JPS58177145A - Process for improving catalytic capacity - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to noble metal catalysts and chemical reactions carried out under the influence of noble metal catalysts.

For chemical reactions to be useful in many industrial processes, they need to be carried out at accelerated rates. For example, a fuel cell is a device that converts the energy of a chemical reaction between a fuel and an oxidant directly into low voltage recoil electrical energy. In order to obtain a high conversion efficiency, the reaction of fuel and oxidant must be carried out in such a way that the amount of energy lost as heat is kept as small as possible (1 hour).At the same time, the reaction rate must be kept at a practical level must be high per minute so that economically useful magnitudes of current can be produced from the cell.Thus, the field of fuel cells (- is similar to that of many other chemical and combustion converters). In order to make such prohis industrially and commercially useful, it is common to use catalysts to accelerate such reactions.

However, in the field of catalysts, many things still remain to be clarified.The addition or omission of elements to compound catalysts is done through trial and error based on the results of past trials. is the current situation.

As an example, in the field of fuel cells, various combinations of catalyst materials are used without any real understanding of why one catalyst is more or less suitable than another. A number of publications have shown that improved results can be obtained, such as U.S. Pat. No. 3.428.490,
No. 3゜506.494, No. 3.615,836, No. 4.127.469@, No. 4.136.059, No. 4
．． No. 137.372, No. 4.137,373 $j,
glI nol, l Jl (i, I
IO) J, 917-1, l
t+', +, u07, No. 4,202,93
See No. 4 and No. 4,316.944.

Therefore, what is needed in the field of chemical reactions carried out by the influence of catalysts is a decisive factor for increasing the reaction rate so that the reaction rate can be improved in the particular field of chemical reactions carried out by the influence of catalysts. The goal is to better understand the

It is an object of the present invention to provide a method for improving catalytic performance in chemical reactions carried out under the influence of platinum catalysts in which the reduction of oxygen is one rate-limiting step. This objective is achieved according to the invention by increasing the rate of the chemical reaction by increasing the electron density of the states in the Fermi units of the platinum catalyst. This method is realized by the /J method, which involves alloying the catalyst, changing the alloy component of the catalyst, or changing the concentration of the alloy component.
Ru.

One and other objects, features and advantages of the present invention will become more apparent in the following description with reference to the drawings.

In the following discussion, when comparisons of catalyst activity are made, they are intended to be comparisons of catalytic activity.

VI4I4 is an arbitrarily defined measure of the effectiveness of a catalyst per unit of catalytically active component.

For example, in a fuel cell using phosphoric acid as the electrolyte, the active activity of the cathode catalyst is 0.90 volts relative to an unpolarized hydrogen/platinum reference electrode in the same electrolyte and at the same temperature and pressure. The state in which the potential is measured is determined by the maximum current (milliampere/'milligram) per weight φ obtained by acid-based reduction. 1. Greater mass activity increases the surface area of the catalyst and also increases the hardness of the catalyst. ：J、Ri冑t）Reru. Intrinsic activity is defined as the oxygen reduction current obtained per medium surface area of the noble metal.

Adsorption and desorption are two important steps during the catalyst process. For a material to be a useful catalyst in a process such as M reduction, the adsorption bonds must not be strong enough to form a stable oxide and there must be enough adsorbed material for the reaction. It shouldn't be so weak that it doesn't exist. Adsorption bonds are controlled in part by the number of electrons, particularly in the d orbitals, which play a role in bond formation within the platinum-oxygen system. If the number of electrons is too large, a stable oxide will be formed if the electron density of the d-state in the beaten Fermi unit (the highest electron energy unit occupied at low temperature) is high 1, If the number of electrons is too low, ie, the electron density of the d-state in the Fermi unit is low, the material will not exhibit any significant catalytic activity.

According to the present invention, the electron density of the 4J state in Fermi units is increased to improve the catalyst activity.

This /i method involves redemption using one or more 1- elements, the -l composition of '1[F] (・?)1, ('+ +
1 and 9 also accept electrons from or donate electrons to the solvent metal (for example, platinum).

As a result, the overlap of the electron orbits of adjacent atoms changes, and the lattice 1 < parameter changes. Accordingly, the electron density of the state in the lumi unit increases or decreases depending on whether electrons are accepted from or donated to the solvent metal.

It has been found that for alloys, the density of states can be reduced by increasing the lattice parameters.

On the other hand, increasing the density of states can be achieved by decreasing the lattice parameters. For example, the catalytic Is activity of platinum is enhanced when platinum is in solid solution alloyed with certain transition metals. Alloying has been found to decrease the lattice parameters and increase the density of states in Fermi units.

Although direct measurement of the electron density of a state is a very difficult measurement requiring complicated equipment, the electron density of a state has been estimated from the increase in paramagnetic susceptibility and from near-edge X-ray absorption measurements. Paramagnetic susceptibility can be determined by conventional methods, e.g.
Review of by iiom F oner
3cientific)nstruw+ents,
30 (1959), p. 548, using a vibrating sample magnetometer. X-ray absorption measurements have also been performed using the conventional method of irradiating a sample with X-rays and measuring the intensity of the X-rays passing through the sample as a function of the energy of the incident X-ray beam. These measurements were made using the Cornell High Energy Synchrotron Facility at Cornell University.

Two examples showed improved catalyst activity.

First, differential scanning calorimetry (measured on a du Pont 990 thermal analyzer in combination with a differential scanning calorimeter) was used to determine the tactile activity during air oxidation! ! The improvement in
The oxidation onset temperature was shown to decrease with increasing electron density of the platinum alloy state (see Example 1). Second, the improvement in catalytic activity during electrochemical oxygen reduction in phosphoric acid at 350° C. (177° C.) shows that the intrinsic activity at 0.9 volts increases with increasing electron density of the state. (see Example 2). This data is shown in the table below.

This was further demonstrated by the decrease in catalytically active acid as the electron density of the states in the Fermi unit of platinum decreased upon alloying with gold. Adding gold to platinum decreases the density of platinum states as reflected by a decrease in paramagnetic susceptibility. The concentration ranges from 50 μA/CI' for platinum to 1 for alloys, as shown in Figure 2.
It decreases to 5 μA/, 1'.

When changing the electron density of a state by alloying, the parent metal must first be selected. Transition metals are preferred as host metals because they have an appreciable density of d-shaped silver in Fermi units. The additive elements are selected on the basis of their atomic size relative to the parent metal so that the lattice parameters and the electron density of the state (as reflected by the paramagnetic susceptibility) can be modified in the right direction.

- Examples are transition metals having a filled d-shaped silver with an appreciable density of d-shaped silver in Fermi units, such as platinum. The catalytically active acid is increased by increasing the catalytic activity in the Fermi units, which can be achieved, for example, by M-commutative alloying with transition metals. Paramagnetic susceptibility measurements (second
(Figure) shows the occurrence of electronic transitions.

The method described herein has been found to be particularly suitable for platinum in chemical reactions such as oxidation reactions and electrochemical M reduction reactions. In any of these reactions, the speed limit slap is W! Contains the reduction of

Two tests were conducted to further demonstrate the improvements provided by the present invention. A solid solution of platinum alloy was formed. It is important to keep in mind that it is important to maintain the face-centered structure of platinum and increase the electron density in that state. Also, 11 per Fermi! It is important to select only alloys whose electron density in the <j state increases upon alloying. The improvement in the electron density of the 4J state in Fermi units can be measured by measuring the paramagnetic susceptibility, which is proportional to the electron density of the state in Fermi units. The density of states can also be determined by measuring that the difference in the X-ray absorption beam areas of alloyed and unalloyed platinum (see Example 3) increases due to the transition from the full WAO state to the empty d state. can be measured.

□ The alloy thus formed was tested during the oxidation of isopropyl alcohol and during electrochemical oxygen reduction to show that the reaction rate and catalytic activity of platinum increases with increasing electron density of the state. .

Example 1 Air saturated with isopropyl alcohol was passed over each catalyst which was gradually heated in a differential scanning force zero meter (D, S, C,). The onset of alcohol oxidation was detected from a sudden surge of heat. The relationship between the temperature corresponding to the onset of oxidation and the paramagnetic susceptibility of platinum and the respective alloy catalysts was plotted. This is shown in FIG.

It is clearly shown that the higher the paramagnetic susceptibility of the alloy, the lower the oxidation onset temperature. This data thus shows that the higher the electron density in the Fermi unit state, the lower the oxidation initiation temperature.

Example 2 Figure 2 shows i in 1g1m! The electrochemical activity of the catalysts for elementary reduction is plotted as a function of their paramagnetic susceptibility. Tests were carried out in a conventional electrochemical cell at 0.9 volts using phosphoric acid as the electrolyte as described above. Again, it is clearly shown that the catalytic activity is high (the electron density of the state with high paramagnetic susceptibility (IAt in Fermi units) is lower than that of platinum. One alloy is platinum with 10% gold.

Figure 2 shows that the catalytic activity of this alloy is lower than that of platinum. In FIG. 3, the relationship between the catalytic activity of oxygen reduction in fsIlI and the difference in the X-ray absorption peak areas of alloyed platinum and unalloyed platinum is plotted.
There is. These measurements were carried out by F. W. Lytle et al. in the Journal of Chemical Physiology.
cs, Volume 70, No. 11 (June 1, 1979) No. 4
849-4855.

This plot also includes monomagnetic alloys, and a direct analogy between paramagnetic susceptibility and state electron density cannot be shown for these alloys, but the This suggests that the correlation holds similarly for these alloys.

A typical process for making the noble metal catalysts of the present invention involves adsorbing e.g. It involves heating the impregnated catalyst. The preferred anion is chromate, as mentioned above, but for other alloys the anionic forms of vanadate, manganate, molybdate and tungstate, respectively, are also used. Although this method is well suited for making unsupported as well as supported alloys, finely divided unsupported precious metals are generally limited to less than 50*'/9 of the precious metal. ing. Therefore, this method is generally best practiced using a supported finely divided negative metal that can be fabricated to a surface area greater than 100-/9 times the negative metal.

Regarding this, U.S. Patent No. 4, which has the same assignee as the present application,
．． See No. 316,944.

Although the invention has been described specifically as using platinum,
Any noble metal can be similarly modified for catalytic performance. Furthermore,
Although the invention has been described with particular application to fuel cells, the invention is equally applicable to any chemical reaction in the chemical industry, pharmaceuticals, automotive, or pollution control. As mentioned above, the present invention is particularly useful as an electrochemical catalyst for the reduction of oxygen. Owing to its high activity, the catalyst according to the invention is particularly suitable for acid fuel cells.

The catalyst according to the present invention can be used not only in fuel cells but also in any process in which @ elementary reduction, especially electrochemical oxygen reduction, is carried out as part of the process.

The present invention has been described in detail with respect to its embodiments.
It will be understood by those skilled in the art that various changes and omissions may be made in form and detail without departing from the scope of the invention.

[Brief explanation of the drawing]

FIG. 1 shows the catalytic activity during the isobutylene oxidation reaction as a function of the state electron density. Figures 2 and 3 are diagrams showing the catalytic activity as a function of state electron density in an electrochemical oxidation reaction. Patent Applicant: United Chiknoroses Corporation

Claims (1)

[Claims] In a process to improve catalytic performance when the reduction of oxygen is one rate-limiting step in the chemical reactions carried out under the influence of platinum catalysts, the electron density of the state in the Fermi unit of platinum catalysts is A process for improving catalyst performance characterized by increasing the rate of a chemical reaction by increasing the rate of a chemical reaction.