TWI400123B - Catalyst and fabrication method thereof - Google Patents

Description

Catalyst and manufacturing method thereof

The present invention relates to a catalyst and a method of manufacturing the same.

In recent years, the energy problem of oil shortage has become increasingly serious, and environmental protection issues have gradually received attention. For environmental and economic considerations, finding alternative green energy has become an urgent task.

In general, a fuel cell uses methanol or hydrogen as an anode fuel, and oxygen as a cathode fuel, which is subjected to a catalytic reaction of a catalyst to decompose and release electric energy, and the product is water and a small amount of carbon dioxide. Compared to traditional power generation methods, the fuel cell's environmental threat can be greatly reduced. In addition, fuel cells have several important features, including high energy conversion efficiency, low operating temperatures, and easy and safe fuel storage, making them one of the major alternative green energy sources of the future.

In order for a fuel cell to work efficiently, a catalyst must be added to the fuel cell module to catalyze the decomposition reaction of the fuel. The metal catalyst currently used in fuel cells is platinum metal (Pt) and its related alloy (Pt-M). The Watanabe team [Journal of the Electrochemical Society 10, 3750 (1999)] proposed in 1999 that different transition metals are doped in platinum, and the Pt-Pt bond length will change depending on the doping metal element and the doping amount. And the oxygen catalytic activity corresponding to it will also be different. The catalytic activity exhibits a volcano curve according to the concentration of various transition metals doped, that is, the bond length of Pt-Pt needs to reach a certain interval to effectively adsorb oxygen molecules on the catalyst, and then Catalytic decomposition of oxygen is carried out.

In 1998, Shin et al. [Journal of Power Source 71, 169 (1998)] proposed that platinum-based catalysts can be operated in a methanol environment, and the catalyst may be inactivated by methanol poisoning. The authors also explored changes in the open circuit voltage of fuel cells in different concentrations of methanol. The results show that the open circuit voltage decreases with increasing methanol concentration, indicating that the higher the methanol concentration, the more the poisoning tendency of the catalyst will increase. Due to the inactivation of methanol poisoning at the surface active site, and thus the catalytic activity of the catalyst is lost, the open circuit voltage and the battery work efficiency are greatly reduced.

In 1998, The study by [Journal of Power Source 74, 211 (1998)] indicates that the Pt-Ru catalyst is subjected to an oxygen reduction reaction in a methanol atmosphere, and the half-wave potential (half-) is compared with the result of performing the test under a sulfuric acid solution alone. The wave potential part will shift to a lower potential due to the participation of methanol. In addition, the oxygen reduction reaction (ORR) polarization spectrum shows that due to the participation of methanol, a significant oxidation current can be observed in the polarization curve, which is caused by methanol oxidation. From this result, it can be seen that in the environment where methanol and oxygen coexist, the two species will compete with the catalytic reaction at the same time, so the adsorption of oxygen on the catalyst will be suppressed by the appearance of methanol, and thus the oxygen cannot be completely decomposed, and Since the oxidation of methanol also consumes a part of the electrons, the catalytic efficiency of the catalyst is greatly reduced, and the efficiency of the battery is greatly reduced without obtaining a sufficient reduction current.

As the price of platinum is rising year by year, it is not easy to consider the economic benefits of platinum in general application to fuel cells. In addition, platinum catalysts cannot be used in methanol fuel cells for a long time due to their own methanol poisoning problems, even platinum-ruthenium (Pt-Ru) catalysts which have been proven to increase the catalytic activity and catalytic stability of platinum. Operating in a methanol environment, it will still lose its activity with methanol, and it will be aggravated by the poisoning of methanol over time. Therefore, during the discharge of the battery, the output of voltage and current will also fluctuate and gradually lose its catalytic ability.

The invention provides a method for manufacturing a catalyst, comprising: mixing a material containing carbon and palladium elements and a compound containing iron into a mixture, wherein the palladium:iron:carbon atomic molar ratio of the mixture is 60-90 : 5 to 40: 2 to 15; and reduction sintering the mixture to form a catalyst.

The present invention also provides a catalyst prepared by the above method.

The present invention also provides a catalyst comprising an alloy of carbon, palladium and iron formed on a carbon support.

The catalyst provided by the invention has the advantages of high redox efficiency and resistance to methanol poisoning.

The invention provides a catalyst and a method of manufacturing the same.

The method for preparing a catalyst of the present invention is simple and rapid, and it is easy to mass-produce a catalyst.

The method of the present invention comprises mixing a material comprising carbon and palladium elements and a compound comprising iron into a mixture, and reducing and sintering the mixture to form a catalyst. The above iron-containing compound includes iron nitrite, iron sulfate or iron chloride. The above palladium-containing compound includes palladium nitrite, palladium sulfate or palladium chloride. Include the above-described compound containing a carbon surface area is between 100m ² / g to 900m ² / g, preferably, from material 200m ² / g to 850m ² / g is. The atomic molar ratio of palladium:iron:carbon in the above mixture is 60 to 90:5 to 40:2 to 15, preferably 70 to 80:10 to 30:5 to 12. In an embodiment, the above reduction sintering step comprises heating the mixture in an environment containing a reducing atmosphere. The above reducing atmosphere includes hydrogen. The reduction temperature can be between 300 ° C and 700 ° C.

The catalyst prepared by the present invention comprises carbon, palladium and iron, preferably an alloy composed of carbon, palladium and iron. The above alloy composed of carbon, palladium and iron may be formed on a carbon support. The catalyst prepared by the invention has the advantages of high redox efficiency and resistance to methanol poisoning, and can be used for a proton exchange membrane fuel cell (PEMFC) catalyst.

The above and other objects, features, and advantages of the present invention will become more apparent and understood.

[Example 1]

Pd/C (Pd 20 wt%) commercially available from E-TEK Co., Ltd. was selected as a reaction catalyst material, and a 0.05 M aqueous solution of ferrous nitrate was used in a stoichiometric ratio to make the molar ratio of Pd:Fe to 90:10. Pd/C was mixed with the ferrous nitrate solution and shaken for 30 minutes to mix well. It was then heated to 80 ° C under an air atmosphere to evaporate the solvent. After the solid obtained after the evaporation to dryness was sufficiently uniformly mixed, the mixture was placed in an alumina crucible, and then the alumina crucible was placed in a tubular furnace. Then pass a reducing atmosphere of H ₂ /N ₂ (5%/95%), heat to 500 ° C at a heating rate of 5 ° C / min and react for 5 hours, then cool to room temperature at a cooling rate of 5 ° C / min . After reduction sintering, the mixture was ground in a mortar to make it a uniform particle powder, thus obtaining a PdCFe/C catalyst.

Figures 6A through 6C are schematic diagrams of catalyst formation derived from the results of the EXAFS formulation (described later). Figure 6A shows an untreated catalyst comprising a carbon substrate (or carbon support) 2 and a palladium metal 4 on the carbon substrate 2. The palladium metal 4 can be adsorbed on the carbon substrate 2. Referring to Figure 6B, it is inferred from the results of the extension x-ray adsorption fine structure (EXAFS) fitting (described later) that the initial catalyst is immersed in the ferrous nitrate solution. After the ferrous nitrate solution 6 is attached thereto, the iron atoms 8 in the ferrous nitrate solution 6 remain on the surface of the palladium metal 4 and the carbon substrate 2. Therefore, referring to FIG. 6C, after the reduction sintering treatment, the iron atom 8 forms an alloy 14 with the palladium metal 4, and the iron atom 8 also forms the iron carbon compound 16 with the carbon substrate 2. In the step of high temperature reduction, since the iron carbon compound has high mobility, it can migrate to the palladium metal 4 to form the PdCFe alloy 12.

[Example 2]

The procedure is the same as in Example 1, wherein the molar ratio of Pd:Fe is 80:20.

[Example 3]

The procedure is the same as in Example 1, wherein the molar ratio of Pd:Fe is 70:30.

[Comparative Example 1]

Untreated Pd/C (Pd 20 wt%) commercially available from E-TEK Corporation of the United States.

[Comparative Example 2]

Pd/C (Pd 20wt%) commercially available from E-TEK Corporation of the United States was placed in an environment of a reducing atmosphere of H ₂ /N ₂ (5%/95%), and heated at a heating rate of 5 ° C/min. The reaction was carried out at 500 ° C for 5 hours, and then cooled to room temperature at a cooling rate of 5 ° C / min.

[Comparative Example 3]

Untreated U.S. E-TEK Corporation commercially available Pt/C (Pt 40 wt%).

[Comparative Example 4]

Pd metal foil.

[Comparative Example 5]

Dispersing Pd/C (Pd 20wt%) commercially available from E-TEK Co., Ltd. in deionized water, and adding a ferrous nitrate precursor to the solution to make the molar ratio of palladium:iron in the solution to 70:30. Then, an aqueous solution of sodium borohydride was added dropwise to reduce the iron to prepare a catalyst.

【test analysis】

The catalyst products of Examples 1 to 3 and Comparative Examples 1 to 2 were identified by a X-ray powder diffractometer, and the results were as shown in Fig. 1a. Compared with the standard PdFe (JCPDS: 65-3253) X-ray powder diffraction pattern, it is understood that the PdCFe/C catalyst prepared in Examples 1 to 3 is a single phase and has a hexagonal structure (a = b = c, α = β = γ = 90°), and the space group is Pm3m.

Fig. 2 is a Pd K-edge absorption spectrum of the catalyst products of Examples 1 to 3 and Comparative Examples 1 to 2. As can be seen from the figure, the oscillation of the X-ray absorption near edge structure (XANES) portion of the PdCFe/C catalyst of the example is almost the same as that of Pd/C (Comparative Example 2). The Pd in the prepared PdCFe/C catalyst finished product exists in a metallic state.

Fig. 3 is a graph showing the Fe K-edge absorption spectrum of the catalyst products of Examples 1 to 3. Compared with the iron foil (Fe foil), the PdCFe/C catalyst obtained in the example has a lower absorption peak intensity of the pre-edge portion (~7, 112 eV), which is due to the formation of Fe and Pd. The alloy causes Fe to lose the symmetry of the original tetrahedron. In addition, comparing the spectrum of the X-ray absorption near edge structure (XANES) section, the energy shift of the PdCFe/C catalyst at an absorbance of 0.5 is between iron foil and iron oxide ( Between Fe ₂ O ₃ or Fe ₃ O ₄ ), Fe in the PdCFe/C alloy catalyst finished product exists in a charge distribution.

Figure 4 is a graph showing the Pd K-edge absorption spectrum extended X-ray absorption fine structure matching map of the PdCFe/C catalyst products of Examples 1 to 3, which are calculated by the two paths of Pd-Fe and Pd-Pd. . The fitting results are in agreement with the theoretical calculations, thus demonstrating that the neighboring atoms around the palladium atoms are palladium atoms and iron atoms.

Figure 5 is a graph showing the X-ray absorption fine structure of the PdCFe/C catalyst of the examples 1 to 3, which is calculated by the three paths of Fe-Pd, Fe-Fe and Fe-C. The result. This fitting result is in agreement with the theoretical calculation results, thus proving that the neighboring atom around the iron atom is a palladium atom, an iron atom and a carbon atom. From the results of Figs. 4 and 5, it can be seen that the PdCFe/C catalyst obtained in the examples of the present invention has both a palladium atom, an iron atom and a carbon atom.

Fig. 7 is a graph showing the polarization curves of the catalyst products obtained in Examples 1 to 3 and Comparative Example 5. It can be seen from the polarization curves that the onset potential of the PdCFe/C catalysts of Examples 1 to 3 is between 0.78 V and 0.82 V, and the half potential increases to a high potential as the amount of iron added increases. Displacement. In other words, in the process of synthesizing the catalyst, the catalyst obtained by reacting with more iron components has a higher redox ability. Compared with the PdCFe/C series catalysts of Examples 1 to 3, the catalytic activity of the catalyst prepared in Comparative Example 5 was significantly decreased.

Figure 9 shows the XRD diffraction pattern of the finished catalyst. As a result, it was revealed that the catalyst of Comparative Example 5 exhibited a diffraction peak of iron oxide at a position of about 38 ⁰ , indicating that the iron system in the catalyst prepared by the reduction method exists in an oxidation state. Figure 10 shows the Fe K-edge X-ray absorption spectrum of the finished catalyst. As can be seen from Fig. 10, the white line strength and the energy shift amount of the catalyst obtained in Comparative Example 5 were higher than those of the PdCFe/C series catalysts of Examples 1 to 3. Further, the XANES pattern of the catalyst of Comparative Example 5 coincided with Fe ₂ O ₃ , and it was confirmed that the iron system in the catalyst obtained by the reduction method exists in an oxidation state, and the catalyst particles do not have a carbon atom contribution.

From the results of Fig. 7, Fig. 9, and Fig. 10, it was confirmed that if the catalyst particles do not have a carbon atom contribution, the catalytic activity can not be improved. The PdCFe/C catalyst prepared by the embodiment of the invention has the contributions of palladium, iron and carbon atoms, and the palladium, iron and carbon atoms are uniformly mixed, so that the redox ability is better.

Fig. 8 is a graph showing the methanol penetration test of the PdCFe/C catalyst finished product prepared in Examples 1 to 3 and the catalyst product prepared in Comparative Example 3. As can be seen from the figure, the PdCFe/C catalyst prepared in Examples 1 to 3 did not exhibit a characteristic peak of methanol oxidation (about 0.8 V to 1 V), and thus the confirmed PdCFe/C catalyst did have a potential against methanol poisoning.

Table 1 shows the Pd K-edge absorption spectrum extension X-ray absorption fine structure matching results of the PdCFe/C catalyst products of Examples 1 to 3. The results showed that the average bond length (R) of Pd-Pd was about 2.7 angstroms, the coordination number of palladium atoms around palladium atoms was about 7.3, and the coordination number of iron atoms was between 0.6 and 2.4. The number of digits increases as the amount of iron precursor added during the reaction increases. In addition, Figure 1b also shows that the Pd-Pd lattice constant in the PdCFe/C catalyst finished product decreases with the addition of iron, which further confirms that the iron is uniformly mixed with the palladium metal, and the iron is doped. The amount of palladium metal will also increase as the amount of iron added increases.

Table 2 shows the Fe K-edge absorption spectrum extension X-ray absorption fine structure matching results of the PdCFe/C catalyst products of Examples 1 to 3. From the results, it is known that the average bond length (R) of Fe-Fe is about 2.5 angstroms, the coordination number of iron atoms around the iron atom is about 1.5, the bond length of Fe-Pd is about 2.7 angstroms, and the palladium atom around the iron atom is matched. The number of bits is about 5, the bond length of Fe-C is about 2.1 angstroms, and the coordination number of carbon atoms around the iron atom is about 2.5. The results of the absorption spectroscopy test showed that the contribution of carbon atoms did occur in the PdCFe/C catalyst finished product obtained in the examples. In addition, the results of the coordination of the coordination number showed that the number of coordination of Fe-Pd bonds did not increase significantly with the increase of iron atoms added during the reaction. To further explore the fine structure, the results will be fitted. Compared with the results of the theoretical random alloy, if it is the coordination number of the alloy structure, it will meet the following equation:

X _Fe : Mo molar fraction

N ^Pd-Fe : Coordination of iron atoms around palladium atoms

X _Pd : Pd molar fraction

N ^Pd-Pd : coordination number of palladium atoms around palladium atoms

The coordination number of the PdCFe/C alloy catalyst of the examples conforms to the above equation, and it is confirmed that the obtained alloy catalyst structure is a disordered alloy structure. Referring to the structural diagrams of Figs. 6A to 6C, in conjunction with the theoretical calculation of the scattered alloy, the alloy catalyst prepared in the examples is an alloy catalyst formed of three elements of palladium, iron and carbon.

Table 3 shows the relative compositions of palladium, carbon and iron in the PdCFe/C ternary alloy catalyst of Examples 1 to 3 estimated from the results of Table 1 and Table 2. As shown in Table 3, the carbon content in the alloy catalyst is between 2.6 and 10.9%, and increases as the amount of iron added during the reaction increases. Referring to Figure 3, the PdCFe/C series catalyst absorption spectra are nearly identical, and the results of Table 2 adaptation have the same trend, confirming that the obtained alloy catalyst is an alloy composed of uniformly mixed palladium, carbon and iron elements. Moreover, the content of iron-carbon compounds increases with the increase of iron content, and the increase of iron-carbon compounds will help to increase the vacancy of the catalyst d-orbital domain to facilitate the adsorption of oxygen on the catalyst surface.

The embodiments of the present invention are merely illustrative of specific embodiments of the invention and are not intended to limit the scope of the invention. It is within the scope of the invention to make any modifications or variations of the invention without departing from the scope of the invention.

2. . . Carbon substrate

4. . . Palladium metal

6. . . Compound solution containing transition metal

8. . . Iron element

12. . . PdCFe alloy catalyst

14. . . PdFe alloy

16. . . FeC compound

Figure 1a shows the X-ray powder diffraction pattern of the finished catalyst.

Figure 1b shows the finished catalyst prepared in accordance with an embodiment of the present invention having a lattice constant calculated from the X-ray powder diffraction pattern.

Figure 2 is a Pd K-edge absorption spectrum of the finished catalyst.

Figure 3 is a graph of the Fe K-edge absorption spectrum of the finished catalyst.

Figure 4 is a graph showing the Pd K-edge EXAFS curve fit spectrum of the finished catalyst prepared in the examples of the present invention.

Figure 5 is a graph showing the Fe K-edge EXAFS curve fit spectrum of the finished catalyst prepared in the examples of the present invention.

6A to 6C are schematic views showing the formation of a catalyst.

Figure 7 is an ORR polarization map of the finished catalyst.

Figure 8 shows the methanol penetration test results for the finished catalyst.

Figure 9 shows the XRD diffraction pattern of the finished catalyst.

Figure 10 is a Fe K-edge X-ray absorption spectrum of the finished catalyst.

2. . . Carbon substrate

4. . . Palladium metal

12. . . PdCFe alloy catalyst

14. . . PdFe alloy

16. . . FeC compound

Claims (13)

A method for producing a catalyst, comprising: mixing a material containing carbon and palladium elements and a compound containing iron into a mixture, wherein the atomic molar ratio of palladium:iron:carbon in the mixture is 60-90:5~ 40:2~15; and reduction sintering the mixture to form a catalyst. The method for producing a catalyst according to claim 1, wherein the mixture has a molar ratio of palladium:iron:carbon of 70 to 80:10 to 30:5 to 12. The method for producing a catalyst according to claim 1, wherein the iron-containing compound comprises iron nitrite, iron sulfate or iron chloride. The method for producing a catalyst according to claim 1, wherein the palladium-containing compound comprises palladium nitrite, palladium sulfate or palladium chloride. The method for producing a catalyst according to claim 1, wherein the carbon-containing compound comprises a material having a surface area of from 100 m ² /g to 900 m ² /g. The method of producing a catalyst according to claim 5, wherein the material has a surface area of from 200 m ² /g to 850 m ² /g. The method for producing a catalyst according to claim 1, wherein the reduction sintering step comprises heating the mixture in an environment containing a reducing atmosphere. The method for producing a catalyst according to claim 7, wherein the reducing atmosphere comprises hydrogen. The method for producing a catalyst according to claim 1, wherein the temperature of the reduction sintering step is from 300 ° C to 700 ° C. The method for producing a catalyst according to claim 1, wherein the catalyst is an alloy composed of carbon, palladium and iron. The method for producing a catalyst according to claim 10, wherein the catalyst is an alloy composed of carbon, palladium and iron formed on a carbon carrier. A catalyst formed by the method of any one of claims 1 to 11. A catalyst comprising an alloy of carbon, palladium and iron formed on a carbon support.