FR2524340A1 - REACTIONS CATALYZED BY NOBLE METALS - Google Patents

Abstract

THE INVENTION CONCERNS REACTIONS CATALYED BY NOBLE METALS. THE CATALYTIC ACTIVITY OF A CATALYST AND THEREFORE THE SPEED OF A REACTION IS INCREASED BY INCREASING THE STATE ELECTRONIC DENSITY AT THE FARM LEVEL OF THE CATALYST. TO THIS EFFECT, A CATALYST OF A NOBLE METAL IS COMBINED WITH MATERIALS INCREASING THE ELECTRONIC DENSITY OF STATE. THE INVENTION IS USED TO IMPROVE THE PERFORMANCE OF NOBLE METAL CATALYZERS SUCH AS PLATINUM IN CATALYZED CHEMICAL REACTIONS IN WHICH THE REDUCTION OF OXYGEN IS A SPEED LIMITING FACTOR.

Description

Reactions catalyzed by noble metals.

  The present invention relates to the field

  noble metal catalysts and chemical reactions

  which are catalyzed by noble metals.

  In order for chemical reactions to be useful in many industrial processes, it is necessary that these reactions take place at an accelerated rate. For example, a fuel cell is a device that directly converts the energy of a chemical reaction between a fuel and an agent

  oxidant into a low voltage direct electric current.

  In order to obtain a high conversion efficiency, it is necessary that the reactions between the fuel and the oxidizing agent take place in such a way that the amount of energy degraded to heat is as low as possible. At the same time, the reaction rates must be high enough to economically produce a useful amount of current

  from a stack with practical dimensions.

  Consequently, in the field of fuel cells

  as in many other chemical processes.

  production processes, in com-

  bustion, etc., we usually incorporate cat-

  lysers which accelerate these reactions so as to make these processes useful both on an industrial scale and

commercial scale.

  However, the technique of catalysis still remains to a large extent a little understood domain Additions and eliminations in catalysts

  combined to a large extent by trial and error

  Generally, the field of fuel cells, for which many patents have been granted for different combinations of catalytic materials giving better results, without really knowing the reasons why catalyst may or may not be better than another

  in this environment (See US patents

  United States of America No. 3,340,097, 3,341,936, 3,380,934,

  3,428,490, 3,506,494, 3,615,836, 4,127,469, 4,136,059,

  4.137 372, 4 137 373, 4 186 110, 4 192 907, 4 202 934

and 4,316,944).

  Consequently, in the field of the reactions

  catalyzed chemicals, it is necessary to better

  take the determining factors ensuring higher reaction rates in domains

  particular catalyzed reactions.

  The present invention relates to a process for improving the efficiency of catalysts and hence the reaction rate in platinum-catalyzed chemical reactions, wherein the reduction of oxygen is a rate-limiting step. improved by increasing the electron density

  Fermi level state of the platinum catalyst.

  For this purpose, it is possible to use certain methods, for example, to combine the catalysts, to vary the alloying components of the catalyst or to vary

  the quantities of these alloy components.

  The features and benefits above

  of the present invention, as well as others appearing

  more clearly on reading the description

  Hereinafter given with reference to the accompanying drawings, in which: FIG. 1 gives a comparison of catalytic activity as a function of the electronic density of state in an isopropanol oxidation reaction; Figures 2 and 3 illustrate the catalytic activity as a function of the electron density

  of state in an electrochemical oxidation reaction.

  In the following description of this

  invention and in the following claims, when

  qulon makes comparisons of catalytic activity, it is understood that these are comparisons of

  mass activity is an arbitrary measure

  of the effectiveness of a catalyst by

  unit weight of the catalytically active component.

  For example, in the case of fuel cells in which phosphoric acid is used as

  electrolyte, the mass activity of the catholyte catalyst

  is defined in milliamperes per mg per cou-

  maximum available after the reduction of

  at 0.90 volts, the potential being measured by

  to a reference electrode not polarized with hydrogen / platinum at the same temperature and under the same pressure in the same electrolyte

  to achieve greater mass activity by increasing

  both the specific surface of the analyzer and

  its specific activity The specific activity

  that is defined by the oxygen reduction current (as specified above) available per unit of

noble metal surface.

  Adsorption and desorption are two important steps in the process

  For a material to be a catalytic converter

  In a process such as oxygen reduction, the adsorption linkages must not be so strong as to form a stable oxide nor must they be so weak as not to have enough absorbed species. for the reaction The adsorption bond is controlled in part by the number of electrons (especially in

  the orbitals d) participating in the formation of

  sounds, for example, in platinum / oxygen systems.

  There is too much electron available

  a high electron density of states d at the Fermi level (the highest level of electron energy occupied at low temperature), a stable oxide is formed while, if there is too few electrons, that is to say a low density

  electronic state of Fermi level, matter.

  shows no significant catalytic activity.

  In accordance with the present invention, the electronic density of states at the Fermi level is increased in order to improve the catalytic activity. This process consists in forming substitutional alloys in

  which one or more elements are used.

  Alloys accept or donate electrons from or to the solvent metal (which is the base metal, for example platinum) and, in turn, modify the lattice parameter since the degree of overlap of the electron orbitals is Neighbor QMS changes This change increases or decreases the electronic density of states at-Fermi level

  following that electrons are accepted from

  given to the latter For an alloy, it was found that the density of state was

  reduced by increasing the reticular parameter.

  On the other hand, we can obtain an increase in

  state density by reducing the reticular parameter -

  For example, the catalytic activity of platinum is improved when it is a solid alloy alloyed with certain transition metals. It has been found that the alloy gives a reduced lattice parameter and

  increased density of states at the Fermi level.

  Since the electronic density of states can be measured directly only with

  great difficulty by a complicated apparatus, the den-

  electronic state is deduced from the

  of paramagnetic susceptibility and

  X-ray absorption measurements near the edges.

  Paramagnetic susceptibility was measured by conventional methods, for example, using a vibrating sample magnetometer as described in "Review of Scientific Instruments", Vol.

  (1959), page 548 by Simon Foner.

  X-ray sorption were also performed

  by adopting classical techniques

  the amount of X-ray energy to which the samples were submitted,

  this energy through the samples and in

  the intensity transmitted as a function of the energy of the incident X-ray beam. Such an experiment can be carried out, for example, at the high synchrotron

  University of Cornell Energy.

  Two examples confirming these results

  carried out by observing the improvement in activity

  catalytic activity both during an air oxidation (see Example 1) by adopting a calorimetry

  differential scanning (measured with the

  "Bridge" thermal lyser "990" together with the differential scanning calorimeter), a method according to which it has been observed that the temperature of the onset of oxidation decreases with increasing the electronic density state of platinum alloys, as well as during an electrochemical reduction of oxygen in phosphoric acid at 1770 ° C. (see Example 2), the method according to which the specific activity at 0.9 volts has increased during the increase of the electronic density of state Ces results are shown in

the table below.

BOARD

  Material Parameter Susceptibility Temp.

  paramagnetic reticulum / active

  lane g of the beginning of the vapor o A dtoxyda of the tion in stack the calori at 0,9 V

meter to (amperes

  scan res / differ cm 2) tiel (O c) Pt 3,923 1,0 x 10-6 153 49,9 PtMo 3,913 10,8 x 10-6 105 82,5 Pt-V 3,872 5,7 x 10-6 124 74.7 Pt-W 3.916 3, 4 x 10-6 143 68.2 Pt-Cr 3.865 Ferromagnetic 138 75.8 Pt-Mn 3.930 Ferromagnetic 72.4 These results were also demonstrated by reducing the electron density of Fermi level of platinum by alloy with gold, giving rise to a reduction in catalytic activity When gold is added to platinum, the state density of platinum is reduced as reflected by the reduction in paramagnetic susceptibility The specific activity decreases from 50 1 amp / cm 2 for platinum to 15 amperes / cm 2 for the alloy as indicated in

figure 2.

  When modifying the electron density of state by alloy, it is advisable to choose in the first place the base metal The transition metals

  are preferred because they have a density

  The alloying element is selected on the basis of its atomic dimension with respect to the base metal in such a way that

  that the reticular parameter and the electron density

  state of affairs (which reflects paramagnetic susceptibility

  tick) can be changed in the right direction.

  An example is a transition metal having non-filled states with an appreciable density of d states at the Fermi level, for example, platinum. Its catalytic activity can be increased by increasing its state density at Fermi A level. this effect, substitutional alloys can be carried out, for example, with transition metals.

  of electrons has been demonstrated by

  paramagnetic susceptibility (Figure 2) indicating that the

  platinum state density increased.

  It has been found that the process described here is particularly suitable with platinum in reactions such as oxidation reactions and

  electrochemical oxygen reduction reactions.

  In both cases, the limiting speed factor

is the reduction of oxygen.

  Two attempts were made to further demonstrate the improvements achieved according to the present invention. Solid solutions were formed.

  platinum alloys It should be noted that it is cri-

  to keep intact the cubic structure to -

  centered faces of platinum while increasing its den-

  It is also important.

  to choose only alloys in which the electronic density of state at the Fermi level increases as a result of the alloy formation The improvement of the electronic density of state at the Fermi level can be determined by measuring the paramagnetic susceptibility which is proportional to the electronic density of state at Fermi level The density of state can also be determined by measuring the difference of the x-ray absorption peaks of platinum alloy and unalloyed platinum (see Example 3) following the transitions between the filled states and the

states of voids.

  The alloys thus formed were then subjected

  to tests during the oxidation of isopropyl alcohol

  and electrochemical reduction of.

  oxygen to demonstrate that the electron density

  that increased state increases the speed of the reaction

  and the catalytic activity of platinum.

Example 1

  Air saturated with isopropyl alcohol was passed over the respective catalyst that was

  heated slowly in a different scanning calorimeter

  ferential The onset of oxidation of alcohol was determined from a sudden influx of heat. A diagram of the temperature corresponding to the onset of oxidation with respect to susceptibility was plotted.

  paramagnetic platinum and catalysts

  This diagram is given in Figure 1.

  It is clear that, the lower the temperature at which the oxidation starts, the lower the alloy is paramagnetic Therefore, the results indicate that, the higher the electronic density of state at Fermi level

  is high, the lower the oxidation temperature.

Example 2

  The attached figure 2 is a diagram of 1 lac-

  electrochemical efficiency of catalysts for the reduction

  of oxygen in phosphoric acid as a function of

  The test was carried out as described above in a conventional 0.9 volt electrochemical cell using phosphoric acid as the electrolyte.

  new evidence that the greater the parametric susceptibility

  gnetic is high (plus the electron density

  at the Fermi level is high), plus the

  The alloy with a lower state density than platinum is platinum with 10% gold. FIG. 2 shows that, for this alloy, the catalytic activity is, in fact,

  than that of platinum Figure 3 is a di-

  of the catalytic activity of the reduction of oxy-

  gene in phosphoric acid with respect to the difference

  This measurement was made following the method described in "Journal of Chemical Physics", Vol. 70, No. 11 (June 1, 1979), pages 4849. -4855 by FW Lytle et al. This diagram includes ferromagnetic alloys suggesting that the correlation of the density of states is also valid for these alloys even though a direct paramagnetic / electron density analogy

  can not be demonstrated for these alloys.

  An example of a catalyst alloy process

  The noble metal compounds of the present invention comprise absorbing, for example, a chromium-containing species, preferably in the anionic form, on the supported noble metal catalyst, and then heating the chromium impregnated catalyst under an atmosphere. reducing agent to activate the formation of the preferred Ltanion alloy mentioned above is

  chromate and, for other alloys, the anionic form

  However, although this method is equally suitable for the manufacture of non-deposited alloys on alloys or alloys deposited on a support, finely divided noble metals not deposited on a support are generally limited to less than 50 m 2 / g of noble metal. As a result, this process is best carried out using noble metals finely divided and deposited on a surface.

  support that can be prepared with special surfaces

  typically greater than 100 m 2 / g of noble metal (see U.S. Patent No.

4,316,944).

  Although the invention has been described specifically

  about platinum, the performance of

  door which noble metal can be improved from the same

  In addition, although the invention is particularly

  can be applied to any chemical reaction involved in the manufacture of chemicals, pharmaceuticals, motor vehicles or in the fight against pollution. it has been indicated,

  the invention is particularly useful for electri-

  trocatalysts for the reduction of oxygen.

  This activity makes these catalysts particularly

  appropriate in an acid fuel cell Any-

  However, the use is by no means limited to fuel cells and the invention can be implemented in any ambient medium in which oxygen reduction and, in particular, electrochemical reduction of the oxygen. oxygen is part

the process involved.

  Although the present invention has been illustrated and described with reference to some of its detailed embodiments, the man of

  profession will understand that various modifications and omissions

  can be envisaged in both form and detail without departing from the

of the invention.

Claims (3)

  1 Process for improving catalyst performance in a catalyzed chemical reaction   platinum and in which the reduction of oxy-   gene is a limiting factor of speed, characterized   in that an increase in the speed of the chemical reaction   that by increasing the electronic density of state at   Fermi level of the platinum catalyst -   Process according to claim 1,   characterized by increasing the electron density   of state by combining the platinum catalyst.   Process according to claim 1,   characterized in that the reaction is a reaction electrochemical oxidation.