 2 O)] (a mixture of α 1 -form and α 2 -form) (0.745 g, 0.43 mmol) obtained in step 2 and dimethyl sulfoxide (dmso, 1 mL, 14.08 mmol) were added to a 49 mL of water and agitated for 4 days at 80° C., and after cooling the resulting reaction liquid, KCl (1.5 g) was added to the reaction liquid, and the reaction liquid was agitated until the added KCl was dissolved therein, and the reaction liquid was allowed to stand for overnight. After filtering out the precipitates generated during standing, KCl (1.5 g) was added to the resulting filtrate. The resulting liquid was agitated for one hour at room temperature, and the resulting solution was allowed to stand for overnight in a refrigerator. The solid matters precipitated during standing were filtered out, a 15 mL of acetone was added to the resulting filtrate, and the liquid was agitated for one hour at room temperature. After filtering out the precipitates generated, a 100 mL of acetone was added to the filtrate, and the resulting liquid was agitated for 30 minutes. The resulting dark-green solution was allowed to stand for overnight in a refrigerator. The solid matters generated during standing were filtered out, were washed with a 100 mL of acetone, and were dried at 70° C. to obtain K 8 [P 2 W 17 O 61 Ru II (dmso)]-4KCl-22H 2 O (yield: 0.59 g, yield percentage: 80% (weight basis). 
       Analysis Results 
       [0034]    IR (KBr): ν=1082 (s), 1015 (m), 946 (m), 906 (s), 820 (vs), 781 (vs), 721 (s) cm −1 . UV/Vis (0.5M KH 2 PO 4 ): λmax=445 nm (ε=1.7×10 3  dm 3 ·mol −1 ·cm −1 ) and 597 nm (ε=2.0×10 3  dm 3 ·mol −1 ·cm −1 ). Cyclic voltammogram (in 0.5M KH 2 PO 4  aqueous solution (pH 4.3): E 1/2 (Ru IV/III )=1341 mV and E 1/2 (Ru III/II )=560 mV.  1 H-NMR (D 2 O): (δ/ppm) 3.16 (s, 3H), 3.08 (s, 3H) (cf. 4.659 for HOD).  13 C-NMR (D 2 O): (δ/ppm) 44.18, 43.02 (cf. 30.103 for (CH 3 ) 2 CO).  31 P-NMR (D 2 O): (δ/ppm) −9.67, −12.84.  183 W-NMR (D 2 O): (δ/ppm) 212.4, 127.4, 35.3, −104.4, −122.4, −127.5, −130.1, −137.1, −154.8, −157.2, −159.0, −169.1, −187.3, −200.9, −204.4, −217.7. Elementary analysis: calculated for K 8 [P 2 W 17 O 61 Ru(C 2 H 6 SO)]-4KCl-22H 2 O: C 0.45; H 0.94; P 1.16; W 58.4; Ru 1.89; K 8.77; Na 0; S 0.60; Cl 2.65%; Measured: C 0.67; H 0.71; P 1.21; W 58.3; Ru 2.05; K 8.94; Na&lt;0.01; S 0.45; Cl 2.54%. Anionic MS (CH 3 CN—H 2 O): calculated m/z value for [P 2 W 17 O 61 Ru(dmso)H 2 K] 4− =1095.9267, measured m/z value= 0  1095.92 47. 
       Synthesis Step 4 
       [0035]    K 8 [P 2 W 17 O 61 Ru(C 2 H 6 SO)] (0.158 g) obtained in synthesis step 3 was mixed with a 5 mL of water, and they were allowed to react in a 100 mL Teflon® inner tube-type autoclave for 20 hours at 170° C. After cooling the resulting reaction liquid, a 20 mL of acetone was added thereto to separate out precipitates. The precipitates were filtered out, were washed with a 20 mL of acetone, and were dried at 70° C. to obtain K 7 α 1 -P 2 W 17 O 61 Ru(H 2 O) (α 1 -form). 
       Analysis 
       [0036]    Cyclic voltammogram (in 0.5M KH 2 PO 4  aqueous solution (pH 4.3), vs. NHE): E 1/2 (Ru V/IV )=1059 mV, E 1/2 (Ru IV/III )=716 mV, and E 1/2 (Ru III/II )=176 mV. 
         [0037]    The analyses of the above intermediate and final products were performed using the following apparatuses and conditions. 
         [0038]    Infrared spectroscopic: analysis (IR): The measurement apparatus used was NICOLET 6700 FT-IR (manufactured by Thermo Fisher Scientific). The measurement was performed by the KBr pellet method. 
         [0039]    Cyclic voltammetry (CV): The measurement apparatus used was CH1620D system (manufactured by BAS Inc.). The measurement temperature was 20° C., the working electrode was glassy carbon, the counter electrode was a platinum wire, the reference electrode was Ag/AgCl (3M NaCl, 203 mV vs. NHE), the initial potential was 403 mV, the switching potential was 1303 mV, and the scan rate was 25 mV/sec. The concentration of the object to be measured was 1 mM in 0.5M KH 2 PO 4  aqueous solution (pH 4.3). 
         [0040]    Ultraviolet-visible spectroscopic analysis (UV/Vis): The measurement apparatus used was 8453 UV-Vis spectrometer (manufactured by Agilent). The measurement temperature was normal temperature (about 20° C.). 
         [0041]    Varian system 500 (500 MHz) spectrometer (Agilent) was used for  1 H-NMR,  13 C-NMR and  31 P-NMR measurements, and ECA500 (500 MHz) spectrometer was used for  183 W-NMR measurements. HOD (4.659 ppm) in D 2 O was used as an internal standard in the measurements of  1 H-NMR spectra, (CH 3 ) 2 CO (30.103 ppm) was used as an external standard in the measurements of  13 C-NMR spectra, 85% H 3 PO 4  (0 ppm) was used as an external standard in the measurement of  183 P-NMR spectra, and saturated Na 2 WO 4  (0 ppm) was used as an external standard in the measurements of  183 W-NMR spectra. 
         [0042]    The elementary analysis was entrusted to Microanalytisches Labor Pascher, an elementary analysis company in Germany. 
         [0043]    For the final product (K 7 α 1 -P 2 W 17 O 61 Ru(H 2 O) (α 1 -form)), a CV measurement was performed in 0.5M H 2 SO 4  aqueous solution. The measurement apparatus used was CHI320D system (manufactured by BAS Inc.). The measurement temperature was 20° C., the working electrode was glassy carbon, the counter electrode was a platinum wire, the reference electrode was Ag/AgCl (3M NaCl 203 mV vs. NHE), the initial potential was 403 mV, the switching potential was 1303 mV, and the scan rate was 25 mV/sec. The concentration of the final product in the 0.5M H 2 SO 4  aqueous solution was 1 mM. 
         [0044]      FIG. 2  shows the electric current-voltage curve (cyclic voltammogram) obtained by the CV measurement for K 7 α 1 -P 2 W 17 O 61 Ru(H 2 O) (α 1 -form). As shown in  FIG. 2 , K 7 α 1 -P 2 W 17 O 61 Ru(H 2 O) (α 1 -form) exhibited reversible redox peaks. The formal redox potentials E 0 ′ (=(cathodic peak potential E pc )+(anodic peak potential E pa )/2) determined from the electric current-voltage curve were +984 mV and +203 mV. Further, the number of electrons participated in the oxidation-reduction, determined from the Nernst equation was respectively 1. 
         [0045]    Three sets of peaks are observed in the cyclic voltammogram in  FIG. 2 . 
         [0046]    The one set of peaks at the right side (higher potential side) is attributed to the redox reaction represented by the following formula: 
         [0000]      [P 2 W 17 O 61 Ru(IV)(H 2 O)] 6   −   +e   − ⇄[P 2 W 17 O 61 Ru(III)(H 2 O)] 7   − 
 
         [0047]    The central one set of peaks is attributed to the redox reaction represented by the following formula: 
         [0000]      [P 2 W 17 O 61 Ru(III)(H 2 O)] 7   −   +e   − ⇄[P 2 W 17 O 61 Ru(II)(H 2 O)] 8   31  
 
         [0048]    The one set of peaks at the left side (lower potential side) is attributed to the redox reaction represented by the following formula: 
         [0000]      [P 2 W 17 O 61 Ru(III)(H 2 O)] 7   − +2 e   − +2H + ⇄H 2 [P 2 W 17 O 61 Ru(II)(H 2 O)] 7   − 
 
         [0049]      FIG. 2  shows that the ruthenium in the polyoxometalate molecule of the present invention has an electric potential sufficient to oxidize hydrogen, and accordingly the polyoxometalate molecule of the present invention has a high hydrogen oxidizing ability. Further, this molecule dissociates hydrogen with ruthenium as an catalytic active site, thereby produces electrons and protons. This molecule can receive the produced electrons on tungsten, whereas this molecule can receive the produced protons on any oxygen in the molecule. Accordingly, the polyoxometalate compound of the present invention has hydrogen dissociation sites and the sites on which the electrons and protons produced by the hydrogen dissociation are received. 
         [0050]    Moreover, it can be seen from the sharp waveform of the cyclic voltammogram that the electron movement and proton movement proceed smoothly. 
       INDUSTRIAL APPLICABILITY 
       [0051]    The catalyst of the present invention is useful in various applications which employ hydrogen oxidation reaction in which hydrogen molecule is oxidized to hydrogen ions, for example, as a hydrogen oxidation catalyst in fuel cell.