Polyaniline stabilized highly dispersed Pt nanoparticles: Preparation,characterization and catalytic properties

Platinum–polyaniline (2%Pt/PANI) composites are prepared using three precursor solutions of H₂PtCl₆ differing in the pH and thus in the type of dominating chloro–aqua or chloro–hydroxo complexes of Pt(IV). Various characterization techniques (X-ray diffraction, FTIR and X-ray photoelectron spectroscopy, and electron microscopy; TEM, HRTEM) were employed in order to elucidate the role of platinum precursor composition in the state of Pt-species formed in the Pt/PANI composites. In all three as-received Pt/PANI composites, platinum in two valence states Pt(IV) and Pt(II) was observed thus showing redox reaction under preparation of composites. The reduction of Pt(IV) is much easier in the precursor solution of alkaline pH, i.e. when the Pt(IV) complexes comprising aqua and/or hydroxo ligands are the dominating ones. Under these conditions the content of Pt(II)-species is ca. 7× higher than that of Pt(IV) ones. Reduction of as-prepared composites by NaBH₄ gives 2%Pt/PANI catalysts of well-dispersed Pt-metal particles of size ranging from 1 nm up to 4 nm. The average diameters (2.24 nm) of Pt-particles was slightly smaller in 2%Pt/PANI catalyst prepared using H₂PtCl₆ solution of acidic pH. Catalytic properties of Pt/PANI composites are tested in liquid phase hydrogenation of alkyne-type reactant, 2-butyne-1,4-diol (B3-D) and in gas-phase hydrodehalogenation of CCl₄. In the former, a classical example of alkyne to alkene and finally alkane hydrogenation, a catalytic behavior of Pt/PANI composites in terms of activity and selectivity was found to be similar to that observed for typical inorganic carrier supported Pt-catalysts. CCl₄ hydrodehalogenation on Pt/PANI leads initially to the formation of chloroform, which is the desired reaction product. However, in the course of reaction liberation of HCl brings about large changes, both in overall activity, as well as the selectivity, giving large amounts of dimeric C₂ products, especially C₂Cl₆. It is speculated that these changes follow from reprotonation of PANI which acquires Brönsted acidity used in dimerization of CCl₃ radicals.