Studies on the Formation of Catalytically Active PGM Nanoparticles from Model Solutions as a Basis for the Recycling of Spent Catalysts

The paper presents basic studies on the precipitation of platinum, palladium, rhodium, and ruthenium nanoparticles from model acidic solutions using sodium borohydride, ascorbic acid, and sodium formate as reducing agents and polyvinylpyrrolidone as a stabilizing agent. The size of the obtained PGM particles after precipitation with NaBH4 solution does not exceed 55 nm. NaBH4 is an efficient reducer; the precipitation yields for Pt, Pd, Ru, Rh are 75, 90, 65 and 85%, respectively. By precipitation with ascorbic acid, it is possible to efficiently separate Pt, Rh, and Ru from Pd from the two-component mixtures. The obtained Pt, Pd, and Rh precipitates have the catalytic ability of the catalytic reaction of p-nitrophenol to p-aminophenol. The morphological characteristic of the PGM precipitates was analyzed by AFM, SEM-EDS, and TEM.


Introduction
PGMs (platinum group metals) are among the most important in the metal industry. The main advantages of PGMs include the ability to catalyze various chemical reactions, chemical and mechanical resistance, and resistance to high temperatures [1]. As a result, the demand for these metals is growing, while their natural sources are increasingly limited. An alternative may be the recovery of metals from secondary resources, including spent chemical and petrochemical catalysts [2,3].
PGMs form the active layer of automotive converters. Platinum and/or palladium are responsible for the oxidation of hydrocarbons and CO, while NO x reduction is carried out with rhodium [4]. In search of economic catalytically active metals, research on replacing the expensive Rh (price in October 2021: 13,800 $/Oz) with a cheaper Ru (price in October 2021: 659 $/Oz) has been carried out [5] due to catalysts containing Ru can be used to reduce NO x [6][7][8]. Therefore, in the present studies, except for Pt, Pd and Rh, also Ru was included.
As the synthesis of PGM nanoparticles (NPs) is likely to have an important role in the future for catalytic reactions because nanoparticles are more active than catalysts of a large size [9], the authors have decided to focus on NP formation from leach solutions from secondary resources. The use of smaller than micrometric particles is advantageous and can improve the catalytic properties of the material and decrease the consumption of PGM due to the extended active surface. PGM NPs can be used as catalysts in various reactions: alcohol oxidation, CO oxidation, ethylene glycol reforming (Pt-NPs) [10][11][12], the Suzuki reaction, hydrogenation of alkene (Pd-NPs) [13,14], Fischer-Tropsch reaction (Ru-NPs) [15] and hydrogenation of benzene and phenylacetylene (Rh-NPs) [16]. Metal nanoparticles (NPs) have found applications as catalysts because of their increased selectivity and activity because the polymer contains hydrophobic carbon chains that extend into the solvents and interact with each other as a steric hindrance effect [40,41].

Synthesis of Nanoparticles
The stabilizing agent PVP was added to the appropriate amount of the PGM precursor and mixed for 10 min. After proper mixing, the reducer was added drop by drop, at the mixing speed of 500 rpm. The pH of the solution was then adjusted to neutral/basic (pH 7-8) with 1 M Na 2 CO 3 . The molar ratio of the metal precursor to the reducing agent and to the stabilizing agent was 1:(1 or 2):(1 or 5) depending on the experiment. The precipitation yield (P) was calculated according to the formula: where m 0 is the initial mass of the metal ions in the solution before precipitation and m p is the mass of the metal ions in the solution after precipitation. The mass change of the metal ions in the solution before and after precipitation was calculated on the basis of the AAS technique (Atomic Absorption Spectrometry) determination of the metal ion concentrations. The samples after precipitation were centrifuged before AAS analysis (time: 15 min, 15,700 rpm).

Catalytic Reaction
The prepared precipitates were subjected to the catalytic reaction of p-nitrophenol (NPh) to p-aminophenol (APh) in the presence of NaBH 4 as a hydrogen ion donor source following the reduction reaction: The reduction of NPh to APh by sodium borohydride is a thermodynamically favorable reaction (E 0 for 4-nitrophenol/4-aminophenol −0.76 V) [42]. The mechanism includes adsorption of both reactants (NPh and BH 4 − ions) on the surface of PGM-NPs before NPh reduction, followed by the production of hydrogen radicals by electron transfer. Hydrogen species are added to adsorbed NPhs to form APh through the formation of the 4-hydroxylaminophenol intermediate and the removal of two water molecules from the nitro group. Finally, the product-APh-is detached from the surface of PGM-NP to provide a place for another catalytic cycle [43,44].
NaOH was added to the resulting solution to raise pH to 11.5 and keep NPh in anionic form. The UV-Vis spectrum of the basic form of NPh has a maximum at a wavelength of 400 nm. The spectra were recorded 25 min after the initiation of the reaction to monitor changes in NPh concentration, i.e., the addition of the reducing agent and the appropriate amount of PGM-NP catalyst. The conversion degree (α NPh ) was calculated from Equation (3): where C 0,NPh is the concentration of NPh before the reaction and C p,NPh is the concentration of NPh after the reaction.

Apparatus
The atomic absorption spectrometer (ContrAA 300, Analytik Jena, Jena, Germany) was used for the measurement of metal ion concentrations in aqueous samples at the following wavelengths: 266.0, 244.8, 343.5, 349.9, nm for Pt(IV), Pd(II), Rh(III), Ru(III) respectively. SEM-EDS (SEM FEI Quanta 250 FEG), digital microscope of the VHX-7000 series (Keyence International, Mechelen, Belgium), Hitachi HT7700 transmission electron microscope (Hitachi, Tokyo, Japan) working in high contrast and high-resolution mode, and AFM (NX10, Park Systems, Mannheim, Germany) were applied to analyze the structure and morphology of nanoparticles obtained of precipitation stages from model solutions. The solutions after reduction were analyzed by UV-Vis (spectrophotometer, Specord 40, Analytik Jena, Jena, Germany) to confirm the presence of nanoparticles.

Effect of PVP Concentration on PGM-NP Formation
The results of the production of palladium particles without a stabilizing agent from a one-component solution (2 mM Pd(II) in 0.1 M HCl) confirmed a positive impact of formic acid (4 mM HCOOH) as a palladium reducing agent. The volume ratio of the precursor and the precipitant was 1:1. The tests were carried out without the addition of a stabilizing agent at two different temperatures, 23 and 50 • C. The images taken under the optical microscope showed that the precipitated particles are metallic (Figure 1a). Additionally, a 3D photo of the obtained sediment was taken, which allowed to estimate the size of the precipitated particles from several dozen to several hundred microns (Figure 1b).
The effect of solution pH on the precipitation yields of PGM without stabilizing agent is shown in Figure 2. In an acidic solution (pH 3.5), the yields of Pd precipitation (Equation (1)) do not exceed 50%, however, as the pH increased to slightly acidic and then to alkaline, the amount of precipitated Pd increases. The increase in temperature significantly affected the precipitation only at pH 6.5, while the precipitation efficiency values at pH 3.5 and 8 are similar. The alkaline pH seems to have a more significant influence on the synthesis of NP than the temperature, which was also observed by other researchers [45].
The studies on Pd precipitation with HCOOH show that the metallic particles, larger than the nanosize, are obtained. Therefore, to obtain Pd-NP, the addition of a PVP stabilizing agent was proposed, as also suggested by other researchers [46][47][48]. The appropriate ratio of the stabilizing agent to the precursor may influence the morphology, structure, and particle size of the reduced material. For this purpose, the influence of the concentration of PVP on the precipitation yield was studied and is shown in Figure 3. respectively. SEM-EDS (SEM FEI Quanta 250 FEG), digital microscope of the VHX-7000 series (Keyence International, Mechelen, Belgium), Hitachi HT7700 transmission electron microscope (Hitachi, Tokyo, Japan) working in high contrast and high-resolution mode, and AFM (NX10, Park Systems, Mannheim, Germany) were applied to analyze the structure and morphology of nanoparticles obtained of precipitation stages from model solutions. The solutions after reduction were analyzed by UV-Vis (spectrophotometer, Specord 40, Analytik Jena, Jena, Germany) to confirm the presence of nanoparticles.

Effect of PVP Concentration on PGM-NP Formation
The results of the production of palladium particles without a stabilizing agent from a one-component solution (2 mM Pd(II) in 0.1 M HCl) confirmed a positive impact of formic acid (4 mM HCOOH) as a palladium reducing agent. The volume ratio of the precursor and the precipitant was 1:1. The tests were carried out without the addition of a stabilizing agent at two different temperatures, 23 and 50 °C. The images taken under the optical microscope showed that the precipitated particles are metallic ( Figure 1a). Additionally, a 3D photo of the obtained sediment was taken, which allowed to estimate the size of the precipitated particles from several dozen to several hundred microns ( Figure 1b).
The effect of solution pH on the precipitation yields of PGM without stabilizing agent is shown in Figure 2. In an acidic solution (pH 3.5), the yields of Pd precipitation (Equation (1)) do not exceed 50%, however, as the pH increased to slightly acidic and then to alkaline, the amount of precipitated Pd increases. The increase in temperature significantly affected the precipitation only at pH 6.5, while the precipitation efficiency values at pH 3.5 and 8 are similar. The alkaline pH seems to have a more significant influence on the synthesis of NP than the temperature, which was also observed by other researchers [45].
(a)  The studies on Pd precipitation with HCOOH show that the metallic particles, larger than the nanosize, are obtained. Therefore, to obtain Pd-NP, the addition of a PVP stabilizing agent was proposed, as also suggested by other researchers [46][47][48]. The appropriate ratio of the stabilizing agent to the precursor may influence the morphology, structure, and particle size of the reduced material. For this purpose, the influence of the concentration of PVP on the precipitation yield was studied and is shown in   The studies on Pd precipitation with HCOOH show that the metallic partic than the nanosize, are obtained. Therefore, to obtain Pd-NP, the addition of a P lizing agent was proposed, as also suggested by other researchers [46][47][48]. The ate ratio of the stabilizing agent to the precursor may influence the morphology, and particle size of the reduced material. For this purpose, the influence of the c  Regardless of the reducer used, in all cases the efficiency of Pd precipitationndecreases with the increasing concentration of PVP. In the case of Pt and Ru, the type of the reducer and the PVP concentration do not significantly affect the precipitation efficiency.
On the one hand, AA is an efficient reducer for platinum (PPt > 80%), on the other hand, AA is an inefficient reducer for palladium (PPd < 10%) (Figure 3a). This phenomenon can be helpful in the separation of platinum from palladium from the aqueous two-component solutions. The efficient reductor for Rh is NaBH4 and the precipitation efficiency of Rh is greater than 80%. The most pronounced change in the precipitation efficiency can be seen for Pd reduced with HCOONa. The reduction efficiency decreases as the molar ratio of PVP increases from 100 (for molar ratio of PGM:PVP:reducer was 1:1:2) to 20% (PGM:PVP:reducer was 1:5:2).

Effect of the Reducer on the Formation of PGM-NP
The influence of the type and concentration of a reducer on the formation of PGM-NP was investigated and the results for the precipitation of PGM from acidic solutions are presented in Table 1.  Regardless of the reducer used, in all cases the efficiency of Pd precipitationndecreases with the increasing concentration of PVP. In the case of Pt and Ru, the type of the reducer and the PVP concentration do not significantly affect the precipitation efficiency.
On the one hand, AA is an efficient reducer for platinum (P Pt > 80%), on the other hand, AA is an inefficient reducer for palladium (P Pd < 10%) (Figure 3a). This phenomenon can be helpful in the separation of platinum from palladium from the aqueous two-component solutions. The efficient reductor for Rh is NaBH 4 and the precipitation efficiency of Rh is greater than 80%. The most pronounced change in the precipitation efficiency can be seen for Pd reduced with HCOONa. The reduction efficiency decreases as the molar ratio of PVP increases from 100 (for molar ratio of PGM:PVP:reducer was 1:1:2) to 20% (PGM:PVP:reducer was 1:5:2).

Effect of the Reducer on the Formation of PGM-NP
The influence of the type and concentration of a reducer on the formation of PGM-NP was investigated and the results for the precipitation of PGM from acidic solutions are presented in Table 1. The concentration of the reducer has no significant influence on the precipitation yield for all PGMs. The most effective reducing agent for all metals is NaBH 4 followed by HCOONa. However, due to the fact that AA shows low efficiency in the precipitation of Pd and Rh and has the potential to separate PGM by precipitation from their mixtures, it was chosen as a weak reducing agent for further research. For two selected reducers (AA and NaBH 4 ) AFM analysis was used to determine the size of the obtained PGM particles. The results are presented in Table 2. All obtained PGM-particles are nanoparticles; their size is below 90 nm. Pt-NPs after precipitation with NaBH 4 are smaller than Pt-NPs after precipitation with AA. Of all samples, the smallest particles of 3 nm are obtained for Pd-NPs using AA or NaBH 4 as reducing agent (molar ratio PGM:PVP:reducer 1:1:2). The largest particles for Ru are obtained after reduction with AA and are around 90 nm (molar ratio of PGM:PVP:reducer 1:1:2). Thus, the type of the reducer used affects in a significant way the size of the particles obtained. The reduction with NaBH 4 and the PGM:PVP:reducer 1:1:2 molar ratio, 7-8 pH, ambient temperature are the best conditions for the formation of Pt, Pd, and Rh-NPs up to 5 nm. The change in the molar ratio of PVP to PGM from 2 to 5 does not result in a significant difference in the size of Rh NPs, while an increase in the NP size of Pt and Pd is observed. On the other hand, the conditions for the effective formation of the smallest Ru-NPs with a size of up to 4 nm cover the reduction with NaBH 4 and the molar ratio of PGM:PVP:reducer 1:1:1, 7-8 pH, ambient temperature. Moreover, in this case, no significant influence of PVP ratio on the size of the obtained particles is observed. AFM images, confirming the particle size, are presented in the Appendix A ( Figure A1).
TEM images are taken for some selected samples to check the structure and compare the particle size estimated with AFM technique (Figure 4). TEM images are taken for some selected samples to check the structure and compare the particle size estimated with AFM technique (Figure 4). TEM images of Pt or Pd-NP are used to determine the structure and size of the obtained particles. Particles precipitated with AA (Figure 4a,c) are spherical and have similar sizes of 7 nm. On the contrary, the particles precipitated with NaBH4 (Figure 4b,d) have a larger size range than these precipitated with AA, which are below 10 nm. In the case of Ru ( Figure A2 in the Appendix), the TEM images do not show clear structures of metal clusters as in the case of Pt, Pd and Rh (Figure 4). It can be seen that agglomerates of Rh-NP were also formed. Despite strong reducing agents, such as NaBH4, affecting the synthesis of smaller nanoparticles compared to weak reducing agents (e.g., AA), due to the high thermodynamic instability and excess surface energy, the synthesized particles can TEM images of Pt or Pd-NP are used to determine the structure and size of the obtained particles. Particles precipitated with AA (Figure 4a,c) are spherical and have similar sizes of 7 nm. On the contrary, the particles precipitated with NaBH 4 (Figure 4b,d) have a larger size range than these precipitated with AA, which are below 10 nm. In the case of Ru ( Figure A2 in the Appendix A), the TEM images do not show clear structures of metal clusters as in the case of Pt, Pd and Rh (Figure 4). It can be seen that agglomerates of Rh-NP were also formed. Despite strong reducing agents, such as NaBH 4 , affecting the synthesis of smaller nanoparticles compared to weak reducing agents (e.g., AA), due to the high thermodynamic instability and excess surface energy, the synthesized particles can undergo nucleation and Ostwald ripening [49]. These phenomena lead to particle agglomeration, which can explain the larger agglomerates in samples after precipitation with NaBH 4 than with AA. The trend of PGM particles to agglomerate has been observed by various researchers under various conditions [50][51][52][53]; it was found that agglomeration (for example of Pt-NPs) positively influenced the catalytic activity of the material in the CO oxidation reaction due to the high density of defects on the surfaces of the catalytic material [53].
Values of the particle size estimated from TEM images indicate that the NPs are much smaller than these estimated on the basis of AFM images. It is possible that the size obtained by AFM corresponded to aggregates in which single nanoparticles could not be observed. Due to a higher resolution of the TEM images than of the AFM ones, it is possible to distinguish between much smaller particles [54].
SEM-EDS analysis was used to confirm the presence of Pt in the metallic form of nanoparticles ( Figure 5).
Molecules 2022, 27, x FOR PEER REVIEW 9 of 21 undergo nucleation and Ostwald ripening [49]. These phenomena lead to particle agglomeration, which can explain the larger agglomerates in samples after precipitation with NaBH4 than with AA. The trend of PGM particles to agglomerate has been observed by various researchers under various conditions [50][51][52][53]; it was found that agglomeration (for example of Pt-NPs) positively influenced the catalytic activity of the material in the CO oxidation reaction due to the high density of defects on the surfaces of the catalytic material [53]. Values of the particle size estimated from TEM images indicate that the NPs are much smaller than these estimated on the basis of AFM images. It is possible that the size obtained by AFM corresponded to aggregates in which single nanoparticles could not be observed. Due to a higher resolution of the TEM images than of the AFM ones, it is possible to distinguish between much smaller particles [54].
SEM-EDS analysis was used to confirm the presence of Pt in the metallic form of nanoparticles ( Figure 5). When the SEM-EDS images are compared with each other, it can be seen that oxygen and Pt do not overlap in the images. This indicates that the particles obtained do not form oxides but only metallic forms. In addition, the images show brighter points that can be attributed to the clusters of metals. Both TEM and SEM confirmed that the metal agglomerates formed during precipitation. A similar situation can be seen for Pd, Rh, and Ru-NPs, the presented images confirm that the obtained NPs are not metal oxides ( Figure A3 in the Appendix). When the SEM-EDS images are compared with each other, it can be seen that oxygen and Pt do not overlap in the images. This indicates that the particles obtained do not form oxides but only metallic forms. In addition, the images show brighter points that can be attributed to the clusters of metals. Both TEM and SEM confirmed that the metal agglomerates formed during precipitation. A similar situation can be seen for Pd, Rh, and Ru-NPs, the presented images confirm that the obtained NPs are not metal oxides ( Figure A3 in the Appendix A).

Two-Component Mixtures
The influence of the presence of two different PGMs in the stock solutions was investigated, and the results of the precipitation of both PGMs from acidic solutions are presented in Table 3. Using precipitation with AA as a reducer, platinum, rhodium or ruthenium can be separated from palladium in two-component mixture. In all cases, the precipitation yield with NaBH 4 is high for both PGMs in two-component mixture, therefore their separation is impossible. TEM images were taken for selected two-component samples ( Figure 6) to compare the structure with single-component nanoparticles (Figure 4). Using precipitation with AA as a reducer, platinum, rhodium or ruthenium can be separated from palladium in two-component mixture. In all cases, the precipitation yield with NaBH4 is high for both PGMs in two-component mixture, therefore their separation is impossible. TEM images were taken for selected two-component samples (Figure 6) to compare the structure with single-component nanoparticles (Figure 4).
The materials shown in the TEM images have a structure similar to single-component nanoparticles ( Figure 6). As in the previous images, the most agglomerated samples are obtained after reduction with NaBH4. TEM shows multilayer particle clusters ( Figure  6a,b,d), as well as long agglomerate chains (Figure 6b). Only Pd/Rh-NPs after precipitation with AA does not form larger agglomerates. The particle size of the Pd/Rh-NPs synthesized in the two-component mixture differs in the range from 1 to 10 nm. However, the Pt/Rh-NPs size is centered more closely around 5 nm.
TEM images of Rh/Ru-NP after precipitation with AA and NaBH4 are presented in Figure A4 in the Appendix. As in the case of a single-component solution with Ru-NP ( Figure A2c,d), the TEM images do not show clear structures and metal clusters, although the Rh was agglomerating in a single mixture. SEM-EDS images show rich, uniform precipitates of Pt-NPs in the sample ( Figure  7a). The images also demonstrate bright structures from Rh particles (Figure 7a,b) which correspond to the metal clusters. On the other hand, Pd particles occur pointwise. By comparing the precipitation yield results and the SEM images, it can be confirmed that both Pt and Pd co-precipitated from Rh using NaBH4 as a reducing agent. Figure A5 in the The materials shown in the TEM images have a structure similar to single-component nanoparticles ( Figure 6). As in the previous images, the most agglomerated samples are obtained after reduction with NaBH 4 . TEM shows multilayer particle clusters (Figure 6a,b,d), as well as long agglomerate chains (Figure 6b). Only Pd/Rh-NPs after precipitation with AA does not form larger agglomerates. The particle size of the Pd/Rh-NPs synthesized in the two-component mixture differs in the range from 1 to 10 nm. However, the Pt/Rh-NPs size is centered more closely around 5 nm.
TEM images of Rh/Ru-NP after precipitation with AA and NaBH 4 are presented in Figure A4 in the Appendix A. As in the case of a single-component solution with Ru-NP ( Figure A2c,d), the TEM images do not show clear structures and metal clusters, although the Rh was agglomerating in a single mixture.
SEM-EDS images show rich, uniform precipitates of Pt-NPs in the sample (Figure 7a). The images also demonstrate bright structures from Rh particles (Figure 7a,b) which correspond to the metal clusters. On the other hand, Pd particles occur pointwise. By comparing the precipitation yield results and the SEM images, it can be confirmed that both Pt and Pd co-precipitated from Rh using NaBH 4 as a reducing agent. Figure A5 in the Appendix A indicates the images of SEM-EDS Pt/Rh-NP and Pd/Rh-NP after precipitation with AA. Pt/Rh-NPs are similar to those precipitated with NaBH 4 and both Pt and Rh rich precipitates are visible. Images of Pd/Rh-NPs reduced with AA show that Pd does not precipitate in a metallic form, but forms a likely soluble Pd compound, which is confirmed by small values of precipitation yield (Table 3).
SEM-EDS images show rich, uniform precipitates of Pt-NPs in the sample ( Figure  7a). The images also demonstrate bright structures from Rh particles (Figure 7a,b) which correspond to the metal clusters. On the other hand, Pd particles occur pointwise. By comparing the precipitation yield results and the SEM images, it can be confirmed that both Pt and Pd co-precipitated from Rh using NaBH4 as a reducing agent. Figure A5 in the Appendix indicates the images of SEM-EDS Pt/Rh-NP and Pd/Rh-NP after precipitation with AA. Pt/Rh-NPs are similar to those precipitated with NaBH4 and both Pt and Rh rich precipitates are visible. Images of Pd/Rh-NPs reduced with AA show that Pd does not precipitate in a metallic form, but forms a likely soluble Pd compound, which is confirmed by small values of precipitation yield (Table 3).

Catalytic Properties of PGM-NPs
An important characterization of the obtained PGM-NPs was the testing of their catalytic activity. An exemplary reduction reaction of NPh to APh was carried out in the presence of the synthesized nanoparticles. The course of the reaction was monitored by registration of UV-Vis spectra. The reference spectra of NPh and APh at pH 11.5 are compared to the spectra of the solutions during reaction after 1 or 30 min of reduction ( Figure  8).

Catalytic Properties of PGM-NPs
An important characterization of the obtained PGM-NPs was the testing of their catalytic activity. An exemplary reduction reaction of NPh to APh was carried out in the presence of the synthesized nanoparticles. The course of the reaction was monitored by registration of UV-Vis spectra. The reference spectra of NPh and APh at pH 11.5 are compared to the spectra of the solutions during reaction after 1 or 30 min of reduction (Figure 8).

Catalytic Properties of PGM-NPs
An important characterization of the obtained PGM-NPs was the testing of their catalytic activity. An exemplary reduction reaction of NPh to APh was carried out in the presence of the synthesized nanoparticles. The course of the reaction was monitored by registration of UV-Vis spectra. The reference spectra of NPh and APh at pH 11.5 are compared to the spectra of the solutions during reaction after 1 or 30 min of reduction ( Figure  8). The maximum wavelength at 400 nm is attributed to NPh (in the sample before the reaction, pH 11.5) which shifted to 313 nm during the first minutes of reaction corresponding to an acidic form of NPh [42,44,55]. A maximum observed at 300 nm for the APh model APh solution is attributed to an acidic form of APh at 1.5 pH (Figure 8a).
A hypsochromic shift of the NPh maximum to lower wavelengths during the reaction was observed. The maximum at 260 nm corresponds to the basic (ionized) form of APh at 11.5 pH; it is clearly visible that, at the beginning of the reduction (after 1 min, Figure 8a), APh is not registered in the UV-Vis spectrum. However, the conversion of nitro groups to amino groups in the course of the reaction and, as a result, the significant decrease in NPh The maximum wavelength at 400 nm is attributed to NPh (in the sample before the reaction, pH 11.5) which shifted to 313 nm during the first minutes of reaction corresponding to an acidic form of NPh [42,44,55]. A maximum observed at 300 nm for the APh model APh solution is attributed to an acidic form of APh at 1.5 pH (Figure 8a).
A hypsochromic shift of the NPh maximum to lower wavelengths during the reaction was observed. The maximum at 260 nm corresponds to the basic (ionized) form of APh at 11.5 pH; it is clearly visible that, at the beginning of the reduction (after 1 min, Figure 8a), APh is not registered in the UV-Vis spectrum. However, the conversion of nitro groups to amino groups in the course of the reaction and, as a result, the significant decrease in NPh concentration decrease (decrease at maximum at 400 nm), are visible after 30 min of reduction ( Figure 8b).
The degree of NPh conversion calculated according to Equation (3) proves that Pd-NPs provide the highest conversion degree of NPh conversion (about 90% after 5 min of reduction) compared to the NPs of other PGMs (Figure 9). The lowest conversion of NPh is reached when Ru particles are used. The order of the decreasing catalytic activity of the NPs is as follows: Pd > Rh~Pt >> Ru.
Research related to the separation of PGM nanoparticles from aqueous solutions by precipitation of PGM precipitation may be used in the future to develop a method to obtain PGM from solutions after leaching spent automotive catalysts. The perspective of recovering metals (especially PGM) from secondary resources is beneficial not only for the environment (less waste), but also for the economy (PGM recycling). It should be emphasized that the content of these valuable metals in waste (spent automotive converters) is several times higher than the content of PGM content in the richest available natural ores available in the world.
The degree of NPh conversion calculated according to Equation (3) proves that Pd-NPs provide the highest conversion degree of NPh conversion (about 90% after 5 min of reduction) compared to the NPs of other PGMs (Figure 9). The lowest conversion of NPh is reached when Ru particles are used. The order of the decreasing catalytic activity of the NPs is as follows: Pd > Rh ~ Pt >> Ru. Research related to the separation of PGM nanoparticles from aqueous solutions by precipitation of PGM precipitation may be used in the future to develop a method to obtain PGM from solutions after leaching spent automotive catalysts. The perspective of recovering metals (especially PGM) from secondary resources is beneficial not only for the environment (less waste), but also for the economy (PGM recycling). It should be emphasized that the content of these valuable metals in waste (spent automotive converters) is several times higher than the content of PGM content in the richest available natural ores available in the world.

Conclusions
NaBH4 is an efficient reducer for all PGMs; the precipitation yields of Pt, Pd, Ru, Rh are 75, 90, 65, and 85%, respectively. The size of the PGM particles after precipitation with NaBH4 solution did not exceed 55 nm and the sizes of the PGM particles of PGM after precipitation with AA were below 90 nm. Precipitation of PGM with AA can be used for the separation of platinum from palladium from aqueous acidic solutions.
The development of the PGM nanoparticle precipitation method is very important for the development of knowledge in the field of separation processes, as it makes it possible to use aqueous solutions containing PGM (e.g., after leaching various waste materials-spent automotive converters) to produce catalytically active materials. The obtained Pt-NP, Pd-NP, and Rh-NP have the catalytic ability to catalyze the reaction of NPh to APh.

Conclusions
NaBH 4 is an efficient reducer for all PGMs; the precipitation yields of Pt, Pd, Ru, Rh are 75, 90, 65, and 85%, respectively. The size of the PGM particles after precipitation with NaBH 4 solution did not exceed 55 nm and the sizes of the PGM particles of PGM after precipitation with AA were below 90 nm. Precipitation of PGM with AA can be used for the separation of platinum from palladium from aqueous acidic solutions.
The development of the PGM nanoparticle precipitation method is very important for the development of knowledge in the field of separation processes, as it makes it possible to use aqueous solutions containing PGM (e.g., after leaching various waste materialsspent automotive converters) to produce catalytically active materials. The obtained Pt-NP, Pd-NP, and Rh-NP have the catalytic ability to catalyze the reaction of NPh to APh.

Conflicts of Interest:
The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are available from the corresponding author.

Appendix A
Atomic force microscope (AFM) images of PGM-NP after precipitation with NaBH4 are presented in Figure A1. TEM images of Rh-NP and Ru-NP after a precipitation with AA and NaBH 4 are presented in Figure A2.
SEM coupled with EDS images of Pd-NP, Rh-NP, and Ru-NP after a precipitation with AA and NaBH 4 are presented in Figure A3.
TEM images of Rh/Ru-NP after a precipitation with AA and NaBH 4 are presented in Figure A4.
SEM coupled with EDS images of Pt/Rh-NP, Pd/Rh-NP, and Rh/Ru-NP after a precipitation with AA and NaBH 4 are presented in Figure A5. TEM images of Rh-NP and Ru-NP after a precipitation with AA and NaBH4 are presented in Figure A2. SEM coupled with EDS images of Pd-NP, Rh-NP, and Ru-NP after a precipitation with AA and NaBH4 are presented in Figure A3. TEM images of Rh/Ru-NP after a precipitation with AA and NaBH4 are presented in Figure A4.  TEM images of Rh/Ru-NP after a precipitation with AA and NaBH4 are presented in Figure A4.   SEM coupled with EDS images of Pt/Rh-NP, Pd/Rh-NP, and Rh/Ru-NP after a precipitation with AA and NaBH4 are presented in Figure A5.