materials Article On the Rate of Interaction of Sodium Borohydride with Platinum (IV) Chloride Complexes in Alkaline Media Magdalena Luty-Błocho 1,* , Marek Wojnicki 1, Edit Csapo 2,3 and Krzysztof Fitzner 1 1 Faculty of Non-Ferrous Metals, AGH University of Science and Technology, Al. A. Mickiewicza 30, 30-059 Krakow, Poland; [email protected] (M.W.); fi[email protected] (K.F.) 2 Department of Physical Chemistry and Materials Science, University of Szeged, Rerrich B. Sqr. 1, 6720 Szeged, Hungary; [email protected] 3 MTA-SZTE Biomimetic Systems Research Group, Department of Medical Chemistry, Faculty of Medicine, University of Szeged, Dóm Sqr. 8, 6720 Szeged, Hungary * Correspondence: [email protected]; Tel.: +48-12-617-4126 Abstract: In this work, sodium borohydride was used as a strong reductant of traces of platinum complex ions. The investigations of the kinetics of redox reaction between platinum(IV) chloride complex ions and sodium borohydride were carried out. For the first time, the kinetic experiments were carried out in a basic medium (pH~13), which prevents NaBH4 from decomposition and suppresses the release of hydrogen to the environment. The rate constants of Pt(IV) reduction to Pt(II) ions under different temperatures and concentrations of chloride ions conditions were determined. In alkaline solution (pH~13), the values of enthalpy and entropy of activation are 29.6 kJ/mol and –131 J/mol K. It was also found that oxygen dissolved in the solution strongly affects kinetics of the reduction process. Using collected results, the reduction mechanism was suggested. For the first time, the appearance of diborane as an intermediate product during Pt(IV) ions reduction was suggested. Moreover, the influence of oxygen present in the reacting solution on the rate of reduction reaction Citation: Luty-Błocho, M.; Wojnicki, was also shown. M.; Csapo, E.; Fitzner, K. On the Rate of Interaction of Sodium Borohydride Keywords: chloride Pt(IV) complex ions; sodium borohydride; chemical reduction; diborane; zero- with Platinum (IV) Chloride order reaction Complexes in Alkaline Media. Materials 2021, 14, 3137. https:// doi.org/10.3390/ma14113137 1. Introduction Academic Editor: Adam Cwudzi´nski One of the most important topics for the past two decades has been the recov- ery of so-called strategic metals (e.g., platinum group metals, rare earths) accompa- Received: 28 April 2021 nied with the production of environmentally friendly waste. It is due to the increase Accepted: 3 June 2021 Published: 7 June 2021 of public awareness and in response to environmental derelicts (for example, Direc- tive 2006/21/EC on the management of waste from extractive industries and amend- Publisher’s Note: MDPI stays neutral ing Directive 2004/35/EC), as well as implementation of UE strategy for raw mate- with regard to jurisdictional claims in rials (https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=CELEX:52011DC0025 (ac- published maps and institutional affil- cessed on 10 May 2021)). These regulations stimulate the demand for new technologies iations. which will allow effective metal recovery and purification of waste solution. Among metals, platinum is important due to many potential applications in catalysis [1], modern chemistry, medicine [2], and engineering. For platinum recovery, several techniques can be used such as extraction [3], leaching [4], sorption [5], precipitation [6], chemical reduction, and biological methods [7]. However, despite the high level of recovery, the methods Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. mentioned above do not guarantee environmentally friendly waste. There are at least This article is an open access article two reasons why it happens. The first is directly related to the recovery process itself, distributed under the terms and which requires the use of hazardous reagents. Such a substance may penetrate into the conditions of the Creative Commons waste solution as an unreacted reagent or as a hazardous product of the reaction. The Attribution (CC BY) license (https:// second is related to the form of recovered metals. Especially, metals at nanoscale may be creativecommons.org/licenses/by/ dangerous for life and difficult to recover. They are just toxic and difficult to remove simply 4.0/). by filtration. Materials 2021, 14, 3137. https://doi.org/10.3390/ma14113137 https://www.mdpi.com/journal/materials Materials 2021, 14, 3137 2 of 14 In this context, we suggest using sodium borohydride as a strong reductant of plat- inum ions. It is known that NaBH4 is hazardous, but as a result of the reaction with metal precursor it can be neutralized to an environmentally ambient form such as boric acid, borax. These compounds are used as food additives [8]. Moreover, the high efficiency of NaBH4 requires an application of a small amount of this reductant. The use of sodium borohydride as a reductant allows for a fast reduction of metal ions and nuclei formation. It in turn promotes the formation of small homogeneous nanoparticles (1–3 nm in diameter) which can be used in any field of science [9–11]. The high efficiency of sodium borohydride and thus its strong reducing property depends strongly on the pH of the reacting solution. It has been already confirmed in the literature [12] that its reducing activity increases with + the increasing concentration of H3O ions in the solution. This fact is related to the rate of decomposition of NaBH4 in water, which accelerates visibly below pH = 9.24. In an aqueous solution, NaBH4 dissociates according to the reaction: + − NaBH4 $ Na + BH4 (1) − and in the presence of water, BH4 hydrolyzes spontaneously [13]: − − BH4 + 4H2O $ B(OH)4 + 4H2 (2) In the course of this reaction, hydrogen evolves rapidly. Taking into account reactions of hydrolysis of sodium borohydride and the hexachloro- platinate (IV) [13–15], the process of reduction reaction of Pt(IV) ions was investigated − under alkaline conditions, which assures the presence of BH4 as the reductant in the 2− solution, as well as [PtCl5(OH)] ions as the metal precursor. Consequently, the reduction process between Pt(IV) ions and NaBH4 in alkaline solution (pH ~ 13) in the simplest form can be described by the reaction: 2− − k1 [PtCl5(OH)] + BH4 ! products (3) where k1 denotes the rate constant of reaction (3). The possible product of this reaction can be either Pt(II) complex or metallic platinum particles. Many studies indicate that the process of nanoparticles formation is fast. However, nobody measured the rate of the processes preceding it, i.e., the reduction of Pt(IV) to Pt(II) ions and next Pt(II) to Pt(0). Moreover, there is no reliable model which can describe these stages. In order to follow how sodium borohydride behaves during the reaction with Pt(IV) ions, we decided to undertake a detailed study of the first step in the process of platinum particles formation, i.e., reduction of Pt(IV) to Pt(II) ions. Moreover, based on the obtained kinetic data, the possible mechanism of PtNPs formation will be suggested. The choice of the experimental conditions was related to the elimination of possible processes that could run parallel to the reduction of Pt(IV) ions with sodium borohydride. 2. Materials and Methods 2.1. Chemicals The base solution of platinum precursor, i.e., 0.076 M solution of H2PtCl6 was obtained from the metallic platinum with purity 99.99% (Mennica–Metale Szlachetne, Radzymin, Poland). For this purpose, the metallic platinum was dissolved in aqua regia (mixture of 36% HCl and 65% nitric acid in volumetric ratio 3:1, p.a. Chemland, Stargard Szczeci´nski, Poland) solution, which was dried four times to remove an excess of nitric acid and at the end in hydrochloride acid to keep the acidic pH of the solution. Before every experiment, the fresh solution of Pt(IV) ions was prepared, then the proper volume of base solution was diluted in deionized water. The proper reducer concentration was obtained by dissolving the powder of sodium borohydride (NaBH4, 96%, Fluka, Loughborough, UK) into 0.1 M solution of NaOH (p.a. Chemland, Stargard Szczeci´nski,Poland), which kept the pH alkaline (∼13) and prevented it from a degradation of NaBH4 [16–18]. In the case of Materials 2021, 14, 3137 3 of 14 experiments performed in deaerated solutions, the water and sodium hydroxide solutions were purged with nitrogen for 30 min. 2.2. Methods In the experiments, the redox reaction was monitored at a fixed wavelength (262 nm) using the stopped-flow spectrophotometer (Applied Photophysics, Leatherhead, UK) working in the UV-Vis range (from 190–900 nm wavelength) with the 1-cm or 0.2-cm 2 optical path. The spectrum of the [PtCl6] ¯ solution in 0.05 M NaOH was recorded to establish the fixed wavelength of 262 nm used as a reference to monitor concentration changes. However, due to the stability diagram (Figure S1, Supplementary materials) this 2 reference refers in fact to [PtCl5(OH)] ¯ species. All kinetic curves, which include about one hundred data points each, were repeated six times to obtain each value of the observed rate constant (k1,obs). 2.3. Dynamic Light Scattering (DLS) In order to determine the presence of a solid phase in the aqueous solution and particles size, the DLS method was applied (Nano Zeta-S, Malvern, UK). 3. Results 3.1. Experimental Conditions The conditions of the conducted experiments are gathered in Table1. Table 1. Conditions of experiments for the reaction between platinum(IV) chloride complex ions and sodium borohydride. Conditions: pH = 12.9 ± 0.2, ionic strength I = 0.05 M (except, chloride addition). Initial Concentration of Reagents, M Temperature, K NaCl Addition, M The stoichiometry of reaction C0, Pt(IV):C0,NaBH4 –– 1:2 298 – 1:1 – – 2:1 – – 3:1 – – – Effect of temperature – 0.05 3.0 288 – – 298 – – 308 – – 318 – Effect of Cl¯ concentration (at constant value of ionic strength I = 0.4 M and [Na+] = 0.4 M) 0.05 3.0 298 0.05 – – 0.10 – – 0.20 – – 0.30 – – 0.40 3.2.