Electrochemical and photochemical studies on Dawson-type [P2W18O62]6-, [S2W18O62]4- and [S2Mo18O62]4- polyoxometalates as catalysts for water oxidation in different media

2017-02-02T02:32:16Z (GMT) by Bernardini, Gianluca
In this thesis is discussed the photochemical behaviour of polyoxometalates in different media with concomitant oxidation of water. Polyoxometalate anions (POMs) are a large group of complex inorganic compounds which are easily prepared in H2O from inexpensive, readily accessible precursors such as MO42- where M = W or Mo. 18WO42- + 32H3PO4 + 23H+ → P2W18O626- + 30H2PO-4 + 18H2O [1] POMs are generally large metal-oxygen clusters that form a unique, structurally diverse class of inorganic materials. The application of polyoxometalates in science is remarkably diverse: from medicine to catalysis, electronic devices to membranes etc. The most important application of polyoxometalates over the past two decades is in catalysis. In fact polyoxometalates often exhibit most of the features of a perfect environmentally benign oxidation catalyst: O2 is the only oxidant used, water can be used as the solvent (depending on the cation), and the catalyst possesses no organic ligands. Often they are also thermodynamically stable toward oxidative degradation. An interesting application for polyoxometalates as catalysts is the production of H2 and oxygen. Some general properties that make polyoxometalates an attractive class of material for catalysis have been reported. Selection of a suitable countercation can make the polyoxometalate soluble in either aqueous or organic solvents. Typically tetrabutylammonium (Bu4N+) is chosen as the countercation for reactions in organic solutions, and Na+ or K+ are commonly chosen for reactions in aqueous solutions. Generally speaking, molybdates are better oxidizing reagents than tungstates. Thus, their reoxidation by dioxygen is very slow, often requiring the use of activated carbon with dioxygen or the use of hydrogen peroxide. It is also possible to electrochemically regenerate the starting material by applying an oxidizing current that is positive enough to oxidize back the molybdenum polyoxometalate. When a polyoxometalate is photolysed by UV and near-visible light in the presence of a suitable electron donor (ED), for example isopropanol, a series of photoreactions result in stepwise reduction of the polyoxometalate and oxidation of the ED. In the case of (CH3)2CHOH the overall mechanism may be represented as follows: 2POMn- + (CH3)2CHOH → 2POM(n+1)- + (CH3)2CO + 2H+ [2] Reduction is accompanied by only minor structural changes as the reducing electrons occupy orbitals that are essentially non-bonding. As reduction proceeds, the POM anions acquire an increasing negative charge thereby encouraging protonation, thus the negative charge is lowered and encourages further reduction. It has been noticed during photochemical experiments that in molecular solvents, photo-reduction with the tungsten based polyoxometalates was achieved only if an efficient electron donor such as isopropanol is present. In contrast, the photo-reduction of the molybdenum based polyoxometalates was achieved in molecular solvents without the addition of isopropanol with water present in the solvent acting as an electron donor: [S2Mo18O62]4- + hν → [S2Mo18O62]*4- [3] 2[S2Mo18O62]*4- + H2O → 2[S2Mo18O62]5- + ½ O2 + 2H+ [4] Photo-reduction of [S2Mo18O62]4- to [S2Mo18O62]5- was confirmed by RDE voltammograms and visually colour change, while the Clark-type electrode recorded the increase of oxygen during irradiation with light. However, quantitative photoreduction of all polyoxometalates studied in this thesis occurred when POMs were dissolved in protic (DEAS, DEAP) or aprotic ([BMIM][BF4] and [BMIM][PF6]) room temperature ionic liquids containing adventitious or deliberately added water. Photo-reduction of POMs was also noted when these ionic liquids were used as electrolyte in molecular solvents. Thus, under these conditions water acted as efficient electrons donor when ionic liquids were employed as neat solvents or dissolved in concentration ≥ 0.05 M in molecular solvents. Ionic liquids represent a relatively new class of solvent that consist entirely of ionic species. They have many fascinating properties which make them of fundamental interest to chemists. Significantly, both the thermodynamics and kinetics of reactions carried out in ionic liquid media differ from those in conventional molecular solvents. The achievement of water oxidation in ionic liquids can be explained by the different nature of water, compared to molecular solvents, in this medium. Thus water oxidation was possible by the different nature of water molecules when surrounded by ionic liquid molecules clusters. It is possible that in ionic liquids the oxygen-hydrogen bond in water molecules is much weaker than in molecular solvents. This characteristic allows the break of this bond with the resultant production of oxygen and protons. The importances of the studies conducted in this thesis are innovative to achieve water oxidation. In this thesis it is proposed a new pathway using ionic liquid media for water oxidation using polyoxometalates that was not presented before my studies. Furthermore, water oxidation was achieved in aqueous solutions containing ionic liquids as electrolytes. This will allows extended electrochemical studies that were not possible in neat ionic liquids.