Dft studies on polyoxopalladates and polyoxometalate-surface composites: from structure to catalysis
- Autores: Zhongling Lang
- Directores de la Tesis: Anna Maria Clotet Romeu (dir. tes.), Josep Maria Poblet Rius (codir. tes.)
- Lectura: En la Universitat Rovira i Virgili ( España ) en 2017
- Idioma: español
- Tribunal Calificador de la Tesis: Josep Manel Ricart Pla (presid.), Laia Vilà Nadal (secret.), Francesc Viñes Solana (voc.)
- Programa de doctorado: Programa de Doctorado en Ciencia y Tecnología Química por la Universidad Rovira i Virgili
- Materias:
- Enlaces
- Tesis en acceso abierto en: hdl.handle.net TDX
- Resumen
Modern computational methods based on density functional theory (DFT) have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of polyoxometalates. Present thesis aims to advance in the knowledge of basic concepts as the templating role of cations in the formation of molecular oxide clusters, as well as in the modelling of polyoxometalates adsorbed on the surfaces and the catalytic performance of POM-surface composites. The latter one is still a challenge because of the size and complexity of the systems to deal with.
Since the discovery of first polyoxopalladate by Ulrich Kortz group in Jacobs University, a rapid development of this family has happened. The number of experimental data available has allowed to developed a systematic computational study (Chapter 3), to determine the role that cations play in the formation of polyoxopalladates. The final aim is to rationalize experimental results and potentially to guide experiments for rational synthesis of new compounds. Through the calculations, it revealed that the capture of a Mq+ ion by a peripheral PdII-oxo shell involves the competition between the mother PdII and the guest metal ions. We have studied in detail how the size and charge of the cation control the formation of MPd12 clusters. The main factors governing the formation of a particular polyanion are the charge and size of the guest cation, the electrostatic-interaction between the cation and the surrounding oxo ligands and the dehydration ability of the cation. The accurate analysis has allowed to predict the stability of new clusters, some of them have been already synthesized and characterized by experiment. The same group reported novel structures that contain Ag-Ag and Ag-Pd contacts. We have demonstrated that real metal-metal bonds are formed by electrostatic confinement.
On the other hand,the catalytic efficacy of POMs has encouraged people to involve them on surfaces or nanoparticles to obtain composite materials with wide involvement in several fields, such as electrocatalysis, stabilization of metal nanoparticles, preparation of devices, etc. In order to gain some computational insights. To begin, the reactivity of a non-classical polyoxomolybdate towards gold reduction process has been explored by using DFT method. We revealed that the Kabanos-type polyoxomolybdate, [Na{(Mo2VO4)3(μ2-O)3(μ2-SO3)3(μ6-SO3)}2]15– can act as sufficient electron resources for completely Au(III)→Au(0) reduction, and the whole reduction process is very favorable from a thermodynamic point of view. In the last four chapters of the thesis we are dedicated to the analysis of POMs interacting with surfaces. In the first and two (chapter 5 and 6), we propose an efficient strategy to study these composites by using an integrated approach based on first principle calculations and a periodic continuum solvation model. The effects of counterions and solvent are discussed. We find that, unlike the modeling of polyoxoanions in solution, the incorporation of counterions in the computational models is crucial for accurately reproducing the electronic properties of the system, even if an implicit solvent is used. This strategy has been successfully applied to the description of highly charged lacunary [PW11O39]7− (PW11) adsorbed on the gold and silver surfaces, which has been revealed to a counterintuitive adsorption mode on the surface. With these factors taken into account, we have explored two important reactive processes. One is the water-gas-shift reaction co-catalyzed by PMo12 and Au(111). In addition to the fact that for the first time a heterogeneous catalyzed reaction containing a POM has been studied computationally in deep, we have found that the POM acts as an electron and proton acceptor reducing significantly the apparent activation energy of WGSR (<10 kcal mol-1). To the best of our knowledge, this is the first time that a so low value has been predicted for this reaction. Another process is to modelling the adsorption of PTiW11 on the TiO2 surface , we have shown that the weak absorption of TiO2 in the visible region can be improved by the coordination of reduced PTiW11 anion to the surface. Simulation of the observed UV-Vis spectrum suggests that the reduced PW11Ti can transfer electrons to TiO2 under visible light irradiation.
