SE439438B - PLATINUM CATALYST AND PROCEDURE FOR ITS PREPARATION - Google Patents

Description

å7s117s4-3 L 10 ° C in the presence of a liquid (or at higher temperatures in the be of a gas) it has been shown to lose surface area. This loss of surface area is particularly noticeable in an acid fuel cell environment, such as in fuel cells, in which phosphoric acid is used as the lyte, operating at temperatures anywhere from 120 ° C and over it. The loss of surface area is noticeable during the first few hours of cell work but continues with slow but steady for a significant period thereafter. A decline in cell function can be directly attributed to this loss of platinum surface area.

One way to reduce this loss is described in American U.S. Pat. No. 4,028,274. According to this publication was oxidized the surface of graphitized carbonaceous particles in the presence of a metal oxidizing catalyst, so that pits were formed in the particles surfaces. The metal oxidation catalyst was then removed and placed the tin was precipitated on the oxidized clays. On the basis of theo- During the use of the catalyst, platinum crystallites migrate over the surface of the carbon and is combined with other platinum crystals liters (ie recrystallization) and thereby loses surface area, it was assumed that the pits in the surface of the support material would hold more securely the platinum crystallites in place, thereby reducing migration and the loss of surface area. Although this method meant an improvement compared to the prior art, it was not full satisfactory, and continued efforts have resulted in the improved process of the present invention. The object of the present invention is thus to provide provide an improved platinum catalyst consisting of platinum crystallites with carbon particles as carriers thereof and more specifically to produce an improved fuel cell electrode using of this catalyst.

Another object of the invention is to reduce recrystallization. carbonation of platinum supported by reduction of platinum migration of the carbon over the surface of the carbonaceous particles below the catalyst subsequent use, for example in a fuel cell.

According to the invention, the method for improvement comprises of a platinum crystallite catalyst with carbon particles such as carrier, that the catalyst is subjected to a precipitation of voice carbon on and around the platinum crystallites.

It has been found that a catalyst prepared as follows described methods and used as an electrode catalyst in a phosphorus acid fuel cell has improved recrystallization properties in that the rate of loss in the particular surface area of the catalyst pp. 7811784-3 during the operation of the fuel cell is significantly reduced.

It is believed that the porous carbon, which precipitates on and around platinum crystallites, strive to more safely "hold" platinum crystals the tallit in place on the coal carrier particle and thereby significantly reduces the ability of the crystallites to migrate across carbonaceous pairs surface of the article during use in a fuel cell. By reduction of the migration speed significantly reduces the speed and conversion socket, with which recrystallization takes place.

The method of precipitating the porous carbon on and around the platinum crystallites before the high temperature treatment are not visible have any decisive importance in the invention, but a preferred The process consists in heating a supported platinum catalyst. in the presence of carbon monoxide gas, so that the carbon monoxide decomposes in the presence of the platinum crystallites (which act as a decomposing split catalyst) to precipitate carbon on and around the same.

Other methods for precipitating porous carbon are described therein following, but the carbon monoxide method is most suitable because of its simplicity and low cost. Essentially, all of those described herein include methods for precipitating the porous carbon heating of the catalyst in the presence of a carbonaceous compound for decomposition division and thereby the formation of porous carbon deposits on and around the platinum crystallites. However, this does not mean that other methods of precipitating carbon cannot be used.

The above and other purposes, features and benefits with the present invention will become more apparent from the following details. description of suitable embodiments thereof, which shown in the accompanying drawing.

Fig. 1 is a graph showing the improved fuel cell function, which is achieved by a catalyst of platinum supported on carbon is subjected to a carbon monoxide heat treatment but not a high temperature treatment according to the present invention.

Fig. 2 is a diagram showing that a catalyst set according to the present invention loses surface area with reduced speed when exposed to fuel conditions. 7Fig. 3 is a graph showing the improved catalytic the activity of a catalyst prepared according to a embodiment of the present invention.

Appropriate procedure for precipitation of porous carbon on and around the platinum crystallites in a carbon-clad platinum catalyst consists of heating the catalyst to the temperature range between 260 and 649 ° C in a carbon monoxide atmosphere. The platinum crystallites act 7811784--3 4- as a catalyst for the decomposition of the carbon monoxide according to jande formula: .. Pt 2CO + heat 3 C + CO 2 (l) Because without the presence of platinum the carbon monoxide will not decomposes in this temperature range, the carbon precipitates only on and around the platinum crystallites. The heat treatment area is coordinated with the time that the catalyst is kept within this temperature area. The important measures are to ensure that the precipitates are strong enough to significantly reduce cera platinum migration, while at the same time they do not become so dense and thick that during the use of the catalyst the reactant the gas and / or liquid cannot easily reach the platinum crystallites.

The carbon deposits must, in other words, be so porous that the function of the catalyst does not suffer to such an extent, that the advantages of the present invention are nullified.

Because it has not been possible to calculate the optimal the thickness or porosity of the carbon which is precipitated, a number of catalyst samples were used using different temperatures tours and times. Electrodes were made from these samples with application of standard technology. Table I below shows samples on the data obtained. Electrode performance data is derived from internal cell samples using 5 x 5 cm cathodes with a catalyst charge of 0.25 - 0.50 mg / cmz of the electrode surface. A conventional anode was used. The catalyst support material consisted of a height surface area carbon black. The electrolyte in these tests consisted of phosphoric acid and the reactant of air and hydrogen.

Of these samples and other information regarding heat the action of the supported platinum catalyst it is assumed that benefits can be expected if the heat treatment temperature is within the range of 260 ° C to 649 ° C and if the temperature is kept within this area for one to sixty minutes. The higher the heat the action temperature is, the shorter the time apparently the temperature must be maintained, and vice versa. From experience know man that temperatures above approx. 649OC causes excessive heat sintering ring of platinum during the heat treatment, ie. the high temperature would in itself lead to significant migration of platinum the crystallites during the carbon monoxide treatment, resulting in a increase in platinum crystallite size which may eliminate the parts, which could otherwise be achieved. A suitable heat treatment 5_ 7811784-3 temperature range is estimated to be between 260 and 427 ° C, with temperatures the tour is held in this area for five to thirty minutes. The best results were obtained by heating to a maximum temperature. rature of approx. 37l ° C and by maintaining at this temperature under approx. 10 minutes.

Fig. 1 shows a diagram in which the time in a fuel cell is dotted on the horizontal axis and the function of the fuel cell tion or performance, expressed in cell voltage, is dotted with the vertical axis. The curves are dotted according to current data.

Curve A is from cell using untreated catalyst, for example Catalyst 1 in Table I. Curves B and C are from cells using catalyst which has been carbon monoxide treated charge at 37 ° C for 10 minutes. It should be noted that although differences in initial performance were not significant, var the treated catalysts are clearly superior in length due to the smaller losses over time in the platinum area.

Carbonated gases or vapors other than carbon monoxide may also be used. used in the present invention. Thus, for example, a carbon hydrogen gas such as methane, acetylene or ethylene can be used, as well benzene, hexene or heptane, to be used in the form of steam.

The formulas below show the reactions that occur in two of these materials: methane - CH4 + heat Pt C + 2H2 (2) benzene - CGH6 + heat Pt 6C + 3H2 (3) As with carbon monoxide, tests shall be performed on each gas to: obtain optimal temperature ranges for the heat treatment and it time, as the temperature must be kept within these ranges. A limitation The number of samples was prepared under different conditions. Example on some electrode test data using these samples is shown in Table II below. Electrode size, catalyst charge and other details of the fuel cell samples were the same as described above joke in connection with Table I. Although these tests show that the carbon can be precipitated using of different gases and vapors, insufficient tests were performed to show suitable and optimal heat treatment conditions for these materials.

In a further method for precipitating porous carbon soaked test a carbon-bearing platinum catalyst for 15-30 minutes with a 'mot Qíišf-Iš * 7811784-5 6- diluted solution of approx. 10% by weight C11H22O11, generally known as sucrose. The catalyst was then dried at a low temperature to remove the water, whereby a coating of sucrose was obtained over the entire catalyst particle. The coated catalyst is heated then in an inert atmosphere (a vacuum could have been used to decompose the sucrose according to the following formula: C11H22011 + heat -) 11C + 11H2O (4) This gave porous carbon deposits over all surfaces of the catalyst, incl. on and around the platinum crystallites, including as on the carbon support material. As with the other designs serve carbon, which precipitates on and around the platinum crystallites, to according to the invention reduce the migration of the platinum crystallites.

Coal that precipitates elsewhere on the carrier does no harm, because the catalyst support material already consists of carbon. Table III shows sample data for electrodes comprising the catalyst prepared using the sucrose decomposition method. Electrode size, catalyst charge and other details in the fuel cell samples were the same as described above in connection with Table I.

It is advisable to ensure that the heat treatment temperature at sucrose decomposition is between 427 and 649oC, and that the temperature is kept in this range between half and six hours. For the most part, the temperature only needs to be maintained until all sucrose has been converted to carbon. When the carbon precipitates about temperature the temperature exceeds 649 ° C, excessive heat sintering of the platinum may occur during the sucrose decomposition occur and nullify the purpose with the invention, which consists in maintaining small platinum crystals color sizes. Temperatures below 427 ° C will not be complete densely carbonate the sucrose. It is believed that each soluble, organic material, which on heating will decompose into carbon, can be used for the purpose of the present invention. Sucrose is such a soluble, organic material. Examples of other rial are phenolic resins, cellulose and polyvinyl alcohol.

It has, quite unexpectedly, turned out that if the catalysts as prepared according to the above description additional heat traded in an inert atmosphere or vacuum to a temperature range from 816 to 132 ° C and the temperature is kept within this area for half to six hours, the resulting is recrystallized the catalyst to an even lesser extent. In addition, which is even more so surprisingly, the activity of platinum itself is increased by this high 1. 7811784-5 peratur treatment. This increase in platinum activity is not apparent related to the fact that the platinum recrystallization in the the cell is also reduced. ' After the porous carbon precipitates on the platinum crystallites, for example by any of the methods described above or any other suitable method, the catalyst is subjected to a high temperature treatment in an inert atmosphere or vacuum. Tests were performed for to determine the optimal heat treatment conditions. A few test data are shown in Table IV below. Electrode size, catalytic load and other details of the fuel cell tests were the same as described above in connection with Table I.

Catalyst l9 was a control catalyst and did not any treatment. Coal did not precipitate before high temperature treatment. the catalysts numbered 20 and 22; this was intended for the purpose of assessing the effectiveness of the present invention. activity. All other catalysts were exposed to 37 ° C carbon monoxide treatment for ten minutes before the heat treatment to the indicated temperature in an inert atmosphere or vacuum.

In all these tests, the heat treatment was performed in an inert manner atmosphere or vacuum by heating the catalyst sample from room temperature to the maximum heat treatment temperature at the rate indicated by the second column of the table. The time, indicated in column 4 of the table, is calculated from the moment the temperature amounts to 899øC. At, for example, 20505 per hour would the catalyst 21 is heated from 899 DEG to 170 DEG C. in one hour and minutes. The rest of the total two and a half hours for a long time the temperature will reach ll70oC. Catalyst The samples 27 and 29 were simply heated to ß99OC with it specified speed and kept at 899oC for one hour. Catalyst Tower 24 was heated to 899 ° C at a rate of 11 ° C per hour and then to 10 ° C at a rate of 222%. Although the time began to count at 893 ° C, it is believed that the benefits are likely to start occurring approximately at 816 ° C. Temperatures over approx. l232OC can provide a degree of heat insertion, which practically eliminates the benefits obtained by heating the treatment. Given this and based on our test results and other experience, it is estimated that a heat treatment within the temperature range 8l6QC to l232 ° C from half an hour to six hours will be beneficial. Of course, lower maximum temperatures will temperatures and slower heating rates to require longer heat treatment times than higher maximum temperatures and faster "šoon Qvazrfi 7811784--3 s. heating rates. Too fast raising of the temperature may cause oxidation of the carbon support due to volatilization of pollutants before they could be removed from the tiklarna. For this reason, it is recommended not to sound the heating rate exceed approx. 334OC per hour. Our data indicates that a suitable heat treatment is within the area 89900 to approx. 960oC_from below from three quarters to one and a half hours.

Due to our experimental experiments, it turned out that even if the high temperature treatment is not essential for the The present invention improves the catalyst in two essential respects. seenden. First, the recrystallization of platinum is reduced during the use of the electrode in a fuel cell even beyond the value obtained by ordinary precipitation of the porous carbon on the platinum crystallites without subsequent exposure to the the lysator for a high temperature treatment. Secondly, the tinacatalysatonm; initial activity, i.e. before it was used in the fuel cell. The advantage of this initial increase is maintained throughout the use of the catalyst.

Initially, attempts were made to achieve reduced platinum recrystallization. call using high temperature treatment but without first precipitate carbon on the platinum crystallites. These attempts did not succeed particularly successful due to excessive heat sintering (ie increase in platinum particle size) caused by the high heat treatment temperatures. In the present invention, theory states that the porous carbon precipitates are significantly reduced the migration rate of platinum crystallites during high temperature the peratur treatments; because the platinum crystallites slowly moving over the surface of the coal, they may find suitable, natural defects in the surface of the carbon support material, which retain platinum particles further reduces migration and recrystallization. during the use of the catalyst in the fuel cell.

Table V shows some surface data for the catalysts 19, 20 and Z1, which have been treated as set forth in Table IV. With the data as shown in Tables IV and V, several observations can be made.

First, it is clear that CO treatment (or other treatment for precipitation of porous carbon on platinum crystalline before the high temperature treatment is important in the present case invention. Secondly, the catalyst 2l, despite the large the improvement in cathode starting voltage between the catalyst 20 and catalyst 21 still lower initial cathode voltage than that e. 7811784-3 untreated catalyst 19, but after only 100 hours of work in a fuel cell, the surface areas of the catalysts 19 and Z1 are almost same. If the catalyst 21 is given additional time in the fuel cell, the catalyst does not exceed that of the untreated catalyst 19 (in Fig. 2) performance and surface area. In the long run, there are benefits to occur as a result of the special treatment, which is lysator 21 obtained. Thirdly, the treatment of lyser 25 that even if the heat treatment temperature is not particularly high, additional time at this temperature results in a significant reduction in initial performance. Finally it can mostly be stated that better results are obtained using lower temperature and less heat treatment time, such as at the preferred temperature range between 899 and 960 ° C for three quarters to an hour and a half.

The diagram in Fig. 2 shows that the present invention for a significant improvement over catalysts, which does not precipitate any carbon on the platinum crystallites. In Fig. 2 is the time spent under fuel cell conditions ticked off the horizontal axis and the mean platinum area on the vertical axis. The curves are based on current data. The with the D denoted curve, untreated platinum is supported on carbon the catalyst. The curve denoted by E is a catalyst, which was the same as the catalyst according to curve D with the exception of that porous carbon precipitates on and around the platinum crystallites according to the carbon monoxide method described above. Catalyst E out- however, was not set for a high temperature treatment in an inert atmosphere. The curve denoted by F is a catalyst which have been CO-treated and then subjected to high-temperature treatment according to a suitable embodiment of the present invention, i.e. it refers to a catalyst which is identical to the catalyst according to curve E but which has also been subjected to a high temperature treatment ~ in an inert atmosphere.

It can be seen that catalysts D and E start with approx the same platinum area, which is larger than the platinum of the catalyst F ytarea. In all cases, the platinum area under the fuel cell is reduced. Operation; however, the surface area of the catalysts E and F is reduced by less speed than the catalyst D. After approx. 200 hours of fuel cell operation also exceeds the platinum surface area of the catalyst F tower E platinum area, This partly explains the fuel cell improved function when using catalyst exposed to carbon precipitation and subsequent heat treatment. ÉQQR Q11ä: .ff :: '_ e 7811784-'5 10.

The diagram in Fig. 3 is also based on experimental data and shows that apart from the reduced recrystallization of platinum ring speed platinum at least in the beginning is in itself more active as a result of the present invention. In this dia- grams, the time in the fuel cell is likewise dotted on the horizontal the shaft and the catalyst activity in the form of the fuel cell 2 on the vertical axis. performance or function at 0.22 A / cm It is immediately apparent that the activity of the catalyst according to the present invention in the beginning is always higher than the untreated the activity of the catalyst (curve G) even though it initially has a smaller platinum area (see Fig. 2). It is assumed that although after a long time of use the catalysts according to curves H and G ends with the same area (which is not the case), the active of the catalyst according to curve H Still to be higher than that of the crude catalyst.

Thus, the present invention relates to an improved catalog lysine of platinum crystallites supported on carbon, containing taking a carbon precipitate on and around the platinum crystallites.

According to a suitable embodiment of the invention, a further improvement of this catalyst is achieved by a subsequent heat treatment. Although beneficial, this heat treatment is not essential to the present invention, which even without it further heat treatment achieves a significant improvement in comparison with known catalysts. Although the invention has been shown and described in connection with preferred embodiments thereof, it is for one skilled in the art clear that various changes and exclusions in form and detail can be undertaken without departing from the inventive concept or slip the frame of the invention. 7811784-3 ll. .uonmwæamumx mâëmm> m mGac © cm>: m mmë> oum mumam> w uwumuasmwn umu um .mwmcm oømnëo »um www naamumbm ik ; mcaawsmnwQ | Ou cwmca 1 aww | 1 www I | mmm mm ama mmw wo ama mmw I | www Nw oma mmm am maa www I | www mm mma == amm | omm ww mma 4% amw | www om 1 ow w fi owa 1 oma vf www 1 omm .Eaa ooa mmaw fl cwmmm m \ NE NEU \ <NN.o mm> E mmnmßm um amavnwuømwoumxwmawcnæmom .Émumqmzmmm q m if I H .m> w nu maæ Ua> aßm ß fl> aßm @a> aßm Ua> aßm @a> aßm Ua> omm Ua> omm ®a> omm © a> own Ua> omm UH> AU O uoummmamumxaaouucom # .caë m .caë om .øaä ma .cañ oa .naë oa .caš m .fi aë om .QaE om .fl aä ma .naš ca .øaš m Gwmca wa H wmë mmm | cm nwuøumnwmëmuv m fi aa fl mmßwm Na aa oa å r-lNf fifl L fl kOßæ uøummæamumm ZOHBQBWHMAMWÅWZZNUHM ZOO 4 Z H aQMm li POOR QUALITY 12. 7811784--3 .novmmwamßmx mEEmw> m mn fl cwømšhm wmå> oum .mhm fi w> m vwumu fi øwwn www .Hm mmmøm wømuâo uum www u flfl mnmåwk * .mc flfl øsmsmn uwwq fl .m> w || Hoummm fl mumx fifi onunomx | I omm Uoßmw u fl> .ø fl â m .mmwnmm m fi wm mm fl: à www .. www Uoomm U fl w ... SE m âöxwm 3 mw ow fi xu.> æ @ 1 Qww nmm flfl * ma .E fl ø oo fi ww fl wnsmmwm ïwa Neu? 35 En,> a mmumuw _ Hø fi uswwømwopmxwm fi wncæmwm mc fl awsmnwm Hoummæ fi mumm MOB4mwQ4B4M D <fl DZAOM mmm 4B4Q4MM EUO lwZOHB HH flfl mmáä 7811784-3 13. mm mw .E fl u cow .md flfl w fl m fl wß: wm flfi .m> © || uoummh fl mumx flfl onucomk Hmm Oommm © H> .c fl ñ om w fl on QHA www oommw @ fl> .SHB om wa mm o: www nwm fi .w 3 .ë fl ø oo fi mm fi wc fl æmmm ¶ one? Nwé m? 2: m \ E N N fi m fl v fl muom mwnmnw | Uovmxmm fl m: fl wmmm mc flfl ucmnmm noummw fl mpmm MOB HHH QQMNQB r .. some QUAIITY 14. 7811784'3 .Uommo fl Hamu Uommæ: mmm mEEHu \ U0NNm muommæ fifl mßßm fi wEEHn \ UO fi HH U . ^ mGmm fl Qmswn | OU cmmcm .m> Uv mcmmwGm: mnmEHw> wmßw ummmmu fl mox ummum Q .uommm mm> mmmum m . @ nmmUnmsmQwEmw> mwa fl m mcmm fl cm fl wnløø Qwmsm la møummw fl mumx fl momunøm ä mmm mm m.m mmm mmm mm, mmm s :: mm> m.m mmm mmm mm m mmm mz m.m mmm m mmm mm mmm mz m.m mmm mmm mm mmm 7 mz m.m mmm mmm mm mmm mz m.m mmm mmm mm mmm mz m.m mmmm mmm \ mmmo mm W mmm mz m.m mmm mmm mm mmm mz m.m mmm mmm mmm mmm mz m.m mmmm mmm mm mmm mz m.m mmmm mmm mmn mmm 1 | I 1 «mm A Eu> ~ Nwlo .när 25 Tmmššmvv Û m .møummwmñwv mwššmmïu m N mmmucwuom .mmwwoëumwmmmmm wmuwmnmm olmmmmmmwmmmswn your fl mmmmmwmm | wmmwmmmmæmwnmuoumm nmmmmmmmwmmwëmmmm lwcønmnmšmmm / m. | mmmmm> Ümšmxmä: mmmmmmmâmmmmmmn mommmmwmmumvm _ .mmammmmmm I0Q0ÉHW> ÜHDM UOHßm .mv fl m / US fl HUGMSQQ .G flfi OH .H.O.w .HGHOßMWMHMNQM QHA-m mm fl ßßwßmm Mwmw fl m fifl FmOmmm EOHSV mmmummmzmmm mommm m zommmmmmmmmmmmmmzzmommmommm mm wzmmnzmom «mmm« mmmmmm > m mmmmmm . 7811784-3 TABLE V EFFECTS OF CO-TREATMENT BEFORE HEAT TREATMENT ON PLATINAY AREA Catalyst il 19 Q <1: 21 Ytarea mz / g Beginning 100 hrs. 140 45 40 34 60 43 d see Table IV for treatment details. _. _.__..._......__.__......._....__.._._._ Pøøn QHA:

Claims (16)

Patent 78: 7811784-3 Platinum catalyst, characterized in that it consists of platinum crystallites with carbon particles as carriers thereof, carbon precipitating 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. A process for producing a platinum catalyst according to claim 1, characterized in that a catalyst of platinum crystallites with carbon particles as a carrier thereof is subjected to a precipitation of porous carbon on and around the platinum crystallites. 3. A process according to claim 2, characterized in that during the precipitation the catalyst is heated in the presence of a carbonaceous compound to decompose the compound and form porous carbon. 4. A process according to claim 3, characterized in that a gaseous or vaporous carbonaceous compound is used and that the catalyst is heated in the presence of the gas or vapor. 5. A process according to claim 4, characterized in that carbon monoxide is used as the carbonaceous compound. 6. A method according to claim 5, characterized in that heating takes place to the temperature range 260 - 649 ° C and that the temperature is kept within this range for one to sixty minutes. 7. A method according to claim 5, characterized in that the heating takes place to the temperature range 260 - 427 ° C and that the temperature is kept within this range for five to thirty minutes. A method according to claim 3, characterized in that the heating takes place to a maximum temperature of 37,100 and that this temperature is maintained for ten minutes. 9. A process according to claim 4, characterized in that a hydrocarbon vapor or gas is used as the carbonaceous compound and that heating takes place to a temperature high enough to decompose the hydrocarbon to precipitate porous carbon on and around the platinum crystallites. Process according to Claim 9, characterized in that n-hexane, heptane, benzene, acetylene, methane or ethylene are used as hydrocarbon. 11. 7811784-3 ll. Process according to Claim 3, characterized in that a water-soluble organic material is used as the carbonaceous material, and that a layer of the water-soluble organic material is applied to the catalyst for the precipitation of carbon, after which the catalyst is heated in an inert atmosphere or vacuum. to decompose the organic material into layers, leaving porous carbon on and around the platinum crystallites. 12. A process according to claim 11, characterized in that for applying a layer of the water-soluble organic material, the catalyst is soaked with an aqueous solution of the organic material and the catalyst is dried to remove the water from the layer. 13. A process according to claim 11 or 12, characterized in that sucrose is used as the carbonaceous material. 14. A method according to claim 13, characterized in that heating takes place to a temperature range of 427 - 649 ° C and that the temperature is kept within this range for half to six hours. Process according to one of Claims 2 to 14, characterized in that the catalyst, after precipitation of carbon, is subjected to a heat treatment in an inert atmosphere or vacuum within a temperature range of 816 to 1232 ° C and that the temperature is kept therein. intervals for half to six hours. 16. A method according to claim 15, characterized in that heating takes place to the temperature range 899 - 96o ° c and that the temperature is kept within this range for three quarters to one and a half hours. : room QUALITY