Heterogeneous and Heterogenized Catalysts for Water Oxidation Reaction as Studied by Means of Sacrificial Oxidant

Hydrogen production from solar-driven water splitting (WS) reaction is considered a promising way to store solar energy. This process can be achieved directly by means of a photo-electrochemical cell (PEC), where light absorption, charge separation and WS occur in a single device; or indirectly, by coupling a photovoltaic device to an electrolyzer. WS is a thermodynamically uphill reaction, formed by two half reactions, i.e. the reduction of protons into H2 and the oxidation of water into O2. From a kinetic point of view, the latter is the most challenging one, being generally considered as the bottleneck for a widespread use of WS for hydrogen production. Thus, regardless of whether WS is carried out in direct or indirect configurations, the efficiency of the water oxidation catalyst (WOC) is a key determinant of the overall energy storage efficiency. The present thesis fits within this context, being focused on the study of different WOCs based on earth abundant elements (i.e. Mn and Co). Different heterogeneous Mn-based WOCs were studied, including: (i) different crystal structures of Mn oxides, both commercial and lab-made (i.e. MnO2, Mn3O4 and Mn2O3); (ii) a calcium manganese oxide (containing Ca2Mn3O8 and CaMn2O4) and (iii) lanthanum manganites (LaMnO3) prepared via sol-gel (SG) and flame spray pyrolysis (FP). On the other hand, the heterogenization of a homogeneous Co based WOC, i.e. the polyoxometalate Na10[Co4(H2O)2(PW9O34)2] (CoPOM), was investigated, as well. CoPOM is known to be a highly active WOC, comprising an active {Co4O4} core stabilized by oxidatively resistant polytungstate ligands. Transferring its solution reactivity to solid substrates is a fundamental step in the realization of a PEC

Heterogeneous and Heterogenized Catalysts for Water Oxidation Reaction as Studied by Means of Sacrificial Oxidant

Hydrogen production from solar-driven water splitting (WS) reaction is considered a promising way to store solar energy. This process can be achieved directly by means of a photo-electrochemical cell (PEC), where light absorption, charge separation and WS occur in a single device; or indirectly, by coupling a photovoltaic device to an electrolyzer. WS is a thermodynamically uphill reaction, formed by two half reactions, i.e. the reduction of protons into H2 and the oxidation of water into O2. From a kinetic point of view, the latter is the most challenging one, being generally considered as the bottleneck for a widespread use of WS for hydrogen production. Thus, regardless of whether WS is carried out in direct or indirect configurations, the efficiency of the water oxidation catalyst (WOC) is a key determinant of the overall energy storage efficiency. The present thesis fits within this context, being focused on the study of different WOCs based on earth abundant elements (i.e. Mn and Co). Different heterogeneous Mn-based WOCs were studied, including: (i) different crystal structures of Mn oxides, both commercial and lab-made (i.e. MnO2, Mn3O4 and Mn2O3); (ii) a calcium manganese oxide (containing Ca2Mn3O8 and CaMn2O4) and (iii) lanthanum manganites (LaMnO3) prepared via sol-gel (SG) and flame spray pyrolysis (FP). On the other hand, the heterogenization of a homogeneous Co based WOC, i.e. the polyoxometalate Na10[Co4(H2O)2(PW9O34)2] (CoPOM), was investigated, as well. CoPOM is known to be a highly active WOC, comprising an active {Co4O4} core stabilized by oxidatively resistant polytungstate ligands. Transferring its solution reactivity to solid substrates is a fundamental step in the realization of a PEC

A simple model for a complex system: Kinetics of water oxidation with the [Ru(bpy)3]2+/S2O82− photosystem as catalyzed by Mn2O3 under different illumination conditions

The [Ru(bpy)3]2+/persulfate photosystem is the most common dye/sacrificial reagent pair used to study the catalyzed water oxidation half-reaction. Recently, we developed a bubbling reactor along with its modelling, and we used it with the aforementioned photosystem to measure the actual rate of reaction (RO 2) over time. In the present work, the same method is employed to investigate the kinetics of the reaction occurring through several steps, i.e. not only water oxidation, but also parasitic reactions due to chemical instability of the intermediate [Ru(bpy)3]3+ species, which degrade over time finally decreasing the reaction rate. O2 evolution as catalyzed by Mn2O3 is examined at three irradiance conditions, and for three different catalyst contents. Qualitatively, whereas the increase of catalyst amount yields the expected increase of O2 production and evolution rate, the increase in irradiance enhances the degradation processes, thus giving a “paradoxical” effect of decreasing the chemical yields. Chemical kinetics are applied, and predictions are compared to experimental data derived from bubbling-reactor. The development of the kinetic model imposing steady state condition on transient species yields as expression of RO 2 a simple linear combination of two exponentials. It is found that the main activity indicators obtained from the bubbling reactor tests can be related in an expression that is independent of the amount catalytic sites, i.e. the ratio between the O2 evolution rate and the total amount of O2 evolved during the reaction. Results show that the ratio between the kinetic constants of the desired (i.e. O2 formation) and undesired path (i.e. dye degradation) decreases at increasing irradiance, evidencing how the role of parasitic reactions, far from being negligible, tends to be overwhelming

Effect of surface area on the rate of photocatalytic water oxidation as promoted by different manganese oxides

Commercial Mn2O3,Mn3O4 and MnO2 and the same after thermal or ball-milling treatments have been examined as catalysts for the photocatalytic water oxidation reaction, using [Ru(bpy)3]2+ as photosensitizer and S2O8 2- as sacrificial electron acceptor. Tests were performed in a bubbling reactor, allowing the calculation of the actual rate of O2 evolution as a function of time from raw data (oxygen flow, concen- tration of dissolved oxygen, DO) through a model able to take into account mass transfer phenomena Hernández et al. [19]. A few parameters are proposed for measuring activity, and comparison among them is made. The activity per unit mass of commercial samples is Mn2O3 > MnO2>Mn3O4, in agreement with the literature. Increase in the surface area brought about by milling correspond, as expected, to a steady increase in activity in the case of Mn3O4, whereas had no effect with Mn2O3. The markedly higher specific surface of Mn2O3 and Mn3O4 samples obtained by thermal treatment of MnO2 and a home-made sample, respectively, correspond surprisingly to activities lower than low surface area ball milled samples. Reasons for this are proposed to be a different nature of the surfaces arrived at, because of the different preparation route. A similar study of the effect of surface area for MnO2 specimens is prevented by their largely amorphous nature. Comparison of present data with those already reported gives further support to the bounty of the model taking into account mass transfer