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Cu3TeO6 (CTO) has been synthesized by hydrothermal synthesis applying different pH values without any template or a calcination step to control the crystalline phase and the morphology of nanoparticles. The physicochemical properties characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, N2 adsorption, X-ray photoelectron spectroscopy, and diffuse reflectance ultraviolet–visible (DRUV–vis) spectroscopy techniques revealed that the pH values significantly influence the crystal growth. In acidic media (pH = 2), crystal growth has not been achieved. At pH = 4, the yield is low (10%), and the CTO presents irregular morphology. At pH = 6, the yield increases (up to 71%) obtaining an agglomeration of nanoparticles into spherical morphology. At basic conditions (pH = 8), the yield increases up to 90% and the morphology is the same as the sample obtained at pH = 6. At high basic conditions (pH = 10), the yield is similar (92%), although the morphology changes totally to dispersed nanoparticles. Importantly, the as-prepared CTO semiconductor presents photocatalytic activity for H2 production using triethanolamine as a sacrificial agent under visible light illumination. The results also revealed that the nanoparticles agglomerated in a spherical morphology with larger surface area presented almost double activities in H2 production compared to heterogeneously sized particles. These results highlight the suitable optoelectronic properties, including optical band gap, energy levels, and photoconductivity of CTO semiconductors for their use in photocatalytic H2 production.

Solar photocatalytic H2 production has drawn an increasing amount of attention from the scientific community, industry, and society due to its use of green solar energy and a photocatalyst (semiconductor material) to produce green H2. Cu-based semiconductors are interesting as photocatalysts for H2 production because Cu is earth-abundant, cheap, and the synthesis of its copper-containing semiconductors is straightforward. Moreover, Cu-based semiconductors absorb visible light and present an adequate redox potential to perform water splitting reaction. Nevertheless, pristine Cu-based semiconductors exhibit low photoactivity due to the rapid recombination of photo-induced electron-hole (e−-h+) pairs and are subject to photo corrosion. To remedy these pitfalls, the Cu semiconductor-based Z-scheme family (Z-schemes and S-schemes) presents great interest due to the charge carrier mechanism involved. Due to the interest of Z-scheme photocatalysts in this issue, the basic concepts of the Z-scheme focusing on Cu-based semiconductors are addressed to obtain novel systems with high H2 photo-catalytic activity. Focusing on H2 production using Cu-based Z-schemes photocatalyst, the most representative examples are included in the main text. To conclude, an outlook on the future challenges of this topic is addressed.

Photocatalysis represents a promising technology that might alleviate the current environmental crisis. One of the most representative photocatalysts is graphitic carbon nitride (g-C3N4) due to its stability, cost-effectiveness, facile synthesis procedure, and absorption properties in visible light. Nevertheless, pristine g-C3N4 still exhibits low photoactivity due to the rapid recombination of photo-induced electron-hole (e−-h+) pairs. To solve this drawback, Z-scheme photocatalysts based on g-C3N4 are superior alternatives since these systems present the same band configuration but follow a different charge carrier recombination mechanism. To contextualize the topic, the main drawbacks of using g-C3N4 as a photocatalyst in environmental applications are mentioned in this review. Then, the basic concepts of the Z-scheme and the synthesis and characterization of the Z-scheme based on g-C3N4 are addressed to obtain novel systems with suitable photocatalytic activity in environmental applications (pollutant abatement, H2 production, and CO2 reduction). Focusing on the applications of the Z-scheme based on g-C3N4, the most representative examples of these systems are referred to, analyzed, and commented on in the main text. To conclude this review, an outlook of the future challenges and prospects of g-C3N4-based Z-scheme photocatalysts is addressed.

[ES] En esta tesis se han estudiado la síntesis y aplicación de clústeres y átomos metálicos aislados de Pd y Pt. Clústeres y átomos aislados han sido sintetizados siguiendo nuevas metodologías en disolución, sin utilizar ligandos orgánicos, y soportándolos en materiales metal-orgánico estructurados (MOFs) o polímeros. Después de la síntesis, las pequeñas especies metálicas se han caracterizado con diferentes técnicas, como espectroscopias ultravioleta-visible, de emisión, de absorción de rayos X, análisis atómico de emisión por plasma de inducción acoplado, difracción de rayos X y microscopía. Los clústeres y los átomos aislados se han utilizado como catalizadores en reacciones de acoplamiento C-C, de oxidación de alcoholes y como agentes antitumorales. La síntesis de dímeros de PdII soportados dentro de las cavidades del MOF ha dado lugar a la formación de complejos supramoleculares de coordinación en los canales del MOF (SCCs@MOF). En los SCCs, los dímeros de PdII representan los centros de coordinación, y ligandos de piridina fluorados y no-fluorados son los conectores entre las unidades bimetálicas. Después de la caracterización, los SCCs@MOF se han empleado como catalizadores en reacciones de acoplamiento C-C de ácidos borónicos y/o alquinos y de oxidación de alcoholes alquílicos. Estos catalizadores se comparan con los respectivos homogéneos, como un complejo cíclico de paladio, previamente sintetizado, o el sistema Pd(OAc)2/ligandos. Además, se mostrarán algunos de los pocos ejemplos de reacciones de acoplamiento C-C y de oxidación catalizadas por SCCs. También se ha llevado a cabo la reacción de oxidación de alcohol bencílicos catalizada por átomos aislados de paladio. Estas pequeñas especies fueron preparadas in situ, dando lugar a uno de los primeros ejemplos de átomos aislados en disolución en ausencia de ligandos. Para averiguar la actividad real de los átomos aislados y poder estabilizarlos, se han soportado en un MOF basado en cisteína y se han utiliz, [CA] En la present tesi doctoral s'ha realitzat un estudi de la síntesi i aplicació de clústers metàl·lics i àtoms aïllats de Pd i Pt com a catalitzadors. Ambdós s'han sintetitzat seguint noves metodologies en dissolució, sense la utilització de lligams i suportant-los en materials metal-orgànic estructurats (MOFs) o polímers. Una vegada sintetitzats, s'ha dut a terme una caracterització minuciosa amb diferents tècniques, com espectroscòpia Ultraviolat-Visible (UV-Vis), d'emissió, d'absorció de raigs X (XAS), d'emissió atòmica per plasma d'acoblament inductiu (ICP-AES), difracció de raigs X i microscòpia. Aquests clústers i àtoms aïllats s'han utilitzat com a catalitzadors en reaccions d'acoblament C-C, en oxidacions d'alcohols i en tractaments antitumorals. Concretament, la síntesi dels dímers de PdII suportats dins de les cavitats del MOF ha donat lloc a la formació de complexos de coordinació supramoleculars dins dels canals del MOF (SCCs@MOF). En els SCCs, els dímers de PdII representen els centres de coordinació, i les diferents unitats bimetàl·liques estan connectades per lligams de piridina fluorats i no fluorats. Després de la seua caracterització, els SCCs@MOF s'han utilitzat com a catalitzadors de reaccions d'acoblament C-C d'àcids borònics i/o alquins i en reaccions d'oxidació d'alcohols alquílics. També s'ha dut a terme la comparació entre els corresponents catalitzadors homogenis, per exemple un complex cíclic de pal·ladi o del sistema Pd(OAc)2/lligams. D'altra banda, mostrem un dels pocs exemples de reaccions d'acoblament C-C i de reaccions d'oxidació catalitzades per SCCs. A més, també es descriu la reacció d'oxidació d'alcohol benzílic catalitzada per àtoms aïllats de Pd. Aquestes espècies menudes es preparen in situ, donant lloc a un dels primers exemples d'àtoms aïllats en dissolució sense lligams. A més a més, per a poder verificar la seua activitat i estabilitzar-los, s'han suportat en MOFs basats amb cisteïna, utilitzant-se en el mateix procés d, [EN] The synthesis and application of tal clusters (MCs) and single atoms catalysts (SACs) of Pd and Pt is explored in this Thesis. Both clusters and single atoms were synthesized by following novel ligand-free procedures in solution, or by supporting them on Metal-Organic Frameworks (MOFs) or on polymers. After their synthesis, these entities were deeply characterized by techniques, such as Ultraviolet-Visible (UV-Vis), emission, X-ray Absorption (XAS) spectroscopies, Induced Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), X-ray Diffraction and microscopy, among others. Finally, MCs or SACs were used as catalyst in homo- or cross-coupling reactions, in alcohol oxidation reactions, and in antitumoral treatments. Especially, the supporting of PdII dimers inside a MOF network allowed the following formation of Supramolecular Coordination Complexes (SCCs) within the MOF channels (SCCs@MOF). In the SCCs, the PdII dimers represent the pivotal centers and different pyridyl ligands, fluorinated and non-fluorinated, embody the linkers between the bimetallic units. After being characterized, the SCCs@MOF were used as catalysts in homocoupling reactions of boronic acids and alkynes, in cross-coupling reactions of alkynes and in alkyl alcohols oxidation. These catalysts were compared with its homogeneous counterparts, i.e., a pre-synthesized Pd metallacycle or the system Pd(OAc)2/ligands. Overall, we show some examples of C-C coupling and oxidation reactions catalyzed by SCCs. Moreover, benzyl alcohols oxidation reaction catalyzed by palladium single atoms is described. The small metal species were prepared in situ, leading to one of the first example of ligand-free single atoms in solution. Afterward, in order to verify the real activity of single atoms and to stabilize them, they were supported inside a cysteine-based MOF and used in the same oxidation process. Finally, the application of Pt clusters (MCs) in antitumoral treatments is portrayed. The clusters were synt

[EN] Supramolecular coordination compounds (SCCs) represent the power of coordination chemistry methodologies to self-assemble discrete architectures with targeted properties. SCCs are generally synthesized in solution, with isolated fully coordinated metal atoms as structural nodes, thus severely limited as metal-based catalysts. Metal-organic frameworks (MOFs) show unique features to act as chemical nanoreactors for the in situ synthesis and stabilization of otherwise not accessible functional species. Here, we present the self-assembly of Pd-II SCCs within the confined space of a pre-formed MOF (SCCs@MOF) and its post-assembly metalation to give a Pd-II-Au-III supra molecular assembly, crystallography underpinned. These SCCs@MOFs catalyze the coupling of boronic acids and/or alkynes, representative multi-site metal-catalyzed reactions in which traditional SCCs tend to decompose, and retain their structural integrity as a consequence of the synergetic hybridization between SCCs and MOFs. These results open new avenues in both the synthesis of novel SCCs and their use in heterogeneous metal-based supramolecular catalysis.