Formation of Catalytically Active Nanoparticles under Thermolysis of Silver Chloroplatinate(II) and Chloroplatinate(IV)

The thermal behaviour of Ag2[PtCl4] and Ag2[PtCl6] complex salts in inert and reducing atmospheres has been studied. The thermolysis of compounds in a helium atmosphere is shown to occur in two stages. At the first stage, the complexes decompose in the temperature range of 350–500 °C with the formation of platinum and silver chloride and the release of chlorine gas. At the second stage, silver chloride is sublimated in the temperature range of 700–900 °C, while metallic platinum remains in the solid phase. In contrast to the thermolysis of Ag2[PtCl6], the thermal decomposition of Ag2[PtCl4] at 350 °C is accompanied by significant heat release, which is associated with disproportionation of the initial salt to Ag2[PtCl6], silver chloride, and platinum metal. It is confirmed by DSC measurements, DFT calculations of a suggested reaction, and XRD. The thermolysis of Ag2[PtCl4] and Ag2[PtCl6] compounds is shown to occur in a hydrogen atmosphere in two poorly separable steps. The compounds are decomposed within 170–350 °C, and silver and platinum are reduced to a metallic state, while a metastable single-phase solid solution of Ag0.67Pt0.33 is formed. The catalytic activity of the resulting nanoalloy Ag0.67Pt0.33 is studied in the reaction of CO total (TOX) and preferential (PROX) oxidation. Ag0.67Pt0.33 enhanced Pt nano-powder activity in CO TOX, but was not selective in CO PROX.


Introduction
The search for new efficient energy sources is an important problem nowadays. One option to solve this problem is to use fuel cells, i.e., devices that convert chemical energy into electrical energy, bypassing the inefficient stage of fuel combustion, which occurs with great losses. Since there is no release of harmful substances into the atmosphere, such elements are eco-friendly, which is very important today due to pressing environmentrelated issues. Therefore, fuel cells are promising and have attracted the attention of many researchers [1,2].
Catalysts are important components of fuel cells; they increase the efficiency of the latter. Various catalysts can be used for low-temperature fuel cells, but highly dispersed platinum catalysts on carbon are currently amongst the most popular ones.
Platinum is widely used in catalytic processes. It was shown in [3,4] that the activity of platinum in the methanol oxidation reaction depends on the particle size and these changes are not monotonic. The substrate and additives also make a significant contribution to the oxidation of methanol. When used as a substrate of transition metal oxides, the rate of Complex compounds of platinum metals are a good precursor for the synthesis of solid solutions of metals. The thermal properties of hexa-and tetra-chloroplatinic salts of ammonium and alkali metals have been studied in detail [28]. Alkali metal tetrachloroplatinate(II) is characterised by a disproportionation reaction in the temperature range of 200-400 • C, leading to the formation of Pt, hexa-chloroplatinate(IV), and alkali metal chloride [29]: 2 M 2 [PtCl 4 ] = Pt + M 2 [PtCl 6 ] + 2 MCl.
Therefore, this work is devoted to the synthesis of Ag 2 [PtCl 6 ] and Ag 2 [PtCl 4 ] compoundsprecursors of the bimetallic nanoalloys with precisely assigned ratio of metals-their thermal properties in inert and reducing atmospheres. Ag 0.67 Pt 0.33 nanoalloy which is final product of the precursor compounds thermolysis has been studied in the reactions of preferential (PROX) and total (TOX) CO oxidation. Its formation was investigated upon heating up to 614 • C by in situ X-ray diffraction.

Photo Stability of Complex Compounds
Silver salts are well known to be unstable and to decompose under the daylight. We conducted an experiment to determine the photo stability of the synthesised salts: some amount samples of Ag 2 [PtCl 4 ] and Ag 2 [PtCl 6 ] were placed in a desiccator with P 2 O 5 in a dark place, while others were left in the light. No mass changes were observed for two months, and no formation of new phases. Furthermore, no decrease in the intensity of the reflections of the initial salts was detected on the diffraction pattern as well.
The additional UV-Vis and PL experiments are described in the Supplementary Materials (S2).
Thus, we can talk about the photo stability of the synthesised compounds in relation to light at least of the Ag 2 [PtCl 6 ].

Thermal Properties of Ag 2 [PtCl 4 ] in a Inert Atmosphere
A total of 12.4% of the mass of the substance is lost during the thermolysis of Ag 2 [PtCl 4 ] in an inert atmosphere (helium) in the temperature range of 340-515 • C, which corresponds to the removal of the Cl 2 molecule, and 87.6% of the initial mass remains in the solid, which corresponds to 2 AgCl and Pt ( Figure 1). This assumption is confirmed by X-ray phase analysis.
Molecules 2022, 27, x FOR PEER REVIEW 3 of 16 maintained for the Ag/Pt molar ratio of 0.4-5.1. There have been two particularly stable compositions identified with Ag/Pt ratios of 1.6 and 0.4. Complex compounds of platinum metals are a good precursor for the synthesis of solid solutions of metals. The thermal properties of hexa-and tetra-chloroplatinic salts of ammonium and alkali metals have been studied in detail [28]. Alkali metal tetra-chloroplatinate(II) is characterised by a disproportionation reaction in the temperature range of 200-400 °C, leading to the formation of Pt, hexa-chloroplatinate(IV), and alkali metal chloride [29]: Therefore, this work is devoted to the synthesis of Ag2[PtCl6] and Ag2[PtCl4] compounds-precursors of the bimetallic nanoalloys with precisely assigned ratio of metals-their thermal properties in inert and reducing atmospheres. Ag0.67Pt0.33 nanoalloy which is final product of the precursor compounds thermolysis has been studied in the reactions of preferential (PROX) and total (TOX) CO oxidation. Its formation was investigated upon heating up to 614 °C by in situ X-ray diffraction.

Photo Stability of Complex Compounds
Silver salts are well known to be unstable and to decompose under the daylight. We conducted an experiment to determine the photo stability of the synthesised salts: some amount samples of Ag2[PtCl4] and Ag2[PtCl6] were placed in a desiccator with P2O5 in a dark place, while others were left in the light. No mass changes were observed for two months, and no formation of new phases. Furthermore, no decrease in the intensity of the reflections of the initial salts was detected on the diffraction pattern as well.
The additional UV-Vis and PL experiments are described in the Supplementary Materials (S2).
Thus, we can talk about the photo stability of the synthesised compounds in relation to light at least of the Ag2[PtCl6].

Thermal Properties of Ag2[PtCl4] in a Inert Atmosphere
A total of 12.4% of the mass of the substance is lost during the thermolysis of Ag2[PtCl4] in an inert atmosphere (helium) in the temperature range of 340-515 °C, which corresponds to the removal of the Cl2 molecule, and 87.6% of the initial mass remains in the solid, which corresponds to 2 AgCl and Pt ( Figure 1). This assumption is confirmed by X-ray phase analysis.  It is noteworthy that intense heat is released at a temperature of 365 • C (at the very beginning of the decomposition of the initial compound) which could be associated with a phase transition or another process that is not accompanied by a change in mass. Samples of the complex Ag 2 [PtCl 4 ] weighing 5-14 mg were investigated in the temperature range from 27 to 380 • C. The thermal anomaly with the exo-effect of 662 ± 12 J/g or 366 ± 7 kJ/mol corresponding to the decomposition process at T onset = 358 ± 1 • C was observed on the temperature dependence of the DSC signal ( Figure 2). Irreversibility and completion of the decomposition process was checked by reheating of samples after the first measurement. It is noteworthy that intense heat is released at a temperature of 365 °C (at the very beginning of the decomposition of the initial compound) which could be associated with a phase transition or another process that is not accompanied by a change in mass.
Samples of the complex Ag2[PtCl4] weighing 5-14 mg were investigated in the temperature range from 27 to 380 °C. The thermal anomaly with the exo-effect of 662 ± 12 J/g or 366 ± 7 kJ/mol corresponding to the decomposition process at Tonset = 358 ± 1 °C was observed on the temperature dependence of the DSC signal ( Figure 2). Irreversibility and completion of the decomposition process was checked by reheating of samples after the first measurement.   It is noteworthy that intense heat is released at a temperature of 365 °C (at the very beginning of the decomposition of the initial compound) which could be associated with a phase transition or another process that is not accompanied by a change in mass.
Samples of the complex Ag2[PtCl4] weighing 5-14 mg were investigated in the temperature range from 27 to 380 °C. The thermal anomaly with the exo-effect of 662 ± 12 J/g or 366 ± 7 kJ/mol corresponding to the decomposition process at Tonset = 358 ± 1 °C was observed on the temperature dependence of the DSC signal ( Figure 2). Irreversibility and completion of the decomposition process was checked by reheating of samples after the first measurement.   In order to confirm this assumption, the model complexes were optimised using the Density Functional Theory (DFT). The calculated enthalpy of the proposed reaction gives good agreement with the experiment (3.6 eV or 345 kJ/mol).
In total, 48.3% of the mass is lost in the temperature range of 650-917 • C, which corresponds to AgCl sublimation, and 39.3% of the substance remains, which corresponds to metallic platinum and apparently to the remaining silver chloride (2% of mass). These calculations were also confirmed by X-ray phase analysis ( Figure 3).

Thermal Properties of Ag 2 [PtCl 4 ] in a Hydrogen Atmosphere
Two poorly separated stages occur during the thermolysis of Ag 2 [PtCl 4 ] in a reducing atmosphere in the temperature range of 100-350 • C. According to the mass loss calculations, these stages involve removal of chlorine molecules ( Figure 4). According to X-ray phase analysis, the final product is an FCC phase with the following unit cell parameter: a = 4.024(4) Å, V/Z = 16.3 Å 3 , crystallite size of 4-6 nm, which corresponds to the composition of Ag 0.67 Pt0. 33  In order to confirm this assumption, the model complexes were optimised using the Density Functional Theory (DFT). The calculated enthalpy of the proposed reaction gives good agreement with the experiment (3.6 eV or 345 kJ/mol).
In total, 48.3% of the mass is lost in the temperature range of 650-917 °C, which corresponds to AgCl sublimation, and 39.3% of the substance remains, which corresponds to metallic platinum and apparently to the remaining silver chloride (2% of mass). These calculations were also confirmed by X-ray phase analysis ( Figure 3).

Thermal Properties of Ag2[PtCl4] in a Hydrogen Atmosphere
Two poorly separated stages occur during the thermolysis of Ag2[PtCl4] in a reducing atmosphere in the temperature range of 100-350 °C. According to the mass loss calculations, these stages involve removal of chlorine molecules ( Figure 4). According to X-ray phase analysis, the final product is an FCC phase with the following unit cell parameter: a = 4.024(4) Å, V/Z = 16.3 Å 3 , crystallite size of 4-6 nm, which corresponds to the composition of Ag0.67Pt0.33 (S3).

Thermal Properties of Ag2[PtCl4] in a Inert Atmosphere
The behaviour of Ag2[PtCl6] during thermolysis in an inert atmosphere (helium) is similar to that of Ag2[PtCl4] with the only difference being a lack of intense heat release at the beginning of decomposition. There is a loss of 22.7% of the mass in the temperature range 370-560 °C, which corresponds to the elimination of two molecules of chlorine, and the remaining mass of 77.3% corresponds to a mixture of platinum and silver chloride in the ratio of 1:2, respectively ( Figures 5 and S4).

Thermal Properties of Ag 2 [PtCl 4 ] in a Inert Atmosphere
The behaviour of Ag 2 [PtCl 6 ] during thermolysis in an inert atmosphere (helium) is similar to that of Ag 2 [PtCl 4 ] with the only difference being a lack of intense heat release at the beginning of decomposition. There is a loss of 22.7% of the mass in the temperature range 370-560 • C, which corresponds to the elimination of two molecules of chlorine, and the remaining mass of 77.3% corresponds to a mixture of platinum and silver chloride in the ratio of 1:2, respectively ( Figures 5 and S4). Above 760 °C, further mass loss occurs which corresponds to the sublimation of silver chloride, while only platinum metal remains in the solid residue.  Above 760 • C, further mass loss occurs which corresponds to the sublimation of silver chloride, while only platinum metal remains in the solid residue.

Thermal Properties of Ag 2 [PtCl 6 ] in a Hydrogen Atmosphere
The first stage of Ag 2 [PtCl 6 ] thermolysis in a reducing atmosphere occurs in the temperature range of 170-260 • C, similar to decomposition in a helium atmosphere: 22.7% of the mass is lost, which corresponds to two molecules of chlorine, and the remaining mass of 77.3% corresponds to a mixture of platinum and silver chloride in the ratio 1:2, respectively ( Figure 6). Above 760 °C, further mass loss occurs which corresponds to the sublimation of silver chloride, while only platinum metal remains in the solid residue.

Thermal Properties of Ag2[PtCl6] in a Hydrogen Atmosphere
The first stage of Ag2[PtCl6] thermolysis in a reducing atmosphere occurs in the temperature range of 170-260 °C, similar to decomposition in a helium atmosphere: 22.7% of the mass is lost, which corresponds to two molecules of chlorine, and the remaining mass of 77.3% corresponds to a mixture of platinum and silver chloride in the ratio 1:2, respectively ( Figure 6). In the temperature range of 260-360 °C, 11.4% of the mass is lost, which corresponds to two Cl atoms; thus, the recovery of silver chloride and the formation of a solid solution of silver and platinum in the ratio of 2:1 occur at this stage. In the temperature range of 260-360 • C, 11.4% of the mass is lost, which corresponds to two Cl atoms; thus, the recovery of silver chloride and the formation of a solid solution of silver and platinum in the ratio of 2:1 occur at this stage.
According to X-ray diffraction data (Figure 7), the initial compound remains unchanged when heated to 130 • C, while at 220 • C wide peaks are observed on the diffraction pattern from the platinum phase FCC and AgCl only. An increase in temperature to 250 • C leads to a decrease in the intensity of AgCl reflections, a simultaneous increase in the intensity of the FCC phase, and a shift of the reflections towards smaller angles, which is associated with atoms of silver entering the structure of platinum and forming a solid solution.
At the end point of thermolysis (350 • C), the sample is an FCC phase with the unit cell parameter: a = 4.012(4) Å, V/Z = 16.2 Å 3 , crystallite size of 3-4 nm as per the XRD data, which corresponds to the composition of Ag 0.67 Pt 0.33 according to the calibration line. Further heating causes an increase in the crystallite size to 4-6 nm (500 • C). At the same time, a peak asymmetry appears on the diffraction patterns, which is characterised by an increase in the half-width from large angles. The value V/Z here increases to 16.4 Å 3 , which may indicate the beginning of solid solution stratification, since the resulting nanoalloy is metastable according to the phase diagram [30]. This assumption is confirmed by EDAX analysis at various points of the sample; the relative content of silver and platinum varies from 48:52 to 96:4 at.%. Thus, a sample obtained in a hydrogen atmosphere at 500 • C includes porous branched agglomerates ( Figure 8). Particles with distinguishable observed interplanar distances can be seen in HRTEM images, which correspond at different points to particles of different composition, from pure silver to a solid solution of Ag 0.25 Pt 0.75 . changed when heated to 130 °C, while at 220 °C wide peaks are observed on the diffraction pattern from the platinum phase FCC and AgCl only. An increase in temperature to 250 °C leads to a decrease in the intensity of AgCl reflections, a simultaneous increase in the intensity of the FCC phase, and a shift of the reflections towards smaller angles, which is associated with atoms of silver entering the structure of platinum and forming a solid solution. At the end point of thermolysis (350 °C), the sample is an FCC phase with the unit cell parameter: a = 4.012(4) Å, V/Z = 16.2 Å 3 , crystallite size of 3-4 nm as per the XRD data, which corresponds to the composition of Ag0.67Pt0.33 according to the calibration line. Further heating causes an increase in the crystallite size to 4-6 nm (500 °C). At the same time, a peak asymmetry appears on the diffraction patterns, which is characterised by an increase in the half-width from large angles. The value V/Z here increases to 16.4 Å 3 , which may indicate the beginning of solid solution stratification, since the resulting nanoalloy is metastable according to the phase diagram [30]. This assumption is confirmed by EDAX analysis at various points of the sample; the relative content of silver and platinum varies from 48:52 to 96:4 at.%. Thus, a sample obtained in a hydrogen atmosphere at 500 °C includes porous branched agglomerates ( Figure 8). Particles with distinguishable observed interplanar distances can be seen in HRTEM images, which correspond at different points to particles of different composition, from pure silver to a solid solution of Ag0.25Pt0.75. The sample obtained at 600 °C is an FCC phase with the unit cell parameter: a = 4.012(4)Å, V/Z = 16.2 Å 3 , and crystallite size of 7-13 nm, which, as for the sample at 350 °C, corresponds to Ag0.67Pt0.33 along the calibration line. The peak profile is symmetrical, which indicates a lack of the solid solution stratification.
In situ synchrotron studies were conducted to better understand the processes occurring during the thermolysis of Ag2[PtCl6] in a hydrogen atmosphere.

In Situ X-ray Diffraction Study of the Ag2[PtCl6] Thermolysis in a Hydrogen Atmosphere
Heating in the in situ mode shows that AgCl peaks (36.3 and 52.3° 2θ) already appear in the diffraction pattern ( Figure 9) at 106 °C, in addition to the peaks of the initial complex compound. As the temperature increases, the intensity of the AgCl peaks increases and reaches a maximum at 185 °C, while the intensity of the peaks of the initial complex compound Ag2[PtCl6] decreases. When the temperature reaches 169 °C, the background of the diffraction pattern rises in the area of the angles corresponding to the FCC reflections of the platinum metal phase, while the peaks of the initial compound are almost invisible. At 185 °C, the intensity of the FCC phase peaks increases; an asymmetry of the peaks is manifested at the same time-a shoulder appears from the side of The sample obtained at 600 • C is an FCC phase with the unit cell parameter: a = 4.012(4)Å, V/Z = 16.2 Å 3 , and crystallite size of 7-13 nm, which, as for the sample at 350 • C, corresponds to Ag 0.67 Pt 0.33 along the calibration line. The peak profile is symmetrical, which indicates a lack of the solid solution stratification.
In situ synchrotron studies were conducted to better understand the processes occurring during the thermolysis of Ag 2 [PtCl 6 ] in a hydrogen atmosphere.

In Situ X-ray Diffraction Study of the Ag 2 [PtCl 6 ] Thermolysis in a Hydrogen Atmosphere
Heating in the in situ mode shows that AgCl peaks (36.3 and 52.3 • 2θ) already appear in the diffraction pattern ( Figure 9) at 106 • C, in addition to the peaks of the initial complex compound. As the temperature increases, the intensity of the AgCl peaks increases and reaches a maximum at 185 • C, while the intensity of the peaks of the initial complex compound Ag 2 [PtCl 6 ] decreases. When the temperature reaches 169 • C, the background of the diffraction pattern rises in the area of the angles corresponding to the FCC reflections of the platinum metal phase, while the peaks of the initial compound are almost invisible. At 185 • C, the intensity of the FCC phase peaks increases; an asymmetry of the peaks is manifested at the same time-a shoulder appears from the side of smaller angles, which indicates the formation of metallic silver particles. With a further temperature increase, AgCl is completely reduced to metallic silver (255 °C) and a set of solid solutions is formed in the Ag-Pt system, as supported by wide peaks with a truncated maximum in the diffraction pattern. Furthermore, the maximum is shifted towards smaller angles up to 614 °C, which is evidence of homogenisation and the formation of a solid solution with a clearly defined ratio of metals.
The sample obtained at 614 °C is an FCC phase with the unit cell parameter a = 4.032(4)Å, V/Z = 16.4 Å 3 , which corresponds to the specified composition of Ag0.67Pt0.33, considering the thermal expansion of metals.

Catalytic Properties
The catalytic properties of Ag0.67Pt0.33 nano-powder in CO TOX and PROX were compared with monometallic Pt and Ag nano-powders. The data on CO TOX is presented at Figure 10. It is observed that Ag activity is relatively low, reaching complete conversion of CO only at ca. 280 °C. It can be seen that over Pt, CO is oxidized at T = 160-200 °C. The addition of Ag significantly increases activity; a complete conversion of CO is already observed at 92 °C. As CO TOX over Pt is inhibited by dense surface coverage by adsorbed CO molecules [31], the positive effect of Ag doping could be associated with the realization of CO oxidation reaction at Ag-Pt interface sites. Adsorbed CO molecules over Pt atoms could react with oxygen atoms dissociatively adsorbed over Ag atoms. The presence of both Ag and Pt at essential quantities onto the surface of Ag0.67Pt0.33 was confirmed by XPS data (See Supplementary Materials). These results are in line with the literature data on advanced properties of Ag-Pt catalysts in CO TOX [15]. With a further temperature increase, AgCl is completely reduced to metallic silver (255 • C) and a set of solid solutions is formed in the Ag-Pt system, as supported by wide peaks with a truncated maximum in the diffraction pattern. Furthermore, the maximum is shifted towards smaller angles up to 614 • C, which is evidence of homogenisation and the formation of a solid solution with a clearly defined ratio of metals.
The sample obtained at 614 • C is an FCC phase with the unit cell parameter a = 4.032(4)Å, V/Z = 16.4 Å 3 , which corresponds to the specified composition of Ag 0.67 Pt 0.33 , considering the thermal expansion of metals.

Catalytic Properties
The catalytic properties of Ag 0.67 Pt 0.33 nano-powder in CO TOX and PROX were compared with monometallic Pt and Ag nano-powders. The data on CO TOX is presented at Figure 10. It is observed that Ag activity is relatively low, reaching complete conversion of CO only at ca. 280 • C. It can be seen that over Pt, CO is oxidized at T = 160-200 • C. The addition of Ag significantly increases activity; a complete conversion of CO is already observed at 92 • C. As CO TOX over Pt is inhibited by dense surface coverage by adsorbed CO molecules [31], the positive effect of Ag doping could be associated with the realization of CO oxidation reaction at Ag-Pt interface sites. Adsorbed CO molecules over Pt atoms could react with oxygen atoms dissociatively adsorbed over Ag atoms. The presence of both Ag and Pt at essential quantities onto the surface of Ag 0.67 Pt 0.33 was confirmed by XPS data (See Supplementary Materials). These results are in line with the literature data on advanced properties of Ag-Pt catalysts in CO TOX [15].
Nano-powders were studied in the CO PROX under reducing conditions. CO 2 and H 2 O were the only products of the oxidation reactions of CO and H 2 , respectively. The formation of methane and other hydrogenation products was not observed. The obtained temperature dependences of CO conversion and selectivity are shown in Figure 11. It is seen that Ag is poorly active in CO PROX, predominantly catalysing H 2 oxidation, and the oxidation of CO on pure Pt begins at T > ≈150 • C. In this case, the effect of blocking the Pt surface by adsorbed CO takes place. Complete conversion of CO is observed at temperatures of 190-240 • C. A further increase in temperature leads to an acceleration of the hydrogen oxidation reaction, which absorbs all the oxygen present in the system. Therefore, the decrease in CO conversion at high temperatures on Pt and other samples is due precisely to the competition between the CO and H 2 oxidation reactions and the oxygen deficiency in the system. Nano-powders were studied in the CO PROX under reducing conditions. CO2 and H2O were the only products of the oxidation reactions of CO and H2, respectively. The formation of methane and other hydrogenation products was not observed. The obtained temperature dependences of CO conversion and selectivity are shown in Figure  11. It is seen that Ag is poorly active in CO PROX, predominantly catalysing H2 oxidation, and the oxidation of CO on pure Pt begins at T > ≈150 °C. In this case, the effect of blocking the Pt surface by adsorbed CO takes place. Complete conversion of CO is observed at temperatures of 190-240 °C. A further increase in temperature leads to an acceleration of the hydrogen oxidation reaction, which absorbs all the oxygen present in the system. Therefore, the decrease in CO conversion at high temperatures on Pt and other samples is due precisely to the competition between the CO and H2 oxidation reactions and the oxygen deficiency in the system.   Nano-powders were studied in the CO PROX under reducing conditions. CO2 and H2O were the only products of the oxidation reactions of CO and H2, respectively. The formation of methane and other hydrogenation products was not observed. The obtained temperature dependences of CO conversion and selectivity are shown in Figure  11. It is seen that Ag is poorly active in CO PROX, predominantly catalysing H2 oxidation, and the oxidation of CO on pure Pt begins at T > ≈150 °C. In this case, the effect of blocking the Pt surface by adsorbed CO takes place. Complete conversion of CO is observed at temperatures of 190-240 °C. A further increase in temperature leads to an acceleration of the hydrogen oxidation reaction, which absorbs all the oxygen present in the system. Therefore, the decrease in CO conversion at high temperatures on Pt and other samples is due precisely to the competition between the CO and H2 oxidation reactions and the oxygen deficiency in the system. Ag0.67Pt0.33 nano-powder is more active in CO PROX, starting oxidising CO at ≈50 °C, reaching the maximum conversion of 81% at 117-130 °C, and further oxidising pre- Ag 0.67 Pt 0.33 nano-powder is more active in CO PROX, starting oxidising CO at ≈50 • C, reaching the maximum conversion of 81% at 117-130 • C, and further oxidising predominantly H 2 (Figure 11a). Properties of Ag 0.67 Pt 0.33 with regard to CO oxidation at T < 100 • C under CO TOX and PROX conditions are close, indicating that the excess of H 2 does not affect the catalyst performance at low temperatures while there is oxygen in reaction mixture. At the same time Ag 0.67 Pt 0.33 oxidises H 2 even at 50 • C, accelerating with increased temperature and becoming the predominant process which consumes all oxygen in the reaction mixture at T > 130 • C.
In summary, Ag 0.67 Pt 0.33 is not a prospective catalyst for CO PROX despite high CO TOX activity due to high activity in the undesirable H 2 oxidation reaction. This is in a good agreement with literature data as we have not found any information on good CO PROX performance of Ag-Pt alloy catalysts.

Synthesis of Initial Compounds and Nanoparticles
The source complexes H 2 [PtCl 6 ] and K 2 [PtCl 6 ] were synthesised following the method from [32]. All the initial reagents had ACS or AR purity grade.
The silver cation deposition reaction of the corresponding complex anion is used to obtain the Ag 2 [PtCl 6 ] and Ag 2 [PtCl 4 ] compounds. These compounds are synthesised with a high yield, which indicates their extremely low solubility. They do not dissolve in toluene, acetone or water. Yield

Characterisation of Synthesised Substances
X-ray diffractometric analysis of the samples was performed on a DRON-RM4 diffractometer (Cu-K α radiation, a graphite monochromator using a reflected beam and a scintillation detector with amplitude discrimination, Burevestnik, Saint Petersburg, Russia). The samples were prepared by applying a suspension in alcohol on the polished side of a fused quartz cuvette. A polycrystalline silicon sample (a = 5.4309 Å) prepared in the same way was used as an external reference. The diffraction patterns were recorded in a step-by-step mode in 2θ angles range of 5 • -120 • .
X-ray phase analysis (XRD) of the thermolysis products was carried out in accordance with the data given in the PDF file for pure substances [33]. Parameters of the metal phases were refined over the entire data array using the Powder Cell 2.4 application programme [34]. The size of the crystallites in the obtained metal powders was estimated from the coherent scattering regions as a result of Fourier analysis of the profiles of single diffraction peaks using the WINFIT 1.2.1 software [35].
The composition of the obtained bimetallic solid solutions was determined using calibration curves of the volume per atom ratio (V/Z, where V is the volume of the unit cell, and Z is the number of structural units in it, that is atoms in this case) depending on the concentration of one of the metals. The calibration curves were plotted from the experimental values of atomic volumes for single-phase solid solutions of known composition given in references for Ag-Pt systems [36][37][38].
Thermogravimetric analysis (TGA) in inert and reducing atmospheres was performed using a NETZSCH TG 209 F1 Iris ® thermal microbalance (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany). The sample weight was~10 mg. Al crucibles were used, with the gas flow rate being 60 mL/min and the heating rate of 10 deg/min within the range 20-600 • C.
Differential scanning calorimeter NETZSCH 204 F1 Phoenix (Erich NETZSCH GmbH & Co. Holding KG, Selb, Germany) was used to study thermodynamic properties. DSC measurements were carried out by heat flow measurement method at a 6 K min −1 heating rate in 25 mL min −1 Ar flux in an unsealed aluminium crucible with lid. Powdered samples were distributed uniformly over the bottom, carefully tamped in aluminium crucibles. The sensitivity of the sample carrier sensors and temperature scale graduation were calibrated by melting and crystal-to-crystal transition measurements of standard samples (Hg, Ga, benzoic acid, KNO 3 , In, Sn, Bi, Pb, and Zn). The baseline signal obtained by heating two empty crucibles was subtracted from the experimental results of the samples. Netzsch Proteus Analysis software was used to determine DSC peak areas and transition temperature values. The transition temperature was defined from the resulting heat flow as intersections of the peak onset with the corresponding baseline. The peak area was determined by integrating the area between the measurement curve and the integral tangential baseline. The values of temperatures T onset and enthalpy ∆H are the average of three measurements.
Microscopic studies were carried out using an electron microscope JEM-2010 (with a resolution of 1.4 Å at an accelerating voltage of 200 kV, JEOL Ltd., Tokyo, Japan). Local energy-dispersive X-ray analysis (EDXA) was performed using an EDX spectrometer (EDAX Co., Tokyo, Japan) equipped with a Si(Li) detector with a resolution of 130 eV. The samples for the TEM study were prepared by ultrasonic dispersing in ethanol and consequent deposition of the suspension upon a "holey" carbon film supported on a copper grid.
The thermal decomposition of the complex salt was studied in the XRD mode in situ using an XRK-900 high-temperature reactor chamber (Anton Paar, Austria) mounted on an X-ray powder diffraction instrument in the precise diffractometry system at the Siberian Synchrotron and Terahertz Radiation Centre (SSTRC, Novosibirsk, Russia), and an OD-3M-350 one-coordinate detector [15]. The detector has 3328 channels covering the 2θ angles range of 30 • . The synchrotron radiation wavelength λ is 1.720 Å. XRD patterns were recorded for 60 s. Samples of the complex compound were loaded into an open holder that allowed gas (hydrogen) to pass through, and then the holder was placed in the reactor chamber.

Catalytic Testing
The CO PROX and TOX tests were performed in a fixed-bed continuous-flow quartz reactor (i.d.: 3 mm) at atmospheric pressure. A portion of 50 mg (particle size less than 50 µm) of the catalysts was placed in the reactor between two filters made of fused needle silica. The temperature was measured by a K-type thermocouple in the middle of the catalyst bed. The experiments were performed with the following feed gas composition (vol. %): 1.0 CO, 1.0 O 2 with H 2 as balance for CO PROX, and 1.0 CO, 1.0 O 2 with He as balance for CO TOX, at WHSV of 80,000 cm 3 g −1 h −1 (STP). Prior to the experiments, the samples were pre-treated in He flow for 1 h at 200 • C before CO TOX testing and in H 2 flow for 1 h at 300 • C for CO PROX testing. Then, the samples were cooled down to 200 • C, He (H 2 ) flow was substituted by the reaction feed and the samples were held for 1 h in the reaction mixture. Afterwards, several cycles of temperature decreasing/increasing were performed: all samples demonstrated reproducible catalytic characteristics.
The inlet and outlet concentrations were determined online using a Chromos GC-1000 chromatograph equipped with molecular sieve 5A and Porapak Q columns and with thermal-conductivity (TCD) and flame-ionisation (FID) detectors (Supplementary Materials, S1). The combination of a methanator (containing a reduced nickel catalyst) and the FID allowed highly sensitive analysis of CO and CO 2 . The relative errors of CO, CO 2 , and O 2 concentrations determination were 0.5%, 0.5%, and 1%, respectively. CO 2 and H 2 O were the only products of the reactions; no CH 4 or other hydrocarbons were observed. In the course of experiments, the carbon balance was controlled with an accuracy of ±1%.
The catalyst performance was characterised by the CO conversion (XCO) and selectivity (SCO) in the case of CO PROX which were calculated by the following equations:

Conclusions
Thermolysis of Ag 2 [PtCl 6 ] and Ag 2 [PtCl 4 ] compounds in a helium atmosphere occurs in two stages. At the first stage, the compounds decompose to form platinum and silver chloride and release gaseous chlorine. At the second stage, silver chloride is sublimated, while platinum metal remains in the solid phase. In contrast to the thermolysis of Ag 2 [PtCl 6 ], the thermal degradation of Ag 2 [PtCl 4 ] at 350 • C is accompanied by disproportionation into Ag 2 [PtCl 6 ], silver chloride, and platinum metal. This assumption is confirmed by the change in the reaction enthalpy in the temperature range of 345-375 • C and the corresponding calculations using the Density Functional Theory (DFT). The calculated enthalpy of the proposed reaction gives good agreement with the experiment (calc. 345 kJ/mol, found 366 kJ/mol).
Thermolysis of Ag 2 [PtCl 6 ] and Ag 2 [PtCl 4 ] compounds in a hydrogen atmosphere occurs in two poorly differentiated stages. At low temperatures (below 350 • C), the compounds decompose, and silver is reduced to a metallic state, while a single-phase solid solution of Ag 0.67 Pt 0.33 is formed in ex situ. An increase in the final temperature of thermolysis to 500 • C initiates stratification of the resulting metastable solid solution.
In situ X-ray study of the Ag 2 [PtCl 6 ] thermolysis in a hydrogen atmosphere indicates that AgCl is formed and metallic platinum is released at the first stage. In the case of further heating, AgCl is reduced to metal, with prolonged homogenisation and the formation of a single-phase solid solution of Ag 0.67 Pt 0.33 at 614 • C.
The catalytic activity of the resulting nanoalloy Ag 0.67 Pt 0.33 is studied in the reaction of CO total (TOX) and preferential (PROX) oxidation. Ag 0.67 Pt 0.33 exceeds Pt and Ag nano-powders in activity in CO TOX, but was not selective in CO PROX. Further research could be directed to transition from nanopowders to supported Ag-Pt catalysts and better particle size control; this will help to boost catalytic activity.