CA1207310A

CA1207310A - Noble metal catalyzed reactions

Info

Publication number: CA1207310A
Application number: CA000423191A
Authority: CA
Inventors: Sudhangshu Bose
Original assignee: United Technologies Corp
Current assignee: RTX Corp
Priority date: 1982-03-31
Filing date: 1983-03-09
Publication date: 1986-07-08
Also published as: NL8300822A; FI831107A0; IL68099A0; IT1160756B; SE8301300D0; AU1255683A; BR8301586A; JPS58177145A; GB2117791A; BE896315A; DK95983D0; FR2524340A1; FI831107A7; DK95983A; SE8301300L; DE3310965A1; FI831107L; NO831099L; IT8320266A0; EG16278A

Abstract

Noble Metal Catalyzed Reactions Abstract A method is disclosed for improving the performance of noble metal catalysts such as platinum in catalyzed chemical reactions where the reduction of oxygen is a rate limiting step. The catalytic activity of the catalyst and thus, the rate of reaction is increased by increasing the electron density of state at the Fermi level of the catalyst. This can be accomplished, for example, by alloying the noble metal catalyst with materials which increase the electron density of state.

Description

Description Noble Metal Catalyzed Reactions Technical Field The field of axt to which thi~ invention pertains is noble metal catalysts and chemical reactions which are noble metal catalyzed.

Background Art In order for chemical reactions to be useful in many industrial processes, it is necessary for ~hese reactions to take place at an accelerated ra~e. For example, a fuel cell i5 a device which converts the energy of a chemical reaction between a fuel and oxidant directly into low voltage, direct current electricity. To obtain a high efficiency of conversîon,.it is necessary that the reactions of th~ fuel and oxidant occur in such mannex that the amount of energy degra~ed into heat is as small as possible. At the same time, the rates of reaction must be high enough to produce, economically, a useful amount of curren~ fr~m a cell of. practical si~e. There-~0 fore, in th~ area o~ fuel cells, as in many other chemical production processes, combustioIl convexters, etc., it is customary to incorporate catalysts which accelerate ~uch reactions to make 5ueh processes industrially and com-mer~ially useful.

2~ ~owever, ~he art of catalysis still remains in large .
degree a little understood area. Additions and elimina- .
tions in compounded catalysts by and large take place i~
a trial and error ma~ner in large measure governed by the results of a previous trial. An example of this is in the fuel cell area, where many patents have been issued on various ~om~inations of catalytic material producing improved result~ wi~h no real recognition of the reasons C-1028 ~

7;~
r one catalyst may or may not perform better than another in this environment. Note U.S. Patent Nos. 3,340,097;

3,341,9~6; 3,380,934; 3,4~8,490; 3,506,494; 3t615,836;

4,127,46g; 4,136rO59; 4,137,372; 4,137,373; 4,186,110;

5 4,192,907; 4,202,934; and 4,316,944.
Accordingly, what is needed in the area of catalyzed chemical reactions is a better understanding of the determinative factors which result in increased reaction rates so ~hat reaction rates in particular areas of catalyzed reactions may ~e improved.

Disclosure of Invention The present invention is directed to a method of improving catalyst performanc~ and thus, rate of reaction in platinum catalyzed chemical reactions, where the reduction of oxygen is a rate limiting ~tep. Such reaction rate is improved by increasing the electron density of state at the Fermi level of the platinum catalyst~ ~his can be accomplished by such methods as alloying the catalysts, varying the alloy components o~ the catalyst, or varying the amounts of alloying components.
The foregoing, and other features and advan~ages of the pxesent in~ention will become more appar~nt in light of ~he followi~g description and a companying drawings~

Brief Description of the Drawings Fig. 1 demonstrates a comparison of catalyst acti~ity as a function of electron density of state in an iso-propanol oxidation reaction.
Figs. 2 and 3 d~monstrate cataiyst activity as a function of electron density of state in an electro-chemlcal oxidation reaction.

_ _ ... . , . . . .. . _ .. ..... . , . . . . ... . . .. .... . . . . . . .. , . _ . _ . . . . .

~2~731~

Best ~ode for Carrying Out the Invention In the following discussion of this invention and in the appended claims, when catalytic activity comparisons are made, they are intended to be comparisons of mass activity~ Mass activity is an arbitrarily defined measure of the effectiveness of a catalyst per unit weight of the catalytically active component. For example, in the case of fuel cells with phosphoric acid as an ele tro-lyte, the mass activity of the cathode catalyst is defined in milliamp~ per milligram as the maximum current available due to oxygen reduction as 0.90 volt, the potential being measured relati~e to an unpolarized hydrogen/platinum reference ele~trode at the same temperature and pressure in the same electrolyte. A greater mass activity can be achieved by either increasing a surface area of the catalyst or by increasing its specific activity. Specific activity is defined as the o~ygen reduction current as specified above which is available per unit surface area of the noble metal.
Adsorption and desorption are two important steps during ~he process of catal~sis. For a material to be a suacessful catalyst in a process like oxygen reduction, the adsorptiQn bon~s should not he so strong as to form a stable oxide, nor should it be so weak as not to have enough adsorbed species to react. The adsorption bond is controlled in part by the number of electrons, parti-cularly in the d-orbitals, taking part in the fonmation of bonds such as in platinum-oxygen systems. If there are too many electrons available r that is a high electronic density of d-like states at the Fermi level (highest occupied electronic energy level at low temperature) a stable oxide forms, while if there are too few electrons, that is low electron density of d-like states at the Fermi level, available!, the material does not show any significant catalytic activity.

73~ao According to the present invention, the electron - density of states at the Fermi level are increased to improve the catalytic activity. The method consists of forming substitutional alloys using two or more elements.
S Alloying elements accept electrons from or donate electrons to the solvent metal (which is the base metal, e.g. plati-num) and in turnj chang~ the lattice parameter because the amount of overlap of the elPctron orbitals of neigh-borins atoms changes. This change will either inc~ease or decrease the ele~tron density of sta~e at the Fermi level depending on whether electrons are accepted from or donate~ to the solvent metal.
For an alloy, the density of state has been found to be reduce~ by increasing the lattice parameter. On the other hand, an increase in the density of state may be achieved by reducing the lattice parameter. For example, the catalytic activity of platinum is enhanced when it is solid solu ~on alloyed with certain transition metals.
The alloying has been f~und to result in reduced lattice parameter and increased density of states at the Fermi level.
Since electron density of states can only be measured directly with great difficulty by complicated apparatus, the elec~ron density o~ states were inferred ~rom enhance-. 25 ment of paramagnetic susceptibility and from near edgex-ray absorption measurements. Paramagnetic susceptibility was measured by conventional methods, for example, using a vibrating sample magnetometer as described in Review of Scientific Instruments, volO 30 tl959), page 548 by Simon Foner. X-ray absorption measurements were also performed utilizing conventional techniques ~y determining the amount of x-ray energy the samples were subjected to, allowing such energy to pass through the samples and measuring the transmitted intensity as a function of the 3S ener~y of the incident x-ray beam. Such testing can be performed at such places as the Cornell High Energy ; r ... . . . ... . . . . ... .. ..

- ~20731~1t Synchrotron acility at Cornell University.
Two examples verifying these results were demonstrated by noting the imprDvement in catalytic activity both in air oxidation (note ~xample 1) using differential scanning calorimetry (measured on a duPont 990 Thermal Analyzer in conjunction with the differential scanning calorimeter) where the tempera~ure of onset of oxidation was seen to decrease with increasing electron density vf state of the platinum alloys an~ in electrochemical oxygen reduction in phosphoric acid at 350F (1i7C) (note Example 2) where the specific activity at O . 9 volt increased on increased electron density of state. Ihis data is demonstrated in the Table below.

- ~ABLE
Temperature ~alf cell Latti~e of Onset of Activlty Parameter Parsmag~etic Oxidatio~ in at 0.9 v 2 15 Material A Susceptibility/gm D.S.C. (C~ ~ amps/cm Pt 3.923 1.0 x 10 6 153 49.9 Pt-Mo 3.913 10.8 x 10 6 105 82.5 Pt-V 3.872 ~.7 ~ lO 6 124 74.7 Pt-W 3.916 3.4 2 10 6 143 68.2 Pt-C~ 3~865 Ferromagnetic 138 75.8 Pt-Mn 3.930 Fer~o~sgnetic --- 72.4 Th~s was further demonstrated by reducing the electron density of state at ~ermi le~el of platinum by alloying with gold resulting in a reduction of catalytic activity. ~n adding gold to the platinum~ the d~nsity of state of the platinum is reduced as reflected by the de-crease in para~agnetic susceptibility. The specific activity decreased from 50 ~ amps/cm for platinum to 15 ~ amps/cm or the alloy as shown in Fig. 2.
When altering the electson density of state by alloying the base metal must be selected first. Transi-- ~2~73~
.. .

tion metals are pr~ferred because they have appreciable density of d-like s$ates at Fermi level, The alloying element is chosen based on its atomic size relative to the base metal so that the lattice parameter and electron density of state (as reflected by paramagnetic suscepti-bility) may ba altered in the right direction. An example is a transition metal having unfilled d-like states with appreciable density of d-like states at Fermi level such as platinum. Its catalytic activity can be increased by increasing i$s density of state at the Fermi level. This may be achieved by substitutional alloying with transition metals, for example. Electron transfer has been shown to occux through paramagnetic susceptibility measurements ~Fig. 2~ indicating that the density of state 15- of the platinum has increased.
The method described herein has been found to be particularly ~uitable with platinum in such things as oxidation reactions and electrochemical oxygen reduction reactions. In both cases, the rate limiting step involves the reduction of oxygen.
Two tests were performad to further demonstrate the improvements according to the present invention. Solid solutions of alloys of platinum were formed. It should be noted that it is critical to keep the platinum face . 25 center cubic structure intact while increasing its electron density of state. It is also important to select o~ly those alloys in which the electronic ~ensity of state at Fermi level increases because of alloying. The improvement in electron density of state at Fermi level may be determined by measuring the paramagnetic suscepti-bili$y which is proportional to electron density of state at Fermi level. The density of state may also be deter-mined by measuring the difference in x-ray absorption peak area of alloyed and unalloyed platinum (note Example 3) tha$ arises because of $ransitions ~rom the filled p-states to the empty d-states.

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The alloys thus formed were then tested in the oxida-tion of isopropyl alcohol and in electrochemical oxygen reduction to demonstrate that the increased electron density of state increased the rate of reaction and cata-lytic activity of the platinum.

.
Example 1 Aix saturated with isopropyl alcohol was passed over the respective catalyst which was slowly heated in a dif-ferential scanning calorimeter (D.S.C.). me onset of oxidation of the alcohol was determined from a sudden surge of heat. The temperature corresponding to the onset of oxidation was plott~d against the paramagnetic susceptibilit~ of platinum and the respective alloy cata-lysts. This is showm in Fig. 1. It is clear that oxida-tion commences at a lower temperature the more para~
magnetic the alloy is. The data thus indicates ~hat the higher electron density of state at Fermi level, the lower the temperature of oxidation.

Example 2 Fig. 2 is a plot of the electrochemical activity of the catalysts for oxygen reduction in phosphoric acid - plotted as a function of their paramagnetic susceptibility.
Testing was performed as described above in a conventional electrochemical cell at 0.9 volt using phosphoric acid as the electrolyte. Again, it is clear that the higher the paramagnetic susceptibility (the higher the electron density of state at Fermi level) the hisher the catalytic activity. An alloy which has a lower density of state than that of platinum is platinum with 10% gold. Fig.
2 shows that for this alloy, th~ catalytic activity is in fact, lower than that of the platinum. ~ig. 3 is a plot OI catalytic activitv of oxygen reduction in phosphoric acid against the difference in x-ray absorption ~L2~73~L(3 peak area of the alloyed platinum and unalloyed platinum.
These measurements were made following the method described in the Journal of Chemical Physics, vol. 70, ~o. 11 (June 1, 1979), pages 4849-4855 by F.W. Lytle et al. This plot includes ferromagnetic alloys suggesting that the density of states correlation does hold for these alloys as well, even though a direct paramagnetic-electron density of state analogy cannot be shown for these alloysO
An exemplary process for alloying noble metal catalysts of this invention comprises adsorbing, for example, a chromium containing species, preferably in the anion form on the supported noble metal catalyst fol-lowed by heating the chromium impregnated catalyst in a reducing atmosphere to promote the alloy formation. The preferred anion as recited is the chromate and for other alloys, the vanadate, manganate, molybdate and tungstate anion form respectively. While this method is equally well suited to making unsupported as w~ll as supported alloys, finely divided unsupported noble metals are limited generally to less than 50 m2/gm of noble metal.
Accordingly, this method is best practiced by using supported finely divided noble metals which can be pre-pared in surface areas generally s~reater than 100 m /gm of noble metal. Note commonly assigned U.S. Patent-~,316,944.
Although this invention has been demonstratedspecifically for platinum, any noble metal can be improved in performance in similar fashion. Furthermore, while this invention has particular applicability to fuel cells, any chemical reactions involved in producing chemical, pharmaceutical automotive, or antipollution reactions have similar applicability And as stated, the invention has particular utility as electrocatalysts for the reduction of o~ygen. This activity makes these catalysts ~3~10 particularly suitable in an acid fuel cell. However, the use is not limited to fuel cells and they can be used in any envixonment where oxygen reduction, and especially . electrochemical oxygen reduction takes place as part of the process.
Although this invention has been shown and described with respect to detailed embodiments thereof, it should be - understood by those skilled in the art that various changes and omissions in form and detail may be made therein without depaxting from the spirit and scope of the inven-tion.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. In a platinum catalyzed chemical reaction where the reduction of oxygen is a rate limiting step, the improve-ment comprising increasing the rate of the chemical reaction by increasing the electron density of state at the Fermi level of the platinum catalyst.

2. The method of claim 1 wherein the electron density of state is increased by alloying the platinum catalyst.

3. The method of claim 1 wherein the reaction is an electrochemical oxidation reaction.