Herstellung und Charakterisierung von Platin-Clustern in Molekularsieben über die Synthese von anionischen Carbonylkomplexen

Abstract

A method to form stable subnanometer platinum clusters was worked out employing decomposition of stoichiometric anionic platinum carbonyl (Chini) complexes synthesized within the cages of supporting zeolites (X, Y, EMT and LTL). These ligand-stabilized metal clusters can serve as potential precursors for the preparation of highly active and selective nanoparticle catalysts of uniform cluster size, if agglomeration and sintering is avoided in the course of ligand removal. The well-defined cage and channel system of molecular sieves provided a suitable matrix for the preparation and stabilization of variably-sized metal clusters of up to a few nanometers in diameter thereby forming a narrow diameter distribution. The ship-in-the-bottle synthesis of carbonyl complexes within zeolite cages was established as promising approach to prepare well-defined zeolite-supported metal clusters. For anionic Pt-carbonyl complexes, the influence of zeolite host (basicity, nature of counter ions) and preparation conditions (water content, partial CO pressure, temperature) on size and stability of the complexes was studied in considerable detail. Based on these results, conditions were worked out for the preparation of extraordinarily stable subnanometer Pt clusters within the channel and cage structure of zeolite hosts. The properties of the clusters were characterized by UV/VIS as well as diffuse reflectance infrared Fourier (DRIFT) spectroscopy and also with XPS and EXAFS techniques.The obtained platinum clusters preserved a low nuclearity as evident by rapid recarbonylation resulting in quantitative regeneration of the initial Chini complex. A novel electron transition was interpreted as size-quantization effect and a strong electron donor capacity of Pt was detected in DRIFT spectra of chemisorbed CO and hydrogen.