Intimate Binding Mechanism and Structure of Trigonal Nickel(I) Monocarbonyl Adducts in ZSM-5 Zeolite—Spectroscopic Continuous Wave EPR, HYSCORE, and IR Studies Refined with DFT Quantification of Disentangled Electron and Spin Density Redistributions along σ and π Channels

Nature and Topology of Metal-Oxygen Binding Sites in Zeolite Materials: 17 O High-Resolution EPR Spectroscopy of Metal-Loaded ZSM-5

Determining structural models is pivotal to the rational understanding and development of heterogeneous catalytic systems. A paradigmatic case is represented by open-shell metals supported on oxides, where the catalytic properties crucially depend on the nature of the metal-oxygen bonds and the extent of charge and spin transfer. Through a combination of selective ¹⁷ O isotopic enrichment and the unique properties of open-shell s-state monovalent Group 12 cations, we derive a site-specific topological description of active sites in an MFI zeolite. We show that just a few selected sites out of all possible are populated and that the relative occupancies depend on the specific properties of the metal, and we provide maps of charge and spin transfer at the metal-oxygen interface. This approach is not restricted to zeotype materials, rather it is applicable to any catalysts supported on oxygen-containing materials.

Room temperature O transfer from N2O to CO mediated by the nearest Cd(i) ions in MFI zeolite cavities

The dominant oxidation state of cadmium is +ii. Although extensive investigations into the +ii oxidation state have been carried out, the chemistry of CdI is still largely underdeveloped. Here, we report a new functionality of cadmium created by the zeolite lattice: room temperature O transfer from N2O to CO mediated by the nearest monovalent cadmium ions in MFI zeolite. Thermal activation of CdII ion-exchanged MFI zeolite in vacuo affords the diamagnetic [CdI-CdI]2+ species with a short CdI-CdI σ bond (2.67 Å). This species generates two CdI˙ sites under UV irradiation through homolytic cleavage of the CdI-CdI σ bond, and the thus-formed nearest CdI˙ sites abstract an O atom from N2O to generate the [CdII-Ob-CdII]2+ core, where Ob means bridged oxygen. This bridging atomic oxygen species is transferred to CO at room temperature, through which CO oxidation and regeneration of the CdI-CdI σ bond then proceed. This is the first example pertaining to the reversible redox reactivity of the nearest monovalent cadmium ions toward stable small molecules. In situ spectroscopic characterization captured all the intermediates in the reaction processes, and these data allowed us to calibrate the density-functional-theory cluster calculations, by means of which we were able to show that the charge compensation requirement at the nearest two Al sites arrayed circumferentially in the 10-membered ring of MFI zeolite creates such novel functionalities of cadmium. The unprecedented reactivity of CdI and its origin are discussed.

EPR approaches to heterogeneous catalysis. The chemistry of titanium in heterogeneous catalysts and photocatalysts

Paramagnetic species are often involved in catalytic or photocatalytic reactions occurring at the solid-gas interface of heterogeneous catalysts. In this contribution we will provide an overview of the wealth and breadth of information that can be obtained from EPR in the characterization of paramagnetic species in such systems, illustrating the advantages that modern pulsed EPR methodologies can offer in monitoring the elementary processes occurring within the coordination sphere of surface transition-metal ions. To do so we selected three representative systems, where titanium ions in low oxidation states act as active catalytic sites, trying to outline the methodological approaches which characterize the application of EPR techniques and the questions that can be answered and addressed relative to the characterization of heterogeneous catalytic materials.

Room temperature stable zinc carbonyl complex formed in zeolite ZSM-5 and its hydrogenation reactivity: a solid-state NMR study

The structure and reactivity of a room temperature stable zinc carbonyl complex in Zn-modified H-ZSM-5 zeolite were revealed by solid-state NMR spectroscopy.