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, concentration 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.