PhD Thesis Defence Presentations - Andriopoulou Chrysanhti

The industrial significance of the oxo-rhenate catalysts has triggered basic research and studies on the nature and functionality of the active sites of such catalysts, and particularly the dispersed surface oxo-rhenate species present. The importance of these catalysts lies in their extensive use in olefin metathesis reactions as well as in hydrodesulfurization, hydrogenation and selective hydrogenation reactions, selective reduction of NO_X and partial oxidation of methanol. To this end, the supported oxo-rhenate catalysts have been studied with a variety of techniques. Raman Spectroscopy is very well suited for studying supported metal oxides and consequently supported rhenium oxide. The molecular nature of this characterization and its ability to discriminate between different molecular configurations that can simultaneously exist in the catalytic system make Raman spectroscopy a very powerful characterization tool.

In the present study we study the molecular structure of rhenium oxide catalysts supported on TiO₂ (P25) with in situ molecular vibrational Spectroscopy (Raman and IR). The (ReO_x)_n/TiO₂ (P25) catalysts with surface densities from 0.16 to 2.5 Re/nm² were prepared by the wet impregnation method and the Equilibrium Deposition Filtration (EDF) method. Oxorhenate catalysts supported on two more oxiic carriers (monoclinic ZrO₂, γ-Al₂O₃) with surface densities up to monolayer coverage were also prepared and studied. The use of Raman Spectroscopy under controlled conditions (in situ) allowed the investigation of structural features and changes occurring under oxidative dehydrated conditions at various temperatures. Combinations of i) Raman with IR Spectroscopy and ii) Raman Spectroscopy with ¹⁸O/¹⁶O isotope substitution experiments provided useful and complementary information for establishing the molecular characteristics of the oxo-rhenate sites. The spectra obtained are adequate to determine the heterogeneity of the deposited rhenium phase and to discuss the vibrational properties and configurations of the various deposited sites (ReO_x) as well as their dependence on temperature, loading and support. The results of isotopic substitution shed further light on the termination configurations of the oxo-Re^vII species.

The temperature-dependent evolution of the configurations and molecular structures of (WO_x)_n and (MoO_x)_n species supported on TiO₂(P25) prepared by the equilibrium deposition filtration (EDF) method, was also studied with a view to shed light on the mechanisms of the thermal dispersion and temperature evolution of the dispersed sites following the stage of filtration and during the stages of drying and calcination. The main aim had been an attempt to illustrate that the EDF method is able (at least in a certain range of submonolayer loadings) to result in designed site speciations and that particular snapshots of the structural/configurational temperature evolution can be captured by in situ Raman Spectroscopy. The question of the possible temperature-dependence of the structure / configuration of the amorphous dispersed phase on supported transition metal oxides systems has been of great concern to the scientific community, and the prevailing view argues that catalyst heating above the Tamman Temperature (T_TAM) which is the half of the melting temperature of the metal oxide activates a surface thermal dispersion and spillover, whereas after the final calcination the "thermodynamically stable form" of the dispersed oxometallic phase will prevail on the surface of the carrier. In the present study, evidence is presented based on in situ Raman Spectroscopy, that controverts the above prevailing view, and it proves that the molecular structure of the dispersed oxometallic phases ((Μο^VIΟ_x)_n, (W^VIΟ_x)_n and (Re^VΙIΟ_x)_n is subject to reversible temperature-dependent equilibria between mono- oxo and poly-oxo (most likely di-oxo) configurations.

The last part of this thesis pertains to a case study of reference compounds on the coordination chemistry of Re^VII in an environment of O ligands and the vibrational properties of Re(=O)_x termination configurations. The dissolution reaction of the low melting and volatile (m.p. 297^oC, b.p 360^oC) Re₂O₇ in molten K₂S₂O₇ and K₂S₂O₇-K₂SO₄ in the full composition range is studied by Raman spectroscopy under static equilibrium in oxidative atmosphere at temperatures of 260–470°C. The reaction of Re₂O₇ with K₂S₂O₇ in the molten phase results in formation of two isomeric Re^VII-oxosulafto complexes at a 1:1 stoichiometry. At low contents of Re₂O₇  and temperatures around 300^OC, the majority species is the mononuclear ReO₃SO₄^-(l) complex, in which Re^VII takes on a tri-oxo termination configuration with 5-fold coordination. At increasing contents of Re₂O₇  and higher temperature the ReO₃SO_4-(l) complex interconverts to its isomeric dinuclear (ReO₂)₂O₂(SO₄)₂²⁻ molten complex where Re^VII takes on a dioxo termination configuration with 6-fold coordination and a double oxygen bridge connecting the two Re atoms. The 2ReO₃SO₄^‑(l)«(ReO₂)₂O₂(SO₄)₂^2-(l) interconversion is reversible and full reinstatement of the individual spectral features for each complex takes place with appropriate control of the temperature. The ReO₃SO₄^‑ and (ReO₂)₂O₂(SO₄)₂^2- molten complexes are useful reference compounds for discerning the termination configuration of dispersed (ReO_x)_n sites in supported catalysts. The molten (ReO₂)₂O₂(SO₄)₂^2- complex reacts with K₂SO₄ giving rise to formation of S₂O₇²⁻ and ReO₄^-.