X-ray Photoelectron Spectroscopy ( XPS ) Study of the Products Formed on Sulfide Minerals Upon the Interaction with Aqueous Platinum ( IV ) Chloride Complexes

The interaction of aqueous solutions bearing platinum-group elements (PGEs) with sulfides is important for understanding the formation and weathering of PGE ore deposits, mineral processing, and synthesis of nanomaterials. Here, the surface species formed upon the contact of the main sulfide minerals (pyrite, pyrrhotite, galena, chalcopyrite and valleriite) with the solutions of H2PtCl6 (pH 1.5, 20 ◦C) have been studied using X-ray photoelectron spectroscopy (XPS). Uptake of Pt increased gradually with increasing interaction time, and depended, as well as the composition of immobilized products, on the mineral nature and the state of its surface, e.g., the chemical pre-treatment. The highest rate of Pt deposition was observed on galena and valleriite and the lowest on pyrite and pyrrhotite. The preliminary moderate oxidation of pyrrhotite promoted Pt deposition, which, however, was hindered under harsh reaction conditions. The pre-oxidation of pyrite in all cases resulted in a decrease of the Pt deposition. Initially, Pt(IV) chloride complexes adsorb onto the mineral surface, and then the reduction of Pt(IV) to Pt(II) and substitution of chloride ions with sulfide groups occur forming sulfides of Pt(II) and then, Pt(IV). The reduction of Pt species to the metallic state was observed at valleriite after 24 h, probably due the negative charge of the sulfide nanolayers of this sulfide-hydroxide composite mineral.

Platinum-group elements are commonly associated with base metal sulfides, particularly pentlandite, pyrrhotite, chalcopyrite, pyrite [15,25,26].Palladium is hosted in base metal sulfides (mainly pentlandite) and occurs both within the crystal lattice and as nanometer-sized inclusions of discrete PGE minerals [27].Platinum usually has low concentrations within base metal sulfides and mainly forms discrete platinum-group minerals (PGMs).Most work has been focused on the experimental study and mathematical modeling of Pt dissolution, and stability of its complex compounds with ligands such as HS − , Cl − , S 2 O 3 2− , OH − , NH 3 [11,12,17,18,22,28,29].At low temperature conditions, acidic pHs and high redox potential, Pt transport by chloride ions is the most suitable [18,30].
In addition to the understanding of mobilization and deposition of PGEs, the interaction of Pt-bearing solutions with base metal sulfides is important for mineral processing of PGEs by heap leaching [31][32][33][34][35][36][37] and concentrator plants [35].Platinum-group elements transfer into solution by leaching (both under technological and natural conditions) can then be taken up on the surface of sulfide minerals.However, in contrast to the vast literature devoted to the interaction of aqueous solutions of gold and silver with sulfides (see, e.g., [38][39][40] and references therein), only limited studies describe the deposition of PGEs, on the surface of pyrite, galena and sphalerite by X-ray photoelectron spectroscopy (XPS) [16,41].By the interaction of the Na 2 PdCl 4 solution (pH 4) with pyrite, sulfide species of Pd(II) are formed at room temperature along with retaining mixed aquaand hydrochloride complexes of Pd(II) [16].Recently [41], the interaction of the acidic solution of H 2 PdCl 4 solution with pyrrhotite and pyrite before and after their preliminarily leaching under different conditions was studied.Palladium-oxyhydroxides are not formed, though it is not completely proved since the differences in the binding energies of the Pd 3d line of oxides and sulfides are not significant [42,43].So far, no studies analyzing Pt species immobilized from aqueous solutions onto sulfide minerals, and kinetics of their interaction have been carried out yet.
The aim of this study to investigate the chemical species of Pt formed on the surface of sulfide minerals, including the preliminarily oxidized ones, upon their contact with aqueous Pt(IV)-chloride complexes.This knowledge is important for understanding processes leading to the loss of PGEs in the processing of sulfide ores, affinage, and the behavior of PGEs in tailing ponds where they are leached under near-surface conditions [5,6].The interaction of metal sulfides with the solutions of Pt-containing complex compounds is of interest for preparation of composite nanomaterials and their potential use in (electro)catalysis, electronics [1][2][3].
For the preliminary oxidation the minerals were prepared in the same manner as for XPS.To oxidize, leaching was made using the solution of 0.25 M FeCl 3 + 1 M HCl during 30 min at 50 • C, and then, the sample was rinsed with the solution of 1 M HCl to remove Fe ions and, consequently, to prevent the formation of the products of Fe(III) hydrolysis.Then, the sample was rinsed with distilled water and placed into the H 2 PtCl 6 solution for a specified period of time, usually, 2-4 h.The electrochemical oxidation was carried out using an Elins P30 SM potentiostat (Elins, Moscow, Russia) in the 1 M HCl solution at a constant potential over 10 minutes.A Pt wire was used as a counter electrode and a saturated Ag/AgCl electrode was used as the reference electrode; all potentials are given relative to that.After the electrochemical oxidation the mineral sample was rinsed with distilled water and used further for depositing Pt.
The XPS were acquired using a SPECS spectrometer equipped with a PHOIBOS 150 MCD-9 analyzer (SPECS, Berlin, Germany) at an electron take-off angle of 90 • employing monochromatic Mg Kα radiation (1253.6 eV) of an X-ray tube operated at 180 W. The analyzer pass energy was 10 eV for narrow scans, and 20 eV for survey spectra.The C 1s peak at 284.45 eV from hydrocarbon contaminations was used as a reference.The Pt 4f 7/2,5/2 , S 2p 3/2,1/2 doublets were fitted, after the subtraction of the Shirley-type background, with the coupled peaks with the Gaussian-Lorentzian peak profiles, the spin-orbit splitting of 3.30 eV and 1.19 eV, and the branching ratios of 0.75 and 0.5, respectively, using the CasaXPS software package (version 2.3.16,Casa Software, Teignmouth, UK).The element concentrations on the surface were determined from the wide scan using the empirical sensitivity coefficients.Some photoelectron spectra were measured at the Russian-German laboratory at the dipole magnet beamline of the synchrotron radiation facility BESSY II (Berlin, Germany).Tapping-mode atomic force microscopy (AFM) studies were conducted using a Solver P-47 multimode scanning probe microscope (Nanotekhnologiya MDT, Moscow, Russia) in air at room temperature.A silicon cantilever with the resonance frequencies of 150-250 kHz was used as a probe.

Analysis of the Surface of Pyrite and Pyrrhotite Using X-ray Photoelectron Spectroscopy and Atomic Force Microscopy
The XPS spectra show that both Pt and Cl can be detected on the surface of sulfide minerals after the aqueous media treatment (Figure 1).Typically, Cl − ions are weakly adsorbed on the metal sulfide surface and/or can easily be removed by washing with water [39].Thus, Cl observed in the spectra largely belongs to adsorbed Pt-Cl complexes.The XPS were acquired using a SPECS spectrometer equipped with a PHOIBOS 150 MCD-9 analyzer (SPECS, Berlin, Germany) at an electron take-off angle of 90° employing monochromatic Mg Kα radiation (1253.6 eV) of an X-ray tube operated at 180 W. The analyzer pass energy was 10 eV for narrow scans, and 20 eV for survey spectra.The C 1s peak at 284.45 eV from hydrocarbon contaminations was used as a reference.The Pt 4f7/2,5/2, S 2p3/2,1/2 doublets were fitted, after the subtraction of the Shirley-type background, with the coupled peaks with the Gaussian-Lorentzian peak profiles, the spin-orbit splitting of 3.30 eV and 1.19 eV, and the branching ratios of 0.75 and 0.5, respectively, using the CasaXPS software package (version 2.3.16,Casa Software, Teignmouth, UK).The element concentrations on the surface were determined from the wide scan using the empirical sensitivity coefficients.Some photoelectron spectra were measured at the Russian-German laboratory at the dipole magnet beamline of the synchrotron radiation facility BESSY II (Berlin, Germany).Tappingmode atomic force microscopy (AFM) studies were conducted using a Solver P-47 multimode scanning probe microscope (Nanotekhnologiya MDT, Moscow, Russia) in air at room temperature.A silicon cantilever with the resonance frequencies of 150-250 kHz was used as a probe.

Analysis of the Surface of Pyrite and Pyrrhotite Using X-ray Photoelectron Spectroscopy and Atomic Force Microscopy
The XPS spectra show that both Pt and Cl can be detected on the surface of sulfide minerals after the aqueous media treatment (Figure 1).Typically, Cl − ions are weakly adsorbed on the metal sulfide surface and/or can easily be removed by washing with water [39].Thus, Cl observed in the spectra largely belongs to adsorbed Pt-Cl complexes.Atomic force microscopy (AFM) is used to visualize immobilized Pt products and shows strong changes of the mineral substrate relief due to surface etching.Platinum species are hardly distinguished, especially after the short periods of Pt deposition.Pyrite is a more convenient mineral for the AFM study since its surface is to a lower extent subjected to changes by chemical attack.Figure 2 illustrates regions with the particles of Pt-containing products, in which particles with the lateral size of 50-150 nm cover about 50% of the surface area.Atomic force microscopy (AFM) is used to visualize immobilized Pt products and shows strong changes of the mineral substrate relief due to surface etching.Platinum species are hardly distinguished, especially after the short periods of Pt deposition.Pyrite is a more convenient mineral for the AFM study since its surface is to a lower extent subjected to changes by chemical attack.Figure 2 illustrates regions with the particles of Pt-containing products, in which particles with the lateral size of 50-150 nm cover about 50% of the surface area.

Pyrrhotite and Pyrite
Figure 3a,b presents the data on atomic concentrations of Pt, S, Cl, Fe and O on the surface of pyrite and pyrrhotite after the interaction with 1 mmol/L H2PtCl6 solutions.The concentrations of Cl and Pt increase with time.The Fe concentration on pyrrhotite gradually decreases with time but it insignificantly changes on pyrite during 24 h.; this is due to various rates of oxidation of these minerals.

Pyrrhotite and Pyrite
Figure 3a,b presents the data on atomic concentrations of Pt, S, Cl, Fe and O on the surface of pyrite and pyrrhotite after the interaction with 1 mmol/L H2PtCl6 solutions.The concentrations of Cl and Pt increase with time.The Fe concentration on pyrrhotite gradually decreases with time but it insignificantly changes on pyrite during 24 h.; this is due to various rates of oxidation of these minerals.Figure 3c shows that an increase of Pt concentration with time is similar on the surfaces of pyrite and pyrrhotite.The deposition rate is relatively high over 3-5 h, and it gradually decreases but does not drop to zero, so the Pt concentration does not reach saturation.The ratios Cl/Pt show the rate of substitution of Cl − ions with sulfide ions and on the influence of this process on the Pt accumulation on the mineral surface.The atomic ratio Cl/Pt is about 1.8 in the initial deposition stage and slightly decreases to 1.5 after 48 h at pyrite.At pyrrhotite, the value of Cl/Pt of approximately 4.3 is apparently higher in the initial stage, and it decreases down to 1.5 over 24 h.

Galena, Chalcopyrite and Valleriite
The results of the surface analysis for galena, chalcopyrite and valleriite are presented in Figure 4 and also in Table S1 (Supplementary materials).The deposition of Pt on galena and valleriite is considerably higher than at pyrite and pyrrhotite.The amounts of the Pt normalized to the sulfide phase, that is the ratios Pt/S (Figure 4b), suggest that valleriite accumulates Pt faster than galena and other minerals.The deposition of Pt on chalcopyrite is 1.5 times slower than on galena, while it is 2 times slower on pyrite and pyrrhotite.The Cl/Pt ratios presented in Figure 4c implies a negative correlation between the surface amount of Cl and the accumulation of Pt on the minerals.
Figure 3c shows that an increase of Pt concentration with time is similar on the surfaces of pyrite and pyrrhotite.The deposition rate is relatively high over 3-5 h, and it gradually decreases but does not drop to zero, so the Pt concentration does not reach saturation.The ratios Cl/Pt show the rate of substitution of Cl − ions with sulfide ions and on the influence of this process on the Pt accumulation on the mineral surface.The atomic ratio Cl/Pt is about 1.8 in the initial deposition stage and slightly decreases to 1.5 after 48 h at pyrite.At pyrrhotite, the value of Cl/Pt of approximately 4.3 is apparently higher in the initial stage, and it decreases down to 1.5 over 24 h.

Galena, Chalcopyrite and Valleriite
The results of the surface analysis for galena, chalcopyrite and valleriite are presented in Figure 4 and also in Table S1 (Supplementary materials).The deposition of Pt on galena and valleriite is considerably higher than at pyrite and pyrrhotite.The amounts of the Pt normalized to the sulfide phase, that is the ratios Pt/S (Figure 4b), suggest that valleriite accumulates Pt faster than galena and other minerals.The deposition of Pt on chalcopyrite is 1.5 times slower than on galena, while it is 2 times slower on pyrite and pyrrhotite.The Cl/Pt ratios presented in Figure 4c implies a negative correlation between the surface amount of Cl and the accumulation of Pt on the minerals.

XPS of the Reference Materials
Photoelectron spectra from reference samples Pt (chloride complexes, Pt sulfide and metallic nanoparticles) precipitated onto highly oriented pyrolytic graphite (HOPG) were preliminarily measured for the reliable assignment of components of Pt 4f spectra.Two components can be revealed in the Pt 4f spectra of pure aqueous H2PtCl6 dried at HOPG, with the component at the binding energy of 72.9 eV being attributable to Pt(II) binding Cl − anions, and the high-energy component at 75.0 eV corresponding to PtCl6 2− ions (Figure 5a).For the samples of Pt-sulfide hydrosol obtained by mixing the solutions of Na2S and H2PtCl6, two signals with the binding energy of ~74 eV and 72.6 eV can be assigned to Pt(IV)-S and Pt(II)-S compounds [51][52][53], respectively (Figure 5b).The binding energies can vary in a small range, depending on the S species bound to Pt.Along with minor lines of sulfite and sulfate which are likely to be products of Na2S oxidation, two low-energy doublets with S 2p3/2 at 162.6 eV and 163.3 eV are found in the S 2p spectra (Figure S1, Supplementary materials).These binding energies are typical for disulfide and polysulfide anions, respectively, or other S species carrying similar local negative charges.In its turn, Pt bound with polysulfide could have the binding energy by several tenth eV lower than Pt-sulfide species; this may result in a shift or broadening in the corresponding Pt 4f spectra.The Pt 4f spectra of the metallic Pt particles obtained via the reduction by formaldehyde show the binding energy of Pt 0 metallic nanoparticles to have the value of about 71.5 eV (Figure 5c).The same energy was found for these Pt nanoparticles deposited on pyrite, indicating the absence of the substrate influence on the chemical shift (Figure 5d).The spectra also contain a minor component

XPS of the Reference Materials
Photoelectron spectra from reference samples Pt (chloride complexes, Pt sulfide and metallic nanoparticles) precipitated onto highly oriented pyrolytic graphite (HOPG) were preliminarily measured for the reliable assignment of components of Pt 4f spectra.Two components can be revealed in the Pt 4f spectra of pure aqueous H 2 PtCl 6 dried at HOPG, with the component at the binding energy of 72.9 eV being attributable to Pt(II) binding Cl − anions, and the high-energy component at 75.0 eV corresponding to PtCl 6 2− ions (Figure 5a).For the samples of Pt-sulfide hydrosol obtained by mixing the solutions of Na 2 S and H 2 PtCl 6 , two signals with the binding energy of ~74 eV and 72.6 eV can be assigned to Pt(IV)-S and Pt(II)-S compounds [51][52][53], respectively (Figure 5b).The binding energies can vary in a small range, depending on the S species bound to Pt.Along with minor lines of sulfite and sulfate which are likely to be products of Na 2 S oxidation, two low-energy doublets with S 2p 3/2 at 162.6 eV and 163.3 eV are found in the S 2p spectra (Figure S1, Supplementary materials).These binding energies are typical for disulfide and polysulfide anions, respectively, or other S species carrying similar local negative charges.In its turn, Pt bound with polysulfide could have the binding energy by several tenth eV lower than Pt-sulfide species; this may result in a shift or broadening in the corresponding Pt 4f spectra.The Pt 4f spectra of the metallic Pt particles obtained via the reduction by formaldehyde show the binding energy of Pt 0 metallic nanoparticles to have the value of about 71.5 eV (Figure 5c).The same energy was found for these Pt nanoparticles deposited on pyrite, indicating the absence of the substrate influence on the chemical shift (Figure 5d).The spectra also contain a minor component from Pt(II)-Cl complexes (73 eV) and a signal at ~72 eV, which is probably due to unidentified product of incomplete reduction of H 2 PtCl 6 .
from Pt(II)-Cl complexes (73 eV) and a signal at ~72 eV, which is probably due to unidentified product of incomplete reduction of H2PtCl6.

Change of Platinum Species in the Deposition Process on Pyrite
Figure 6 shows Pt 4f spectra for pyrite interacted with the 1 mmol/L H2PtCl6 solution.The fitting of the spectra suggests that Pt on the surface is in the form of Pt(II) and Pt(IV) bound both with S and Cl, the proportions of which depending on the deposition duration.The high energy component with the binding energy of 75.2 eV corresponding to the adsorbed complexes of tetravalent Pt is present during the first two hours of the reaction (~25% of total Pt), and then it almost vanishes.Amount of the Pt(IV) species is significantly lower on pyrite than on pyrrhotite (Figures 6 and 7).The share of Pt(II)-Cl species having the binding energy of about 73 eV stays in the range of 30-40%, and thus adsorbed chloride complexes prevail during a 24 h reaction.The Pt 4f7/2 component arising afterwards at 74.1 eV can be attributed to Pt(IV) sulfide, possibly PtS2 (about 18% of total Pt).The further course of the reaction results in a noticeable decrease of the Pt-chloride species and increase in the amount of Pt(II) sulfide (~72.5 eV).However, the photoelectron spectra acquired using synchrotron radiation with the photon excitation energy of 250 eV (Figure S2, Supplementary materials) having a higher surface sensitivity show slightly different composition of the surface products.The main difference consisting in the presence of the component at 75.0 eV from Pt(IV)-Cl species with the relative intensity up to 8%.Thus, independent of the reaction time, a small amount of the adsorbed PtCl6 2− ions seems to be always present on the mineral surfaces.

Change of Platinum Species in the Deposition Process on Pyrite
Figure 6 shows Pt 4f spectra for pyrite interacted with the 1 mmol/L H 2 PtCl 6 solution.The fitting of the spectra suggests that Pt on the surface is in the form of Pt(II) and Pt(IV) bound both with S and Cl, the proportions of which depending on the deposition duration.The high energy component with the binding energy of 75.2 eV corresponding to the adsorbed complexes of tetravalent Pt is present during the first two hours of the reaction (~25% of total Pt), and then it almost vanishes.Amount of the Pt(IV) species is significantly lower on pyrite than on pyrrhotite (Figures 6 and 7).The share of Pt(II)-Cl species having the binding energy of about 73 eV stays in the range of 30-40%, and thus adsorbed chloride complexes prevail during a 24 h reaction.The Pt 4f 7/2 component arising afterwards at 74.1 eV can be attributed to Pt(IV) sulfide, possibly PtS 2 (about 18% of total Pt).The further course of the reaction results in a noticeable decrease of the Pt-chloride species and increase in the amount of Pt(II) sulfide (~72.5 eV).However, the photoelectron spectra acquired using synchrotron radiation with the photon excitation energy of 250 eV (Figure S2, Supplementary materials) having a higher surface sensitivity show slightly different composition of the surface products.The main difference consisting in the presence of the component at 75.0 eV from Pt(IV)-Cl species with the relative intensity up to 8%.Thus, independent of the reaction time, a small amount of the adsorbed PtCl 6 2− ions seems to be always present on the mineral surfaces.

Change of Platinum Species in the Deposition Process on Pyrrhotite
About 80% of Pt immobilized on the pyrrhotite surface (Figure 7) is represented by divalent Pt bound to Cl and S, with the former being dominant for short deposition (e.g., 60% vs. 22% after 2 h), and the latter increasing with time and prevailing after 24 h reaction (36% vs. 40%).The amount of Pt(IV) tends to decrease within the range of 30% to 20% with increasing the interaction time.Concurrently, the component at 75.2 eV shifts towards 74.1 eV, suggesting that the Pt(IV)-Cl adsorbates transform to Pt(IV)-S species.Both the reduction and sulfidization of immobilized Pt(IV)-Cl species are faster on pyrite than pyrrhotite, despite pyrite being much more chemically inert.At the same time, it should be taken in mind that the total uptake of Pt is somewhat higher at pyrrhotite.

Change of Platinum Species in the Deposition Process on Pyrrhotite
About 80% of Pt immobilized on the pyrrhotite surface (Figure 7) is represented by divalent Pt bound to Cl and S, with the former being dominant for short deposition (e.g., 60% vs. 22% after 2 h), and the latter increasing with time and prevailing after 24 h reaction (36% vs. 40%).The amount of Pt(IV) tends to decrease within the range of 30% to 20% with increasing the interaction time.Concurrently, the component at 75.2 eV shifts towards 74.1 eV, suggesting that the Pt(IV)-Cl adsorbates transform to Pt(IV)-S species.

Change of Platinum Species in the Deposition Process on Pyrrhotite
About 80% of Pt immobilized on the pyrrhotite surface (Figure 7) is represented by divalent Pt bound to Cl and S, with the former being dominant for short deposition (e.g., 60% vs. 22% after 2 h), and the latter increasing with time and prevailing after 24 h reaction (36% vs. 40%).The amount of Pt(IV) tends to decrease within the range of 30% to 20% with increasing the interaction time.Concurrently, the component at 75.2 eV shifts towards 74.1 eV, suggesting that the Pt(IV)-Cl adsorbates transform to Pt(IV)-S species.Both the reduction and sulfidization of immobilized Pt(IV)-Cl species are faster on pyrite than pyrrhotite, despite pyrite being much more chemically inert.At the same time, it should be taken in mind that the total uptake of Pt is somewhat higher at pyrrhotite.Both the reduction and sulfidization of immobilized Pt(IV)-Cl species are faster on pyrite than pyrrhotite, despite pyrite being much more chemically inert.At the same time, it should be taken in mind that the total uptake of Pt is somewhat higher at pyrrhotite.

Influence of the Preliminary Modification of the Mineral Surface on the Platinum Deposition
One can expect that the state of a mineral surface is modified, for example, the weathering and mineral processing could affect the interaction with Pt-bearing solutions.The preliminary oxidative leaching of pyrrhotite in the 0.25 M FeCl 3 + HCl solution (30 min at 50 • C) leads to a significant increase in the amount of the deposited Pt (Figure 8a).The pre-treatment in 1 M HCl under the non-oxidative dissolution conditions (−50 mV, 10 min), which produces a massive metal-deficient layer on the pyrrhotite surface [44], also results in an increased amount of Pt on the surface but to a smaller extent than after the oxidative treatment (Figure 8).After the anodic pre-oxidation at 0.5 V (region of potentials of the passive dissolution of pyrrhotite) a significant decrease in the Pt deposition, with Pt/S being two times lower as compared to unreacted pyrrhotite is observed.

Influence of the Preliminary Modification of the Mineral Surface on the Platinum Deposition
One can expect that the state of a mineral surface is modified, for example, the weathering and mineral processing could affect the interaction with Pt-bearing solutions.The preliminary oxidative leaching of pyrrhotite in the 0.25 М FeCl3 + HCl solution (30 min at 50 °С) leads to a significant increase in the amount of the deposited Pt (Figure 8а).The pre-treatment in 1 M HCl under the nonoxidative dissolution conditions (−50 mV, 10 min), which produces a massive metal-deficient layer on the pyrrhotite surface [44], also results in an increased amount of Pt on the surface but to a smaller extent than after the oxidative treatment (Figure 8).After the anodic pre-oxidation at 0.5 V (region of potentials of the passive dissolution of pyrrhotite) a significant decrease in the Pt deposition, with Pt/S being two times lower as compared to unreacted pyrrhotite is observed.Figure 8. Pt/S and Pt/Cl atomic ratios for pyrrhotite (left panels) and pyrite (right panels) after various chemical pre-treatment followed by 2 h interaction with 1 mmol/L H2PtCl6 solution.
Figure 9 shows the Pt 4f spectra and forms of Pt deposited on the preliminarily oxidized pyrrhotite; the Pt concentration on the oxidized pyrite is low, so the spectra were of poor quality and are not shown here.On the base of these data, we hypothesize that the enhanced uptake of Pt at pyrrhotite mainly as the sulfide species is due to excessive S in the Fe-depleted surface layers produced by the chemical pre-treatment [44,45].The Fe depletion and S enrichment of the reacted pyrite are much lower [44], and the oxidation promotes mainly increasing surface concentration of adsorbed chloride ions, which probably retard the adsorption of the Pt complex ions.Figure 9 shows the Pt 4f spectra and forms of Pt deposited on the preliminarily oxidized pyrrhotite; the Pt concentration on the oxidized pyrite is low, so the spectra were of poor quality and are not shown here.On the base of these data, we hypothesize that the enhanced uptake of Pt at pyrrhotite mainly as the sulfide species is due to excessive S in the Fe-depleted surface layers produced by the chemical pre-treatment [44,45].The Fe depletion and S enrichment of the reacted pyrite are much lower [44], and the oxidation promotes mainly increasing surface concentration of adsorbed chloride ions, which probably retard the adsorption of the Pt complex ions.

Platinum Species on Different Sulfide Minerals
In the Pt 4f spectra of galena (Figure 10

Platinum Species on Different Sulfide Minerals
In the Pt 4f spectra of galena (Figure 10) two distinct components are shown one with the binding energy of 72.The spectra of Pt collected from valleriite after 4 h of interaction with the H 2 PtCl 6 solution are rather similar to galena, with the two doublets being somewhat wider and contribution of the chloride complex being lower.Surprisingly, metallic Pt of ~60% from the entire amount of Pt, is found to form on valleriite as the reaction progresses, which is evidenced by the appearance of the Pt 4f 7/2 peak at 71.5 eV.The remainder is Pt(II) bound with sulfur (the binding energy of 72.3 eV), while contributions of Pt-Cl species is lower.Noteworthy, the Pt 4f bands are overlapped with Cu 3p band at ~75 eV, which is weak for valleriite.Chalcopyrite, has almost the same composition as sulfide nanolayers in valleriite, but the uptake of Pt is much lower, with Pt-Cl species dominating and Pt(IV) species notably contributing especially in the initial stages; rather intense Cu 3p line overlaps the Pt 4f spectra, making difficult their unanimous fitting.

Discussion
The process of Pt immobilization onto sulfide minerals appears to proceed via a sequence of stages.The first is the adsorption of Pt-chloride complexes.The following stages include the substitution of chloride ions with sulfide (and, maybe, di-and polysulfide) anions and reduction of tetravalent Pt to divalent Pt, and to metallic state in the case of valleriite.It is possible that there exist more Pt products in addition to those described above, including intermediates of the substitution reactions, involving some OH − or/and H 2 O ligands, etc.However, exact compositions of the Pt species cannot be unequivocally derived from the above XPS data because of contributions of numerous surface S-, Cl-, O-bearing entities at the sulfide substrates and errors of the fitting procedures.
As it was demonstrated for pyrrhotite and pyrite, the total and relative contents of Pt products differ as a function not only of the reaction time but also of mineral nature, the state of its surface, in particular, a measure of oxidation and composition of the modified metal sulfide.The key factor seems to be the activity of the surface sulfur anions involved in the reduction of Pt(IV) and the substitution of Cl − ions in the coordination sphere of Pt.In a number of cases, e.g., for galena and pyrrhotite, Pt complexes may be reduced by aqueous bisulfide ions (hydrogen sulfide) formed due to acidic dissolution of the metal sulfides.Both these effects are related with reactivity of metal sulfide minerals, which is commonly known to increase in the order pyrite < chalcopyrite < valleriite < galena, pyrrhotite, that generally agrees with increasing the uptake of Pt (Figure 5).Furthermore, oxidation of pyrrhotite, including in ambient air, results in its passivation [54][55][56], while both non-oxidative and oxidative preliminary leaching causes the formation of more active over-stoichiometric sulfide, di-and polysulfide anions, which facilitate the fixation of Pt via the reduction and sulfidization reaction.
The formation of Pt(IV)-S species in the next stages can be explained by oxidation of the Pt(II)-S by di-and polysulfide groups as reported in refs [57,58].The oxidative capability of the -S-S-species arising at the sulfide mineral surfaces upon their prolonged oxidation seems to be in fact mediated by Fe(III) entities and oxygen.
The largest uptake of Pt was observed at valleriite, the most important mineral of so-called "coppery" ores of the Noril'sk deposits (about 7% of the total resources), which form veins in sedimentary and metamorphic rocks on the margins of the massive magmatic ores and is known for increased contents of PGEs [59,60].Valleriite is a sulfide-hydroxide composite mineral where the sulfide nanolayers with the composition CuFeS n close to chalcopyrite alternate with brucite-like hydroxide nanolayers [Mg,Al(OH) 2 ] [61 -63].The hydroxide layers are believed to have positive charge due to the substitution of Mg 2+ for Al 3+ , and so the sulfide nanolayers are negative to support electro neutrality [54], but this has not been confirmed experimentally yet.We believe that the localization of electron density at the sulfide layers makes them a strong reducing agent, which is capable of transforming Pt compounds to the metallic state over its immobilization.This mechanism corroborates with the fact that chalcopyrite being the structural analogue of the sulfide nanolayers and a precursor of valleriite in the geochemical processes, exhibits much weaker reducing properties towards Pt(II,IV) species.

1.
The interaction of the aqueous solutions of H 2 PtCl 6 with the surface of sulfide minerals results in the deposition of Pt on their surface.The amount of the deposited Pt increases with time.

2.
The highest rate of Pt uptake is observed on galena and valleriite.

3.
The preliminary moderate oxidation and non-oxidative leaching of pyrrhotite creating metal-deficient surface layers usually promotes Pt deposition.In the case of pyrite, the preliminary oxidation decreases the amount of the deposited Pt. 4.
The main Pt phases immobilized on sulfide minerals are sulfides and chloride complexes of Pt(II), and, to a smaller extent, chloride complexes of tetravalent Pt; no Pt(IV) was observed on galena and valleriite.As the reaction progresses, the amount of Pt(IV) usually decreases while the amount of the sulfide forms increases.

5.
Metallic Pt was found to form only on valleriite, probably owing to the negative charge localized at sulfidic nanolayers.6.
The di-and polysulfide surface species arising on sulfide minerals upon their oxidation are capable of oxidizing immobilized Pt(II)-S to Pt(IV)-S species.

Figure 1 .
Figure 1.Fragments of the X-ray photoelectron survey spectra of pyrite and pyrrhotite after the interaction with 1 mmol/L H2PtCl6 + 0.03 mol/L HCl solution for 4 h.

Figure 1 .
Figure 1.Fragments of the X-ray photoelectron survey spectra of pyrite and pyrrhotite after the interaction with 1 mmol/L H 2 PtCl 6 + 0.03 mol/L HCl solution for 4 h.

Figure 2 .
Figure 2. Atomic force microscopy images of the pyrite surface after the interaction with the solution of H2PtCl6 for 14 h; (a) relief, (b) phase contrast.

Figure 3 .
Figure 3. Atomic concentrations of Pt, Cl, S, O and Fe on the surface of (a) pyrrhotite and (b) pyrite and the atomic concentration ratios of (c) Pt/S and (d) Cl/Pt on pyrrhotite and pyrite after different interaction time with the 10 −3 М H2PtCl6 solution.

Figure 2 .
Figure 2. Atomic force microscopy images of the pyrite surface after the interaction with the solution of H 2 PtCl 6 for 14 h; (a) relief, (b) phase contrast.

3. 2 . 14 Figure 2 .
Figure3a,b presents the data on atomic concentrations of Pt, S, Cl, Fe and O on the surface of pyrite and pyrrhotite after the interaction with 1 mmol/L H 2 PtCl 6 solutions.The concentrations of Cl and Pt increase with time.The Fe concentration on pyrrhotite gradually decreases with time but it insignificantly changes on pyrite during 24 h.; this is due to various rates of oxidation of these minerals.

Figure 3 .
Figure 3. Atomic concentrations of Pt, Cl, S, O and Fe on the surface of (a) pyrrhotite and (b) pyrite and the atomic concentration ratios of (c) Pt/S and (d) Cl/Pt on pyrrhotite and pyrite after different interaction time with the 10 −3 М H2PtCl6 solution.

Figure 3 .
Figure 3. Atomic concentrations of Pt, Cl, S, O and Fe on the surface of (a) pyrrhotite and (b) pyrite and the atomic concentration ratios of (c) Pt/S and (d) Cl/Pt on pyrrhotite and pyrite after different interaction time with the 10 −3 M H 2 PtCl 6 solution.

Figure 4 .
Figure 4. Atomic concentrations of (a) Pt, and the ratios (b) Pt/S, and (c) Cl/Pt at the surface of sulfide minerals after 2 h interaction with 1 mmol/L H2PtCl6 solution.

Figure 4 .
Figure 4. Atomic concentrations of (a) Pt, and the ratios (b) Pt/S, and (c) Cl/Pt at the surface of sulfide minerals after 2 h interaction with 1 mmol/L H 2 PtCl 6 solution.

Figure 6 .
Figure 6.(а) Normalized Pt 4f spectra of pyrite treated with 1 mmol/L H2PtCl6 solution for 2 h, 4 h, 24 h and 48 h, and (b) distribution of the surface Pt species.

Figure 7 .
Figure 7. (а) Normalized Pt 4f spectra for pyrrhotite samples after 2, 4, 12 and 24 h of the interaction with 1 mmol/L H2PtCl6 solution, and (b) corresponding distribution of the Pt species.

Figure 6 .
Figure 6.(a) Normalized Pt 4f spectra of pyrite treated with 1 mmol/L H 2 PtCl 6 solution for 2 h, 4 h, 24 h and 48 h, and (b) distribution of the surface Pt species.

Figure 6 .
Figure 6.(а) Normalized Pt 4f spectra of pyrite treated with 1 mmol/L H2PtCl6 solution for 2 h, 4 h, 24 h and 48 h, and (b) distribution of the surface Pt species.

Figure 7 .
Figure 7. (а) Normalized Pt 4f spectra for pyrrhotite samples after 2, 4, 12 and 24 h of the interaction with 1 mmol/L H2PtCl6 solution, and (b) corresponding distribution of the Pt species.

Figure 7 .
Figure 7. (a) Normalized Pt 4f spectra for pyrrhotite samples after 2, 4, 12 and 24 h of the interaction with 1 mmol/L H 2 PtCl 6 solution, and (b) corresponding distribution of the Pt species.

Figure 8 .
Figure 8. Pt/S and Pt/Cl atomic ratios for pyrrhotite (left panels) and pyrite (right panels) after various chemical pre-treatment followed by 2 h interaction with 1 mmol/L H 2 PtCl 6 solution.

Figure 9 .
Figure 9. (a) Normalized XPS Pt 4f spectra of pyrrhotite after the deposition of Pt from 1 mmol/L H2PtCl6 solution onto unreacted surface and onto the surface preliminarily treated in 0.25 mol/L FeCl3 + HCl during 30 min, 1 mol/L HCl at the potential of −50 mV, and at the potential of 500 mV (10 min), and (b) corresponding diagram of the distribution of the Pt forms.
) two distinct components are shown one with the binding energy of 72.3 eV and contribution of 65%, and another at 73.4 eV amounting of 35%, almost independent of the deposition time.The first component corresponds to Pt(II)-S species, and the second originates from Pt(II) atoms bound to Cl − , apparently in adsorbed PtCl4 2− ions.Neither tetravalent Pt nor metallic Pt were observed on galena under the given conditions.

Figure 10 .
Figure 10.(a) XPS Pt 4f spectra of the sulfide minerals reacted with 1 mmol/L H2PtCl6 solution and (b) corresponding distribution diagrams of the platinum forms.

Figure 9 .
Figure 9. (a) Normalized XPS Pt 4f spectra of pyrrhotite after the deposition of Pt from 1 mmol/L H 2 PtCl 6 solution onto unreacted surface and onto the surface preliminarily treated in 0.25 mol/L FeCl 3 + HCl during 30 min, 1 mol/L HCl at the potential of −50 mV, and at the potential of 500 mV (10 min), and (b) corresponding diagram of the distribution of the Pt forms.

3. 5 . 14 Figure 9 .
Figure 9. (a) Normalized XPS Pt 4f spectra of pyrrhotite after the deposition of Pt from 1 mmol/L H2PtCl6 solution onto unreacted surface and onto the surface preliminarily treated in 0.25 mol/L FeCl3 + HCl during 30 min, 1 mol/L HCl at the potential of −50 mV, and at the potential of 500 mV (10 min), and (b) corresponding diagram of the distribution of the Pt forms.
3 eV and contribution of 65%, and another at 73.4 eV amounting of 35%, almost independent of the deposition time.The first component corresponds to Pt(II)-S species, and the second originates from Pt(II) atoms bound to Cl − , apparently in adsorbed PtCl4 2− ions.Neither tetravalent Pt nor metallic Pt were observed on galena under the given conditions.

Figure 10 .
Figure 10.(a) XPS Pt 4f spectra of the sulfide minerals reacted with 1 mmol/L H2PtCl6 solution and (b) corresponding distribution diagrams of the platinum forms.

Figure 10 .
Figure 10.(a) XPS Pt 4f spectra of the sulfide minerals reacted with 1 mmol/L H 2 PtCl 6 solution and (b) corresponding distribution diagrams of the platinum forms.
Supplementary Materials:The following are available online at http://www.mdpi.com/2075-163X/8/12/578/s1,FigureS1:XPS the S 2p spectra from a droplet of sol formed in the reaction H 2 PtCl 6 + H 2 S dried on pyrolytic graphite, FigureS2: SR photoelectron Pt 4f spectra of the surfaces of pyrrhotite (a) and pyrite (b) after the interaction with solution of 1 mmol/L H 2 PtCl 6 + 0.03 mol/L HCl for 12 h.TableS1: Atomic concentrations of the elements on the surfaces of different sulfide minerals after 2 h interaction with 1 mmol/L M H 2 PtCl 6 .
Author Contributions: A.R. and Y.M. conceived and designed the study; A.R. prepared samples for all experiments; M.L. performed synthetic, AFM experiments; A.R. and Y.M. conducted XPS measurements; A.R. and Y.M. wrote the paper.