CH638693A5 - PLATINUM CATALYST AND METHOD FOR THE PRODUCTION THEREOF. - Google Patents

Description

The invention relates to a platinum catalyst, in particular to a platinum catalyst with carbon particles as a carrier, and to a process for its production.

Platinum is a known catalyst that is used in electrochemical cells. The electrode performance in a cell is directly related to the platinum surface, which can be achieved by various reacting components within the cell. This fact, combined with the high cost of platinum, led to considerable efforts to bring platinum into a usable form with a maximum surface area per unit weight of platinum. The basic solution was and still is to bring the platinum onto the surface of suitable particles called carriers. Coal or carbon particles and graphite particles are common platinum carriers in the field of fuel cells. There are several known techniques for depositing small platinum particles on such supports. For example, the carrier can be dispersed in an aqueous solution of chloroplatinic acid, dried and exposed to hydrogen. Some other techniques are described in U.S. Patent Nos. 3,857,737,3,440,107,3,470,019 and 3,532,556.

With these techniques, platinum crystals can be distributed on the surfaces of the carrier particles so that there is a large surface area for the platinum. "

If platinum on carbon is used at temperatures above 100 ° C in the presence of a liquid (or at higher 4s temperatures in the presence of a gas), it has been found to lose surface area. This loss of surface area is particularly pronounced in an acidic fuel cell environment, e.g. Fuel cells that use phosphoric acid as an electrolyte and operate at temperatures of 120 ° C and above - During the first few hours of cell operation, the loss of surface area is enormous, but continues slowly but steadily for a considerable time afterwards. A loss of cell power is directly attributable to this loss of platinum surface.

A method of reducing this loss of surface area is described in U.S. Patent No. 4,028,274. Thereafter, the surfaces of graphitized carbon carrier particles were oxidized in the presence of an oxidizing metal catalyst 6o to form scars in the particle surfaces. The oxidizing metal catalyst was then removed and the platinum was deposited on the oxidized particles. Based on the theory that during the use of the catalyst the platinum crystallites migrate over the carbon surface and unite (i.e. recrystallize) with other platinum crystallites, whereby surface is lost, it was assumed that the scars in the surface of the carrier material prevent the platinum crystallites

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would hold in place, reducing migration and loss of surface area. Although this method was an improvement in the prior art, it was not entirely satisfactory and continued efforts led to the improved method according to the invention.

It is therefore the main object of the invention to produce an improved platinum-on-carbon catalyst, in particular an improved fuel cell electrode using this catalyst. The invention is intended to recrystallize the platinum supported on the carbon by reducing the migration of the platinum across the surface of the carbon support particles when the catalyst is later used, e.g. in a fuel cell.

Therefore, in the method according to the invention for improving a catalyst from platinum crystallites on carbon, deposition is carried out on and around the platinum crystallites.

It has been shown that a catalyst produced by the processes described here and used as an electrode catalyst in a phosphoric acid fuel cell has improved recrystallization properties, namely in that the rate of loss of specific catalyst surface is considerably reduced during fuel cell operation.

Presumably, the porous carbon deposited on and around the platinum crystallites holds the platinum crystals more securely in place on the carbon support particle, thereby significantly reducing the tendency of the crystallites to travel across the surface of the carbon support particle during use in a fuel cell. By reducing the rate of migration, the rate and extent to which recrystallization occurs are significantly reduced.

The method of depositing porous carbon on and around the platinum crystallites prior to high temperature heat treatment appears to be uncritical to the invention, but a preferred method is to heat a supported platinum catalyst in the presence of carbon monoxide gas so that the carbon monoxide is in the vicinity of the platinum crystallites (which act as a decomposition catalyst) with carbon being deposited on and around them. Other porous carbon deposition methods are mentioned below, but the carbon monoxide method is preferred because of its simplicity and low cost. In all of the methods described here for the deposition of porous carbon, the catalyst is preferably heated in the presence of a carbon-containing compound to decompose the compound to form porous carbon deposits on the platinum crystallites and around them. However, this does not mean that other carbon deposition techniques are ineffective.

These and other objects, features and advantages of the invention will become more apparent from the following detailed description of preferred embodiments, as illustrated in the figures; show it:

1 is a graphical representation of the improved fuel cell performance achieved by carbon monoxide / heat treatment of a platinum on carbon catalyst, but without the high temperature heat treatment according to the invention;

2 is a graphical representation which makes it clear that a catalyst produced according to the invention loses surface area more slowly under fuel cell conditions, and

Fig. 3 is a graph illustrating the improved ka-15 analytical activity of a catalyst made according to the invention.

A preferred method of depositing porous carbon on and around platinum crystallites of a platinum on supported catalyst is to heat the catalyst to a temperature in the range of 260 to 649 ° C in a carbon monoxide atmosphere. The platinum crystallites act as a catalyst for the decomposition of the carbon monoxide according to the following reaction equation:

25th

2 CO + heat

Pt

'C "T CO2

CD

Since carbon monoxide does not decompose in the absence of platinum in this temperature range, the carbon is only deposited on the platinum crystallites and around them. The range of heat treatment is coordinated with the time the catalyst is held in the temperature range. Critical issues are to ensure that the carbon deposits are heavy enough to significantly reduce platinum migration, 35 while at the same time not being so dense and thick that the reacting gas and / or liquid cannot easily reach the platinum crystallites while the catalyst is in use can In other words, the carbon deposits must be porous so that the performance of the catalyst does not suffer so much that the advantages of the invention are nullified.

Since it was not possible to calculate the optimal thickness or porosity of the carbon to be deposited, a series of catalyst samples were produced using different temperatures and times. Electrodes were made from these samples using standard techniques. The following Table I shows samples of the data obtained. The electrode performance data are from sub-normal cell tests using 5 x 5 cm cathodes with a catalyst amount of 0.25 to 0.50 mg / cm 2 electrode surface. A conventional anode was used. The catalyst support material was a high surface area carbon black. The electrolyte in these tests was phosphoric acid and the reaction components were air and hydrogen.

Table I

Influence of CO treatment temperature and time on platinum surface and initial performance in H3P04 fuel cells

catalyst

treatment

(Temperatures ± 14 ° C)

Initial cathode potential mV at 0.22 A / cm2

Pt surface area, m2 / g initially

100 h

1*

2nd

3rd

4th

without

5 min at 260 10 min at 260 15 min at 260

680-687 ** 682-691 ** 680-691 ** 685

120-140 **

139

125

40-50

46

53

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4th

Table I (continued)

Catalyst treatment initial cathode Pt surface, m2 / g

(Temperatures + 14 ° Q potential initially 100 h mV at 0.22 A / cm2

5 60 min at 260 686 113 51

6 60 min at 260 686 120 62

7 5 min at 371 687

8 10 min at 371 695 131 64

9 10 min at 371 695 131 66

10 15 min at 371 683 - -

11 30 min at 371 678

12 5 min at 816 621 - -

* Control catalyst, i.e. without CO treatment

** Areas are the result of multiple tests using the same catalyst.

From these tests and other information related to heat treatment of supported platinum catalysts, it is estimated that benefits can be seen when the heat treatment temperature is in the range of 260 to 649 ° C and is held in this range for 1 to 60 minutes. Apparently, the higher the heat treatment temperature, and the shorter the time that the temperature should be maintained, and vice versa. It is known from past experience that temperatures above about 649 ° C lead to excessive thermal sintering of the platinum during the heat treatment. In other words, the high temperature would cause a significant migration of the platinum crystallites during the carbon monoxide treatment, which would increase the size of the platinum crystallites and nullify advantages that would otherwise be achievable. A preferred temperature range for the heat treatment is 260 to 427 ° C, the temperature being held in this range for 5 to 30 minutes. Best results have been obtained by heating to a maximum temperature of about 371 ° C and holding at that temperature for 10 minutes.

1 shows a graph in which the time in a fuel cell is plotted on the abscissa and the fuel cell power, expressed in cell voltage, is plotted on the ordinate. The curves have arisen from actual data. Curve A is from a cell with an untreated catalyst, such as Catalyst 1 from Table I. Curves B and C are from cells with a catalyst that is at 371 ° C for 10 minutes with carbon monoxide

20 had been treated. It should be noted that, although differences in initial performance were insignificant, the treated catalysts were clearly superior over a long period of time due to the lower loss of platinum surface over time.

2s Carbon-containing gases or vapors other than carbon monoxide can also be used in the practice of the invention. For example, a gaseous hydrocarbon such as methane, acetylene or ethylene can be used, as can benzene, hexene or 30 heptane which should be used in vapor form. The following reaction equations represent the reactions that occur with these two materials:

Pt

Methane (CH4) + heat ► C + 2HZ (2)

Benzene (CeH6) + heat 6 C + 3H2 (3)

As with carbon monoxide, tests would have to be carried out for each gas to find the optimal temperature ranges for the heat treatment and the length of time for which the temperature in the ranges would be maintained. A number of samples were carried out under different conditions. Examples of some electrode test data using these samples are given in Table II 45 below. Electrode size, catalyst build, and other aspects of the fuel cell tests were as previously described in Table I.

Table II

Performance and surface data for hydrocarbon treated catalyst

Catalyst treatment initial cathode platinum surface, m2 / g

Potential initially 100 h mV at 0.22 A / cm2

13 * without 680-687 ** 140 43

14 witches, 5 min at 360 ° C 672-683 ** 133 54

15 benzene, 5 min at 427 ° C 680 - -

* Control catalyst, i.e. without treatment

** Areas are the result of multiple tests using the same catalyst.

While these tests show that the carbon is porous under another method of deposition

Using various gases and vapors deposited 65 carbon, a platinum-on-carbon catalyst could be used until 15, but they were not yet preferred and 30 minutes in an aqueous solution of about 10 wt .-%

optimal heat treatment conditions for this Mate Ct iH22Oh, commonly known as sucrose,

recognize rialien. gives way. The catalyst was then at a low temperature

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Dried to remove water, leaving a sucrose coating on all of the catalyst particles. The coated catalyst was then heated in an inert atmosphere (vacuum could have been used) to decompose the sucrose according to the following reaction equation:

CnHa-jOn + heat - »11 C + 11H20 (4)

This leaves porous carbon deposits on all surfaces of the catalyst, including on and around the platinum crystallites, as well as simply on the carbon support material. As with the other embodiments, the carbon deposited on and around the platinum crystallites serves the purpose of reducing platinum migration. s crystallites according to the teachings of the invention. Carbon that is deposited elsewhere on the support does no harm since the catalyst support material is already carbon. Table III shows test data for electrodes with a catalyst made by the sucrose decomposition method. Electrode size, amount of catalyst and other aspects of the fuel cell tests were the same as described in connection with Table I.

Table III

Performance and surface data for sucrose treated catalyst

Catalyst treatment initial cathode platinum surface, m2 / g

Potential initially 100 h 400 h mV at 0.22 A / cm2

16 * without 685 140 56 43

17 30 min at 493 ° C 685 110 70 58

18 30 min at 593 ° C 681 -

* Control catalyst, i.e. no treatment.

In order to recognize advantages, the temperature of the heat treatment for the decomposition of sucrose should be between 427 and 649 ° C and held for half a to 6 hours. In general, the temperature need only be maintained until all of the sucrose has been converted to carbon. If carbon is deposited when the temperature exceeds 649 ° C, excessive thermal sintering of the platinum can occur during the decomposition of sucrose and nullify the purpose according to the invention, which is to obtain small platinum crystallite sizes. Temperatures below 427 ° C do not charr the sucrose completely. It is believed that any soluble organic material that decomposes to carbon when heated can be used for the purposes of the present invention. Sucrose is one such soluble organic material. Examples of other materials are phenolic resins, cellulose and polyvinyl alcohol.

Quite unexpectedly, it has been found that if the catalysts prepared as disclosed above are further heat treated in an inert atmosphere or in vacuum in a temperature range of 816 to 1232 ° C and the temperature is held in this range for half a to 6 hours, the resultant Catalyst recrystallized even less. It is even more surprising that the activity of the platinum itself is increased by this high temperature treatment. This increase in platinum activity does not appear to be related to the fact that platinum recrystallization in the fuel cell is also reduced.

Has the porous carbon been deposited on the platinum crystallites, e.g. B. by one of the above methods or by any other suitable method, the

The catalyst is then subjected to a high temperature heat treatment in an inert atmosphere or in vacuum. 30 tests were carried out to determine the optimal heat treatment conditions. Some of this test data is shown in Table IV below. Electrode size, amount of catalyst and other aspects of the fuel cell tests were the same as described above in connection with Table I.

Catalyst 19 was the control catalyst and received no treatment. Carbon was not deposited in the case of catalysts 20 and 22 before the high-temperature heat treatment, for the reason that it was possible to assess the effectiveness which can be achieved according to the invention. All other catalysts were subjected to a carbon monoxide treatment at 371 ° C. for 10 minutes before the heat treatment in an inert atmosphere or in vacuo at the stated temperature.

In all of these tests, the heat treatment was carried out in an inert atmosphere or in vacuum by heating the catalyst sample from room temperature to the maximum heat treatment temperature at the speed indicated in the second column of the table. The time given in column 4 of the table counts from the moment the temperature reaches 899 ° C. For example, at 205 ° C / h, the catalyst 21 would be heated from 899 to 1170 ° C in 1 hour 20 minutes. The remaining time of the 2.5 hours 55 total time would be at 1170 ° C. Catalyst samples 27 and 29 were simply heated to 899 cc at the rate indicated and held at that temperature for 1 hour. Catalyst 24 was heated at 111 ° C to 899 ° G and then at 222 ~ C to 1038 ° C.

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Table IV

Influence of the catalyst heat treatment on the initial cathode performance in an H3P04. Fuel cell (unless otherwise stated, all catalysts were subjected to a 10-minute CO treatment at 371: C before the heat treatment)

Catalyst heating-up maximum heat time of heat rate treatment treatment "(° C / h) temperature (h)

(= C)

Heat treatment atmosphere

Initial cathode potential (mV at 0.22 A / cm2)

19 *

-

-

-

685

20 "

210

1177

2.5

n2

630

21st

210

1177

2.5

n2

680

22b

167

950

1.0

n2

680

23

167

950

1.0

n2

710

24th

III / 222c

1038

1.0

n2

690

25th

111

954

6.0

n2

674

26

iii

916

2.0

n2

695

27th

111

899

1.0

n2

710

28

iii

960

1.0

n2

701

29

167

899

1.0

vacuum

714

30th

250

910

1.0

Ar

708

* Control catalyst - no CO treatment or heat treatment

a starting at 899 ° C

b no carbon deposition before the heat treatment (i.e. no CO treatment) = 111 ° C / h up to 899 ° C; 222 ° C / h from 899 to 1038 ° C.

Although the counting time started at 899 ° C, the 30 tems (i.e., the increase in platinum particle size) begin to

Advantages to grow at around 816 ° C. gently due to the high temperatures of the heat treatment

Temperatures above about 1232 ° C can cause thermal lung. With regard to the invention it is assumed that

Cause sintering to such an extent that all of the porous carbon deposits

Advantages caused by heat treatment actually speed of the platinum crystallites during the high-temperature

will be niece. Taking this temperature heat treatment into consideration, slow it down considerably and, based on the test results obtained, if the platinum crystallites move slowly over the surface and further attempts to heat treat in the temper of carbon, you will find suitable natural temperature ranges from 816 to 1232 ° C for half to 6 hours defects in the surface of the carbon carrier material

the advantageous. Of course, deeper maximum temperatures than that which hold the platinum particles and require the migration and re-

temperatures and slower heating rates longer crystallization after use of the catalyst in the combustion

Reduce heat treatment times as higher maximum temperatures.

and heating rates. Too rapid an increase in temperature, oxidation of the carbon support by Table V may provide some information about the catalyst surface volatilization of contaminants before they have escaped for the catalysts 19, 20 and 21 that have been treated to escape from the carbon particles. as indicated in Table IV. From the data in the table. Therefore a heating rate of not high IV and V can be seen from the following: At first it turns out to be about 334 ° C / h recommended. The data obtained clearly show that the CO treatment (or another treatment against a preferred heat treatment in the temperature range for depositing porous carbon on the platinum crystal range from 899 to about 960 ° C. for% to 1Vi hours) the high temperature heat treatment should be for. ■ ... 50 the invention is critical. Second, the catalyst has 21

On the basis of the test tests, it was found, despite the great improvement in the cathode start - that the high-temperature heat treatment, although it is still a low potential between catalyst 20 and 21 - is not necessary for the invention, but the catalyst's initial cathode potential than the untreated kata improved in two ways. First, the recrystallizer 19 is used, but after only 100 hours of operation in an ionization of the platinum while using the electrode 5s fuel cell, the surfaces of the catalysts 19 in a fuel cell are reduced even more than and 21 are almost the same. If the fuel cell is used for a long time by simply separating porous carbon onto it, the catalyst 21 outperforms the untreated platinum crystallite catalyst without subsequent high-temperature heat in terms of output and surface (cf. FIG. 2). This shows me treatment of the catalytic converter, and secondly, the advantages in long-term operation are increased as a result of the special starting activity of the platinum catalytic converter (i.e. prior to the use that the catalytic converter 21 has experienced. Thirdly, the use in the fuel cell). The advantage of this and the treatment of the catalyst 25 is that even if the catchable increase remains not particularly high during the entire use heat treatment temperature, the catalyst is retained. For a long time at this temperature, there was a considerable initial effort. Finally, it can be concluded generally using a high temperature heat that better results can be obtained when using treatment, but without first depositing carbon at lower temperatures and with shorter heat treatment - the platinum crystallites. These tests were not obtained at the time when the preferred temperature was particularly successful because of the too strong thermal range from 899 to 960 ° C at V * to 1 Vi hours.

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Table V

Influence of the CO treatment on the platinum surface before the heat treatment

catalyst

Surface, m2 / g initially

100 h

19d 20d 21d

140 40 60

45 34 • 43

1 cf. Table IV for details of treatment.

The graphical representation of FIG. 2 makes it clear that the invention provides a significant improvement over catalysts which have no carbon deposition on the platinum crystallites. 2, the time elapsed under fuel cell conditions is plotted on the abscissa and the average platinum surface area on the ordinate. The curves are based on actual data. The curve marked D relates to untreated platinum-on-carbon catalyst. The curve marked E relates to a catalyst which was the same as for curve D, except that porous carbon had been deposited on and around the platinum crystallites by the carbon monoxide method described above. However, the catalyst E had not been subjected to a high temperature heat treatment in an inert atmosphere. The curve labeled F relates to a catalyst which was treated with CO and then subjected to the high-temperature heat treatment according to a preferred embodiment of the invention. That is, it refers to a catalyst which is identical to the catalyst of curve E, but which has also been subjected to high-temperature heat treatment in an inert atmosphere.

The catalysts D and E start recognizably with approximately the same platinum surface, which is larger than the platinum surface of the catalyst F. In all cases, the platinum surface decreases during fuel cell operation; the surface of the catalysts E and F decreases more slowly than that of the catalyst D. After about 200 hours

Fuel cell operation exceeds the platinum surface of the catalyst F the surface of the catalyst D. After about 2000 hours of fuel cell operation, the surface of the catalyst F also exceeds the platinum surface of the catalyst E. This partly explains the improved performance of the fuel cell, the catalyst subjected to carbon separation and subsequent heat treatment used.

3 is also based on experimental data and shows that, apart from the reduced recrystallization rate of the platinum, at least initially the platinum is more active in itself and as a result of the invention. In this diagram the time in the fuel cell is again plotted on the abscissa and 15 the catalyst activity, expressed as fuel cell performance at 0.22 A / cm2, on the ordinate. It can be seen immediately that the activity of the catalyst produced according to the invention (curve H) is at the beginning and always higher than the activity of the untreated catalyst (curve G), despite the fact that it has a smaller platinum surface area at the beginning (cf. Fig. 2). Even if the catalysts of curves H and G finally have the same surface (which is not the case) after a long period of use, the activity of the catalyst represented by the Kur-25 ve H is still higher than that of the untreated catalyst.

The invention thus discloses an improved platinum crystallite on carbon catalyst with carbon deposition on and around the platinum crystallites. According to a preferred embodiment of the invention, a further improvement of this catalyst can be achieved by subsequent heat treatment. However, this heat treatment, although advantageous, is not necessary according to the invention, even in the absence of this additional heat treatment it provides a considerable improvement over the catalysts of the prior art.

Although the invention has been shown and described in terms of preferred embodiments, those skilled in the art will of course be able to make various other changes and omissions in form and details without departing from the spirit and scope of the invention.

s

3 sheets of drawings

Claims (15)

638 693 PATENT CLAIMS 1. Platinum catalyst, characterized in that it has pia tin krf s talli ta on carbon particles as a carrier, wherein carbon has been deposited on and around the crystallites to reduce the migration of the crystallites on the surface of the carbon particles during subsequent treatment or use of the catalyst is. Process for the production of a platinum catalyst according to claim 1, characterized in that the catalyst made of platinum crystallites on carbon particles as a carrier is subjected to the deposition of porous carbon on the platinum crystallites and around them. 3. The method according to claim 2, characterized in that in the Abscheidimg the catalyst is heated in the presence of a carbon-containing compound to decompose it and to form porous carbon. 4. The method according to claim 3, characterized in that a gaseous or vaporous carbon-containing compound is used and the catalyst is heated in the presence of the gas or steam. 5. The method according to claim 4, characterized in that carbon monoxide is used as the carbon-containing compound. 6. The method according to claim 5, characterized in that when heated to a temperature of 260 to 649 ° C and the temperature is maintained in this range for 1 to 60 minutes. 7. The method according to claim 5, characterized in that heated to 260 to 427 ° C and the temperature is held in this range for 5 to 30 minutes. 8. The method according to claim 3, characterized in that heated to a maximum of 371 ° C and this maximum temperature is maintained for 10 minutes. 9. The method according to claim 4, characterized in that a hydrocarbon vapor or gas is used as the carbon-containing compound and is heated to a sufficiently high temperature for the decomposition of the hydrocarbon for the deposition of porous carbon on the platinum crystallites and around them. 10. The method according to claim 9, characterized in that as the hydrocarbon n-hexene, heptane, benzene, 5 acetylene, methane or ethylene is used. 11. The method according to claim 3, characterized in that a water-soluble organic material is used as the carbon-containing material and a layer of the water-soluble organic for the deposition of the carbon 10 material applied to the catalyst and this is then heated in an inert atmosphere or in vacuo to decompose the organic material in the layer, leaving porous carbon on and around the platinum crystallites. 1S 12. The method according to claim 11, characterized in that to apply a layer of the water-soluble organic material, the catalyst is soaked in an aqueous solution of the organic material and dried to remove the water from the layer. 20th 13. The method according to any one of claims 11 or 12, characterized in that sucrose is used as the carbon-containing material. 14. The method according to claim 13, characterized in 25 net that to a temperature in the range of 427 to 649 ° C heated and the temperature is held in this range for half a to t hours. 15. The method according to any one of claims 2 to 14, characterized in that the catalyst after the carbon 30 subjected to a heat treatment in an inert atmosphere or in vacuum to a temperature in the range from 816 to 1232 CC and the temperature is maintained in this range for half a to 6 hours. 16. The method according to claim 15, characterized in 3s net that to a temperature in the range of 899 to 960 ° C heated and the temperature in this range is maintained for 0.75 to 1.5 hours.