RU2515529C1 - Catalyst for selective oxidation of carbon monoxide in mixture with ammonia and method for production thereof (versions) - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: described is a catalyst for selective oxidation of carbon monoxide in a mixture with ammonia, containing 0.7-1.2 wt % gold, 0.8-5.0 wt % Fe³⁺ and a crystalline theta-modification of aluminium oxide (θ-Al₂O₃) - the balance. Described are methods of producing said catalyst.

EFFECT: obtaining a catalyst with high activity and selectivity in oxidation of CO while reducing activity in ammonia conversion.

3 cl, 1 tbl, 10 ex

Description

The invention relates to the field of catalytic purification of exhaust gases of vehicles, in particular to a catalyst for the selective oxidation of carbon monoxide (CO) in a mixture with ammonia containing gold, a compound of iron and aluminum oxide, and to a method for producing it, and can be used for exhaust gas purification systems engines operating in a lean mixture. Such systems will find application in the purification of motor vehicle exhausts.

At present, in order to reduce fuel consumption by vehicles, more economical engines working in a lean mixture (turbocharged gasoline engines, diesel engines) are being developed and introduced, and this leads to an increase in the content of nitrogen oxides (NO _x ) in motor vehicle exhausts, because . the three-way catalyst (TWC) catalysts currently in use are inactive under these conditions and cannot efficiently reduce the resulting nitrogen oxides to N3. Therefore, it is necessary to use other auto-exhaust neutralization systems operating in the conditions of depleted fuel-air mixtures (FA). Recently, a passive selective ammonia reduction system (PASS) has been proposed [CH Kim, K. Perry, M. Viola, W. Li, K. Narayanaswamy, SAE Technical Paper 2011-01-0306 (2011) 452 p.], The action of which is based on following principles.

1. Periodically, the engine runs on enriched fuel assemblies with a small, compared with stoichiometry, reduction in the air-fuel ratio. In this case, ammonia is formed at TWC.

2. The resulting ammonia is absorbed (adsorbed) by a catalyst for selective reduction of nitrogen oxides (SCR), located after TWC.

3. In subsequent engine lean cycles, the resulting NO _{x is} converted to environmentally friendly nitrogen on an SCR catalyst using adsorbed ammonia.

Since SCR catalysts are effective at temperatures below 400 ° C, it is diluted with air from outside to lower the temperature of the ammonia-containing gas. However, simultaneously with ammonia, a large amount of toxic carbon monoxide (CO) is formed, which does not oxidize on the SCR catalyst, moreover, it poisons it and enters the environment. One way to eliminate this drawback is to place an additional catalyst for selective CO oxidation in the presence of ammonia between the TWC and SCR catalysts, which provides a high degree of CO oxidation to carbon dioxide with minimal ammonia conversion. The operating temperatures of the selective CO oxidation catalyst should be as low as possible and not exceed 400 ° C.

The literature describes and patented many catalysts for the oxidation of CO in the exhaust of automobile engines [N.M. Popova. Catalysts for vehicle exhaust gas purification. Publishing House "Science", Alma-Ata, 1987; R.M. Heck, R.J. Farrauto. Catalytic air pollution control. Commercial Technology. Van Nostrand Reinhold, Division of International Thomson Publishing Inc., New-York, 1995], in which Pt, Pd, Rh. The disadvantages of these catalysts are low efficiency with a high content of CO and high activity in the oxidation of ammonia [Chang-Mao Hung, Cordierite-supported Pt-Pd-Rh ternary composite for selective catalytic oxidation of ammonia, Powder Technology 200 (2010) 78-83], which leads to the simultaneous oxidation of CO and ammonia in mixtures.

Known alumina-iron oxide catalysts for the oxidation of carbon monoxide containing 1.9-5.3% of iron in the form of oxide, the rest is gamma-alumina [Taskina S.Yu., Kirichenko O.A. and Chermashentseva O.K. Oxidation of methane, butane and carbon monoxide on Al-Fe-O oxide catalysts. React Kinet. Catal. Lett., 49, 2 (1993) 235-240]. Catalysts are prepared by impregnating the moisture content of alumina with solutions of iron oxalate complexes, followed by drying and heat treatment at 350-400 ° C. The disadvantage of these catalysts is the elevated temperature of CO oxidation.

Catalysts are described in the literature containing 5% gold on gamma alumina modified with magnesium oxide promoted with iron oxides with the following atomic ratio Au: Mg: Fe = 1: 5: 5 [RJH Grisel, CJ Weststrate, A. Goossens, MWJ Crajé, AM van der Kraan, BE Nieuwenhuys, Oxidation of CO over Au / MO _x / Al ₂ O ₃ multi-component catalysts in a hydrogen-rich environment. Catal. Today 72 (2002) 123-132]. Catalysts are prepared by sequentially applying gamma alumina to magnesium oxide, gold, and iron oxide. The application of a gold precursor to alumina is carried out by urea precipitation from solutions of hydrochloric acid (HAuCl ₄ ) when heated, followed by drying and heat treatment in a stream of hydrogen-containing inert at 300 ° C. Promotion with iron oxides was carried out by impregnating the catalysts of the composition Au / MgO / Al ₂ O _{3 with} solutions of iron nitrate, followed by heat treatment at 400 ° C.

Known catalyst for the oxidation of CO composition FeO _x / Au / Al ₂ O ₃ [RJH Grisel, BE Nieuwenhuys. A comparative study of the oxidation of CO and CH ₄ over Au / MO / Al ₂ O ₃ . Catalysis Today 64 (2001) 69-81], containing a carrier (gamma alumina), a catalytic component (gold, 5%) and a promoter (iron oxide) at an atomic ratio of Au: Fe = 1, which corresponds to the mass content of iron 1, four%. This catalyst was prepared by sequentially depositing gold and iron compounds on gamma alumina. First, a gold precursor is precipitated onto gamma alumina from a solution of hydrochloric acid using urea as a precipitant. To do this, prepare an aqueous solution of hydrochloric acid containing the required amount of gold, with a pH of approximately 4.0, and add it to gamma alumina. An excess of urea is added to the suspension, the suspension is heated to 70-75 ° C and maintained at these temperatures until the pH reaches 8. After that, the suspension is cooled and filtered. The resulting sample is washed from chloride ions, dried and reduced in hydrogen at 300 ° C. In a similar manner, an iron precursor is applied to the prepared Au / Al ₂ O ₃ sample, dried at 80 ° C and calcined at 400 ° C for 30 minutes. The disadvantage of the catalyst is the high gold content and too high activity of the catalyst in the oxidation of ammonia at temperatures below 400 ° C. The disadvantage of the method of producing the catalyst is the need for twice a long deposition of urea in suspension, filtering and washing, as well as an increase in the size of gold particles from 2.7 nm to 6.7 nm after the second deposition, which reduces the efficiency of use of gold.

Gold-containing (1.35 wt.% Au) CO oxidation catalysts are known in which the alumina support is preliminarily promoted with iron oxide (1.8-7.2 wt.%) [X. Zou, J. Xu, S. Qi, Z. Suo, L. An, F. Li. Effects of preparation conditions of Au / FeO _x / Al ₂ O ₃ catalysts prepared by a modified two-step method on the stability for CO oxidation. J Natural Gas Chemistry 20 (2011) 41-47], and adopted by us for the prototype. Promotion is carried out by impregnating the gamma modification of alumina (specific surface 133 m ² / g) in terms of moisture capacity with a solution of nitrate or iron (III) chloride, followed by drying at 110 ° C and calcination at 450 ° C for 2 hours. Gold is applied by impregnating the moisture content of the promoted support with a solution of hydrochloric acid previously adjusted to pH = 6–9 by adding 0.5–1 M KOH. After 2 hours, the impregnated carrier is immersed for 24 hours in a dilute ammonia solution (pH <10), after which the sample is separated from the solution, washed repeatedly with distilled water, dried at 60 ° C and calcined (or reduced in hydrogen) at 300 ° C for 1 hour The disadvantage of this catalyst is its high activity in the oxidation of ammonia. The disadvantage of the method of preparation of the catalyst is a large loss of gold in its preparation (10 relative%), the need for a long exposure in a solution of ammonia.

The objective of the present invention is to increase the activity and selectivity of the catalyst in the oxidation of carbon monoxide in the presence of ammonia, as well as the development of a method for its production.

The problem is achieved by the proposed catalyst for the selective oxidation of carbon monoxide in a mixture with ammonia, including gold, an iron compound and aluminum oxide, according to the invention, the catalyst contains a crystalline theta modification of aluminum oxide (θ-Al ₂ O ₃ ) in the following ratio, wt. %:

Au 0.7-1.2 Fe ³⁺ 0.8-5.0 θ-Al ₂ O ₃ rest

and the proposed methods for its preparation. The first method is carried out by applying an iron compound to a crystalline theta modification of aluminum oxide with further heat treatment of the obtained Fe / θ-Al ₂ O ₃ sample at a temperature of 350-550 ° C, followed by deposition of gold on it by precipitation of its precursor and reduction to metallic gold. The second method for the preparation of the catalyst is carried out by applying an iron compound to active alumina, which is a gamma or delta modification, followed by calcination at a temperature of 950-1000 ° C and applying gold to the thus obtained Fe / θ-Al ₂ O ₃ sample by deposition of its precursor, followed by reduction to metallic gold.

Thus, the difference of the first method is the deposition of an iron compound on the finished crystalline theta modification of alumina (θ-Al ₂ O ₃ ).

The second method involves the step of applying the iron compound by impregnating the active alumina, which is a gamma or delta modification, with an aqueous solution of the iron compound and a subsequent calcination step at a temperature of 950-1000 ° C until polymorphic conversion of aluminum oxide into the theta modification with simultaneous interaction with Fe ³⁺ ions and their stabilization at the θ-Al ₂ O ₃ surface centers.

Gold is deposited using urea, alkali or other precipitants. In order to completely precipitate gold and increase its dispersion, the deposition of a gold precursor is preferably with urea.

A distinctive feature of the proposed catalyst is the use of alumina with a crystalline structure of theta modification promoted by iron ions, and gold as its active component, as well as their quantitative content, which is 0.8-5 wt.% Iron, 0, 7-1.2 wt.% Gold, theta alumina - the rest, which allowed to increase the activity and selectivity of the catalyst in the oxidation of CO and at the same time reduce the activity in the oxidation of ammonia.

In the presence of other, in addition to theta, modifications of alumina, the selectivity of the catalyst decreases. In the absence or at a lower concentration of gold, the activity and selectivity of the catalyst in the oxidation of CO decreases. An increase in gold concentration of more than 1.2 wt.% Is impractical, since it slightly increases the activity of the catalyst and, moreover, leads to a decrease in selectivity. An increase in iron concentration of more than 5 wt.% Causes a polymorphic conversion of active aluminum oxides directly into alpha-modification, which leads to a loss of activity and selectivity. When the iron concentration is less than 0.8 wt.% Decreases selectivity in the oxidation of CO.

It should be noted that gold-containing catalysts on theta-modified alumina promoted by iron compounds were not offered at all in catalysis.

A distinctive feature of the proposed methods for the preparation of the catalyst is the use of a theta modification of aluminum oxide promoted with iron ions as a carrier.

As the starting iron compound, salts commonly used to deposit iron oxides on substrates, for example, nitrate, acetate, chloride, oxalate and oxalate complexes, can be used. As the starting gold compound, hydrochloric acid and its salts commonly used for depositing gold nanoparticles on substrates, as well as other compounds, for example, acetate and gold halides, can be used.

The technical result of the present invention is the creation of a catalyst for the selective oxidation of carbon monoxide in a gas mixture with ammonia with increased activity and selectivity in the oxidation of CO while reducing activity in the conversion of ammonia.

The invention meets the criterion of "novelty", as in the well-known scientific, technical and patent literature there is no complete set of features characterizing the invention. The catalysts containing gold deposited on the theta modification of aluminum oxide promoted by iron, according to our analysis of the scientific and technical literature, as well as abstracts and patent descriptions, were not used for the selective oxidation of CO mixed with ammonia.

The invention also meets the criterion of "inventive step", since, in contrast to the known catalysts for the oxidation of carbon monoxide, containing two active components, one of which is gold particles deposited on a carrier consisting of gamma particles of alumina and particles of oxide deposited on them iron offered catalyst contains as carrier alyumozhelezooksidnogo modification theta alumina with Fe ³⁺ ions inclusions predominantly in the form of surface exchange or isolated o-bound ions; the activity of such a system in the reaction of selective oxidation of carbon monoxide in the presence of ammonia, as well as the method of their synthesis proposed by the present invention have not been described previously. Moreover, the results of studies of the catalytic activity of systems containing both gold, iron in an oxide form and aluminum oxide presented in the literature do not imply the possibility of a decrease in activity in the oxidative conversion of ammonia, but, on the contrary, an increase in activity in the oxidation of ammonia when promoting active alumina with compounds was promoted iron [SD Lin, AC Gluhoi, BE Nieuwenhuys. Ammonia oxidation over Au / MO _x / γ-Al ₂ O ₃ -activity, selectivity and FTIR measurements. Catalysis Today 90 (2004) 3-14].

The tests of the catalysts in the oxidation of CO were carried out in a flow unit with exposure at a constant temperature to constant values of CO conversion. The composition of the reaction mixture: СО 4.5 vol.%, NH ₃ 1.5 vol.%, О ₂ 22.5 vol.%, Helium - the rest. The volumetric feed rate of the reaction mixture is 6000 h ^-1 . The activity was compared at temperatures reaching 50 and 95% CO conversion: the lower the temperature, the more active the catalyst. The selectivity of the catalysts in the oxidation of CO was evaluated by the temperature difference (T ₁₀ NH ₃ -T ₉₅ CO) of the detected conversion of ammonia (10% conversion) and almost complete CO oxidation (95% conversion). In the case of selective oxidation of CO, this difference has positive values, and a negative value indicates the simultaneous oxidation of ammonia in a mixture with CO.

The phase composition of the carriers was analyzed on a DRON-2 diffractometer in Cu Kα radiation. The completeness of the deposition of gold is controlled by iodometric titration of a sample of a solution from a suspension [S.I. Ginzburg, K.A. Kladyshevskaya. Guidelines for the chemical analysis of platinum metals and gold. Moscow, Science, 1965].

The invention is further illustrated by examples, not limiting its scope.

Example 1. A sample of the claimed composition. A sample of 3.00 g of theta alumina with a specific surface area of 95 m ² / g is impregnated with a moisture content of the required amount (1.7 ml) of a 0.25 M solution of iron nitrate, dried on a rotary evaporator at 80 ° C. The dried sample is calcined in a muffle furnace at a temperature of 550 ° C for 4 hours. Gold obtained in this way is modified with iron oxide. The application of gold is carried out by a known method of deposition using urea. To do this, 3.00 g of modified alumina is dispersed in 290 ml of distilled water and heated to 80 ° C. Then, with vigorous stirring, 0.75 ml of a 0.204 M solution of hydrochloric acid (HAuCl ₄ ), 0.90 g of urea are added to the suspension, and stirring is continued at 80 ° C for 4 hours. The deposition rate of gold is 99%. After this, the suspension is cooled to room temperature, the sample is separated from the solution by centrifugation, washed with 50 ml of 0.01 M NH ₄ OH and decarbonized distilled water to remove chlorine ions, while the precipitate is also separated by centrifugation. The catalysts are dried in air to a free-flowing state, and then in an oven at 60 ° C. Before testing for activity, the catalyst is calcined in a muffle at 300 ° C for 2 hours. The iron content in the catalyst is 0.8 wt.%, Gold - 1.0 wt.%. The prepared catalyst is tested in the reaction of selective oxidation of CO in a mixture with ammonia. The test results are shown in the table.

Example 2. A sample of the claimed composition. Similar to example 1, but using a 1.57 M solution of ammonium trioxalatoferrate, and calcination is carried out at a temperature of 350 ° C. The composition and test results of the catalyst are shown in the table.

Example 3. A sample of 3.00 g of gamma-alumina with a specific surface area of 115 m ² / g is impregnated with the required capacity (2.18 ml) of a 0.25 M solution of iron nitrate, dried on a rotary evaporator at 80 ° C. The dried sample is calcined in a muffle furnace at 1000 ° C. for 2 hours. The phase composition of the obtained carrier is θ-Al ₂ O ₃ . Gold obtained in this way is modified with iron oxide. The application of gold is carried out analogously to example 1 by precipitation using urea, but the volume of the HAuCl ₄ solution is 0.90 ml. The composition and test results of the catalyst are shown in the table.

Example 4. Similar to example 3, but as the starting compound of iron, ammonium trioxalatoferrate is taken in the form of a 1.25 M solution, calcination is carried out at a temperature of 950 ° C, and the volume of the HAuCl solution is ₄ 0.53 ml. The composition and test results of the catalyst are shown in the table.

The following examples 5-10 are given for comparison and describe the preparation and test results of samples with exorbitant calcination temperatures (Examples 5 and 6), a prototype of the composition and method of preparation of the catalyst (Example 7) and samples with exorbitant composition values (Examples 8-10) . The heat treatment temperature of the modified alumina and the results of tests of activity and selectivity in the oxidation of CO are given in the table.

Example 5. Similar to example 4, but the calcination temperature of the modified alumina is 900 ° C. The prepared catalyst is tested in the reaction of selective oxidation of CO in a mixture with ammonia. The phase composition of the modified carrier, as well as the chemical composition and test results of the catalyst are shown in the table.

Example 6. Similar to example 4, but the calcination temperature of the modified alumina is 1050 ° C. The phase composition of the modified carrier, as well as the chemical composition and test results of the catalyst are shown in the table.

Example 7 (prototype). Promotion is carried out by impregnating a weighed portion (2.20 g) of gamma modification of alumina (specific surface 130 m ² / g) with a moisture capacity of 1.38 M solution of iron nitrate (solution volume 1.44 ml), followed by drying at 110 ° C 14 hours and annealing at 450 ° C for 2 hours. Gold is applied by impregnating the promoted carrier with a solution of hydrochloric acid, previously adjusted to pH = 6.8 by adding 1 M KOH. To prepare this solution, 0.6 ml of 1 M KOH is added to 0.8 ml of a 0.204 M solution of hydrochloric acid. After 2 hours, the impregnated carrier is immersed for 24 hours in a dilute ammonia solution (pH = 9.8), after which the sample is separated from the solution, washed repeatedly with distilled water, dried at 60 ° C for 14 hours and calcined in a muffle at 300 ° C for 1 hour The deposition rate of gold is 90%. The prepared catalyst is tested in the reaction of selective oxidation of CO in a mixture with ammonia. The composition and test results of the catalyst are shown in the table.

Example 8. A sample of 2.00 g of gamma-alumina with a specific surface area of 115 m ² / g is impregnated with the required capacity (1.45 ml) of a 0.15 M solution of iron nitrate, dried on a rotary evaporator at 80 ° C. The dried sample is calcined in a muffle furnace at 1000 ° C. for 2 hours. The phase composition of the obtained carrier is θ-Al ₂ O ₃ . Gold obtained in this way is modified with iron oxide. The application of gold is carried out analogously to example 1 by precipitation using urea. For this, 2.00 g of modified alumina is dispersed in 20 ml of distilled water and heated to 80 ° C. Further, with vigorous stirring, 1.25 ml of 8.16 · 10 ^-3 M solution of hydrochloric acid (HAuCl ₄ ), 0.088 g of urea are added to the suspension and stirring is continued at 80 ° C for 4 hours. The deposition rate of gold is 99%. After this, the suspension is cooled to room temperature, the sample is separated from the solution by centrifugation, washed with 50 ml of 0.01 M NH ₄ OH and decarbonized distilled water to remove chlorine ions, while the precipitate is also separated by centrifugation. The catalysts are dried in air to a free-flowing state, and then in an oven at 60 ° C. Before testing for activity, the catalyst is calcined in a muffle at 300 ° C for 2 hours. The composition and test results of the catalyst are shown in the table.

Example 9. Similar to example 1, but theta-alumina is not promoted with an iron compound. The composition and test results of the catalyst are shown in the table.

Example 10. Similar to example 3, but gold is not applied. The composition and test results of the catalyst are shown in the table.

The examples 1-2 show that the catalysts prepared according to the first method on an iron-promoted theta modification of alumina allow selective oxidation of carbon monoxide in the presence of ammonia and achieve almost complete oxidation of CO to carbon dioxide without significant ammonia conversion. Gold-containing catalysts on iron-promoted theta alumina modifications prepared according to the second method (Examples 3-4) have higher activity and selectivity in CO oxidation. When the production method is changed (raising or lowering the heat treatment temperature), catalysts are obtained with a small difference in the temperatures of the complete oxidation of CO and the substantial conversion of ammonia, i.e. low selectivity (comparative examples 5 and 6). The known catalyst prepared according to the prototype (comparative example 7) has a high activity in the oxidation of both CO and ammonia, which leads to their simultaneous oxidation, and with the complete oxidation of CO, the ammonia conversion is almost 100%, and therefore cannot be used for the selective oxidation of CO in the presence of ammonia. When the concentrations of iron and gold are less than the declared limits, the catalytic activity and selectivity of the catalyst are reduced (comparative example 8). On a gold-containing catalyst prepared on unpromoted theta-Al ₂ O ₃ iron (comparative example 9), the oxidation of CO and ammonia proceeds simultaneously, and with almost complete oxidation of CO, ammonia conversion reaches 40%. Iron-promoted theta-Al ₂ O ₃ (comparative example 10), but not containing gold, has a significantly lower activity and selectivity in CO oxidation than the catalyst of the claimed composition. Only the totality of all the claimed features of the invention allows to achieve the goal and to obtain a highly active and selective catalyst for the oxidation of CO in a mixture with ammonia.

Thus, the technical result of the present invention is to increase the selectivity in the oxidation of CO in a mixture with ammonia by increasing the activity of the catalyst in the oxidation of CO while reducing the activity in the conversion of ammonia.

The present invention can be applicable in various systems and devices used in the purification of motor vehicle exhausts. The proposed catalyst preparation methods make it possible to obtain highly effective carbon monoxide oxidation catalysts capable of selectively oxidizing CO in mixtures with ammonia at low temperatures without ammonia conversion, and the presence of a large temperature interval between the areas of CO and ammonia oxidation expands the design possibilities for the development of appropriate ammonia purification devices for CO. The proposed method for producing a catalyst is technologically advanced, does not require specific equipment, and in the case of using ammonium trioxalatoferrate, it is also environmentally friendly, which together will allow organizing large-scale catalyst production on the basis of existing chemical industry facilities.

Table The activity and selectivity of catalysts in the oxidation of carbon monoxide mixed with ammonia. Example Chemical composition, % The promoted alumina calcination temperature Al ₂ O ₃ phase T ₅₀ CO (° C) T ₉₅ CO (° C) T ₁₀ NH3 (° C) T ₁₀ NH3-T ₉₅ CO (° C) Fe Au Example 1 0.8 1,0 550 Theta 205 240 295 +55 Example 2 5,0 1,0 350 Theta 185 200 270 +70 Example 3 1,0 1,2 1000 Theta 165 210 370 +160 Example 4 5,0 0.7 950 Theta 185 210 300 +90 Example 5 5,0 1,0 900 Delta theta 180 225 260 +35 Example 6 5,0 1,0 1050 Theta Alpha 205 245 270 +25 Example 7 5,0 1.35 450 Gamma 210 300 135 -165 Example 8 0.6 0,083 1000 Theta 400 470 300 -170 Example 9 0 1,0 - Theta 240 290 220 -70 Example 10 1,0 0 1000 Theta 385 470 350 -120 ^and T ₅₀ CO and T ₉₅ CO - the temperature of reaching 50% and 95% conversion of carbon monoxide, respectively; T ₁₀ NH3 - temperature to achieve 10% ammonia conversion.

Claims (3)

1. The catalyst for the selective oxidation of carbon monoxide in a mixture with ammonia, including gold, a compound of iron and alumina, characterized in that it contains crystalline theta modification of alumina (θ-Al ₂ O ₃ ) in the following ratio, wt.% :
Au 0.7-1.2 Fe ³⁺ 0.8-5.0 θ-Al ₂ O ₃ rest. 2. A method of producing a catalyst for the selective oxidation of carbon monoxide in a mixture with ammonia according to claim 1, which is carried out by applying an iron compound to a crystalline theta modification of aluminum oxide (θ-Al ₂ O ₃ ) and heat treatment of the obtained sample Fe / θ-Al ₂ O ₃ at a temperature of 350-550 ° C, followed by the deposition of gold on it by precipitation of its precursor, which is reduced to metallic gold. 3. The method of producing a catalyst for the selective oxidation of carbon monoxide in a mixture with ammonia according to claim 1, which is carried out by applying an iron compound to active alumina, which is a gamma or delta modification, followed by calcination at a temperature of 950-1000 ° C and applying gold to the resulting Fe / θ-Al ₂ O ₃ sample by precipitating its precursor, followed by reduction to metallic gold.