Reduction of Pt(II) by H₂:  Effects of Citrate and NaOH and Reaction Mechanism

Abstract

Aqueous solutions of Pt(II) (aged PtCl4 2- solutions) are reduced by hydrogen in the absence of a stabilizer and in the presence of stabilizing sodium citrate or/and sodium hydroxide. The rate of reduction is measured optically and conductometrically, and the colloidal nanoparticles, which are finally formed, are studied by high-resolution electron microscopy. In dilute solutions, the rate of reduction is proportional to the square of the Pt(II) concentration. In the presence of citrate and NaOH, the reduction occurs more slowly. A mechanism, in which intermediate free Pt atoms do not appear, is discussed. Monovalent platinum is postulated as the first intermediate, and an autocatalytic contribution to the reduction is observed. Particle growth in a chain reaction is also discussed. In the presence of NaOH, the reaction occurs in two steps: formation of a hydroxilated precursor and reduction of the latter. Depending on the concentration of the stabilizing additives, colloids consisting of polyhedra of different shape (cubic, tetrahedral, and triculent octahedral) are formed. Stabilization by citrate plus NaOH yields particularly long lifetimes of the colloidal solutions.

Synthesis and characterization of silver/montmorillonite/chitosan bionanocomposites by chemical reduction method and their antibacterial activity

“…The use of a strong reducing agent, such as NaBH 4 , results in tiny particles that are well dispersed. 29 Nowadays, a succession of chemical reductants can been used for the synthesis of noble metal nanoparticles which contain NaBH 4 35 formaldehyde, 36 formamide, 37 Tollens' reagents, 38 ethylene glycol, 39 ethanol, 40 citrate, 32,40 aniline, 41 polyaniline, 42 or ascorbic acid. 43 Layered silicates, eg, montmorillonite (MMT), have been used in polymer nanocomposites with a significant improvement in mechanical properties.…”

Nanostructured Heterogeneous Catalysts: Well Defined Platinum Nanoparticles Supported on Alumina. Preparation, Characterization and Application to the Selective Hydrogenation of Buta-1,3-diene

“…CTAB probably forms micellar species since its concentration in solution (around 5 mM in the final solution) is higher than the Critical Micellar Concentration (0.8-1.1 mM) [37]. When the aged H 2 PtCl 6 solution is added, the Pt 4+ cation could be surrounded by different coordination shells since the ageing of the solution is supposed to lead to the partial or complete substitution of chloride ligands by hydroxides ones [21,22,38]: (1), an interaction between CTAB and the platinate salt could be envisaged. Such an interaction could lead to {Pt 4+ -CTAB} moieties as it was previously demonstrated for palladium [14].…”

“…Tetrahedra were also obtained using polyvinylpyrrolydone (PVP), adjusting the pH permitted to form up to 75% of tetrahedra [19,20]. Platinum nanoparticules with well defined morphologies were obtained using citrate as a structure directing agent [21,22]. So, without or with low amounts of citrate, cubes and truncated octahedra were observed and when the proportion of citrate was raised, the proportion of tetrahedra was increased.…”

The unique electrochemical reactivity of small metal nanoparticles.

“…139 They conclude that the electrode has a larger influence on smaller Au NPs due to more points of interaction. This work is relevant to the early findings where small metal clusters showed a positive shift in oxidation potential 112,127 instead of the predicted negative shift and is similar to the phenomenon of underpotential deposition. 140 More recently, Kuo and Hwang showed by voltammetry that the oxidation potential of Ag NPs depends on the shape and follows the order of cubes > spheres > decahedrons > triangular plates.…”

“…Previously, Henglein and Giersig showed that the reduction of PtCl4 2-occurs more slowly in the presence of citrate and NaOH. 127 It is well-known that pH plays a role in the synthesis of citrate-coated metal NPs. Peng and coworkers described the influence of pH on the size/shape distribution and the size/shape evolution of Au NPs.…”