A fundamental study of Pt impregnation of carbon: Adsorption equilibrium and particle synthesis

Abstract

To determine whether the method of “strong electrostatic adsorption” (SEA) can be extended to the preparation of carbon-supported Pt catalysts, a series of carbons of different type (activated, black, and graphitic) with different surface areas and points of zero charge (PZC) has been studied. Cationic Pt tetraammine, [(NH3)4Pt]2+, was adsorbed over low- and mid-PZC carbons in the high pH range, while anionic hexachloroplatinate (IV), [PtCl6]2−, was adsorbed over high-PZC carbons in the low pH range. Adsorption equilibrium was determined by measuring pH and metal concentration in the impregnation solution before and after contacting with the carbon supports. Filtered, dried materials were reduced in hydrogen, and the Pt particle size was characterized by Z contrast imaging.Electrostatic adsorption occurs at short contact time (1h) for both Pt anions and cations. The adsorptive behavior of all carbons of like PZC is the essentially the same, independent of type and surface area. There are pH optima at which electrostatic adsorption is strongest; a sharp maximum occurs for anions in the low pH range at pH 2.9, while the high pH optimum for cations is pH 12. To attain the required final values, pH buffering by the surface, a phenomenon not sufficiently appreciated in the literature, has to be overcome. Particles synthesized by SEA are normally in the 1–2nm range and are as small as or smaller with narrower size distributions than by other methods, especially at high metal loadings.Results also reveal a longer time scale, reductive mechanism that occurs with Pt(IV) chlorides over carbon at low pH, which mitigates the need for precise pH control if the contact time is long, and might explain the small particle sizes obtained by dry impregnation with the [PtCl6]2− complex. This mechanism will be more fully explored in future work.