RU2635111C1 - Catalyst of oxidating combustible gases, method of its production and method of synthesis of iridium-containing connection-predecessor - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: catalyst contains nanoparticles of precious metal compounds, such as platinum, palladium, and iridium, with the molar ratio of elements (Pt+Pd):Ir equal to 1:x, where x ranges from 0.02 to 0.67 on porous media with a specific surface area of the pores from 50 to 500 m²/g. A method of producing a catalyst and a method of making compound-predecessor of iridium are also provided.

EFFECT: invention makes it possible to produce a catalyst characterized by a high stability of the catalytic properties with long service lives.

16 cl, 9 dwg, 1 tbl, 8 ex

Description

The group of inventions relates to the field of inorganic chemistry, and more particularly to compositions and methods for producing catalysts and inorganic precursor compounds for the synthesis of catalysts.

The claimed group of inventions is intended for the manufacture of a catalyst for the oxidation of combustible gases, which is an integral part of the sensitive elements (CHE) of gas sensors of the thermocatalytic principle of operation, and the use of the proposed precursor compound allows the creation of sensitive elements with improved characteristics, in particular with a more stable signal throughout the entire operating time .

In addition to the manufacture of catalysts for CE thermocatalytic sensors, the proposed group of inventions can be used to create a wide range of catalytic materials designed for the processing of hydrocarbons and the neutralization of combustible gases, in particular in the manufacture of catalysts for the afterburning of exhaust gases of machines and units operating on natural gas.

Platinum group metals and their oxides are promising catalysts for the oxidation of methane and other combustible gases due to their high activity at fairly low temperatures (400-500 ° C). Therefore, they are often used in the manufacture of sensitive elements of thermocatalytic sensors for combustible gases and vapors.

The most commonly used metals are palladium and platinum, with platinum being in the catalyst in metallic form, and palladium in metallic and oxide. In order for the catalyst to have high activity and stability of properties, it is necessary that its composition includes both oxide and metal phases [Kinnunen N. et al. "Role of the Interface between Pd and PdO in Methane Dissociation" // The Journal of Physical Chemistry C, 2011, 115, 19197-19202].

Other platinum metals can also be used in the manufacture of catalysts for the oxidation of methane and other combustible gases. In the case of bulk catalysts for the oxidation of combustible gases, it was shown that the addition of iridium to a platinum-based catalyst can increase its activity due to the stabilization of the PtO ₂ phase [N. Ohtsuka, “The Oxidation of Methane at Low Temperatures Over Zirconia-Supported Pd, Ir and Pt Catalysts and Deactivation by Sulfur Poisoning” // Catal. Lett., 2011, 141, 413-419]. Thus, iridium is a promising additive that can improve the properties of the oxidation catalysts of methane and other combustible gases.

A known method (analogue) of the manufacture of a catalyst for the complete oxidation of hydrocarbons, in particular a catalyst for oil reforming (US 3718578 A). The invention discloses a method of manufacturing a catalyst containing from 0.01 to 3 mass. % Pt; from 0.01 to 5 mass. % Sn; from 0.001 to 1 mass. % Ir supported on a solid porous support, preferably based on Al ₂ O ₃ . In an analogue, as in the present invention, the deposition of a catalyst of the indicated composition on a support is carried out by sequential or one-time impregnation of the porous support with solutions of precursor compounds containing Pt and Ir. As the precursor compounds Pt and Ir, H ₂ IrCl ₆ , IrCl ₃ , IrBr ₃ , (NH ₄ ) ₂ IrCl ₆ , (NH ₄ ) ₂ IrBr ₆ containing iridium, and H ₂ PtCl ₆ , Pt (NH _{3 can be used)} ) ₄ Cl ₂ , (NH ₄ ) ₂ PtCl ₆ containing platinum. After applying one or all of the necessary solutions of the precursor compounds to the porous carrier, the obtained samples are dried at a temperature of 93-260 ° C. If the method of successive impregnation of a porous support is used, then the drying step under the previously indicated conditions should follow after the application of each of the solutions. After the drying stage, there follows the stage of annealing of the obtained samples in a hydrogen atmosphere at a temperature of 300-700 ° C or in the presence of oxygen at temperatures up to 700 ° C. The authors of this patent indicate that annealing in the atmosphere of purified hydrogen at temperatures of 300-500 ° C leads to the most positive results.

Along with the fact that this invention discloses a method of manufacturing a hydrocarbon oxidation catalyst containing both platinum and iridium, this catalyst and its production method have several disadvantages, especially when used to obtain catalysts that are part of the thermocatalytic sensors.

1) As a result of the preparation of the catalyst, various compounds of the starting metals, in particular, compounds such as oxides, halides and sulfides, can form on the surface of the solid porous support. The proposed catalyst is obtained at sufficiently low temperatures of heat treatment, however, its disadvantages include the presence on the surface of the support of ions such as halogen ions, sulfide ions, etc., which adversely affect the activity and stability of the catalysts for the complete oxidation of hydrocarbons.

2) The process of annealing the obtained samples in a hydrogen atmosphere, which results in the decomposition of the compounds of the precursors, is costly and time-consuming. This process is incompatible with the existing technology for the manufacture of thermocatalytic sensors, which includes heating the SE with the deposited catalyst to 400-700 ° C in air by passing an electric current through a measuring and heating platinum wire. The heating of the iridium precursor compounds indicated in the analogue invention in the temperature range of 400-700 ° C in air will obviously lead to the formation of chlorine-containing iridium phases that do not have catalytic activity. Moreover, the temperature range allowed for the decomposition of the precursor compounds is limited by a temperature of 700 ° C, which is associated with the characteristics of the measuring and heating platinum spiral.

3) Annealing in hydrogen, necessary for the decomposition of the precursor compounds of platinum and iridium, adds an additional technological step in the manufacture of catalysts, requires the use of additional equipment.

There is another method (analog) of the manufacture of a catalyst for the complete oxidation of hydrocarbons (WO 02078840). The composition of the catalyst includes at least one manganese oxide MnO _x ; at least one component containing metal from a subgroup of platinum (0.1-8 wt.%); at least one component containing cobalt and / or at least one component containing a metal from a subgroup of alkali, alkaline earth or rare earth metals. This catalyst is applied to a porous solid support, which most often acts as a mixture of low-temperature phases Al ₂ O ₃ or a mixture of low-temperature phases Al ₂ O ₃ and MnO _x (5-95 wt.% MnO _x ). The application of the catalyst on a solid porous support is carried out by sequential impregnation. This method involves the sequential application of solutions of precursor compounds of various metals, between which drying is carried out at a temperature of about 80 ° C and the final sequential washing of the samples with a solution of NaOH and water for a long time. As the precursor compounds of precious metals, compounds such as H ₂ IrCl ₆ , IrCl ₃ , (NH ₄ ) ₂ IrCl ₆ containing iridium can be used; H ₂ PtCl ₆ , Pt (NH ₃ ) ₄ Cl ₂ , (NH ₄ ) ₂ PtCl ₆ , H ₂ PtBr ₆ , Pt (NH ₃ ) ₄ (OH) ₂ , Pt (NH ₃ ) ₂ (NO ₂ ) ₂ , containing platinum; (NH ₄ ) ₂ PdCl ₆ , (NH ₄ ) ₂ PdCl ₄ , Pd (NH ₃ ) ₄ Cl ₂ , PdCl ₂ , Pd (NO ₃ ) ₂ containing Pd. The proposed catalyst has several significant disadvantages.

1) The process of applying the precursor compounds to a solid porous support is very complex and multi-stage and involves several stages of sequential washing with a solution of NaOH and water. During these stages, it is difficult to control the composition of the catalyst, the content of noble metals in it.

2) The use of NaOH during the manufacture of the catalyst can lead to its contamination with sodium ions, which can lead to a decrease or loss of catalytic activity.

3) The catalyst manufacturing method disclosed in this application, in particular the need for multiple washes, is technologically difficult to manufacture with the manufacture of thermocatalytic sensors.

Another catalyst for the complete oxidation of hydrocarbons is known (WO 2014136279) and a method for its preparation, selected as a prototype. The composition of the catalyst includes Pt from 0.5 to 2 mass. %; Ir from 0.5 to 2 mass. % deposited on a solid porous carrier, which is used as an oxide or a mixture of oxides, for example α-Al ₂ O ₃ or a mixture of α-Al ₂ O ₃ , γ-Al ₂ O ₃ , ZrO ₂ , La ₂ O ₃ and TiO ₂ . A method of applying a catalyst to a porous support is to impregnate the support with solutions of noble metal precursor compounds. In this patent, iridium compounds such as IrCl ₃ and Ir (NO ₃ ) ₃ and platinum compounds such as PtCl ₄ and Pt (NH ₃ ) ₂ (NO ₃ ) _{2 were} considered as precursor compounds. The impregnation method consists of several stages: applying solutions of the precursor compounds to a porous carrier, drying at a temperature of about 80 ° C for 3 hours, holding in an aqueous NaOH solution for 20 hours, restoring the obtained samples with hydrazine in the liquid phase, washing the samples with distilled water , final drying at a temperature of about 80 ° C for 3 hours. However, this method also has a number of serious disadvantages.

1) As in the case of the previous analogue invention, the catalyst deposition process is characterized by multi-stage and high energy intensity. During a large number of stages, it is not possible to control the content of precious metals in the catalyst.

2) The use of NaOH, as in the previous analogue method, can lead to contamination of the catalyst with sodium ions, which entails a decrease or loss of catalytic activity.

3) The precursor compound Ir (NO ₃ ) ₃ can produce an iridium oxide phase upon decomposition in air at a temperature of 400-700 ° C, however, it is prone to hydrolysis in an aqueous solution, which over time can lead to precipitation and a change in concentration.

4) The precursor compound Pt (NH ₃ ) ₂ (NO ₃ ) ₂ can produce phases of metal platinum and / or platinum oxide when decomposed in air at a temperature of 400-700 ° C; however, it has low solubility in water, which requires the use of large volumes of a solution of this compound.

5) The use of hydrazine as a reducing agent leads to the reduction of precursor compounds to metal phases, while the presence of platinum group metal oxides is necessary to obtain catalysts with high catalytic activity.

6) The use of solution methods for the reduction of precursor compounds is difficult compatible with the technology for the manufacture of catalysts in the composition of thermocatalytic sensors.

In connection with the foregoing, the development of a method for producing an oxidation catalyst for methane and other combustible gases and a new iridium precursor compound suitable for the manufacture of catalysts by thermal decomposition in air is an important task of modern science and technology.

In the present invention, the term "thermocatalytic sensor" we mean the design of the sensor, which includes a pair of sensitive elements connected to the Wheatstone bridge circuit and located in the reaction chamber, limited from the environment by a porous gas exchange filter and a buffer chamber, which are designed to create a laminar flow of the analyzed gas to the sensor [Karpov EF, Basovsky B.I. Control of ventilation and degassing in coal mines. Reference manual. M .: Nedra, 1994].

The working sensitive element (SE) consists of a heating and measuring platinum spiral immured in porous ceramics, consisting mainly of low-temperature crystalline modifications of aluminum oxide or other metal oxides, on the surface of which catalyst particles are deposited. The composition of the catalyst preferably includes Pt, Pd and PdO. In the framework of this invention, we propose a method of manufacturing a catalyst with the addition of iridium and a precursor compound of iridium, which is necessary for introducing iridium into the composition of the catalyst without significantly changing the manufacturing technology of SE, which is currently used in the production of thermocatalytic sensors. The structure of the comparative SE coincides with the structure of the working SE except that the comparative SE does not contain catalyst particles.

In the present invention, the following meanings are given to the applicable terms:

- “catalyst” - a material that changes the rate of a chemical reaction, but is not part of the reaction products, in particular, causing an increase in the rate of reaction of methane oxidation with atmospheric oxygen. In the framework of this invention, as a catalyst, particles are meant consisting of Pt, Pd, Ir and / or their alloy and PtO ₂ , PdO and IrO ₂ located on the outer surface and in the pores of the catalyst carrier. A catalyst particle may contain either one of the above components, or several components in any combination;

- “catalyst activity” is a characteristic of a catalyst expressing its property to change the rate of a chemical reaction, within the framework of the invention, to accelerate the oxidation of methane and other combustible gases with atmospheric oxygen;

- “nanoparticles” - particles of material whose volume does not exceed 5⋅10 ⁵ nm ³ . In this case, the volume of particles can be calculated as the volume of the ellipse V = 4 / 3⋅π⋅a⋅b⋅c, where 2a, 2b, 2c are the linear dimensions of the particle in three mutually perpendicular directions;

- “alloy” - a material homogeneous within the phase boundary, consisting of a mixture of platinum, palladium or iridium atoms in the form of metals. In the framework of this invention, the alloy may consist of any pair of the listed metals, or contain all three metals in any molar ratio;

- "porous catalyst support" - a material containing pores, due to the presence of which its surface area is 50-500 m ² / g, on the surface and in the pores of which the catalyst nanoparticles are located;

- “low-temperature modifications of Al ₂ O ₃ ” - crystalline modifications of Al ₂ O ₃ formed during firing at a temperature of not more than 1000 ° C of phases of the composition AlO _x (OH) _at ⋅zH ₂ O and including phases γ-Al ₂ O ₃ , δ-Al ₂ O ₃ , θ-Al ₂ O ₃ , χ-Al ₂ O ₃ , κ-Al ₂ O ₃ , η-Al ₂ O ₃ ;

- "precursor compounds" - compounds of noble metals, in particular platinum, palladium and iridium, which are dissolved and applied to the surface of the catalyst carrier and then subjected to treatment in solutions of various compositions and / or heating in atmospheres of various compositions to form catalyst nanoparticles;

- "complex compounds" - neutral molecules or ions that are formed as a result of the addition of neutral molecules or other ions called ligands to a metal ion.

The objective of the present invention is to develop a catalyst for complete oxidation, which is characterized by high stability of catalytic properties during long service life.

The problem is solved by the proposed combustible gas oxidation catalyst containing nanoparticles of noble metal compounds such as platinum, palladium and iridium, with a molar ratio of elements (Pt + Pd): Ir equal to 1: x, where x varies in the range from 0.02 to 0.67 deposited on a porous support with a specific surface area of 50 to 500 m ² / g.

Preferably, nanoparticles of noble metal compounds with a volume of less than ⁵ × 10 ⁵ nm ^{3 are used} .

Preferably, the molar ratio of the elements Pt: Pd is 1: y, where y varies from 2 to 4. The best result is achieved when the molar ratio of the elements Pt: Pd: Ir = 1: 3: 1.

Preferably, Al ₂ O _{3 is} used as the porous support. The best result is achieved when using a porous support, which is a low-temperature modification of Al ₂ O ₃ .

Preferably, PdO, IrO ₂ , PtO ₂ and / or Pt, and / or Pd, and / or Ir, and / or their alloy are used as noble metal compounds. Moreover, for the presence of catalytic properties, it is important that the metal phase itself exist, which can consist of both Pt, Pd, and Ir separately, and of two or three-component alloys of these metals of various compositions.

Preferably, the total total content of noble metals Pt, Pd and Ir in the composition consisting of a catalyst and a porous catalyst support is from 0.01 to 35 mol. %

Also, the problem is solved by a method of manufacturing a catalyst, comprising preparing an aqueous solution containing platinum and palladium precursor compounds and a solution containing an iridium precursor compound in a protic solvent; alternately applying these solutions with drying after each application at a temperature of 80-120 ° C until the total total content of noble metals Pt, Pd and Ir in the composition consisting of a catalyst and a porous catalyst support is from 0.01 to 35 mol. % and calcining the resulting composition in air at a temperature of not more than 700 ° C until complete decomposition of the precursor compounds to platinum, palladium and iridium compounds, including PdO, IrO ₂ , PtO ₂ and / or Pt, and / or Pd, and / or Ir, and / or their alloys, wherein the iridium precursor compound includes oxalate ions and iridium (III) and is prepared by dissolving hydrated iridium (IV) oxide and / or iridium (III) hydroxide in an oxalic acid solution with a concentration of 10 or more up to 50 mass. % in proton solvent.

Preferred are the precursors of platinum, palladium and iridium, which are capable of forming stable solutions for at least 30 days. The precursor compounds forming unstable solutions in which separation or precipitation occurs during storage are unsuitable for use in the synthesis of catalysts, since this will not ensure the constancy of the catalyst within one batch. The best result is achieved using solutions of the compounds [Pt (NH ₃ ) ₄ ] (NO ₃ ) ₂ , [Pd (NH ₃ ) ₄ ] (NO ₃ ) ₂ , H ₃ [Ir (C ₂ O ₄ ) ₃ ].

It is preferable to use water or a mixture of water and ethyl alcohol with an ethyl alcohol content up to 95 vol.% As a proton solvent. %

It is preferable to use oxalic acid taken in at least a 9-fold molar excess relative to the number of moles of iridium.

The problem is also solved by the method of manufacturing a precursor compound of iridium, which includes oxalate ions and iridium (III), which consists in dissolving hydrated iridium (IV) oxide and / or iridium (III) hydroxide in an oxalic acid solution with a concentration of from 10 to 50 mass. % in proton solvent.

It is preferable to use oxalic acid taken in at least a 9-fold excess relative to the amount of mole of iridium.

It is preferable to use water or a mixture of water and ethyl alcohol with an ethyl alcohol content up to 95 vol.% As a proton solvent. %

The technical result achieved by the claimed invention is to improve the operational characteristics of the catalyst, namely, to reduce the rate of degradation of the catalyst system (from 0.8% to 0.3% per day) based on nanoparticles of PdO and Pt, Pd and / or their alloy, deposited on a porous carrier with a specific surface area of 50-500 m ² / g due to the introduction of iridium in its composition. The technical result is achieved due to the fact that the introduction of iridium leads to a change in the composition of the catalyst and the formation of phases IrO ₂ and / or PtO ₂ , Ir and / or alloy in the Pt-Pd-Ir system. The introduction of iridium and its presence in oxide and / or metal form ensures the formation of active centers more resistant to degradation, which extends the life of the catalyst. In addition, the technical result is achieved by using as a carrier for the catalyst a porous material with a specific surface area of 50-500 m ² / g. A material based on low-temperature crystalline modifications of Al ₂ O ₃ , including γ-Al ₂ O ₃ , δ-Al ₂ O ₃ , θ-Al ₂ O ₃ , χ-Al ₂ O ₃ , κ- Al ₂ O ₃ , η-Al ₂ O. These modifications of Al ₂ O _{3 are} characterized by a small crystallite size, which provides a high specific surface area on which the catalyst nanoparticles are placed. On the surface of low-temperature modifications of Al ₂ O ₃ there is a significant amount of acid sites that interact with catalyst nanoparticles and prevent their sintering during operation.

The technical result is achieved through the use of a precursor compound, which is a complex compound of iridium with oxalate ions, which does not contain any elements other than Ir, C, O, N. The advantage of this precursor compound is that it undergoes conversion to oxide iridium (IV) during annealing in air at temperatures of 300-700 ° C.

In the manufacture of catalysts for sensitive elements of thermocatalytic sensors, the constancy of their catalytic activity during long-term operation is of great importance. Moreover, oxidation catalysts for combustible gases based on noble metals such as palladium, platinum and iridium are characterized by a gradual decrease in activity. To ensure the stability of the activity of the catalysts in thermocatalytic sensors, they are held at operating temperature. The best result for a catalyst containing a total of 0.33 mol. % of platinum, palladium and rhodium in the ratio of Pt: Pd: Ir, equal to 1: 3: 1, is achieved by aging at 420-450 ° C for 28 days: the activity drift does not exceed 1% per month.

The high stability of the properties of the catalyst is especially important when it is used in sensitive elements of thermocatalytic sensors, the analytical signal of which is determined by the release of heat during the oxidation of methane and other combustible gases on the surface of the catalyst. The stability of the catalytic properties corresponds to a small change in the thermal effect of the reaction over time, which, in turn, provides a small change in the sensitivity of the sensors with a long operating time.

Brief Description of the Drawings

The invention is illustrated by graphs and drawings.

In FIG. 1 is an X-ray diffraction pattern of a commercially available catalyst support having a specific surface area according to the passport of 200 m ² / g, demonstrating that this support consists of two crystalline phases related to the low-temperature phases of alumina: θ-Al ₂ O ₃ and γ- Al ₂ O ₃ .

In FIG. 2 shows the data of thermogravimetric analysis of the platinum precursor compound - [Pt (NH ₃ )] ₄ (NO ₃ ) ₂ ⋅ 2H ₂ O - in the air, demonstrating that it decomposes at a temperature of 250 ° C. According to mass loss calculations, the final decomposition product is platinum metal.

In FIG. Figure 3 shows the data of thermogravimetric analysis of the palladium precursor compound - [Pd (NH ₃ )] ₄ (NO ₃ ) ₂ ⋅H ₂ O - in an air atmosphere, demonstrating that it decomposes at a temperature of 300 ° C. According to mass loss calculations, the final decomposition product at 300 ° C is palladium metal, and the mass gain at a temperature of more than 400 ° C corresponds to the conversion of a part of palladium to PdO when heated in air.

In FIG. 4 presents the results of IR spectroscopy of an aqueous solution of a compound of the precursor of iridium, confirming the formation of a complex compound of iridium with an oxalate ion or ions.

In FIG. Figure 5 presents the data of thermogravimetric analysis of the precursor compound of iridium in the air, demonstrating that the main mass loss occurs up to 290 ° C.

In FIG. Figure 6 presents the results of x-ray phase analysis (x-ray diffraction pattern obtained using Cu K _α radiation) of the iridium precursor compound after heat treatment at 450 ° C for 24 hours, demonstrating that the IrO ₂ phase is the decomposition product of the iridium precursor compound in air.

In FIG. 7 presents the results of x-ray phase analysis (x-ray obtained using Cu K _α radiation), demonstrating that the catalyst deposited on a porous support contains crystalline IrO ₂ and / or PtO ₂ , PdO and metal phases. The presence of several icons corresponding to different compounds above one peak indicates that each of these compounds has a peak at a given angle and, therefore, may be contained in the sample. The presence of peaks that do not coincide with the peaks of other compounds unambiguously proves the presence of PdO, IrO _2, and metal phases. The metal phases can be represented by Pt, Pd, Ir or their alloys in any molar ratio; An exact determination of what the metal phases consist of for catalysts of this type is not possible due to the close parameters of the Pt, Pd, Ir crystal lattices, small particle sizes and the resulting significant broadening of the reflections of the metal phases in the x-ray diffraction pattern.

The position of all PtO ₂ peaks coincides with the position of some IrO ₂ peaks; therefore, it is impossible to draw an unambiguous conclusion about the presence of this compound from X-ray phase analysis data. Therefore, to confirm the presence of PtO ₂ , Raman spectroscopy was further used.

In FIG. Figure 8 presents the results of Raman spectroscopy (laser wavelength was 514 nm), demonstrating the simultaneous presence of the oxide phases IrO ₂ , PtO ₂ and PdO in the composition of the catalyst. The presence of peaks in the experimental graph is correlated with the position of the peaks of the oxide phases, which are indicated by labels, which are known from the literature and are arranged in a row for each phase under the experimental graph.

In FIG. Figure 9 presents the results of studying the sensitivity of thermocatalytic sensors: comparing the properties of a Pd and Pt based catalyst with a Pt: Pd molar ratio of 1: 3 and a Pt, Pd and Ir based catalyst with a Pt: Pd: Ir molar ratio of 1: 3 :one. In the case of a thermocatalytic sensor with a catalyst containing iridium, there is a faster exit to a constant value and a more stable signal.

The claimed invention discloses a catalyst for the oxidation of combustible gases with improved performance, in particular with a reduced rate of degradation of catalytic activity, a method for its preparation and a method for the synthesis of a precursor compound containing iridium. The catalyst preparation method is based on applying a solution of precursor compounds containing platinum, palladium and a precursor compound containing iridium on the surface of a porous support with a specific surface area of 50-500 m ² / g based on low-temperature modifications of Al ₂ O ₃ . As a carrier for such a catalyst, any alumina having the indicated surface area and phase composition can be used. As a rule, the X-ray diffraction pattern of such a carrier is characterized by the presence of peaks of low-temperature modifications of Al ₂ O ₃ with significant broadening, which corresponds to the presence of stacking faults in the crystal structure of the carrier, the small size of crystallites and the high specific surface area available for applying catalyst nanoparticles (Fig. 1).

After each application of a solution of precursor compounds, the resulting composition is dried at a temperature of 80-120 ° C to a water content of not more than 10 mass. %, and after reaching the required content of platinum, palladium and iridium, it is subjected to heat treatment in air at a temperature of 300-700 ° C to form catalyst nanoparticles. The formation of catalyst particles is established by X-ray diffraction analysis and Raman spectroscopy by the presence of IrO ₂ , PtO ₂ , PdO phases and a metal phase, which can be represented by Pt, Pd, Ir or their alloys. In the synthesis of the catalyst, it is necessary to use such precursor compounds that decompose to the platinum group metals or their oxides under the indicated annealing conditions. Another requirement for the precursor compounds is the good solubility in water or water-ethanol mixtures and the stability of the resulting solutions in order to ensure the constancy of the properties of the catalyst in the manufacture of large batches of catalysts and sensitive sensors. In the case of platinum and palladium, such precursor compounds are known and commercially available, these are [Pt (NH ₃ )] ₄ (NO ₃ ) ₂ ⋅ 2H ₂ O and [Pd (NH ₃ )] ₄ (NO ₃ ) ₂ ⋅ H ₂ O The platinum precursor compound decomposes at 250 ° C (FIG. 2), with a mass loss of 52% corresponding to the formation of platinum metal. The palladium precursor compound decomposes to palladium metal at a temperature of 300 ° C, while the mass loss is 66%, which corresponds to the formation of palladium metal. With a further increase in temperature, mass gain corresponding to the partial oxidation of Pd to PdO is observed (Fig. 3).

In the case of iridium, most commercially available compounds contain chlorine or other halogens, and in some cases, alkali metal ions. The presence of these impurities can lead to a decrease in catalytic activity. At the same time, their removal during annealing in air at temperatures of 300-700 ° C is impossible. In this regard, in our solution, as the iridium precursor compound, it is proposed to use the complex iridium precursor compound with an oxalate ion. The method of preparation of the iridium oxalate complex is based on the dissolution of hydrated iridium (IV) oxide or iridium (III) hydroxide in an aqueous or aqueous-alcoholic solution of oxalic acid. It is preferable to use freshly precipitated hydrated iridium (IV) oxide and iridium (III) hydroxide, since they dissolve in oxalic acid solutions. The best result is achieved when hydrated iridium (IV) oxide and iridium (III) hydroxide obtained by precipitation from a solution of H ₂ IrCl ₆ ⋅ Н ₂ О, (NH ₄ ) ₂ IrCl ₆ , Na ₂ [IrCl ₆ ] ⋅6H ₂ O or IrCl ₃ ⋅xH ₂ O, respectively, by adding a 10% aqueous NaOH solution dropwise to pH = 10, followed by washing with deionized water. Washing is carried out by subjecting the mother liquor with sediment to centrifugation in a centrifuge; any centrifuge models and any centrifugation conditions are suitable for centrifugation, which ensure the precipitation of hydrated iridium (IV) oxide and iridium (III) hydroxide on the bottom of the tank in which the suspension is located during centrifugation. After centrifugation is completed, the mother liquor is carefully drained, avoiding loss of sediment, and the precipitate is again dispersed in deionized water and centrifuged. To achieve an optimal result, washing must be performed at least 3 times.

For dissolution, a solution of oxalic acid with a concentration of from 10 to 50 masses is used. % in a protic solvent, while the acid solution is taken in excess in relation to the number of moles of iridium (at least a nine-fold excess in relation to moles of iridium). To accelerate dissolution, a suspension of hydrated iridium (IV) oxide or iridium (III) hydroxide in an oxalic acid solution is boiled. After dissolving hydrated iridium (IV) oxide or iridium (III) hydroxide, the resulting solution is filtered through a glass porous filter No. 1, 2 or 3 (classes 160, 100 and 40 according to GOST 9775-69) or another glass porous filter with similar characteristics, and then evaporated. Since the precursor compound has an extremely high solubility, the large excess of oxalic acid used to dissolve the precipitate can be separated by evaporating the solution at 90 ° C before the crystallization of oxalic acid, and then cooling to room temperature and filtering off the crystals formed. In this case, the iridium precursor compound remains in the mother liquor. Since the solubility of oxalic acid is 120 g per 100 g of water at 100 ° C and 10 g per 100 g of water at 20 ° C, this method allows you to separate up to 90% of excess oxalic acid from the solution. Then the solution of the precursor compound can be evaporated or, conversely, diluted to the concentration required for the synthesis. To ensure the long-term stability of the solution of the iridium precursor compound and to provide the concentration necessary for the synthesis, water or an ethanol-water mixture is added. An excess of unreacted oxalic acid does not interfere with the use of a solution of the iridium precursor compound to produce an iridium-containing catalyst. The iridium precursor compound is isolated from the aqueous-alcoholic solution and purified from excess oxalic acid by recrystallization.

It should be noted that the small amount of oxalic acid that may be present in the solution does not interfere with the use of the solution of the precursor compound.

The solution of the precursor compound is highly stable during storage, there are no visible changes (color change, turbidity, precipitation) for at least 1 month. This is achieved due to the fact that iridium in solution is associated with oxalate ions in a complex compound. The formation and existence of the iridium precursor complex compound in solution is confirmed by the presence of bands at 1700 and 1674 cm ^-1 , corresponding to C — O bonds in oxalate ions bound to the iridium atom (Fig. 4).

The iridium precursor compound decomposes in air at a temperature of not more than 290 ° C (Fig. 5). The decomposition product is IrO ₂ , which is confirmed by the data of x-ray phase analysis (Fig. 6).

When applying solutions of precursor compounds, followed by drying and annealing, a catalyst is formed, in which crystalline phases IrO ₂ and / or PtO ₂ , PdO, Pt, and / or Pd, and / or Ir, and / or their alloy (see example 5 , Fig. 7 and 8). The introduction of iridium leads to the formation of a PtO ₂ phase, which was not observed upon decomposition of the precursor compound [Pt (NH ₃ )] ₄ (NO ₃ ) ₂ ⋅ 2H ₂ O and which is absent in the catalyst based on palladium and platinum, which does not contain iridium. The presence / presence of this phase provides an improvement in the functional characteristics of the catalyst.

The concentration of the solutions of the precursor compounds does not matter, since these solutions are still dried, as a result of which they are concentrated first and only then the water is completely removed. When applying a low metal content, a very low concentration of the solution can be used, and the upper concentration limit is limited only by the solubility of the precursor compounds.

The synthesis of a catalyst with a reduced rate of degradation containing nanoparticles of metals, alloys and oxide phases of iridium, palladium and platinum is carried out using solutions of the complex compounds of the precursors of palladium and platinum of the composition [Pd (NH ₃ ) ₄ ] (NO ₃ ) ₂ and [Pr ( NH ₃ ) ₄ ] (NO ₃ ) ₂ and iridium precursor compounds. For the synthesis of the catalyst, an aqueous solution is prepared containing [Pd (NH ₃ ) ₄ ] (NO ₃ ) ₂ and [Pt (NH ₃ ) ₄ ] (NO ₃ ) ₂ , taken in the required molar ratio, and a solution of the iridium oxalate complex. The preferred molar ratio of Pt: Pd is 1: 3 and the preferred concentrations are 0.2 M for [Pt (NH ₃ ) ₄ ] (NO ₃ ) ₂ and 0.6 M for [Pd (NH ₃ ) ₄ ] (NO ₃ ) ₂ . Then, a solution of the iridium precursor compound in a proton solvent is prepared, the preferred concentration is 0.2 M. These solutions are applied to the porous carrier in turn and dried after each application at a temperature of 120 ° C until the target content of platinum, palladium and iridium in the catalyst composition is reached. The optimal target metal content is in the range from 0.1 to 35 mol. %

After applying the precursor compounds to the porous carrier, the resulting composition is fired in air. The advantage of using these precursor compounds is that they decompose to oxide and / or metal phases in air at temperatures of 300-700 ° C. This allows the catalyst to be manufactured by a single-stage treatment in air, bypassing the stage of reduction of platinum, palladium and iridium to metals. In the case of the manufacture of catalysts for sensitive elements of thermocatalytic sensors, this advantage allows the application of solutions of precursor compounds directly on a porous carrier connected to a heating element, and then heating and decomposition of precursor compounds by passing an electric current through a heating element.

In the case of creating catalyst-based thermocatalytic sensors, an improvement in the functional characteristics of the catalyst is expressed in the fact that the thermocatalytic sensor has a more stable signal, the value of which is set after a certain period of signal decrease. It should be noted that signal stabilization is observed both during long-term operation of the thermocatalytic sensor, and during alternating periods of operation and storage. In the case of an iridium-containing catalyst, there is no uncontrolled increase in the sensitivity of the sensor after a storage period (see example 8, Fig. 9).

The following are examples of specific embodiments of the invention, which illustrate the invention, but in no way limit its scope.

Example 1

To 1 g of Na ₂ [TrCl ₆ ] ⋅6H ₂ O dissolved in 20 ml of water was added dropwise a 10% aqueous NaOH solution to pH = 10, controlling the pH of the solution using a pH meter. The resulting suspension of hydrated iridium (IV) oxide was placed in a plastic centrifugation container and centrifuged for 15 minutes at a speed of 8000 rpm, which led to the complete precipitation of the hydrated iridium (IV) oxide precipitate on the bottom of the centrifugation vessel. The mother liquor above the precipitate was carefully drained, and the hydrated iridium (IV) oxide precipitate was dispersed in 25 ml of deionized water by stirring with a glass rod and centrifugation was repeated again. The cycle, including dispersion of the precipitate, centrifugation, draining the liquid, was repeated 3 times, then the precipitate of hydrated iridium (IV) oxide was dispersed in 20 ml of deionized water.

To a 20 ml suspension of hydrated iridium (IV) oxide in deionized water containing 1.79 x 10 ^-3 mol Ir, a solution of 1.2 g of oxalic acid in 12 ml of water was added and boiled for 20 hours until the precipitate of hydrated oxide was completely dissolved iridium (IV). The solution was evaporated before the formation of H ₂ C ₂ O ₄ crystals, cooled to 20 ° C and filtered through a No. 3 glass porous filter. 3 ml of ethyl alcohol (95%) was added to the filtrate, and then the volume was adjusted with deionized water to 5 ml. The resulting solution of the precursor compound can be diluted with water or ethanol to the desired iridium concentration and used to synthesize the catalyst.

Example 2

To 0.53 g of IrC1 ₃ ⋅xH ₂ O dissolved in 20 ml of water was added dropwise a 10% aqueous NaOH solution to pH = 10, controlling the pH of the solution using a pH meter. The resulting suspension of iridium (III) hydroxide was centrifuged and washed as in Example 1, and after washing it was dispersed in 20 ml of deionized water.

To a suspension of iridium (III) hydroxide in water containing 1.79-10 ^-3 mol Ir was added 12 ml of an aqueous solution of oxalic acid containing a solution of 1.2 g of oxalic acid in 12 ml of water, and boiled for 20 hours until completely dissolved iridium (III) hydroxide precipitate. The solution was evaporated before the formation of H ₂ C ₂ O ₄ crystals, cooled to 20 ° C and filtered through a No. 3 glass porous filter. 10 ml of ethanol (95%) was added to the filtrate and evaporated again at a temperature of 30 ° С, as a result of which crystals of the precursor compound formed, which were filtered on a No. 3 glass porous filter. Crystals of the iridium precursor compound were obtained from this solution by evaporation of the solution at a temperature of 30 ° C, cooling to 0 ° C and filtering on a No. 3 glass porous filter.

Example 3

The preparation of a solution of precursor compounds containing platinum and palladium consists in dissolving 0.067 g of [Pd (NH ₃ ) ₄ ] (NO ₃ ) ₂ ⋅H ₂ O and 0,034 g of [Pt (NH ₃ ) ₄ ] (NO ₃ ) ₂ ⋅ 2H ₂ O in a protic solvent, which was used as 10 ml of water.

Example 4

The preparation of a solution of the precursor compound containing iridium is that 0.0341 g of the compound obtained in Example 2 was taken and dissolved in 10 ml of water.

Example 5

A catalyst sample with a Pt: Pd: Ir molar ratio of 1: 3: 1 and a metal to carrier molar ratio of 5% was obtained by alternately applying a solution of Pt and Pd precursor compounds and a solution of Ir precursor compound. In the first step, an aqueous solution of the precursor compounds Pt and Pd was prepared (Example 3). The resulting solution was applied to 0.5 g of a porous carrier containing θ-Al ₂ O ₃ and γ-Al ₂ O ₃ phases by evaporation of the mixture obtained to dryness using a rotary evaporator and drying at 80 ° C. An aqueous solution of the iridium precursor compound (Example 4) was then prepared in a volume of 10 ml. This solution was applied onto an alumina support with the platinum and palladium precursor compounds obtained in the first step by evaporation of the resulting mixture by dryness using a rotary evaporator and drying at 80 ° C. After that, the alumina powder was heat treated with Pt, Pd, and Ir metal precursor compounds deposited on it as follows: first, short-term annealing at 670 ° C for 1 min, then long-term annealing at 450 ° C for 10 hours . At the end of the heat treatment experiment, the phase composition of this sample was established, which includes PdO, IrO ₂ , PtO ₂ and the metal phase.

Example 6

Similar to example 5, the difference is that a catalyst was prepared with a molar ratio of Pt: Pd: Ir equal to 1: 3: 2 and a total molar content of metals equal to 5%. Qualitatively, the phase composition of the obtained catalyst sample is identical to the phase composition of the catalyst from example 5.

Example 7

On the sensitive element of the thermocatalytic combustible gas sensor, the ceramic shell of the platinum measuring and heating coil of which is made of low-temperature modifications of Al ₂ O ₃ - phases θ-Al ₂ O ₃ and γ-Al ₂ O ₃ , two drops of an aqueous solution [Pd ( NH ₃ ) ₄ ] (NO ₃ ) ₂ ⋅H ₂ O and [Pt (NH ₃ ) ₄ ] (NO ₃ ) ₂ ⋅ 2H ₂ O containing 0.6 M Pd and 0.2 M Pt, and two drops each a solution of a precursor compound of iridium containing 0.2 M Ir. The total molar content of metals in the catalyst was 0.33 mol. % In the intervals between the application of the drops, the sensing element was dried at a temperature of 120 ° C. Then the sensing element was heated to 700 ° C by passing an electric current and kept at 450 ° C for 28 days.

Example 8

Tests of the performance properties of a series of ten CE made by the method described in Example 7 were carried out. The measurements were carried out in an atmosphere of a test gas methane-air mixture, while the working CE (containing the catalyst) and the comparative CE (without the catalyst) were connected to the Wheatstone bridge circuit . During the tests, the CEs were energized; the temperature at which the catalyst was 430-470 ° C. Sensitivity measurement was carried out every 1-30 days (Fig. 9).

The graph of the dependence of sensitivity on time clearly shows that at the initial stage there is a decrease in sensitivity due to the fact that the composition and structure of the catalyst comes into thermodynamic equilibrium. Then the signal is stabilized. To stabilize the SE signal with an iridium-containing catalyst, aging is necessary for 28 days, after which the sensitivity drop does not exceed 9.7% per month or 0.3% per day. It is worth noting that these values are an upper estimate, since the rate of decrease in sensitivity decreases with the time of operation of the catalyst.

It is clearly seen that the SE signal with an iridium-containing catalyst and the SE signal with a catalyst manufactured using standard technology, which contains PdO, Pt, Pd and / or their alloy, are generally close at the initial stage. To predict the sensitivity values over long operating times, the dependences of sensitivity on time were approximated by the exponential function y = A⋅exp (-x / t) + у ₀ . From the parameters of this function, we calculated the rate of decrease in sensitivity and other characteristics of CEs, shown in Table 1. After exposure to CEs made using standard technology for 28 days, the rate of decrease in sensitivity is 25% per month or 0.8% per day, which is significantly exceeds the corresponding values for ChE with an iridium-containing catalyst. The sensitivity reduction rate after 28 day aging, averaged over 1 year, for the catalyst containing iridium was 0.9% per month (0.03% per day), while for the reference samples without iridium - 3.5% in month (0.11% per day).

It should also be noted the advantage of SE with an iridium-containing catalyst, which is expressed in the fact that after a long period of inactivity the sensitivity value does not change, while in the case of SE with an iridium-free catalyst, an increase in sensitivity occurs.

Claims (16)

1. The catalyst for the oxidation of combustible gases, characterized in that it contains nanoparticles of compounds of noble metals, such as platinum, palladium and iridium, with a molar ratio of elements (Pt + Pd): Ir equal to 1: x, where x varies in the range from 0 , 02 to 0.67 deposited on a porous carrier with a specific pore surface area of 50 to 500 m ² / g. 2. The catalyst according to claim 1, characterized in that the used nanoparticles of compounds of noble metals with a volume of less than ⁵ × 10 ⁵ nm ³ . 3. The catalyst according to claim 1, characterized in that the molar ratio of the elements Pt: Pd is 1: y, where y varies from 2 to 4. 4. The catalyst according to claim 1, characterized in that the molar ratio of the elements is Pt: Pd: Ir = 1: 3: 1. 5. The catalyst according to claim 1, characterized in that the porous carrier is Al ₂ O ₃ . 6. The catalyst according to claim 1, characterized in that the porous carrier is a low-temperature modification of Al ₂ O ₃ . 7. The catalyst according to claim 1, characterized in that the compounds of noble metals include PdO, IrO ₂ , PtO ₂ and / or Pt, and / or Pd, and / or Ir, and / or their alloy. 8. The catalyst according to claim 1, characterized in that the total total content of noble metals Pt, Pd and Ir in the composition consisting of a catalyst and a porous support for the catalyst is from 0.01 to 35 mol. % 9. A method of manufacturing a catalyst according to claim 1, comprising preparing an aqueous solution containing platinum and palladium precursor compounds and a solution containing an iridium precursor compound in a protic solvent; alternately applying these solutions with drying after each application at a temperature of 80-120 ° C until the total total content of noble metals Pt, Pd and Ir in the composition consisting of a catalyst and a porous catalyst support is from 0.01 to 35 mol. % and calcining the obtained sample in air at a temperature of not more than 700 ° C until complete decomposition of the precursor compounds to platinum, palladium and iridium compounds, including PdO, IrO ₂ , PtO ₂ and / or Pt, and / or Pd, and / or Ir and / or their alloy, wherein the iridium precursor compound includes oxalate ions and iridium (III) and is prepared by dissolving hydrated iridium (IV) oxide and / or iridium (III) hydroxide in an oxalic acid solution with a concentration of 10 or more up to 50 wt. % in proton solvent. 10. The method according to claim 9, characterized in that use solutions of the compounds of the precursors of platinum, palladium and iridium, stable for at least 30 days. 11. The method according to claim 9, characterized in that as the compounds for the preparation of solutions of the precursor compounds of platinum, palladium and iridium, [Pt (NH ₃ ) ₄ ] (NO ₃ ) ₂ , [Pd (NH ₃ ) ₄ ] ( NO ₃ ) ₂ , H ₃ [Ir (C ₂ O ₄ ) ₃ ]. 12. The method according to claim 9, which consists in the fact that as a proton solvent use water or a mixture of water and ethyl alcohol with an ethyl alcohol content of up to 95 vol. % 13. The method according to claim 9, wherein the oxalic acid is taken in at least a 9-fold molar excess relative to the amount of mole of iridium. 14. A method of manufacturing a precursor compound of iridium for use in the method according to claim 9, which includes oxalate ions and iridium (III), which consists in dissolving hydrated iridium (IV) oxide and / or iridium (III) hydroxide in an oxalic solution acid with a concentration of from 10 to 50 wt. % in proton solvent. 15. The method according to 14, which consists in the fact that oxalic acid is taken in at least a 9-fold molar excess relative to the number of moles of iridium. 16. The method according to 14, which consists in the fact that as a proton solvent use water or a mixture of water and ethyl alcohol with an ethyl alcohol content of up to 95 vol. %

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