JPH02207844A - Catalyst for electrolytic reduction and production of reduced compound with this catalyst - Google Patents

Abstract

PURPOSE:To obtain a useful reduced compd. at a reduction voltage nearly equal to theoretical voltage by using a catalyst contg. at least one kind of metal selected among Pt, Pd, Ir, Rh, Fe, Ni and Co and at least one kind of metal selected among Pb, Cu, Zn and Cd. CONSTITUTION:At least one kind of metal selected among Pt, Pd, Ir, Rh, Fe, Ni and Co has a lower hydrogen overvoltage than the conventional catalyst for reduction. At least one kind of metal selected among Pb, Cu, Zn and Cd has a hydrogen over voltage fit to initiate a prescribed reduction reaction. When a catalyst for electrolytic reduction contg. both the selected metals is used, a useful reduced compd. is obtd. at a reduction voltage nearly equal to theoretical voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a binary or multi-component electrolytic reduction catalyst for converting various compounds into corresponding reduced compounds by electrolytic reduction reaction at low voltage.

(Prior art and its problems) The rapid increase in demand for fossil energy has already led to the release of carbon dioxide that exceeds the natural reduction cycle, causing global warming due to acid rain or the greenhouse effect of carbon dioxide accumulated in the atmospheric layer. This problem is predicted to become even more serious in the future. Electrochemical reduction of carbon dioxide gas, carbon monoxide, etc. is an important matter for protecting nature and restoring them to useful resources by human hands.

Until now, all the electrode catalysts that have been studied for this reduction reaction are single metal electrodes (e.g. lead, zinc, cadmium, etc.) with high hydrogen overvoltage (Electrochemistry Vol. 56, p. 323 (1).
988)). and methane useful on copper electrodes, for example.
It has also been found that ethylene, etc. can be obtained with a current efficiency of 20-30% (Chemistry Letters, p. 1695 (1)
985)). However, these catalysts require a voltage that is 1v or more higher than the theoretically required electrolytic reduction voltage, and are accompanied by significant electrical energy loss. This is an inevitable result of using copper, lead, etc., which require a voltage higher than the theoretical voltage, in order to avoid the generation of hydrogen, which has a reduction voltage approximately equal to the theoretical reduction voltage of carbon dioxide gas, etc.

However, the process of reducing carbon dioxide gas, etc. must be an energy-saving reaction for its purpose.

(Object of the invention) The object of the present invention is to provide a catalyst for electrolytic reduction that makes it possible to obtain a useful reduced compound at a reduction voltage approximately equal to the theoretical voltage, and a method for producing the reduced compound using the catalyst. shall be.

(Means for solving the problems) The present invention provides platinum, palladium, iridium, rhodium,
at least one first catalyst metal selected from the group consisting of iron, nickel, and cobalt; and at least one second catalyst metal selected from the group consisting of lead, copper, zinc, and cadmium.
The present invention relates to an electrolytic reduction catalyst comprising a catalytic metal, and a method for producing a reduced compound using the catalyst.

The present invention will be explained in detail below.

In the present invention, the first catalyst metal of the present invention, that is, at least one metal selected from the group consisting of platinum, palladium, iridium, rhodium, iron, nickel, and cobalt, has a hydrogen overvoltage lower than that of conventional reduction catalysts. a point having a hydrogen overvoltage, and the second catalyst metal, namely lead, copper, zinc,
At least one metal selected from the group consisting of cadmium has a hydrogen overvoltage suitable for causing the desired reduction reaction, as evidenced by its use in conventional reduction reactions. This method is characterized by the fact that by using both metals in combination, a reduction reaction that normally occurs only at a higher hydrogen overvoltage occurs at a low electrolytic voltage.

The first catalytic metal has a significantly lower hydrogen overvoltage than the second catalytic metal, in other words, it generates hydrogen at a lower voltage. Therefore, if these first catalyst metals are used alone in a reduction reaction of carbon dioxide, etc., only hydrogen will be generated.
Almost no carbon dioxide reduction product can be obtained.

Generation of hydrogen on the first catalyst metal occurs when two hydrogen atoms (H) adsorbed on adjacent metal atoms on the surface combine to form a hydrogen molecule (H2). On the other hand, when only the second catalyst metal is used, no hydrogen generation occurs at the same electrolysis voltage as the first catalyst metal because the hydrogen overvoltage is high.

Therefore, the adjacency relationship between atoms on the surface of the first catalyst metal is
When the hydrogen atoms are separated by the second catalyst metal, even if hydrogen atoms are adsorbed on the first catalyst metal, bonding between the adsorbed hydrogen atoms is prevented, and as a result, hydrogen generation is significantly inhibited.

゛-In order to reduce a reducible compound such as an acid gas, it is essential that a hydrogen atom be added to it.

When both the catalyst metals coexist on the catalyst surface, when the reducible compound approaches or adsorbs the two catalyst metals, the hydrogen atoms adsorbed on the adjacent first catalyst metal easily reach the reducible compound. As a result, the reducible compound can be reduced and converted into the corresponding reduced compound at a very low reduction voltage without hydrogen evolution.

In the present invention, the ratio of both catalyst metals is 5 to 9 on one side.
The ratio can be set to any proportion within the range of 5 at. Preferably less.

These catalytic metals in the present invention are usually coated on an electrode substrate, but it is also possible to further adsorb the other catalytic metal onto an electrode substrate coated with one of the first or second catalytic metal and formed with a base. Alternatively, an alloy consisting of both catalyst metals may be coated on the electrode substrate.

Furthermore, since the current efficiency changes depending on the ratio of both catalyst metals on the substrate, in other words, the coverage of one of the catalyst metals or the alloy component ratio, it is preferable to set the coverage depending on each reduction reaction.

Using such an electrolytic reduction catalyst of the present invention, reducible compounds such as carbon dioxide, carbon oxides, carboxylic acids, and albitides can be converted to corresponding reducing compounds such as methane,
Reducing to formic acid and alcohols such as methanol 5
In order to do this, the catalyst is coated on the surface of the cathode substrate in the cathode chamber of an electrolytic cell which is divided into an anode chamber and a cathode chamber by a diaphragm, and the reducible compound is contained in the electrolyte as a solid,
Alternatively, if it is a liquid, its solution, preferably an aqueous solution, is added, or if it is a gas, it is added by bubbling or the like and electrolyzed to obtain the corresponding reduced compound. The material of the diaphragm and other electrolytic conditions depend on the type of the reducible compound, but those conventionally used may be used as they are.

(Example) The present invention will be described in more detail based on the following examples, but the present invention is not limited to the examples.

Example 1 10 cells were placed in an electrolytic cell with an anode chamber and a cathode chamber separated by a Nafion membrane.
A sulfuric acid aqueous solution containing −' mol of copper ions is added, and the surface of the platinum black-coated platinum wire mesh electrode is coated with a coverage of θcu>1.0 in the electrolyte.
After coating the copper atoms with , the electrode is held at a predetermined equilibrium potential and a part of the coating is removed to form a copper atom in which a plurality of copper atoms with various coverage ratios θ and U values less than 1 are atomically dispersed. We created an atomic adsorption electrode. The coverage of adatoms on the electrode was determined electrochemically using a hydrogen adsorption voltamodrum. , while suppressing the elution of the adatoms, the electrolyte was
．． Substitute with 5M potassium hydrogen carbonate aqueous solution and -1,OV while circulating and bubbling carbon dioxide gas with a glass pump.
(vs, RHE), potentiostatic electrolysis was performed at about 10°C. The electrolytic voltage at this time was 0.5 V to IV lower than that of conventional single steel. After the reduction was completed, the product was identified by gas chromatography and liquid chromatography. In this case, the electrolyte volume is 2
5rn! , the gas phase volume was 93 mf.

In the copper atom adsorption electrode of this example, methane (
CH,, ○) and sodium formate ion (HCOO-,
) occurred selectively. Each coverage rate (θCu)
The relationship between the current efficiency (η1) and the generated current efficiency (η1) excluding hydrogen was as shown in FIG. 0 for sodium formate ion. <0.4.0.7<θcu<1.0
It was found that it was hardly detected in the region where 0.4<θcu<0.7 and was selectively generated in the region of 0.4<θcu<0.7. On the other hand, although there is some variation, the current efficiency for methane production is θ. It was found that the increase is almost proportional to 8.

Example 2 Using lead, which has a higher hydrogen overvoltage than copper in Example 1, as an adsorbent, the relationship between coverage and current efficiency was investigated. Example 1
By the same operation as above, lead adsorbed atoms were dispersed and precipitated on a black platinum wire mesh in a perchloric acid aqueous solution to prepare a reduction electrode. Reduction was carried out in the same manner as in Example 1 at 25° C. with a reduction potential of -500 mV.

This reduction voltage is 1 to 1 compared to the case of conventional single lead electrodes.
It was 1.5V low. This lead atom adsorption electrode selectively generated methanol, which was hardly generated on conventional electrodes with a large hydrogen overvoltage. Coverage rate θ2. and the current efficiency of producing methanol (・) in the liquid phase and hydrogen (mu) in the gas phase as the second
Shown in the figure. 0.2<θ2. <0.6, η1 (hydrogen)≦0.6, and at the same time η! (methanol) increases, θ1. -0.5, methanol production current efficiency is 30
% or more. θ1. The reaction current decreased with increasing .

In the region of θPI<0.6, the total amount of current applied was set to 150 coulombs, whereas in the region of θpb>0.6, the total amount of current applied was set to 100 coulombs. As θpb increases, η1
(methanol) decreases (θpb = around 0.5 to 0.6), θ1. It increased again as the number increased. This result is
This suggests that the reduction reaction to methanol proceeds in stages.

In addition, the data shown in parentheses in the figure show the results of reduction using a lead atom adsorption electrode at θpbζ0.95, and the data marked with (Pb) using a pure lead electrode at an IV high voltage. In the latter case, no methanol was observed to be produced, and formic acid was mainly produced with an efficiency of over 80%.

However, as mentioned above, by having only a few tens of percent of platinum atoms present, the IV can also be reduced to 5% with a low reduction voltage.
It was possible to generate it with an efficiency of about 0%. This fact suggests that carbon dioxide gas is catalytically reduced by adsorbed hydrogen on the platinum sites.

Example 3 Chloroplatinic acid and lead chloride were dissolved in a 1 molar sulfuric acid aqueous solution so that the atomic ratio of platinum to lead was 2:1, and both metals were electrodeposited on a lead electrode immersed in the same wave.

As a result of compositional analysis of the precipitate, the ratio of elementary atoms in each element matched the elemental atomic ratio added to the solution within the analytical error range.

Carbon dioxide gas was reduced on these electrode catalysts under the same experimental conditions as in Example 2, and the main product methanol was reduced to a current efficiency of 15%.
Obtained with.

(Effects of the Invention) In the present invention, as an electrolytic reduction catalyst, a first catalyst metal having a low hydrogen overvoltage and on which adsorbed hydrogen atoms are likely to be generated, and a second catalyst metal having an electrolytic voltage in a range suitable for electrolytic reduction are used. are used in combination to inhibit contact between the first catalyst metals.

Therefore, hydrogen atoms generated on the first catalyst metal are preferably prevented from combining with other hydrogen atoms to generate hydrogen molecules, almost eliminating hydrogen generation during electrolysis and reducing the energy necessary for hydrogen generation. It becomes possible to save money.

In addition, the reducible compound adsorbed on the catalyst metals or approaching the substrate is reduced by the adsorbed hydrogen atoms existing in the surroundings in a normal electrolytic reaction, and the corresponding reduced compound can be obtained.

Furthermore, since the method for producing a reduced compound according to the present invention also uses a combination of the catalyst metals, it is possible to produce the reduced compound with low energy.

[Brief explanation of the drawing]

FIG. 1 shows the copper coverage θ in Example 1 of the present invention. 1
FIG. 2 is a graph showing the relationship between the current efficiency η1 of methane and formate ion production, and the lead coverage θp in Example 2.
It is a graph showing the relationship between b and current efficiency η1 for hydrogen and methanol production. Patent Applicant: Tanaka Kikinzoku Kogyo Co., Ltd. Masahiro Watanabe Taifeng Kassokubo (117%)

Claims (2)

[Claims] (1) Platinum, palladium, iridium, rhodium, iron,
For electrolytic reduction, comprising at least one first catalyst metal selected from the group consisting of nickel and cobalt, and at least one second catalyst metal selected from the group consisting of lead, copper, zinc, and cadmium. catalyst. (2) At least one metal selected from the group consisting of platinum, palladium, iridium, rhodium, iron, nickel, and cobalt is added to the cathode substrate in the cathode chamber of the electrolytic cell, which is divided into an anode chamber and a cathode chamber by a diaphragm. a cathode coated with an electrolytic reduction catalyst comprising one catalyst metal and at least one second catalyst metal selected from the group consisting of lead, copper, zinc, and cadmium; A method for producing a reduced compound, which comprises reducing a reducible compound and converting it into a corresponding reduced compound.