Experimental Study and Modeling of the UV–Vis and Infrared Spectra of the [VO(O₂)Hheida]⁻ Complex Dissolved in Water

Combined theoretical and experimental studies of the [VO­(O₂)­Hheida]⁻ anion dissolved in water that may serve as a functional model for vanadium haloperoxidase enzymes have been performed. The geometrical structure and absorption and vibrational spectra of this system have been evaluated within the framework of density functional theory (DFT). The obtained theoretical results on the equilibrium structure and optical spectra are in quite good agreement with the experimental data. With the aid of the combination of UV–visible spectroscopy and electronic structure calculations, it has been revealed that, in the apparent absorption spectra of the [VO­(O₂)­Hheida]⁻ anion, the highest in energy band corresponds to a ligand to metal electron excitation, while the band with a maximum at 430 nm arises from the peroxo group. The calculations also reproduce quite well the positions, intensities and the grouping of frequencies in the near-infrared (NIR) spectra. The visualization of the calculated vibrations in the energy range of 400–1100 cm^–1 has been presented

Exploring the Structure of Paramagnetic Centers in SBA-15 Supported Vanadia Catalysts with Pulsed One- and Two-Dimensional Electron Paramagnetic Resonance (EPR) and Electron Nuclear Double Resonance (ENDOR)

Using pulsed EPR and ENDOR, the full set of <i>g</i> matrix and vanadium hyperfine parameters of the persistent paramagnetic V⁴⁺ center in “as prepared” and H₂ reduced SBA-15 supported VO_<i>x</i> catalysts has been measured. The determination of relative signs of the vanadium hyperfine tensor elements by ENDOR using orientation selection allowed an unambiguous extraction of the isotropic part of this interaction. This allowed for identification of the persistent V⁴⁺ center as a surface exposed deprotonated vanadium site. The same site topology was found for oxidized and H₂ reduced catalysts, thus indicating that the identified sites represent catalytically active centers. Hyperfine interaction with distant protons indicates formation of an oligomeric structure even for samples with vanadium loadings of less than 2 wt %. This conclusion is confirmed by applying 2D EPR for measuring the hyperfine interaction with neighboring vanadium atoms, covalently linked to reduced V⁴⁺ sites. Hence, application of 2D EPR enabled us to directly identify the previously proposed V–O–V structural motif on SiO₂ supported VO_<i>x</i> catalysts for the first time

Probing Metal–Support Interaction in Reactive Environments: An in Situ Study of PtCo Bimetallic Nanoparticles Supported on TiO₂

Our recent surface characterization studies of extended and nanosized PtCo alloys under hydrogen and oxygen atmospheres, indicated significant and reversible surface segregation in response to the gas phase environment [J. Phys. Chem. Lett. 2011, 2, 900]. In the present communication, an insight into the effect of the support on the PtCo alloy stability is attempted. A model PtCo/TiO₂ interface is investigated under reducing, oxidizing, and catalytic reaction conditions using ambient pressure X-ray photoelectron and absorption spectroscopies (APPES and NEXAFS respectively). Encapsulation of PtCo by the TiO₂ support was observed upon vacuum annealing. Upon oxidation/reduction conditions, a mixture of CoO_<i>y</i> (1 ≤ <i>y</i> < 1.33), TiO₂, and mixed Co_<i>x</i>Ti_<i>y</i>O_<i>z</i> phases with Pt located in the subsurface was formed. TiO₂ was found to be remarkably stable under the temperature and pressure conditions used here (up to 620 K, 0.2 mbar), with titanium remaining always in the Ti⁴⁺ state. The interplay between the gas atmosphere and the surface is limited to modifications of the cobalt oxidation state. However, in contrast to the observations on the unsupported PtCo alloy, neither oxidation of CoO to Co₃O₄ in O₂ nor full reduction to metallic Co under various reducing agents (H₂, CH₃OH), occurred. Synchronized changes of the binding energy position of core level photoelectron peaks in response to the gas phase are related to the band-bending development at the gas/solid interface. This documents the direct coupling of the electronic properties and the gas phase chemical potential of a chemically functional material useful as catalyst or gas sensing device