Preparation method of platinum-rhodium alloy powder for nitrogen oxide sensor electrode

Abstract

The present invention provides a preparation method of platinum-rhodium alloy powder for nitrogen oxide sensor electrodes, which includes: performing a chemical co-reduction step on a platinum metal precursor and a rhodium metal precursor to obtain a composite powder, wherein the particle size range of the composite powder is 50-100 nm; performing a molten salt heat treatment step on the composite powder to obtain sub-micron platinum-rhodium alloy powder for nitrogen oxide sensor electrodes. The platinum-rhodium alloy powder prepared by the method of the present invention increases the uniformity of the three-phase interface between the metal catalytic functional phase and the zirconia ion electrolyte substrate in the electrode paste, improves the catalytic ability of the electrode, and at the same time appropriately increases the particle size of the platinum-rhodium alloy particles, so that the prepared electrode paste has better fluidity, reduces the problem of easy wire breakage during the subsequent sintering process, and improves the yield and aging life of the sensor.

Description

Technical Field

[0001] The present invention relates to the technical field of nitrogen oxide sensors, and particularly to a preparation method of platinum-rhodium alloy powder for nitrogen oxide sensor electrodes. Background Art

[0002] The chip-type nitrogen oxide sensor is a device for detecting the content of nitrogen oxides (NO x ) in the engine exhaust gas. It usually consists of a zirconia (ZrO2) substrate and a nitrogen oxide sensitive electrode material. The nitrogen oxide sensor separates the nitrogen oxides in the exhaust gas through an electrode catalytic reaction and converts the measurement of nitrogen oxides into the measurement of O2 decomposed from the nitrogen oxides. The sensor chip has two working chambers. There is an oxygen pump electrode in the first chamber. By applying a certain voltage to the oxygen pump, the oxygen in the exhaust gas is first removed to avoid the decomposition of nitrogen oxides. The second chamber reduces the nitrogen oxides in the exhaust gas to N2 and O2 through a reducing agent, and then calculates the content of nitrogen oxides by detecting the concentration of the decomposed oxygen.

[0003] The platinum-rhodium electrode is used as a catalytic electrode for accelerating the decomposition of nitrogen oxides in the nitrogen oxide sensor and plays a decisive role in the response speed and sensitivity of the sensor. Therefore, the performance of the platinum-rhodium electrode is one of the most critical factors determining the performance of the nitrogen oxide sensor. In the prior art, mainly a mixture of platinum and rhodium powders is used as the functional phase of the catalytic electrode. In this catalytic electrode, the two metals of platinum and rhodium are in a sub-micron separated contact state, and the catalytic effect on low-concentration nitrogen oxides is poor. The concentration of nitrogen oxides in the existing automobile exhaust gas is generally below 2000 ppm. Therefore, the electrode prepared from the above-mentioned mixed powder shows weak synergistic catalytic performance and poor resolution in application.

[0004] Therefore, it is necessary to design a preparation method of platinum-rhodium alloy powder for nitrogen oxide sensor electrodes to solve the above problems. Summary of the Invention

[0005] The purpose of the present invention is to provide a preparation method of platinum-rhodium alloy powder for nitrogen oxide sensor electrodes. The platinum-rhodium alloy powder prepared by this method has good dispersibility and high degree of alloying, increases the uniformity of the three-phase interface between the metal catalytic functional phase and the zirconia electrolyte substrate in the electrode, and improves the catalytic ability of the electrode. At the same time, this method improves the particle size and particle size uniformity of the platinum-rhodium alloy particles, so that the electrode paste prepared has better fluidity and compactness, and reduces the problem of easy wire breakage during the subsequent sintering process.

[0006] To achieve the above purpose, the present invention adopts the following technical scheme: A preparation method of platinum-rhodium alloy powder for nitrogen oxide sensor electrodes, which includes:

[0007] Perform a chemical co-reduction step on a platinum metal precursor and a rhodium metal precursor to obtain a composite powder, which includes platinum elemental powder and rhodium elemental powder. The weight ratio of the platinum elemental powder to the rhodium elemental powder is 0.8 - 1.2, and the particle size range of the composite powder is 50 - 100 nm;

[0008] Perform a molten salt heat treatment step on the composite powder to obtain a platinum-rhodium alloy powder for a nitrogen oxide sensor at the submicron level, and the purity of the platinum-rhodium alloy powder for the nitrogen oxide sensor is ≥99%.

[0009] As a further improved technical solution of the present invention, the chemical co-reduction step includes:

[0010] S1. Dissolve chloroplatinic acid and rhodium trichloride in a solvent to form a mixed solution. In the mixed solution, the mass concentration of chloroplatinic acid is 1% - 5%, and the mass concentration of rhodium trichloride is 1% - 5%. At room temperature, add a reducing agent to the mixed solution, stir and react for 5 - 30 min, wash and dry to obtain the composite powder.

[0011] As a further improved technical solution of the present invention, in step S1, the reducing agent is lithium aluminum hydride, lithium borohydride, sodium borohydride or potassium borohydride. The addition amount of the reducing agent is 5 - 15 times the total weight of chloroplatinic acid and rhodium trichloride. The reducing agent is added in the form of a solution, and the solution concentration of the reducing agent is 0.5 - 2 M.

[0012] As a further improved technical solution of the present invention, the molten salt heat treatment step includes:

[0013] S2. Add an inorganic mixed salt and a solvent to the composite powder until the inorganic mixed salt is completely dissolved. After stirring evenly, heat to evaporate the solvent until the inorganic mixed salt crystallizes out;

[0014] S3. Put the material obtained in S2 into a furnace, perform a heating and melting treatment, then cool, wash and dry to obtain the platinum-rhodium alloy powder.

[0015] As a further improved technical solution of the present invention, in step S2, the solvent is water or a volatile polar organic solvent. The inorganic mixed salt is a mixture of one or more of metal halides, nitrates, and sulfates. The metal is an alkali metal or an alkaline earth metal. The melting point of the inorganic mixed salt is 130 - 300 °C, and the working temperature of the inorganic mixed salt is 400 - 700 °C.

[0016] As a further improved technical solution of the present invention, in step S2, the inorganic mixed salt is sodium nitrate - potassium nitrate, calcium chloride - potassium chloride, potassium nitrate - sodium nitrite - sodium nitrate or sodium chloride - potassium chloride - calcium chloride; in step S3, the heating rate of the material is 0.5 - 1 °C / s, the heat treatment temperature is 450 - 550 °C, and the heat treatment time is 2 - 4 h.

[0017] As a further improved technical solution of the present invention, it further includes:

[0018] S4. Mix the platinum-rhodium alloy powder obtained in S3 with water and zirconia balls, then carry out ball milling, washing and drying to obtain the finished platinum-rhodium alloy powder; wherein, the addition amount of water is 3 to 10 times the total weight of the platinum-rhodium alloy powder, the addition amount of the zirconia balls is 2 to 5 times the total weight of the platinum-rhodium alloy powder, the particle size of the zirconia balls is 0.5 to 1.0 mm, and the ball milling time is 20 - 30 h.

[0019] As a further improved technical solution of the present invention, a surfactant is added during the ball milling process, and the addition amount of the surfactant is 0.5% - 1% of the total weight of the platinum-rhodium alloy powder.

[0020] As a further improved technical solution of the present invention, the particle shape of the platinum-rhodium alloy powder is spherical or nearly spherical, the particle size range is 0.8 - 1.2 μm, and the weight ratio of platinum element to rhodium element in the platinum-rhodium alloy powder is 1:(0.5 - 2).

[0021] The purpose of the present invention also lies in providing a slurry for a nitrogen oxide sensor electrode, which has a high degree of alloying, increases the binding of the metal catalytic functional phase and the zirconia ion electrolyte three-phase interface, and improves the catalytic ability of the electrode.

[0022] To achieve the above purpose, the present invention adopts the following technical solution: A slurry for a nitrogen oxide sensor electrode includes the above-mentioned platinum-rhodium alloy powder for a nitrogen oxide sensor electrode. The preparation method of the electrode slurry is as follows:

[0023] Mix the following raw materials according to the following weight percentages: 0.2 wt% - 1 wt% of an inorganic binder phase, 10 wt% - 20 wt% of an organic binder carrier, 50 wt% - 70 wt% of platinum-rhodium alloy powder, 5 wt% - 10 wt% of zirconia powder; wherein: the platinum-rhodium alloy powder is a catalytic functional phase, its particle size distribution is 0.8 - 1.2 μm, and the loose bulk density is 2.3 g / cm 3 , the particle shape is spherical or nearly spherical; the zirconia powder is zirconia doped with 5% mol yttrium oxide (5YSZ), the particle size is 300 nm, and the specific surface area is 2.4 m 2 / g; the weight of the organic resin in the organic binder carrier is 5% - 10%, and the rest is an organic solvent. The organic resin is selected from ethyl cellulose and / or acrylic resin, and the organic solvent is selected from one or a mixture of terpineol, turpentine, butyl carbitol, dibutyl phthalate, etc.; the inorganic binder phase is preferably alumina or silica.

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] 1. The present invention combines chemical co-reduction with molten salt heat treatment, greatly improving the alloying degree of the platinum-rhodium alloy powder (≥99%). Compared with the mixed powder of platinum nanopowder and rhodium nanopowder, since the particles after high-degree alloying of platinum and rhodium are atomically homogeneous phases, there is no difference in the position of catalytic points. Therefore, the four-phase interface of traditional platinum particles, rhodium particles, zirconia, and gas is reduced to the three-phase interface combination of platinum-rhodium alloy particles, zirconia, and gas, optimizing the platinum-rhodium synergistic catalytic efficiency and improving the catalytic ability of the electrode.

[0026] 2. The present invention also further improves the particle density and particle size uniformity of the platinum-rhodium alloy, so that the prepared electrode paste has better fluidity, and further improves the densification of the fired electrode, thus reducing the problem of easy wire breakage during sintering and improving the qualification rate and reliability of the nitrogen oxide sensor chip.

[0027] 3. Compared with traditional solid-phase heat treatment, the molten salt in the present invention does not participate in the reaction but only serves as a reaction medium. During the reaction process, first, due to the formation of the molten salt fluid, the fluidity of the reaction components is enhanced and the diffusion rate is significantly increased. Therefore, the reaction temperature is significantly reduced and the reaction time is greatly shortened. The heat transfer in the molten salt method relies on the liquid molten salt. Since the molten salt penetrates between the generated alloy powder particles, by selecting the type of molten salt and regulating the surface energy and interfacial energy difference between and inside the powder particles, the excessive agglomeration bonding between composite particles can be effectively prevented, avoiding the abnormal growth due to the direct close contact of particles during the solid-phase heat treatment process. Therefore, it is also easier to control the shape and size of the powder particles. At the same time, due to the solid-liquid interfacial tension effect, alloy particles with an approximate spherical shape can be obtained. Therefore, the molten salt heat treatment can obtain platinum-rhodium alloy powder with a more regular morphology and more uniform particle size. Thus, the electronic paste prepared from the platinum-rhodium alloy powder has a uniform three-phase interface and controllable catalytic active points after sintering, so that the catalytic electrode is stable and controllable and has a low zero-point potential. Description of the Drawings

[0028] Figure 1 It is the X-ray diffraction pattern of the composite powder A in Example 1.

[0029] Figure 2 It is the X-ray diffraction pattern of the platinum-rhodium alloy powder A1 in Example 1.

[0030] Figure 3 It is the scanning electron micrograph of the composite powder A in Example 1.

[0031] Figure 4 It is the scanning electron micrograph of the platinum-rhodium alloy powder A1 in Example 1.

[0032] Figure 5 It is the test data of the catalytic electrode prepared from the platinum-rhodium electrode paste B1 in Example 1 applied to the nitrogen oxide sensor chip.

[0033] Figure 6 Test data of the catalytic electrode prepared from the platinum-rhodium electrode paste B4 in Comparative Example 2 applied to the nitrogen oxide sensor chip. Detailed implementation manners

[0034] In order to make the objectives, technical solutions and advantages of the present invention clearer, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Example 1

[0035] Prepare platinum-rhodium alloy powder A1 for the nitrogen oxide sensor electrode:

[0036] Step 1: Weigh 20 g of chloroplatinic acid and 22 g of rhodium trichloride in a beaker, add 100 ml of deionized water to dissolve them into a mixed solution; at room temperature, drop a sodium borohydride solution into the mixed solution and continuously stir. The molar concentration of the sodium borohydride solution is 1 M, the dropping amount is 500 ml, and the dropping speed is 3 ml / min. After the sodium borohydride solution is dropped, continue to stir and react for 10 min; after the chemical co-reduction reaction is completed, wash and dry the product, and the drying temperature is 60 °C to obtain composite powder A; please refer to Figure 1 the XRD pattern of the composite powder A obtained in this example shown in the figure. Each diffraction peak of the composite powder A corresponds one by one to the standard cards PDF#05-0685 and PDF#04-0802, and there are only separate diffraction peaks of platinum and rhodium, indicating that the composite powder A includes platinum elemental powder and rhodium elemental powder; the weight ratio of platinum elemental powder to rhodium elemental powder in the composite powder A is 0.875. Please refer to Figure 3 As shown in the figure, it is the scanning electron microscope image of the composite powder A obtained in Step 1. It can be seen that the platinum and rhodium particles in the composite powder A are nanoscale. Specifically, the particle size range of the composite powder is 50-100 nm.

[0037] Step 2: Mix and stir 75 g of calcium chloride and 75 g of potassium chloride solids with 200 g of deionized water to completely dissolve the solids and obtain an inorganic mixed salt solution; transfer 10 g of composite powder A and the inorganic mixed salt solution to a porcelain crucible, stir evenly, and heat the mixed material at 120 °C to gradually evaporate the water. Stop heating when a large amount of crystallization appears; further, put the porcelain crucible into an oven and dry it at 110 °C until it no longer loses weight;

[0038] Step 3: Put the material obtained in Step 2 into a molten salt heating furnace for heat treatment. Specifically, the molten salt heating furnace is heated to 500 °C at a heating rate of 0.8 °C / s and kept warm for 3 h. During the heating and keeping warm process, adjacent platinum and rhodium nanoparticles gradually combine in the molten salt fluid and complete the alloying process; then cool it, and ultrasonically wash the heat-treated material with a large amount of deionized water 5-10 times and dry it at 60 °C;

[0039] Step 4: Mix the material obtained in Step 3 with water with a weight 5 times that of the material, zirconia balls with a diameter of 1 mm with a weight 3 times that of the material, and 1% polyethylene glycol, transfer the mixture to a ball mill tank for ball milling and dispersion, perform ball milling for 24 h at a rotation speed of 250 rpm, then wash with deionized water 3 - 5 times, and dry at 60 °C to obtain highly dispersed platinum-rhodium alloy powder A1.

[0040] Please refer Figure 2 to the XRD pattern of the platinum-rhodium alloy powder A1 obtained in this example as shown. It can be seen that each diffraction peak of the platinum-rhodium alloy powder A1 corresponds one by one to the standard card PDF#27 - 0504, and there are no separate diffraction peaks of platinum or rhodium or other impurity peaks, indicating that the platinum-rhodium alloy powder A1 has achieved high alloying, and the purity of the platinum-rhodium alloy powder is ≥99%, that is, the proportion of the alloy phase in the platinum-rhodium alloy powder is ≥99%. Please refer Figure 4 to the SEM image of the platinum-rhodium alloy powder A1 as shown. It can be seen that the particle size of the platinum-rhodium alloy powder A1 can reach the sub-micron level. Specifically, the particle size of the platinum-rhodium alloy powder A1 is distributed in the range of 0.8 - 1.2 μm, and the apparent density is 2.3 g / cm 3 .

[0041] Furthermore, prepare platinum-rhodium electrode paste B1 using the platinum-rhodium alloy powder A1:

[0042] The platinum-rhodium electrode paste B1 is prepared from the following raw materials according to weight percentages: 80 wt% of the platinum-rhodium alloy powder A1, 5 wt% of zirconia, 0.5 wt% of alumina, and 14.5 wt% of an organic binder carrier. Among them, the zirconia powder is zirconia doped with 5% mol yttrium oxide (5YSZ), its particle size is 300 nm, and the specific surface area is 2.4 m 2 / g; in other embodiments, the inorganic binder phase can also be selected from inorganic powders such as silica and talc powder; the organic binder is a mixture of an organic resin and an organic solvent, where the content of the organic resin is 8%, and the rest is the organic solvent. The organic resin is selected as ethyl cellulose, and the organic solvent is selected as terpineol. Mix the above components and perform grinding on a three-roll grinder, with a grinding fineness of 5 μm, to obtain the platinum-rhodium electrode paste B1. Example 2

[0043] Prepare platinum-rhodium alloy powder A2 for the nitrogen oxide sensor electrode:

[0044] Step 1: The same as in Example 1, to obtain the composite powder A;

[0045] Step 2: Mix 75 g of sodium nitrate and 75 g of potassium nitrate solids with 200 g of deionized water and stir to completely dissolve the solids, obtaining an inorganic mixed salt solution; transfer 10 g of composite powder A and the inorganic mixed salt solution to a porcelain crucible, stir evenly, and heat the mixed materials at 120 °C to gradually evaporate the water. Stop heating when a large amount of crystallization appears. Further, place the porcelain crucible in an oven and dry it at 110 °C until it no longer loses weight;

[0046] Step 3: Put the material obtained in Step 2 into a molten salt heating furnace for heat treatment. Specifically, heat the molten salt heating furnace to 550 °C at a heating rate of 1.0 °C / s, hold for 2.5 h, and then cool. Wash the heat-treated material ultrasonically 5 - 10 times with a large amount of deionized water and dry at 60 °C to obtain platinum-rhodium alloy powder;

[0047] Step 4: Consistent with Example 1, obtain highly dispersed platinum-rhodium alloy powder A2.

[0048] Match each diffraction peak of the XRD pattern of the platinum-rhodium alloy powder A2 obtained in this example with the standard card PDF#27 - 0504 one by one. There are no separate diffraction peaks of platinum or rhodium or other impurity peaks, and the platinum-rhodium alloy powder A2 is also submicron-sized. The purity of the platinum-rhodium alloy powder A2 ≥ 99%.

[0049] Further, prepare platinum-rhodium electrode paste B2 using the platinum-rhodium alloy powder A2: Consistent with Example 1, obtain platinum-rhodium electrode paste B2. Comparative Example 1

[0050] Prepare platinum-rhodium composite powder A3:

[0051] Step 1: Mix 75 g of calcium chloride and 75 g of potassium chloride solids with 200 g of deionized water and stir to completely dissolve the solids, obtaining an inorganic mixed salt solution; transfer a total of 10 g of nano-platinum powder and nano-rhodium powder and the inorganic mixed salt solution to a porcelain crucible, stir evenly, and heat the mixed materials at 120 °C to gradually evaporate the water. Stop heating when a large amount of crystallization appears. Further, place the porcelain crucible in an oven and dry it at 110 °C until it no longer loses weight; among them, both the nano-platinum powder and the nano-rhodium powder are commercially available, with a particle size range of 50 - 100 nm, and the nano-platinum powder and the nano-rhodium powder are added in equal weight ratios.

[0052] Step 3: Put the material obtained in Step 2 into a molten salt heating furnace, heat to 500 °C at a heating rate of 0.8 °C / s, hold for 3 h, and then cool. Wash ultrasonically 5 - 10 times with a large amount of deionized water and dry at 60 °C;

[0053] Step 4: Mix the material obtained in Step 3 with 5 times water, 3 times of zirconia balls with a diameter of 1 mm, and 1% polyethylene glycol, transfer it to a ball mill tank for ball milling and dispersion, ball mill for 24 h at a rotation speed of 250 rpm, then wash it 3 - 5 times with deionized water, and obtain highly dispersed platinum-rhodium composite powder A3 after drying.

[0054] Further, prepare platinum-rhodium electrode paste B3: It is generally the same as the preparation method in Example 1, except that the platinum-rhodium alloy powder A1 in Example 1 is replaced with platinum-rhodium composite powder A3 to obtain platinum-rhodium electrode paste B3. Comparative Example 2

[0055] Prepare platinum-rhodium composite powder A4:

[0056] Step 1: The same as in Example 1 to obtain composite powder A;

[0057] Step 2: The same as in Example 1;

[0058] Step 3: Put the material obtained in Step 2 into a molten salt heating furnace for heat treatment. Specifically, heat the molten salt heating furnace to 850 °C at a heating rate of 0.8 °C / s, keep it warm for 3 h, then cool it, and ultrasonically wash the heat-treated material 5 - 10 times with a large amount of deionized water and dry it at 60 °C;

[0059] Step 4: The same as in Example 1 to obtain platinum-rhodium composite powder A4.

[0060] Further, prepare platinum-rhodium electrode paste B3: It is generally the same as the preparation method in Example 1, except that the platinum-rhodium alloy powder A1 in Example 1 is replaced with platinum-rhodium composite powder A4 to obtain platinum-rhodium electrode paste B4. Comparative Example 3

[0061] Prepare platinum-rhodium electrode paste B5: It is generally the same as the preparation method in Example 1, except that 80 wt% of the platinum-rhodium alloy powder A1 in Example 1 is replaced with a mixed powder of 40 wt% nano platinum powder and 40 wt% nano rhodium powder to obtain platinum-rhodium electrode paste B4.

[0062] Conduct performance tests on the platinum-rhodium electrode pastes of Example 1, 2 and Comparative Examples 1, 2, 3:

[0063] Viscosity test: Use a Broodfield #52 / 1.5 rpm viscometer to measure the viscosities of platinum-rhodium electrode pastes B1, B2, B3, B4 and B5 at 25 °C.

[0064] Print patterns of lines with a length of 10 cm and a width of 0.1 cm on the zirconia green ceramic film substrate with platinum-rhodium electrode pastes B1, B2, B3, B4 and B5 respectively, and then sinter at 1500 °C for 2 h. Use a multimeter to measure the resistance after sintering. Please refer to Table 1 for the above test results.

[0065] Table 1 Performance Parameter Test Table of Platinum-Rhodium Electrode Pastes B1, B2, B3, B4 and B5

[0066] B1 B2 B3 B4 B5 Slurry viscosity (Pa·s) 198 195 204 212 200 Resistivity (Ω·cm) <![CDATA[2.2×10 -4 > <![CDATA[2.3×10 -4 > <![CDATA[2.8×10 -4 > <![CDATA[3.4×10 -4 > <![CDATA[3.3×10 -4 >

[0067] Measure the electrodes sintered from the platinum-rhodium electrode pastes B1 and B2 obtained in Examples 1 and 2. The electrical conductivity is 100%, and the resistivity is as low as 2.2×10 -4 Ω·cm; Measure the electrodes sintered from the platinum-rhodium electrode paste B4 obtained in Comparative Example 3. The electrical conductivity is 30%, and the resistivity is 3.4×10 -4 Ω·cm, indicating that there is a risk of easy wire breakage in the application of the electrode paste of Comparative Example 3 in preparing electrodes. Measure the electrodes sintered from the platinum-rhodium electrode pastes B3 and B5. Their electrical conductivity and resistivity are both between B1 and B4.

[0068] In addition, the present invention applies the catalytic electrodes prepared from the platinum-rhodium electrode pastes B1, B2, B3, B4 and B5 to the nitrogen oxide sensor chip, Figure 5 and Figure 6 are the test data graphs of B1 and B4 respectively. Among them, the abscissa is time (s), and the ordinate is current (nA). The larger the peak value of the ordinate, the higher the electrode activity. It can be seen from the chip test data that the chip made of the platinum-rhodium electrode paste B4 has a chip current of about 180 nA in air. In the NO atmosphere, the chip IP2 (the sensor limit current of the actual concentration of NO x reflects the response limit of the sensor to the change of NO x concentration) is 915 nA; in addition, the chip IP2 of the chips prepared from the pastes in other comparative examples is lower than 1000 nA; while the chip current in air of the chip made of the platinum-rhodium electrode paste B1 of the present invention is close to zero. In the NO atmosphere of the same concentration, the chip IP2 reaches 3100 nA, and the catalytic activity of the electrode is greatly improved, which can ensure that the sensor can accurately monitor and control the nitrogen oxide content in automobile exhaust.

[0069] The above embodiments are only used to illustrate the present invention and do not limit the technical solutions described in the present invention. The understanding of this specification should be based on those skilled in the art. Although this specification has described the present invention in detail with reference to the above embodiments, those skilled in the art should understand that those skilled in the art can still modify the present invention or make equivalent substitutions. All technical solutions and their improvements that do not depart from the spirit and scope of the present invention shall be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing platinum-rhodium alloy powder for nitrogen oxygen sensor electrodes, characterized in that: include: A platinum metal precursor and a rhodium metal precursor are subjected to a chemical co-reduction step to obtain a composite powder, wherein the composite powder comprises a platinum element powder and a rhodium element powder, wherein the weight ratio of the platinum element powder to the rhodium element powder is 0.8-1.2, and the particle size of the composite powder is in the range of 50-100 nm; the chemical co-reduction step comprises: S1, dissolving chloroplatinic acid and rhodium trichloride in a solvent to form a mixed solution, wherein the mass concentration of chloroplatinic acid is 1% to 5%, and the mass concentration of rhodium trichloride is 1% to 5%; adding a reducing agent to the mixed solution at room temperature, stirring the reaction for 5 to 30 minutes, washing and drying to obtain the composite powder; the reducing agent is lithium aluminum hydride, lithium borohydride, sodium borohydride or potassium borohydride, and the amount of the reducing agent added is 5 to 15 times the total weight of chloroplatinic acid and rhodium trichloride; The composite powder is subjected to a molten salt heat treatment step to obtain a submicron platinum-rhodium alloy powder for nitrogen oxygen sensor electrodes, wherein the purity of the platinum-rhodium alloy powder for nitrogen oxygen sensor electrodes is ≥ 99%; The molten salt heat treatment step comprises: S2, adding an inorganic mixed salt and a solvent to the composite powder until the inorganic mixed salt is completely dissolved, stirring evenly and heating to evaporate the solvent until the inorganic mixed salt crystals are precipitated, wherein the solvent is water or a volatile polar organic solvent, the inorganic mixed salt is a mixture of one or more of a metal halide, a nitrate, and a sulfate, the metal is an alkali metal or an alkaline earth metal, the melting point of the inorganic mixed salt is 130 to 300° C., and the working temperature of the inorganic mixed salt is 400 to 700° C.; S3. Put the material obtained in S2 into a furnace for heating and melting treatment, and then cool, wash and dry to obtain platinum-rhodium alloy powder. The material heating rate is 0.5-1°C / s, the heat treatment temperature is 450-550°C, and the heat treatment time is 2-4h.

2. The method for preparing platinum-rhodium alloy powder for nitrogen oxygen sensor electrodes according to claim 1, characterized in that: Step S1 In the method, the reducing agent is added in the form of a solution, and the concentration of the reducing agent solution is 0.5-2M.

3. The method for preparing platinum-rhodium alloy powder for nitrogen oxygen sensor electrode according to claim 1, characterized in that: In step S2, the inorganic mixed salt is sodium nitrate-potassium nitrate, calcium chloride-potassium chloride, potassium nitrate-sodium nitrite-sodium nitrate or sodium chloride-potassium chloride-calcium chloride.

4. The method for preparing platinum-rhodium alloy powder for nitrogen oxygen sensor electrode according to claim 1, characterized in that: Also includes: S4. Mix the platinum-rhodium alloy powder obtained in S3 with water and zirconium oxide balls, and then perform ball milling, washing and drying to obtain a finished platinum-rhodium alloy powder; wherein the amount of water added is 3 to 10 times the total weight of the platinum-rhodium alloy powder, the amount of the zirconium oxide balls added is 2 to 5 times the total weight of the platinum-rhodium alloy powder, the particle size of the zirconium oxide balls is 0.5 to 1.0 mm, and the ball milling time is 20 to 30 hours.

5. The method for preparing platinum-rhodium alloy powder for nitrogen oxygen sensor electrode according to claim 4, characterized in that: A surfactant is added during the ball milling process, and the amount of the surfactant added is 0.5% to 1% of the total weight of the platinum-rhodium alloy powder.

6. The method for preparing platinum-rhodium alloy powder for nitrogen oxygen sensor electrodes according to claim 1, characterized in that: The particle shape of the platinum-rhodium alloy powder is spherical or nearly spherical, and the particle size ranges from 0.8 to 1.2 μm. The weight ratio of the platinum element to the rhodium element in the platinum-rhodium alloy powder is 1:(0.5 to 2).

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