Metal solution precursors: their role during the synthesis of MoVTeNb mixed oxide catalysts

[EN] Synthesized via the slurry method and activated at high temperature (873 K), MoVTeNb multimetallic mixed oxides are applied to catalyze the oxidative dehydrogenation of ethane to ethylene (ODHE). Mixed oxides typically contain M1 and M2 crystalline phases, the relative contribution of these phases and the respective catalytic behaviour being notably influenced by the preparation conditions of the metallic aqueous solution precursor, given the complexity of the chemical interactions of metal species in solution. Thus, detailed in situ UV-vis and Raman studies of the chemical species formed in solution during each step of the synthetic procedure are presented herein. The main role of vanadium is to form decavanadate ions, which interact with Mo species to generate an Anderson-type structure. When niobium oxalate solution is added into the MoVTe solution, a yellow-coloured gel is immediately formed due to a common ion effect. When liquid and gel phases are separated, the M1 crystalline phase is produced solely from the gel phase. Attention is also devoted to the influence and role of each metal cation (Mo, V, Te and Nb) on the formation of the active M1 crystalline phase and the catalytic behaviour in the ODHE. The catalyst constituted mostly of M1 crystalline phase is able to convert 45% of the fed ethane, with a selectivity to ethylene of around 90%.This work was financially supported by the Instituto Mexicano del Petroleo (IMP) Project D.61010. EMF thanks CONACyT Mexico and IMP. JMLN thanks DGICYT in Spain (Project CTQ2015-68951-C3-1-R).Sánchez-Valente, J.; Maya-Flores, E.; Armendariz-Herrera, H.; Quintana-Solorzano, R.; López Nieto, JM. (2018). Metal solution precursors: their role during the synthesis of MoVTeNb mixed oxide catalysts. Catalysis Science & Technology. 8(12):3123-3132. https://doi.org/10.1039/c8cy00750kS31233132812Ushikubo, T., Oshima, K., Kayou, A., Vaarkamp, M., & Hatano, M. (1997). 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Synthesis, spectroscopy and characterization of titanium dioxide based photocatalysts for the degradative oxidation of organic pollutants

Tese de doutoramento. Engenharia Química e Biológica. Faculdade de Engenharia. Universidade do Porto. 20

Synthesizing Efficient Quasi-one-dimension Titanium Dioxide Nanocatalyst for Enhanced Photocatalytic Degradation of Aqueous Organic Pollutants and Hydrogen Production

This dissertation is focused on synthesizing Q1D TiO2-based nanocatalysts for degrading aqueous organic pollutants and producing H2. A facile alkaline hydrothermal process was used to synthesize Q1D TiO2 under different hydrothermal synthesis factors (reaction temperature, NaOH concentration and TiO2 precursor concentration). The hydrothermal synthesis factors significantly affected the Q1D TiO2 phase structure, crystal size, specific surface area (SSA), bandgap, photocatalytic activities. A Box-Behnken design (BBD) model was used to optimize the hydrothermal factors for synthesizing Q1D TiO2 with maximum photodegradation rate and H2 production rate. The optimized Q1D TiO2 with maximum photodegradation rate was further enhanced with partially reduced graphene oxide (RGO) (designated as GT) for degrading aqueous hazardous pollutants. The study also examined the impact of the RGO atomic oxygen-to-carbon (O/C) ratio on GT photocatalytic activities. The highest photocatalytic activity was observed when the RGO atomic O/C ratio was 0.130±0.003. Next, the GT photocatalyst was enhanced with Ag NPs (designated as Ag-GT). The highest photocatalytic activity was observed for a silver content of 10 wt% in the photocatalyst film. Finally, an atmospheric pressure plasma jet (APPJ) was employed to synthesize micrometer thick Ag nanoparticles modified TiO2 (Ag-TiO2) coatings, presenting a core-shell structure for degrading RhB and trace pharmaceutical compounds using a solar light source. The Ag-TiO2 coatings were characterized having a porous anatase phase, improved charge separation and visible light absorption. The highest photodegradation rate was observed for a silver content of 0.4wt% in the composite. Cette thèse porte sur la synthèse des nanocatalyseur à base de TiO2 Q1D (quasi-unidimensionnel) pour dégrader des polluants organiques aqueux et pour produire de l’hydrogène. Un processus hydrothermal alcalin simple a été utilisé pour synthétiser TiO2 Q1D avec des paramètres de synthèse différents (la température, la concentration de NaOH de précurseur TiO2). Ces paramètres affectent la structure de phase de TiO2 Q1D, la taille du cristal, la surface spécifique (SS), l\u27énergie du gap et les activités photocatalytiques qui en résultent. Le modèle Boîte-Behnken (BBD) est utilisé pour optimiser les paramètres hydrothermaux permettant d’obtenir un taux de photodégradation maximal des polluants étudiés, et un taux de production d’H2 maximal. Le TiO2 Q1D optimisé a été davantage amélioré à l’aide d’oxyde de gràphène partiellement réduit (RGO) pour obtenir le catalyseur désigné GT. L’impact du rapport atomique O/C du RGO sur l’activité catalytique du GT est étudié. La valeur maximale d’activité photocatalytique est obtenu pour O/C =0.130±0.003. Puis, le GT est amélioré avec les NP d’Ag (Ag-GT). L’activité maximale est obtenue pour 10%d’Ag. Enfin un jet plasma atmosphérique (APPJ) a été employé pour synthétiser des couches minces nanométriques de TiO2 avec inclusion des NP d’Ag de structure cœur-coquille pour la dégradation du colorant RhB et des différents des produits pharmaceutiques à l’aide d’un simulateur solaire. Les couches composites minces (Ag-TiO2) sont très poreuses de phase anatase avec une absorption dans le visible et une amélioration de la séparation des charges. La valeur maximale de la photo-dégradation a été obtenue pour 0.4% d’Ag dans la couche composite

Multifunctionality of crystalline MoV(TeNb) M1 oxide catalysts in selective oxidation of propane and benzyl alcohol

Propane oxidation at 653-673 K and benzyl alcohol oxidation at 393 K over phase-pure MoV(TeNb) M1 oxide catalysts were studied to gain insight into the multiple catalytic functions of the surface of the M1 structure. Electron microscopy and X-ray diffraction confirmed the phase purity of the M1 catalysts. Propane oxidation yields acrylic acid via propene as intermediate, while benzyl alcohol oxidation gives benzaldehyde, benzoic acid, benzyl benzoate, and toluene. The consumption rates of benzyl alcohol and propane level in the same range despite huge difference in reaction temperature, suggesting high activity of M1 for alcohol oxidation. Metal-oxygen sites on the M1 surface are responsible for the conversion of the two reactants. However, different types of active sites and reaction mechanisms may be involved. Omitting Te and Nb from the M1 framework eliminates acrylic acid selectivity in propane oxidation, while the product distribution in benzyl alcohol oxidation remains unchanged. The results suggest that the surface of M1 possesses several types of active sites that likely perform a complex interplay under the harsh propane oxidation condition. Possible reaction pathways and mechanisms are discussed