Immobilized metal complexes on nanostructures with shape-selective properties and well-defined and highly uniform adsorption sites, such as zeolites or nanostructured metal oxides, have drawn wide attention because of their novel size-dependent properties and their potential applications for catalysis, biochemistry, fine and pharmaceutical chemistry, chemical sensing, microelectronic devices, and environmental technology.1 There is a striking lack of understanding on the anchoring and stabilization processes of well-defined metal centers, due in part to the complexity and size of the metal particles that are used as precursors. Rhodium-based catalysts are commonly used, among many other reactions, for the partial oxidation of methane to form syngas.2 However, the structure and stability of the rhodium active centers are not well understood. The anchoring of homogenous catalysts with simple structures on zeolites provides an opportunity to investigate well-defined active metal centers.3 The goal of this research was to investigate the surface chemistry during the reaction of three well-defined rhodium precursors with different rhodium oxidation states and ZSM-5 with different Si/Al ratios and the formation of their corresponding well-defined active rhodium centers by using an ensemble of in-situ spectroscopic techniques. Syntheses and transfers of samples were performed with exclusion of air and moisture. The Rh/ZSM-5 samples, containing 1 wt % Rh, were synthesized by forming a slurry of either Rh+(CO)2(acac) (acac is C5H7O2), Rh22+(O2CCH3)4, or Rh3+(acac) in toluene with ZSM-5 with Si/Al ratios of 23 and 50. The samples were characterized by XRD and TEM before and after the immobilization of the rhodium centers. In situ characterization of the samples was performed by IR, Raman, and XAS during different temperature and flow treatments to determine the structure and stability of the well-defined rhodium centers. In this work, we will present infrared, Raman, and XAS spectra providing evidence of the formation of Rh+, Rh2+, and Rh3+ active sites and Al– and Si–acac surface species. Time-resolved XANES spectroscopy was used to characterize the oxidation states of the immobilized rhodium centers during flowing He, H2, and CH4 treatment. Similarly, EXAFS spectroscopy was used to postulate a structural model of the immobilized rhodium catalysts. A discussion on the influence of the zeolite Si/Al ratio on the immobilization of the rhodium precursors will be presented. References
1. J. Guzman and B. C. Gates, Dalton Trans. 2003, 3303–3318.
2. Y. Liu, W. P. Fan, W. Z. Weng, H. L. Wan, J. Of Molecular Catalysis A: Chemical. 2005, 239, 193-200.
3. E. Ivanova, M. Mihaylov, F. Thibault-Starzyk, M Daturi, K. Hadjiivanov, J. Catal. 2005, 236, 168–171.