SUPPORTED METAL-CATALYSTS - PREPARATION, CHARACTERIZATION, AND FUNCTION .1. PREPARATION AND PHYSICAL CHARACTERIZATION OF PLATINUM CATALYSTS

The preparation, by impregnation (I), co-crystallization (C), and metal vapour deposition (M), of a range of silica-, alumina-, and molybdena-supported platinum catalysts is described. These materials (and the supports) have been thoroughly characterised in both pre- and postreduction states using a wide range of physical techniques including electronic spectroscopy, TGA, TPR, XPS, HRTEM. and EXAFS. UV-visible spectroscopic data for the impregnated materials is consistent with the presence of octahedral Pt(IV) environments in the prereduction state. Pt/silica (I) is essentially chloride-free after reduction, whereas Pt/alumina (I) retains a significant amount of chloride which is associated with the support rather than the metal. Pt/silica (M) is chloride-free and Pt/alumina (M) contains no more than the residual chloride initially associated with the alumina support. TGA and TPR data are largely as expected and, in the case of the molybdena-supported catalysts, are consistent with hydrogen bronze formation upon reduction. XPS data are consistent with the presence of Pt(0) in Pt/alumina (M) and the reduction of platinum to the metallic state in the case of Pt/silica (I). In contrast, with Pt/alumina (I), Pt/molybdena (I), and Pt/molybdena (C), the metal retains a significant δ+ character in the postreduction state. In the case of Pt/alumina (I), this behaviour is best interpreted in terms of a metal-support interaction, whereas that observed with the molybdena supports is consistent with hydrogen bronze formation. XPS, TGA, and TPR data show a high degree of internal consistency with respect to bronze stoichiometries. HRTEM and EXAFS data are self-consistent and, with the exception of Pt/molybdena (I), the platinum crystallites are extremely highly dispersed, with size distributions in the order Pt/alumina (M) ∼ Pt/silica (M) < Pt/molybdena (C) < Pt/alumina (I) < Pt/silica (I) ≪ Pt /molybdena (I). Models for the platinum sites in Pt/alumina (I) and Pt/silica (I) have been developed. © 1993 Academic Press, Inc.