Formation of Pt particles in Y-type zeolites: The influence of coexchanged metal cations

Abstract

Samples of platinum, supported on NaY, HNaY, and M NaY (M = Ca 2+ , La 3+ , or Fe 2+ ) , have been studied by temperature-programmed reduction (TPR), desorption (TPD), and extended X-ray absorption fine structure (EXAFS) to investigate the mechanism of the formation of Pt particles and the influence of H + and added metal cations on it. Size and location of Pt particles in a zeolite matrix were found to be strongly dependent on the distribution of Pt ions between the supercages and the sodalite cages, which is controlled by the calcination temperature ( T c) prior to the reduction. At low T c (e.g., 360 °C), the majority of the Pt 2+ ions are in the supercages; after reduction small particles (5–10 Å) are formed which are also located in the supercages. This is also true for Pt Y with or without coexchanged multivalent cations. The Pt 0O 2− distance is 2.65 Å as derived from the k 1-weighted EXAFS functions. At medium T c (e.g., 450 °C), the Pt 2+ ions are distributed between supercages and sodalite cages. In this case, the Pt 2+ ions in the supercages are reduced first to form small particles which become nucleation sites for Pt 0 atoms leaving the sodalite cages above 400 °C. Eventually, these Pt particles can become larger than the supercages. At high T c (e.g., 550 °C), most Pt 2+ ions migrate to sodalite cages and require a high reduction temperature ( T r). The reduced Pt 0 atoms then migrate from sodalite cages to supercages, and in the absence of nucleation sites they finally coagulate on the external surface of the zeolite crystals to form large Pt particles. However, the presence of coexchanged multivalent cations, e.g., Fe 2+, which can effectively block sodalite cages and hexagonal prisms, thus forcing Pt 2+ ions to stay in supercages, can prevent the formation of large particles on the external surface of zeolites even at high T c. The evolution of hydrogen at T max = 450 °C, caused by oxidative reaction of Pt 0 atoms with hydroxyl groups during TPD, as indicated by FTIR, can be used to determine the presence of Pt in sodalite cages.