Preparation of Highly Dispersed Platinum Particles in HZSM-5 Zeolite: A Study of the Pretreatment Process of [Pt(NH3)4]2+

Abstract

The pretreatment of [Pt(NH3)4]2+, ion exchanged into zeolite HZSM-5, was studied using temperature-programmed techniques and diffuse-reflectance UV/VIS spectroscopy. The pretreatment is an important part of the preparation of bifunctional Pt/HZSM-5 catalysts, which are active in the hydroisomerization of alkanes. Pretreatment in a helium atmosphere leads to autoreduction and the conversion of part of the NH3ligands into H2and N2. Pretreatment in an oxygen-containing atmosphere leads to the oxidation of the NH3ligands into N2, N2O, and H2O. Temperature-programmed pretreatment with oxygen gives rise to three reaction-rate maxima if the sample is not dried, and the conclusion is that [Pt(NH3)3(H2O)]2+is formed as an intermediate in the first step. If temperature-programmed pretreatment with oxygen is carried out on a previously dried sample, oxidation of the ligands is a two-step process: three of the NH3ligands are oxidized in one rapid step and the last ligand is oxidized at a higher temperature. Careful pretreatment in an oxygen-containing atmosphere of [Pt(NH3)4]HZSM-5 leads to the formation of the pale green PtHZSM-5 in which platinum is present as Pt2+coordinated to the zeolite framework. Small platinum particles are formed by subsequent reduction of these platinum ions in flowing hydrogen.