TWI412404B - Catalysts and methods for manufacturing the same - Google Patents

Abstract

The present invention relates to a catalyst and a forming method thereof. The catalyst according to the invention comprises an alloy catalyst which absorbs on a carbon carrier. The alloy catalyst is composed of the following components: 50atom%-98atom% of palladium, 2atom%-30atom% of cobalt, and 0.01atom%-less than 5atom% of molybdenum. If the proportion of the molybdenum of the alloy catalyst is larger than or equal with 5atom%, activity and durability of the alloy catalyst are reduced.

Description

Catalyst and its formation method

The present invention relates to catalysts, more particularly to the composition ratio and formation method of the catalyst, and to the use of the catalyst in fuel cells.

A direct methanol fuel cell (DMFC) is a power supply device that operates at a low temperature using a fuel directly. Since it is operated under low temperature conditions, it is necessary to use a catalyst to achieve an ideal power generation efficiency. Since proton exchange membrane fuel cells (PEMFC) mainly use hydrogen as a fuel source, and then pure hydrogen has difficulty in transportation and storage, most of the recombiners are used to convert methanol or other hydrocarbon fuels into required fuel gases. These fuel gases often carry impurities such as carbon monoxide (CO) or carbon dioxide (CO ₂ ), so direct methanol fuel cells (DMFCs) that are simply reacted directly are valued. The current development is mainly divided into active and passive. The output power of the active fuel cell system is usually designed to be more than 10W. It is suitable for fixed products and is generally operated at low methanol concentration (1M). Passive fuel cell systems have been developed for low power and below 1W product requirements, and are suitable for portable products, typically operating at high methanol concentrations (5M). Although platinum is often used as an electrode material and has good electrochemical properties, the metal platinum content is limited and the price is high, which makes the practical application difficult, even if a second metal is added to form Pt-M as a cathode contact. The medium reduces the amount of platinum used, but in active or passive systems, it still faces the problem that the anode methanol penetrates to the cathode and poisons the cathode catalyst by carbon monoxide (CO), resulting in reduced catalyst activity or even complete loss of catalytic ability. Therefore, the cheaper palladium is gradually gaining attention. However, palladium itself has poor catalytic activity. At present, some catalysts have been explored for palladium as the main component, but most of the catalysts obtained have low oxygen reduction ability, or low load, low load during coating because It is necessary to apply a thick catalyst layer and face quality problems.

U.S. Patent No. 7,739,910 uses synthetic PtM to reduce costs, but it still cannot avoid the poisoning problem caused by methanol penetration. At present, it is generally studied that the catalyst alloy of Pd system is more than the low load amount. This is because the low load is easy to disperse and synthesize, and the better activity can be achieved. However, in practical applications, it is necessary to apply a thick catalyst layer to cause quality. Passing the problem makes the practical application difficult. Although PdCo synthesized by US Pat. No. 7,632,601 can achieve activity similar to Pt, the loading of the catalyst synthesis is only about 20% by weight to 30% by weight, and thus there are still deficiencies. US7498286 is a carbon-supported PdCoMo. Although the activity and stability are improved, the metal loading is about 20 wt%, so it is difficult to practically apply to the DMFC system. In addition to this, this document forms a PdCo/C on a carbon support (XC-72R) using an ammonium hexachlororpalladate and cobalt nitrate, and then uses an Impregnation Method to Mo. (ammonium heptamolybdate) is impregnated with PdCo/C as a two-step synthesis method. The elemental composition ratio is Pd between 60 and 80 atom%, Co is between 10 and 30 atom%, and Mo is between 5 and 15 atom%. The disadvantage of this method is a two-step process with a high Mo content.

In summary, there is an urgent need for a new catalyst composition and system to solve the above problems.

An embodiment of the present invention provides a catalyst comprising a carbon support; and an alloy catalyst adsorbed on the surface of the carbon support, wherein the alloy catalyst is from 50 atom% to 98 atom% of palladium, and 2 atom% to 30 atom% of cobalt. And a composition of molybdenum of 0.01 atom% to less than 5 atom%.

Embodiments of the present invention provide a method of forming a catalyst. The carbon support is first dispersed in ethylene glycol to form a carbon support dispersion. The carbon support may be activated carbon, carbon black, carbon nanoparticle, carbon nanotube, carbon nanofiber, furnace black, graphite carbon black, graphite, or a combination thereof. In an embodiment of the invention, the carbon support has a surface area between 10 m ² /g and 2000 m ² /g. If the surface area of the carbon support is too small, the amount of catalyst loading will decrease and the catalyst particles will be unevenly dispersed on the carrier. If the surface area of the carbon support is too large, it usually has more micropores, which does not contribute much to the dispersion, and the pores are easily blocked by the catalyst.

An aqueous solution of a palladium salt, a cobalt salt, and a molybdenum salt is then added to the carbon carrier dispersion. In one embodiment, the palladium salt may be PdCl ₂ , Pd ^‧ (NO ₃ ) ₂ , Pd(NH ₃ )Cl ₂ ‧H ₂ O, Pd ^‧ (C ₂ H ₃ O ₂ ) ₂ , Pd (C ₅ H ₇ O ₂ ) ₂ , Pd(CN) ₂ , PdSO ₄ or a combination thereof, the cobalt salt may be Co(NO ₃ ) ₂ ‧6H ₂ O, CoCl ₂ ‧6H ₂ O, Co(C ₂ H ₃ O ₂ ) _{2, 2CoCO 3 .3Co (OH)} 2 .H 2 O, CoCO 3 .Co (OH) 2, CoSO 4 ‧7H 2 O, CoSO 4 ‧H 2 O or a combination thereof, the molybdenum may be (NH _{4) 6} Mo ₇ O ₂₄ ‧4H ₂ O, (NH ₄ ) ₂ MoO ₄ , MoCl ₅ , MoCl ₃ ‧3H ₂ O, Mo(C ₂ H ₃ O ₂ ) ₂ or a combination thereof.

The synthesis method may be an impregnation method, an incipient wetness impregnation method, a sol-gel method, a sputtering method or a chemical reduction method, etc., and the present invention synthesizes a catalyst by an impregnation method and a chemical reduction method using ethylene glycol as a solvent.

The pH of the aqueous solution of ethylene glycol is adjusted to reduce and adsorb the palladium salt, the cobalt salt, and the molybdenum salt to the carbon support to form an alloy catalyst. In an embodiment of the invention, the pH of the aqueous solution of ethylene glycol is adjusted to between 8 and 13. If the pH of the aqueous solution of ethylene glycol is too high, the surface potential of the catalyst will not easily adsorb to the surface of the carbon material. If the pH of the aqueous solution of ethylene glycol is too low, the synthesized catalyst particles become large, reducing the active center of the surface catalyst.

The above solution is filtered, and the filter cake is washed to obtain an alloy catalyst. Subsequent heat treatment of the alloy catalyst in a reducing atmosphere reduces the oxide layer on the surface of the catalyst and helps to improve its stability. In an embodiment of the invention, the reducing atmosphere is hydrogen, or a mixed gas of hydrogen and blunt gas. The above-mentioned inert gas may be argon gas, nitrogen gas, or a combination thereof. In an embodiment of the invention, the temperature at which the heat treatment is carried out under a reducing atmosphere is between 200 and 750 °C. If the temperature of the heat treatment is too high, the particles aggregate into large particles, resulting in a decrease in the active center and a decrease in activity. If the temperature of the heat treatment is too low, the degree of catalyst alloying is insufficient to make the stability insufficient.

After the above steps, a PdCoMo alloy catalyst can be formed and adsorbed on the carbon support. In an embodiment of the invention, the weight ratio of the alloy catalyst to the carbon carrier is between 40:60 and 80:20, and in another embodiment, the weight ratio of the alloy catalyst to the carbon carrier is between 40:60 and 70. :30. If the proportion of the alloy catalyst is too high, the aggregation of the alloy catalyst particles will result in a decrease in the actual use rate of the catalyst. If the proportion of the alloy catalyst is too low, it is necessary to apply a thick catalyst layer when applying the MEA to cause a quality problem. The alloy catalyst is composed of 50 atom% to 98 atom% of palladium, 2 atom% to 30 atom% of cobalt, and 0.01 atom% to less than 5 atom% of molybdenum. In another embodiment, the alloy catalyst system is 75- 95 atom% palladium, 5-25 atom% cobalt, 0.03-3 atom% molybdenum. If the amount of molybdenum is less than 0.01 atomic %, the Mo effect will be insignificant and the activity will be similar to PdCo. If the amount of molybdenum is greater than or equal to 5%, the activity and stability of the alloy catalyst are lowered.

The above alloy catalyst adsorbed on the carbon support can be used as a cathode of the fuel cell. For the production method of the fuel cell and the cathode, reference is made to US7498286.

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

[Examples]

Synthesis method of catalyst sample:

Ketjen Black ECP300 was dispersed as a carbon carrier in ethylene glycol and stirred with a magnet for 30 minutes to form a carbon carrier dispersion. The desired PdC12, Co(NO ₃ ) ₂ ‧6H ₂ O and (NH ₄ ) ₆ Mo ₇ O ₂₄ ‧4H ₂ O were dissolved in an aqueous NaCl solution to form a metal salt aqueous solution. After the aqueous metal salt solution was added dropwise to the above carbon carrier dispersion, stirring was continued for 10 minutes and then ultrasonically oscillated for 30 minutes. The pH of the shaken solution was then adjusted with NaOH (1 M) dissolved in ethylene glycol. The lye was stirred with a magnet under argon, and the lye was heated under reflux at 150 ° C for 2 hours to reduce the adsorption of the metal salt in the lye to the carbon support. The above lye was cooled to room temperature, filtered, and the cake was washed to obtain a crude product (PdCoMo/C). The crude product was then heat treated (350 ° C) in a mixed atmosphere of 2% hydrogen and 98% argon for two hours to obtain the final product: an alloy catalyst (PdCoMo/C) adsorbed on a carbon support.

Elemental analysis of catalyst samples:

The elemental composition ratio of PdCoMo was measured by an energy dispersive analyzer (EDS).

Prepare test samples:

Disperse 0.07 g of the catalyst prepared in the following examples or comparative examples in 19 g of deionized water with 0.0524 g of Nafion In the solution, after the ultrasonic wave was evenly oscillated, 10 μL of the catalyst was dispersed to the glassy carbon electrode. After the above catalyst dispersion is dried, the catalyst properties can be tested according to different conditions.

Test Conditions:

First, the above-mentioned catalyst-coated electrode was placed in an aqueous 0.5 MH ₂ SO ₄ solution, and nitrogen gas was passed through a sulfuric acid aqueous solution to perform CV activation scanning for 20 cycles. Then, oxygen gas was introduced into an aqueous sulfuric acid solution, and the electrode was rotated at a rotation speed of 2,500 rpm to carry out a redox reaction activity test of the catalyst.

Anti-methanol test conditions:

The above-mentioned electrode-coated electrode was placed in an aqueous 0.5 MH ₂ SO ₄ solution, and nitrogen gas was passed through an aqueous sulfuric acid solution for 20 cycles of CV activation scanning. Next, the electrode was taken out and placed in a 1 M aqueous methanol solution, oxygen was passed through an aqueous methanol solution, and the electrode was rotated at a rotation speed of 2,500 rpm to carry out a redox reaction activity test of the catalyst.

Endurance test:

The above electrode-coated electrode was placed in a 0.5 MH ₂ SO ₄ aqueous solution, and nitrogen gas was passed through a sulfuric acid aqueous solution to perform CV activation scanning for 20 cycles. Next, oxygen was introduced into the aqueous sulfuric acid solution, and the electrode was rotated at a rotation speed of 2,500 rpm for a while, and then the catalyst was subjected to a discharge test at a constant voltage of 0.7 V.

Anti-methanol endurance test:

The above-mentioned electrode-coated electrode was placed in an aqueous 0.5 MH ₂ SO ₄ solution, and nitrogen gas was passed through an aqueous sulfuric acid solution for 20 cycles of CV activation scanning. Next, the electrode was taken out and placed in a 1 M aqueous methanol solution, oxygen was introduced into an aqueous methanol solution, and the electrode was rotated at a rotation speed of 2,500 rpm for a while, and then a catalyst discharge test was performed at a constant voltage of 0.7 V.

Example 1

The alloy catalyst PdCoMo was synthesized according to the catalyst sample synthesis method. The atomic ratio of Pd, Co and Mo is 92.59:6.97:0.44, and the weight ratio of alloy catalyst to carbon carrier is 64:36. The activity of the catalyst in a 0.5 M aqueous solution of sulfuric acid and a voltage of 0.75 V was 68 A/g, and the activity at a voltage of 0.5 M aqueous sulfuric acid and a voltage of 0.8 V was 46 A/g. The activity of the catalyst in a 1 M aqueous methanol solution at a voltage of 0.75 V was 47 A/g, and the activity in a 1 M aqueous methanol solution and a voltage of 0.8 V was 27 A/g. The activity of the catalyst after being placed in a 0.5 M aqueous sulfuric acid solution for 3 hours was 38 A/g, and the activity after being placed in a 1 M aqueous methanol solution for 3 hours was 17 A/g.

Example 2

A PdCoMo alloy catalyst having an atomic ratio of Pd, Co and Mo of 95.11:4.86:0.03 was synthesized, and the weight ratio of the alloy catalyst to the carbon carrier was 64:36. The activity of the catalyst in a 0.5 M aqueous solution of sulfuric acid and a voltage of 0.75 V was 63 A/g, and the activity in a 0.5 M aqueous sulfuric acid solution and a voltage of 0.8 V was 40 A/g. The activity of the catalyst in a 1 M aqueous methanol solution at a voltage of 0.75 V was 45 A/g, and the activity in a 1 M aqueous methanol solution and a voltage of 0.8 V was 33 A/g. The activity of the catalyst after being placed in a 0.5 M aqueous sulfuric acid solution for 3 hours was 28 A/g, and the activity after being placed in a 1 M aqueous methanol solution for 3 hours was 26 A/g.

Example 3

A PdCoMo alloy catalyst having an atomic ratio of Pd, Co and Mo of 91.73:5.58:2.69 was synthesized, and the weight ratio of the alloy catalyst to the carbon carrier was 62:38. The activity of the catalyst in a 0.5 M aqueous sulfuric acid solution at a voltage of 0.75 V was 60 A/g, and the activity at a voltage of 0.5 M sulfuric acid aqueous solution and a voltage of 0.8 V was 37 A/g. The activity of the catalyst after being placed in a 0.5 M aqueous solution of sulfuric acid for 3 hours was 21 A/g.

Example 4

The PdCoMo alloy catalyst having an atomic ratio of Pd, Co and Mo of 81.67:13.95:4.38 was synthesized, and the weight ratio of the alloy catalyst to the carbon carrier was 64:36. The activity of the catalyst in a 0.5 M aqueous solution of sulfuric acid and a voltage of 0.75 V was 47 A/g, and the activity at a voltage of 0.5 M aqueous sulfuric acid solution and a voltage of 0.8 V was 32 A/g. The activity of the catalyst in a 1 M aqueous methanol solution at a voltage of 0.75 V was 38 A/g, and the activity in a 1 M aqueous methanol solution and a voltage of 0.8 V was 26 A/g.

Example 5

A PdCoMo alloy catalyst having an atomic ratio of Pd, Co and Mo of 88.38:11.61:0.01 was synthesized, and the weight ratio of the alloy catalyst to the carbon carrier was 64:36. The activity of the catalyst in a 0.5 M aqueous sulfuric acid solution at a voltage of 0.75 V was 61 A/g, and the activity at a voltage of 0.5 M sulfuric acid aqueous solution and a voltage of 0.8 V was 37 A/g. The activity of this catalyst after placing it in a 0.5 M aqueous sulfuric acid solution for 3 hours was 17 A/g.

Comparative example 1

A PdCoMo alloy catalyst having an atomic ratio of Pd, Co and Mo of 86.15:7.84:6 was synthesized, and the weight ratio of the alloy catalyst to the carbon carrier was 61:39. The activity of the catalyst in a 0.5 M aqueous solution of sulfuric acid and a voltage of 0.75 V was 47 A/g, and the activity at a voltage of 0.5 M aqueous sulfuric acid solution and a voltage of 0.8 V was 32 A/g. The activity of the catalyst in a 1 M aqueous methanol solution at a voltage of 0.75 V was 32 A/g, and the activity in a 1 M aqueous methanol solution and a voltage of 0.8 V was 14 A/g. The activity of the catalyst after being placed in a 0.5 M aqueous sulfuric acid solution for 3 hours was 6.8 A/g, and the activity after being placed in a 1 M aqueous methanol solution for 3 hours was 8 A/g.

Comparative example 2

A PdCoMo alloy catalyst having an atomic ratio of Pd, Co, and Mo of 73.3:13.5:13.2 was synthesized, and the weight ratio of the alloy catalyst to the carbon carrier was 61:39. The activity of the catalyst in a 0.5 M aqueous sulfuric acid solution at a voltage of 0.75 V was 28 A/g, and the activity at a voltage of 0.5 M sulfuric acid aqueous solution and a voltage of 0.8 V was 15 A/g. The activity of the catalyst in a 1 M aqueous methanol solution at a voltage of 0.75 V was 22 A/g, and the activity in a 1 M aqueous methanol solution at a voltage of 0.8 V was 12 A/g. The activity of the catalyst after being placed in a 0.5 M aqueous sulfuric acid solution for 3 hours was 5.3 A/g, and the activity after placing in a 1 M aqueous methanol solution for half an hour was 1.5 A/g.

Comparative example 3

A PdCo alloy catalyst having an atomic ratio of Pd to Co of 91.65: 8.35 was synthesized, and the weight ratio of the alloy catalyst to the carbon carrier was 61:39. The activity of the catalyst in a 0.5 M aqueous sulfuric acid solution at a voltage of 0.75 V was 26 A/g, and the activity at a voltage of 0.5 M sulfuric acid aqueous solution and a voltage of 0.8 V was 8 A/g. The activity of the catalyst in a 1 M aqueous methanol solution at a voltage of 0.75 V was 22 A/g, and the activity in a 1 M aqueous methanol solution and a voltage of 0.8 V was 5 A/g. The activity of this catalyst after placing it in a 0.5 M aqueous sulfuric acid solution for 2.5 hours was 2.3 A/g.

Comparative example 4

Pt catalyst is a commercial catalyst. The activity of the catalyst in a 0.5 M aqueous sulfuric acid solution at a voltage of 0.75 V was 41 A/g, and the activity at a voltage of 0.5 M sulfuric acid aqueous solution and a voltage of 0.8 V was 35 A/g. The activity of the catalyst in a 1 M aqueous methanol solution at a voltage of 0.75 V was 0 A/g, and the activity in a 1 M aqueous methanol solution and a voltage of 0.8 V was 0 A/g. The activity of the catalyst after being placed in a 0.5 M aqueous sulfuric acid solution for 3 hours was 20 A/g, and the activity of the aqueous methanol solution placed in 1 M was 0 A/g.

The comparison of the activity of the catalyst of the above examples and the comparative examples under sulfuric acid is shown in Fig. 1, and the activity comparison chart under methanol is shown in Fig. 2, and the durability under sulfuric acid is shown in Fig. 3, The durability under methanol is shown in Fig. 4.

It can be seen from the above that the catalytic metal loading in this case can reach 60 wt%, which solves the problem that the general chemical reduction method cannot achieve high load or high load but has poor activity. In addition, the composition ratio of Mo element in this case is lower than that of the previous case. When it is 5 atom% or less, activity, stability, and resistance to methanol poisoning are better. As the proportion of Mo increases, the activity and stability tend to decrease. When the proportion of Mo reaches 13 atom%, the activity is conservatively estimated to be only one-half of the catalyst with a Mo content of 5 atomic % or less. The stability is three. Only one-fifth of the hour is left.

While the invention has been described above in terms of several preferred embodiments, it is not intended to limit the invention, and the invention may be modified and modified without departing from the spirit and scope of the invention. The scope of the invention is defined by the scope of the appended claims.

1 is a comparison chart of activities of a catalyst having different Mo contents at a voltage of 0.5 M sulfuric acid and a voltage of 0.75 V in an embodiment of the present invention;

2 is a comparison chart of activities of a catalyst having different Mo contents in a 1 M methanol and a voltage of 0.75 V in an embodiment of the present invention;

Figure 3 is a graph comparing the durability of a catalyst having a different Mo content to 0.5 M sulfuric acid in an embodiment of the present invention;

Figure 4 is a graph comparing the durability of a catalyst having different Mo contents in 1 M methanol in the examples of the present invention.

Claims (9)

A catalyst comprising: a carbon support; and an alloy catalyst adsorbed on the surface of the carbon support, wherein the alloy catalyst is from 50 atom% to 98 atom% palladium, 2 atom% to 30 atom% cobalt, And a composition of molybdenum of 0.01 atom% to less than 5 atom%, wherein the total of palladium, cobalt, and molybdenum is 100 atom%. The catalyst according to claim 1, wherein the weight ratio of the carbon carrier to the alloy catalyst is between 40:60 and 80:20. The catalyst according to claim 1, wherein the carbon carrier comprises activated carbon, carbon black, carbon nanoparticle, carbon nanotube, carbon nanofiber, furnace black, graphite carbon black, graphite, or the like. combination. The catalyst according to claim 1, wherein the carbon carrier has a surface area of between 10 m ² /g and 2000 m ² /g. The catalyst described in claim 1 is applied to the cathode of a fuel cell. A method for forming a catalyst comprises: dispersing a carbon carrier in ethylene glycol to form a carbon carrier dispersion; adding an aqueous solution of a palladium salt, a cobalt salt, and a molybdenum salt to the carbon carrier dispersion to form a polyethylene An aqueous alcohol solution; adjusting the pH of the aqueous solution of the ethylene glycol to reduce and adsorb the palladium salt, the cobalt salt, and the molybdenum salt to the carbon support to form an alloy catalyst; and heat treating the alloy catalyst in a reducing atmosphere, Wherein the alloy catalyst is composed of 50 atom% to 98 atom% of palladium, 2 atom% to 30 atom% of cobalt, and 0.01 atom% to less than 5 atom% of molybdenum, wherein the total of palladium, cobalt and molybdenum It is 100 atom%. The method for forming a catalyst according to claim 6, wherein in the step of adjusting the pH of the aqueous solution of the ethylene glycol, the pH is between 8 and 13. The method for forming a catalyst according to claim 6, wherein the reducing atmosphere comprises hydrogen or a mixed gas of hydrogen and a gas. The method for forming a catalyst according to claim 6, wherein the temperature of the alloy catalyst is heat treated under the reducing atmosphere to be between 200 and 750 °C.