Reduction of Pt(II) by H-2: Effects of citrate and NaOH and reaction mechanism

Aqueous solutions of Pt(II) (aged PtCl42- solutions) are reduced by hydroge n in the absence of a stabilizer and in the presence of stabilizing sodium citrate or/and sodium hydroxide. The rate of reduction is measured opticall y and conductometrically, and the colloidal nanoparticles, which are finall y formed, are studied by high-resolution electron microscopy. In dilute sol utions, the rate of reduction is proportional to the square of the Pt(II) c oncentration. In the presence of citrate and NaOH, the reduction occurs mor e slowly. A mechanism, in which intermediate free Pt atoms do not appear, i s discussed. Monovalent platinum is postulated as the first intermediate, a nd an autocatalytic contribution to the reduction is observed. Particle gro wth in a chain reaction is also discussed. In the presence of NaOH, the rea ction occurs in two steps: formation of a hydroxilated precursor and reduct ion of the latter. Depending on the concentration of the stabilizing additi ves, colloids consisting of polyhedra of different shape (cubic, tetrahedra l, and triculent octahedral) are formed. Stabilization by citrate plus NaOH yields particularly long lifetimes of the colloidal solutions.