Patent Application
20130177839
The present invention relates to a reduction catalyst containing palladium-gold alloy.
The palladium-supported catalysts in which palladium or palladium alloy with the other metals is supported on a supporting material such as alumina, carbon, etc. have been put to practical use as a heterogenious catalyst in a wide range of application such as hydrogenation of olefins, acetylene, nitro groups, ketones, aldehydes, nitriles, etc., oxidation of hydrogen, hydrocarbons, carbon monoxide, etc., and oxidative acetoxylation reaction of olefins.
As such palladium supported catalysts have been disclosed, for example, Pd—C catalysts and Pd—Al2O3 catalysts, and these catalysts can be used for hydrogenation reactions under elevated temperature and increased pressure (Non-Patent Document 1).
Non-Patent Document 1: Shigeo Nishimura, Yuzuru Takagi, “Catalytic Hydrogenation Reaction, Application in Organic Synthesis”, Apr. 10, 1987, published by Tokyo Kagaku Dojin Co. Ltd., p. 34-37
Conventional palladium-supported catalysts are low in reducibility and are insufficient in selectivity in some reactions. Thus, an object of the present invention is to provide a palladium-supported catalyst which is excellent in reducibility and exhibits a high conversion ratio and preferably an excellent selectivity when used in a hydrogenation reaction.
As the results of intensive researches, the present inventors have noted an alloying degree of an alloy as an index for evaluating the microstructure of the alloy, and have completed the present invention.
Namely, the present invention provides reduction catalyst which comprises conductive carbon and palladium-gold alloy supported on the said carbon, wherein an alloying degree of the said alloy is in a range of from 50 to 100%.
The reduction catalyst according to the invention is an oxygen-reduction catalyst in one embodiment, and is a hydrogenation catalyst in other embodiment The oxygen-reduction catalyst is a cathode catalyst for polymer electrolyte fuel cells in one embodiment.
Further, the present invention provides a cathode catalyst for polymer electrolyte fuel cells, wherein the cathode catalyst is used.
Further, the present invention provides a method for reducing oxygen which comprises contacting the reduction catalyst according to the present invention with oxygen.
Further, the present invention provides a method for hydrogenating an organic compound having reducible functional groups, which comprises contacting the reduction catalyst according to the present invention with the organic compound having the reducible functional groups. Examples of the reducible functional groups include, for example, carbon-carbon double bond-containing groups (such as alkenyl group), carbon-carbon triple bond-containing groups (such as arkynyl group), nitro group, carbonyl group and cyano group. Examples of the organic compounds include, for example, olefins, acetylene, nitro compounds, ketones, aldehydes, and nitriles.
Further, the present invention provides use of the reduction catalyst according to the present invention as an oxygen-reduction catalyst.
Further, the present invention provides use of the reduction catalyst according to the present invention as a hydrogenation catalyst.
The reduction catalyst according to the present invention is excellent in reducibility, and thus is useful as, for example, an oxygen-reduction catalyst, and further as a cathode catalyst for polymer electrolyte fuel cell. A cathode according to the invention in which the cathode catalyst is used is useful for polymer electrolyte fuel cell which are expected to be used as stationary power source unit for household use and mobile power supply system for automobiles. The reduction catalyst according to the present invention is also useful as a hydrogenation catalyst which exhibits a high conversion ratio and moreover an excellent selectivity in, for example, a reaction for hydrogenating an aliphatic nitrile compound to convert it into a primary amine.
The invention will be explained below in more detail.
1-1. Formation of Palladium-Gold Alloy-Supported Carbon Catalyst
Suitable conductive carbon support materials include, for example, acetylene black- or furnace black-based carbon powder. The mol ratio of palladium to gold in the supported palladium-gold alloy is preferably from 9.5:0.5 to 7:3.
The palladium-gold alloy-supported carbon can be formed by the following procedures;
Through the above procedures, particles consisting of a palladium-gold alloy can be formed on the conductive carbon support.
In the present description, the alloying degree of a palladium-gold alloy refers to an index determined as described below from the X-ray diffraction (XRD) data, specifically the measured values of the peak top position as a diffraction angle 2θ of the (220) plane and accordingly calculated value of lattice spacing d. In other word, it is possible to determine the crystal structure of a palladium-gold alloy catalyst from the X-ray diffraction pattern by calculating the lattice spacing d from the characteristic diffraction angle 2θ, wherein the deviations of the lattice spacing d of the empirically prepared palladium-gold alloy from those of pure palladium and pure gold are calculated. Other deviation values are calculated by using the lattice spacing d of the ideal palladium-gold alloy, pure palladium and pure gold in the same manner. Finally, the ratio of those deviations between the empirical and the ideal al toys allocated proportionately to the mol fractions of palladium and gold present in the catalyst according to Vegard's Law; and the resulting value is used as an index of the alloying degree of the whole catalyst.
First, data used as standard are shown in Table 1. Those data were obtained from XRD patterns of pure palladium (Pd content: 100 mol %) and pure gold (Pd content: 0 mol %) respectively at wavelength of 1.540598 angstrom (Cu—Kα) by using Bragg's condition:
2d sin θ=nλ
wherein, n=1, λ=1.540598 angstrom
An approximation graph indicating the theoretical values of lattice spacing d of each alloy were drawn in the range of Pd content between 0 to 100% based on the assumption that the lattice spacing of alloy changes linearly depending on Pd content in mol % (
Based on the above results, the alloying degree was defined from the theoretical and the observed value of d as follows. Namely, the alloying degree of each component metal is calculated by the following equation, in which the alloying degree of palladium-gold alloy is defined as an allocation of the alloying degree in proportion to the mol fraction of each component metal.
In the above equation, Pd mol fraction is calculated from the equation:
(Pd content)/100, and Au mol fraction was calculated from the equation: 1−(Pd mol fraction).
Meanwhile, the alloying degree of component metal in the above equation is calculated from the following equation:
Alloying degree (%) of component metal=(|d(measured)−d(standard data)|/{d(theoretical)−d(standard data)}×100
In the above equation, the d(standard data) means the value d shown in Table 1. Namely, when the component metal is palladium, d(standard data=1.3754 angstrom, and when the component metal is gold, d(standard data)=1.4420 angstrom.
Meanwhile, d(theoretical) in the above equation is the value calculated from the approximation curve shown in Table 1. For example, when Pd content is 90 mol %, d(theoretical)=1.3821 angstrom (see Table 2).
Further, in the above equation, d(measured) is the value calculated from Bragg's condition based on the results of XRD measurement. In the case where the peak of the (220) plane is observed as a single peak in the XRD, the value is calculated from the position of such single peak. On the contrary, in the case where the peaks of the (220) plane is observed as split two peaks in the XRD, the value is calculated by allocating the peak at higher angle to palladium and the peak at lower angle to gold.
For example, in the case where a palladium-gold alloy with Pd content of 90 mol % provides an XRD pattern shown in
In the different case where a palladium-gold alloy with Pd content of 90 mol % provides an XRD pattern shown in
Alloying degree (%) of palladium-gold alloy=0.9×11.94+0.1×21.04=13.85.
The alloying degree can be controled as follows Namely, in the “1-1. Formation of palladium-gold alloy-supported carbon” in the above, an alloying degree can be controled by adjusting pH during addition of the reducing agent. The pH is preferably in a range from 9.0 to 13.0, more preferably from 10.0 to 13.0, further more preferably from 11.0 to 13.0. If the pH is within the above-mentioned range, the alloying degree of the resulting palladium-gold alloy can be readily controled to a value within a range from 50 to 100%. For adjusting the pH, conventional pH adjusting chemicals can be used without particular limitation. Examples of the pH adjusting chemical include, for example, potassium hydroxide, sodium hydroxide, potassium carbonate and sodium carbonate.
The reduction reactions for which the catalyst of this invention can be used include, but not limited to, for example, oxygen-reduction reactions; hydrogenation reactions of olefins, acetylene, nitro groups, ketones, aldehydes or nitriles; hydrogenation decomposition reactions of benzyl ethers, benzyl esters, or benzyl oxycarbonyl groups.
In the case where the catalyst of this invention is an oxygen-reduction catalyst used for an oxygen-reduction reaction, examples thereof include cathode catalyst for polymer electrolyte fuel cell.
In the case where the catalyst of the invention is a hydrogenation catalyst used for a hydrogenation reaction, examples of the hydrogenation include, but not limited to, the reaction in which hydrogen is added to aliphatic nitrile compound to convert it to primary amine. Examples of the aliphatic nitrile compound include an aliphatic nitrile compound represented by a general formula: R1—CN, and examples of the primary amine include a primary amine represented by a general formula: R1—CH2—NH2. In these formulas, R1 denotes an unsubstituted or substituted alkyl group. Examples of R1 include alkyl groups of 1 to 20 carbon atoms, preferably 3 to 10 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, t-butyl, pentyl, hexyl and octyl groups; and substituted alkyl groups in which a part or all of the hydrogen atoms in such alkyl groups are substituted with halogen atoms such as fluorine atoms, bromine atoms or chlorine atoms, hydroxyl group, amino group, carbonyl group, oxo group (i.e. ═O), alkoxy group or thiol group, such as chloromethyl group, bromoethyl group and 3,3,3-trifluoropropyl group.
The catalytic hydrogenation reaction to convert a nitrile to an amine proceeds in the presence of various metal catalysts (Co, Ni, Rh, Pd, Pt, etc.). However, it is known that secondary amines and tertiary amines easily generate in addition to generation of primary amines as shown by the chemical formula shown below. The ratio of these amines varies depending on reaction conditions such as catalyst type, temperature, pressure, solvent and the like. Therefore, the selection of catalyst type and reaction conditions is important to obtain primary amines as the main product. For example, in a reaction at higher temperature, condensation reaction accompanied by elimination of ammonia is promoted, and thus secondary amines and tertiary amines are predominantly produced. On the other hand, in the presence of excess ammonia, an equilibrium of the step of the reaction between imine intermediate and primary amine to produce secondary amine is suppressed, whereby resulting in decrease of production of secondary amines and tertiary amines. In other example, by conducting the reaction in acid solvent or acylation solvent such as acetic anhydride, primary amines can be predominantly produced. It is believed that such result comes from the formation of ammonium salt from the acid and the primary amine and thus addition of the primary amine to imine intermediates is suppressed.
The hydrogenation catalyst according to the present invention can remarkably enhance selectivity of primary amine synthesis in reaction for converting aliphatic nitrile compounds to amines by addition of hydrogen.
Catalyst layer is formed on gas diffusion layer to prepare an anode and a cathode, and then an ion-exchange membrane is placed between the anode and the cathode with the catalyst layer inside, which is pressed to form membrane-electrode assembly (MEA). The both electrodes are formed by the following preparation method.
For preparing the gas diffusion layer, conductive carbon is blended with a dispersing agent, an aqueous solution of Teflon (registered trade mark) is added thereto for retention of water-repellency, the resulting mixture is applied to conductive carbon sheet, and the resulting product is dried.
Thereafter, the product is molded by a hot-pressing machine to a sheet-shaped product.
For preparing the catalyst layer, catalyst is blended with pure water, Nafion solution or the like is added thereto according to a conventional method, and after further blending, the resulting mixture is coated on the gas diffusion layer.
Then, after drying, the catalyst layer is formed on the gas diffusion layer.
An electrode sheet is thus prepared.
The present invention will be explained below by way of examples and comparative example which are not intended to restrict the invention.
Using 2.5 L of pure water, 44 g of acetylene black used as catalyst support was dispersed therein, and the resulting dispersion was heated to 95° C. to give a uniform suspension of the catalyst support. Separately, 500 mL of pure water was prepared, and sodium chloropalladate in an amount of 15.8 g as palladium metal basis and chloroauric acid in an amount of 3.3 g on as gold metal basis were dissolved in the water to give a mixed solution of palladium salt and gold salt. Then the mixed solution of palladium salt and gold salt was added dropwise to the suspension of the catalyst support to give a renewed suspension. The renewed suspension was cooled to room temperature. Then formaldehyde and potassium hydroxide were dissolved in 500 mL of pure water to give a reducing solution, and the reducing solution was added dropwise over 60 minutes to the suspension which had been cooled to room temperature. After dropwise addition, the resulting mixture was heated to conduct reduction.
Thus, particles of palladium and gold were supported on the surface of the acetylene black as the support material.
After cooling the reaction liquid, solid matter was filtered off and rinsed. Thereafter, drying was effected at about 70° C. In the atmosphere, and the solid matter was pulverized to give a catalyst in which the palladium-gold alloy was supported on the acetylene black used as a support.
Incidentally, samples A to D were prepared by adjusting the pH of the reducing solution to 13.0, 11.0, 10.0 and 8.0, respectively.
In the present preparation of the catalyst, the theoretical mol ratio of palladium to gold was 9:1, and the contents of the supported elements relative to the weight of the catalyst were 25% by weight of palladium and 5.2% by weight of gold according to ICP analysis.
Incidentally, catalysts having a theoretical mol ratio of palladium to gold of 8:2 and 7:3 were also prepared.
XRD measurement was conducted on the samples A to D obtained in the procedure (1) above to give X-ray diffraction patterns. Upon observation of the peaks of the (220) plane in these XRD patterns, it was shown that samples A to C have a single peak while sample D has two peaks. In sample D, the peak at higher angle was dealt as corresponding to palladium and the peak at lower angle was dealt as corresponding to gold. From the peak position of the (220) plane, an alloying degree was calculated. The results are shown in Table 5.
In order to evaluate the oxygen reducibility of palladium-gold alloy-supported carbon catalysts, the rotating disk electrode measurement of the samples A-D was conducted. As the evaluation apparatus was used electrochemical measurement system (HZ-3000) (Hokuto Denko Corporation). The apparatus is composed of rotating electrodes, control units and electrolysis cells all manufactured by Hokuto Denko Corporation. Preparation of the electrode was conducted by applying catalyst to the glassy carbon disk electrode attached to the rotating electrode according to the following procedures. Incidentally, as a reference catalyst, 30wt. % palladium-supported carbon catalyst (referred to as sample E) was used.
Evaluation conditions are as follows:
As an index of oxygen reducibility, an oxygen reduction current at the potential of 0.85 V vs. RHE was used. A value of oxygen reduction current of each catalyst is shown in Table 6.
(*)palladium-supported carbon catalyst
As shown in Table 6, a palladium-gold alloy-supported carbon catalyst having an alloying degree of as high as 96.02% or 85.41% (sample A or B) exhibits a higher oxygen reduction potential compared with a palladium-supported carbon catalyst and is excellent in oxygen reducibility (
It is considered that, in a palladium-gold alloy-supported carbon catalyst having a low alloying degree, the unalloyed gold particles which have low oxygen reducibility exist near the palladium particles, which suggests the possibility that gold particles hinder the oxygen-reduction reaction conducted by palladium. On the other hand, it is considered that, in the palladium-gold alloy-supported carbon catalyst having a high alloying degree, the electronic state of the palladium is changed by the alloying, whereby a superior oxygen reducibility is exhibited compared with the case where palladium is used alone.
A hydrogenation reaction of valeronitrile was carried out under the conditions shown in Table 7.
Valeronitrile (3.4 g) was used as a model substrate. The substrate and the solvent 50 mL, were introduced into a reaction vessel to give a suspension. Then, a catalyst was introduced thereto in an amount corresponding to 0.44 mol % when reduced to the content of the metals in the catalyst relative to the substrate, and the resulting mixture was stirred in the hydrogen atmosphere (0.15 MPa=1.48 atm) at 50° C. Three hours after, the resulting reaction liquid was partitioned between pure water and hexane and extracted. (when acetic acid was used as a solvent, the partition/extraction was effected after preliminarily neutralization with a caustic soda solution). Finally, an appropriate amount of nitrobenzene that is an inner standard substance was added to the upper layer (hexane layer) for sampling. The resulting sample was analyzed by gas chromatography (GC). Incidentally, the hydrogenation reaction of valeronitrile proceeds according to the following reaction scheme and generates amines 1 to 3 shown in the scheme.
In Table 8 are shown the combinations of catalyst and solvent that were used, as well as the results of the measurements of conversion ratio and selectivity. In the table, the conversion ratio is a weight ratio of the weight of consumed substrate to the weight of the substrate before the reaction, and the selectivity is shown by a mol ratio of amines 1,2 and 3.
(*)MeOH: methanol, THF: tetrahydrofuran, AcOH: acetic acid
At first, a 10 wt % Pd/carbon was used as a catalyst, and MeOH, THF or AcOH was used as a reaction solvent to carry out a hydrogenation reaction. As the results, it was revealed that a raw material remained and an aimed primary amine was not produced at all (Experiments 1 and 2). On the other hand, a dramatic enhancement in conversion ratio and selectivity was observed in the reaction in AcOH (Experiment 3). It is considered that the result comes from the suppression of the addition of a primary amine to a synthetic intermediate imine, as previously explained.
Then a hydrogenation reaction was carried out in a solvent of AcOH using a 30 wt % Pd/carbon, which resulted in an activity equivalent to that in Experiment 3 (Experiment 4). On the other hand, hydrogenation of the nitrile did not proceed at all with a 30 wt % Au/carbon catalyst (Experiment 5). From this result, it was revealed that gold does not have a reduction activity on cyano groups at all.
Then, a measurement was conducted using a 25wt % Pd-5.2wt % Au/carbon catalyst having an alloying degree of as high as 96.02% (the Sample A). It was found that the selectivity of a primary amine increased to 72% (Experiment 7). in order to confirm the effect of this alloy, a measurement was conducted similarly using a 25wt % Pd-5.2wt % Au/carbon catalyst having an alloying degree of as low as 12.85% (the Sample D). It was revealed that the selectivity decreased largely to 29% (Experiment 8). It is considered that the result comes from blocking of the activity points of palladium by inactive gold particles that are not alloyed. A similar measurement was conducted by mixing 30 wt % Pd/carbon and 30 wt % Au/carbon in such amounts to give supported amounts of 25 wt % Pd and 5.2 wt % Au relative to the weight of the catalyst, and the results obtained were similar to those obtained by using the 25 wt % Pd-5.2 wt % Au/carbon catalyst having a low alloying degree (Experiments 6 and 8). From these observations, it is considered that a mere mixture of palladium and gold provides a low activity whereas alloying of palladium and gold provides an enhanced selectivity of primary amine synthesis as a result of change of the electronic state of palladium.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-240341 | Oct 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/074715 | 10/26/2011 | WO | 00 | 3/18/2013 |
